December 2010 Lithosphere Highlights
The December 2010 Lithosphere analyzes tectonic histories across the Llano Uplift, Texas; activity along the ~85-mile-long Kern Canyon fault, southern Sierra Nevada; deformed mantle materials in the Twin Sisters ultramafic body of Washington State; a giant granitic intrusion called the Sahwave Intrusive Suite near Reno, Nevada; the Socorro Magma Body, New Mexico; gravity anomalies on and offshore of the Antarctic continent; and the shallow upper mantle stratification of the “Lehmann” and “X” discontinuities.
Contrasting Grenville-aged tectonic histories across the Llano Uplift, Texas: New evidence for deep-seated high-temperature deformation in the western uplift
J.S.F. Levine and S. Mosher, Dept. of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas 78712, USA. Pages 399-410.
The Llano Uplift of central Texas, with rocks over a billion years in age, is comprised of two portions (east and west) which have experienced different tectonic histories. The entire uplift was involved in a continent-continent collision, but the western uplift is interpreted to have been at greater depth in the Earth’s crust at the time of this collision, based on observations during geologic mapping as well as from looking at rocks under the microscope. As a result, the rocks record higher temperatures in the western portion and in some areas they even started to melt. The two portions of the uplift also indicate different directions of movement, with the eastern and western portions showing opposite orientations of folds and overall movements during collision. As a result of these differences, Levine and Mosher suggest that this area may have experienced a tectonic history similar to the Alps with two areas at different depths in the Earth’s crust with opposite directions of motion during a continent-continent collision.
Late Quaternary slip rate on the Kern Canyon fault at Soda Spring, Tulare County, California
Colin B. Amos et al., William Lettis & Associates, Inc., Walnut Creek, California 94596, USA. Pages 411-417.
Colin B. Amos of William Lettis & Associations and colleagues present new results on recent fault activity along the Kern Canyon fault, which stretches for roughly 85 miles north-south within the southern Sierra Nevada mountains of California. This work is part of a larger study of the fault commissioned by the U.S. Army Corps of Engineers (USACOE) Dam Safety Assurance Program to define earthquake hazards to the dams that impound Lake Isabella. The two dams that form this lake provide flood control and water to the southern San Joaquin Valley and the city of Bakersfield, California. To aid in this study, the USACOE commissioned an airborne topographic survey of the fault zone, which utilized state of the art laser-ranging, called LiDAR, to image the fault zone in areas of dense vegetation and rugged terrain typical of the remote Kern Canyon. This survey provided critical information on the location of previously undiscovered ground-surface breaks, or fault scarps, along the Kern Canyon fault that formed during large, pre-historic earthquake events. These scarps were not identified prior to this survey and provided evidence for fault activity that had eluded researchers over the past century. This study uses a subset of the LiDAR survey to quantify the rate of movement of the Kern Canyon fault over the past about 20,000 years at a site called Soda Spring, where the fault scarp cuts large piles of glacial debris, called moraines, that formed during the last major ice age. Amos et al., use modern geochemical techniques to precisely date the formation of these moraines and calculate an average rate of fault movement of at least 0.2 mm/yr over this time at Soda Spring. Although individual earthquakes on the Kern Canyon fault may shift the ground surface up to a meter and a half nearly instantaneously, the average slip rate presented here includes the time period between earthquakes, which ranges between hundreds and thousands of years for the Kern Canyon fault. Overall, the results of this study will be incorporated into a broader seismic hazard evaluation that includes the results of fault-trenching studies near Lake Isabella, with the overall goal of understanding and mitigating seismic hazards to the surrounding population and area.
Field-based constraints on finite strain and rheology of the lithospheric mantle, Twin Sisters, Washington
Basil Tikoff et al., Dept. of Geoscience, University of Wisconsin-Madison, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA. Pages 418-422.
Basil Tikoff of the University of Wisconsin-Madison and colleagues present direct finite strain and rheological estimates of naturally deformed mantle materials using field observations in the Twin Sisters ultramafic body of Washington State.
Sahwave Batholith, NW Nevada: Cretaceous arc flare-up in a basinal terrane
Nicholas J. Van Buer and Elizabeth L. Miller, Dept. of Geological and Environmental Sciences, Stanford University, 450 Serra Mall, Bldg. 320, Stanford, California 94305-2115, USA. Pages 423-446.
New geologic mapping shows that granitic rocks like those found throughout the Sierra Nevada Mountains can also be found in the desert ranges of northwest Nevada. In particular, Nicholas J. Van Buer and Elizabeth L. Miller of Stanford University discovered one giant granitic intrusion, effectively a frozen blob of magma, that covers over 400 square miles about an hour northeast of Reno, Nevada, USA. This intrusion, which Van Buer and Miller call the Sahwave Intrusive Suite, was active about ninety million years ago, and may have underlain a supervolcano. It took about four million years for this large volume of magma to accumulate — early batches of magma cooled around the outside edges, while new batches of magma were added to the molten interior, resulting in a concentric pattern of different granitic rocks. Very similar intrusions of the same age occur along the crest of the Sierra Nevada, for example underlying Mount Whitney and much of Yosemite National Park. Some geologists have suggested that these giant intrusions were formed when easily-melted parts of the continental crust were shoved from the east into the melting zone above a subducting oceanic plate, but there is no evidence for easy-to-melt continental crust near the Sahwave Intrusive Suite. They speculate instead that extra water from the subducting oceanic plate may help melt rocks at the base of the crust to create these giant intrusions.
New data reflect on the thermal antiquity of the Socorro magma body locale, Rio Grande Rift, New Mexico
Marshall Reiter et al., New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA. Pages 447-543.
The Socorro Magma Body was discovered by Allan Sanford and his coworkers at New Mexico Tech almost four decades ago. It is now recognized as a large volume, pancake-shaped, mid-crustal sill, uniquely lacking active volcanism. New heat flow data by Marshall Reiter of the New Mexico Bureau of Geology and Mineral resources and colleagues, combined with seismic data and geomorphic studies, suggest that the Socorro Magma Body is the latest in a sequence of basaltic crustal intrusions (totaling more than 600 m), occurring in this region of the Rio Grande rift over the past about 1 to 3 million years. These studies and recent modeling of thermally induced uplift for the region imply historic uplift is related to the present mid-crustal magma body on a time scale of about 1 thousand years.
Gravity anomalies of the Antarctic lithosphere
John G. Weihaupt et al., Dept. of Geology, University of Colorado, Denver, Colorado 80217, USA. Pages 454-461.
This study by John G. Weihaupt of the University of Colorado at Denver and colleagues use field-based over-snow traverses and airborne and Earth-orbiting satellite remote sensing to reveal regions of unusually low magnitude gravity signals on and offshore of the Antarctic continent. Three potential sources for these gravity anomalies have been identified, namely the Earth’s deep mantle, the Earth’s lithosphere, and the Earth’s crust lying at the top of the lithosphere. Examinations focused on heat from within the earth and heat from radioactivity, on density variations caused by low density sedimentary accumulations or rock-crushing from tectonic or meteoroid impact events, and on topographic variations beneath the Antarctic continental ice sheet are the leading hypotheses for explanation of these unusual gravitational phenomena.
Deciphering shallow mantle stratification through information dimension
D.S. Ramesh et al., National Geophysical Research Institute (Council of Scientific and Industrial Research), Hyderabad 500007, India. Pages 462-471.
A longstanding problem confronting seismologists is the unambiguous detection of shallow upper mantle stratification in the general depth range of 180 to 330 km that go by the nomenclature “Lehmann” and “X” discontinuities. These upper mantle seismic boundaries are sporadic in their occurrence, unlike the well-resolved 410-km and 660-km, which have a global presence. Though compressional to shear-wave converted phases (Ps) have emerged as powerful tools to detect vertical layering inside Earth, they fail to unequivocally image shallow mantle layers owing to interference by multiply reflected arrivals. Invoking concepts based on information theory, D.S. Ramesh of India’s National Geophysical Research Institute and colleagues have developed a new approach to resolving the outstanding issue of unambiguous detection of shallow mantle stratigraphy. They compute cluster entropy and estimate the attendant cluster information dimension associated with primary converted phases arising from target depth discontinuities and the unwanted interfering multiples. Three diagnostics distilled from their analyses bring out the distinct and mutually consistent characteristics of target depth boundaries that differ from those of the multiples. This constitutes a robust measure to discriminate between the two inherently divergent wave types which otherwise show apparent similarities in the measurement space. The concept of observation of seismological data in the generic space rather than in the observational frame is still nascent in geophysical applications. This research is likely to open new vistas in earth sciences with a potential to pinpoint the origin of shallow mantle layering that is presently besieged by competing hypotheses.
View abstracts for the complete issue of LITHOSPHERE at http://lithosphere.gsapubs.org/current.dtl.
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