The Titri Fault System: Quaternary-active faults near the leading edge of the Otago reverse fault province

The Titri Fault System is a 58 km long, northeast-striking, southeast-dipping, reverse fault system situated near the outer (seaward) edge of the Otago reverse fault province, the outboard zone of the Pacific-Australian convergent plate boundary. Geological mapping and seismic reflection profiles show that the fault system is composed of a master fault and frontal, Quaternary-active strands. The master fault-the major range-bounding fault-is the fault on which the majority of both the middle-Late Cretaceous normal faulting and late Cenozoic reverse faulting took place. The relatively straight map trace, and a fault plane exposure at Snowdrift Quarry of 65degreesE, indicates a steep fault plane, but is proposed to be listric at depth. Frontal strands, up to 2 km west of the master fault, variably deform alluvial fans, indicating the strands have different Quaternary histories. Map traces of the frontal strands indicate a shallower dip than the master fault and are therefore proposed to be footwall shortcut thrusts. Seismic reflection profiles and drillhole data indicate that a Holocene silt-sand wedge is not disturbed across the fault system in the southern Taieri Basin area. This is consistent with an absence of deformation of the youngest alluvial fans along the entire fault trace, and collectively indicates that the Titri Fault System has not ruptured in the Holocene. A scarp displacing the second-youngest alluvial fan set near the southern end of the fault system is the youngest evidence of movement. Correlation of fan formation with major glacial periods indicates this scarp probably dates from the middle of the Last Glacial period. In contrast, the last episode of widespread deformation probably occurred during the Last Interglacial period, or even earlier. Variation in total throw across the fault, the shape of the re-exposcd Late Cretaceous erosion surface on basement of the hanging-wall block, and variable Quaternary rupture histories for the frontal strands are presented as evidence that the Titri Fault System is segmented. Three segments are proposed, separated by boundaries at the position of the entrances to two antecendent gorges crossing the hanging wall (Taieri and Tokomairiro Rivers). In addition, the parallel, reverse Castle Hill Fault to the south may be acting as a fourth segment.