Late Oligocene–Early Miocene Detachment Faulting in Crystalline Basement, Mecca Hills, CA, Documented With Zircon (U–Th)/He Date-eU-Visual Radiation Damage Patterns

Zircon (U–Th)/He (zircon He) thermochronometry and visual radiation damage relationships reveal the timing and patterns of Orocopia Schist and North American basement exhumation in the Mecca Hills, CA. Zircon crystals from these units were selected to maximize the spectrum in visual metamictization and effective U (eU) concentrations, and thus He retentivity, in each sample. Zircon He dates (n = 31 analyses) from six of seven samples are ∼24 Ma regardless of eU, despite a large spread in eU (>2,800 ppm) across all grains. One sample (n = 7 analyses) yields a mean ∼65 Ma date. Uniform zircon He dates over a range of eU concentrations and their preserved visual metamictization indicate these grains experienced a thermal history that induced complete He loss, but did not anneal the damage. This is consistent with recent work that indicates zircon radiation damage anneals at higher temperatures than parameterized in current damage annealing models. The zircon He data patterns, together with new zircon U-Pb results and thermal history modeling, reveal the two basement units in the Mecca Hills exhumed simultaneously at ∼24 Ma, likely by the Orocopia Mountains detachment fault. This requires both basement units lie in the footwall of the detachment, and implies the breakaway zone of the Orocopia Mountains detachment fault was west of the present-day Mecca Hills basement exposures and has been translated by the San Andreas fault. Combined with prior work, our data suggest large-magnitude late Oligocene–early Miocene extension in the southwestern Cordillera was more widespread than previously recognized.