More than 2,000 m of Paleozoic and Mesozoic rocks are present in the Sagamore Canyon–Slaughterhouse Spring area, New York Mountains, California. Although recrystallized to marble, the rocks can be subdivided into formations typical of platform units east of the Cordilleran geosyncline. These formations range from the Cambrian Tapeats Sandstone, which rests unconformably on Precambrian crystalline rocks, to the Upper Mississippian–Pennsylvanian–Permian(?) Bird Spring Formation. A calc-silicate and quartzite unit unconformably overlies the Bird Spring and is tentatively correlated with the Triassic Moenkopi Formation. Mesozoic siliceous metavolcanic and metasedimentary rocks unconformably overlie the Moenkopi(?).

Precambrian crystalline rocks, Paleozoic marble, and Mesozoic(?) calc-silicate rocks and quartzite are thrust eastward over Mesozoic metavolcanic and metasedimentary rocks. The allochthon is structurally complex; it consists of structures belonging to at least 10 deformational events. Correlation of these deformational events with better-dated events in the Clark Mountains suggests that both early and late Mesozoic folding, thrusting, and high-angle faulting affected the allochthonous rocks. All folds and thrust faults are intruded by a Late Cretaceous pluton (K-Ar dated at 71.7 ± 0.8 m.y. B.P).

Regionally, allochthonous rocks of the New York Mountains probably are in the easternmost thrust plate of the Cordilleran fold and thrust belt, which is in a position analogous to that of the Keystone thrust plate farther north. The thrust plate contains Precambrian crystalline rocks and platform Paleozoic rocks and includes structures of probably both early and late Mesozoic age. These relationships demonstrate the complete crosscutting of the Paleozoic geosyncline by Mesozoic structures.

The high-angle Slaughterhouse fault juxtaposes autochthonous Precambrian crystalline rocks and deformed and intruded Paleozoic and Mesozoic rocks. Structural relations in adjacent areas suggest that this fault can be traced more than 45 km, but no rocks can be matched across it. We speculate that the fault has strike-slip displacement and may represent a Mesozoic strike-slip fault associated with Mesozoic subduction and arc magmatism.

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