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.
Highlights
Subduction earthquakes and tsunami pose a significant hazard to coastal communities around the Pacific (e.g. Moore et al, 2011)
anisotropy of magnetic susceptibility technique (AMS) measures the bulk fabric of dia, para- and ferromagnetic minerals in a sample and it is difficult to assess the contributions of different mineral phases to the bulk signal
FORC diagrams of high coercivity intervals (Fig. 3a) are characterized by closed contours with a mean coercivity of ∼60 mT and partial ARM (pARM) acquisition curves show a wide distribution of coercivity, with all of the samples being dominated by magnetic mineral phases with coercivities of 30 mT or more
Summary
Subduction earthquakes and tsunami pose a significant hazard to coastal communities around the Pacific (e.g. Moore et al, 2011). The interrelation between non-destructive earthquakes, stress build-up and release is not fully understood yet In particular it is not known what role splay- and shallow subduction thrust faults play in the propagation and release of seismic energy. Studies that aim to reconstruct type and distribution of strain within accretionary prisms often overlook the effect of ductile strain and texture development in the soft sediments.
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