Seven million years of strike‐slip and related off‐fault deformation, northeastern Marlborough fault system, South Island, New Zealand

Abstract

The Marlborough fault system (MFS), South Island, New Zealand is remarkable for the oblique‐slip nature of its active faulting and can be placed in a framework of Pacific‐Australian (Pac‐Aus) plate motions and geodetic strain. New Zealand illustrates changing degrees of upper plate slip partitioning in an obliquely convergent margin that varies along strike in crustal type and subduction geometry. In this paper, we focus on the NE part of this transcurrent fault system, especially the Awatere fault, which is the only Marlborough fault to offset a major Neogene basin. Strike slip on an ancestral eastern strand of this fault began after ∼7.4–6.4 Ma; later vertical axis rotation may have caused it to be abandoned in favor of the currently active strand. Since ∼7 Ma, at least 34±10 km of dextral slip has accrued on the Awatere fault, <6 km of slip has accrued on two nearby strands, and <4 km has accrued on the currently active coastal strand. Comparison of Miocene with late Quaternary offsets suggests that rates of strike slip on the fault have been nearly constant or have possibly decelerated. We interpret spatial and temporal patterns of strike slip in the MFS to reflect southward migration of transform motion resulting from increased Pac‐Aus plate convergence. Late Neogene to present‐day shortening directions in NE South Island are indicative of a partially slip‐partitioned margin. Slip partitioning diminishes southward as the underlying subduction interface becomes strongly coupled, suggesting a simple mechanical relationship between these processes.