ABSTRACT Miocene–Pliocene volcanism around Lake Tahoe, California/Nevada, USA, part of the Southern Ancestral Cascades arc, ceased at around 3 Ma as the southern edge of the subducting Juan de Fuca plate migrated north of the region. Post–3 Ma, arc volcanism continued north of Lake Tahoe, but modern subduction and arc volcanism now occur only north of the Lassen volcanic center. Miocene–Pliocene Tahoe arc lavas appear to include an older mantle source component that is not common in Quaternary Lassen arc rocks. The goal of this work was to investigate how magma sources and/or volcanic processes transitioned in the northern Sierra Nevada between Lake Tahoe and Lassen. The Sierra Nevada between Lake Tahoe and Lassen, or the North Sierra segment of the Ancestral Cascades, includes eroded remnants of Ancestral Cascades volcanic rocks, including lava flow complexes, intrusions, and landslide/debris-flow deposits. Lava samples from the North Sierra segment include calc-alkaline basalts to dacites, with rare rhyolites. All North Sierra segment lavas exhibit normalized incompatible-element patterns with negative Nb, Ta, and Ti anomalies and positive Pb, Sr, and Ba anomalies. The North Sierra segment is geochemically split into two parts: a northern group including lavas from the Susanville area, and a southern group consisting of arc rocks from the Portola, Sierraville, Henness Pass, and Sagehen areas. With the exception of the Sagehen area, the North Sierra segment shows little variation in radiogenic isotope ratios with SiO 2 , indicating that assimilation of crustal rocks was outweighed by liquid-crystal crystallization during magma evolution. Trace-element and isotopic ratios in mafic rocks of the northern group are more typical of Lassen area Quaternary volcanic rocks, whereas those of southern group mafic rocks are more typical of Miocene–Pliocene arc lavas of the Lake Tahoe area. The isotopic distinction between Lassen-like and Tahoe-like arc lavas is likely controlled by basement age and lithology, where Lassen-like magmas were derived largely by mantle wedge melting and Tahoe-like magmas were primarily partial melts of metasomatized Sierran lithospheric mantle. The Susanville area represents the “transition zone” between these two geochemically distinct primary magma sources.

ABSTRACT Hafnium isotope ratios in Late Jurassic zircon from the Summit Gabbro provide geochemical evidence for rifting at ca. 148 Ma through the southern Sierra Nevada arc crust. Previous evidence for intra-arc extension includes a linear distribution of early Mesozoic volcano-sedimentary deposits located within the regional footprint of the ~600-km-long Independence dike swarm. In this study, latest Jurassic rifting through the entire thickness of the arc crust is supported by an abrupt, 40-εHf i -unit isotopic pull-up that records the incorporation of depleted-mantle partial melts into basaltic magmas that rapidly migrated into the upper arc crust. Isotopic context for this latest Jurassic pull-up is provided by Permian through Cretaceous zircon xenocrysts sampled by ca. 85–78 Ma dikes and sills that sampled the crust underlying the eastern Sierra Nevada range crest. Summit Gabbro bodies define a northwest-trending lineament paralleling the Kern Plateau shear zone, a geometry consistent with previous interpretations that the Kern Plateau shear zone originated as a late Paleozoic sinistral transform fault that evolved to a normal fault accommodating early Mesozoic intra-arc extension. New zircon U-Pb geochronology data coupled with field observations and whole-rock geochemistry serve as the basis to reclassify Summit gabbros and diorites as latest Jurassic and to motivate definition of the bimodal Summit igneous complex (151.0 ± 1.7–146.1 ± 1.2 Ma; N = 15) as also including coeval gabbro-granite dikes of the Osa Creek ring complex and the newly defined Tübatulabal hypabyssal-volcanic series. Tight age constraints support correlation of the Summit igneous complex to the regionally extensive, ca. 148 Ma Independence dike swarm. Multiple lines of evidence support latest Jurassic Sierran arc magmagenesis within a north-northwest–trending, sinistral transtensional regime, including: (1) subparallel orientations of the Independence dike swarm and aligned Summit Gabbro intrusions; (2) overlapping and bimodal compositions in both suites; and (3) the dramatic Hf isotope pull-up in zircons from Summit gabbro-diorites, recording a short-lived increase in the proportion of isotopically primitive mafic magmas intruding into the shallow arc crust. Though clearly differentiated, the composition of one gabbroic dike is the most primitive composition reported in the Sierra Nevada arc to date. The footprint of an intra-arc graben produced by early Mesozoic extension in eastern California is constrained by the Independence dike swarm, the Kern Plateau shear zone, and the Summit igneous complex, along with distributions of Triassic–Jurassic volcano-sedimentary deposits. Coupling the history of motion along the Kern Plateau shear zone with evidence for local extension beginning in the Permian–Triassic arc, we hypothesize that Mesozoic intra-arc faulting was strongly influenced by north-northwest–trending structures inherited from a mid- to late Paleozoic sinistral transpressional-transtensional plate boundary that extended for thousands of kilometers along the western boundary of Laurentia. We assemble a Permian–Jurassic time line of intra-arc extension within the plate margin locally transitioning from sinistral transform to convergent, one that culminated in latest Jurassic rifting through the entire crust underlying the Kern Plateau.