Article

Composition and Evolution of the Ancestral South Sandwich Arc: Implications for the flow of Deep Ocean Water and Mantle through the Drake Passage Gateway

Authors:
To read the full-text of this research, you can request a copy directly from the authors.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

... Others assume that the oldest dated arc volcanic rocks dredged from the Scotia Sea floor (~28.5-33 Ma; Barker, 1995;Dalziel et al., 2013) indicate that subduction initiated in the Scotia Sea region around 34 Ma (Crameri et al., 2020;Pearce et al., 2014). Because there is no plate convergence at that time, Pearce et al. (2014) assumed that subduction initiated spontaneously and immediately led to ocean basin formation in the upper plate. ...
... Ma; Barker, 1995;Dalziel et al., 2013) indicate that subduction initiated in the Scotia Sea region around 34 Ma (Crameri et al., 2020;Pearce et al., 2014). Because there is no plate convergence at that time, Pearce et al. (2014) assumed that subduction initiated spontaneously and immediately led to ocean basin formation in the upper plate. But absence of evidence for older arc volcanism from such a scarcely sampled and poorly accessible region does not provide a conclusive argument against older subduction. ...
... It has been proposed that the eastern part of the slab is related to ongoing South Sandwich subduction, while a more westerly located high velocity zone represents a fossil part of the slab that formed at an Ancestral South Sandwich subduction zone (Beniest and Schellart, 2020). In this interpretation, the Ancestral South Sandwich subduction zone became inactive in the Miocene after the arrival of the South America-Antarctica Ridge in the southern segment of the trench (Pearce et al., 2014), which led to slab tearing and the onset of oceanic spreading in the East Scotia Basin, separating the South Sandwich arc from the Ancestral South Sandwich arc (Govers and Wortel, 2005;Pearce et al., 2014). The Ancestral South Sandwich subduction zone is the equivalent of our Endurance-South Sandwich subduction zone and these interpretations of the tomography are consistent with our reconstruction. ...
Article
Full-text available
During evolution of the South Sandwich subduction zone, which has consumed South American Plate oceanic lithosphere, somehow continental crust of both the South American and Antarctic plates have become incorporated into its upper plate. Continental fragments of both plates are currently separated by small oceanic basins in the upper plate above the South Sandwich subduction zone, in the Scotia Sea region, but how fragments of both continents became incorporated in the same upper plate remains enigmatic. Here we present an updated kinematic reconstruction of the Scotia Sea region using the latest published marine magnetic anomaly constraints, and place this in a South America-Africa-Antarctica plate circuit in which we take intracontinental deformation into account. We show that a change in marine magnetic anomaly orientation in the Weddell Sea requires that previously inferred initiation of subduction of South American oceanic crust of the northern Weddell below the eastern margin of South Orkney Islands continental crust, then still attached to the Antarctic Peninsula, already occurred around 80 Ma. Subsequently, between ~71–50 Ma, we propose that the trench propagated northwards into South America by delamination of South American lithosphere: this resulted in the transfer of delaminated South American continental crust to the overriding plate of the South Sandwich subduction zone. We show that continental delamination may have been facilitated by absolute southward motion of South America that was resisted by South Sandwich slab dragging. Pre-drift extension preceding the oceanic Scotia Sea basins led around 50 Ma to opening of the Drake Passage, preconditioning the southern ocean for the Antarctic Circumpolar Current. This 50 Ma extension was concurrent with a strong change in absolute plate motion of the South American Plate that changed from S to WNW, leading to upper plate retreat relative to the more or less mantle stationary South Sandwich Trench that did not partake in the absolute plate motion change. While subduction continued, this mantle-stationary trench setting lasted until ~30 Ma, after which rollback started to contribute to back-arc extension. We find that roll-back and upper plate retreat have contributed more or less equally to the total amount of ~2000 km of extension accommodated in the Scotia Sea basins. We highlight that viewing tectonic motions in a context of absolute plate motion is key for identifying slab motion (e.g., rollback, trench-parallel slab dragging) and consequently mantle-forcing of geological processes.
... These volcanic edifices are interpreted to represent a remnant volcanic arc (Barker et al., 1982), which Dalziel et al. (2013a) termed the ancestral South Sandwich arc (ASSA; Fig. 2). Pearce et al. (2014) determined that the ASSA had a life span of ~20 Myr and characterised its magmatic history from a limited number of dredged samples. This paper examines the age and geochemistry of a new suite of dredged samples from Discovery Bank of the South Scotia Ridge, previously interpreted to represent a sector of the ASSA (Pearce et al., 2014). ...
... Pearce et al. (2014) determined that the ASSA had a life span of ~20 Myr and characterised its magmatic history from a limited number of dredged samples. This paper examines the age and geochemistry of a new suite of dredged samples from Discovery Bank of the South Scotia Ridge, previously interpreted to represent a sector of the ASSA (Pearce et al., 2014). However, other workers have suggested that Discovery Bank represents stretched continental crust (Galindo-Zaldívar et al., 2002;Civile et al., 2012), or is underlain by a segment of the Cretaceous Pacific Margin Anomaly intrusive belt (e.g. ...
... The Scotia Plate is predominantly formed of oceanic crust created on the West Scotia Ridge and on the western flank of the East Scotia Ridge (Fig. 1), as well as a central zone of oceanic crust of potentially Cretaceous sea floor determined from geophysical differences to the surrounding crust (Eagles, 2010). The ancestral South Sandwich arc developed during the Oligocene and Miocene (Dalziel et al., 2013a;Eagles and Jokat, 2014;Pearce et al., 2014) in the central Scotia Sea region from South Georgia to Jane Bank (Fig. 2). The formation of oceanic crust of the Scotia Plate initiated during the Late Eocene and marked the transition from eastward oriented subduction of the Phoenix Plate beneath the South America-Antarctic Peninsula Pacific margin to the severance of the land bridge and rifting and dispersal of continental fragments, combined with westward dipping subduction at the Weddell Sea front Fig. 1). ...
Article
The ancestral South Sandwich arc (ASSA) records evidence of Oligocene – Miocene intra-oceanic island arc volcanism in the central Scotia Sea and potentially formed an important topographic barrier to deep ocean currents during the early development of the Antarctic Circumpolar Current. New geochemistry and geochronology of dredged basaltic samples from multiple sites across Discovery Bank, in the southern part of the ASSA, provide key information about this poorly understood volcanic arc in the Scotia Sea. Two new ⁴⁰Ar³⁹Ar ages confirm volcanism was active in the Discovery Bank segment from 14 to 10 Ma, overlapping with the initial phase of spreading along the East Scotia Ridge. These ages are younger than previously determined for the ASSA and the island arc chain would have influenced deep ocean pathways in the southern Scotia Sea during the mid-Miocene. Geochemical analysis indicates that magmatism was derived from a depleted asthenospheric source, modified by subduction, akin to the present day South Sandwich island arc. Evidence from across Discovery Bank suggests that arc volcanism developed on pre-existing basaltic crust potentially related to Weddell Sea subduction, although the submerged bank is dominated by Miocene arc volcanic rocks. Evidence for an intra-oceanic island arc setting is also provided by the identification and imaging of a linear chain of rear-arc seamounts, characteristic of several volcanic centres in the present day South sandwich island arc. Mafic volcanic rocks, including ultramafic lithologies, from the neighbouring Bruce Bank topographic high have also been investigated and are demonstrated to have been generated in a different tectonic setting to the volcanic rocks of Discovery Bank. They are interpreted to be associated with the opening of Scan Basin during the Late Eocene, or potentially distal evidence of the gabbroic Pacific Margin Anomaly.
... Towards the CSS, the Bouguer gravity anomaly becomes lower ( Fig. 2a) and the magnetic fabric changes ( Fig. 2b) as it loses its distinct striped pattern common to mafic oceanic crust. Samples recovered in this area are still mafic but range from tuffs, to basaltic andesite to clastic volcanic material (samples 116, 117, 118, 120 and 122, supplementary table 1, Pearce et al. (2014)). The flat bathymetry ( Fig. 1) suggests that part of the oceanic basement is covered with oceanic sediments. ...
... In the very southeast and east of the CSS several seamounts are aligned in a semi-arcuate way (Fig. 1). This feature has been interpreted as part of the Eocene-early Oligocene Ancestral South Sandwich Arc (ASSA, Fig. 5, Pearce et al., 2014), an ancient volcanic arc. Around the seamounts the Bouguer gravity anomaly is lower than in the CSS (Fig. 2a), because the seamounts are topographically higher, or because the underlying crust is less mafic in composition, for example oceanic island arc crust, which has a lower density than oceanic plateau crust (Tetreault and Buiter, 2014). ...
... Around the seamounts the Bouguer gravity anomaly is lower than in the CSS (Fig. 2a), because the seamounts are topographically higher, or because the underlying crust is less mafic in composition, for example oceanic island arc crust, which has a lower density than oceanic plateau crust (Tetreault and Buiter, 2014). The few dredges around the ASSA recovered mafic rocks varying from basalts to basaltic andesites and a few dacites in the northeast (samples 126, 127, 128, 120, 130 and 131, supplementary table 1, Pearce et al. (2014), Saunders and Tarney (1979)). In the CSS, the areas with a high Bouguer gravity anomaly ( Fig. 2a) are mapped as outcropping basaltic andesites (Fig. 5), whereas areas with a flat bathymetry and a lower Bouguer gravity anomaly ( Fig. 2a) are mapped as oceanic sediments that overly the basement (Fig. 5). ...
Article
Full-text available
The Scotia Sea is one of the tectonically most complex and least understood back-arc basins on Earth, which partly results from its remote location making the acquisition of data challenging. Here, we provide a review of current, publicly available geophysical and geological data in the Scotia Sea realm, including magnetic, Bouguer gravity anomaly, high-resolution bathymetric, heat flow and reflection seismic data and rock type data from cored and dredged samples. With this inter-disciplinary data-set, we performed an offshore geological mapping exercise that allowed us to identify lithologies in the predominantly submerged Scotia Sea domain. Cross-sections combining crustal structure and mantle tomography enabled us to address some of the still persisting geological challenges in this tectonically complex area. The data-review revealed that basalt is the dominant lithology in the Scotia Sea area, occupying most of the West and East Scotia Sea (WSS and ESS). Andesitic and more felsic lithologies are identified in the Central Scotia Sea (CSS) and the northern East Scotia Sea (ESS). Mesozoic/Palaeozoic metamorphic/sedimentary lithologies are reported from the highs along the North and South Scotia Ridges (NSR and SSR). These highs originate from a land bridge that, until the late-Mesozoic, connected the South American and Antarctic continents. Scarcely available and contradicting data prevent the age determination of several structural units surrounding and located on the Scotia plate, but our mapping exercise allowed us to confirm the presence of the early Oligocene - late Miocene Ancestral South Sandwich Arc (ASSA) in the east of the CSS, setting the minimum age of the older segment of the CSS crust to Eocene-earliest Oligocene. Three cross-sections cross-cutting the Scotia Sea reveal two high velocity zones, indicating cold mantle material. One is situated below the structural highs along the SSR, which we interpreted as remnant slab material of the ASSA and another below South Georgia and the CSS, which we interpreted as the front of the slab that currently subducts at the South Sandwich Trench (SSaT). The neutral velocity discontinuity below the WSS implies mantle conditions of a recently extinct spreading centre. The upper mantle low velocity anomaly below the CSS is interpreted as warmer toroidal flows around the slab edges of the subducting plate at the SSaT. We have demonstrated that the geological and geophysical data publicly available today allows us to create offshore geological maps in remote, inaccessible offshore domains. This is a less time-consuming, economically advantageous exercise, which re-uses existing geological and geophysical data for a new purpose. It is the most data-inclusive study there is today of the Scotia Sea region and serves as a guideline for future expeditions targeting the CSS and the structural features along the NSR to identify their age and origin. A georeferenced version of the map is provided in the supplementary material.
... Subduction-related or supra-subduction zone ophiolites include those in which oceanic lithosphere formed in the extended upper plate of a subduction zone either in a forearc, arc or back-arc position. Among these, subduction-initiation ophiolites, formed in the forearc during the inception and early stages of intra-oceanic subduction, are those that have been defined most recently (Stern and Bloomer, 1992) and our understanding has developed rapidly in the last decade (e.g., Lázaro et al., 2016;Maffione et al., 2015;Pearce and Robinson, 2010;Pearce et al., 2014;Stern et al., 2012;Turner et al., 2014;Whattam and Stern, 2011). This knowledge has hugely benefitted from the study of in situ dredged and drilled basement lithologies of the archetypical Izu-Bonin-Mariana (IBM) forearc (Ishizuka et al., 2011(Ishizuka et al., , 2014Pearce et al., 2015;Reagan et al., 2010;Ribeiro et al., 2013Ribeiro et al., , 2015, which has even led to the definition of a new chemostratigraphic model for subduction initiation magmatism, involving: 1) forearc basalts (FAB; Reagan et al., 2010); 2) boninites and island arc tholeiites (IAT); and 3) island arc tholeiite to calc-alkaline arc magmas. ...
... Insights into the nature and provenance of the slab component can be obtained from variations of the Nd-Pb radiogenic isotopes, which are customarily used in petrogenetic studies of arc volcanic rocks as they are sensitive tracers of slab additions (e.g., Haase et al., 2002;Moghadam et al., 2014;Pearce et al., 2014;Ribeiro et al., 2013). The Maimón Fm. is part of the so-called IAT volcanic suite of the Caribbean, which records the incipient to mature stages of SW-dipping subduction of the Proto-Caribbean (Atlantic) lithosphere beneath the Greater Antilles island arc in the early Cretaceous (e.g., Boschman et al., 2014;Escuder-Viruete et al., 2014;Pindell et al., 2012). ...
... Actually, the development and preservation of the entire set of geochemical lithotypes linked to subduction initiation in a given ophiolite depend on many petrological and geodynamical parameters related to the subduction setting itself and the mechanisms of obduction. Some of these parameters are the temperature of the subducting crust (cold vs hot subduction), the development vs absence of trench rollback and its rate and extent, and the mode and substrate of ophiolite emplacement (Tethyan vs Cordilleran ophiolite types) (e.g., Leng et al., 2012;Pearce and Robinson, 2010;Pearce et al., 2014;Ribeiro et al., 2015). A possible explanation of the different extents of slab fluids input in Group 1 Maimón basalts compared to IBM FAB is that, in the case of the Greater Antilles paleo-arc, the release of fluids and/or melts from the downgoing slab may have occurred at shallower depths and at faster rates due to hot subduction (Pearce and Robinson, 2010), as discussed below in the framework of the early evolution of the Caribbean island arc. ...
Article
Metamorphosed basalts, basaltic andesites, andesites and plagiorhyolites of the Early Cretaceous, probably pre-Albian, Maimón Formation, located in the Cordillera Central of the Dominican Republic, are some of the earliest products of the Greater Antilles arc magmatism. In this article, new whole-rock element and Nd-Pb radiogenic isotope data are used to give new insights into the petrogenesis of the Maimón meta-volcanic rocks and constrain the early evolution of the Greater Antilles paleo-arc system. Three different groups of mafic volcanic rocks are recognized on the basis of their immobile element contents. Group 1 comprises basalts with compositions similar to low-Ti island arc tholeiites (IAT), which are depleted in light rare earth elements (LREE) and resemble the forearc basalts (FAB) and transitional FAB-boninitic basalts of the Izu-Bonin-Mariana forearc. Group 2 rocks have boninite-like compositions relatively rich in Cr and poor in TiO2. Group 3 comprises low-Ti island arc tholeiitic basalts with near-flat chondrite-normalized REE patterns. Plagiorhyolites and rare andesites present near-flat to subtly LREE-depleted chondrite normalized patterns typical of tholeiitic affinity. Nd and Pb isotopic ratios of plagiorhyolites, which are similar to those of Group 1 and 3 basalts, support that these felsic lavas formed by anatexis of the arc lower crust.
... Data grid from Sandwell and Smith (1997). crust created on the West Scotia Ridge and on the west side of the East Scotia Ridge, as well as a central zone probably consisting of oceanic crust of potentially Cretaceous age (Eagles, 2010) which is overlain by the ancestral South Sandwich arc (Dalziel et al., 2013a;Eagles and Jokat, 2014;Pearce et al., 2014). Formation of oceanic crust of the Scotia Plate led to the opening of the oceanic and mantle gateway, the Drake Passage (Fig. 1). ...
... Eagles, 2010) who, based on geophysical evidence, suggested it has a distinct history to the otherwise Eocene -Recent Scotia Plate. Eagles (2010) and Eagles and Jokat (2014) interpreted the central Scotia Sea as Cretaceous oceanic crust, whilst Dalziel et al. (2013b) and Pearce et al. (2014) suggested that it could represent Cretaceous oceanic crust overlain by a Cenozoic volcanic arc. Dubinin et al. (2016) interpreted the central Scotia Sea as extended continental crust. ...
Article
Full-text available
Understanding the tectonic evolution of the Scotia Sea is critical to interpreting how ocean gateways developed during the Cenozoic and their influence on ocean circulation patterns and water exchange between the Atlantic and Southern oceans. We examine the geochronology and detrital age history of lithologies from the prominent, submerged Barker Plateau of the North Scotia Ridge. Metasedimentary rocks of the North Scotia Ridge share a strong geological affinity with the Fuegian Andes and South Georgia, indicating a common geological history and no direct affinity to the Antarctic Peninsula. The detrital zircon geochronology indicates that deposition was likely to have taken place during the mid – Late Cretaceous. A tonalite intrusion from the Barker Plateau has been dated at 49.6 ±0.3Ma and indicates that magmatism of the Patagonian–Fuegian batholith continued into the Eocene. This was coincident with the very early stages of Drake Passage opening, the expansion of the proto Scotia Sea and reorganization of the Fuegian Andes. The West Scotia Ridge is an extinct spreading centerthat shaped the Scotia Sea and consists of seven spreading segments separated by prominent transform faults. Spreading was active from 30–6Ma and ceased with activity on the W7 segment at the junction with the North Scotia Ridge. Reinterpretation of the gravity and magnetic anomalies indicate that the architecture of the W7 spreading segment is distinct to the other segments of the West Scotia Ridge. Basaltic lava samples from the eastern flank of the W7 segment have been dated as Early – mid Cretaceous in age (137–93Ma) and have a prominent arc geochemical signature indicating that seafloor spreading did not occur on the W7 segment. Instead the W7 segment is likely to represent a downfaulted block of the North Scotia Ridge of the Fuegian Andes continental margin arc, or is potentially related to the putative Cretaceous Central Scotia Sea.
... km in depth are traced in the bottom topography. Based on the results of dredging and detailed bathymetric surveying, Pierce et al. [53] suggest that these uplifts are relics of an island arc ( Figs. 5a and 6). The central province is separated from the microcontinent of South Georgia Island by the Endurance Basin, 3.5-4.0 ...
... The Deep Basin and smaller basins in the zone of recent diffuse spreading to the south of the Discovery and Herdman banks [32], as well as the Bruce and Hesperida basins, are examples of pull-apart basins along the southern boundary of the central province ( Fig. 5a). The Powell Basin, pertaining to the category [28]; (13) dredging station, after [35]; (14) dredging station, after [53]. U n c o r r e c t e d a u t h o r c o p y No. 1 2016 DUBININ et al. of minor riftogenic structural elements, is located to the southwest of the central province between the South Orkney microcontinent and the northeastern end of the Antarctic Peninsula ( Fig. 1). ...
Article
Full-text available
Western, central, and eastern provinces are recognized in the Scotia Sea. They are distinguished by their bottom topography, geophysical characteristics, and crustal structure, which record their different origin and evolution. The western province is characterized by the oceanic crust that formed on the West Scotia Ridge, where active spreading may have ceased as a result of a collision between propagating rift and the structural barrier of the thick continental lithosphere of the Falkland Plateau. The central province is a series of blocks mainly composed of continental crust that subsided to various depths depending on the degree of extension in the course of rifting. These blocks are separated by local areas with oceanic crust formed due to the breakup of the continental crust and diffusive spreading. These areas are characterized by deep bottom and high values of Bouguer anomalies. The southern framework of the central province consists of subsided continental blocks and microcontinents divided by small spreading-type basins formed by lithospheric extension complicated by strike-slip faulting. The eastern province is composed of oceanic crust formed on the backarc spreading East Scotia Ridge. The results of density analysis, analog, and numerical simulations allowed us to explain some features of the structure and evolution of these provinces. The insight into tectonic structure of the provinces and their evolution allowed us to recognize several types of riftogenic basins differing in geodynamics, age, and geological and geophysical characteristics.
... Исходя из результатов драгирования и детальной батиметрической съемки поднятий, Дж. Пирс с соавторами [53] предполагают, что данные поднятия являются реликтами островной дуги (см. рис. ...
... Цифры в кружках см. на рис. 5. 1-3 -континентальная кора (показана градациями серого в зависимости от степени утонения -чем светлее, тем кора более утонена); 4 -океаническая кора; 5 -древние островные дуги; 6 -транспрессивно-транстенсивные зоны; 7 -кора неясного строения и генезиса, предположительно, сильно утоненная континентальная кора; 8 -отдельные малые поднятия неясного генезиса; 9 -предположительно океаническая кора котловины Шота; 10 -крупные линеаменты; 11 -малые линеаменты; 12 -линейные магнитные аномалии по [28]; 13 -станции драгирования по [35]; 14 -станции драгирования по [53] континентов, которые в отдельные интервалы времени могли независимо вращаться относительно материковой части Антарктики или других континентальных блоков, и, следовательно, были отделены от них дивергентными, конвергентными или трансформными границами плит небольшой протяженности. В настоящее время наблюдается надвигообразование вдоль южной границы микроконтинента о. ...
... Data grid from Sandwell and Smith (1997). crust created on the West Scotia Ridge and on the west side of the East Scotia Ridge, as well as a central zone probably consisting of oceanic crust of potentially Cretaceous age (Eagles, 2010) which is overlain by the ancestral South Sandwich arc (Dalziel et al., 2013a;Eagles and Jokat, 2014;Pearce et al., 2014). Formation of oceanic crust of the Scotia Plate led to the opening of the oceanic and mantle gateway, the Drake Passage (Fig. 1). ...
... Eagles, 2010) who, based on geophysical evidence, suggested it has a distinct history to the otherwise Eocene -Recent Scotia Plate. Eagles (2010) and Eagles and Jokat (2014) interpreted the central Scotia Sea as Cretaceous oceanic crust, whilst Dalziel et al. (2013b) and Pearce et al. (2014) suggested that it could represent Cretaceous oceanic crust overlain by a Cenozoic volcanic arc. Dubinin et al. (2016) interpreted the central Scotia Sea as extended continental crust. ...
Article
Full-text available
Understanding the tectonic evolution of the Scotia Sea is critical to interpreting how ocean gateways developed during the Cenozoic and their influence on ocean circulation patterns and water exchange between the Atlantic and Southern oceans. We examine the geochronology and detrital age history of lithologies from the prominent, submerged Barker Plateau of the North Scotia Ridge. Metasedimentary rocks of the North Scotia Ridge share a strong geological affinity with the Fuegian Andes and South Georgia, indicating a common geological history and no direct affinity to the Antarctic Peninsula. The detrital zircon geochronology indicates that deposition was likely to have taken place during the mid-Late Cretaceous. A tonalite intrusion from the Barker Plateau has been dated at 49.6 ± 0.3 Ma and indicates that magmatism of the Patagonian-Fuegian batholith continued into the Eocene. This was coincident with the very early stages of Drake Passage opening, the expansion of the proto Scotia Sea and reorganization of the Fuegian Andes. The West Scotia Ridge is an extinct spreading center that shaped the Scotia Sea and consists of seven spreading segments separated by prominent transform faults. Spreading was active from 30-6 Ma and ceased with activity on the W7 segment at the junction with the North Scotia Ridge. Reinterpretation of the gravity and magnetic anomalies indicate that the architecture of the W7 spreading segment is distinct to the other segments of the West Scotia Ridge. Basaltic lava samples from the eastern flank of the W7 segment have been dated as Early-mid Cretaceous in age (137-93 Ma) and have a prominent arc geochemical signature indicating that seafloor spreading did not occur on the W7 segment. Instead the W7 segment is likely to represent a downfaulted block of the North Scotia Ridge of the Fuegian Andes continental margin arc, or is potentially related to the putative Cretaceous Central Scotia Sea.
... Such process could hence lead to the contamination of the mantle below the Proto-Caribbean slab with North American-derived sediments containing the ancient zircons. During the embryonic stages of the Caribbean island arc (Fig. 8d), the sinking of the Proto-Caribbean slab into the asthenospheric mantle and its subsequent roll-back would have triggered the rising of deeper mantle materials contaminated with the detrital zircons of interest in the fore-arc of the arc system (McGowan et al., 2015;Griffin et al., 2016); also the transfer via return and slab-window flow of Proto-Caribbean (Atlantic) asthenospheric mantle to fill the newly created mantle wedge (Torró et al., 2017a; see also Pearce andRobinson, 2010 andPearce et al., 2014). Further, Pindell et al. (2012) propose that during Neocomian times, any material entering the subduction channel in western Mexico was potentially dragged southeastward throughout the inter-American transform along which the subduction of the Proto-Caribbean took place (Fig. 8c); therefore, that material would have found itself in the hanging wall of the new westdipping subduction zone right in the moment of the proto-Caribbean subduction initiation. ...
Article
Full-text available
Little mineralogical evidence is left of the recycling of continental and oceanic crust into the mantle at subduction zones. Zircon, because of its exceptional robustness, is probably the only surviving phase and the best mineral tracer of this global-scale process. This article combines new in-situ U-Pb dating and O and Hf isotope analyses on Cretaceous (co-magmatic) and pre-Cretaceous (inherited) zircons separated from Albian-Aptian arc-related igneous rocks from the Dominican Republic. The O and Hf systematics of Cretaceous zircons reflect derivation from predominantly juvenile sources and variable mixing with evolved melts, as expected for an oceanic island-arc. Inherited zircons yield U-Pb ages between 256 and 2923 Ma (n = 219). Most studied inherited zircons are Permian-Carboniferous, peaking at ca. 300 Ma, and formed in continental crust to judge from mineral inclusions (including quartz, orthoclase, muscovite and apatite), d18 O (6.16 to 12.67 ‰) and εHf(t) (-11.9 to 1.12) values. Ordovician to Proterozoic zircons yield d18 O between 4.93 and 9.21 ‰ and εHf(t) between-19.9 and 8.5 and hence crystallized in equilibrium with either mantle-derived (juvenile) magmas or magmas derived from melting of supracrustal rocks. For Archean zircons, d18 O between 6.98 and 7.94 ‰ and εHf(t) between-18.5 and-1.6 might reflect contribution of supracrustal materials in the parent magmas. We suggest that arc magmas picked up the inherited zircon cargo from their mantle sources. Accounting paleogeographic and paleo-tectonic constraints, Mexican and Colombian terranes are identified as the potential primary (magmatic) and secondary (sedimentary) sources for the studied inherited zircons. We envisage that inherited zircons were transported downward to the mantle beneath the Caribbean plate as detrital grains on top of the subducting Proto-Caribbean and/or Pacific slabs.
... In the case of the Scotia Sea, volcanic activity would be readily expected in the eastern regions, because they constitute the active parts of the Scotia Arc. Pirie Bank volcanic activity has previously been attributed to events occurring at 11 Ma (Dalziel et al., 2013;Eagles & Jokat, 2014;Pearce et al., 2014), an age similar to that of the magmatic activity affecting the western basins . Recent volcanic events are likewise present along the northern Antarctic Plate, as represented by the Bransfield Strait spreading (1.3 Ma; Lawver et al., 1995) and the north Powell Basin volcanism (3.9 Ma; Barber et al., 1991). ...
Article
Dove Basin, a small oceanic domain located within the southern Scotia Sea, evidences a complex tectonic evolution linked to the development of the Scotia Arc. The basin also straddles the junction between the main Southern Ocean water masses: the Antarctic Circumpolar Current (ACC), the Southeast Pacific Deep Water (SPDW) and the Weddell Sea Deep Water (WSDW). Analysis of multichannel seismic reflection profiles, together with swath bathymetry data, reveals the main structure and sediment distribution of the basin, allowing a reconstruction of the tectonostratigraphic evolution of the basin and assessment of the main bottom water flows that influenced its depositional development. Sediment dispersed in the basin was largely influenced by gravity-driven transport from adjacent continental margins, later modified by deep bottom currents. Sediments derived from melting icebergs and extensive ice-sheets also contributed to a fraction of the basin deposits. We identify four stages in the basin evolution which ―based on regional age assumptions― took place during the early Miocene, middle Miocene, late Miocene-early Pliocene, and late Pliocene-Quaternary. The onsets of the ACC flow in Dove Basin during the early Miocene, the WSDW flow during the middle Miocene, and the SPDW during the late Miocene were influenced by tectonic events that facilitated the opening of new oceanic gateways in the region. The analysis of Dove Basin reveals that tectonics is a primary factor influencing its sedimentary stacking patterns, the structural development of new oceanic gateways permitting the inception of deep-water flows that have since controlled the sedimentary processes.This article is protected by copyright. All rights reserved.
... This is reflected in the rough seafloor fabric close the flanks of the present-day extinct ridge(Fig. 1).Dalziel et al. (2013a) andPearce et al. (2014) relate intraplate volcanism in the Central Scotia Plate to low-angle subduction of part of the Northeast Georgia Rise large igneous province beneath the Scotia Sea. This subduction occurs at the South Sandwich Trench, the eastern boundary of a young and small Sandwich plate(Fig. ...
Chapter
Study of the tectonic development of the Scotia Sea region started with basic lithological and structural studies of outcrop geology in Tierra del Fuego and the Antarctic Peninsula. To nineteenth- and early twentieth-century geologists, the results of these studies suggested the presence of a submerged orocline running around the margins of the Scotia Sea. Subsequent increases in detailed knowledge about the fragmentary outcrop geology from islands distributed around the margins of the Scotia Sea, and later their interpretation in the light of the plate tectonic paradigm led to large modifications in the hypothesis such that by the present day the concept of oroclinal bending in the region persists only in vestigial form. Of the early comparative lithostratigraphic work in the region, only the likenesses between Jurassic–Cretaceous basin floor and fill sequences in South Georgia and Tierra del Fuego are regarded as strong enough to be useful in plate kinematic reconstruction by permitting the interpretation of those regions’ contiguity in mid-Mesozoic times. Marine and satellite geophysical data sets reveal features of the remaining, submerged, 98 % of the Scotia Sea region between the outcrops. These data enable a more detailed and quantitative approach to the region’s plate kinematics. In contrast to long-used interpretations of the outcrop geology, these data do not prescribe the proximity of South Georgia to Tierra del Fuego in any past period. It is, however, possible to reinterpret the geology of those two regions in terms of the plate kinematic history that the seafloor has preserved.
... Each node is represented by the ancestral distribution before the cladogenesis event using ancestral range reconstruction based on the best-fitting model (DIVALIKE+J) and is associated with one or more of the 10 geographic locations depicted in the map at the bottom right (letters A to J and color codes); areas at branch tips represent the current range of species. Past temperature of the Southern Ocean is represented by the white graph behind the phylogeny (22,23); onset of the strengthening of the ACC, by the dashed red line (24,25). (Top Right) Ecological niche disparity through time (DTT) for penguins, with the phylogeny projected onto niche parameter space on the y-axis (maximum surface water temperature) with predicted niche occupancy (PNO) over time (x-axis) reconstructed for internal nodes. ...
Article
Full-text available
Penguins are the only extant family of flightless diving birds. They currently comprise at least 18 species, distributed from polar to tropical environments in the Southern Hemisphere. The history of their diversification and adaptation to these diverse environments remains controversial. We used 22 new genomes from 18 penguin species to reconstruct the order, timing, and location of their diversification, to track changes in their thermal niches through time, and to test for associated adaptation across the genome. Our results indicate that the penguin crown-group originated during the Miocene in New Zealand and Australia, not in Antarctica as previously thought, and that Aptenodytes is the sister group to all other extant penguin species. We show that lineage diversification in penguins was largely driven by changing climatic conditions and by the opening of the Drake Passage and associated intensification of the Antarctic Circumpolar Current (ACC). Penguin species have introgressed throughout much of their evolutionary history, following the direction of the ACC, which might have promoted dispersal and admixture. Changes in thermal niches were accompanied by adaptations in genes that govern thermoregulation and oxygen metabolism. Estimates of ancestral effective population sizes (N e ) confirm that penguins are sensitive to climate shifts, as represented by three different demographic trajectories in deeper time, the most common (in 11 of 18 penguin species) being an increased N e between 40 and 70 kya, followed by a precipitous decline during the Last Glacial Maximum. The latter effect is most likely a consequence of the overall decline in marine productivity following the last glaciation.
... The Antarctic continent was geologically separated from South America 33 MYA (Zachos et al. 2001), and its marine ecosystems were further isolated by the establishment of the Antarctic Circumpolar Current. Subsequent events, like the Drake Passage deep opening (24 MYA), the deepening of the Scotian Arc (around 14 MYA ago) and the intensification of seaice formation (around 5 MYA), triggered the cooling of the sea surface temperatures and reinforced the flow of deep-ocean water (Dalziel et al. 2013;Kemp et al. 2010;Pearce et al. 2014;Pfuhl and McCave 2005). Fossil records indicate that most marine species of warm-water origin disappeared from the Antarctic after this separation and cooling intensification (Griffiths et al. 2013). ...
Article
Full-text available
The establishment of non-indigenous species in the Antarctic, an ecosystem isolated for millions of years, could dramatically alter its unique and endemic biota. In coastal waters, calcified species (e.g., echinoderms, gastropods, bivalves) of benthic communities will be particularly vulnerable to shell-crushing (i.e., durophagous) predators such as crabs. The magnitude of changes in the community structure of shallow Antarctic waters potentially produced by such non-indigenous predators will depend on the innate vulnerability of these species (e.g., shell characteristics) and their potential to respond to novel threats (e.g., behavior, shell thickening). This study aims to evaluate the potential interaction between shell-crushing predators and the limpet Nacella concinna, an endemic Antarctic species and one of the most abundant and conspicuous gastropods in intertidal and shallow subtidal zones of the Antarctic. First, we showed that the king crab Lithodes santolla, a representative species of a group of crabs likely to invade Antarctic waters, was able to break the shell of N. concinna and consume it in the laboratory. We then assessed the shell-breaking force of N. concinna living in four Antarctic habitats (two intertidal and two subtidal) and found wide variation in this trait. Finally, we examined shell-breaking force of a subantarctic congener, N. deurata, which naturally coexists with shell-crushing predators in its native range, and found its shell-breaking force to be similar to the strongest populations of the Antarctic species. Taking into account the crushing claw force of crabs and their high consumption rate of limpets, N. concinna will be highly vulnerable to this kind of durophagous predators and may have limited reaction norms for increasing any inducible defenses such as the thickening and hardening of the shell or changes in their behavior in the face of the almost inevitable invasion of shell-crushing predators into the Antarctic marine ecosystem.
... Lineage 5 is a low-K tholeiitic trend (Fig. 13); this includes glass shards from Mt Burney and South Sandwich Islands volcanic arc (Hubberten et al., 1991;Pearce et al., 2014). This lineage overlaps the composition of some tephra layers found in Siple Dome, Vostok, and Talos Dome ice cores. ...
Article
Explosive volcanoes from the southern Andes are able to disperse ash over wide areas of the Southern Hemisphere, potentially as far as Antarctica. With the aim of improving correlations between sources and tephra in southernmost South America and, possibly, Antarctica, this work presents new field, textural and geochemical data on tephra layers from southern Patagonia and Tierra del Fuego (Argentina and Chile). Major- and trace-element data, obtained on single glass shards allowed to identify tephra sources in Late Glacial-Holocene eruptions from Hudson, Reclus and Mt Burney volcanoes, located in the Southern and Austral Volcanic Zone of the Andean Cordillera. Twelve new radiocarbon age determinations of charcoals, peats and soils have further constrained the correlations between the studied tephra layers and known eruptions from Hudson, Mt Burney and Reclus volcanoes. Therefore, this study contributes to expand the geochemical dataset on volcanic glasses valuable for tephra correlations in South America, and improves the current tephrostratigraphic framework of this region. Furthermore, we revised literature data by compiling a database including Neogene-Quaternary volcanic tephra found in Antarctic ice cores, marine sediments, blue ice and continental outcrops as well as tephra produced by volcanic sources located in Antarctica and circum-Antarctic areas. This revision shows that Antarctic tephra can be correlated with confidence to Antarctic and circum-Antarctic (South Shetlands and South Sandwich Islands) volcanic sources, whereas correlations with South American sources are arguable, and a complete geochemical fingerprinting is needed for validation.
... Thus, the Carboniferous to Permian magmatic rocks of the Bogda Range were most likely derived from metasomatized depleted mantle sources with variable inputs of subducted sediments. On the Th-Zr/117-Nb/16 (Wood et al., 1979) and Nb/Yb vs Th/Yb (Pearce et al., 2014) discrimination diagrams (Fig. 10), most volcanic rocks consistently plot in the field of volcanic arcs except a few Late Carboniferous and Early Permian basalts (Fig. 10c and e). Therefore, the Carboniferous to Early Permian magmatic rocks in the Bogda Range were most likely formed in a supra-subduction zone. ...
Article
The Late Paleozoic magmatic evolution of the Bogda Range (Chinese North Tianshan) is important for understanding the accretionary history of the Central Asian Orogenic Belt. We investigated the Carboniferous and Lower Permian volcanic and sedimentary sequences of the Daheyan section, southern Bogda Range, and present new zircon U-Pb ages and whole-rock geochemical data for the volcanic rocks. One Carboniferous rhyolite is dated at 298 ± 8 Ma; a Permian basalt yielded many Proterozoic zircon xenocrysts, and its maximum age (∼297 Ma) is constrained by the detrital zircon ages of the sandstone that stratigraphically underlies it. These volcanic rocks belong to calc-alkaline series. We further synthesize previous geochronological, geochemical and isotopic data of magmatic and sedimentary rocks in the Bogda Range. The available data indicate that the magmatism occurred continuously from 350 Ma to 280 Ma. A comprehensive analysis allows us to propose that: (1) the Carboniferous to Early Permian magmatic rocks of the Bogda Range generally show consistent arc-type features; (2) increasing mantle input through time suggests intra-arc extension in a supra-subduction zone; (3) the localized occurrence of Early Permian alkaline pillow basalts and deep water sediments close to the major shear zone advocate a transtensional crustal thinning during the transition from Carboniferous convergence to Early Permian transcurrent tectonics; (4) occurrence of a large number of Proterozoic zircon xenocrysts in the Late Paleozoic magmatic rocks, and Proterozoic detrital zircons in the coeval clastic sediments suggest a continental or transitional basement of the Bogda Arc; (5) subduction in the Bogda area terminated prior to the deposition of Middle Permian terrestrial sediments.
Article
Full-text available
The Cenozoic development of the Scotia Sea and opening of Drake Passage evolved in a complex tectonic setting with sea‐floor spreading accompanied by the dispersal of continental fragments and the creation of rifted oceanic basins. The post‐Eocene tectonic setting of the Scotia Sea is relatively well established, but Late Mesozoic paleo‐locations of many continental fragments prior to dispersal are largely unknown, with almost no geological control on the submerged banks. Detrital zircon analysis of dredged metasedimentary rocks of Bruce Bank from the South Scotia Ridge demonstrates a geological continuity with the South Orkney microcontinent (SOM) and also a clear geological affinity with the Trinity Peninsula Group metasedimentary rocks of the Antarctic Peninsula and components of the Cordillera Darwin Metamorphic Complex of Tierra del Fuego. Kinematic modelling indicate an Antarctic Plate origin for Bruce Bank and the SOM is the most plausible setting, prior to translation to the Scotia Plate during Scotia Sea opening.
Article
We are beginning to appreciate that the origin of the modern Antarctic marine fauna is related to a series of key events throughout the Cenozoic era. In the first of these, the mass extinction at the Cretaceous–Palaeogene boundary (66 Ma) reset the evolutionary stage and led to a major radiation of modern taxa in the benthic realm. Although this took place in a greenhouse world, there is evidence to suggest that the radiation was tempered by the seasonality of primary productivity, and this may be a time‐invariant feature of the polar regions. Although there could well have been a single, abrupt extinction event at c. 34 Ma, there is also evidence to suggest a phased extinction of various taxa over a period of millions of years. Important new molecular phylogenetic data are indicating that a wide variety of both benthic and pelagic taxa radiated shortly after a second major phase of cooling at c. 14 Ma. Such a phenomenon is linked to a series of major palaeoceanographic changes, which in turn led to a proliferation of diatom‐based ecosystems. Although the modern benthic marine fauna can be traced back some 45–50 Myr, a substantial component of the modern pelagic one may be less than 14 Myr old. The latter is also characterized by assemblages of high abundance but comparatively low species richness and evenness. A distinctive signature of low diversity but high dominance within Antarctic marine assemblages was maintained by the interplay between temperature and primary productivity throughout the Cenozoic.
Article
Full-text available
We investigate the effect of the westerly rotation of the lithosphere on the active margins that surround the Americas and find good correlations between the inferred easterly-directed mantle counterflow and the main structural grain and kinematics of the Andes and Sandwich arc slabs. In the Andes, the subduction zone is shallow and with low dip, because the mantle flow sustains the slab; the subduction hinge converges relative to the upper plate and generates an uplifting doubly verging orogen. The Sandwich Arc is generated by a westerly-directed SAM (South American) plate subduction where the eastward mantle flow is steepening and retreating the subduction zone. In this context, the slab hinge is retreating relative to the upper plate, generating the backarc basin and a low bathymetry single-verging accretionary prism. In Central America, the Caribbean plate presents a more complex scenario: a) To the East, the Antilles Arc is generated by westerly directed subduction of the SAM plate, where the eastward mantle flow is steepening and retreating the subduction zone. b) To the West, the Middle America Trench and Arc are generated by the easterly-directed subduction of the Cocos plate, where the shallow subduction caused by eastward mantle flow in its northern segment gradually steepens to the southern segment as it is infered by the preexisting westerly-directed subduction of the Caribbean Plateau. In the frame of the westerly lithospheric flow, the subduction of a divergent active ridge plays the role of introducing a change in the oceanic/continental plate’s convergence angle, such as in NAM (North American) plate with the collision with the Pacific/Farallon active ridge in the Neogene (Cordilleran orogenic type scenario). The easterly mantle drift sustains strong plate coupling along NAM, showing at Juan de Fuca easterly subducting microplate that the subduction hinge advances relative to the upper plate. This lower/upper plate convergence coupling also applies along strike to the neighbor continental strike slip fault systems where subduction was terminated (San Andreas and Queen Charlotte). The lower/upper plate convergence coupling enables the capture of the continental plate ribbons of Baja California and Yakutat terrane by the Pacific oceanic plate, transporting them along the strike slip fault systems as para-autochthonous terranes. This Cordilleran orogenic type scenario, is also recorded in SAM following the collision with the Aluk/Farallon active ridge in the Paleogene, segmenting SAM margin into the eastwardly subducting Tupac Amaru microplate intercalated between the proto-Liquiñe-Ofqui and Atacama strike slip fault systems, where subduction was terminated and para-autochthonous terranes transported. In the Neogene, the convergence of Nazca plate with respect to SAM reinstalls subduction and the present Andean orogenic type scenario.
Preprint
Full-text available
The plate tectonic cycle produces chemically distinct mid-ocean ridge basalts (MORB) and arc volcanics, with the latter enriched in fluid-mobile elements and depleted in Nb owing to fluxes from the subducted slab. Basalts from back-arc basins (BABB), with intermediate compositions, show that the subduction flux can escape the arc. Hence it is puzzling why arc signatures have rarely been recognized in MORB. Here we report the first MORB samples with distinct arc signatures, akin to BABB, from the Arctic Gakkel Ridge. A new high precision dataset for 576 Gakkel samples suggests a pervasive subduction influence. This influence can also be identified in Atlantic and Indian MORB with a “BABB filter”, but is nearly absent in Pacific MORB. This global distribution reflects the control of a “subduction shield” that has surrounded the Pacific Ocean for 180Myr. Statistics suggest that a flux equivalent to ~ 13% of output at arcs is incorporated into the convecting upper mantle.
Article
The mountainous, glaciated island of South Georgia is the crest of one of the most isolated fragments of continental crust on Earth. It is located approximately 1700 km east of the southern termination of the Andean Cordillera of South America. The island is primarily composed of Lower Cretaceous turbidites, the infill of a marginal basin floored by stretched continental and ophiolitic crust. Remnants of a volcanic arc are preserved on offshore islets to the southwest. The Pacific hinterland of the southernmost Andes is missing in Tierra del Fuego, terminating at a submarine escarpment forming the continental margin immediately east of Cape Horn. The arc and back-arc basin infill rocks of South Georgia correspond exactly to part of the missing Cordilleran hinterland. The mechanism of transport of the South Georgia microcontinent eastward relative to South America remains obscure, but likely involved some form of ‘escape tectonics’ during mid- to Late Cretaceous counterclockwise rotation of the arc that led to closure and inversion of the marginal basin.
Article
Full-text available
The plate tectonic cycle produces chemically distinct mid-ocean ridge basalts and arc volcanics, with the latter enriched in elements such as Ba, Rb, Th, Sr and Pb and depleted in Nb owing to the water-rich flux from the subducted slab. Basalts from back-arc basins, with intermediate compositions, show that such a slab flux can be transported behind the volcanic front of the arc and incorporated into mantle flow. Hence it is puzzling why melts of subduction-modified mantle have rarely been recognized in mid-ocean ridge basalts. Here we report the first mid-ocean ridge basalt samples with distinct arc signatures, akin to back-arc basin basalts, from the Arctic Gakkel Ridge. A new high precision dataset for 576 Gakkel samples suggests a pervasive subduction influence in this region. This influence can also be identified in Atlantic and Indian mid-ocean ridge basalts but is nearly absent in Pacific mid-ocean ridge basalts. Such a hemispheric-scale upper mantle heterogeneity reflects subduction modification of the asthenospheric mantle which is incorporated into mantle flow, and whose geographical distribution is controlled dominantly by a “subduction shield” that has surrounded the Pacific Ocean for 180 Myr. Simple modeling suggests that a slab flux equivalent to ~13% of the output at arcs is incorporated into the convecting upper mantle.
Article
The Scotia Arc, situated between South America and Antarctica, is one of the Earth’s most important ocean gateways and former land bridges. Understanding its structure and development is critical for the knowledge of tectonic, paleoenvironmental and biological processes in the southern oceans and Antarctica. It extends from the Drake Passage in the west, where the Shackleton Fracture Zone forms a prominent, but discontinuous, bathymetric ridge between the southern South American continent and the northern tip of the Antarctic Peninsula to the active intra-oceanic volcanic arc forming the South Sandwich Island in the east. The tectonic arc comprises the NSR to the north and to the south the South Scotia Ridge, both transcurrent plate margins that respectively include the South Georgia and South Orkney microcontinents. The Scotia and Sandwich tectonic plates form the major basin within these margins. As the basins opened, formation of first shallow sea ways and then deep ocean connections controlled the initiation and development of the Antarctic Circumpolar Current, which is widely thought to have been important in providing the climatic conditions for formation of the polar ice-sheets. The evolution of the Scotia Arc is therefore of global palaeoclimatic significance. The Scotia Arc has been the focus of increasing international research interest. Many recent studies have stressed the links and interactions between the solid Earth, oceanographic, palaeoenvironmental and biological processes in the area. This special issue presents new works that summarize significant recent research results and synthesize the current state of knowledge for the Scotia Arc.
Article
Full-text available
Reports the results of a magnetostratigraphic study of the sediments recovered at Site 701, a deep-water site located within the gateway, and Site 702, a shallow-water site located near the crest of the Islas Orcadas Rise. The sequence of Pliocene-Pleistocene reversals observed at Site 701 is readily correlated with the Brunhes Chron through Chron C3A. The polarity sequence observed in the very weakly magnetized middle to upper Eocene nannofossil chalks recovered from Hole 702B is correlated with Chrons C18 through C21. The correlation of the polarity sequences at these two sites provides a temporal framework for these sediments and makes it possible to calibrate southern high-latitude biostratigraphic datums to the geomagnetic polarity time scale. -from Authors
Article
Full-text available
A reconnaissance survey of the little-known E S Scotia Ridge was carried out aboard the RRS Shackleton in March 1976. The region shows considerable topographic relief, being occupied by several discrete crustal blocks with tops at 350-2200m depth and steep sides reaching 5000m in places. The blocks are strongly magnetized at shallow depth, and recent sediment cover is thin and discontinuous. Three dredge hauls from steep scarps on two of the blocks yielded a homogeneous collection of lavas and agglomerates. Chemical analyses show the lavas to be low-K tholeiites virtually indistinguishable from rocks of the active S Sandwich island arc, and K-Ar dates of 12-20Ma have been obtained from representative samples. We conclude that the blocks sampled, as well as other similar features in the region, were part of an intraoceanic island arc, the Discovery arc, similar to the S Sandwich arc but active before the present episode of back-arc extension, which began 7-8Ma ago. By analogy with present-day behavior of the Sandwich plate, earlier subduction was probably coupled to back-arc extension; such a spreading feature located in the central Scotia Sea would explain the anomalous crustal structure and complex magnetic character of that region. Subduction beneath this S part of the Discovery arc may have stopped because of its failure to consume oceanic lithosphere of the Antarctic plate. Such a situation would arise after collision of a section of the SW Atlantic spreading ridge with the trench.-AuthorsDept of Geol Sci, Univ of Birmingham, Birmingham, B15 2TT, UK.
Article
Full-text available
Northeast Georgia Rise is located on inferred oceanic crust that is considered Albian in age and to have formed during the separation of Africa and South America. Basalt overlying a weathered regolith was recovered at Site 698 and a basaltic substratum at other sites is inferred from the downhole variation in pore-water chemistry. The provenance of a 2-m-thick gravel bed containing abundant clasts of continental lithologies displaced into lower Oligocene ooze at Site 699 is an enigma. We infer that at least part of the Northeast Georgia Rise was formed at a spreading center by excessive volcanism. At least two episodes of deformation have subsequently modified the topography of the rise. -from Authors
Article
Full-text available
In the study of geochemical mass balances at subduction zones, the composition of the mantle wedge prior to additions from the slab is a critically important yet poorly constrained parameter. Deconvolving the influence of ancient versus modern enrichments is particularly difficult, especially when considering elements that are highly mobile. Here we provide an alternative approach, using less mobile elements, and a filter to remove the effects of recent slab additions. We provide new Hf isotope data for 30 Mariana Trough (MT) backarc basin lavas. Once filtered, Hf and Nd isotope ratios are highly correlated, of Indian mid-oceanic ridge basalt character, and display variations similar to ocean ridges of comparable lengths. The isotopic variability observed in this "ambient mantle" provides a new paradigm for the interpretation of the varied volcanic products of the arc. Thus, shoshonites associated with the northern termination of the backarc basin rift axis reflect the interaction of a subducted sediment melt with an isotopically enriched mantle source. In contrast, the large volcanoes of the Central Island province have a consistent offset in Nd isotope compositions from the MT array resulting from fluid addition. Existing data for smaller edifices in the submarine portion of the arc have larger variations resulting from fluid addition on a more local scale. We suggest that the similar characterization of ambient mantle elsewhere may help to resolve many conflicting geochemical observations in arc lavas worldwide.
Article
Full-text available
Petrographic and geochemical investigations were carried out on 21 ash layers from four sites of ODP Legs 113 and 114 in the southern Atlantic Ocean. With the help of geochemical data and petrographic characterization three rock series can be distinguished for stratigraphically different ash layers from Site 701 (Leg 114) located east of the South Sandwich Island Arc, whereas the Leg 113 tephras from the southern slope of the South Orkney Microcontinent belong to another magmatic series. Geochemical correlation of the Leg 113 tephras with possible source areas indicates that they were probably erupted from the Antarctic Peninsula. The Miocene ashes from Site 701 are probably derived from the now-extinct Discovery Arc, the precursor of the South Sandwich Islands. The Pliocene ashes from the site show some affinity with the South Shetland Islands, although the available data do not permit a clear correlation. The Quaternary ashes from Site 701 display a chemistry typical of island-arc tholeiites and are therefore most probably derived from eruptions on the South Sandwich Islands. Because of their distant position the southern Andes seem to be rather improbable as a potential source region for the tephra layers investigated.
Article
Full-text available
The Bismarck Sea region of Papua New Guinea is marked by recent arc–continent collision giving rise to a highly dynamic tectonic environment, characterised by complex plate interactions that are yet to be fully understood. We present a new crustal and upper mantle crustal architecture model for northeastern Papua New Guinea and western New Britain that reveals complex tectonic geometries of overprinting slab subduction and partial continental subduction, resulting in a unique setting in which to investigate along-arc magmatic variation. Earthquake hypocentre databases are combined with detailed topography and seafloor structure together with geology and regional-scale gravity to unravel the sub-surface structure of northeastern Papua New Guinea. These data are used in conjunction with an updated 3-D slab map of the region to propose a new interpretation of the area whereby Australian continental crust extends as an underthrust block beneath the accreted Finisterre Terrane. The subducting continental crust combined with slab stagnation has resulted in a complex pattern of arc-related geochemical signatures from east to west along the Bismarck arc. In the east, where the Solomon Sea plate is subducting beneath New Britain, the sedimentary component is low, whereas in the west, the arc volcanics exhibit a greater sedimentary component, consistent with subduction of Australian crustal sediments. As a result, a new plate reconstruction is provided for the region together with a forward-looking reconstruction of the Papuan peninsula, the Solomon Sea plate and New Britain that illustrates that the same process will likely be repeated in some 5–10 m.y.
Article
Full-text available
pdf , The following values have no corresponding Zotero field: ID - 513
Chapter
Full-text available
The Scan Basin is a small oceanic basin located in the southern central Scotia Sea, north of the Bruce Passage (BP) which represents the main gateway between the Weddell Sea and the Scotia Sea. A seismic stratigraphic analysis has been carried out on multichannel seismic reflection profiles to determine the Miocene to present evolution of the basin. Five seismic units are identified. The oldest unit (Sc5) was deposited during seafloor spreading in the Scan Basin. The upper four units represent the post-spreading deposits and show three major evolutionary stages: A) pre-BP opening (unit Sc-4); B) BP opening (units Sc3 and Sc2) and C) post-BP opening (unit Sc1). Stage B occurred once the BP was deep enough to allow Weddell Sea Deep Water (WSDW) into the Scotia Sea. This led to the development of large, northwards-migrating contourite drifts. Stage C developed during the present-day sinistral transcurrent compressive regime.
Article
Full-text available
New high precision PIMMS Hf and Pb isotope data for 14-28 Ma basalts recovered during ODP Leg 187 are compared with zero-age dredge samples from the Australian-Antarctic Discordance (AAD). These new data show that combined Nd-Hf isotope systematics can be used as an effective discriminant between Indian and Pacific MORB source mantle domains. In particular, Indian mantle is displaced to lower εNd and higher εHf ratios compared to Pacific mantle. As with Pb isotope plots, there is almost no overlap between the two mantle types in Nd-Hf isotope space. On the basis of our new Nd-Hf isotope data, we demonstrate that Pacific MORB-source mantle was present near the eastern margin of the AAD from as early as 28 Ma, its boundary with Indian MORB-source mantle coinciding with the eastern edge of a basin-wide arcuate depth anomaly that is centered on the AAD. This observation rules out models requiring rapid migration of Pacific MORB mantle into the Indian Ocean basin since separation of Australia from Antarctica. Although temporal variations in isotopic composition can be discerned relative to the fracture zone boundary of the modern AAD at 127°E, the distribution of different compositional groups appears to have remained much the same relative to the position of the residual depth anomaly for the past 30 m.y. Thus significant lateral flow of mantle along the ridge axis toward the interface appears unlikely. Instead, the dynamics that maintain both the residual depth anomaly and the isotopic boundary between Indian and Pacific mantle are due to eastward migration of the Australian and Antarctic plates over a stagnated, but slowly upwelling, slab oriented roughly orthogonal to the ridge axis. Temporal and spatial variations in the compositions of Indian MORB basalts within the AAD can be explained by progressive displacement of shallower Indian MORB-source mantle by deeper mantle having a higher εHf composition ascending ahead of the upwelling slab. Models for the origin of the distinctive composition of the Indian MORB-source based on recycling of a heterogeneous enriched component that consist of ancient altered ocean crust plus
Article
Full-text available
The processes of arc initiation at the margin of an oceanic plateau are remarkably well preserved along the southern coastline of eastern Costa Rica and western Panama. We present new results of a combined tectonostratigraphic and petrologic study with which protoarc initiation (75-73 Ma) at the margin of an oceanic plateau (89-85 Ma) is documented. Dykes of protoarc igneous rocks within the plateau and occurrences of protoarc igneous rocks are widely distributed. These types of field observations, geochemical data, and paleontologic ages for Late Cretaceous to Eocene fore-arc rocks of the Golfito Complex and Azuero Marginal Complex (southern Costa Rica and western Panama) provide the first direct evidence that a Coniacian-early Santonian oceanic plateau forms the arc basement. Stratigraphic and geochemical constraints from Golfito and Azuero indicate subduction initiation in south Central America, associated with geochemically distinctive suprasubduction igneous rocks, occurred in the late Campanian along the margin of the newly defined Azuero Plateau. Overall, the Golfito Complex and Azuero Marginal Complex provide a significant opportunity for exploration of petrologic mechanisms linking some oceanic plateaus to the growth of continents. The Azuero Plateau may extend further toward the Colombian Basin and forms thickened Caribbean crust. It served as a nucleus for accretion of additional oceanic plateaus, seamounts, and oceanic islands of Pacific origins.
Article
Full-text available
This contribution focusses on terminal stage subduction, often triggered by continent-continent or arc- continent collision. The landlocked basin setting of the Mediterranean region provides unique opportunities to study terminal stage subduction and its consequences. We use seismic tomography results on lithosphere and upper mantle structure as a source of information on plate boundary structure, and concentrate on the lithospheric scale aspects. Combining this structural information with process-oriented numerical modelling studies and regional observations, we present a D model for convergent plate boundary evolution after collision, in which slab detachment and the formation of tear or STEP (Subduction-Transform-Edge-Propagator; see R. Govers and M.J.R. Wortel, EPSL, 236, 505- 523, 2005) faults are key elements. A STEP fault laterally decouples subducting lithosphere from non- subducting lithosphere in a scissor type of fashion. It enhances the ability of a slab to retreat through the mantle flow around the edge of the subducted slab. In this way collision and back-arc extension may occur in close proximity. In our study area this specifically pertains to collision along the north African margin, STEP formation in easterly direction, CCW rotation of the southern Apennines slab and the opening of the Tyrrhenian Sea. Vertical tearing of subducted lithosphere may play an important role as well, but is probably not crucial. Similar processes are likely to have occurred in the eastern Mediterranean. On the basis of the good agreement between the Mediterranean-based model and the evolution of the Tonga-Fiji region we expect that the model may shed light on other complex convergent plate boundary regions, as well. In summary: Upon continental (or arc-continent) collision, along-trench variations in lithospheric properties of the subducting lithosphere may lead to disruption and segmentation of the subduction system. Following slab detachment along limited segments of a convergent plate boundary, the development of STEP faults is expected. These faults contribute to an increase in arc curvature within plate boundary segments. This contributes to the sinuous geometry of long subduction systems such as in the western and southwest Pacific.
Article
Full-text available
The Bonin Ridge and trench slope preserves the geological record of subduction initiation and subsequent evolution of the Izu–Bonin–Mariana (IBM) arc. Diving and dredging in this region has revealed a bottom to top stratigraphy of: 1) mantle peridotite, 2) gabbroic rocks, 3) a sheeted dyke complex, 4) basaltic pillow lavas, 5) boninites and magnesian andesites, 6) tholeiites and calcalkaline arc lavas. This forearc stratigraphy is remarkably similar to that found in other IBM forearc localities and many ophiolites. Zircon U–Pb ages obtained here for gabbros are 51.6–51.7Ma. The overlying basalts have 40Ar/39Ar ages of 48–52Ma. A forearc basalt from the Mariana forearc near Guam produced a similar 40Ar/39Ar age of 51.1Ma. The collective geochronology of igneous rocks from throughout the IBM system now indicates that the first basaltic magmatism at subduction initiation was produced by decompression melting of the mantle and took place at 51–52Ma. The change to flux melting and boninitic volcanism took 2–4m.y., and the change to flux melting in counterflowing mantle and “Normal” arc magmatism took 7–8m.y. This evolution from subduction initiation to arc normalcy occurred nearly simultaneously along the entire length of the IBM subduction system. Mesozoic rocks found in the deep Bonin forearc suggest that the overriding plate at subduction initiation consisted of Mesozoic terranes and subduction preceded the opening of most or all of the Philippine Sea basins. The contemporaneousness of IBM forearc magmatism with the major change in plate motion in Western Pacific at ca. 50Ma suggests that the two events are intimately linked.
Article
Full-text available
Recent diving with the JAMSTEC Shinkai 6500 manned submersible in the Mariana fore arc southeast of Guam has discovered that MORB-like tholeiitic basalts crop out over large areas. These “fore-arc basalts” (FAB) underlie boninites and overlie diabasic and gabbroic rocks. Potential origins include eruption at a spreading center before subduction began or eruption during near-trench spreading after subduction began. FAB trace element patterns are similar to those of MORB and most Izu-Bonin-Mariana (IBM) back-arc lavas. However, Ti/V and Yb/V ratios are lower in FAB reflecting a stronger prior depletion of their mantle source compared to the source of basalts from mid-ocean ridges and back-arc basins. Some FAB also have higher concentrations of fluid-soluble elements than do spreading center lavas. Thus, the most likely origin of FAB is that they were the first lavas to erupt when the Pacific Plate began sinking beneath the Philippine Plate at about 51 Ma. The magmas were generated by mantle decompression during near-trench spreading with little or no mass transfer from the subducting plate. Boninites were generated later when the residual, highly depleted mantle melted at shallow levels after fluxing by a water-rich fluid derived from the sinking Pacific Plate. This magmatic stratigraphy of FAB overlain by transitional lavas and boninites is similar to that found in many ophiolites, suggesting that ophiolitic assemblages might commonly originate from near-trench volcanism caused by subduction initiation. Indeed, the widely dispersed Jurassic and Cretaceous Tethyan ophiolites could represent two such significant subduction initiation events.
Article
Full-text available
Joint inversion of isochron and flow line data from the flanks of the extinct West Scotia Ridge spreading center yields five reconstruction rotations for times between the inception of spreading prior to chron C8 (26.5 Ma), and extinction around chron C3A (6.6–5.9 Ma). When they are placed in a regional plate circuit, the rotations predict plate motions consistent with known tectonic events at the margins of the Scotia Sea: Oligocene extension in Powell Basin; Miocene convergence in Tierra del Fuego and at the North Scotia Ridge; and Miocene transpression at the Shackleton Fracture Zone. The inversion results are consistent with a spreading history involving only two plates, at rates similar to those between the enclosing South America and Antarctica plates after chron C5C (16.7 Ma), but that were faster beforehand. The spreading rate drop accompanies inception of the East Scotia Ridge back-arc spreading center, which may therefore have assumed the role of the West Scotia Ridge in accommodating eastward motion of the trench at the eastern boundary of the Scotia Sea. This interpretation is most easily incorporated into a model in which the basins in the central parts of the Scotia Sea had already formed by chron C8, contrary to some widely accepted interpretations, and which has significant implications for paleoceanography and paleobiogeography.
Article
Full-text available
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.
Article
Full-text available
[1] Abstract: The application of multiple collector inductivelycoupled plasma source mass spectrometry(MC-ICPMS) to 176 Lu- 176 Hf and 92 Nb- 92 Zr chronometryhas been hampered bycomplex Zr-Hf purification procedures that involve multiple ion exchange column steps. This studypresents a single-column separation procedure for purification of Hf and Lu byion exchange using Eichrom 1 LnSpec resin. The sample is loaded in pure HCl, and element yields are not dependent on the sample matrix. For 92 Nb- 92 Zr chronometry, a one-column procedure for purification of Zr using Biorad 1 AG1- 8 resin is described. Titanium and Mo are completelyremoved from the Zr, thus enabling accurate 92 Zr measurements. Zirconium and Nb are quantitativelyseparated from rock samples using Eichrom Ln-Spec resin, allowing measurements of Zr/Nb with a precision of better than ±5% (2s). The Ln-Spec and anion resin procedures maybe combined into a three-column method for separation of Zr-Nb, Hf, Ta, and Lu from rock samples. For the first time, this procedure permits combined isotope dilution measurements of Nb/Ta, Zr/Hf, and Lu/Hf using a mixed 94 Zr- 176 Lu- 180 Hf- 180 Ta tracer. Analytical protocols for Zr and Hf isotope measurements using the Micromass Isoprobe, a second generation,
Article
Full-text available
The nature of back-arc extension in the East Scotia Sea is re-examined with the use of an enlarged geophysical data set. Well developed oceanic magnetic lineations confirm that the present spreading episode started about 8 Ma ago, that spreading is asymmetric, and that the total rate increased from 50 to 70 mm/a about 1.5 Ma ago. Most of the currently active South Sandwich volcanic island arc lies upon ocean floor only 6-8 Ma old and generated at the current spreading ridge. Subsequent extension has not modified the curvature of the arc. East--west magnetic lineations of Miocene age in the Central Scotia Sea and contemporaneous low-K arc tholeiites dredged from the eastern South Scotia Ridge (Discovery Bank) indicate a regime of coupled subduction and back-arc extension preceding that occurring now. A speculative model involving a series of collisions of parts of this earlier Discovery trench with ridge crest sections of the South American--Antarctic plate boundary explains the transformation of this earlier regime into the present, self-contained Sandwich plate regime. The considerable small-scale variability observed in the back-arc region may be seen as an inevitable consequence of the action of the ridge--trench collision mechanism. The entire Scotia Sea could have formed by a similar kind of back-arc extensional modification of the South American--Antarctic plate boundary.
Article
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
Article
The South Sandwich Islands and associated seamounts constitute the volcanic arc of an active subduction system situated in the South Atlantic. We introduce a map of the bathymetry and geological setting of the South Sandwich Islands and the associated East Scotia Ridge back-arc spreading centre that consists of two sides: side 1, a regional overview of the volcanic arc, trench and back-arc, and side 2, detailed maps of the individual islands. Side 1 displays the bathymetry at scale 1:750 000 of the intra-oceanic, largely submarine South Sandwich arc, the back-arc system and other tectonic boundaries of the subduction system. Satellite images of the islands on side 2 are at scales of 1:50 000 and 1:25 000 with contours and main volcanological features indicated. These maps are the first detailed topological and bathymetric maps of the area. The islands are entirely volcanic in origin, and most have been volcanically or fumarolically active in historic times. Many of the islands are ice-covered, and the map forms a baseline for future glaciological changes caused by volcanic activities and climate change. The back-arc spreading centre consists of nine segments, most of which have rift-like morphologies.
Chapter
This chapter reviews the tectonic evolution of the East Scotia Sea, testing and extending previously published conclusions in light of the additional and expanded data sets now available. The East Scotia Sea floor was generated behind the east-migrating South Sandwich Trench, at a spreading center now lying along 30°W. On its western flank, lineated magnetic anomalies are identified out to at least anomaly 5 (10–11 Ma) and probably out to anomaly 5B (ca. 15 Ma). Spreading was essentially symmetric at about 27 mm/year from 15 Ma to about 5–7 Ma, then slowly accelerated. From 4 Ma to 1.7 Ma, spreading was at 50 mm/year and slightly asymmetric. Since 1.7 Ma, spreading has been up to 15% asymmetric, favoring accretion to the arc flank, within an overall rate of 65 mm/year. Asymmetry is confined within segments bounded by fracture zones that in some cases were created only at 1.7 Ma. A relation between asymmetric spreading, segmentation, and ridge migration seems likely. The median valley is between 6 and 20 km wide and exceptionally is up to 1200 m deep, but usually is smaller and the ridge flanks smooth, as is typical of faster spreading. The ridge crest depth is 500 m or more deeper than the global MOR average. Before 3–4 Ma the ridge was rougher and probably the ridge crest shallower.
Article
The South Sandwich Islands are one of the world's classic examples of an intra-oceanic arc. Formed on recently generated back-arc crust, they represent the earliest stages of formation of arc crust, and are an excellent laboratory for investigating variations in magma chemistry resulting from mantle processes, and generation of silicic magmas in a dominantly basaltic environment. Two volcanoes are examined. Southern Thule in the south of the arc is a complex volcanic edifice with three calderas and compositions that range from mafic to silicic and tholeiitic to calc-alkaline. It is compared to the Candlemas-Vindication edifice in the north of the arc, which is low-K tholeiitic and strongly bimodal from mafic to silicic. Critically, Southern Thule lies along a cross-arc, wide-angle seismic section that reveals the velocity structure of the underlying arc crust. Trace element variations are used to argue that the variations in both mantle depletion and input of a subducted sediment component produced the diverse low-K tholeiite, tholeiite and calc-alkaline series. Primitive, mantle-derived melts fractionally crystallized by c. 36% to produce the most Mg-rich erupted basalts and a high-velocity cumulitic crustal keel. Plagioclase cumulation produced abundant high-Al basalts (especially in the tholeiitic series), and strongly influenced Sr abundances in the magmas. However, examination of volumetric and geochemical arguments indicates that the silicic rocks do not result from fractional crystallization, and are melts of amphibolitic arc crust instead.
Article
The bulk composition of the continental crust throughout geological history is thought by most previous workers to be andesitic. This assumption of an andesitic bulk composition led to an early hypothesis by Taylor (1967) that the continental crust was created by are magmatism. This hypothesis for the origin of continental crust was challenged by several authors because: (i) the mean rate of are crust addition obtained by Reymer and Schubert (1984) is too small to account for some certain phases of rapid crustal growth; and (ii) the bulk composition of ocean island arcs, the main contributor to the Archean and early Proterozoic crust, is basaltic rather than andesitic (Arculus 1981; Pearce et al. 1992). New data fi om the Northern Izu-Bonin are are presented here which support the Taylor (1967) hypothesis for the origin of the continental crust by andesitic arc magma. A geological interpretation of P. wave crustal structure obtained fi om the Northern Izu-Bonin are by Suyehiro et al. (1996) indicates that the are crust has four distinctive lithologic layers: fi om top to bottom: (i) a 0.5-2-km-thick layer of basic to intel mediate volcaniclastic, lava and hemipelagite (layer A); (ii) a 2-5-km-thick basic to intermediate volcaniclastics, lavas and intrusive layer (layer B); (iii) a 2-7-km-thick layer of felsic (tonalitic) rocks (layer C); and (iv) a 4-7-km-thick layer of mafic igneous rocks (layer D). The chemical composition of the upper and middle part of the northern Izu-Bonin are is estimated to be similar to the average continental crust by Taylor and McLennan (1985). The rate of igneous addition of the Northern Izu-Bonin are since its initial 45-Ma magmatism was calculated as 80 km(3)/km per million years. This rate of addition is considered to be a reasonable estimate for all arcs in the western Pacific. Using this rate, the global rate of crustal growth is estimated to be 2.96 km(3)/year which exceeds the average rate of crustal growth since the formation of the Earth (1.76 km(3)/year). Based on this estimate of continental growth and the previously documented sediment subduction and tectonic erosion rate (1.8 km(3)/year, von Heune & Scholl 1991), several examples of growth curves of the continental crust are presented here. These growth curves suggest that at least 50% of the present volume of the continental crust can be explained by are magmatism. This conclusion indicates that are magmatism is the most important contributor to the formation of continental crust, especially at the upper crustal level.
Article
The volcanoes of the South Sandwich island arc follow three distinct series: low-K tholeiitic (followed by Zavodovski, Candlemas, Vindication, Montagu and Bristol), tholeiitic (followed by Visokoi, Saunders and Bellinghausen) and calcalkaline (followed by Leskov, Freehand and part of Cook and Thule). Flux calculations indicate that the percentage contribution of the subduction component to the mantle source of all three series varies from undetectable (e.g. Zr) through small (e.g. Nd=20%) and moderate (e.g. La, Ce, Sr=50–80%) to dominant (e.g. Pb, K, Ba, Rb, Cs >90%) with little change along the arc. Isotope systematics (Pb, Nd, Sr) show that this subduction component obtains a greater contribution from altered oceanic crust than from pelagic sediment. Elements for which the subduction contribution is small show that the mantle is already depleted relative to N-MORB mantle (equivalent to loss of an ∼2⋅5% melt fraction) before melting beneath the arc. After addition of the subduction component, dynamic melting of this depleted mantle then causes the variations in K that distinguish the three series. The estimated degree of partial melting (∼20%) is slightly greater than that beneath ocean ridges, though geothermometry suggests that the primary magma temperature (∼1225°C) is similar to that of primary MORB. About half of the melting may be attributed to volatile addition, and half to decompression. Dynamic melting involving three-dimensional, two-phase flow may be needed to explain fully the inter-island variations.
Article
The central Scotia Sea, located between the South American and Antarctic plates, is an inte- gral part of the marine conduit that permits eastward deep-water flow from the Pacific Ocean to the Atlantic Ocean. The geologic history of the central Scotia Sea is therefore critical for a full understanding of the initiation and subsequent evolution of the complete, deep Antarctic Circumpolar Current, widely believed to have been a key factor in the history of Antarctic glaciation. Here, we present new evidence on the nature and age of the central Scotia Sea floor. Multibeam surveys and the first dredged samples indicate that a now-submerged remnant volcanic arc may have formed a barrier to deep eastward oceanic circulation until after the mid-Miocene climatic optimum. Inception and development of a full deep Antarctic Circum- polar Current may therefore have been important, not in the drop in global temperatures at the Eocene-Oligocene boundary as long surmised, but in the subsequent late Miocene global cooling and intensification of Antarctic glaciation.
Article
Results are reported from seven heat flow stations in small basins of the southern part of the central Scotia Sea (CSS), undertaken in order to determine basement ages. The basins are small, which makes magnetic anomaly-based ages ambiguous and preserves basin subsidence that may have been anomalous as a result of local factors. The fact that these small basins formed in a back-arc setting adds additional uncertainty to depth-based age estimates. The results confirm that basin extension commenced in the Eocene, and indirectly support a relatively young, backarc origin for the northern CSS, but do not affect previously published suggestions of the age of onset of the Antarctic Circumpolar Current.
Article
The Scotia Sea in the South Atlantic holds a prominent position in geodynamics, because it has been proposed as a potential outlet of asthenosphere from under the shrinking Pacific into the mantle beneath the opening Atlantic. Shear wave splitting and geochemical studies have previously tested this hypothesis. Here, we take a different approach by calculating present-day dynamic topography of the region in search for a systematic trend in dynamic topography decreasing from west to east in response to a flow-related pressure gradient in the sublithospheric mantle. To this end, we reconstruct the kinematic history of the Scotia Sea, which is characterized by complex back-arc spreading processes active on a range of time scales. Our plate reconstructions allow us to derive an oceanic age-grid and to calculate the associated residual (dynamically maintained) topography of the Scotia Sea by comparing present-day isostatically corrected topography with that predicted from our reconstruction. The results provide no indication for a systematic trend in dynamic topography and we conclude that the material needed to supply the growing subatlantic mantle must be derived from elsewhere.
Article
A minimum-complexity tectonic reconstruction, based on published and new basin opening models, depicts how the Scotia Sea grew by Cenozoic plate divergence, dismembering a Jurassic sheared margin of Gondwana. Part of the Jurassic–early Cretaceous ocean that accreted to this margin forms the core of the Central Scotia Plate, the arc plate above a trench at the eastern end of the Scotia Sea, which migrated east away from the Antarctic and South American plates. A sequence of extensional basins opened on the western edge of the Central Scotia Plate at 50–30 Ma, decoupled from the South American Plate to the northwest by slow motion on a long transform fault. Succeeding the basins, seafloor spreading started around 30 Ma on the West Scotia Ridge, which propagated northwards in the 23–17 Ma period and ceased to operate at 6 Ma. The circuits of plate motions inside and outside the Scotia Arc are joined via rotations that describe Antarctic–Central Scotia plate motion in Powell Basin until 20 Ma, and along the South Scotia Ridge thereafter. The modelled relative motion at the northern edge of the Scotia Sea is thus constrained only by the plate circuit, but nonetheless resembles that known coarsely from the geological record of Tierra del Fuego. A paleobathymetric interpretation of nine time slices in the model shows Drake Passage developing as an intermediate-depth oceanographic gateway at 50–30 Ma, with deep flow possible afterwards. Initially, this deep flow would have been made tortuous by numerous intermediate and shallow barriers. A frontal pattern resembling that in the modern Scotia Sea would have awaited the clearance of significant barriers by continuing seafloor spreading in the Scotia Sea at ~ 18.5 Ma, at Shag Rocks Passage, and after 10 Ma southeast of South Georgia.
Article
New U–Th–Ra, major and trace element, and Sr–Nd–Pb isotope data are presented for young lavas from the New Britain and Western Bismarck arcs in Papua New Guinea. New Britain is an oceanic arc, whereas the latter is the site of an arc–continent collision. Building on a recent study of the Manus Basin, contrasts between the two arcs are used to evaluate the processes and timescales of magma generation accompanying arc–continent collision and possible slab detachment. All three suites share many attributes characteristic of arc lavas that can be ascribed to the addition of a regionally uniform subduction component derived from the subducting altered oceanic crust and sediment followed by dynamic melting of the modified mantle. However, the Western Bismarck arc lavas diverge from the Pb isotope mixing array formed by the New Britain and the Manus Basin lavas toward elevated 208Pb/204Pb. We interpret this to reflect a second and subsequent addition of sediment melt at crustal depth during collision. 238U and 226Ra excesses are preserved in all of the lavas and are greatest in the Western Bismarck arc. High-Mg andesites with high Sr/Y ratios in the westernmost arc are attributed to recent shallow mantle flux melting at the slab edge. Data for two historical rhyolites are also presented. Although these rhyolites formed in quite different tectonic settings and display different geochemical and isotopic compositions, both formed from mafic parents within millennia.
Article
The earliest subaerially exposed magmatic products of the Fiji–Tonga–Kermadec (FTK) arc are preserved in the Yavuna Group of Viti Levu, Fiji, and cobbles from ‘Eua, Tonga. They are similar in age and magma types to the earliest rocks of the Izu–Bonin–Mariana (IBM) arc. In Fiji they include typical island arc tholeiitic (IAT), boninitic (BON), and MORB-like early arc tholeiitic (EAT) pillow lavas that are interpreted as products of flux- and decompression-melting which occurred simultaneously during subduction initiation. Although the oldest rocks in the southwest Pacific (FTK) and the northwest Pacific (IBM) arcs are generally similar, they differ in two important respects. First, all magma types erupted simultaneously in the SW Pacific whereas a similar assemblage may have erupted sequentially in IBM. Second, the primary mantle wedge was “Pacific” in isotopic character in the SW Pacific, but “Indian” in the NW Pacific.
Article
Rocks with the geochemical characteristics of melts derived directly from subducted lithosphere are present in some modern island and continental arcs where relatively young and hot lithosphere is being subducted. These andesites, dacites, and sodic rhyolites or their intrusive equivalents are usually not associated with parental basaltic magmas. It is shown here that the trace-element geochemistry of these magmas is consistent with a derivation by partial melting of the subducted slab, and in particular that subducting lithosphere younger than 25 Myr seems to be required for slab melting to occur.
Article
A large portion of Earth's crust is formed at convergent plate boundaries that are accompanied by the subduction of sediments that can contain evolved crust-derived detritus. Partial melting of such sediments can strongly affect the trace element and isotope geochemistry of new arc rocks. Here, we present high-precision Lu-Hf-Zr concentration data and Hf isotope compositions for a series of volcanic rocks from the Banda arc, East Indonesia, to quantify the transfer of subducted Hf to the Banda arc crust and address the influence of recycled Hf in subduction zones on the Hf isotope systematics of arc rocks.
Article
Closure of the Pacific Ocean basin by the convergence of its surrounding plates, some of which have deep continental roots, implies that there is net mass flux out of the mantle under the Pacific. Here we report on a shear-wave splitting study designed to test the prediction that there should be flow around its southern margin. Our results show no evidence for present-day flow around the tip of southern South America. Instead, the results suggest present-day flow directions in the southern Atlantic that parallel the South American absolute plate motion direction, even under Antarctica. The results also provide evidence for absolute plate motion driven by the basal drag of ocean basin-scale mantle flow, and suggest that ~200 km thick flow boundary layers exist under South America and Antarctica, and also demonstrate that mantle flow directions cannot be reliably inferred from present-day plate morphology.
Article
The North Scotia Ridge is a series of islands and submarine ridges extending 2000 km from Tierra dei Fuego to South Georgia in the western South Atlantic. The ridge forms the elevated northern tectonic margin of the Scotia Sea, and accommodates E-W sinistral strike-slip motion at the South American-Scotia plate boundary. Existing studies have shown that the northern flank of the North Scotia Ridge is a large and continuous accretionary prism, formed during presumed mid-late Cenozoic N-S convergence. In this study, we present long-range side-scan sonar (GLORIA) images and seismic reflection profiles which show the structural style of the accretionary prism for the first time. The youngest accreted sediments show a uniform fabric of initial deformation (symmetric-gently asymmetric folds of 1-4 km wavelength), which has been subsequently disrupted at shallower depths by additional shortening and uplift. Between 52 degrees 45'W and 50 degrees 30'W, the deformation front is exposed at the sea floor, and the Falkland Trough retains the appearance of an active convergent margin. Elsewhere, however, the deformation front is buried beneath younger, undeformed drift sediments indicating that convergence has ceased. GLORIA sonographs also show geological features consistent with current-control of sedimentation, non-deposition, and erosion beneath the Antarctic Circumpolar Current. In particular, this study describes current-influenced sedimentation in the Falkland Trough, and sleep-sided, eroded depressions and diffuse slope-parallel fabric on the elevated Falkland Plateau.
Article
Remnant arcs are the submarine ridges which lie behind active island-arc systems. In simple cases, these ridges are bounded by scarp systems on both flanks, have volcaniclastic aprons on the rear flank, and are similar in composition to frontal arcs. Simple remnant arcs form as the rifted remnants of frontal arcs and are left behind as inter-arc basins widen. After creation, remnant arcs subside several kilometers. The simple morphology of some remnant arcs is obscured by sedimentation, by isostatic adjustment, and by tectonic reactivation. Occasionally, the polarity of an active arc system reverses, and, on the rear flank of the frontal arc, a new trench forms which consumes marginal basin crust. This reversed configuration results in arc-arc collisions and more complex forms of remnant arcs. The collision of a reversed arc with remnant arcs or continental margins is a likely explanation for the thrusting of oceanic crust over continental crust. Active and remnant arcs in which these processes are now taking place are located between New Guinea and the Solomon Islands. The oceanic crust on the leading edge of the New Britain ridge (reversed frontal arc) is colliding with New Guinea.
Article
New swath bathymetry plus existing geophysical data reveal that the direction of West Philippine Basin (WPB) seafloor spreading rotated 100° counter-clockwise between 49 Ma and 33 Ma. The curvilinear and multi-stranded Mindanao Fracture Zone separates the WPB from the Palau Basin to the south. WPB opening was contemporaneous with, and behind, early Izu-Bonin-Mariana (IBM) subduction, whose arc volcanism began by 50 Ma. This produced over 1000 km of arc parallel spreading (or stretching plus magmatism) in the Mariana segment of the Eocene IBM arc/forearc. The initial IBM subduction cut across, rather than followed, pre-existing structures (remnant arcs, fracture zones and spreading fabric). New models of subduction initiation are required to reproduce such characteristics.
Article
Tectonic features at the earth's surface can be used to test models for mantle return flow and to determine the geographic pattern of this flow. A model with shallow return and deep continental roots places the strongest constraints on the geographical pattern of return flow and predicts recognizable surface manifestations. Because of the progressive shrinkage of the Pacific (averaging 0.5 km²/yr over the last 180 m.y.) this model predicts upper mantle outflow through the three gaps in the chain of continents rimming the Pacific (Carribbean, Drake Passage, Australian-Antartic gap). In this model, upper mantle return flow streams originating at the western Pacific trenches and at the Java Trench meet south of Australia, filling in behind this rapidly northward-moving continent and provding an explanation for the negative bathymetric and gravity anomalies of the 'Australian-Antarctic-Discordance'. The long-continued tectonic movements toward the east that characterize the Caribbean and the eastenmost Scotia Sea may be produced by viscous coupling to the predicted Pacific outflow through the gaps, and the Caribbean floor slopes in the predicted direction. If mantle outflow does not pass through the gaps in the Pacific perimeter, it must pass beneath three seismic zones (Central America, Lesser Antiles, Scotia Sea); none of these seismic zones shows foci below 200 km. Mantle material flowing through the Caribbean and Drake Passage gaps would supply the Mid-Atlantic Ridge, while the Java Trench supplies the Indian Ocean ridges, so that deep-mantle upwellings need not be centered under spreading ridges and therefore are not required to move laterally to follow ridge migrations. The analysis up to this point suggests that upper mantle return flow is a response to the motion of the continents. The second part of the paper suggest driving mechanism for the plate tectonic process which may explain why the continents move.
Article
Mount St. Helens, 50 km to the west of Mount Adams and the main Cascade volcanic chain, is only 80 km above the subducting oceanic lithosphere. The elevated temperatures off the subducting slab, because of the close proximity of the Juan de Fuca Ridge to the trench,may induce slab melting at a depth of ˜80 km. Dacites from Mount St. Helens have geochemical compositions off magmas that are derived by direct partial melting of metamorphosed basalts at high pressure, i.e., relatively high AI (Al2O3 > 15% at 70% SiO2), low Y and Yb (because of garnet and amphibole stability in the source), low Sc, and high Sr and Eu. Trace element modeling of the partial melting of mid-oceanic ridge basalt (MORB) from the Juan de Fuca Ridge that yields a hornblende eclogite residue can reproduce the Mount St. Helens data (results off the model are quite distinct from data derived from the Mount Adams volcanic rocks). In contrast, Mount Adams is ˜135 km above the subducting slab and is associated with normal arc magmatism believed to be derived from the mantle above the subducting plate. The Cascade are has been active in its present locality, because of oblique subduction, for the past 7 m.y. The major volcanoes along the arc have existed for at least 500 ka, but Mount St. Helens has existed for
Article
With the cessation of subduction along the western margin of Antarctica, Mesozoic calc-alkaline activity gradually gave way in the Cenozoic to more alkaline volcanism associated with an extensional regime. Calc-alkaline volcanism persisted well into the Tertiary in the South Shetland Islands and has started to develop in the Quaternary in the South Sandwich Islands, though most of the Pliocene-Recent products of this group are of island-are tholeiite affinity. The Cenozoic volcanic rocks of the Ross Sea and Marie Byrd provinces are generally highly undersaturated basanitoids, alkali basalts and phonolites. In contrast, those of the more northerly parts of the Antarctica Peninsula and its off-lying islands are for the most part mildly alkaline or transitional. However, Paulet Island, the youngest volcano on the northeast side of the Peninsula, is distinctly more alkalic than its Pliocene predecessors. Deception Island, distinctive on account of its strongly sodic differentiates, is probably connected with residual back-arc extension along Bransfield Strait.
Article
Oceanic volcanoes offer abundant evidence of changes in their elevations through time. Their large-scale motions begin with a period of rapid subsidence lasting hundreds of thousands of years caused by isostatic compensation of the added mass of the volcano on the ocean lithosphere. The response is within thousands of years and lasts as long as the active volcano keeps adding mass on the ocean floor. Downward flexure caused by volcanic loading creates troughs around the growing volcanoes that eventually fill with sediment. Seismic surveys show that the overall depression of the old ocean floor beneath Hawaiian volcanoes such as Mauna Loa is about 10 km. This gross subsidence means that the drowned shorelines only record a small part of the total subsidence the islands experienced. In Hawaii, this history is recorded by long-term tide-gauge data, the depth in drill holes of subaerial lava flows and soil horizons, former shorelines presently located below sea level. Offshore Hawaii, a series of at least 7 drowned reefs and terraces record subsidence of about 1325 m during the last half million years. Older sequences of drowned reefs and terraces define the early rapid phase of subsidence of Maui, Molokai, Lanai, Oahu, Kauai, and Niihau. Volcanic islands, such as Maui, tip down toward the next younger volcano as it begins rapid growth and subsidence. Such tipping results in drowned reefs on Haleakala as deep as 2400 m where they are tipped towards Hawaii. Flat-topped volcanoes on submarine rift zones also record this tipping towards the next younger volcano. This early rapid subsidence phase is followed by a period of slow subsidence lasting for millions of years caused by thermal contraction of the aging ocean lithosphere beneath the volcano. The well-known evolution along the Hawaiian chain from high to low volcanic island, to coral island, and to guyot is due to this process. This history of rapid and then slow subsidence is interrupted by a period of minor uplift lasting a few hundred thousand years as the island migrates over a broad flexural arch related to isostatic compensation of a nearby active volcano. The arch is located about 190±30 km away from the center of volcanic activity and is also related to the rejuvenated volcanic stage on the islands. Reefs on Oahu that are uplifted several tens of m above sea level are the primary evidence for uplift as the islands over-ride the flexural arch. At the other end of the movement spectrum, both in terms of magnitude and length of response, are the rapid uplift and subsidence that occurs as magma is accumulated within or erupted from active submarine volcanoes. These changes are measured in days to years and are of cm to m variation; they are measured using leveling surveys, tiltmeters, EDM and GPS above sea level and pressure gauges and tiltmeters below sea level. Other acoustic techniques to measure such vertical movement are under development. Elsewhere, evidence for subsidence of volcanoes is also widespread, ranging from shallow water carbonates on drowned Cretaceous guyots, to mapped shoreline features, to the presence of subaerially-erupted (degassed) lavas on now submerged volcanoes. Evidence for uplift is more limited, but includes makatea islands with uplifted coral reefs surrounding low volcanic islands. These are formed due to flexural uplift associated with isostatic loading of nearby islands or seamounts. In sum, oceanic volcanoes display a long history of subsidence, rapid at first and then slow, sometimes punctuated by brief periods of uplift due to lithospheric loading by subsequently formed nearby volcanoes.
Article
Detailed analysis of marine magnetic profiles from the western part of the East Scotia Sea confirms continuous, organized back-arc spreading since at least 15 Ma ago. In the eastern part of the East Scotia Sea, the South Sandwich arc lies on crust that formed at the back-arc spreading centre since 10 Ma ago, so older back-arc crust forms the basement of the present inner forearc. Interpretations of two multichannel seismic reflection profiles reveal the main structural components of the arc at shallow depth, including evidence of trench-normal extension in the mid-forearc, and other features consistent with ongoing subduction erosion. The seismic profile interpretations have been used to constrain simple two-dimensional gravity models. The models were designed to provide constraints on the maximum possible thickness of the arc crust, and it is concluded that this is 20 and 19.2 km on the northern and southern lines, respectively. On the northern line the models indicate that the forearc crust cannot be much thicker than normal oceanic crust. Even with such thin crust, however, the magmatic growth rate implied by the cross-section of the arc crust is within the range recently estimated for two other arcs that have been built over a much longer interval.
Article
We reassess the applicability of the thermal plate cooling model to the subsidence of the North Pacific, Atlantic and North Indian Ocean Basins. We use a new numerical plate model in which the thermophysical parameters of the lithosphere vary with temperature according to the results of laboratory experiments, and the ridge temperature structure is consistent with the thickness of the oceanic crust. We first attempt to exclude thickened crust from our data set, and then to exclude swells and downwellings by masking regions of the data that remains that have significant gravity anomalies when there exists a clear regional correlation between intermediate-wavelength gravity and topography. We find that the average variation of depth with age is consistent with conventional half-space models until about 90 Myr. Thereafter, the departure from the half-space cooling curve is more rapid than predicted using simple conductive plate cooling models. The depth-age curves in the Pacific and Atlantic show ~250 m of temporary shallowing between the ages of 90-130 Myr, a result consistent with the outcome of experiments on the initiation of small-scale boundary layer convection. The results do not change significantly if the estimated component of the gravity arising from plate cooling is subtracted prior to calculation of the correlation between gravity and topography. A 90-km-thick conductive plate is nevertheless a reasonable model for the average temperature structure of the oldest part of the Pacific ocean lithosphere. In the Pacific, the broad topographic undulations associated with the Line Island Swell, the Hawaiian Swell and surrounding basins have correlated gravity anomalies and an admittance of approximately 30 mGal km-1 and are likely to result from convective circulation in the upper mantle. In the Northeast Atlantic, the intermediate-wavelength admittance over the Cape Verde swell is similar; in the Northwest Atlantic over the Bermuda Swell it is slightly larger but not as well constrained.
Article
Minor, trace-element and Sr-isotope data are compiled for Tertiary-Recent orogenic andesites of three tectonic settings: (1) oceanic island arcs (divided into low-K and “other” andesites); (2) continental island arcs and thin continental margins; and (3) thick continental margins (Andean). From (1) to (3), there is a steady, overlapping increase in La, Ce, ΣREE, La/Yb, La/Y, Th, Th/U, Zr/Y, Hf/Yb and Ni/Co and a decrease in K/La, P/La, Sc/Cr and Sc/Ni. Optimum discriminationis achieved with: (a) plots of La/Yb vs. Th and La/Yb vs. Sc/Ni and (b) cluster analysis using the critical parameters listed above. Shoshonitic andesites and high-Mg andesites (boninites) are considered separately. Orogenic andesites are distinguished from non-orogenic andesites by high contents of Al, low contents of Ti, Zr, REE, Y, Nb, Ta and Ga, and the absence of pronounced negative Eu anomalies.