Variations In Lithospheric Strength Research Articles

Lithospheric- and crustal-scale controls on variations in foreland basin development in the Northern Alpine Foreland Basin

The Northern Alpine Foreland Basin (i.e., Molasse Basin) developed due to flexural subsidence from slab- and topographic loading during continental collision between the Adriatic and European plates. Previous studies highlight a diachronous transition from underfilled- to overfilled conditions in the Molasse Basin in response to orogen-parallel variations in flexural subsidence and sediment supply. In this contribution, we investigate the lithospheric- and crustal-scale orogenic process(es) that generated this diachronous transition. For this, we conducted a tectonostratigraphic analysis of the Molasse Basin. Firstly, we constructed a 3D geological model to derive the architecture of the European margin during the Eocene onset of flexure. Second, 2D/3D reflection seismic- and borehole data were used to characterise the spatiotemporal development of depositional environments and syn-flexural normal fault kinematics in the German Molasse. Our data show a stepwise rather than continuous eastward migration of the underfilled- to overfilled transition. Furthermore, syn-flexural fault kinematics document a Rupelian to early Burdigalian northward younging trend and higher cumulative Cenozoic offsets in the Eastern German Molasse (< 230 m) compared to the Western German Molasse (< 150 m). This implies a west-to-east increase in the curvature of bending of the European plate, induced by along-strike variations in the magnitude of applied loads and/or European lithospheric strength variations. These variations drove lateral variations in accommodation space and sediment supply. Subsequently, this led to the diachronous underfill-to-overfilled transition in the Molasse Basin. Taken together, we suggest that the diachronous transition was driven by spatiotemporal variations in the thickening of the orogenic wedge supported by slab detachment, promoted by subduction- and collision of the irregular European margin.

The stability of cratons is controlled by lithospheric thickness, as evidenced by Rb-Sr overprint ages in granitoids

The ancient cores of modern continents, cratons, are the oldest blocks of “stable” lithosphere on Earth. Their long-term survival relies on the resistance of their underlying thick, strong, and buoyant mantle keels to subsequent recycling. However, the effect of substantial geographical variations in keel thickness on the post-assembly behaviour and mass movement within these continental cores remains unknown. Here, we demonstrate that the spatial distribution of fluid-reset in-situ Rb-Sr ages for Paleo-Mesoarchean (3.6–2.8 billion years ago; Ga) granitoids of the Pilbara Craton, Australia shows remarkable correlation with independently-constrained lithospheric thickness models. Without craton-wide heating/magmatic events, these anomalously young Rb-Sr ages document episodes of fluid infiltration into granitoid complexes as a response to lithospheric reactivation by far-field stresses. This correlation implies that craton-wide fluid mobilization triggered by extra-cratonic Neoarchean to Mesoproterozoic (2.8–1.0 Ga) tectonic events is facilitated by variations in lithospheric strength and thickness. Compared to areas of older overprints, the two-thirds of the craton comprised of younger reset ages is underlain by comparatively thin lithosphere with higher susceptibility to reactivation-assisted fluid flow. We propose that even the strongest, most pristine cratons are less stable and impermeable than previously thought, as demonstrated by the role of granitoid complexes and cratons as selective lithospheric “sponges” in response to minor tectonic forces. Therefore, variations in lithospheric thickness, likely attained before cratonization, exert a crucial control on billions of years of fluid movement, elemental redistribution and mineralization within ancient continental nuclei.

Evidence for Stress Localization Caused by Lithospheric Heterogeneity From Seismic Attenuation

AbstractThe Wyoming Craton underwent tectonic modifications during the Laramide Orogeny, which resulted in a series of basement‐cored uplifts that built the modern‐day Rockies. The easternmost surface expression of this orogeny ‐ the Black Hills in South Dakota ‐ is separated from the main trend of the Rocky Mountains by the southern half of the Powder River Basin, which we refer to as the Thunder Basin. Seismic tomography studies reveal a high‐velocity anomaly which extends to a depth of ∼300 km below the basin and may represent a lithospheric keel. We constrain seismic attenuation to investigate the hypothesis that variations in lithospheric thickness resulted in the localization of stress and therefore deformation. We utilize data from the CIELO seismic array, a linear array that extends from east of the Black Hills across the Thunder Basin and westward into the Owl Creek Mountains, the BASE FlexArray deployment centered on the Bighorn Mountains, and the EarthScope Transportable Array. We analyze seismograms from deep teleseismic events and compare waveforms in the time‐domain to characterize lateral variations in attenuation. Bayesian inversion results reveal high attenuation in the Black Hills and Bighorn Mountains and low attenuation in the Thunder and Bighorn Basins. Scattering is rejected as a confounding factor because of a strong anticorrelation between attenuation and the amplitude of P wave codas. The results support the hypothesis that lateral variations in lithospheric strength, as evidenced by our seismic attenuation measurements, played an important role in the localization of deformation and orogenesis during the Laramide Orogeny.

Lithospheric strength variations and seismotectonic segmentation below the Sea of Marmara

The Sea of Marmara is a tectonically active basin that straddles the North Anatolian Fault Zone (NAFZ), a major strike-slip fault that separates the Eurasian and Anatolian tectonic plates. The Main Marmara Fault (MMF), which is part of the NAFZ, contains an approximately 150 km long seismotectonic segment that has not ruptured since 1766. A key question for seismic hazard and risk assessment is whether or not the next rupture along this segment is likely to produce one major earthquake or a series of smaller earthquakes. Geomechanical characteristics such as along-strike variations in rock strength may provide an important control on seismotectonic segmentation. We find that variations in lithospheric strength throughout the Marmara region control the mechanical segmentation of the MMF and help explain its long-term seismotectonic segmentation. In particular, a strong crust that is mechanically coupled to the upper mantle spatially correlates with aseismic patches, where the MMF bends and changes its strike in response to the presence of high-density lower crustal bodies. Between the bends, mechanically weaker crustal domains that are decoupled from the mantle indicate a predominance of creeping. These results are highly relevant for the ongoing debate regarding the characteristics of the Marmara seismic gap, especially in view of the seismic hazard (Mw > 7) in the densely populated Marmara region.

Seismic Strain Rate and Flexure at the Hawaiian Islands Constrain the Frictional Coefficient

AbstractFlexure occurs on intermediate geologic timescales (∼1 Myr) due to volcanic‐island building at the Island of Hawaii, and the deformational response of the lithosphere is simultaneously elastic, plastic, and ductile. At shallow depths and low temperatures, elastic deformation transitions to frictional failure on faults where stresses exceed a threshold value, and this complex rheology controls the rate of deformation manifested by earthquakes. In this study, we estimate the seismic strain rate based on earthquakes recorded between 1960 and 2019 at Hawaii, and the estimated strain rate with 10−18–10−15 s−1 in magnitude exhibits a local minimum or neutral bending plane at 15 km depth within the lithosphere. In comparison, flexure and internal deformation of the lithosphere are modeled in 3D viscoelastic loading models where deformation at shallow depths is accommodated by frictional sliding on faults and limited by the frictional coefficient (μf), and at larger depths by low‐temperature plasticity and high‐temperature creep. Observations of flexure and the seismic strain rate are best‐reproduced by models with μf = 0.3 ± 0.1 and modified laboratory‐derived low‐temperature plasticity. Results also suggest strong lateral variations in the frictional strength of faults beneath Hawaii. Our models predict a radial pattern of compressive stress axes relative to central Hawaii consistent with observations of earthquake pressure (P) axes. We demonstrate that the dip angle of this radial axis is essential to discerning a change in the curvature of flexure, and therefore has implications for constraining lateral variations in lithospheric strength.

Cenozoic intraplate tectonics in Central Patagonia: Record of main Andean phases in a weak upper plate

Contraction in intraplate areas is still poorly understood relative to similar deformation at plate margins. In order to contribute to its comprehension, we study the Patagonian broken foreland (PBF) in South America whose evolution remains controversial. Time constraints of tectonic events and structural characterization of this belt are limited. Also, major causes of strain location in this orogen far from the plate margin are enigmatic. To unravel tectonic events, we studied the Cenozoic sedimentary record of the central sector of the Patagonian broken foreland (San Bernardo fold and thrust belt, 44°30′S-46°S) and the Andes (Meseta de Chalia, 46°S) following an approach involving growth-strata detection, U-Pb geochronology and structural modeling. Additionally, we elaborate a high resolution analysis of the effective elastic thickness (Te) to examine the relation between intraplate contraction location and variations in lithospheric strength. The occurrence of Eocene growth-strata (~44–40Ma) suggests that contraction in the Andes and the Patagonian broken foreland was linked to the Incaic phase. Detection of synextensional deposits suggests that the broken foreland collapsed partially during Oligocene to early Miocene. During middle Miocene times, the Quechua contractional phase produced folding of Neogene volcanic rocks and olistostrome deposition at ~17Ma. Finally, the presented Te map shows that intraplate contraction related to Andean phases localized preferentially along weak lithospheric zones (Te<15km). Hence, the observed strain distribution in the PBF appears to be controlled by lateral variations in the lithospheric strength. Variations in this parameter could be related to thermo-mechanical weakening produced by intraplate rifting in Paleozoic-Mesozoic times.

Lithospheric strength variations in Mainland China: Tectonic implications

AbstractWe present a new thermal and strength model for the lithosphere of Mainland China. To this purpose, we integrate a thermal model for the crust, using a 3‐D steady state heat conduction equation, with estimates for the upper mantle thermal structure, obtained by inverting a S wave tomography model. With this new thermal model and assigning to the lithospheric layers a “soft” and “hard” rheology, respectively, we estimate integrated strength of the lithosphere. In the Ordos and the Sichuan basins, characterized by intermediate temperatures, strength is primarily concentrated in the crust, when the rheology is soft, and in both the crust and upper mantle, when the rheology is hard. In turn, the Tibetan Plateau and the Tarim basin have a weak and strong lithosphere mainly on account of their high and low temperatures, respectively. A comparison of temperatures, strength, and effective viscosity variations with earthquakes distribution and their seismic energy released indicates that both the deep part of the crust and the upper mantle of the Tibetan Plateau are weak and prone to flow toward adjacent areas. The high strength of some of the tectonic domains surrounding Tibet (Tarim, Ordos, and Sichuan basins) favors the flow toward the weak western part of South China block.

Rheology of the lithosphere beneath the central and western Tien Shan

AbstractThe distribution of crustal velocity over a 800 by 400 km region of the Tien Shan shows strain rates with principal contractional axes aligned perpendicular to the mountain chain and with negligible velocity gradients in the along‐strike direction. This configuration allows us to describe the dynamics by a force balance along profiles perpendicular to the chain. The velocities on each of these profiles fit, with root‐mean‐square misfits of ∼1 mm/yr, a constant rate of contractional strain. Gravitational potential energy per unit area varies along these profiles by at least 3 and maybe 5 TN m−1; the insensitivity of contractional strain rates to those variations implies a lower bound of ∼1022 Pa s on the effective viscosity of the lithosphere. Seismic tomography shows variations in P and S wave speeds that imply large lateral temperature variations in crust and upper mantle of the region. If the ductile portion of the lithosphere were deforming by the mechanisms of high‐temperature creep, such temperature variations would be accompanied by variations in strength of 1 or 2 orders of magnitude. The observed velocity profiles allow variations in lithospheric strength by no more than a factor of 2 to 4, implying that the strength‐controlling portion of the lithosphere cannot be strongly sensitive to temperature. Lithospheric strength profiles that incorporate flow laws for low‐temperature plasticity of olivine reproduce both the effective viscosity and the insensitivity to lateral temperature variation that are observed in the Tien Shan. Plastic deformation of dry, pyroxene‐rich lower crust may also contribute to such a temperature‐insensitive strength profile.

On the robustness of estimates of mechanical anisotropy in the continental lithosphere: A North American case study and global reanalysis

Lithospheric strength variations both influence and are influenced by many tectonic processes, including orogenesis and rifting cycles. The long, complex, and highly anisotropic histories of the continental lithosphere might lead to a natural expectation of widespread mechanical anisotropy. Anisotropy in the coherence between topography and gravity anomalies is indeed often observed, but whether it corresponds to an elastic thickness that is anisotropic remains in question. If coherence is used to estimate flexural strength of the lithosphere, the null-hypothesis of elastic isotropy can only be rejected when there is significant anisotropy in both the coherence and the elastic strengths derived from it, and if interference from anisotropy in the data themselves can be plausibly excluded. We consider coherence estimates made using multitaper and wavelet methods, from which estimates of effective elastic thickness are derived. We develop a series of statistical and geophysical tests for anisotropy, and specifically evaluate the potential for spurious results with synthetically generated data. Our primary case study, the North American continent, does not exhibit meaningful anisotropy in its mechanical strength. Similarly, a global reanalysis of continental gravity and topography using multitaper methods produces only scant evidence for lithospheric flexural anisotropy.

Thrust belts of the southern Central Andes: Along-strike variations in shortening, topography, crustal geometry, and denudation

The Andean fold-and-thrust belts of west-central Argentina (33°S and 36°S), above the normal subduction segment, present important along-strike variations in mean topographic uplift, structural elevation, amount and rate of shortening, and crustal root geometry. To analyze the controlling factors of these latitudinal changes, we compare these parameters and the chronology of deformation along 11 balanced crustal cross sections across the thrust belts between 70°W and 69°W, where the majority of the upper-crustal deformation is concentrated, and reconstruct the Moho geometry along the transects. We propose two models of crustal deformation: a 33°40′S model, where the locus of upper-crustal shortening is aligned with respect to the maximum crustal thickness, and a 35°40′S model, where the upper-crustal shortening is uncoupled from the lower-crustal deformation and thickening. This degree of coupling between brittle upper crust and ductile lower crust deformation has strong influence on mean topographic elevation. In the northern sector of the study area, an initial thick and felsic crust favors the coupling model, while in the southern sector, a thin and mafic lower crust allows the uncoupling model. Our results indicate that interplate dynamics may control the overall pattern of tectonic shortening; however, local variations in mean topographic elevation, deformation styles, and crustal root geometry are not fully explained and are more likely to be due to upper-plate lithospheric strength variations.

Lateral variation of the strength of lithosphere across the eastern North China Craton: New constraints on lithospheric disruption

The structure and rheology of the lithosphere can provide constraints on our understanding of the geodynamic processes in the Earth's interior. The lateral heterogeneity of the strength and rheological structure of the lithosphere are closely correlated with the lateral variation in the thickness of the seismogenic layer. It is still unclear how geodynamic process(es) are linked with this lateral variation in the geophysical properties of the lithosphere. In order to understand the possible links between geodynamical process and lateral variation of lithosphere strength, we compiled crustal velocity and density structure, the elastic thickness of the lithosphere, and the temperature field, and then construct a northeast–southwest transect of the lithosphere across the eastern part of the North China Craton (NCC) with an overall length of 1660km. Along the lithosphere-scale transect, we observe that (a) a thinner (80–120km) Archean lithosphere at the North China Plain (NCP), (b) a lateral variation of the seismogenic brittle layer, which is about 15km thick beneath the Qinling-Dabie orogenic belt (QDOB) and the Yanshan orogenic belt (YSOB), and about 25km thick under the NCP. Together, these characteristics represent an excellent illustration of the scale of geodynamic processes necessary to accommodate lateral variations in lithospheric strength and the contribution of these to the mechanism of disruption of the Archean NCC. The thick seismogenic layer beneath the NCP can be formed due to: (1) deeper fault penetration beneath the NCP than beneath the QDOB and the YSOB; (2) the thick brittle layer is preserved after delamination of the lower part of the thickened crust; (3) thickening by changes to brittleness of the brittle/ductile transition layer (BDTL). With constraints of above mentioned geophysical properties of the lithosphere, we propose that (1) only the first and third processes can successfully explain the lateral variation of the thickness of the seismogenic layer across the eastern part of the NCC, and (2) the extensional tectonics and thermal erosion (notwithstanding the delamination of the lithosphere) play their significant roles in the disruption of the NCC.

Modification of the lithospheric stress field by lateral variations in plate‐mantle coupling

The presence of deeply penetrating continental roots may locally increase the magnitude of basal shear tractions by up to a factor of 4 compared to a layered viscosity structure. Here we examine how these increases in mantle‐lithosphere coupling influence stress patterns in the overlying elastic lithosphere. By coupling a mantle flow model to a model for the elastic lithosphere, we show that the amplification of mantle tractions beneath cratons increases elastic stress magnitudes by at most a factor of only 1.5 in a pattern not correlated to local basal traction changes. This disconnect is explained by the transmission of elastic stresses across large distances, which makes them sensitive to regionally‐averaged changes in basal tractions, but not local variations. Our results highlight the importance of regional variations in lithospheric strength, which could allow stress patterns to more closely match regional changes in basal shear.

The effects of a lateral variation in lithospheric strength on foredeep evolution: Implications for the East Carpathian foredeep

Lateral variations in lithospheric strength have been adopted often in flexural modeling (both 2D and 3D) to better fit the observed basement deflections, typically supported by gravity data. This approach provides essentially a “snap-shot” of the role of lithosphere strength in determining the present day geometry. In contrast, we investigate and quantify the effects of a lateral change in lithospheric strength on the evolution of the foredeep in front of an advancing orogen. Transitions in lithospheric strength are common in the foreland of orogens and show large variations in the width of the transition zone and the strength difference. Former passive margins, for instance, will display strength changes distributed over several tens to hundreds of kilometers. Other transitions may originate from juxtaposition or accretion of pieces of lithosphere with different properties and may be characterized by a much smaller width than former passive margins. In our modeling, a constant load, representing an advancing orogenic belt, is displaced towards and across a transition from a weak to a strong plate in a 2D elastic thin plate model. The effect of different transition widths and strength contrasts on foredeep geometry and bending stress is investigated. Interference of flexural wavelengths across the transition affects foredeep geometry by causing rapid basin widening, oscillation of the bulge and volume increase. The bending stresses are found to concentrate and amplify around the strength transition. Large transition gradients, i.e. large strength contrast or small transition width, cause the highest rates of change. Basin widening caused by the orogenic load advancing towards the transition between the East European Craton and the Moesian Platform, appears to control the Sarmatian transgression over the East Carpathian foreland in Romania.

Recovering lateral variations in lithospheric strength from bedrock motion data using a coupled ice sheet‐lithosphere model

A vertically integrated two‐dimensional ice flow model was coupled to an elastic lithosphere‐Earth model to study the effects of lateral variations in lithospheric strength on local bedrock adjustment. We used a synthetic bedrock profile and a synthetic climate to model a characteristic ice sheet through an ice age cycle. Realistic differences in lithospheric strength altered the local bedrock adjustment up to 100 m, the ice extent by tens of kilometers, and the ice volume by several percent. Hence, when modeling ice sheets, it is essential to include information on lithospheric structure. In addition, we used the coupled ice flow–lithosphere model to construct synthetic bedrock motion time series to assess their potential in resolving lithospheric structure. Inverse experiments showed that the model can resolve lateral variations in lithospheric strength from these bedrock motion time series, provided that we have data from both sides of a lateral transition in lithospheric strength. The inversion that solved for a lateral transition was able to find a solution that was consistent with all data, even if they were noisy. In the presence of lateral variations in lithospheric strength, there was no solution to the inverse problem for which all data were modeled correctly by a uniform lithospheric model. The synthetic data showed no significant sensitivity to the location of the transition. Hence we require information from independent sources, such as seismology or gravity, about the locations of transitions in lithospheric strength.

Modes of basin (de)formation, lithospheric strength and vertical motions in the Pannonian-Carpathian system: inferences from thermo-mechanical modelling

Abstract After a rapid multiphase evolution and a transition from passive to active rifting during late Early Miocene to Pliocene times, the Pannonian Basin has been subjected to compressional stresses leading to gradual basin inversion during Quaternary times. Stress modelling demonstrates the significance of the interaction of external plate-boundary forces and the effect of gravitational stresses caused by continental topography and crustal thickness variation. Flexural modelling and fission-track studies have elucidated the complex interplay of flexural downloading during collision, followed by rapid unroofing by unflexing and isostatic rebound of the lithosphere. The stretching and subsidence history of the Pannonian Basin, the temporal and spatial evolution of the flexure of the Carpathian lithosphere, and the lithospheric strength of the region reflect a complex history of this segment of the Eurasia-Africa collision zone. The polyphase evolution of the Pannonian-Carpathian system has resulted in strong lateral and temporal variation in thermo-mechanical properties in the area. Modelling results suggest that, as a whole, the Pannonian Basin has been an area of pronounced lithospheric weakness since Cretaceous time, shedding light on the high degree of strain localization in this region. This basin, the hottest in continental Europe, has a lithosphere of extremely low rigidity, making it prone to multiple tectonic reactivations. Another feature is the noticeable absence of lithospheric strength in the mantle lithosphere of the Pannonian Basin. Modelling studies suggest pronounced lateral variations in lithospheric strength along the Carpathians and their foreland, which have influenced the thrust load kinematics and post-collisional tectonic history. The inferences and models discussed in this paper are constrained by a large geophysical database, including seismic profiles, gravity and heat-flow data.

Influence of lithospheric thickness variations on 3‐D crustal velocities due to glacial isostatic adjustment

Predictions of 3‐D crustal velocities driven by glacial isostatic adjustment (GIA) have generally been based on spherically symmetric Earth models. We adopt a finite‐volume formulation to explore the impact of lateral variations in elastic plate strength, including lithospheric thickness changes across the continent‐ocean interface and plate boundary weak zones, on these predictions. Weak zones introduce horizontal rate perturbations with a plate scale coherency and amplitudes reaching 1–2 mm/yr; radial velocity perturbations can be as large, but are geographically isolated to the weak zones (specifically, the North Atlantic Ridge). A discontinuity in ocean‐continent lithospheric thickness significantly impacts rates along continental margins (order 1 mm/yr for radial rates and generally about half this for tangential rates). We conclude that lateral variations in lithospheric strength should be included in future GIA analyzes of space‐geodetic survey results and in assessing the impact of GIA on the stability of geodetic reference frames.

Role of the 3-D distributions of load and lithospheric strength in orogenic arcs: polystage subsidence in the Carpathians foredeep

It has been widely documented that the depth of foredeeps does not always reflect the topography of the neighboring orogens. In many cases, the topographic load is insufficient to explain basin subsidence. Such is the case of the SE Carpathians where an anomalously deep (almost 13 km) foreland basin has evolved since the Middle Miocene (Badenian). A peculiar feature of this basin is its position relative to the orogen. In contrast to typical foredeeps, which deepen towards the belt, the maximum depth of this basin is 10–20 km out of the orogen. The subsidence in the Carpathians Bend foreland is characterized by two stages: the first is Middle Miocene (Badenian) in age and is related to NE–SW extension when fault-bounded basins were formed. Modeling shows that the foreland underwent small pre-orogenic uniform thinning. The modeling also predicts <100 m post-rift subsidence in accordance with the regional unconformity observed at the Middle/Upper Miocene (Badenian/Sarmatian) boundary. The second subsidence stage follows rifting and is caused by flexural loading of the Carpathians nappes. According to the planform flexural modeling results, the location of depocenter in front of the Carpathians Bend in the latter contractional stage can be accounted for by the present topography if lateral variations in lithospheric strength are taken into account. Since the depth of the predicted basin is half of what is actually observed, an extra-load is required, which is equivalent to 500–800 m in terms of extra topography. In this case, the predicted basin corresponds with the observed geometry in terms of position/shape and depth, the latter depending on the magnitude of intraplate stresses as well. The modeling also suggests that the flexural fore-bulge of the Carpathians system is represented by the uplifted Dobrogea. Our explanation for the large subsidence recorded by the SE Carpathians foredeep highlights the control exerted by lateral changes in lithospheric strength on 3-D subsidence patterns in arcuate orogenic belts.

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.

Deformation in transcurrent and extensional environments with widely spaced weak zones

Previous mechanical models of the western U.S. have concluded that plate boundary forces cannot generate far‐field deformation. Such models have ignored preexisting large‐scale lithospheric strength variations, an assumption that appears to be inconsistent with seismically determined variations in lithospheric structure. We have formulated a three‐dimensional viscous flow model with imposed plate motions, but include lateral zones of low viscosity. These models show that strain rates are concentrated in weak zones with adjacent blocks experiencing little deformation. Deformation can extend far inboard of plate boundaries, contrary to the result of previous studies with rheologically homogeneous plates, and apparently compatible with the variation is seismic velocity and GPS determined deformations in western U.S. These results suggest that plate boundary forces cannot be neglected in the deformation of the western U.S., including the Cenozoic extension of the Basin and Range Province.

Thermochronology of high heat‐producing crust at Mount Painter, South Australia: Implications for tectonic reactivation of continental interiors

The Mount Painter Region in the Northern Flinders Ranges, South Australia, contains a Mesoproterozoic gneissic complex characterized by extraordinary heat production (∼16 μW m−3), resulting in the development of elevated middle‐upper crustal thermal gradients through much of the Paleozoic. Early Paleozoic deformation and metamorphism attained amphibolite facies (&gt;500°C) in the deepest parts of the metamorphic pile (∼10–12 km) during the ∼500 Ma Cambro‐Ordovician Delamerian orogeny. The subsequent thermal history of these rocks is assessed through new K/Ar and40Ar/39Ar age measurements on amphiboles and micas and multiple‐diffusion‐domain thermal modeling of K‐feldspar40Ar/39Ar data. The preferred interpretation of these data is that the deepest rocks were at ∼500°C until around 430 Ma, requiring average upper crustal thermal regimes of the order of 40°C km−1for at least 70 million years. At around 430 Ma, and again at 400 Ma, the terrane underwent periods of moderately fast cooling, possibly separated by a period of isothermal residence. Following cooling at 400 Ma, the terrane entered a second period of relative tectonic quiescence remaining essentially isothermal until ∼330 Ma. This long residence near the closure temperature of biotite resulted in variable argon loss from biotites in the 400 Ma to 330 Ma interval. Tectonic and thermal stability was terminated by a further period of moderately fast cooling (∼4°–8°C Ma−1) in the interval between 330 and 320 Ma. The three cooling episodes at around 430 Ma, 400 Ma, and 330 Ma are interpreted to be the result of exhumation resulting in a combined minimum of 6–7 km of denudation. We attribute this exhumation to the Alice Springs orogeny, a major intraplate tectonothermal event known throughout central Australia but not previously recognized as a significant tectonic event in the Adelaide Fold Belt. These new data provide compelling evidence that thermally modulated variations in lithospheric strength control the distribution of intraplate deformation at the continental scale.