Variations In Effective Elastic Thickness Research Articles

Spatial variations in effective elastic thickness and loading ratio in the Indo-Burma subduction zone based on the joint inversion of Bouguer coherence and admittance

Our knowledge of the mechanical structure and geodynamic evolution of the Indo-Burma Subduction (IBS) Zone is currently limited. The effective elastic thickness (Te) reflect the thermo-rheological characteristics of the subduction zone that aid in understanding the long-term deformation and geodynamic evolution. In the present study, we employed joint inversion of wavelet admittance and coherence to obtain the spatial variation in Te and the loading structure (F) over IBS based on satellite gravity and topography data. The spatial pattern of Te and F exhibit a remarkable correlation with the tectonic features of IBS. The Indo-Burma ranges (IBR) and the Central Myanmar basins (CMB) in the Burma Terrain (BT) are characterized by high lithospheric strength (Te = 35–40 km). In comparison, low lithospheric strength (Te = 15–20 km) prevails over the Shan Plateau (SP). High F (0.6–0.8) values over the Bay of Bengal and the SP indicate the dominance of the internal loading, while medium F (0.5–0.6) over the CMB suggest mixed loading, probably related to the combined effect of thick sediments and dense downgoing slabs. Comparative analysis of Te with shear wave velocity and curie point depth indicates that the thermal state is crucial in controlling the lithosphere strength. Further, the absence of modern interplate seismic activity over the strong coupling zones characterized by high Te in BT suggests that the Indo-Burma megathrust is locked.

Spatial variations of effective elastic thickness of Eastern Arabia: Implications for lithospheric weakening in the Oman-UAE mountain belt

The continental lithosphere of the Arabian Plate has evolved through a long and complex tectonic history and comprises major geological features such as an ophiolite complex, Cenozoic basalt extrusion, and Proterozoic basement outcrops. In eastern Arabia, an NW-SE to N-S oriented orogenic belt is present in Oman and the United Arab Emirates (UAE). The mountain belt formed due to the Late Cretaceous obduction of the Semail ophiolite, where a thick slab of oceanic crust and upper mantle was emplaced on the rifted Arabian continental margin. However, the mechanical strength of the lithosphere over the entire region is poorly known. In this study, we employ Bouguer gravity anomaly and topographic data, along with data on crustal properties obtained from teleseismic analyses, to assess the spatial variations in the effective elastic thickness (Te) of the eastern Arabian Plate. The joint inversion of coherence and admittance yielded Te values ranging from 10 to 50 km and an initial subsurface-to-surface load ratio (F) of 0.1 to 0.8. Te values in the southern part of eastern Arabia are low (< 20 km), while the northeastern regions of Saudi Arabia, the UAE, and Qatar exhibit intermediate to high Te values ranging from 30 to 45 km. The Oman-UAE mountain belt is associated with an intermediate Te of 30 km in the northern segment and a low Te of 20 km in the southern segment. The reduction in Te observed along the strike of the mountain belt suggests weakening of the lithosphere during the obduction of the Semail ophiolite. Te correlates well with both the seismic thickness of the lithosphere (Ts) and the decrease in shear-wave velocity at a depth of 150 km. The seismogenic thickness in eastern Arabia, however, is about 15 km, which is significantly less than Te and shows that both the crust and upper mantle contribute to the strength of the continental lithosphere. The Ts/Te ratio ranges from 1 to 9, with a maximum peak at 3–5, which is similar to the results observed for the oceanic lithosphere.

Crustal structure and tectonic evolution of Greater Maldive Ridge, Western Indian Ocean, in the context of plume-ridge interaction

The Greater Maldive Ridge (GMR), consisting of the Maldive Ridge (MR) and Deep Sea Channel (DSC) region, is the N-S trending, middle segment of Chagos-Laccadive Ridge system. This ridge system is widely considered to be the repository representing the interaction between Central Indian Ridge spreading centre and Reunion plume. The present study investigates in detail, spatial variations in effective elastic thickness (Te), isostasy and crustal structure of GMR using high resolution satellite derived gravity, residual geoid and bathymetry data. Estimated Te values along the GMR from 2D and 3D flexural modelling ranges from 6.5–16.5 km with comparatively lower Te values over MR (7–9 km) and slightly higher values over the DSC region (>10 km). Geoid to Topography Ratio computed for two wavelength bands shows almost similar kind of variation along the ridge with a maximum value of 1.4 m/km in the DSC region, decreasing northwards to 0.6 m/km over MR. Integrating results from the present study with crustal thickness, Moho undulations and Curie depth along the entire length of the GMR suggest that MR was formed in the vicinity of spreading centre while DSC region was under a long transform fault which has given rise to the gap zone between Chagos Bank and Maldive Ridge during plume-ridge interaction.

Spatial variations of effective elastic thickness of the lithosphere in the Okinawa Trough

The Okinawa Trough is an active back-arc basin of the Ryukyu island arc-trench system caused by the Phillippine Sea plate subduction. Its unique location makes it attractive to study the lithosphere of continental margins, and it is of great significance to understand the dynamic mechanism of the trench-arc-basin convergent plate boundary. In this paper, we developed the spectral correlation method to calculate the effective elastic thickness of the lithosphere in the study area. The results show that the effective elastic thickness of the lithosphere is distributed in the range of 9–30 km and the lithosphere strength gradually decreases accordingly. Comparing the spatial variations of effective elastic thickness with the data sets of tectonic activities, we conclude that: The spatial variations of the effective elastic thickness have the negative correlation with that of the heat flow. The thermal tectonic activities of the trough gradually become intense from north to south. There are close links between the spatial variations of the effective elastic thickness and the distribution earthquakes. Most of earthquakes are distributed in the relatively lower effective elastic thickness areas or steep effective elastic thickness gradient zones, which indicate that the weak lithosphere and areas with steep change of effective elastic thickness are prone to accumulate and then release tectonic stresses causing earthquakes. The total characteristics of tectonic activities of the Okinawa Trough indicate a strong trend from north to south in the deep region, and the opposite in the shallow region.

Effective elastic thickness of Zealandia and its implications for lithospheric deformation

Zealandia was once located at the active Gondwana margin and experienced complicated tectonic evolution from continental breakup to subduction and oblique slipping along the plate boundary. Here, we investigate the lithospheric thermal and rheological properties via spatial variations in effective elastic thickness (Te). Using the topography, sediment thickness and gravity data, the fan wavelet coherence method is employed to recover Te values. The effects of dynamic topography and gravity in the subduction zone on the estimated Te are considered and removed. The final results reveal that the Te variation pattern generally coincides well with the tectonic regimes. Relatively high Te values up to 22 km are observed in North Island and the southernmost part of South Island, while central and northern South Island are dominated by smaller Te values (~15 km) similar to those of the surrounding submerged plateaus. We interpret the relatively high Te values as indicating the combined lithospheric strength due to the contacting plates in the subduction zone. To the north along the Tonga-Kermadec subduction zone, the lithosphere surprisingly exhibits anomalously high Te values, ranging from 22 to 36 km, which contradicts the geological evidence of normal faults developed in the incoming Pacific Plate and the magmatism in the overlying Australian Plate that generally contribute to low lithospheric strength. We speculate that the high-Te zone is related to the large bathymetry and gravity anomalies in the bending plate and controlled by non-isostatic factors. Finally, we find widespread low Te values covering the entire arcs and oceanic basins in northern Zealandia, which may indicate that regional lithospheric thinning associated with delamination weakened the integrated strength of the lithosphere.

Isostasy and crustal structure of the Chagos–Laccadive Ridge, Western Indian Ocean: Geodynamic implications

The Chagos–Laccadive Ridge (CLR), a prominent linear aseismic ridge in the Western Indian Ocean is believed to be a trace of the Réunion hotspot. In order to understand the mode of emplacement of this ridge and the nature of the underlying crust, we carried out three-dimensional (3D) flexural modelling and coherence analysis of satellite-derived gravity and bathymetry data along the ridge. The analysis revealed variations in Effective Elastic Thickness ( $$T_{\mathrm{e}}$$ ) along the CLR. While the northernmost part of CLR is associated with low $$T_{\mathrm{e}}$$ of 3 km with a subsurface to surface loading ratio (f) of 1, towards the south, the Maldive Ridge and the Chagos Bank have a fairly uniform $$T_{\mathrm{e}}$$ of 8–10 km with a very low loading ratio f of 0.1–0.2. We consider the Laccadive Ridge as a continental sliver possessing underplated magmatic rocks caused by the Réunion hotspot volcanism. The Maldive Ridge and the Chagos Bank appear to have emplaced on a lithosphere of intermediate strength possibly on the flanks of the Central Indian Ridge.

Spatial variations of effective elastic thickness of the lithosphere in Central America and surrounding regions

As a proxy for long-term lithospheric strength, the effective elastic thickness (Te) can be used to understand the relationship between lithospheric rheology and geodynamic evolution of complex tectonic settings. Here we present, for the first time, high-resolution maps of spatial variations of Te in Central America and surrounding regions from the analysis of the coherence between topography and Bouguer gravity anomaly using multitaper and wavelet methods. Regardless of the technical differences between the two methods, there is a good overall agreement in the spatial variations of Te recovered from both methods. Although absolute Te values can vary in both maps, the qualitative Te structure and location of the main Te gradients are very similar. The pattern of the Te variations in Central America and surrounding regions agrees well with the tectonic provinces in the region, and it is closely related to major tectonic boundaries, where the Middle American and Lesser Antilles subduction zones are characterized by a band of high Te on the downgoing slab seaward of the trenches. These high Te values are related to internal loads (and in the case of the southernmost tip of the Lesser Antilles subduction zone also associated with a large amount of sediments) and should be interpreted with caution. Finally, there is a relatively good correlation, despite some uncertainties, between surface heat flow and our Te results for the study area. These results suggest that although this area is geologically complex, the thermal state of the lithosphere has profound influence on its strength, such that Te is strongly governed by thermal structure.

Spatial variations of effective elastic thickness over the Ninetyeast Ridge and implications for its structure and tectonic evolution

We present new data on the strength of oceanic lithosphere along the Ninetyeast Ridge (NER) from two independent methods: spectral analysis (Bouguer coherence) using the fan wavelet transform technique, and spatial analysis (flexure inversion) with the convolution method. The two methods provide effective elastic thickness (T-e) patterns that broadly complement each other, and correlate well with known surface structures and regional-scale features. Furthermore, our study presents a new high resolution database on the Moho configuration, which obeys flexural isostasy, and exhibit regional correlations with the T-e variations. A continuous ridge structure with a much lower T-e value than that of normal oceanic lithosphere provides strong support for the hotspot theory. The derived T-e values vary over the northern (higher T-e similar to 10-20 km), central (anomalously low T-e similar to 0-5 km), and southern (low T-e similar to 5 km) segments of the NER. The lack of correlation of the T-e value with the progressive aging of the lithosphere implies differences in thermo-mechanical setting of the crust and underlying mantle in different parts of the NER, again indicating diversity in their evolution. The anomalously low T-e and deeper Moho (similar to 22 km) estimates of the central NER (between 0.5 degrees N and 17 degrees S) are attributed to the interaction of a hotspot with the Wharton spreading ridge that caused significant thermal rejuvenation and hence weakening of the lithosphere. The higher mechanical strength values in the northern NER (north of 0.5 degrees N) may support the idea of off-ridge emplacement and a relatively large plate motion at the time of volcanism. The low T-e and deeper Moho (similar to 22 km) estimates in the southern part (south of 17 degrees S) suggest that the lithosphere was weak and therefore younger at the time of volcanism, and this supports the idea that the southern NER was emplaced on the edge of the Indian plate. (C) 2013 Elsevier B.V. All rights reserved.

Spatial variations in effective elastic thickness in the Western Pacific Ocean and their implications for Mesozoic volcanism

We have used free-air gravity anomaly and bathymetric data, together with a moving window admittance technique, to determine the spatial variation in oceanic elastic thickness, T e , in the Western Pacific ocean. Synthetic tests using representative seamounts show that T e can be recovered to an accuracy of ± 5 km for plates up to 30 km thick, with increased accuracy of ± 3 km for T e ≤ 20 km. The Western Pacific has a T e range of 0–50 km, with a mean of 9.4 km and a standard deviation of 6.8 km. The T e structure of the region is dominated by relatively high T e over the Hawaiian-Emperor Seamount Chain, intermediate values over the Marshall Islands, Gilbert Ridge, and Marcus-Wake Guyots, and low values over the Line Islands, Mid-Pacific Mountains, Caroline Islands, Shatsky Rise, Hess Rise, and Musician Seamounts. Plots of T e at sites with radiometric ages suggest that T e is to first order controlled by the age of the lithosphere at the time of loading. In areas that backtrack into the South Pacific Isotopic and Thermal Anomaly (SOPITA), T e may be as low as the depth to the 180 ± 120 °C isotherm at least locally. In the northern part of the study area including the Hawaiian-Emperor Seamount Chain, T e correlates with the depth to 310 ± 120 °C. These best-fitting isotherms imply peak rates of volcanism during 100–120 Ma (Early Cretaceous) and 140–150 Ma (Late Jurassic). The corresponding addition of 8 × 10 6 km 3 and 4 × 10 6 km 3 of volcanic material to the surface of the oceanic crust would result in long-term sea-level rises of 20 m and 10 m respectively. The Late Jurassic volcanic event, like the later Early Cretaceous event, appears to have influenced the tectonic evolution of the Pacific plate convergent boundaries, resulting in increased volcanism and orogenesis.

Effective elastic thickness variations along the Andean margin and their relationship to subduction geometry

We present a new map of the spatial variations in effective elastic thickness, Te, along the Andes estimated using Bouguer coherence. The Te variations reflect interactions between subducting slab and preexisting terrane structure. In the forearc, conductive cooling of the continent by the subducting slab exerts primary control on rigidity, resulting in Te that is highest (∼40 km) where the oceanic lithosphere is oldest and coldest (∼20°S). In the central Andes, Te is relatively low (∼20 km) along the volcanic chain and the Altiplano and Puna plateaus. We interpret this weakening to reflect a high geothermal gradient maintained by advective magmatic processes, a shallow and hot asthenosphere, and a very weak lower crust throughout this region. East of the plateaus, high Te delineates underthrusting of the Brazilian shield. Finally, north and south of the plateaus, flat subduction areas are characterized by high Te, high shear wave velocity, thick thermal lithosphere, and low heat flow, indicating that continental lithosphere there is thicker, colder, and stronger. On the basis of these relationships we suggest that variations in slab dip along the margin relate to variations in structure of the continental lithosphere. In particular, we propose that upper plate structure influences the width and viscosity of the asthenospheric wedge, which control the suction moment responsible for the subduction angle at depths ≥70–100 km. For example, when oceanic lithosphere subducts beneath thin continental lithosphere, the low‐viscosity asthenosphere allows the slab to detach from the continent and sink into the mantle at normal angles. However, when oceanic lithosphere subducts close or beneath thick and strong continental lithosphere, the asthenospheric wedge narrows and corner flow drags high‐viscosity mantle from the base of the thick (&gt;150 km), cold continent into the wedge. Suction forces increase with both narrowing of the wedge and its increasing viscosity. We estimate the asthenospheric viscosity underneath thick, cold continents to be ≥1020 Pa s, sufficient to induce flat subduction. Later, after prolonged hydration and weakening of the continent's base, asthenospheric flow into the wedge may resume, allowing the slab to sink again into the mantle at normal angles.

Effective elastic thickness Te of the lithosphere in western Canada

A stochastic relationship between topography and Bouguer gravity is used to calculate high‐resolution variations in effective elastic thickness, Te, of the lithosphere in western Canada. The topography‐gravity coherence is calculated using a two‐dimensional, maximum‐entropy‐based spectral estimator. This method allows for smaller data windows and provides Te determinations with higher spatial resolution than standard Fourier spectral estimators. Our analysis shows significant variations in Te in western Canada. Te increases from ∼20–40 km in the weak, young portions of the Cordillera to 100 km and greater in the strong, old Canadian Shield. Te estimates are in good agreement with lithospheric temperatures calculated from surface heat flow and radioactive heat generation data. Our calculated Te distribution also shows strong correlation with other thermally related geophysical parameters, such as lithospheric age, regional heat flow, seismicity, seismic properties, and the stress field. Consequently, we infer that lithospheric temperatures exert a primary control on large‐scale variations in Te. Collectively, the correlations readily explain why the Craton continues to be stable and undeformed, whereas the Cordillera has continued to be deformed through the Cenozoic. An exception is the Wopmay Orogen, which includes the easternmost part of the northern Cordillera. There Te is ∼90 km, although the surface heat flow is ∼90 mW/m2. We infer that the high heat flow in this region is caused primarily by very high radioactive heat generation in the upper crust and that deep lithospheric temperatures are moderately low as expected from its age and long‐term geological stability.

Conditions for flow in the continental crust

Earthquake focal depth distributions and effective elastic thickness variations on the continents suggest that in old cold regions such as shields, the lower continental crust can be stronger than the upper mantle, and imply that the upper mantle can flow even when the lower crust does not. However, in other places substantial flow in the lower crust does occur, particularly near extensional metamorphic core complexes. We suggest that the lower crust only flows when its viscosity is significantly reduced, either by heating from igneous intrusions or by the addition of water. In both cases, the important characteristics of the flow are a consequence of the limited time over which the viscosity reduction occurs. The timescale is controlled either by conductive cooling of intrusions, or by the separation of melt from its matrix, which removes the water.

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.

A re-assessment of focal depth distributions in southern Iran, the Tien Shan and northern India: do earthquakes really occur in the continental mantle?

SUMMARY We investigate the depth distribution of earthquakes within the continental lithosphere of southern Iran, the Tien Shan and northern India by using synthetic seismograms to analyse P and SH body waveforms. In the Zagros mountains of southern Iran, earthquakes are apparently restricted to the upper crust (depths of <20 km), whereas in the Tien Shan and northern India they occur throughout the thickness of the continental crust, to depths of ∼40–45 km. We find no convincing evidence for earthquakes in the continental mantle of these regions, in spite of previous suggestions to the contrary, and question whether seismicity in the continental mantle is important in any part of the world. In some regions, such as Iran, the Aegean, Tibet and California, seismicity is virtually restricted to the upper continental crust, whereas in others, including parts of East Africa, the Tien Shan and northern India, the lower crust is also seismically active, although usually less so than the upper crust. Such variations cannot reliably be demonstrated from published catalogue or bulletin locations, even from ones in which depth resolution is generally improved. In contrast to the oceanic mantle lithosphere, in which earthquakes certainly occur, the continental mantle lithosphere is, we suggest, virtually aseismic and may not be significantly stronger than the lower continental crust. These variations in continental seismogenic thickness are broadly correlated with variations in effective elastic thickness, suggesting that the strength of the continental lithosphere resides in the crust, and require some modification to prevalent views of lithosphere rheology.

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.

Soft plates and hot spots: Views from Afar

The uplifted Ethiopian plateau region encompasses amagmatic rift basins and basins with nascent seafloor spreading. Rift segments in the Main Ethiopian rift and southern Afar show a distinct structural segmentation, with a south to north reduction in the length, width, and spacing of fault zones; rift segments in northern Afar, where extension exceeds 100%, show a magmatic segmentation. The objectives of remote sensing, gravity, and modeling studies of the Ethiopian plateau area are (1) to summarize morphological patterns along the length of the rift system and (2) to relate variations in effective elastic thickness of the lithosphere within the Ethiopian plateau region to tectonics. Inverse models of new and existing Bouguer gravity data from the rifted regions of the uplifted plateau constrained by seismic data are used to relate the variations in the geometry of the along‐axis structural and magmatic segmentation to variations in flexural rigidity. We use the wavelength dependence of the coherence between gravity and topography to estimate the flexural rigidity, or, equivalently, the effective elastic plate thickness (Te), of the lithosphere. Estimates of Te range from 17 ± 2 km to 5 ± 3/−2 km within the 300‐km‐wide Afar Depression. These estimates of Te within the rift are considerably less than values found beneath the uplifted but largely unfaulted plateau to the west (Te ≥ 56 km). These results show that the transition from a “continental segmentation” to “oceanic segmentation” corresponds to a decrease in rift basin segment length and the separation of faults and magmatic centers, an increase in magmatic construction, and a marked decrease in effective elastic thickness. We suggest that the length scales of extensional segments prior to the onset of seafloor spreading are controlled primarily by plate strength, at least in rift areas affected by mantle plumes.

Lithosphere dynamics and tectono-stratigraphic evolution of the Mesozoic Betic rifted margin (southeastern Spain)

Quantitative subsidence analysis demonstrates that Mesozoic sediments in the eastern Betic Cordilleras have been deposited on a thermally subsiding, rifted margin. The dynamics of rifting are related to regional trans-tension in the Atlantic realm. Modelling of this phase of rifting at the southern Iberian margin shows that the associated stretching of the lithosphere significantly altered its geometrical and mechanical structure and exerted a major control on subsequent convergence tectonics. The Mesozoic stretching led to the formation of a ramp at the margin due to strong lateral variations in the effective elastic thickness (EET) of the lithosphere and the thickness of the crust, in a direction perpendicular to the margin. The existence of a ramp structure in the Late Mesozoic at the Iberian margin, which is also supported by stratigraphic and structural data, is important for the style of subsequent deformation of the lithosphere in Late Cretaceous-Tertiary times. The lateral variations in EET inferred from forward modelling of rifted margin stratigraphy could explain the observed multiple collapse of the Betic margin and the partitioning of deformation between the converging units involved in the Betic orogeny. Modelling of depth-dependent palaeo-rheological profiles of the margin shows that the mechanical properties of the lithosphere after stretching differed distinctly from those before stretching. Lithospheric deformation in the Betic Cordilleras during the Tertiary orogeny seems, therefore, to be largely controlled by the previous rifting phase. A comparison of the palaeo and present-day rheologies in terms of estimates for EET during Late Cretaceous and present times indicates that the mechanical properties of the lithosphère underlying the Betic orogen were also affected by rifting during Late Oligocene-Aquitanian times. This phase of rifting is coeval with extension in the Valencia trough, the Gulf of Lions and the West European rift system, which supports a causal relation with regional lithospheric processes. The lateral and spatial variations in thermo-mechanical properties of the lithosphere underlying the Betics, therefore, reflect the dynamics of lithospheric extension on a larger scale.

Variations in effective elastic thickness of the North American lithosphere

THE isostatic response of the lithosphere to loading can often be successfully represented by a model in which the weight of topographic features on the Earth's surface is supported in part by stresses within a flexed elastic plate overlying an inviscid asthenos-phere, and in part by the buoyancy forces associated with the deflection of density interfaces accompanying flexure1,2. Previous estimates3–10 of the apparent thickness of the elastic plate in North America, based on the shapes of sedimentary basins or the deformation accompanying slip on faults, range from a few to more than a hundred kilometres—or equivalently, flexural rigidities ranging from about 1021 to 1025 N m−1. Here we use a technique for estimating flexural rigidity that is not limited to sedimentary basins to map variations in the effective elastic thickness of the North American lithosphere. The effective elastic thickness ranges from a minimum of ∼4 km in the Basin and Range Province to >100 km in the Precambrian core of the continent, supporting the idea that flexural rigidity increases with time since the last thermal event11.