The source of volcanic material in the Upper Triassic Chinle Formation on the Colorado Plateau has long been speculated upon, largely owing to the absence of similar-age volcanic or plutonic material cropping out closer than several hundred kilometers distant. These strata, however, together with Upper Triassic formations within El Antimonio and Barranca Group sedimentary rocks in northern Sonora, Mexico, yield important clues about the inception of Cordilleran magmatism in Triassic time. Volcanic clasts in the Sonsela Member of the Chinle Formation range in age from ca. 235 to ca. 218 Ma. Geochemistry of the volcanic clasts documents a hydrothermally altered source region for these clasts. Detrital zircons in the Sonsela Member sandstone are of similar age to the clasts, as are detrital zircons from the El Antimonio and Barranca Groups in Sonora. Most noteworthy about the Colorado Plateau Triassic zircons, however, are their Th/U ratios, which range from ~1 to 3.5 in both clast and detrital zircons. Thorium/uranium ratios in the Sonoran zircons, in contrast, range from ~0.4 to ~1. These data, together with rare-earth-element geochemistry of the zircons, shed light on likely provenance. Geochemical comparisons support correlation of clasts in the Sonsela Member with Triassic plutons in the Mojave Desert in California that are of the same age. Zircons from these Triassic plutons have relatively low Th/U ratios, which correspond well with values from El Antimonio and Barranca Group sedimentary rocks, and support derivation of the strata, at least in part, from northern sources. The Sonsela Member zircons, in contrast, match Th/U values obtained from Proterozoic through Miocene volcanic, volcaniclastic, and plutonic rocks in the eastern and central Mojave Desert. Similarly, rare-earth-element compositions of zircons from Jurassic ignimbrites in the Mojave Desert, though overlapping those of zircons from Mojave Desert plutons, also closely resemble those from Sonsela Member zircons. We use these data to speculate that erosion of Triassic volcanic fields in the central to eastern Mojave Desert shed detritus that became incorporated into the Chinle Formation on the Colorado Plateau.

Abstract The eastern California shear zone is an important component of the Pacific–North America plate boundary. This region of active, predominantly strike-slip, deformation east of the San Andreas fault extends from the southern Mojave Desert along the east side of the Sierra Nevada and into western Nevada. The eastern California shear zone is thought to accommodate nearly a quarter of relative plate motion between the Pacific and North America plates. Recent studies in the region, utilizing innovative methods ranging from cosmogenic nuclide geochronology, airborne laser swath mapping, and ground penetrating radar to geologic mapping, geochemistry, and U-Pb, 40 Ar/ 39 Ar, and (U-Th)/He geochronology, are helping elucidate slip rate and displacement histories for many of the major structures that comprise the eastern California shear zone. This field trip includes twelve stops along the Lenwood, Garlock, Owens Valley, and Fish Lake Valley faults, which are some of the primary focus areas for new research. Trip participants will explore a rich record of the spatial and temporal evolution of the eastern California shear zone from 83 Ma to the late Holocene through observations of offset alluvial deposits, lava flows, key stratigraphic markers, and igneous intrusions, all of which are deformed as a result of recurring seismic activity. Discussion will focus on the constancy (or non-constancy) of strain accumulation and release, the function of the Garlock fault in accommodating deformation in the region, total cumulative displacement and timing of offset on faults, the various techniques used to determine fault displacements and slip rates, and the role of the eastern California shear zone as a nascent segment of the Pacific–North America plate boundary.

The Western North American Volcanic and Intrusive Rock Database (NAVDAT— http://navdat.geo.ku.edu and http://navdat.geongrid.org ) is a relational database that serves as a repository for chemical and age data for igneous rocks in this region and allows, for the first time, the voluminous, high-quality, igneous rock data generated over the past thirty years to be integrated into time-space-composition models of western North American igneous activity. As in other continental regions, the analysis of geochemical, geochronological, and geospatial data from igneous rocks has played a critical role in the understanding of the tectonic evolution of western North America. However, the seminal, regional-scale studies of the tectonic evolution of this region remain primarily those undertaken in the 1970s, prior to the explosion in the number of high-quality chemical and isotopic studies of North American igneous rocks that occurred during subsequent decades. There has been little concerted effort to integrate these new data into regional tectonic models of western North America, due in large part to the lack of a unified, high-quality database for igneous rocks in this region. As a result, the potential that these data have for providing new fundamental insights into the tectonic evolution of the continent remains largely untapped. The NAVDAT effort is designed to remedy this situation. At its core, NAVDAT is a Web-accessible petrologic database with a schema compatible with existing petrologic databases (GEOROC, PetDB), but with an emphasis on a structure that will easily allow these data to be queried and manipulated as functions of age, composition, and geographic position. The NAVDAT user interface includes a number of advanced query options including both graphical and map-plotting analysis tools as well as close linking between data and reference information for the source of the data. NAVDAT allows investigations of continent-scale patterns in magmatism and provides the means to address how local variations in the age, composition, and location of igneous rocks fit into a regional context. The latter ability will produce more thorough and rigorous assessments of the relationships between volcanism and lithospheric deformation, including continental mantle delamination, the controls of subducted slab orientation on space-time-composition patterns of magmatism, and the role of mantle plumes in continental evolution. NAVDAT also will provide basic information on volcanic hazards and geothermal resources in western North America. The overall goal is for NAVDAT to become a rich data tool, an important part of the national geoinformatics effort, and a functioning component of an overall national geoscience cyberinfrastructure.

Full article available in PDF version

Full article available in PDF version

Full article available in PDF version