Styles of Deformation in Ishtar Terra and Their Implications

Ishtar Terra, the highest region on Venus, appears to have characteristics of both plume uplifts and convergent belts. Magellan imagery over longitudes 330°-30°E indicates a great variety of tectonic and volcanic activity, with large variations within distances of only a few 100 km. The most prominent terrain types are the volcanic plains of Lakshmi and the mountain belts of Maxwell, Freyja, and Danu. The belts appear to have marked variations in age. There are also extensive regions of tessera in both the upland and outboard plateaus, some rather featureless smooth scarps, flanking basins of complex extensional tectonics, and regions of gravitational or impact modification. Parts of Ishtar are the locations of contemporary vigorous tectonics and past extensive volcanism. Ishtar appears to be the consequence of a history of several 100 m.y., in which there have been marked changes in kinematic patterns and in which activity at any stage has been strongly influenced by the past. Ishtar demonstrates three general properties of Venus: (1) erosional degradation is absent, leading to preservation of patterns resulting from past activity; (2) many surface features are the responses of a competent layer less than 10 km thick to flows of 100 km or broader scale; and (3) these broader scale flows are controlled mainly by heterogeneities in the mantle. Ishtar Terra does not appear to be the result of a compression conveyed by an Earthlike lithosphere. But there is still doubt as to whether Ishtar is predominantly the consequence of a mantle upflow or downflow. Upflow is favored by the extensive volcanic plain of Lakshmi and the high geoid: topography ratio; downflow is favored by the intense deformation of the mountain belts and the absence of major rifts. Both could be occurring, or have recently occurred, with Lakshmi the most likely locus of upflow and Maxwell the main locus of downflow. But doubts about the causes of Ishtar will probably never be resolved without circularization of the Magellan orbit to obtain a more detailed gravity field.