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.

High-resolution topography data acquired with lidar (light detection and ranging) technology are revolutionizing the way we study Earth surface processes. These data permit analysis of the mechanisms that drive landscape evolution at resolutions not previously possible yet essential for their appropriate representation. Unfortunately, the volume of data produced by the technology, software requirements, and a steep learning curve are barriers to lidar utilization. To encourage access to these data we use Keyhole Markup Language (KML) and Google Earth to deliver lidar-derived visualizations of these data for research and educational purposes. Display of full-resolution images derived from lidar in the Google Earth virtual globe is a powerful way to view and explore these data. Through region-dependent network linked KML (a.k.a., super-overlay), users are able to access lidar-derived imagery stored on a remote server from within Google Earth. This method provides seamless, Internet-based access to imagery through the simple download of a small KML-format file from the OpenTopography Facility portal. Lidar-derived imagery in Google Earth is the most popular product available via OpenTopography and has greatly enhanced the usability and thus impact of these data. Users ranging from scientists to K–12 educators have downloaded KML files ~12,000 times during the first eight months of 2011. The overwhelming usage of these data products demonstrates the impact of this simple yet novel approach for delivering easy to use lidar data visualizations to Earth scientists, students, and the general public.

The Neogene Boca basin, located 15 km northeast of Truckee, California, records the depositional and deformational history for the late Miocene–Pliocene period in this part of the northern Sierra Nevada. This study consists of fine-scale analysis of the well-exposed Neogene sedimentary rocks in an otherwise poorly exposed area of the northern Sierra Nevada. The Neogene Boca basin sedimentary section is >500 m thick and dips generally west to southwest. Four distinct lithologic intervals are deposited unconformably over lahars and intermediate lavas of the Miocene Kate Peak Formation. An ~180-m-thick section of conglomerate and conglomeratic litharenite represents a generally southwest directed fluvial system that existed from at least 4.4 Ma (interval I). This is overlain by and locally interfingered with a ca. 4.38 Ma basalt flow of Boca Hill. Above this basalt, an ~107-m-thick section of quartz wacke and siltstone deposits represents a deltaic system controlled by local volcanic topography from ca. 4.4 to 4.1 Ma (interval II). Conformably above interval II, an ~122-m-thick section of silty diatomite deposits with interbedded tephra and litharenite represents a lacustrine environment from ca. 4.1 to 2.7 Ma (interval III). Overlying the diatomite along a disrupted surface, a >91-m-thick section of medium- to coarse-grained litharenite and cobble conglomerate represents an abrupt change in depositional environment, to a west directed fluvial system (interval IV). Pliocene westward tilting and change in base level began during deposition of interval IV (ca. 2.7 Ma) and prior to eruption of the Boca Ridge Formation (ca. 2.61 Ma). Four orientations of large faults (>0.1 m displacement) are distributed evenly across the basin: (1) northeast to north-northeast striking sinistral faults; (2) northwest to north-northwest striking dextral faults; (3) west to west-northwest striking oblique-reverse faults; and (4) other fault orientations that have apparent motions not included in these categories. Strike-slip faulting is thought to have occurred during tilting of the Neogene section. The distributed conjugate strike-slip faults in the rocks of Boca basin accommodated east-southeast directed extension and south-southwest directed contraction. These new stratigraphic and structural data provide information on late Miocene–Pliocene deformation at the eastern edge of the Sierra Nevada. The Boca basin appears to have been an isolated basin controlled by volcanic topography. A late Miocene deformation event is not recorded in Boca basin; however, a Pliocene event is recorded in the termination of deposition and deformation of the section through tilting, incision, and distributed faulting. Pliocene deformational style is consistent with generally east-west extension associated with westward encroachment of the Basin and Range or northward migration of normal faults at Lake Tahoe. The structural data cannot disprove migration of Walker Lane deformation into the Sierra Nevada but merely show that this did not occur in the area occupied by the Neogene Boca basin. The Pliocene deformation event coincided with local eruption of high-potassium lavas and a regional base-level change, and it may represent rollback of the Juan de Fuca plate after ca. 3 Ma.