TASMANTE CRUISE - SWATH-MAPPING AND UNDERWAY GEOPHYSICS SOUTH AND WEST OF TASMANIA

The 1994 Tasmante swath-mapping and reflection seismic cruise covered 200 000 km(2) of sea floor south and west of Tasmania. The survey prov ided a wealth of morphological, structural and sedimentological inform ation, in an area of critical importance in reconstructing the break-u p of East Gondwana. The west Tasmanian margin consists of a non-deposi tional continental shelf less than 50 km wide and a sedimented contine ntal slope about 100 km wide. The adjacent 20 km of abyssal plain to t he west is heavily sedimented, and beyond that is lightly sedimented E ocene oceanic crust formed as Australia and Antarctica separated. The swath data revealed systems of 100 m-deep downslope canyons and large lower-slope fault-blocks, striking 320 degrees and dipping landward. T hese continental blocks lie adjacent to the continent ocean boundary ( COB) and are up to 2500 m high and have 15 degrees-20 degrees scarps. The South Tasman Rise (STR) is bounded to the west by the Tasman Fract ure Zone extending south to Antarctica. Adjacent to the STR, the fract ure zone is represented by a scarp up to 2000 m high with slopes of 15 -20 degrees. The scarp consists of continental fault-blocks dipping la ndward. Beyond the scarp to the west is a string of sheared parallel h ighs, and beyond that is lightly sedimented Oligocene oceanic crust 42 00-4600 m deep with distinct E-W spreading fabric. The eastern margin of the bathymetric STR trends about 320 degrees and is structurally co ntrolled. The depression between it and the continental East Tasman Pl ateau (ETP) is heavily sedimented; its western part is underlain by th inned continental crust and its central part by oceanic crust of Late Cretaceous to Early Tertiary age. The southern margin of the STR is fo rmed by N-S transform faults and south-dipping normal Faults. The STR is cut into two major terrains by a N-S fracture zone at 146 degrees 1 5' E. The western terrain is characterised by rotated basement blocks and intervening basins mostly trending 270 degrees-290 degrees. The ea stern terrain is characterised by basement blocks and intervening stri ke-slip basins trending 300 degrees-340 degrees. Recent dredging of ba sement rocks suggests that the western terrain has Antarctic affinitie s, whereas the eastern terrain has Tasmanian affinities. Stretching an d slow spreading between Australia and Antarctica was in a NW directio n from 130-45 Ma, and fast spreading was in a N-S direction thereafter . The western STR terrain was attached to Antarctica during the early movement, and moved down the west coast of Tasmania along a 320 degree s shear zone, forming the landward-dipping continental blocks along th e present COB. The eastern terrain either moved with the western terra in, or was welded to it along the 146 degrees 15' E fracture zone in t he Early Tertiary. At 45 Ma, fast spreading started in a N-S direction , and after some probable movement along the 146 degrees 15' E fractur e zone, the west and east STR terrains were welded together and became part of Australia.