LATE CRETACEOUS INTRUSION AND EXTENSIONAL EXHUMATION OF THE CADIZ VALLEY BATHOLITH, IRON MOUNTAINS, SOUTHEASTERN CALIFORNIA

New geochronology, thermochronology, and structural studies clarify the age and origin of mylonitic gneiss in the roof zone of the Cadiz Valley batholith. The Iron Mountains expose a spectacular cross section of the roof of the Cadiz Valley batholith; mapped by D.M. Miller and K.A. Howard, monzogranite is overlain by a roof zone that consists of screens of Precambrian (?) gneiss, amphibolite, and schist, separated by Cretaceous sills. Within the roof zone, a solid-state deformation fabric increases in intensity upward from the structurally lowest wallrock screen to the highest sill, and forms a >1.3 km stack of mylonitic rocks. Measured foliation has a mean strike and dip of 8, 15 W, and stretching lineation has mean trend and plunge of 261, 15. Kinematic and deformation mechanism studies demonstrate top-to-the-east non-coaxial shear, and temperatures decreasing during deformation from amphibolite to greenschist facies conditions. Beneath the shear zone in the uppermost ~200 meters of porphyritic monzogranite, a magmatic foliation is concordant to overlying wallrock screens and solid-state foliation, and a sparse magmatic lineation is subparallel to overlying mylonitic lineation. New ion microprobe ²³⁸U/²⁰⁶Pb dates on zircon allow discrimination of Late Cretaceous magmatic and Proterozoic inherited components. Tera-Wasserburg concordia lower intercepts, uncorrected for common lead, yield ages of 75.4 ± 2.1 for the porphyritic monzogranite (core of range) and 75.6 ± 1.7 for the porphyritic granodiorite gneiss (structurally highest sill). Four ⁴⁰Ar/³⁹Ar analyses of biotite, from 3 km intervals along a 9 km lineation-parallel transect, yield a consistent progression of isochron ages that are younger toward the east - from 66.6 ± 0.3 Ma (west) to 59.1 ± 0.7 Ma (east). The inferred systematic progressive exhumation from west to east is consistent with an initial component of east dip to the shear zone; downdip top-to-the-east kinematics and U-Pb zircon and ⁴⁰Ar/³⁹Ar biotite ages indicates shearing is extensional and of late Cretaceous age. Similar to many other 72 to 75 Ma granitoids in the southwest Cordillera, intrusion is interpreted as synextensional. West tilt is attributed to footwall flexure associated with the Miocene breakaway to the Colorado River extensional corridor detachment fault system.