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Exhumed Himalayan-type syntaxis in the Grenville orogen, northeastern Laurentia
Abstract
A deep-seated analog of the syntaxis developed in the Tibetan Plateau occurs in the Grenville Orogen of eastern Laurentia. During the final assembly of Rodinia, Amazonia collided with Laurentia and produced a series of large, conjugate, transcurrent, shear systems and pervasive strike-slip deformation that overprinted compressional structures related to the Ottawan Orogeny (the last orogenic phase of what is considered Grenvillian). A northeast-striking dextral system at least 35-km wide developed in the Reading Prong of New York (locally known as the Hudson Highlands), New Jersey, and Pennsylvania. U-Pb SHRIMP zircon geochronology and Ar/Ar thermochronology on the lowest grade cataclasites constrain the age of movement between 1008 and 876 Ma. A 60-km-wide, east-west striking, sinistral shear system developed across the central Adirondack Highlands. This system overprints rocks with granulite-facies metamorphic assemblages containing ca. 1050 Ma metamorphic zircons and is cut by a swarm of 950 Ma leucogranites. The timing, geometric relationships, and shear sense of the Adirondacks and Reading Prong shear systems suggest a conjugate system within a syntaxis with bulk compression directed ENE–WSW. This tectonic scenario invokes a component of strike-parallel deformation during the Ottawan Orogeny and provides a kinematic mechanism for an otherwise enigmatic, synchronous, late (ca. 930 Ma) extensional event including the Carthage–Colton mylonite zone in the northwest Adirondacks and Canada.
... The major exposed part of the orogen occurs in the Grenville Province, including the contiguous Adirondack Lowlands and Highlands, as well as in a series of smaller fragments preserved as inliers of Grenvillian basement along the length of the Appalachians (Fig. 1). Prior to about 2000, detailed geochemical and geochronological studies of the central and northern Grenvillian inliers in the Appalachians were few (e.g., Drake, 1984;Ratcliffe et al., 1991;Volkert and Drake, 1999), but the situation has changed since then (e.g., Aleinikoff et al., 2000;Gates et al., 2004;Gorring et al., proper and adjacent Appalachian inliers. ...
... The range of 40 Ar/ 39 Ar hornblende plateau ages in the New Jersey Highlands inlier determined in this study (947-914 Ma) overlaps with that determined from the contiguous Hudson Highlands (ca. 924-915 Ma; Dallmeyer et al., 1975;Gates et al., 2004). It is also similar to results from the Adirondack Highlands Gneiss Complex (ca. ...
... The slow rate of post-peak Ottawan cooling of 1.9-3.5°C/Myr recorded in granulite-facies rocks of the New Jersey Highlands is comparable to cooling rates of 1-4°C/Myr determined in other Grenvillian high-grade gneiss complexes (e.g., Hudson Highlands, Dallmeyer et al., 1975;Gates et al., 2004;Adirondack Highlands, Onstott and Peacock, 1987;Streepy et al., 2004; part of the Green Mountains inlier, Sutter et al., 1985;Spear and Harrison, 1989; and the Ottawa River and Mékinac gneiss complexes Cosca et al., 1991;Culshaw et al., 1991;Reynolds et al., 1995;Rivers, 2012;Rivers et al., 2012;Schneider et al., 2013;Soucy La Roche et al., 2015). Based on these studies, a slow rate of cooling after the Ottawan metamorphic peak can be considered a signature of Grenvillian high-grade gneiss complexes. ...
... The Adirondack region has been the focus of cutting-edge petrologic inquiry for decades (Bohlen et al., 1985;Storm and Spear, 2005) but its structure has received less attention (cf. McLelland, 1984;Gates et al., 2004). Recognizing and characterizing the major lithotectonic domains is critical to placing the Adirondack region into tectonic context within the southern Grenville Province. ...
... Recognizing and characterizing the major lithotectonic domains is critical to placing the Adirondack region into tectonic context within the southern Grenville Province. The Piseco Lake shear zone (PLSZ, Fig. 2) bisects the width of the Adirondack Mountains (Gates et al., 2004), and delineates a major lithotectonic discontinuity within the region (Fakundiny et al., 1994). As noted here, the width (> 20 km), high finite strain (Gates et al., 2004), and lithologic discontinuities across it, suggest that the PLSZ delineates a major structural boundary between two distinct terranes: the Adirondack Highlands and rocks to the south of the PLSZ, herein called the Southern Adirondack Terrane. ...
... The Piseco Lake shear zone (PLSZ, Fig. 2) bisects the width of the Adirondack Mountains (Gates et al., 2004), and delineates a major lithotectonic discontinuity within the region (Fakundiny et al., 1994). As noted here, the width (> 20 km), high finite strain (Gates et al., 2004), and lithologic discontinuities across it, suggest that the PLSZ delineates a major structural boundary between two distinct terranes: the Adirondack Highlands and rocks to the south of the PLSZ, herein called the Southern Adirondack Terrane. ...
Synthesis of new and existing field, structural, geochemical, isotopic, and geochronologic data in the Adirondack Highlands of the southern Grenville Province indicate a suite of arc plutonic rocks (Piseco Lake granite suite) was intruded into basement rocks from ca. 1205–1180 Ma due to northward subduction beneath the Laurentian margin. Although rocks of similar age and origin are found in the Adirondack Lowlands and elsewhere in the Highlands, they occur in the greatest volume and are most intensely deformed in the Piseco Lake Shear Zone (PLSZ). This structure separates the Adirondack Highlands from ca. 1350-1300 Ma tonalitic gneisses in the southern Adirondacks. Tectonites in the PLSZ record the deformation of a continental arc batholith during collision with an outboard terrane which was accreted to Laurentia during the Shawinigan Orogeny. Internal to the PLSZ, a 10–15 km wide, subvertical zone of subhorizontally lineated L-S and L-tectonites (Piseco Lake gneisses) was derived primarily from megacrystic arc plutonic protoliths and documents sinistral relative motion. Intrusion of the arc granitoids, deformation and anatexis (ca. 1180–1160 Ma) throughout the entire Adirondack region immediately preceded and overlapped the intrusion of the anorthosite-mangerite-charnockite-granite (AMCG) suite at ca. 1165-1145 Ma in the Adirondack Highlands. We hypothesize that delamination or slab detachment, in combination with an irregular Laurentian margin, triggered disruption of the underlying lithosphere, allowed ascent of enriched asthenosphere, and facilitated the intrusion of voluminous AMCG rocks in the Adirondacks north of the PLSZ. Other large anorthosite massif complexes in the southern Grenville Province, including the Morin and Lac St. Jean bodies near the Laurentian margin, have also been linked to Shawinigan convergent structures and were an important part of accretionary processes documented in the Grenville Orogen.
... Several large (>20-km across) structural domes cored by rheologically rigid anorthosite lie within the zone. Kinematic investigations indicate that this zone is dominated by sinistral shear strain (Chiarenzelli et al., 2000;Gates et al., 2004). There are a number of large-scale features, such as drag folds and rotated mega (giga-) clasts, which are consistent with the abundant meso-and micro-scale kinematic indicators. ...
... There are a number of large-scale features, such as drag folds and rotated mega (giga-) clasts, which are consistent with the abundant meso-and micro-scale kinematic indicators. Kinematic indicators include S-C fabrics, shear bands, and rotated porphyroclasts (Gates et al., 2004). This east-west, broad zone is bounded by shear zones that traverse portions of the southern Adirondacks. ...
... In the southern Adirondacks, there is a zone (10-20 km wide) of spectacular L-S and L>S tectonite with a general east-west map pattern ( Figure 2). This deformation zone was designated by Gates et al. (2004) the Piseco Lake shear zone (PLSZ) based upon its inclusion of the Piseco dome and antiform of earlier workers (Cannon, 1937;Glennie, 1973;McLelland, 1984;Wiener et al., 1984), but also upon the extent of penetrative fabrics general shear fabrics found well beyond the core of the antiform. Throughout the PLSZ, rocks of mostly granitic composition contain intense foliation and lineation, as described by Cannon (1937) and McLelland (1984). ...
... The modern tectonic analog of the Grenville orogeny is the Himalayas (Dewey and Burke, 1973;Windley, 1986;McLelland et al., 2001). As noted by Gates et al. (2004), the Grenville terrane is one of the longest and most deeply exhumed exhumed orogens on Earth. The granulite-facies of the Grenville orogen was produced at a depth of 25 km. ...
... Ga. Some zircons from the Hudson Highlands ( Figure 1) have a 2.0 Ga age, and suggest that the Hudson Highlands are "exotic" in the Grenville province and represent part of the Amazonian craton (in Gates et al., 2004). ...
... Interestingly, the southern part of the Adirondacks is a roughly E-W-trending orocline of ductile shear zones, folds, and domes produced as a syntaxis during the collision with Amazonia at the end of the Grenville orogeny (Gates et al., 2004). However, these Grenville structures do not define the modern topography of the Adirondacks. ...
... The Mesoproterozoic Grenville Province is a portion of one of the Earth's major orogenic belts that extends for thousands of kilometers and across several continents, often invoking Himalayan analogues (Gates et al., 2004). Our understanding of the Grenville is hampered by its age, complexity, and deep exhumation. ...
... In addition, work by others, including Wasteneys et al. (1999) and Peck et al. (2004), based on a variety of tectonic and isotopic arguments, strongly suggests northwest-directed subduction prior to Shawinigan orogenesis. Thus we tentatively suggest that the ARS represents melt generated and emplaced during, or just prior to, Shawinigan orogenesis within the attenuated Laurentian margin during northwest-directed subduction, perhaps along the present location of the Piseco Lake shear zone, which was modifi ed by later reactivation during Ottawan orogeny (Gates et al., 2004;Valentino et al., 2008). ...
... Collision of an outboard arc remnant and collapse of a series of small backarc basins can explain the limited extent of Shawinigan orogenesis, and thus that of the ARS and HGG, and provide a mechanism and setting conducive for delamination. Models invoking docking of an outboard continental mass (e.g., Amazonia) are not necessarily required, and we suggest that the terminal collision with Amazonia occurred during the Ottawan phase of the Grenvillian orogeny (Gates et al., 2004;Hoffman, 1991;Rivers, 2008), rather than during the Shawinigan orogeny (cf. Hanmer et al., 2000). ...
The Antwerp-Rossie metaigneous suite (ARS) represents arc magmatism related to closure of the Trans-Adirondack backarc basin during Shawinigan collisional orogenesis (ca. 1200-1160 Ma). The ARS is of calc-alkaline character, bimodal, and lacks intermediate compositions. Primarily intruding marble and pelitic gneiss, the ARS is spatially restricted to the Adirondack Lowlands southeast of the Black Lake fault. On discrimination diagrams, the ARS samples plot primarily within the volcanic arc granite fields. Incompatible elements show an arc-like signature with negative Nb, Ta, P, and Zr and positive Cs, Pb, La, and Nd anomalies relative to primitive mantle. Neodymium model ages (T-DM, depleted mantle model) range from 1288 to 1634 Ma; the oldest ages (1613-1634) and smallest epsilon Nd (epsilon(Nd)) values are found in proximity to the Black Lake fault, delineating the extent of Laurentia prior to the Shawinigan orogeny. The epsilon Nd values at crystallization (1200 Ma) plot well below the depleted mantle curve. Geochemical and isotopic similarities to the Hermon granitic gneiss (HGG) (ca. 1182 Ma) and differences from the Hyde School Gneiss-Rockport Granite suites (1155-1180 Ma) suggest that arc plutonism rapidly transitioned into A-type AMCG (anorthosite-mangerite-charnockite-granite) plutonism. Given the short duration of Shawinigan subduction, apparently restricted extent of the ARS (Adirondack Lowlands), location outboard of the pre-Shawinigan Laurentian margin, intrusion into the Lowlands supracrustal sequence, bimodal composition, and recent discovery of enriched mantle rocks in the Lowlands, it is proposed the ARS formed as a consequence of subduction related to closure of a backarc basin that once extended between the Frontenac terrane and the Southern Adirondacks.
... Supracrustal rocks are typically found in narrow, attenuated, linear belts and are often highly disrupted, intruded, and interleaved with metaigneous rocks. A number of large structural domes are also apparent and are defi ned by upward arching foliation trends (DeWaard and Romey, 1969;Chiarenzelli et al., 2000;Gates et al., 2004). The overall picture is one of intense deformation and translation such that primary geologic relationships are rarely preserved. ...
... Just north of the study area there is an east-west arching belt of highly deformed marbles and charnockitic gneisses. Some of these rocks may be traced around the northern fl ank of Snowy Mountain Dome (deWaard and Romey, 1969;Gates et al., 2004;Valentino and Chiarenzelli, 2008) into the Chimney Mountain area, across a large brittle fault within Indian Lake, whose offset is unknown (Isachsen et al., 1990). ...
... The overprinting of this earlier fabric can be seen in several areas. Gates et al. (2004) and Valentino and Chiarenzelli (2008) document the overprinting and folding of an earlier fabric along the fl anks of the Snowy Mountain Dome, 10 km to the west. A similar relationship can be seen at Chimney Mountain where an early foliation defi ned by the alignment of micas and quartzofeldspathic lenses is folded about shallowly plunging folds. ...
The rocks at Chimney Mountain provide a rare glimpse into primary intrusive relations and exceptionally well-preserved pre-Shawinigan metasedimentary rocks in the Adirondack Highlands despite a strong Ottawan thermal overprint. A near vertical contact between granite (ca. 1172 Ma) and a shallowly dipping and structurally intact sequence of quartzose to calc-silicate metasedimentary rocks is exposed on the southern flank of Chimney Mountain in the Central Adirondacks. The contact is marked by foliation truncation and a meta-somatic aureole with randomly orientated porphyroblasts of enstatite rimmed by anthophyllite (max. 5 cm) and phlogopite (max. 2 cm), and a zone of granular, quartz-rich rock. The granite is non-to weakly foliated and has a shallow, north-plunging, mineral lineation as do the metasedimentary rocks. Zircons separated from a diopsidebearing quartzite (82% SiO(2); 0.75 m thick) are of variable size (up to 400 mu m), equant, and contain, on average, >1000 ppm uranium. Scanning electron microscope investigation indicates that there is little variation in a uniformly dark cathodoluminescence response, no discernible cores or rims, few inclusions, and partially faceted to round morphologies. Zircon U-Pb sensitive high-resolution ion microprobe (SHRIMP II) ages of 1042 +/- 4 Ma and 1073 +/- 15 Ma are coincident with Ottawan metamorphic ages from the Adirondack Highlands. Zircons from the intrusive granite yield large cores with typical anorthosite-mangerite-charnockite-granite (AMCG) ages (ca. 1171.6 +/- 6.3 Ma) and sparse, thin younger rims (ca. 1060-1090 Ma) readily distinguishable by cathodoluminescence. Despite the younger zircon ages, the metasedimentary rocks and their fabric must predate the crosscutting granite. The thermal effect of the Ottawan event was likely enhanced by volatile fluxing and resulted in recrystallization and resetting of zircons in the metasedimentary rocks. However, it had limited effects on zircons in the granite and produced only thin metamorphic rims emphasizing the importance of local geochemical conditions to the response of zircon to metamorphism. Elzevirian or Shawinigan fabrics are preserved as the dominant foliation; the lineation and folding is likely late (post-1170 Ma) Shawinigan or Ottawan (ca. 1050 Ma). Titanites from the same metasedimentary sequence yield a range of (238)U/(206)Pb ages from 969 to 1077 Ma, with a maximum probability age of 1035 Ma, similar to other titanites in the Adirondack Highlands. Ottawan paleotemperatures, estimated by zirconium in titanite thermometry, range from 787 to 818 degrees C.
... Gates (1995Gates ( , 1998 and Gates and Costa (1998) proposed a major late Grenvillian dextral strike-slip shearing event in the Reading Prong. This shearing was constrained to discrete faults, such as the Ramapo and Reservoir Faults and Indian Hill shear zone (Gates et al., 2004) (Fig. 3), which were active well after peak Grenville tectonism and to lower temperatures. ...
... Zircons from three samples of gneiss from the fi eld area analyzed at the sensitive high-resolution ion microprobe (SHRIMP) lab at the Geological Survey of Canada (GSC) to obtain U-Pb ages (Gates et al., 2004). Sample G-5 is from a semi-pelitic gneiss layer within the metasedimentary lithofacies, sample G-2 is from the quartzofeldspathic gneiss body located west of the New York Thruway, and sample G-1 was collected from a small diorite body (Lake Tiorati diorite). ...
... The cores are subround to round and strongly zoned. Rim ages range between 1000 and 1030 Ma with a cluster between 1005 and 1010 Ma, and these rim ages are interpreted to refl ect the age of regional metamorphism (Gates et al., 2004). Zircon core ages span a range between 1100 and 2800 Ma with the highest frequency between 1500 and 1800 Ma (Fig. 5A). ...
A new multidisciplinary research collaboration to study the western Hudson Highlands, New York, has unraveled a complex Rodinian tectonic history that will be illustrated by visiting key locations on this trip. New sensitive high-resolution ion microprobe (SHRIMP) data demonstrates a cryptic suture between a ca. 1.2–1.1 Ga island arc and sedimentary rocks from a deeply incised craton (Amazonia?). The 1.05 Ga collision between these two terranes produced westward-directed fold nappes, granulite facies metamorphism and the dominant subhorizontal gneissic foliation. Tectonic surge granite sheets were emplaced into the nappes. Bimodal (diorite and granite) plutons intruded the area prior to the onset of a steeply SE-dipping 35-km wide dextral shear system that resulted from tectonic escape. Extensive iron remobilization and mineralization accompanied the shearing and post-kinematic pegmatite plutons mark the end of activity at ca. 980 Ma.
... Several large structural domes in the central Adirondack Mountains (15-20-km across) are interpreted to have resulted from fold interference (Kusky and Loring, 2001) or to have developed as the result of sinistral transpression during Proterozoic orogenic events (Gates et al., 2004;Fig. 1). ...
... 1). The pronounced lineaments that trace north-northeast from Piseco Lake, eventually merge near the southern reaches of Indian Lake ( Fig. 2) and cross-cut the Snowy Mountain structural dome (DeWaard and Romey, 1969;Gates et al., 2004;. The Snowy Mountain dome ( Fig. 8) is underlain by AMCG suite rocks with megacrystic anorthosite in the core containing plagioclase crystals commonly up to 20 cm in diameter. ...
... On Chimney Mountain, east of the lake, the intrusive contact between weakly to non-foliated granite of the AMCG suite and intensely deformed quartzites and calc-silicates has been recently documented (Chiarenzelli et al., 2011). DeWaard and Romey (1969) and Gates et al. (2004) interpreted the eastern half of Snowy Mountain dome to be the country rock that received the anorthosite, gabbro, and charnockite that makes up the core of the dome, with the eastern side of the dome displaced downward along a late brittle fault that is concealed by Indian Lake (Kush et al., 2006). This inferred fault appears on the New York State geologic map, and is also shown to splay toward the south in the direction of Piseco Lake. ...
An integrated magnetic gradiometry and structural analysis was conducted on three lakes in the southern Adirondacks Mountains, New York, in order to develop a geometric and kinematic model for concealed and long lived faults that transect the Proterozoic basement structures, offset Paleozoic strata to the south, and may be associated with the development of the post-Paleozoic cratonic dome (the Adirondack dome). Two lakes occur along the trace of two of the most prominent topographic lineaments that have been proposed to be faults in the southern Adirondack Mountains, and a third lake is located at the apparent fault intersection. Hinkley Lake occurs over the east–west trending lineament that corresponds to the trace of the Prospect fault. Indian Lake resides in a set of north–northeast trending pronounced lineaments that transect an anorthosite-cored structural dome and are inferred to be faults on the NYS geologic map. Piseco Lake is immediately adjacent to the intersection of the two proposed fault zones (Prospect and Indian Lake fault zones). Magnetic surveys were conducted on all three lakes, resulting in anomaly maps. Accompanying two dimensional geologic models for Hinkley and Piseco Lake were produced. At Piseco Lake, field evidence supports a brittle deformation history with sinistral-normal displacement. A similar deformation history is consistent with field data collected at Indian Lake. Correlation of the two dimensional magnetic models resulted in a sinistral, releasing-bend fault geometry beneath Piseco Lake, and the fault truncation of a granitic gneiss cored antiform for the subsurface geology of Hinkley Lake. The magnetic data and models suggest that Piseco Lake resides over a sinistral, pull-apart structure with sufficient throw to preserve the lowermost Paleozoic strata that once covered much of the Adirondack dome. This would account for both sinistral strike-slip and normal displacement on the interpreted faults, at Piseco and Indian Lakes, and rotation of structural blocks as the result of fault interaction. Relative timing, regional relationships, and ties to published ages suggest that the distribution of the faults is a relic of Neoproterozoic Iapetan rifting, Paleozoic displacement, and Mesozoic uplift of the Adirondack dome. Finally there is probable correlation of these faults with modern seismic activity.
... 1350-1220 Ma ages, which has been interpreted as reflecting a restricted basin in the Adirondacks at this time (Heumann et al., 2006;Bickford et al., 2008). A similarly restricted range of detrital ages has been reported for a semi-pelite in the Hudson Highlands (Gates et al., 2004), which may indicate that this style of sedimentation extends to other Grenville inliers and parts of the Allochthonous Monocyclic Belt. ...
... Quartzite 02NJ43 from the Bloomingdale graphite deposit contains the youngest observed zircon population, which yields a 1005 ± 11 Ma metamorphic 206 Pb/ 207 Pb age. This age is identical to the 1007 ± 4 Ma metamorphic rim age from a semi-pelite sample from the Hudson Highlands (Gates et al., 2004), which is contiguous to the north with the New Jersey Highlands. Although relatively rare in the Adirondacks, Rigolet metamorphic ages such as these are well known from the New Jersey Highlands (Volkert et al., 2010). ...
The southern Grenville Province is made up of metaigneous and metasedimentary rocks that were formed and metamorphosed as part of the Mesoproterozoic active margin of Laurentia. Grenville Supergroup quartzites from the Adirondack Highlands (New York), the Morin terrane (Quebec), and New Jersey Highlands were analyzed to determine detrital and metamorphic zircon U-Pb ages and to help constrain Mesoproterozoic tectonics. These Grenville quartzites have broadly similar detrital age populations as each other, suggesting similar sedimentary source regions and depositional settings in back-arc basin environments at the Laurentian Margin during Geon 12. All samples contain detrital zircon with 1.45–1.25 Ga ages, consistent with proximal Grenville source regions. They also contain detrital ages that point to derivation from the Mazatzal (1.7–1.6 Ga), Yavapai (1.8–1.7 Ga), and Penokean (1.9–1.8Ga) orogens of the North American Midcontinent, and have a diverse population of Archean ages. These results and published data from Adirondack Lowlands metasediments (Chiarenzelli et al., 2015, 2017) show distinctly Laurentian sources, and the related and likely connected nature of the sedimentary histories of the Adirondack Highlands and Lowlands, Morin terrane and New Jersey Highlands. Metamorphic zircon formation is common in high-grade quartzites, and ages are variable between samples depending on the metamorphic history of each terrane and the melt productivity of different rocks (Peck et al., 2010a). Adirondack Highlands samples experienced metamorphic zircon grown during the Shawinigan orogeny and anorthosite-suite emplacement (1.19–1.14 Ga). Morin terrane samples are from the Morin Shear Zone, which was active and caused metamorphic zircon growth during the Ottawan phase of the Grenvillian orogeny (1.09–1.02 Ga). The New Jersey Highlands sample has metamorphic zircon formed during the Rigolet phase of the Grenvillian orogeny (1010–980 Ma).
... Oroclinal bending usually involves two deformation phases: the first creates the fold-and-thrust belt and the second bends it. A syntaxis is an abrupt bend in an otherwise geometrically straight orogen (GATES et al. 2004). MARSHAK (2004) used the term recess to describe a syntaxis (Fig. 3a). ...
... Therefore, it is not an orocline in the sense of CAREY (1955,1958). Analyzing salients (GRAY and STAMTAKOS 1997;MUKUL andMITRA 1998) andsyntaxes (BUTLER et al. 1989;GATES et al. 2004) is an important step toward understanding the geodynamics of fold-and-thrust belts (POBLET and LISLE 2011). In many cases curved fold-and-thrust belts are regarded as oroclines, such as the Patagonian-Fuegian Andes (DALZIEL and ELLIOT 1973) and the Cantabrian Arc (GUTIÉRREZ-ALONSO et al. 2012). ...
The South Limón fold-and-thrust belt, in the back-arc area of southern Costa Rica, is characterized by a 90° curvature of the strike of the thrust planes and is therefore a natural laboratory for the analysis of curved orogens. The analysis of curved fold-and-thrust belts is a challenge because of the varying structural orientations within the belt. Based on seismic reflection lines, we created a 3-D subsurface model containing three major thrust faults and three stratigraphic horizons. 3-D kinematic retro-deformation modeling was carried out to analyze the spatial evolution of the fold-and-thrust belt. The maximum amount of displacement on each of the faults is (from hinterland to foreland); thrust 1: 800 m; thrust 2: 600 m; thrust 3: 250 m. The model was restored sequentially to its pre-deformational state. The strain history of the stratigraphic horizons in the model was calculated at every step. This shows that the internal strain pattern has an abrupt change at the orogenic bend. Contractional strain occurs in the forelimbs of the hanging-wall anticlines, while a zone of dilative strain spreads from the anticline crests to the backlimbs. The modeling shows that a NNE-directed transport direction best explains the structural evolution of the bend. This would require a left-lateral strike-slip zone in the North to compensate for the movement and thereby decoupling the South Limón fold-and-thrust belt from northern Costa Rica. Therefore, our modeling supports the presence of the Trans-Isthmic fault system, at least during the Plio-Pleistocene.
... Oroclinal bending usually involves two deformation phases: the first creates the fold-and-thrust belt and the second bends it. A syntaxis is an abrupt bend in an otherwise geometrically straight orogen (GATES et al. 2004). MARSHAK (2004) used the term recess to describe a syntaxis (Fig. 3a). ...
... Therefore, it is not an orocline in the sense of CAREY (1955,1958). Analyzing salients (GRAY and STAMTAKOS 1997;MUKUL and MITRA 1998) and syntaxes (BUTLER et al. 1989;GATES et al. 2004) is an important step toward understanding the geodynamics of fold-and-thrust belts (POBLET and LISLE 2011). In many cases curved fold-and-thrust belts are regarded as oroclines, such as the Patagonian-Fuegian Andes (DALZIEL and ELLIOT 1973) and the Cantabrian Arc (GUTIÉRREZ-ALONSO et al. 2012). ...
The South Limón fold-and-thrust belt, in the back-arc area of southern Costa Rica, is characterized by a 90° curvature of the strike of the thrust planes and is therefore a natural laboratory for the analysis of curved orogens. The analysis of curved fold-and-thrust belts is a challenge because of the varying structural orientations within the belt. Based on seismic reflection lines, we created a 3-D subsurface model containing three major thrust faults and three stratigraphic horizons. 3-D kinematic retro-deformation modeling was carried out to analyze the spatial evolution of the fold-and-thrust belt. The maximum amount of displacement on each of the faults is (from hinterland to foreland); thrust 1: 800 m; thrust 2: 600 m; thrust 3: 250 m. The model was restored sequentially to its pre-deformational state. The strain history of the stratigraphic horizons in the model was calculated at every step. This shows that the internal strain pattern has an abrupt change at the orogenic bend. Contractional strain occurs in the forelimbs of the hanging-wall anticlines, while a zone of dilative strain spreads from the anticline crests to the backlimbs. The modeling shows that a NNE-directed transport direction best explains the structural evolution of the bend. This would require a left-lateral strike-slip zone in the North to compensate for the movement and thereby decoupling the South Limón fold-and-thrust belt from northern Costa Rica. Therefore, our modeling supports the presence of the Trans-Isthmic fault system, at least during the Plio-Pleistocene.
... The extent of post-Ottawan magmatism in the northeastern North American Grenville is quite widespread and occurs nearly continuously from the north-central Appalachians northward to Canada. In the Hudson Highlands, U-Pb zircon ages of postorogenic intrusive rocks include the 1010 ± 6 Ma Canada Hill Granite (Aleinikoff and Grauch, 1990), 1008 ± 4 Ma Lake Tioroti diorite (Gates et al., 2004), and felsic pegmatites dated at 965 ± 10 Ma (Grauch and Aleinikoff, 1985). Elsewhere regionally, felsic igneous rocks in the Southern Green Mountains and Chester Dome in Vermont and Massachusetts have provided U-Pb zircon ages of 965 ± 4 Ma to 945 ± 7 Ma (Karabinos and Aleinikoff, 1990), and in the Adirondack Highlands a U-Pb zircon age of 935 ± 9 Ma (McLelland et al., 2001). ...
... Gravitational collapse in the northern Appalachians was followed by pervasive regional extension along crustal-scale shear zones at ca. 945 Ma, with the interval between 1045 and 945 Ma representing a transition from a compressional tectonic setting to an extensional one (Streepy et al., 2004). Gates et al. (2004) propose that postorogenic extension in the north-central Appalachians at ≤1008 Ma resulted in the development of regionally pervasive right-lateral strike-slip shear zones. Post-collisional events commonly end with emplacement of alkaline bimodal suites related to transpressioinal or transtensional faulting (Bonin et al., 1998;Turner et al., 1992). ...
Postorogenic rocks are widespread in Grenville terranes of the north-central Appalachians where they form small, discordant, largely pegmatitic felsic intrusive bodies, veins, and dikes, and also metasomatic calcic skarns that are unfoliated and postdate the regional 1090 to 1030 Ma upper amphibolite- to granulite-facies metamorphism related to the Grenville (Ottawan) Orogeny. Zircons from magmatic and nonmagmatic rocks from northern New Jersey and southern New York were dated to provide information on the regional tectonomagmatic and metallogenic history following Ottawan orogenesis.
... The granulite-facies, sinistral Moose River Plain shear zone wraps around the Snowy Mountain dome then extends east toward Gore Mountain (but exposure is lost). Chiarenzelli et al. (2000), Gates et al. (2004), and Valentino et al. (2008) suggest that sinistral transpression along the Moose River Plain shear zone was in part responsible for exhumation of the anorthosite-cored Snowy Mountain dome. A similar concept is used to explain the evolution of migmatitic gneiss domes in the southern Karakoram (northwestern Himalaya, Pakistan), where molten middle crust was exhumed diapirically along vertical structures parallel to the crustal-scale Karakoram fault in the compressional regime of the Himalayan orogen (Mahéo et al., 2004). ...
This study combines field observations, mineral and whole-rock geochemistry, phase equilibrium modeling, and U-Pb sensitive high-resolution ion microprobe (SHRIMP) zircon geochronology to investigate sillimanite-bearing felsic migmatites exposed on Ledge Mountain in the central Adirondack Highlands (New York, USA), part of an extensive belt of mid-crustal rocks comprising the hinterland of the Mesoproterozoic Grenville orogen. Phase equilibrium modeling suggests minimum peak metamorphic conditions of 960–1025 °C and 11–12.5 kbar during the Ottawan orogeny—significantly higher pressure-temperature conditions than previously determined—followed by a period of near-isothermal decompression, then isobaric cooling. Petrography reveals abundant melt-related microstructures, and pseudosection models show the presence of at least ~15%–30% melt during buoyancy-driven exhumation and decompression. New zircon data document late Ottawan (re)crystallization at ca. 1047 ± 5 to 1035 ± 2 Ma following ultrahigh-temperature (UHT) metamorphism and anatexis on the retrograde cooling path. Inherited zircon cores give a mean date of 1136 ± 5 Ma, which suggests derivation of these felsic granulites by partial melting of older igneous rocks. The ferroan, anhydrous character of the granulites is similar to that of the ca. 1050 Ma Lyon Mountain Granite and consistent with origin in a late- to post-Ottawan extensional environment. We present a model for development of a late Ottawan migmatitic gneiss dome in the central Adirondacks that exhumed deep crustal rocks including the Snowy Mountain and Oregon anorthosite massifs with UHT Ledge Mountain migmatites. Recognition of deep crustal meta-plutonic rocks recording UHT metamorphism in a migmatite gneiss dome has significant implications for crustal behavior in this formerly thickened orogen.
... Although U-Pb detrital zircon with ages of 1037 6 12 to 1029 6 9 Ma that were derived from the erosion of local granite sources are reported from paragneiss in the northern Blue Ridge (Aleinikoff et al. 2013), there is an absence of ca. 1050 Ma detrital zircon ages in paragneiss from the New Jersey Highlands (Peck et al. 2019), Hudson Highlands (Gates et al. 2004), the Adirondack Highlands (Heumann et al. 2006;Chiarenzelli et al. 2015;Peck et al. 2019), and the Canadian Grenville Province . The age of the Assunpink Creek Granite is 1041 6 6 Ma, and given likely uncertainties presumably overlaps with the ca. ...
New zircon U–Pb geochronologic data from the Grenville-age Trenton Prong provide information on the age of magmatism, timing of metamorphism, and post-metamorphic history of the inlier. Diorite gneiss (1318 ± 13 Ma) of the Colonial Lake Suite temporally correlates to magmatic arc sequences that formed along the eastern margin of Laurentia at <1.4 Ga. Metasedimentary gneisses yielded detrital zircon ages of ca. 1319-1133 Ma and ca. 1370-1207, consistent with sediment derived from a similar local source of Laurentian affinity. A small population of zircon (either detrital or igneous in origin) in one sample yielded ages of ca. 1074-1037 Ma. Possible interpretations for their formation are explored. Ca. 1060 Ma overgrowths on zircon in the northern part of the inlier constrain the timing of granulite-facies metamorphism to the Ottawan phase of the Grenvillian Orogeny. The undeformed Assunpink Creek Granite (1041 ± 6 Ma) intruded country rocks as small bodies of late-orogenic syenogranite. It provides a minimum age for amphibolite-facies metamorphism and Ottawan orogenesis elsewhere in the inlier. Regionally, zircon rim ages of ca. 1010–960 Ma record continued thermal activity during the Rigolet phase of the orogen that resulted in migmatization of paragneiss at ca. 1004 Ma and juxtaposition of upper- and mid-crustal rocks during orogenic collapse. The lithologic ages and tectonic history of the Trenton Prong correlate to those in other Appalachian Mesoproterozoic inliers, and parts of the Canadian Grenville Province, confirming it is not an exotic terrane that was accreted to eastern Laurentia during Grenvillian orogenesis.
... The granulite-facies, sinistral Moose River Plain shear zone wraps around the Snowy Mountain dome then extends east toward Gore Mountain (but exposure is lost). Chiarenzelli et al. (2000), Gates et al. (2004), and Valentino et al. (2008) suggest that sinistral transpression along the Moose River Plain shear zone was in part responsible for exhumation of the anorthosite-cored Snowy Mountain dome. A similar concept is used to explain the evolution of migmatitic gneiss domes in the southern Karakoram (northwestern Himalaya, Pakistan), where molten middle crust was exhumed diapirically along vertical structures parallel to the crustal-scale Karakoram fault in the compressional regime of the Himalayan orogen (Mahéo et al., 2004). ...
... In the Highlands, similarly-aged (ca. 1200-1170 Ma) rocks of arc chemistry are found within the Piseco Lake Shear Zone, which separates the central and southern Adirondacks (Gates et al. 2004). From 1165-1155 Ma enormous quantities of anorthosite and associated felsic or granitic members of the AMCG were intruded into the Adirondack Highlands (McLelland et al. 2010b). ...
... The undeformed Mount Eve Granite yielded SHRIMP U-Pb zircon ages of 1020-1019 Ma that place a firm temporal constraint on the close of Ottawan orogenesis in the New Jersey Highlands and contiguous areas following continental collision. Small volumes of postorogenic felsic intrusions were emplaced between 1005 and 986 Ma (Volkert et al., 2005) as the region underwent thermal relaxation and localized extension along post-collisional strike-slip faults (Gates et al., 2004;Gorring et al., 2004). ...
Rare exposures of orthoamphibole mafic (Oam) gneiss of Mesoproterozoic age in the north-central Appalachians are confined to the northwestern New Jersey Highlands where they form thin lens-shaped bodies composed of gedrite and sparse anthophyllite, oligoclase (An13–An20), biotite, magnetite, and local fluorapatite, rutile, and ilmenite. The gneiss is penetratively foliated and has sharp, conformable contacts against enclosing supracrustal paragneiss and marble. Orthoamphibole mafic gneiss is characterized by low SiO2 (48 ± 2.5 wt%), CaO (1.9 ± 1.3 wt%), and high Al2O3 (18 ± 1.2 wt%), Fe2O3 (10.5 ± 1.6 wt%), and MgO (12 ± 2.3 wt%). Trace element abundances overlap those of unaltered amphibolites in the study area and, coupled with δ¹⁸O values of 9.45 ± 0.6‰ (VSMOW) from gedrite separates, support an origin from a basalt protolith.
The geochemical and isotopic data are consistent with the formation of Oam gneiss through sea floor hydrothermal alteration of basalt at low temperature of 150–200 °C. Mass-balance calculations indicate gains during alteration mainly in MgO and Al2O3 and losses in CaO, Sr, and light rare earth elements. Our results are compatible with the pre-metamorphic alteration of the basalt protoliths through chloritization and plagioclase dissolution that produced a Mg-rich and Ca-poor rock. Subsequent metamorphism of this chlorite-rich rock to the current mineral assemblage of Oam gneiss took place at ca. 1045 Ma, during the Ottawan phase of the Grenvillian Orogeny. The close spatial association in the study area of Oam gneiss bodies and sulfide occurrences suggests an affinity to the style of mineralization associated with volcanogenic massive sulfide (VMS)-type deposits.
... The Chimney Mountain area is located ~10 km east of Snowy Mountain Dome within the northwestern corner of the Thirteenth Lake Quadrangle (Krieger, 1937). Snowy Mountain Dome is a small anorthosite body mantled by rocks of the AMCG suite (DeWaard and Romey, 1969;Gates et al., 2004). Its eastern flank is cut by a long, linear NNE-trending fault whose trace extends within the linear valley now occupied by Indian Lake (Figure 2). ...
A particularly well preserved, shallowly dipping, and apparently intact sequence of
metasedimentary rocks (Chimney Mountain Metasedimentary Sequence – CMMS)
occurs on the western summit of Chimney Mountain near Indian Lake in the Central
Adirondack Highlands. These rocks are exposed along the headwall of a post-glacial
landslide scarp. Diopsidic quartzites (up to 82% SiO2), calc-silicate gneisses and
diopsidite, and rusty biotite-quartz-plagioclase gneisses are interlayered on the dcmmeter-
scale and dip shallowly (<30o) to the north. These rocks overlie a thick, folded
sequence of supracrustal gneisses dominated by calc-silicates.
The unusual state of preservation, relative lack of disruptive intrusive bodies, orientation,
and overall appearance differs from the underlying supracrustal gneisses and suggested
the possibility of a unique dynamothermal history or origin for the CMMS. However,
detailed field observations indicate that the CMMS and granitic gneiss that intrudes them
share a shallow, north-plunging mineral lineation suggesting both rocks were deformed
during the same event. In addition, mineral assemblages are typical of local granulite
facies gneisses. The contact between granitic gneiss and the CMMS is marked by a
contact zone up to 10 m or more in thickness in which large (1-5 cm), randomly oriented,
anthophyllite porphyroblasts grow across an earlier foliation and compositional layering.
Zircons separated from a quartz-rich layer (0.75 m thick) of the CMMS yield U-Pb
SHRIMP ages of 1042+/-4 Ma and 1073+/-15 Ma. The zircons are large (up to 400 um),
equant, and contain, on the average, >1000 ppm Uranium. Scanning electron microscope
investigation indicates that there is little cathodoluminscence response, no discernible
cores or rims, few inclusions, and partially faceted to round morphologies. Uranium to
Thorium ratios fall between those typically measured in metamorphic and igneous
crystals. Titanites from the same rock yield an age of 941+/-46 Ma, considerably
younger than other post-Grenville cooling ages of titanite in the Highlands. Zircons from
the granitic gneiss yield cores with typical AMCG ages (ca. 1140-1170 Ma) and younger
rims (ca. 1060-1090 Ma) readily distinguishable by cathodoluminscence. These data and
observations suggest the zircons analyzed from a diopsidic quartzite layer in the CMMS
are either completely recrystallized, and preserve no evidence of their provenance, or
grew during metamorphic reactions during the peak of Ottawan metamorphism.
Several lines of evidence suggest that the zircons analyzed were detrital but have been
completely recrystallized and isotopically reset during granulite facies metamorphism.
This was facilitated by geochemical conditions and fluid fluxing (largely CO2)
accompanying diopside formation.
... The Grenville orogen records the as- sembly of the Rodinia supercontinent with the collision of the Amazon craton and its overriding of areas that comprise modern eastern North America ( Dalziel et al., 1994). The U-Pb zircon ages from the Adirondacks show sedimentary and volcanic protolith ages of 1.3 to 1.2 Ga, whereas some zircons from the Hudson Highlands ( Figure 1) have 2.0 Ga ages and suggest that the Hudson Highlands are exotic and represent part of the Amazonian craton ( Gates et al., 2004).The super- Grenville cover sequence is readily separable into the (1969,1980; see discussion in Landing et al., 2007a). The Paraprioniodus costatus interval conodonts (Ethington and Clark, 1981, = fauna 4 of Sweet et al., 1971) in the upper Providence Island Formation (E. ...
... The oldest rocks in the Adirondack region, 1350e1300 Ma tonalitic gneisses, occur in the Southern Adirondack Terrane (Fig. 14.2;McLelland and Chiarenzelli, 1990). An east-west trending, left-lateral, strike-slip shear zone, the Piseco Lake shear zone (Gates et al., 2004), separates the Southern Adirondack Terrane from the adjacent portion of the Highlands dominated by massif anorthosite and related rocks . Similar, contemporaneous, tonalitic gneisses also occur in the eastern Adirondacks (McLelland and Chiarenzelli, 1990) and in nearby Vermont (Ratcliffe et al., 1991). ...
Detrital zircons in quartz-rich lithologies from the deformed and metamorphosed Mesoproterozoic Adirondack Lowlands in northern New York, part of the Grenville Province, have been used to document sediment provenance and basin evolution, and provide initial temporal constraints on sedimentation. Despite the effects of Shawinigan (c. 1200–1150 Ma) and Elzevirian (c. 1240–1225 Ma) orogenesis, zircon grains recovered from quartz-rich lithologies from the Grenville Supergroup largely retain detrital morphology and isotopic systematics, while displaying minimal metamorphic effects. The ages obtained constrain deposition of the entire sequence between c. 1276 and 1255 Ma. Changes in provenance track the evolution of the basin from the initial rift-drift phase, foredeep development, and transition to final basin fill and response to initial pre-Elzevirian compression. The relative lack of zircons derived from the Superior Province and predominance of those from south-central Laurentia during deposition of the sequences indicates the influence of an intervening oceanic basin to the north.
... The granulite-facies, sinistral Moose River Plain shear zone wraps around the Snowy Mountain dome then extends east toward Gore Mountain (but exposure is lost). Chiarenzelli et al. (2000), Gates et al. (2004), and Valentino et al. (2008) suggest that sinistral transpression along the Moose River Plain shear zone was in part responsible for exhumation of the anorthosite-cored Snowy Mountain dome. A similar concept is used to explain the evolution of migmatitic gneiss domes in the southern Karakoram (northwestern Himalaya, Pakistan), where molten middle crust was exhumed diapirically along vertical structures parallel to the crustal-scale Karakoram fault in the compressional regime of the Himalayan orogen (Mahéo et al., 2004). ...
... The sedimentary successions that predate Rodinia, found inboard of the convergent margin, for which detrital zircon age spectra are available include the Siamarnekh Formation (Wheeler, 1964;Spencer et al., 2015), Wakeham Group (Madore et al., 1998;van Breemen and Corriveau, 2005), Seal Lake Group (Reardon et al., 2009), Siamarnekh Formation (Spencer et al., 2015), Appalachian Inliers (Carrigan et al., 2003;Gates et al., 2004;Ownby et al., 2004), and Composite Arc Belt (Sager-Kinsman and Parrish, 1993;Friedman and Martignole, 1995;Wodicka et al., 1996;Corrigan and van Breemen, 1997). These successions show multimodal age spectra that include zircon grains derived from both the crystalline basement of the cratonic interior (>1500 Ma) but also younger populations associated with magmatism related to subduction zone(s) preceding the Grenville orogeny (ca. ...
Changing patterns in detrital provenance through time have the ability to resolve salient features of an orogenic cycle. Such changes in the age spectrum of detrital minerals may be attributable to fluctuations in the geodynamic regime (e.g., opening of seaways, initiation of subduction and arc magmatism, and transition from subduction to collisional tectonics with arrival of exotic crustal material). This changing geodynamic regime leads to a variety of sedimentary responses driven by basin formation, transition from rift to drift sedimentation, or inversion and basement unroofing. Detrital zircon grains within sedimentary rocks chart the aforementioned processes by the presence of older detrital zircon populations during basement unroofing events, followed by a successive younging in the detrital zircon age signature either through arrival of young island arc terranes or the progression of subduction magmatism along a continental margin. Hence, the response within the detrital zircon cargo to the geodynamic environment can be visualized in their changing age patterns. However, such patterns are often cryptic and evaluated on the basis of visual comparisons. In an effort to enhance objectivity in the diagnosis of the sedimentary response to the orogenic cycle, we illustrate the utility of a multidimensional scaling approach to detrital zircon age spectra. This statistical tool characterizes the "dissimilarity" of age spectra from various sedimentary successions, but it importantly also charts this measure through time. We present three case studies in which multidimensional scaling reveals additional useful information on the style of basin evolution within the orogenic cycle. The Albany-Fraser orogen in Western Australia and Grenville orogen (sensu stricto) in Laurentia demonstrate clear patterns in which detrital zircon age spectra become more dissimilar with time. In stark contrast, sedimentary successions from the Mesoproterozoic to Neoproterozoic North Atlantic region reveal no consistent pattern. Rather, the North Atlantic region reflects a signature consistent with significant zircon age communication due to a distal position from the orogenic front, oblique translation of terranes, and complexity of the continental margin. This statistical approach provides a mechanism to connect the evolutionary patterns of detrital zircon age spectra to the geodynamics of an orogenic system, which in many cases is a direct function of proximity to the orogenic front.
... Some of the high temperature magnetite deposits marked by olivine and high temperature minerals were formed during this event. Gates et al. (2004) describe a steep, NE-striking dextral strike-slip shear system in the western Hudson Highlands. The exposed shear system is > 35 km wide and composed of an anastomosing system of 1-2 km wide individual shear zones. ...
... Those associated with the Shawinigan orogeny (ca. 1180 Ma) form a west-to east-trending arcuate belt of highly deformed rocks 20-30 km wide (the Piseco Lake Shear Zone) that separates the Southern Adirondacks from the remainder of the Adirondack Highlands (Gates et al. 2004). By ca. ...
Pegmatites in New York State are largely restricted to areas of exposed Precambrian basement rocks, including the Adirondack Mountains and the Hudson Highlands. In the Adirondacks, they were emplaced into rocks that were metamorphosed during the Grenville orogeny to either the upper amphibolite facies (Adirondack Lowlands) or the granulite facies (Adirondack Highlands). Their mineral assemblages range from simple to complex, with localized occurrences of: (a) Be-, Al-, and B-rich species such as beryl, chrysoberyl, sillimanite, dumortierite, and tourmaline-supergroup minerals, (b) rare-element-and rare-earth-element-bearing minerals such as columbite-(Fe), uranopolycrase, fergusonite-(Y), uraninite, xenotime-(Y), and monazite-(Ce), (c) Ca-, F-, and K-dominant amphiboles, (d) phosphates (fluorapatite, isokite, wagnerite), (e) sulfides (bismuthinite, molybdenite), and (f) tungstates (scheelite). The pegmatite bodies from southern New York State and from some of the southern Adirondack locations display mineralogical zoning, typically with a quartz-rich core. Post-emplacement metamorphic features are observed in pegmatites in the northwestern Adirondack Lowlands, but are negligible or obscured in most of those from the southern and eastern Adirondacks. On the basis of the metamorphic grade of the host rocks, all New York pegmatites belong to the Abyssal class, whereas on the basis of mineralogy and inferred tectonic setting, they show affiliations to the NYF and mixed NYF-LCT pegmatite families. Pegmatites in the Adirondack Lowlands are related to the calc-alkaline arc magmatism of the Antwerp-Rossie granite suite and yield U-Pb zircon crystallization ages that indicate intrusion during the late Shawinigan orogeny (similar to 1195 Ma); currently there are no known pegmatites related to the Ottawan and Rigolet orogenies in the Lowlands. Pegmatites in the Highlands yield U-Pb zircon crystallization ages that correspond to: (a) emplacement during the late Elzevirian (similar to 1222 Ma), and metamorphism during the late Shawinigan (similar to 1178 Ma) orogenies; (b) emplacement during the early (1098 Ma) and mid to late Ottawan orogeny (similar to 1062-1025 Ma); (c) intrusion during the early Rigolet orogeny (1009-1003 Ma), and finally, (d) intrusion related to a thermal pulse at 949 Ma, possibly associated with the Cathead Mountain leucocratic dike swarm (935 +/- 9.2 Ma). Few, if any, granitic pegmatites were intruded between 1178 and 1098 Ma in the Adirondack Highlands. The main pegmatite bodies in the Adirondack Highlands occur in association with extensional A-type granites such as the Hawkeye and Lyon Mountain Granite suites. These rocks were emplaced in response to orogenic collapse following the Ottawan orogeny. At present, relatively little can be concluded about the tectonic setting of pegmatites in southern New York.
... In the Composite Arc Belt, detrital zircons are variably discordant and range in age from around 3200 to 1250 Ma with the bulk of the concordant analyses at the younger end of this spectrum (Fig. 5) and a secondary concentration around 1800 Ma, which does not appear on the probability plot because of discordance of analyses (Sager-Kinsman & Parrish 1993;Friedman & Martingnole 1995;Wodicka et al. 1996;Corriveau & Morin 2000;Davidson et al. 2002). Detrital zircons from paragneisses of Grenville basement inliers within the Appalachian orogen are dominated by ages in the range 1350-1175 Ma (Fig. 5) with minor peaks at 2050 and 1500 Ma (Gates et al. 2004;Ownby et al. 2004). ...
Tectonic processes associated with supercontinent cycles result in a variety of basin types, and the isotopic dating of detrital minerals within sedimentary sequences assists palaeogeographical reconstructions. Basins located along the Laurentia-Baltica margin prior to assembly of Rodinia at 1.2-1.0 Ga are dominated by zircon detritus derived from contemporaneous magmatic arcs. Basins formed during assembly are also dominated by zircon detritus with ages similar to that of sediment accumulation, reflecting syn-collisional magmatism and rapid exhumation of the developing Grenville-Sveconorwegian orogen. Post-collision intracratonic basins lack input front syn-depositional magmatism, and are dominated by significantly older detritus derived from the mountain range as well as its foreland. Basins formed during late Neoproterozoic to Cambrian breakup of Rodinia are divisible into two types. Those within the Caledonides lie on the Grenville Sveconorwegian foreland and incorporate Archaean and Palaeoproterozoic detritus derived from the cratonic interior and Mesoproterozoic detritus derived from the eroded remnants of the orogen. In the Appalachian orogen, such basins are dominated by Mesoproterozoic detritus with older detritus forming only a minor component, suggesting restricted input from the cratonic interior as a result of either the Grenville orogen still forming a drainage divide or the formation of rift shoulders.
... Almost 500 Ma of geological time are unrepresented on the northeast Laurentian craton. This interval follows the c. 1.0 Ga Grenville Orogeny, a late stage in the assembly of the Rodinia supercontinent that featured collision of the Amazon Shield with a region that now comprises much of the eastern United States and adjacent Canada (Dalziel, Salada & Gahagan, 1994; Gates et al. 2004). Subsequent geological events in northern New York and adjacent Canada are only recorded much later in the Ediacaran with fragmentation of Rodinia, and the development of a triple junction that defined the Quebec Reentrant, New York Promontory, and Ottawa–Bonnechere aulocogen (discussed in Section 2). ...
The discovery of a fossiliferous interval (Altona Formation, new unit) under the Potsdam Formation requires a new geological synthesis of a large part of the northeast Laurentian craton. Potsdam sandstones can no longer be regarded as the oldest sedimentary unit on the middle Proterozoic Grenville orogen in northern New York and adjacent Quebec and Ontario. The thickest Potsdam sections (to 750 m) in the east Ottawa–Bonnechere aulocogen have been explained by deposition with normal faulting possibly associated with Ediacaran rifting (c. 570 Ma) that led to formation of the Iapetus Ocean. However, sparse trilobite faunas show a terminal early Cambrian–middle middle Cambrian age of the Altona, and indicate much later marine transgression (c. 510 Ma) of the northeast Laurentian craton. Altona deposition was followed by rapid accumulation of lower Potsdam (Ausable Member) sandstone in the middle–late middle Cambrian. The Altona–Ausable succession is probably conformable. The Altona is a lower transgressive systems tract unit deposited on the inner shelf (sandstone, reddish mudstone, and carbonates) followed by aggradation and the deposition of highstand systems tract, current cross-bedded, in part terrestrial(?), feldspathic Ausable sandstone. Unexpectedly late Altona transgression and rapid Ausable deposition may reflect renewed subsidence in the Ottawa–Bonnechere aulocogen with coeval (terminal early Cambrian) faulting that formed the anoxic Franklin Basin on the Vermont platform. Thus, the oldest cover units on the northeast New York–Quebec craton record late stages in a cooling history near an Ediacaran triple junction defined by the Quebec Reentrant and New York Promontory and the Ottawa–Bonnechere aulocogen.
... reviews by Yin, 2006Yin, , 2009; and references therein), the India-Eurasia collision has become the archetypal continent-continent collision serving as a younger actualistic template for the study of older, no longer active orogens (e.g. Dewey and Burke, 1973;Hoffman, 1980;Gates et al., 2004;Jacobs and Thomas, 2004;St-Onge et al., 2006;Labrousse et al., 2010). Given the global interest and applicability of the Himalayan-Karakoram-Tibetan orogen to the understanding of ancient analogues worldwide, it is somewhat surprising that the age of initiation of this collision remains a subject of much discussion. ...
The Ladakh batholith is part of the > 2500 km long Trans-himalayan batholith that forms the southern margin of the Asian plate and is unconformably overlain by the post-collision Indus Molasse Group sedimentary rocks. We present new U–Pb ID-TIMS zircon ages from a host hornblende-bearing granodiorite (57.7 ± 0.2 Ma) and a later intrusive leucocratic granite dyke (47.1 ± 0.1 Ma) from the Ladakh batholith at Chumathang in northeast Ladakh, India. Subduction-related granodioritic magmatism in eastern Ladakh is dominantly of late Paleocene–early Eocene age. The age of the Chumathang dyke gives a maximum age constraint on Indus Molasse Group basin formation along the northern margin of the Indus Suture Zone and a minimum age constraint on the India–Asia collision. Together with the age of youngest marine sedimentary rocks in the suture zone (Nummulitic Limestone; 50.5 Ma) we propose that by late Ypresian–early Lutetian (early Eocene) time, the two continents had collided, sedimentation in the suture zone became purely continental and subduction-related igneous intrusions had ceased.
... Transpressional deformation has been proposed for other shear zones in the region (e.g. Streepey et al., 2001;Gates et al., 2004;Mezger et al., 1993;Busch et al., 1997;Martignole and Friedman, 1998), and also has been suggested for the areas near the BLSZ (Baird and Shrady, 2009) so our results may lend further support to transpressional models for the region. ...
Abstract
The Antwerp-Rossie metaigneous suite (ARS) represents arc magmatism related to closure of the Trans-Adirondack backarc basin during Shawinigan collisional orogenesis (ca. 1200–1160 Ma). The ARS is of calc-alkaline character, bimodal, and lacks intermediate compositions. Primarily intruding marble and pelitic gneiss, the ARS is spatially restricted to the Adirondack Lowlands southeast of the Black Lake fault. On discrimination diagrams, the ARS samples plot primarily within the volcanic arc granite fields. Incompatible elements show an arc-like signature with negative Nb, Ta, P, and Zr and positive Cs, Pb, La, and Nd anomalies relative to primitive mantle. Neodymium model ages (TDM, depleted mantle model) range from 1288 to 1634 Ma; the oldest ages (1613–1634) and smallest epsilon Nd (εNd) values are found in proximity to the Black Lake fault, delineating the extent of Laurentia prior to the Shawinigan orogeny. The epsilon Nd values at crystallization (1200 Ma) plot well below the depleted mantle curve. Geochemical and isotopic similarities to the Hermon granitic gneiss (HGG) (ca. 1182 Ma) and differences from the Hyde School Gneiss–Rockport Granite suites (1155–1180 Ma) suggest that arc plutonism rapidly transitioned into A-type AMCG (anorthosite-mangerite-charnockite-granite) plutonism. Given the short duration of Shawinigan subduction, apparently restricted extent of the ARS (Adirondack Lowlands), location outboard of the pre-Shawinigan Laurentian margin, intrusion into the Lowlands supracrustal sequence, bimodal composition, and recent discovery of enriched mantle rocks in the Lowlands, it is proposed the ARS formed as a consequence of subduction related to closure of a backarc basin that once extended between the Frontenac terrane and the Southern Adirondacks
... Furthermore, the St Boniface Quartzite sample is biased in favour of igneous-looking zircons as the initial purpose of the study was to constrain the maximum age of deposition (Corrigan & van Breemen 1997). Other potential Laurentian metasedimentary source areas include the Grenville inliers of the Appalachian Belt, with detrital zircon data indicating input predominantly of c. 1350-1175 Ma detritus and in situ metamorphic overgrowths constraining deposition to pre-1007 4 Ma (Gates et al. 2004;Cawood et al. 2007). ...
Synopsis
Whole-rock Sm–Nd model ages of 1601 Ma (lower Laggan Formation) and 1674 Ma (upper Blackrock Formation) suggest a late Palaeoproterozoic, early Mesoproterozoic source for the metasediments of the Bowmore Sandstone Group, central Islay, SW Scotland. In situ U–Pb detrital zircon (SIMS) analyses show that the group was deposited after 1003 Ma ± 19 Ma in a basin dominated by Mesoproterozoic detritus, with some Palaeoproterozoic material and minimal Archaean input. The data are consistent with Laurentian sources, mostly Grenville (possibly including reworked sedimentary material from the southwestern Grenville Province), but with some input from Ketilidian, Labradorian, Pinwarian and Archaean terranes. Previous correlations with the Torridonian and the upper Dalradian are no longer tenable given the paucity of Archaean detritus in the Bowmore Sandstone Group. However, the data are comparable to parts of the Dalradian Grampian Group, in particular, the Glen Spean Subgroup (Upper Grampian Group, Strathtummel succession). It is proposed that the Bowmore Sandstone Group was deposited in an eastern Grampian Group basin, similar to the Strathtummel and Cromdale basins, which are also dominated by Grenville detritus.
... The ages are considerably younger than those of the youngest orogenic process (Ottawan) in the Grenvillian System (1.06-1.02 Ga), according to recent papers (Wasteneys et al., 1997;Gates et al., 2004;Slagstad et al., 2005;Heumann et al., 2006). Thus, although the Cariris Velhos processes could have been part of the general worldwide fusion of Rodinia, it is more likely they were subsequent to this and largely, if not entirely, independent, as proposed for Tonian events in East Greenland (Watt and Thrane, 2001). ...
The Borborema Province in northeastern South America is a typical Brasiliano-Pan-African branching system of Neoproterozoic orogens that forms part of the Western Gondwana assembly. The province is positioned between the São Luis-West Africa craton to the north and the São Francisco (Congo-Kasai) craton to the south. For this province the main characteristics are (a) its subdivision into five major tectonic domains, bounded mostly by long shear zones, as follows: Médio Coreaú, Ceará Central, Rio Grande do Norte, Transversal, and Southern; (b) the alternation of supracrustal belts with reworked basement inliers (Archean nuclei + Paleoproterozoic belts); and (c) the diversity of granitic plutonism, from Neoproterozoic to Early Cambrian ages, that affect supracrustal rocks as well as basement inliers. Recently, orogenic rock assemblages of early Tonian (1000–920 Ma) orogenic evolution have been recognized, which are restricted to the Transversal and Southern domains of the Province.
Besides a few isolated rift basins along the Neoproterozoic eastern Laurentian margin, the Appalachian Palaeozoic strata rest unconformably on Mesoproterozoic crystalline basement, reflecting a ca. 550+ Ma hiatus. However, a nearly 3 km deep experimental geothermal borehole through the Appalachian basin at Cornell University, New York, unexpectedly encountered deformed Neoproterozoic greenschist facies phyllites and metavolcanics (Cayuta Formation) beneath the Cambrian non-conformity. Magnetic modelling shows they are in the 5000 km2 fault-bounded Ithaca basin, proposed to be associated with Iapetan rifting. The volcanism and metamorphism appear linked to ca. 600–650 Ma thermal events related or unrelated to extension, but requiring uplift and denudation of 5–10 km of rock prior to deposition of the Cambrian Potsdam Group. The Ithaca basin is >350 km inboard of the Laurentian Iapetus margin, indicating a broad Neoproterozoic rift zone for at least the NY and PA portion of eastern Laurentia.
A model to encourage the utilization of geoscience educational resources within public nature-type parks is being developed in Harriman-Bear Mountain-Sterling Forest State Park, Hudson Highlands, New York using multimedia techniques. The model uses a videotape to pique peoples interest and encourages them to visit a web site which in turn encourages them to see the rocks in the field. The videotape was produced by combining footage shot in the park with footage of prime examples of geologic features and custom animations to create an entertaining yet informative presentation. The fast-paced video is designed to inspire interest and enthusiasm in the geology of the park. Interested people are encouraged to visit an interactive web site. It provides the scientific background to the features in a stepwise manner of increasing complexity from simple pictures to advanced geochemistry, isotope geochemistry and structural geology. This allows visitors to find their level of interest and comfort. There are suggested general and topical field trips and a form to apply for a group permit.
Determining the relationship among crustal blocks within an orogen is a key factor in understanding the architecture and construction of that orogen. Within the large, mid-Proterozoic Grenville Province, the relationship between the Adirondack Lowlands and the adjacent Frontenac terrane is ambiguous. Review of previous work demonstrates that the Adirondack Lowlands have different plutonic suites, a lower grade of metamorphism, and a different geochemical signature. However, the timing and kinematics of deformation in the Lowlands, and their relation to major orogenic events, have not previously been well constrained, making comparisons with the Frontenac terrane difficult. On the northwestern edge of the Adirondack Lowlands, detailed structural analysis of upper-amphibolite grade migmatites and marbles reveals two penetrative deformation phases. Interference of F-1 and F-2 folds results in Type 3 fold interference patterns and is sufficient to produce the regional map patterns. The Noname ductile shear zone, a 0.5-2-km-wide northeast-striking steep ductile shear zone with subvertical lineation, developed during D-2. The steep geometry of the Noname ductile shear zone, paired with consistent sinistral kinematic indicators only found in subhorizontal surfaces, indicate that kinematics for D-2 was sinistral transpression. Sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG) U-Pb zircon geochronology from three granitic samples that have well-defined relationships with D-1 and D-2 indicates that both deformation phases developed through continuous or progressive deformation during ca. 1185-1145 Ma. Zircon geochronology from a quartzite, and the presence of melt during all deformation phases, demonstrate that metamorphism was synchronous with deformation. This work reveals that the Shawinigan orogeny (1190-1140 Ma) developed the dominant structural features observed in the northwest Adirondack Lowlands. These structures are the result of the northward collision of a rifted slice of the Laurentian margin (Adirondis) into previously accreted terranes on the margin of Laurentia. Shawinigan deformation of the Adirondack Lowlands may have outlasted that of the Frontenac terrane across any potential terrane-bounding shear zone. While Frontenac terrane and Adirondack Lowlands geology are sufficiently distinct to warrant separate terrane designation, evidence is lacking to indicate that a suture exists between them.
Massive magnetite veins formed during hydrothermal mineralization within northeast striking dextral shear zones in Proterozoic-age crystalline bedrock of the western Hudson Highlands. The veins formed in an open fracture system in right step-over dilational jogs during the late stages of movement. Acidic metamorphic fluids derived from metavolcanic country rock and saturated with iron, flushed through fractures, reacted with wall rock, and exchanged chemical species. Buffered by the composition of the local country rock, fluids migrated and mixed along the fault during ‘seismic pumping’ events. The fluids deposited mineral assemblages in the fractures that reflect the changing flux, fluid buffering, and/or physical conditions.
The Cambrian–Ordovician carbonate platform units on the New York promontory of eastern Laurentia reflect the south tropical location of the area. The slow subsidence of the region through much of the Cambrian–Ordovician meant that strong eustatic rises and falls defined unconformity-bound carbonate formations. These depositional sequences aid in paleoocean-ographic reconstruction as they correlate with organic-rich dysoxic–anoxic mudstones on the Laurentian continental slope. Eustatic rise increased insolation as epeiric seas covered the plat-form and produced climate maximums with reduced deep-water circulation. The oldest car-bonate platform unit (Forestdale Marble and equivalents, upper Lower Cambrian) overlies rift facies deposited with the Rodinia breakup and origin of the Iapetus Ocean and marks the transition to a passive margin. Drowning of the Forestdale platform occurred, and the over-lying anoxic black mudstone (Moosalamoo Phyllite) abruptly shoals up into tidalite sand-stone (Cheshire Formation). This depositional history records a decreased rate of sea level rise as the Cheshire Formation continued to onlap middle Proterozoic basement. Super-Cheshire Cambrian carbonate platform units in the northern Appalachian are mostly hydrothermally dolomitized, record eustatic highs (Dunham, Winooski, and Little Falls Formations), and corre-late with black mudstone macroscale units on the slope (Browns Pond and Hatch Hill dysoxic– anoxic intervals). The latest Early Cambrian Hawke Bay regression ended carbonate platform deposition of the Dunham Formation, led to quartz arenite or red shaly dolostone offlap or shoaling deposits on the platform, and was coeval with oxic green mudstone on the con-tinental slope (Hawke Bay oxic interval in Taconian allochthons). Subsequent Middle Cam-brian eustatic rise is recorded by dolostone (Winooski and upper Stissing), but carbonate deposition was again suppressed as quartz sand swept toward the shelf margin (Danby For-mation) coincident with cratonic transgression by the upper Potsdam Formation (uppermost Middle Cambrian–lower Upper Cambrian). Post-Potsdam deposition was carbonate dominated 451
The New York-Alabama (NY-AL) lineament, recognized in 1978, is a magnetic anomaly that delineates a fundamental though historically enigmatic crustal boundary in eastern North America that is deeply buried beneath the Appalachian basin. Data not in the original aeromagnetic data set, particularly the lack of any information available at the time to constrain the southern continuation of the anomaly southwest of Tennessee, left the source of the lineament open to conjecture. We use modern digital aeromagnetic maps to fill in these data gaps and, for the first time, constrain the southern termination of the NY-AL lineament. Our analysis indicates that the lineament reflects a crustal-scale, right-lateral strike-slip fault that has displaced anomalies attributed to Grenville orogenesis by similar to 220 km. Palinspastic restoration of this displacement rearranges the trace of the Grenville belt in southern Rodinia and implies only passive influence on later-formed Appalachian structures. The precise timing of dextral movement on the NY-AL structure is not resolvable from the existing data set, but it must have occurred during one of, or combinations of, the following events: (1) a late, postcontractional (post-Ottawan) stage of the Grenville orogeny; (2) late Neoproterozoic to Cambrian rifting of Laurentia; or (3) right-slip reactivation during the late Neoproterozoic-Cambrian rifting of Laurentia, or during Appalachian movements. Our palinspastic reconstruction also implies that the host rocks for modern earthquakes in the Eastern Tennessee Seismic Zone are metasedimentary gneisses, and it provides an explanation for the spatial location and size of the seismic zone.
Discontinuous sills of felsic gneiss in the interior and western margin of the Berkshire massif and granite sills on the eastern margin of the massif were correlated by Ratcliffe (1984a, 1984b, 1985) and Ratcliffe and Hatch (1979), and interpreted by them as syntectonic anatectic melts that intruded Taconic thrusts. We dated three felsic gneiss sills and two granite sills in an attempt to constrain the age of Taconic thrusting, but discovered that the sills are not coeval. Rather, they were intruded during two widely separated episodes, one during the Mesoproterozoic at approximately 1000 Ma and the other during the Silurian at approximately 430 to 435 Ma. The 1000 Ma sills of felsic gneiss in the interior of the massif are located in Mesoproterozoic units and many of the mapped Taconic thrusts within the massif closely follow the distribution of this unit, here informally called the felsic gneiss of Harmon Brook. These 1000 Ma sills formed during the Ottawan or Rigolet orogeny and they have no connection to the Taconic orogeny. The 430 to 435 Ma granite sills along the eastern margin of the massif, informally called the granite of Becket Quarry, are found in both Mesoproterozoic basement and the Neoproterozoic Hoosac Formation. The sills are too young to have intruded during the Ordovician Taconic orogeny, but they may have formed during later faulting near the contact between Mesoproterozoic basement and Neoproterozoic cover rocks. The Tyringham Gneiss is one of the most common Mesoproterozoic units in the Berkshire massif. Zircons from the Tyringham Gneiss contain cores with oscillatory zoning and thin unzoned rims. The weighted average of eight 206Pb/238U analyses from the cores is 1179 /- 9 Ma, whereas nine spot analyses from the rims yield an age of 1004 /- 9 Ma. We interpret these two ages to represent the crystallization of the Tyringham Gneiss protolith and a subsequent high grade metamorphism, coeval with the intrusion of the felsic gneiss of Harmon Brook. The western contact between Mesoproterozoic rocks of the Berkshire massif and underlying Early Paleozoic rocks is clearly a thrust, but there is no independent evidence that movement occurred during the Taconic orogeny; displacement may also have occurred during the Silurian Salinic or the Devonian Acadian orogeny. Many contacts mapped as Taconic thrusts within the Berkshire massif follow the distribution of the 1000 Ma felsic gneiss of Harmon Brook. The age of the sills is clearly incompatible with this interpretation, and evidence for faulting along these mapped thrusts is lacking. Instead of being deformed into an imbricate stack, the massif behaved as a rigid block during Paleozoic uplift. Finally, the age of granite sills along the eastern margin of the massif does not constrain the basement-cover contact to be a Taconic thrust, as previously interpreted. The contact may be a Silurian fault, possibly related to extension and the opening of the Connecticut Valley trough as a back-arc basin. According to this model, the magma for the granite sills was generated above a west-dipping subduction zone under the Laurentian margin, which developed after the Taconic orogeny.
The northwest-dipping Carthage-Colton shear zone, located near the eastern margin of the Grenville province in northern New York, separates the Adirondack Lowlands of the Metasedimentary Belt from the Adirondack Highlands of the Granulite Terrane. Published U/Pb sphene ages are 1156-1103 Ma in the Lowlands and 1050-982 Ma in the Highlands, indicating an ∼100 m.y. offset in metamorphic ages across the boundary. Our reported hornblende 40Ar-39Ar ages are ∼1060 Ma in the Lowlands and ∼950 Ma in the Highlands. This confirms the offsets in sphene ages across the Carthage-Colton shear zone and indicates an average cooling rate in both the Lowlands and the Highlands of 1°-2°/ m.y. over ∼100 m.y. Biotite 40Ar-39Ar ages from this study yield no apparent age difference, thus approximating the time of final Carthage-Colton shear-zone extension. These 40Ar-39Ar data suggest that extensional motion along the Carthage-Colton shear zone occurred between 950 and 920 Ma, postdating the latest recorded compressional activity between 1060 and 1030 Ma and orogenic collapse between 1045 and 1030 Ma in the Metasedimentary Belt. Extensional motion occurring at least 100 m.y. after the last compressional event is probably not related to orogenic collapse, but rather may be related to another, as yet undefined, extensional event in eastern Laurentia during Late Proterozoic time.
We have analyzed shear wave splitting recorded by portable and permanent broadband and long-period stations located in the eastern United States. Teleseismic shear waves (SKS, SKKS, and PKS) were used to retrieve the splitting parameters: the orientation of the fast wave polarization plane varphi and the delay time deltat. In total, 120 seismic events were processed, allowing for more than 600 splitting measurements. Within the Appalachians, stations located in the western (external) part are characterized by deltat~1 s and varphi trending N50°-70°E in the south and central regions and N30°-40°E in the north, closely following the trend of the orogenic belt in these areas. The transition region between north and central is characterized by deltat~1-1.3 s and by E-W trending varphi that are at a high angle to the regional geologic trend. Measurements at two stations located in the eastern (internal) part of the belt indicate very weak anisotropy. The large-scale pattern of anisotropy is not consistent with that predicted for simple asthenospheric flow beneath the plate. Splitting along the southern and eastern margins of the continent is consistent with that expected for Grenvillian deformation, an alternative model of asthenospheric flow around the cratonic keel cannot be ruled out. Within the cratonic core, the correlation between deltat and lithospheric thickness suggests a lithospheric anisotropy. Smaller-length-scale variations also argue for a significant contribution of lithospheric structures. The fabric responsible for shear wave splitting may have formed during tectonic episodes that affected the eastern United States, i.e., the Grenville and Appalachian orogenies and the subsequent rifting of the North Atlantic Ocean. Our observations in the western Appalachians suggest that the anisotropy may be preserved since the Grenvillian orogeny. The absence of detectable splitting in the two stations in the eastern Appalachians is attributed to the igneous intrusions related to the Atlantic rifting. The measurements in the transition between the northern and central southern Appalachians, constitute an intriguing anomaly, whose E-W varphi have little obvious relation to the regional surface geology. We suggest two possible causes: (1) the local dominance of asthenospheric flow, motivated by the proximity of a pervasive low-velocity anomaly and (2) lithospheric deformation in a transcontinental strike-slip fault zone active during the Appalachian collision.
A laterally extensive zone of southeast-dipping marble mylonites in the Central Metasedimentary Belt (CMB) of the southern Ontario Grenville (the "Bancroft shear zone") records southeast-directed extension and a displacement of about 10-12 km. Calcite-graphite carbon isotope thermometry indicates that the maximum temperature at which shearing occurred was ca. 450 °C. In contrast, carbon isotope fractionations (Delta13Ccc-gr) within the marble protolith consistently give temperatures of 650-700 °C. The Bancroft shear zone postdates 1060 Ma and older high-grade shear zones in the Central Gneiss Belt and the CMB that are associated with northwest-directed thrusting. Comparison of the temperature data from the retrograde marble mylonites with cooling curves for the CMB gives a late Grenvillian age range of 935-1010 Ma for the extensional event. The geometric relation between thrusting and normal faulting in the CMB is comparable to that in parts of the Himalayas and southern Tibet, and similarly we interpret late normal faulting in the CMB to be a result of gravitational collapse of a thrust thickened crust. In contrast to the Himalaya-Tibet region, however, accretion in the Grenville becomes older in the direction of regional thrusting.
U-Pb zircon studies of metamorphosed igneous rocks in the Adirondack Mountains have yielded preliminary ages within the range 1420-990 Ma. Several geochronologically and geochemically distinct episodes of igneous intrusion and at least one pre-granulite facies dynamothermal metamorphic event are documented. This information is consistent with recent field and geochronological studies throughout the Grenville province and suggests that a complex sequence of events occurred in the Adirondack Mountains prior to the widespread deformation and metamorphism commonly attributed to the ˜1100-1000 Ma Ottawan phase of the Grenvillian orogenic cycle.
Some of the most useful criteria for the deduction of the sense of shear are summarized for use in areas where unequivocal field evidence is lacking. Apparently conflicting evidence from rotated pressure-shadow regions around porphyroclasts and porphyroblasts is clarified. The use of quartz-crystallographic fabric asymmetry to deduce the shear sense in the bulk rock should be treated with caution and used only together with detailed microstructural observations.
Based on lithological, structural and geophysical characteristics, the Proterozoic Grenville Orogen of southern Ontario and New York has been divided into domains that are separated from each other by ductile shear zones. In order to constrain the timing of metamorphism, U-Pb ages were determined on metamorphic and igneous sphenes from marbles, calc-silicate gneisses, amphibolites, granitoids, skarns and pegmatites. In addition, U-Pb ages were obtained for monazites from metapelites and for a rutile from an amphibolite. These mineral ages constrain the timing of mineral growth, the duration of metamorphism and the cooling history of the different domains that make up the southern part of the exposed Grenville Orogen. Based on the ages from metamorphic minerals, regional and contact metamorphism occurred in the following intervals:Central Granulite Terrane:Adirondack Highlands: 1150 Ma; 1070–1050 Ma; 1030–1000 MaCentral Metasedimentary Belt:Adirondack Lowlands 1170–1130 MaFrontenac domain 1175–1150 MaSharbot Lake domain ca. 1152 MaFlzevir domain: 1240 Ma; 1060–1020 MaBancroft domain: ca. 1150 Ma; 1045–1030 MaCentral Gneiss Belt: ca. 1450 Ma; ca. 1150 Ma; 1100–1050 MaGrenville FrontTectonic Zone ca. 1000 Ma.Combination of mineral ages with results from thermobarometry indicates that metamorphic pressures and temperatures recorded by thermobarometers were reached polychronously in the different domains that are separated by major shear zones. Some of these shear zones such as the Robertson Lake shear zone and the Carthage-Colton shear zone represent major tectonic boundaries. The Grenville Orogen is made up of a collage of crustal terranes that have distinct thermal and tectonic histories and that were accreted laterally by tectonic processes analogous to those observed along modern active continental margins. The subsequent history of the orogen is characterized by slow time-integrated cooling rates of 31C/Ma and denudation rates of 12040m/Ma.
A possible palaeogeographic link between the Grenville and Rondonia-Sunsas provinces is reinforced by the timing of thermometamorphic and plutonic events. Equivalent terms of regional geological time scales are: Ketilidian = Transamazonian; Palaeohelikian or Elsonian = Rondonian-San Ygnacio; Neohelikian or Grenvillian = Sunsas; Hadrynian = Brasiliano. Remobilization of the older crust is another feature common to both provinces in Helikian times. Felsic volcanics extruded 1650 million years ago in the eastern Grenville Province show 1.6 Ga analogues in the Roosevelt lava flows of the Amazon Craton. Also a recently defined 1470–1500 Ma Pinwarian event in the Grenville shows correlatives in the final stages of evolution of the Rio Negro-Juruena belt.
The Grenville Province formed the southeastern limit of Proterozoic Laurentia and is composed of lithologic units that range in age from Archean to Late Mesoproterozoic. Reworked continuations of Archean, Early and Middle Paleoproterozoic orogens extend at the surface and at depth well into the southwestern and central Grenville Province, but are absent in the northeast. There is evidence for the existence of an active margin, with subduction-accretion and arc formation, along southeastern Laurentia for >400 Ma from the Late Paleoproterozoic to the Late Mesoproterozoic. Major juvenile crustal additions to the Laurentian margin, comprising Andean-style, calc-alkaline magmatic arcs and coeval inboard backarc deposits, occurred between ∼ 1.71 and 1.61, 1.51 and 1.42, and 1.40 and 1.23 Ga. Backarc magmatic and (or) sedimentary products document variable degrees of backarc extension and basin formation. Examples include the coeval oceanic and continental backarc basin settings of Hastings/Frontenac and Wakeham/Seal Lake groups respectively during geon 12, and the incipient continental backarc extensional settings of the Michael-Shabogamo dyke swarm during geon 14 and of alkali granite and anorthosite complexes during geons 13 and 12. Arc magmatism was followed by accretionary orogenesis during the Labradorian (∼ 1680-1660 Ma), Pinwarian (∼ 1500-1450 Ma) and Elzevirian (∼ 1250-1190 Ma) orogenies, respectively, resulting in substantial growth of Laurentia. A result of this growth is that the ages of most major units tend to young towards the southeast of the Grenville Province, except for those that formed inboard of the continent margin in a backarc setting. The continent-continent Grenvillian Orogeny took place between ∼ 1.19 and 0.98 Ga and comprised three distinct pulses of crustal shortening at ∼ 1.19-1.14, 1.08-1.02 and 1.00-0.85 Ga, separated by periods of extension. The loci of the earlier (Shawinigan and Ottawan) pulses of crustal shortening were in the hinterland of the orogen, whereas the latest (Rigolet) pulse caused northwesterly propagation of the orogen into its foreland. Periods of crustal extension during the Grenvillian Orogeny were coeval with emplacement of mafic magmas and anorthosite complexes, implying that large quantities of mantle magma and heat had access to the base of the previously thickened orogenic crust. This scenario is compatible with extensional collapse of the orogen following delamination or convective removal of the lower lithosphere, as suggested previously by others. An inference from these conclusions is that anorthosite complexes formed in two contrasting extensional tectonic environments in southeastern Laurentia during the Mesoproterozoic, that is, in areas of backarc extension inboard from an active continental-margin magmatic arc and within a collisional orogen during periods of tectonic collapse and rising isotherms.
A simple analogy is made between the tectonics of Asia and deformation in a rigidly indented rigid-plastic solid. India is analogous to the indenter and the great strike-slip faults correspond to slip lines. For various indentation geometries, the sense and linearity (or curvature) of strike-slip faults, convergence at the Burma arc and the existence of the Himalayan Burman Syntax, the conjugate strike-slip faults in Mongolia and the extension at the Baikal and Shansi graben can be predicted. Given the horizontal force necessary to support Tibet, an average shear stress of a few to several hundred bars along faults in Asia is predicted, corresponding to the yield stress of rigid-plastic material.
The western Grenville province (Proterozoic) and the western Himalaya (Mesozoic-Cainozoic) have remarkably similar tectonic patterns. Both contain island arc volcanic successions which were intruded by Andean-type batholiths following deformation attributed to arc-collision. Subsequently they were affected by deformation correlated with continental collision. Both arc-batholith belts (the CMB and the Kohistan belt, respectively) are bordered by straightening zones containing refoliated, lineated porphyroclastic gneisses and major thrust stacks. By analogy with the Himalaya it is suggested that the northwestward-directed thrusting in the central gneiss belt of the Grenville formed as a result of post-collisional deformation of the margin of the CGB continent, caused by continuing southeastward motion and indentation of the continent against the arc-batholith of the CMB. (Author's abstract)-J.M.H.
There are two distinct tectonic events, early contractional and late dextral transpressional, within the larger designation of the Grenvillian Orogen in the Hudson Highlands of southern New York. The rocks of the Hudson Highlands are ca. 1.3 Ga, and are interpreted to have formed in an island arc or magmatic arc as volcanics, volcaniclastics, and sedimentary rocks. A continental collision terminated this segment of the geologic history and resulted in a major mountain belt. Deformation during this event is characterized by tight recumbent folds that resulted from westward-directed fold nappe emplacement. The predominant gneissic foliation was developed during granulite facies metamorphism, migmatization, and extensive pegmatite intrusion. Subsequent to this event, there was an episode of intrusion of dioritic, and locally anorthositic dikes and stocks. These bodies provide a time marker between the two orogenic events. The second event produced discrete zones of ductile deformation with a consistent dextral strike-slip shear sense and crenulation cleavage. This event is interpreted to result from tectonic escape resulting from a collision to the north of the study area. Steeply southeast to northwest dipping 1-3 km wide zones of mylonite display extensive shallow (15-20°) northeast trending lineations, S-C fabrics, asymmetric boudins, asymmetric intrafolial folding, and rotated porphyroclasts locally. These zones appear to have been active during a period of rapid cooling. Late in their history, the shear zones crossed the brittle-ductile transition and became dilational. Fracture systems formed along the zones and were progressively mineralized with hedenbergite, scapolite, magnetite, and quartz, phlogopite, hornblende, hematite, and calcite locally. Upright, gentle to open, and generally symmetric, north-trending mesoscopic to megascopic folds formed adjacent to the shear zones. The entire area was intruded by extensive pegmatite dikes and locally by granitic plutons. Pegmatites are common within the mineralized shear zones where they contain xenoliths of fault rock.
Fundamental zones of crustal weakness that were established in the northern Reading Prong, central Appalachians during Grenvillian orogenesis were reactivated multiple times during subsequent tectonic events. The rocks in these fault zones contain complexly overprinted mylonites, cataclasites, and breccias with distinct kinematics and mineralizations. By studying the similarities and differences of rocks from three different fault zones across the Reading Prong of eastern Pennsylvania, New Jersey and the southeastern New York, the tectonic succession and a relative chronology can be determined. As many as seven phases of reactivation can be identified within the major Ramapo, Reservoir, and Morgan Hill fault zones. These weak zones between the rigid crystalline basement blocks exhibit these multiple and intense deformational episodes because they absorbed the majority of the strain that was transmitted through the basement rocks during the subsequent intense tectonic events. The cover sequence was also heavily deformed during these events. The crystalline blocks between the major faults, on the other hand, show relatively little post peak-Grenville deformation. The only evidence of these events are minor faulting and several phases of jointing. The rigid crystalline blocks simply adjusted to the renewed stresses by movement along their boundaries. Depending upon stress and metamorphic conditions, the previously formed mylonites and cataclasites were either sheared in ductile or semi-brittle deformation or fragmented into physically separated blocks.
Sensitive high-resolution ion microprobe (SHRIMP) analyses of zircons from four charnockitic gneisses and a ferrodiorite dike belonging to the anorthosite-mangerite-charnockite-granite (AMCG) suite of the Adirondack Mountains are consistent with prior multigrain U-Pb zircon determinations that indicated an emplacement age of ca. 1150-1160 Ma for other units of the suite. This result differs from recent assertions that Adirondack anorthosites were emplaced at ca. 1050 Ma. Each of the four charnockites discussed in this paper is intimately associated with adjacent massif anorthosite, with which they commonly exhibit mutually crosscutting and usually gradational relationships. Xenoliths of anorthosite in the charnockitic rocks provide a minimum age for the associated anorthosite. The presence of xenocrysts of blue-gray andesine in the charnockites reflects plucking from a coeval, but not wholly consolidated, anorthositic magma. Ages determined for the four charnockitic rocks are 1176 ± 9 Ma (Minerva), 1154 ±17 Ma (Gore Mountain), 1174 ±25 Ma (Snowy Mountain), and 1164 ± 11 Ma (Diana Complex). Results also indicate an age of 1156 ± 7 Ma for a ferrodiorite dike that crosscuts, and is thought to be comagmatic with, the Oregon Dome anorthosite massif. These five new ages fall within a restricted interval of ca. 1150-1170 Ma that is in close agreement with similar AMCG magmatism in the central and eastern Grenville Province and, as a result, reinforce the proposal that this magmatism took place in response to orogen collapse and delamination following a widespread collisional event at ca. 1200-1160 Ma.
A new stratigraphic and structural synthesis is presented for Precambrian rocks of the Adirondack Mountains, New York, an amphibolite-granulite facies terrane in the l.l-b.y.-old Grenville province exposed in a dome on the North American craton. The geology of the Adirondacks appears to be explicable in terms of a stratigraphic sequence that has been subjected to multiple folding, metamorphism, and intrusive activity. This stratigraphic sequence is correlated across the entire width of the Adirondacks and westward into Ontario. Recognition of the widespread nature of this stratigraphic sequence has resulted in a coherent structural framework for the Adirondacks, consisting of two stages of nappe formation followed by three stages of upright to overturned folding. Both widespread intrusive activity and subsequent mylonitization of intrusive rocks occurred mostly during the second phase of nappe formation. The stratigraphy of the Adirondacks is interpreted to consist of an older granitic basement, referred to as the Piseco Group, overlain unconformably by a metamorphosed clastic/carbonate sequence, referred to as the Oswegatchie Group in the northwest and the Lake George Group in the east. The oldest recognized formation in the Piseco Group is the Pharaoh Mountain Gneiss, consisting of charnockitic and granitic gneiss. This unit is overlain in many places by the Alexandria Bay Gneiss, consisting of pink leucogranitic gneiss. The Alexandria Bay Gneiss is equivalent to the Brant Lake Gneiss in the eastern Adirondacks. The basal formation of the metasedimentary rocks of the Oswegatchie Group is the Baldface Hill Gneiss. This thin and discontinuous unit consists of garnet-sillimanite gneiss and quartzite. The overlying Poplar Hill Gneiss consists of biotite-quartzplagioclase gneiss that contains granitic portions. Rocks of the Baldface Hill and Poplar Hill may represent metamorphosed basal quartz sand and conglomerate, shale, shaly arkose, and possibly reworked Fe- and Al-rich regolith that was formed by weathering of the basement prior to deposition of the cover rocks. The Baldface Hill and Poplar Hill are equivalent to the Eagle Lake Gneiss of the Lake George Group. Overlying these thin basal clastic deposits of the Oswegatchie Group is the Gouverneur Marble that consists of five members, two of which contain three subdivisions within them. Member A at the base consists of thick, calcific, dolomitic, and siliceous marbles. Member B is a thin, pyritic biotite schist. Member C consists of interbedded siliceous marbles, quartzites, and calc-silicate rocks. Member D consists of well-layered calcareous gneiss, and Member E, only locally present, is a quartz-feldspar granulite. To the east, the Gouverneur Marble correlates with carbonate rocks of the Cedar River, Blue Mountain Lake, and Paradox Lake Formations, and correlates via facies changes to metamorphosed calcareous clastics of the Cranberry Lake, Sacandaga, Tomany Mountain, and Springhill Pond Formations. The previously defined upper and lower marble may be stratigraphically equivalent in the Northwest Lowlands and possibly in the Adirondack Highlands. The Pleasant Lake Gneiss overlies the Gouverneur Marble and consists largely of migmatitic gneiss equivalent to the Treadway Mountain Formation of the Lake George Group. K-feldspar megacrystic granitic gneisses overlie the Pleasant Lake Gneiss. These rocks are equivalent to the Lake Durant Formation in the Lake George Group and probably represent intrusive sheets. Anorthosite, charnockite, hornblende granite, and gabbro successively intruded the metamorphosed sedimentary rocks and themselves were later metamorphosed and deformed. Mangerite-charnockite suites that mantle anorthosite contain xenoliths of anorthosite and are thought to be produced by partial melting of Pharaoh Mountain Gneiss by heat from the anorthosite. Megacrystic hornblende granitic gneisses intrude various formations of the Lowlands and Highlands but show gross structural concordance. Five phases of folding affected all stratigraphic units, but only the last four phases affected the intrusive rocks. The first phase of folding resulted in northwest-directed nappes and formation of regional foliation and lineation. A second phase of isoclinal folding folded the regional foliation and lineation. Intrusion of anorthosite, charnockite, hornblende granite, and gabbro accompanied second-phase folding, as well as local mylonitization of charnockite and local thrusting. The third-phase folds are upright to overturned and responsible for the "grain" of the Adirondacks. The axial traces of these folds form an arc convex to the north that swings continuously from N70°W to eastwest to N45°E from south to northwest. Peak 1.1 to 1.02-b.y.-old granulite facies metamorphism outlasted third-phase folding in the Adirondack Highlands and secondphase folding in the Lowlands. Fourth-phase, northwest-trending folds are open and best developed in the northwestern Adirondacks, where they are associated with retrograde metamorphism and possibly with intrusion of diabase dikes at mid-amphibolite facies. Fifth-phase, north-northeast-trending folds are open and best developed in the Adirondack Highlands, where they are associated with retrograde metamorphism and with 930-m.y.-old pegmatite dikes. The fourth- and fifth-phase folds interfere with third-phase folds to produce dome and basin map patterns. Hook and heart and anchor map patterns result from interference of the later folds with first- or second-phase isoclinal folds.
The N-NE-trending Adirondack dome exposes an oblique section of middle to lower Proterozoic crust with metamorphic conditions ranging from about 600 MPa and 550oC in the NW Lowlands to about 800 MPa and 800oC in the central Highlands. Metamorphosed platform sediments, migmatites and basal leucogranitic gneiss underlie most of the Lowlands, whereas the Highlands are dominated by granitic, charnockitic and anorthositic gneiss with intervening synclines of metasediment. Five phases of folding, from isoclinal to open, have affected the region. Strong elongation lineations parallel early fold axes and indicate regional ductile rotational strain with a tentatively assigned E-overW sense of shear similar to that in other parts of the Grenville Province.-from Authors
The Mylonite Zone (MZ) is a major, ductile deformation zone in the Sveconorwegian orogen (Baltic Shield) of southwestern Sweden and southeastern Norway which has a strike length of over 400 km and an across-strike width which often exceeds 5 km. It is an orogen-parallel deformation zone which formed under retrogressive metamorphic conditions relative to the higher-grade structures in the surrounding crustal units. The MZ marks a conspicuous metamorphic break in the area south of lake Vänern and a distinct lithological break in the area north of this lake. Regional metamorphic considerations suggest that its surface exposure represents an oblique section through the crust with deeper levels exposed along the southern parts of the zone and shallower levels exposed farther north.
Both single and multigrain U-Pb zircon thermal ionization mass spectrometry (TIMS) as well as sensitive high resolution ion microprobe (SHRIMP 11) dating of two suites of Adirondack granites have yielded ages constraining the principal tectonomagmatic events of the Ottawan Orogeny to the interval ca 1090-1035 Ma. The earliest of these consists of mildly A-type hornblende granites of the Hawkeye granite suite, multigrain samples of which define a tight age cluster of ca 1103-1093 Ma. Assemblages and fabrics in this suite demonstrate that it experienced the high-grade effects of the Ottawan Orogeny, thereby fixing the maximum age of the latter at ca 1090 Ma. The second suite consists of Lyon Mt. Granitic Gneiss, six samples of which cluster tightly at ca 1060-1045 Ma. Fabrics associated with this suite indicate a late- to post-tectonic origin thus fixing the minimum age for the Ottawan Orogeny. Especially critical are two samples of ca 1047 Ma fayalite granite that are essentially undeformed and must post-date tectonism. In addition, an undeformed pegmatite dike yields an age of 1034 +/- 8 Ma confirming the termination of Ottawan orogenesis by that time. It is suggested that the genesis of the Hawkeye suite is related to athenospheric heating of the crust due to far-field effects of contemporaneous magmatic events at the Midcontinent rift. Lyon Mt. Granitic Gneiss is interpreted as the result of deep crustal melting following delamination of the overthickened Ottawan orogen. Together with the results of metamorphic investigations, these two suites define a counterclockwise P-T-t loop for the Adirondacks during the Ottawan Orogeny. Geochronological and tectonic investigations from the Grenville Province of Canada and the northern Blue Ridge Province of the Appalachians demonstrate the presence of strong Ottawan deformation, magmatism, and metamorphism in these areas and emphasize the large scale and marked intensity of this event. In Canada, the crust responded to the Ottawan collision principally by imbricating into large northwest-directed thrust slices rather than the fold nappes of the Adirondacks. This is consistent with the apparent absence in the Canadian foreland of Hawkeye age magmatism and resultant theological weakening of the crust.
Extensive terranes of basement reactivation are interpreted as resulting from crustal thickening following continental collision. It is suggested that terranes, such as the Grenville Province and much of the Variscan orogenic belt in Europe, have their modern analog in the Tibetan Plateau. The Tibetan Plateau is underlain by a continental crust between 60 and 80 km thick and is characterized by extensive high-potash Neogene vulcanism. Following T. H. Green's arguments that partial melting of a dioritic lower crust may yield potassic granitic liquids and refractory anorthositic residues, we consider that continental collision is followed by crustal thickening, to accommodate further plate convergence, with ensuing partial melting of the lower crust. At high structural levels, silicic-potassic ignimbrites are extruded in intermontane basin-horst terranes, with subjacent granite plutons. At deeper levels, a dry refractory lower crust consisting of pyroxene granulites and anor-thosites is generated.
Radiometric age determinations of a granitic gneiss give an age of at least 1350 m.y.-K.A.R.
Vertical to subvertical planes of shear within the active crustal
deformation field in Tibet align with fast directions of shear-wave
polarization. This observation suggests that the present-day velocity
gradient tensor field within the crust of Tibet correlates with the
lithospheric mantle velocity gradient tensor field beneath Tibet. This
inference requires the following. (1) The [100] axes of olivine are
aligned within established zones of lithospheric mantle fabric or
anisotropy, built up through finite strain. (2) Present-day shear in the
mantle lithosphere is occurring within these established zones of
fabric, parallel to the direction of olivine [100] axis alignment. (3)
The zones of mantle lithospheric fabric, or zones of mantle shear, are
aligned with zones of crustal shear (faults). The correlation of crustal
and mantle strain fields most simply results from the fact that both
crust and mantle lithosphere are under the influence of similar velocity
boundary conditions. Furthermore, the observations confirm the
distributed nature of lithospheric mantle deformation beneath
east-central Tibet and suggest that left-lateral shear has been the
dominant component of finite shear there.
Movement directions on major shear zones in the 1100-1000 Ma Sveconorwegian belt of Baltica indicate oblique sinistral convergence during the main Sveconorwegian collision event, whereas shear zones in the Grenville belt indicate convergence orthogonal to the belt. A model for the drift history of Baltica suggests that the ~80° rotation away from Laurentia at ~1200 Ma was followed by collision ~1050 Ma with a third continent (South America?) that may have already docked with Laurentia along the main Grenville suture. Subsequent movement of Baltica toward Laurentia ended with docking along the northern Britain and eastern Greenland margin at ~1000 Ma. At ~950 Ma, sinistral transform motion took place along the Laurentian margin and may have been responsible for early (pre-750 Ma) orogen-parallel movement directions in the Caledonides of Scotland and Norway, linked with a regional thermal event producing widespread granites and pegmatites at 800-750 Ma. -from Author
The Adirondack Mountains are characterized by three major events that took place during the interval ca. 1350-1000 Ma. The earliest of these is the arc-related Elzevirian Orogeny (ca. 1350-1185 Ma) during which substantial volumes of juvenile calc-alkaline crust were added to the Adirondacks as well as to the northwest segment of the Central Metasedimentary Belt. Data from the southwestern United States as well as from Ireland and Baltica indicate that Elzevirian magmatism and orogeny were of global dimensions. Within the southwestern sector of the Grenville Province, the Elzevirian Orogeny culminated at ca. 1185 Ma when accretion of all outboard terranes was completed. Compressional orogeny related to this convergence resulted in overthickened crust and lithosphere which subsequently delaminated giving rise to orogen collapse and AMCG magmatism that swept southeastward from the Frontenac Terrane into the Adirondack Highlands during the interval ca. 1180-1130 Ma. Localized compressional events within neighboring parts of the Grenville Province emphasize the continued existence of contraction during this interval, although crustal extension caused local in sedimentary basins in which were deposited the Flinton and the St. Boniface Groups.
The Protogine Zone (PZ) forms the border between the Svecofennian and Sveconorwegian domains of southern Sweden. Intruded in the Zone are three generations of mafic dykes dated 1510 Ma, 1180 Ma and 930 Ma. A palaeomagnetic study of these dykes in the central and southern parts of the PZ yields two main directions of magnetization: ‘A’ (D = 291°, I = −78° , N = 31) and ‘B’ (D = 129°, I = 40°, N = 13). These directions give palaeopoles on the Sveconorwegian Loop of the Fennoscandian apparent polar wander path (APWP). A third, poorly defined group, ‘C’, yields a mean declinationof 185° and inclination of 9°, with the corresponding palaeopole situated on the c. 1500–1600 Ma part of the APWP. Two other sites yield palaeopoles located in the 1200 Ma part of the path. The data imply that the Sveconorwegian orogeny caused a remagnetization of the two older dyke generations and that this remagnetization was complete in the southern part of the PZ, while a few dykes retained their original magnetization in the central part of the Zone. Either the difference between the ‘A’ and ‘B’ directions record appreciable APW movement during differential cooling of the PZ, or the ‘A’ and ‘B’ directions may represent an asymmetric reversal of the Earth's magnetic field. In the latter case they may be contemporary and have implications for the size and form of the Sveconorwegian Loop of the Fennoscandian APWP.
Some of the most useful criteria for the deduction of the sense of shear are summarized for use in areas where unequivocal field evidence is lacking. Apparently conflicting evidence from rotated pressure-shadow regions around porphyroclasts and porphyroblasts is clarified. The use of quartz-crystallographic fabric asymmetry to deduce the shear sense in the bulk rock should be treated with caution and used only together with detailed microstructural observations. -Authors
Laurentia, the rift-bounded Precambrian nucleus of North America, may have broken out from a Neoproterozoic supercontinent between East and West Gondwana. Several lines of evidence suggest that the Appalachian margin of Laurentia subsequently collided with the proto-Andean margin of the amalgamated Gondwana supercontinent in different relative positions during early and mid-Paleozoic time, in route to final docking against northwest Africa to complete the assembly of Pangea. Hence the Appalachian and Andean orogens may have originated as a single mountain system. The overall hypothesis retains the same paleomagnetic and paleobiogeographic controls as previous global reconstructions for the Paleozoic Era. Laurentia-Gondwana collisions may help to explain contemporaneous unconformities in the Paleozoic sedimentary cover of the Laurentian, Gondwanan, and Baltic cratons.
Plane indentation experiments on unilaterally confined blocks of plasticine help us to understand finite intracontinental deformation and the evolution of strike-slip faulting in E Asia. Several large left-lateral strike-slip faults may have been activated successively, essentially one at a time. The experiments suggest that the penetration of India into Asia has rotated (approx= 25o) and extruded (approx= 800km) Indochina to the SE along the then left-lateral Red River fault in the first 20-30Ma of the collision. This process can account for the opening of the S China Sea before late Miocene time. Extrusion tectonics then migrated N, activating the Altyn Tagh fault as a second major left-lateral fault and moving S China hundreds of km to the E. As this occurred, Indochina kept rotating clockwise (as much as 40o), but the sense of motion reversed on the Red River and other strike-slip faults in the S. Opening of the Mergui basin and Andaman Sea (up to the present) also appears to be a simple kinematic consequence of the extrusion. Recent rifts in NE China and Yunnan may be considered incipient analogs of the S China and Andaman Seas. Other Tertiary tectonic features such as the sedimentary basins of the Gulf of Thailand may be explained as collisional effects, if one uses our experiments as a guide. The experiments also suggest that a major left-lateral strike-slip fault and rift system will propagate across the Tien Shan, Mongolia, and Baikal to the Sea of Okhotsk.-Authors
Bedrock geologic studies in the epicentral areas of the 1983 Goodnow (mb = 5.1) and 1985 Ardsley (mL = 4.0) earthquakes in New York State suggest that seismogenic intraplate faults have subtle, but clearly recognizable, expressions in crystalline bedrock along surface extrapolations of these well-defined earthquake ruptures. Both ruptures are small and totally confined to the subsurface. The 1983 Goodnow rupture and the 1985 Ardsley rupture are correlated with the Catlin Lake fault zone and the Dobbs Ferry fault zone, respectively. Although both structures had been recognized by other authors as brittle structures prior to the recent earthquakes, they had not been mapped as faults because of their small accumulated offsets. Our interpretation of these features as the surface expressions of seismogenic structures is based on: 1.(1) the spatial correlation of the rupture plane, as defined by aftershock hypocenters, with a prominent fracture-controlled topographic lineament, and2.(2) observations of mesoscopic brittle structures along the lineaments that reflect the larger scale structure and, more importantly, are consistent with both the rupture orientation and sense of slip determined from seismic data. In both study areas mesoscopic structures indicate that fault formation at pre-existing joints was important in the overall evolution of the fault zones. The mesoscopic-scale observations are in agreement with studies of fault zone formation and growth in crystalline rock, and provide information on the geometry, segmentation, kinematics, and evolution of the fault zones. Fault zone segmentation seems to be controlled by wavelengths of folds in the pre-existing ductile structure.
Unequivocal kinematic indicators in cataclasites and mylonites of the Reservoir fault zone, New Jersey, show consistent dextral strike-slip shear sense and provide evidence for an extensive strike-slip event subsequent to the peak of the Grenvillian (Ottawan) Orogeny. The fault zone occurs in layered granitic gneiss and minor amphibolite of Precambrian age in the New Jersey Highlands (northern Reading Prong). The fault rocks underwent extensive synkinematic hydrothermal retrogression that produced amphibole-rich assemblages from all previous phases. Retrogression of the protolith shows sequential reactions to assemblages dominated by F-magnesiohastingsite, Cl-magnesiohastingsite, ferro-actinolite, actinolite, and finally actinolite + chlorite. Although deformation was purely brittle within the granitic protolith (quartz shows marginal plasticity), reaction enhanced ductility and plasticity of some phases produced S-C mylonites in amphibole mineralized fault rocks. C planes are composed of fine aligned grains of ferro-actinolite and actinolite and define tails on σ-type amphibole porphyroclasts. The porphyroclast cores are composed of F-magnesiohastingsite and/or Cl-magnesiohastingsite and commonly define S-planes. Some porphyroclast cores show brittle pull-apart textures and SEM analysis shows that virtually all have micro-brecciated rims that are indurated with epitaxial amphibole overgrowths from a more retrograde assemblage. C′ planes are locally developed and defined by fine-grained chlorite and actinolite. All kinematic indicators show a consistent dextral strike-slip shear sense.
The Proterozoic Grenville Province in Canada is widely regarded as a compressional orogen in which thrust transport occurred towards the northwest, perpendicular to the orogenic front. New structural data from an area near Parry Sound, Ontario, lead to a re-evaluation of this interpretation. Nappe structures in this area are consistent with northeast- rather than northwest-directed thrusting and imply that the southwestern part of the Province experienced an early, sinistral oblique collision which predated the emplacement of the 1.24 Ga Sudbury Dike swarm. Two nappe piles, which contain different rock types and have distinct stacking sequences, are separated by a lateral ramp or ductile tear fault that coincides with the interior of the Parry Sound Domain of Davidson et al. (1982). Early nappes and associated structures (F1) are overprinted by NW-SE-striking upright folds (F2) that formed between about 1.16 and 1.12 Ga. These later folds partially reorient rotated feldspar porphyroclast systems that formed during nappe emplacement, and have axes that are consistently parallel to a well-developed mineral lineation. The orientation of this lin lineation reflects the two-stage deformation history of the area rather than the thrusting direction during a single orogenic event.
Multiply deformed metamorphic rocks of the southern Adirondacks exhibit a pronounced linear fabric consisting of elongate, flat ribbons oriented parallel to early fold axes and lying within foliation planes. The ribbons, which are often monomineralic, consist of quartz, feldspar and mafic minerals. As seen in transition from least to most deformed rocks, these ribbons appear to be the result of elongation of grains, or grain aggregates, in response to a regional rotational strain which also rotated early fold axes into parallelism with the lineation. A consistent sense of asymmetry of feldspar tails with respect to foliation suggests that simple shear was the dominant component of strain. The long dimension of these ribbons is believed to mark the maximum elongation direction (X) of the finite strain ellipsoid.
Porphyroclasts of relatively strong minerals in mylonites commonly have an internal monoclinic shape symmetry defined by tails of dynamically recrystallized material. The geometry of a porphyroclast and its tails, called a ‘porhyroclast system’, can serve as a valuable indicator of the sense of vorticity. Porphyroclast systems have been divided into σ- and δ-types on the basis of the geometry of the tails. σ-Types have wedge-shaped recrystallized tails whose median lines lie on opposite sides of a reference plane parallel to the tails and containing the symmetry axis for the system. σ-Types are further subdivided into a σa-types, in which the porphyroclast is isolated in a relatively homogeneous matrix, and σb-types, in which the porphyroclast system is associated with a shear band foliation in the matrix. δ-Types typically have narrow recrystallized tails whose median lines cross the reference plane adjacent to the porphyroclast. Consequently, embayments of matrix material occur adjacent to the porphyroclasts and the tails display characteristic bends.
Two types of foliations are commonly developed in mylonites and mylonitic rocks: (a) S-surfaces related to the accumulation of finite strain and (b) C-surfaces related to displacement discontinuities or zones of relatively high shear strain. There are two types of S-C mylonites. Type I S-C mylonites, described by Berthé et al., typically occur in deformed granitoids. They involve narrow zones of intense shear strain which cut across (mylonitic) foliation.Type II S-C mylonites (described here) have widespread occurrence in quartz-mica rocks involved in zones of intense non-coaxial laminar flow. The C-surfaces are defined by trails of mica ‘fish’ formed as the result of microscopic displacement discontinuities or zones of very high shear strain. The S-surfaces are defined by oblique foliations in the adjacent quartz aggregates, formed as the result of dynamic recrystallization which periodically resets the ‘finite-strain clock’. These oblique foliations are characterized by grain elongations, alignments of segments of the grain boundary enveloping surfaces, and by trails of grains with similar c-axis orientations.Examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of S-C tectonites, and a deviation from the general tradition of purely geometric analysis of foliation and time relationships. Kinematic indicators such as those discussed here allow the recognition of kilometre-scale zones of intense non-coaxial laminar flow in crustal rocks, and unambiguous determination of the sense of shear.
PALAEOMAGNETIC and structural data on the Grenville Province in North America are best explained if the Grenvillian `orogeny' was caused by right-lateral simple shear of the entire Province. Relative displacement of the edges of the belt is estimated at 200-300 km.
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