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Schubert, G, Sandwell DT.  1995.  A Global Survey of Possible Subduction Sites on Venus. Icarus. 117:173-196.   10.1006/icar.1995.1150   AbstractWebsite

About 10,000 km of trenches in chasmata and coronae have been identified as possible sites of retrograde subduction on Venus. All the sites have narrow deep trenches elongate along strike with arcuate planforms, ridge-trench-outer rise topographic profiles typical of terrestrial subduction zones, large outer rise curvatures >10(-7) m(-1), fractures parallel to the strike of the trench on the outer trench wall and outer rise, and no cross-strike fractures across the trench. Both the northern and southern margins of Latona Corona are possible subduction sites. Identification of a major graben between the two principal outer ridges in southern Latona Corona is evidence of back-are extension in the corona; the amount of extension is estimated to be more than 2-11 km. The moment exerted by the ridges of southern Latona Corona is insufficient to bend the lithosphere into the observed outer rise shape; a negatively buoyant subducted or underthrust slab is needed. Depending on the unknown trench migration rate, lithospheric subduction can make a significant contribution to mantle cooling on Venus. Venusian chasmata could have a dual character. They may be propagating rifts near major volcanic rises, and subduction trenches far from the rises in the lowlands. Subduction and rifting may occur in close proximity on Venus, unlike on Earth. Rifting induced by hotspots on Venus may be necessary to break the lithosphere and allow subduction to occur. Such a process could result in gradual lithospheric subduction or global, episodic overturn of the lithosphere. (C) 1995 Academic Press, Inc.

Schubert, G, Moore WB, Sandwell DT.  1994.  Gravity over Coronae and Chasmata on Venus. Icarus. 112:130-146.   10.1006/icar.1994.1174   AbstractWebsite

The global spherical harmonic model of Venus' gravity field MGNP60FSAAP, with horizontal resolution of about 600 km, shows that most coronae have little or no signature in the gravity field. Nevertheless, some coronae and some segments of chasmata are associated with distinct positive gravity anomalies. No corona has been found to have a negative gravity anomaly. The spatial coincidence of the gravity highs over four closely spaced 300- to 400-km-diameter coronae in Eastern Eistla Regio with the structures themselves is remarkable and argues for a near-surface or lithospheric origin of the gravity signals over such relatively small features. Apparent depths of compensation (ADCs) of the prominent gravity anomalies at Artemis, Latona, and Heng-o Coronae are about 150 to 200 km. The geoid/topography ratios (GTRs) at Artemis, Latona, and Heng-o Coronae lie in the range 32 to 35 m km(-1). The large ADCs and GTRs of Artemis, Latona, and Heng-o Coronae are consistent with topographically related gravity and a thick Venus lithosphere or shallowly compensated topography and deep positive mass anomalies due to subduction or underthrusting at these coronae. At arcuate segments of Hecate and Parga Chasmata ADCs are about 125 to 150 km, while those at Fatua Corona, four coronae in Eastern Eistla Regio, and an arcuate segment of Western Parga Chasma are about 75 km. The GTRs at Fatua Corona, the four coronae in eastern Eistla Regio, and the arcuate segments of Hecate, Parga, and Western Parga Chasmata are about 12 to 21 m km(-1). The ADCs and GTRs of these coronae and arcuate chasmata segments are generally too large to reflect compensation by crustal thickness variations. Instead, they suggest compensation by thermally induced thickness variations in a moderately thick (approximate to 100 km) lithosphere. Alternatively, the gravity signals at these sites could originate from deep positive mass anomalies due to subduction or underthrusting. Weighted linear least squares fits to GTR vs h (long-wavelength topography) data from Heng-o and Fatua Coronae, the four coronae in eastern Eistla Regio, and the arcuate segments of Hecate, Parga, and western Parga Chasmata are consistent with compensation by thermally induced thickness variations of a dense lithosphere above a less dense mantle; the fits imply an average lithosphere thickness of about 180 km and an excess lithospheric density of about 0.5 to 0.7%. Gravity anomalies at the arcuate segments of Dali and Diana Chasmata that form Latona Corona, at Artemis Chasma, and other arcuate segments of Parga and Hecate Chasmata occur on the concave sides of the arcs. By analogy with gravity anomalies of similar horizontal scale (600 km-several thousand kilometers) on the concave sides of terrestrial subduction zone arcs, which are due in large part to subducted lithosphere, it is inferred that the gravity anomalies on Venus are consistent with retrograde subduction at Artemis Chasma, along the northern and southern margins of Latona Corona, and elsewhere along Parga and Hecate Chasmata. (C) 1994 Academic Press, Inc.

Johnson, CL, Sandwell DT.  1994.  Lithospheric Flexure on Venus. Geophysical Journal International. 119:627-647.   10.1111/j.1365-246X.1994.tb00146.x   AbstractWebsite

Topographic flexural signatures on Venus are generally associated with the outer edges of coronae, with some chasmata and with rift zones. Using Magellan altimetry profiles and grids of venusian topography, we identified 17 potential flexure sites. Both 2-D cartesian, and 2-D axisymmetric, thin-elastic plate models were used to establish the flexural parameter and applied load/bending moment. These parameters can be used to infer the thickness, strength and possibly the dynamics of the venusian lithosphere. Numerical simulations show that the 2-D model provides an accurate representation of the flexural parameter as long as the radius of the feature is several times the flexural parameter. However, an axisymmetric model must be used to obtain a reliable estimate of load/bending moment. 12 of the 17 areas were modelled with a 2-D thin elastic plate model, yielding best-fit effective elastic thicknesses in the range 12 to 34 km. We find no convincing evidence for flexure around smaller coronae, though five possible candidates have been identified. These five features show circumferential topographic signatures which, if interpreted as flexure, yield mean elastic thicknesses ranging from 6 to 22 km. We adopt a yield strength envelope for the venusian lithosphere based on a dry olivine rheology and on the additional assumption that strain rates on Venus are similar to, or lower than, strain rates on Earth. Many of the flexural signatures correspond to relatively high plate-bending curvatures so the upper and lower parts of the lithosphere should theoretically exhibit brittle fracture and flow, respectively. For areas where the curvatures are not too extreme, the estimated elastic thickness is used to estimate the larger mechanical thickness of the lithosphere. The large amplitude flexures in Aphrodite Terra predict complete failure of the plate, rendering mechanical thickness estimates from these features unreliable. One smaller corona also yielded an unreliable mechanical thickness estimate based on the marginal quality of the profile data. Reliable mechanical thicknesses found by forward modelling in this study are 21 km-37 km, significantly greater than the 13 km-20 km predictions based on heat-flow scaling arguments and chondritic thermal models. If the modelled topography is the result of lithospheric flexure, then our results for mechanical thickness, combined with the lack of evidence for flexure around smaller features, are consistent with a venusian lithosphere somewhat thicker than predicted. Dynamical models for bending of a viscous lithosphere at low strain rates predict a thick lithosphere, also consistent with low temperature gradients. Recent laboratory measurements indicate that dry crustal materials are much stronger than previously believed. Corresponding time-scales for gravitational relaxation are 10(8)-10(9) yr, making gravitational relaxation an unlikely mechanism for the generation of the few inferred flexural features. If dry olivine is also found to be stronger than previously believed, the mechanical thickness estimates for Venus will be reduced, and will be more consistent with the predictions of global heat scaling models.

Sandwell, DT, Schubert G.  1992.  Flexural Ridges, Trenches, and Outer Rises around Coronae on Venus. Journal of Geophysical Research-Planets. 97:16069-16083.   10.1029/92JE01274   AbstractWebsite

High-resolution altimetry collected by the Magellan spacecraft reveals trench and outer rise topographic signatures around major coronae (e.g. Eithinoha, Heng-0, 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 die 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-0, 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-0, 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-0 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 increase.

McKenzie, D, Ford PG, Johnson C, Parsons B, Sandwell D, Saunders S, Solomon SC.  1992.  Features on Venus Generated by Plate Boundary Processes. Journal of Geophysical Research-Planets. 97:13533-13544.   10.1029/92JE01350   AbstractWebsite

Various observations suggest that there are processes on Venus that produce features similar to those associated with plate boundaries on Earth. Synthetic aperture radar images of Venus, taken with a radar whose wavelength is 12.6 cm, are compared with GLORIA images of active plate boundaries, obtained with a sound source whose wavelength is 23 cm. Features similar to transform faults and to abyssal hills on slow and fast spreading ridges can be recognized within the Artemis region of Venus but are not clearly visible elsewhere. The composition of the basalts measured by the Venera 13 and 14 and the Vega 2 spacecraft corresponds to that expected from adiabatic decompression, like that which occurs beneath spreading ridges on Earth. Structures that resemble trenches are widespread on Venus and show the same curvature and asymmetry as they do on Earth. These observations suggest that the same simple geophysical models that have been so successfully used to understand the tectonics of Earth can also be applied to Venus.