DOWN-DIP CHANGES IN DIP, FRICTIONAL STRENGTH, AND SLIP BEHAVIOR OF THE ACTIVE MAI’IU LOW-ANGLE FAULT, SUCKLING-DAYMAN METAMORPHIC CORE COMPLEX, PAPUA NEW GUINEA

The Mai’iu fault has a Holocene slip rate of 11.7 ±3.5 mm/y and is one of the clearest examples of an active low-angle normal fault on Earth. Along most of its trace it dips ~20° N. Slip has exhumed a little-eroded, abandoned fault surface >29 km wide, which crests southward over >3 km-high mountains to define the Suckling-Dayman dome. North of the trace, aligned microseismicity delineates fault activity zone that dips 30-40° at depths of 12-25 km. The fault has been deformed into a convex-up shape as a result of “rolling-hinge” bending of its footwall.

During the last several Myr, mafic mylonitic rocks in the footwall have been exhumed from 20-25 km and ~425 °C. These are overprinted by a <2 m-thick zone of mafic fault rocks which we have characterised using microstructural, mineralogical, geochronological, and frictional data. From bottom to top, the fault rocks include: 1) foliated cataclasite cut by pseudotachylite veins; 2) a layer of ultracataclasite; and 3) a layer of saponite- and corrensite-rich gouges. The units comprise an exhumational time sequence.

In mylonites, paleopiezometry based on calcite recrystalized grain-size in late deformed veins records differential stresses of 90-120 MPa (at T >250 °C). Fault rock layer 1) contains pseudotachylites dated by ⁴⁰Ar/³⁹Ar to be ~2.2 Ma, consistent with seismogenesis at depths of ~10-12 km, and calcite twinning studies yield stresses of ~150-160 MPa. Layer 2) contains authigenic K-feldspar: at T of 150-250 °C it has a friction coefficient (μ) of ~0.6-0.7, and is rate-weakening, indicating a tendency for seismic slip. The shallow, clay-rich gouges (layer 3) are frictionally weak, with a m of 0.13-0.28 at T <150°C, and rate-strengthening, which promotes creep. Mohr-Coulomb modelling based on the surface dip of an active (~45°) and an inactive (mostly 5-10°) fault bounding a rider block, together with 3D modelling of their geometries, suggests that the shallow parts of the Mai’iu fault are frictionally weak, with a μ of ~0.21-0.25, consistent with field and experimental observations.

In summary, dip, strength, frictional properties and slip behavior of the Mai’iu fault vary with depth: it is weakest and most prone to creep near the surface, where the fault is poorly oriented; and strongest near the brittle-ductile transition, where it dips moderately and nucleates earthquakes.