Bedrock erosion and relief production in the northern Flinders Ranges, Australia

Cosmogenic 10Be concentrations in exposed bedrock surfaces and alluvial sediment in the northern Flinders Ranges reveal surprisingly high erosion rates for a supposedly ancient and stable landscape. Bedrock erosion rates increase with decreasing elevation in the Yudnamutana Catchment, from summit surfaces (13.96 ± 1.29 and 14.38 ± 1.40 m Myr-1), to hillslopes (17.61 ± 2.21 to 29.24 ± 4.38 m Myr-1), to valley bottoms (53.19 ± 7.26 to 227.95 ± 21.39 m Myr-1), indicating late Quaternary increases to topographic relief. Minimum cliff retreat rates (9.30 ± 3.60 to 24.54 ± 8.53 m Myr-1) indicate that even the most resistant parts of cliff faces have undergone significant late Quaternary erosion. However, erosion rates from visibly weathered and varnished tors protruding from steep bedrock hillslopes (4.17 ± 0.42 to 14.00 ± 1.97 m Myr-1) indicate that bedrock may locally weather at rates equivalent to, or even slower than, summit surfaces. 10Be concentrations in contemporary alluvial sediment indicate catchment-averaged erosion at a rate dominated by more rapid erosion (22.79 ± 2.78 m Myr-1), consistent with an average rate from individual hillslope point measurements. Late Cenozoic relief production in the Yudnamutana Catchment resulted from (1) tectonic uplift at rates of 30-160 m Myr-1 due to range-front reverse faulting, which maintained steep river gradients and uplifted summit surfaces, and (2) climate change, which episodically increased both in situ bedrock weathering rates and frequency-magnitude distributions of large magnitude floods, leading to increased incision rates. These results provide quantitative evidence that the Australian landscape is, in places, considerably more dynamic than commonly perceived

Landscape responses to intraplate tectonism: Quantitative constraints from Be-10 nuclide abundances

Cosmogenic 10Be concentrations in bedrock and alluvium combined with structural studies provide a novel approach for identifying neotectonic forcing of landscape evolution in mildly deforming continental interiors. Measured 10Be concentrations in the Flinders Ranges indicate rapid and spatially variable rates of bedrock erosion in a catchment that has incurred at least three large, surface-rupturing earthquakes since ∼ 67 ka. 10Be-derived erosion rates are lower where late Quaternary neotectonic activity is reduced or absent, implying that 10Be concentration may act as a 'tracer' for disequilibrium landscapes responding to recent tectonism. Mechanisms for elevated erosion rates include (1) headward migration of fault-generated bedrock knickpoints and resultant oversteepening of stream profiles and catchment hillslopes and (2) liberation of bedrock material from catchment hillslopes via co-seismic shaking. Despite climatic influences on sediment production and transport, this study shows that tectonism can provide a dominant control on bedrock erosion rate and relief production in unglaciated mountain belts, even in intraplate settings where rates of crustal deformation are mild and earthquake activity is episodic

Geomorphic and cosmogenic nuclide constraints on escarpment evolution in an intraplate setting, Darling Escarpment, Western Australia

The ~900km long Darling Scarp in Western Australia is one of the most prominent linear topographic features on Earth. Despite the presence of over-steepened reaches in all westerly flowing streams crossing the scarp, and significant seismic activity within 100km of the scarp, there is no historical seismicity and no reported evidence for Quaternary tectonic displacements on the underlying Darling Fault. Consequently, it is unclear whether the scarp is a rapidly evolving landform responding to recent tectonic and/or climatic forcing or a more slowly evolving landform. In order to quantify late Quaternary rates of erosion and scarp relief processes, we obtained measurements of the cosmic-ray produced nuclide beryllium-10 (10Be) from outcropping bedrock surfaces along the scarp summit and face, in valley floors, and at stream knickpoints. Erosion rates of bedrock outcrops along the scarp summit surface range from 0·5 to 4·0mMyr-1. These are in the same range as erosion rates of 2·1 to 3·6mMyr-1 on the scarp face and similar to river incision rates of 2·6 to 11·0mMyr-1 from valley floor bedrock straths, indicating that the Darling Scarp is a slowly eroding 'steady state' landform, without any significant contemporary relief production over the last several 100kyr and possibly several million years. Knickpoint retreat rates determined from 10Be concentrations at the bases of two knickpoints on small streams incised into the scarp are 36 and 46mMyr-1. If these erosion rates were sustained over longer timescales, then associated knickpoints may have initiated in the mid-Tertiary to early Neogene, consistent with early-mid Tertiary marginal uplift. Ongoing maintenance of stream disequilibrium longitudinal profiles is consistent with slow, regional base level lowering associated with recently proposed continental-scale tilting, as opposed to differential uplift along discrete faults. Cosmogenic 10Be analysis provides a useful tool for interpreting the palaeoseismic history of intraplate near-fault landforms over 105 to 106 years

Geomorphology reveals active decollement geometry in the central Himalayan seismic gap

The similar to 700-km-long ``central seismic gap'' is the most prominent segment of the Himalayan front not to have ruptured in a major earthquake during the last 200-500 yr. This prolonged seismic quiescence has led to the proposition that this region, with a population >10 million, is overdue for a great earthquake. Despite the region's recognized seismic risk, the geometry of faults likely to host large earthquakes remains poorly understood. Here, we place new constraints on the spatial distribution of rock uplift within the western similar to 400 km of the central seismic gap using topographic and river profile analyses together with basinwide erosion rate estimates from cosmogenic Be-10. The data sets show a distinctive physiographic transition at the base of the high Himalaya in the state of Uttarakhand, India, characterized by abrupt strike-normal increases in channel steepness and a tenfold increase in erosion rates. When combined with previously published geophysical imaging and seismicity data sets, we interpret the observed spatial distribution of erosion rates and channel steepness to reflect the landscape response to spatially variable rock uplift due to a structurally coherent ramp-flat system of the Main Himalayan Thrust. Although it remains unresolved whether the kinematics of the Main Himalayan Thrust ramp involve an emergent fault or duplex, the landscape and erosion rate patterns suggest that the decollement beneath the state of Uttarakhand provides a sufficiently large and coherent fault segment capable of hosting a great earthquake