The Cenozoic normal fault system exposed in the Chemehuevi Mountains of the southern Cordillera provides constraints on the initiation angle and geometry of an extensional fault system that has accommodated extreme crustal stretching. There, three stacked, brittle, low-angle normal faults that formed at depths as great as 10-12 km cut gently down section northeastward through deformed Proterozoic and Mesozoic crystalline basement. Hanging-wall blocks are displaced relatively northeastward. The upper crust above the Chemehuevi detachment fault was pulled apart along high-angle normal faults that rotated to more gentle dips through time. In contrast, rocks of originally mid-crustal affinity in the footwall were only gently rotated and accommodated minor extension (<2%) by normal and strike-slip faulting, local ductile shearing, and dike emplacement. These relationships imply that the upper and middle crust as a whole extended nonuniformly. Structural constraints on the initiation angle of the detachment faults in the Chemebuevi Mountains are based on a wide variety of observations, including fault rock type and associated mineral deformation mechanisms, orientation and crosscutting relations of syntectonic dikes and faults, and the metamorphic grade of footwall rocks to the regionally developed normal fault system. In each case, the initial dip of the fault is limited to <30°.

Application of 40Ar/39Ar and fission-track thermochronology to rocks in the footwall of the Chemehuevi detachment fault system provides further constraints on the timing and initiation angle of regional detachment faulting. At the onset of extension between 22 and 24 Ma, granitic rocks exposed in the southwestern and northeastern portions of the footwall were at ∼200 °C and 350-400 °C, respectively, separated by a distance of some 23 km down the known slip direction. This gradual increase in temperature with original depth is attributed to the gentle southwest tilting of broadly planar pre-extension isothermal surfaces and constrains the exposed part of the Chemehuevi detachment fault to have had a regional dip initially of about 15° to 30°. The fault system apparently cut gently down through the upper crust, to a minimum depth of ∼10-12 km, the deepest exposed parts of the system today, and was domed from midcrustal depths and locally denuded during continued slip. Together the structural and thermochronologic data confirm the suggestion that faults accommodating large-magnitude slip can be initiated and move within the seismogenic regime at moderate to low angles (that is, ≤30°).

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