TECTONICS OF SMALL TESSERA ISLETS ON VENUS

Two orthogonal fracture sets pattern many Venusian bar-like tessera bl ocks. The fracture pattern consists of parallel main edge faults which may have strike-slip displacements, and pronounced dense surface patt erns of parallel and high-angle tessera faults with complex cross-cutt ing relations. Dimensions and regularity of the complex pattern indica te a certain block-wide uniformity of processes involved. The two orth ogonal fracture sets within bar-like tessera blocks can be understood in terms of tensional uppermost crust. The bar-like blocks were first caused by primary parallel faulting which was then followed by formati on of two orthogonal high-angle fault sets of more local, multiphase r elaxational deformation within the blocks. Complex cross-cutting relat ions of fractures of the two perpendicular sets call be interpreted as results of chocolate tablet boudinage of surface rocks underlied by a low-viscous intralithospheric, or intracrustal, layer. Progressive or superposed high angle fracturing has characterized this relaxational deformation. Few fractures which connect over block boundaries from or into surrounding terrains indicate connections to wider regional stre sses which were less important during orthogonal fracturing. Copyright (C) 1996 Elsevier Science Ltd