ABSTRACT This field trip traverses a cross section of northern Sierra Nevada geology and landscape along two major corridors, Highway 49 (Yuba Pass) and Highway 70. These highways, and adjacent roadways, offer roadcuts, outcrops, and overviews through diverse pre-Cenozoic metamorphic rocks along the Laurentian margin, Mesozoic batholithic rocks, and Miocene volcanic rocks. Observing this array of rocks on a single trip provides an opportunity to examine the progression of tectonic forces in this region since the Paleozoic Era. Inspiration for this trip is a 1:100,000-scale geologic map and geophysical maps of the Portola 30′ × 60′ quadrangle that integrate decades of published and unpublished mapping with new geophysical data. The quadrangle map will seamlessly depict a geologically complex region along the boundary between the Sierra Nevada and Basin and Range provinces, dominated by transtensional tectonics of the Walker Lane. This field trip highlights many of the major units of the geologic map and will also feature new geochronological data on plutonic rocks.

Neogene sediments in a structural and geomorphic high in the southwestern Honey Lake basin represent lacustrine deposition from 3.7 to 2.9 Ma, interrupted once by a significant lowstand. Tephras in the upper section are 3.26 Ma and 3.06 Ma. A thick debris-flow bed, truncated by an erosional surface and overlain concordantly by a thin interval of subaerial sediments, is evidence for lake-level fall at ca. 3.4 Ma. The dominant structure is a broad east-southeast–plunging anticline cut by several sets of faults. These include northwest-striking dextral and northeast-striking sinistral strike-slip faults and a conjugate set of west-northwest–striking thrust faults; all are consistent with north-south shortening. Mutually crosscutting relationships between faults, and tilt fanning of the dextral faults, indicate that tightening of the anticline was synchronous with faulting. A Quaternary strand of the dextral Honey Lake fault crops out near the northern end of the exposure, suggesting that the cause of the local shortening and uplift was a contractional stepover between two strands of the Honey Lake fault. The Neogene section limits this faulting to some time after 2.9 Ma. The Honey Lake basin lies at the intersection of the Walker Lane with the Sierran frontal fault system. Although the timing of tectonic disruption was roughly consistent with passage of the triple junction to the west and with uplift and exhumation of several nearby basins, the described deformation seems to be directly related to dextral faulting, dating the propagation of a strand of the Honey Lake fault through the southwestern Honey Lake basin.

The Honey Lake fault zone is one of four major, northwest-striking dextral faults that constitute the northern Walker Lane in northwestern Nevada and northeastern California. Global positioning system (GPS) geodetic data indicate that the northern Walker Lane accommodates ~10%–20% of the dextral motion between the North American and Pacific plates. Regional relations suggest that dextral movement in the Honey Lake area began ca. 6–3 Ma. Five 31.3–25.3 Ma ash-flow tuffs, totaling ~250 m in thickness, were distinguished in a paleovalley in the Black Mountain area of the Diamond Mountains, southwest of the Honey Lake fault. Four of these tuffs, totaling ~200 m in thickness, also occupy a paleovalley in the Fort Sage Mountains northeast of the fault. On the basis of the similar tuff sequences, we infer that the Diamond and Fort Sage Mountains contain offset segments of a once-continuous, westerly trending late Oligocene paleovalley. Paleomagnetic data from the 25.3 Ma Nine Hill Tuff indicate negligible vertical-axis rotation in the Diamond and Fort Sage Mountains. Correlation of the paleovalley segments in the Diamond and Fort Sage Mountains suggests 10–17 km of dextral displacement across the Honey Lake fault. About 10 km of offset is favored on the basis of constraints near the southeast end of the fault. The spread of possible offset values implies long-term slip rates of ~1.7–2.8 mm/yr for a 6 Ma initiation, and ~3.3–5.7 mm/yr for a 3 Ma initiation. These rates are comparable to slip rates inferred from Quaternary fault studies and GPS geodesy.

Abstract The magmatic arc represented by the crystalline core of the North Cascades (Cascades core) reached a crustal thickness of >55 km in the mid-Cretaceous. Eocene collapse of the arc was marked by migmatization, magmatism, and exhumation of deep-crustal (9-12 kb) rocks at the same time as subsidence and rapid deposition in nearby transtensional nonmarine basins. The largest region of deeply exhumed rocks, the migmatitic Skagit Gneiss Complex, consists primarily of leucocratic, biotite tonalite orthogneiss intruded between ca. 76-59 Ma and 50-45 Ma. Well-layered biotite gneiss is also widespread. U-Pb (isotope dilution-thermal ionization mass spectrometry) dating of zircon and monazite from trondhjemitic leucosome and biotite gneiss mesosome indicates that metamorphism and melt generation/crystallization occurred at least intermittently from ca. 71 to 47 Ma, and the youngest U-Pb dates overlap Ar/Ar (biotite, muscovite) dates, compatible with rapid cooling. Mesoscopic to map-scale, gently plunging, upright folds have hinge lines subparallel to orogen-parallel (NW-SE) lineations in the Skagit Gneiss Complex, and are as young as 48 Ma. Eocene top-to-northwest flow occurred in parts of the complex. The gently to moderately dipping foliation, subhorizontal lineation, and constrictional domains are compatible with ductile transtension linked to dextral-normal displacement on the Ross Lake fault system, the northeastern boundary of the Cascades core. On the south flank of the core, sediments were deposited in part at ca. 51 Ma in the Swauk basin and shortly afterward folded, and then intruded by 47 Ma Teanaway basaltic dikes. Extension taken up by these dikes ranges from ~10% to 43%. Extension directions from Teanaway and other Eocene dikes are arc-parallel to arc-oblique. The shallow-crustal extension direction is counterclockwise (mostly 10°-30°) to the ductile flow direction, implying decoupling of brittle and ductile crust; however, some coupling is supported by the temporal coincidence between basin formation and partial melting and ductile flow, and the upright folding of both the Skagit Gneiss Complex and Swauk basin. Arc-oblique to arc-parallel flow probably resulted in part from dextral shear along the plate margin, along-strike gradients in crustal thickness, and thermally controlled rheology.

The Lovejoy basalt represents the largest eruptive unit identified in California, and its age, volume, and chemistry indicate a genetic affinity with the Columbia River Basalt Group and its associated mantle-plume activity. Recent field mapping, geochemical analyses, and radiometric dating suggest that the Lovejoy basalt erupted during the mid-Miocene from a fissure at Thompson Peak, south of Susanville, California. The Lovejoy flowed through a paleovalley across the northern end of the Sierra Nevada to the Sacramento Valley, a distance of 240 km. Approximately 150 km 3 of basalt were erupted over a span of only a few centuries. Our age dates for the Lovejoy basalt cluster are near 15.4 Ma and suggest that it is coeval with the 16.1–15.0 Ma Imnaha and Grande Ronde flows of the Columbia River Basalt Group. Our new mapping and age dating support the interpretation that the Lovejoy basalt erupted in a forearc position relative to the ancestral Cascades arc, in contrast with the Columbia River Basalt Group, which erupted in a backarc position. The arc front shifted trenchward into the Sierran block after 15.4 Ma. However, the Lovejoy basalt appears to be unrelated to volcanism of the predominantly calc-alkaline Cascade arc; instead, the Lovejoy is broadly tholeiitic, with trace-element characteristics similar to the Columbia River Basalt Group. Association of the Lovejoy basalt with mid-Miocene flood basalt volcanism has considerable implications for North American plume dynamics and strengthens the thermal “point source” explanation, as provided by the mantle-plume hypothesis. Alternatives to the plume hypothesis usually call upon lithosphere-scale cracks to control magmatic migrations in the Yellowstone–Columbia River basalt region. However, it is difficult to imagine a lithosphere-scale flaw that crosses Precambrian basement and accreted terranes to reach the Sierra microplate, where the Lovejoy is located. Therefore, we propose that the Lovejoy represents a rapid migration of plume-head material, at ~20 cm/yr to the southwest, a direction not previously recognized.

Abstract Within the western Great Basin, a system of dextral strike-slip faults accommodates a significant fraction of the North American–Pacific plate motion. The northern Walker Lane in northwest Nevada and northeast California occupies the northern terminus of this fault system and is one of the youngest and least developed parts of the North American–Pacific transform plate boundary. Accordingly, the northern Walker Lane affords an opportunity to analyze the incipient development of a major strike-slip fault system. In northwest Nevada, the northern Walker Lane consists of a discrete ~50-km-wide belt of overlapping, curiously left-stepping dextral faults, whereas a much broader zone of disconnected, widely-spaced northwest-striking faults characterizes northeast California. The left steps accommodate little shortening and are not typical restraining bends. The left-stepping dextral faults may represent macroscopic Riedel shears developing above a nascent lithospheric-scale transform fault. Strands of the northern Walker Lane terminate in arrays of northerly striking normal faults in the northwestern Great Basin and along the eastern front of the Sierra Nevada. These relations suggest that dextral shear in the northern Walker Lane is transferred to ~NW-SE extension in the Great Basin. Offset segments of a west-trending Oligocene paleovalley suggest ~20–30 km of cumulative dextral slip across the northern Walker Lane. Strike-slip faulting began between 3 and 9 Ma, indicating a long-term slip rate of ~2–10 mm/yr, which is compatible with GPS geodetic observations of the current strain field .