DEFORMATION OF THE COOKE’S RANGE DRIVEN BY SUBDUCTION OF THE CONJUGATE HESS RISE

The Cookes Range, 15 km north of Deming, NM, is one of the easternmost expressions in the SWUS of flat slab subduction and subsequent rollback of the Farallon plate beneath North America (NA). Tectonic reconstructions of the Conjugate Hess Rise (CHR) indicate it is embedded within the Farallon plate and moving N60E beneath the NA in the vicinity of the Cookes Range at 60-50 Ma. Geologic mapping and structural analysis of Goat Ridge located in the SW portion of the range, show a north-northeast striking thrust fault (Goat Ridge Thrust; GRT) within the Mississippian Lake Valley Fm. The GRT is characterized by a 20-10 m-thick damage zone with several sets of conjugate thrust faults. Fault striations, ridges, grooves, and chatter marks indicate that the GRT accommodates top-to-NE slip. Several andesite-dacite dikes have geochemical affinities to the pluton of Cookes Peak with a preliminary U-Pb zircon date by Amato et al. (2018) of 39.1 Ma; our work on a different location of this pluton gave only xenocrystic grains at ~1650 Ma, similar zircons analyzed from a granite from Fluorite Ridge to the SE of Goat Ridge Amato et al. (2018). The dikes in the area strike NE, consistent with maximum compressive stress-oriented NE during their emplacement (tentatively considered ~40 Ma). Geodynamic reconstructions of the Farallon plate and the CHR show the CHR to be located to the immediate south of the Cooke’s Range at 60-50 Ma, at a depth >180km. This depth is likely greater than the thickness of the North American lithosphere, we suggest that Laramide-age shortening in the Cooke’s Range arises from edge stress imposed on the North American plate by the Farallon plate at the trench rather than from basal shear imposed on the base of the North American lithosphere.