SYNCHRONOUS COMPRESSIONAL PULSES IN EXTENSIONAL BASINS

Such paradoxes as synchronous compression and tension in extensional s edimentary basins, and synchronous deformation of intracratonic basins and eustatic sea-level falls, can each be related to pulsed changes i n curvature of the Earth's surface. Australia is everywhere in a state of significant, relatively uniform, upper crustal horizontal compress ion, approximately perpendicular to the continental margins and genera lly directed towards its centre. The compression commenced in the Plio cene but was absent in the Late Miocene. During the evolution of the G ippsland and Bass basins under extensional stress from the Late Jurass ic to Recent, seven pulses of similarly directed compression have been identified from the Campanian to Recent. Time equivalent compressiona l pulses can also be demonstrated in such widely separated areas as No rthwest and Northeast Australia, Northwest Europe and Central America. Nine earlier pulses of Early Triassic to Albian age are similarly syn chronous global events and another twenty-three pulses inferred since the Neoproterozoic (800 Ma) are probably similarly globally synchronou s. Earth expansion (with subduction) can explain these observations si nce it causes the surface curvature to decrease resulting in 'flatteni ng' of the semi-rigid crust. Flattening results in lower crustal exten sion which generates sedimentary basins while contemporaneous upper cr ustal compression inverts and folds the sediments. Since globally sync hronous compressional pulses may reflect expansion in the ocean basins there is a correlation of such pulses with major eustatic sea-level l owstands. Any site of expansion within the earth is transmitted to the semi-rigid crust as a volume increase by the whole gravitating-fluid, oblate spheroid. This results in ubiquitous synchronous compressional pulses in the upper crust detectable on seismic reflection data not o nly at basin scale, but also in individual petroleum-producing structu res. (C) 1997 Elsevier Science Ltd.