SUPERPOSITION OF 80-55 MA HIGH P-T (0.7 GPA/750 °C) METAMORPHISM BY MID-TERTIARY DETACHMENT FAULTING IN THE WHIPPLE MOUNTAINS, SE CALIFORNIA

Most ductile shear zones in the mid-Tertiary Cordilleran extensional core complexes are 10s-100s m thick, but the Whipple detachment system in SE California departs from this norm by exposing a >4-km shear zone in the footwall. To address this, we conducted a systematic geochronological investigation coupled with P-T studies on the shear-zone rocks. U-Pb detrital-zircon dating of 5 mylonitic-gneiss samples yield similar U-Pb age spectra dominated by a 1.4-1.7 Ga ages. However, the spectra also contain younger ages from 300 Ma to 30 Ma. The young ages could have resulted from a protracted thermal state promoting new zircon formation in the metamorphic rocks. Alternatively, the other ages may contract at least the youngest age of some protolith components in the shear zone. Petrological studies of garnet-bearing mylonitic-gneiss samples that were collected from the same sequence of the mylonitic gneiss from which we obtained our detrital zircon age spectra indicate the peak metamorphic condition of P=0.6-0.8 GPa and T=750-800°C. U-Th dating of monazite inclusions in garnets from the same rock samples yield an older age of ~982 Ma and a range of younger ages from 80 Ma to 55 Ma. We tentatively interpret the younger monazite ages of 80-55 Ma to date peak metamorphism of the garnet schist in the Whipple ductile shear zone (WDSZ). A gabbro-block sample yields a U-Pb zircon age cluster at ~1.05 Ga, which we interpret as the intrusion age and 1.3-1.4 Ga zircons, which likely represent inherited grains. The gabbro samples contain a few 70-80 Ma zircons, which may have formed during a younger thermal event. Although the ages of syn-kinematic plutons in the footwall and syn-extensional volcanic rocks in the hanging wall constrain the Whipple extensional system to have been active between ~26 Ma and ~14 Ma, our new geochronological ages show that the mid-Tertiary WDSZ may have been a product of superposed Laramide (80-55 Ma) and mid-Tertiary (26-14 Ma) deformation, which may explain the >4 km thickness without exposing its base. The high-temperature (~750°C) metamorphism at 80-55 Ma from the monazite-in-garnet inclusion ages appear to be incompatible with flat subduction, which would have produced a cooler geothermal gradient at middle crustal depths where P=~0.7 Ga as obtained in this study. More detailed work is needed to address this issue.