POST-LATEST TRIASSIC THRUST EMPLACEMENT OF THE GOLCONDA ALLOCHTHON, SONOMA RANGE, NEVADA

The Golconda thrust is defined as the basal thrust along which the Havallah sequence was transported as part of the Golconda Allochthon (GA) onto the western margin of North America. The emplacement of the GA is commonly linked to the Late Permian-Early Triassic Sonoma orogeny based on a Permo-Triassic unconformity observed only on Golconda hanging wall rocks. Detailed mapping of structural features in the Sonoma Range suggests a much younger, possibly Jurassic, age of final emplacement of the GA. The interaction of the Golconda thrust with the Jurassic Clear Creek thrust system (CCTS) provides a useful timing constraint. The CCTS is a west-vergent thrust system that places lower Paleozoic rocks above Paleozoic and Triassic age rocks. One strand of the CCTS, the Sonoma thrust, places the Ordovician Valmy Formation onto the Devonian Harmony, Permian Edna Mountain, and Late Triassic (Norian) Grass Valley Formations. The trace of the thrust continues southward where it is truncated by the Golconda thrust in the Clearwater Canyon. This crosscutting relationship implies that the Golconda thrust is younger than the Sonoma thrust and the age of final Golconda thrusting in the Sonoma Range is post-Late Triassic. Another timing constraint is the presence of the Early Triassic (?) Tallman fanglomerate unconformably on Golconda footwall rocks and structurally beneath the Golconda thrust. The presence of Triassic rocks structurally beneath the Havallah sequence rocks is not just a local phenomenon; rather it occurs at a number of places in Nevada. By convention, however, these thrusts have not been regarded as part of the Golconda thrust system, but rather as part of a younger thrust belt not associated with the emplacement of the GA. Although essentially complete Permo-Triassic closure of the Havallah basin and structural emplacement of the GA may have occurred further south along the western margin of North America, Sonoman-age contraction may have resulted only in internal shortening and partial closure at the latitude of central and northern Nevada. Final emplacement of the GA in its type locality at Golconda Summit and the adjacent Sonoma Range apparently did not occur until post-Late Triassic time as part of the east-directed Jurassic contractional belt widely recognized throughout Nevada.