NEW GEOLOGIC AND THERMOCHRONOLOGIC CONSTRAINTS ON THE MIOCENE TECTONIC EVOLUTION OF THE HARCUVAR METAMORPHIC CORE COMPLEX, WEST ARIZONA

Cordilleran metamorphic core complexes are interpreted as unique modes of large-magnitude crustal extension that result in the exhumation of mylonitic mid-crustal rocks along a currently low-angle normal (detachment) fault. The processes by which core complexes form remain controversial, including the initial dip of the detachment fault, the magnitude and rate of slip, and the role of mid-crustal flow. We present data from the Harcuvar core complex in western Arizona that provides new insight on its tectonic evolution.

There is significant evidence that shallow structural levels SW of the mylonitic front in the Harcuvar core complex experienced major Miocene tilting. Miocene volcanic and sedimentary rocks that unconformably overly footwall granite dip 30-45° SW. In addition, ⁴⁰Ar/³⁹Ar K-feldspar multiple diffusion domain (MDD) thermal modeling suggests that a 150-200 °C thermal gradient was present across 10-15 km of the nonmylonitic footwall just prior to Miocene extension, further supporting tilting of this part of the footwall. In contrast, MDD models from deeper structural levels NE of the mylonitic front show more uniform Miocene temperatures across >30 km in the slip direction. These results suggest that the detachment fault initiated with a listric-style geometry with a moderate to steep dip in the brittle upper crust that shallowed dramatically or was replaced by a low-angle ductile shear zone in the middle crust.

K-feldspar MDD models from the footwall also record a systematic delay in the inception of rapid Miocene footwall cooling in the slip direction. The earliest rapid cooling occurs at ca. 20.5 Ma in SW (shallow) footwall, which likely records the initiation of rapid fault slip. However, the inception of cooling systematically youngs northeastwards to ca. 15 Ma in the northeastern part of the footwall. The age of the inception of rapid cooling vs. distance in the slip direction data are linearly correlated with a slope of 7.4 km/m.y., which is similar to slip rate estimates derived from low-temperature thermochronology. These data suggest that rapid cooling occurred later in the NE footwall and that this process was directly linked to fault slip. These results may also support the interpretation that the detachment fault initiated with a listric geometry in the Early Miocene.