ISOTOPIC RECORD OF FLUID-FAULT INTERACTION IN MODERN AND ANCIENT EXTENSIONAL SYSTEMS (Invited Presentation)

Fluids play a fundamental role in the short- and long-term activity of fault systems and their isotopic composition traces fluid sources and fluid-rock interaction history. In modern settings, thermal springs issuing along deeply penetrating faults provide windows to deep-seated fluids influenced by active tectonics. Past diagenetic alteration preserved in exhumed fault zones informs the sources of fluids, the timing of flow, and multiple scales fluid-fault interaction. We present isotopic and companion geochemical data from hot springs emanating from the Cordillera Blanca detachment (CBD) fault in the Peruvian Andes and exhumed fault-zone mineralization along the Hurricane Fault (HF) in the western U.S. Results illustrate the variability of temporal and spatial fluid-fault records in continental extensional systems.

The CBD is an active, orogen-parallel 200-km west-dipping detachment fault that has exhumed the Cordillera Blanca massif since ~6 Ma in a flat-slab subduction setting. Hot springs up to 89°C emanate from the detachment zone and yield ³He/⁴He ratios up to 2 R_A, indicative of ~25% mantle derived helium that is enigmatic for this amagmatic arc segment. Based on spring stable isotope geochemistry (C, H, O, Cl) and cation geothermometry, fault-related fluids are a mix of meteoric and deep-seated fluids ascending from a 200 – 260°C hydrothermal system. These data imply the presence of deeply-sourced slab and mantle-derived fluids in the CBD, and the role of these fluids in detachment structural evolution is under investigation.

The HF is an active, 250-km long intercontinental normal fault along the boundary between the Colorado Plateau and the Basin and Range province. Diagenetic alteration preserved in the fault and footwall damage zone includes calcite and hematite veins, calcite-cemented deformation bands, and redox features along fractures. Veins show evidence of multiple fluid events including laminations, cross-cutting veins, and textures suggestive of crack-seal cycles. Calcite stable isotope and fluid-inclusion microthermometry results indicate that basin brines, deeply circulated meteoric waters, and possible magmatic-hydrothermal fluids have influenced the HF. Preliminary vein U-Th ages from ~86 – 113 ka document punctuated fluid-flow events during the late Pleistocene.