Influence of variable decoupling between vertically separated fault populations on structural inheritance – The Laminaria High, NW Shelf of Australia

Abstract

When extension events are greatly separated in time, older faults may be buried and stratigraphically separated from newly developing faults at shallower depths. During rifting, the buried structures may reactivate and propagate upwards to be expressed within the shallow system. The degree of linkage between structural levels determines the influence that the deeper structures can exert over the geometry and evolution of the incipient fault system. In this study we use 3D seismic reflection data to examine how a deep fault population across the Laminaria High, NW shelf of Australia influences the development of a younger fault system at shallow depths. These fault populations are non-parallel and decoupled across a mechanically weak interval. The majority of shallow faults are not linked to those at depth. However, the reactivation and upward propagation of some of the deeper faults produce anomalously oriented structures at shallow depths, hard-linked to the deeper structures. One fault in particular shows along-strike variability, with the deep segment reactivated and present at shallow depths in the west. To the east, the shallow and deep fault segments become decoupled across the mechanically weak interval, although some soft-linkage and strain transfer still occurs. We suggest that this switch in the degree of coupling along the fault is due to the geometry of the deeper structure, with the transition corresponding to a prominent relay ramp. We show how the geometry of a deeper fault may affect its propensity to reactivate during subsequent extensional events, ultimately affecting the degree of structural inheritance that is expressed within younger, shallower fault populations.