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Showing papers in "Tectonics in 1997"


Journal ArticleDOI
TL;DR: In this paper, a model for the Miocene evolution of the Betic-Rif mountain belts is proposed, which is compatible with the evolution of rest of the western Mediterranean.
Abstract: In recent years, the origin of the Betic-Rif orocline has been the subject of considerable debate. Much of this debate has focused on mechanisms required to generate rapid late-orogenic extension with coeval shortening. Here we summarize the principal geological and geophysical observations and propose a model for the Miocene evolution of the Betic-Rif mountain belts, which is compatible with the evolution of the rest of the western Mediterranean. We regard palaeomagnetic data, which indicate that there have been large rotations about vertical axes, and earthquake data, which show that deep seismicity occurs beneath the Alboran Sea, to be the most significant data sets. Neither data set is satisfactorily accounted for by models which invoke convective removal or delamination of lithospheric mantle. Existing geological and geophysical observations are, however, entirely consistent with the existence of a subduction zone which rolled or peeled back until it collided with North Africa. We suggest that this ancient subducting slab consequently split into two fragments, one of which has continued to roll back, generating the Tyrrhenian Sea and forming the present-day Calabrian Arc. The other slab fragment rolled back to the west, generating the Alboran Sea and the Betic-Rif orocline during the early to middle Miocene.

682 citations


Journal ArticleDOI
TL;DR: In this article, the authors describe the Cenozoic geological evolution of the Bolivian Altiplano, using a revised chronostratigraphy and an analysis of the crustal and lithospheric structure.
Abstract: The Bolivian Altiplano, in the Central Andes of South America, is part of the second largest high plateau on Earth. It is an elongate region of subdued relief, ∼1.2 × 105 km2 and ∼4 km above sea level, bounded by the Eastern Cordillera and volcanic arc (Western Cordillera). Here the crust is up to ∼75 km thick. We describe the Cenozoic geological evolution of this region, using a revised chronostratigraphy and an analysis of the crustal and lithospheric structure. Crustal shortening and magmatic addition and, locally, sedimentation are the main mechanisms of Cenozoic crustal thickening, leading to nearly 4 km of surface uplift since the Paleocene. Addition of mafic melts appears to be a first-order mechanism of Cenozoic crustal growth, contributing ∼40% of the crustal thickening beneath the volcanic arc. Removal of the basal part of the lithosphere may have caused two episodes of widespread arc and behind-arc mafic volcanism, at ∼23 Ma and 0 – ∼5 Ma, contributing to the surface uplift. The Altiplano originated as a sedimentary basin, several hundred kilometers wide, between the proto-Western Cordillera and a narrow zone of uplift (proto-Eastern Cordillera) farther east. The latter zone formed by inversion of the center of a wide lacustrine or marine Cretaceous - Paleocene basin close to sea-level at ∼45 Ma. A thickness of 2–4 km of Paleogene continental elastics accumulated in the proto-Altiplano basin. Subsequently, in the Oligocene, we estimate that this region and the western margin of the Eastern Cordillera were technically shortened ∼22% (∼65 km), resulting in ∼9 km of average crustal thickening. The Altiplano basin was rejuvenated at ∼25 Ma and subsequently flooded with up to 8 km thickness of detritus eroded from the uplifting Eastern and Western Cordilleras. Between ∼25 and 5 Ma, folding and thrusting in the western margin of the Eastern Cordillera migrated westward into the center of the Altiplano basin, essentially terminating deposition, except in local subbasins, and accommodating ∼13% (∼30 km) of shortening and an estimated ∼7 km of average crustal thickening. Subsequently, there has been strike-slip deformation and limited local thrusting (< 5 km of shortening). Geomorphological and geochronological evidence for 1.5–2 km of surface uplift of this region since the Late Miocene suggests ∼14 km of lower crustal thickening beneath an essentially rigid “lid”, and can be explained by ∼100–150 km of underthrusting of the Brazilian shield and adjacent regions beneath the eastern margin of the Central Andes. The present subdued relief in the Altiplano may be a result of ductile flow in the lower crust and sedimentation and erosion in an internal drainage basin.

257 citations


Journal ArticleDOI
TL;DR: In this paper, the trishear model is generalized to include a variety of fault propagation to slip ratios and fault propagation from a flat decollement, and the results show continuous rotation of the forelimb with the characteristic development of cumulative wedges within growth strata.
Abstract: In contrast to kink band migration modeling methods, trishear numerical models produce fault propagation folds with smooth profiles and rounded hinges. Modeled fold hinges tighten and converge downward, within a triangular zone of distributed deformation which is focused on the fault tip. Such features have been reported from field studies and are also seen in analogue models of compressional deformation. However, apart from its initial application to Laramide folds, little quantitative work has been undertaken on trishear fault propagation folding in other settings. In addition, no study has been undertaken into the growth strata which might be associated with such structures. This paper uses an equivalent velocity description of the geometric model of trishear, together with models of erosion and sedimentation, to investigate trishear fault propagation folding of both pregrowth and growth strata. The trishear model is generalized to include a variety of fault propagation to slip ratios and fault propagation from a flat decollement. The models show continuous rotation of the forelimb with the characteristic development of cumulative wedges within growth strata. When total slip on a structure is high, the model predicts overturned pregrowth and growth strata. During the initial stages of deformation, beds in the forelimb thicken but later thin when they become steep or overturned. The effect of variations in fault propagation to slip ratios on two-dimensional finite strain in the models is assessed by the use of initially circular strain markers. High fault propagation to slip (p/s) ratios lead to narrow zones of high finite strain, while lower p/s ratios lead to more ductile deformation and broader zones of high strain. In all cases, hanging wall anticlines and footwall synclines originate as early ductile folds which are later cut by the propagating fault. Modeled structures are compared with natural examples.

248 citations


Journal ArticleDOI
TL;DR: In this paper, the authors show that extensive Neogene Patagonian plateau lavas are syncollisional or postcollisional in age, with eruptions of both sequences migrating northeastward at 50 to 70 km/Ma.
Abstract: Extensive Neogene Patagonian plateau lavas (46.5° to 49.5°S) southeast of the modern Chile Triple Junction can be related to opening of asthenospheric “slab windows” associated with collisions of Chile Rise segments with the Chile Trench at ≈ 12 Ma and 6 Ma. Support comes from 26 new total-fusion, whole rock 40Ar/39Ar ages and geochemical data from back arc plateau lavas. In most localities, plateau lava sequences consist of voluminous, tholeiitic main-plateau flows overlain by less voluminous, 2 to 5 million year younger, alkalic postplateau flows. Northeast of where the ridge collided at ≈12 Ma, most lavas are syncollisional or postcollisional in age, with eruptions of both sequences migrating northeastward at 50 to 70 km/Ma. Plateau lavas have ages from 12 to 7 Ma in the western back arc and from 5 to 2 Ma farther to the northeast. Trace element and isotopic data indicate main-plateau lavas formed as larger percentage melts of a garnet-bearing, oceanic island basalt (OIB) -like mantle than postplateau lavas. The highest percentage melts erupted in the western and central plateaus. In a migrating slab window model, main-plateau lavas can be explained as melts that formed as upwelling, subslab asthenosphere which flowed around the trailing edge of the descending Nazca Plate and then interacted with subduction-altered asthenospheric wedge and continental lithosphere. Alkaline, postplateau lavas can be explained as melts generated by weaker upwelling of subslab asthenosphere through the open slab window. Thermal problems of high-pressure melt generation of anhydrous mantle can be explained by volatiles (H2O and CO2) introduced by the subduction process into slab window source region(s). An OIB-like, rather than a mid-ocean ridge basalt (MORB) -like source region, and the lack of magmatism northeast of where ridge collision occurred at ≈13 to 14 Ma can be explained by entrainment of “weak” plume(s) or regional variations in an ambient, OIB-like asthenosphere.

242 citations


Journal ArticleDOI
TL;DR: A detailed analysis of the structural and metamorphic records of one of the main Alboran Domain complexes, however, plainly reveals a complex tectonic evolution prior to the development of the Miocene arc/back arc system, which includes a major intraorogenic extensional event as mentioned in this paper.
Abstract: In the western Alpine system, Neogene extensional tectonics triggered the development of marine basins on the concave side of tight orogenic arcs, as happened within the Alboran Crustal Domain, the hinterland of the Gibraltar Arc. A detailed analysis of the structural and metamorphic records of one of the main Alboran Domain complexes, however, plainly reveals a complex tectonic evolution prior to the development of the Miocene arc/back arc system, which includes a major intraorogenic extensional event. This large-scale subvertical shortening, that can be assessed from the PT paths of representative tectonic units, was subsequent to the continental crust subduction inferred from high pressure-low temperature mineral asssemblages. The crustal section was thinned in nearly isothermal conditions, its thickness being reducted to at least 1/3 of the initial value. Yet still before the Miocene, a second contractional event led to the overthrusting of high-grade metamorphic rocks over other low-grade rocks, accompanied by subordinate overturning of the metamorphic zones. Since migration of the Gibraltar Arc is roughly balanced by crustal spreading in the back arc, available data concerning Miocene extension suggest that the Alboran Domain can be restored to its appropriate position several hundred kilometers to the east. Thus a collision belt that underwent significant intraorogenic extension must have existed in what is now the western South-Balearic basin.

192 citations


Journal ArticleDOI
TL;DR: Seismic reflection data acquired in the vicinity of Isla Mocha across the southern coast of Chile image structures formed along the continental margin and reveal an episodic history of accretion, nonaccretion, and possibly erosion as discussed by the authors.
Abstract: Seismic reflection data acquired in the vicinity of Isla Mocha across the southern coast of Chile image structures formed along the continental margin and reveal an episodic history of accretion, nonaccretion, and possibly erosion. Structures formed at the toe of the continental slope suggest frontal accretion of ¾ to 1 ¾ km of trench fill. Seismic images also reveal that a small accretionary wedge, 20–30 km wide, abuts the truncated continental metamorphic basement that extends seaward from beneath the shelf. The small size of the accretionary wedge on three profiles examined here is not consistent with a long history of accretion with the current deformational style, as current rates of frontal accretion could have accumulated all of the existing accretionary wedge in less than 1–2 m.y. This is a small fraction of convergence history along this margin, and the current accretionary mode has not been consistently maintained in the past. The Isla Mocha region is located between the temperate climate of central Chile and the glacial climate of southern Chile, and climatic conditions in this region have likely fluctuated sufficiently to cause significant variation in trench sediment supply. Accretionary and nonaccretionary or erosional episodes are probably linked to temporal variations in trench sediment thickness, as suggested by observations along the Chile margin. Currently, thick trench sediment correlates with accretion along the southern Chile margin, and thin trench sediment correlates with nonaccretion/tectonic erosion as near the Chile Ridge and from the Juan Fernandez Ridge to northern Chile. The Isla Mocha region also lies 900 – 1000 km north of the Chile triple junction, and the Chile Ridge lies approximately 2000 km to the west and has not yet collided and affected the margin near Isla Mocha. This part of the precollision zone provides an excellent reference to examine the effects of Chile Ridge collision in the development of the Chile margin. The most apparent effect of subduction of the buoyant, young crust of the Chile Ridge is a shallow trench that is nearly devoid of sediment. Consequently, the triple junction is undergoing nonaccretion or erosion, and the accretionary complex near the triple junction remains smaller than to the north or south because the current phase of rapid accretion elsewhere in the trench has bypassed the triple junction region. The interplay of subduction zone processes, such as trench sedimentation and ridge collision, has resulted in an episodic development of the margin and produced a discontinuous record of convergence history within the accretionary wedge.

190 citations


Journal ArticleDOI
TL;DR: In this paper, a simple hypothesis is proposed to explain the occurrence of localized zones of tectonic deformation and seismicity within intraplate regions subjected to relatively uniform far-field tectonics stresses.
Abstract: A simple hypothesis is proposed to explain the occurrence of localized zones of tectonic deformation and seismicity within intraplate regions subjected to relatively uniform far-field tectonic stresses. In most intraplate regions (especially continental shield areas and old oceanic basins), temperatures in the lower crust and upper mantle are quite low so that the upper mantle is cold and strong. In these regions, significant lithospheric deformation does not occur because the cumulative strength of the lithosphere far exceeds the magnitude of plate-driving forces. If lower crust and upper mantle temperatures are relatively high, however, plate-driving forces are largely supported by the upper crust because the lower crust and upper mantle are relatively weak. In this case, the regions can deform relatively rapidly because the cumulative strength of the lithosphere is comparable in magnitude to that of the forces acting on the lithosphere. In this paper, we apply this hypothesis to the New Madrid seismic zone and the surrounding central and eastern United States. Within the seismic zone, the heat flow appears to be slightly elevated (about 60 mW/m2) relative to the background regional value of 45 mW/m2. Calculated crustal geotherms and laboratory-derived ductile flow laws suggest that the lower crust and upper mantle are sufficiently weak within the seismic zone that intraplate stresses are largely transmitted through the upper crust and deformation can occur at relatively rapid rates for this intraplate area. In marked contrast, in the surrounding area where the heat flow is relatively low, cumulative lithospheric strength appears to far exceed the plate-driving force, and the tectonic stress is carried in both the crust and upper mantle. Thus the marked contrast in seismicity between the seismic zone and the surrounding area appears largely because of heat flow and whether or not the lower crust and upper mantle support an appreciable fraction of the plate-driving forces.

174 citations


Journal ArticleDOI
TL;DR: In this article, an upper plate tectonics related to subduction of the ridge was studied by an international group of geoscientists in the two-degree segment offshore Valparaiso, extending from the shelf edge seaward across the eastern end of Juan Fernandez Ridge.
Abstract: Near the latitude of Valparaiso, Chile, a fundamental change in configuration of the Benioff Zone, volcanic arc activity, and the structure of the continental margin occurs opposite the subducting Juan Fernandez Ridge. Upper plate tectonics related to subduction of the ridge were studied by an international group of geoscientists in the two-degree segment offshore Valparaiso, extending from the shelf edge seaward across the eastern end of Juan Fernandez Ridge. Near the O'Higgins group of seamounts, the Juan Fernandez Ridge strikes northeast rather than continuing its east-west trend across the Pacific Basin. The ridge uplifts the upper plate, and sediments of the Valparaiso Basin are deformed against its southern flank. This deformation is consistent with the southward migration required by an oblique trending ridge and the nearly trench-normal vector of plate convergence. In the trench axis, the ridge forms a basement barrier behind which sediments 2.5 km deep have ponded. The lower slope over the ridge appears eroded, whereas the margin not yet affected by ridge subduction is fronted by an accretionary prism about 25 km wide. Nazca Plate relief clearly influences tectonism of the margin where it subducts beneath thin continental crust; its relation to deeper processes segmenting the Andean Orogen appears to involve prior tectonic events.

167 citations


Journal ArticleDOI
TL;DR: In this paper, the authors show that back arc extension in Mediterranean back arcs began in the late Oligocene or early Miocene, but the pressure-temperature-time (P-T-t) paths of eclogite-facies rocks exposed in these areas indicate that a major part of the overburden, several tens of kilometers, has been removed from above these rocks prior to the Oligo-Miocene.
Abstract: Eclogite-facies rocks exposed in Mediterranean back arcs are delimited from above by low-angle normal faults and detachments. Nevertheless, our work demonstrates that these extensional structures associated with back arc extension played only a limited role in removing the overburden from above the eclogites. Extension in Mediterranean back arcs began in the late Oligocene or early Miocene, but the pressure - temperature - time (P-T-t) paths of eclogite-facies rocks exposed in these areas indicate that a major part of the overburden, several tens of kilometers, has been removed from above these rocks prior to the Oligo-Miocene. We show that the time period bracketed between the peak of eclogite metamorphism (Eocene in the central Aegean, probably Upper Cretaceous in Corsica and the Betics) and the onset of back arc extension in the Oligo-Miocene was characterized by thrust faulting. In the central Aegean, Corsica, and the Betics, eclogite-bearing units were partly unroofed and then overthrusted lower-pressure units. We emphasize that, with one exception (Tinos island, Greece), the entire inventory of extensional contacts operated subsequently to the overthrusting of the eclogites above the lower-grade sequences. Thus Mediterranean back arc extension lags behind a major part of the denudation process, and is superposed on orogenic wedges that contain eclogite-facies rocks at relatively shallow structural levels. We emphasize that the mode of occurrence of eclogites in Mediterranean back arc regions involves a continuum of in-situ crustal accretion below the eclogites, widespread P-T paths that show cooling or isothermal decompression, and lower-grade rocks at the bottom of the structural pile. Thus instead of reflecting whole - crust back arc extension, the tectonic style associated with the denudation of Mediterranean eclogites better fits an active accretionary-wedge setting. This is similar to the mode of occurrence of eclogite-facies rocks in mountain belts, such as the western Alps, where decompression was synorogenic and back arc extension played no role.

165 citations


Journal ArticleDOI
TL;DR: In this paper, the authors argue that the WSW directed low-angle normal faults formed during large-scale extension in connection with important westward arc migration, and that the superposition of these two systems resulted in a chocolate tablet megastructure.
Abstract: The Gibraltar arc, which closes the westernmost part of the Mediterranean basin, is a Miocene A-type subduction arc formed by the continental collision of various pre-Miocene terranes in the major zone of collision between the Iberian and African cratons. The hanging-wall block, known as the Alboran domain, has undergone more than 300 km migration from a more easterly position, where it was the continuation of the Alpine Cretaceous-Paleogene orogen. Contemporaneous with thin-skinned thrusting in the footwall, the Alboran domain underwent two episodes of nearly orthogonal extension in which extensional systems developed with directions of extension varying from a NNW-SSE system, orthogonal to the belt axis, in the late Burdigalian-Langhian to a WSW directed orogen-parallel one in the Serravallian. The superposition of these two systems resulted in a chocolate tablet megastructure. This extensional pattern is not satisfactorily explained in previously proposed models for the evolution of the arc. Orthogonal extension is plausible in a process of the gravitational collapse of an overthickened crust; nevertheless, orogen-parallel extension is more difficult to explain in this context. We advocate that the WSW directed low-angle normal faults formed during large-scale extension in connection with important westward arc migration. The driving force of extension in a general context of convergence is controversial and varies between a convective removal model and a delamination model. Constraints on both the timing and the kinematics of extension, as presented in this paper, seem to support the contribution of both mechanisms. Convective removal may have started the process, but continued N-S convergence could have resulted in westward tectonic escape and asymmetric lateral inflow of asthenospheric material accompanying lithospheric delamination.

162 citations


Journal ArticleDOI
TL;DR: In this paper, the thermomechanical evolution of a midcrustal ductile duplex in central Australia has been reconstructed through space and time using 40Ar/39Ar thermochronology, flow stress estimates, cross-sectional restoration of dislocation creep microstructures, and microstructural and structural analysis.
Abstract: The thermomechanical evolution of a midcrustal ductile duplex in central Australia has been reconstructed through space and time using 40Ar/39Ar thermochronology, flow stress estimates, cross-sectional restoration of dislocation creep microstructures, and microstructural and structural analysis. A critical aspect of this analysis is the identification of populations of white micas in quartzite mylonites that have neocrystallized below their closure temperature and which record the time when ductile deformation ceased. In dating these micas the mylonitic microstructures have effectively been dated. The time-temperature history of the duplex has been constrained through multidomain thermal modeling of K-feldspar 40Ar/39Ar data. The modeling demonstrates that a temperature gradient existed across the duplex during its formation. The concept of microstructural continuity during ductile deformation has great potential for elucidating the kinematic evolution of ductile duplexes. Mapping of the deformation mechanisms and recrystallized grain sizes of quartzites deformed under greenschist facies conditions has been used to evaluate tectonic offsets that occurred after microstructural freezing. This analysis shows that the duplex formed as a forward propagating thrust system accommodating ∼60 km of convergence between the upper and lower plates of the megathrust, with a significant fraction of the displacement occurring after microstructural freezing. Finally, using the data as input to published flow laws for quartz aggregates provides a strain rate history for the duplex. Although uncertainties are clearly large, the timing of highest-estimated strain rates during duplex evolution does, indeed, correlate with the highest rates of convergence between the upper and lower plates of the megathrust system (according to regional cooling history studies) and with coeval sedimentation in adjoining molasse basins.

Journal ArticleDOI
TL;DR: In this paper, small-scale modeling was performed to examine the effects of the superposition of two successive extensional phases from orthogonal to oblique (type 1) and from oblique-to-orthogonal (type 2).
Abstract: Small-scale modeling was performed to examine the effects of the superposition of two successive extensional phases from orthogonal to oblique (type 1) and from oblique to orthogonal (type 2). In both the type 1 and type 2 models, faults produced during the first stage strongly control fault development during the second stage. In type 1 models, the oblique faults developed during the second oblique phase are confined within a first-phase graben, whereas in type 2 models the oblique faults, produced during the first phase, continue to develop during orthogonal extension and connect with each other to give sigmoidal fault blocks. Type 1 models are compared with the structural setting of the Ethiopian Rift; the evolution of the rift is related to a recent extensional event, whose principal direction of stretching trends at around 50° to preexisting major normal faults. Type 1 laboratory models are fairly comparable to the northern sector of the Ethiopian Rift, referred to here as MER. They account for both the development of the en echelon oblique faults of the Wonji Fault Belt and the sinistral shear gradient running parallel to the eastern border of the MER, which formed during an oblique rifting extension. The statistical analysis of the whole Ethiopian Rift fault pattern by reference to the experimental data allows the determination of a N100°–N110° mean direction of stretching.

Journal ArticleDOI
TL;DR: The Rosy Finch shear zone (RFSZ) represents an example of synmagmatic, strike-slip tectonics in the east central Sierra Nevada and provides information on transpressional tectonic within magmatic arcs as discussed by the authors.
Abstract: The Rosy Finch shear zone (RFSZ) represents an example of synmagmatic, strike-slip tectonics in the east central Sierra Nevada and provides information on transpressional tectonics within magmatic arcs. The RFSZ (1–4 km wide and 80 km long) is contained in Late Cretaceous granitoids (92–83 Ma) of the Mono Pass Intrusive Suite. Dextral strike-slip deformation is indicated by a continuous band of orthogneiss, mylonitic, and cataclastic deformation, characterized by subvertical foliations with subhorizontal lineations. The style of dextral shearing in the Rosy Finch shear varies along strike, from wide zones of ductile deformation in the youngest plutons to narrow zones of cataclastic deformation in the older plutons. Deformation of the youngest granitoids is synmagmatic, as constrained by both field observations and isotopic studies. Dextral shearing occurs concurrently in contemporaneous Intrusive Suites to the north and south (Tuolumne and Mount Whitney) suggesting strike-slip movement along the axis of active plutonism. Field studies also suggest that contraction across the arc acted concurrently with strike-slip movement, consistent with a transpressional setting for the Late Cretaceous Sierra Nevada magmatic arc. Comparison of the field data with strain models of oblique plate convergence suggests that the RFSZ is a preserved ductile signature of transcurrent motion of strike-slip partitioning within a transpressional orogenic system. Recent plate reconstructions indicate a switch from sinistral to dextral oblique convergence at ∼95 Ma, which is consistent with the timing constraints on the dextral movement on the RFSZ.

Journal ArticleDOI
TL;DR: In this article, structural and 40Ar/39Ar data from the Santa Rosa area, southeastern Sonora shed light on the magnitude and timing of extensional tectonism.
Abstract: New structural and 40Ar/39Ar data from the Santa Rosa area, southeastern Sonora shed light on the magnitude and timing of extensional tectonism. Episodic magmatic activity from the Late Cretaceous to the Miocene is recorded by a 3–4 km thick structural/stratigraphic section that includes a composite Laramide granodiorite batholith and its andesitic wall rocks, early to middle Tertiary rhyolite ignimbrites and silicic domes, and late Oligocene to middle Miocene mafic lavas interbedded with tuffaceous sediment and fanglomerate. The older units are complexly faulted and steeply tilted by several generations of normal faults. Palinspastic reconstructions indicate cumulative extension of ∼90%. Early gently dipping, NW striking normal faults document major extension oriented −N50°E. Angular unconformities and growth fault relations within Oligo-Miocene sequences bracket this extension between 26 and ∼20 Ma. Younger, widely spaced NS to NNW trending, high-angle normal faults cut the previously faulted and tilted sections and produced modest (10–15%) E-W extension that is largely bracketed between 20 and 17 Ma. The magnitude of Neogene extension in this region is greater and the timing is older than previously recognized. Reconnaissance of other areas between the Sierra Madre Occidental and the coast at this latitude suggests that most extension in Sonora occurred between ∼27 and 12 Ma, while remnants of the Farallon plate were still being subducted. Plate-tectonic models that predict substantial extension in Sonora during proto-Gulf (∼10 – 5 Ma) transtensional deformation may need to be reevaluated. An alternative model, presented here, is that Baja California began moving with Pacific plate motion shortly after the ∼11 Ma termination of subduction and that northwest motion of Baja relative to mainland Mexico might total 500 km, distributed across the previously extended Oligo-Miocene magmatic arc.

Journal ArticleDOI
TL;DR: In this paper, a suite of 74 new apatite and zircon fission track results have been obtained from the Nevado-Filabride Complex and these have been used to define regional cooling patterns for the complex.
Abstract: The Internal Zone of the Betic Cordillera, SE Spain, consists of a nappe stack of three complexes, the deepest of which is the Nevado-Filabride Complex. The zone is separated from the overlying Alpujarride Complex by a crustal scale shear zone that has variously been interpreted as a thrust or an extensional detachment. A suite of 74 new apatite and zircon fission track results have been obtained from the Nevado-Filabride Complex and these have been used to define regional cooling patterns for the complex. Rapid cooling (105°C–200°C Ma−1) is spatially related to the tectonic contact with the overlying Alpujarride Complex-Cooling to near-surface temperatures occurred first in the east (Sierra de los Filabres) during the mid-Serravallian (12±1 Ma) and was completed by the early Tortonian (9–8 Ma) in the west (Sierra Nevada). There is no correlation between fission track age and sample elevation. These results are consistent with tectonic unroofing of this complex, a finding that favors extension as the mechanism by which the two complexes were brought into contact. Extension spans the middle and earliest upper Miocene (12–8 Ma) in the study area and therefore lasted much longer than previously documented. A hypothesis is advanced which links oblique convergence between the Iberian plate and the Betic Internal Zones, resulting in crustal contraction at depth, with orogen parallel extension in the middle and upper crust.

Journal ArticleDOI
TL;DR: In this paper, the ages of white mica fabrics were derived from Rb-Sr analysis of white micas whose microstructural and chemical characteristics indicate that they crystallized or recrystallized during shear fabric formation.
Abstract: Ages of deformation have been obtained by Rb-Sr analysis of white micas whose microstructural and chemical characteristics indicate that they crystallized or recrystallized during shear fabric formation. Since white micas commonly define deformation fabrics in medium-grade metamorphic rocks, these ages can be directly related to structural geometries with regional context. This direct method contrasts with estimates of midcrustal deformation ages derived from cooling histories because it does not rely on assumptions about the thermal structure of the crust. It does require that the dated minerals attained isotopic equilibrium with the dominant Sr reservoir at temperatures lower than the closure temperature. This resetting was apparently achieved during dynamic recrystallization of white micas in greenschist-facies metasediments and metagranitoid units in the western Alps. The results suggest that the Sr isotopic composition of the new mica is buffered by the coexisting high-Sr phases (calcite, feldspar or epidote) via the grain boundary network. High-strain rocks from the Entrelor shear zone system of the western Alps have yielded indistinguishable white mica Rb-Sr ages along 30 km of individual and kinematically linked shear zones. The age of the back-thrusting event is constrained at 34±1Ma, the age yielded by the younger generation of synkinematically crystallized white micas. This event was short-lived, involving at least 20 km of shortening in ∼1 m.y. or less. An earlier, variably overprinted component, dating from 38 to 37 Ma, has been identified in the mica fabric, but its kinematic significance is uncertain. This method of dating strain fabrics offers a powerful tool for tectonic studies, since isotopic resetting can be directly linked to structural geometries, microstructural textures, and PT conditions. It allows testing of kinematic models in orogens and can provide important information on the rates of geological processes in the crust.

Journal ArticleDOI
TL;DR: In this paper, a finite element representation of the thin viscous sheet model is used to approximate the deformation field in the India-Asia collision and compare model crustal thickness distributions with that inferred from a local isostatic model of topography.
Abstract: The India-Asia collision has caused crustal thickening in Tibet by at least a factor of 2. In the last 20–30 Myr of this collision the Tian Shan mountain range has also been reactivated. The Tarim Basin, however, shows little internal deformation. We describe a series of numerical experiments which constrain the effective lithospheric strength parameters of the Tarim Basin and the Tian Shan in the context of a thin viscous sheet model. We use a finite element representation of the thin viscous sheet model to approximate the deformation field in the India-Asia collision and compare model crustal thickness distributions with that inferred from a local isostatic model of topography. The experiments show that a strong Tarim Basin, while undergoing little internal deformation, transfers strain to the Tian Shan, producing significant crustal thickening in the Tian Shan region. Based only on diagnostic parameters such as the maximum thickening in the Tian Shan and the minimum in the Tarim Basin, the principal features of the topography can be approximately reproduced using models in which either the Tarim Basin is strong, or the Tian Shan is weak, or both. For a rheological model in which the stress versus strain-rate exponent is n = 3 the strength coefficient for the strong Tarim Basin model, VTarim is between about 1.7 and 2. For the weak Tian Shan model the relative strength coefficient VTian is between about 0.65 and 0.75. If both strong Tarim and weak Tian Shan are included, there is a trade-off between the required values of VTarim and VTian and the shape of the predicted crustal thickness profiles better matches the observed profiles. The steep topographic slope on the southern margin of the Tarim Basin requires that it is anomalously strong, while the rapid decrease of topographic height to the north of the Tian Shan requires that it is anomalously weak. Similar conclusions are obtained with a rheological model based on n = 10. Simplified rheological models of the lithosphere show that the variations in lithospheric strength may be explained by changes to the Moho temperature of the order of 10° to 30°C.

Journal ArticleDOI
TL;DR: In this article, the authors show that the maximum horizontal compression directions obtained from 110 stations show a mean σ1 orientation of 083°, indicating that the E-W-directed compressive stress was transferred through the uppermost brittle crust of the earlier extended Pannonian region into the Eastern Alps.
Abstract: Kinematic and paleostress data constrain a Late Miocene E-W compressional event that affected the entire Alpine-Carpathian-Pannonian system after 9 Ma and prior to 5.3 Ma. The maximum horizontal compression directions obtained from 110 stations show a mean σ1 orientation of 083°. Deformation is mainly strike-slip. E-W directed compression followed Early to Middle Miocene upper plate extension in the Pannonian Basin which was caused by the retreating subduction boundary in the outer Carpathians. This compressional event terminated Early to Middle Miocene eastward lateral extrusion of the Eastern Alps and reverted strike-slip faults which bounded earlier extruded wedges such as the Salzachtal-Ennstal fault, the Periadriatic fault, the Mur-Murz-Vienna Basin fault system, and strike-slip faults in the Western Carpathians. E-W compression caused the reorientation of the extension direction of the Alpine crustal stack. East-directed tectonic unroofing of the metamorphic domes in the central Eastern Alps terminated between 9 and 6 Ma and Early to Middle Miocene orogen-parallel E-W extension switched to Late Miocene N-S extension which parallels modern topographic slopes. In the Pannonian Basin the change from synrift extension to Late Miocene compression during the postrift phase caused the pronounced postrift subsidence by stress induced downward flexure of the loaded lithosphere. We relate Late Miocene E-W compression to coeval late-stage west- directed subduction of the European plate below the Eastern Carpathians. Slab pull of the subducted plate caused subduction roll back and upper plate extension of the Pannonian area up to the Middle Miocene. Subduction slowed down and ceased when up to 60-km-thick buoyant continental crust entered the subduction zone. During the short period of continued convergence E-W -directed compressive stress was transmitted across the subduction boundary into the upper plate. Compressive stress was transferred through the uppermost brittle crust of the earlier extended Pannonian region into the Eastern Alps, up to 1400 km behind the subduction zone. E-W compression terminated during the Pliocene when the Pliocene to recent NNW-SSE compressive stress field was established in Central Europe.

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TL;DR: The most widely accepted model for the opening of Canada Basin invokes 66° of counterclockwise rotation of Arctic Alaska and Chukotka away from the Canadian Arctic in Early Cretaceous time as mentioned in this paper.
Abstract: The most widely accepted model for the opening of Canada Basin invokes 66° of counterclockwise rotation of Arctic Alaska and Chukotka away from the Canadian Arctic in Early Cretaceous time. Late Paleozoic structural trends and paleogeography have been used in support of the rotation hypothesis. Recent refinements in the ages of Paleozoic tectonic events in Arctic Alaska, Yukon, and the Canadian Arctic Islands provide new controls on correlations of late Paleozoic paleogeography and raise doubts about whether the Paleozoic tectonics of the Arctic Alaska-Yukon region necessitate a rotational reconstruction of Arctic Alaska against the Canadian Arctic Islands. A rotational restoration of Arctic Alaska requires the Alaskan and Canadian margins to be conjugates of comparable age and evolution. The rift-drift transition age for the Alaskan margin is most likely Hauterivian (Early Cretaceous), but for the Canadian Arctic margin it is most likely post-Albian (mid-Cretaceous). Crustal structure data from the Beaufort Sea continental margin in Canada define a rifted margin segmented by fracture zones which constrain the kinematics of ocean spreading to be northwestward, perpendicular to that required by the rotation hypothesis but subparallel to that suggested by seismic velocity anisotropy in the upper mantle. The Alaska-Chukotka rotation hypothesis also fails to account for up to 600 km of continental overlap upon restoration of 66° of rotation and the absence of any accommodating contractional structures in northern Yukon and adjacent Northwest Territories. Because the Alaska-Chukotka rotation hypothesis fails to account for much of the available data, senous doubt is cast on its viability. An existing multistage tectonic model for the evolution of Canada and Makarov basins is summarized as an example of a model which can account for the existing data.

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TL;DR: In this article, the authors propose a model in which subduction of thin sedimented plates concentrates shear strains in a narrow zone, generating melanges like the McHugh in accretionary complexes.
Abstract: Controls on accretion of flysch and melange terranes at convergent margins are poorly understood. Southern Alaska's Chugach terrane forms the outboard accretionary margin of the Wrangellia composite terrane, and consists of two major lithotectonic units, including Triassic-Cretaceous melange of the McHugh Complex and Late Cretaceous flysch of the Valdez Group. The contact between the McHugh Complex and the Valdez Group on the Kenai Peninsula is a tectonic boundary between chaotically deformed melange of argillite, chert, greenstone, and graywacke of the McHugh Complex and a less chaotically deformed melange of argillite and graywacke of the Valdez Group. We assign the latter to a new, informal unit of formational rank, the Iceworm melange, and interpret it as a contractional fault zone (Chugach Bay thrust) along which the Valdez Group was emplaced beneath the McHugh Complex. The McHugh Complex had already been deformed and metamorphosed to prehnite-pumpellyite facies prior to formation of the Iceworm melange. The Chugach Bay thrust formed between 75 and 55 Ma, as shown by Campanian-Maastrichtian depositional ages of the Valdez Group, and fault-related fabrics in the Iceworm melange that are cut by Paleocene dikes. Motion along the Chugach Bay thrust thus followed Middle to Late Cretaceous collision (circa 90–100 Ma) of the Wrangellia composite terrane with North America. Collision related uplift and erosion of mountains in British Columbia formed a submarine fan on the Farallon plate, and we suggest that attempted subduction of this fan dramatically changed the subduction/accretion style within the Chugach accretionary wedge. We propose a model in which subduction of thinly sedimented plates concentrates shear strains in a narrow zone, generating melanges like the McHugh in accretionary complexes. Subduction of thickly sedimented plates allows wider distribution of shear strains to accommodate plate convergence, generating a more coherent accretionary style including the fold-thrust structures that dominate the outcrop pattern in the Valdez belt. Rapid underplating and frontal accretion of the Valdez Group caused a critical taper adjustment of the accretionary wedge, including exhumation of the metamorphosed McHugh Complex, and its emplacement over the Valdez Group. The Iceworm melange formed in a zone of focused fluid flow at the boundary between the McHugh Complex and Valdez Group during this critical taper adjustment of the wedge to these changing boundary conditions.

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TL;DR: In this article, the authors analyzed the relationship between the forces experienced by the upper plate and the kinematics of subduction, not only in terms of a vertical cross section but also by taking into account the along-arc dimension.
Abstract: In order to gain quantitative insight into the forces that control the present-day stress field and pattern of horizontal motions in the Aegean region, we adopt a forward model approach. Mechanisms that have been proposed to explain the present-day tectonics are represented, to first approximation, in terms of the expected force distributions. On the basis of a thin elastic shell representation of the Aegean lithosphere we then calculate the stress field and displacements associated with these force sets and compare the results with observations of the state of stress based on geological studies of fault kinematics, strain as expressed by earthquake focal mechanisms, and horizontal motions measured by means of satellite geodesy. The two most commonly cited mechanisms suggested to control the Aegean tectonics, (1) the push exerted by the westward moving Anatolian block and (2) forces associated with the Hellenic subduction zone, are first analyzed separately. It is found that the westward Anatolian push alone does not explain the observed prevalence of tensional stress. This is also expressed in a mismatch between the displacements computed to result from the westward push and the observed horizontal velocities. Regarding the forces on the Aegean overriding margin, we find that a model with outward pulling forces of uniform magnitude, acting normal to the arc from the SW Peloponnese to Rhodes, yields a stress field that matches the observed pattern of tension to a large extent. This distribution of forces is consistent with the notion of gravitational spreading of the Aegean lithosphere. The pattern of seismic strain of the overriding margin may evidence the occurrence of a small additional resistive force. Although the prevalence of tension thus appears due mainly to the subduction-related forces, other aspects of the stress field and also the pattern of horizontal motions indicate that these forces act in combination with westward push. Our model results allow us to address the relation between the forces experienced by the upper plate and the kinematics of subduction, not just in terms of a vertical cross section but also by taking into account the along-arc dimension.

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TL;DR: In this paper, surface and subsurface data image contrasting structural styles for the two flanks of the Venezuela Andes are presented. Andean regional profiles have been balanced and constrained by an inversion of the gravimetric data.
Abstract: Surface and subsurface data image contrasting structural styles for the two flanks of the Venezuela Andes. In the north, a flexural basin developed in Neogene times between the Andes and the Lake Maracaibo. North verging thrusts are mainly detached in the pre-Cretaceous substratum and form a deeply buried antiformal stack. Secondary decollement levels occur both in the Upper Cretaceous and Tertiary strata, accounting for the passive roof thrust of a conventional frontal triangle zone. In the south, the Barinas basin hardly compares with a flexural basin. It is largely dominated by both north and south verging basement-involving structures. Paleogene normal faults are locally inverted, and Caribbean nappes are frequently reactivated or refolded by younger oblique Neogene Andean structures. Seismic profiles in this area also attest to the strong Neogene structural inversion of Upper Jurassic-Lower Cretaceous grabens. Hercynian, or more likely Early Paleozoic structures account for local preexisting crustal heterogeneities, reactivated during both the Tethyan rifting and the Andean deformations. Two trans-Andean regional profiles have been balanced and constrained by an inversion of the gravimetric data. They imply a progressive deepening of the northern Moho and a south dipping subduction of the infracontinental lithospheric mantle of the Maracaibo block. The shortening for both sections averages 60 km. Palinspastic restorations assume a relative cylindricity for the deep crustal architecture of the Andes and minimize the possible effects of a progressive right-lateral escape of the Maracaibo block with respect to stable South America along the Bocono Fault. Strain partitioning during Neogene oblique convergence induced surficial thrust fronts parallel to the plate boundary, strike-slip motion in the allochthon along the Bocono Fault, and an asymmetric subduction (wedging) at depth.

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TL;DR: The Grenville orogenic cycle, between ∼ 1190 and 980 Ma, involved accretion of magmatic arcs and/or continental terranes to the Laurentian craton as mentioned in this paper.
Abstract: The Grenville orogenic cycle, between ∼ 1190 and 980 Ma, involved accretion of magmatic arcs and/or continental terranes to the Laurentian craton. A transect across the western Central Gneiss Belt, Georgian Bay, Ontario, which crosses the boundary between parautochthonous and allochthonous units at an inferred orogenic depth of 20–30 km, offers some insights on the thermal and mechanical behavior of the lower crust during the development of the Grenville orogen. Prior to Grenvillian metamorphism, this part of Laurentia consisted largely of Meso-proterozoic (∼ 1450 Ma) granitoid orthogneisses, granulites, and subordinate mafic and supracrustal rocks. Grenvillian convergence along the transect began with transport of the previously deformed and metamorphosed (∼ 1160 Ma) Parry Sound domain over the craton sometime between 1120 Ma and 1080 Ma. This stage of transport was followed by out-of-sequence thrusting and further convergence along successively deeper, foreland-propagating ductile thrust zones. A major episode of extension at ∼ 1020 Ma resulted in southeast directed transport of allochthonous rocks along the midcrustal Shawanaga shear zone. The final stage of convergence involved deformation and metamorphism in the Grenville Front Tectonic Zone at ∼ 1000–980 Ma. Peak metamorphism along most of the transect at 1065–1045 Ma followed initial transport of allochthonous rocks over the craton by 15–35 m.y. Regional cooling, which postdated peak metamorphism by >70 m.y., was probably delayed by the combined effects of late-stage extension and convergence. Transport of allochthons at least 100 km over the craton was accomplished along a weak, migmatitic decollement; further propagation of the orogen into the craton followed partial melting and weakening of parautochthonous rocks below this decollement. Extensional deformation was associated with distributed ductile flow, the formation of regional transverse folds with axes parallel to the stretching direction, and reactivation of the allochthon-parautochthon thrust boundary as an extensional decollement. The extensional lower crustal flow was likely the primary cause of the subhorizontal attitude of many structures and seismic reflectors in this part of the Central Gneiss Belt.

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TL;DR: A high-resolution 3D reconstruction of the 25m.y.-old central Swiss Molasse Basin reveals two sedimentary domains separated by a ∼5km wide floodplain this article.
Abstract: A high-resolution three-dimensional reconstruction of the 25-m.y.-old central Swiss Molasse Basin reveals two sedimentary domains separated by a ∼5-km-wide flood-plain. The proximal domain of the basin attained a width of 20 km, and its basement is steeply flexed (6°-7° dip). Petrographic data indicate that it was filled by sediment from the Rigi dispersal system derived from the central Alps of eastern Switzerland and by locally sourced bajadas. In contrast, the distal sedimentary domain, located farther north, was gently dipping (<2°) and was filled by the meandering Lac Leman and Honegg dispersal systems. Chronological data reveal that sedimentation in the northern proximal part of the basin started at ∼27 Ma, when sediment supply to the basin started to increase. Deflection of the foreland plate at ∼25 Ma is successfully simulated by flexural modeling of the thrust load and the sediment load. The model reveals that the Lac Leman and Honegg dispersal systems are located on a buried flexural bulge. Furthermore, it shows that burial and suppression of the flexural bulge at ∼27 Ma as well as an increase of the basin wavelength were controlled by the contemporaneous increase in the sediment supply rate of the Rigi system. The model presented suggests that the tectonic subsidence of the Molasse Basin was mainly controlled by tectonic events in the northern part of the orogen, within ∼70 km distance from the tip of the orogenic wedge. Crustal thickening in this part of the orogen is reflected in the proximal Molasse by sedimentary cycles characterized by an increase in the sediment accumulation rates up section and by the presence of locally sourced bajada fans at the top of each cycle. Although south vergent back thrusting along the Insubric Line ∼150 km south of the foreland basin contributed little to flexure, it resulted in an increase of the sediment supply to the foreland basin. This is reflected in the Molasse by coarsening and thickening upward trends, an increase of the basin wavelength, basinward shifts of the depocenters of the dispersal systems, and uplift and erosion of the proximal basin border.

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TL;DR: In this paper, structural relationships, stratigraphic data, and previously published metamorphic data indicate that basement-involved contraction occurred during latest Cretaceous to early Tertiary time.
Abstract: Brittle fault arrays, regional structural patterns, and marine multichannel seismic reflection profiles across the Magallanes basin near 53°S latitude show contrasting styles of superposed extensional, contractional, and transtensional deformation. Below the northeastern end of a NE-SW transect of the basin, seismic reflection profiles show deepening levels of pre-Late Jurassic basement and Upper Jurassic to Lower Cretaceous volcanic horizons toward the southwest. This trend is associated with apparent normal fault offsets that may be attributed to a widespread Late Jurassic-Early Cretaceous extensional event. Contrastingly, below the southwestern end of Magallanes basin subsurface depths of basement and volcanic horizons shallow to the southwest. Integration of seismic reflection data with published seismic refraction data, geologic mapping, and satellite gravity anomaly data suggest that this shallowing trend is controlled structurally by craton-vergent, basement-involved contractional deformation. This style of shortening contrasts with thin-skinned styles of thrusting in the northern and central parts of the basin on Tierra del Fuego. Structural relationships, stratigraphic data, and previously published metamorphic data indicate that basement-involved contraction occurred during latest Cretaceous to early Tertiary time. Overprinting all contractional structures, steep Tertiary strike-slip and oblique-slip faults in the central and southern parts of the Magallanes basin display oblique-normal (mostly down-to-the-southwest) offsets across WNW-ESE trending segments of the South American-Scotia transform plate boundary on the South American continent. Newly discovered features along the plate boundary segment include sediment-filled grabens and half grabens up to 7 km wide that occur between subparallel fault splays in the Magallanes fault zone. Kinematic analyses of fault-slip data indicate that sinistral transtensional motion occurred within this fault zone between the eastern arm of the Straits of Magellan and central Tierra del Fuego. Together these data indicate that (1) the southernmost Andes were under compression during rapid cooling and exhumation of unique high-pressure (∼8 kbars) basement rocks now exposed in the Darwin Metamorphic Complex (DMC) near 55°S latitude and (2) final exhumation of basement rocks in the DMC occurred during a shift away from compression toward transtension during late Tertiary crustal relaxation. We suggest that preexisting structural anisotropies formed during basement-involved contractional deformation and the initiation of seafloor spreading south of South America during the Oligocene controlled late Tertiary sinistral transtensional patterns in the Magallanes fault zone.

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TL;DR: The Bilila-Mtakataka fault is the longest continuous normal fault segment that we know of on the continents, and it supports the suggestion that the thickness of the seismogenic layer is the fundamental control on the scale of geological structures that form within it as discussed by the authors.
Abstract: Some parts of the east African rift system have deeper earthquakes (30–40 km), a larger effective elastic thickness (∼35 km), and wider half grabens (∼50 km) than are typical in other regions of continental extension. One such region is the southern part of the western branch of the east African rift in Malawi. In this region we describe a normal fault scarp that is up to 15 m high and continuous over a distance of more than 100 km. It represents the latest increment of slip, perhaps in a single earthquake, on a fault (the Bilila-Mtakataka fault) with a total offset of about 1000 m. It is the longest continuous normal fault segment that we know of on the continents, and it supports the suggestion that the thickness of the seismogenic layer is the fundamental control on the scale of geological structures that form within it. A consequence of this association is the possibility of very large (Mw 8.0), though infrequent, normal faulting earthquakes in east Africa.

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TL;DR: In this paper, geochemical and isotopic data for Nisutlin assemblage metasedimentary rocks and Anvil assemblages greenstones from the Teslin tectonic zone of the northern Canadian Cordillera are presented.
Abstract: We present geochemical and isotopic data for Nisutlin assemblage metasedimentary rocks and Anvil assemblage greenstones from the Teslin tectonic zone of the northern Canadian Cordillera. This study aims to establish the tectonic setting of formation for the sedimentary and basaltic protoliths of these highly deformed and metamorphosed rocks and thereby place constraints on the origin of these enigmatic rocks for which differing tectonic models have been proposed. For the Nisutlin assemblage metasedimentary rocks, the geochemical and isotopic data show that two widely different source regions contributed detritus to the original sediments. One source region was felsic, upper crustal material with Nd isotopic compositions compatible with ultimate derivation from the North American continent (depleted mantle model age (TDM) 2.5–2.8 Ga). The second source region is deduced to be chemically primitive crust (basaltic-andesitic) with a short crustal residence history (<0.9 Ga). For the Anvil assemblage greenstones, immobile trace element abundances are dissimilar to within-plate, oceanic arc and normal mid-ocean ridge basalts and similar to calc-alkaline basalts in active continental margin settings. We interpret the paleosetting for the Nisutlin assemblage to be at the outermost margin of the ancestral North American continent, in an area which received detrital input from the distal North American craton in the early Paleozoic. However, this area also received detritus from a chemically and isotopically juvenile magmatic arc source, a source type not known from Paleozoic metasedimentary rocks from the miogeoclinal sequence. On the basis of similar Nd isotopic relationships recorded elsewhere in deformed Paleozoic rocks of the orogen, we infer that these geochemical signals reflect tectonic processes of regional extent. The trace element geochemistry of the Anvil assemblage greenstones does not support a correclation with known Paleozoic greenstones of the Slide Mountain terrane, which some tectonic models have advocated.

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TL;DR: In this paper, the authors analyzed the large-scale topography, structure, and morphology across the central part of the DSR between Lake Kinneret and the Gulf of Elat and showed a distinct asymmetrical topographic pattern across the rift axis.
Abstract: The Dead Sea Rift (DSR) is one of the deepest continental depressions on the Earth's surface and is the best example of a continental rift lying along a transform plate boundary (the Dead Sea Transform). We systematically analyze the large-scale topography, structure, and morphology across the central part of the DSR between Lake Kinneret and the Gulf of Elat and show a distinct asymmetrical topographic pattern across the rift axis. The topography analysis uses a Digital Terrain Model (DTM) of Israel and adjacent areas to plot a series of 64 profiles perpendicular to the rift axis. The profiles show that the eastern side is topographically higher than the western side and that its overall shape resembles an uplifted shoulder; the lower western side resembles an arch. This analysis also reveals along-strike variations in the topography that allow us to subdivide the central DSR into five segments of similar topography. The large-scale structure across the DSR is investigated by a series of 10 geological cross sections drawn perpendicular to the rift axis along the five segments. On the basis of the stratigraphic record and the geological history of the region, we identify a regional marker (Top Eocene Sequence) to trace the rift-related structure. This marker shows that the structure parallels the topographic asymmetry across the rift axis: the rift's eastern margin is uplifted toward the axis, whereas the rift's western margin is downflexed toward the axis and defines a wide asymmetrical monocline. Our analyses indicate that (1) the large-scale asymmetry across the DSR reflects a wide half-graben structure (30–60 km wide), (2) the rift's eastern margin reflects broad regional uplift along the rift, and (3) the western side arching is a subsidary structure that follows the main rift structure.

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TL;DR: In this paper, a six-step reconstruction of the South Pyrenean foreland fold-and-thrust belt in Spain delineates the topographic slope, basal dtcollement angie, internal deformation, and thrust- front advance from the Early Eocene until the end of contractional deformation in the Late Oligocene.
Abstract: A six-step reconstruction of the South Pyrenean foreland fold-and-thrust belt in Spain delineates the topographic slope, basal dtcollement angie, internal deformation, and thrust- front advance from the Early Eocene until the end of contractional deformation in the Late Oligocene. Style of thrust- front advance, dip of the basal dtcollement, slope of the upper surface, and internal deformation are decoupled and not simply related. Internal deformation increased, decreased, and maintained surface slope angle at different stages. From the onset to the cessation of deformation, the basal dtcollement angle decreased overall suggesting translation of the thrust belt onto stronger crust with time. Taper angle of the Pyrenean thrust wedge was fundamentally controlled by the flexural rigidity of the lower plate, the relative rate of creation of structural relief in the rear versus the front of the wedge, the extent of deposition of eroded material within the deforming wedge, and the taper of the pretectonic stratigraphic wedge.

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TL;DR: In this paper, a transpressive orogenic belt consisting from north to south of a nascent subduction zone (South Caribbean deformed belt), a volcanic arc (Leeward Antilles arc), a “hinterland” with high-pressure (P)/low temperature (T) metamorphic rocks (Cordillera de la Costa belt), and a southern nonmetamorphic, foreland fold and thrust belt (Serrania del Interior).
Abstract: The Caribbean/South American plate boundary zone in northeastern Venezuela is a transpressive orogenic belt consisting from north to south of a nascent subduction zone (South Caribbean deformed belt), a volcanic arc (Leeward Antilles arc), a “hinterland” with high-pressure (P)/low temperature (T) metamorphic rocks (Cordillera de la Costa belt), and a southern nonmetamorphic, foreland fold and thrust belt (Serrania del Interior). The geometry, style, and orientation of mid-Cretaceous to Tertiary synmetamorphic deformation structures (D1) in the hinterland are compatible with formation in a right-oblique subduction or collision zone in which displacement partitioning has occurred. Late Oligocene to Recent right-oblique convergence resulted in the emplacement of the arc and hinterland on the passive South American margin and the formation of the foreland fold and thrust belt (D2); the displacements between the Caribbean and South American plates are partitioned as well. Both D1 and D2 deformations are diachronous: they are older in the west and younger in the east and related to the eastward passage of the Caribbean plate with respect to South America. The ascent, decompression, and exhumation of the high-P/low-T metamorphic rocks occurred in two stages: the first in the Cretaceous by arc-parallel extension (D1) and the second in Neogene time by thrusting (D2) and subsequent erosion.