No abstract

To what degree low‐angle normal faults (LANFs) deform by a “rolling‐hinge” mechanism is still debated for continental metamorphic core complexes (MCCs). The Mai'iu fault in SE Papua New Guinea is one of the best preserved and fastest slipping active continental LANFs on Earth, providing an ideal setting in which to evaluate footwall deformation and doming in MCCs. We analyzed structural field data from the exhumed slip surface and subjacent footwall of the Mai'iu fault, together with geomorphic data interpreted from aerial photographs and GeoSAR‐derived digital terrain models. The exhumed part of the Mai'iu fault forms a smooth, continuous surface, traced at least 28 km in the slip direction. The fault emerges from the ground near sea level with a northward dip of ≤22°N and flattens southward over the crest of the Suckling‐Dayman Dome. Its most southern mapped portion dips ~12°S. Geomorphic and structural evidence indicates updip tectonic transport of the footwall and progressive back‐tilting of the exposed part of the fault and the underlying foliation through >26°. We infer that antithetic (northside‐up) dip slip on an array of steep‐dipping faults striking parallel to the Mai'iu fault accommodated some of the exhumation‐related inelastic bending of the footwall. The exhuming footwall was subject to late‐stage slip‐parallel contractional strain as recorded by a postmetamorphic crenulation foliation that strikes parallel to the curved Mai'iu fault trace, by folds of bedding in a large rider block that is stranded on the current footwall and by strike‐parallel warps in the exhumed fault surface. Geodynamic modeling predicts the observed footwall strain.

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 Summary In 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.

• Fault rock microstructures reveal slow-to-fast slip on an active detachment fault that dips 15-24° at the Earth's surface • Pseudotachylites, foliated cataclasites, and ultracataclasites developed in a zone of mixed mode, seismic-to-aseismic slip behavior • Frictionally weak saponite in fault gouge promotes slip on the most poorly oriented, surficial part (<24°) of the Mai'iu fault Accepted Article This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as

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.