Effective Elastic Plate Thickness Research Articles

Intraplate stress distribution within the Indian Plate: Insights from finite element modelling

The Indian Plate is bounded by different types of plate boundaries like divergent, convergent/collisional, transform and consists of oceanic and continental lithosphere. Due to such tectonic settings, the intraplate tectonic stress distribution across the Indian Plate is very complex. Further, available in-situ stress information over the Indian Plate is relatively sparse; therefore, numerical modelling of maximum horizontal compressive stress (SHmax) is needed to comprehend regional stress patterns. In the present work, stress simulations are carried out to explore the effects of rigidity, lithospheric thickness, and boundary forces on the distribution of intraplate stress across the Indian Plate. The rigidity is derived from effective elastic plate thickness (Te), which is estimated through joint inversion of admittance and coherence using the wavelet transform approach. The stress field is simulated for four cases for different combinations of Te, lithospheric thickness and tectonic forces and compared with in-situ stress observations. The analysis of models shows that the magnitude of SHmax is primarily controlled by boundary forces, Te and scaled by lithospheric thickness; and its orientations are mostly governed by the applied tectonic forces and rigidity. Spatial distribution of earthquakes correlated well with the von-Mises stress and high von-Mises stresses are accumulated at earthquake prone regions. The stress field and von-Mises stress over major geological features are also analyzed and discussed. Apart from boundary forces, it is inferred that Te is one of the most significant factors which controls the stress field distribution.

Estimating geodynamic model parameters from geodetic observations using a particle method

SUMMARY Bayesian-based data assimilation methods integrate observational data into geophysical forward models to obtain the temporal evolution of an improved state vector, including its uncertainties. We explore the potential of a variant, a particle method, to estimate mechanical parameters of the overriding plate during the interseismic period. Here we assimilate vertical surface displacements into an elementary flexural model to estimate the elastic thickness of the overriding plate, and the locations and magnitudes of line loads acting on the overriding plate to produce flexure. Assimilation of synthetic observations sampled from a different forward model than is used in the particle method, reveal that synthetic seafloor data within 150 km from the trench are required to properly constrain parameters for long wavelength solutions of the upper plate (i.e. wavelength ∼500 km). Assimilation of synthetic observations sampled from the same flexural model used in the particle method shows remarkable convergence towards the true parameters with synthetic on-land data only for short to intermediate wavelength solutions (i.e. wavelengths between ∼100 and 300 km). In real-data assimilation experiments we assign representation errors due to discrepancies between our incorrect or incomplete physical model and the data. When assimilating continental data prior to the 2011 Mw Tohoku-Oki earthquake (1997–2000), an unrealistically low effective elastic plate thickness for Tohoku of ∼5–7 km is estimated. Our synthetic experiments suggest that improvements to the physical forward model, such as the inclusion of a slab, a megathrust interface and viscoelasticity of the mantle, including accurate seafloor data, and additional geodetic observations, may refine our estimates of the effective elastic plate thickness. Overall, we demonstrate the potential of using the particle method to constrain geodynamic parameters by providing constraints on parameters and corresponding uncertainty values. Using the particle method, we provide insights into the data network sensitivity and identify parameter trade-offs.

Mechanical strength and nature of lithospheric deformation beneath the rifted sedimentary basins of the east India passive margin

The study of gravity signatures corresponding to sediment load over the passive margins is helpful to constrain the mechanical properties of continental rift systems as well as the adjacent ocean basins. Seismic reflection, free-air gravity and well information from the East India Passive Margin (EIPM) are investigated through spectral approach and 2D process-oriented gravity modelling for quantifying effective elastic plate thickness (Te) to understand the rift mechanisms and the sedimentary basins evolution. The characteristics of lithosphere within the major structural domains along the margin are quite variable. A narrow necking zone (40 km) in the southern segment, and approximately a 70 km hyper-thinned and exhumed domains are inferred in the central segment, while a narrow hyper-thinned domain (30–35 km) is observed in the northern segment of the margin. The southern segment of EIPM, adjacent to Southern Granulite Terrain has low Te values ranging from 5 to 10 km. The segment in the central margin, offshore Dharwar Craton has moderate Te values of 15–20 km. And the northern segment adjacent to onshore Eastern Ghats Mobile Belt has relatively high mechanical strength with Te values ranging from 25 to 35 km. The pattern of Te values and extent of major structural domains along the margin indicates that the southern, central and northern segments evolved as three different modes of rifting such as transform, hyperextended and hypoextended, respectively. In the oceanic domain, the lithospheric strength broadly increases with age following the thermal induced cooling plate model (600 °C isotherm). The thick sedimentary basin of the Bay of Bengal had attained the high value of elastic thickness (Te > 45 km) in the last 23 Myrs. The significant rheological strengthening of the oceanic lithosphere may be attribute to the following reasons: 1) increase in shear-wave velocity anomaly at a depth of 150 km and upper mantle density, 2) change in plate curvature caused by excessive deposition of Bengal Fan sediments in the deep ocean basins and subduction of oceanic plate under the Eurasian plate near the Bengal Fan and 3) modification of stress state of the lithosphere as a convergent interaction of the Indian plate with SE Asia.

Intra-trench variations in flexural bending of the subducting Pacific Plate along the Tonga-Kermadec Trench

We conducted a detailed analysis of along-trench variations in the flexural bending of the subducting Pacific Plate at the Tonga-Kermadec Trench. Inversions were conducted to obtain best-fitting solutions of trench-axis loadings and variations in the effective elastic plate thickness for the analyzed flexural bending profiles. Results of the analyses revealed significant along-trench variations in plate flexural bending: the trench relief (W0) of 1.9 to 5.1 km; trench-axis vertical loading (V0) of −0.5×1012 to 2.2×1012 N/m; axial bending moment (M0) of 0.1×1017 to 2.2×1017 N; effective elastic plate thickness seaward of the outer-rise region (\(T_{\rm{e}}^{\rm{M}}\)) of 20 to 65 km, trench-ward of the outer-rise (\(T_{\rm{e}}^{\rm{m}}\)) of 11 to 33 km, and the transition distance of 20 to 95 km. The Horizon Deep, the second greatest trench depth in the world, has the greatest trench relief (w0 of 5.1 km) and trench-axis loading (V0 of 2.2×1012 N/m); these values are only slightly smaller than that of the Challenger Deep (W0 of 5.7 km and V0 of 2.9×1012 N/m) and similar to that of the Sirena Deep (W0 of 5.2 km and V0 of 2.0×1012 N/m) of the Mariana Trench, suggesting that these deeps are linked to great flexural bending of the subducting plates. Analyses using three independent methods, i.e., the \(T_{\rm{e}}^{\rm{M}}\)/\(T_{\rm{e}}^{\rm{m}}\) inversion, the flexural curvature/yield strength envelope analysis, and the elasto-plastic bending model with normal faults, all yielded similar average Te reduction of 28%–36% and average Te reduction area \({S_{\Delta {T_{\rm{e}}}}}\) of 1 195–1 402 km2 near the trench axis. The calculated brittle yield zone depth from the flexural curvature/yield strength envelope analysis is also consistent with the distribution of the observed normal faulting earthquakes. Comparisons of the Manila, Philippine, Tonga-Kermadec, Japan, and Mariana Trenches revealed that the average values of \(T_{\rm{e}}^{\rm{M}}\) and \(T_{\rm{e}}^{\rm{m}}\) both in general increase with the subducting plate age.

Flexural bending curvature and yield zone of subducting plates

ABSTRACT We quantify flexural deformation of subducting oceanic plates at a global array of 15 ocean trenches, using a new approach of modelling spatial variations in flexural bending shape and curvature. The investigated trenches are chosen to represent a diverse range of subducting plate age of 24–150 Ma, including the Middle America, Peru, Chile, West and East Aleutian, Sumatra, North and South Philippine, Tonga, Kermadec, Kuril, Japan, Izu-Bonin, and South and North Mariana. The studied trenches show systematic intra- and inter-trench variability in the calculated flexural bending curvature, stress distribution, extensional brittle yield zone, and effective elastic plate thickness Te of the subducting plates. We find that subducting plate age is a critical factor controlling the bending curvature and the corresponding extensional yield zone. The width, depth, and area of the extensional yield zone are all calculated to increase systematically with the subducting plate age. The newly-developed curvature analysis can yield continuously varying apparent Te(x) from the trench axis to outer rise. The calculated extensional yield zones from the curvature analysis are in general consistent with the observed normal faulting earthquakes of magnitude≥6.0 at the 15 global trenches. Our analyses also reveal that the five deepest regions of the global trenches, i.e. the Challenger, Horizon, Serena, and Scholl Deeps and Galathea Depth, are associated with relatively large flexural bending and calculated yield zones comparing to their respective adjacent trench segments. The Serena and Challenger Deeps of the S. Mariana trench are calculated to have the largest flexural bending among the five deeps.

Crustal underplating and effective elastic plate thickness of the Laxmi Ridge, northern Arabian Sea

Laxmi Ridge, a prominent basement high feature with negative gravity anomaly in the northern Arabian Sea, is generally considered as stretched continental crust. However, recently it is proposed that the ridge could be an extinct spreading ridge. Ship-borne and satellite-altimetry derived bathymetry, gravity and geoid data over the Laxmi Ridge are analysed and modelled to determine effective elastic plate thickness (Te) and crustal structure beneath the ridge. The estimation of Te and compensation mechanism of the ridge are examined based on 3D coherence and admittance analysis considering surface and subsurface loads which involve computation of coherence and admittance between Mantle Bouguer Anomaly (MBA) and sediment corrected bathymetry. The best fit model reveals that Te is about 3–4 km and amplitude of subsurface load at Moho is approximately equal to the surface load. Low value of Te implies elastically weak lithospheric plate which can be attributed to either weakening of lithosphere by re-heating and underplating caused by the then nearby Réunion Hotspot or volcanic emplacement on the young oceanic crust. Further, all along the length of the Laxmi Ridge, the MBA is negative and its magnitude reaches up to 115 mGal, which indicates either presence of thickened crust or less dense mantle beneath the ridge. 2D crustal models suggest that underplating material is prevalent all along the Laxmi Ridge. High amplitude (>400 nT) magnetic anomaly over the southern part of the ridge reveals several lineations in WNW-ESE direction suggesting magma outpouring along week zones. Geoid to topographic ratio over the ridge is negative (−1.98 m/km), possibly indicating shallow subsurface load (<20 km) with small wavelength (<200 km). Based on present study and available geophysical results, we infer that the Laxmi Ridge is a stretched continental sliver which is underplated, intruded by volcanics and isostatically compensated by Airy Model of Compensation.

Elasto-plastic deformation and plate weakening due to normal faulting in the subducting plate along the Mariana Trench

We investigated variations in the elasto-plastic deformation of the subducting plate along the Mariana Trench through an analysis of flexural bending and normal fault characteristics together with geodynamic modeling. Most normal faults were initiated at the outer-rise region and grew toward the trench axis with strikes mostly subparallel to the local trench axis. The average trench relief and maximum fault throws were measured to be significantly greater in the southern region (5 km and 320 m, respectively) than the northern and central regions (2 km and 200 m). The subducting plate was modeled as an elasto-plastic slab subjected to tectonic loading at the trench axis. The calculated strain rates and velocities revealed an array of normal fault-like shear zones in the upper plate, resulting in significant faulting-induced reduction in the deviatoric stresses. We then inverted for solutions that best fit the observed flexural bending and normal faulting characteristics, revealing normal fault penetration to depths of 21, 20, and 32 km beneath the seafloor for the northern, central, and southern regions, respectively, which is consistent with the observed depths of the relocated normal faulting earthquakes in the central Mariana Trench. The calculated deeper normal faults of the southern region might lead to about twice as much water being carried into the mantle per unit trench length than the northern and central regions. We further calculated that normal faulting has reduced the effective elastic plate thickness Te by up to 52% locally in the southern region and 33% in both the northern and central regions. The best-fitting solutions revealed a greater apparent angle of the pulling force in the southern region (51–64°) than in the northern (22–35°) and central (20–34°) regions, which correlates with a general southward increase in the seismically-determined dip angle of the subducting slab along the Mariana Trench.

Dominant influence of volcanic loading on vertical motions of the Hawaiian Islands

Rates of island vertical motion above intra-plate hotspots record the processes that arise from interactions between lithospheric plates and mantle plumes. To assess the contribution of mantle and lithospheric processes to surface motion above the Hawaiian hotspot, we compare simple models of lithospheric deformation to vertical motion rates measured from dated paleoshorelines and tide gauge records in the Hawaiian Islands. Our analysis shows that observed uplift and subsidence rates mainly record the flexural response of the lithosphere to volcanic loads. The effective elastic plate thickness that best fits the spatial distribution of subsidence and uplift rates is ∼40 km, consistent with previous estimates based on total vertical deflection.Because volcanic loading dominates the vertical motion signal, Hawaiian Islands appear to follow a predictable trajectory of vertical motion when they reside within one flexural half-wavelength of the active volcanic center. Islands initially subside at rapid and decreasing rates in the first ∼1 Myr following their construction, uplift relatively slowly ∼1–2.5 Myr following their construction, and eventually subside again, but at slow rates, within ∼5 Myr of their construction. This observed pattern of uplift and subsidence is consistent with the pattern of vertical motion predicted to result from volcanic loading at the Hawaiian hotspot.Lithospheric migration over the long-wavelength topographic swell associated with the Hawaiian hotspot has a comparatively minor influence on island uplift and subsidence. Its contribution to island vertical motion is not readily observed in our data, with the possible exception of some uplift observed in the past ∼500 kyr on O‘ahu that might correspond to non-steady state behavior of the Hawaiian plume.

Variations in oceanic plate bending along the Mariana trench

We quantify along-trench variations in plate flexural bending along the Mariana trench in the western Pacific Ocean. A 3-D interpreted flexural deformation surface of the subducting Pacific Plate was obtained by removing from the observed bathymetry the effects of sediment loading, isostatically-compensated topography based on gravity modeling, age-related lithospheric thermal subsidence, and residual short-wavelength features. We analyzed flexural bending of 75 across-trench profile sections and calculated five best-fitting tectonic and plate parameters that control the flexural bending. Results of analysis revealed significant along-trench variations: the trench relief varies from 0.9 to 5.7 km, trench-axis vertical loading (−V0) from −0.73×1012 to 3.17×1012 N/m, and axial bending moment (−M0) from 0.1×1017 to 2.7×1017 N. The effective elastic plate thickness seaward of the outer-rise region (TeM) ranges from 45 to 52 km, while that trench-ward of the outer-rise (Tem) ranges from 19 to 40 km. This corresponds to a reduction in Te of 21–61%. The transition from TeM to Tem occurs at a breaking distance of 60–125 km from the trench axis, which is near the outer-rise and corresponds to the onset of observed pervasive normal faults. The Challenger Deep area is associated with the greatest trench relief and axial vertical loading, while areas with seamounts at the trench axis are often associated with more subtle trench relief, smaller axial vertical loading, and greater topographic bulge at the outer-rise.

Thermal models of dyke intrusion during development of continent–ocean transition

A consensus has emerged in recent years from a variety of geoscientific disciplines that extension during continental rifting is achieved only partly by plate stretching: dyke intrusion also plays an important role. Magma intrusion can accommodate extension at lower yield stresses than are required to extend thick, strong, unmodified continental lithosphere mechanically, thereby aiding the breakup process. Dyke intrusion is also expected to heat and thereby weaken the plate, but the spatial extent of heating and the effect of different rates of magmatic extension on the timescales over which heating occurs are poorly understood. To address this issue, a numerical solution to the heat-flow equation is developed here to quantify the thermal effects of dyke intrusion on the continental crust during rifting. The thermal models are benchmarked against a priori constraints on crustal structure and dyke intrusion episodes in Ethiopia. Finite difference models demonstrate that magmatic extension rate exerts a first-order control on the crustal thermal structure. Once dyke intrusion supersedes faulting and stretching as the principal extensional mechanism the crust will heat and weaken rapidly (less than 1 Ma).In the Main Ethiopian Rift (MER), the majority of present-day extension is focused on ∼20 km-wide Quaternary-Recent axial magmatic segments that are mostly seismogenic to mid-crustal depths and show P-wave seismic velocities characteristic of heavily intruded continental crust. When reviewed in light of our models, these observations require that no more than half of the MER's extension since ∼2 Ma has been achieved by dyke intrusion. Magmatic heating and weakening of the crust would have rendered it aseismic if dyke intrusion accounted for the entire 6 mm/yr extension rate. In the older, faster extending (16 mm/yr) Red Sea rift (RSR) in Afar, dyke intrusion is expected to have had a more dramatic impact on crustal rheology. Accordingly, effective elastic plate thickness and Moho depth in the Danakil region of northernmost Afar are markedly reduced and seismicity is shallower than in the MER. Thermally driven variations in crustal rheology over time in response to dyke intrusion thus play an important role in the development of continent–ocean transition.

Crustal structure of the Khartoum Basin, Sudan

The crustal structure of the northern part of the Khartoum Basin has been investigated using data from 3 permanent seismic stations within 40km of Khartoum and two modeling methods, H–k stacking of receiver functions and a joint inversion of receiver functions and Rayleigh wave group velocities. The Khartoum Basin is one of several Mesozoic rift basins in Sudan associated with the Central African Rift System. Results from the H–k-stacking indicate that crustal thickness beneath the Khartoum Basin ranges between 33 and 37km, with an average of 35km, and that the crustal Vp/Vs ratio ranges from 1.74 to 1.81, with an average of 1.78. From the joint inversion of receiver functions and Rayleigh wave group velocities, we obtained similar results for Moho depth, as well as an average shear wave velocity of 3.7km/s for the crust. These results provide the first seismic estimates of Moho depth for a basin in Sudan. When compared to average crustal thickness for unrifted Proterozoic crust in eastern Africa, our results indicate that at most only a few km of crustal thinning may have occurred beneath the Khartoum Basin. This finding is consistent with estimates of effective elastic plate thickness, which indicate little modification of the Proterozoic lithosphere beneath the basin, and suggests that there may be insufficient topography on the lithosphere–asthenosphere boundary beneath the Sudanese basins to channel plume material westward from Ethiopia.

How rigid is Europe's lithosphere?

The integrated strength distribution and variations of the effective elastic plate thickness (Te) have been estimated for the European lithosphere based on thermal and rheological data for the crust and upper mantle. The new results show a significant spatial variability demonstrating that both ‘jelly sandwich’ and ‘crème brûlée’ models might be valid depending on lithospheric physical conditions. In most of Europe crustal strength provides a relatively large contribution (∼50%) to the lithospheric strength. Western Europe appears mostly characterized by mechanically decoupled lithospheric layers, low strength and Te &lt; 30 km. The contribution of the mechanically strong mantle to Te is low in most parts of western Europe. No clear relationship between Te and thermal age is found in the continent: the values for the tectonic provinces older than 85 Ma are significantly smaller than theoretically expected for their age and crustal thickness, whereas the opposite is true for the younger provinces.

Dislocation models of interseismic deformation in the western United States

The GPS‐derived crustal velocity field of the western United States is used to construct dislocation models in a viscoelastic medium of interseismic crustal deformation. The interseismic velocity field is constrained by 1052 GPS velocity vectors spanning the ∼2500‐km‐long plate boundary zone adjacent to the San Andreas fault and Cascadia subduction zone and extending ∼1000 km into the plate interior. The GPS data set is compiled from U.S. Geological Survey campaign data, Plate Boundary Observatory data, and the Western U.S. Cordillera velocity field of Bennett et al. (1999). In the context of viscoelastic cycle models of postearthquake deformation, the interseismic velocity field is modeled with a combination of earthquake sources on ∼100 known faults plus broadly distributed sources. Models that best explain the observed interseismic velocity field include the contributions of viscoelastic relaxation from faulting near the major plate margins, viscoelastic relaxation from distributed faulting in the plate interior, as well as lateral variations in depth‐averaged rigidity in the elastic lithosphere. Resulting rigidity variations are consistent with reduced effective elastic plate thickness in a zone a few tens of kilometers wide surrounding the San Andreas fault (SAF) system. Primary deformation characteristics are captured along the entire SAF system, Eastern California Shear Zone, Walker Lane, the Mendocino triple junction, the Cascadia margin, and the plate interior up to ∼1000 km from the major plate boundaries.

Isostatic response of the Laccadive Ridge from admittance analysis of gravity and bathymetry data

The Laccadive Ridge (L-R), trending roughly parallel to the west coast of India, is an intriguing segment of the northernmost Chagos-Laccadive Ridge (C-L-R) system. Although crustal nature and isostatic response of the southern C-L-R is well known, there are no similar studies on the L-R. In the present study, the isostatic response of the lithosphere beneath the L-R is estimated so as to characterize its crustal nature, total crustal as well as effective elastic plate thickness and mode of compensation. Twelve gravity and bathymetry profiles across the ridge were analyzed using linear transfer function and forward model techniques. The observed admittance function within the diagnostic waveband of 250 < λ > 80 km (0.025 < k > 0.080 km−1) fits well with (i) the Airy model whose average crustal thickness (Tc) and density are 17 ± 2 km and 2.7 × 103 kg m−3, respectively, and (ii) the thin plate flexure model of isostasy with an effective elastic plate thickness (Te) of 2–3 km. The estimated average crustal thickness and density are in good agreement with published seismic refraction results over the ridge. The results of the present study support an Airy model of isostasy for the L-R. The low Te value, in view of other published results in the study area, suggests stretched and loaded continental lithosphere of the L-R during the evolution of the western continental margin of India.

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.

3D strength and gravity anomalies of the European lithosphere

Tectonic studies show that intraplate Europe is more active than might be expected from its tectonic setting. We employ integrative analysis of various datasets, which provides new understanding of deformation processes in response to different types of loading. A 3D strength model of the lithosphere, which is based on existing structural, compositional and thermal models of the lithosphere, forms the basis for this study. Boundaries of the crustal layers are determined from existing seismic data. Constraints on the thermal structure of the lithosphere are obtained from heat flow studies and upper mantle seismic tomography. The calculated strength distribution is used to quantify spatial variations in effective elastic plate thickness (EET) of the European lithosphere. A comparison with seismic and geodetic strain rates provides new insights into the localization of intraplate deformation. Another important indication of lithospheric strength is provided by the residual mantle gravity anomalies obtained after removing the crustal contribution from the observed gravity field. We have found significant spatial variations in rigidity within the Central and Western European lithosphere, in contrast with a more uniform strong lithosphere of the East European Platform. These variations are consistent with differences in the density structure of the mantle lithosphere reflected in mantle gravity anomalies. Western Europe is mostly characterized by negative mantle gravity anomalies and low EET values (generally ≤ 20 km), whereas the opposite is true for Eastern Europe. Large differences exist also for specific tectonic units: e.g. a pronounced contrast in lithosphere properties is found between the strong Adriatic plate and the weak Pannonian Basin area, as well as between the Baltic Shield and the North Sea rift system. We demonstrate that the European Cenozoic Rift System (ECRIS), which is characterized by low EET values and negative mantle gravity anomalies, localizes neotectonic deformation manifested by relatively high geodetic and seismic strain rates.

Variation in effective elastic plate thickness of the East Africa lithosphere

We use new and existing Bouguer gravity data to characterize the long‐term flexural rigidity of the lithosphere beneath East Africa. Four tectonic provinces are characterized by distinct effective elastic plate thicknesses. These are the Tanzanian Craton, the Proterozoic Mobile Belts, the Paleogene rift basins in Sudan and Kenya, and the Cenozoic East African Rift System. Beneath the Tanzanian craton the elastic plate thickness is ∼75 km, while the Pan African Mobile Belts and the Ubendian Belts are ∼55 and ∼65 km, respectively. The Paleogene rift basins exhibit an elastic plate thickness varying between 40 and 45 km. The effective elastic plate thickness within the faulted East African Rift System ranges from 14 km in the north to 33 km in the south. The minimum is found beneath the active rift of the Afar depression. The north to south along‐rift axis variation in effective elastic plate thickness can be attributed to contrasts in the mechanical strength of the lithosphere. This is consistent with the north‐south decrease in extensional velocity and the northward decrease in the focal depths of the earthquakes, reflecting a northward thinning in the brittle layer of the crust. The maximum elastic plate thickness is found beneath the Tanzanian craton, while the surrounding Mobile Belts have relatively moderate elastic plate thicknesses. The high effective elastic plate thickness of the Tanzanian craton suggests that this part of the lithosphere is rheologically competent compared to the surrounding Mobile Belts. The craton effectively resists deformation, even though it is located within a broad zone of an east‐west extensional tectonic regime. Consequently, it has altered the direction of the rift propagation into the warmer and weaker lithosphere of the surrounding Mobile Belts.

Lateral variations of effective elastic thickness of the subducting Philippine Sea plate along the Nankai trough

We investigated lateral variations of effective elastic thickness of the subducting Philippine Sea plate along the Nankai trough. For this purpose, we applied two-dimensional thin elastic plate bending theory to several profiles almost perpendicular to the trough axis, correcting the effect of thick marine sediments in the Shikoku Basin. The 15 topographic profiles were selected from 61 profiles and were fit to respective theoretical deflection curves by minimization of the L1 norm. Classifying the Shikoku Basin into three regions, namely, the western, the middle, and the eastern regions, we obtained the results that the effective elastic plate thickness tends to decrease gradually from the east of the Ryukyu-Palau ridge to the Kinan seamount chain. This is consistent with heat flow data, seafloor age, formation process of the Shikoku Basin, length of the slab subducted from the Nankai trough, and spatial distribution of subcrustal earthquakes associated with subduction of the Philippine Sea plate.

Tectonic modification of axial crustal structure: Evidence from spectral analyses of residual gravity and bathymetry of the Mid-Atlantic Ridge flanks

Bathymetry and gravity data of the northern Atlantic suggest that oceanic crustal structure on the flanks of the Mid-Atlantic Ridge is strongly modified by tectonic extension at spreading ridge segments. The seafloor along the zero-age isochron is systematically more elevated and the crust thicker near the segment midpoint than at the ends, suggesting focused magmatic accretion at segment centers. In contrast, seafloor older than 2–3 Ma is usually the shallowest at inside corners of ridge-offset intersections, where positive residual gravity anomalies indicate tectonically thinned crust. Coherence between residual gravity and bathymetry for zero-age crust is found at wavelength greater than ∼20 km, which reflects the characteristic length of ridge segments, and is consistent with an effective elastic plate thickness of 1–3 km. No significant coherence, however, is observed on isochron profiles at ages of 2.5 Ma and older at all wavelengths. Flow-line profiles display coherence at wavelength greater than ∼5 km, yielding an estimated effective elastic plate thickness of only 0.3–0.5 km. Synthetic fault models suggest that the flow-line coherence may be controlled by the relatively close spacing of fault-generated abyssal hills rather than reflecting the true elastic thickness of unfaulted lithosphere. Together these results indicate that the crustal structure of the zero-age crust is modified strongly by tectonic extension along the rift valley walls of the slow-spreading Mid-Atlantic Ridge.

Localization of gravity and topography: constraints on the tectonics and mantle dynamics of Venus

We develop a method for spatio-spectral localization of harmonic data on a sphere and use it to interpret recent high-resolution global estimates of the gravity and topography of Venus in the context of geodynamical models. Our approach applies equally to the simple spatial windowing of harmonic data and to variable-length-scale analyses, which are analogous to a wavelet transform in the Cartesian domain. Using the variable-length-scale approach, we calculate the localized RMS amplitudes of gravity and topography, as well as the spectral admittance between the two fields, as functions of position and wavelength. The observed admittances over 10 per cent of the surface of Venus (highland plateaus and tessera regions) are consistent with isostatic compensation of topography by variations in crustal thickness, while admittances over the remaining 90 per cent of the surface (rises, plains and lowlands) indicate that long-wavelength topography is dominantly the result of vertical convective tractions at the base of the lithosphere. The global average crustal thickness is less than 30 km, but can reach values as large as 40 km beneath tesserae and highland plateaus. We also note that an Earth-like radial viscosity structure cannot be rejected by the gravity and topography data and that, without a mechanical model of the lithosphere, admittance values cannot constrain the thickness of the thermal boundary layer of Venus. Modelling the lithosphere as a thin elastic plate indicates that at the time of formation of relief in highland plateaus and tesserae, the effective elastic plate thickness, Te, was less than 20 km. Estimates of Te at highland rises are consistently less than 30 km. Our inability to find regions with Te > 30 km is inconsistent with predictions made by a class of catastrophic resurfacing models.