Millennial Erosion Rates Research Articles

Bedrock gorge incision via anthropogenic meander cutoff

Abstract Bedrock river-gorge incision represents a fundamental landscape-shaping process, but a dearth of observational data at >10 yr timescales impedes understanding of gorge formation. I quantify 102 yr rates and processes of gorge incision using historical records, field observations, and topographic and image analysis of a human-caused bedrock meander cutoff along the North Fork Fortymile River in Alaska (USA). Miners cut off the meander in 1900 CE, abruptly lowering local base level by 6 m and forcing narrowing and steepening of the channel across a knickpoint that rapidly incised upstream. Tectonic quiescence, consistent rock erosivity, and low millennial erosion rates provide ideal boundary conditions for this 102 yr gorge-formation experiment. Initial fast knickpoint propagation (23 m/yr; 1900–1903 CE) slowed (4 m/yr; 1903–1981 CE) to diffusion (1981–2019 CE) as knickpoint slope decreased, yielding an ~350-m-long, 6-m-deep gorge within the pre–1900 CE channel. Today, diffusion dominates incision of a 500-m-long knickzone upstream of the gorge, where sediment transport likely limits ongoing adjustments to the anthropogenic cutoff. Results elucidate channel width, slope, discharge, and sediment dynamics consistent with a gradual transition from detachment- to transport-limited incision in fluvial adjustment to local base-level lowering.

Impact of fault damage on eastern Tibet topography

AbstractTectonic deformation can influence spatiotemporal patterns of erosion by changing both base level and the mechanical state of bedrock. Although base-level change and the resulting erosion are well understood, the impact of tectonic damage on bedrock erodibility has rarely been quantified. Eastern Tibet, a tectonically active region with diverse lithologies and multiple active fault zones, provides a suitable field site to understand how tectonic deformation controls erosion and topography. In this study, we quantified erosion coefficients using the relationship between millennial erosion rates and the corresponding channel steepness. Our work shows a twofold increase in erosion coefficients between basins within 15 km of major faults compared to those beyond 15 km, suggesting that tectonic deformation through seismic shaking and rock damage significantly affects eastern Tibet erosion and topography. This work demonstrates a field-based, quantitative relationship between rock erodibility and fault damage, which has important implications for improving landscape evolution models.

Erosion after an extreme storm event in an arid fluvial system of the southern Atacama Desert: an assessment of the magnitude, return time, and conditioning factors of erosion and debris flow generation

Abstract. The contribution of an individual extreme storm event to long-term erosion rates has been estimated for the first time in the Atacama Desert. A mean erosion of 1.3 mm has been calculated for the March 2015 event that impacted the southernmost part of the Atacama Desert. The estimated erosion is consistent with millennial erosion rates and the previously reported return times of high-sediment-discharge events in the study area. This is significant because erosion rates, related to events of high sediment discharge in arid fluvial systems, are difficult to measure with sediment loading due to destruction of gauges by devastating flash floods and therefore have not been directly measured yet. During the March 2015 storm, debris flows were reported as the main sediment transport process, while gullies and channels erosion were the main source of sediments that generated debris flows reaching the tributary junctions and the trunk valleys. Sediment yield at tributary outlets is highly dependent on the ability of catchments to store sediments in stream networks between storms. The largest tributary catchments, the high hydrological hierarchy, the low topographic gradient and the gentle slopes are the most determining factors in generating debris flows capable of reaching alluvial fans in any storm event from large sediment volumes stored in the stream networks. Our findings better assess the susceptibility to debris flow of arid catchments, which is significant for the southernmost valleys of the Atacama Desert because human settlements and industries are mostly established in alluvial fans.

Late Cenozoic cooling history of the central Menderes Massif: Timing of the Büyük Menderes detachment and the relative contribution of normal faulting and erosion to rock exhumation

Based on new thermochronological data and 10Be-derived erosion rates from the southern part of the central Menderes Massif (Aydın block) in western Turkey, we provide new insights into the tectonic evolution and landscape development of an area that undergoes active continental extension. Fission-track and (U-Th)/He data reveal that the footwall of the Büyük Menderes detachment experienced two episodes of enhanced cooling and exhumation. Assuming an elevated geothermal gradient of ~50°C/km, the first phase occurred with an average rate of ~0.90km/Myr in the middle Miocene and the second one in the latest Miocene and Pliocene with a rate of ~0.43km/Myr. The exhumation rates between these two phases were lower and range from ~0.14 to ~0.24km/Myr, depending on the distance to the detachment. Cosmogenic nuclide-based erosion rates for catchments in the Aydın block range from ~0.1 to ~0.4km/Myr. The similarity of the erosion rates on both sides of the Aydın block (northern and southern flank) indicate that a rather symmetric erosion pattern has prevailed during the Holocene. If these millennial erosion rates are representative on a million-year timescale they indicate that, apart from normal faulting, erosion in the hanging wall of the Büyük Menderes detachment fault did also contribute to the exhumation of the metamorphic rocks.

Decoupling of long-term exhumation and short-term erosion rates in the Sikkim Himalaya

Understanding the relative strengths of tectonic and climatic forcing on erosion at different spatial and temporal scales is important to understand the evolution of orogenic topography. To address this question, we quantified exhumation rates at geological timescales and erosion rates at millennial timescales in modern river sands from 10 sub-catchments of the Tista River drainage basin in the Sikkim Himalaya (northeast India) using detrital apatite fission-track thermochronology and cosmogenic 10Be analyses, respectively. We compare these rates to several potential geomorphic or climatic forcing parameters. Our results show that millennial erosion rates are generally higher and spatially more variable than long-term exhumation rates in Sikkim. They also show strongly contrasting spatial patterns, suggesting that the processes controlling these rates are decoupled. At geological timescales, exhumation rates decrease from south to north, with rates up to 1.2±0.6 mm/yr recorded in southwest Sikkim and as low as 0.5±0.2 mm/yr in the northernmost catchment. Long-term exhumation rates do not correlate with any geomorphic or climatic parameter. We suggest they are tectonically controlled: high rates in southwest Sikkim may be linked to the building of the Lesser Himalaya Rangit Duplex, whereas low rates in north Sikkim are consistent with cessation of extensional exhumation along the South Tibetan Detachment after 13 Ma. The highest apparent erosion rates recorded by cosmogenic nuclides (∼5 mm/yr) occur in catchments spanning the Main Central Thrust Zone, but these appear to be strongly influenced by recent landsliding. High millennial erosion rates (1–2 mm/yr) also occur in north Sikkim and may be climatically driven through strong glacial inheritance of the landscape, as attested by high channel-steepness values close to the maximum extent of glaciers during the Last Glacial Maximum. In contrast, variations in rainfall rate do not seem to strongly influence either millennial erosion or long-term exhumation rates in Sikkim.

Deciphering the driving forces of erosion rates on millennial to million‐year timescales in glacially impacted landscapes: An example from the Western Alps

In many regions, tectonic uplift is the main driver of erosion over million‐year (Myr) timescales, but climate changes can markedly affect the link between tectonics and erosion, causing transient variations in erosion rates. Here we study the driving forces of millennial to Myr‐scale erosion rates in the French Western Alps, as estimated from in situ produced cosmogenic 10Be and a newly developed approach integrating detrital and bedrock apatite fission‐track thermochronology. Millennial erosion rates from 10Be analyses vary between ~0.27 and ~1.33 m/kyr, similar to rates measured in adjacent areas of the Alps. Significant positive correlations of millennial erosion rates with geomorphic measures, in particular with the LGM ice thickness, reveal a strong transient morphological and erosional perturbation caused by repeated Quaternary glaciations. The perturbation appears independent of Myr‐scale uplift and erosion gradients, with the effect that millennial erosion rates exceed Myr‐scale erosion rates only in the internal Alps where the latter are low (<0.4 km/Myr). These areas, moreover, exhibit channels that clearly plot above a general linear positive relation between Myr‐scale erosion rates and normalized steepness index. Glacial erosion acts irrespective of rock uplift and thus not only leads to an overall increase in erosion rates but also regulates landscape morphology and erosion rates in regions with considerable spatial gradients in Myr‐scale tectonic uplift. Our study demonstrates that climate change, e.g., through occurrence of major glaciations, can markedly perturb landscape morphology and related millennial erosion rate patterns, even in regions where Myr‐scale erosion rates are dominantly controlled by tectonics.

Slope and climate variability control of erosion in the Andes of central Chile

Climate and topography control millennial-scale mountain erosion, but their relative impacts remain matters of debate. Confl icting results may be explained by the infl uence of the erosion threshold and daily variability of runoff on long-term erosion. However, there is a lack of data documenting these erosion factors. Here we report suspended-load measurements, derived decennial erosion rates, and 10 Be-derived millennial erosion rates along an exceptional climatic gradient in the Andes of central Chile. Both erosion rates (decennial and millenial) follow the same latitudinal trend, and peak where the climate is temperate (mean runoff ~500 mm yr ‐1 ). Both decennial and millennial erosion rates increase nonlinearly with slope toward a threshold of ~0.55 m/m. The comparison of these erosion rates shows that the contribution of rare and strong erosive events to millennial erosion increases from 0% in the humid zone to more than 90% in the arid zone. Our data confi rm the primary role of slope as erosion control even under contrasting climates and support the view that the infl uence of runoff variability on millennial erosion rates increases with aridity.

Spatial controls on erosion in the Three Rivers Region, southeastern Tibet and southwestern China

Global data suggest that erosion rates variously scale with steepness or climate forcing (precipitation or glacial excavation), but the relative influence of these factors has proven difficult to assess without comparisons from a single location. A new suite of detrital 10Be data from the Three Rivers Region, SE Tibet is used to examine the relative importance of rainfall and relief in predicting patterns of erosion rates across a region with a strong gradient in exhumation. The data reveal millennial erosion rates vary by two orders of magnitude, from 0.01 to 8 mm/yr across a regional gradient in exhumation rates inferred from previous thermochronology and cosmogenic nuclide data to the west and east of the study region. The new millennial erosion rates mirror the pattern of decreasing exhumation rates from west to east across the region, with the highest rates in the lower Salween River drainage and the lowest rates in the Yangtze River drainage. Erosion rates in the Mekong and Salween River drainages are correlated with mean local relief whereas in the Yangtze River drainage they are correlated most strongly with mean annual rainfall. The tectonic setting of this region, with a strong west to east gradient in exhumation rates which we infer to mirror a gradient in rock uplift, seems to exert a stronger control on erosion rate patterns than rainfall or relief.

Landscape form and millennial erosion rates in the San Gabriel Mountains, CA

It has been long hypothesized that topography, as well as climate and rock strength, exert first order controls on erosion rates. Here we use detrital cosmogenic 10Be from 50 basins, ranging in size from 1 to 150 km 2, to measure millennial erosion rates across the San Gabriel Mountains in southern California, where a strong E–W gradient in relief compared to weak variation in precipitation and lithology allow us to isolate the relationship between topographic form and erosion rate. Our erosion rates range from 35 to 1100 m/Ma, and generally agree with both decadal sediment fluxes and long term exhumation rates inferred from low temperature thermochronometry. Catchment-mean hillslope angle increases with erosion rate until ∼ 300 m/Ma, at which point slopes become invariant with erosion rate. Although this sort of relation has been offered as support for non-linear models of soil transport, we use 1-D analytical hillslope profiles derived from existing soil transport laws to show that a model with soil flux linear in slope, but including a slope stability threshold, is indistinguishable from a non-linear law within the scatter of our data. Catchment-mean normalized channel steepness index increases monotonically, though non-linearly, with erosion rate throughout the San Gabriel Mountains, even where catchment-mean hillslope angles have reached a threshold. This non-linearity can be mostly accounted for by a stochastic threshold incision model, though additional factors likely contribute to the observed relationship between channel steepness and erosion rate. These findings substantiate the claim that the normalized channel steepness index is an important topographic metric in active ranges.