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

Abstract

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 nonplastically deformed actinolite grains inherited from the fine-grained (6–35 µm) metabasaltic protolith indicates that shearing at depth was controlled by diffusion creep accompanied by grain-boundary sliding of these grains together with chlorite neo-crystallization at T > 275°C–370°C. In a foliated cataclasite unit at shallower crustal levels (T ≈ 150°C–275°C), metasomatic reactions accompanied fluid-assisted mass transfer processes that 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.