The northern Sacramento Mountains, southwest United States. Part II: Exhumation history and detachment faulting

Thermochronologic and thermobarometric data reveal the timing, distribution and intensity of thermal events associated with detachment faulting in the Sacramento Mountains metamorphic core complex. In the northwest Sacramento Mountains, cooling rates of c. 100°C Ma⁻¹ are associated with Late Cretaceous plutonism followed by cooling of the crust by thermal conduction. Post-Late Cretaceous cooling slowed to c. 1–6°C Ma⁻¹. Finally, the region records average cooling rates of 38–53°C Ma⁻¹ between c. 20 and 15 Ma. In contrast, the thermal profile of the northeast Sacramento Mountains is dominated by syntectonic Tertiary plutonism followed by very rapid cooling. A granodioritic suite intruded at c. 680°C and c. 3 kbar at c. 20 Ma, records cooling to <100°C by c. 15 Ma. Such rapid cooling and exhumation suggests that unroofing by tectonic denudation was the driving mechanism for the final cooling. The similarity of the miocene cooling profiles between these two areas clearly suggests that the Sacramento Mountains experienced a regional cooling event associated with tectonic unroofing driven by regional Miocene crustal extension. Estimates of the initial angle of the Sacramento Mountains detachment fault using palaeothermal gradients suggest that it was active at a dip of 25°.