The Rattlesnake Valley and Oriflamme Canyon plutons: Key temporal markers in the Jurassic and Cretaceous development of the transition zone of the Peninsular Ranges batholith
Along ~33°N latitude, we mapped a segment of the transition zone of the Peninsular Ranges batholith. Based on structural fabric and plutonic ages, we subdivided the mapped area into western and eastern domains. The structural fabric of the western domain is characterized by brittle to brittle-ductile faults overprinting an earlier-formed ductile fabric dominated by a series of upright tight to isoclinal F1 folds, and S1, a spaced to continuous pervasively developed cleavage and gneissic foliation. Faults overprinting F1 and S1 within the western domain include the conjugate dextral Green Valley and sinistral Indian Exhibit faults. The ca. 130 Ma Rattlesnake Valley pluton crosscuts F1 and S1 and is not cut by these faults. The brittle faults within the western domain do not extend eastward beyond the Sunrise Highway fault, the frontal fault defining the western boundary of the eastern domain and the Sunrise Highway–Oriflamme Canyon shear zone. The Sunrise Highway fault is represented by an ~1–3-m-thick protomylonitic rind along the margin of the Rattlesnake Valley pluton. The Sunrise Highway–Oriflamme Canyon shear zone includes a tabular and platy segment of the Julian Schist, the ca. 116 Ma protomylonitic Oriflamme Canyon pluton, and a poorly exposed high-grade segment of the Harper Creek Gneiss. S2, within the Julian Schist segment, is a penetrative, NW-striking, steeply NE-dipping phyllonitic cleavage. It can be traced eastward into the syntectonic Oriflamme Canyon pluton, where it is represented by protomylonitic foliation. C-surfaces dipping ~30°–40° eastward consistently deflect S-surfaces in an east-over-west sense. Following development of the Sunrise Highway–Oriflamme Canyon shear zone, between ca. 94 Ma and ca. 80 Ma, a large segment of the Harper Creek Gneiss was thrust westward over the Oriflamme Canyon pluton along the brittle Chariot Canyon fault. As reported in the literature, during the Late Jurassic to Early Cretaceous, the Farallon and North American plates converged in an oblique sinistral fashion, and then sometime between broadly ca. 140 Ma and ca. 125 Ma, convergence between the two plates became nearly orthogonal. This period of normal convergence continued until ca. 115 Ma. We speculate that (1) the tight to isoclinal upright folds and S1 cleavage characteristic of the western domain are the intra-arc record of the transition from oblique sinistral convergence to orthogonal convergence; (2) during and following emplacement of the Rattlesnake Valley pluton, the intra-arc strain field produced by the interaction of the Farallon and North American plates became weaker; and (3) the formation of the Sunrise Highway–Oriflamme Canyon shear zone was the result of heat and fluids softening the areas immediately adjacent to the intruding Oriflamme Canyon pluton, and thus weakening them to the point that they yielded to the horizontal contraction imposed by normal convergence. As suggested by other investigators, brittle movement along the Chariot Canyon fault some 22–36 m.y. following the development of the Sunrise Highway–Oriflamme Canyon shear zone was likely generated by removal of lithospheric mantle during Laramide shallow subduction. This process destabilized the overlying crust and triggered erosion, localizing brittle shortening along the Chariot Canyon fault.