Progressive localisation of strain during the evolution of a normal fault population

Abstract

The evolution of a syn-sedimentary normal fault population has been established from the analysis of a high-quality 3D seismic reflection survey from the Timor Sea, offshore NW Australia. Growth of the fault population, which initiated ca. 6Ma ago, occurred in three distinct but temporally overlapping stages. The initial stage of fault population evolution was characterised by rapid growth of fault length in the first 1–2Ma of extension. Displacement rates of individual faults were established during this stage and were constant throughout subsequent fault growth. The second stage involved amplification of fault displacements on existing fault traces with minimal fault propagation. During the third stage, shortening of the traces of active faults and high mortality rates for small faults resulted in a net decrease in fault trace length on successively younger horizons. This stage was accompanied by a declining extension rate.Fault throw populations have constant slopes on each syn-faulting horizon; however an up-sequence shallowing of the slope of length and geometric moment populations demonstrates a progressive concentration of strain onto fewer and larger faults through time. If extension were to continue beyond the present value of 4.5%, this strain localisation might culminate in the formation of a single active through-going fault.