Chronology of Pluton Emplacement and Regional Deformation in the southern Sierra Nevada batholith, California

Abstract

Cretaceous plutonic rocks of the southern Sierra Nevada batholith between latitudes 35.5°N and 36°N lie in a strategic position that physically links shallow, subvolcanic levels of the batholith to lower-crustal (~35 km deep) batholithic rocks. This region preserves an oblique crustal section through the southern Sierra Nevada batholith. Prior studies have produced large U/Pb zircon data sets for an aerially extensive region of the batholith to the north of this area and for the lower-crustal rocks of the Tehachapi complex to the south. We present a large set of new U/Pb zircon age data that ties together the temporal relations of pluton emplacement and intra-arc ductile deformation for the region. We define five informal intrusive suites in the area based on petrography, structural setting, U/Pb zircon ages, and patterns in initial ^(87)Sr/^(86)Sr (Sr_i). Two regionally extensive intrusive suites, the 105–98 Ma Bear Valley suite and 95–84 Ma Domelands suite, underlie the entire southwestern and eastern regions of the study area, respectively, and extend beyond the limits of the study area. A third regionally extensive suite (101–95 Ma Needles suite) cuts out the northern end of the Bear Valley suite and extends for an unknown distance to the north of the study area. The Bear Valley and Needles suites are tectonically separated from the Domelands suite by the proto–Kern Canyon fault, which is a regional Late Cretaceous ductile shear zone that runs along the axis of the southern Sierra Nevada batholith. The 105–102 Ma Kern River suite also lies west of the proto–Kern Canyon fault and constitutes the subvolcanic plutonic complex for the 105–102 Ma Erskine Canyon sequence, an ~2-km-thick silicic ignimbrite- hypabyssal complex. The 100–94 Ma South Fork suite lies east of the proto–Kern Canyon fault. It records temporal and structural relations of high-magnitude ductile strain and migmatization in its host metamorphic pendant rocks commensurate with magmatic emplacement. Integration of the U/Pb age data with structural and isotopic data provides insights into a number of fundamental issues concerning composite batholith primary structure, pluton emplacement mechanisms, compositional variations in plutons, and the chronology and kinematics of regional intra-arc ductile deformation. Most fundamentally, the popular view that Sierran batholithic plutons rise to midcrustal levels (~20–15 km) and spread out above a high-grade metamorphic substrate is rendered obsolete. Age and structural data of the study area and the Tehachapi complex to the south, corroborated by seismic studies across the shallow-level Sierra Nevada batholith to the north, indicate that felsic batholithic rocks are continuous down to at least ~35 km paleodepths and that the shallower-level plutons, when and if they spread out, do so above steeply dipping primary structures of deeperlevel batholith. This steep structure reflects incremental assembly of the lower crust by multiple magma pulses. Smaller pulses at deeper structural levels appear to be more susceptible to having source isotopic and compositional signatures modified by assimilation of partial melt products from metamorphic framework rocks as well as previously-plated-out intrusives. Higher-volume magma pulses appear to rise to higher crustal levels without substantial compositional modifications and are more likely to reflect source regime characteristics. There are abundant age, petrographic, and structural data to indicate that the more areally extensive intrusive suites of the study area were assembled incrementally over 5–10 m.y. time scales. Incremental assembly involved the emplacement of several large magma batches in each (~50 km^2-scale) of the larger plutons, and progressively greater numbers of smaller batches down to a myriad of meter-scale plutons, and smaller, dikes. The total flux of batholithic magma emplaced in the study area during the Late Cretaceous is about four times that modeled for oceanic-island arcs. Integration of the U/Pb zircon age data with detailed structural and stratigraphic studies along the proto–Kern Canyon fault indicates that east-side-up reverse-sense ductile shear along the zone was operating by ca. 95 Ma. Dextralsense ductile shear, including a small reverse component, commenced at ca. 90 Ma and was in its waning phases by ca. 83 Ma. Because ~50% of the southern Sierra Nevada batholith was magmatically emplaced during this time interval, primarily within the east wall of the proto–Kern Canyon fault, the total displacement history of the shear zone is poorly constrained. Stratigraphic relations of the Erskine Canyon sequence and its relationship with the proto–Kern Canyon fault suggest that it was ponded within a 102–105 Ma volcano-tectonic depression that formed along the early traces of the shear zone. Such structures are common in active arcs above zones of oblique convergence. If such is the case for the Erskine Canyon sequence, this window into the early history of the "proto–Kern Canyon fault" could preserve a remnant or branch of the Mojave–Snow Lake fault, a heretofore cryptic hypothetical fault that is thought to have undergone large-magnitude dextral slip in Early Cretaceous time. The changing kinematic patterns of the proto–Kern Canyon fault are consistent with age and deformational relations of ductile shear zones present within the shallow-level central Sierra Nevada batholith, and with those of the deep-level exposures in the Tehachapi complex. This deformational regime correlates with fl at-slab segment subduction beneath the southern California region batholithic belt and resultant tilting and unroofing of the southern Sierra Nevada batholith oblique crustal section. These events may be correlated to the earliest phases of the Laramide orogeny.

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