We integrate new stratigraphic, structural, geochemical, geochronological, and magnetostratigraphic data on Cenozoic volcanic rocks in the central Sierra Nevada to arrive at closely inter-related new models for: (1) the paleogeography of the ancestral Cascades arc, (2) the stratigraphic record of uplift events in the Sierra Nevada, (3) the tectonic controls on volcanic styles and compositions in the arc, and (4) the birth of a new plate margin. Previous workers have assumed that the ancestral Cascades arc consisted of stratovolcanoes, similar to the modern Cascades arc, but we suggest that the arc was composed largely of numerous, very small centers, where magmas frequently leaked up strands of the Sierran frontal fault zone. These small centers erupted to produce andesite lava domes that collapsed to produce block-and-ash flows, which were reworked into paleocanyons as volcanic debris flows and streamflow deposits. Where intrusions rose up through water-saturated paleocanyon fill, they formed peperite complexes that were commonly destabilized to form debris flows. Paleo-canyons that were cut into Cretaceous bedrock and filled with Oligocene to late Miocene strata not only provide a stratigraphic record of the ancestral Cascades arc volcanism, but also deep unconformities within them record tectonic events. Preliminary correlation of newly mapped unconformities and new geochronological, magnetostratigraphic, and structural data allow us to propose three episodes of Cenozoic uplift that may correspond to (1) early Miocene onset of arc magmatism (ca. 15 Ma), (2) middle Miocene onset of Basin and Range faulting (ca. 10 Ma), and (3) late Miocene arrival of the triple junction (ca. 6 Ma), perhaps coinciding with a second episode of rapid extension on the range front. Oligocene ignimbrites, which erupted from calderas in central Nevada and filled Sierran paleocanyons, were deeply eroded during the early Miocene uplift event. The middle Miocene event is recorded by growth faulting and landslides in hanging-wall basins of normal faults. Cessation of andesite volcanism closely followed the late Miocene uplift event. We show that the onset of Basin and Range faulting coincided both spatially and temporally with eruption of distinctive, very widespread, high-K lava flows and ignimbrites from the Little Walker center (Stanislaus Group). Preliminary magnetostratigraphic work on high-K lava flows (Table Mountain Latite, 10.2 Ma) combined with new 40 Ar/ 39 Ar age data allow regional-scale correlation of individual flows and estimates of minimum (28,000 yr) and maximum (230,000 yr) time spans for eruption of the lowermost latite series. This work also verifies the existence of reversed-polarity cryptochron, C5n.2n-1 at ca. 10.2 Ma, which was previously known only from seafloor magnetic anomalies. High-K volcanism continued with eruption of the three members of the Eureka Valley Tuff (9.3–9.15 Ma). In contrast with previous workers in the southern Sierra, who interpret high-K volcanism as a signal of Sierran root delamination, or input of subduction-related fluids, we propose an alternative model for K 2 O-rich volcanism. A regional comparison of central Sierran volcanic rocks reveals their K 2 O levels to be intermediate between Lassen to the north (low in K 2 O) and ultrapotassic volcanics in the southern Sierra. We propose that this shift reflects higher pressures of fractional crystallization to the south, controlled by a southward increase in the thickness of the granitic crust. At high pressures, basaltic magmas precipitate clinopyroxene (over olivine and plagioclase) at their liquidus; experiments and mass-balance calculations show that clinopyroxene fractionation buffers SiO 2 to low values while allowing K 2 O to increase. A thick crust to the south would also explain the sparse volcanic cover in the southern Sierra compared to the extensive volcanic cover to the north. All these data taken together suggest that the “future plate boundary” represented by the transtensional western Walker Lane belt was born in the axis of the ancestral Cascades arc along the present-day central Sierran range front during large-volume eruptions at the Little Walker center.

We have examined the deformation associated with a right-releasing stepover along the dextral Walker Lane belt where it traverses Wild Horse Mesa in eastern California. We use a micropolar inversion of both seismic focal mechanism and fault-slickenline data and compare the results to the micropolar deformation parameters inferred from paleomagnetically determined block rotations and GPS velocities. The focal mechanisms, fault-slickenlines, and GPS velocities all show horizontal shear with a consistent ENE–WSW to E–W maximum extension-rate axis ( d 1 ). A subset of data shows crustal thinning with a similarly oriented d 1 . We interpret these results as a reflection of divergent strike-slip (i.e., transtensional) boundary conditions in a negative flower structure developed in the right-releasing stepover. The fault-slickenline data also show a crustal thickening solution that we attribute to the local accommodation of block rotations. Paleomagnetic data demonstrate clockwise-looking-down rotations of 12.0° ± 2.6° (68% confidence limits) in ca. 3 Ma volcanic rocks, relative to the same rocks outside the stepover. Assuming rotations took 2–3 m.y. gives average microspins (block rotation rates) of 4.0° ± 0.9°/m.y. to 6.0° ± 1.3°/m.y. GPS velocities define a current macrospin (half the continuum rotation rate) of 3.9° ± 0.6°/m.y. to 6.1° ± 1.5°/m.y. These spin components are consistent with expectations for transtension. Our calculations of relative vorticity W from the GPS and paleomagnetic data are generally consistent with values obtained from the inversion of the fault-slickenline data, but the uncertainties in the data do not permit a definitive test of these results.