Is mixed brittle-ductile deformation simultaneous or successive? Insights from the strontium isotope systematics of deformation structures in a heterogenous fault zone

Modern concepts of fault strength account for compositional and hence rheological heterogeneity of natural fault zones. An important implication of heterogeneity is strain partitioning between stronger and weaker phases and a gradual transition from brittle to ductile deformation. While a broader brittle-ductile transition zone is consistent with deformation fabrics in fault zones, it remains uncertain how broad the transition zone is and whether deformation fabrics record truly simultaneous brittle-ductile deformation or successions of brittle and ductile deformation driven by temporal changes in temperature, strain rate, and/or pore fluid pressure. Here we present data from a major fault zone of the Permo-Triassic Khao-Khwang fold-and-thrust belt, Thailand. The fault zone is located in marine calcareous shales with Permian limestones in the foot- and hanging wall. Thrusting occurred at peak temperatures of ~200°C. Deformation fabrics document frictional sliding, cataclasis, mineral veining, and pressure solution in the main fault zone and local mylonitization at the base of the hanging wall limestone. Thus, the fault zone records competing brittle and ductile deformation and incipient strain partitioning between the shales and limestones, consistent with a broad brittle-ductile transition zones due to rheological heterogeneity. Radiogenic strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) of syntectonic vein carbonates and calc-mylonites, tell, however, a slightly different story. Vein carbonates from the fault zone all have Permian seawater-like ⁸⁷Sr/⁸⁶Sr ratios documenting a rock-buffered pore fluid during brittle deformation. By comparison, the calc-mylonites record less radiogenic ⁸⁷Sr/⁸⁶Sr ratios hinting at a mantle source. The Sr ratios are similar to those of syntectonic mafic dykes that intruded Khao-Khwang fold-and-thrust belt. We interpret the distinct Sr ratios of the calc-mylonite to document that mylonitization was triggered by migration of hot magmatic fluids. In this case, strain partitioning between the shales and limestone were likely insignificant without the impact of the hot fluids, although the limestones were already close to the brittle-ductile transition at ambient temperatures of ~200°C. Taken together, our findings suggest that the transition from frictional deformation and pressure solution in shales to ductile shearing in limestones might be rather abrupt than gradual.