|31 July 2009
GSA Release No. 09-38
Director of Education, Communication, & Outreach
AUGUST 2009 LITHOSPHERE HIGHLIGHTS
Boulder, CO, USA - The fourth issue of LITHOSPHERE is now available online and in print. Articles ask and answer four main questions: "What’s going on beneath the Juan de Fuca slab?"; "What's down the SAFOD borehole?"; "What did the long-term borehole strain records of the Parkfield Earthquake Prediction Experiment record?"; and "What are the mechanisms responsible for map-view curvature over a range of scales for most active and ancient mountain belts?"
Highlights are provided below, and abstracts for the issue are available for review at http://lithosphere.gsapubs.org/content/1/4. Representatives of the media may obtain complementary copies of articles by contacting Christa Stratton at . Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to LITHOSPHERE in articles published. Contact Christa Stratton for additional information or assistance.
Non-media requests for articles may be directed to GSA Sales and Service, .
Subducted oceanic asthenosphere and upper mantle flow beneath the Juan de Fuca slab
Raymond M. Russo, University of Florida, Geological Sciences, P.O. Box 112120, 241 Williamson Hall, Gainesville, Florida 32611, USA. Pages 195-205.
Oceanic plates are underlain by a weak layer -- the asthenosphere -- characterized by slow seismic velocities from ~100-250 km depth, but what happens to this asthenosphere when the oceanic plate above it subducts? Seismic waves from earthquakes in the Juan de Fuca plate, now subducting beneath North America just offshore of Washington and Oregon states, show that the asthenosphere beneath the Juan de Fuca slab develops two distinct layers: one with mantle flow trends parallel to the subduction trench, and a second, deeper layer with flow fabrics parallel to the motion of the Juan de Fuca plate. The upper mantle flow layer parallel to the Juan de Fuca subduction trench must develop when the lithosphere subducts, probably due to compression of the weak asthenosphere as it is overridden by North America.
Arkosic rocks from the San Andreas Fault Observatory at Depth (SAFOD) borehole, central California: Implications for the structure and tectonics of the San Andreas fault zone
Sarah Draper Springer et al., 4505 Old Main Hill, Logan, Utah 84322-4505, USA. Pages 206-226.
Springer et al. marshal a wide range of data collected in the San Andreas Fault Observatory at Depth (SAFOD) project, part of the National Science Foundation's Earthscope program, to determine the nature and source of a sequence of rocks encountered in the borehole. The SAFOD borehole is a vertical hole for 2.2 km, and it then bends to the northeast and intersects the San Andreas fault at a depth of 3.2 km. Along the inclined part of the hole there is a nearly 1-km-wide section of sandstones and conglomerates, the presence of which was not predicted in the geologic models of the area. Springer et al. use geologic studies of the rock cuttings, evidence from geophysical tools in the hole, and a dating method to suggest that these rocks are part of a sequence of 65-55-million-year-old sedimentary rocks that were deposited across much of western California while the area was the coastal margin. These rocks were subsequently offset as the granitic rocks west of the San Andreas fault rotated into place and by slip along the San Andreas fault and related faults.
Parkfield revisited: I. Data retrieval
Cinna Lomnitz, Universidad Nacional Autonoma de Mexico, Instituto de Geofisica, Ciudad Universitaria, Mexico, D.F. 04510, Mexico; and Chao-jun Zhang. Pages 227-234.
The Parkfield Earthquake Prediction Experiment (1985-2004) was designed to monitor stress accumulation in the lithosphere related to an impending earthquake on the San Andreas fault. However, no precursory signals were detected prior to the 2004 Parkfield earthquake (M6.0). In this paper, Lomnitz and Zhang reexamine the long-term borehole strain records at Parkfield. They find that they are consistent with a stationary tectonic stress field on the order of 55 MPa in the direction of the fault. This is the first measurement of far-field tectonic stresses from borehole strainmeter records. It suggests that logarithmic creep strains from boreholes can be used to interpret the state of stress in the lithosphere surrounding the San Andreas fault.
Anisotropy of magnetic susceptibility in weakly deformed red beds from the Wyoming salient, Sevier thrust belt: Relations to layer-parallel shortening and orogenic curvature
Arlo B. Weil, Bryn Mawr College, Dept. of Geology, 101 North Merion Ave., Bryn Mawr, Pennsylvania 19010, USA; and Adolph Yonkee. Pages 235-256.
Most active and ancient mountain belts display map-view curvature over a range of scales, yet mechanisms responsible for developing such curvature remain incompletely understood. Determining the origins of curved mountain belts is critical for understanding the tectonic and paleogeographic evolution of continents. At the root of this problem is when and how mountains acquire curvature during complex and protracted deformation histories. By integrating anisotropy of magnetic susceptibility (AMS), structural, and paleomagnetic studies in the Wyoming salient of the Sevier mountain belt Weil and Yonkee have been able to constrain its 3-D kinematic evolution and interpret processes responsible for producing the belt's present-day architecture. The Wyoming salient began with minor primary curvature, which then underwent progressive secondary rotation penecontemporaneous with mountain building. Rotation was related to curvature of fault slip directions, differential shortening, and wrenching. Processes that gave rise to this kinematic evolution include (1) variations in initial thickness and strength of foreland basin-fill stratigraphy, (2) feedback with basins that were formed in front of, and eventually incorporated into, the growing mountain belt, and (3) interaction with foreland uplifts along the salient ends. Weil and Yonkee's approach of integrating AMS, regional structural, and paleomagnetic studies can be applied to other mountain belts to better understand the processes that produce curved orogens.