The Sierra Nevada of California is the longest and tallest mountain range in the co-terminus USA, and has long been regarded as topographically very young (, 6 Ma); however, recent work has provided evidence that the range is very old (.80 Ma), and represents the western shoulder of a Tibetan-like plateau (the Nevadaplano) that was centred over Nevada. A great deal of effort has been invested in applying modern laboratory and geophysical techniques to understanding the Sierra Nevada, yet some of the most unambiguous constraints on the Sierran landscape evolution are derived from field studies of dated strata preserved in the palaeochannels/palaeocanyons that crossed the range in Cenozoic time. Our work in the Sierra Nevada suggests that neither endmember model is correct for the debate regarding youth vs. antiquity of the range. Many features of the Cenozoic palaeocanyons and palaeochannels reflect the shape of the Cretaceous orogen, but they were also affected by Miocene tectonic and magmatic events. In the central Sierra Nevada, we infer that the inherited Cretaceous landscape was modified by three Miocene tectonic events, each followed by ,2 -5 Myr of subductioninduced magmatism and sedimentation during a period of relative tectonic quiescence. The first event, at about 16 Ma, corresponds to the westward sweep of the Ancestral Cascades arc front into the Sierra Nevada and adjacent western Nevada. We suggest that this caused thermal uplift and extension. The second event, at about 11 -10 Ma, records the birth of the 'future plate boundary' by transtensional faulting and voluminous high-K volcanism at the western edge of the Walker Lane belt. The third event, at about 8 -7 Ma, is associated with renewed range-front faulting in the central Sierra, and rejuvenation and beheading of the palaeocanyons. Volcanic pulses closely followed all three events, and we tentatively infer that footwall uplift of the Sierra Nevada occurred during all three events. By analogy with the , 11 Ma event, we speculate that high-K volcanic rocks in the southern part of the range mark the inception of yet a fourth pulse of range-front faulting at 3 -3.5 Ma, which resulted in a fourth tilting and crestal uplift event. Cenozoic rocks along the western edge of the Nevadaplano record the following variation, from the central to the northern Sierra: decrease in crustal thickness (and presumably palaeoelevation), decrease in palaeorelief and attendant decrease in coarse-grained fluvial-and mass-wasting deposits, and greater degree of encroachment by Walker Lanerelated faults beginning at 10 -11 Ma. By mapping and dating Cenozoic strata in detail, we show that what is now the Sierra Nevada was, at least in part, shaped by the Miocene structural and magmatic events.

We show here that transtensional rifting along the eastern boundary of the Sierra Nevada microplate (Walker Lane rift) began by ca. 12 Ma in the central Sierra Nevada (USA), within the ancestral Cascades arc, triggering voluminous high-K intermediate volcanism (Stanislaus Group). Flood andesite (i.e., unusually large-volume effusive eruptions of intermediate composition) lavas erupted from fault-controlled fi ssures within a series of grabens that we refer to as the Sierra Crest graben-vent system. This graben-vent system includes the following.1. The north-northwest-south-southeast Sierra Crest graben proper consists of a single 28-km-long, 8-10-km-wide full graben that is along the modern Sierra Nevada crest between Sonora Pass and Ebbetts Pass (largely in the Carson-Iceberg Wilderness). This contains fi ssure vents for the high-K intermediate lavas.2. A series of north-northwest-south-southeast half-grabens on the western margin of the full graben, which progressively disrupted an ancient Nevadaplano paleochannel that contains the type section of Stanislaus Group (Red Peak-Bald Peak area). These Miocene half-grabens are as much as 15 km west of the modern Sierra Nevada crest, and vented high-K lavas from point sources.3. Series of northeast-southwest grabens defi ne a major transfer zone along the northeast side of the Sierra Crest graben. These extend as much as ~30 km from the modern range crest down the modern Sierra Nevada range front, in a zone ~30 km wide, and vented high-K lavas and tuffs of the Stanislaus Group from point sources. Rangefront north-south and northeast-southwest faults to the south of that, along the southeast side of the Sierra Crest graben, did not vent volcanic rocks (although they ponded them); those will be described elsewhere.We present evidence for a dextral component of slip on the north-northwest-southsoutheast normal faults, and a sinistral component of slip on the northeast-southwest normal faults. The onset of transtension immediately preceded the high-K volcanism (within the analytical error of 40 Ar/ 39 Ar dates), and triggered the deposition of a debris avalanche deposit with a preserved volume of ~50 km 3 . The grabens are mainly fi lled with high-K lava fl ows, ponded to thicknesses of as much as 400 m; this effusive volcanism culminated in the development of the Little Walker caldera over a relatively small part of the fi eld. Trachydacite outfl ow ignimbrites from the caldera also became ponded in the larger graben-vent complex, where they interfi ngered with high-K lavas vented there, and escaped the graben-vent complex on its west margin to fl ow westward down two paleochannels to the western foothills.The Sierra Crest graben-vent system is spectacularly well exposed at the perfect structural level for viewing the controls of synvolcanic faults on the siting and styles of feeders, vents, and graben fi lls under a transtensional strain regime in an arc volcanic fi eld.

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The eastern escarpment of the Sierra Nevada (USA) forms one of the most prominent topographic and geologic features in the Cordillera, yet the timing and nature of fault displacements along it remain relatively poorly known. The central Sierra Nevada range front is an ideal place to determine the structural evolution of the range front because it has abundant dateable Cenozoic volcanic rocks. The Sonora Pass area of the central Sierra Nevada is particularly good for reconstructing the slip history of rangefront faults, because it includes unusually widespread and distinctive high-K volcanic rocks (the ca. 11.5-9 Ma Stanislaus Group) that serve as outstanding strain markers. These include the following, from base to top.(1) The Table Mountain Latite (TML) consists of voluminous trachyandesite, trachybasaltic andesite, and basalt lava flows, erupted from fault-controlled fi ssures in the Sierra Crest graben-vent system. (2) The Eureka Valley Tuff consists of three trachydacite ignimbrite members erupted from the Little Walker caldera. These ignimbrites are interstratifi ed with lava fl ows that continued to erupt from the Sierra Crest grabenvent system, and include silicic high-K as well as intermediate to mafi c high-K lavas. The graben-vent system consists of a single ~27-km-long, ~8-10-km-wide approximately north-south graben that is along the modern Sierran crest between Sonora Pass and Ebbetts Pass, with a series of approximately north-south half-grabens on its western margin, and an ~24-km-wide northeast transfer zone emanating from the northeast boundary of the graben on the modern range front south of Ebbetts Pass. In this paper we focus on the structural evolution of the Sonora Pass segment of the Sierra Nevada range front, which we do not include in the Sierra Crest graben-vent complex because we have found no vents for high-K lava fl ows here. However, we show that these faults localized the high-K Little Walker caldera.We demonstrate that the range-front faults at Sonora Pass were active before and during the ca. 11.5-9 Ma high-K volcanism. We show that these faults are dominantly approximately north-south down to the east normal faults, passing northward into a system of approximately northeast-southwest sinistral oblique normal faults that are on the southern end of the ~24-km-wide northeast transfer zone in the Sierra Crest graben-vent complex. At least half the slip on the northsouth normal faults on the Sonora Pass range front occurred before and during eruption of the TML, prior to development of the Little Walker caldera. It has previously been suggested that the range-front faults formed a right-stepping transtensional stepover that controlled the siting of the Little Walker caldera; we support that interpretation by showing that synvolcanic throw on the faults increases southward toward the caldera. The Sonora Pass-Little Walker caldera area is shown here to be very similar in structural style and scale to the transtensional stepover at the Quaternary Long Valley fi eld. Furthermore, the broade...

We integrate new stratigraphic, structural, geochemical, geochronological, and magnetostratigraphic data on Cenozoic volcanic rocks in the central Sierra Nevada to arrive at closely interrelated 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 fl ows, which were reworked into paleocanyons as volcanic debris fl ows and streamfl ow deposits. 2 Busby et al.