Subduction,mega-shear systems and Late Palaeozoic basin development in the African segment of Gondwana

 Basins within the African sector of Gondwana contain a Late Palaeozoic to Early Mesozoic Gondwana sequence unconformably overlying Precambrian basement in the interior and mid-Palaeozoic strata along the palaeo-Pacific margin. Small sea-board Pacific basins form an exception in having a Carboniferous to Early Permian fill overlying Devonian metasediments and intrusives. The Late Palaeozoic geographic and tectonic changes in the region followed four well-defined consecutive events which can also be traced outside the study area. During the Late Devonian to Early Carboniferous period (up to 330 Ma) accretion of microplates along the Patagonian margin of Gondwana resulted in the evolution of the Pacific basins. Thermal uplift of the Gondwana crust and extensive erosion causing a break in the stratigraphic record characterised the period between 300 and 330 Ma. At the end of this period the Gondwana Ice Sheet was well established over the uplands. The period 260–300 Ma evidenced the release of the Gondwana heat and thermal subsidence caused widespread basin formation. Late Carboniferous transpressive strike-slip basins (e.g. Sierra Australes/Colorado, Karoo-Falklands, Ellsworth-Central Transantarctic Mountains) in which thick glacial deposits accumulated, formed inboard of the palaeo-Pacific margin. In the continental interior the formation of Zambesi-type rift and extensional strike-slip basins were controlled by large mega-shear systems, whereas rare intracratonic thermal subsidence basins formed locally. In the Late Permian the tectonic regime changed to compressional largely due to northwest-directed subduction along the palaeo-Pacific margin. The orogenic cycle between 240 and 260 Ma resulted in the formation of the Gondwana fold belt and overall north–south crustal shortening with strike-slip motions and regional uplift within the interior. The Gondwana fold belt developed along a probable weak crustal zone wedged in between the cratons and an overthickened marginal crustal belt subject to dextral transpressive motions. Associated with the orogenic cycle was the formation of mega-shear systems one of which (Falklands-East Africa-Tethys shear) split the supercontinent in the Permo-Triassic into a West and an East Gondwana. By a slight clockwise rotation of East Gondwana a supradetachment basin formed along the Tethyan margin and northward displacement of Madagascar, West Falkland and the Gondwana fold belt occurred relative to a southward motion of Africa. Received: 2 October 1995 / Accepted: 28 May 1996     

Relation between alternations of uplift and subsidence revealed by Late Cenozoic fluvial sequences and physical properties of the continental crust

Reversals in vertical crustal motion, alternations between uplift and subsidence over time scales of hundreds of thousands of years or more, have been identified in Late Cenozoic fluvial sequences in many regions worldwide. They form a class of fluvial archive that is distinct from the extreme stability observed in Archaean cratons and the monotonic uplift or subsidence that is widely observed in other regions. Such alternations between uplift and subsidence are characteristic of regions of Early or Middle Proterozoic crust, where the initial crustal consolidation included the development of a thick ‘root’ of mafic material at the base of the crust; the present study focuses on localities with this crustal type in the USA and eastern Europe. It has previously been suggested on the basis of uplift modelling that this style of crustal behaviour occurs only in regions where the mobile lower‐crustal layer is relatively thin. This study supports this conclusion on the basis of independent geothermal calculations, which indicate that such alternations between uplift and subsidence occur where the mobile lower‐crustal layer is ≤～7 km thick. An understanding of this phenomenon, in relation to the understanding of vertical crustal motions induced by surface processes (and thus by climate change) in general, therefore requires analysis of the properties and dynamics of the mobile lower‐crustal layer; detailed analysis of fluvial sequences thus contributes unique information in this area.     

盆地岩石圈结构与油气成藏及分布

本文综述了大陆岩石圈研究现状和克拉通盆地、裂谷盆地和前陆盆地的岩石圈结构特征,指出在古裂谷、褶皱带或区域性深断裂等陆壳构造薄弱带上发育起来的多期叠合盆地,具有很好的含油气前景。大型含油气盆地往往存在地幔上隆、地壳减薄和地壳内低速层,盆地基底沉降与盖层沉积厚度较大。适度的后期构造活动改造和岩浆活动有利于沉积盆地内油气生成与保存。     

The Southern Urals. Decoupled evolution of the thrust belt and its foreland: a consequence of metamorphism and lithospheric weakening

An analysis is presented of the mechanisms of tectonic evolution of the southern part of the Urals between 48N and 60N in the Carboniferous–Triassic. A low tectonic activity was typical of the area in the Early Carboniferous — after closure of the Uralian ocean in the Late Devonian. A nappe, ≥10–15 km thick, overrode a shallow-water shelf on the margin of the East European platform in the early Late Carboniferous. It is commonly supposed that strong shortening and thickening of continental crust result in mountain building. However, no high mountains were formed, and the nappe surface reached the altitude of only ≤0.5 km. No high topography was formed after another collisional events at the end of the Late Carboniferous, in the second half of the Early Permian, and at the start of the Middle Triassic. A low magnitude of the crustal uplift in the regions of collision indicates a synchronous density increase from rapid metamorphism in mafic rocks in the lower crust. This required infiltration of volatiles from the asthenosphere as a catalyst. A layer of dense mafic rocks, 20 km thick, still exists at the base of the Uralian crust. It maintains the crust, up to 60 km thick, at a mean altitude 0.5 km. The mountains, 1.5 km high, were formed in the Late Permian and Early Triassic when there was no collision. Their moderate height precluded asthenospheric upwelling to the base of the crust, which at that time was 65–70 km thick. The mountains could be formed due to delamination of the lower part of mantle root with blocks of dense eclogite and/or retrogression in a presence of fluids of eclogites in the lower crust into less dense facies.

The formation of foreland basins is commonly attributed to deflection of the elastic lithosphere under surface and subsurface loads in thrust belts. Most of tectonic subsidence on the Uralian foreland occurred in a form of short impulses, a few million years long each. They took place at the beginning and at the end of the Late Carboniferous, and in the Late Permian. Rapid crustal subsidence occurred when there was no collision in the Urals. Furthermore, the basin deepened away from thrust belt. These features preclude deflection of the elastic lithosphere as a subsidence mechanism. To ensure the subsidence, a rapid density increase was necessary. It took place due to metamorphism in the lower crust under infiltration of volatiles.

The absence of flexural reaction on the Uralian foreland on collision in thrust belt together with narrow-wavelength basement deformations under the nappe indicate a high degree of weakening of the lithosphere. Such deformations took also place on the Uralian foreland at the epochs of rapid subsidences when there was no collision in thrust belt. Weakening of the lithosphere can be explained by infiltration of volatiles into this layer from the asthenosphere and rapid metamorphism in the mafic lower crust. Lithospheric weakening allowed the formation of the Uralian thrust belt under convergent motions of the plates which were separated by weak areas.     

From oceanic plateaus to allochthonous terranes: Numerical modelling

Large segments of the continental crust are known to have formed through the amalgamation of oceanic plateaus and continental fragments. However, mechanisms responsible for terrane accretion remain poorly understood. We have therefore analysed the interactions of oceanic plateaus with the leading edge of the continental margin using a thermomechanical–petrological model of an oceanic-continental subduction zone with spontaneously moving plates. This model includes partial melting of crustal and mantle lithologies and accounts for complex rheological behaviour including viscous creep and plastic yielding. Our results indicate that oceanic plateaus may either be lost by subduction or accreted onto continental margins. Complete subduction of oceanic plateaus is common in models with old (> 40 Ma) oceanic lithosphere whereas models with younger lithosphere often result in terrane accretion. Three distinct modes of terrane accretion were identified depending on the rheological structure of the lower crust and oceanic cooling age: frontal plateau accretion, basal plateau accretion and underplating plateaus.Complete plateau subduction is associated with a sharp uplift of the forearc region and the formation of a basin further landward, followed by topographic relaxation. All crustal material is lost by subduction and crustal growth is solely attributed to partial melting of the mantle.Frontal plateau accretion leads to crustal thickening and the formation of thrust and fold belts, since oceanic plateaus are docked onto the continental margin. Strong deformation leads to slab break off, which eventually terminates subduction, shortly after the collisional stage has been reached. Crustal parts that have been sheared off during detachment melt at depth and modify the composition of the overlying continental crust.Basal plateau accretion scrapes oceanic plateaus off the downgoing slab, enabling the outward migration of the subduction zone. New incoming oceanic crust underthrusts the fractured terrane and forms a new subduction zone behind the accreted terrane. Subsequently, hot asthenosphere rises into the newly formed subduction zone and allows for extensive partial melting of crustal rocks, located at the slab interface, and only minor parts of the former oceanic plateau remain unmodified.Oceanic plateaus may also underplate the continental crust after being subducted to mantle depth. (U)HP terranes are formed with peak metamorphic temperatures of 400–700 °C prior to slab break off and subsequent exhumation. Rapid and coherent exhumation through the mantle along the former subduction zone at rates comparable to plate tectonic velocities is followed by somewhat slower rates at crustal levels, accompanied by crustal flow, structural reworking and syndeformational partial melting. Exhumation of these large crustal volumes leads to a sharp surface uplift.     

Late Cenozoic uplift of the eastern United States revealed by fluvial sequences of the Susquehanna and Ohio systems: coupling between surface processes and lower-crustal flow

Fluvial and karstic data sets indicating Late Cenozoic surface uplift in the eastern United States are modelled for the first time using a technique, incorporating coupling between surface processes and flow in the lower continental crust, which has been extensively used for modelling similar data sets elsewhere in the world, notably in Europe. Distinct phases of lower-crustal flow forcing, starting in the early Middle Miocene, Late Pliocene, and late Early Pleistocene, are evident, as observed elsewhere, and relate to combinations of cyclic surface loading (by sea-level variations and ice loads) and to variations in regional erosion rates, as elsewhere. However, the detailed uplift histories inferred are rather different from those indicated in many other regions, notably Europe, primarily because of different properties of the crust. In particular, in the Late Proterozoic/Phanerozoic crust of the Appalachians, the mobile lower-crustal layer seems to be relatively thick, causing a prolonged uplift response for each phase of lower-crustal flow forcing, whereas in the Early Proterozoic crust of the Yavapai crustal province farther west, underlain by a thick basal mafic layer, this mobile layer is much thinner, leading to a very different response consisting of abrupt alternations of uplift and subsidence, as is also observed in other regions of Early Proterozoic crust. Another important difference relative to western and central Europe is the much smaller number of terraces in the eastern US rivers that have been studied. The general applicability of this type of physics-based modelling technique is thus confirmed.     

Lithospheric Thermal Isostasy of North Continental Margin of the South China Sea

Accompanied with rifting and detaching of the north continental margin of the South China Sea,the ernst and the lithosphere become thinner away from the continental margin resulting from the tectonic activities,such as tensile deformation,thermal uplift,and cooling subsidence,etc..Integrated with thermal,gravimetric,and isostatic analysis techniques,based on the seismic interpretation of the deep penetration seismic soundings across the northern margin of the South China Sea,we reconstructed the lithospheric thermal structure and derived the variation of the crust boundary in the east and west parts of the seismic profde by using gravity anomaly data.We mainly studied the thermal isostasy problems using the bathymetry of the profiles and calculated the crust thinning effect due to the thermal variety in the rifting process.The results Indicate that the thermal isostasy may reach 2.5 kin,and the compositional variations in the ilthospheric density and thickness may produce a variation of 4.0 kin.Therefore,the compositional isostatic correction is very important to recover the relationship between surface heat flow and topography.Moreover,because of the high heat flow characteristic of the continental margin,building the model of lithospheric geotherm in this region is of great importan for studying the Cenozoic tectonic thermal evolution of the north passive continental margin of the South China Sea.     

Recent crustal uplift of Precambrian cratons: Key patterns and possible mechanisms

Precambrian cratons cover about 70% of the total continental area. According to a large volume of geomorphological, geological, paleontological, and other data for the Pliocene and Pleistocene, these cratons have experienced a crustal uplift from 100-200 m to 1000-1500 m, commonly called the recent or Neotectonic uplift. Shortening of the Precambrian crust terminated half a billion years ago or earlier, and its uplift could not have been produced by this mechanism. According to the main models of dynamic topography in the mantle, the distribution of displacements at the surface is quite different from that of the Neotectonic movements. According to seismic data, there is no magmatic underplating beneath most of the Precambrian cratons. In most of cratonic areas, the mantle lithosphere is very thick, which makes its recent delamination unlikely. Asthenospheric replacement of the lower part of the mantle lithosphere beneath the Precambrian cratons might have produced only a minor part of their Neotectonic uplifts. Since the above mechanisms cannot explain this phenomenon, the rock expansion in the crustal layer is supposed to be the main cause of the recent uplift of Precambrian cratons. This is supported by the strong lateral nonuniformity of the uplift, which indicates that expansion of rocks took place at a shallow depth. Expansion might have occurred in crustal rocks that emerged from the lower crust into the middle crust with lower pressure and temperature after the denudation of a thick layer of surface rocks. In the dry state, these rocks can remain metastable for a long time. However, rapid metamorphism accompanied by expansion of rocks can be caused by infiltration of hydrous fluids from the mantle. Analysis of phase diagrams for common crustal rocks demonstrates that this mechanism can explain the recent crustal uplift of Precambrian cratons.     

The final rifting evolution at deep magma-poor passive margins from Iberia-Newfoundland: a new point of view

In classical rift models, deformation is either uniformly distributed leading to symmetric fault bounded basins overlying stretched ductile lower crust (e.g. pure shear McKenzie model) or asymmetric and controlled by large scale detachment faulting (simple shear Wernicke model). In both cases rifting is considered as a mono-phase process and breakup is instantaneous resulting in the juxtaposition of continental and oceanic crust. The contact between these two types of crusts is often assumed to be sharp and marked by a first magnetic anomaly; and breakup is considered to be recorded as a major, basin wide unconformity, also referred to as breakup unconformity. These classical models, are currently challenged by new data from deep rifted margins that ask for a revision of these concepts. In this paper, we review the pertinent observations made along the Iberia-Newfoundland conjugate margins, which bear the most complete data set available from deep magma-poor margins. We reevaluate and discuss the polyphase nature of continental rifting, discuss the nature and significance of the different margin domains and show how they document extreme crustal thinning, retardation of subsidence and a complex transition into seafloor spreading. Although our study is limited to the Iberia-Newfoundland margins, comparisons with other margins suggest that the described evolution is probably more common and applicable for a large number of rifted margins. These new results have major implications for plate kinematic reconstructions and invite to rethink the terminology, the processes, and the concepts that have been used to describe continental rifting and breakup of the lithosphere.     

Late Cretaceous–Cenozoic evolution of the North German Basin—results from 3-D geodynamic modelling

The Late Cretaceous–Cenozoic evolution of the North German Basin has been investigated by 3-D thermomechanical finite element modelling. The model solves the equations of motion of an elasto-visco-plastic continuum representing the continental lithosphere. It includes the variations of stress in time and space, the thermal evolution, surface processes and variations in global sea level.The North German Basin became inverted in the Late Cretaceous–Early Cenozoic. The inversion was most intense in the southern part of the basin, i.e. in the Lower Saxony Basin, the Flechtingen High and the Harz. The lower crustal properties vary across the North German Basin. North of the Elbe Line, the lower crust is dense and has high seismic velocity compared to the lower crust south of the Elbe Line. The lower crust with high density and high velocity is assumed to be strong. Lateral variations in lithospheric strength also arise from lateral variations in Moho depth. In areas where the Moho is deep, the upper mantle is warm and the lithosphere is thereby relatively weak.Compression of the lithosphere causes shortening, thickening and surface uplift of relatively weak areas. Tectonic inversion occurs as zones of preexisting weakness are shortened and thickened in compression. Contemporaneously, the margins of the weak zone subside. Cenozoic subsidence of the northern part of the North German Basin is explained as a combination of thermal subsidence and a small amount of deformation and surface uplift during compression of the stronger crust in the north.The modelled deformation patterns and resulting sediment isopachs correlate with observations from the area. This verifies the usefulness and importance of thermomechanical models in the investigation of intraplate sedimentary basin formation.     

大型花岗岩基形成和演化的深部动力学过程：滴水构造、钾质火山作用与地表地质过程

以美国内华达山脉复合岩基为例,系统评述了与大型花岗岩基的形成、演化相关的深部地球动力学过程及构造地貌学响应.在大陆岛弧环境下,基性岩浆的底侵作用促使下地壳发生角闪岩脱水部分熔融,在岩基根部形成高密度的石榴辉石岩,岩基根部最终发生重力失稳,形成滴水构造;在地貌上反映为滴水构造对应区域的沉降和相应的张性构造,在岩浆作用上则表现为软流圈地幔上涌和残余富集岩石圈地幔的低程度部分熔融,形成钾质火山岩.这种高度动态的深部动力学过程是维持大型花岗岩基地区较高高程或促使这些区域高程骤然增加的重要因素.     

Plate and intraplate processes of Hercynian Europe during the late paleozoic

It is speculated that until Late Carboniferous time the region of Hercynian Europe was occupied by an elongated island arc system underlain by a segment of continental crust. In the Upper Carboniferous, two subduction zones are assumed to have extended from the north and south beneath Hercynian Europe. An extensive zone of hot, partially molten upper mantle lay above and between these, and diapiric uprise of portions of this material led to separation of mafic magmas, widespread partial melting in the lower and middle crust, high temperature-low pressure metamorphism in crustal rocks, and regional uplift and extension of the crust, as indicated by intermontane troughs and their associated volcanic rocks.In Visean to Westphalian time Hercynian Europe collided with both the large neighbouring plates North America-Europe and Africa. During these diachronous collisions and owing to reduced rigidity of the relatively hot island arc crust, the irregular continental margins of the larger and thicker continental plates induced oroclinal bending of Hercynian Europe. After the collision processes had been terminated, processes of upper mantle activity continued, causing further crustal uplift and even, enhanced crustal extension for several tens of million years into the Lower Permian. Decline of the upper mantle activity beneath Hercynian Europe is indicated by crustal subsidence and formation of a peneplain in Permian time followed by the Upper Permian transgression of both the Zechstein sea and the Tethys sea which mark the end of the Hercynian geodynamic cycle.     

半地堑盆地演化机制的粘弹塑数值模拟

利用新近完成的粘弹塑构造模拟软件包对盆地的动力学演化进行了一系列模拟。文中主要概述不同厚度的上地壳中由高角度平面正断层界定的半地堑盆地的演化模拟。模拟时上地壳被考虑成具有Byerlee型强度包络 ,并且位于无粘性基底之上 ,盆地中由密度比地壳密度小的沉积物充填。计算了以一定增量逐渐拉伸上地壳层时各个阶段的非静岩应力 (Nonlitho staticstress)、塑性破裂 (Plasticfailure)分布及挠曲剖面 (Flexureprofile)。塑性变形使得有效弹性厚度减小。到切穿破裂出现以前 ,断层断距一直增加 ,之后 ,断距基本停止增加。所以 ,地壳强度使沉降量和隆升量均有极限。上地壳层厚度和沉积物密度是控制盆地宽度和极限深度的两个重要因素 ,上地壳层厚度增加或者沉积物密度加大都使盆地宽度和深度增大。模拟结果可以解释一些大陆裂谷盆地的宽度和沉积深度。     

Paleolithospheric structure revealed by continental geoid anomalies

Lithospheric geoid anomalies record changes in elevation and potential energy experienced by continental lithosphere. Estimates of local isostatic equilibration and potential energy, in tandem with lithosphere-related geoid anomalies, can be used to estimate paleolithospheric thickness, providing a clearer understanding of how and why continental topography is developed. We employ several simplifying assumptions about the crustal and mantle lithosphere density and structure (and readily acknowledge that our results are therefore first-order approximations) to predict the pre-orogenic structure of the lithosphere. At the outset we emphasize that while this approach does not provide an exhaustive evaluation of the deformation mechanism, it does serve to quantify the relative role played by the variations in the crustal and upper mantle components of the lithosphere. In this way we are able to use independent measurement of lithospheric geoid anomalies, current (post-orogenic) elevation and lithospheric structure, and paleoelevation information to estimate topographic development and structural support over time. Application of this technique to the southwestern United States indicates that the uplift of the Colorado Plateau is the result of processes in both the crust and mantle lithosphere and that the lithosphere of the pre-orogenic Southern Basin and Range was thinned relative to the Northern Basin and Range and Colorado Plateau. Although we use the southwestern U.S. as an example, this method can help constrain uplift mechanisms for any region for which the structure and geoid anomaly of the modern lithosphere is well understood.     

Two layer model of lithospheric compression and uplift/exhumation in an intracratonic setting: an example from the Cooper–Eromanga Basins,Australia

Seismic reflection profiles indicate the compressive nature of the structural style associated with the major uplift events in the Cooper–Eromanga Basins. Inversion geometries and reactivated features attest to a period of compression during Late Triassic–Early Jurassic times. In the Eromanga Basin, compressional structural styles associated with Late Cretaceous–Tertiary are apparent. Many of the Late Cretaceous–Tertiary structures coincide with exhumation highs in Late Cretaceous–Tertiary times. The two-layer lithospheric compression model is considered as the most complete explanation of both the uplift of areas subject to compression and crustal thickening, and of the regional uplift of areas not subject to any apparent Late Cretaceous–Tertiary compression. In the model, compression and thickening in the lower lithosphere is decoupled and laterally displaced from that in the upper crust. Thickening of the mantle lithosphere without thickening of the overlying crust can account for the initial subsidence then uplift of not inverted platform areas. The opening of the Tasman Sea and the Coral Seas can lead to stress transmission in the interior of the continent. These stresses are likely to generate uplift but cannot explain the distribution of uplift in areas not subject to compression.     

Numerical modeling of eastern Tibetan-type margin: Influences of surface processes,lithospheric structure and crustal rheology

The eastern Tibetan margin is characterized by a steep topographic gradient and remarkably lateral variations in crustal/lithospheric structure and thermal state. GPS measurements show that the surface convergence rate in this area is strikingly low. How can such a mountain range grow without significant upper crustal shortening? In order to investigate the formation mechanism of the eastern Tibetan-type margins, we conducted 2D numerical simulations based on finite difference and marker-in-cell techniques. The numerical models were constrained with geological and geophysical observations in the eastern Tibetan margin. Several major parameters responsible for topography building, such as the convergence rate, the erosion/sediment rate, and the presence of partially molten crust, were systematically examined. The results indicate that the presence of partially molten material in the middle/lower crust can make a positive contribution to the formation of steep topography, but it is not a necessary factor. A steep topographic gradient may be a characteristic feature when a thin lithosphere with thick crust converges with a thick lithosphere with thin crust. In the context of a high erosion rate, the Longmen Shan range still gains and maintains its steep high topography to the present. This could be explained by exerting a large push force on Tibet side. Our numerical experiments suggest that topographic characteristic across the eastern Tibetan-type margins is mainly derived from isostatic equilibration forces and intensive convergence between two continental lithospheres with totally different rheological properties.     

Formation of sedimentary basins of graben type by extension of the continental crust

A mechanism for causing graben-like subsidence by crustal stretching in response to tension is suggested, based partly on previous hypotheses of Vening Meinesz, Artemjev and Artyushkov, Bott and Fuchs. The mechanism requires rheological subdivision of the crust into a brittle upper layer about 10–20 km thick overlying a ductile lower crust. The brittle layer responds to tension by normal faulting and wedge subsidence; the ductile layer responds by local or regional thinning and by lateral flow of material from beneath the subsiding wedge causing complementary uplift by horst formation or elastic upbending. A graben width of between 30 and 60 km is predicted in absence of basement inhomogeneity. Computations of the energy budget indicate that sedimentary basins of more than 5 km thickness can form by the mechanism provided that water pressure reduces the friction on the faults. The mechanism can explain relatively rapid beginning and end of subsidence, and spasmodic sinking may occur. A wide variety of observed graben-like basins can be explained by the hypothesis, including classical rift valleys and the Mesozoic basins of UK and the North Sea, but it is inapplicable to broad unfaulted cratonic or shelf subsidence.     

How Alpine or Himalayan are the Central Andes?

 Although non-collisional mountain belts, such as the Andes, and collisional mountain belts, such as the Alps and the Himalayas–Tibet, have been regarded as fundamentally different, the Central Andes share several features with the Himalayas–Tibet. The most important of these are extremely thickened (≥70 km) continental crustal roots supporting high plateaus and mountain fronts characterized by large basement thrusts. The main prerequisite for very thick crustal roots and extreme mountainous topography appears to be large-scale underthrusting of continental crust of normal thickness, irrespective of whether the crustal thrusts are antithetic with respect to subduction as in the Andes, or synthetic with respect to preceding subduction of oceanic lithosphere as in the Himalayas. In both cases sole thrusts near the base of the continental crust nucleated in thermally anomalous zones of the hinterland and then propagated across ramps into shallower detachments located within thick sedimentary or metasedimentary cover rocks. In contrast to the Central Andes and the Himalayas, the Alps are characterized by intracrustal detachment which allowed both the subduction of lower crust and a stacking of relatively thin upper crustal slivers, which make up a narrow mountain chain with a more subdued topography. Received: 10 August 1998 / Accepted: 1 March 1999     

The gravity field of the U.S. Atlantic continental margin

Approximately 39,000 km of marine gravity data collected during 1975 and 1976 have been integrated with U.S. Navy and other available data over the U.S. Atlantic continental margin between Florida and Maine to obtain a 10 mgal contour free-air gravity anomaly map. A maximum typically ranging from 0 to +70 mgal occurs along the edge of the shelf and Blake Plateau, while a minimum typically ranging from −20 to −80 mgal occurs along the base of the continental slope, except for a −140 mgal minimum at the base of the Blake Escarpment. Although the maximum and minimum free-air gravity values are strongly influenced by continental slope topography and by the abrupt change in crustal thickness across the margin, the peaks and troughs in the anomalies terminate abruptly at discrete transverse zones along the margin. These zones appear to mark major NW—SE fractures in the subsided continental margin and adjacent deep ocean basin, which separate the margin into a series of segmented basins and platforms. Rapid differential subsidence of crustal blocks on either side of these fractures during the early stages after separation of North America and Africa (Jurassic and Early Cretaceous) is inferred to be the cause of most of the gravity transitions along the length of margin. The major transverse zones are southeast of Charleston, east of Cape Hatteras, near Norfolk Canyon, off Delaware Bay, just south of Hudson Canyon and south of Cape Cod.Local Airy isostatic anomaly profiles (two-dimensional, without sediment corrections) were computed along eight multichannel seismic profiles. The isostatic anomaly values over major basins beneath the shelf and rise are generally between −10 and −30 mgal while those over the platform areas are typically 0 to +20 mgal. While a few isostatic anomaly profiles show local 10–20 mgal increases seaward of the East Coast Magnetic Anomaly (ECMA: inferred to mark the ocean-continent boundary), the lack of a consistent correlation indicates that the relationship of isostatic gravity anomalies to the magnetic anomalies and the ocean—continent transition is variable.Two-dimensional gravity models have been computed for two profiles off Cape Cod, Massachusetts and Cape May, New Jersey, where excellent reflection, refraction and magnetic control appear to define 10 and 12 km deep sedimentary basins beneath the shelf, respectively and 10 km deep basins beneath the rise. The basins are separated by a 6–8 km deep basement ridge which underlies the ECMA and appears to mark the landward edge of oceanic crust. The gravity models suggest that the oceanic crust is between 11 and 18 km thick beneath the ECMA, but decreases to a thickness of less than 8 km within the first 20–90 km to the southeast. In both profiles, the derived crustal thickness variations support the interpretation that the ECMA occurs over the ocean-continent boundary. The crust underlying the sedimentary cover appears to be 12 to 15 km thick on the landward side of the ECMA and gradually thickens to normal continental values of greater than 25 km within the first 60 to 110 km to the northwest. Multichannel seismic profiles across platform areas, such as Cape Hatteras and Cape Cod, indicate the ocean-continent transition zones there are much narrower than profiles across major sedimentary basins, such as the one off New Jersey.     

Crustal Uplift in the Longmen Shan Mountains Revealed by Isostatic Gravity Anomalies along the Eastern Margin of the Tibetan Plateau

This study examines the relationship between high positive isostatic gravity anomalies (IGA), steep topography and lower crustal extrusion at the eastern margin of the Tibetan Plateau. IGA data has revealed uplift and extrusion of lower crustal flow in the Longmen Shan Mountains (the LMS). Firstly, The high positive IGA zone corresponds to the LMS orogenic belt. It is shown that abrupt changes in IGA correspond to zones of abrupt change of topography, crustal thickness and rock density along the LMS. Secondly, on the basis of the Airy isostasy theory, simulations and inversions of the positive IGA were conducted using three-dimensional bodies. The results indicated that the LMS lacks a mountain root, and that the top surface of the lower crust has been elevated by 11 km, leading to positive IGA, tectonic load and density load. Thirdly, according to Watts’s flexural isostasy model, elastic deflection occurs, suggesting that the limited (i.e. narrow) tectonic and density load driven by lower crustal flow in the LMS have led to asymmetric flexural subsidence in the foreland basin and lifting of the forebulge. Finally, based on the correspondence between zones of extremely high positive IGA and the presence of the Precambrian Pengguan-Baoxing complexes in the LMS, the first appearance of erosion gravels from the complexes in the Dayi Conglomerate layer of the Chengdu Basin suggest that positive IGA and lower crustal flow in the LMS took place at 3.6 Ma or slightly earlier.