|25 Sept. 2012
GSA Release No. 12-70
October LITHOSPHERE Delivered Online
Boulder, Colo., USA – The October issue of Lithosphere covers geology in Wyoming, USA; the California Coast Ranges, USA; the Alpine Fault, New Zealand; the South Atlantic seafloor; the central Himalaya in Nepal; and Sidekan, Kurdistan Region, Iraqi Zagros suture zone. Topics and methods include tectonics, orogeny, hazards, paleogeography, trigonometrics, multiple-point data analysis, LiDAR, oceanic isostasy, computer modeling, and spectroscopy.
LITHOSPHERE is the newest bimonthly publication of The Geological Society of America, printing in February, April, June, August, and October, and December each year. Abstracts are available at lithosphere.gsapubs.org. Representatives of the media may obtain complimentary copies of LITHOSPHERE articles by contacting Kea Giles at .
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The impact of vertical-axis rotations on shortening estimates
Aviva J. Sussman et al., Earth and Environmental Sciences, MS-D443, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA. First published on 7 Aug. 2012, doi: 10.1130/L177.1
Aviva Sussman and colleagues define the components of the total displacement field, describe the diagnostic and suggestive features associated with vertical-axis rotations, and apply trigonometric map-view calculations to estimate the amount of shortening contributed by such rotations. They then apply their approach to the Wyoming salient and show that previous estimates of shortening there may contain up to 14% error.
Superimposed extension and shortening in the southern Salinas Basin and La Panza Range, California: A guide to Neogene deformation in the Salinian block of the central California Coast Ranges
Joseph P. Colgan, 345 Middlefield Road, MS 973, U.S. Geological Survey, Menlo Park, California 94025, USA. Lithosphere, October 2012, v. 4, p. 411-429, first published on August 7, 2012, doi: 10.1130/L208.1
Joseph P. Colgan and colleagues synthesized data from geologic maps, wells, seismic-reflection profiles, potential-field interpretations, and low-temperature thermochronology to refine current understanding of late Cenozoic extension and shortening in the Salinian block of the central California Coast Ranges. The central California Coast Ranges are dominated by faults of the San Andreas system, and were formed by ongoing crustal shortening in an approx. 100-km-wide zone stretching from the San Joaquin Valley to the coast across the San Andreas fault.
Scale dependence of oblique plate-boundary partitioning: New insights from LiDAR, central Alpine fault, New Zealand
Nicolas C. Barth et al., Southern Illinois University, Geology, 1249 Lincoln Dr., MC 4324, Carbondale, IL 62958, USA. Lithosphere, October 2012, v. 4, p. 435-448, doi: 10.1130/L201.1
In the past 10 years, airborne light detection and ranging (LiDAR) technology has become an effective tool for identifying active faults and assessing seismic hazards in a wide range of environments, including urban areas, grassland and scrub, and densely vegetated forests. In particular, the ability of LiDAR to image the land surface beneath thick vegetative covers at a resolution of centimeters to meters has allowed the topographic features of these landscapes to be examined in revolutionary new ways. The prospect of a 30% chance of a surface-rupturing, approx. magnitude 8 Alpine fault earthquake in the next 50 years poses a significant hazard in New Zealand, and, according to researchers Nicolas Barth and colleagues, a better understanding of surface rupturing along the Alpine fault is sorely needed. By incorporating interpretations of newly acquired LiDAR data with aerial photo interpretation and decades of previous geologic mapping, Barth and colleagues explore the significance of several orders of magnitude of shallow transpressional partitioning observed on the oblique-slip central Alpine fault and propose a new model for the geometry of structures observed at multiple scales.
Non-Pratt component of oceanic isostasy
James A. Conder, Southern Illinois University, Geology, 1249 Lincoln Dr., MC 4324, Carbondale, IL 62958, USA. First published 5 Sept. 2012, doi: 10.1130/L229.1
For the past 40 years, oceanic lithosphere has been understood to cool and subside away from mid ocean ridges to a Pratt-like isostasy condition. However, James Conder shows that in the presence of an overlying fluid, a dynamic response within the Earth's mantle is also necessary to reach equilibrium. Similar to an icecap on a continent, the addition of seawater on top of subsiding lithosphere drives a small degree of flow in the asthenosphere to accommodate the excess mass accumulated on top. The mantle flow is systematically driven from beneath younger seafloor towards older seafloor. This new understanding of seafloor subsidence means that while about two-thirds of oceanic isostasy is accommodated by Pratt isostasy, the remaining one-third must be accommodated by mantle flow.
Pulsed deformation and variable slip rates within the central Himalayan thrust belt
Delores M. Robinson, Dept. of Geological Sciences, University of Alabama, Tuscaloosa, Alabama 35487 USA; and Nadine McQuarrie. First published on 5 Sept. 2012; doi: 10.1130/L204.1
The Himalaya are the highest and most imposing mountains on the surface of the planet. Glaciers and snow cover the high Himalaya, while forests and steep ravines define the regions immediately to the south. Understanding how these mountains evolved over time gives us a fundamental connection to how they are currently growing and changing. Delores Robinson and Nadine McQuarrie use data from years of field work in remote far western Nepal in combination with computer modeling to determine the evolution of the Himalayan Mountains from 25 million years ago to the present. Because erosion constantly removes rock from the growing mountain range, documenting the rocks, structure and topography millions of years ago is daunting. However, some of these eroded rocks become stored in sedimentary basins and provide data for understanding when faults moved, rocks cooled and sediment was deposited. Robinson and McQuarrie use these data to construct models that provide an estimate for rates of erosion and deformation for seven different time frames. They concluded that averaged over millions of years, the rate of erosion was essentially constant. However, the rate of deformation varied through time. Highest rates of deformation occurred on large faults that accommodated 100-plus km of motion between the colliding Indian and Asian continents at about 16 to 25 and about 10 to 13 million years ago. Even though many researchers assume an average rate of deformation, Robinson and McQuarrie found that there are periods of time when deformation was fast and periods of time when deformation was slow.
Recognition of Late Cretaceous Hasanbag ophiolite-arc rocks in the Kurdistan Region of the Iraqi Zagros suture zone: A missing link in the paleogeography of the closing Neotethys Ocean
S.A. Ali et al., School of Earth and Environmental Sciences, University of Wollongong, Wollongong, NSW 2522, Australia. First published on 4 June 2012, doi: 10.1130/L207.1
This paper by S.A. Ali of the University of Wollongong and colleagues is (a) innovative because it provides first evidence of a "missing" Cretaceous arc assemblage in the Iraqi segment of the Zagros orogenic belt; (b) provocative because it challenges current ideas concerning Zagros evolution and anatomy; and (c) timely because there is much current literature on the neighboring Iranian segment of the Zagros orogen, whereas new information from Iraq is lacking.