Estimates Of Effective Elastic Thickness Research Articles

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

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

Reconsidering Effective Elastic Thickness Estimates by Incorporating the Effect of Sediments: A Case Study for Europe

AbstractIn the present study we analyzed the influence of density heterogeneity in the sedimentary cover on estimates of the effective elastic thickness (EET) of the lithosphere based on a cross‐spectral analysis of gravity and topography data. The fan wavelet coherence technique was employed to calculate EET for most of Europe and adjoining southern mountain belts. We employed Bouguer gravity anomalies and topography corrected for the effect of density variations within sediments. Correcting for sediments considerably suppresses the effect of unexpressed subsurface loads and substantially reduces EET estimates in areas with negligible topography variations as it was demonstrated for North Europe and East European Platform. The results show a good correspondence between the EET patterns and tectonic fragmentation of Europe and better agree with independent estimates based on the strength model of the lithosphere. Therefore, considering of the effect of sediments is essential for correct determinations of EET in flat areas.

Estimates of effective elastic thickness at subduction zones

The effective elastic thickness (Te) is an important parameter that characterizes the long-term strength of the lithosphere. Estimates of Te at subduction zones have important tectonic and geodynamic implications, providing constraints for the strength of the oceanic lithosphere at a short-term scale. We estimated Te values in several subduction zones worldwide by using models including both surface and subsurface loads from the analysis of free-air gravity anomaly and bathymetric data, together with a moving window admittance technique (MWAT). Tests with synthetic gravity and bathymetry data show that this method is a reliable way to recover Te of oceanic lithosphere. Our results show that there is a noticeable reduction in the effective elastic thickness of the subducting plate from the outer rise to the trench axis for most studied subduction zones, suggesting plate weakening at the trench-outer rise of the subduction zones. These subduction zones have Te range of 6–60 km, corresponding to a wide range of isotherms from 200 to 800 °C. Different trenches show distinct patterns. The Caribbean, Kuril-Japan, Mariana and Tonga subduction zones show predominantly high Te. By contrast, the Middle America and Java subduction zones have a much lower Te. The Peru-Chile, Aleutian and Philippine subduction zones show considerable scatter. The large variation of the isotherm for different trenches does not show clear relationship with plate weakening at the outer rise.

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

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

The long‐wavelength admittance and effective elastic thickness of the Canadian Shield

AbstractThe strength of the cratonic lithosphere has been controversial. On the one hand, many estimates of effective elastic thickness (Te) greatly exceed the crustal thickness, but on the other the great majority of cratonic earthquakes occur in the upper crust. This implies that the seismogenic thickness of cratons is much smaller than Te, whereas in the ocean basins they are approximately the same, leading to suspicions about the large Te estimates. One region where such estimates have been questioned is the Canadian Shield, where glacial isostatic adjustment (GIA) and mantle convection are thought to contribute to the long‐wavelength undulations of the topography and gravity. To date these have not been included in models used to estimate Te from topography and gravity which conventionally are based only on loading and flexure. Here we devise a theoretical expression for the free‐air (gravity/topography) admittance that includes the effects of GIA and convection as well as flexure and use it to estimate Te over the Canadian Shield. We use wavelet transforms for estimating the observed admittances, after showing that multitaper estimates, which have hitherto been popular for Te studies, have poor resolution at the long wavelengths where GIA and convection predominate, compared to wavelets. Our results suggest that Te over most of the shield exceeds 80 km, with a higher‐Te core near the southwest shore of Hudson Bay. This means that the lack of mantle earthquakes in this craton is simply due to its high strength compared to the applied stresses.

An explanation for the age independence of oceanic elastic thickness estimates from flexural profiles at subduction zones, and implications for continental rheology

Most properties of oceanic lithosphere are widely observed to be dependent on the age of the plate, such as water depth, heat flow, and seismogenic thickness. However, estimates of the ‘effective elastic thickness' of oceanic lithosphere based on the deflection of the plate as it enters a subduction zone show little correlation with the age of the incoming lithosphere. This paradox requires reconciliation if we are to gain a full understanding of the structure, rheology, and behaviour of oceanic lithosphere. Here, we show that the permanent deformation of the plate due to outer-rise faulting, combined with uncertainties in the yield stress of the lithosphere, the in-plane forces transmitted through subduction zones, and the levels of noise in bathymetric and gravity data, prevents simple elastic plate modelling from accurately capturing the underlying rheological structure of the incoming plate. The age-independent estimates of effective elastic thickness obtained by purely elastic plate modelling are therefore not likely to represent the true rheology of the plate, and hence are not expected to correspond to the plate age. Similar effects may apply to estimates of elastic thickness from continental forelands, with implications for our understanding of continental rheology.

Reappraisal of the effective elastic thickness for the sub-Andes using 3-D finite element flexural modelling, gravity and geological constraints

Estimates of effective elastic thickness (Te) for the western portion of the South American Plate using, independently, forward flexural modelling and coherence analysis, suggest different thermomechanical properties for the same continental lithosphere. We present a review of these Te estimates and carry out a critical reappraisal using a common methodology of 3-D finite element method to solve a differential equation for the bending of a thin elastic plate. The finite element flexural model incorporates lateral variations of Te and the Andes topography as the load. Three Te maps for the entire Andes were analysed: Stewart & Watts (1997), Tassara et al. (2007) and Pérez-Gussinyé (2007). The predicted flexural deformation obtained for each Te map was compared with the depth to the base of the foreland basin sequence. Likewise, the gravity effect of flexurally induced crust—mantle deformation was compared with the observed Bouguer gravity. Te estimates using forward flexural modelling by Stewart & Watts (1997) better predict the geological and gravity data for most of the Andean system, particularly in the Central Andes, where Te ranges from greater than 70 km in the sub-Andes to less than 15 km under the Andes Cordillera. The misfit between the calculated and observed foreland basin subsidence and the gravity anomaly for the Marañon basin in Peru and the Bermejo basin in Argentina, regardless of the assumed Te map, may be due to a dynamic topography component associated with the shallow subduction of the Nazca Plate beneath the Andes at these latitudes.

Estimativa da espessura elástica efetiva da litosfera do sul do cráton são francisco usando dados da missão Grace

This paper presents the estimates of effective elastic thickness (T e) from free-air anomalies/geoid undulations and topography data in the southern Sao Francisco Craton, by using admittance function technique. The gravity field elements have been calculated from geopotential model GGM02C (GRACE mission). The observed admittance was computed by dividing the cross-spectra of the free-air anomalies/geoid undulations by the power of topography. Theoretical admittances were estimated based on two hypothesis of flexural compensation mechanism: surface topographic loads or loads beneath of lithosphere. The best fit was obtained by using the second hypothesis, were anegative density contrast (located at depth of 150 km) suggested 45 km for T e value. This low valiue of elastic thickness, combined with high heat flow and positive geoid undulations, can be explained by the presence of density perturbations in the mantle, which are associated with the chemical depletion and thermal anomalies, both are correlated with the complex geodynamic evolution of this important cratonic area in the South American plate.

The flexural rigidity of Fennoscandia: reflection of the tectonothermal age of the lithospheric mantle

Estimates of effective elastic thickness ( Te), a measure of the strength of the plate, have shown that older and cool cratonic regions are characterised by a stronger lithosphere (higher Te values) than areas which have been tectonically reworked. We use geophysical data and information from xenoliths in volcanic rocks from Fennoscandia and show that the strength of the lithosphere reflects the tectonothermal age of the lithospheric mantle. Fennoscandia is made up of the Baltic shield (1.9–3.0 Ga) and the Caledonian Orogen of Norway and northern Sweden (400–500 Ma). The shield is characterised by an old, thick and cold lithosphere in contrast to the Caledonides where the lithosphere is hotter and thinner. Our geophysical analysis results show a regional variation in elastic plate thickness from 8 km to 70 km, or equivalently, a flexural rigidity between 0.4×10 22 and 3×10 24 Nm. These results suggest that the lithosphere is strongest in the relatively stable Archaean Province, weaker in the regions characterised by Proterozoic crustal formation, and lowest in the tectonically reworked and deformed Caledonian belt. Within the study area, there is a direct correlation between lithosphere strength, the age of the last major tectonothermal event registered in the crust and lithospheric mantle composition. These broad correlations reflect thinner and more fertile lithosphere, and higher geothermal gradients, beneath regions of progressively younger crust.

Extensional basin geometry and the elastic lithosphere

Although the morphology and dimensions of continental rift basins vary considerably worldwide, one aspect is similar; tectonically active rifts are bordered on one or both sides by relatively long (tens of kilometres) normal fault systems (termed border faults) that largely control basin morphology. We compile data constraining the geometry of border faults within the tectonically active East African Rift system, and evaluate these results with respect to variations in thickness of the elastic lithosphere. Border–fault lengths greater than 75 km occur in regions with deep crustal seismicity and relatively high estimates of effective elastic thickness ( T e derived from forward and inverse models of gravity and topography data ( T e > 25 km). Most East African border faults cross–cut pre–existing structures and basement foliations, although segments of the longest faults (greater than 80 km) reactivate Precambrian shear zones or structural fabrics. From observations in East Africa, comparisons with data from the Aegean and Baikal Rifts, and considerations of the rheology of continental lithosphere, we propose that the elastic lithosphere determines the length, width and style of faults within East Africa, and perhaps other continental rifts.

Lateral variations of thermo-mechanical properties in the Tyrrhenian–northern Apennine region

We present lithospheric strength profiles calculated for a transect running from the Island of Corsica, in the west, to the Apennine foredeep in the east. Strength profiles are constructed at locations selected on the basis of the availability of seismic, heat flow, gravity data and earthquake foci distribution. Rheological modelling points to the presence of strong lateral variations in the thermo-mechanical state of the lithosphere. The Tyrrhenian–Tuscany area has a low rigidity in relation with the intense stretching active from the Early Miocene, and with very high heat flow associated with asthenospheric rise. On the contrary, hinterland (Corsica) and Apenninic foreland areas (Adriatic area) show a higher level of lithospheric strength. Effective elastic thickness estimates confirm this scenario, exhibiting a strong reduction from the northern Tyrrhenian to the inner Apennines and then an abrupt increase in correspondence of the Adriatic plate. The distribution of earthquake focal depths confirms the presence of a brittle rheology in the first kilometres of the crust in the Tuscany area, whereas close to the plate boundary much lower thermal gradients, partly induced by the westward-plunging Adriatic plate, enable seismic activity to occur at larger depths.

The effect of sedimentary cover on the flexural strength of continental lithosphere

The factors that control the flexural rigidity — or effective elastic thickness (EET) — of continental lithosphere have been extensively studied over the past two decades. Using EET estimates derived from the analysis of topography, basin structures and gravity anomalies, several authors1,2,3,4,5 have shown that crustal thickness, geothermal gradient, strain rate, rheology and plate curvature all affect the flexural strength of continents. Recognition that certain combinations of these parameters result in a significant reduction of flexural strength caused by decoupling of the crust and the upper mantle3,5 has been a critical step in understanding why many continental areas have estimated EETs that are thin compared with the total mechanical thickness of the continental lithosphere5. Here we develop a semi-analytical model of the EET through a parametrization of the yield stress envelope6,7 that includes the effects of crust–mantle decoupling. We perform a detailed comparison of EET estimates at foreland basins and mountain belts to values predicted by our model and find that, to predict the EET estimates successfully, we need to take into account the effect of the sediment cover and to use a strong plagioclase-controlled rheology. The effect of sediment cover is to weaken the lithosphere because of the lower density of sediments relative to crystalline crust5,8,9 and by thermally insulating the lower crust9,10,11.

Variations in the along‐axis segmentation of the Afar Rift system

Both oceanic and continental rifts show regular along‐axis segmentation, but the relationship, if any, between the two is poorly understood. The tectonically active East African Rift system encompasses systematic along‐axis variations in extension and magmatism, making it possible to explore the links between strain, magmatism, and the length scales of faulting and depositional systems during rift development. We summarize Quaternary along‐axis segmentation within the Afar Rift system, which is transitional to seafloor spreading, and compare it with segmentation in a “continental” rift sector, the Main Ethiopian Rift (MER) system to the south. We use high‐resolution satellite imagery calibrated by field studies and digital topography data to delineate faults and magmatic centers in unmapped areas and to compare with existing geological reports. From south to north we see a significant and systematic decrease in the lengths of young basin‐bounding normal faults (∼50 km long in MER to ≤ 15 km long in northern Afar), in the lengths and widths of rift basins (from 50 to 100 km long × 30 to 80 km wide in MER to ≤ 20 km long × ≤ 5 km wide in northern Afar), and in the relief of the uplifted rift flanks surrounding the basins (from ≥ 1000 m high in MER to ≤ 100 m high in northern Afar). Erosional escarpments along the older, outer rift margins of Afar reveal long, high border fault segments similar to those found in the less evolved MER, suggesting that the Afar Rift was originally bounded by much longer faults. In northern Afar, there is a drastic increase in the volume of Quaternary basalts, with segmentation now dominated by 50‐ to 80‐km‐long volcanic ridges, which are similar in size, morphology, and spacing to the second order, nontransform offset segmentation of slow‐spreading mid‐oceanic ridges. The along‐axis changes in rift segmentation are concomitant with decreasing crustal thickness (i.e., cumulative strain), decreasing effective elastic thickness estimates, and increasing magma supply. These temporal and spatial patterns suggest that segmented continental rifts can evolve into segmented oceanic rifts dominated by mantle sourced magmatic processes.

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

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

Flexural ridges, trenches, and outer rises around coronae on Venus

High‐resolution altimetry collected by the Magellan spacecraft reveals trench and outer rise topographic signatures around major coronae (e.g. Eithinoha, Heng‐O, Artemis, and Latona). In addition, Magellan synthetic aperature radar images show circumferential fractures in areas where the plates are curved downward. Both observations suggest that the lithosphere around coronae is flexed downward by the weight of the overriding coronal rim or by the negative buoyancy of subducted lithosphere. We have modelled the trench and outer rise topography as a thin elastic plate subjected to a line load and bending moment beneath the corona rim. The approach was tested at northern Freyja Montes where the best fit elastic thickness is 18 km, in agreement with previously published results. The elastic thicknesses determined by modelling numerous profiles at Eithinoha, Heng‐O, Artemis, and Latona are 15, 40, 37, and 35 km, respectively. At Eithinoha, Artemis, and Latona where the plates appear to be yielding, the maximum bending moments and elastic thicknesses are similar to those found at the Middle America, Mariana, and Aleutian trenches on Earth, respectively. Estimates of effective elastic thickness and plate curvature are used with a yield strength envelope model of the lithosphere to estimate lithospheric temperature gradients. At Heng‐O, Artemis, and Latona, temperature gradients are less than 10 K/km, which correspond to conductive heat losses of less than one half the expected average planetary value. We propose two scenarios for the creation of the ridge, trench, and outer rise topography: differential thermal subsidence and lithospheric subduction. The topography of Heng‐O is well matched by the differential thermal subsidence model. However, at Artemis and Latona the amplitudes of the trench and outer rise signatures are a factor of 5 too large to be explained by thermal subsidence alone. In these cases we favor the lithospheric subduction model wherein the lithosphere outboard of the corona perimeter subducts (rolls back) and the corona diameter increases.

Seasat observations of flexure: Evidence for a strong lithosphere

Outer rises seaward of subduction zones arise due to flexure of the oceanic lithosphere. A mechanical model of this flexure is used to predict the shape of the associated geoid signal. These predicted geoid signals are compared with Seasat altimeter profiles over outer rises adjacent to nine major subduction zones. The nine zones include the Aleutian, Mariana, Izu-Bonin, Puerto Rico/Lesser Antilles, Peru-Chile, Middle America, South Sandwich, Philippine, and Kuril. The comparisons with Seasat data provide estimates of effective elastic thickness, h e , and maximum bending moment, M m for each zone. It is shown that: 1. (1)these thicknesses, h c , are approximately proportional to the square root of lithospheric age 2. (2)the maximum bending moments, M m, , increase roughly as the cube of thickness, h c . These relations, (1) and (2), are both very similar to those predicted by a layered rheology model (brittle/elastic ductile) of the lithosphere derived from mechanical experiments on rocks. According to this model, thermally activated creep is the mechanism of ductile flow in the lower lithosphere; hence an isotherm (740° ± 50°) defines the bottom of the lithosphere. Thus, old oceanic lithosphere may have significant strength to depths in excess of 60 km or about half that of the thermal lithosphere.