MODEL OF FORMATION OF ISHTAR-TERRA, VENUS

For more than a decade, the radar mapping of Venus surface has reveale d that it results from a complex volcanic and tectonic history, especi ally in the northern latitudes. Ishtar Terra (0 degrees E-62 degrees E ) consists of a high plateau, Lakshmi Planum, surrounded by highlands, Freyja Montes to the north and Maxwell Montes to the east. The latter is the highest relief of Venus, standing more than 10 km in elevation . The high resolution of Magellan radar images (120-300 m) allows us t o interpret them in terms of tectonics and propose a model of formatio n for the central part of Ishtar Terra. The detailed tectonic interpre tations are based on detailed structural and geologic cartography. The geologic history of Ishtar Terra resulted from two distinct, opposite tectonic stages with an important, transitional volcanic activity. Fi rst, Lakshmi Planum, the oldest part of Ishtar Terra is an extensive a nd complexly fractured plateau that can be compared to a terrestrial c raton. Then the plateau is partially covered by fluid lava flows that may be similar to Deccan traps, in India. Second, after the extensiona l deformation of Lakshmi Planum and its volcanic activity. Freyja and Maxwell Montes formed by WSW-ENE horizontal crustal shortening. The la tter produced a series of NNW-SSE parallel, sinuous, folds and imbrica ted structures that overlapped Lakshmi Planum westward. So these mount ain belts have the same structural characteristics as terrestrial fold -and-thrust belts. These mountain belts also display evidence of a lat e volcanic stage and a subsequent period of relaxation that created gr abens parallel to the highland trend, especially in Maxwell Montes. Co pyright (C) 1996 Elsevier Science Ltd