Effective Elastic Thickness Te Research Articles

Quantitative estimation of the effective elastic thickness around the Burma Plate and correlation analysis of its influencing factors

The Burma Plate is a microplate that extends along the boundary between the Indian and Eurasian plates. It is characterized by an extraordinarily complex lithospheric tectonic setting, resulting from the continental collision in the north, the oceanic crustal subduction in the south, and the large amount of sediment from the Tibetan Plateau. The lithospheric strength is a key to understanding the tectonic evolution of the Burma Plate. In this study, we use topography and gravity disturbance data to estimate the spatial distribution of effective elastic thickness Te, which is a measure of lithospheric strength. The Tevalues range from ∼10 km to 80 km, with higher values in the Indian Plate than those in the other regions. The non-isostatic flexural effects of sediment loading and subducting slab pull can bias the Te estimation, with maximum reductions of ∼50 km and ∼10 km, respectively. The consistent distributions of the Te and the shear wave velocity anomaly ΔVsat 100 km depth suggest that the lithospheric strength is generally controlled by the thermal structure of the upper mantle. Meanwhile, the Te variations are highly related to the geometry of the subducting Indian Plate along the collision and subduction zones, indicating that the plate tectonics play a dominant role in determining the lithospheric strength of the Burma Plate.

Lithospheric strength of the Anatolian plateau and implications for strong earthquake activity in Turkey

On February 6, 2023, the doublet earthquake including two main shocks with magnitudes MW7.8 and MW7.5, occurred near the western side of the East Anatolian Fault at the southeast boundary of the Anatolian Plateau in Turkey. Based on the WGM2012 Bouguer gravity anomaly data and the Etopo1 topography data, this study first introduced a joint inversion of admittance and coherence functions and used the Bayesian optimal parameter estimation method to obtain the effective elastic thickness Te and loading ratio F of the lithosphere for various tectonic units in the Anatolian Plateau. Secondly, we discussed the characteristics and influencing factors of the lithospheric mechanical strength and analyzed its relationship with seismic activity. The lithospheric mechanical strength of the Anatolian Plateau showed clear lateral heterogeneity and a "weak-strong-weak" spatial pattern from east to west, reflecting various tectonic processes. At last, the strong seismic activity was found where the lithospheric strength was low in the Anatolian Plate. We also incorporated GPS strain rate and other results to investigate the tectonic background and primary causes of the MW7.8 and MW7.5 doublet earthquakes in Turkey. The results have a good insight into urban safety design in the Turkish region, including post-disaster rehabilitation, earthquake hazard assessment, and loss reduction.

Lithospheric strength and stress revisited: Pruning the Christmas tree

Whether lithospheric stress can reach the maximum level predicted by the Christmas-tree strength envelope is a fundamental question but with controversial answers. There is little controversy that a deforming lithosphere in high heat flow regions is likely critically stressed, i.e., at full yield at all depths, as described by the envelope. But different conceptual frameworks offer opposite views for very cold lithosphere, either at full yield or far below yield. Here, we use simple numerical models to investigate stresses in end-member cold cratonic lithosphere (e.g., Canadian Craton) in comparison with end-member warm plate-boundary lithosphere (e.g., Canadian Cordillera). The two key elements of our modelling are (1) that lithospheric stress builds up elastically with horizontal tectonic loading not only in the elastic–frictional brittle regime but also in the viscoelastic ductile regime, and (2) that the stress level is limited by the available tectonic force. In a cratonic lithosphere, the limiting tectonic force is sustained by competent rock material over a large depth range, represented by the competent thickness Tc that exceeds 90 km. The lithosphere undergoes mostly elastic deformation at a stress level of a few tens of MPa. While weakly stressed strong lithosphere can still produce limited earthquakes at shallow depths due to structural and stress heterogeneity, the lithospheric stress under horizontal tectonic loading is theoretically predicted to be orders of magnitude lower than predicted by the Christmas-tree envelope. Stresses in a real lithosphere may substantially deviate from this theoretical level because of spatiotemporal variations in rheology and structure. For example, the stress memory of past loading history in cold lithosphere may or may not be erased by more recent tectonic stresses. Because much of previous scientific debates on lithospheric stress levels and comparison with seismicity were focused on topographically induced flexural stress, we also investigate the effect of vertical loading. We show that the effective elastic thickness Te derived from the flexural response is a reasonable proxy for Tc derived from horizontal tectonic loading; a very large Te such as > 80 km is generally associated with very low tectonic stress far below yield. However, the flexure-induced bending stress is not directly comparable with seismicity because it may either enhance or suppress seismogenic stress in the crust.

Variations of the effective elastic thickness reveal tectonic fragmentation of the Antarctic lithosphere

To this day, little is known about thermal and rheological properties of the Antarctic lithosphere. We derive the effective elastic thickness Te as a proxy for these parameters by using ice thickness, bedrock topography and a combination of new satellite and high resolution terrestrial gravity data. Cross-spectral analysis based on the fan wavelet technique was employed to calculate Te variations by means of admittance and coherence techniques. Our results confirm a clear tectonic division of Antarctica with predominantly high values in East Antarctica (EANT) (Te~60–80km) and low values in West Antarctica (WANT) (Te~5–20km). For the Transantarctic Mountains separating these provinces, we found Te to be around 10km along the whole chain which is comparable to WANT. Apart from this general division, we found fragmentation of the lithosphere within these provinces. Especially EANT doesn't represent a single lithospheric block but shows strong variations of Te. The highest values are found around the Aurora Subglacial Basin (Te~90km) and in Dronning Maud Land (Te~80km). The minimum value of Te within EANT (~15km) is found in the Lambert Graben. Such a low value can be associated with active rifting in the Permian-Triassic, strong localized erosion or possibly the effect of a Cenozoic mantle plume. According to the coherence calculations, the weak zone extends to the Gamburtsev Subglacial Mountains (GSM), showing a distinct decrease of Te to 25–30km. Thus, this weak channel divides the previously mentioned strong blocks. However, the admittance analysis gives relatively high values (Te~70km) for the GSM. Based on the analysis of the wavelength-dependent admittance and coherence results and misfits for several principal locations, we give some preference to the coherence based values.

Temperature control of continental lithosphere elastic thickness, Te vs Vs

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

Reply to comment by W. P. Schellart on “The thermal structure of subduction zone back arcs”

[1] Currie and Hyndman [2006] documented the evidence that the majority of continental back arcs are uniformly hot with thin lithospheres over regional extents of hundreds of kilometers, even if where is no thermally significant extension. Using a number of independent observations, we showed that eight such current subduction zones have similar thermal regimes, with Moho temperatures greater than 800 C (35–40 km depth) and lithosphere thicknesses of 60 km. The one exception, the Peru flat slab region of South America, supports our model explanation. We then presented the hypothesis that the elevated back-arc temperatures have a common mechanism: efficient transport of heat by vigorous thermal convection in hydrated, low-viscosity back-arc upper mantle. [2] The paper was structured such that the observations indicating regionally high back-arc temperatures were separate and independent from our proposed model. The evidence for high uniform temperatures seems secure; the discussion is on the explanation. Schellart [2007] appears to agree that the back arcs included in our study are characterized by hot, thin lithospheres but does not agree with our interpretation of the underlying cause. His main criticisms are that (1) local tectonics, in particular extension, may account for the high temperatures in some areas, and (2) three-dimensional mantle flow and complex slab dynamics may produce elevated temperatures. [3] We acknowledge, as stated in the original paper, that a number of site-specific processes may affect the thermal regime at each back arc. We also recognize that nearly all back-arc regions have undergone at least some past extension, as described by Schellart [2007]; notable exceptions are central Alaska and the northern Canadian Cordillera. The key question is whether or not this extension is the primary cause of the observed uniformly hot thermal regimes. [4] Schellart [2007] argues that high surface heat flow at some back arcs is due to recent extension, including parts of the Sunda (Borneo) and Ryukyu (Korea) back arcs. We note that our compilation used a number of independent observational constraints on lithosphere temperatures, not just surface heat flow. Mantle temperatures determined using only surface heat flow data are highly uncertain, due to the point sampling and measurement uncertainties, and uncertainties in the thermal parameters used to calculate the geotherm. Other temperature constraints on upper mantle thermal structure include seismic velocities from refraction Pn and tomography, mantle xenolith thermobarometry, effective elastic thickness Te, etc. For each back arc, the upper mantle temperatures from all constraints are in good agreement and show no significant lateral variation on a regional scale. This is especially illustrated by the uniform low upper mantle seismic velocities for different back arcs [Currie and Hyndman, 2006, and references therein]. [5] Extension affects the thermal regime by locally decreasing lithosphere thickness and by inducing upwelling of hot underlying mantle. Modeling studies have shown that such extension should only affect lithosphere temperatures and surface heat flow within 100 km of the extended region [Morgan, 1983; Alvarez et al., 1984], and for continental crust, extension may actually decrease surface heat flow due to thinning of the crustal radioactive heat generation [Waples, 2001]. In addition, limited local extension should not significantly perturb the deep thermal region. For the back arcs in our compilation there is no evident upper mantle temperature increase in more strongly extended regions, suggesting that a more regional phenomenon is required to produce regionally elevated temperatures. We also note that extension can only occur if the lithosphere is sufficiently hot that it is weak enough to fail under available extensional stresses. A normal cool lithosphere is too strong to be deformed by plate tectonic forces [e.g., Hyndman et al., 2005]. We suggest that back-arc extension is a consequence of high temperatures and associated weak lithosphere, and not the primary cause. [6] Schellart [2007] also suggests a thermal role for a number of upper mantle dynamical processes, including along-margin mantle flow associated with a slab edge, slab retreat/advance, and slab break-off. We agree that each of these factors may affect the back-arc thermal regime in some locations. However, these are local phenomena. If they are thermally important, they should be evident in lateral variations in mantle seismic velocity and other deep temperature indicators. Lateral variations are not apparent in the current observational data. [7] As stated in the original paper [Currie and Hyndman, 2006, paragraph 71], we do not rule out local factors in JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, B11408, doi:10.1029/2007JB005415, 2007 Click Here for Full Article

Re-examination of the magnetic lineations of the Gascoyne and Cuvier Abyssal Plains, off NW Australia

This thesis addresses causes of two forms of secondary Hawaiian volcanism: rejuvenated onshore eruptions and offshore Hawaiian Arch flows. It is proposed that secondary volcanism is generated as a direct consequence of lithospheric flexural uplift that surrounds new volcanic shields as they grow. This uplift causes decompression of the underlying asthenosphere, which is assumed to be chemically and isotopically heterogeneous, near its solidus, and derived from the Hawaiian mantle plume. Uplift is modeled as the axisymmetric response of an elastic plate to a (volcanic) point load that grows linearly in time. To model flow in the asthenosphere, the rate of flexure of the lithosphere is taken as the upper boundary condition on an isoviscous, incompressible, fluid half-space. The first feature of secondary volcanism this model explains is the observed spatial gap between secondary volcanism and active shields. Best agreement is found with the majority of the observed spatial gaps with a lithosphere of effective elastic thickness Te=25-35 km. Secondly, this work demonstrates that the flexural model can produce observed crustal production if some magma focusing toward individual eruption sites occurs from the mantle over an area two to ten times the eruption area. The third feature this model addresses is that secondary lavas are isotopically distinct from shield lavas. In this model, melting the same two-component mantle forms the secondary and shield lavas, but the components are sampled by melting at rates that differ between the locations as predictable functions of depth. Flexural decompression produces melts that preferentially sample the mantle component that begins melting shallowest and which is associated with Sr and Nd isotope ratios most like those of secondary lavas. Melting in the center of a mantle plume is assumed to generate shield volcanism and is predicted to preferentially sample the component that begins melting deepest which is associated with Sr and Nd isotope ratios more similar to shield lavas. Models therefore successfully predict the observed mean difference in 87Sr/86Sr and 143Nd/144Nd compositions between the secondary and shield lavas. The fourth feature addressed is that secondary lavas are alkalic and shield lavas are dominantly tholeiitic. To explain this difference, the mean extent of partial melting is computed, and it is found that a model plume composed mostly of depleted peridotite (90%) and some pyroxenite (10%) will yield a lower extent of melting for secondary lavas than shield lavas. This particular model assumes lithospheric thicknesses (90-100 km) and plume potential temperatures (mean of 1550 °C) that are consistent with independent studies of the Hawaiian hotspot. Thus, asthenospheric melting by flexural decompression is a viable mechanism of intraplate volcanism, which can explain many general characteristics of secondary Hawaiian volcanism.

Wavelet transform mapping of effective elastic thickness and plate loading: Validation using synthetic data and application to the study of southern African tectonics

We present a new wavelet transform method to map spatial variations in effective elastic thickness Te and plate loading ratio f. The method assumes a model of thin plate flexural isostasy to describe the mechanical response of the lithosphere to vertical loading. In this model, the rheological properties of the lithosphere are aggregated into the effective elastic thickness Te of an equivalent thin plate overlying an inviscid fluid. A number of methods have been developed to map spatial variations in Te in an attempt to assess regional patterns of flexural strength. Our new method first obtains local coherence and local admittance through wavelet cross‐spectral analysis of surface topography and Bouguer gravity anomaly. Wavelet coherence is used to obtain the local characteristic wavelength, which is a function of both Te and the degree of relative loading f of the plate by subsurface loads and surface loads (loading is the combined effect of erosion, sedimentation, intrusion, faulting, and metamorphism). Wavelet admittance, in a normalized form, is used to resolve this f − Te ambiguity, and maps are made of the spatial variations in Te and f. We carry out extensive tests of the wavelet method on simulated topography and Bouguer gravity anomaly data that we generate through finite difference simulations of flexural isostasy with spectrally realistic loads and simple spatial variations in Te. These tests demonstrate that the wavelet inversion method is reasonably robust to uncertainties in loading and in crustal thickness and is able to recover Te to within ±25–50% of its correct value. We apply the wavelet method to southern Africa and recover estimates of Te principally in the range 25–50 km, in good agreement with existing estimates from forward modeling and Fourier coherence analyses. We find that our Te estimates often exceed estimates of crustal thickness, pointing to a strong upper mantle in parts of southern Africa.

Estimating Te in the presence of internal loads

Many attempts have been made to estimate the effective elastic thickness Te of the continental lithosphere from the relationship between the Fourier transforms of gravity and topography. If internal loads are ignored, a simple model with values of Te of between 5 and 25 km fits most of the observations. If internal loads with topographic expression are present, the simple model of topographic loading underestimates the true value of Te, potentially by a large factor. This difficulty can be avoided by using the coherence γB2 between the Bouguer gravity and topography to estimate Te. However, erosion and sedimentation can produce loads that are not associated with any topography. If Te is estimated from γB2 when such loads are important, the value of Te is overestimated, again potentially by a large factor. The only obvious solution to these problems is to use a simple model that can describe all three types of loads, surface loads, internal loads with topographic expression, and loads with no topographic expression, and to estimate their contributions at the same time as Te is determined. The parameters of such a model are estimated from several data sets and show that there are no significant internal loads with topographic expression in any of the seven continental regions discussed here and that none give values of Te greater than 25 km.

Estimates of Europa's ice shell thickness from elastically‐supported topography

The thickness of Europa's solid ice shell is uncertain, and has important implications for Europa's habitability and thermal history, and the design of future spacecraft missions. Here we obtain an estimate of the ice shell thickness from observations of a plateau SW of Cilix impact crater. Stereo topographic profiles suggest that the plateau is flexurally supported, with an effective elastic thickness te of 6−2+5 km. For a conductive temperature profile this te value implies a solid ice shell thickness of 15−9+20 km; if the shell is convecting, this estimate is a lower bound. Combined with independent estimates, we infer a probable shell thickness of ≈25 km. The shell thickness is likely to be uniform over the entire satellite.

Effective elastic thickness and heat flux estimates on Ganymede

We identify sites of apparent flexural uplift at rift zone boundaries on Ganymede using Galileo stereo‐derived topography. The estimated effective elastic thickness te is 0.9–1.7 km for a nominal Young's modulus of 1 GPa. Using a viscoelastic model of the ice crust we find that the temperature defining the base of the elastic layer is <185 K for likely strain rates. The inferred local heat flux during deformation is less than 245 mW m−2, and probably close to 100 mW m−2. The stresses required to cause fault motion are around 1 MPa. Both the high heat flux and the high stresses are consistent with estimates of these quantities during an episode of transient tidal heating in Ganymede's past.

Isostatic response of the Australian lithosphere: Estimation of effective elastic thickness and anisotropy using multitaper spectral analysis

Gravity and topography provide important insights regarding the degree and mechanisms of isostatic compensation. The azimuthally isotropic coherence function between the Bouguer gravity anomaly and topography evolves from high to low for increasing wavenumber, a diagnostic that can be predicted for a variety of lithospheric loading models and used in inversions for flexural rigidity thereof. In this study we investigate the isostatic response of continental Australia. We consider the effects of directionally anisotropic plate strength on the coherence. The anisotropic coherence function is calculated for regions of Australia that have distinctive geological and geophysical properties. The coherence estimation is performed by the Thomson multiple‐Slepian‐taper spectral analysis method extended to two‐dimensional fields. Our analysis reveals the existence of flexural anisotropy in central Australia, indicative of a weaker N‐S direction of lower Te. This observation is consistent with the suggestion that the parallel faults in that area act to make the lithosphere weaker in the direction perpendicular to them. It can also be related to the N–S direction of maximum stress and possibly the presence of E–W running zones weakened due to differential sediment burial rates. We also demonstrate that the multitaper method has distinct advantages for computing the isotropic coherence function. The ability to make many independent estimates of the isostatic response that are minimally affected by spectral leakage results in a coherence that is more robust than with modified periodogram methods, particularly at low wavenumbers. Our analysis elucidates the reasons for discrepancies in previous estimates of effective elastic thickness Te of the Australian lithosphere. In isotropic inversions for Te, we obtain values that are as much as a factor of 2 less than those obtained in standard inversions of the periodogram coherence using Bouguer gravity and topography but greater than those obtained by inversions that utilize free‐air rather than Bouguer gravity and ignore the presence of subsurface loads. However, owing to the low spectral power of the Australian topography, the uncertainty on any estimate of Te is substantial.

Estimates of the effective elastic thickness of the continental lithosphere from Bouguer and free air gravity anomalies

In oceanic regions the effective elastic thickness Te of the lithosphere is generally estimated from the free air gravity field and approximately corresponds to the depth of the 450°C isotherm. In continental regions the Bouguer anomaly is commonly used and gives thicknesses for shields as great as 130 km, where geotherms reach temperatures of 800–1000°C. If this result were correct, it would require the continental lithosphere to be able to support elastic stresses at considerably higher temperatures than the oceanic lithosphere can. However, detailed examination of the free air and Bouguer anomalies over continents shows that values of Te obtained from Bouguer anomalies over shields are often upper bounds, rather than estimates, because most of the short‐wavelength topography has been removed by erosion, and what remains is incoherent with both the free air and Bouguer gravity anomalies. The present study uses free air anomalies, and topography as a load whose geometry is known. The main importance of subsurface density contrasts and their associated gravity anomalies is that they reduce the signal to noise ratio, without affecting the estimated value of Te. No estimates of Te from free air gravity and topographic data from North America, East Africa, Australia, the Indian peninsula, and the former Soviet Union exceed 25 km, and all are smaller than the seismogenic thickness. The largest estimate, of 42 km, comes from profiles across the Himalayan foredeep. The gravity in this region may be affected by dynamic effects. There is therefore no evidence that the continental lithosphere can support elastic stresses for geological times at temperatures above about 350°C.

Strength and rheology of the western U.S. Cordillera

Effective elastic thickness Te depends primarily on temperature, composition, and state of stress of the lithosphere. In this paper, we examine high‐resolution spectral estimates of Te and their relationships to regional heat flow, age of the lithosphere, seismic properties, stress orientations, and earthquake focal depths of the western U.S. Cordillera. The relationship of Te to heat flow indicates that ductile flow accommodates long‐term (∼106 to 108 years) isostatic response at different levels of the crust and upper mantle, depending principally on age (and, by implication, bulk composition) of the lithosphere. Isostatic response is primarily controlled by the upper mantle in Archean lithosphere of the middle Rocky Mountains, whereas Te depends on lower crustal flow in Early Proterozoic lithosphere of the Colorado Plateau. The Yellowstone‐Snake River Plain volcanic field and significantly extended regions in the Basin‐Range and northern Rocky Mountains are associated with latest Proterozoic aged lithosphere and indicate middle to upper crustal control of long‐term Te. We also show that azimuthal variations of Te reflect deviatoric stress in the lithosphere. Te is found empirically to approximate the 95th percentile focal depth of background seismicity. The latter relationship is inconsistent with brittle‐ductile control of focal depth, indicating that another rheological transition (e.g., from stick‐slip to stable sliding frictional behavior) is responsible. Tectonic and structural relationships expand upon the hypothesis that the geographic distribution of tectonic features depends fundamentally on spatial variations in strength of the lithosphere. Moreover, we find a spatial correlation of the Intermountain Seismic Belt to a marked transition in Te, implying that forces responsible for this active seismic zone are derived from local buoyancy anomalies rather than from current‐day plate boundary interactions.

Gravity anomalies, isostasy, and mantle flow at the East Pacific Rise crest

Bathymetry and gravity data obtained during a detailed Hydrosweep survey of the southern East Pacific Rise from 7°S to 9°S are used to investigate isostasy at the axis of a fast spreading mid‐ocean ridge. In particular, we examine the manner in which the bathymetric crestal high is supported and how this support varies along the axis within the 160‐km‐long 7°12'S‐8°38'S ridge segment. The crestal high stands about 400 m above the adjacent ridge flanks and has a nearly constant minimum axial depth for a distance of 140 km. The summit is broad and flat, and an axial summit caldera is present for the entire length of the ridge segment. However, the width and cross‐sectional shape of the crestal high vary systematically along the axis. It is broad with gentle slopes in the center of the segment but becomes progressively narrower and steeper toward the ends of the ridge segment. The ridge crest is marked by a free‐air gravity anomaly high about 15–20 km wide with an amplitude of 10–15 mGal relative to the ridge flanks. Mantle Bouguer anomalies vary systematically along the axis with minimum values found near the center of the segment. The axial mantle Bouguer anomalies thus do not reflect the axial depth but are correlated with changes in the cross‐sectional area of the crestal bathymetric high. The effects of cooling and subsidence away from the axis were removed from the bathymetry and free‐air anomalies to isolate residual topographic and gravity anomalies associated with the ridge crest. The residual crestal bathymetric high was modeled as a flexural feature resulting from the upward buoyant load of a region of low density material centered beneath the axis. The lithosphere was treated as a broken plate, either with a constant flexural rigidity or an effective elastic thickness Te which grows at a rate proportional to the square root of distance from the axis. The best fitting values of Te for the constant rigidity case are in the range of 0.3–0.6 km. For the growing plate model, Te increases at a rate of 0.2–0.3 km½. The gravity constrains the mass deficiency to extend to a depth of 20–30 km for both lithospheric models. We interpret this low‐density material as a region of partial melt feeding magma to the ridge axis. The best fitting density anomalies imply that a 4–9% melt fraction is present beneath the crestal high. Upwelling of melt to the axis is thus confined to a narrow zone within about 10 km of the axis. The mass deficiency and thus the upwelling partial melt are not distributed evenly along the ridge axis but rather are concentrated in the central portion of the ridge segment. It thus appears that differences between the along‐axis gravity and depth patterns observed at slow spreading and at fast spreading ridges are not the result of a change from three‐dimensional, focused upwelling at slow spreading ridges to two‐dimensional sheetlike upwelling at fast‐spreading ridges. Rather, the differences in axial gravity and depth between fast and slow spreading ridges reflect differences in the efficiency of the shallow along‐axis magma distribution.

Sediment blanketing and the flexural strength of extended continental lithosphere

AbstractThe flexural rigidity of the oceanic lithosphere is strongly dependent on its temperature structure at the time of loading. It is commonly assumed that the depth to the 450°C isotherm defines the effective elastic thickness Te of the lithosphere. However, recent gravity studies across the Baltimore Canyon and Nova Scotian margins suggest that temperature may play a more complicated role in controlling the mechanical strength of extended continental lithosphere. For example, the flexural strength of the Baltimore Canyon margin (with sediment thicknesses of ≅ 15 km) appears to be controlled by the depth to the 150°C isotherm whereas the strength of the Nova Scotian margin (with sediment thicknesses cf ≅ 10 km) is controlled by the depth to the 250°C isotherm. The apparent correlation between sediment thickness and controlling isotherm suggests that sediment blanketing may play a role in modifying the flexural strength of extended continental lithosphere. This hypothesis was investigated by simulating the sedimentation history of a margin as a Gaussian function in which sedimentation peak and rate are determined by the mean and standard deviation of the function. The temperature structure of the lithosphere is continually modified as sediments are deposited on, and incorporated into the temperature structure of, the underlying lithosphere. Given a ‘starting’ value of Te defined by the degree of extension of the lithosphere, the modification of Te appears to be directly proportional to the sedimentation rate and cumulative sediment thickness, and inversely proportional to the time at which the sedimentation rate is a maximum. The first‐order consequence of sediment blanketing is to reduce the cooling rate of the lithosphere relative to cooling in the absence of sediments. At thermal equilibrium, the initial value of Te is reduced by the cumulative sediment thickness. Local isostatic conditions (i. e. Te≅ 0) can only be approached when the sedimentation rate is unrealistically high (> 1000 m/Myr) during the rift or early post‐rift phase of basin development. However, while these early loads may be locally compensated, any subsequent loads will be regionally compensated. Thus, it is unlikely that the low present‐day flexural strengths interpreted from the Baltimore Canyon and Nova Scotian passive continental margins are a consequence of sediment blanketing.

Mechanical strength of extended continental lithosphere: Constraints from the Western Rift System, East Africa

Although regional isostasy generally is associated with continental lithospheric compression and foreland basin formation, local isostatic compensation commonly is assumed in models of extensional basin formation. The assumption of negligible lithospheric strength during rifting often is justified on the basis of: (1) high heat flow and temperatures produced by elevating the lithosphere ‐ asthenosphere boundary and (2) fracturing of the crust and lithosphere by normal faults. By modeling the development of rift basins within the Western rift system of East Africa and their associated free air gravity anomalies, we assess the role of basin‐producing normal faults in modifying the flexural strength of extended lithosphere. Heat flow and seismicity data from the East African plateau region indicate that the Western rift system located on the western side of the plateau developed in old, cold continental lithosphere. These relatively narrow (40–70 km wide), but deep, basins are bounded along one side by high‐angle border faults that penetrate to lower crustal levels, as indicated by seismicity data. Along the length of the Western rift system, depth to pre‐rift basement and rift flank topography vary between basins from 1 to 8 km and from 1 to 2 km respectively, with deeper basins generally correlating with higher flanks. Comparison of model predictions with topography and free air gravity profiles reveals that the basin depth and the flank height in the majority of the Western rift basins studied can be explained simply by small heaves (3–10 km) across the border fault and with significant flexural strength of the lithosphere maintained during extension. Where both observed basin depth and flank height could not be reproduced, basins were located adjacent to eruptive volcanic centers active in Miocene‐Recent times. In these areas, basin depth, rift flank elevation, and free air gravity anomaly may be modified by magmatic underplating of the crust. Estimates of effective elastic thickness Te obtained from our forward models vary between 17 and 38 km. These Te estimates are consistent with previous values obtained through the wavenumber domain correlation of Bouguer gravity and topography data. We find that the lithosphere beneath East Africa maintains significant flexural strength in extension despite localized, but intense, fracturing of the crust by normal faults that penetrate to lower crustal levels. Therefore the assumption of local isostasy (Te ∼ 0) appears invalid within the youthful Western rift system.