title: Tectonic Evolution of the Tucki Mountain Metamorphic Core Complex, Southeastern California: Evidence for Late Cretaceous Extension within the Sevier-Laramide Orogen
Creators: name: Barba, William
Publisher: name: University of Nevada, Las Vegas
Year: 2020

description: The exhumation of Cordilleran metamorphic core complexes from mid-crustal depths in the western U.S. Basin and Range Province has been classically associated with Cenozoic extension. However, recent studies reveal that extensional tectonic exhumation of numerous metamorphic core complexes initiated during the Late Cretaceous. New geologic mapping and structural analysis integrated with geochronology and thermochronology data provide evidence for a Late Cretaceous extensional shear zone, previously interpreted as Miocene in age, within the Tucki Mountain metamorphic core complex on the western flank of Death Valley, California. The shear zone occurs within the immediate footwall of the Miocene-Pliocene Tucki Mountain detachment fault system where middle and upper greenschist facies Kingston Peak and Noonday Formations display a penetrative top-NNW shear fabric. The Skidoo Pluton and other leucocratic dikes are involved with this deformation and yield LA-ICP-MS zircon U-Pb crystallization ages of 84.46 +0.6/-0.97 Ma and 94.10 +1.82/-1.34 Ma, respectively. Deformation temperatures of the mylonitic rocks were estimated through petrographic observation of the dominant quartz and feldspar recrystallization mechanisms, which suggest temperatures ca. 400-500°C. To constrain the age of shearing, deformation temperatures were compared with muscovite and K-feldspar 40Ar/39Ar thermochronology data from several samples along a transect parallel to the shear direction. The majority of muscovite step-heating analyses show well-defined age gradients, a departure from linearity or a “knee” in the Arrhenius data, and a strong correlation between the log(r/r0) plots and their corresponding age spectra. These observations are highly characteristic of multiple diffusion domain (MDD) behavior and led us to apply the MDD method to both muscovite and K-feldspar, which when combined, allows for the recovery of a continuous thermal history from ~425-150°C. MDD modeling of Late Cretaceous igneous rocks show cooling through 400°C between ~85-70 Ma and continued cooling at rates of 10-30°C/m.y. down to 250°C by no later than ~55 Ma. This constrains the timing of fabric development to Late Cretaceous in age, as is supported by a 72 Ma intrusion (40Ar/39Ar age) that we interpret to be synkinematic with respect to top-NNW shear. Furthermore, this Late Cretaceous shear fabric is developed along the axial planar foliation of the Tucki Mountain nappe structure, stretching lineations are co-linear with measured nappe fold hinge line orientations, and the top-NNW kinematics are consistent on both upright and overturned limbs. These features suggest a close relationship between extension and nappe formation. Intrusive relationships and 40Ar/39Ar thermochronology demonstrate that the Late Cretaceous shear fabric is superimposed on an earlier Late Jurassic foliation. Therefore, we propose a two-stage model for the evolution of the Tucki Mountain Nappe where Late Jurassic contractional structures are developed, highly modified, and reoriented during top-NNW Late Cretaceous extension, thus forming the present-day nappe structure that records sub-vertical shortening and sub-horizontal, NNW extensional flow. The Tucki Mountain metamorphic core complex is one of several “Cenozoic” core complexes that, after detailed reexaminations, now show evidence for initial Late Cretaceous unroofing and adds to the growing body of evidence for widespread Late Cretaceous extensional tectonism throughout the Sevier-Laramide Orogen.