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The Seismic Structure of Island Arc Crust
Abstract
Intraoceanic island arcs are considered to be fundamental building blocks of continental crust that are accreted during arc–continent
collision. P wave velocity models derived from wide-angle seismic surveys can constrain the thickness and composition of arc
crust. The variations of P wave velocity with depth of the Aleutian, Izu-Bonin-Mariana, Lesser Antilles, Solomon, South Sandwich,
and Tonga island arcs are compared, and the unextended Aleutian arc is contrasted in detail with the Izu-Bonin-Mariana arc-back-arc
system, which has been variably subject to extension and arc rifting. The Aleutian arc is interpreted to be 35km thick along
much of its eastern section, while close to the volcanic line the Izu arc is 26–35km thick, the Bonin arc 10–22km thick,
and the Mariana arc 16–24km thick, with these variations in thickness primarily related to the amount of extension that has
affected the different segments of the arc. Both wide-angle refraction and normal incidence reflection surveys indicate that
the crust–mantle transition can extend 4–10km beneath the top-Moho reflector used to determine most crustal thicknesses.
At depths greater than 8–10km, i.e., a confining pressure of ~0.2GPa, all surveyed island arcs exhibit higher seismic velocities
than continental crust, and are thus on average more mafic. However, at depths less than 8–10km, P wave velocities in island
arcs generally fall within the broad range of values corresponding to continental crust. These upper crustal velocities are
consistent with the presence of tonalitic rocks, but at shallow depths felsic rocks cannot be readily discriminated from more
mafic rocks with elevated porosity. Nevertheless lateral variations in seismic velocity along the Izu-Bonin arc on the scale
of ~50km can be correlated with the chemistry of arc volcanoes, suggesting a link between seismic velocity, crustal composition,
and the magmatic evolution of the arc. Prior to arc–continent collision, sedimentary rocks derived from the approaching continent
can accumulate across the forearc and in the back-arc basin, and may reach thicknesses as great as 12km.
... While high-pressure melting of basaltic crust was likely the major process of continental crust growth during the Archean (Castillo, 2012;Martin et al., 2005;Moyen and Martin, 2012;Stern and Scholl, 2010), the processes of post-Archean crustal growth involve a significant proportion of crustal recycling (Jagoutz, 2010;Moyen et al., 2021). However, seismic velocity profiles of intra-oceanic island-arc crust revealed low-velocity middle crust supposedly composed of intermediate to felsic igneous rocks (Calvert, 2011). Combined with evidence from exposed ancient island arcs, it is assumed that large portions of siliceous crust can be formed within intra-oceanic arcs, where no continental crust is available to be recycled (DeBari and Greene, 2011;Greene et al., 2006;Kawate and Arima, 1998). ...
... It has been suggested that low-pressure intra-oceanic arc granitoids (LP IOAG) may form large portions of island-arc middle crust worldwide (DeBari and Greene, 2011;Greene et al., 2006;Jagoutz and Kelemen, 2015). Such rocks have been found in the middle crust of several exposed ancient island arcs (DeBari and Greene, 2011;D'Souza et al., 2016;Greene et al., 2006;Kawate and Arima, 1998) and are suspected to be present also in modern island arcs since seismic imaging revealed low-velocity (6.0-6.5 km⋅s − 1 ) P-wave layers in their middle crust (Calvert, 2011;DeBari and Greene, 2011;Leat et al., 2007). Therefore, LP IOAG provide key information for a better understanding of the processes of siliceous crust formation from juvenile (isotopically depleted) sources and therefore contribute to the long-lasting and vigorous debate on the dynamics of Earth's crust growth. ...
... In this respect, intra-oceanic island arcs, where juvenile melts with a negligible assimilation of matured crust are generated, represent a natural laboratory for the study of the prime principle of crustal growth. While the melts that reach the surface are dominantly mafic (Gill, 1981), seismic profiles across active intraoceanic arcs reveal that the middle crust is rather intermediate to felsic in its composition (Calvert, 2011), hence probably composed mainly of tonalitic-trondhjemitic plutons (DeBari and Greene, 2011). As the middle crust is only rarely exposed in modern island arcs, we are reliant on the preserved ancient intra-oceanic arcs. ...
The processes leading to the formation of siliceous (intermediate to felsic) magmatic rocks from juvenile sources within intra-oceanic island arcs govern the formation of continental crust and tempos of crustal growth. To unravel the predominant process responsible for the formation of the intermediate to felsic intra-oceanic arc crust, we performed inverse geochemical modelling using major elements and fluid-immobile trace elements. Subvolcanic trondhjemite from a volcanic arc – Davle volcanic complex (DVC) situated in the Teplá–Barrandian unit, Bohemian Massif – that formed during the Neoproterozoic Avalonian–Cadomian orogeny was used as a proxy for the most felsic endmember of the tonalite–trondhjemite suite. The DVC trondhjemite that follows the calc-alkaline trend is enriched in several fluid-mobile elements such as Cs, Ba and U, but depleted in high field strength elements (Nb, Ti) and lithophile elements (e.g., K, Rb, Sr and Th), and exhibits flat rare earth element (REE) primitive mantle-normalized distributions with slightly negative Eu anomalies. Together with the largely radiogenic Hf-Nd isotopic signatures (εNd + 6.0 to +8.7 and εHf + 10.2 to +11.7), these data indicate formation in an intra-oceanic arc setting with no significant assimilation of older crustal material. In general, the composition of the studied trondhjemite markedly resembles that of other intra-oceanic arc-derived tonalites and trondhjemites worldwide, formed at low pressures below the garnet stability field. These are herein referred to as “low-pressure intra-oceanic arc granitoids” or “LP IOAG”. The performed inverse geochemical modelling revealed that dehydration partial melting or fractional crystallization of a mafic source can produce a primitive tonalitic melt. Consequently, fractional crystallization of that melt accounts for the compositional heterogeneity observed within the tonalite–trondhjemite suite. We conclude that LP IOAG, commonly forming large portions of intra-oceanic arc middle crust, can form in a nearly closed system with a negligible contribution of subduction-related melts or assimilation of older siliceous crust. Therefore, our findings suggest that the formation of LP IOAG within oceanic crust may represent the predominant process of post-Archean crustal growth.
... However, their use as analogues for continental crust generation is controversial due to the fact that seismic studies of island arcs suggest that they are generally dominated by basaltic compositions [Holbrook et al., 1999;Fliedner and Klemperer, 2000;Shillington et al., 2004;Calvert, 2011, Gazel et al., 2015 and where andesitic melts are produced, they are generally depleted in incompatible elements compared to average continental crust estimates [Kelemen et al., 1995;Kelemen et al., 2003;Gazel et al., 2015]. Therefore, in order to elucidate the processes that produce juvenile continental crust in subduction systems, focus must be directed toward arcs where the volcanic output resembles juvenile continental crust produced in the Archean. ...
... These observed variations in seismic velocities between the Miocene Arc and the modern arc are consistent with earlier seismic velocity models from earthquake tomography [Husen et al., 2003]. Furthermore, compilations of seismic velocities in modern intraoceanic arcs [Calvert, 2011], which range in bulk composition from intermediate to mafic, show that the Miocene Arc is similar to other modern volcanic arcs; whereas, the active arc is closer to continental crust estimates. ...
... A) Locations of the 1D velocity-depth profiles. 2) Results of the different tomographic inversions to compare the seismic velocity signatures of the active arc (dark blue), the Miocene Arc (light blue), and the Caribbean oceanic crust (purple) with velocities of continental crust (red; Christensen and Mooney, 1995) and other modern arcs (Calvert, 2011). Note that in the lower crust the Miocene Arc is more similar to other modern oceanic arcs and Caribbean oceanic crust compared to the modern arc. ...
... Some geophysical studies have found evidence for the presence of sizeable bodies of magma in the upper mantle (50–90 km depth) beneath large continental arc volcanoes such as Klyuchevskoy and Katmai (Matumoto, 1971; Utnasin et al., 1975) that provide support for deep level differentiation. However, numerous studies in the Cascades, Tonga and Vanuatu have consistently failed to find any evidence for the existence of magma bodies (see summaries by Calvert, 2011; Iyer, 1984). Moreover, when crustal magma chambers have been observed beneath arc volcanoes, they tend to be small (100s to 1000s m 3 ) and shallow (typically 1–3 km depth) and, by implication, inferred to be largely transitory features (Dvorak and Dzurisin, 1997; Iyer, 1984; Marsh, 1989). ...
... What may be more significant is that 1-D seismic velocity profiles indicate that abundant silicic material (e.g. tonalite with P wave velocities b6.5 km s −1 ) is only present at depths shallower than 9 km in intra-oceanic arcs (Calvert, 2011). Strikingly, comparison of Fig. 1a–c shows that there is a significant decrease in P wave velocities for the depth at which the thermobarometry suggests that the majority of silicic arc lavas have been produced (i.e. ...
... H 2 O will become vapor-saturated. The grey field on (c) encompasses 1D P wave velocity profiles from a broad range of island arcs indicating that voluminous felsic crust only exists at depths shallower than ~8 km (modified from Calvert, 2011). For data sources see Appendix 1. ...
A number of currently popular models for the genesis of evolved arc-magmas (from basaltic andesite to dacite) invoke repeated intrusion, partial-melting and differentiation at the base of the crust. However, several observations suggest that this may be the exception rather than the norm: (1) geobarometry often indicates shallow pressure (0.1-0.3 GPa) evolution; (2) incongruent melting of amphibolite at elevated pressures should yield magmas in equilibrium with high pressure phases like garnet, but rare earth element patterns almost ubiquiously preclude this; (3) compositionally-zoned caldera forming eruptions suggest differentiation at near surface depths; (4) U-series data most commonly indicate differentiation over millennia time-scales. This requires rapid cooling that, in turn, is most easily explained by relatively small magma volumes undergoing crystal fractionation within the shallow (i.e. cool) crust. To further test these ideas, we combined published experimental-data for liquidus equilibria with appropriate silicic arc-magma compositions. On projections of the ternary liquidus system nepheline-silica-olivine, recent data for Tongan silicic lavas plot either on or close to low-pressure (1 atm) cotectics for the rocks’ phenocryst phases, suggesting low-pressure differentiation. Using our own and published data from arc volcanoes around the world we find that the majority are consistent with differentiation at shallow depths, regardless of total crustal thickness. Combined with the typical timescales of differentation, we estimate that the volumes of magma stored during differentiation in shallow crustal zones are usually on the order of only a few km3. There is also a clear role for mixing and recharge that involves magmas that are more deeply-sourced and primitive in character (typically evolved basalts and basaltic andesites). Whether the latter differentiated in the lower-crust or at the crust/mantle boundary has important implications for the constitution and average composition of arc crust. At present, we can only conclude that evidence for more silicic arc-magma generation at these depths is generally lacking.
... Intraoceanic arcs provide modern analogues for the "subduction model" of new continental crust, in this case both subducting and overriding plates have an oceanic origin (Rudnick & Gao, 2003). However, their use as analogues for continental crust generation is controversial due to the fact that seismic studies of island arcs suggest that they are generally dominated by basaltic compositions (Calvert, 2011;Gazel et al., 2015;Holbrook et al., 1999;Shillington et al., 2004) and where andesitic melts are produced, they are generally depleted in incompatible elements compared to average continental crust estimates (Gazel et al., 2015;Kelemen, 1995;Kelemen, Hanghj, et al., 2003;Kelemen, Yogodzinski, et al., 2003). Therefore, in order to elucidate the processes that produce juvenile continental crust in subduction systems, focus must be directed toward arcs where the volcanic output resembles juvenile continental crust produced in the Archean. ...
... These observed variations in seismic velocities between the Miocene Arc and the modern arc are consistent with earlier seismic velocity models from earthquake tomography (Husen et al., 2003). Furthermore, compilations of seismic velocities in Geochemistry, Geophysics, Geosystems modern intraoceanic arcs (Calvert, 2011), which range in bulk composition from intermediate to mafic, show that the Miocene Arc is similar to other modern volcanic arcs, whereas the active arc is closer to continental crust estimates. ...
... Below the Mariana frontal arc seismic velocities range from 5.0 to 6.5 km/s above 12-km depth and increase to 6.6-7.4 km/s between 12 and 20 km and are >7.6 km/s between 20 and 30 km (Calvert et al., 2008). These velocities suggest a more mafic composition for the deep sections of the Christensen & Mooney, 1995) and other modern arcs (Calvert, 2011). Note that in the lower crust the Miocene Arc is more similar to other modern oceanic arcs and Caribbean oceanic crust compared to the modern arc. ...
En la falda suroeste de la cordillera de Talamanca aflora un batolito del Mioceno Superior con composiciones de gabro-dioriticas a graníticas pertenecientes al Grupo Intrusivo Talamanca, el cual es uno de los pocos cuerpos intrusivos del mundo desarrollado en un arco de islas. En general los granitos y granodioritas corresponden a series calco alcalinas hasta alcalinas, mientras que los gabros y las dioritas son más cálcicos. Los intrusivos son metalumínicos a ligeramente peralumínicos y correlacionados a un ambiente tectónico de arco de islas, presentan valores de SiO2 y Mg# entre los rangos de una corteza continental (55-75% y 40-60 respectivamente). El patrón de elementos traza de los granitos y granodioritas es similar al de la corteza continental superior. El Grupo Intrusivo Talamanca se caracteriza por presentar plagioclasas con valores de An entre 12-52% (andesinas y oligoclasas), feldespatos alcalinos clasificados principalmente como sanidina, piroxenos cálcicos, hornblendas enriquecidas en magnesio y micas del tipo biotita. Durante el Mioceno se da un evento volcánico correlacionable temporalmente con la Formación la Cruz, el cual queda registrado en las brechas volcanoclásticas de la Formación Curré, formación en la que se dio el emplazamiento de los intrusivos produciendo metamorfismo de contacto localmente. Los intrusivos se encuentran sobreyacidos por rocas volcánicas andesíticas y basálticas con tendencia adakítica (altas relaciones Sr/Y y empobrecimiento en HREE) que se correlacionan con una fusión directa de la corteza oceánica y son de edad Plioceno. Estas lavas se encuentran compuestas por plagioclasas entre 43-80% de anortita (principalmente bitownitas), piroxenos cálcicos (pigeonitas y augitas) con valores de Wo entre 20-48%, pocos ortopiroxenos tipo enstantita y olivinos enriquecidos en Mg (Fo entre 88,2-89,7%) del tipo crisolita asociados a un origen mantélico. La cordillera de Talamanca ha tenido un intenso levantamiento (1 km/Ma para los últimos 3 Ma), el cual es contrarrestado por una fuerte erosión dando lugar a la depositación de sedimentos al pie de la cordillera, reflejados en la Formación Valle de El General y los abanicos aluviales holocénicos. Estos abanicos aluviales se encuentran deformados por la Falla Buenos Aires, la cual presenta una componente inversa, tiene un rumbo NWW-SEE para la zona y se asocia al régimen de esfuerzos horizontales compresivos producidos por la subducción del levantamiento de Coco.
... Despite these variations, average arc crust has a dominantly mafic composition in all intraoceanic arcs studied to date [Calvert, 2011]. While each arc is unique in the combination of variables that drive lava chemistry, a few specific variables help explain compositional variance and the evolution of island arc material to more continental-like compositions. ...
... While each arc is unique in the combination of variables that drive lava chemistry, a few specific variables help explain compositional variance and the evolution of island arc material to more continental-like compositions. For example, the Izu-Bonin arc, which contains areas of intermediate material, has seen a relatively large amount of extension compared to the Aleutian arc, which has little extension and is dominantly mafic [Calvert, 2011]. Volcano-scale variations of seismic velocity along the Izu-Bonin arc have been attributed to differing stages of continental growth [Kodaira et al., 2007]. ...
... [25] On average, velocities across the Costa Rican volcanic front are at the high-velocity extreme of bulk continental crust and slightly lower than modern island arc velocities (Figure 9). Seismic velocities less than 6.5 km/s characterize crust above a depth of 13 km at the volcanic front, whereas crust in the Izu-Bonin and Aleutian arcs reach average velocities greater than this at depths of only 8-10 km [Calvert, 2011]. Velocities of <6.5 km/s are too low to be gabbroic and are commonly associated with tonalitic/ intermediate compositions [e.g., Calvert, 2011;Kawate and Arima, 1998;Suyehiro et al., 1996]. ...
arcs are proposed to be essential building blocks for the crustal growth
of continents; however, island arcs and continents are fundamentally
different in bulk composition: mafic and felsic, respectively. The
substrate upon which arcs are built (oceanic crust versus large igneous
province) may have a strong influence on crustal genesis. We present
results from an across-arc wide-angle seismic survey of the Costa Rican
volcanic front which test the hypothesis that juvenile continental crust
is actively forming at this location. Travel-time tomography constrains
velocities in the upper arc to a depth of ~15 km where average
velocities are <6.5 km/s. The upper 5 km of crust is constrained by
velocities between 4.0 and 5.5 km/s, which likely represent sediments,
volcaniclastics, flows, and small intrusions. Between 5 and 15 km depth,
velocities increase slowly from 5.5 to 6.5 km/s. Crustal thickness and
lower crustal velocities are roughly constrained by reflections from an
inferred crust-mantle transition zone. Crustal thickness beneath the
volcanic front in Costa Rica is ~40 km with best-fit average
lower-crustal velocities between 6.8 and 7.1 km/s. Overall, velocities
across the arc in central Costa Rica are at the high-velocity extreme of
bulk continental crust velocities and are lower than modern island arc
velocities, suggesting that continental compositions are created at this
location. These data suggest that preexisting thick crust of the
Caribbean Large Igneous Province has a measurable effect on bulk
composition. This thickened arc crust may be a density filter for mafic
material and thereby support differentiation toward continental
compositions.
... In backarc basins that form by arc rifting, each rifting event splits the arc so that fragments of the older arc are carried trenchward, leaving remnant arcs in the backarc region (e.g., in the IBM arc, the Palau-Kyushu Ridge and West Mariana Ridge). As a result, an arc that has undergone arc-normal or longitudinal extension to form a backarc basin will contain thinner crust than does an un-rifted arc (Calvert, 2011). Rearrangement of the arc volcanic front and backarc geometry in this manner can lead to an intraoceanic arc having a sedimentary record (preserved in the forearc) that spans a much longer time interval than the activity of the volcanic arc in its present location (Clift, 1995). ...
... Zagorevski and van Staal (2011) noted that the surface manifestation of these soft and hard collision styles depends considerably on the erosion level of the orogen. Interpreting how substantially collision altered either plate, and with what timing, is challenging not only because of surficial exposure but also because arc crust is inherently variable and complex (Calvert, 2011;DeBari and Greene, 2011), because collision timing and geometry are complicated by promontories and embayments in most margins (Brown et al., 2011b), and because compressive and extensional forces can cause great spatial variation in the collision-zone morphology (Whitmore et al., 1997). Collision timing is almost always diachronous along strike because of oblique convergence directions and non-linear continental margins. ...
... If original backarc-rifted crust were to survive collision and become incorporated into an orogen relatively intact, one might expect to find repetition of arc rocks with similar ages (Calvert, 2011); we are not aware of any such well preserved examples in the geologic record. Even without age repetition, geochemical signatures imply a backarc origin for some ophiolite crust in accreted terranes (such as the Ordovician Solund-Stavfjord ophiolite complex of Norway; (Furnes et al., 2012), with geochemistry indicating the clearest distinction between rifted backarc crust and the arc massif (Hawkins and Melchior, 1985;Pearce and Stern, 2006;Dilek and Furnes, 2011). ...
Records of ancient intraoceanic arc activity, now preserved in continental suture zones, are commonly used to reconstruct paleogeography and plate motion, and to understand how continental crust is formed, recycled, and maintained through time. However, interpreting tectonic and sedimentary records from ancient terranes after arc–continent collision is complicated by preferential preservation of evidence for some arc processes and loss of evidence for others. In this synthesis we examine what is lost, and what is preserved, in the translation from modern processes to the ancient record of intraoceanic arcs.
... Evidence shows that the crust and upper mantle structures of the Mariana subduction zone have been strongly modified by tectonic deformation, including bending-related normal faults (Zhou et al., 2015;Zhou and Lin, 2018), the distribution of earthquakes (Emry et al., 2014;Emry and Wiens, 2015), the serpentinized upper mantle with a thickness of 24 ± 5 km (Cai et al., 2018) in the subducting plate, and serpentinized mud volcanoes (Oakley et al., 2007) and magmatic activities (Calvert, 2011) in the overriding plate. Moreover, a number of wide-angle seismic profiles across the IBM subduction zone have been obtained to detect the variations in the crust and upper mantle structures and to analyze the formation of the subduction zone (Suyehiro et al., 1996;Takahashi et al., 2008Takahashi et al., , 2009Christeson et al., 2016). ...
... Mean 1-D velocity-depth profiles beneath on the overriding plate are also extracted from the forward and inverse models and compared with typical arc crust (Calvert, 2011) (Fig. 12c and 12d). The 1-D velocity-depth curves of the forearc blocks (FA) can fall into the shaded region, indicating an island arc-type crust. ...
... The preservation of such a structure against possible delamination processes is explained by the low density of amphibole gabbros compared to mantle peridotites and pyroxenites (cf. Figure 5c). The seismic signature of the IVZ + upper IGB section is consistent with that of the mafic oceanic crust (Calvert, 2011), that of the continental and oceanic paleo-island arcs (Behn & Kelemen, 2006;Guo et al., 2020;Jagoutz & Behn, 2013;Tibaldi et al., 2013), and that of embryonal continental crust produced at volcanic arcs (Shillington et al., 2004) (Figure 5b). But the pyroxene hornblendites contain sufficient amounts of garnet and clinopyroxene to contribute to the positive gravity anomaly ( Figure 2) and could represent the hidden cumulate fraction of the IVZ sequence (∼32 km thick; Figures 6 and 8), in line with experimental observations of cumulate line of descent produced by fractional crystallization of hydrous magmas Ulmer et al., 2018). ...
... Data are based on: (I) laboratory constraints for gabbro (0-3 km), serpentinized harzburgite, pyroxenite, and lherzolite (3-15 km), and thermodynamic calculations for (II) anhydrous ultramafic rocks, (III) anhydrous gabbros, and (IV) hydrous lherzolite and gabbros of the IVZ (3-50 km). V P data of the Aleutian arc(Shillington et al., 2004), average of oceanic arcs(Calvert, 2011), and oceanic and continental paleo-arc systems(Behn & Kelemen, 2006;Guo et al., 2020;Jagoutz & Behn, 2013;Tibaldi et al., 2013) are also reported. Brown bars indicate values as uncertainties in V P model grid points (every 15 km of depth) ofDiehl et al. (2009). ...
One of the few near‐complete continental crustal sections exposed on Earth's surface is the Ivrea‐Verbano Zone (Western Alps, Italy), which is considered as a petro‐geophysical reference of the continental lithosphere. Exposed peridotite slivers embedded in lower crustal rocks at the surface and large density, seismic velocity anomalies of the Ivrea Geophysical Body in the sub‐surface suggest that mantle‐like rocks are located as shallow as a few kilometres depth, but the actual composition of the rocks producing these anomalies is unknown. Here we investigate how the published seismological and new gravimetric data in the location of Valsesia could be reconciled with petrologic data and models of the Ivrea‐Verbano Zone. We use the Perple_X software to calculate densities and compressional wave velocities for a range of possible deep crustal rock types. We argue that amphibole gabbros (< 18 km depth) and pyroxene hornblendites (> 18 km depth) provide the best fit to the joint geophysical and petrologic constraints, whereas residual ultramafic rocks and anhydrous gabbros are inconsistent with the existing data. This indicates that the Ivrea Geophysical Body beneath the Valsesia area in the Ivrea‐Verbano Zone preserves the structure of an igneous complex formed during magmatic underplating from the crystallization of hydrous mafic magmas. This would imply melting of a damp mantle source that produced a continental crust of an original thickness of up to ∼48 km in the Permian, of which ∼30 km are exposed at Earth's surface today.
... The calculated seismic properties of the Gangdese arc deep crust agree with those of the averaged continental crust (Fig. 13) (Christensen and Mooney 1995). Additionally, the P-wave speeds (V P ) of the Gangdese arc deep crust are lower than that of active oceanic arcs (Fig. 13) (Calvert 2011). The Wolong granites have constant V P of ~ 6.25 km/s and density of ~ 2.70 kg/m 3 from ~ 17 to 21 km depth (Fig. 13). ...
... Mineral density and seismic velocity are calculated using pressures derived from geothermobarometry of the observed mineral compositions and temperature calculated along steady-state 60 mW/ m 2 geotherm. Comparison of calculated V P profiles with the seismic wave velocities for the average continental crust (Christensen and Mooney 1995), oceanic arc crust (Calvert 2011), and calculated velocities depth-structures of the Kohistan (Jagoutz and Behn 2013), Talkeetna (Behn and Kelemen 2006), and Famatinian arcs (Tibaldi et al. 2013) indicating that the Late Cretaceous Gangdese arc lower crust underwent significant density sorting. ...
Arc lower crust plays a critical role in processing mantle-derived basaltic melts into the intermediate continental crust, yet can only be studied indirectly or in exposed arc sections. Compared with the relatively well-studied oceanic arc sections (e.g., Kohistan and Talkeetna), the composition and formation mechanisms of continental arc lower crust remain less clear. Here we present a geochronological and geochemical study on the Lilong Complex and the Wolong granitoids from the Gangdese arc deep crustal section in southern Tibet. The Lilong Complex is composed of the early (85–95 Ma) mafic-intermediate sequence and late (85–86 Ma) ultramafic sequence. The Lilong crustal section exposed crustal depth extending from ~ 42 to 17 km based on the geobarometry. The mafic-intermediate sequence is a damp (low H2O) igneous differentiation sequence characterized by the subsequent appearance of pyroxene → plagioclase → amphibole → biotite. The ultramafic sequence represents a wet igneous differentiation sequence composed of olivine → pyroxene → amphibole → plagioclase. The 74–84 Ma Wolong granitoids were formed by fractional crystallization of wet magma and intra-crustal assimilation. Calculated seismic properties of the Gangdese deep arc crust are comparable to the average continental crust at a similar depth. The average composition of the Gangdese arc lower crust is basaltic andesite with SiO2 of ~ 54 wt%. The highly incompatible elements in the Gangdese arc lower crust are systematically higher than those of the oceanic arc and are comparable with the estimates of lower continental crust, suggesting continental arc magmatism significantly contributes to the formation of continental crust.
... However, some oceanic island arcs are built on fragments of continental crust, most notably Japan, and can eventually be-come accreted terranes, and those special cases are included. Island arc chains are geographically curvilinear, spanning hundreds of kilometers along strike and about 100 km in width (Calvert, 2011). The topography of island arcs is quite striking, with the elevation rising from sea floor to sometimes a couple of kilometers above sea level over just 10 or 20 km distance. ...
... There is a noticeable variation in crustal thickness and structure of modern island arcs between arc systems and even along strike within arc systems (Calvert, 2011) (Fig. 3), which can be attributed to the level of maturity in arc crustal evolution , the amount of back arc extension (Nishizawa et al., 2007), and the magmatic production rate (Christeson et al., 2008). Mature island arc systems, such as the Izu-Bonin-Mariana system, have three crustal layers which were developed by partial melting of the initial immature basaltic arc crust . ...
Allochthonous accreted terranes are exotic geologic units that originated from anomalous crustal regions on a subducting oceanic plate and were transferred to the overriding plate by accretionary processes during subduction. The geographical regions that eventually become accreted allochthonous terranes include island arcs, oceanic plateaus, submarine ridges, seamounts, continental fragments, and microcontinents. These future allochthonous terranes (FATs) contribute to continental crustal growth, subduction dynamics, and crustal recycling in the mantle. We present a review of modern FATs and their accreted counterparts based on available geological, seismic, and gravity studies and discuss their crustal structure, geological origin, and bulk crustal density. Island arcs have an average crustal thickness of 26 km, average bulk crustal density of 2.79 g cm-3, and three distinct crustal units overlying a crust-mantle transition zone. Oceanic plateaus and submarine ridges have an average crustal thickness of 21 km and average bulk crustal density of 2.84 g cm-3. Continental fragments presently on the ocean floor have an average crustal thickness of 25 km and bulk crustal density of 2.81 g cm-3. Accreted allochthonous terranes can be compared to these crustal compilations to better understand which units of crust are accreted or subducted. In general, most accreted terranes are thin crustal units sheared off of FATs and added onto the accretionary prism, with thicknesses on the order of hundreds of meters to a few kilometers. However, many island arcs, oceanic plateaus, and submarine ridges were sheared off in the subduction interface and underplated onto the overlying continent. Other times we find evidence of terrane-continent collision leaving behind accreted terranes 25-40 km thick. We posit that rheologically weak crustal layers or shear zones that were formed when the FATs were produced can be activated as detachments during subduction, allowing parts of the FAT crust to accrete and others to subduct. In many modern FATs on the ocean floor, a sub-crustal layer of high seismic velocities, interpreted as ultramafic material, could serve as a detachment or delaminate during subduction.
... Recently, Calvert (2011) reviewed island arc seismic velocity models and discussed two examples as evolutionary end members: the Aleutian arc, initiated in the Eocene and not significantly affected by extension and rifting, and the Izu-Ogasawara (Bonin)-Mariana (IBM) arc-backarc system, also initiated in the Eocene but variably affected by localized extension and two episodes of arc rifting and backarc spreading. Calvert (2011) divided both arc crusts into three parts based on their Vp and crustal thickness proportions and showed that their middle and lower crusts have higher seismic velocities than those of typical continental crust. Moreover, the Aleutian middle crust has Vp of 6.5 to 7.3 km/s, which is significantly higher than that of the IBM middle crust (6.0 to 6.8 km/s). ...
... The Vp models of the Daito Ridges show a closer match to those of the IBM arc and Kyushu-Palau Ridge, which are immature paleo-island arcs, than they do to those of the mature Japan island arc. Moreover, the velocities of the middle and lower crusts are not as high as those of the Aleutian arc, which may be a result of extension and rifting in this region (Calvert 2011). Future studies on S wave velocity structure are expected to provide more information about the material properties of these paleo-island arcs. ...
Three large bathymetric highs (from north to south: the Amami Plateau, the Daito Ridge, and the Oki-Daito Ridge) originating from paleo-island arcs characterize the northwestern end of the Philippine Sea plate. We obtained 10 seismic refraction and multi-channel seismic reflection profiles across and along these bathymetric highs and obtained P wave velocity (Vp) models of the crust and the uppermost mantle. Although there are large variations in the crustal structure throughout this region, these bathymetric highs usually have a middle crust with Vp of 6.3 to 6.8 km/s, a lower crust with Vp of 6.8 to 7.2 km/s, a Pn velocity of 7.6 to 7.8 km/s, and a total crustal thickness of 15 to 25 km. These features are similar to those of the Izu-Ogasawara (Bonin)-Mariana island arc and the Kyushu-Palau Ridge, which are immature paleo-island arcs. However, the crust at the southwestern part of the Oki-Daito Ridge contains a relatively thin middle crust and a smaller total crustal thickness compared with other ridges in this region. In addition, we identified a deep reflector beneath the ridge, with these properties indicating a different origin, such as intraplate volcanism.
... Recently, Calvert (2011) reviewed island arc seismic velocity models and discussed two examples as evolutionary end members: the Aleutian arc, initiated in the Eocene and not significantly affected by extension and rifting, and the Izu-Ogasawara (Bonin)-Mariana (IBM) arc-backarc system, also initiated in the Eocene but variably affected by localized extension and two episodes of arc rifting and backarc spreading. Calvert (2011) divided both arc crusts into three parts based on their Vp and crustal thickness proportions and showed that their middle and lower crusts have higher seismic velocities than those of typical continental crust. Moreover, the Aleutian middle crust has Vp of 6.5 to 7.3 km/s, which is significantly higher than that of the IBM middle crust (6.0 to 6.8 km/s). ...
... The Vp models of the Daito Ridges show a closer match to those of the IBM arc and Kyushu-Palau Ridge, which are immature paleo-island arcs, than they do to those of the mature Japan island arc. Moreover, the velocities of the middle and lower crusts are not as high as those of the Aleutian arc, which may be a result of extension and rifting in this region (Calvert 2011). Future studies on S wave velocity structure are expected to provide more information about the material properties of these paleo-island arcs. ...
Three large bathymetric highs characterize the northwestern end of the
Philippine Sea plate. They are the Amami Plateau, Daito Ridge and
Oki-Daito Ridge from north to south and their origins seem to be
paleo-island arcs and/or intraplate volcanoes. We carried out ten
seismic refraction and multi-channel seismic (MCS) reflection profiles
across and along these bathymetric highs and obtained P-wave velocity
(Vp) models of the crust and uppermost mantle. In each investigation in
2004-2006, we shot a tuned airgun array with a volume of 8,040 cubic
inches (132 liters) at an interval of 200 m (90 sec) for the wide-angle
seismic profiles and at 50 m for the MCS (480 channels, 60 folds)
profiles. Ocean bottom seismographs (OBSs) were deployed at an average
interval of 5 km. The total number of the OBSs amounted to 1300 and
profile length was about 6300 km for the ten profiles. The dense and
high quality OBS data were modeled by a tomographic inversion,
two-dimensional ray tracing and comparison with synthetic seismograms.
The crustal structure of the Amami Plateau varies in horizontal
direction, especially in upper crust. The mid crust with Vp=6.0-6.8 km/s
in the east of the plateau has a thickness of about 4 km, rather thicker
than that in the west with deeper water depths. Vp of the lower crust is
6.8-7.2 km/s. Travel times of PmPs give the maximum depth of the Moho of
19 km beneath the plateau. Pn velocity is about 7.6 km/s, significantly
slower than 8 km/s of a typical oceanic uppermost mantle. The
characteristic of the Daito Ridge crust can be divided into the north
and south of the ridge. The materials with Vp <=6.3 km/s and a
thickness of 6-11 km exist in the southern part, while the materials
with Vp >=6.3 km/s ascend to near the seafloor in the northern area.
This feature may show the evidence for rifting between the Daito Ridge
and Amami Plateau. The Moho depth increases from north to south and the
maximum crustal thickness is about 20 km in the western part of the
ridge with a wide shallow-water area. The lower curst and upper mantle
velocities are 6.8-7.2 and 7.6-7.9 km/s, respectively. The Oki-Daito
Ridge in the broad sense consists of three parts, the Oki-Daito Rise,
Oki-Daito Plateau and Oki-Daito Ridge in the narrow sense. The Oki-Daito
Plateau has the thickest crust of 20-25 km among them. The crust shows a
structure similar to that of the Daito Ridge. On the other hand, the
Oki-Daito Rise at the southwestern part of the ridge has very thin mid
crust and the crustal thickness is ~10 km, which is different from not
only the Daito Ridge but also a typical oceanic crust. Deep reflector
estimated at around 50 km below the sea bottom may correspond to
intraplate volcanic activities in the Oki-Daito Rise area. The Oki-Daito
Ridge in the narrow sense shows a narrower ridge topography compared to
the other two parts, but has a thick crust of 20-23 km.
... These regions represent key locations for the creation and destruction of continental crust, due to the interaction of middle to lower crust with the upper mantle [Rudnick and Fountain, 1995]. Recent studies admit the importance of seismic velocity profiles of crust to mantle sections, in particular to clarify seismic imaging of active island arcs [i.e., Suyehiro et al., 1996;Holbrook et al., 1999;Kitamura et al., 2003;Kodaira et al., 2007aKodaira et al., , 2007bTakahashi et al., 2008;Calvert et al., 2008;Brown et al., 2009;Kono et al., 2009;Calvert, 2011;Calvert and McGeary, 2012]. On the one hand, it is imperative to know the physical and chemical signature of the different rocks and fluids of island arc systems. ...
... The Kohistan paleo-island arc represents a rather thick arc, with a lower crustal arc root deeper than 30 km . The lower crust of modern thick arcs, like the Aleutian and the northern Izu-Bonin arcs, can be up to 15 km thick [Miller and Christensen, 1994;Holbrook et al., 1999;Shillington et al., 2004;Kodaira et al., 2007aKodaira et al., , 2007bCalvert, 2011;Calvert and McGeary, 2012]. Shallow arc roots at depths <25 km are found in the southern Izu-Bonin-Mariana arc system, where the lower crust is >10 km thick . ...
[1] P-wave velocities (Vp) have been measured in the laboratory and calculated using thermodynamic modeling for seven representative rock samples from the lower crust to mantle section of the Kohistan paleo-island arc. Lower crustal rocks comprise plagioclase-rich gabbro, garnet-bearing gabbro, and hornblendite; mantle rocks comprise garnetite, pyroxenite, websterite, and dunite. Measurements were performed at confining pressures up to 0.5 GPa and temperatures up to 1200°C. Vp were also calculated using rock major element chemistry with the Perple_X software package. Calculated Vp match closely the laboratory measurements. At depths representative for the arc root, Vp of upper mantle rocks vary from 7.7–8.1 km/s, whereas the lower crustal rocks have velocities between 6.9–7.5 km/s. P-wave anisotropy is small, with exceptions of sheared gabbros. Measured and calculated seismic properties are consistent with, and complement a growing database of published seismic properties from the Kohistan arc. In the light of such data, we discuss seismic imaging of present-day island arcs. Intermediate Vp (7.4–7.7 km/s) in arc roots can be explained by pyroxenites and garnet-bearing mafic rocks. Strong seismic reflectors may be related to garnetites (8.0–8.2 km/s).
... Also, there are orogenic gold deposits such as the Kalatongke, Tokuzibayi and Donbastau copper-nickel (Christensen and Mooney, 1995): 1, continental arcs; 2, global average for continent; 3, orogens; 4, extended crust; 5, shields and platforms; 6, rifts. The grey shaded area represents the range of velocities inferred in island arcs from wide-angle seismic surveys (Calvert, 2011). ...
Altaids in the Central Asian Orogenic Belt (CAOB) is one of the world’s largest orogenic belts containing mineral deposits. Together with the Junggar terrain they open an important window to study the Paleozoic tectonic evolution of the CAOB. In this paper, we analyze a 637-km-long wide-angle refraction/reflection seismic profile across the Altai-Eastern Tianshan orogenic belt in the southern Altaids, conducted in September 2018 using 10 large explosive charges fired in drilled holes. We use a traveltime inversion method to reconstruct the lithospheric P-wave velocity structure along the profile. The lithosphere is composed of a 43-55-km-thick crust, a ∼10-km-thick crust-mantle transition layer beneath the Altai Mountain, and a ∼25-km-thick layer of lithospheric mantle. The results clearly reveal: a prominent Moho uplift beneath the Yemaquan Island Arc, two major crustal-scale low-velocity anomalies (LVAs) beneath the Yemaquan Arc and Bogda Mountain, and three high-velocity anomalies (HVAs) near the surface around the Kalatongke, Yemaquan and Kalatage mining areas. We hypothesize that the subduction of the Paleo-Asian Ocean occurred with strong mantle upwelling. We suggest that continued compression of the Paleo-Asian Ocean causes the delamination of lithosphere, as well as asthenospheric material upwelling and magma underplating into the crust. Consistently, Paleozoic mafic-ultramafic rocks and mantle-derived minerals related to gold, copper and nickel deposits, are widely extended in the area. Our results show that the P-wave velocity-depth curves for deeper depths (>30 km) in the southern Altai and Junggar Basin are close to those of the continental arcs and global continent average. Despite powerful Paleozoic subduction activity, orogeny and volcanism strongly modified the lower crust in the region, part of ancient continental crust was still preserved below the southern Altai and Junggar Basin. In addition, the upper part (depth 5–30 km) of the velocity-depth curve for the Junggar Basin is close to that of the Costa Rica volcanic front and the British Columbia accreted terrain, suggesting that Paleozoic orogenic activity has intensively reconstructed the upper-middle crust beneath the Junggar Basin.
... avoid problems in distinction between juvenile and pre-extensional oceanic crust. While the estimates depend on the assumed thickness of pre-extensional crystalline basement, its variations in normal oceans and island arcs are usually small (White et al., 1992;Calvert, 2011), but the adopted thickness of 30 km for the CO-type BABs is a strong generalization since extended continental crust may be highly variable in basement thickness. ...
This global study of 31 off-shore back-arc basins and subbasins (BABs) identifies their principal characteristics
based on a broad spectrum of geophysical and subduction-related parameters. My synthesis is used to identify
trends in the evolution of back-arc basins for improving our understanding of subduction systems in general. The
analysis, based on the present plate configuration, demonstrates that geophysical characteristics and fate of the
back-arc basins are essentially controlled by the tectonic type of the overriding plate, which controls the lithosphere
thermo-compositional structure and rheology. The type of the plate governs the length of the extensional
zone in back-arc settings along the trench, the efficiency of lithosphere stretching, and the crustal structure,
buoyancy and bathymetry of the BABs. Subduction dip angle apparently controls the location of the slab melting
zone and the efficiency of slab roll-back with feedback links to other parameters. By the tectonic nature of the
overriding plate (the downgoing plate is always oceanic) the back-arc basins are split into active BABs formed by
ocean-ocean, arc-ocean, and continent-ocean convergence, and extinct back-arc basins. By geophysical characteristics,
BABs formed on continental plates are subdivided into active BABs with and without seafloor spreading,
and extinct BABs are subdivided into the Pacific BABs, possibly formed on oceanic plates, and the non-Pacific
BABs with reworked continental or arc fragments.
Six types of BABs are distinctly different. Extension of the overriding oceanic plate above a steeply dipping old
oceanic plate, preferentially subducting nearly westwards, forms large deep back-arc basins with a thin oceanictype
crust. In contrast, BABs on the overriding continental or arc plates form at small opening rates and often by
shallow subduction of younger oceanic plates with a random subduction orientation; these BABs have small
sizes, shallow bathymetry, and hyperextended or transitional ~20 km thick arc- or continental-type crust typical
of passive margins. The presence of a 2–5 km thick high-Vp lowermost crustal layer, characteristic of BABs in all
settings, indicates the importance of magmatic underplating in the crustal growth.
Conditions required for the initiation of a back-arc basin and transition from stretching to seafloor opening depend
on the nature of the overriding plate. BABs formed on oceanic plates always evolve to seafloor spreading.
BABs formed on continental or arc plates require long spreading duration with large (>8 cm/y) opening rates
and a large crustal thinning factor of 2.8–5.0 to progress from crustal extension to seafloor spreading; on the present
Earth such transition does not happen in the back-arc basins formed behind a shallow subduction (<45o) of
a young (<40 My) oceanic plate.
The nature of the overriding plate also determines the fate of back-arc basins after termination of lithosphere
extension: the extinct Pacific back-arc basins with oceanic-type crust evolve towards deep old “normal” oceans,
while the shallow non-Pacific BABs with low heat flow and thick crust are likely to preserve their continental or
arc affinity. BABs do not follow the oceanic cooling plate model predictions. Distinctly different geophysical signatures
for mid-ocean ridge spreading and for back-arc seafloor spreading are caused by principally different dynamics.
... The remnant intra-oceanic arc is a rectangular box 210 km wide, which is the mean width for modern intra-oceanic arcs (Calvert, 2011;Stern, 2010). The thermal structure and initial strength profile of the remnant intra-oceanic arc are set the same as the host oceanic subducting crust-lithosphere (Figure 2c), as done by Leng and Gurnis (2015). ...
We investigate how the mechanical properties of intra-oceanic arcs affect the collision styleand associated stress-strain evolution with buoyancy-driven models of subduction that accurately reproducethe dynamic interaction of the lithosphere and mantle. We performed a series of simulations only varyingthe effective arc thickness as it controls the buoyancy of intra-oceanic arcs. Our simulations spontaneouslyevolve into two contrasting styles of collision that are controlled by a 3% density contrast between the arcand the continental plate. In simulations with less buoyant arcs (15–31 km; effective thickness), we observearc-transference to the overriding plate and slab-anchoring and folding at the 660 km transition zone thatresult in fluctuations in the slab dip, strain-stress regime, surface kinematics, and viscous dissipation. Afterslab-folding occurs, the gravitational potential energy is dissipated in the form of lithospheric flow causinglithospheric extension in the overriding plate. Conversely, simulations with more buoyant arcs (32–35 km;effective thickness) do not lead to arc-transference and result in slab break-off, which causes an asymptotictrend in surface kinematics, viscous dissipation and strain-stress regime, and lithospheric extension in theoverriding plate. The results of our numerical modeling highlight the importance of slab-anchoring and foldingin the 660 km transition zone on increasing the mechanical coupling of the subduction system.
... Faster velocities (up to 6-16 cm/yr) are only observed in narrow slabs (≤1,500 km) with curved, convex geometries, such as the Oligocene to Holocene Scotia and Calabrian subduction zones. Additionally, three-dimensional numerical modeling of continental ribbons with horizontally stratified lithosphere (as observed in arc crust; Calvert, 2011) show that horizontal-axis folding and thickening of the lithosphere is more probable than vertical-axis rotation of an orocline (Smith et al., 2021). Thus, it is improbable that the Stikinia subduction zone was subject of wholesale angular rotation in the Early Jurassic, as implied in some recent models Logan & Mihalynuk, 2014). ...
End‐on arc collision and onset of the northern Cordilleran orogen is recorded in Late Triassic to Jurassic plutons in the Intermontane terranes of Yukon, and in development of the synorogenic Whitehorse trough (WT). A synthesis of the extensive data set for these plutons supports interpretation of the magmatic and tectonic evolution of the northern Intermontane terranes. Late Triassic juvenile plutons that locally intrude the Yukon‐Tanana terrane represent the northern extension of arc magmatism within Stikinia. Early Jurassic plutons that intrude Stikinia and Yukon‐Tanana terranes were emplaced during crustal thickening (200–195 Ma) and subsequent exhumation (190–178 Ma). The syn‐collisional magmatism migrated to the south and shows increasing crustal contributions with time. This style of magmatism in Yukon contrasts with coeval, juvenile arc magmatism in British Columbia (Hazelton Group), that records southward arc migration in the Early Jurassic. Exhumation and subsidence of the WT in the north were probably linked to the retreating Hazelton arc by a sinistral transform. East of WT, Early Jurassic plutons intruded into Yukon‐Tanana record continued arc magmatism in Quesnellia. Middle Jurassic plutons were intruded after final enclosure of the Cache Creek terrane and imbrication of the Intermontane terranes. The post‐collisional plutons have juvenile isotopic compositions that, together with stratigraphic evidence of surface uplift, are interpreted to record asthenospheric upwelling and lithospheric delamination. A revised tectonic model proposes that entrapment of the Cache Creek terrane was the result of Hazelton slab rollback and development of a sinistral transform fault system linked to the collision zone to the north.
... Coeval andesitic flows and volcanosediments that are interbedded with immature arc-derived sediments and detrital continental material eroded from the Saharan meta-craton, relying on field observations ( Fig. 1-9; Perret et al., 2021). Such an architecture is also well-known in both modern-day and paleo-island arcs (e.g., Calvert, 2011). The geometry of the system likely led to high rate of sedimentation and diversity in the nature of sediment influx, as suggested by previous studies (Abdelrahman, 1993;Bailo et al., 2003). ...
Gold is a siderophile element preferentially concentrated in the Earth’s mantle and core. The understanding of mechanisms leading to its transfer towards the crust requires the study of the geodynamical evolution of juvenile crust portions, i.e., crystallized from melt directly extracted from the mantle. The gold mineral system thus combines the deciphering of crustal fertilization geodynamical processes and subsequent transient tectono-metamorphic setting(s) favorable for remobilization of this metal-enriched reservoir and formation of mineralized occurrences. This approach is applied to the Keraf and Atmur-Delgo sutures which are part of the Arabian-Nubian shield, the world-largest track of Neoproterozoic juvenile crust and one of the main Pan-African gold provinces. Although these structures are considered as insightful windows into the assembly of
Gondwana in Sudan and host major gold deposits, their coupled geodynamical-metallogenic evolution remains poorly understood.
We report the existence of two magmatic flare-ups with suprachondritic Hf and Nd signals, recording the build-up and maturation of island arcs along these suture zones between 840-810 Ma and 760-655 Ma. The ~185 Myr-long lifespan of the island arc described along the Keraf suture and its predominant juvenile nature likely accounted for crustal gold fertilization at the regional scale. The earliest crustal growth stage, only recorded along the Keraf suture, is coeval to a minor magmatic-hydrothermal gold event. Both the Keraf and Atmur-Delgo sutures keep record of the second island arc flare-up.
The matching between field and laboratory data exemplifies the district- to microscopic-scale spatial continuity of structural control on later gold-bearing structures described in this study. On one hand, a gold event occurred between 755-725 Ma, during high-strain progressive deformation expressed by sheath folding and the formation of linear ore shoots under lower amphibolite facies metamorphism. It likely relates to tectonic accretion of the sedimentary wedge at the Atmur-Delgo intra-oceanic subduction site. At the microscopic scale, the ore formation involved the syn-metamorphic remobilization of the gold budget contained in earlier sulfide generations. On another hand, several orogenic gold deposits display intrusion-hosted mineralized extension veins, highlighting
a strong rheological control on ore deposition. The fault-valve mechanism induced the formation of mineralized veins, triggered by episodic and localized reactivation of crustal strike-slip shears within the Keraf suture zone until 550 Ma. Gold events described along the Atmur-Delgo and Keraf sutures are the oldest vein-type gold and youngest orogenic gold episodes reported throughout the Arabian- Nubian shield to this day, respectively.
The mineral system approach therefore enables district-scale ore targeting by translating magmatic processes and tectono-metamorphic settings into environments and geological features (e.g., relics of an island arc and/or an accretionary wedge, presence of late collisional strike-slip shears) favoring the formation of gold occurrences with very distinct structural, geochemical and timing characteristics in the western Arabian-Nubian shield. This alternative view of ore deposits sharply contrasts with the typological strategy which targets a single class of ore occurrence with an effectiveness limited to the deposit scale.
... The remnant intra-oceanic arc is a rectangular box 210 km wide, which is the mean width for modern intra-oceanic arcs (Calvert, 2011;Stern, 2010). The thermal structure and initial strength profile of the remnant intra-oceanic arc are set the same as the host oceanic subducting crust-lithosphere (Figure 2c), as done by Leng and Gurnis (2015). ...
Arc-continent collision is the process by which intra-oceanic arc crust is accreted to continental margins and the most important mechanism that enables the growth of the continental crust since Phanerozoic times. We use numerical visco-plastic mechanical models to explore how crustal-mantle dynamics control the evolution of the stress regime in continental margins during arc-continent collision. We performed a series of simulations only varying the thickness of the arc as it has been suggested to control the density profile and rheology of intra-oceanic arcs and therefore the dynamics of collision. Modelling results show that arc-continent collision can evolve into two contrasting mechanisms: i) arc transference in thin arcs (15-31km in thickness); and ii) slab break-off in thick arcs (32-35 km in thickness). In turn, these two contrasting mechanisms trigger the partition of stress into extension in the continental margin and compression towards the subducting plate. We interpret that the partitioning in stress into compression and extension in all simulations is caused by a gravity-driven flow, which equilibrates the contrasts in gravitational potential energy (GPE) stored in the lithosphere during collision and episodes of lithospheric thickening. We argue that this gravity-driven flow applies a horizontal gravitational force (body force) directed from the collided arc towards the subducting plate (compressional) and the continental margin (extensional). Finally, we conclude that the large-scale mantle return flow emerged from slab-anchoring facilitates the stress partitioning by enhancing: i) compression and lithospheric thickening; and ii) the contrast in GPE between the accreted arc and the continental margin during collision. In the particular case of thin arc simulations, slab-anchoring also promotes further compression-thickening during post-collision and further storage of GPE.
... Although the crustal velocity structure is variable in magmatic arc settings, a high-velocity lower crust is a common characteristic (Figure 5b; Calvert, 2011;Contreras-Reyes et al., 2011;Shillington et al., 2004). Our observations of the velocity structure and the MCS data, together with the affinity of basement samples as well as the subduction context inferred from tomographic models, agree with a magmatic arc type crust flooring the EAB (Figures 5h and 7) (Booth-Rea et al., 2007Duggen et al., 2008;Gómez de la Peña et al., 2018;Hoernle et al., 1999). ...
In continental settings, seismic failure is generally restricted to crustal depth. Crustal structure is therefore an important proxy to evaluate seismic hazard of continental fault systems. Here we present a seismic velocity model across the Gibraltar Arc System, from the Eurasian Betics Range (South Iberian margin), across offshore East Alboran and Pytheas (African margin) basins, and ending onshore in North Morocco. Our results reveal the nature and configuration of the crust supporting the coexistence of three different crustal domains: the continental crust of the Betics, the continental crust of the Pytheas Basin (south Alboran Basin) and onshore Morocco, and a distinct domain formed of magmatic arc crust under the East Alboran Basin. The magmatic arc under the East Alboran Basin is characterized by a velocity structure containing a relatively high‐velocity lower crust (~7 km/s) bounded at the top and base by reflections. The lateral extension of this crust is mapped integrating a second perpendicular wide‐angle seismic profile along the Eastern Alboran basin, together with basement samples, multibeam bathymetry, and a grid of deep‐penetrating multichannel seismic profiles. The transition between crustal domains is currently unrelated to extensional and magmatic processes that formed the basin. The abrupt transition zones between the different crustal domains support that they are bounded by crustal‐scale active fault systems that reactivate inherited structures. Seismicity in the area is constrained to upper‐middle crust depths, and most earthquakes nucleate outside of the magmatic arc domain.
... The 4.5-6.5 km/s V P of the western Raukumara Peninsula is consistent with the Mesozoic Torlesse graywacke (e.g., Christensen & Okaya, 2007). P wave velocities of the lower crust beneath the Bay of Plenty increase from 6.5 to 7.1 km/s, which is ∼0.5 km/s faster than global averages of continental crust at 15-to 20-km depth (Christensen & Mooney, 1995) and slower than oceanic island arcs (e.g., Calvert, 2011). Similar lower crust V P is observed in the Ryukyu arc, where extended continental crust is intruded by arc magmas (Arai et al., 2018). ...
Relationships between extensional tectonics and magmatism are ubiquitous in continental
rifts and oceanic spreading centers. Yet few studies document interactions between extensional faults and
mantle melts in volcanic arcs. We constrain the crustal structure of the extensional offshore Taupo
Volcanic Zone (TVZ) from a marine multichannel and wide-angle seismic experiment. The TVZ crust
thins from >26 km to ∼18–19 km across ∼50 km in the Bay of Plenty. Elevated P wave velocities in the
lower crust indicate mafic additions. Magmatic sills between 4- and 15-km depth lie beneath listric normal
faults in a ∼40-km-wide active rift zone. P wave velocities in the middle and upper crust along the arc front
are ∼0.3–0.5 km/s slower than in the adjacent crust, indicating a possible thermal anomaly imparted by
heat from magmatic intrusions. We propose that rifting in the offshore TVZ is partially compensated by
intrusions and assisted by thermal weakening of the lithosphere.
... The 1-D velocity-depth profiles extracted from the final model at selected M.D. (a) Comparison with velocity values of other island arcs(Calvert, 2011). The observed velocity profiles of our study area tend to fall outside the velocity envelope of other island arcs. ...
The southernmost Mariana margin lacks a mature island arc and thus differs significantly from the central-north Mariana and Izu-Bonin margins. This paper presents a new P-wave velocity model of the crust and uppermost mantle structure based on a 349-km-long profile of wide-angle reflection/refraction data. The active source seismic experiment was conducted from the subducting Pacific plate to the overriding Philippine plate, passing through the Challenger Deep. The subducting plate has an average crustal thickness of ~6.0 km with Vp of 7.0±0.2 km/s at the base of the crust and low values of only 5.5–6.9 km/s near the trench axis. The uppermost mantle of the subducting plate is characterized by low velocities of 7.0–7.3 km/s. The overriding plate has a maximum crustal thickness of ~18 km beneath the forearc with Vp of ~7.4 km/s at the crustal bottom and 7.5–7.8 km/s in the uppermost mantle. A zone of slight velocity reduction is imaged beneath the Southwest Mariana Rift (SWMR) that is undergoing active rifting. The observed significant velocity reduction in a near-trench crustal zone of ~20-30 km in the subducting plate is interpreted as a result of faulting-induced porosity changes and fracture-filling fluids. The velocity reduction in the uppermost mantle of both subducting and overriding plates is interpreted as mantle serpentinization with fluid sources from dehydration of the subducting plate and/or fluid penetration along faults.
... The calculated depths of formation for the gabbros range from 21 to 26 km while a depth of 15-32 km was calculated for the clinopyroxenites. These depths equate to the approximate mantle-crust boundary underlying island arcs, such as the Izu-Bonin-Mariana Arc (Suyehiro et al., 1996;Calvert, 2011) and Tonga-Kermadec arc (Shor Jr. et al., 1971;Crawford et al., 2003). ...
Some arc magmas reside in the uppermost mantle and the lower crust. Their deep-seated behavior determines the composition of magmas that erupt at the surface. Mafic-ultramafic xenoliths newly found in Sabtang island, Batanes group of islands of the Luzon arc record subarc processes. The xenolith suite is comprised mainly of dunites, orthopyroxenites, clinopyroxenites, hornblendites, and gabbros, all hosted in basaltic to andesitic lavas. Petrographic characteristics suggest the metasomatic formation of orthopyroxenites and hornblendites from dunites and clinopyroxenites, respectively. The apparently primary minerals are homogeneous in composition. Olivine is relatively magnesian (Fo 82–90 ) and chromian spinel is rich in Cr# (=Cr/[Cr + Al], around 0.7) in dunites. Clinopyroxene is relatively magnesian (Mg# = Mg/[Mg + Fe ²⁺ ] = 0.73–0.93) in clinopyroxenites and gabbros, and plagioclase is highly anorthitic (An 89–98 ) in the gabbros. The primary mineral assemblage reflects crystallization of olivine and spinel followed by clinopyroxene all occurring in the uppermost mantle and lower crust of the Luzon arc. The orthopyroxenes and amphiboles were metasomatically produced at the expense of olivine and clinopyroxene, respectively. Clinopyroxene in the xenoliths is in equilibrium with the magmas that formed the Sabtang volcanics. They have relatively elevated contents of large-ion lithophile elements and light-rare-earth elements, which suggest derivation from an enriched mantle. The Sabtang xenoliths evidence the very active modification of the subarc mantle-crust boundary zone by mantle-derived magmas and slab-derived melts/fluids so that the mineral assemblage of the resultant rocks is similar to that of the predominant recent magma.
... Instead, what is present is, essentially, a transition zone within the lower crust of modern arcs that grades into mantle lithologies. For example, Calvert et al. (2008) and Calvert (2011) quote data from the Mariana arc and oceanic arcs older than c. 20 Ma, suggesting that the 'Moho' boundary is a c. 2 km-thick zone where seismic velocities transition from 7.4 to 7.7 km s −1 . Jagoutz & Kelemen (2015) reviewed arc seismic velocity and depth data, and concluded that they reveal a picture of arc structure with a gradual seismic velocity transition from c. 6.5 to 7 km s −1 between 10 and 20 km depth, and a crust that rarely gets to velocities >8 km s −1 until depths of >50 km. ...
The Kohistan–Ladakh terrane, northern Pakistan/India, offers a unique insight into whole-arc processes.
This research review presents summaries of fundamental crustal genesis and evolution models. Earlier
work focused on arc sequence definition. Later work focused on holistic petrogenesis. A new model emerges of
an unusually thick (c. 55 km) arc with a c. 30 km-thick batholith. Volatile-rich, hornblende ± garnet ± sediment
assimilation-controlled magmatism is predominant. The thick batholith has a complementary mafic–ultramafic
residue. Kohistan crustal SiO2 contents are estimated at >56%. The new-Kohistan, silicic-crust model contrasts
with previous lower SiO2 estimates (c. 51% SiO2 crust) and modern arcs that imply <35 km crustal thicknesses
and arc batholith thicknesses of c. 7 km. A synthetic overview of Kohistan–Ladakh volcanic rocks presents a
model of an older, cleaved/deformed Cretaceous volcanic system at least 800 km across strike. The Jaglot–
Chalt–Dras–Shyok volcanics exhibit predominant tholeiitic-calc-alkaline signatures, with a range of arc-related
facies/tectonic settings. A younger, post-collisional, Tertiary silicic volcanic system (the Shamran–Dir–Dras-
2–Khardung volcanics) lie unconformably upon Cretaceous basement, and erupted within an intra-continental
tectonic setting. Kohistan–Ladakh tectonic model controversies remain. In essence, isotope-focused researchers
prefer later (Tertiary) collisions, whilst structural field-geology-orientated researchers prefer an older (Cretaceous)
age for the Northern/Shyok Suture.
... Beneath the Dongsha Rise, the velocities vary from 6.4 km/s to 7.6 km/s in the lower crust, consistent with the HVL previously reported to exist in this region [Nissen et al., 1995a;Yan et al., 2001;Wei et al., 2011] and exceed 5.5 km/s in the upper crust. The envelope of arc crustal velocity functions, calculated by Calvert [2011], is indicated by the shaded lavender area in Figure 15b. Our 1-D velocity-depth function of the Dongsha Rise falls within this envelope, justifying the arc crust interpretation. ...
We present a 2-D seismic tomographic image of the crustal structure along the OBS2012 profile, which delineates the Moho morphology and magmatic features of the northeastern South China Sea margin. The image was created by forward modelling (RayInvr) and travel time tomographic inversion (Tomo2D). Overall, the continental crust thins seaward from ~27 km to ~21 km within the continental shelf across the Zhu I Depression and Dongsha Rise, with slight local thickening beneath the Dongsha Rise accompanying the increase in the Moho depth. The Dongsha Rise is also characterized by ~4–7-km-thick high-velocity layer (HVL) (~7.0–7.6 km/s) in the lower crust and exhibits a relatively high velocity (~5.5–6.4 km/s) in the upper crust with a velocity gradient lower than those of the Zhu I Depression and Tainan Basin. Across the continental slope and continent–ocean transition (COT), which contain the Tainan Basin, the crust sharply thins from 20 km to 10 km seaward and a ~2–3 km-thick HVL is imaged in the lower crust. We observed that volcanoes are located only within the COT but none exist in the continental shelf; the Dongsha Rise exhibits a high magnetic anomaly zone and different geochemical characteristics from the COT. Based on those observations, we conclude that the HVL underlying the COT is probably extension-related resulting from the decompression melting in the Cenozoic, whereas the HVL beneath the Dongsha Rise is probably arc-related and associated with the subduction of the paleo-Pacific plate. These findings are inconsistent with those of some previous studies.
... Our study focuses on lithosphere within the Mariana subduction zone (Fig. 1), the strongly arcuate segment of the IBM arc system south of the Ogasawara Plateau extending towards the deepest point on the planet, Challenger Deep. Since the initiation of subduction at ~50 Ma [Taylor, 1992], multiple episodes of crustal extension, rifting, and back-arc spreading have produced ridge, trough, arc, and trench features within the system [e.g., Stern et al., 2003;Calvert, 2011]. Some of the oldest remaining oceanic lithosphere in the western Pacific is subducted at the Mariana Trench, transporting crustal rocks, fluids, and igneous seamounts from the Pacific Plate beneath the overriding Philippine Sea Plate. ...
This thesis examines the structure of Pacific oceanic lithosphere that has been modified by post-formation magmatism in order to better understand the processes of secondary magmatic evolution of the lithosphere, which can have global-scale implications for oceanic and atmospheric chemistry. In the western Pacific, widespread Cretaceous magmatism has modified oceanic lithosphere over hundreds of millions of square kilometers. Seismic models of the upper crust from within the Jurassic Quiet Zone and the crust and upper mantle near the Mariana Trench reveal crust that is locally thickened via focused extrusive volcanism and crust that is modestly but uniformly thickened over broad regions. These distinct modes of magmatic emplacement suggest the operation of both focused and diffuse modes of melt transport through the lithosphere. Analysis of seismic observations from Guaymas Basin, in the Gulf of California, endeavor to advance our understanding of sill-driven alteration of sediments, an important consequence of secondary magmatism. We show that seismically imaged physical disruption to sediments due to igneous sill intrusion can be related to changes in sediment physical properties that reflect alteration processes. We also show how sill thickness can be estimated, enabling alteration intensity to be related to sill thickness in a variety of settings.
... fractional crystallization and anatexis of infracrustal gabbroic rocks) can give rise to a significantly thickened arc crust and a weakening of its lithospheric structure (Boutelier et al., 2003). Such matured IOAS have been demonstrated by indirect geophysical investigations in modern environments (Izu-Bonin-Marianna or Aleutian arc; Calvert, 2011;Tatsumi et al., 2008) and are relatively in agreement with exceptionally preserved paleo-arc sequences (Talkeetna and Kohistan oceanic arc remnants; Behn and Kelemen, 2006;Burg, 2011;Garrido et al., 2007;DeBari and Greene, 2011). Furthermore, IOAS may potentially show trench parallel heterogeneities as observed in the active Izu-Bonin-Marianna (IBM) intra-oceanic arc by intense geochemical variations along strike and also by seismic velocity models (Takahashi et al., 2007;Stern et al., 2003). ...
... Thick mafic melts formed new basal crust, and provided the thermal engines for the secondary melts that rose closer to the surface. That the lower crust of active intra-oceanic island arcs is similarly mafic is shown by seismology (Calvert, 2011). These mafic rocks, like MORBs and oceanic-island rocks, may all be products of re-fertilization of depleted upper mantle that began in Archean or Hadean time. ...
This review evaluates and rejects the currently dominant dogmas of
geodynamics and geochemistry, which are based on 1950s–1970s
assumptions of a slowly differentiating Earth. Evidence is presented for
evolution of mantle, crust, and early Moho that began with fractionation
of most crustal components, synchronously with planetary accretion, into
mafic protocrust by ~ 4.5 Ga. We know little about Hadean crustal
geology (> 3.9 Ga) except that felsic rocks were then forming, but
analogy with Venus, and dating from the Moon, indicate great shallow
disruption by large and small impact structures, including huge
fractionated impact-melt constructs, throughout that era.
Located in northern Dominican Republic, the Early Cretaceous Rio Boba mafic‐ultramafic plutonic sequence constitutes a lower crust section of the Caribbean island arc, made up by gabbroic rocks and subordinate pyroxenite. Modal compositions, mineral chemistry, whole‐rock compositions and thermobarometric calculations indicate that pyroxenites and gabbronorites represent a cumulate sequence formed by fractionation of tholeiitic magmas with initially very low H2O content in the lower crust of the arc (0.6–0.8 GPa). Melts evolved along a simplified crystallization sequence of olivine → pyroxenes → plagioclase → Fe‐Ti oxides. The magmatic evolution of the Rio Boba sequence and associated supra‐crustal Puerca Gorda metavolcanic rocks is multi‐stage and involves the generation of magmas from melting of different sources in a supra‐subduction zone setting. The first stage included the formation of a highly depleted substrate as result of decompressional melting of a refractory mantle source, represented by a cumulate sequence of LREE‐depleted island arc tholeiitic (IAT) and boninitic gabbronorites and pyroxenites. The second stage involved volumetrically subordinate cumulate troctolites and gabbros, which are not penetratively deformed. The mantle source was refractory and enriched by a LILE‐rich hydrous fluid derived from a subducting slab and/or overlying sediments, and possibly by a LREE‐rich melt. The third stage is recorded in the upper crust of the arc by the Puerca Gorda “normal” IAT protoliths, which are derived from an N‐MORB mantle source enriched with a strong subduction component. This magmatic evolution has implications for unraveling the processes responsible for subduction initiation and subsequent building of the Caribbean island arc.
As an interoceanic arc, the Kyushu-Palau Ridge (KPR) is an exceptional place to study the subduction process and related magmatism through its interior velocity structure. However, the crustal structure and its nature of the KPR, especially the southern part with limited seismic data, are still in mystery. In order to unveil the crustal structure of the southern part of the KPR, this study uses deep reflection/refraction seismic data recorded by 24 ocean bottom seismometers to reconstruct a detailed P-wave velocity model along the ridge. Results show strong along-ridge variations either on the crustal velocity or the thickness of the KPR. P-wave velocity model is featured with (1) a crustal thickness between 6–12 km, with velocity increases from 4.0 km/s to 7.0 km/s from top to bottom; (2) high gradient (∼1 s−1) in the upper crust but low one (<0.2 s−1) in the lower crust; (3) a slow mantle velocity between 7.2 km/s and 7.6 km/s in the uppermost mantle; and (4) inhomogenous velocity anomalies in the lower crust beneath seamounts. By comparing with the mature arc in the Izu-Bonin-Mariana arc in the east, this study suggests the southern part of KPR is a thicken oceanic crust rather than a typical arc crust. The origin of low velocities in the lower crust and upper mantle may be related with crustal differentiation, which implies advanced crustal evolution from normal oceanic crust to partly thicken oceanic crust. High velocities in the lower crust are related to the difference in magmatism.
The South Sandwich Volcanic Arc is one of the most remote and enigmatic arcs on Earth. Sporadic observations from rare cloud-free satellite images—and even rarer in situ reports—provide glimpses into a dynamic arc system characterised by persistent gas emissions and frequent eruptive activity. Our understanding of the state of volcanic activity along this arc is incomplete compared to arcs globally. To fill this gap, we present here detailed geological and volcanological observations made during an expedition to the South Sandwich Islands in January 2020. We report the first in situ measurements of gas chemistry, emission rate and carbon isotope composition from along the arc. We show that Mt. Michael on Saunders Island is a persistent source of gas emissions, releasing 145 ± 59 t day−1 SO2 in a plume characterised by a CO2/SO2 molar ratio of 1.8 ± 0.2. Combining this CO2/SO2 ratio with our independent SO2 emission rate measured near simultaneously, we derive a CO2 flux of 179 ± 76 t day−1. Outgassing from low temperature (90–100 °C) fumaroles is pervasive at the active centres of Candlemas and Bellingshausen, with measured gas compositions indicative of interaction between magmatic fluids and hydrothermal systems. Carbon isotope measurements of dilute plume and fumarole gases from along the arc indicate a magmatic δ13C of − 4.5 ± 2.0‰. Interpreted most simply, this result suggests a carbon source dominated by mantle-derived carbon. However, based on a carbon mass balance from sediment core ODP 701, we show that mixing between depleted upper mantle and a subduction component composed of sediment and altered crust is also permissible. We conclude that, although remote, the South Sandwich Volcanic Arc is an ideal tectonic setting in which to explore geochemical processes in a young, developing arc.
The Bear Valley Intrusive Suite (BVIS) in the Southernmost Sierra Nevada exposes a trans-crustal magmatic system that spans emplacement pressures from 3-10 kbars, and was emplaced at all crustal levels between 100.1-101.5 Ma. As such, it represents an unparalleled snapshot of magmatic processes within continental arc crust. In this study we present new field observations combined with whole rock geochemistry that show a fundamental dichotomy within the BVIS. The lower crust of the BVIS is dominantly composed of mafic cumulates that preserve originally shallow to horizontal magmatic fabrics, while the middle and upper crust is dominantly composed of voluminous homogeneous tonalites with steep fabrics. Using a stochastic model of melt fractionation and extraction, we show that these observations strongly constrain the P-T paths along which BVIS magmas must have been emplaced: to create the observed abrupt transition from mafic lower crust to felsic middle and upper crust, evolving melts must cool nearly isobarically in the lower crust before being rapidly emplaced in the upper crust along near-isothermal paths. Our modeling results show that the BVIS magmas must have cooled below 900 °C near 7 kbars depth.
These modeling results additionally require that the BVIS was emplaced into an unusually warm lower crust. The Sierra Nevada Batholith is typically characterized by felsic crust with low seismic velocities between 6.0-6.5 km/s to at least 30-35 km depth, significantly deeper than the observed transition at ∼28 km depth in the BVIS to mafic cumulates with calculated velocities >7.0 km/s. Given this observation, we conclude that the bulk of the Sierra Nevada Arc magmas must have stalled, cooled and differentiated at greater depths than the BVIS magmas.
The polygenic suite of on- and off-craton mantle xenoliths from the ensemble of cratons reveals the admixed and/or interstratified nature of the depleted and fertile mantle beneath the Indian shield. Most cratons are reactivated and exhibit decoupling of the crust and mantle post-Proterozoic. The SCLM beneath exhibits accretion and reworking to varying degrees. The cratons to the south of the CITZ are older and have a more evolved crustal structure than the ones to the north. The Conrad is less prominent beneath the Bundelkhand Craton, and the lower crust has a larger component of magmatic cumulates.
Island arc magmatism is considered a major mechanism of continental crustal growth; hence, the variation in island arc magma production rates and its influencing factors are of great importance. The along-arc strike variation in the island arc magma production rate is important for elucidating whether the subduction zone temperature is a controlling factor on arc magmatism. No back-arc spreading or ridge subduction is occurring in the intraoceanic subduction region of the Aleutian subduction zone, and the along-arc temperatures vary due to differences in the subducting plate age, convergence rate, and slab dip. Therefore, this is an ideal region to study the correlation between magma production rates and subduction zone temperatures. However, previous studies estimated the magma production rates at only a few locations due to the insufficient coverage of seismic data. Here, we employ the gravity inversion method based on density modelling and seismic data constraints to calculate the present-day arc crustal thickness and then obtain the magmatic thickness by removing the pre-existing crustal thickness from the present-day arc crustal thickness. Finally, the magma production rates along the main intraoceanic subduction region of the Aleutian Arc are mapped by estimating the island arc magmatic thickness and the most recent subduction inception dating result. In addition, the subduction zone temperatures and mantle melt amounts along five profiles across the Aleutian Arc are calculated. The magma production rate variation is highly correlated with the mantle melt amount. The slab temperature controls the extent of the slab dehydration, which is essential for mantle wedge melting, and the mantle wedge temperature controls the size of the hydrated mantle melting region and the melting fraction therein. Therefore, as the subduction zone temperature controls the mantle melt amount, we suggest that the subduction zone temperature is the first-order controlling factor on the magma production rate.
Seismic investigations of the structure of the Earth's crust are summarized with representative results derived from seismic refraction, seismic reflection, surface waves, seismic tomography, receiver functions, and ambient noise analyses. These results are complemented by laboratory seismic measurements, as well as nonseismic data, including gravity, aeromagnetics, geoelectrical studies, heat flow, and studies of exposed deep crustal sections, xenoliths, and geochemistry. Regional and global seismic crustal models provide comprehensive descriptions of the crust at continental and worldwide scales. Seismic and nonseismic models of the crust provide essential data for understanding the composition, structure, and evolution of the Earth.
The Mid Black Sea High comprises two en echelon basement ridges, the Archangelsky and Andrusov ridges, that separate the western and eastern Black Sea basins. The sediment cover above these ridges has been characterized by extensive seismic reflection data, but the crustal structure beneath is poorly known. We present results from a densely sampled wide-angle seismic profile, coincident with a pre-existing seismic reflection profile, which elucidates the crustal structure. We show that the basement ridges are covered by approximately 1–2 km of pre-rift sedimentary rocks. The Archangelsky Ridge has higher pre-rift sedimentary velocities and higher velocities at the top of basement (c. 6 km s−1). The Andrusov Ridge has lower pre-rift sedimentary velocities and velocities less than 5 km s−1 at the top of the basement. Both ridges are underlain by approximately 20-km-thick crust with velocities reaching around 7.2 km s−1 at their base, interpreted as thinned continental crust. These high velocities are consistent with the geology of the Pontides, which is formed of accreted island arcs, oceanic plateaux and accretionary complexes. The crustal thickness implies crustal thinning factors of approximately 1.5–2. The differences between the ridges reflect different sedimentary and tectonic histories.
Subduction zone mantle wedge temperatures impact plate interaction, melt generation, and chemical recycling. However, it has been challenging to reconcile geophysical and geochemical constraints on wedge thermal structure. Here we chemically determine the equilibration pressures and temperatures of primitive arc lavas from worldwide intra-oceanic subduction zones and compare them to kinematically driven thermal wedge models. We find that equilibration pressures are typically located in the lithosphere, starting just below the Moho, and spanning a wide depth range of ∼25 km. Equilibration temperatures are high for these depths, averaging ∼1300°C. We test for correlations with subduction parameters and find that equilibration pressures correlate with upper plate age, indicating overriding lithosphere thickness plays a role in magma equilibration. We suggest that most, if not all, thermobarometric pressure and temperature conditions reflect magmatic re-equilibration at a mechanical boundary, rather than reflecting the conditions of major melt generation. The magma re-equilibration conditions are difficult to reconcile, to a first order, with any of the conditions predicted by our dynamic models, with the exception of subduction zones with very young, thin upper plates. For most zones, a mechanism for substantially thinning the overriding plate is required. Most likely thinning is localised below the arc, as kinematic thinning above the wedge corner would lead to a hot forearc, incompatible with forearc surface heat flow and seismic properties. Localised sub-arc thermal erosion is consistent with seismic imaging and exhumed arc structures. Furthermore, such thermal erosion can serve as a weakness zone and affect subsequent plate evolution. This article is protected by copyright. All rights reserved.
We acquired 27 wide-angle seismic profiles to investigate variation in crustal structure along the Kyushu-Palau Ridge (KPR), a 2600-km-long remnant island arc in the center of the Philippine Sea plate; 26 lines were shot across the strike of the KPR at 13 degrees-31 degrees N, and one was shot along the northernmost KPR. The derived P-wave velocity (Vp) models show that the KPR has a crustal thickness of 8-23 km, which is thicker than the neighboring backarc basin oceanic crusts of the West Philippine Basin to the west and the Shikoku and Parece Vela Basins to the east. While the KPR crust consists mainly of lower crusts with a Vp of 6.8-7.2 km/s, the thicker crust contains a thick middle crust with Vp of 6.0-6.8 km/s. In general, the KPR crust is thicker in the north than in the south. The uppermost mantle velocities just below the KPR bathymetric highs are lower than 8.0 km/s and are commonly associated with a slightly high Vp of 7.2 km/s at the base of the crust. Large amplitude reflection signals are sometimes observed at far offsets on several lines suggesting the existence of several reflectors at depths of 23-40 km in the mantle beneath the KPR. The characteristics of these reflections are similar to these observed beneath the Izu-Ogasawara (Bonin) island arc, the tectonically conjugate arc of the KPR before backarc basin spreading. Very thin crust and high Pn velocities characterize the transition between the KPR and the eastern basins, which is probably a relic of the initial stage of the rifting. West of the KPR, the crust varies in structure from north to south as a result of the different tectonic settings in which it evolved.
The Annieopsquotch accretionary tract (AAT) comprises a thrust stack of Lower to Middle Ordovician arc and backarc terranes that were accreted to the composite Laurentian margin of Iapetus during the Middle to Late Ordovician. Geological relationships suggest that the constituent terranes of the AAT initially formed outboard of the composite Laurentian margin in an extensional arc that underwent multiple rifting episodes prior to its accretion. The initiation of AAT magmatism led to the development of Tremadocian to Floian supra-subduction zone ophiolites (481 to 477 Ma) with organized ridges indicated by the presence of well-developed sheeted dyke complexes. This spreading centre propagated through a fragment of Laurentian crust and separated it from the composite Laurentian margin. This Laurentian crust fragment then formed the basement to subsequent Floian to Darriwilian AAT arc magmatism. The Floian arc (473 to 468 Ma) underwent extensive rifting indicated by organized spreading in the Lloyds River backarc basin, which was floored by juvenile backarc ophiolitic crust (472 Ma). The establishment of the Darriwilian arc (467 to 462 Ma) was in part coeval with yet another stage of rifting. Darriwilian magmatism is characterised by significant along-strike variability, ranging from continental to primitive calcalkaline arc to tholeiitic backarc-like magmatism. The diversity of Darriwilian magmatism can be attributed to fragmentation and magmatic reworking of Laurentian-derived basement along strike in the same arc undergoing disorganized spreading. The development of the AAT is interpreted to be similar to that of the modern Izu – Bonin – Mariana arc in the western Pacific.
The composition of much of Earth's lower continental crust is enigmatic. Wavespeeds require that 10-20% of the lower third is mafic, but the available heat-flow and wavespeed constraints can be satisfied if lower continental crust elsewhere contains anywhere from 49 to 62 wt% SiO2. Thus, contrary to common belief, the lower crust in many regions could be relatively felsic, with SiO2 contents similar to andesites and dacites. Most lower crust is less dense than the underlying mantle, but mafic lowermost crust could be unstable and likely delaminates beneath rifts and arcs. During sediment subduction, subduction erosion, arc subduction, and continent subduction, mafic rocks become eclogites and may continue to descend into the mantle, whereas more silica-rich rocks are transformed into felsic gneisses that are less dense than peridotite but more dense than continental upper crust. These more felsic rocks may rise buoyantly, undergo decompression melting and melt extraction, and be relaminated to the base of the crust. As a result of this refining and differentiation process, such relatively felsic rocks could form much of Earth's lower crust.
We review data and recent research on arc composition, focusing on the relatively complete arc crustal sections in the Jurassic Talkeetna arc (south central Alaska) and the Cretaceous Kohistan arc (northwest Pakistan), together with seismic data on the lower crust and uppermost mantle. Whereas primitive arc lavas are dominantly basaltic, the Kohistan crust is clearly andesitic and the Talkeetna crust could be andesitic. The andesitic compositions of the two arc sections are within the range of estimates for the major element composition of continental crust. Calculated seismic sections for Kohistan and Talkeetna provide a close match for the thicker parts of the active Izu arc, suggesting that it, too, could have an andesitic bulk composition. Because andesitic crust is buoyant with respect to the underlying mantle, much of this material represents a net addition to continental crust. Production of bulk crust from a parental melt in equilibrium with mantle olivine or pyroxene requires processing of igneous crust, probably via density instabilities. Delamination of dense cumulates from the base of arc crust, foundering into less dense, underlying mantle peridotite, is likely, as supported by geochemical evidence from Talkeetna and Kohistan. Relamination of buoyant, subducting material - during sediment subduction, subduction erosion, arc-arc collision, and continental collision - is also likely.
Thin oceanic crust is formed by decompression melting of the upper mantle at mid-ocean ridges, but the origin of the thick and buoyant continental crust is enigmatic. Juvenile continental crust may form from magmas erupted above intra-oceanic subduction zones, where oceanic lithosphere subducts beneath other oceanic lithosphere. However, it is unclear why the subduction of dominantly basaltic oceanic crust would result in the formation of andesitic continental crust at the surface. Here we use geochemical and geophysical data to reconstruct the evolution of the Central American land bridge, which formed above an intra-oceanic subduction system over the past 70 Myr. We find that the geochemical signature of erupted lavas evolved from basaltic to andesitic about 10 Myr ago—coincident with the onset of subduction of more oceanic crust that originally formed above the Galápagos mantle plume. We also find that seismic P-waves travel through the crust at velocities intermediate between those typically observed for oceanic and continental crust. We develop a continentality index to quantitatively correlate geochemical composition with the average P-wave velocity of arc crust globally. We conclude that although the formation and evolution of continents may involve many processes, melting enriched oceanic crust within a subduction zone—a process probably more common in the Archaean—can produce juvenile continental crust.
The collision of continental crust of the Eurasian Plate with the overriding Luzon arc in central Taiwan has led to compression, uplift and exhumation of rocks that were originally part of the Chinese rifted margin. Though the kinematics of the fold-thrust belt on the west side of the orogen has been described in detail, the style of deformation in the lower crust beneath Taiwan is still not well understood. In addition, the fate of the Luzon arc and forearc in the collision is also not clear. Compressional wave arrival times from active-source and earthquake seismic data from the Taiwan Integrated GEodynamic Research (TAIGER) program constrain the seismic velocity structure of the lithosphere along transect T5, an east-west corridor in central Taiwan. The results of our analysis indicate that the continental crust of the Eurasian margin forms a broad crustal root beneath central Taiwan, possibly with a thickness of 55 km. Compressional seismic velocities beneath the Central Range of Taiwan are as low as 5.5 km/s at 25 km depth, whereas P-wave seismic velocities in the middle crust on the eastern flank of the Taiwan mountain belt average 6.5-7.0 km/s. This suggests that the incoming sediments and upper crust of the Eurasian Plate are buried to mid-crustal depth in the western flank of the orogen before they are exhumed in the Central Range. To the east, the Luzon arc and forearc are deformed beneath the Coastal Range of central Taiwan. Fragments of the rifted margin of the South China Sea that were accreted in the early stages of the collision form a new backstop that controls the exhumation of Eurasian strata to the west in this evolving mountain belt.
A long-standing theory for the genesis of continental crust is that it is formed in subduction zones. However, the observed seismic properties of lower crust and upper mantle in oceanic island arcs differ significantly from those in the continental crust. Accordingly, significant modifications of lower arc crust must occur, if continental crust is indeed formed from island arcs. Here we investigate how the seismic characteristics of arc crust are transformed into those of the continental crust by calculating the density and seismic structure of two exposed sections of island arc (Kohistan and Talkeetna). The Kohistan crustal section is negatively buoyant with respect to the underlying depleted upper mantle at depths exceeding 40 kilometres and is characterized by a steady increase in seismic velocity similar to that observed in active arcs. In contrast, the lower Talkeetna crust is density sorted, preserving only relicts (about ten to a hundred metres thick) of rock with density exceeding that of the underlying mantle. Specifically, the foundering of the lower Talkeetna crust resulted in the replacement of dense mafic and ultramafic cumulates by residual upper mantle, producing a sharp seismic discontinuity at depths of around 38 to 42 kilometres, characteristic of the continental Mohorovičić discontinuity (the Moho). Dynamic calculations indicate that foundering is an episodic process that occurs in most arcs with a periodicity of half a million to five million years. Moreover, because foundering will continue after arc magmatism ceases, this process ultimately results in the formation of the continental Moho.
Crustal growth at convergent margins can occur by the accretion of
future allochthonous terranes (FATs), such as island arcs, oceanic
plateaus, submarine ridges, and continental fragments. Using geodynamic
numerical experiments, we demonstrate how crustal properties of FATs
impact the amount of FAT crust that is accreted or subducted, the type
of accretionary process, and the style of deformation on the overriding
plate. Our results show that (1) accretion of crustal units occurs when
there is a weak detachment layer within the FAT, (2) the depth of
detachment controls the amount of crust accreted onto the overriding
plate, and (3) lithospheric buoyancy does not prevent FAT subduction
during constant convergence. Island arcs, oceanic plateaus, and
continental fragments will completely subduct, despite having buoyant
lithospheric densities, if they have rheologically strong crusts. Weak
basal layers, representing pre-existing weaknesses or detachment layers,
will either lead to underplating of faulted blocks of FAT crust to the
overriding plate or collision and suturing of an unbroken FAT crust. Our
experiments show that the weak, ultramafic layer found at the base of
island arcs and oceanic plateaus plays a significant role in terrane
accretion. The different types of accretionary processes also affect
deformation and uplift patterns in the overriding plate, trench
migration and jumping, and the dip of the plate interface. The resulting
accreted terranes produced from our numerical experiments resemble
observed accreted terranes, such as the Wrangellia Terrane and Klamath
Mountain terranes in the North American Cordilleran Belt.
Most primitive arc melts are basaltic in composition, yet the bulk continental crust, thought to be generated in arcs, is andesitic. In order to produce an andesitic crust from primitive arc basalts, rocks complementary to the andesitic crust have to be fractionated and subsequently removed, most likely through density sorting in the lower arc crust. The Kohistan Arc in northern Pakistan offers a unique opportunity to constrain the composition and volume of material fluxes involved in this process. In a lower crustal section >10 km cumulates (dunites, wehrlites, websterites, clinopyroxene-bearing garnetites and hornblendites, and garnet-gabbros) are exposed that are 0.1–0.3 g/cm3 denser than the underlying mantle. The cumulates combine with the andesitic bulk Kohistan Arc crust to reproduce the major and trace element composition of primitive basaltic arc melts. Our petrochemical analysis suggests that fractionation and subsequent foundering of wehrlites+ultramafic hornblende–garnet–clinopyroxene cumulates+garnet-gabbros is a viable mechanism for producing andesitic crust from a calc-alkaline/tholeiitic primitive high-Mg basalt. The mass of the foundered material is approximately twice that of the arc crust generated. For an overall andesitic arc composition, we estimate a magma flux into the arc (11–15 km3/yr) about three times the rate of arc crust production itself. Foundering fluxes of cumulates (6.4–8.1 km3/yr) are one third to half those of the globally subducted oceanic crust (~19 km3/yr). Hence, the delaminate forms a volumetrically significant, albeit refractory and depleted geochemical reservoir in the mantle. Owing to its low U/Pb and high Lu/Hf the foundered material evolves with time to a reservoir characterized by unradiogenic Pb and highly radiogenic Hf isotopes, unlike any of the common mantle endmembers defined by OIB chemistry. The unradiogenic Pb of the foundered arc cumulates could counterbalance the radiogenic Pb composition of the depleted mantle. The predicted highly radiogenic Hf (at rather unradiogenic Nd) of the foundered material can explain the εHf–εNd systematics observed in some abyssal peridotites and mantle xenoliths.
This paper focuses on the processes of arc rifting in the context of the volcanic, structural, and sedimentologic evolution of the Izu-Bonin-Mariana arc-trench system. Sedimentation patterns were directly influenced by the productivity of the proximal arc volcanoes, with volcanic lulls recorded by hemipelagic interbeds. Many arc segments go through a cycle of (1) frequent volcanism before and during rifting; (2) reduced and/or less disseminated volcanism during latest rifting and early backarc spreading, as new frontal arc volcanoes are being constructed and growing to sea level; and (3) increasingly vigorous volcanism during middle and late stage backarc spreading, and until the next rift cycle begins. Differences in plate boundary forces at the ends, more than in the middle, of volcanic arcs may significantly influence their proclivity to rift. -from Author
It is proposed that many summit basins along the Aleutian Arc form from
the clockwise rotation of blocks of the arc massif. Summit basins are
arc-parallel grabens or half-grabens formed within the arc massif and
are commonly located near or along the axis of late Cenozoic volcanism.
Geomorphically, the Aleutian Arc appears to consist of contiguous
rhombic blocks of varying size, tens to hundreds of kilometers in
length. The boundaries between adjacent blocks are delineated by
fault-controlled canyons that cut the southern slope of the arc
transverse to its regional trend. Evidence that these blocks have
rotated clockwise is provided by the triangular-shaped summit basins
bordering the blocks to the north, oblique physiographic trends, offsets
in the summit platform, and broad deflections in the southern slope of
the arc. We present a model for block rotation that involves translation
of blocks parallel to an arc. It is suggested that block rotation, which
appears to have accelerated in late Cenozoic time, is linked to (1) a
shift in the Euler pole for the Pacific plate, (2) the consequential
start-up of late Cenozoic volcanism, (3) improved interplate coupling
instigated by sediment flooding of the Aleutian Trench, and (4) westward
subduction of northeast striking segments of the inactive Kula-Pacific
Ridge.
Compressional seismic travel times from a relatively sparse wide-angle
data set hold key information on the structure of a 800 km long section
of the central Aleutian arc. Since the source and receiver locations
form a swath along the arc crest that is ˜50 km wide, we trace
rays in 3-D for a collection of 8336 seismic refraction and reflection
arrivals. We investigate variations in seismic velocity structure
parallel to the Aleutian arc, assuming that our result represents
average crustal structure across the arc. We explore seismic velocity
models that consist of three crustal layers that exhibit smooth
variations in structure in the 2-D vertical plane. We consider the
influence of additional constraints and model parameterization in our
search for a plausible model for Aleutian arc crust. A tomographic
inversion with static corrections for island stations reduces the data
variance of a 1-D starting model by 91%. Our best model has seismic
velocities of 6.0-6.5 km/s in the upper crust, 6.5-7.3 km/s in the
middle crust, and 7.3-7.7 km/s in the lower crust and a total crustal
thickness of 35-37 ± 1 km. A resolution analysis shows that
features having a horizontal scale less than 20 km cannot be imaged, but
at horizontal length scales of ˜50 km most model features are well
resolved. The study indicates that the Aleutian island arc crust is
thick compared to other island arcs and strongly stratified and that
only the upper 60% of the arc crust has seismic velocities that are
comparable to average seismic velocities in continental crust.
The Izu-Bonin-Mariana (IBM) arc system extends 2800km from near Tokyo,
Japan to Guam and is an outstanding example of an intraoceanic
convergent margin (IOCM). Inputs from sub-arc crust are minimized at
IOCMs and output fluxes from the Subduction Factory can be more
confidently assessed than for arcs built on continental crust. The
history of the IBM IOCM since subduction began about 43 Ma may be better
understood than for any other convergent margin. IBM subducts the oldest
seafloor on the planet and is under strong extension. The stratigraphy
of the western Pacific plate being subducted beneath IBM varies simply
parallel to the arc, with abundant off-ridge volcanics and
volcaniclastics in the south which diminish northward, and this seafloor
is completely subducted. The Wadati-Benioff Zone varies simply along
strike, from dipping gently and failing to penetrate the 660 km
discontinuity in the north to plunging vertically into the deep mantle
in the south. The northern IBM arc is about 22km thick, with a felsic
middle crust; this middle crust is exposed in the collision zone at the
northern end of the IBM IOCM. There are four Subduction Factory outputs
across the IBM IOCM: (1) serpentinite mud volcanoes in the forearc, and
as lavas erupted from along (2) the volcanic front of the arc and (3)
back-arc basin and (4) from arc cross-chains. This contribution
summarizes our present understanding of matter fed into and produced by
the IBM Subduction Factory, with the intention of motivating scientific
efforts to understand this outstanding example of one of earth's most
dynamic, mysterious, and important geosystems.
We acquired coincident wide-angle and multi-channel seismic reflection data along four profiles perpendicular to the Kyushu-Palau Ridge (KPR) between 15°N and 20°N on the Philippine Sea plate. The crustal thickness beneath the KPR, which is a remnant arc created in the Late Eocene, varies along the strike from 8 to approximately 20 km and is always thicker than the adjacent oceanic crust of the West Philippine Basin to the west and the Parece Vela Basin to the east. The thickest crust among the four profiles, which is primarily due to a thickening of the lower crust, is found where the KPR adjoins Oki-no-Tori-Shima Island. There is no clear evidence of the thick (>5 km) middle crustal layer with a P-wave velocity of 6.0-6.5 km/s that has been inferred beneath the conjugate rifted counterpart of the Izu-Ogasawara(Bonin)-Mariana Island-arc. Our results suggest that the crust of the KPR at 15-21°N represents a less mature island arc crust relative to that further north along the ridge where a mid-crustal layer of 6 km/s has been reported.
Intra-oceanic arcs are the simplest type of subduction,systems in that they occur where overridding plates of subduction zones consist of oceanic rocks, contrasting with arcs built on continental margins. They comprise,some,40% of the subduction,margins,of the Earth. The better-known examples include the Izu-Bonin-Mariana arc, the Tonga- Kermadec arc, the Vanuatu arc, the Solomon arc, the New Britain arc, the western part of the Aleutian arc, the South Sandwich arc and the Lesser Antilles arc. They are thought to represent the first stage in the generation of continental crust from oceanic materials. They are generally more inaccessible than continental arcs, but, for a variety of reasons, provide insights into processes in subduction,zones that are impossible or difficult to glean from the better-studied continental arcs. Intra-oceanic arcs typically have a simpler crustal structure than arcs built on continental crust, although there are significant differences between examples. Geochemically, magmas erupted in intra-oceanic arcs are not contaminated by
Group velocities of Rayleigh waves recorded at a long-period seismograph station on Amchitka Island were obtained for mixed tectonic paths across the Pacific and the Aleutian Island Arc Ridge. The mixed-path group velocities for periods between 20 and 60 sec were then separated into pure-oceanic and pureridge path group velocities. The group velocities for the pure paths along the Aleutian ridge are on the average 0.36 km/sec lower than those for the purely oceanic paths across the Pacific. Inversion of the pure-ridge group velocities yields almost continental shear velocities in the crust, a very gradual crust-mantle transition at depths between 20 and 40 km, a thin lithospheric lid of uppermost mantle material between 30 and 70 km with relatively low maximum shear velocities approaching 4.4 km/sec, and a very pronounced low-velocity zone at depths below 70 km with an average shear velocity of 4.1 km/sec. The computed shear velocity structure beneath the Aleutian Ridge is compared to models for other tectonically active and stable regions.
Clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones. Based on a critical appraisal of the geophysical and geological literature, 26 parameters are estimated for each of 39 modern subduction zones. To isolate causal relationships among these parameters, multivariate analysis is applied to this data set. This analysis yields empirical quantitative relations that predict strain regime and strike-slip faulting in the overriding plate, maximum earthquake magnitude, Benioff zone length, slab dip, arc-trench gap, and maximum trench depth. Excellent correlation is found between length of the Benioff zone and the product of convergence rate and age of the downgoing slab. This relationship is consistent with the conductive heating model of Molnar et al. (1979), if the model is modified in one respect. The rate of heating of the slab is not constant; it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate. The structural style in the overriding plate is determined by its stress state. Though the stress state of overriding plates cannot be quantified, one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional (class 1, back-arc spreading) to strongly compressional (class 7, active folding and thrusting). This analysis indicates that strain class is probably determined by a linear combination of convergence rate, slab age, and shallow slab dip. Interplate coupling, controlled by convergence rate and slab age, is an important control on strain regime and the primary control on earthquake magnitude. Arc-parallel strike-slip faulting is a common feature of convergent margins, forming a forearc sliver between the strike-slip fault and trench. Optimum conditions for the development of forearc slivers are oblique convergence, a compressional environment, and a continental overriding plate. The primary factor controlling presence of strike-slip faulting is coupling; strongly oblique convergence is not required. The rate of strike-slip faulting is affected by both convergence obliquity and convergence rate. Maximum trench depth is a response to flexure of the underthrusting plate. The amount of flexural deflection at the trench depends on the vertical component of slab pull force, which is very sensitive to slab age and shallow slab dip. Shallow slab dip conforms to the cross-sectional shape of the overriding plate, which is controlled by width of the accretionary prism and duration of subduction. Deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow. Much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike-slip faulting and strain regime. Empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones.
A new model for the earliest stages in the evolution of subduction zones is developed from recent geologic studies of the Izu-Bonin-Mariana (IBM) arc system and then applied to Late Jurassic ophiolites of Cailfornia. The model accounts for several key observations about the earliest stages in the evolution of the IBM system: (1) subduction nucleated along an active transform boundary, which separated younger, less-dense lithosphere in the west from older, more-dense lithosphere to the east; (2) initial arc magmatic activity occupied a much broader zone than existed later; (3) initial magmatism extended up to the modern trench, over a region now characterized by subnormal heat flow; (4) early are magmatism was characterized by depleted (tholeiitic) and ultra-depleted (boninitic) magmas, indicating that melting was more extensive and involved more depleted mantle than is found anywhere else on earth; (5) early arc magmatism was strongly extensional, with crust forming in a manner similar to slow-spreading ridges; and (6) crust production rates were 120 to 180 km³/km-Ma, several times greater than for mature arc systems. These observations require that the earliest stages of subduction involve rapid retreat of the trench; we infer that this resulted from continuous subsidence of denser lithosphere along the transform fault. This resulted in strong extension and thinning of younger, more buoyant lithosphere to the west. This extension was accompanied by the flow of water from the sinking oceanic lithosphere to the base of the extending lithosphere and the underlying asthenosphere. Addition of water and asthenospheric upwelling led to catastrophic melting, which continued until lithosphere subsidence was replaced by lithospheric subduction. Application of the subduction-zone infancy model to the Late Jurassic ophiolites of California provides a framework in which to understand the rapid formation of oceanic crust with strong arc affinities between the younger Sierran magmatic arc and the Franciscan subduction complex, provides a mechanism for the formation and subsidence of the Great Valley forearc basin, and explains the limited duration of high-T, high-P metamorphism experienced by Franciscan mélanges.
The Tanzawa plutonic complex (TPC), central Japan, is a suite of tonalitic-gabbroic plutons exposed in a globally unique arc-arc collision zone, where an active intraoceanic Izu-Bonin-Mariana (IBM) arc is colliding against the Honshu arc. The TPC has been widely accepted as an exposed middle crust section of the IBM arc, chiefl y because of geochemical similarities between the TPC and IBM rocks and previously reported precollisional Miocene K-Ar ages. However, new zircon U-Pb ages show that the main pulse of TPC magmatism was syncollisional and that plutons were emplaced rapidly and cooled soon after Pliocene collision. Trace element compositions of TPC zircon show distinctively elevated Th/Nb ratios compared to zircon from other noncollisional IBM silicic plutonic rocks, indicating the involvement of continental sediments from the Honshu arc in their magma genesis.
Results from the 1994 Aleutian Seismic experiment delineate basic oceanic arc crustal architecture, constrain magmatic flux rates and bulk arc composition, and address questions of continental crustal genesis. Here we present results from a transect across protocontinental crust of the westernmost Alaska Peninsula (line A3) and compare this structure to a purely oceanic arc transect farther west. Arc crustal structure is similar along these two transects. Magmatic accretion occurs at the top and bottom of preexisting oceanic crust as a 5- to 10-km-thick upper crustal carapace of low velocity (2-5.8 km s-1) volcaniclastics, flows and small plutons, and a mafic lower crustal underplate (~7.0 km s-1) of variable thickness, for a maximum arc crust thickness of ~25-30 km. Lateral lower crustal velocity gradients and high velocities (>7.5 km s-1) beneath the forearc suggest dominantly vertical lower crustal accretion above a focused melt source and a forearc underlain by little magmatic crust but rather partially intruded and/or serpentinized mantle. The ratio of upper to lower crustal volume is ~1, and the total arc crust volume implies a magmatic flux of ~67 km3 km-1 m.y.-1, more than twice previous estimates for this arc and global productivity. The crust is thinner and more mafic than continental crust, and it lacks a massive tonalitic upper crust characteristic of the continents. An interpreted accumulation of upper crustal carapace material at midcrustal depths on line A3 has a velocity of ~6.4 km s-1, suggesting an intermediate composition. Accretionary complex terranes consisting of accumulations of this type material would thus have bulk compositions similar to continental crust.
New seismic wide-angle data from the eastern Aleutian Islands show a mafic composition and a 30-km-thick island-arc crust. Traveltimes of P and S refracted arrivals and prominent crustal and mantle reflectors observed to offsets of over 300 km were used to derive velocity models for the eastern Aleutian Arc between the islands of Atka and Unimak using a three-dimensional finite difference modeling and tomography code. We interpret the highest crustal P wave velocities of 7.2-7.4 km/s between about 12 and about 22 km depth to the south and north of the main volcanic line as remainders of preexisting oceanic crust into which arc magma is intruded. Apart from the pieces of oceanic crust, the velocities suggest an overall mafic composition for the arc, composed mainly of metabasalts, diorite and diabase in the upper crust, and garnet-granulite or amphibolite-to-hornblendite in the lower crust. Reflected arrivals from the subducting Pacific plate at depths of 45-55 km beneath the fore-arc, together with Pn, show a mantle wedge with P wave velocities as low as 7.4 km/s, increasing with depth to about 8.1 km/s with an average of about 7.7 km/s. A mantle composition that grades from mainly pyroxenite (probably ultramafic cumulates) near the Moho to dunite at greater depths best explains these observed velocities.
A new model for the earliest stages in the evolution of subduction zones is developed from recent geologic studies of the Izu-Bonin-Mariana (IBM) arc system and the applied to Late Jurassic ophiolotes of California. The model accounts for several key observations which require that the earliest stages of subduction involve rapid retreat of the trench; this resulted from continuous subsidence of denser lithosphere along the transform fault. This resulted in strong extension and thinning of younger, more buoyant lithosphere to the west. This extension was accompanied by the flow of water from the sinking oceanic lithosphere to the base of the extending lithosphere and the underlying asthenosphere. Addition of water and asthenospheric upwelling led to catastrophic melting, which continued until lithosphere subsidence was replaced by lithosphere subduction. -from Authors
The author offers an overview of the complex phenomenon of andesite genesis to help clarify the long-standing problem and to identify profitable areas for future research. It is considered that conventional explanations better account for more of the data than do the more elegantly simple theories produced by plate tectonic theory. -R.A.H.
The Mariana Trough is an actively spreading, crescent-shaped, backarc basin located in the western Pacific between the active Mariana volcanic island arc and the West Mariana Ridge, a remnant arc. The geologic evolution of the Mariana Trough varies along strike of the basin from the initial opening phase in the north to the mature seafloor spreading phase in the central latitudes. The opening of the basin began with an initial period of stretching and collapse of the preexisting arc followed by development of ridge/ transform structures along an active volcanic and tectonic zone on the eastern side of the basin. Eventually a true spreading center developed within the basin as the principal volcanic and tectonic zone diverged from the active volcanic front. The current along-strike variations in rifting/ spreading history define distinct geographic regions: the northern rifting apex, the central spreading basin, and the southern platform.
Despite significant advances in marine streamer design, seismic data are often plagued by coherent noise having approximately linear moveout across stacked sections. In an attempt to understand the characteristics that distinguish such noise from signal, three general mechanisms that might produce such noise patterns on stacked sections are examined. Of the many processing tools available, moveout filtering is best for suppressing the noise while preserving signal. Our data example demonstrates that although it is more costly, moveout filtering of the unstacked data is particularly effective because it conditions the data for the critical data-dependent processing steps of predictive deconvolution and velocity analysis. -after Authors
Basaltic and andesitic rocks of the Miocene Tanzawa Group underwent low- to medium-grade metamorphism around the quartz diorite body. Mineral chemistry and phase relations of Ca-Al silicates, actinolite and tschermakitic hornblende in the greenschist facies and the prehnite-pumpellyite facies rocks have been investigated. Suggests that the presence of synmetamorphic deformation involving rotational components, is due to the diapiric upwelling of the Tanzawa quartz diorite.-from English summary
The Shikoku Basin is a backarc basin occupying the northern part of the Philippine Sea Plate. The recent Continental Shelf Survey Project of Hydrographic Department, Maritime Safety Agency of Japan reveals interesting features of the basin genesis. Based on detailed mapping of magnetic anomalies and topography, a scenario of evolution of the Shikoku Basin from 30 Ma to 15 Ma is proposed. The history of Shikoku Basin is divided into five stages as follows - rifting, NNW-SSE opening, N-S opening, NW-SE opening, and volcanism and deformation after opening. -from Authors
The nearly continuous recovery of 0.5 km of generally fresh, layer 3 gabbroic rocks at Hole 735B, especially near the bottom of the section, presents scientists an unusual opportunity to study the detailed elastic properties of the lower oceanic crust. Extending compressional-wave and density shipboard measurements at room pressure, V p and V s were measured at pressures from 20 to 200 MPa using the pulse transmission method. All of the rocks exhibit significant increases in velocity with increasing pressure up to about 150 MPa, a feature attributed to the closing of microcrack porosity. Measured velocities reflect the mineralogical makeup and microstructures acquired during the tectonic history of Hole 735B. Most of the undeformed and unaltered gabbros are approximately 65:35 plagioclase/ clinopyroxene rocks plus olivine or oxide minerals, and the observed densities and velocities are fully consistent with the Voigt-Reuss-Hill (VRH) averages of the component minerals and their proportions. Depending on their olivine content, the predominant olivine gabbros at 200 MPa have average V p = 7.1 ± 0.2 km/s, V s = 3.9 ± 0.1 km/s, and grain densities of 2.95 ± 0.5 g/cm 3 . The less abundant iron-titanium (Fe-Ti) oxide gabbros average V p = 6.75 ± 0.15 km/s, V s = 3.70 ± 0.1 km/s, and grain densities of 3.22 ± 0.05 g/cm 3 , reflecting the higher densities and lower velocities of oxide minerals compared to olivine. About 30% of the core is plastically deformed, and the densities and directionally averaged velocities of these shear-zone tectonites are generally consistent with those of the gabbros, their protoliths. Three sets of observations indicate that the shear-zone metagabbros are elastically anisotropic: (1) directional variations in V p , both vertical and horizontal and with respect to foliation and lineation; (2) discrepancies among V p values for the horizontal cores and the VRH averages of the component minerals and their mineral proportions, suggesting preferred crystallographic orientations of anisotropic minerals; and (3) variations of V s of up to 7%, with polarization directions parallel and perpendicular to foliation. Optical inspection of thin sections of the same samples indicates that plagioclase feldspar, clinopyroxene, and amphibole typically display crystallographic-preferred orientations, and this, plus the elastic anisotropy of these minerals, suggests that preferred orientations are responsible for much of the observed anisotropy, particularly at high pressure. Alteration tends to be localized to brittle faults and brecciated zones, and typical alteration minerals are amphibole and secondary plagioclase, which do not significantly change the velocity-density relationships.
Presents a critique of the models proposed to exlain the formation of back arc basins: those marginal basins located behind active or inactive trench systems and whose origin is inferred to be subduction related. The three main classes of models proposed to explain back arc basin formation are mantle diapirism, induced aesthenospheric convection, and global plate kinematics. The first two classes of models fail to explain the temporal and spatial distribution of back arc basins. The third class of models proposes that back arc basins should form whenever global plate interactions require divergence between the overriding plate and the trench line. However, the tectonic setting of several back arc basins suggests that they represent more than just a passive response to kinematic boundary conditions. -from Authors
Reports a study of the distribution of volcanoes in 16 active plate margins, corresponding to a total of 479 volcanic systems. The active volcanic arcs are found to have a ribbon geometry with an average length/width ratio of around 10. The shape of the volcanic arc is compared to the shape of the associated trench by projecting one onto the older. The projection direction agrees with the direction of plate convergence. In each arc, the distribution of volcano spacing is best represented by a Gamma distribution which corresponds to randomly generated points in the same geometrical conditions. In order to explain how such distributions may be generated, the gravitational instability of a layer of buoyant liquid which is fed at a constant rate at the bottom of denser fluid is investigated. -from Authors
As island arc rifting evolves to mature back-arc spreading, the nature
of melt generation and mode of crustal accretion may vary in response to
the interplay of different subduction-related processes and conditions,
including (1) changes in mantle dynamics from flux-melting and
buoyancy-driven upwelling at the arc volcanic front to decompression
melting driven by plate separation at back-arc spreading centers; (2)
re-circulation of refractory material through arc and back-arc melting
regimes by mantle wedge corner flow; (3) changes in the locus of
magmatic centers relative to the arc volcanic front; (4) variable locus
of initial rifting and breakup; (5) spatially varying rheology
attributable to mantle wedge hydration gradients with distance from the
slab; (6) slab subduction rate, dip, and length. We discuss the possible
influence of these factors on crustal accretion processes in light of
observations from intra-oceanic back-arc basins, with particular focus
on new compilations of swath bathymetry and sidescan imagery from the
Lau Basin. In the Lau Basin south of 18°S, the active spreading
centers undergo large changes in morphology and crustal characteristics
as they separate from the arc volcanic front. Ahead of the southern
limit of organized seafloor spreading, a broad area of high acoustic
backscatter indicates a wide, "distributed" form of crustal accretion.
Parts of the western Lau Basin have been previously interpreted as
remnants of a tectonically rifted preexisting arc. The swath mapping
data show, however, that western basin morphology is similar to that
formed by the sites of currently active magmatic crustal accretion to
the east. These observations support a revised model of Lau Basin
evolution in which essentially the entire back-arc basin is formed by
magmatic crustal accretion, but crustal thickness and morphology reflect
the changing locus of the magmatic centers with respect to a mantle
wedge of varying chemical fertility and rheology. Compared to mid-ocean
settings, the observations imply an expanded range of crustal accretion
variables in arc-proximal magmatic centers in which seafloor morphology
is more indicative of mantle wedge chemistry than spreading rate.
The process by which continental crust has formed is not well understood, though such crust mostly forms at convergent plate margins today. It is thus imperative to study modern intra-oceanic arcs, such as those common in the western Pacific Ocean. New seismic studies along the representative Izu-Bonin intra-oceanic arc provide unique along-strike images of arc crust and uppermost mantle to complement earlier, cross-arc lithospheric profiles. These reveal two scales (1000-10 km scale) of variations, one at the scale of the Izu versus Bonin (thick versus thin) arc crust, the other at the intervolcano (˜50 km) scale. These images show that: (1) the bulk composition of the Izu-Bonin arc crust is more mafic than typical continental crust, (2) the middle crust with seismic velocities similar to continental crust is predominantly beneath basaltic arc volcanoes, (3) the bulk composition beneath basaltic volcanoes changes little at thick and thin arc segments, and (4) a process to return lower crustal components to the mantle, such as delamination, is required for an arc crust to evolve into continental crust. Continued thickening of the Izu-Bonin crust, accompanied by delamination of lowermost crust, can yield velocity structure of typical continental crust.
At ocean margins where two plates converge, the oceanic plate sinks or is subducted beneath an upper one topped by a layer of terrestrial crust. This crust is constructed of continental or island arc material. The subduction process either builds juvenile masses of terrestrial crust through arc volcanism or new areas of crust through the piling up of accretionary masses (prisms) of sedimentary deposits and fragments of thicker crustal bodies scraped off the subducting lower plate. At convergent margins, terrestrial material can also bypass the accretionary prism as a result of sediment subduction, and terrestrial matter can be removed from the upper plate by processes of subduction erosion. Sediment subduction occurs where sediment remains attached to the subducting oceanic plate and underthrusts the seaward position of the upper plate's resistive buttress (backstop) of consolidated sediment and rock. Sediment subduction occurs at two types of convergent margins: type 1 margins where accretionary prisms form and type 2 margins where little net accretion takes place. At type 2 margins (∼19,000 km in global length), effectively all incoming sediment is subducted beneath the massif of basement or framework rocks forming the landward trench slope. At accreting or type 1 margins, sediment subduction begins at the seaward position of an active buttress of consolidated accretionary material that accumulated in front of a starting or core buttress of framework rocks. Where small-to-medium-sized prisms have formed (∼16,300 km), approximately 20% of the incoming sediment is skimmed off a detachment surface or decollement and frontally accreted to the active buttress. The remaining 80% subducts beneath the buttress and may either underplate older parts of the frontal body or bypass the prism entirely and underthrust the leading edge of the margin's rock framework. At margins bordered by large prisms (∼8,200 km), roughly 70% of the incoming trench floor section is subducted beneath the frontal accretionary body and its active buttress. In rounded figures the contemporary rate of solid-volume sediment subduction at convergent ocean margins (∼43,500 km) is calculated to be 1.5 km³/yr. Correcting type 1 margins for high rates of terrigenous seafloor sedimentation during the past 30 m.y. or so sets the long-term rate of sediment subduction at 1.0 km³/yr. The bulk of the subducted material is derived directly or indirectly from continental denudation. Interstitial water currently expulsed from accreted and deeply subducted sediment and recycled to the ocean basins is estimated at 0.9 km³/yr. The thinning and truncation caused by subduction erosion of the margin's framework rock and overlying sedimentary deposits have been demonstrated at many convergent margins but only off northern Japan, central Peru, and northern Chile has sufficient information been collected to determine average or long-term rates, which range from 25 to 50 km³/m.y. per kilometer of margin. A conservative long-term rate applicable to many sectors of convergent margins is 30 km³/km/m.y. If applied to the length of type 2 margins, subduction erosion removes and transports approximately 0.6 km³/yr of upper plate material to greater depths. At various places, subduction erosion also affects sectors of type 1 margins bordered by small- to medium-sized accretionary prisms (for example, Japan and Peru), thus increasing the global rate by possibly 0.5 km³/yr to a total of 1.1 km³/yr. Little information is available to assess subduction erosion at margins bordered by large accretionary prisms. Mass balance calculations allow assessments to be made of the amount of subducted sediment that bypasses the prism and underthrusts the margin's rock framework. This subcrustally subducted sediment is estimated at 0.7 km³/yr. Combined with the range of terrestrial matter removed from the margin's rock framework by subduction erosion, the global volume of subcrustally subducted material is estimated to range from 1.3 to 1.8 km³/yr. Subcrustally subducted material is either returned to the terrestrial crust by arc-related igneous processes or crustal underplating or is lost from the crust by mantle absorption. Geochemical and isotopic data support the notion that upper mantle melting returns only a small percent of the subducted material to the terrestrial crust as arc igneous rocks. Limited areal exposures of terrestrial rocks metamorphosed at deep (>20–30 km) subcrustal pressures and temperatures imply that only a small fraction of subducted material is reattached via deep crustal underplating. Possibly, therefore much of the subducted terrestrial material is recycled to the mantle at a rate near 1.6 km³/yr, which is effectively equivalent to the commonly estimated rate at which the mantle adds juvenile igneous material to the Earth's layer of terrestrial rock.
Seismic refraction studies in the Melanesian Borderland (the area between Australia on the west and New Zealand and Tonga on the east) show extreme diversity of crustal structure. The Lord Howe rise and the Norfolk ridge are topped by thick sediments and have deep crustal roots and thick layers of material with the same compressional wave velocity as the Australian continental crust. The Kermadec and Lau ridges, on the other hand, have structure and velocities typical of normal island arcs. The South Fiji basin structurally resembles an oceanic area with additional sedimentary fill. The western part of the Fiji plateau has thin sediments and has the structure of an area of normal deep-ocean basin that has been uplifted two kilometers. All the features are compatible with the hypothesis that the area has been disrupted and fragments of continental material have been separated from the Australian mass.
We present the results of an onshore–offshore wide-angle refraction and reflection experiment off Miyagi, in the central Japan Trench forearc region. There are two rupture zones of large interplate earthquakes here: the landward rupture zone and the trenchward rupture zone. To examine the influence of plate boundary geometry on the distributions of the rupture zones, we determined reflector geometries from reflections. The subducting oceanic plate increases its dip from about 5° to 13° around 143.2°E. This bending point in the oceanic plate corresponds to the eastern edge of the trenchward rupture zone. Moreover, another bending point may be present at approximately 142.3°E, which corresponds to the eastern edge of the landward rupture zone. The coincidence between bending points in the oceanic plate and the edges of rupture zones suggests that changes in the plate boundary geometry may affect rupture propagation of interplate earthquakes in the direction of plate convergence.
The seismic structure of the slow-spreading Atlantic ridge is reviewed using results constrained by synthetic seismogram modelling and all available seismic determinations of the crustal structure of fracture zones. The variation of velocity with depth in normal oceanic crust is characterized by a series of velocity gradients: large (1-2 s -1) gradients in the upper crust and more gentle ones (~0.1 s -1) in the lower crust, with a variable crust-mantle transition. Petrologic changes, modified by hydrothermal circulation, metamorphic alteration and by cracks and fractures control the seismic structure. Superimposed on the 'typical' crustal structure are marked lateral variations caused by: (i) crustal formation in areas away from a mature spreading centre (e.g. aseismic ridges, the oceanic-continental transition); (ii) accretion within transform faults, generating low-velocity material of greatly attenuated or variable thickness; (iii) ageing of the young crust as it moves away from the spreading centre; (iv) localized and generally small variations in regions away from fracture zones caused by spatial and temporal changes in the igneous and tectonic activity. The observed variations in seismic structure can be explained by a model of the spreading centre consisting of a string of independently accreting segments separated by transform faults every 50-80 km. Restricted magma supply at the ends of the segments may result in accretion of thinned crust in transforms whilst faulting lowers the seismic velocity and allows the deep penetration of water and possibly serpentinization at depth. Growth faulting within the median valley causes the isovelocity contours in the lower crust to be rather flatter than the surface expression shown by the basement relief. Episodic accretion in the spreading centre results in small-scale lateral variability in the extrusive carapace.
