GLOBALLY SYNCHRONOUS COMPRESSIONAL PULSES IN EXTENSIONAL BASINS: IMPLICATIONS FOR HYDROCARBON EXPLORATION

Abstract

It is possible to interpret many continental stresses on the Global Stress Map (Zoback, 1992) in terms of plate tectonics. Plate tectonics on a constant radius earth predicts a state of zero stress in Australia, except for northerly to northeasterly compression along the northern margin where Australia interacts with the Pacific Plate. However, the continent is everywhere in a state of significant horizontal compression, generally directed towards its centre.

In southeastern Australia the current maximum horizontal compressional stress is directed northwestwards. While Gippsland Basin and Bass Basin developed under extensional stress from the Late Jurassic to Recent, there have been pulses of similarly directed compression in the Pliocene to Recent, Mid Miocene, Early Miocene, Late Eocene to Early Oligocene, Early Eocene, Paleocene, Campanian, Late Albian to Early Cenomanian, Aptian and Valanginian(?).

Most of these pulses can also be demonstrated in such widely separated areas as the Carnarvon Basin in northwestern Australia, the Capricorn and Surat/Bowen Basins in eastern Australia, southern England, the Viking Graben in the North Sea and Pacific Guatemala. Pulses in the Portlandian, Callovian, Early Jurassic, Late Triassic and Mid Triassic appear to be similarly synchronous while two events in the Early Permian have been recognised also.

Near-surface compressional pulses contemporaneous with lower crustal extension can be explained by continental flattening on an expanding earth. Such an interpretation is consistent with the centrewards horizontal compressional stresses observed in the Australian continent since at least the Late Triassic.

Since an expansion pulse results in increased ocean basin capacity, compressional pulses have a strong tendency to coincide with the major sea level falls on the Haq et al (1987) global eustatic cycle chart.

The orientations of horizontal compressional stresses appear to have varied little since the Late Triassic. If a basin axis is approximately perpendicular to those stresses the basin may record all compression pulses. However, repeated compression sub-parallel to a basin axis may induce movement on wrench faults which can be a threat to seal integrity.