Boulder, Colo., USA - In the latest issue of The Geological Society of America journal Lithosphere: Learn more about the Great Slave Lake shear zone in northwest Canada (open access article); the tectonic development of the Tibetan Plateau; and two flysch belts. Also in this issue: an open-access review article on crustal melting, ductile flow, and deformation in mountain belts.
Abstracts are online at http://lithosphere.
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Mantle roots of major Precambrian shear zones inferred from structure of the Great Slave Lake shear zone, northwest Canada
D.B. Snyder, Geological Survey of Canada, firstname.lastname@example.org; and B.A. Kjarsgaard. Lithosphere, v. 5, p. 539-546, http://lithosphere.
Mountain belt structure from the early Earth rarely survives. New seismic and geochemical data from Northwest Canada have provided insight into the deep structure of the 2 billion year old Thelon mountains that formed, similar to the Himalaya, when the 4.0-2.8 billion year old Slave and 3.2-2.1 billion year old Rae subcontinents collided. The new observations are interpreted to indicate that no ocean separated these sub-continents as they converged, and final structural geometries suggest that these blocks wedged apart one another at multiple levels to depths of 250 km. Analogous to the modern San Andreas fault zone, the new observations suggest that hundreds of inferred strike-slip offset along the Macdonald Fault strand of the Great Slave Lake shear zone penetrates on vertical faults only a few tens of kilometers into the Earth; fault dips are shallow below that depth and inherited from early stages of convergence. The composite geometry of the wedges and shallow fault dips imply that known economic diamond kimberlite deposits and the associated diamond source reservoirs at 150-220 km depths associated with the Slave sub-continent today underlie the entire former mountain belt and the East Arm of Great Slave Lake.
Tectonic development of the northeastern Tibetan Plateau as constrained by high-resolution deep seismic-reflection data
Rui Gao, UCLA, Earth & Space Sciences, Los Angeles, CA 90095-1567, USA; Haiyan Wang, An Yin (corresponding author: email@example.com, firstname.lastname@example.org), Shuwen Dong, Zhaoyang Kuang, Andrew V. Zuza, Wenhui Li, and Xiaosong Xiong. Lithosphere, v. 5, p. 555-574, http://lithosphere.
The Tibetan plateau rises about 5 km on average above the sea level. However, what makes up its crust that leads to the formation this high altitude plateau on Earth is not clear. This paper uses a high-resolution seismic image to elucidate this problem. A key discovery is that the faults responsible for the plateau formation are restricted within the crust and does not cut into the mantle.
Two flysch belts having distinctly different provenance suggest no stratigraphic link between the Wrangellia composite terrane and the paleo-Alaskan margin
Chad P. Hults, U.S. Geological Survey, 4200 University Dr., Anchorage, Alaska 99508, USA, email@example.com; and Frederic H. Wilson, Raymond A. Donelick, and Paul B. O'Sullivan. Lithosphere, v. 5, p. 575-594, http://lithosphere.
Jurassic to Cretaceous flysch lying between the Wrangellia composite terrane and the paleo-Alaskan margin obscures the northern boundary of the terrane, but the provenance of the flysch may hold the key to unraveling the timing and location of accretion of the terrane, which has been a controversy for many decades. Combining traditional provenance techniques and detrital zircon geochronology, Chad Hults and colleagues compile previously published provenance data and contribute new targeted provenance data to test contradictory models of accretion. The authors present a compelling argument that there is no unambiguous provenance link between the paleo-Alaskan margin and the Wrangellia composite terrane. This new interpretation, combined with geophysical models and structural evidence, constrains the location of the northern boundary of the Wrangellia composite terrane and supports Late Cretaceous accretion of the terrane.
Crustal melting, ductile flow, and deformation in mountain belts: Cause and effect relationships
Mike Searle, Dept. Earth Sciences, Oxford University, South Parks Road, Oxford OX1 3AN, UK. Lithosphere, v. 5, p. 547-554, http://lithosphere.
From the abstract: "Exhumed sections of migmatites are beautifully exposed in the middle crust of old orogens such as the Proterozoic Wet Mountains of Colorado and young Tertiary-active orogens such as the Himalaya and Karakoram. Migmatites and leucogranites occur both on a regional scale (e.g., Greater Himalayan Sequence) and along more restricted shear zones and strike-slip faults (e.g., Karakoram, Jiale, and Red River faults). Melting and deformation are clearly diachronous across orogenic belts over space and time, yet in general, deformation must precede regional metamorphism and melting in order to thicken the crust and increase pressure and temperature."