Abstract
Abstract. Individual, large thrusting earthquakes can cause hundreds to thousands of years of exhumation in a geologically instantaneous moment through landslide generation. The bedrock landslides generated are important weathering agents through the conversion of bedrock into mobile regolith. Despite this, orogen-scale records of surface uplift and exhumation, whether sedimentary or geochemical, contain little to no evidence of individual large earthquakes. We examine how earthquakes and landslides influence exhumation and surface uplift rates with a zero-dimensional numerical model, supported by observations from the 2008 Mw 7.9 Wenchuan earthquake. We also simulate the concentration of cosmogenic radionuclides within the model domain, so we can examine the timescales over which earthquake-driven changes in exhumation can be measured. Our model uses empirically constrained relationships between seismic energy release, weathering, and landsliding volumes to show that large earthquakes generate the most surface uplift, despite causing lowering of the bedrock surface. Our model suggests that when earthquakes are the dominant rock uplift process in an orogen, rapid surface uplift can occur when regolith, which limits bedrock weathering, is preserved on the mountain range. After a large earthquake, there is a lowering in concentrations of 10Be in regolith leaving the orogen, but the concentrations return to the long-term average within 103 years. The timescale of the seismically induced cosmogenic nuclide concentration signal is shorter than the averaging time of most thermochronometers (>103 years). However, our model suggests that the short-term stochastic feedbacks between weathering and exhumation produce measurable increases in cosmogenically measured exhumation rates which can be linked to earthquakes.
Highlights
Surface uplift of a mountain range is controlled by the balance of additive uplift processes and removal of material by surface, typically fluvial, processes
If shaking can produce similar volumes of landsliding regardless of how much bedrock or regolith is on the hillslopes, landslide deposits in a mountain range with widespread regolith will contain less fresh bedrock, as regolith will make up a greater proportion of material mobilised by the earthquake
The decline of coseismic landslide regolith production (CLRP) with existing regolith thickness is reminiscent of soil production functions described for soil mantled landscapes (Heimsath et al, 1997), and by analogy, we term the non-linear relationship between regolith production rate and the average depth of weathering by landslides seen here a “coseismic landslide regolith production function” (CLRPF)
Summary
Surface uplift of a mountain range is controlled by the balance of additive uplift processes and removal of material by surface, typically fluvial, processes. Earthquakes produce rock uplift and importantly generate exhumation via landsliding (Avouac, 2007; Keefer, 1994; Marc et al, 2016a). Existing mass balances on single (Parker et al, 2011) or sequences (Li et al, 2014, 2019; Marc et al, 2016b) of earthquakes demonstrate that landslide volumes of large thrust earthquakes are comparable to, and may exceed, rock uplift. Earthquakes are not the only rock uplift process in mountain belts; aseismic mechanisms such as viscous and elastic crustal deformation (Meade, 2010; Simpson, 2015), lithospheric delamination (Hales et al, 2005; Molnar et al, 1993), and isostatic rock uplift (Molnar, 2012; Molnar et al, 2015) can contribute.
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