Boulder, Colo., USA - In an addition to Geosphere's ongoing themed issue series, "Geodynamics and Consequences of Lithospheric Removal in the Sierra Nevada, California," Craig H. Jones of the University of Colorado Boulder and colleagues examine the seismological study of the entire extent of the U.S. Sierra Nevada range using seismograms collected in the Sierra Nevada EarthScope field experiment from 2005 to 2007.
The southern Sierra Nevada is known to have unusually thin crust for mountains with such high elevations (peaks higher than 4 km/14,000 ft, and average elevations near 3 km/10,000 ft). Jones and his team use measurements of the arrival times of seismic waves (called P-waves) from earthquakes around the globe to image the earth under the Sierra Nevada and neighboring locations.
Their results reveal that the entire eastern Sierra overlies low-velocity upper mantle and lacks the dense, quartz-poor lower crust that they say must have existed 80 million years ago when the granites of the range were created.
Jones and colleagues write that this missing dense material probably was removed within the past 10 million years. "Previous workers," they note, "have suggested it might be within a high-velocity mantle anomaly under the southeastern San Joaquin Valley," which is "the right size to be the old, dense rock previously under the eastern Sierra."
They argue, however, that the geometry and extent of earth within the anomaly does not appear to be consistent with it being a piece of old subducted ocean floor. This would mean that a long strip of dense rock under the Sierra somehow deformed into a steeply plunging ellipsoid at the southwestern edge of the range. This conclusion suggests that the range rose within the past 10 million years as this dense material fell away to the west and south. Finally, Jones and colleagues note that something similar might be underway at the northern edge of the range.
Three other new articles posted online on 25 April are highlighted below. All GEOSPHERE articles available at http://geosphere.
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P-wave tomography of potential convective downwellings and their source regions, Sierra Nevada, California
Craig H. Jones et al., Dept. of Geological Sciences and Cooperative Institute for Research in Environmental Sciences, University of Colorado at Boulder, Colorado 80309-0216, USA. Published online 25 April 2014; http://dx.
Early Cretaceous construction of a structural culmination, Eureka, Nevada, U.S.A.: Implications for out-of-sequence deformation in the Sevier hinterland
Sean P. Long et al., Nevada Bureau of Mines and Geology, Mail Stop 178, University of Nevada, Reno, Nevada 89557, USA. Published online 25 April 2014; http://dx.
Documenting the timing relationships between deformation in the frontal and distal parts of mountain belts is fundamental for understanding the dynamics of how they are constructed. In this study, Sean P. Long and colleagues present a new geometric and timing model for deformation in the distal part of the Mesozoic Cordilleran mountain belt in central Nevada. The model is based on new geologic mapping, cross-sections that illustrate the modern and pre-deformed geometry, new age dates from volcanic and sedimentary rocks, and assessment of regional field relationships. Their results reveal that Cretaceous deformation was taking place in the distal part of the mountain belt in Nevada at the same time as deformation in the frontal part in Utah. Long and colleagues use this new timing relationship, along with the predictions of models for the dynamic behavior of the construction of mountain belts, to propose that this deformation in Nevada and Utah was genetically linked.
Paleomagnetic results from the eastern Caliente-Enterprise zone, southwestern Utah: Implications for initiation of a major Miocene transfer zone
Michael S. Petronis et al., Environmental Geology, Natural Resource Management Dept., New Mexico Highlands University, Las Vegas, New Mexico 87701, USA. Published online 25 April 2014; http://dx.
The Caliente-Enterprise zone (CEZ) in southwestern Utah (USA) is a 20-50-km-wide, east-northeast-trending, left lateral transfer zone that displaces north-south-trending crustal blocks of the eastern Basin and Range Province to the west. Results of recent detailed geologic mapping and new geochronologic data in the area allow us to extend previous paleomagnetic studies into the easternmost CEZ. New paleomagnetic results reveal significant components of counterclockwise vertical axis rotation. If the rotation estimates are viable, authors Michael S. Petronis and colleagues suggest that this component of deformation involves much of the upper crust, and they furthermore propose that the boundary of the eastern CEZ extends farther east than previously envisioned to within a few kilometers of the breakaway with the Colorado Plateau. The transitional zone between the eastern CEZ and Colorado Plateau is therefore abrupt and occurs within a narrow zone near Cedar City, Utah. As this part of the Hurricane Fault system remains active, they postulate a present-day seismic hazard risk based on ancient faulting and crustal deformation dating back millions of years.
Influence of pre-Andean history over Cenozoic foreland deformation: Structural styles in the Malargüe fold-and-thrust belt at 35° S, Andes of Argentina
José F. Mescua et al., IANIGLA (Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales), Centro Científico Tecnológico Mendoza, Av. Ruiz Leal s/n, Parque General San Martín, Mendoza, 5500, AP 330, Argentina. Published online 25 April 2014; http://dx.
This work studies the tectonic evolution of a transect across the Andes of Mendoza, Argentina (35 degrees S) using a combination of structural geology and geophysical techniques. The authors describe the structural styles displayed by this Andean segment, reconstruct the crustal thickness along the transect prior to the uplift of the Andes, and compare the tectonic shortening and present crustal features at the regional scale. The results underscore the importance of the reactivation of old structures in the Andes.