Abstract

Determinations of the original dip of the Newberry Mountains detachment fault of the early Miocene Mojave extensional belt, taken from fission-track thermochronology, geologic observations, and the most reasonable values for the early Miocene geothermal gradient, support two general models: (1) a two-segment fault consisting of a moderately dipping upper segment (≤30°-41°) and a lower, shallowly dipping (≤15°-19°) segment, and (2) a single-segment fault that formed at an angle of <20°-27°. However, a steeper dip (≥45°) for the entire fault is feasible only if the geothermal gradient was low (≤13°/km) prior to extension. Fission-track thermochronology data may also better determine the temperature conditions of the detachment during dip modification caused by footwall unloading. The change from an original higher dip to the present low-angle and arched geometry occurred at low temperatures (<∼175 °C).

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