Browsing by Author "Kohn, BP"

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    New constraints on the geometry and kinematics of active faults in the Hinterland of the Northwest Himalaya
    (American Geophysical Union, 2014-12-15) Morell, KD; Sandiford, M; Rajendran, CP; Fink, D; Kohn, BP
    The geometry and kinematics of the active, and potentially seismogenic, fault structures within the hinterland of the Himalaya have proven challenging to constrain in the past, primarily because active faults in this region tend to be buried beneath the subsurface and active seismicity often does not align with surficially mapped fault traces. Here we present a series of complementary datasets, including results from low temperature thermochronology, basin-wide erosion rates from 10Be concentrations, and topographic and longitudinal profile analyses, that place constraints on the spatial distribution of fault-related rock uplift and erosion across a ~400-km long region of the lower and high Himalaya of northwest India. Results from our analyses reveal that hillslope morphology and channel steepness are relatively invariant parallel to strike but vary significantly across strike, with the most prominent and abrupt variations occurring at the physiographic transition between the lower and high Himalaya (PT2), near the axial trace of the ramp-flat transition in the Main Himalayan Thrust (MHT). The cross-strike changes in geomorphology observed across the PT2 correlate with an order of magnitude northward increase in basin-wide erosion rates (~0.06-0.8 mm/a) and a corresponding decrease in apatite (~5-2 Ma) and zircon (U-Th)/He (~10-2 Ma) cooling ages. Combined with published geophysical and seismicity data, we interpret these results to reflect spatial variations in rock uplift and exhumation induced by a segment of the MHT ramp-flat system that is at least ~400 km long and ~125 km wide. The relatively young (U-Th)/He ages (<10 Ma) greater than 20 km south of the MHT ramp-flat transition preliminarily suggest that the kinematics of this system are best explained by a model which incorporates an accreting duplex on the MHT ramp but additional forthcoming analyses, including thermal modeling, will confirm if this hypothesis is robust.
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    Quantifying the response of Australian landscapes to climatic and tectonic forcing using cosmogenic isotope analysis
    (18th INQUA Congress, 2011-07-21) Fink, D; Quigley, M; Kohn, BP
    Cosmogenic 10Be concentrations in earth surface materials offer novel information on the rate and spatial patterns of bedrock erosion and basin-wide sediment generation which together influence continental denudation. This opens an opportunity to address the vexing question of how continental landscapes respond to climate and tectonic forcing. One hypothesis is that for equivalent long-term climatic variables, variations in tectonic activity will be reflected in bedrock and sediment 10Be concentrations such that more tectonically active landscapes will yield mean lower 10Be concentrations. We test this hypothesis across the Australian continent by sampling non-glaciated bedrock outcrops and stream sediments from 4 selected ‘end-member’ regions of contrasting climatic (large precipitation, temperature gradients) and tectonic (fault deformation, paleo-earthquake activity) variables with similar granitic lithologies. Localities included tropical northern Queensland and west coast Tasmania (>3000 mm/y), contrasting with late Quaternary neo-tectonically modified regions of the Flinders Ranges, South Australia (<200 mm/y), and tectonically passive but arid Port Hedland and the Darling Scarp, WA. Preliminary cosmogenic-derived erosion rates in regions proximal to active faults with surface ruptures in the last 100 kyr are 1-2 orders of magnitude higher than erosion rates in tectonically inactive regions, implying co-seismic rock fracturing and mass movement exert first order control on bedrock erosion rates on the 100 kyr timescale. Erosion rates derived from subhorizontal bedrock surfaces (and associated stream sediments) spanning Australia's most diverse climates with respect to annual precipitation and temperature gradients are typically <5 m/Ma, implying minimal climatic control on weathering of non-soil mantled bedrock. Cosmogenic nuclide data from across the continent provide quantitative evidence for strong tectonic and weak climatic influence on bedrock erosion. Copyright (c) 2011 INQUA 18
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    Quantitative resolution of the debate over antiquity of the central Australian landscape: implications for the tectonic and geomorphic stability of cratonic interiors
    (Elsevier, 2004-02-28) Belton, DX; Brown, RW; Kohn, BP; Fink, D; Farley, KA
    We report the first measure of long- (∼100 Myr) and short- (∼1 Myr) term denudation rates from key geologically stable landforms in the Davenport Range, central Australia. These landforms have previously been assigned a Cambrian age, which arguably places them amongst the oldest persistent landforms on the continent, if not on Earth. Our results from combined apatite fission track thermochronology and in situ cosmogenic radionuclide analyses using 10Be and 26Al show that while average exhumation rates are low, the denudation history for this cratonic region is incompatible with extreme, sub-aerial longevity and long-term tectonic and geomorphic stability. Our revised model for the landscape evolution of this region is consistent with one of maximum burial prior to and during the Mesozoic, followed by a phase of kilometre-scale exhumation that was largely complete by the beginning of the Cainozoic. We suggest that a similar process of burial and exhumation has probably been responsible for the sub-aerial preservation of seemingly ancient landforms elsewhere in Australia.© 2004 Elsevier B.V.
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    Scanning force microscopy of 129Iodine surface impact structures in muscovite, zircon and apatite as proxies for damage of simulated fission fragments in solids
    (Elsevier B.V., 2013-04-20) Kohlmann, F; Kohn, BP; Gleadow, AJM; Siegele, R
    Using artificially created swift, heavy ions of 129I with a typical fission-fragment mass and specific energies which are common during the spontaneous fission of 238U, the response in minerals commonly used in geochronological dating such as muscovite, zircon and apatite, has been evaluated by the means of Atomic Force Microscopy (AFM). The surface impact structures form hillocks with greater diameters but smaller heights for low-energetic ions and smaller diameters but greater heights for high-energetic ions. The observed hillock widths range from 22 ± 2 nm for low-energetic ions to 17 ± 1 nm for the highest energy in mica. Furthermore, the dimensions formed in apatite for low and high-energy ions range from 53 ± 2 nm to 27 ± 2 nm and from 64 ± 3 nm to 28 ± 1 nm in zircon. With increasing 129I particle energy hillock heights increase, but diameters decrease regardless of mineral orientation. The crystallographic orientation of apatite and zircon also seems to have no effect on hillock dimensions, which are identical for perpendicular and parallel to the c-axis oriented mineral grains. The results support the “Compound Spike” track formation model of Chadderton (2003), which combines the Coulomb ion explosion model with a thermal spike model. © 2013 Elsevier Ltd.