MULTIPHASE DEFORMATIONAL HISTORY OF THE SADDLE ISLAND METAMORPHIC CORE COMPLEX IN THE NORTHERN COLORADO RIVER EXTENSIONAL CORRIDOR, NEVADA

The Saddle Island metamorphic core complex is a unique structure in southern Nevada whose timing and kinematics of brittle and ductile deformation are not well constrained. Saddle Island hosts a low angle detachment fault known as the Saddle Island Fault that separates a brittley deformed upper plate and a lower plate with ductile mylonitic fabrics. Previous studies centered on the timing and kinematics of deformation between these features on Saddle Island have conflicting interpretations between the two end member models: (1) a single phase of Miocene deformation where the mylonites represent the down dip continuation of normal faults at the brittle ductile transition or (2) a two stage reactivation where the Miocene low angle normal fault exhumed an older mylonitic shear zone. In this study we test the structural models for metamorphic core complex formation by constraining the timing of ductile shearing in the lower plate and upper crustal extension in the upper plate. New field and microtextural analysis, zircon and apatite U-Pb petrochronology, and detrital zircon U-Pb geochronology provide constraints on the kinematics and absolute ages of lower plate deformation and upper plate extension and basin deposition. Our results support the multi-phase deformation model. The first phase is recorded by the lower plate’s amphibolite facies mylonites with top to the southwest shear which yield deformational apatite U-Pb ages of 53-55 Ma. The second phase is constrained by Miocene maximum depositional age (~20 Ma) of the upper plate sedimentary units and cross-cutting relationships of the low angle detachment fault brackets brittle deformation to <13.4 Ma. These new ages have implications to the paleogeography and deformational history of southern Nevada, specifically to the deformation of the Kingman Uplift, a cryptic Laramide topographic feature. The deformational age of the ductile mylonitic fabric correlates to crustal cooling ages of the region, and the sedimentary provenances best correlate to the sedimentary units bordering the Kingman Uplift. We interpret that the ductile mylonitic fabric of Saddle Island is the record of an Eocene mid-crustal shear zone that accommodated Laramide uplift and created topographic relief for subsequent erosion and isostatic uplift of the Kingman Uplift into the Miocene.