Recognition of active strike-slip faulting from high-resolution marine seismic reflection profiles: Eastern Marlborough fault system, New Zealand

Major strike-slip faults within the eastern part of the Marlborough fault s ystem of New Zealand extend offshore beneath the continental shelf, into th e southern end of the Hikurangi margin. Six major submarine faults, each te ns of kilometers in length and exhibiting the three-dimensional structural characteristics typical of strike-slip deformation zones, are mapped in det ail using closely spaced high-resolution seismic-reflection profiles. Each fault displaces late Quaternary sediments that have been interpreted within the framework of the sequence stratigraphic model and the established hist ory of glacio-eustatic sea-level cyclicity. Some of the faults are associat ed with strike-slip microearthquake focal mechanisms. The faults are within a complex structural high that underlies much of the outer continental shelf, and within the Flaxbourne basin between the struct ural high and the coast. The plan-view pattern of faulting recorded in the Quaternary sediments reflects only the major surface traces that have propa gated upsection as contemporaneous sedimentation has blanketed most of the evolving structures, Regionally, the late Quaternary faults compose two gro ups that bound blocks with rhomboid surface areas on a scale of tens of squ are kilometers to hundreds of square kilometers, One group strikes typicall y between 023 degrees and 057 degrees, i.e., 22 degrees-56 degrees from the plate motion vector (079 degrees), and includes dextral, oblique-slip thru st faults as well as inherited, steeply dipping, pre-Pliocene strike-slip f aults that bound thick sedimentary basins, The second group comprises possi bly young (<1 Ma), steeply dipping strike-slip faults that commonly exhibit normal-slip separation, and strike between 067 degrees and 085 degrees, su bparallel to the azimuth of the instantaneous Pacific-Australian plate moti on vector.