Ellen P. Metzger

Geochemistry and Tectonic Setting of the Ophiolitic Ingalls Complex, North Cascades, Washington: Implications for Correlations of Jurassic Cordilleran Ophiolites

This study synthesizes new and existing chemical data to interpret the tectonic setting of the Late Jurassic Ingalls Complex of the Northwest Cascades, Washington, and to compare it with other Cordilleran ophiolites of similar age. Mafic rocks of the Ingalls Complex represent a spectrum of magma types including normal‐type and enriched mid‐ocean ridge basalt, within‐plate basalt, and island arc basalt. Based on mafic magma chemistry, compositions of Cr‐spinels in associated peridotite, and field relations, the Ingalls Complex is a suprasubduction zone ophiolite formed in a back‐arc basin cut by an oceanic fracture zone. Regional tectonic relations also support this hypothesis. Our synthesis of available chronologic, geochemical, and stratigraphic data for all Jurassic Cordilleran ophiolites shows that the Ingalls Complex has similarities and differences with each. The most compelling likeness is between the Ingalls Complex and Josephine ophiolite, both of which exhibit a wide range of magmatic affinities, Cr‐spinel compositions that are consistent with a suprasubduction zone origin in a back‐arc basin setting, and positions inboard of a coeval Jurassic arc.

Detrital zircon geochronology of Neoproterozoic–Lower Cambrian passive-margin strata of the White-Inyo Range, east-central California: Implications for the Mojave–Snow Lake fault hypothesis

Correlation of lithotectonic packages across major transcurrent structures is critical to understanding the tectonic evolution of the North American continental margin. Detrital zircon geochronology of uppermost Proterozoic to Lower Paleozoic miogeoclinal strata from the White-Inyo Mountains permits evaluation of: (1) the age and provenance of these metasediments and (2) a model for truncation of the passive margin along a postulated large-magnitude Cretaceous dextral shear zone, i.e., the Mojave–Snow Lake fault. U-Pb ages of detrital zircons from the Neoproterozoic Wyman Formation, the uppermost Proterozoic Reed Dolomite (Hines Tongue Member), and clastic strata of the Lower Cambrian Deep Springs, Campito (Montenegro Member), Poleta, and Harkless formations reflect ultimate derivation from the adjacent 1.7–pre-1.8 Ga Mojavia terrane and/or 1.7–1.8 Ga Yavapai continental basement, with subsidiary sources in both the ca. 1.4 Ga Yavapai-Mazatzal anorogenic granitoids and the >2.5 Ga North American craton, and a small proportion of 1.0–1.3 Ga grains, most likely reworked from Grenville clastic wedge deposits. Detrital zircon age spectra from the Lower Cambrian Andrews Mountain Member of the Campito Formation are unique in comparison with the remainder of the studied section, containing a major age peak centered at ca. 1.1 Ga and a subsidiary Lower Cambrian age peak, permitting calculation of a ca. 527 ± 12 (2σ) Ma maximum depositional age. These features, in addition to abundant detrital magnetite-ilmenite grains, reflect a distal source for these rocks, most likely from the ca. 1.1 Ga Pikes Peak batholith and/or the Midcontinent rift and ca. 0.53 Ga bimodal intrusions of the Oklahoma-Colorado aulacogen. In terms of zircon age distribution, allochthonous metamorphic pendants in the Snow Lake terrane of the central Sierra Nevada batholith are most similar to those of stratigraphically equivalent units in the Death Valley region and, to lesser degrees, the White-Inyo section, and the Mojave Desert region. Given the similarity and relative proximity of Death Valley facies assemblages to the Snow Lake terrane, we suggest that the latter was not transported northward from the Mojave Desert region and instead represents footwall assemblages of a late Early to early Middle Jurassic low-angle normal fault system, probably along the outer transform-truncated margin of the Last Chance thrust stack. This model implies a few tens of kilometers of offset, in contrast to the hundreds of kilometers required by the Mojave–Snow Lake fault hypothesis.

Aluminous Xenoliths in Miocene Andesite, Central California Coast Ranges: Magma-Crust Interaction in a Subduction-Transform Transitional Setting

Non-foliated, Al-rich inclusions in Miocene andesite dikes and plugs cropping out in the Dowdy Ranch area, central Diablo Range, record two stages of high-temperature metamorphism, as well as complex chemical interactions between the inclusions and their mafic magma host. Both xenoliths and andesitic host show marked compositional zoning and partial resorption of minerals. The first stage of subsolidus contact metamorphism produced andalusite + alkali-feldspar + Na-plagioclase + muscovite + biotite + quartz (± actinolite or epidote?) in xenoliths of the originally quartzofeldspathic wall rock. P-T conditions attending this event were approximately 600°C, ˜2 kbar, with Pfluid approaching Ptotal. Capture of fragments of metamorphosed wall rock by ascending mafic magma resulted in partial fusion of the xenoliths, loss of silica and alkalis to the liquid, and calcium-enrichment in the restitic, increasingly aluminous inclusions. Second-stage neoblastic assemblages are sillimanite + hercynite-rich spinel + corundum + Ca-plagioclase ± Mg-rich biotite or phlogopite ± orthopyroxene. Physical conditions in the xenoliths accompanying decompression partial melting were approximately 700-750°C and ˜1 kbar (with Pfluid < Ptotal). The presence of corundum and ZnO-poor spinel is consistent with a restitic residue produced by dehydration melting under high-temperature, silica-deficient conditions. The parageneses developed in the Al-rich inclusions were an upper crustal thermal response to Miocene passage of the Mendocino triple junction at this latitude.

Sustainability: Why the Language and Ethics of Sustainability Matter in the Geoscience Classroom.

ABSTRACT Because challenges to sustainability arise at the intersection of human and biophysical systems they are inescapably embedded in social contexts and involve multiple stakeholders with diverse and often conflicting needs and value systems. Addressing complex and solution-resistant problems such as climate change, biodiversity loss, and environmental degradation thus demands not only a scientific understanding of Earth systems, but consideration of the underlying human values, institutions, and norms that drive unsustainable ways of living. The search for solutions amidst a multiplicity of players and an array of potential outcomes inevitably leads to ethical quandaries. The purpose of this commentary is to synthesize perspectives from the geosciences and philosophy to provide a rationale for including the ethical dimensions of sustainability in geoscience education and to clarify the nature and ethics of sustainability. Drawing on an approach developed in the book Living Well Now and in the Future: Why Sustainability Matters (Curren and Metzger, 2017), we outline a way to conceptualize sustainability that bridges scientific and ethical perspectives and present four fundamental principles of sustainability ethics derived from our analysis and from core commitments of common morality. We supply a compilation of relevant teaching approaches and materials to help geoscience educators connect the enumerated concepts and principles to classroom practice and we conclude with a call for further cross-disciplinary conversations among geoscientists, philosophers, and social scientists who share a commitment to including sustainability concepts and ethics in their teaching.

Preserving Opportunity: A Précis of Living Well Now and in the Future: Why Sustainability Matters

Abstract This article is a précis of the book, Living well now and in the future: Why sustainability matters. It provides an overview of the book, focusing especially on its conceptualization of the nature and normative dimensions of sustainability. The latter include its formulation of an ethic of sustainability and eudaimonic theory of justice. Some central claims are that the fundamental normative concern of sustainability is the long-term preservation of opportunity to live well, and that the conceptualization of preservation of opportunity should be focused on the satisfaction of basic psychological needs associated with fulfillment of potential.

The Ongoing Educational Anomaly of Earth Science Placement

The geosciences have traditionally been viewed with less “academic prestige” than other science curricula. Among the results of this perception are depressed K-16 enrollments, Earth Science assignments to lower-performing students, and relegation of these classes to sometimes under-qualified educators, all of which serve to confirm the widely-held misconceptions. An Earth Systems course developed at San José State University demonstrates the difficulty of a standard high school Earth science curriculum, while recognizing the deficiencies in pre-college Earth science education. Restructuring pre-college science curricula so that Earth Science is placed as a capstone course would greatly improve student understanding of the geosciences, while development of Earth systems courses that infuse real-world and hands-on learning at the college level is critical to bridging the information gap for those with no prior exposure to the Earth sciences. Well-crafted workshops for pre-service and inservice teachers of Earth Science can help to reverse the trends and unfortunate “status” in geoscience education.

Composition of sediment records late Quaternary paleogeographic evolution of Santa Clara Valley, California

Gravel and sand samples from alluvium in five wells in the Santa Clara Valley, California (USA), have particle compositions that help constrain patterns of Quaternary basin evolution and sediment dispersal within the valley. Samples were collected from depths as great as 307 m, and paleomagnetic results obtained by other workers suggest that the sediment sampled ranges in age to ca. 800 ka. The gravel contains common to abundant Mesozoic graywacke and metavolcanic rocks; less common clast types indicate derivation from other Mesozoic rocks. Clasts having lithologies that match Cenozoic source rocks are minor in amount. The abundance of metavolcanic rocks, as well as serpentinite, blueschist, and eclogite, among the less common gravel clasts, generally increases with depth in each well. Sand samples contain a variety of mostly metamorphic heavy minerals, including some distinctive blueschist facies minerals. Blue amphibole and chrome spinel are rare to common in the sand, and their abundance also increases with depth. The most useful clasts for determining the location of the sediment source are the metavolcanic rocks and some of the less common clasts. Metavolcanic rocks, common in modern outcrops only in the Santa Cruz Mountains south and west of the Santa Clara Valley, also are common in the gravel. In contrast, with only a few exceptions at shallow levels in the northeasternmost wells, distinctive gravel clasts with sources east of the valley are nearly absent. These observations suggest that most of the sediment was derived from the south and west. Thus, the topographic asymmetry of the valley, with its modern axial drainage east of the valley center, evidently has persisted for at least the past 800 k.y. The increased abundance of pebbles of serpentinite, blueschist, and eclogite and of sand-sized particles of blue amphibole and chrome spinel with depth suggests a change in source through time. The source for these clasts in the deeper levels, and perhaps also some of the metavolcanic clasts, evidently was a basement high within the valley that was gradually buried as the basin filled with sediment. The gravel in the wells has a composition different from that of any previously described outcrops of Pliocene and Pleistocene nonmarine gravels, including the Santa Clara Formation, exposed around the margins of the Santa Clara Valley. This difference suggests that the Santa Clara Formation and similar units are not present in the parts of the valley penetrated by the wells.