|17 May 2012
GSA Release No. 12-38
Visualizing the Imprints of Past and Present Earth Dynamics
Boulder, Colo., USA – New Lithosphere articles posted online 16 May 2012 report on (1) seismic anisotropy measured beneath 14 broadband stations in southeastern India; (2) why geoscientists should persist in their efforts to reach and study such spectacular sub-sea geologic features as the Mariana Trench (recently explored by film director James Cameron) and how "land geologists" can help this effort by studying on-land equivalents like ophiolites; and (3) pressures and melting temperatures of sediments deeply buried in Earth's mantle.
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Seismic anisotropy beneath the eastern Dharwar Craton
Sunil Kumar Roy et al., National Geophysical Research Institute, Seismic Hazard Group, Hyderabad 500007, India. Posted online 16 May 2012; doi: 10.1130/L198.1.
Seismic anisotropy is an intrinsic property of the Earth that imparts a directional dependence to the velocity of elastic waves and carries imprints of past and present deformation. Due to this phenomenon, a shear wave passing through an anisotropic medium gets polarized in a particular direction and splits into two orthogonal waves, with one wave traveling faster than the other. Analysis of the nature and difference in the arrival times of the fast and slow waves registered at a seismic station enables Sunil Kumar Roy and colleagues to parameterize anisotropy in terms of the delay time and fast polarization direction. They estimate the nature of anisotropy beneath 14 broadband stations in southeastern India, utilizing the core refracted (SKS, SKKS) and direct S waves to obtain a total of 113 high-quality measurements of delay time and fast polarization direction. The delay time between the fast and slow axes tend to cluster around 0.8 s, slightly lower than that observed globally for continental shield regions (~1 s). The fast directions at a majority of stations are in accordance with the present-day motion of the Indian plate, suggesting that the shear at the base of the Indian lithosphere is the primary cause of anisotropy. Interestingly, this study also brings out the effect of anisotropy frozen in the lithosphere due to past tectonic episodes. For example, stations in the vicinity of the east coast of India reveal a coast parallel trend, suggesting that anisotropy in the underlying medium may be the imprint of continental rifting that separated India from the rest of Gondwana.
To understand subduction initiation, study forearc crust: To understand forearc crust, study ophiolites
R.J. Stern et al., Geosciences Dept., The University of Texas at Dallas, Richardson, Texas 75083-0688, USA. Posted online 16 May 2012; doi: 10.1130/L183.1.
Subduction is the process by which seafloor (oceanic crust and upper mantle) is returned to Earth's interior. Subduction is what powers the plates and thus may be the most important solid Earth process. Subduction results in spectacular geologic features, including "island arc" volcanoes like those of the U.S. Cascades and trenches like the Mariana Trench, which has recently been explored by film director James Cameron. As a result of studying many convergent plate margins around the world, geoscientists have a good understanding of how mature subduction zones operate but know far less about how new subduction zones form. In this paper, R.J. Stern and colleagues emphasize the importance of studying the igneous rocks of the ~100-mile-wide "forearc" region, which lies between the arc volcanoes and the trench, for understanding how new subduction zones are generated. Forearc igneous rocks preserve an outstanding record of how new subduction zones form, but direct study is difficult because forearcs are buried beneath younger sediments and often lie in the deepest parts of the ocean, where deep-sea studies require expensive research vessels and submersibles. This article explores why geoscientists must continue to study in situ forearcs and how land geologists can help this effort by studying on-land equivalents of forearc crust known as "ophiolites." Ophiolites are found on all continents, and they are important targets for geoscientific study because they present an opportunity for better understanding of the composition and origin of forearc crust and how new subduction zones form.
Melting of metasedimentary Rocks at Ultrahigh Pressure -- Insights from Experiments and Thermodynamic Calculations
H.-J. Massonne and T. Fockenberg, Institut für Mineralogie und Kristallchemie, Universität Stuttgart, Azenbergstrasse 18, D-70174 Stuttgart, Germany. Posted online 16 May 2012; doi: 10.1130/L185.1.
H.-J. Massonne and T. Fockenberg use high pressure experiments at temperatures of 950 to 1400 degrees Celsius to simulate the melting of sediments deeply buried into Earth's mantle by geodynamic processes. Experimental pressures were at and above 3 GPa (greater than or equal to 100 km Earth depths). Temperatures close to 1000 and 1100 degrees Celsius at 3 GPa and 5 GPa, respectively, were sufficient to produce initial melts from the selected rocks. At a temperature of about 350 degrees Celsius above these temperatures, the rocks were completely molten. In addition, the melting was modeled by complex calculations using thermodynamic data for minerals and melt. Both methods resulted in initial melt compositions rich in water and potassium. With rising temperatures the melts become granitic with garnet plus coesite plus or minus kyanite as remaining solid phases. The new data were applied to natural diamondiferous rocks with sedimentary whole-rock compositions from the Erzgebirge in central Europe and the Kokchetav Massif in northern Kazakhstan. According to previously reported microfabrics etc. of these rocks, pointing to their partial melting and crystallization of diamond from the melt, the rocks would have been as hot as 1400 degrees Celsius (Erzgebirge) and 1200 degrees Celsius (Kokchetav Massif) once, probably at pressures of around 7 GPa. Furthermore, Massonne and Fockenberg conclude that granitic melts could also have been produced in deep mantle regions and not exclusively in lower portions of Earth's crust in the past.