Km Of Crustal Shortening Research Articles

Buried thrust belt front of the western Central Andes of northern Chile: Style, age, and relationship with basement heterogeneities

We present a regional-scale study combining field, seismic, and limited borehole information to analyze the structural characteristics of the thrust belt front of the western Central Andes at 21°S. This region, located in northern Chile, consists of the easternmost part of the Coastal Cordillera, the Pampa del Tamarugal, and the western Domeyko Cordillera (or Chilean Precordillera). On the surface, a series of N–S-oriented folds alternating between monoclines and narrow anticline and syncline folds containing Mesozoic, Upper Cretaceous, and Cenozoic rocks form part of the main structural styles, which are frequently bound by west-verging reverse faults that form the basement of the deeper structural levels of the upper crust. The interpretation of a regional seismic grid composed of N–S, NE–SW, and WNW–ESE-oriented 2-D seismic profiles revealed the presence of several blind west-verging inversion anticlines that define the orogenic front of the Central Andes at this latitude. These anticline folds are related to the partial and/or total positive reactivation of ancient east-dipping Jurassic normal basement faults bounding half-graben structures and sub-basins developed within the Tarapacá Basin. Our results suggest that the thrust belt front in this region is preferably located under the Pampa del Tamarugal, and not along the topographic culmination of the western Precordillera as traditionally proposed. New tectonic restorations of 2-D structural models suggest that, during tectonic inversion, between 3 and 4 km of crustal shortening were accommodated by vertical motion along the inverted normal faults. The presence of Upper Cretaceous synorogenic deposits covering the Jurassic syn-rift deposits on the inversion anticlines indicates that both basin inversion and formation of the western thrust belt front began during the Late Cretaceous, and not during the Eocene “Incaic” tectonic phase as previously suggested by regional studies developed in the area.

Paleomagnetism of Eocene red-beds in the eastern part of the Qiangtang Terrane and its implications for uplift and southward crustal extrusion in the southeastern edge of the Tibetan Plateau

A primary magnetic component with the acquisition time of 56.0–43.2 Ma was isolated between 580 °C and 685 °C from the Eocene Gonjo and Ranmugou Formations in the eastern part of the Qiangtang Terrane, Tibetan Plateau. The tilt corrected site-mean direction is Ds=35.5°, Is=29.3°, k=45.9, α95=3.2°. The site-mean inclination increased from 29.3° to 41.6° after multiple inclination shallowing corrections, giving a paleopole of 57.9°N/192.1°E, A95=2.9°. Comparison of the Paleogene paleomagnetic data for the Qiangtang Terrane and Lhasa Terrane reveals that both terranes experienced latitudinal crustal shortening before 54–43 Ma, indicating the uplift of southern and central Tibet in the early Eocene. Subsequently, since 35.4 ± 2.4 Ma, the north of the Qiangtang Terrane experienced ∼1300 ± 410 km of crustal shortening, indicating the uplift of the northern Tibetan Plateau. The Lhasa Terrane and Qiangtang Terrane have not experienced further crustal shortening since the late Eocene, and the southeastern part of Tibet cannot have provided abundant crustal material to accommodate the significant crustal southward extrusion in the southeastern edge of the Tibetan Plateau. The crust of the Tengchong Terrane and Shan–Thai Block did not experience significant southward extrusion since the late Oligocene–Early Miocene. The Indochina Block was situated in the north of the Qiangtang Terrane before the Oligocene, and since the early Oligocene, the Indochina Block began to experience southward extrusion from the north of the Qiangtang Terrane, which absorbed part of the crustal shortening in the north of the Qiangtang Terrane.

Flexural bending of the Zagros foreland basin

We constrain and model the geometry of the Zagros foreland to assess the equivalent elastic thickness of the northern edge of the Arabian plate and the loads that have originated due to the Arabia–Eurasia collision. The Oligo-Miocene Asmari formation, and its equivalents in Iraq and Syria, is used to estimate the post-collisional subsidence as they separate passive margin sediments from the younger foreland deposits. The depth to these formations is obtained by synthesizing a large database of well logs, seismic profiles and structural sections from the Mesopotamian basin and the Persian Gulf. The foreland depth varies along strike of the Zagros wedge between 1 and 6 km. The foreland is deepest beneath the Dezful embayment, in southwest Iran, and becomes shallower towards both ends. We investigate how the geometry of the foreland relates to the range topography loading based on simple flexural models. Deflection of the Arabian plate is modelled using point load distribution and convolution technique. The results show that the foreland depth is well predicted with a flexural model which assumes loading by the basin sedimentary fill, and thickened crust of the Zagros. The model also predicts a Moho depth consistent with Free-Air anomalies over the foreland and Zagros wedge. The equivalent elastic thickness of the flexed Arabian lithosphere is estimated to be ca. 50 km. We conclude that other sources of loading of the lithosphere, either related to the density variations (e.g. due to a possible lithospheric root) or dynamic origin (e.g. due to sublithospheric mantle flow or lithospheric buckling) have a negligible influence on the foreland geometry, Moho depth and topography of the Zagros. We calculate the shortening across the Zagros assuming conservation of crustal mass during deformation, trapping of all the sediments eroded from the range in the foreland, and an initial crustal thickness of 38 km. This calculation implies a minimum of 126 ± 18 km of crustal shortening due to ophiolite obduction and post-collisional shortening.

GPS velocities and the construction of the Eastern Cordillera of the Colombian Andes

AbstractGPS velocities across the northeast trending Eastern Cordillera of Colombia show oblique convergence at 8.8 ± 1.7 mm/yr, consisting of 8.0 ± 1.7 mm/yr of right‐lateral strike‐slip shear along the mountain range and 3.7 ± 0.3 mm/yr of northwest southeast shortening. Faster convergence occurs only at the northeast end of the Cordillera, where its eastern edge trends northwest and the highest mountains lie. The strike‐slip shear corroborates geologic work suggesting such movement southwest and northeast of the range. Given the ~200 km width of the Eastern Cordillera, the ~100–150 km of crustal shortening inferred from balanced cross sections and implied by recent estimates of crustal thickness would require ~25–40 Myr of shortening at ~4 mm/yr. The present‐day GPS measurements, therefore, are inconsistent with the inference, based on paleobotanical observations that the entire Eastern Cordillera rose 1500–2500 m since 3–6 Ma and called for a different interpretation of those data.

Paleomagnetic evidence for ∼4000 km of crustal shortening across the 1 Ga Grenville orogen of North America

Research Article| December 01, 2015 Paleomagnetic evidence for ∼4000 km of crustal shortening across the 1 Ga Grenville orogen of North America Henry C. Halls Henry C. Halls Department of Chemical and Physical Sciences, University of Toronto Mississauga, 3359 Mississauga Road, Mississauga, Ontario L5L 1C6, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information Henry C. Halls Department of Chemical and Physical Sciences, University of Toronto Mississauga, 3359 Mississauga Road, Mississauga, Ontario L5L 1C6, Canada Publisher: Geological Society of America Received: 23 Jul 2015 Revision Received: 25 Sep 2015 Accepted: 27 Sep 2015 First Online: 09 Mar 2017 Online Issn: 1943-2682 Print Issn: 0091-7613 © 2015 Geological Society of America Geology (2015) 43 (12): 1051–1054. https://doi.org/10.1130/G37188.1 Article history Received: 23 Jul 2015 Revision Received: 25 Sep 2015 Accepted: 27 Sep 2015 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Henry C. Halls; Paleomagnetic evidence for ∼4000 km of crustal shortening across the 1 Ga Grenville orogen of North America. Geology 2015;; 43 (12): 1051–1054. doi: https://doi.org/10.1130/G37188.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Paleomagnetic data from the Grenville Province of North America suggest that Laurentia underwent ∼4000 ± 1000 km of crustal shortening from ca. 1040–1020 Ma to 980 Ma as a result of the Grenville orogeny, in which Laurentia was in collision with Amazonia. A corollary is that the size and scale of shortening affect the assembly of continental fragments that include Laurentia, which before 1 Ga may have been significantly larger in area than now by thousands of square kilometers, caused by crustal shortening in a broadly northwest-southeast direction by the Grenville orogeny. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The Moho beneath western Tibet: Shear zones and eclogitization in the lower crust

The Tibetan Plateau is formed by continuing convergence between Indian and Asian plates since ∼50 Ma, involving more than 1400 km of crustal shortening. New seismic data from western Tibet (the TW-80 experiment at 80°E) reveal segmentation of lower crustal structure by the major sutures, contradicting the idea of a mobile lower crust that flows laterally in response to stress variations. Significant changes in crustal structure and Moho depth occur at the mapped major tectonic boundaries, suggesting that zones of localized shear on sub-vertical planes extend through the crust and into the upper mantle. Converted waves originating at the Moho and at a shallower discontinuity are interpreted to define a partially eclogitized layer that extends 200 km north of the Indus–Yarlung Suture Zone, beneath the entire Lhasa block at depths of between 50 and 70 km. This layer is thinner and shallower to the north of the Shiquanhe Fault which separates the northern Lhasa block from the southern part, and the degree of eclogitization is interpreted to increase northward. The segmentation of the Tibetan crust is compatible with a shortening deformation rather than shear on horizontal planes. Unless the Indian-plate mantle lithosphere plunges steeply into the mantle beneath the Indus–Yarlung suture, leaving Indian-plate crust accreted to the southern margin of Tibet, then it too must have experienced a similar shortening deformation.

Regional structure and kinematic history of the Sevier fold-and-thrust belt, central Utah: Discussion

[DeCelles and Coogan (2006)][1] summarize the structure and evolution of the Late Jurassic to Paleocene Sevier orogenic belt in central Utah, inferring at least 220 km of crustal shortening on the basis of a regional balanced cross section. Difficulties arise in an otherwise excellent synthesis in

00/02425 A process and apparatus for treating solid fuel materials

LITHOPROBE is Canada's national, collaborative, multidisciplinary geoscience project directed toward an enhanced understanding of how the North American continent has evolved. Research in its eight transects, which span the country and geological time, are spearheaded by seismic reflection studies. These, combined with other studies, provide new insight into the varied tectonic processes that have been active in forming the continent. The Archean Kapuskasing Structural Zone, a prominent feature which cuts obliquely across east-west Superior Province structures, has been imaged as a low-angle thrust sheet above a major décollement, along which 55–70 km of crustal shortening has occurred to bring mid-crustal rocks to the surface. The shortening was accomplished by brittle faulting and erosion at levels above 20 km and ductile shear and flow in the lower crust. The GLIMPCE transect in Lake Huron shows a spectacular series of east-dipping crustal-scale reflections that coincide with the Grenville Front Tectonic Zone. A two-stage model involves terrane collision with the cratonic margin in the Early Proterozoic followed by crustal-scale stacking and ramping associated with the Middle Proterozoic Grenvillian orogeny. In Lake Superior, GLIMPCE data reveal the structure of the Middle Proterozoic mid-continent rift that almost splits North America. The Lithoprobe East transect covering the Paleozoic Newfoundland Appalachians reveals three lower crustal. blocks, defined seismically on the basis of reflectivity characteristics imaged along the seismic profiles. Reflective upper crust can be related to geological structures at the surface; the thin-skinned tectonics implied have yet to be reconciled with the block tectonics of the lower crust. Results from the southern Canadian Cordillera show that the Mesozoic-Cenozoic east-west shortening in the eastern and central Cordillera occurred by compressional deformation forming crustal-scale antiforms that overrode a westward thinning cratonic North America; and in the western Cordillera by subduction processes that have tectonically eroded the lower lithosphere of accreted terranes and left underplated sediments and oceanic lithosphere. The reflection Moho has highly variable characteristics as imaged within transects and among different transects. Crustal and Moho reflectivity is caused by a wide range of features, including fault/shear zones, lithologie contacts, compositional layering, fluids in zones of high porosity, and anisotropy.

The arc of the western Alps in the light of geophysical data on deep crustal structure

Recently, two international deep seismic campaigns in the western and central Alps (Etude Continentale et Océanique par Reflexion et Refraction Sismique ‐ Progetto Strategico Crosta Profonda, ECORS‐CROP [Roure et al. 1996b]; and National Research Program 20, NRP‐20 [Pfiffner et al. 1997]) have been completed. Here we present a synoptic interpretation of the wealth of geophysical data about deep crustal structure of the Alps collected during the past 40 years, including the two above mentioned geophysical projects. The tectonic reinterpretation of the ECORS‐CROP high‐resolution seismic transect is based on an evaluation of the deep crustal structure by integrating new and literature data on surface geology. Combined with previously published interpretations regarding the central Alps [Schmid et al., 1996], this study reveals substantial differences in geometry and kinematics between transects across the western and central Alps, respectively. At depth the transition between the central and western Alps is marked by the western limits of an Adriatic lower crustal wedge‐shaped structure and the northeastern limits of a similar structure made of European lower crustal material. At the surface it coincides with a corridor of dextral strike‐slip along the Tonale and Simplon lines. In addition, the thickness of the seismogenic zone shows a remarkable variation from over 40 km beneath the Penninic realm of the western Alps to less than 20 km in the central Alps. The formation of the western Alpine arc was initiated during convergence and collision before 35 Myr ago, when the Adriatic micro‐plate, moving northward with respect to the European foreland, caused sinistral transpression in the western Alps while the central and eastern Alps underwent head‐on convergence and collision. During the post‐collisional stage, i.e., after 35 Myr ago, the arcuate shape of the western Alps was accentuated by WNW‐directed movement and anticlockwise rotation of the Adriatic microplate, decoupled from the central and eastern Alps along the Tonale‐Simplon dextral shear zone. This led to wedging of lower crustal slices both in the western Alps and in the central Alps. The new tectonic interpretation of the ECORS‐CROP transect allows a kinematic model to be established for crustal shortening in the western Alps during the past 35 Myr. The Ivrea mantle plays the role of a backstop in our tectonic model. We distinguish three episodes of post‐collisional crustal shortening along the ECORS‐CROP transect: From 35 to 30 Ma the Brianconnais basement was back‐thrusted over the Gran Paradiso units, leading to 30 km of crustal shortening (first episode). In the early Miocene, movements concentrated mainly along the Penninic frontal thrust and resulted in about 60 km shortening (second episode). Post‐12 Ma shortening within the external massifs is associated with folding in the Jura mountains when the crust was shortened by an additional 30 km (third episode).

Late Cenozoic tectonic evolution of the southern Chinese Tian Shan

Structural, sedimentological, magnetostratigraphic, and 40Ar/39Ar thermochronological investigations were conducted in the southern Chinese Tian Shan. On the basis of our own mapping and earlier investigations in the area, the Late Cenozoic southern Tian Shan thrust belt may be divided into four segments based on their style of deformation. From west to east, they are (1) Kashi‐Aksu imbricate thrust system, (2) the Baicheng‐Kuche fold and thrust system, (3) the Korla right‐slip transfer system, and (4) the Lop‐Nor thrust system. The westernmost Kashi‐Aksu system is characterized by the occurrence of evenly spaced (12–15 km) imbricate thrusts. The Baicheng‐Kuche and Korla systems are expressed by a major north dipping thrust (the Kuche thrust) that changes its strike eastward to become a NW striking oblique thrust ramp (the Korla transfer zone). The Lop Nor system in the eastern‐most part of the southern Chinese Tian Shan consists of widely spaced thrusts, all involved with basement rocks. Geologic mapping and cross‐section construction suggest that at least 20–40 km of crustal shortening with a horizontal shortening strain of 20–30% has occurred in the southern Chinese Tian Shan during the late Cenozoic. These estimates are minimum because of both conservative extrapolation of the thrust geometries and partial coverage of the thrust belt by the cross sections. The timing of initial thrusting is best constrained in the Kuche basin where crustal shortening may have occurred at 21–24 Ma, the time of a major facies transition between lacustrine and braided‐fluvial sequences constrained in general by biostratigraphy and in detail by magnetostratigraphy. This estimate represents only a minimum age, as development of thrusts in the southern Chinese Tian Shan may have propagated southward toward the foreland. Thus the sedimentary record only represents the southernmost and therefore youngest phase of thrusting. If our estimate of timing for the thrust initiation (21–24 Ma) is correct, using the estimated magnitude of shortening (20–40 km) and shortening strain (20–30%), the averaged rates of late Cenozoic horizontal slip and shortening strain are 1–1.9 mm yr−1 and 2.9–4.5 × 10−16 s−1, respectively. Our reconnaissance 40Ar/39Ar thermochronological analysis in conjunction with earlier published results of apatite fission track analysis by other workers in the Chinese Tian Shan suggests that the magnitude of Cenozoic denudation is no more than 10 km, most likely less than 5 km. We demonstrate via a simple Airy‐isostasy model that when the thermal effect on changes in surface elevation is negligible, determination of the spatial distribution and temporal variation of both horizontal shortening strain and denudation becomes a key to reconstructing the elevation history of the Tian Shan. Using this simple model, the loosely constrained magnitude of crustal‐shortening strain and denudation in the southern Chinese Tian Shan implies that it may have been elevated 1.0–2.0 km since the onset of Cenozoic thrusting.

Latest Cretaceous-Tertiary Tectonic Evolution of Northern Yukon and Adjacent Arctic Alaska

Regional crustal shortening in northern Yukon and northeastern Alaska occurred episodically from the latest Cretaceous to the late Miocene, with a major culmination occurring in the Paleocene to middle Eocene, and a secondary culmination in the late Miocene. Structural trends are predominantly north to northeastward in northern Yukon and adjacent east-central Alaska, and generally east-west along the Brooks Range. The early Tertiary trend is strongly arcuate in the Beaufort fold belt and adjacent onshore areas. The continuity of structures southward from the Beaufort Sea region through northern Yukon and east-central Alaska supports the interpretation that the structures north of 65°N form a single orogenic entity. The Beaufort Sea region is 1000 km from the nearest plate margin. Northward shortening across Arctic Alaska and the northern Canadian cordillera reflects the convergent component of Kula (later, Pacific)-North America interactions throughout the Tertiary. North- to northeast-trending structures of latest Cretaceous to early Tertiary age in northern Yukon and east-central Alaska accommodate shortening of 180-240 km, and reflect Eurasia-North America convergence. The strongly arcuate offshore Beaufort fold belt and similar structures in adjacent northernmost Yukon and northeast Alaska were formed by the complex interplay of three factors: shortening of northern Yukon between Arctic Alaska and the craton to produce a north-trending orogenic welt; northward displacements propagated from the Kula plate margin; and local boundary conditions imposed by lithology and crustal structure, which aided lateral escape of the deforming supracrustal succession northward into the Beaufort Sea. In central Alaska, any kinematic linkage between the Kaltag fault in the west and the Tintina fault of the northern cordillera is more complex than was previously assumed. A new regional tectonic reconstruction of northern Yukon-Alaska quantifies the tectonic shortening in central Alaska south of the Kaltag and Tintina faults in an area where tectonic shortening is difficult to quantify due to complex geology. Complex deformation accommodated by folding, thrust and strike-slip faulting, and/or tectonic rotations accounts for an estimated 460 km of crustal shortening, approximately equivalent in magnitude to the total Tintina fault displacement of at least 450 km. The foreland of the Brooks Range, the Beaufort fold belt, and the northern cordillera contain proven petroleum basins. This regional synthesis validates a model of orogenic shortening for the Beaufort fold belt and provides a unifying tectonic setting for oil and gas plays throughout the region. The latest Cretaceous-Tertiary structural evolution of the Brooks Range, Beaufort, and northern Yukon fold belts is a case study of the temporally and kinematically complex far-field deformation arising from the convergence of four major tectonic plates.

Thermal and structural evolution of the intracratonic Arltunga Nappe Complex, central Australia

The thermal and structural evolution of a Paleozoic thrust system in central Australia, the Arltunga Nappe Complex, has been examined through field mapping, petrographic, microstructural and chemical analyses, K/Ar and 40Ar/39Ar thermochronology, and multi‐diffusion‐domain (time‐temperature) modeling of K‐feldspar 40Ar/39Ar data. The Arltunga Nappe Complex contains three distinct tectonic units, bounded by regional faults and shear zones, that have undergone contrasting deformational and metamorphic histories. The cooling history of the three tectonic units has been constrained by K/Ar and 40Ar/39Ar isotopic age data from K‐feldspars, micas, and amphiboles. Results are consistent with a piggyback history of crustal imbrication spanning over 100 m.y., with the hottest rocks uplifted and cooled first and with imbrication acting progressively toward the foreland. Implications for the intracratonic Paleozoic tectonics of central Australia are (1) uplift within the structurally highest tectonic unit was initiated before 400 Ma; (2) continued piggyback imbrication, footwall failure, and uplift and exhumation of the crust resulted in duplexing in the lowest structural unit at approximately 325–300 Ma; (3) the contrast in thermal history between the tectonic units indicates that once widely separated crustal segments were juxtaposed during thrusting above a hinterland dipping megathrust, resulting in ∼85 km of crustal shortening.

Structural evolution of a modern arc‐continent collision in Papua New Guinea

The Finisterre Mountains and western Solomon Sea of northern Papua New Guinea are the site of an active, oblique arc‐continent collision. Comparison of structures along the length of the collision zone provides a history of its structural evolution. Here we compare the results of structural mapping in the Finisterre Mountains, where collision is in an advanced state, with side scan sonar images and potential field data from the western Solomon Sea, which illuminate the structure of the juvenile collision zone. An accretionary wedge complex, composed of a southwestward younging stack of imbricate thrust sheets, developed along a westward extension of the modern New Britain Trench prior to collision. This accretionary wedge crops out in the Finisterre Mountains as the Erap Structural Complex. Consequences of collision were doubling of the crustal thickness to 50–52 km, rapid up‐lift of the Finisterre terrane, and initiation of a major out‐of‐sequence thrust that displaced the Finisterre Volcanics over younger sediments of the accretionary wedge. This out‐of‐sequence thrust is mapped in the Finisterre Mountains as the Wongat Thrust, and it is recognized in the western Solomon Sea on side scan sonar images. A second generation out‐of‐sequence thrust formed after substantial uplift had subaerially exposed the collision zone. This thrust crosscuts the Wongat Thrust, forming a window of accretionary wedge sediments bounded to both the NE and SW by Finisterre Volcanics. This younger out‐of‐sequence thrust is the Gain Thrust of the Finisterre Mountains. At least 15 km of crustal shortening has occurred across the Wongat Thrust at 146°30′E longitude, and the Gain Thrust has accommodated up to 7.4 km of shortening in the same area. It is probable that the well‐lithified Finisterre Volcanics serve as the backstop for the accretionary wedge/collision complex that is forming in the collision zone of the western Solomon Sea. Potential field models indicate that this backstop is arcward dipping. Thrusting of the back‐stop material over the accretionary wedge, as we observe along the Wongat Thrust, is expected for wedges with arcward dipping backstops. A series of NE–SW trending cross faults, perpendicular to the dominant structural trend, exist in the collision zone. These faults have components of left‐lateral strike‐slip and SE down dip‐slip motion. It is likely that the Finisterre terrane is being segmented along these faults, with the segmentation caused by differential resistance to convergence between the downgoing continental crust of the collision zone and the oceanic crust of the subduction zone to the east. The origin of the faults is uncertain. Previous authors believed that they formed during Paleogene volcanic activity in the Finisterre terrane. The colinearity of several of these cross faults with major mapped faults on the downgoing plate raises the possibility that the Finisterre Range cross faults are upper plate manifestations of lower plate structures and, as such, reveal strong interplate coupling in the collision zone.

Gravity modelling of the lithosphere in the Eastern Alpine-Western Carpathian-Pannonian Basin region

Gravity models illustrate changes in the degree of continental convergence in the Eastern Alpine-Western Carpathian region, and modifications to the lithosphere due to the plate convergence and subsequent Pannonian Basin extension. Analysis of the continental collision zone incorporates a kinematic model of ocean basin closure, whereby gravity anomalies and topography are viewed as part of a continuum of continental crustal shortening, erosion and isostatic rebound. Thick crust and high topography in the Eastern Alps, along with a broad Bouguer anomaly of −140 mGal amplitude, are consistent with about 175 km of crustal shortening, followed by 10 km of isostatic rebound. Eastward, crustal thicknesses and gravity anomaly widths and amplitudes are less, so that only about 50 km of continental crustal shortening and 4 km of rebound occurred in the Western Carpathians. Preservation of thick flysch deposits and small isostatic rebound are attributable to the high-density, shallow mantle of the intact continent-ocean transition zone. Seismic delay time studies have suggested that, relative to the average thickness of the region, the lithosphere thickens by about 70 km beneath the Eastern Alps and thins by about 60 km under the Pannonian Basin. In both regions, gravity anomalies cannot be explained fully without considering this large relief on the lithosphere/asthenosphere boundary. The Eastern Alpine crustal root, which extends 15 km below the average depth for the region, overcompensates the topography and results in gravity anomalies that are 40 mGal lower than those observed; the extra 70 km of lithosphere provides excess mass that achieves isostatic equilibrium and accounts for the 40 mGal difference. Observed gravity anomalies and local isostasy are also consistent with thin crust and thin lithosphere beneath the Pannonian Basin, whereby the 60 km of extra asthenosphere provides a large part of the compensation for the elevated mantle. Regional cross sections suggest that shallowing of the lithosphere/asthenosphere boundary, associated with Pannonian Basin extension, has propagated northward beyond the Carpathians, to within the European Platform. Crustal thinning, however, appears to be confined to exotic terranes of the Carpathian interior, so that crustal structure in the Eastern Alps and Outer Carpathians is a remnant of the earlier collision orogen.

Variations in continental crustal structure in Canada from LITHOPROBE seismic reflection and other data

LITHOPROBE is Canada's national, collaborative, multidisciplinary geoscience project directed toward an enhanced understanding of how the North American continent has evolved. Research in its eight transects, which span the country and geological time, are spearheaded by seismic reflection studies. These, combined with other studies, provide new insight into the varied tectonic processes that have been active in forming the continent. The Archean Kapuskasing Structural Zone, a prominent feature which cuts obliquely across east-west Superior Province structures, has been imaged as a low-angle thrust sheet above a major décollement, along which 55–70 km of crustal shortening has occurred to bring mid-crustal rocks to the surface. The shortening was accomplished by brittle faulting and erosion at levels above 20 km and ductile shear and flow in the lower crust. The GLIMPCE transect in Lake Huron shows a spectacular series of east-dipping crustal-scale reflections that coincide with the Grenville Front Tectonic Zone. A two-stage model involves terrane collision with the cratonic margin in the Early Proterozoic followed by crustal-scale stacking and ramping associated with the Middle Proterozoic Grenvillian orogeny. In Lake Superior, GLIMPCE data reveal the structure of the Middle Proterozoic mid-continent rift that almost splits North America. The Lithoprobe East transect covering the Paleozoic Newfoundland Appalachians reveals three lower crustal. blocks, defined seismically on the basis of reflectivity characteristics imaged along the seismic profiles. Reflective upper crust can be related to geological structures at the surface; the thin-skinned tectonics implied have yet to be reconciled with the block tectonics of the lower crust. Results from the southern Canadian Cordillera show that the Mesozoic-Cenozoic east-west shortening in the eastern and central Cordillera occurred by compressional deformation forming crustal-scale antiforms that overrode a westward thinning cratonic North America; and in the western Cordillera by subduction processes that have tectonically eroded the lower lithosphere of accreted terranes and left underplated sediments and oceanic lithosphere. The reflection Moho has highly variable characteristics as imaged within transects and among different transects. Crustal and Moho reflectivity is caused by a wide range of features, including fault/shear zones, lithologie contacts, compositional layering, fluids in zones of high porosity, and anisotropy.

Lithoprobe: new perspectives on crustal evolution

Lithoprobe is Canada's national, collaborative, multidisciplinary earth science research program directed toward an enhanced understanding of how the North American continent evolved. Research in its eight transects or study areas, which span the country from Vancouver Island to Newfoundland and geological time from 4 Ga to the present, is spearheaded by seismic reflection surveys. These, combined with many other studies, are providing new insight into the varied tectonic processes that have been active in forming the continent. Results from the Southern Cordillera transect show that Mesozoic crustal growth occurred in the central and eastern Cordillera by the accretion and amalgamation of exotic terranes, the collision of which resulted in the generation of crustal-scale antiforms and duplexes. After the principal periods of compression, this area was affected by a major episode of extension that led to the unroofing of the metamorphic core complexes. Farther to the west, past and present subduction processes have eroded the lower lithosphere of accreted terranes and left underplated sediments and oceanic lithosphere. The Lithoprobe East transect, covering the Paleozoic Newfoundland Appalachians and Mesozoic rifted Atlantic margin, reveals three lower crustal blocks, each with distinctive reflection signatures on marine seismic data. Structures of the geologically established tectono-stratigraphic domains, imaged clearly by new onshore reflection data, sole at upper crustal to mid-crustal levels, suggesting that much of the surface stratigraphy is allochthonous to the lower crustal blocks. At the ocean–continent transition, interpretations suggest underplating of thinned continental crust by basaltic melt during the rifting process.In Lake Superior, data from the Great Lakes International Multidisciplinary Program on Crustal Evolution (GLIMPCE) transect reveal the complex structures of the late Middle Proterozoic Keweenawan rift, which is up to 35 km deep, that almost split North America. The GLIMPCE data in Lake Huron show a spectacular series of east-dipping crustal-scale reflections that coincide with the Grenville front tectonic zone. These and other data have led to a two-stage model involving collision of an exotic terrane with the southern Superior cratonic margin in the late Early Proterozoic followed by stacking–crustal penetrating imbrication and ramping associated with the Middle Proterozoic Grenvillian orogeny. The Archean Kapuskasing structural zone, a prominent northeast-trending feature that cuts obliquely across the dominant east-west structures of the Superior Province, is interpreted as a thin thrust sheet, soled by a variably reflective décollement, above which about 70 km of crustal shortening has occurred to bring mid-crustal to lower crustal rocks to the surface, and below which the Moho deepens. The shortening may have been accomplished by brittle faulting and erosion at levels above 20 km and ductile folding or faulting in the lower crust. Preliminary studies in the Archean Abitibi greenstone belt indicate that two major fault zones, the Larder Lake–Cadillac and Porcupine–Destor, which host significant mineralization, were generated by crustal-scale thrust and (or) strike-slip tectonics. Archean crustal sections are as structurally diverse and complex as their Proterozoic and Phanerozoic counterparts. The reflection Moho has highly variable characteristics as imaged within transects and among different transects. Crustal and Moho reflectivity observed in the various transects is caused by a wide range of features, including fault–shear zones, lithologic contacts, compositional layering, fluids in zones of high porosity, and anisotropy.

Crustal-scale balanced cross sections through the Variscan fold belt, Germany: the central EGT-segment

Abstract This paper presents crustal-scale, geometrically and isostatically balanced cross sections through the Rhenohercynian and Saxothuringian zones of the Variscan fold belt of Germany. The cross sections are based on surface geological data and on results of the DEKORP deep reflection profiles. They were constructed by means of computer-assisted, iterative forward modelling. The Rhenohercynian zone, corresponding to the Externides of the Variscan belt, developed out of a some 300 km wide rifted continental margin sedimentary prism, limited to the south by the narrow Giessen oceanic basin. During Carboniferous imbrication this shelf sequence is stripped of its basement above one or more decollements and shortened by some 150 km. Closure of the Giessen ocean and emplacement of the ophiolite bearing Giessen nappe entailed a minimum additional 200 km of crustal shortening. The structure of the model correlates well with the present outcrop pattern of rock units, their metamorphic history, the seismic and magnetotelluric architecture of the crust, and its present regional distribution of thickness. The deeper crust beneath the Rhenish Massif is probably devoid of extremely weak decollement horizons and pronounced rheological stratification. In the southward adjacent Saxothuringian zone, nappe development involving the stacking of crustal slices during Middle Devonian to Visean times accounts for a further 170 km of crustal shortening above an infracrustal, partly blind thrust system with flat-ramp-geometry. Total crustal shortening in the analyzed profiles is estimated to be of the order of 520 km; this figure is a conservative value. Prominent uncertainties include the unknown importance of crustal wrenching, the state of strain within thrust units, the distribution of strain within the lower crust, and major changes in thrusting directions during the course of orogenic history.

Budget of crustal shortening and subduction of continental crust in the Alps

The stratigraphy, surface structure, and deep structure of the Alps permit, but do not require, that a significant amount of European continental crust was subducted into the mantle beneath Adria, the fragment of continental lithosphere now comprising much of Italy and the Adriatic Sea floor. The present thin crust in much of Europe suggests that the crust that was stretched in Mesozoic time to form the southern margin of Europe was initially thinner than normal. The stratigraphic history of sediment deposited on that margin before its collision with Adria suggests that the crustal basement on which much of this material was deposited had become particularly thin during the formation of the margin. Most of the crustal basement underlying the European margin and involved in the collision and the formation of the chain probably was thinner than 20 km, and much of it may have been thinner than 10 km. Profiles of the present deep structure of the Alps yield a range of possible values for the the average cross‐sectional area of crust involved in the collision that permit as much as 280 km of crustal shortening, if this crust was as thin as implied by the stratigraphy. Thus, although subduction of hundreds of kilometers of continental crust into the mantle beneath Adria could have occurred, the budget of crustal material involved in the formation of the Alps does not require such crustal subduction.

Gravity anomalies and flexure of the Brazilian Shield beneath the Bolivian Andes

Seismological and geological data suggest that the Brazilian Shield is now underthrusting beneath the sub-Andes. In order to constrain the minimum amount of shortening that results from this underthrusting, we analyzed Bouguer gravity anomalies over the shield and the Bolivian Andes, assuming that they are due primarily to variations in crustal thickness that in turn are due to the flexure of an effectively elastic plate (Brazilian Shield) loaded by the weight of the Andean mountain range. Gravity anomalies show that Airy-type isostatic equilibrium does not exist; the sub-Andes are overcompensated and the Chaco Plain to the east is undercompensated. The flexure of the plate can help explain both the overcompensation and the undercompensation if the Brazilian Shield behaves as an essentially elastic plate that extends at least 150 km, and probably 200 km, beneath the sub-Andes and the Cordillera Oriental and if the flexural rigidity of the plate is between 0.1 and 2 × 10 24 N m. The fit of observed and calculated anomalies, however, is far from perfect, and additional sources of short-wavelength anomalies appear to be present. Assuming that the sediments and sedimentary rocks in the sub-Andes and Cordillera Oriental were deposited on the Brazilian Shield and that later they were overthrust eastward onto one another causing the present topography and the deflection of the plate, we computed a family of estimates for the minimum amount of crustal shortening. These estimates depend strongly on the amount of sedimentary rock deposited before deformation, which does not seem to be well constrained by published geologic mapping. Nevertheless, the calculations suggest that at least 100 km of crustal shortening have occurred, and probably more, implying that the Andes may have been built mostly by crustal shortening.

The Laramide orogeny: Evidence from cocorp deep crustal seismic profiles in the wind river mountains, wyoming

Deep crustal reflection data that are critical for the interpretation of Laramide structure have been obtained by the Consortium for Continental Reflection Profiling (COCORP). The Laramide orogeny, which occurred from the late Cretaceous to early Eocene, is characterized in Wyoming by large uplifts of Precambrian basement, commonly flanked by reverse faults. The attitude of these faults at depth has been a major tectonic problem and is very important for deciding whether horizontal or vertical crustal movements were primarily responsible for the basement uplifts. COCORP has run 158 km of deep seismic reflection profiles (recording to 20-sec two-way travel time) across the southeastern end of the Wind River Mountains, the largest of these Laramide uplifts. Reflections from the thrust fault flanking the Wind River uplift can be clearly traced on the profiles to at least 24-km depth and possibly as deep as about 36 km with a fairly uniform apparent dip of 30°–35°. Other reflection events subparallel to the main Wind River thrust are present in the seismic profiles and may represent other faults. There is at least 21 km of crustal shortening along the thrust. There is no evidence in the reflection profiles for large-scale folding of the basement; the Wind River Mountains were formed predominantly by thrust movements. Gravity anomalies in the Wind River Mountains can be modeled by a thrust that displaces dense material in the lower crust. If the thrust ever cut the Moho, the effect is not observed in the gravity today. A proposed model for the presence of uplifted basement in Wyoming invokes a shallowly dipping, subducted Farallon plate beneath the North American continent; drag between the two plates localized compressional stresses in an area over 800 km into the North American plate causing large thrusts to develop. The earth's crust seems to have fractured as a fairly rigid plate