Geology of the northeast Sacramento mountains, California
A multidisciplinary investigation into the timing, distribution, and intensity of tectonothermal events has resulted in an understanding of the tectonic evolution of the northeast Sacramento Mountains, in particular, and of the northern Sacramento Mountains in general. The application of geologic, structural, geochemical, and thermochronologic techniques has provided the relative and absolute timing of crustal and tectonic processes, leading to the development of a petrogenetic model for the evolution of the Sacramento Mountains metamorphic core complex. The metamorphic core complex was uplifted and cooled during Miocene detachment faulting (-23- 12 Ma). The Eagle Wash Intrusive Complex, a calc-alkalic granodioritic intrusion, was emplaced during detachment faulting at -20 Ma, at -3 kb and ~680°C. The EWIC records rapid cooling and uplift following emplacement, at rates of >100°C/Ma and 1.5-3 km/Ma, respectively. The EWIC was below ~100°C (the closure temperature of fission tracks in apatite) by 15 Ma. The structural and thermal history for the syntectonic intrusive suite is best explained via an evolving simple shear zone. The EWIC was intruded into, or proximal to, a mylonitic shear zone. The SW dip of the myonitic foliation in the EWIC could represent a primary feature of the shear zone, or the capture of a relatively older feature by a younger detachment fault splay. The later interpretation is consistent with the thermochronologic data, which suggests that faulting continued in the east after its termination in the west. Using the thermochonologic data to develop a thermal profile of the crust, the angle of faulting was calculated to be <30°C. The slip-rate associated with the detachement fault, though poorly constrained, was determined to be -4 mm/yr. This value is about half that determined from other core complexes and suggests that extension was slower here than elsewhere in the region.