Tectonic Controls on Himalayan Denudation?

Abstract

The effect of late-Cenozoic climate cooling on global erosion rates is intensely debated (e.g., Willenbring & Jerolmack, 2016). First, there is a lack of consensus on the use of the sedimentary record as a proxy for erosion (Straub et al., 2020; Willenbring & von Blanckenburg, 2010) and second, the paucity of data in available paleo-erosion rate records makes differentiation between tectonic and climatic forcings difficult (Schildgen et al., 2018; Whipple, 2009). More recently, Mandal et al. (2021) analyzed cosmogenic 10Be concentrations in foreland sediments from the north-western Indian Himalaya dating back 6 Ma. Their detailed erosion record suggests an approximately one-million-year periodicity in erosion rates with a gradual increase towards the present. The authors relate the inferred quasi-cyclic pattern in erosion rates to the tectonic growth of the Himalaya, rather than to changes in climate. Their findings suggest that tectonic accretion processes dominate erosion rate changes over the late-Cenozoic, and may thus confound climatic interpretations of erosion rate histories in collisional orogenic settings globally. Cosmogenic nuclides (especially 10Be) have been shown to be a robust metric of erosion rates (Granger & Schaller, 2014). However, applying the technique to ancient sediment is not trivial. The cosmogenic nuclide inventory in an ancient sediment parcel will be a function of the mean erosion rate, altitude, and latitude of the sediment's source area, and of its pathway through the sediment routing system into the depositional sink. At the source, changes in erosion rate will manifest in changes in nuclide concentration, moderated by the response time of the cosmogenic nuclide “clock.” The latter, in turn, depends on the nuclide's half-life, the amplitude and duration of forcing, and the amount of buffering occurring within the landscape (Granger & Schaller, 2014) (Figure 1a). As the sediment parcel moves through the colluvial and fluvial systems, its cosmogenic nuclide inventory may increase, decrease, or stay constant. Further, the degree of change in nuclide concentration will also depend on the production rate and half-life of each cosmogenic nuclide (Figure 1a). Protracted sediment transport may result in an increase in nuclide concentrations, whereas reworking of deeply buried and old sediment will incorporate material with depleted inventories of shorter-lived nuclides such as 26Al, 36Cl, and 14C, but with unchanged inventories of longer-lived or stable nuclides, such as 10Be and 21Ne (Fülöp et al., 2020; Vermeesch et al., 2010). Changes to nuclide inventories may also occur in the depositional sink. Following abandonment, cosmogenic nuclide accumulation will continue until the overburden becomes thick enough to completely shield cosmic rays and may also occur as the stratigraphic section is reworked and exposed at the surface by subsequent geological processes (Straub et al., 2020). Lastly, substantial loss of nuclides via radioactive decay will occur in the sink, and, depending on the deposition age, may result in the complete depletion of shorter-lived radionuclides (Figure 1a). Changes in cosmogenic nuclide inventories during source-to-sink sediment transfer. (a) Cosmogenic nuclides accumulate in surficial rocks in proportion to the rate of erosion. Following detachment from bedrock, the cosmogenic nuclide inventory in a parcel of sediment moving from the source region undergoing erosion to the depositional sink may increase, decrease, or stay constant, depending on the characteristics of the sediment routing system, the rate of deposition in the sink, and the nuclide's half-life. (b) Multiple cosmogenic nuclide pairs measured in ancient fluvial sediment from a sedimentary section may provide independent constraints on the alteration of the cosmogenic nuclide signal during sediment transport and deposition in addition to providing controls on deposition age. It is impossible to unravel the complex histories of sediment grains as they travel from the erosional source areas to the depositional sinks from the abundance of a single cosmogenic nuclide (e.g., 10Be). To minimize environmental signal buffering associated with complex sediment routing systems, Mandal et al. (2021) target a proximal foreland basin that is, intimately linked to the eroding hinterland. The high erosion rates characterizing tectonically active mountain ranges (Codilean et al., 2018) mean that cosmogenic 10Be response times to changes in erosion rates are short in these settings. Further, despite prolific sediment buffering occurring in the source regions of Himalayan rivers (Blöthe & Korup, 2013), there is evidence that the 10Be record of mountain erosion may be effectively transmitted with minimal alteration (Wittmann & von Blanckenburg, 2016). Nevertheless, important limitations remain. First, there is no independent control on paleo-source-area characteristics or lag times (Figure 1b). Second, how the 10Be signal might be altered once sediment is deposited in the sink is also largely unconstrained, as a proximal foreland basin may include hiatuses or unsteady accumulation rates that directly reflect the hinterland tectonics and climate (Straub et al., 2020). The envelope of accumulation rates that are determinable from Mandal et al.’s magnetostratigraphic record does expand at time scales less than 1.3 million years, as would be expected for a strong periodicity among hiatuses in the stratigraphic section itself (Sadler & Strauss, 1990). Mandal et al. (2021) identify fluctuations in erosion rate that approach long timescales at which several interrelated environmental forcings might be dominant: Milankovitch grand cycles of climate change, source area uplift, and the tectonic accommodation of resulting sediments in a non-marine setting. In the proximal Himalayan foreland, the authors apply cosmogenic 10Be analysis and magnetostratigraphy for independent dating of locally unsteady, upstream erosion and downstream accommodation rate, respectively. They point to the imbricate thrust faults of the upstream duplex structures as plausible drivers of correspondingly pulsed uplift. The challenges, going forward, are to find an independent means to date individual fault histories and to better resolve the potentially complex sediment pathways. Numerical experiments have suggested that climate-driven increases in erosion rate would be out of phase with accommodation rate in the adjacent basins (Whipple, 2009), leading to two questions pertinent for this study: do empirical erosion rates, tectonic pulses and sediment accumulation rates share the same periodicity? Are they in-phase or out-of-phase? The ways landscapes respond to tectonic or climatic perturbations will depend on the perturbation itself and on how the erosional and depositional parts of the landscape are linked by the sediment-routing system (Allen, 2008). The ability to identify changes in erosion rates through time will depend on the temporal resolution of the erosion rate record and on the capacity of the sediment-routing system to buffer environmental signals (e.g., Lenard et al., 2020). Settling the debate on the links between late-Cenozoic cooling, tectonics, and global erosion rates will require paleo-erosion-rate records with a high temporal resolution (e.g., Mariotti et al., 2021) and approaches that leverage the power of multiple cosmogenic nuclide pairs to decipher the complex histories of sediment as they travel from the erosional source to the depositional sink.

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