STRUCTURAL AND THERMAL CONSTRAINTS ON THE INITIATION ANGLE OF DETACHMENT FAULTING IN THE SOUTHERN BASIN AND RANGE - THE CHEMEHUEVI MOUNTAINS CASE-STUDY

The Cenozoic normal fault system exposed in the Chemehuevi mountains o f the southern Cordillera provides constraints on the initiation angle and geometry of an extensional fault system that has accommodated ext reme crustal stretching. There, three stacked, brittle, low-angle norm al faults that formed at depths as great as 10-12 km cut gently down s ection northeastward through deformed Proterozoic and Mesozoic crystal line basement. Hanging-wall blocks are displaced relatively northeastw ard. 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 extensio n (<2%) by normal and strike-slip faulting, local ductile shearing, an d dike emplacement. These relationships imply that the upper and middl e crust as a whole extended nonuniformly. Structural constraints on th e initiation angle of the detachment faults in the Chemehuevi Mountain s are based on a wide variety of observations, including fault rock ty pe and associated mineral deformation mechanisms, orientation and cros scutting relations of syntectonic dikes and faults, and the metamorphi c grade of footwall rocks to the regionally developed normal fault sys tem. In each case, the initial dip of the fault is limited to <30-degr ees.Application of Ar-40/Ar-39 and fission-track thermochronology to r ocks in the footwall of the Chemehuevi detachment fault system provide s further constraints on the timing and initiation angle of regional d etachment faulting. At the onset of extension between 22 and 24 Ma, gr anitic rocks exposed in the southwestern and northeastern portions of the footwall were at approximately 200-degrees-C and 350-400-degrees-C , respectively, separated by a distance of some 23 km down the known s lip direction. This gradual increase in temperature with original dept h is attributed to the gentle southwest tilting of broadly planar pre- extension isothermal surfaces and constrains the exposed part of the C hemehuevi detachment fault to have had a regional dip initially of abo ut 15-degrees to 30-degrees. The fault system apparently cut gently do wn through the upper crust, to a minimum depth of approximately 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 wi thin the seismogenic regime at moderate to low angles (that is, less-t han-or-equal-to 30-degrees).