Strain partitioning across a subduction thrust fault near the deformation front of the Hikurangi subduction margin, New Zealand: A magnetic fabric study on IODP Expedition 375 Site U1518

Abstract

Understanding the distribution of strain along thrust and splay faults in active accretionary systems is crucial to understand the mechanical properties of the sediments and the strength of the fault zone and its slip behavior. This paper investigates the distribution of strain through sediment compaction and texture development across the Pāpaku fault, a major splay fault near the deformation front of the Hikurangi subduction margin, New Zealand using the anisotropy of magnetic susceptibility technique (AMS). International Ocean Discovery Program Site U1518 penetrated hanging wall, fault zone and footwall sequences to a maximum depth of 484.9 meters below seafloor. A total of 330 discrete samples was subjected to AMS measurements and magnetic remanence data used to reconstruct the axial orientation of each sample in a geographic reference frame. The AMS display distinct fabric differences between hanging wall, through the fault zone and footwall domains, demonstrating that strain is partitioned across the fault zone. Hanging wall sequences show a strike-parallel northeasterly lineation of Kmax and weakly prolate shapes, typical for a component of northeast-southwest lateral shortening. In contrast, footwall sequences are more oblate and show a clustering of Kmax in northerly direction. This demonstrates that strain in the footwall is dominated by gravitational loading, however a component of sub-horizontal east-westerly strain, parallel to the convergence direction of the Pacific Plate exists. Strain decoupling between hanging- and footwall sequences occurs near the top of the Pāpaku fault zone. Differences in the degree of magnetic susceptibility between footwall sediments incorporated into the fault zone, and the underlying undeformed footwall sequences are indicative for the progressive dewatering of the underconsolidated footwall sequences.