GRAVITY-ANOMALIES, COMPENSATION MECHANISMS, AND THE GEODYNAMICS OF WESTERN ISHTAR TERRA, VENUS

Pioneer Venus line-of-sight orbital accelerations have been used to calculate the geoid and vertical gravity anomalies for western Ishtar Terra on various planes of altitude z0. The apparent depth of isostatic compensation at z0 = 1400 km is 180 +/- 20 km based on the usual method of minimum variance in the isostatic anomaly. We seek to explain this observation, as well as the regional elevation, peripheral mountain belts, and inferred age of western Ishtar Terra in terms of one of three broad geodynamic models. Under local mantle upwelling, western Ishtar is the surface expression of a hotspot which produces topography both dynamically and by volcanic construction. This model holds that the mountain belts form as a result of incipient downward mantle return flow. An upwelling with its centroid at 160-km depth and buoyancy stress 100 MPa can satisfy the observed long-wavelength geoid and topography provided that the crust-mantle density contrast does not control the response of the crust (so that late-time subsidence does not occur) and that the mantle viscosity does not increase markedly with depth. Mantle upwelling produces a combination of strike-slip motion and azimuthal extension on the uplift itself; radial compression is restricted to the surrounding plains. Under local mantle downwelling, cold sinking mantle beneath western Ishtar induces shear tractions on the crust which thicken and elevate it; volcanism is potentially the result of basal crustal remelting. The formation of mountain belts is assumed to be a by-product of crustal thickening. A deeply seated downwelling (300-700 km, buoyancy stress 200-300 MPa) can satisfy the geoid and central topography provided that the crust can respond separately to mantle flow and that a more viscous lower mantle exists. The predicted surface strains during intermediate stages of the uplift are in qualitative agreement with observations (azimuthally oriented thrusts at the uplift margin). Incorporation of the effective viscosities of diabase and olivine suggests that uplift above a plume will be rapid, but the thermal gradient over downgoing flow must be at least as high as the expected mean value for Venus (approximately 20 K/km) in order to attain substantial uplift within several hundred million years. A third scenario, regional compression, may be distinguished from the second in that western Ishtar is simply a locus of strain accumulation and crustal thickening from regional stress fields; neither broad downward mantle flow nor lithospheric subduction need occur. However, both the source of stress and geoid anomaly are problematic. If the topography of western Ishtar is assumed to be Airy compensated, then the geoid signature must arise from unrelated mantle density anomalies. In this case, the lithosphere must be decoupled from mantle flow or else the mantle must be effectively rigid in order to suppress dynamic topography. A more complex model than those analyzed in detail here is needed to produce both the correct surface strains and apparent compensation depth. We favor upwelling models in which a mantle plume has flattened out and the surface has been subsequently deformed. Western Ishtar Terra must be distinguished from other paradigm hotspots on Venus by different environmental conditions or by being in a unique stage of hotspot evolution. Further quantitative investigations of the interaction of mantle shear tractions with a laterally heterogenous lithosphere are required.