Elastic Thickness Of Lithosphere Research Articles

Plan‐view curvature of foreland basins and its implications for the palaeostrength of the lithosphere underlying the western Alps

In zones of continental collision, three methods can be used to calculate the strength of the underthrust lithosphere: (1) a forward model approach to the Bouguer gravity field, (2) an inverse model of the gravity and topography using admittance techniques, or (3) a forward model of the stratigraphic infill of the foreland basin to estimate the cross‐sectional profile of the downflexed plate. The use of reconstructed stratigraphy has the potential to yield values for the equivalent elastic thickness (Te) of the cratonic lithosphere at varying slices in geological time, and hence enable an insight into the longer term (10–50 Myr) mechanical behaviour of the continental lithosphere.Calculations of Te based on isopachs of foreland basin stratigraphy use sea level as a reference line to estimate the basement deflection, and therefore are limited to using stratigraphy which records shallow marine or coastal sedimentation. A new empirical approach is applied to evaluating ancient Te values using the reconstructed palaeocurvature of the basin in plan view. The radius of curvature of 12 curvilinear foreland basins is plotted against their documented Te values and shows a linear relationship. The maximum Te value for a given radius of curvature can also be plotted as a straight line. The palaeocurvature of reconstructed basins can then be compared with the plots, and estimates of likely maximum Te values may be obtained.During Eocene times, the underfilled foreland basin of the Alps was characterized on its cratonic edge by the deposition of Nummulite‐rich limestones. Palaeogeographical reconstructions of the Nummulitic Limestones enable estimates of the palaeocurvature of the cratonic margin of the Alpine foreland basin during the Eocene. By comparing this value with the curvature of documented basins, it is possible to suggest that the European lithosphere underlying the western Alps had an effective elastic thickness of no greater than 17 km during the Eocene. It has been suggested that the transition in the depositional state of the Alpine foreland basin from an underfilled to a filled state during middle Oligocene times was linked to a thickening of the continental lithosphere associated with the effective ramp of the Tethyan passive margin. The Te value of less than 17 km during the underfilled stage combined with a value of 10±5 km for the later filled stage at 17 Ma does not lend support to this hypothesis.

Effective elastic thickness of the continental lithosphere in South Africa

We have estimated the effective elastic thickness of the continental lithosphere beneath South Africa using the coherence technique. This involves (1) estimating the coherence between Bouguer gravity anomalies and topography in the spectral domain and (2) comparing the coherence with that predicted by an elastic plate model that flexes under loads placed on, within and beneath the lithosphere. The depth to the base of this elastic layer which gives the best root‐mean‐square fit between estimated and predicted coherence is the effective elastic thickness (Te). Two major tectonic provinces, namely, the Archean Kaapvaal Craton and the Mesoproterozoic Namaqua‐Natal Mobile Belt (which together form the Kalahari Craton in southern Africa), are found to have Te values of 72 km and 38 to 48 km, respectively. There is indication from the coherence data that over the Kalahari Craton, topographic features with equivalent wavelengths less than ∼200–300 km are supported by the rigidity of the lithosphere while features with wavelengths greater than 700 km are compensated. It is implied from the present findings that each of the two South African tectonic provinces can be considered as separate coherent domains. On the basis of geotherms from both provinces, the effective elastic thicknesses obtained point to a lithospheric basal temperature of about 600°C. Geologic and geophysical considerations suggest that the contrast in flexural rigidity of the lithosphere between the two provinces can be attributed to the combined effects of compositional and thickness differences of the lithosphere, as well as variation in present‐day asthenospheric heat flow. Our Te estimates of the Namaqua‐Natal Mobile Belt are similar to those of the Mesoproterozoic Grenville belt, corroborating tectonic models which suggest that these two belts may once have been part of the same mobile belt. In contrast, the Te estimate of the Kaapvaal Craton fall below average global Archean cratonic values; the reasons for this are not clear.

Glacial Rebound of the British Isles-Iii. Constraints On Mantle Viscosity

SUMMARY Observations of sea-level change since the time of the last glacial maximum provide important constraints on the response of the Earth to changes in surface loading on time-scales of 103-104 years. This response is conveniently described by an effective elastic lithospheric thickness and effective viscosities for one or more mantle layers. Considerable trade-off between the parameters describing these layers can occur, and different combinations can give rise to comparable predictions of sea-level change. In particular, the trade-off between lithospheric thickness and upper-mantle viscosity can be important, and for any reasonable value for the lithospheric thickness a corresponding mantle viscosity structure can be found that gives a plausible comparison of sealevel predictions with observations. In particular, thin-lithosphere models will lead to low estimates for the upper-mantle viscosity, while thick-lithosphere models lead to high viscosity values. However, either solution may represent only a local minimum in the model parameter space, and may not correspond to the optimum solution. It becomes important, therefore, that in the inversion of observational data, a comprehensive search is conducted throughout the entire model-parameter space, to ensure that the solution identified does indeed correspond to the optimum solution. The sea-level data for the British Isles lend themselves well to such an inversion because of the relatively high quality of the data, the good geographic distribution of the data relative to the former ice sheet, and reasonable observational constraints on the dimensions of the former ice sheet and on its retreat. Furthermore, because of the contribution to the sea-level signal from the distant ice sheets, as well as from the melt-water load, the observational data base for the region also has some resolving power for the viscosity of the deeper mantle. The parameter space explored is defined by up to five mantle layers, the lithosphere of effective elastic thickness D,, and a series of upper-mantle layers, i = 2-4, extending down to depths of 200, 400 and 670 km, respectively, each of viscosity qi, and a lower-mantle layer of viscosity qlm extending down to the coremantle boundary. The range of parameters explored is 30 < D, I 120 km, 3 x lOI9 Iqi

Isostatic compensation of Ishtar Terra, Venus

We have used spherical harmonic representations of the Venus topography and geopotential, obtained from Magellan data, to evaluate isostatic support in several areas within the Ishtar Terra highlands, including the Lakshmi plateau, its surrounding mountain belts, namely Akna and Freyja, and Maxwell Montes, and the Fortuna Tessera province. We find that topography in Ishtar is largely isostatically compensated (&gt;80%). Regional geoid‐topography variations in the subregions can be explained by a combination of Airy (crustal thickening) and thermal (lithospheric thinning) mechanisms, provided Venus has a thick reference thermal lithosphere (∼300–400 km). With the exception of eastern Fortuna, low elevation areas (h &lt; 3–4 km above the mean planetary radius, MPR) with large geoid‐topography ratios (GTR) seem to be associated, to various degrees, with thermal isostasy, whereas the higher areas (h &gt; 4 km above MPR) with small GTRs are almost certainly Airy compensated via thickened crust. Relatively large (&gt;60 km) total Airy crustal thicknesses obtained in the western Ishtar mountain belts, together with a probable basalt‐eclogite phase change, suggest a possible silicic composition for these structures, provided they are older than ∼25–50 Ma. Lakshmi Planum seems essentially thermally supported, with the thermal lithosphere thinned to ∼100 km. We suggest, as one possibility, that the lithospheric thinning process under Lakshmi is delamination of a dense eclogite lower lithosphere layer into the mantle. The decrease in GTR observed in Ishtar between Lakshmi to the west (GTR ∼ 20 m/km), Maxwell and west Fortuna (GTR ∼ 8 m/km), and eastern Fortuna (GTR ∼ 4 m/km) may correspond to a decay in thermal compensation attributed to lithospheric delamination, which would be fairly recent (∼100 Ma) in Lakshmi, partially decayed in west Fortuna, and absent in east Fortuna, where a mostly Airy‐supported topography is essentially relaxed with no thermal uplift. Alternatively, if surficial concentrations in radiogenic elements were prevalent throughout the crust, partial melting of a thickened crust could account for the thermal uplift in Lakshmi and west Fortuna. The zero‐elevation basaltic crustal thickness H ∼ 24 km obtained for the east Fortuna Tessera region may be representative of the ambient crustal thickness in the Venus lowlands. Our findings support multicomponent models for tectonic and volcanic activity in Ishtar. The thick ambient crust and thermal lithosphere implied by this study agree with observational constraints such as support of extreme elevations, large topographic slopes, unrelaxed craters, and the thick elastic lithosphere suggested by flexure studies. If the ambient thermal lithosphere on Venus were to be relatively thin (∼100–200 km), with a cold mantle and radiogenic elements concentrated in the crust, then thermal evolution on Venus may be in quasi‐steady state, with the geodynamic evolution in monotonic decline. However, if the ambient thermal lithosphere is very thick (∼300–400 km), as suggested by our thermal model fits, then it is consistent with the predictions of strongly unsteady state thermal evolution models and an interior which is currently heating up. This would support the view that catastrophic resurfacing on Venus might be episodic.

The effective elastic thickness (Te) of continental lithosphere: What does it really mean?

It is well accepted that the lithosphere may exhibit nonzero mechanical strength over geological time and space scales, associated with the existence of non‐lithostatic (deviatoric) stress. The parameter that characterizes the apparent strength of the lithosphere is the flexural rigidity D, which is commonly expressed through the effective elastic thickness (Te) of the lithosphere. Estimates of Te for oceanic lithosphere approximately follow the depth to a specific isotherm (∼600°C), which marks the base of the mechanical lithosphere. The physical meaning and significance of the effective elastic thickness for continents are still enigmatic, because for continental lithosphere estimates of Te bear little relation to specific geological or physical boundaries. Although high observed values of Te (70–90 km for cratons) can be partly explained by the present‐day temperature gradients, the low values (10–20 km), in general, cannot. In addition, the elastic plate models are self‐inconsistent in that they mostly predict intraplate stresses high enough to lead to inelastic (brittle or ductile) deformation, according to data of rock mechanics. To provide a basis for a physically consistent unified interpretation of the observed variations of Te for continental and oceanic lithosphere, we developed an analytical and numerical approach that allows direct treatment of Te in terms of the lithospheric rheology, thermal structure, and strain/stress distribution. Our technique is based on finding true inelastic and equivalent (effective) elastic solutions for the problem of deformation of the lithosphere with realistic brittleelasto‐ductile rheology. We show that the thermal state (thermotectonic age) of the lithosphere is only one of at least three equally important properties that determine apparent values of Te. These other properties are the state of the crust‐mantle interface (decoupling of crust and mantle), the thickness and proportions of the mechanically competent crust and mantle, and the local curvature of the plate, which is directly related to the bending stresses. The thickness of the mechanically competent crust and the degree of coupling or decoupling is generally controlled by composition of the upper and lower crust, total thickness of the crust, and by the crustal geotherm. If decoupling takes place, it permits as much as 50% decrease of Te, compared with Te implied from conventional thermal profiles. Comparison of the theoretically predicted Te with inferred values for different regions suggests that the lower crust of most continental plates has a low‐temperature activation rheology (such as quartz) which permits crust and mantle decoupling. The curvature of the plate depends on the rheological structure and on the distribution of external loads applied to the plate (e.g., surface topography, sediment fill, and plate‐boundary forces). Bending stresses created by major mountain belts are large enough to cause inelastic deformation (brittle failure and a ductile flow) in the underlying plate, which, in turn, leads to a 30 to 80% decrease of Te beneath such belts and less beneath the adjacent regions. The boundary forces and moments (e.g., due to the slab pull, etc.) lead to more localized but even stronger reductions in Te (e.g., plate necking in subduction zones). Our approach provides a feedback between the “observed” Te and rheology, allowing to constrain the lithospheric structure from estimates of Te.

Viscosity estimates for the crust and upper mantle from patterns of lacustrine shoreline deformation in the Eastern Great Basin

The deformed shorelines of Lake Bonneville constitute a classic source of information on lithospheric elastic thickness and upper mantle viscosity. We describe and apply a new model to a recently augmented data set. New data better constrain both the complex spatio‐temporal pattern of the lake load and the crustal deformation response to that load. The history of lake level fluctuations has been significantly refined and somewhat modified. This is due to both more radiocarbon dates from within the Bonneville basin and to an improved calibration of the radiocarbon timescale itself. The data which constrain the crustal deformation pattern consist of ages and shoreline elevations from several hundred points which sample three major levels of Lake Bonneville and corresponding elevations from the high stands of three smaller lakes situated to the west of Lake Bonneville. The data from the smaller lakes help elucidate the pattern of deflection which occurred beyond the edge of the big lake. The geometry of the Earth model incorporates an arbitrary number of layers overlying a half‐space, and the rheology of each level can accommodate an arbitrary number of Maxwell viscoelastic elements in parallel. The inverse modeling comprises three complementary approaches: for the simplest configurations, we performed a direct search of the parameter space and delineated the irregular boundary of the subspace of acceptable models. For more complex configurations, we constrained the elastic parameters to their seismically determined values and then solved for viscosity versus depth profiles by either expressing the log(viscosity) versus log(depth) profile as a series of specially constructed orthogonal polynomials, or by allowing each of 8–10 layers (plus the half‐space) to have an independently determined viscosity. We found that the data do not strongly support (nor can they conclusively exclude) a more complex rheology than simple Maxwell viscoelasticity. The orthogonal polynomial solution exhibits an essentially monotonic decrease in viscosity with depth. The most rapid change occurs at shallow depths, decreasing from 1023 Pa s at 3 km to 1020 Pa s at 30 km. The decrease is much more gradual below, with only another factor of 5 decrease between 30 and 300 km depth. The unconstrained solution exhibits a rapid decrease in viscosity with depth from 2×1024 Pa s in the top 10 km to 4×1017 Pa s at a depth of 40 km. A nearly isoviscous asthenospheric region extends from 40 to 150 km and is underlain by a mantle lithospheric region with increased viscosity (2×1020 Pa s) extending from 150 to 300 km depth and by a uniform viscosity (1019 Pa s) half‐space below.

Estimating Lithospheric Properties at Atla Regio, Venus

Magellan spherical harmonic gravity and topography models are used to estimate lithospheric properties at Atla Regio, Venus, a proposed hotspot with dynamic support from mantle plume(s). Global spherical harmonic and local representations of the gravity field share common properties in the Atla region in terms of their spectral behavior over a wavelength band from ∼2100 to ∼700 km. The estimated free-air admittance spectrum displays a rather featureless long-wavelength portion followed by a sharp rise at wavelengths shorter than about 1000 km. This sharp rise requires significant flexural support of short-wavelength structures. The Bouguer coherence also displays a sharp drop in this wavelength band, indicating a finite flexural rigidity of the lithosphere. A simple model for lithospheric loading from above and below is introduced (D. W. Forsyth, 1985, J. Geophys. Res. 90, 12,623-12,632) with four parameters: f, the ratio of bottom loading to top loading; Z m , crustal thickness; Z l, depth to bottom loading source; and T e, elastic lithosphere thickness. No model is found that fits simultaneously the estimated free-air admittance and Bouguer coherence across the entire spectrum. A dual-mode compensation model is introduced in which the shorter wavelengths (λ ⪉ 1000 km) might be explained best by a predominance of top loading by the large shield volcanoes Maat Mons, Ozza Mons, and Sapas Mons, and the longer wavelengths (λ ⪊ 1500 km) might be explained best by a deep depth of compensation, possibly representing bottom loading by a dynamic source. A Monte Carlo inversion technique is introduced to thoroughly search out the four-space of the model parameters and to examine parameter correlations in the solutions. It is shown that simultaneous inversion on admittance and Bouguer coherence in the short-wavelength band constrains the elastic thickness to a range of approximately 40 to 50 km. Introduction of the free-air coherence as an observable in the inversion narrows the estimates of the other three parameters ( f = 0.10 ± 0.01, Z m = 30 ± 13 km, Z l = 7 ± 5 km) and provides a solution T e = 45 ± 3 km. In the long-wavelength band, Z l is found to be about 150 km, and this is the only parameter that can be estimated robustly. Crustal thickness, Z m , is unbounded, and the solutions are degenerate in that there is no discrimination between top and bottom loading. Acceptable elastic thicknesses range from 0 to 140 km. The elastic thickness results from both wavelength bands are corroborated in the spatial domain by examining residual standard deviations for a range of T e values. The results found here are consistent with the results of Smrekar (1994) for wavelengths longer than 600 km. Values of Z l are consistent with other analyses that exclude wavelengths longer than about 2,000 km. Inclusion of wavelengths longer than about 3000 km leads to compensation depths of approximately 200 km. Moment-curvature relationships based on yield strength envelopes are used to constrain temperature gradient. Average curvature is obtained from the flexurally deformed Moho surface by Fourier transform techniques. Surface temperature gradients are obtained in the range 7-10 K km -1, depending on the strain rate adopted. The relationship between elastic thickness and temperature gradient is calibrated by comparing elastic thickness estimates of two East African hotspots with measured heat flow. The excess heat flow at the East African hotspots over the African continental mean, if applicable to Venus, can be used to show that the globally averaged heat flow on Venus must be much lower than the Earth-scaled value predicts. A thick (∼350-km) ambient thermal lithosphere, as proposed by A. B. Kucinskas and D. L. Turcotte in this issue, and interpretable from the long-wavelength depth of compensation at Atla Regio, provides an acceptable background temperature gradient. When it is added to the excess heat flow or temperature gradient, the predicted heat flow at Atla Regio is consistent with the moment-curvature results. The thermal lithosphere beneath Atla Regio may be about 100 km thick and this answer can be obtained by two independent methods: from the ambient thermal lithosphere thickness and simple elevation scaling (result dependent on long gravity wavelengths) and from the elastic thickness estimate converted to temperature gradient (result dependent on short gravity wavelengths). Either Venus is considerably deficient in heat sources relative to Earth, or the thermal lithosphere is overthickened in response to an earlier episode of significant heat loss from the planet.

The Isostatic State of Mead Crater

We have analyzed high-resolution Magellan Doppler tracking data over Mead crater, using both line-of-sight and spherical harmonic methods, and have found a negative gravity anomaly of about 4-5 mgal (at spacecraft altitude, 182 km). This is consistent with no isostatic compensation of the present topography; the uncertainty in the analysis allows perhaps as much as 30% compensation at shallow depths (∼25 km). This is similar to observations of large craters on Earth, which are not generally compensated, but contrasts with at least some lunar basins which are inferred to have large Moho uplifts and corresponding positive Bouguer anomalies. An uncompensated load of this size requires a lithosphere with an effective elastic lithosphere thickness greater than 30 km. In order for the crust-mantle boundary not to have participated in the deformation associated with the collapse of the transient cavity during the creation of the crater, the yield strength near the top of the mantle must have been significantly higher on Earth and Venus than on the Moon at the time of basin formation. This might be due to increased strength against frictional sliding at the higher confining pressures within the larger planets. Alternatively, the thinner crusts of Earth and Venus compared to that of the Moon may result in higher creep strength of the upper mantle at shallower depths.

Moho geometry beneath the eastern margin of the Andes, northwest Argentina, and its implications to the effective elastic thickness of the Andean foreland

The seismically constrained shape of the Moho beneath the eastern margin of the northwest Argentine Andes (24°S) is interpreted in the context of regional isostasy to constrain the flexural rigidity of the lithosphere in this actively deforming orogen. Seismic travel time residuals from regional intermediate depth earthquakes are inverted for three‐dimensional variations in the Moho depth beneath a portable seismic network spanning the eastern margin of the Puna plateau and the Santa Barbara foreland system. Beneath the easternmost Santa Barbara ranges, the Moho is relatively flat and lies at 40–45 km depth. Farther west, beneath the eastern margin of the Puna, the Moho dips 10–15° to the west before flattening out at a depth of 60–65 km beneath the plateau. The Moho inversion model and coincident gravity data constrain the flexural rigidity of the foreland lithosphere to be between 1021 and 1022 m. The corresponding effective elastic thickness of 6–12 km is a factor of 2 to 4 less than that estimated in previous published studies for the Bolivian Altiplano and Subandean ranges farther to the north. These results indicate that in contrast to the Bolivian Andes, the foreland lithosphere in northwest Argentina does not provide significant flexural support to the plateau margin, and the eastern margin of the Andes is nearly in a state of local isostatic compensation. The north to south decrease in the effective elastic thickness of the Andean lithosphere supports published seismic wave attenuation studies which indicate that south of 23°S the lithospheric thickness decreases beneath the plateau and foreland.

Fault Model of the 1891 Nobi Earthquake from Historic Triangulation and Leveling.

The faulting associated with the 1891 Nobi earthquake, central Honshu, is re-examined on the basis of horizontal displacement vectors determined between the periods 1882/1899-1950/1961. These vectors are modelled in terms of two components: (1) strain accumulation and release along the Nankai Trough and Suruga Trough, and (2) postseismic relaxation following the 1891 earthquake. We assume a rheological model of the central Honshu crust and upper mantle consisting of a 33-km thick elastic lithosphere overlying a 130 km thick viscoelastic sublithosphere. The modelling demonstrates that the observed displacement vectors are dominated by post-seismic displacements following the Nobi earthquake. The Nukumi-Neodani faults are inferred to have accommodated 7-8 m of left lateral strike-slip displacement, in good agreement with those estimates based on the observed surface rupture. In addition, we identify two northeast-southwest trending blind thrust faults, together comprising a 70-km-long region of compressional deformation, each of which accommodated several meters of reverse slip during the Nobi earthquake. These thrust faults consist of a 20-km-long western segment and a 50-km-long eastern segment. The western thrust begins near the southern termination of the strike-slip faulting component, has shallow (31°) dip, is buried by 15 km and has a prominent coseismic signal in the leveling data. Examination of the topography east of Nagoya suggests that the eastern thrust is buried 5-10 km below the surface and represents the subterranean continuation of the exposed thrust fault on the eastern flank of the Kiso Range to the northeast. The model of strain accumulation and release includes the various effects of the 1854 Ansei and 1944 Tonankai earthquakes, as well as the effect of steady state Philippine Sea interplate convergence. A model consisting of only tectonic strain accumulation and release along the Philippine Sea interplate boundary provides a poor fit to the data. The modelling suggests that the 1944 rupture did not extend beyond the Atsumi Peninsula. The ob-served displacement vectors and an independent set of Suruga Bay uplift data contain a moderate signal due to 1854 postseismic displacements. Considering that this signal has tradeoffs with the slip accumulation rate in Suruga Bay, this enables us to constrain the central Honshu sublithospheric viscosity to lie near 2×1019 Pa·s.

An episodic hypothesis for Venusian tectonics

It is suggested that episodic plate tectonics occurs on Venus; episodes of rapid plate tectonics are separated by periods of surface quiescence. For the last 500 ± 200 m.y. it is postulated that the surface of Venus has been a single rigid plate that has been thickening due to conductive cooling. A near‐uniform surface age is consistent with observed crater densities and the relatively small number of craters modified by surface tectonics or embayed by lava flows. A lithosphere that has conductively thickened for some 500 m.y. has a thickness of about 300 km, nearly an order of magnitude greater than the thickness associated with steady state conductive heat loss. Such a thick lithosphere can support the high topography and associated gravity anomalies on Venus as well as the unrelaxed craters; studies of lithospheric flexure at coronae are also consistent with a thick elastic lithosphere. Incipient subduction associated with large coronae may represent the onset of a new episode of rapid plate tectonics. On the Earth, 75–90% of mantle heat transport is attributed to the creation of new oceanic lithosphere at ocean ridges. This process is not operative on Venus. This paper suggests that episodic plate tectonics on Venus constitutes the primary mechanism for mantle heat transport on that planet.

On the thermo‐mechanical evolution of continental lithosphere

Elastic thickness of continental lithosphere is modified by both mechanical (thickening and thinning) events and thermal evolution. A simple thermo‐mechanical model can account for the processes of tectonic alteration and subsequent thermal reequilibration of lithospheric thermal structure. Mechanical thinning or thickening of the lithosphere directly affects its thickness and flexural rigidity. After a tectonic event, thermal reequilibration proceeds at a rate dependent upon the amount of thinning or thickening and total lithospheric thickness, previous tectonic history, and the heat flux at the base of the lithosphere. Heat flux at the base of the lithosphere is controlled by the efficiency of convective mantle heat delivery. Rapid heat delivery to the base of the lithosphere retards lithospheric thickening, whereas slow conductive heat delivery allows lithospheric thickening at the maximum rate. The relative importance of thermal effects to mechanical alterations of lithospheric elastic thickness depends on the time scale of concern. Very thin lithosphere undergoes rapid thermal evolution such that cooling between tectonic events significantly thickens the lithosphere. For thick lithosphere, conductive heat transport is so slow that only minor changes in thermal structure and no significant changes in elastic thickness take place between tectonic events. Model results with infinite stretching agree with observations of oceanic lithospheric thickness evolution, showing a divergence from a cooling half‐space at about 70 m.y. Model results also agree with independent estimates of continental thermal evolution in the absence of tectonic events. On the longest time scales approaching the age of the Earth, thermal reequilibration is attained, and any tectonic effects are overprinted. The time scale at which the transition between mechanically and thermally dominated elastic thickness evolution occurs depends on the heat transport efficiency of the convecting mantle. For high Nusselt numbers, thermal effects take longer to become important Even for lower Nusselt numbers, mechanical effects still dominate on time scales of 108 years. Crustal radiogenic heat production reduces the equilibrium lithospheric thermal and elastic thickness. It also introduces a dependence of equilibrium thickness on tectonic crustal thinning or thickening. Model results indicate that on time scales of thrust sheet and sedimentary loading in foreland basins, mechanical effects dominate the evolution of elastic thickness except for very thin lithosphere. The model can be applied to the lithosphere beneath foreland basins so that observed basin geometry can be used to constrain tectonic history prior to loading by thrust sheets and basin sediments.

Tectonic Patterns and Regional Stresses near Venusian Coronae

Many Venusian coronae exhibit distinctive patterns of faults; in particular, we find the tectonic patterns of one such corona similar to the striking patterns of the grabens surrounding Alba Patera on Mars. Pursuing this similarity, we perform a stress analysis analogous to one that successfully explained the Alba Patera pattern and in so doing determined a value for the local regional stress. Local stress fields of Venusian coronae are assumed to be created by either a Gaussian axially symmetric upwarp or a cylindrical load due to accumulation of volcanic materials on an elastic lithosphere. The local coronal stresses are combined with varying amounts of a uniform regional stress field for different elastic lithospheric thicknesses. From the magnitudes and orientations of the resultant principal stresses, the predicted patterns of surface tectonic features, i.e., graben and thrust fault chains, are plotted. These patterns are compared to those of coronae on Magellan images. Matching the patterns thus determines the regional stress and elastic lithospheric thickness about the coronae. Comparison of predicted to observed tectonic patterns for three specific coronae leads to estimated regional stresses of approximately 0.1-0.6 kbar and effective elastic lithospheric thicknesses consistent with 10 ± 5 km, although one area may have had a lithosphere as thin as 2 km during corona formation.

The effective elastic thickness of the lithosphere and the evolution of foreland basins

AbstractThe elastic thickness of the continental lithosphere is strongly ‘bi‐modal’. Foreland basins reflect this bi‐modality with narrow, deep basins (e.g. Apennines) and wide, shallow ones (e.g. Ganges). The bi‐modal distribution cannot be explained by thermal models which describe the relationship between elastic thickness and plate age in the oceans, suggesting the involvement of factors other than secular cooling. The high values (80–90 km) are consistent with present‐day temperature gradients of the cratons and the scatter within cratons may be explained by changes in the radiogenic heat production. The low values (10–20 km), however, are more difficult to explain. Foreland basins develop by flexure in front of thrust/fold loads as they advance over former passive margins and onto the cratons. Recent studies suggest that passive margins are underlain by highly attenuated crust and lithosphere which has a low elastic thickness that remains low for long periods (&gt; 108 yr) of time. Foreland basins may inherit these low values as they migrate over a passive margin. Stratigraphic modelling suggests that the low elastic thicknesses would have a profound effect on the development of foreland basins predicting as they do the asymmetry, the pattern of onlap and, the transition from the ‘underfilled’ to ‘overfilled’ phase. Why stretched crust and lithosphere is so weak on long time‐scales is enigmatic. Rifting, however, seems to proceed in such a way that the strong uppermost part of the crust is effectively ‘de‐coupled’ from any support that it might otherwise receive from the strong underlying mantle.

Mapping elastic lithospheric thickness variations in Canada

The variation of elastic lithospheric thickness within the Canadian landmass is determined using the coherence between surface topography and Bouguer gravity anomalies. Estimates of elastic thickness (or, equivalently, rigidity) are derived by minimizing the difference between the observed coherence and that predicted by an isostatic model consisting of a thin elastic plate which is subject to surface and subsurface loading and overlies a fluid substratum (the asthenosphere). Estimates of the elastic thickness vary from 17–18 km in coastal Labrador and Baffin Island to > 150 km near Lake Winnipeg, Manitoba. A broad correlation exists between lithospheric thickness and age. Exceptions to this correlation are areas of reduced plate thickness in North Atlantic coastal regions where magmatic activity associated with rifting and the formation of a passive margin may have been sufficient to reduce the plate strength but not enough to reset the thermal age of the basement rocks. Areas of greater plate thickness than that predicted by thermal cooling occur in the continental interior and may reflect lithospheric thickening due to the chemical and mechanical effects accompanying continental agglomeration.

Speculations on the origin and evolution of the Utopia‐Elysium Lowlands of Mars

The origin of the overall low and variable topographic character of the northern plains is a fundamental aspect of the Martian crustal dichotomy. A model for the development of the Utopia‐Elysium lowlands in eastern Mars in terms of the long‐term evolution of large, overlapping impact basins is presented. The effects of relaxation of basin topography, volcanic flooding and loading of the basins with subsequent subsidence and downwarping, and the growth of a long‐lived major tectonovolcanic center in the overlap region between the proposed Utopia and Elysium impact basins are qualitatively described. In an early hot, thin (&lt;50 km thick) lithosphere with low mantle viscosity (&lt;1022 P), relaxation of topographic relief was probably rapid (&lt;104 years) by comparison with the likely time between the two largest impacts (&gt;106 years). The later Utopia Basin impact drastically altered the structure of the earlier Elysium Basin, uplifting portions of what had been the central depression. The heating and fracturing due to the impacts was most intense in the region of overlap, which probably provided an easy conduit for volcanic eruptions in this area. Loading by Early Hesperian and older basin volcanics caused widespread subsidence which may have extended beyond the basin rim and which was responsible for the generally low character of the northern plains, despite the minimum 2–3 km volcanic fill present there. Prolonged volcanism (through the Early Amazonian) would likely keep the overlap region hotter longer, opposing later general subsidence of the impact basin floor where it had cooled. Slower cooling and thickening of the lithosphere in the overlap region is consistent with lower estimates of the Early‐Middle Amazonian elastic lithosphere thickness (50 km) in central Elysium, by comparison with inferred greater thickness in the Early Hesperian near the edge of the Utopia Basin at Isidis (&gt;120 km). Prolonged volcanic construction in the overlap region was also probably responsible for the great height of the Elysium tectonovolcanic complex by comparison with the other portions of the Utopia‐Elysium lowland, where most Volcanics are Hesperian in age. We suggest this model may be applicable to the origin of the northern lowlands elsewhere on Mars and that overlapping large impacts may be important in the establishment of the fundamental topographic and crustal dichtomy and the origin of major tectonovolcanic complexes on the planet.

Lithospheric flexure beneath the Freyja Montes Foredeep, Venus: Constraints on lithospheric thermal gradient and heat flow

Analysis of Venera 15 and 16 radar images and topographic data from the Freyja Montes region on Venus suggest that this mountain belt formed as a result of a sequence of underthrusts of the lithosphere of the North Polar Plains beneath the highlands of Ishtar Terra, The Freyja Montes deformation zone consists, south to north, of a linear orogenic belt, an adjacent plateau, a steep scarp separating the plateau from the North Polar Plains, a linear depression at the base of the scarp, and an outer rise. The topographic profile of the depression and outer rise are remarkably similar to that of a foreland deep and rise formed by the flexure of the underthrusting plate beneath a terrestrial mountain range. We test the lithospheric flexure hypothesis and we estimate the effective thickness Te of the elastic lithosphere of the underthrusting portion of the North Polar Plains by fitting individual topographic profiles to deflection curves for a broken elastic plate. The theoretical curves fit the observed topographic profiles to within measurement error for values of flexural rigidity D in the range (0.8–3) × 1022 N m, equivalent to Te in the range 11–18 km. Under the assumption that the base of the mechanical lithosphere is limited by the creep strength of olivine, the mean lithospheric thermal gradient is 14–23 K/km. The portion of the North Polar Plains to the immediate north of Freyja Montes stands at a regional elevation very near the modal elevation for the planet, so the inferred elastic lithosphere thickness and thermal gradient may be at least approximately representative of a significant fraction of the Venus lithosphere. That the inferred thermal gradient is similar to the value expected for the global mean gradient on the basis of scaling from Earth provides support for the hypothesis that simple conduction dominates lithospheric heat transport on Venus relative to lithospheric recycling and volcanism.

Horizontal motions due to post‐glacial rebound

North American horizontal post‐glacial rebound velocities are computed for three five‐layered, incompressible, spherical Maxwell earth models, using a simple glacial loading cycle centered on Hudson Bay. The reference earth model has a 120‐km thick elastic lithosphere and upper‐ and lower‐mantle viscosities of 1021 Pa·s and 2 × 1022 Pa·s, respectively. The second model has a lower viscosity upper‐mantle (5 × 1020 Pa·s) and a higher viscosity lower‐mantle (1023 Pa·s), and the third has a 250‐km thick lithosphere. The reference model and the low‐viscosity upper‐mantle model calculations are in good agreement with uplift and gravity data, although they may somewhat overestimate the present‐day central uplift rate. The thick lithosphere model fares less well because it fails to exhibit the well‐pronounced peripheral bulge sinking that is required by eastern North America tide gauge data. The reference model generally has the largest horizontal rebound velocities, reaching a maximum of 4 mm/yr towards Hudson Bay at 2400 km from the load center. The effect of a thick lithosphere or of a low‐viscosity upper‐mantle is to reduce present‐day horizontal velocities, especially in the formerly glaciated region. The sensitivity of horizontal motions to lithospheric thickness and upper‐mantle viscosity may assist in discriminating between different models of the earth's rheology once sufficient high‐quality geodetic data exist.

Topography and Geoid Undulations Caused By Small-Scale Convection Beneath Continental Lithosphere of Variable Elastic Thickness

Summary The effect of subcontinental upper mantle convection on topography, gravity and geoid undulation is studied with special regard to the effective elastic thickness of the continental lithosphere (hel). We designed a numerical model of the upper mantle-lithosphere system consisting of a constant viscosity fluid overlayed by a partly elastic plate. the boundary tractions (stresses) at the top of the convecting mantle model were applied to the lithosphere model for which we used the thin plate approximation. the total thickness of the lithosphere was kept constant to 150 km while the elastic thickness was varied between 0 and 150 km. the aspect ratio of the convecting mantle was varied between 0.7 and 3 and the Rayleigh number between 104 and 106. Our calculations demonstrate the effect of a thick elastic lithosphere on the surface observables: peak to peak averaged amplitude of topography, geoid undulation and gravity are strongly reduced and their spectra are low pass filtered. For an aspect ratio less than 1 and a very thick elastic lithosphere topography tends to zero and while geoid undulation and gravity may even become negative above the up welling plume. the spectral relation of geoid or gravity to topography (admittance) is nearly unaffected for hie, <70km. For Aie 70km admittance values for all wavelengths generally decrease with increasing elastic thickness of the plate and may become negative. Geoid admittance for the basic mode increases with larger aspect ratio, while gravity admittance for hie <100 km slightly decreases. Associating higher Rayleigh numbers of the convecting fluid with lower viscosity, an increase in Rayleigh number leads to considerable decrease of the surface observables. However, admittance values are only slightly affected and decrease approximately linearly with increasing Rayleigh number. Comparison of our results with observations of the topography spectrum and the free air gravity admittance for North America suggests that large aspect ratio cells beneath continents are unlikely. However, convection cells of aspect ratio 1 or less underlying an elastic lithosphere of at least 100 km thickness is in agreement with data. the Rayleigh number should be as high as 106.

Tepev Mons on Venus: Morphology and elastic bending models

Tepev Mons is a large volcanic structure of about 250 km in diameter with an elevation of 5 km above the surroundings, located at the southwestern edge of Bell Regio. It is surrounded by a moat with a depth of about 0.5 km. If this moat is considered to be caused by bending of the lithosphere due to the load of the volcano, then elastic bending models give limits for the effective flexural rigidity FR and the effective elastic thickness of the lithosphere L: 2 x 1023 Nm ≲ FR ≲ 3 x 1024 Nm and 30 km ≤ L ≤ 100 km. High flexural rigidities are associated with small depressions and large thicknesses of the lithosphere and vice versa.