Mechanical Implications of Creep and Partial Coupling on the World's Fastest Slipping Low-angle Normal Fault in Southeastern Papua New Guinea

Abstract

We use densely spaced campaign GPS observations and laboratory friction experiments on fault rocks from one of the world's most rapidly slipping low-angle normal faults, the Mai'iu fault in Papua New Guinea, to investigate the nature of interseismic deformation on active low-angle normal faults. GPS velocities reveal 8.3 ± 1.2 mm/year of horizontal extension across the Mai'iu fault, and are fit well by dislocation models with shallow fault locking (above 2 km depth), or by deeper locking (from ~5-16 km depth) together with shallower creep. Laboratory friction experiments show that gouges from the shallowest portion of the fault zone are predominantly weak and velocity-strengthening, while fault rocks deformed at greater depths are stronger and velocity-weakening. Evaluating the geodetic and friction results together with geophysical and microstructural evidence for mixed-mode seismic and aseismic slip at depth, we find that the Mai'iu fault is most likely strongly locked at depths of ~5-16 km and creeping updip and downdip of this region. Our results suggest that the Mai'iu fault and other active low-angle normal faults can slip in large (M w > 7) earthquakes despite near-surface interseismic creep on frictionally stable clay-rich gouges. Plain Language SummaryIn regions of extension, where tectonic plates pull apart, the Earth's crust breaks along fractures, or "normal faults", that allow parts of the crust to slip past each other. Many of these faults intersect the Earth's surface at a steep angle, but some anomalously low-angle normal faults are oriented at a shallower angle to the surface. Faults can slip during infrequent fast earthquakes or through slower gradual fault creep. Because active low-angle normal faults are rare and typically have low long-term slip-rates, it is not clear whether they cause large earthquakes or creep gradually. Using two approaches, this study addresses whether earthquakes occur on one of the fastest-slipping of these types of faults, the Mai'iu fault in Papua New Guinea. One approach uses GPS measurements to track patterns of displacement of the Earth's surface near the Mai'iu fault over 3 years. Surface displacements confirm that the Mai'iu fault slips actively and are used to constrain models of fault slip at depth. The second approach uses laboratory experiments on rocks from the Mai'iu fault zone to test whether these rocks tend to slip unstably in earthquakes, or creep stably under conditions similar to those in the fault zone. Laboratory results show that rocks from the shallowest parts of the fault tend to creep stably, while deeper fault rocks tend to slip unstably. Combining laboratory, geological, and GPS results to map slip behaviors to different fault zone depths, we find that the Mai'iu fault most likely creeps near the Earth's surface but can generate larger earthquakes at greater depths.

Slow‐to‐Fast Deformation in Mafic Fault Rocks on an Active Low‐Angle Normal Fault, Woodlark Rift, SE Papua New Guinea

“…The gouge samples contain angular to sub‐rounded mono‐ and polyphase clasts of epidote (Ø ≈ 4 ± 3 µm), actinolite (Ø ≈ 4 ± 3 µm), titanite (Ø ≈ 3 ± 2 µm), and albite (Ø ≈ 3 ± 2 µm), as well as clinopyroxene, quartz, and calcite (<2 µm) embedded in a phyllosilicate matrix (Figures 8a and 8c). Based on XRD and EDS data, the 2 µm fraction of this matrix consists mainly of saponite (Table ; see also Biemiller et al., 2020). Polyphase clasts are up to 1 cm in diameter and include fragments of mylonite, foliated cataclasite, and fine‐grained K‐rich fault rocks that resemble the ultracataclasite in texture and composition (Figure 2g, 8a, 8b, and 8d; Tables and ).…”

“…Precipitation of albite, calcite and quartz in fractures and pores strengthened the rock, especially in the light‐colored folia (e.g., Richard et al., 2014). Such reaction‐strengthening would slow down rates of dissolution–precipitation creep (i.e., case of partial locking; see Biemiller et al., 2020) by increasing the mass transfer distance (e.g., Rutter, 1983; late interseismic period, Figure 11d). Increased sealing of the fractures with calcite may have led to a positive feedback loop, whereby the well‐cemented, feldspar‐rich light‐colored folia formed frictionally strong asperities that accumulated stress elastically and ultimately yielded by brittle, cataclastic deformation (cf.…”

“…Schematic curve of maximum (static) differential stress versus depth for the Mai'iu fault is based on Mizera (2019) and Biemiller et al. (2020) indicating a stress peak in the velocity weakening cataclasite units. Red star on this strength curve indicates the depth of dated pseudotachylite veins assuming a dip slip rate of ∼10 mm/yr and a convex‐upward shaped fault dipping 30–40° at depth.…”

“…Microseismicity may occur within asperities encompassed by otherwise creeping material. In contrast, the shallower inferred zone of pseudotachylite formation (10–12 km depth) is interpreted to be currently locked and therefore microearthquake‐free (Biemiller et al., 2020).…”

“…A further 12 mylonite and foliated cataclasite samples were subjected to electron probe microanalysis (EPMA) at the Victoria University of Wellington. A collection of 15 fault rock samples comprising ultracataclasites and gouges were analyzed with X‐ray powder diffraction (XRD) at the Centre for Australian Forensic Soil Science (CAFSS), in Urrbrae, South Australia (see also Biemiller et al., 2020). Finally, whole rock major element concentrations of four mafic fault rocks including mylonite and gouge were analyzed using X‐ray fluorescence (XRF) at the University of Waikato.…”