Slip on the active Mai’iu low-angle normal fault in Papua New Guinea that dips 15–24° at the surface has exhumed in its footwall a single, continuous fault surface across a >25 km-wide dome. Derived from a metabasaltic protolith, the fault zone consists of a ≤3 m-thick zone of gouges and cataclasites that overprint a structurally underlying carapace of extensional mylonites. Detailed microstructural and geochemical data, combined with chlorite-based geothermometry, reveal changing deformation processes and conditions in the Mai’iu fault rocks as they were exhumed. The microstructure of non-plastically deformed actinolite grains inherited from the fine-grained (6–35 µm) basaltic protolith indicate that shearing at depth was controlled by diffusion creep accompanied by grain-boundary sliding of these grains together with chlorite neo-crystallization at T≥270–370°C. In a foliated cataclasite unit at shallower crustal levels (T≈150–270°C), fluid-assisted mass transfer and metasomatic reactions accommodated aseismic, distributed shearing; pseudotachylites and ultracataclasites in the same unit indicate that such creep was punctuated by episodes of seismic slip—after which creep resumed. At the shallowest levels (T≤150°C), gouges contain abundant saponite, a frictionally weak mineral that promotes creep on the shallowest-dipping (≤24°), most poorly oriented part of the Mai’iu fault. Our field, microstructural and geochemical data of freshly exhumed fault rocks support geodetic, seismological, and geomorphic evidence for mixed seismic-to-aseismic slip on this active low-angle normal fault.

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/yr 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 > 7) earthquakes despite near-surface interseismic creep on frictionally stable clay-rich gouges.

Quantifying lithospheric strength is essential to better understand seismicity in continental regions. We estimate differential stresses and principal stress orientations driving rapid slip (~10 mm/yr) on the active Mai’iu low-angle normal fault (dipping 15–24° at the Earth’s surface) in Papua New Guinea. The fault’s mafic footwall hosts a well-preserved sequence of mylonite, foliated cataclasite, ultracataclasite and gouge. In these fault rocks, we combine stress inversion of fault-slip data and paleostress analysis of syntectonically emplaced calcite veins with microstructural and clumped-isotope geothermometry to constrain a syn-exhumational sequence of deformation stresses and temperatures, and to construct a stress profile through the exhumed footwall of the Mai’iu fault. This includes: 1) at ~12–20 km depth (T≈275–370°C), mylonites accommodated slip on the Mai’iu fault at low differential stresses (>25–135 MPa) before being overprinted by localized brittle deformation at shallower depths; 2) at ~6–12 km depth (T≈130–275°C) differential stresses in the foliated cataclasites and ultracataclasites were high enough (>150 MPa) to drive slip on a mid-crustal portion of the fault (dipping 30–40°), and to trigger brittle yielding of mafic footwall rocks in a zone of mixed-mode seismic/aseismic slip; and 3) at the shallowest crustal levels (T<150°C) on the most poorly oriented part of the Mai’iu fault (dipping ~20–24°), slip occurred on frictionally weak clay-rich gouges (μ≈0.15–0.38). Subvertical σ1 and subhorizontal σ3 parallel to the extension direction, with σ1≈σ2 (constriction), reflect vertical unloading and 3-D bending stresses during rolling-hinge style flexure of the footwall.