Boulder, Colo., USA – Studies in this Geology posting cover direct dating of brittle fault activity along the Dead Sea fault zone in Northern Israel; onset of the last deglaciation of valley glaciers in southern Patagonia; cutting-edge techniques, including NanoSIMS ion mapping, to identify the microbial metabolism involved in ooid cortex formation; slope failure at Scripps Canyon, California; and the continuing and relatively quick uplift of the U.S. Sierra Nevada, which gains 1-2 mm per year in elevation.
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U-Th dating of striated fault planes
Perach Nuriel et al., School of Earth Sciences, University of Queensland, Brisbane, QLD 4072, Australia. Posted online 27 April 2012; doi: 10.1130/G32970.1.
Direct dating of brittle fault activity is of fundamental importance to tectonic reconstructions and paleoseismic studies. One way to address this issue is by constraining the timing of fault striations, but this requires a better understanding of the striation formation mechanism and associated precipitation processes. Perach Nuriel and colleagues present results from a microstructural, geochemical, and geochronological study of calcite precipitates within striated fault planes from the Dead Sea fault zone in northern Israel. They recognize four types of coexisting precipitates: calcite cement in dilation breccia, calcite in striated groove morphology, calcite gouge associated with hydraulic fracturing and pressure solution, and calcite coating of the fault surface. The geochemistry of the calcite studied indicates that precipitation occurred at different stages of the seismic cycle (co-seismic and inter-seismic). Using Uranium-thorium (U-Th) dating of samples from three adjacent fault planes, Nuriel and colleagues delineate four well-defined deformation ages of between 60 and 220 thousand years old. They conclude that these ages constrain the timing of activity along the Dead Sea fault zone in northern Israel and argue that a similar methodological approach could potentially shed light on the timing of deformation in other brittle fault zones.
Northern Hemisphere forcing of the last deglaciation in southern Patagonia
Daniel S. Murray et al., Dept. of Geoscience, University of Wisconsin, Madison, Wisconsin 53706, USA. Posted online 27 April 2012; doi: 10.1130/G32836.1.
Daniel S. Murray and colleagues investigate the onset of the last deglaciation of valley glaciers in southern Patagonia and what forced ice to retreat. They show that glaciers began to retreat about 19,000 years ago, well before the rise in atmospheric carbon dioxide. Using a global climate model, Murray and colleagues are able to link this glacier retreat from regional warming to the retreat of Northern Hemisphere ice sheets. Ice sheet retreat in response to changes in Earth's orbit around the Sun caused a slowing of ocean circulation from meltwater discharge that resulted in heat being trapped in the Southern Hemisphere. This trapped heat triggered the last deglaciation of southern Patagonia.
Going nano: A new step toward understanding the processes governing freshwater ooid formation
Muriel Pacton et al., Geological Institute, ETH-Zürich, Rämistrasse 101, 8092 Zürich, Switzerland. Posted online 27 April 2012; doi: 10.1130/G32846.1.
Ooids are well-rounded sand grains composed of a nucleus encompassed by poorly to well-developed concentric micritic laminae. Results presented here by Muriel Pacton and colleagues challenge the standard hypothesis that ooids are indicators of turbulent hydrodynamic conditions by showing microbes as the main agent in ooid cortex formation in a quiescent environment. Pacton and colleagues combine cutting-edge techniques (i.e., NanoSIMS ion mapping, scanning electron microscopy imaging and analysis, and secondary ion mass spectrometry delta-13C and delta-18O isotopic analyses) to identify the microbial metabolism involved in ooid cortex formation. The combined elemental mapping and stable isotope study of freshwater ooids indicate that lamina formation is the result of the mineralization of organic substances produced by photosynthetic microbes. This study illustrates the importance of physico-chemical conditions versus organo-mineralization in determining the distribution, abundance, and cortical mineralogy of oolitic sands throughout the Phanerozoic stratigraphic record of carbonate accumulation. These new data further highlight the advantage of using a nano-scale approach to better discern between the various biotic and abiotic processes linked to carbonate precipitation.
Dynamics of dilative slope failure
Yao You et al., Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78713, USA. Posted online 27 April 2012; doi: 10.1130/G32855.1.
We usually associate slope failure of sediments with avalanches, but the case of Scripps Canyon, California, USA, where the mechanical properties of the sediments rule out the possibility of avalanche, does not fit this conventional wisdom. Aided with experiments, Yao You and colleagues propose a new model for an unconventional type of slope failure that can explain the case of Scripps Canyon. They further show that this type of slope failure is much more common than previously assumed and plays an important role in shaping the landscape on the Earth and other planetary bodies like Mars.
Contemporary uplift of the Sierra Nevada, western United States, from GPS and InSAR measurements
William C. Hammond et al., Nevada Geodetic Laboratory, Nevada Bureau of Mines and Geology, and Nevada Seismological Laboratory, University of Nevada, Reno, Nevada 89557, USA. Posted online 27 April 2012; doi: 10.1130/G32968.1.
The Sierra Nevada range in California and Nevada, western United States, stands more than 3 km tall and is the cumulative effect of millions of years of mountain building. New research using space geodetic methods shows that the uplift of the Sierra Nevada continues to this day, adding between 1 mm per year in elevation. To achieve this unprecedented accuracy, William C. Hammond and colleagues analyzed over a decade of GPS and space-based radar data to estimate the long-term trends in crustal uplift. While the uplift rates are very slow on human time scales, they can build the modern elevation of the Sierra Nevada in just a few million years, which is relatively quick compared to estimates based on some geological techniques. Thus, these measurements offer new constraints on the age of the modern Sierra Nevada and suggest that the elevation is relatively young. The data were obtained from GPS networks supported by the National Science Foundation, including the EarthScope Plate Boundary Observatory. Radar data were obtained from the European Space Agency with support from NASA. This research was funded by NASA, the U.S. National Science Foundation, and Natural Environment Research Council of the UK.
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Geology