Boulder, CO, USA - Lithosphere examines a record through time as seen in crystal remnants from Greenland's crust; two main tectonic events contributing to the evolution of the northwestern Red Sea; the workings of the Fish Lake Valley fault in eastern California; and how long and how quickly the Dadu, Yalong, and Yangtze Rivers have been carving gorges into the Tibetan Plateau.
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Oxygen isotopes in detrital zircons: Insight into crustal recycling during the evolution of the Greenland Shield
Chris Kirkland, Senior Geochronologist, Geological Survey of Western Australia, Mines and Petroleum, 100 Plain Street, East Perth, Western Australia 6004, Australia. Pages 3-12.
Samples of the eroded crystal remnants of Greenland's crust range in age from 3900 to 900 million years old. The oxygen isotopic signal locked within these grains provides a record through time of the degree to which material produced or altered near Earth's surface has been incorporated into melts. This essentially serves as a proxy for the degree of recycling of sediments (and altered rocks) into magmas through time. The data set illustrates a progressive increase in crustal recycling from the Archean to the present day and supports models for increasing continental crust volume through time. The data set also indicates that the early Earth heavy oxygen isotopic signal from the Jack Hills is not unique to Western Australia but is also evident in Greenland. Such early heavy oxygen isotopic signals imply liquid water from at least 3900 million years ago.
Structural setting of Cretaceous pull-apart basins and Miocene extensional folds in the Quseir-Umm Gheig region, northwestern Red Sea, Egypt
Mohamed Ahmed Abd El-Wahed, Tanta University, Geology Department, Faculty of Science, El Geish Street, Tanta, 31527, Egypt. Pages 13-32.
Abd El-Wahed et al. examine the evolution of the northwestern Red Sea, Egypt, by study of the Quseir-Umm Gheig subbasin. The subbasin records two main tectonic events. The first event is related to the development of Late Cretaceous synclinal basins due to sinistral movement along the reactivated Najd fault system. The second event is related to the sinistral movement along the NNE-SSW Aqaba-Dead Sea transform and dextral movement along the Queih and Hamrawin shear zones. This movement was synchronous with northeast extension of the Red Sea.
Rates of extension along the Fish Lake Valley fault and transtensional deformation in the Eastern California shear zone-Walker Lane belt
Plamen N. Ganev, Univ. of Southern California, Dept. of Earth Sciences, 3651 Trousdale Parkway, Los Angeles, CA 90089, USA. Pages 33-49.
The Fish Lake Valley fault is an important component of the eastern California shear zone, helping accommodate both right-lateral slip and extension along the Pacific-North America plate boundary. Ganev et al. determined new rates for the extensional component of fault slip at four locations along the fault using airborne laser swath mapping (ALSM; also known as LiDAR) and cosmogenic nuclide surface exposure geochronology. The new rates from ALSM surveys and dating of offset alluvial fans indicate a northward increase in extension from about 0.1 to about 0.7 mm/yr over the past 100,000 years or so. Comparison of these long-term rates with short-term measurements of deformation from GPS data suggest that approximately half of the regional extension is accommodated by the Fish Lake Valley fault. When summed with published rates of extension for other extensional faults at the same latitude, the Fish Lake Valley fault data indicate that long-term and short-term rates of extension are commensurate with each other. The northward increase in extension rates is opposite the previously determined pattern of right-lateral slip rates along the Fish Lake Valley fault, likely reflecting a diffuse extensional transfer zone that relays slip to the northeast along this important section of the Pacific-North America plate boundary.
Regional incision of the eastern margin of the Tibetan Plateau
William Ouimet, Amherst College, Geology, 11 Barrett Hill Road, Amherst, MA 01002, USA. Pages 50-63.
Deep river gorges in western Sichuan, China, reflect the growth and uplift of the Tibetan plateau and subsequent incision by major rivers in the region. In this paper, Ouimet et al. explore how fast the Dadu, Yalong, and Yangtze Rivers on the eastern margin of the Tibetan Plateau have incised into bedrock and when they began doing so. The latter allows researchers to say something about the uplift of the Tibetan plateau, since a requisite for deep river gorges is elevated regional topography. The technique they use is apatite and zircon (U-Th)/He thermochronometry, which quantifies the amount of time it takes a particular rock to go from being 2-5 km beneath Earth's surface to being exposed at the surface. Their results indicate that a broad area within the eastern margin of the Tibetan Plateau encompassing the Dadu River adjacent to the Sichuan Basin and Yangtze River ~450 km to the west was being incised by 10 million years ago and that incision into high topography has been relatively constant and uniform in the region throughout the late Cenozoic (with rates between 0.3 and 0.4 km/m.y.). This has important implications for understanding the evolution of the region, and more generally, for studies aiming to better understand feedbacks among topography, the rate and style of tectonic deformation, climate, and surface processes and/or erosion in orogen evolution.