Geosphere examines volcanic zones, the Sierra Nevada, and Utah's Confusion Range

(Press-News.org) Contact information: Kea Giles
kgiles@geosociety.org
Geological Society of America
Geosphere examines volcanic zones, the Sierra Nevada, and Utah's Confusion Range Boulder, Colo., USA – New Geosphere papers posted online 14 Jan. cover the San Joaquin Basin in California, the Catalan Volcanic Zone in Spain, the Taupo Volcanic Zone of New Zealand, the Confusion Range of west-central Utah, and the southern U.S. Sierra Nevada. Two articles add to the "Geodynamics and Consequences of Lithospheric Removal in the Sierra Nevada, California" collection. The paper on the Confusion Range may provide a basis for new petroleum exploration in the region.

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Pliocene–Quaternary subsidence and exhumation of the southeastern San Joaquin Basin, California, in response to mantle lithosphere removal
M. Robinson Cecil et al., Tectonics Observatory at the California Institute of Technology, Pasadena, California 91125-2300, USA; robinson.cecil@csun.edu. Posted online 14 Jan. 2014, http://dx.doi.org/10.1130/GES00882.1. Special issue: Geodynamics and Consequences of Lithospheric Removal in the Sierra Nevada, California.

From the abstract: Thermomechanical models of mantle lithosphere removal from beneath the southern Sierra Nevada region, California, predict a complex spatiotemporal pattern of vertical surface displacements. M. Robison Cecil and colleagues evaluate these models by using (U-Th)/He thermochronometry, together with other paleothermometry estimates, to investigate such topographic transients. We target Tertiary strata from the Kern arch, a crescent-shaped active uplift located in the southeastern San Joaquin Basin, along the western flank of the southern Sierra Nevada.

Phreatomagmatic volcanism in complex hydrogeological environments: La Crosa de Sant Dalmai maar (Catalan Volcanic Zone, NE Spain)
Dario Pedrazzi et al., Institute of Earth Sciences Jaume Almera, ICTJA-CSIC, Group of Volcanology, SIMGEO (UB-CSIC) Lluis Sole i Sabaris s/n, 08028 Barcelona, Spain; dpedrazzi@ictja.csic.es. Posted online 14 Jan. 2014, http://dx.doi.org/10.1130/GES00959.1.

The volcano of La Crosa de Sant Dalmai (northeastern Iberian Peninsula) formed on the northern border of La Selva Basin. It is the result of a single eruptive episode characterized by four consecutive eruptive phase with alternating magma-water interaction. This volcano is an example of the way in which the edifice develops in a mixed setting characterized by a hard and soft basement with heterogeneities and differences in the hydrogeological structure of the area, and aquifer levels with different hydraulic properties and fracturing patterns. These differences clearly influenced the way in which the magma-water interaction occurred throughout the eruption and, consequently, the style of the eruption and the resulting deposits. The results obtained here can be extrapolated to other phreatomagmatic volcanoes of similar characteristics, determining the eruptive style and associated hazards in this type of volcanism.

Evolution of the intra-arc Taupo-Reporoa Basin within the Taupo Volcanic Zone of New Zealand
D.T. Downs et al., School of Environment, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand; d.downs@auckland.ac.nz; ddowns84@hotmail.com. Posted online 14 Jan. 2014, http://dx.doi.org/10.1130/GES00965.1.

Our research highlights the variety of processes that are occurring in the world's most active volcanic arc: the Taupo Volcanic Zone. Some of the processes, such as tectonism, volcanism, and magmatism, are related in ways that are not fully understood, particularly as to how they affect basins that host economic and cultural assets. D.T. Downs and colleagues undertook field and laboratory studies to understand the rocks in the region and the rates at which the tectonic, volcanic, and magmatic processes occur. Their study shows that these arc processes have resulted in faster than normal basin development during eruptive episodes and slow evolution during non-volcanic episodes. Some parts of the basin also tend to undergo infilling and migration that are not uniform through time or across the entire region. This research shows that there is a lot that geoscientists still do not understand about the way that the Taupo Volcanic Zone operates or the rates at which basins may form.

The Confusion Range, west-central Utah: Fold-thrust deformation and a western Utah thrust belt in the Sevier hinterland
David C. Greene, Department of Geosciences, Denison University, Granville, Ohio 43023, USA; greened@denison.edu. Posted online 14 Jan. 2014, http://dx.doi.org/10.1130/GES00972.1.

A new study by David C. Greene of the Confusion Range in west-central Utah indicates that sedimentary rocks in the range are deformed in a zone of concentrated folding and thrust faulting that resulted in about six miles (10 km) of east-directed horizontal shortening. The Confusion Range is part of a newly recognized zone of deformation termed the western Utah thrust belt that is at least 80 miles (130 km) in length. The rock types and structures in the thrust belt are similar to areas of petroleum production in central Utah, and the new structural interpretations presented in this paper may provide a basis for new petroleum exploration in the region.

Internal deformation of the southern Sierra Nevada microplate associated with foundering lower lithosphere, California
Jeffrey Unruh et al., Lettis Consultants International, Inc., 1981 North Broadway, Suite 330, Walnut Creek, California 94596, USA; unruh.jeff@gmail.com. Posted online 14 Jan. 2014, http://dx.doi.org/10.1130/GES00936.1. Special issue: Geodynamics and Consequences of Lithospheric Removal in the Sierra Nevada, California.

From the abstract: Quaternary faulting and background seismicity in the southern Sierra Nevada microplate are concentrated east and south of the Isabella anomaly, a high-velocity body in the upper mantle interpreted to be lower Sierra lithosphere that is foundering into the asthenosphere. Jeffrey Unruh and colleagues analyze seismicity in this region to evaluate patterns of upper crustal deformation above and adjacent to the Isabella anomaly. Earthquakes in the southern Sierra and San Joaquin Valley were relocated using joint hypocentral inversion and double-difference techniques, and groups of focal mechanisms were inverted for the components of a reduced deformation rate tensor. The deformation field derived from this analysis reveals two distinct departures from horizontal plane strain associated with distributed northwest-directed dextral shear east of the Pacific plate: (1) heterogeneous extension and crustal thinning in the high Sierra and western foothills east of the Isabella anomaly; and (2) pronounced counterclockwise rotation of the principal strains from regional trends in the southwestern Sierra Nevada and across the Kern Arch. Based on comparison with a three-dimensional tomographic model, the extension in the southern Sierra is spatially associated with relatively thinner crust and anomalous low P-wave speeds in the upper mantle (40󈟆 km depth range) directly east of the Isabella anomaly. These relations suggest that seismogenic crustal thinning is localized above upwelling asthenosphere that is replacing foundering lithosphere.

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