Effective Elastic Thickness Values Research Articles

Variations of the effective elastic thickness evidence for a wide diffusive boundary between the North American and Eurasian plates in Siberia

Northeastern Eurasia is one of the least explored regions in the world. Very little geophysical data is available for this inaccessible area. Even the exact location of the plate boundary between Eurasia and North America remains a subject of ongoing debate. The effective elastic thickness (EET) of the lithosphere is a proxy for lithospheric strength and can provide insight into the thermal regime and tectonic processes. We have computed a high-resolution map of the EET for northeastern Eurasia using the fan wavelet coherence technique applied to the Bouguer gravity anomalies and topography/bathymetry data, appropriately adjusted to account for the influence of density variations within sediments. The results obtained provide insights into different tectonic regimes within this predominantly understudied region. In particular, we identify the boundary between the Eurasian and North American plates in Siberia as a rheologically weak diffusive zone extending from the Verkhoyansk and Sette-Daban Ranges to the eastern boundary of the Chersky Range. Unlike the Sette-Daban and Verkhoyansk Ranges, which were formed by plate collision and have an EET of 30–50 km, other mountainous regions have much lower EET values, usually less than 15 km. These areas have recently experienced tectonic activity that has weakened the lithosphere.

The mechanical behaviour of the lithosphere beneath the Chad Basin and the Bangui Magnetic Anomaly. Insights from Moho depth and effective elastic thickness estimates

We used high-resolution gravity data to analyse crustal structure beneath the Bangui Magnetic Anomaly (BMA) and the Chad Basin within the North-Central African region. Moho depths and effective elastic thickness values, calculated respectively by a regularized non-linear inversion method and by analysis of the coherence function between gravity and topography, have been interpreted with reference to seismic data, previous gravity studies and regional geology. Our aim is to determine the crustal architecture and to analyse its mechanical behaviour. Estimated Moho depths range from 30 to 43 km in the North-Central African region. Beneath the uplift zones (Adamawa-Yade plateau, Ouaddaï, Tibesti, Hoggar, Tuareg shield) and the Congo Craton, the Moho depths range between 35 and 37 km. These values can be interpreted by tectonic and seismic events recorded along the regional shear zones; but also to the remobilization of the Precambrian basement. Analysis of the coherence function between the Bouguer gravity and the topography shows two different domains of rigidity: the highest values (100 km < Te < 180 km) are observed in Central African Republic, Chad, Niger, Algeria, and Egypt; the lowest values (20 < Te < 100 km) are observed in Cameroon, Nigeria, and Sudan. The lithosphere beneath the Chad Basin has a higher rigidity, similar to that observed in Central African Republic. In Chad and the Central African Republic, there is a positive correlation between the Te and the Moho depths; this indicates that cratonic remnant (Chad craton) beneath the Chad Basin is well preserved and covered by the sediments. The low Te values in Cameroon and the constant value in the Moho depth, are a proof of the reactivation of the Precambrian basement. This reactivation is related to the tectonic, seismic and thermal events occurred during the Eburnean and Pan-African orogenies. The contrast between the Te values in Cameroon compared to others regions (Chad, Central African Republic, Niger, Egypt, Algeria) show the magnitude of these events on the crustal architecture. This being defined by the presence of cratonic cores, the Cameroon Volcanic Line (CVL) and hot spots as well as large regional shear zones, explain the fact that thermo-tectonic and seismic events that occurred at the northern edge of the Congo Craton in Cameroon, are more intense than in the Central African Republic. So according to the geodynamic process in Cameroon and Central African Republic, related to the hot spots along the CVL, thermo-tectonic and seismic events recorded along the regional shear zones (Sanaga Shear Zone and the Central Cameroon Shear Zone) and the Congo Craton, the BMA is associated to two different geotectonic contexts. The sources of the BMA are closely linked to the Congo Craton; they bottom should not exceed −37 km depth. The present results agree with previous studies which allow us to understand the evolution of the Saharan metacraton and the preservation of cratonic remnants (Murzuq, Al-Kufrah, Chad); similarly the estimated Moho depths is an important step that would serve as an input to model the sources of the Bangui Magnetic Anomaly.

Strength and elastic thickness variations in the Arabian Plate: A combination of temperature, composition and strain rates of the lithosphere

The Arabian Plate shows a strong asymmetry between its Shield and Platform, in terms of topography, seismic velocity and density structure of the upper mantle. This asymmetry also results in significant rheological differences between these blocks, as revealed by the effective elastic thickness (EET) estimates, obtained using a spectral gravity method. However, these estimates may be biased due to various factors. Therefore, other approaches based on a direct rheological modeling of the lithospheric structure should be employed to verify these results. In this study, we use a recent model of the lithosphere, based on an integrative interpretation of the gravity field and seismic tomography, to correct an initial thermal model obtained from the inversion of seismic velocity, assuming a uniform composition. The results are used together with the most recent crustal model of the Arabian Plate to construct two alternative models of strength and EET of the lithosphere. The first model (Model I) assumes a constant value of 10−15s−1 for the strain rates. In the second model (Model II), we used the strain rates obtained from a global mantle flow model. Model I confirms the asymmetry in the rigidity of the Shield and Platform. In contrast, Model II shows that the influence of the variable strain rates causes a significant increase in the strength and EET of the central and eastern part of the Shield and in contrast to previous studies, reveals that most of the Arabian Plate is a long-term stable tectonic feature, predominantly characterized by large EET values (≥70km).

Interpreting the Coseismic Uplift and Subsidence of the Longmen Shan Foreland Basin System during the Wenchuan Earthquake by a Elastic Flexural Model

AbstractThe coseismic surface uplift of the Longmen Shan (LMS) created an instantaneous topographic load over the western margin of the Sichuan Basin, where surface subsidence, decreasing eastward, has been measured using several methods, such as GPS, SAR and levelling. Using an elastic flexural model, we aim to interpret the coseismic surface uplift and subsidence, and constrain the effective lithospheric elastic thickness (Te) of the Sichuan Basin. Using different effective elastic thickness values for the Sichuan Basin, a series of subsidence curves were computed by the elastic flexure model equation for a broken elastic plate. The curves, produced by models using an effective elastic thickness of 30–40 km, provided the best fit to the general pattern of observed coseismic subsidence of the Sichuan Basin. However, the calculated subsidence (∼40–70 cm) at the front of the LMS is evidently lower than the observed values (∼100 cm), suggesting that the effective elastic thickness therein should be lower. These results indicate that the lithospheric strength may decrease westward from the Sichuan Basin to the LMS.

Lithospheric structure of Venus from gravity and topography

There are many fundamental and unanswered questions on the structure and evolution of the venusian lithosphere, which are key issues for understanding Venus in the context of the origin and evolution of the terrestrial planets. Here we investigate the lithospheric structure of Venus by calculating its crustal and effective elastic thicknesses (Tc and Te, respectively) from an analysis of gravity and topography, in order to improve our knowledge of the large scale and long-term mechanical behaviour of its lithosphere. We find that the venusian crust is usually 20–25km thick with thicker crust under the highlands. Our effective elastic thickness values range between 14km (corresponding to the minimum resolvable Te value) and 94km, but are dominated by low to moderate values. Te variations deduced from our model could represent regional variations in the cooling history of the lithosphere and/or mantle processes with limited surface manifestation. The crustal plateaus are near-isostatically compensated, consistent with a thin elastic lithosphere, showing a thickened crust beneath them, whereas the lowlands exhibit higher Te values, maybe indicating a cooler lithosphere than that when the venusian highlands were emplaced. The large volcanic rises show a complex signature, with a broad range of Te and internal load fraction (F) values. Finally, our results also reveal a significant contribution of the upper mantle to the strength of the lithosphere in many regions.

Characteristics of the outer rise seaward of the Manila Trench and implications in Taiwan–Luzon convergent belt, South China Sea

The outer rise on the distal periphery of a subduction system is caused by emplacement of an accreted load onto the flexed oceanic lithosphere. By examining the bathymetry and free-air gravity anomaly data collected by satellite observations and marine reflection seismic data collected during the TAIGER project, we demonstrate the characteristics of the flexural outer rise seaward of the Manila Trench. The region of the outer rise on the westernmost periphery of the Manila subduction system is characterized by the positive free-air gravity anomaly seaward parallel to the Manila Trench and the morphological rise at the south of the Manila subduction system. A flexure simulation is performed based on the flexural profiles along the southern Manila Trench-outer system and the resulting effective elastic thickness values may provide an alternative aspect for the spreading rates of the South China Sea basin. Since both the western periphery of the Taiwan collision belt and Manila subduction belt are dominated by the strain regime of extension of flexural origin, it appears that the strain regime of flexural extension associated with the flexural forebulge of the Western Taiwan Foreland Basin to the north, and the strain regime of flexural extension associated with the outer rise seaward of the Manila Trench to the south are meridionally interconnected. This revised understanding of the strain regime of flexural extension origin west of the Taiwan–Luzon convergent belt provides an alternative point of view on the strain regime offshore SW Taiwan.

Multitaper coherence method for appraising the elastic thickness of the Indonesian active continental margin

We estimate the mechanical strength of the Indonesian active continental margin. Values of effective elastic thickness (Te) were obtained for azimuthally averaged coherence measurements γ 2 ( | k | ) between Bouguer gravity and topography using a multitaper method with Hermite tapers. The T e estimates reveal uniform low strength (4–7 km) beneath the entire Indonesian continental margin. The 2D coherence function γ 2 ( | k | ) estimated with the help of MTM reveals varying anisotropy in the Indonesian margin and illustrates how the results correlate with the maximum horizontal stress orientations (SH max), heat flow measurements, sediment thickness and shear wave speed measurements. The sediment thickness and heat flow in these regions do not appear to influence the strength of the Indonesian margin. We have inferred that low effective elastic thickness could have been fossilized on a much younger oceanic plate at the time of loading.

Strength of the lithosphere and strain localisation in the Baikal rift

We use previously published estimates of Moho and lithosphere–asthenosphere boundary (LAB) depths combined with a 3D thermal model to infer variations of the effective elastic thickness (EET) of the lithosphere in the Baikal rift zone. The predicted continuous EET map is validated using EET values from previous forward flexural models across separate profiles. This map presents a present-day snapshot of the lithosphere mechanical behaviour. It suggests that the cratonic keel bounding the NW part of the rift is characterized by a strong lithosphere where the crust and mantle are mechanically coupled, whereas the more recent (Palaeo-Mesozoic) fold zone located SE of the rift is significantly weaker. Predicted spatial EET variations correlate well with the spatial distribution of earthquakes: where EET abruptly drops from high (~ 50 km) to low (~ 25 km) values, epicentre distribution also changes from very localised to diffuse, suggesting that the degree of crust–mantle coupling responsible for EET drops also controls strain localisation. Finally, we suggest that even though the current thermal state of the lithosphere does not favour partial melting near the LAB depth, the latter could have been promoted in case of moderately hotter lithosphere (+ 120 °C compared to the temperature at the current base). This could be the case during the earlier volcanic period, which spanned from the Oligocene to the Pleistocene.

Variations in elastic thickness in the Canadian Shield

We have compared different spectral methods (the standard windowed Fourier transform, the multitaper method (MTM), and the maximum entropy method (MEM)) to determine the coherence between Bouguer gravity and topography and estimate the variations of the effective elastic thickness (Te) of the lithosphere. These methods differ significantly in spatial resolution (1000 to 2000 km) determined by the width of the windows required to resolve the wavelength of transition from low to high coherence. The tests with synthetic data show that the standard deviation on elastic thickness estimates is high with the three methods but that the mean value obtained by the maximum entropy is close to the input value, while the other methods underestimate the elastic thickness.In the Canadian Shield, the effective elastic thickness values vary from ≈40 km to >100 km over distances less than the width of the moving window. There is no clear geographical or geological pattern in these variations as high and low elastic thickness values are found near the edge as well as in the center of the continent, and in all provinces regardless of their age. Correlation between Te and the heat flow field is weak. The values of Te being much higher beneath Hudson Bay than the Williston Basin are consistent with models of basin evolution and seismic estimates of mantle temperatures.

Elastic thickness and isostatic coherence anisotropy in the South Indian Peninsular Shield and its implications

We present measurements of the mechanical strength of the lithosphere of the South Indian Peninsular Shield (SIPS). Values of Effective Elastic Thickness (Te) were obtained from azimuthally averaged coherence measurements γ2(∣k∣) between Bouguer gravity and topography using a multitaper method (MTM) and a mirrored periodogram method (MPM). Values of Te from MTM (11–16 km) are consistently lower than those from MPM (20–31 km) and are contained within the seismogenic thickness (Ts), substantiating earlier measurements in shield regions that have been tectonically inactive for a long time. The 2D coherence function γ2(k) calculated with the MTM reveals little mechanical anisotropy in the SIPS. We conduct analyses of different provinces within the SIPS and show how the results correlate with maximum horizontal stress orientations (σHmax) and heat flow measurements. Local processes appear to influence the strength of the SIPS lithosphere more profoundly than processes occurring at the plate boundary.

3-D flexural modelling of the Silurian Baltic Basin

The Baltic Basin (BB) is situated on the western part of the East European Craton (EEC). It was established initially as a passive margin basin in response to the breaking apart of the Rodinia megacontinent during the Latest Precambrian–Early Cambrian times. Thereafter, it has been suggested that the Late Ordovician–Silurian basin subsidence was generated by foreland bending of the western margin of the EEC as a result of the Caledonian convergence and collision of continental plates, namely Baltica, Laurentia, and Eastern Avalonia. This is based on the regional basin architecture, which indicates accelerating subsidence, rapid deepening of the basin, and significant thickening of the sedimentary succession towards the Tornquist–Teisseyre Zone (TTZ), along the southwestern margin of the EEC, during the Silurian. However, the very broad extent of the Silurian BB is not obviously explicable by a model of simple foreland flexure. This problem has been investigated by quantitative modelling of Caledonian foreland flexure, east of the TTZ, using a 3-D finite difference technique. The results are interpreted in terms of their implications for the rheology of the EEC and the geometry and temporal evolution of Caledonian orogenic loading. Assuming higher values of effective elastic thickness (EET) for the EEC than for Caledonian lithosphere, the model results demonstrate that the evolution of the BB cannot be explained by the effects of orogenic loads lying only on the west of the present Caledonian Deformation Front (CDF). Furthermore, the inferred shape of the BB deflection is not easily reproduced, even allowing lateral changes of EEC lithosphere EET. Rather, additional loads east of the CDF (and TTZ) were necessary, in order to replicate the shape and amplitude of the BB as a flexural basin. These are tentatively interpreted to be dynamic loads related to mantle flow above an eastward dipping Silurian subduction zone.

Three-dimensional flexural modelling of the Ebro Basin�(NE Iberia)

The Ebro Basin, the southern foreland basin of the Pyrenees, has undergone a complex evolution in which, apart from the Pyrenees, the Iberian Range and the Catalan Coastal Ranges have played an important role, both as sediment sources and as basin confining structures. The deflected basement underlying the Ebro Basin dips north, suggesting a lithospheric-scale control on the structure of this basin. This is compatible with the results of subsidence analyses, which show that the study area is not in a local mode of isostatic compensation. In order better to understand the mechanisms that led to the present configuration of the Ebro Basin, and particularly the relevance of the various kinds of (un)loading (e.g. surrounding fold-and-thrust belts, basin topography, subsurface loads), we carried out a 3-D kinematic modelling study that accounts for the flexural state of the lithosphere, subjected to various loads applied at its lateral boundaries, and the sedimentary fill of the basin. We also included the effect of Neogene extensional tectonics along the eastern basin margin, which is related to the opening of the Valencia Trough. We show the suitability of the 3-D lithospheric-scale flexural modelling approach to the study of NE Iberia. Modelling results point to a relatively strong lithosphere in this area, with values of effective elastic thickness ranging from 10 to 35 km in the Ebro Basin, increasing towards the Pyrenees. We also find that the topographic (tectonic) load itself is insufficient to explain the observed basement deflection. Thus an extra subsurface load beneath the Pyrenees, corresponding to the underthrusted Iberian lithosphere, is required. The effect of lithospheric stretching in the Valencia Trough on the Ebro Basin is appreciable only in its eastern part, where the lithosphere was uplifted. This had considerable repercussions on the sedimentary and erosional regime of the Ebro Basin. We have analysed the link between the stretching-related, tectonically uplifted areas and the erosional patterns observed onshore northeast Iberia.

Effective elastic thickness of the northern Australian continental lithosphere subducting beneath the Banda orogen (Indonesia): inelastic failure at the start of continental subduction

Pliocene-Recent continent-island arc collision of the northern Australian continental lithosphere across the Banda orogen from Roti to the Kai Plateau (∼121–137°E longitude) has formed an underfilled foreland basin within the Timor–Tanimbar–Aru Trough. Continental collision on northern Australian lithosphere is most advanced near central Timor Island in terms of shortening and absorbing the forearc basin (Savu Basin) within the accretionary prism. Australian continental lithosphere north of area around central Timor Island is believed to be detached from the oceanic lithosphere. Effective Elastic Thickness (EET) of the northern Australian continental lithosphere from Roti to the Kai Plateau are derived using an elastic half-beam model. Modeled deflection is matched to the seafloor bathymetry and the marine complete 3D Bouguer gravity anomalies. The EET varies from 27 to 75 km across the northern Australian continental lithosphere from Roti to Kai Plateau when the thickness of the elastic half-beam is kept constant. The highest EET values lies near central Timor. From the shelf to beneath the Banda orogen, the EET of the northern Australian continental lithosphere is reduced from ∼90 to ∼30 km when the thickness of the elastic half-beam is allowed to vary down dip. Elastic half-beam modeling approximates the Banda orogen as a triangular load and hidden subsurface loads as end-point loads. Wider triangular loads modeling the load contribution from Banda orogen need higher values of EET. Such an observation highlights the role of high EET in thin-skinned collisional tectonics by promoting the support of wider accretionary prisms by parts of foreland basins with higher EET. Variations in EET may result from inelastic yielding in the northern Australian continental lithosphere. Oroclinal bending of the Australian continental lithosphere in the east, from Tanimbar to the Kai Plateau, may create additional yielding and further decrease the EET. Change in EET occurred at the start of continental subduction in the late Miocene–early Pliocene boundary due to change in curvature of the northern Australian lithosphere near the shelf-slope, both in map and cross-sectional view. Evidence for the inelastic yielding of the northern Australian continental lithosphere near the present-day shelf-slope at the continental subduction is found in: (1) the maximum change of EET near shelf-slope in laterally variable EET calculations, and (2) in the almost cessation of normal faulting in late Miocene–early Pliocene seen on seismic reflection data from northern Australian continental shelf-slope. Elastic half-beam models across central Timor do not require end-point loads and may imply that there are no slab pull forces from the oceanic lithosphere that are acting on the leading edge of the subducted northern Australian continental lithosphere.

Kinematic modelling of the West Antarctic Rift System, Ross Sea, Antarctica

In this study, we investigate the thermo-mechanical controls on the formation of the Ross Sea basin (Antarctica) and the uplift of the adjacent Transantarctic Mountains (TAM) rift shoulder, which started in the Late Cretaceous and continued until the present time. Quantitative forward modelling has been performed along three 700 to 800 km long East–West offshore profiles, extended inland to the front of the TAM. The modelling is constrained by an extensive database of multichannel seismic (MCS), refraction seismic, Ocean Bottom Seismographs (OBS), and gravity data. MCS data is tied to well stratigraphy from DSDP leg 28, CIROS-1 and MSTSS-1. Quantitative estimates of uplift of the TAM are provided by previous work on apatite fission track analysis. We incorporate the finite strength of the lithosphere in basin formation models using the concept of the level of necking. Lateral variations of necking level and associated bulk rheological properties (with necking levels ranging from 15 km in the northernmost, to 20 km in the central and 23 km to the southernmost profiles) are required to explain the observed crustal geometries. High values of effective elastic thickness (more than 30 km) and pre-rift lithospheric thicknesses (220–230 km), indicating a cold lithosphere in a pre-rift cratonic setting, are consistent with elevation of the rift shoulder. The western parts of the profiles appear to have unusually high stretching values without the development of oceanic crust. Inferred average values of stretching factors vary from 2.3 to 2.8, equivalent with extension in the Ross Sea of 115% to 140%. The modelling result for the uplift of the TAM predicts a late Cretaceous tectonic uplift of about 1.5 to 2.0 km, and a Cenozoic uplift of about 1.3 km restricted to southern Victoria Land.

Lithosphere folding: Primary response to compression? (from central Asia to Paris basin)

We examine the role of lithosphere folding in the large‐scale evolution of the continental lithosphere. Analysis of the record of recent vertical motions and the geometry of basin deflection for a number of sites in Europe and worldwide suggests that lithospheric folding is a primary response of the lithosphere to recently induced compressional stress fields. Despite the widespread opinion, folding can persist during long periods of time independently of the presence of many inhomogeneities such as crustal faults and inherited weakness zones. The characteristic wavelengths of folding are determined by the presence of young lithosphere in large parts of Europe and central Asia and by the geometries of the sediment bodies acting as a load on the lithosphere in basins. The proximity of these sites to the areas of active tectonic compression suggests that the technically induced horizontal stresses are responsible for the large‐scale warping of the lithosphere. Wavelengths and persistence of folding are controlled by many factors such as rheology, faulting, time after the end of the major tectonic compression, nonlinear effects, and initial geometry of the folded area. In particular, the persistence of periodical undulations in central Australia (700 Myr since onset of folding) or in the Paris basin (60 Myr) long after the end of the initial intensive tectonic compression requires a very strong rheology compatible with the effective elastic thickness values of about 100 km in the first case and 50–60 km in the second case.

Flexural expression of European continental lithosphere under the western outer Carpathians

We present the results of an analysis of the flexural down bending of the European lithosphere under the western outer Carpathians for a grid of five profiles with a lateral spacing of about 100 km crossing the Carpathians and its foreland. Seismic sections, including two deep seismic reflection lines, and numerous wells allowed us to quantify the flexural curvature of the lithosphere. In addition, gravity data are used to obtain independent constraints. Two evolutionary scenarios involving subduction/underthrusting of passive margin and a phase change in the lower crust are investigated in a context of relatively flat Moho under the area. Furthermore, recent uplift, expressing itself in erosional features observed along the distal margin of the foredeep, is quantified. The generally modest Bouguer gravity anomaly points to nonisostatic processes causing this uplift. Steeply dipping basement under the Carpathians to about 10 km depth at the location of the Pieniny Klippen Belt is explained by relatively weak lithosphere, which is expressed in the predicted low values of effective elastic thickness of 16 km decreasing locally to 3 km. The low strength of the European lithosphere in this region is probably caused by mechanical weakening controlled by partial yielding due to high flexural curvature. The weakest parts of the lithosphere along the profiles, corresponding to low effective elastic thicknesses, are concentrated in the areas of maximum bending. Predicted flexural bending stresses as a function of bending moment and effective elastic thickness increase dramatically under the outer zone of the Carpathian belt, explaining reactivations of older basement structures in these regions.

Spatial variability of tidal gravity anomalies and its correlation with the effective elastic thickness of the lithosphere

Associations of the Earth tidal gravity response to physical properties of the lithosphere have been attempted at least for the last four decades. Although experimental data suggest this association, rigorous models have not yet been proposed. In this work, statistical tests are performed on the available World Gravity Earth Tides data set. Autocorrelation analysis shows that the M2 tidal gravity anomalies (TGAs) are significantly correlated up to a distance of about 500 km, with an approximately exponential correlation decay. The analysis of the latitudinal dependence of the anomalies shows that the anomaly variance, estimated inside of different latitude bands, follows a cos 4 ϕ curve within the ±45° latitude interval and defines the noise level for the M2 gravity anomaly data set. The regression analysis between M2 TGA and the lithosphere effective elastic thickness (EET) estimates shows that these quantities are significantly correlated, with a correlation coefficient of −0.82. The wide range of TGA and EET values, combined with a good global distribution of the data used in the regression analysis, makes the regression equation suitable to be used as a predictor for EET values in areas where M2 TGA data exist and meet the required quality criteria.

The role of pre-existing faults in basin evolution: constraints from 2D finite element and 3D flexure models

Abstract The lithospheric driving forces which cause intraplate basin deformation are relatively constant over large areas. Consequently, lateral variations in deformation and stress and strain concentrations seem to be primarily caused by (pre-existing) heterogenities in the rheological signature of the continental lithosphere underlying the sedimentary basins. In this paper, we explore the weak character of upper crustal faults and their control on basin shape for a number of case studies on intraplate Phanerozoic basin settings, using a 2D finite element and a 3D flexure model. Of key importance is the integration of seismic data and field observations with the tectonic modelling, allowing the investigation of deformation processes and their expressions on different scales, operating on different levels of the lithosphere. Finite element models for sub-basin scales, incorporating weak upper crustal faults, predict strong control of these weak zones on local stress distributions and subsequent deformation, in agreement with observed deformation patterns. Slip along upper crustal faults control stress distribution and subsequent faulting in overlying sedimentary rocks. The effect of weak upper crustal fault movements on basin-wide (regional) upper crustal flexure is looked at in two case studies on: (1) extensional tectonics in the Lake Tanganyika Rift Zone (East Africa); and (2) compressional tectonics in the Central System and Tajo Basin (Central Spain). Both settings indicate that basement warpings are controlled by large amounts of slip along so-called weak crustal-scale border faults, which are mostly planar. Adopting border fault displacements in the 3D flexure model, the results indicate low effective elastic thickness (EET) values in a range of 3–7 km for induced basement deflection patterns in accordance with observations. The low EET values most likely reflect a (partly) decoupling of upper crustal and subcrustal deformation, facilitated by the weak lower crust, and in agreement with standard rheological assumptions for Phanerozoic lithosphere. In contrast, the inferred weakness of upper crustal faults relative to surrounding rock is not evident from uniform rheological assumptions. However, observations of reactivations of faults which are not preferably aligned with the stress field, and reactivations of basin deformation on long time-scales are in support of this feature.

Palaeo-elevation and effective elastic thickness evolution at mountain ranges: inferences from flexural modelling in the Eastern Pyrenees and Ebro Basin

The results are presented of a two-dimensional flexural modelling study of the lithosphere underlying the southern Pyrenees and the Ebro Basin. The modelling is based on a crustal-scale balanced cross-section along a profile through the north-eastern part of the Iberian plate. Two time slices of the structural evolutions of the mountain chain are modelled: the present day configuration and the configuration at Middle Lutetian time (47 Ma), where important structural and sedimentological changes are observed. The flexure model incorporates lateral variations in the effective elastic thickness (EET) of the lithosphere. The present day deflection in the profile is simulated using boundary forces and a northward decreasing EET that varies from 30 to 11 km. Models for Middle Lutetian times indicate EET values of 26-18 km in the northern part of the profile, assuming that the EET at the distal margin of the Ebro Basin has not significantly changed since Middle Lutetian times. These higher values for the EET at Middle Lutetian times suggest that the effect of the Cretaceous extensional phase on the present day flexural rigidity is small and, therfore, the inferred northward decreasing rigidity is predominantly related to the Pyrenean collision. Flexural modelling provides also constraints for the palaeo-elevation of the inner part of the chain. Including the assumption that the EET at the distal margin of the Ebro Basin has not significantly changed since Middle Lutetian times, the model predicts a maximum palaeo-elevation of ∼2000 m, which is in agreement with geological observations concerning the relation between basin-fill and palaeo-elevation.

Evidence for Active Hotspots on Venus from Analysis of Magellan Gravity Data

The 500-Myr average crater retention age for Venus has raised questions about the present-day level of tectonic activity. In this study we examine the relationship between the gravity and topography of four large volcanic swells, Beta, Atla, Bell, and Western Eistla Regiones, for clues about their stage of evolution. The Magellan line-of-sight gravity data are inverted using a point mass model of the anomalous mass to solve for the local vertical gravity field. Spectral admittance calculated from both the local gravity inversions and a spherical harmonic model is compared to three models of compensation: local compensation, a "flexural" model with local and regional compensation of surface and subsurface loads, and a "hotspot" model of compensation that includes top loading by volcanoes and subsurface loading due to a deep, low density mass anomaly. The coherence is also calculated in each region, but yields an elastic thickness estimate only at Bell Regio. In all models, the long wavelengths are compensated locally. The long-wavelength estimates of the deep compensation depth for Bell, Atla, Western Eistla, and Beta Regiones are 125, 175, 200, and 225 km, respectively, with an error of approximately ± 35 km. The flexural model is rejected primarily because it gives values of effective elastic thickness of approximately 100 km, which is at least a factor of two larger than estimates of effective elastic thickness obtained from the coherence spectra at Bell Regio and from other studies that model the topographic expression of flexure at Bell Regio and in other tectonic settings globally. Assuming a crustal thickness of 30 km, the fit to the hotspot model at Atla Regio gives an effective elastic thickness of 30 ± 5 km. Atla Regio is interpreted as an active hotspot because of the deep compensation depth and a strong subsurface loading signature. At Bell Regio, effective elastic thickness is 30 ± 5 km at short wavelengths and 50 ± 5 km at long wavelengths, using a crustal thickness of 10 km. The 30-km value is interpreted as the effective elastic thickness at the time when the volcanoes are emplaced; the 50-km value is believed to reflect the present-day effective elastic thickness. Bell Regio has a relatively shallow compensation depth, 125 km, and a weak bottom-to-top-loading ratio determined from the coherence spectra. These results may indicate a relatively old, possibly inactive plume. The data at Beta and Western Eistla Regiones are of relatively poor quality and do not permit detailed interpretations. These areas are interpreted as active hotspots because of their large compensation depths, greater than typical of the rest of the planet.