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