|31 July 2013
GSA Release No. 13-47
August 2013 Lithosphere Concentrates on China, the Himalaya, India, and North America
Boulder, Colorado, USA - The complete August 2013 issue of Lithosphere is now available online. Papers covering the lithosphere of China and Tibet dominate the issue, with articles on the Himalayan kingdom of Bhutan and the Idaho, USA, Snake River plain as well. The issue also features an article on diamond prospecting in India, a numerical modeling study, and an open access research focus article that asks, "Is it possible to predict the past?"
Abstracts are online at http://lithosphere.gsapubs.org/content/early/recent. Representatives of the media may obtain complimentary copies of Lithosphere articles by contacting Kea Giles 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 your articles or blog posts. Contact Kea Giles for additional information or assistance. Non-media requests for articles may be directed to GSA Sales and Service, .
Highlights are provided below.
Late-stage foreland growth of China’s largest orogens (Qinling, Tibet): Evidence from the Hannan-Micang crystalline massifs and the northern Sichuan Basin, central China
Z. Yang et al., Freiberg, Sachsen 09596, Germany. Lithosphere, August 2013, v. 5, p. 420-437, first published on 10 May 2013, http://dx.doi.org/10.1130/L260.1
This paper addresses the timing of final growth of China's largest orogens: the Qin Mountains (Qinling) and the Tibetan Plateau. In particular, Yang and colleagues investigate when the front of the Qinling-orogen fold-thrust belt was emplaced, and when the northern Sichuan basin was affected by the eastward growth of the Tibetan Plateau. From the late Middle Jurassic to the early Late Cretaceous enhanced cooling and exhumation, with rates of 1.2-2.5 degrees Celsius per million years and 0.04-0.10 mm/yr, record propagation of the Qinling orogen into its leading foreland. Negligible cooling/exhumation since the Late Cretaceous mark the end of the formation and the subsequent persistence of a low-relief landscape that occupied extensive parts of central China; cooling and exhumation rates of 0.38-0.70 degrees Celsius per million years and <0.02 mm/yr characterize this tectonic stagnation period. Accelerated cooling (4-5 degrees Celsius per million years) since the Late Miocene (9 to 13 million years ago) signifies involvement of the southern Qinling into the eastward growing Tibetan Plateau.
Imaging mantle lithosphere for diamond prospecting in southeast India
Subrata Das Sharma and Durbha Sai Ramesh, National Geophysical Research Institute (Council of Scientific & Industrial Research), Hyderabad 500007, India. Lithosphere, August 2013, v. 5, p. 331-342, first published on 10 May 2013, http://dx.doi.org/10.1130/L269.1
The search for diamonds in ancient land masses of the globe remains a passion with geologists and the general public alike. Diamonds formed deep within the bowels of the earth (>150 km) are transported to the surface by their carrier rocks, the kimberlites and lamproites. Owing to their rarity in occurrence and very limited spatial expression, these delicate carriers of the precious diamonds are rather difficult to locate on Earth's surface. Therefore, the quest to find diamonds employing scientific techniques that are both simple and cheap becomes desirable. Toward the unfulfilled dream of discovering diamonds in a routine manner, Das Sharma and Ramesh propose a suitable and cost-effective reconnaissance technique that can be deployed as a quick-search tool over large areas. Acknowledging that a thick lithosphere together with a large carbon source is pre-requisite for diamond genesis and stability, they demonstrate that such a conducive setting for diamond formation indeed exists in southeast India covering a wide area of ~2.5 by 105 square kilometers. They also identify that a complex velocity layering in the mid lithosphere could be another principal diagnostic in this context. Integration of deep images obtained mainly from passive seismology and P-T estimates from xenoliths constrained by geological, geochemical geochronological and heat flow observations from the region enable them to suggest that even rift valleys, often assumed barren, could yield the elusive diamonds.
Geochronological and geochemical constraints for a Middle Paleozoic continental arc in northern margin of the Tarim Block: Implications for the Paleozoic tectonic evolution of the South Chinese Tianshan
Wei Lin et al., Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China. Lithosphere, August 2013, v. 5, p. 355-381, first published on 10 May 2013, http://dx.doi.org/10.1130/L231.1
Structural, geochemical, and geochronological work has been conducted on the South Chinese Tianshan orogenic belt. Middle Paleozoic granitic and volcanic rocks in the northern margin of Tarim Block have been identified. Geochemical data suggest that these Devonian igneous rocks are metaluminous, high-K calc-alkaline felsic volcanic and plutonic rocks. All the samples display relatively enriched LREE contents, weak or no negative Eu anomalies and are relatively depleted in Nb, Ta, Ti, and P, but show enrichment in K, Rb, and Ba. In situ zircon Hf isotopic results show a positive range of εHf (t). These geochemical and isotopic features suggest that this magmatic event probably occurred in an Andean-type arc. Combined with zircon U-Pb and Hf isotopic studies on the basement rocks, the results indicate multi-phase Precambrian continental growth and minor involvement of basement in the Devonian arc magmatism. Finally, the structural, geochemical and geochronological data are integrated into a geodynamic model of the South Chinese Tianshan that emphasizes south-directed subduction of the South Tianshan Ocean along the northern margin of the Tarim Block during the Early to Middle Paleozoic.
Fine crustal structure beneath the junction of the southwest Tian Shan and Tarim Basin, NW China
Rui Gao et al., Institute of Geology, Chinese Academy of Geological Sciences, No. 26 Baiwangzhuang street, Beijing, 100037, China. Lithosphere, August 2013, v. 5, p. 382-392, first published on 3 June 2013, http://dx.doi.org/10.1130/L248.1.
China occupies a large and geological complex region of central and eastern Asia and holds the key to resolving many basic problems in lithospheric structures and links between these and natural resources. In order to investigate the fine crustal structure as well as the relationships between deep and shallow structures beneath the junction between the western part of the South Tian Shan and the northwest margin of the Tarim craton, a 121 km long active source deep seismic reflection profile, TT2007, was completed in 2007. The author talked about the crustal and upper mantle geometry beneath the Junction of the Southwest Tian Shan and Tarim Basin, NW China. The goal of this paper was to establish a framework for interpreting the evolutionary processes that occurred within this complex region.
Evidence for Pliocene-Quaternary normal faulting in the hinterland of the Bhutan Himalaya
B.A. Adams et al., Arizona State University, School of Earth and Space Exploration, ISTB4, Room 795, 781 E. Terrace Road, Tempe, AZ 85287, USA. Lithosphere, August 2013, v. 5, p. 438-449, first published on 3 June 2013, http://dx.doi.org/10.1130/L277.1.
Compressional forces in this setting cause the building of high mountains as rock is thrust up and over other packages of rock along faults. In this study, B.A. Adams and colleagues explored a regional landform thought to be an expression of active rock uplift in the eastern Himalayan country of Bhutan -- "the last Himalayan Kingdom." While their field investigation did identify a brittle fault zone in this location, their data-constraining of the thermal history of this structure suggests that it is an extensional normal fault and not a contractional thrust fault as might otherwise be expected. Thermal histories of minerals around the fault zone also demonstrate that the fault has been active in the last 100,000 years. The age and motion of this fault imply a recent reorganization of faults in the eastern Himalaya, which has implications for active construction of the Himalayan mountains in Bhutan and local seismogenic hazard.
Constraints on Cenozoic tectonics in the southwestern Longmen Shan from low-temperature thermochronology
K.L. Cook et al., German Research Center for Geosciences GFZ, Helmoltz-Zentrum Potsdam, Telegrafenberg, Potsdam, Brandenburg 14473, Germany. Lithosphere, August 2013, v. 5, p. 393-406, first published on 3 June 2013, http://dx.doi.org/10.1130/L263.1.
The Longmen Shan mountains in western Sichuan Province, China, mark the eastern boundary of the Tibetan Plateau and have been the site of two deadly earthquakes in the past five years, most recently on 20 April 2013. But despite the importance of these mountains, both in terms of seismic hazard, and for understanding the evolution of the Tibetan Plateau, the uplift history of much of the range has remained poorly understood. K.L. Cook and colleagues provide a detailed discussion of the geology of the southwestern section of the range and present a set of cooling age data that provides new insight into the past 15 million years of uplift. Their data allow them to identify which faults have been most important in the uplift of the range, and how the faults of the southwestern Longmen Shan are related to the faults that ruptured in the 2008 earthquake. As the 20 April 2013 earthquake occurred in the studied area, the team is also able to place the earthquake in context with the local geology.
Extension-driven right-lateral shear in the Centennial shear zone adjacent to the eastern Snake River Plain, Idaho
S.J. Payne et al., Idaho National Laboratory, P.O. Box 1625, MS 2203, Idaho Falls, Idaho 83415, USA. Lithosphere, August 2013, v. 5, p. 407-419, first published on 3 June 2013, http://dx.doi.org/10.1130/L200.1
S.J. Payne and colleagues evaluate global positioning system (GPS) surface velocities and gravitational potential energy (GPE) variations to assess the causes of right-lateral shear in the Centennial shear zone, a NE-trending accommodation zone between the extensional Centennial tectonic belt (Montana-Idaho) and volcanic terrain of the eastern Snake River Plain (Idaho). They test the hypothesized “bookshelf” faulting model and find that the normal faults in the Centennial tectonic belt do not accommodate distributed dextral shear.
Autogenic cycles of channelized fluvial and sheet flow and their potential role in driving long-runout gravel progradation in sedimentary basins
T.M. Engelder and J.D. Pelletier, Department of Geosciences, University of Arizona, 1040 E. Fourth Street, Tucson, Arizona 85721, USA. Lithosphere, August 2013, v. 5, p. 343-354, first published on 10 May 2013, http://dx.doi.org/10.1130/L274.1
The paleoslope estimation method uses a threshold-shear-stress criterion, together with field-based measurements of median grain size and channel depth in alluvial gravel deposits, to calculate the threshold paleoslopes of alluvial sedimentary basins. Threshold paleoslopes are the minimum slopes that would have been necessary to transport sediment in those basins. In some applications of this method, inferred threshold paleoslopes are sufficiently steeper than modern slopes that large-magnitude tectonic tilting must have occurred in order for sediments to have been transported to their present locations. In this paper, T.M. Engelder and J.D. Pelletier argue that autogenic cycles of channelized fluvial and sheet flow in alluvial sedimentary basins result in spatial and temporal variations in the threshold slope of gravel transport that can, under certain conditions, cause gravel to prograde out to distances much longer than previously thought possible based on paleoslope estimation theory (i.e., several hundred kilometers or more from a source region). They test this hypothesis using numerical models for two types of sedimentary basins: (1) an isolated sedimentary basin with a prescribed source of sediment from upstream, and (2) a basin dynamically coupled to a postorogenic mountain belt.
Is it possible to predict the past?
Chris Paola, Department of Earth Sciences, University of Minnesota, Minneapolis, Minnesota 55455, USA. Lithosphere, August 2013, v. 5, p. 450-451, http://dx.doi.org/10.1130/RF.L005.1
There are many reasons for reconstructing past environments as accurately and quantitatively as possible. As part of understanding our planet’s history, it is an end in itself. Once we know, for example, that we are dealing with an ancient river system, it is natural to begin taking its measure: how deep was the flow? How wide, how fast? Was this a major trunk stream or a minor offshoot? Where are we in the larger scheme of things, i.e. the sediment-routing system? These quantitative questions seem fairly natural — probably most curious lay people would readily understand why we ask them. This article is OPEN ACCESS online.