Evolution of an active continental detachment fault: clumped isotope thermometry of syntectonic calcite, Mai'iu fault, SE Papua New Guinea

Detachment faults that bound continental metamorphic core complexes typically record slip magnitudes of tens of kilometers—sufficient to exhume crustal rocks in their footwall from below the brittle-ductile transition. However, initiation and slip on low-angle (dip <30°) normal faults are at odds with the predictions of Coulomb failure during horizontal extension. What allows low-angle normal faults to acquire large displacements? Key obstacle to addressing this question is the scarcity of presently exposed active detachment faults. With a strike length of >60 km and a dip of 16°–21°at the surface, the active Mai’iu low-angle normal fault in SE Papua New Guinea has self-exhumed a smooth and corrugated footwall fault surface of >29 km width in the extension direction. Progressive strain localization preserved relicts of older-formed fault rocks in structurally lower positions on the fault surface including, from bottom-to-top, non-mylonitic schists through mylonites to cataclasites and ultracataclasites. The rapidly exhumed metabasaltic footwall of the Mai'iu fault contains multiple generations of deformed calcite veins that crosscut the sequentially formed fault rock units. Microstructural and stable and clumped isotope data of the syntectonic calcite are combined to reconstruct a profile of crustal strength with depth.

Clumped isotope thermometry of calcite in non-mylonitic schists and mylonites (n=8) yielded temperatures of 150-200°C. These temperatures are well below peak metamorphic temperatures of the metabasalt and mostly below the temperature range estimated by calcite twin morphologies in the non-mylonitic schists and mylonites (250-400°). Thus, they do not document calcite crystallization, but represent blocking of isotope reordering in calcite during cooling, However, calcite veins in cataclasites (n=3) record T=130-160°C, mostly below calcite blocking temperatures, and thus may be interpreted as true calcite precipitation or recrystallization temperatures. 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. At ~6-12 km depth (T=135-270°C) differential stresses in the foliated cataclasites and ultracataclasites were high enough (>150 MPa) to drive slip on mid-crustal portion of the fault (dipping 30-40°).

Carbon isotope ratios of calcite veins from all fault rocks are within a narrow range: õ¹³C_Cc = +2.1 to -2.6‰, typical of marine carbonates. However, their õ¹⁸O_Cc values are more variable and span distinct ranges for individual rock types: non-mylonitic metamorphic rocks, 25 to 27.5‰ (n=5), mylonites, 21 to 24‰ (n=9) and cataclasites, 21.5 to 22.5‰ (n=2). The foliation-parallel calcite-rich seams from the non-mylonitic schist were derived from intercalations of pelagic limestones, metamorphosed together with their host metabasalt. Moving upwards into the fault-zone mylonites and cataclasites, both isotope ratiosdecrease sharply, suggesting that CO₂ derived by breakdown of organic matter was dissolved in groundwater introduced into the damage zone of the Mai’iu fault and mixed with the local metamorphic fluids.