Abstract - Fault reactivation is observed in many basins worldwide and has been extensively studied, yet the details of how earlier displacement accumulation impacts on later faulting is often unresolved. We address this question using seismic reflection data (25,000km of 2D and 4,000km2 of 3D seismic data with up to 16 interpreted seismic horizons tied to 35 wells), from the Southern Taranaki Basin, which contains sedimentary rocks up to 8km thick that record multiple phases of faulting and folding since ca 85Ma. As is common in New Zealand many early formed Late Cretaceous to Palaeocene normal faults in the Southern Taranaki Basin were inverted during Miocene shortening. We find that all normal faults with maximum vertical displacements over 300 milliseconds (TWTT) were inverted while faults smaller than this were not. In the majority of cases, the entire length of the initial normal fault was inverted with the approximate location of the maximum displacement and the proportion of the total (basin wide) strain budget accommodated on the fault being similar between deformational episodes. These observations suggest that few new large (>300 milliseconds) faults were initiated in the Miocene and that the size (displacement and length) of each normal fault is a key determinant for whether it will be reactivated and for the size of the subsequent reverse fault. It is proposed that larger normal faults (>300 milliseconds) are reactivated along their entire lengths at the onset of shortening with the reverse fault system largely utilising the pre-existing system from an early stage. Under these conditions a hierarchy of fault length was established rapidly and longer faults accrue a greater proportion of the total strain budget perhaps because their greater dimensions or weakness locally promotes strain localisation at the expense of smaller faults in the system or the creation of new faults.
Abstract of talk given to:
Geosciences New Zealand Annual Conference, Geological Society of New Zealand, Christchurch, November 2013.