Grujic, Djordje
Permanent URI for this collectionhttps://hdl.handle.net/10222/22087
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Item Open Access The South Tibetan detachment system facilitates ultra rapid cooling of granulite facies rocks in Sikkim Himalaya(American Geophysical Union, Washington, DC, 2013-03) Kellett, Dawn A.; Grujic, Djordje; Coutand, Isabelle; Cottle, John; Mukul, MalayItem Open Access An insight into the breakup of Gondwana: Identifying events through low-temperature thermochronology from the basement rocks of Madagascar(2004-06) Seward, D.; Grujic, Djordje; Schreurs, G.Fission track analysis was applied to the Precambrian suites of Madagascar in order to identify the lower-temperature cooling histories and their relationships to the Phanerozoic events that affected the island. Apatite ages range from 431 to 68 Ma, and zircon ages range from 452 to 238 Ma. Thermochronologically, the island can be divided into a southern, central, and northern region each with a subdivision on an east-west basis. The southern region is sharply separated from the central region by strongly contrasting apparent apatite ages over the northwest-southeast striking Ranotsara Shear Zone (RSZ). The change in apparent ages over the RSZ is indicative of later reactivation along younger brittle faults. The central region has the oldest ages of the island and has a diffuse contact to the third region northward. Along the entire western margin of the Precambrian basement initial Paleozoic exhumation was followed by heating (burial by sediments) during Jurassic and Cretaceous times. A decrease in ages along the eastern margin from 119 to 68 Ma coincides with the predicted positions of the Marion hot spot after effects of erosion are considered. On the other hand, these ages may represent progressive opening of the margin in a southward direction together with associated denudation of the rift shoulder. The eastern part of the central region has remained very stable since at least Devonian times, undergoing only long-term very slow exhumation at rates of 1-5 m/Myr.Item Open Access Probing the depths of the India-Asia collision: U-Th-Pb monazite chronology of granulites from NW Bhutan(2011-03) Warren, C. J.; Grujic, Djordje; Kellett, D. A.; Cottle, J.; Jamieson, R. A.; Ghalley, K. S.Rocks metamorphosed to high temperatures and/or high pressures are rare across the Himalayan orogen, where peak metamorphic conditions recorded in the exposed metamorphic core, the Greater Himalayan Sequence (GHS), are generally at middle to upper amphibolite facies. However, mafic garnet-clinopyroxene assemblages exposed at the highest structural levels in Bhutan, eastern Himalaya, preserve patchy textural evidence for early eclogite-facies conditions, overprinted by granulite-facies conditions. Monazite hosted within the leucosome of neighboring granulite-facies orthopyroxene-bearing felsic gneiss yields LA-MC-ICP-MS U-ThPb ages of 13.9 +/- 0.3 Ma. Monazite associated with sillimanite-grade metamorphism in granulite-hosting migmatitic gneisses yields U-Th-Pb rim ages between 15.4 +/- 0.8 Ma and 13.4 +/- 0.5 Ma. Monazite associated with sillimanite-grade metamorphism in gneiss at structurally lower levels yields U-Pb rim ages of 2117 Ma. These data are consistent with Miocene exhumation of GHS material from a variety of crustal depths at different times along the Himalayan orogen. We propose that these granulitized eclogites represent lower crustal material exhumed by tectonic forcing over an incoming Indian crustal ramp and that they formed in a different tectonic regime to the ultrahigh-pressure eclogites in the western Himalaya. Their formation and exhumation in the Miocene therefore do not require diachroneity in the timing of the initial India-Asia collision. Citation: Warren, C. J., D. Grujic, D. A. Kellett, J. Cottle, R. A. Jamieson, and K. S. Ghalley (2011), Probing the depths of the India-Asia collision: U-Th-Pb monazite chronology of granulites from NW Bhutan, Tectonics, 30, TC2004, doi:10.1029/2010TC002738.Item Open Access Using small, temporary seismic networks for investigating tectonic deformation: Brittle deformation and evidence for strike-slip faulting in Bhutan(2007-07) Velasco, A. A.; Gee, V. L.; Rowe, C.; Grujic, Djordje; Hollister, L. S.; Hernandez, D.; Miller, K. C.; Tobgay, T.; Fort, M.; Harder, S.We processed data from a small, five-station temporary seismic network deployed from January 2002 until March 2003 within the Kingdom of Bhutan. We detected, associated, and located approximately 2,100 teleseismic, regional, and local events; approximately 900 were not in the United States Geological Survey (USGS) Earthquake Data Report catalog. We supplemented our data for these 900 events with data from the Global Seismographic Network (GSN) stations in the region. After relocation of these events, we focused on approximately 175 events that occurred near or within the borders of Bhutan. We reviewed each solution, manually timing the P- and S-waves for each event, and inverted for event locations and an average 1-D velocity model for the region. Our model was tested with other models appropriate for the region. We found a high amount of microseismicity throughout southern Bhutan and almost no seismicity under northern Bhutan and southern Tibet. Our results showed that analysis of data from small in-country seismic networks resulted in new scientific findings. In this case, we found the crust under southern Bhutan brittlely deforming, and there was evidence for strike-slip faulting, supporting previous) results for the region.Item Open Access Exhumation and uplift of the Shillong plateau and its influence on the eastern Himalayas: New constraints from apatite and zircon (U-Th-[Sm])/He and apatite fission track analyses(2007-12) Biswas, Subrata; Coutand, Isabelle; Grujic, Djordje; Hager, Christian; Stoeckli, Daniel; Grasemann, BernhardNo abstract available.Item Open Access New insight into the South Tibetan detachment system: Not a single progressive deformation(2012-03) Kellett, Dawn A.; Grujic, DjordjeLow-angle normal faults (LANF), typically regarded as accommodating crustal or lithospheric extension, may also form during lithospheric shortening. The best-studied system of syn-contractional LANFs is the South Tibetan detachment system, a network of low-angle normal sense faults and shear zones that formed coevally with and parallel to south-vergent thrusts during lithospheric shortening accompanying development of the Himalayan orogen. In the eastern Himalaya, there are several across-strike exposures of the South Tibetan detachment system. We present new structural and thermometry data from the eastern Himalaya that demonstrate that the South Tibetan detachment system cannot have formed as a single progressive structure. We characterize and distinguish two distinct structural and tectonic components within the currently recognized system: (1) an extensive diffuse, sheared layer that formed the boundary between strong upper crust and weak, southward-flowing middle crust, and (2) a network of brittle-ductile LANFs that locally exhume, partly excise and overprint the earlier mylonite zone at the topographic break between the Himalayan orogen and the Tibetan plateau. The sheared layer, not a LANF, formed the boundary between upper and middle crust during ductile flow of the middle crust and is extensively exposed in the Himalaya at the base of klippen of upper crustal rocks preserved in Bhutan, along the crest of the Himalaya where it has been excised and exhumed by the brittle-ductile extrusion LANFs, and bounding the cores of the North Himalayan gneiss domes.Item Open Access Experimental Melting of Biotite Plus Plagioclase Plus Quartz Plus Or Minus Muscovite Assemblages and Implications for Crustal Melting(1995-08) GARDIEN, V.; THOMPSON, AB; Grujic, Djordje; ULMER, P.In order to understand the role of mica-rich rocks as a source of granite magmas, a series of melting experiments was performed on two different starting materials. The first composition is a model biotite gneiss consisting of 30 wt % biotite, 30 wt % plagioclase, and 40 wt % quartz. The second composition is a model two-mica pelites consisting of 15 wt % biotite, 15 wt % muscovite, 30 wt % plagioclase, and 40 wt % quartz. Experiments were performed under vapor-absent conditions at 1.0 GPa and between 750 degrees and 950 degrees C. With only biotite in the starting material the volume of melt is always less than 15 vol % below 900 degrees C and reaches 25 vol % at 950 degrees C. In experiments that involve both biotite and muscovite in the starting material, the melt proportion increases up to 28 vol % at 825 degrees C and reaches 60 vol % at 950 degrees C. For the biotite-plagio clase-quartz (BPQ) assemblage, the solidus is located at 800 degrees C at 1.0 GPa. The melting reaction produces a metaluminous granitic liquid and leaves a residuum consisting of garnet + biotite + orthopyroxene + plagioclase + quartz. In addition, the experiments show that at 1.0 GPa biotite can be stable above 950 degrees C. With both micas in the starting material (BPQM), the solidus at 1.0 GPa is located at 750 degrees C. The melting reactions produce a peraluminous granitic liquid and leave a residuum of garnet + sillimanite + biotite + quartz + plagioclase + Kfeldspar in experiments below 900 degrees C. At 950 degrees C the residuum consists of garnet + orthopyroxene + biotite + plagioclase. The melt fraction is determined by the proportions of the hydrous phases and of the amount of feldspar relative to quartz. Mineral modes of the source rocks, particularly the amount of quartz, are at least as important as the amount of available H2O in controlling the melt fraction generated during crustal anatexis.