Elastic Thickness Of Lithosphere Research Articles

Mechanisms for oscillatory true polar wander

Palaeomagnetic studies of Palaeoproterozoic to Cretaceous rocks propose a suite of large and relatively rapid (tens of degrees over 10 to 100 million years) excursions of the rotation pole relative to the surface geography, or true polar wander (TPW). These excursions may be linked in an oscillatory, approximately coaxial succession about the centre of the contemporaneous supercontinent. Within the framework of a standard rotational theory, in which a delayed viscous adjustment of the rotational bulge acts to stabilize the rotation axis, geodynamic models for oscillatory TPW generally appeal to consecutive, opposite loading phases of comparable magnitude. Here we extend a nonlinear rotational stability theory to incorporate the stabilizing effect of TPW-induced elastic stresses in the lithosphere. We demonstrate that convectively driven inertia perturbations acting on a nearly prolate, non-hydrostatic Earth with an effective elastic lithospheric thickness of about 10 kilometres yield oscillatory TPW paths consistent with palaeomagnetic inferences. This estimate of elastic thickness can be reduced, even to zero, if the rotation axis is stabilized by long-term excess ellipticity in the plane of the TPW. We speculate that these sources of stabilization, acting on TPW driven by a time-varying mantle flow field, provide a mechanism for linking the distinct, oscillatory TPW events of the past few billion years.

The effective elastic thickness of the continental lithosphere: Comparison between rheological and inverse approaches

Following the release of global continental effective elastic thickness (Te) maps obtained using different approaches, we now have the opportunity to provide better constraints onTe.We improve previous estimates ofTederived from thermo‐rheological models of lithospheric strength (orTer) using new equations that consider variations of the Young's Modulus in the lithosphere. These new values are quantitatively compared with those obtained from an inverse approach (orTei) based on a comparison of the spectral coherence between topography and gravity anomalies with the flexural response of an equivalent elastic plate to loading. The two models show in general a good agreement, having equal means (at the 95% significance level) in about half of the continental areas. In other regionsTeiexceedsTerin about 65% of the data points, showing thatTeiprovides an upper bound onTe.The two data sets have a similar range, but demonstrate different distributions.Terhas a bimodal distribution, with the two peaks representative of the cratons and of the areas outside of them. In contrast,Teihas more uniform distribution without predominant peaks. Our models show higher similarities in the Meso‐Cenozoic orogens than in the Archaean and Proterozoic shields and platforms, due to the methods employed. For the regions with the most robust determinations ofTerandTei, the relationship between them is close to linear. The results of this work can be used for further studies on the mechanical properties of the lithosphere.

Lithospheric flexure in the Sichuan Basin and Longmen Shan at the eastern edge of Tibet

The mountain range at the steep eastern edge of the Tibetan Plateau, the Longmen Shan, was deformed by a Mw7.9 earthquake with oblique thrust and strike‐slip motion in 2008. The tectonic processes and structure of the lithosphere beneath this range have been controversial. Gravity measurements reflect the distribution of mass within the Earth, including the large load of rock above the geoid in mountain ranges. We investigate the response of the lithosphere to the load of the Longmen Shan and estimate the flexural rigidity or effective elastic thicknessTeusing new gravity data acquired by recent satellites and the combined GOCO2S and EIGEN‐6c datasets. The free‐air gravity anomalies over the Longmen Shan show that its mass is supported by flexure of the adjacent Sichuan Basin lithosphere, similar to the flexural support of the Himalayas. Analysis of a stacked profile of the free‐air anomalies shows that the effective elastic thickness of the basin lithosphere is greater than 10 km, but has a broad minimum misfit function with no upper limit on the thickness. Two‐dimensional admittance analysis shows theTe of easternmost Tibet is very low, approximately 7 km.

Density and lithospheric thickness of the Tharsis Province from MEX MaRS and MRO gravity data

Radio science tracking of Mars Express (MaRS experiment) and Mars Reconnaissance Orbiter produced high resolution Martian gravity data. Applying localized spectral analysis on the new gravity data sets, we study the surface density and lithospheric elastic thickness in the Tharsis province. The gravity signal is predicted with geophysical models either including bottom loading (top/bottom model) or taking into account the loading history (top/top model). Volcanic shields are mainly composed of high‐density lava but their construction could have begun with lower density lava, with the exception of Ascraeus Mons which has a top of lower density. Buoyant bottom loading may have been present in the form of a mantle plume under Olympus Mons. The elastic thickness was much larger at Olympus Mons than at other volcanoes, suggesting large spatial variations of heat flux during the Hesperian. Alternatively, small elastic thicknesses could be artifacts reflecting the presence of very localized high‐density crustal intrusions beneath the volcanoes. Thaumasia highlands were probably supported by a mantle plume at the time of formation. In Valles Marineris, top/bottom models predict low densities and large elastic thicknesses, in conflict with the basaltic rock composition and Hesperian age of the valley. Dense mafic dikes underlie the western part of the valley. The top/top model serves to test another scenario in which the trough is formed with sedimentary infilling removed much later by erosion, the elastic thickness increasing in between. At the large volcanoes, the relation between gravity and topography is anisotropic probably because of density variations.

Influence of lithosphere‐asthenosphere interaction on the overriding lithosphere in a subduction zone: Numerical modeling

Several factors influence the topography of the overriding plate in a subduction zone: mantle wedge flow caused by motion of the descending slab, lithosphere elasticity, and interaction between the solid lithosphere and the viscous asthenosphere. This paper presents the results of new subduction modeling that incorporates both elastic/elastoplastic deformation in the overriding plate and viscous deformation in the asthenosphere. The effects of fluid‐solid coupling and interaction were considered using an arbitrary Lagrangian‐Eulerian technique that solves for the continuous deformation of fluid flow in contact with the rigid plate. Modeling results show that thicker lithosphere, which is harder to deform, contributes to the development of broadly shallower back‐arc basins. As lithospheric strength decreases, its ability to withstand outside force (fluid load) resulting from flow in the mantle wedge also declines, leading to the development of deep and narrow basins. Large‐scale deformation of the lithosphere modifies both the mantle wedge geometry and the wedge flow pattern, which in turn increases the fluid load and thereby enhances topographic variation. Generally, the effect of interaction is enhanced with lithospheric deformation. The elastoplastic model also leads to a weaker lithosphere and a correspondingly larger deflection in the lithosphere. Similarly to previous studies, back‐arc basins are deeper when mantle viscosity is higher. For a 50 km thick elastic lithosphere, the lithosphere‐asthenosphere interaction is negligible when the asthenospheric viscosity is less than 4 × 1019 Pa s. Increasing the rate of subduction or decreasing the dip angle will also lead to the development of deeper basins.

Upper mantle viscosity and lithospheric thickness under Iceland

Deglaciation during the Holocene on Iceland caused uplift due to glacial isostatic adjustment. Relatively low estimates for the upper mantle viscosity and lithospheric thickness result in rapid uplift responses to the deglaciation cycles on Iceland. The relatively high temperatures of the upper mantle under the newly formed mid-ocean ridge under Iceland are responsible for the low upper mantle viscosity values. In this study, estimates for lithospheric thickness and upper mantle viscosity under Iceland from glacial isostatic adjustment studies are complemented by a microphysical modelling approach using the theoretical temperature distribution under mid-ocean ridges combined with olivine diffusion and dislocation creep flow laws. The lithospheric thickness (27–40 km) and upper mantle viscosity (2 × 10 18–10 19 Pa s) outcomes for the upper mantle recent glaciation under the Vatnajökull glacier are consistent with previous reports of viscosity and lithospheric thickness from glacial isostatic adjustment studies. A combination of a 40 km thick elastic lithosphere and an average upper mantle viscosity of 5 × 10 18 Pa s would suggest that the upper mantle under Iceland is most likely dry. The earlier and larger Weichselian glaciation event (∼10,000 BP) on Iceland is predicted to have had a slightly larger upper mantle viscosity ∼10 19 Pa s and a lithospheric thickness of ∼100 km. Large lateral variations in upper mantle viscosity and especially lithospheric thickness are expected for Iceland perpendicular to the ridge axis.

Mars without the equilibrium rotational figure, Tharsis, and the remnant rotational figure

We use a revised partitioning of the planet figure into equilibrium and nonequilibrium contributions that takes into account the presence of an elastic lithosphere to study the Martian gravity field and shape. The equilibrium contribution is associated with the present rotational figure, and the nonequilibrium contribution is dominated by Tharsis and a remnant rotational figure supported by the elastic lithosphere that traces the paleopole location prior to the formation of Tharsis. We calculate the probability density functions for Tharsis' size and location, the paleopole location, and the global average thickness of the elastic lithosphere at the time Tharsis was emplaced. Given the observed degree‐3 spherical harmonic gravity coefficients, the expected Tharsis center location is 258.6 ± 4.2°E, 9.8 ± 0.9°N, where the uncertainties represent the 90% confidence interval. Given this Tharsis center location and the observed degree‐2 spherical harmonic gravity coefficients, the expected paleopole location prior to the emplacement of Tharsis is 259.5 ± 49.5°E, 71.1−14.4+17.5°N, and the expected elastic lithospheric thickness at the time of loading is 58−32+34km. Our estimated paleopole colatitude implies 18.9−17.5+14.4°of true polar wander (TPW) driven by the emplacement of Tharsis, in disagreement with previous studies that invoke large TPW. The remnant rotational figure is visible in both the nonequilibrium degree‐2 geoid (areoid) without Tharsis and the nonequilibrium degree‐2 topography without Tharsis. The remnant rotational figure is also visible in the total nonequilibrium geoid without Tharsis, but it is not visible in the total nonequilibrium topography without Tharsis due to the strong signal of the north‐south dichotomy. Shorter wavelength geological features become significantly more visible in the geoid with the removal of the long wavelength contributions of the equilibrium rotational figure, Tharsis, and the remnant rotational figure. Removal of the equilibrium rotational figure and Tharsis from the topography reveals a better defined north‐south dichotomy boundary.

A climatic trigger for a major Oligo-Miocene unconformity in the Himalayan foreland basin

[1] Subsidence in foreland basins is modulated by the size of the flexural load, the elastic thickness of the foreland lithosphere, the nature of the sedimentary fill, and the rate of convergence. Basal “forebulge unconformities” are well known from these basins, but here we focus on a major unconformity in the Himalayan foreland, which removed much of the Oligocene–lower Miocene. This is a critical time in the Himalaya because it spans the period of initial rapid exhumation of high-grade metamorphic rocks and the start of motion on the Main Central Thrust. We show that the synchronous timing of unconformity and rapid Greater Himalayan exhumation may be explained by the same trigger, enhanced erosion. We estimate that accelerating erosion between 24 and 20 Ma would have removed ∼1.5 km more rock from the Greater Himalaya than would have been the case had erosion remained at the slower Paleogene rates. During this time erosion must have temporarily exceeded rock uplift rates. By transferring rock from the Himalaya to the Indian Ocean the load is reduced and the Indian Plate is flexed less, so that the depth of the foreland basin shallows to a new equilibrium state. We use a flexural model to estimate that erosional unloading could have caused uplift of 400–500 m of the basin at the range front, extending 70–150 km into the basin. Intensification of the Asian summer monsoon around the Oligo-Miocene boundary (∼24 Ma) is the most likely trigger of the stronger erosion. Similar unconformities are seen at 15–16 Ma and in the Pleistocene also, linked to faster erosion at these times.

Internal structure of Planum Boreum, from Mars advanced radar for subsurface and ionospheric sounding data

An investigation of the internal structure of the ice‐rich Planum Boreum (PB) deposit at the north pole of Mars is presented, using 178 orbits of Mars advanced radar for subsurface and ionospheric sounding data. For each radargram, bright, laterally extensive surface and subsurface reflectors are identified and the time delay between them is converted to unit thicknesses, using a real dielectric constant of 3. Results include maps of unit thickness, for PB and its two constituent units, the stratigraphically older basal unit (BU) and the stratigraphically younger north polar layered deposits (NPLD). Maps of the individual units' surface elevation are also provided. Estimates of water ice volume in each unit are (1.3 ± 0.2) × 106 km3 in PB, (7.8 ± 1.2) × 105 km3 in the NPLD, and (4.5 ± 1.0) × 105 km3 in the BU. No lithospheric deflection is apparent under PB, in agreement with previous findings for only the Gemina Lingula lobe, which suggests that a thick elastic lithosphere has existed at the north pole of Mars since before the emplacement of the BU. The extent of BU material in the Olympia Planum lobe of PB is directly detected, providing a more accurate map of BU extent than previously available from imagery and topography. A problematic area for mapping the BU extent and thickness is in the distal portion of the 290°E–300°E region, where MARSIS data show no subsurface reflectors, even though the BU is inferred to be present from other lines of evidence.

Geophysical limitations on the erosion history within Arabia Terra

The Arabia Terra region, an area of ∼1 × 107 km2 lying south of the hemispheric dichotomy boundary and centered at (25E, 5N), is a unique physiographic province with topography and crustal thickness intermediate between those of the southern highlands and northern lowlands. Previous workers have identified numerous morphological indicators suggestive of erosion. Using altimetry data returned by the Mars Orbiter Laser Altimeter (MOLA) on the Mars Global Surveyor (MGS) along with gravity data from the Mars Reconnaissance Orbiter (MRO), we place geophysical constraints on the amount of erosion permitted within Arabia Terra. Admittance estimates using a multitaper, spatiospectral localization approach provide a best fit to the observations through degree 50 at an elastic lithosphere thickness of 15 km. The elevation difference between Arabia Terra and the highlands would require as much as 5 km of erosion in certain areas to yield the current topography, neglecting the effects of subsequent flexure. However, incorporating flexural rebound requires substantially more erosion, up to 25 km, in order to reproduce the elevation and crustal thickness deficit of Arabia Terra. Such a large amount of erosion would result in exterior flexural uplift surpassing 1 km and gravity anomalies exceeding observations by ∼60 mGal. Consequently, it is unlikely that Arabia Terra was formed from surface erosion alone. We determine that no more than 3 × 107 km3 of material could have been removed from Arabia Terra, while 1.7 × 108 km3 of erosion is required to explain the observed crustal thickness.

On the spatial variability of the Martian elastic lithosphere thickness: Evidence for mantle plumes?

The present day elastic lithosphere thickness at the Martian north pole has recently been constrained to De > 300 km and this is a factor of 3–4 larger than elastic thickness estimates for other Amazonian surface features like the Tharsis volcanoes. Here we present a model for the Martian elastic lithosphere thickness which takes the locally varying crustal thickness, the local concentration of heat‐producing elements, as well as variations of strain rate into account. The model predicts De = 196 km at the north pole today, whereas elastic thicknesses at the Tharsis volcanoes are best compatible with middle to late Amazonian loading ages. Therefore, although a large degree of spatial heterogeneity can be explained by the presented model, large elastic thicknesses in excess of 300 km cannot be reproduced. In order to fit all elastic thickness estimates derived from observations, mantle heat flux at the north pole needs to be reduced by 35%. However, this can only be reconciled with a bulk chondritic concentration of heat‐producing elements in the Martian interior if the excess heat is deposited elsewhere. Therefore, this argues for the presence of recently active mantle plumes, possibly underneath Tharsis. The size and strength of such a plume can be constrained by the elastic thickness at the Tharsis Montes and maximum average heat flux between 8 and 24 mW m‐2, corresponding to a central peak heat flux of 40 to 120 mW m−2, is consistent with the observations. Such a plume would leave a clear signature in the surface heat flux and should be readily detectable by in situ heat flux measurements.

Constraining models of postglacial rebound using space geodesy: a detailed assessment of model ICE-5G (VM2) and its relatives

Using global positioning system, very long baseline interferometry, satellite laser ranging and Doppler Orbitography and Radiopositioning Integrated by Satellite observations, including the Canadian Base Network and Fennoscandian BIFROST array, we constrain, in models of postglacial rebound, the thickness of the ice sheets as a function of position and time and the viscosity of the mantle as a function of depth. We test model ICE-5G VM2 T90 Rot, which well fits many hundred Holocene relative sea level histories in North America, Europe and worldwide. ICE-5G is the deglaciation history having more ice in western Canada than ICE-4G; VM2 is the mantle viscosity profile having a mean upper mantle viscosity of 0.5 × 1021 Pa s and a mean uppermost-lower mantle viscosity of 1.6 × 1021 Pa s; T90 is an elastic lithosphere thickness of 90 km; and Rot designates that the model includes (rotational feedback) Earth's response to the wander of the North Pole of Earth's spin axis towards Canada at a speed of ≈1° Myr−1. The vertical observations in North America show that, relative to ICE-5G, the Laurentide ice sheet at last glacial maximum (LGM) at ≈26 ka was (1) much thinner in southern Manitoba, (2) thinner near Yellowknife (Northwest Territories), (3) thicker in eastern and southern Quebec and (4) thicker along the northern British Columbia–Alberta border, or that ice was unloaded from these areas later (thicker) or earlier (thinner) than in ICE-5G. The data indicate that the western Laurentide ice sheet was intermediate in mass between ICE-5G and ICE-4G. The vertical observations and GRACE gravity data together suggest that the western Laurentide ice sheet was nearly as massive as that in ICE-5G but distributed more broadly across northwestern Canada. VM2 poorly fits the horizontal observations in North America, predicting places along the margins of the Laurentide ice sheet to be moving laterally away from the ice centre at 2 mm yr−1 in ICE-4G and 3 mm yr−1 in ICE-5G, in disagreement with the observation that the interior of the North American Plate is deforming more slowly than 1 mm yr−1. Substituting VM5a T60 for VM2 T90, that is, introducing into the lithosphere at its base a layer with a high viscosity of 10 × 1021 Pa s, greatly improves the fit of the horizontal observations in North America. ICE-4G VM5a T60 Rot predicts most of the North American Plate to be moving horizontally more slowly than ≈1 mm yr−1, in agreement with the data. ICE-5G VM5a T60 Rot well fits both the vertical and horizontal observations in Europe. The space geodetic data cannot distinguish between models with and without rotational feedback, in the vertical because the velocity of Earth' centre is uncertain, and in the horizontal because the areas of the plate interiors having geodetic sites is not large enough to detect the small differences in the predictions of rotational feedback going across the plate interiors.

Mantle convection controls the observed lateral variations in lithospheric thickness on present‐day Mars

Sounding radar observations of the north polar layered deposits on Mars suggest an elastic lithosphere thickness of at least 300 km in that region, significantly larger than any prior estimate of lithospheric thickness on Mars. Previous studies interpreted this result as requiring either that Mars is significantly sub‐chondritic in its abundance of radioactive elements, or alternatively that there are large lateral variations in lithospheric thickness. We find that the required depletion of radioactivity is geochemically unlikely. On the other hand, stagnant lid convection naturally produces factor of two or more lateral variations in both mantle heat flux and lithospheric thickness and is the preferred explanation for the polar elastic thickness measurement. Our mantle plume models, which have previously been shown to explain observations of present‐day magma production on Mars, can also explain the new elastic thickness observations.

Temporal Gravity Variations near Shrinking Vatnajökull Ice Cap, Iceland

Repeated gravity measurements were carried out from 1991 until 1999 at sites SE of Vatnajokull, Iceland, to estimate the mass flow and deformation accompanying the shrinking of the ice cap. Published GPS data show an uplift of about 13 ± 5 mm/a near the ice margin. A gravity decrease of –2 ± 1 μGal/a relative to the Hofn base station, was observed for the same sites. Control measurements at the Hofn station showed a gravity decrease of –2 ± 0.5 µGal/a relative to the station RVIK 5473 at Reykjavik (about 250 km from Hofn). This is compatible, as a Bouguer effect, with a 10 ± 3 mm/a uplift rate of the IGS point at Hofn and an uplift rate of ~20 mm/a near the ice margin. Although the derived gravity change rates at individual sites have large uncertainties, the ensemble of the rates varies systematically and significantly with distance from the ice. The relationship between gravity and elevation changes and the shrinking ice mass is modelled as response to the loading history. The GPS data can be explained by 1-D modelling (i.e., an earth model with a 15-km thick elastic lithosphere and a 7·1017 Pa·s asthenosphere viscosity), but not the gravity data. Based on 2-D modelling, the gravity data favour a low-viscosity plume in the form of a cylinder of 80 km radius and 1017 to 1018 Pa·s viscosity below a 6 km-thick elastic lid, embedded in a layered PREM-type earth, although the elevation data are less well explained by this model. Strain-porosity-hydrology effects are likely to enhance the magnitude of the gravity changes, but need verification by drilling. More accurate data may resolve the discrepancies or suggest improved models.

Implications of large elastic thicknesses for the composition and current thermal state of Mars

The martian elastic lithosphere thickness T e has recently been constrained by modeling the geodynamical response to loading at the martian polar caps and T e was found to exceed 300 km at the north pole today. Geological evidence suggests that Mars has been volcanically active in the recent past and we have reinvestigated the martian thermal evolution, identifying models which are consistent with T e > 300 km and the observed recent magmatic activity. We find that although models satisfying both constraints can be constructed, special assumptions regarding the concentration and distribution of radioactive elements, the style of mantle convection and/or the mantle's volatile content need to be made. If a dry mantle rheology is assumed, strong plumes caused by, e.g., a strongly pressure dependent mantle viscosity or endothermic phase transitions near the core–mantle boundary are required to allow for decompression melting in the heads of mantle plumes. For a wet mantle, large mantle water contents of the order of 1000 ppm are required to allow for partial mantle melting. Also, for a moderate crustal enrichment of heat producing, elements the planet's bulk composition needs to be 25 and 50% sub-chondritic for dry and wet mantle rheologies, respectively. Even then, models resulting in a globally averaged elastic thicknesses of T e > 300 km are difficult to reconcile with most elastic thickness estimates available for the Hesperian and Amazonian periods. It therefore seems likely that large elastic thicknesses in excess of 300 km are not representative for the bulk of the planet and that T e possibly shows a large degree of spatial heterogeneity.

Coupled afterslip and viscoelastic flow following the 2002 Denali Fault, Alaska earthquake

Models of postseismic deformation following the 2002 M 7.9 Denali Fault, Alaska earthquake provide insight into the rheologic structure of the Alaskan lithosphere and the physical processes activated following a large earthquake. We model coseismic GPS displacements and 4 yr of postseismic GPS position time-series with a coupled model of afterslip on the fault in the lithosphere and distributed viscous flow in the asthenosphere. Afterslip is assumed to be governed by a simplified version of a laboratory-derived rate-strengthening friction law that is characterized with a single parameter, σ(a − b), where σ is the effective normal stress on the fault and a − b is a dimensionless empirical parameter. Afterslip is driven by coseismic stress changes on the fault generated by the main shock. The lithosphere is modelled as an elastic plate overlying a linear, Maxwell, viscoelastic asthenosphere. We devise a scheme to simultaneously estimate the distributions of coseismic slip and afterslip, friction parameters, lithosphere thickness and asthenosphere viscosity. The postseismic GPS time-series are best reproduced with a 45–85 km thick elastic lithosphere overlying an asthenosphere of viscosity 0.6–1.5 × 1019 Pa s. The 45–85 km elastic lithosphere thickness is greater than or equal to the average crustal thickness in the region suggesting that distributed postseismic flow occurs largely within the mantle while postseismic deformation in the crust is confined largely to slip on a discrete fault penetrating the entire crust and perhaps into the uppermost mantle. For typical laboratory values of a − b of order 10−2 at temperatures corresponding to the inferred depth of afterslip, the estimated effective normal stress on the fault is ∼50 MPa, which is about an order of magnitude lower than effective normal stresses at hydrostatic pore pressure. Previous studies showed that models with linear Newtonian rheology cannot reproduce the observed GPS time-series but that models incorporating non-linear or biviscous flow of the mantle do fit the data. We show that a model with afterslip governed by a non-linear fault zone rheology coupled to Newtonian mantle flow is sufficient to reproduce the GPS time-series.

Temperature control of continental lithosphere elastic thickness, Te vs Vs

The elastic thickness of the continental lithosphere is closely related to its total strength and therefore to its susceptibility to tectonic deformation and earthquakes. Recently it has been questioned whether the lithosphere thickness and strength are dependent on crustal and upper mantle temperatures and compositions as predicted by laboratory data. We test this dependence regionally by comparison in northwestern North America of the effective elastic thickness Te, from topography–gravity coherence, with upper mantle temperatures mapped by shear wave tomography velocities Vs and other temperature indicators. The Te values are strongly bimodal as found globally, less than 20 km for the hot Cordillera backarc and over 60 km for the cold stable Craton. These Te correspond to low Vs beneath the Cordillera and high Vs beneath the Craton. Model temperature-depth profiles are used to estimate model Te for comparison with those observed. Only limited areas of intermediate thermal regimes, i.e., thermotectonic ages of ~ 300 Ma, have intermediate Te that suggest a weak lower crust over a stronger upper mantle. There are large uncertainties in model Te associated with composition, water content, strain rate, and decoupling stress threshold. However, with reasonable parameters, model yield stress envelopes correspond to observed Te for thermal regimes with 800–900 °C at the Cordillera Moho and 400–500 °C for the Shield, in agreement with temperatures from Vs and other estimators. Our study supports the conclusion that lithosphere elastic thickness and strength are controlled primarily by temperature, and that laboratory-based rheology generally provides a good estimate of the deformation behaviour of the crust and upper mantle on geological time scales.

Gravity anomaly, lithospheric structure and seismicity of Western Himalayan Syntaxis

A compiled gravity anomaly map of the Western Himalayan Syntaxis is analysed to understand the tectonics of the region around the epicentre of Kashmir earthquake of October 8, 2005 (Mw = 7.6). Isostatic gravity anomalies and effective elastic thickness (EET) of lithosphere are assessed from coherence analysis between Bouguer anomaly and topography. The isostatic residual gravity high and gravity low correspond to the two main seismic zones in this region, viz. Indus–Kohistan Seismic Zone (IKSZ) and Hindu Kush Seismic Zones (HKSZ), respectively, suggesting a connection between siesmicity and gravity anomalies. The gravity high originates from the high-density thrusted rocks along the syntaxial bend of the Main Boundary Thrust and coincides with the region of the crustal thrust earthquakes, including the Kashmir earthquake of 2005. The gravity low of HKSZ coincides with the region of intermediate–deep-focus earthquakes, where crustal rocks are underthrusting with a higher speed to create low density cold mantle. Comparable EET (∼55 km) to the focal depth of crustal earthquakes suggests that whole crust is seismogenic and brittle. An integrated lithospheric model along a profile provides the crustal structure of the boundary zones with crustal thickness of about 60 km under the Karakoram–Pamir regions and suggests continental subduction from either sides (Indian and Eurasian) leading to a complex compressional environment for large earthquakes.

Integrated seismic, flexural and gravimetric modelling of the Coastal Cordillera Thrust Belt and the Guárico Basin, North-Central Region, Venezuela

We constructed two integrated geodynamic transects for the eastern and western areas of the Coastal Cordillera Thrust Belt and the Guárico Basin (North-Central region of Venezuela), based on information from depth-converted reflection seismic cross-sections, surface geology, Bouguer anomaly and deep refraction seismic profiling. These geodynamic transects were modelled in order to: (1) understand the geodynamic processes responsible for the formation of the Guárico Basin, (2) quantitatively determine the contribution of the Coastal Cordillera thrust sheet loading in the generation of the basin fill (i.e. the effective elastic thickness of the northern part of the South American lithosphere, the total shortening of the thrust belt and the maximum subsidence of the basin) and (3) characterize the crustal structure and Moho depth in North-Central Venezuela. We used flexural modelling to obtain important quantitative information related to the total Middle Eocene to Recent shortening of the Coastal Cordillera, the total subsidence observed in the Guárico Basin and the effective elastic thickness of the South American lithosphere in northern Venezuela. We conclude that the load of the Coastal Cordillera on the South American lithosphere is sufficient to generate the subsidence observed in the Guárico Basin. The total shortening and subsidence obtained in the western transect are 44 km and 5 km, whereas they are 10 km and 7 km, in the eastern transect, respectively. The effective elastic thickness of the South American lithosphere obtained from the modelling is 25 km. The seismically-constrained deep structure of North-Central Venezuela indicates that Moho reaches a maximum depth of 45 km under the Guayana Shield and shallows to the north, to 35 km beneath the Guárico Basin. According to the gravity models, Moho is deeper under the Coastal Cordillera Thrust Belt than expected from flexural modelling, pointing to the existence of a crustal root more than 35 km deep. The Moho gets shallower (30 km) towards the Caribbean Sea.

Geodetic strain across the San Andreas fault reflects elastic plate thickness variations (rather than fault slip rate)

The interseismic velocity field provided by geodetic methods is generally interpreted in the framework of a thick elastic lithosphere with a slipping fault at depth. Because lateral variations of lithospheric rheology play a key role in determining the geological strain distribution, I examine the idea that interseismic strain rate variations also occur in response to lateral variations in the elastic thickness of the lithosphere. Using a stress balance principle and some simplifying assumptions, I show using a 1D model that elastic thickness is inversely proportional to strain rate for the simple case of pure strike-slip faulting. Elastic thickness computed on three profiles crossing the San Andreas fault system (SAFS) suggests that the distribution of interseismic strain rate is compatible with a thick elastic lithosphere in the Great Basin-Sierra Nevada province and on the Pacific plate. Conversely, a thin plate with a shallow asthenosphere is needed on the SAFS to explain its high strain rate. A 2.5D Finite Element model of interseismic strain in the Carrizo Plain region in Central California shows how known vertical and horizontal variations of elastic properties refine 1D model predictions. In a case of a multiple fault system, I point out that the interseismic velocity is not causally tied to faults slip rate. Therefore, analysing the velocity field across the SAFS cannot reliably provide faults slip rate distribution as previously claimed. Rather, the apparent correlation between geologic slip rate and interseismic strain may only indicate that the elastic thickness plays a dominant role in controlling fault strength. Finally, I suggest that interseismic geodetic strain could be a new way to infer effective elastic plate thickness on the continents.