Boulder, CO, USA - October Lithosphere articles cover diverse locations: the South Tibetan detachment in the eastern Himalaya of Bhutan; metamorphic rocks in the Aegean region; the Volcanic Tableland near Bishop, California, USA; and the Puna Plateau of northwest Argentina. Methods include sensitive high resolution ion microprobe examination of U-Pb ages and characteristics of zircons in the Himalaya; one-dimensional modeling of the thermal evolution of a subducted continental margin; and detailed field analyses to understand deformation and lithospheric foundering.
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View abstracts for the complete issue of Lithosphere at http://lithosphere.gsapubs.org/current.dtl.
Miocene structural reorganization of the South Tibetan detachment, eastern Himalaya: Implications for continental collision
Dawn A. Kellett et al., Dalhousie University, Earth Sciences, Life Sciences Building, Halifax, Nova Scotia B3H 4J1, Canada. Pages 259-281.
Tiny but resilient accessory minerals such as zircon are especially useful for studies of collisional orogenesis. They can survive several mountain building cycles, recording burial and exhumation, as well as metamorphism, magmatism, and deformation. Here Kellet et al. use the high spatial and analytical resolution of the SHRIMP (sensitive high resolution ion microprobe) to examine the U-Pb age and trace element characteristics of growth rims on zircon grains within leucogranites that crystallized in the Miocene within the Himalayan orogen. After crystallizing, these leucogranites were deformed in the South Tibetan detachment system (STDS), an orogen-parallel normal-sense shear zone. The proposed role of this shear zone in the development of the Himalayan orogen varies widely in the literature. This study is the first to unravel the timing of the multiple STDS components in the eastern Himalaya of Bhutan. Zircon ages obtained in their study indicate that part of the STDS became abandoned during a tectonic reorganization at about 12-10 million years ago. This information allows researchers to discriminate those models of continental collision that can accommodate such reorganization, a step towards understanding the type example for continental collision, the Himalayan orogen.
P/T ratio in high-pressure rocks as a function of dip and velocity of continental subduction
Frederic Gueydan et al., Universite Rennes 1, Geosciences Rennes, UMR CNRS/INSU 6118, Campus de Beaulieu, Bat 15, Rennes, 35000, France. Pages 282-290.
High-pressure low-temperature (HP-LT) metamorphic rocks that belong to the same orogen commonly show alignment of their peak pressure and related temperature within a P/T diagram, defining a P/T ratio. In the Aegean region, for example, two metamorphic belts of different ages, the Eocene Cycladic blueschists and the Miocene Cretan blueschists, show contrasting P/T, with a colder P/T ratio for the Cretan blueschists. One-dimensional modeling of the thermal evolution of a subducted continental margin shows that subduction velocity and dip angle control the value of the P/T ratio. On these bases, the variations of P/T ratios in the Aegean region reflect variations through time of subduction dip and velocity. Eocene subduction for Cycladic blueschists burial occurred at a rate of 1.5 cm/a, while subduction velocity during Cretan blueschists formation is found to be 2.75 cm/a. Because the convergence rate between Africa and Eurasia is constant and around 1-1.5 cm/a at these times, the active southward roll back of the Aegean slab during the Oligo-Miocene likely explains the larger subduction velocity for the Cretan HP-LT rocks. These results exemplify the use of this new modeling approach as a proxy to quantify dip and velocity of continental subduction from the P/T ratio of high-pressure rocks.
Deformation analysis of tuffaceous sediments in the Volcanic Tableland near Bishop, California
Ronald Nelson McGinnis et al., Southwest Research Institute, Dept. of Earth, Material, and Planetary Sciences, 6220 Culebra Road, San Antonio, TX 78238, USA. Pages 291-304.
Small-scale brittle faults and fractures that cut bedded tuffaceous sediments of variable textures and grain sizes were studied in a 110-m-long cutbank exposure of poorly consolidated sediments at the southern erosional boundary of the Volcanic Tableland, Owens Valley, California. This study was motivated by the need to evaluate potential length scales for lateral flow in nonwelded bedded tuffs and tuffaceous sediments at Yucca Mountain, Nevada—the site of a potential high-level radioactive waste repository. Small-displacement (<20 cm) faults and fractures are strongly clustered and spatially correlated with larger faults (displacements >20 cm). Vertical fractures are present throughout the exposure, but fracture frequency is generally highest in the vicinity of larger faults. Fault zones are characterized by grain-size reduction and discrete slip surfaces, the number of which increases with increasing displacement. A semiquantitative stress field interpretation, based on tectonic constraints and reconstruction of overburden thicknesses, yields a simple history of burial, deformation, and exhumation under continuous tectonic extension. McGinnis et al. interpret the deformation to include shear (faults), hybrid (faults, nonvertical fractures), and tensile (vertical fractures) failure of the tuffaceous sediments under conditions of low overburden stress (<2.5 MPa). The intersecting network of faults and fractures is characterized by grain comminution, cementation, and fracture dilation. These features, in conjunction with stratigraphic layering, likely produce anisotropic permeability, where maximum permeability is parallel to fault and fracture strike.
Mafic volcanism on the Puna Plateau, NW Argentina: Implications for lithospheric composition and evolution with an emphasis on lithospheric foundering
Scott Drew et al., The Ohio State University, School of Earth Sciences, 125 South Oval Mall, 275 Mendenhall Laboratory, Columbus, OH 43210, USA. Pages 305-318.
Lithospheric foundering has become a popular physiochemical explanation in the earth science community for increased elevation, extensional stress, mafic volcanism, and other geologic activity far from convergent plate margins. The Puna Plateau of northwest Argentina is one of the original locales exhibiting such phenomena explained by removal of lower lithosphere. Here Drew et al. revisit and revise the explanation for said geologic activity, especially the mafic volcanism. They find that partial or piecemeal lithospheric foundering is the most appropriate explanation for the tectonomagmatic activity in the region. They show that full-scale delamination of the lower lithosphere is not always the case and that removal of lithospheric material is a complex process that needs further exploration and evaluation.
Journal
Lithosphere