GEOLOGICAL MAPPING APPROACH IN LOW-ANGLE THRUSTS: A LESSON LEARNED FROM THE MUDDY MOUNTAIN THRUST, NV, USA

Low angle thrust faults represent orogenic mountain-building episodes in the lithosphere, with potential for causing devastating earthquakes. Movement along a low angle basal detachment fault (dip angle < 30°) is unlikely without high pore pressure, which represents a weakening mechanism needed for fault slip, earthquake triggering, heat transport, fluid and chemical cycling. Detailed field mapping targeting the origin, development and timing sequence of lithological units and structures is necessary for understanding mechanical behaviour in thrusts. We undertook this project to study slip weakening mechanisms in orogenic thrusts. The Muddy Mountain Thrust in southern Nevada, USA, is part of the regional mid-late Cretaceous Sevier orogenic front and exposes evidence of paleoseismic slip. The thrust juxtaposed an imbricated Paleozoic carbonate sequence above Jurassic and Cretaceous sandstones, molasse and conglomerates. Our mapping reveals the thrust structure, including a shattered hangingwall and footwall separated by an intensely deformed fault core, containing fault breccias and gouge of both footwall and hangingwall origin. Molasse and conglomerates with clast of both carbonate and sandstone found unconformably in incised channels cut into the footwall Aztec sandstone can imply syn-thrusting deposition in front of leading thrust wedge. Damage in the fault zone during the Cretaceous is offset by steep normal faults, which were probably reactivated during the Neogene. Around the same time, the Horse Spring Formation limestone was deposited. We compiled map and cross-section using initial field data along with topographic, remote sensing and previous work datasets. Our map and cross-section show the distribution of the litho-tectonic units and structures associated with thrusting within a 15km² area. Furthermore, the mapping provided new limits on the timing relationship between lithologies and structures. Thus, leading observational constraints toward developing a new tectonic model for low angle thrusting.