Boulder, Colo., USA - Highlights include several studies based in the U.S. Sierra Nevada, including a description of "magma fingers" and the formation of granite in the high Sierra crest near Yosemite National Park. Other studies investigate knickzones in the South Fork of the Eel River, California; the Rodgers Creek-Maacama fault system in the northern California Coast Ranges and its relation to the San Andreas fault; and the frequency and severity of destructive debris flows in the Pacific Northwest.
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Formation and transfer of stoped blocks into magma chambers: The high temperature interplay between focused porous flow, cracking, channel flow, host rock anisotropy, and regional deformationScott R. Paterson et al., Dept. of Earth Sciences, University of Southern California, Los Angeles, California 90089, USA. Posted online 15 Feb. 2012; doi: 10.1130/GS680.1.
When rocks melt at depth in the earth and form magma, this magma rises through the overlying crust, during which time it must displace the preexisting crust to make a pathway for the magma. One means of doing so is called magmatic stoping -- the formation of and movement through magma of older host rock pieces. However, the prevailing view that stoping occurs by rapid heating, thermal shattering, and collapse into magmas may not be the dominant process of block formation and displacement in the now frozen magma represented by plutonic rocks in the Sierra Nevada, California. Detailed studies in the Sierra Nevada by Scott R. Paterson of the University of Southern California at Los Angeles and colleagues suggest that block formation often occurs by the following incremental process: (1) low stress sites develop in host rock leading to planar zones of increased porosity; (2) focused porous flow of first felsic melts followed by intermediate melts occur in these higher porosity zones leading to growth of "magma fingers," which in turn lead to increased porosity and loss of host rock cohesion; and (3) connection of magmatic "fingers," resulting in the formation of dike-like channels in which magma flow facilitated removal of all host rock material in these planar zones. Once formed, blocks were initially displaced by repeated magma injections along these channels often resulting in unidirectional growth of these zones creating local magmatic, sheeted complexes along block margins. Free block rotation occurred when sufficient non-layered magma surrounded the host block: in some cases segments of former sheeted zones remain attached to rotated blocks. Thus, this study indicates that a number of processes may drive block formation, some of which are rapid, such as thermal shattering, and others that occur over longer durations, such as incremental magma pulsing and formation of sheeted complexes and regional deformation.
Knickpoint and knickzone formation and propagation, South Fork Eel River, northern CaliforniaMelissa A. Foster, Dept. of Geological Sciences and Institute for Arctic and Alpine Research (INSTAAR), Campus Box 450, University of Colorado, Boulder, Colorado 80309, USA; and Harvey M. Kelsey. Posted online 15 Feb. 2012; doi: 10.1130/GS700.1.
The South Fork of the Eel River, California, USA, displays a prominent steep section (knickzone) in the upper portion of the channel, which deviates from the expected geometry of a river in equilibrium. This peculiarity could represent a perturbation to the system that migrates upstream with time. Authors Melissa A. Foster of the University of Colorado at Boulder and Harvey M. Kelsey of Humboldt State University examine two tributaries to the South Fork Eel River, located downstream from this steep section, to document the presence of knickzones within tributary basins and explore their correlation to the South Fork Eel River knickzone. In the study area, Standley and Bear Pen Creeks, Foster and Kelsey identify 107 major knickzones. The distribution of knickzones throughout the two basins indicates that the channels are responding to an event of base-level fall along the South Fork Eel River. However, most of the knickzones they identify in Standley and Bear Pen Creeks do not correlate with the current, prominent knickzone along the South Fork Eel River. Foster and Kelsey believe knickzone distribution within the study area indicates that there have been multiple instances of base-level fall along the South Fork Eel River.
Petrogenetic connections between ash-flow tuffs and a granodioritic to granitic intrusive suite in the Sierra Nevada arc, CaliforniaA.P. Barth et al., Dept. of Earth Sciences, Indiana University-Purdue University Indianapolis, 723 West Michigan Street, Indianapolis, Indiana 46202, USA. Posted online 15 Feb. 2012; doi: 10.1130/GS737.1.
Magmatic processes at convergent plate margins lead to the formation of explosive volcanoes and ultimately to the formation of granite, hidden from view deep within the volcanic plumbing system. Along the high Sierra crest east of Yosemite National Park in California, a tilted section of the crust has exposed the volcanic superstructure and granite core of an ancient explosive volcano, providing a crucial window into the volcanic plumbing system. Field and geochemical evidence shows that the formation of granite common to the Sierra Nevada was part of the cyclical evolution of a long-lived magma system that also fed explosive volcanic eruptions, blanketing the landscape with ash and pumice. This part of the Sierra Nevada provides an important region for understanding the deep roots of magmatic systems that created the granitic mountains of California and feed explosive eruptions on the modern Earth.
Evolution of the Rodgers Creek-Maacama right-lateral fault system and associated basins east of the northward-migrating Mendocino Triple Junction, northern CaliforniaRobert J. McLaughlin et al., U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA. Posted online 15 Feb. 2012; doi: 10.1130/GS682.1.
The Rodgers Creek-Maacama fault system in the northern California Coast Ranges takes up substantial right-lateral motion within the wide transform boundary between the Pacific and North America plates, over a slab window that has opened northward beneath the Coast Ranges. This study by Robert J. McLaughlin of the U.S. Geological Survey and colleagues with the California Geological Survey traces the evolution of this fault system with time since its inception about seven million years ago and establishes amounts and rates of long term slip for the individual fault members and the contribution of the system as a whole to the long-term displacement history of the transform margin east of the San Andreas fault. The fault system traverses Neogene volcanic rocks of the Tolay and Sonoma volcanic fields, and their eruption histories span the timing of fault system initiation and reorganizations. This relation allows McLaughlin and colleagues to use geochronology and tephrochronology of the volcanics to constrain fault displacements and slip rates as well as to constrain the time of formation and deformation of depositional basins formed during faulting. Their study establishes that the Rodgers Creek-Maacama fault system has contributed at least 44-53 km of right-lateral displacement to the East Bay fault system south of San Pablo Bay since seven million years ago at a minimum rate of 6.1 to 7.8 mm/yr. McLaughlin and colleagues suggest that frequent along-strike and successional changes in fault and basin geometries seen in the Rodgers Creek-Maacama fault system are a response to ongoing adjustments associated with lengthening of the transform margin in the Mendocino Triple Junction region far to the north, and to northward transit of a major releasing bend in the northern San Andreas fault itself.
Early Cenozoic topography, morphology, and tectonics of the northern Sierra Nevada and western Basin and RangeElizabeth J. Cassel et al., Dept. of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas 78712, USA. Posted online 15 Feb. 2012; doi: 10.1130/GS671.1.
Debate surrounds the past elevations and recent evolution of the northern Sierra Nevada and western Basin and Range in California and Nevada. Elizabeth J. Cassel of The University of Texas at Austin and colleagues present a number of methods that integrate different scales of observation, from local studies of ancient river valleys and their sediments to regional studies of the sources of those sediments and past elevations, at 48 to 25 million years ago, across northern California and Nevada, to gain a better understanding of the landscape evolution of the region and to assess the possible plate tectonic and climatic drivers for that evolution. Changes in plate tectonics in western North America, primarily, along with fluctuations in sea level and climate, drove cyclic deposition of river and floodplain sediments in the Sierra Nevada from 48 to 33 million years ago. Volcanic outflow ashes, erupted from volcanoes in central Nevada between 31 and 25 million years ago, prove that ancient rivers drained west across what is now the crest of the Sierra Nevada. Glass from these ashes recorded the variation in the hydrogen isotopic composition of ancient precipitation (31-28 million years ago), which is affected by topography. The variation in isotopic compositions across the northern Sierra Nevada suggest that the region had elevations similar to or slightly higher than the modern range, up to 3.0 plus or minus 0.3 kilometers. Isotopic compositions across the Basin and Range indicate that there was either a gradual increase or little change in mean elevation from west to east across that region, suggesting the existence of an ancient high elevation plateau, similar to the modern Andean Altiplano, with a steep western flank and elevations of 3-3.5 km.
Wilson cycles, tectonic inheritance, and rifting of the North American Gulf of Mexico continental marginAudrey D. Huerta, Dept. of Geological Sciences, Central Washington University, Ellensburg, Washington 98926-7418, USA; and Dennis L. Harry. Posted online 6 Mar. 2012; doi: 10.1130/GS725.1.
Continental rifting associated with breakup of the ancient supercontinents Pangea and Gondwana led to formation of the Gulf of Mexico between 215 and 165 million years ago. On the northern Gulf margin, as on many passive continental margins, rifting followed the trend of an older and largely eroded mountain system, the nearly 315-million-year-old Ouachita orogen. The Ouachita orogen is exposed in the Ouachita Mountains of western Oklahoma and eastern Arkansas and in the Marathon Uplift in west Texas. Between west Texas and Oklahoma and between Arkansas and southern Alabama the Ouachita orogen is buried in the subsurface. The Ouachita orogen follows the trend of and older continental rift system and an even older orogen. Audrey D. Huerta of Central Washington University and Dennis L. Harry of Colorado State University use Finite Element computer simulations of continental rifting to explore how preexisting tectonic features such as the Ouachita orogen influenced the style of rifting during opening of the Gulf of Mexico. Their computer models show that the major features of the northern Gulf continental margin and coastal plain are directly related to the presence of a fossil subduction zone and accreted volcanic arc present beneath the Ouachita orogen. These features include the Interior Salt Basin, outboard unextended Wiggins Arch, and the unusually broad region of extension beneath the coastal plain and continental shelf.
Periglacial debris-flow initiation and susceptibility and glacier recession from imagery, airborne LiDAR and ground-based mappingStephen T. Lancaster et al., College of Earth, Ocean, and Atmospheric Sciences and Institute for Water and Watersheds, Oregon State University, Corvallis, Oregon 97331, USA. Posted online 6 Mar. 2012; doi: 10.1130/GS713.1.
Climate changes in the Pacific Northwest, USA, may cause both retreat of alpine glaciers and increases in the frequency and magnitude of storms delivering rainfall at high elevations absent significant snowpack. Both of these changes may affect the frequency and severity of destructive debris flows (i.e., rapidly flowing mixtures of mud, boulders, and water) initiating at high elevations on the region's tallest volcanoes. A better understanding of debris-flow susceptibility on the slopes of these volcanoes is therefore warranted. Stephen T. Lancaster of Oregon State University and colleagues used field mapping and remote sensing data, including airborne laser altimetry, to locate and characterize initiation sites of six debris flows that occurred during a so-called "atmospheric river" event (i.e., a warm wet storm) on Mount Rainier, Washington, in November 2006. They also analyzed data from prior studies to identify six more debris flows that occurred in 2001 to 2005. All of these debris flows had initiation sources at the heads of gullies near the termini of glaciers, and all debris-flow initiation sites were located within areas exposed by glacier retreat in the last century. Pre- and post-2006 gully width measurements from remote sensing, field observations of gully banks, and elevation changes calculated from repeated laser altimetry all indicate that debris flows were initiated by distributed sources, including bank mass failures, related to erosion by overland flow of water. The initiation sites were found to occupy restricted ranges of simulated glacial meltwater flow, slope angle, and minimum distance to an area of recent (1994 to 2008) glacier retreat. A model based on similarity to mapped initiation sites identifies the heads of most gullies, including all sites of known debris-flow initiation, as high-susceptibility areas, but the current version does not appear to differentiate between areas of varying gully-head density or between debris-flow and no-debris-flow gullies. The model and field data, despite limitations, do provide insight into debris-flow processes, as well as feasible methods for mapping and assessment of debris-flow susceptibilities on areas nearby glaciers in the Cascade Range.
The geology of the Tama Kosi and Rolwaling valley region, East-Central NepalKyle P. Larson, Dept. of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Saskatchewan, S7N 5E2, Canada. Posted online 6 Mar. 2012; doi: 10.1130/GS711.1.
The Tama Kosi and Rolwaling area of east-central Nepal is underlain by the exhumed mid-crustal core of the Himalaya. The geology of the area consists of Greater Himalayan sequence phyllitic schist, paragneiss, and orthogneiss that generally increase in metamorphic grade from biotite or garnet assemblages to sillimanite-grade migmatite up structural section. All metamorphic rocks are pervasively deformed and commonly record top-to-the-south sense shear. The top of the Greater Himalayan sequence in the mapped area is marked by an undeformed, pegmatitic leucogranite stock. Researcher Kyle P. Larson of the University of Saskatchewan notes that the geology of the mapped area appears similar to that observed in the adjacent, better-studied Everest region.
A terrestrial LiDAR-based workflow for determining three-dimensional slip vectors and associated uncertaintiesPeter O. Gold et al., Dept. of Geology, University of California, Davis, California 95616, USA. Posted online 6 Mar. 2012; doi: 10.1130/GS714.1.
Three-dimensional (3-D) slip vectors recorded by displaced landforms are difficult to constrain across complex fault zones, and the uncertainties associated with such measurements become increasingly challenging to assess as landforms degrade over time. Peter O. Gold of the University of California at Davis and colleagues approach this problem from a remote sensing perspective by using terrestrial laser scanning (TLS) and 3-D structural analysis. They have developed an integrated TLS data collection and point-based analysis workflow that incorporates accurate assessments of aleatoric and epistemic uncertainties using experimental surveys, Monte Carlo simulations, and iterative site reconstructions. In a case study, they measured slip vector orientations at two sites along the rupture trace of the 1954 Dixie Valley earthquake (central Nevada, United States), yielding measurements that are the first direct constraints on the 3-D slip vector for this event. These observations are consistent with a previous approximation of net extension direction for this event.
Cenozoic volcanism in the Sierra Nevada and Walker Lane, California, and a new model for lithosphere degradationKeith Putirka et al., Dept. of Earth and Environmental Sciences, California State University, 2576 E. San Ramon Avenue, MS/ST24, Fresno, California 93740-8039, USA. Posted online 6 Mar. 2012; doi: 10.1130/GS728.1.
Volcanic rock and mantle xenolith compositions in the Sierra Nevada (western United States) contradict a commonly held view that continental crust directly overlies asthenosphere beneath the Sierran range front, and that ancient continental mantle lithosphere was entirely removed in the Pliocene. Instead, space-time trends show that the Walker Lane is the principle region of mantle upwelling and lithosphere removal in eastern California, that lithosphere loss follows the migration of the Mendocino Triple Junction, and that the processes of lithosphere removal are not yet complete beneath the Sierra Nevada and its range front. Keith Putirka of California State University and colleagues present key evidence by analysis of volcanic rock compositions and propose a new model of lithosphere degradation, where asthenosphere or its partial melts pervasively invade the continental mantle lithosphere beneath the Walker Lane. They note that this process is now nearly complete beneath Coso, and is migrating west, so that it is only partly complete at the southern Sierra range front, or within the Sierra Nevada, at any latitude. According to Putirka and colleagues, this model of intermixed asthenosphere and lithosphere better explains the compositions of volcanic rocks and their included xenoliths, and the remarkably consistent S-wave receiver function data, which show a 70-km-thick lithosphere beneath the Sierra Nevada. If the upper mantle is warm CML, permeated by partial melts, this model may also explain low P- and S-wave velocities.
Source: Geological Society of America