Carbonate deformation through the brittle-ductile transition: The case of the SW Helvetic nappes, Switzerland

Carbonate deformation through the brittle-ductile transition (BDT) remains incompletely documented in the field. We therefore investigate the exhumation of the SW-Helvetic nappe stack using a new multi-method approach that integrates optical observations with a revised nomenclature, thermochronology, stable isotopes, and clumped isotope thermometry, aiming to constrain the time-temperature history of BDT deformation processes in carbonates. Single grain (U-Th)/He zircon and apatite fission track ages establish new burial/exhumation trajectories from different nappes, allowing us to infer the thermal history of the Rawil Depression. This prominent doubly-plunging hinge zone between the Mont Blanc and Aar Crystalline Massifs underwent postnappe faulting, now constrained to the Tortonian-Early Pliocene, reflected in differential exhumation rates of -0.2 km/Myr between the most depressed area and its easternmost side. Calculating and modelling the rockbuffered clumped isotope temperatures (ranging from -250 degrees C to 55 degrees C), we indirectly date the BDT processes, exemplified by the Rezli Fault. On this structure, possible shear heating during mylonitisation is indirectly dated at 18-15.5 Ma. Progressive embrittlement started around 11-9 Ma at temperatures of about 150-110 degrees C. Lower clumped isotope temperatures correspond to recent brittle faulting between 9 and 5 Ma. Our results on the regional evolution can be applied to analogous seismogenic carbonate-rich crustal sections.