Boulder, Colo., USA - The rise of the Tibetan plateau -- the largest topographic anomaly above sea level on Earth -- is important for both its profound effect on climate and its reflection of continental dynamics. In this study published in GSA Bulletin, Katharine Huntington and colleagues employ a cutting-edge geochemical tool -- "clumped" isotope thermometry -- using modern and fossil snail shells to investigate the uplift history of the Zhada basin in southwestern Tibet.
Views range widely on the timing of surface uplift of the Tibetan Plateau to its current high (~4.5 km) over more than 2.5 square kilometers. Specifically, interpretations differ on whether the modern high elevations were recently developed or are largely a continuation of high elevations developed prior to Indo-Asian collision in the Eocene.
Clumped isotope temperatures of modern and fossil snail shells record changing lake water temperatures over the last nine million years. This is a reflection of changes in surface temperature as a function of climate and elevation change. A key to their Zhada Basin paleo-elevation reconstructions is that Huntington and colleagues were able to contextualize them with sampling of modern and Holocene-age tufa and shells from a range of aquatic environments.
Huntington and colleagues find that the Zhada basin was significantly colder from three to nine million years ago, implying a loss of elevation of more than one kilometer since the Pliocene. While surprising given the extreme (~4 km) elevation of the basin today, the higher paleo-elevation helps explain paleontological evidence of cold-adapted mammals living in a high-elevation climate, and is probably the local expression of east-west extension across much of the southern Tibetan Plateau at this time.
Huntington and colleagues note that future studies could improve on their own initial "calibration" work with year-round monitoring of water temperature and a focus on specific taxa and their micro-habitat preferences.
High Late Miocene-Pliocene elevation of the Zhada basin, SW Tibetan plateau, from carbonate clumped isotope thermometry
K.W. Huntington et al., Dept. of Earth and Space Sciences, University of Washington, Seattle, Washington 98195, USA. Published online 7 Aug. 2014; http://dx.doi.org/10.1130/B31000.1.
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Stratigraphic and microfossil evidence for a 4,500-year history of Cascadia subduction zone earthquakes and tsunamis at Yaquina River estuary, Oregon, USA
N.A. Graehl et al. (Kelsey, corresponding author), Dept. of Geology, Humboldt State University, Arcata, California 95524, USA. Published online 7 Aug. 2014; http://dx.doi.org/10.1130/B31074.1.
New research on the central Oregon coast reveals that 11 subduction zone earthquakes and associated tsunamis have affected the Newport, Oregon, USA, area in the past 4,500 years. This research adds to a growing body of knowledge that implicates great subduction zone earthquakes recurring on average every 420-580 years. Each earthquake is accompanied by abrupt land-level lowering of the coastal region of at least half a meter and by incursion of tsunami waves to the Yaquina Bay region.
Evolution of the central Garlock fault zone, California: A major sinistral fault embedded in a dextral plate margin
Joseph E. Andrew et al., Dept. of Geology, University of Kansas, Lawrence, Kansas 66045, USA. Published online 26 Aug. 2014; http://dx.doi.org/10.1130/B31027.1.
The slip rate of the Garlock fault in southeastern California has accelerated through time to the modern rate of 9 mm per year. Left-lateral strike-slip displacement of 64 km on the Garlock fault began 11 million years ago as an accommodation structure to Basin and Range extension. The Garlock fault subsequently changed geometry and rates when dextral shear initiated inboard of the San Andreas plate boundary. The Garlock fault evolved to a structurally more complex fault zone to accommodate the transversely oriented dextral shear. Dextral shear across the Garlock fault is accommodated by internal deformation within the Garlock fault zone via lateral extrusion, uplift, folding, internal block rotation, and counterclockwise rotation of its trace.
Confirmation of a low pre-extensional geothermal gradient in the Grayback normal fault block, Arizona: Structural and AHe thermochronologic evidence
M.S. Wong et al., Dept. of Geology, Colgate University, Hamilton, New York 13346, USA. Published online 7 Aug. 2014; http://dx.doi.org/10.1130/B31033.1.
Many models of continental extension and rifting call upon crustal heating from magmatism or other processes to trigger and localize rifting (active rift models). However, it is often difficult to measure the thermal structure of the crust directly for ancient rift zones, so such models have been difficult to evaluate. M.S. Wong and colleagues present new (U-Th)/He apatite thermochronology from the Grayback tilted normal fault block in the North American Basin and Range extensional province confirming that a very low geothermal gradient (14 to 17 degrees Celsius per kilometer) existed just prior to major Tertiary extension. This result challenges the notion that reheating of the crust was a significant driving force of crustal extension in the Basin and Range.
U-Pb geochronology and global context of the Charnian Supergroup, UK: Constraints on the age of key Ediacaran fossil assemblages
S.R. Noble et al., Natural Environment Research Council Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottinghamshire, NG12 5GG, UK. Published online 26 Aug. 2014; http://dx.doi.org/10.1130/B31013.1.
The Charnian Supergroup of Central England is a globally significant fossil locality where Precambrian complex life-forms are well preserved. The Neoproterozoic Ediacaran Period was a seminal time in the evolution of multicellular life. Important among these Precambrian complex organisms were the Rangeomorphs and their possible taphomorphs (ivesheadiamorphs). Unfortunately, the timing of key biological events relating to the evolution of these organisms is relatively poorly constrained due to a paucity of high-precision dates worldwide. New U-Pb geochronology data presented here by Stephen R. Noble and colleagues contribute significantly to the temporal constraints on these organisms, showing that Charnian fossils (older than 569 to 557 million years) partly overlap in age with the other key Rangeomorph locality in the paleo-region (Mistaken Point, Newfoundland), and also with macro-organism communities from very different environmental settings (e.g. White Sea, Russia). Together, this indicates that paleoenvironmental differences rather than temporal control are a first-order control on biota composition at this time.
Sinkholes, pit craters, and small calderas: Analog models of depletion-induced collapse analyzed by computed X-ray microtomography
S. Poppe et al., Dept. of Geography, Earth System Science, Vrije Universiteit Brussel (VUB), Pleinlaan 2, B-1050 Brussels, Belgium. Published online 26 Aug. 2014; http://dx.doi.org/10.1130/B30989.1.
In nature, sub-surface bodies of molten or ductile rock might become abruptly depleted, resulting in the collapse of the overlying rock and the sometimes catastrophic formation of a "hole in the ground" at surface. These are termed sinkholes in karst terrains and pit-craters or calderas on volcanoes. For the first time, S. Poppe and colleagues use computed X-ray micro-tomography (μCT), similar to the imaging of patients using CT-scanners, to image laboratory-scale models of such collapse process. Their simulations illustrate how collapse rates within the collapsing rock are varying in space and time, with the highest rates before and shortly after the surface depression is formed. The collapsing material also undergoes a volumetric expansion. On volcanoes, such "bulking" of the rocks overlying an emptying magma chamber may at least partially explain why the volume of the erupted lava is commonly larger compared to that of the surface depression. This insight is of great use to seismic or volcano observatories who will face such collapse events in the future.
Impacts of catastrophic volcanic collapse on the erosion and morphology of a distal fluvial landscape: Hautapu River, Mount Ruapehu, New Zealand
M. Tost et al., Volcanic Risk Solutions, Massey University, Palmerston North 4442, New Zealand. Published online 26 Aug. 2014; http://dx.doi.org/10.1130/B31010.1.
Debris avalanches caused by the collapse of volcanic flanks may permanently change the surrounding landscape and its drainage systems. Deposits of a two- to three-cubic-kilometer debris avalanche exposed along the Hautapu River about 50 km southeast of Mount Ruapehu, New Zealand, reflect the largest known collapse event of the stratovolcano, followed by a regrowth phase that produced pyroclastic eruptions and pumice-rich lahars. The collapse most likely occurred due to magmatic unrest during the shift from a glacial to an interglacial climate. The debris avalanche inundated an area of more than 260 square kilometers, and was channelized within the proto-Hautapu catchment. The subsequent pumiceous mass-wasting events continued to be confined to the proto-Hautapu River for another approx. ten thousand years. At present the volcaniclastic deposits form a distinctive plateau on the highest topographic elevation within the river valley, while the Hautapu River incises into the underlying softer late Pliocene sediments.
Sevier belt exhumation in central Utah constrained from complex zircon (U-Th)/He data sets: Radiation damage and He inheritance effects on partially reset detrital zircons
William R. Guenthner et al., Dept. of Geology, University of Illinois, Urbana-Champaign, Champaign, Illinois 61820, USA. Published online 26 Aug. 2014; http://dx.doi.org/10.1130/B31032.1.
In this manuscript, William Guenthner and colleagues provide new data that demonstrate the age of mountain building processes in the mountain ranges of western Utah. The authors rely upon radiometric dating and a new understanding of nanoscopic processes occurring in the mineral zircon to record some of the oldest ages for mountain uplift in this portion of the Rocky Mountains. Their new data provide support for previous interpretations that the Rocky Mountains in Utah were being uplifted at least 120 million years ago.
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