|02 March 2011
GSA Release No. 11-15
Director of Education, Communication, & Outreach
Lithosphere: New Research Posted 10 February
Boulder, CO, USA - Lithosphere is now regularly posting pre-issue publication content -- finalized papers ready to go to press and not under embargo. GSA invites you to sign up for e-alerts and/or RSS feeds to have access to new journal content the minute it is posted online. Go to http://www.gsapubs.org/cgi/alerts and enter your e-mail address to manage your subscriptions for pre-issue postings, tables of contents alerts, and more. The following LITHOSPHERE articles were published online 10 Feb. 2011.
Keywords: Archean, East Indian Shield, xenoliths, SKS, anisotropy, South America, Grand Canyon, zircons, Paleozoic sandstones.
Highlights are provided below. View abstracts for these LITHOSPHERE papers at http://lithosphere.gsapubs.org/content/early/recent. Abstracts will be removed from this page once they are published as part of an issue. The article's doi number will remain consistent.
Representatives of the media may obtain complementary copies of LITHOSPHERE articles by contacting Christa Stratton at the address above. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to LITHOSPHERE in articles published.
Non-media requests for articles may be directed to GSA Sales and Service, .
Mafic granulite xenoliths from the East Indian Shield: Evidence for recycled continental crust in the Archean mantle
S. Bhattacharya et al., Indian Statistical Institute, 203 B.T. Road, Kolkata 700108, India. Published online 10 Feb. 2011; doi: 10.1130/L120.1.
An amazing wealth of information can be found in a few pieces of rocks on the east coast of India. These rocks tell the story of two gigantic volcanisms 3.3 and 2.5 billion years ago. Such magmas originating from the deep interior of Earth must have battled through great hazards during the long journey from antiquity to the present. Still, these minor rock-pieces preserved information of their parentage -- some characteristics of their source. The most interesting information of their parentage is that pieces of continent were mixed in with the source -- i.e., a mixed parentage. This, according to S. Bhattacharya of the Indian Statistical Institute and colleagues, implies the existence of a continent even prior to 3.3 billion years in this part of the globe that could have been part of the original supercontinent.
Upper-mantle seismic anisotropy from SKS splitting in the South American stable platform: A test of asthenospheric flow models beneath the lithosphere
Marcelo Assumpção et al., Institute of Astronomy, Geophysics, and Atmospheric Sciences, University of São Paulo (USP), Rua do Matão 1226, São Paulo, SP 05508-090, Brazil. Published online 10 Feb. 2011; doi: 10.1130/L99.1.
The rocks of Earth's upper mantle are usually anisotropic -- that is, the velocity of the seismic waves varies slightly depending on the direction of the seismic vibrations. This anisotropy is due to the preferential orientation of the olivine, the main mineral constituent of the upper mantle. This preferential orientation is caused by deformation of the rocks during the evolution of the upper mantle. It can be measured with the SKS wave (an S, or transverse, wave that crosses Earth's core and arrives at the recording station almost vertically), which is split into two separate waves by the anisotropic upper mantle. The anisotropy in the continental upper mantle has been interpreted as due to the present deformation related to the plate motion and asthenospheric flow, or due to past deformation that occurred during the last major cycles of mountain building. In this study by Marcelo Assumpção of the University of Sao Paolo and colleagues, additional measurements of SKS splitting in the central part of South America, especially in the poorly sampled Amazonian region, indicates that the directions of the fast polarization of the SKS waves tend to be roughly parallel to the absolute motion of the South American plate with some local deviations around cratonic keels. This may imply that a large contribution to the anisotropy in the South American upper mantle comes from deformation in the asthenosphere related to the present motion of the plate as opposed to lithospheric deformation in past orogenesis.
Detrital zircon U-Pb geochronology of Paleozoic strata in the Grand Canyon, Arizona
George E. Gehrels et al., Dept. of Geosciences, University of Arizona, Tucson, Arizona 85721, USA. Published online 10 Feb. 2011; doi: 10.1130/L121.1.
In this study by George E. Gehrels of the University of Arizona and colleagues, U-Pb ages of detrital zircons in Paleozoic sandstones from the Grand Canyon yield new information about the origin of the sandstones and the paleogeography of North America during their time of deposition. Lower sandstones, which accumulated during Cambrian and Devonian time, record derivation of sand mainly from the transcontinental arch, a mountainous ridge that trended southwest-northeast across the central part of North America. Mid-level sandstones, beginning in Mississippian time, received some sand from the interior portion of the continent, but most sediment was derived from the central or northern portion of the Appalachian Mountains. Upper sandstones of the Grand Canyon, including the spectacular red beds of the Supai Group and white cliffs of the Coconino Sandstone, were shed from both the distant Appalachian Mountains and the nearby Ancestral Rocky Mountains, both of which were uplifted due to the collision of North America with Africa and South America. Transport of sediment from the Appalachian Mountains was accomplished by a combination of westward flow in large river systems and southwestward motion in the belt of northeasterly trade winds.