THERMOTECTONIC EVIDENCE FOR TWO-STAGE EXTENSION ON THE TRINITY DETACHMENT SURFACE, EASTERN KLAMATH MOUNTAINS, CALIFORNIA

We present integrated fission-track and (U-Th)/He analysis of a reconnaissance suite of 8 apatite samples along a transect crossing the footwall of the Trinity Detachment Surface, a variably preserved low-angle extensional structure in the Eastern Klamath Mountains of California. These thermochronological data display significant variation when plotted along the extension lineation evident on exposed remnants of the fault, and the combined ages, together with the fission track length variation observed, require two discrete episodes of exhumation associated with extension on this structure. Consistent mid-Cretaceous fission-track ages indicate continuation of earlier slip on the detachment surface and/or erosional exhumation to bring upper greenschist facies footwall rocks into the brittle upper crustal domain. This phase of exhumation continued until c. 80 Ma, terminating while currently exposed samples were still 2 to 3 km below the surface. Final exhumation of the footwall block occurred in a second discrete episode initiating at c. 20 to 23 Ma, contemporaneous with development or re-activation of extension on the La Grange Fault to the south.