In the introduction to his paper, Hanson provides a succinct history of the area, as paraphrased here: Santa Clara Valley is a long, narrow (240 square miles), trough-like coastal watershed that borders the southern end of San Francisco Bay, extending about 35 miles southeast from there. The watershed principally drains parts of Santa Clara and San Mateo counties. Santa Clara Valley has experienced the typical evolution of land- and water-use development in the western United States, with a transition from an agricultural and ranching economy to one based on urban services and industry. In the first half of the twentieth century, the valley was intensively cultivated for fruit and truck crops, but subsequent development has included urbanization and industrialization, so that the area is now commonly known as "Silicon Valley."
Hanson says that the valley underwent extensive groundwater development from the early 1900s through the mid-1960s. This development caused groundwater level declines of more than 200 feet and induced regional subsidence of as much as 12.7 feet from the early 1900s to the mid-1960s. As with other coastal aquifer systems, Hanson notes, "the possibility exists that the combined effects of land subsidence and seawater intrusion will result in large water-level declines."
The San Francisco Water Department started delivering imported water to several north county cities in the early 1950s. In the 1960s, the Santa Clara Valley Water District (SCVWD) began importing surface water into the valley to help meet growing demands and to reduce the area's dependence on groundwater. The combination of reduced groundwater pumping and this artificial recharge has caused groundwater levels to recover to near their predevelopment levels, and this, in turn, has arrested the land subsidence, says Hanson, noting, "Currently, the water purveyors in the Santa Clara Valley, in conjunction with SCVWD, would like to meet the water demand in the basin while limiting any potential for additional land subsidence."
Even though extensive studies have been completed in the Santa Clara Valley, there were no comprehensive three-dimensional hydrologic, geologic, and geochemical data that would allow the delineation of the hydrologic framework that controls the distribution and movement of the water resources in the Santa Clara Valley. Hanson's article summarizes the hydrologic framework of the valley using data obtained from nine new monitoring-well sites and various supply wells in combination with a detailed groundwater-surface-water model.
The synthesis of this framework is based on a sequence of interdisciplinary studies between the U.S. Geological Survey and the Santa Clara Valley Water District. The framework components, as summarized in Hanson's article, include the hydrogeologic structure of the valley, groundwater budgets, the role of climate cycles, the nature of stream-aquifer interactions, distribution and nature of groundwater pumpage, effects of land subsidence, the distribution of artificial recharge, geochemical characteristics of the aquifers and wells, and the overall water-resource management issues relevant to the sustainable and conjunctive use of the groundwater and surface water resources of the Santa Clara Valley.
FEATURED ARTICLE
Hydrologic framework of the Santa Clara Valley, California
R.T. Hanson, U.S. Geological Survey, San Diego, California, USA. Published online on 13 May 2015; http://dx.doi.org/10.1130/GES01104.1. Themed issue: A New Three-Dimensional Look at the Geology, Geophysics, and Hydrology of the Santa Clara ("Silicon") Valley.
Other GEOSPHERE articles (see below) cover such topics as The evolution of the Colorado River system, USA; The first comprehensive imaging and mapping of the fault network beneath Pyramid Lake, Nevada, USA; and A field-based study of the 2010 Mw 7.2 El Mayor-Cucapah earthquake surface rupture, Mexico.
All GEOSPHERE articles available at http://geosphere.gsapubs.org/. Representatives of the media may obtain complimentary copies of GEOSPHERE articles by contacting Kea Giles at the address above. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOSPHERE in articles published. Non-media requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org.
Importance of groundwater in propagating downward integration of the 6-5 Ma Colorado River system: Geochemistry of springs, travertines, and lacustrine carbonates of the Grand Canyon region over the past 12 Ma
L.C. Crossey et al., University of New Mexico, Albuquerque, New Mexico, USA. Published online on 13 May 2015; http://dx.doi.org/10.1130/GES01073.1. Themed issue: CRevolution 2: Origin and Evolution of the Colorado River System II.
The Colorado River is central to modern concerns over climate change, water resources, and public policy in the parched western U.S. struggling under a decade-long severe drought. The geologic origins of this iconic river system are the focus of vigorous scientific debate. This paper examines the record of sedimentary carbonates (including lakes and spring deposits) and finds that groundwater played an important role in the persistence and integration of a chain of lakes that evolved into the Colorado River. Geochemical studies of sedimentary carbonates provide a rich record of past hydrology and climate conditions. Specifically, The Hualapai Limestone (deposited 6 to 12 million years ago) and the Bouse Formation (deposited 4.8 to 5.6 million years ago) record an evolving paleohydrologic system that led to the first arrival of the Colorado River to the Gulf of California about five million years ago. L.C. Crossey and colleagues use comparisons between modern and ancient carbonates, including spring-deposited travertines in the Grand Canyon, to test our models. Debate about whether the earliest Colorado River interacted with a marine estuary near Yuma continues, but the authors find that southern Blythe basin 87Sr/86Sr values of approx. 0.710 to 0.711 could be produced by 25% to 75% sea water mixed with river water in a delta/marine estuary system. Thus, Sr isotope data do not preclude a marine connection for the earliest Colorado River deposits.
New constraints on fault architecture, slip rates, and strain partitioning beneath Pyramid Lake, Nevada
Amy Kendra Eisses et al., Nevada Seismological Laboratory, University of Nevada, Reno, Nevada, USA. Published online on 13 May 2015; http://dx.doi.org/10.1130/GES00821.1. Themed issue: Origin and Evolution of the Sierra Nevada and Walker Lane.
High-resolution seismic reflection data collected in Pyramid Lake, Nevada, give important insights into fault structure within the lake basin. Fault systems beneath Pyramid Lake contribute to seismic hazards near Reno, Nevada, and provide information about fault architecture and how slip is released throughout time. The imaged stratigraphic layers beneath Pyramid Lake show that the strike-slip Pyramid Lake fault dies along the southwestern shoreline and gives way to the Lake Range normal fault that runs along the eastern shoreline; the northwestern portion of the lake is characterized by a series of short dip-slip faults that form a nascent shear zone. This study is the first to comprehensively image and map the complex fault network beneath the lake.
Geologic and structural controls on rupture zone fabric: A field-based study of the 2010 Mw 7.2 El Mayor-Cucapah earthquake surface rupture
Orlando J. Teran et al., Centro de Investigacion Cientifica y de Educacion Superior de Ensenada, Carretera Tijuana-Ensenada, Zona Playitas, Ensenada, Baja California, México. Published online 13 May 2015; http://dx.doi.org/10.1130/GES01078.1.
From the abstract: Teran and colleagues systematically mapped (scales greater than 1:500) the surface rupture of the 4 April 2010 Mw (moment magnitude) 7.2 El Mayor-Cucapah earthquake through the Sierra Cucapah (Baja California, northwestern Mexico) to understand how faults with similar structural and lithologic characteristics control rupture zone fabric, which is here defined by the thickness, distribution, and internal configuration of shearing in a rupture zone. Fault zone thickness and master fault dip are strongly correlated with many parameters of rupture zone fabric. Wider fault zones produce progressively wider rupture zones, and both of these parameters increase systematically with decreasing dip of master faults. ... The results from this study show that the measureable parameters that define rupture zone fabric allow for testing hypotheses concerning the mechanics and propagation of earthquake ruptures, as well as for siting and designing facilities to be constructed in regions near active faults.
Complex structural and fluid flow evolution along the Grenville Front, west Texas
Ben R. Davis and Sharon Mosher, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas, USA. Published online 13 May 2015; http://dx.doi.org/10.1130/GES01098.1.
The Grenville orogeny, a Precambrian mountain-building event, affected rocks from Canada through Texas and into Mexico and is associated with the assembly of the supercontinent Laurentia. This paper examines the deformational evolution of Precambrian sedimentary rocks exposed in talc mines near the town of Van Horn, Texas. The study outlines four phases of deformation with changing stress directions and associated fluid flow along faults and proposes a tectonic model for Grenville orogenesis in Texas These rocks represent the only exposed Grenville age foreland rocks in North America and are key to understanding the evolution of the southern margin of Laurentia.
Creating three-dimensional channel bodies in LiDAR-integrated outcrop characterization: A new approach for improved stratigraphic analysis
Hiranya Sahoo, University of New Orleans, New Orleans, Louisiana, USA; and Nahid D. Gani, Western Kentucky University, Bowling Green, Kentucky, USA. Published online 14 May 2015; http://dx.doi.org/10.1130/GES01075.1.
Being a ubiquitous transporting agent on the surface of Earth, rivers carry sediments that get deposited over time and space. The occurrence of this deposition and its preservation in Earth's history are generally configured at a range of scales, to which this paper brings out novel three-dimensional anatomy of channel sand body for earth-science enthusiasts across communities. Results of this paper render significant to our socioeconomic vitality, because they not only aid to better understand landscape evolution by river dynamics, but also bring potential for industrial application like for the petroleum industry.
Provenance signature of changing plate boundary conditions along a convergent margin: Detrital record of spreading-ridge and flat-slab subduction processes, Cenozoic forearc basins, Alaska
Emily S. Finzel et al., University of Iowa, North Capitol Street, Iowa City, Iowa 52242, USA. Published online on 13 May 2015; http://dx.doi.org/10.1130/GES01029.1.
The effects of flat or shallow subduction of anomalous oceanic plates upon basins located on the upper plate of a subduction zone are investigated using zircon grains from ancient sandstones. The new data suggest that when a spreading ridge was subducted beneath southern Alaska about 50 to 60 million years ago, new sediment sources for the continental margin basins were established from far inboard and north of the present day Alaska Range. Subsequently, as an oceanic plateau was subducted beneath the margin of southern Alaska beginning about 30 million years ago, the creation of new topography above the shallow slab caused a contraction of the sediment source area that has consequently become restricted to only basin-proximal regions.
Initial rupture and displacement on the Altyn Tagh fault, northern Tibetan Plateau: Constraints based on residual Mesozoic to Cenozoic strata in the western Qaidam Basin
Feng Cheng et al., Key Laboratory of Orogenic Belts and Crustal Evolution, Peking University, Beijing, China. Published online on 13 May 2015; http://dx.doi.org/10.1130/GES01070.1.
As the major strike-slip fault on the northern Tibetan plateau, the Altyn Tagh Fault, holds important implications for unraveling the Cenozoic growth history of the entire Tibetan plateau. Despite its significance, the initial timing and kinematic patterns of movement along the Altyn Tagh Fault remain highly debated. By correlating the lithology and sedimentary features of the Cenozoic strata along the Altyn Tagh Fault, and analyzing the Mesozoic strata in three sections (the Tula, Anxi, and Caishiling) along the fault, we find that these strata record the northward migration of the Qaidam basin and can be identified as the piercing points for defining the Cenozoic offset of the Altyn Tagh Fault. Specifically, the Tula section records the early Eocene position of the western Qaidam basin (e.g., Caishiling section), while the Anxi section documents its middle Miocene position. Based on this, we estimate approx. 170 km of offset occurred between the early Eocene (ca. 49 Ma) to middle Miocene (ca. 15 Ma) epochs and approx. 190 km of offset occurred between the late Miocene epoch and the present day, yielding approx. 360 km of total offset along the Altyn Tagh Fault during the Cenozoic period. This estimate implies that motion along the Altyn Tagh Fault has accelerated in recent time from an average sinistral slip rate of approx. 5.0 mm/yr between the early Eocene (ca. 49 Ma) to middle Miocene (ca. 15 Ma) epochs to an accelerated rate of approx. 12.6 mm/yr from the middle Miocene epoch to the present day.
Mesozoic magmatism and timing of epigenetic Pb-Zn-Ag mineralization in the western Fortymile mining district, east-central Alaska: Zircon U-Pb geochronology, whole-rock geochemistry, and Pb isotopes
Cynthia Dusel-Bacon et al., U.S. Geological Survey, 346 Middlefield Road, MS 901, Menlo Park, California 94025, USA. Published online on 13 May 2015; http://dx.doi.org/10.1130/GES01092.1.
From the abstract: The Mesozoic magmatic history of the North American margin records the evolution from a more segmented assemblage of parautochthonous and allochthonous terranes to the more cohesive northern Cordilleran orogenic belt. We characterize the setting of magmatism, tectonism, and epigenetic mineralization in the western Fortymile mining district, east-central Alaska, where parautochthonous and allochthonous Paleozoic tectonic assemblages are juxtaposed, using sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon geochronology, whole-rock geochemistry, and feldspar Pb isotopes of Mesozoic intrusions and spatially associated mineral prospects. New SHRIMP U-Pb zircon ages and published U-Pb and 40Ar/39Ar ages indicate four episodes of plutonism in the western Fortymile district: Late Triassic (216-208 Ma), Early Jurassic (199-181 Ma), mid-Cretaceous (112-94 Ma), and Late Cretaceous (70-66 Ma). ... We interpret displacement on the northeast-trending faults to be a far-field effect of dextral translation along Late Cretaceous plate-scale boundaries and faults that were roughly parallel to the subsequently developed Denali and Tintina fault systems, which currently bound the region.
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