Using Syntectonic Calcite Veins To Reconstruct The Strength Evolution Of An Active Low-Angle Normal Fault, Woodlark Rift, Se Papua New Guinea

Quantifying the strength evolution of faults that cut the lithosphere is essential to better understand seismicity in continental regions. We estimate differential stresses and principal stress orientations driving rapid slip of similar to 10 mm/yr on the active Mai'iu low-angle normal fault (LANF), SE Papua New Guinea. The fault's mafic footwall hosts a well-preserved sequence of mylonite, (ultra-)cataclasite, 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 active Mai'iu LANF. This includes: (a) at similar to 12-20 km depth (T approximate to 275-370 degrees 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; (b) at similar to 6-12 km depth (T approximate to 130-275 degrees 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 degrees), and to trigger brittle yielding of mafic footwall rocks in a zone of mixed-mode seismic/aseismic slip; and (c) at shallower crustal depths (T < 150 degrees C; depth <6 km) on the most poorly oriented segment of the Mai'iu LANF (dipping similar to 22 degrees), slip occurred on frictionally weak clay-rich gouges (mu approximate to 0.15-0.38). Subvertical sigma(1) and subhorizontal sigma(3) parallel to the extension direction, with sigma(1) approximate to sigma(2) (constriction), reflect vertical unloading and 3-D bending strain during rolling-hinge style flexure of the footwall.