STRUCTURAL AND STRATIGRAPHIC AGE CONSTRAINTS OF THE INSKIP FORMATION, EAST RANGE, NEVADA: IMPLICATIONS FOR MESOZOIC TECTONICS OF WESTERN NORTH AMERICA

The latest Paleozoic and Early Mesozoic orogenic activity along the western margin of the North American continent have long been believed to reflect arc-continent interactions. However, for the Nevada portion of this continental margin, very few arc type rocks are preserved of the correct age, and structural and stratigraphic position. The Inskip Formation of north-central Nevada is composed of a metavolcano-sedimentary sequence that may represent the easternmost component of an upper Paleozoic or Mesozoic arc.

The Inskip Formation has been split into upper and lower units; both previously were interpreted as upper Paleozoic in age. The Lower Inskip remains upper Paleozoic in age and is interpreted to be part of the Havallah sequence. However, new U-Pb zircon geochronology from a tuff collected in the Upper Inskip is early Triassic in age (249.08 +/- 0.14 Ma). Based on this new age constraint and lithostratigraphic correlation, the Upper Inskip is here assigned to the Limerick Member of the Koipato Group.

Originally recognized in the Tobin Range, the Sonoma orogeny was defined as weakly deformed Koipato Group rocks unconformably overlying the strongly deformed Havallah sequence, which in turn had been thrust over lower Paleozoic rocks of the Antler orogeny and its overlap sequence. The Golconda Thrust is the basal thrust for the Sonoma orogeny. With the reassignment of the Upper and Lower Inskip to the Koipato Group and the Havallah sequence respectively, the same relationship exists in the north-cental East Range as does in the Tobin Range. In the East Range the contact between the Lower Inskip and the lower Paleozoic rocks of the Valmy Formation is interpreted to represent a segment of the Golconda Thrust. These new data however, do not provide a minimum age constraint for the emplacement of the Golconda allochthon, which could still be a post-Koipato event.