LARAMIDE SHORTENING IN THE CLARK MOUNTAIN THRUST COMPLEX

The most accepted geodynamic driver for the Laramide orogeny is slab flattening from the combined effects of progressive increase in plate convergent velocities and an increase in slab buoyancy associated with the embedded Shatsky Rise conjugate. The tectonic and magmatic record in the eastern Mojave Desert – within the proposed pathway of the subducted Shatsky conjugate – has been an enigma; unlike the fold-thrust belt further north, the last well-documented contractional deformation in the southernmost fold-thrust belt (Clark Mountain thrust complex – CMTC) is ~ 100-90 Ma and documented Mojave Laramide features include numerous Late Cretaceous plutons and extensional shear zones. New data from the CMTC suggests that Laramide contraction has been hiding in plain sight. The CMTC comprises at least 3 major thrust systems from east to west: the Keaney-Mullusk Mine, Mesquite Pass, Mescal, and Winters Pass thrusts. The Winters Pass thrust in the Mesquite Mountains, previously inferred to be Late Jurassic, carries mylonitic Proterozoic basement rocks over Pahrump Group metasedimentary rocks. The basal Mesquite Pass thrust in the NW Clark Mountains forms the base of a fold nappe cored by Proterozoic basement. Here we report preliminary microstructural and kinematic analyses and geochronology of the Winters Pass thrust and Mesquite Pass mylonites. Both mylonites record top-NE shear at greenschist facies conditions. Quartz underwent bulge and subgrain rotation dynamic recrystallization by primarily basal <a> slip, and feldspar deformed brittley and by dissolution creep. Secondary muscovite, albite, and quartz from the extensive breakdown of feldspars grew synkinematically in dilatant sites and commonly forms bands parallel to foliation. Preliminary furnace step-heating ⁴⁰Ar/³⁹Ar geochronology on synkinematic muscovite from the mylonites provide direct geochronologic constraints on mylonitic deformation; we interpret the ages as growth ages not cooling ages. Muscovite ages for the Winters Pass thrust mylonites are ~73 Ma and ages for the basal Mesquite Pass thrust are ~81 Ma. These data suggest that the CMTC records a backwards-breaking thrust sequence from 100-90 Ma (Keaney-Mollusk Mine, west) to 73 Ma (Winters Pass, east). Additional study is needed to confirm these preliminary results, which would have significant implications for Laramide geodynamic models.