TECTONIC INTERPRETATIONS OF CENTRAL ISHTAR-TERRA (VENUS) FROM VENERA-15 16 AND MAGELLAN FULL-RESOLUTION RADAR IMAGES/

For more than a decade, the mapping of Venus has revealed a surface th at has had a complex volcanic and tectonic history, especially in the northern latitudes. Detailed morphostructural analysis and tectonic in terpretations of Central Ishtar Terra, based both on Venera 15/16 and Magellan full-resolution radar images, have provided additional insigh t to the formation and evolution of Venusian terrains. Ishtar Terra, c entred at 0-degrees-E longitude and 62-degrees-N latitude, consists of a broad high plateau, Lakshmi Planum, partly surrounded by two highla nds, Freyja and Maxwell Montes, which have been interpreted as orogeni c belts based on Venera 15 and 16 data. Lakshmi Planum, the oldest par t of Ishtar Terra, is an extensive and complexly fractured plateau tha t can be compared to a terrestrial craton. The plateau is partially co vered by fluid lava flows similar to the Deccan traps in India, which underwent a late stage of extensional fracturing. After the extensiona l deformation of Lakshmi Planum Freyja and Maxwell Montes were created by regional E-W horizontal shortening that produced a series of N-S f olds and thrusts. However, this regional arrangement of folds and thru sts is disturbed locally, e.g. the compressive deformation of Freyja M ontes was closely controlled by parallel WNW-ESE-trending left-lateral shear zones and the northwestern part of Maxwell Montes seems to be e xtruded laterally to the southwest, which implies a second oblique thr ust front overlapping Lakshmi Planum. These mountain belts also show e vidence of a late volcanic stage and a subsequent period of relaxation that created grabens parallel to the highland trends, especially in M axwell Montes.