Formation of pluton roofs, floors, and walls by crack opening at Split Mountain, Sierra Nevada, California

Abstract

Spectacularly exposed contact relations of the Split Mountain interpluton screen, Sierra Nevada batholith, indicate that its bounding plutons grew by injection of dikes. The screen is composed of Cambrian metasedimentary rocks and ranges from a few tens of meters to ∼500 m thick. On Split Mountain, the screen is bounded by gently dipping intrusive contacts with two Jurassic plutons, the overlying Tinemaha Granodiorite and underlying leucogranite of Red Mountain Creek. Field relations indicate that both plutons grew mainly by opening of subhorizontal crack systems and that stoping was at most a minor process. The intrusive contacts are sharp and cut across wall-rock structures that reflect pre–165 Ma tectonic shortening. Wall-rock xenoliths are extremely rare in both plutons. Contrary to previous descriptions, neither pluton has an exposed steep wall, but later tectonic deformation locally produced steep contacts. Westward, the Split Mountain screen enters the subvertical Sawmill Lake mylonitic shear zone, which sheared it downward to a subvertical dip. On the north side of the Red Mountain Creek pluton, the primary intrusive contact dips gently, but a zone of high-angle faults steps the contact incrementally downward toward the north and east from near the range crest to the eastern range front. These faults were not previously recognized and the resulting map pattern has been misconstrued to reflect northward steepening of the contact. The Cretaceous Lamarck Granodiorite intrudes the western contact of the vertical section of the screen along a locally concordant contact. However, elsewhere the intrusive contact dips more gently and sharply cuts across the screen and the shear zone. Concordant segments of the intrusive contact reflect opening of fractures that were guided by the shear zone fabric. The Split Mountain interpluton screen thus owes its shape and its location between several plutons to the opening of fractures that admitted the bounding intrusions. The screen originally formed in the Jurassic as a subhorizontal body between the vertically stacked, broadly laccolithic Tinemaha and Red Mountain Creek plutons. After part of the screen was tectonically sheared into a subvertical orientation, the Lamarck Granodiorite invaded steep fractures concordant with the shear zone.