Variations In Denudation Research Articles

Denudation rates and tectonic geomorphology of the Spanish Betic Cordillera

The tectonic control on landscape morphology and long-term denudation is largely documented for settings with high uplift rates. Relatively little is known about the rates of geomorphic response in areas of low tectonic uplift. Here, we evaluate spatial variations in denudation of the Spanish Betic Cordillera based on cosmogenic 10Be-derived denudation rates. Denudation rates are compared to published data on rock uplift and exhumation of the Betic Cordillera to evaluate steady-state topography. The spatial patterns of catchment-wide denudation rates (n=20) are then analysed together with topographic metrics of hillslope and channel morphology. Catchments draining the Betic ranges have relatively low denudation rates (64±54 mmkyr−1), but also show large variation as they range from 14 to 246 mm kyr−1. Catchment-wide denudation is linearly proportional to the mean hillslope gradient and local relief. Despite large spatial variation in denudation, the magnitude and spatial pattern of denudation rates are generally consistent with longer-term local uplift rates derived from elevated marine deposits, fission-track measurements and vertical fault slip rates. This might be indicative of a steady-state topography where rock uplift is balanced by denudation.

Large spatial and temporal variations in Himalayan denudation

In the last decade growing interest has emerged in quantifying the spatial and temporal variations in mountain building. Until recently, insufficient data have been available to attempt such a task at the scale of large orogens such as the Himalaya. The Himalaya accommodates ongoing convergence between India and Eurasia and is a focal point for studying orogen evolution and hypothesized interactions between tectonics and climate. Here we integrate 1126 published bedrock mineral cooling ages with a transient 1D Monte-Carlo thermal–kinematic erosion model to quantify the denudation histories along ~2700km of the Himalaya. The model free parameter is a temporally variable denudation rate from 50Ma to present. Thermophysical material properties and boundary conditions were tuned to individual study areas. Monte-Carlo simulations were conducted to identify the range of denudation histories that can reproduce the observed cooling ages. Results indicate large temporal and spatial variations in denudation and these are resolvable across different tectonic units of the Himalaya. More specifically, across >1000km of the southern Greater Himalaya denudation rates were highest (~1.5–3mm/yr) between ~10 and 2Ma and lower (0.5–2.6mm/yr) over the last 2My. These differences are best determined in the NW-Himalaya. In contrast to this, across the ~2500km length of the northern Greater Himalaya denudation rates vary over length scales of ~300–1700km. Slower denudation (<1mm/yr) occurred between 10 and 4Ma followed by a large increase (1.2–2.6mm/yr) in the last ~4Ma. We find that only the southern Greater Himalayan Sequence clearly supports a continuous co-evolution of tectonics, climate and denudation. Results from the higher elevation northern Greater Himalaya suggest either tectonic driven variations in denudation due to a ramp-flat geometry in the main décollement and/or recent glacially enhanced denudation.

Global climate and tectonic controls on the denudation of glaciated mountains

Significant uncertainty exists concerning the efficiency of alpine glacial erosion relative to fluvial and hillslope processes. Latitudinal variations in temperature are important for determining the extent of glaciers, as are the rates of tectonic uplift that influence the elevation (and hence temperatures) that glaciers can form. The acute sensitivity of glacial erosion to temperature has complicated previous interpretations because temperatures must be cool enough to maintain ice yet warm enough to allow glacial sliding. Here we quantify the influence of climate and tectonics on glacial landscape evolution with a coupled glacial, fluvial, and hillslope landscape evolution model that systematically explores variations in rock-uplift rate and periodic variations in climate (i.e. glacial–interglacial periods) over million-year time scales. Emphasis is placed on understanding when a particular climate is either more (e.g. “buzzsaw” conditions) or less erosive than its preglacial landscape. Results indicate that the erosional efficiency of glaciers varies as a function of latitudinal controlled temperature and rock-uplift rate. An order of magnitude increase in erosion rates occurs in some scenarios for both localized (valley bottom) erosion and short-term (one glacial period) durations of glaciation. However, when averaged over the entire landscape for 2Ma, increases in glacial erosion are typically less than double that of the preglacial landscape. In some scenarios, average glacial erosion rates are less than preglacial rates due to either small, inefficient glaciers or extensive cold-based glaciation. Model predictions are compared with a compilation of long-term denudation rates from glaciated mountain ranges and indicate models perform well at explaining patterns of glacial erosion efficiency. The findings presented here have clear implications for the impact of glaciations on the evolution of landscapes including: (1) the climatic “window” in which glaciers are more erosive compared to pre-glacial rates; (2) spatial and temporal variations in denudation that can lead to pulses of erosion; and (3) predictions of glacial erosional efficiency at different latitudes. We conclude that latitudinal and elevation dependent variations in temperature control the efficiency of glacial denudation and explain discrepancies between previous studies.

Spatial and temporal variations in denudation of the Wasatch Mountains, Utah, USA

We evaluate spatial and temporal variations in denudation of the north-central Wasatch Mountains, Utah, by determining catchment-wide denudation rates with 10Be concentrations in alluvial sediment and comparing these rates with previously published data on rock uplift and exhumation of the range. Catchments draining the range front show relatively little variation in denudation rate (0.07–0.17 mm/yr), while steeper (mean hillslope gradient >30°) catchments in the core of the range show larger variation (0.17–0.79 mm/yr). We attribute the larger spatial variation in catchment-wide denudation in the core of the range to landsliding of hillslopes at threshold gradients; faster denudation in this region may signify landscape adjustment to late Pleistocene glaciations. The mean denudation rate for all catchments (0.2 mm/yr) is generally consistent with longer-term exhumation rates derived from thermochronometers and with shorter-term vertical fault displacement rates, suggesting that denudation of the north-central Wasatch has been roughly steady, or decreasing slightly, over the past 5 m.y. Although 10Be-based catchment-wide denudation rates are sensitive to localized geomorphic processes and events, overall, they appear to reflect the larger tectonic forces that have driven denudation of the Wasatch Mountains over longer time scales.

Apatite (U – Th)/He age constraints on the Mesozoic and Cenozoic evolution of the Bathurst region, New South Wales: evidence for antiquity of the continental drainage divide along a passive margin

The apatite (U – Th)/He and fission track thermochronometers are combined to constrain the Mesozoic and Cenozoic denudational history of the Bathurst region in New South Wales. New apatite (U – Th)/He ages across the continental drainage divide range from 83 ± 14 to 114 ± 18 Ma (±2σ), supporting the evidence from previously published apatite fission track (AFT) data that the region underwent a period of rapid denudation during the mid-Cretaceous. The apatite He-derived thermal histories constrain the amount of erosion in the area and suggest that spatial variations in denudation of the Bathurst Batholith correspond to three broad regions with different histories. The area of the continental drainage divide separates the eastern flank where denudation was rapid at 120 – 90 Ma, and >3 km of crust was removed in about 30 million years, from the western flank, where denudation was less rapid and less prominent. The coincidence of some of the AFT and He ages with the timing of continental extension and Tasman Sea opening suggests that the rapid denudation east of the continental drainage divide was associated with the rifting processes. Such an interpretation means that the escarpment that bounds the Blue Mountains on the east was not formed at the time of rifting and may be a later topographic feature, connected with exhumation of an earlier structural feature and/or related to post-Mesozoic tectonic activity. The evolution of the area west of the continental drainage divide is characterised by rates of denudation higher than on the rest of the plateau but lower than on the eastern flank in the Bathurst area. The reason for this pattern is still unclear. It is possible that faults of the Lachlan Fold Belt were reactivated during the Cretaceous extension and later, but no faults are mapped across the batholith. Overall, the data imply that the continental margin in the Bathurst region evolved by plateau downwearing pinned to the continental drainage divide. This means in turn that the continental drainage divide is a longstanding topographic feature, dating from before Tasman Sea breakup.

Latitudinal Variation of Denudation in the Evolution of the Bolivian Andes

Latitudinal gradients in topography, relief, climate, and deformation have been used to suggest that climate-driven erosion has exerted a first order control on the development of the central Andes. We synthesize the spatial and temporal variations in denudation across the eastern Bolivian Andes (14-22°S) from new and existing estimates to test whether physical evidence exists to support the hypothesis that erosion influences thrust belt evolution. Basin-morphometry, channel network indices, climate, and longitudinal river profiles indicate a northward increase in relative relief, fluvial incision, and denudation. Short-term denudation-rate averages from landslide mapping and sediment-flux data range from 1 to 9 mm/yr in the north compared to 0.3 to 0.4 mm/yr in the south. Long-term denudation-rate estimates from thermochronology, cosmogenic radionuclides, foreland basin sediment volumes, stra- tigraphy, paleoerosion surface degradation, and balanced cross sections range from 0.04 to 1.6 mm/yr with rates up to more than twice as fast in the north when comparing estimates from the same method. The shorter-term denudation rates exhibit the greatest variance. Our denudation synthesis shows that an along-strike disparity in denudation (greater in the north) has existed throughout the Holocene and perhaps existed since as early as the late Miocene. Our denudation synthesis also suggests that the disparity and denudation rates have increased to the present. Correlations between the thrust belt geology, geometry, geomorphology, climate, and kinematics of the orogenic wedge provides a case study in observing the regional scale interactions between uplift, climate, and erosion. We conclude that the denudation history, uplift history, and tectonic-geomorphic correlations suggest that models of the evolution of the Bolivian Andes should incorporate a latitudinal erosion gradient for the last 10 kyrs to perhaps 10 Myrs.

Mesozoic regolith and palaeolandscape features in southeastern Australia: Significance for interpretations of denudation and highland evolution

The long‐term survival of regolith and landscape features of Mesozoic origins in southeastern Australia is supported by the sedimentary record, pre‐volcanic remnants, oxygen‐isotope signatures of secondary minerals, and the associated geomorphological context. The recognition of these ancient landscape remnants reflects the establishment of a major paradigm in Australian landscape studies after early interpretations emphasised a predominantly Late Cenozoic landscape history. The Mesozoic regolith and landscape remnants constrain interpretations of the evolution of the highlands in this region, indicating that Late Mesozoic and Cenozoic denudation did not extend beyond the complete removal of Mesozoic landscape features. Apatite fission track thermochronology interpretations invoke kilometre‐scale denudation levels across the southeastern highlands during the mid‐Cretaceous, which at first may appear contradictory to the geomorphological evidence of restricted denudation. Rather than necessarily being mutually exclusive interpretations, possible scenarios allowing for the preservation of Mesozoic palaeosurfaces along with kilometre‐scale denudation in the mid‐Cretaceous include: (i) exhumation of palaeosurfaces from beneath a thick sedimentary cover; (ii) truncation of formerly more extensive weathering profiles; and (iii) local‐scale variations in denudation. Local‐scale variations in denudation may not have been detected in previous studies due to dangers associated with overextending regional extrapolations and interpretations, as well as a tendency for the field basis of the different denudation models to emphasise different parts of the landscape. Field studies from areas where there has been localised deep incision into Mesozoic landscape remnants highlight the problem. Geomorphological studies have tended to emphasise areas of relative stability, featuring the preservation of regolith materials and the associated long‐term landscape record. In contrast, apatite fission track thermochronology interpretations may tend to emphasise a regional thermal history related to maximum denudation. Local variations in denudation leading to the preservation of Mesozoic palaeosurfaces have mainly been facilitated by localised lithological and structural controls on stream base‐levels and knickpoints, tectonic setting, and sedimentary and volcanic burial. When palaeolandscape interpretations are considered at the local scale, arguments proposing long‐term stability based on palaeolandscape remnants and apatite fission track thermochronology interpretations of large‐scale denudation may therefore not necessarily be contradictory.

Temporal cycles of karst denudation in northwest Georgia, U.S.A.

AbstractTime patterns of karst denudation in northwest Georgia (U.S.A.) were investigated at three spring sites for 12 months and at five stream sites for 10 years. Rainfall was evenly distributed and showed no significant seasonality. At the springs, as well as the streams, water hardness was largely controlled by discharge. At the springs, soil pCO2 and water pH were strongly correlated (r + ‐0·69 to ‐0·83). Solute transport in spring waters was highly seasonal, with two conduit flow springs removing more limestone in the winter, and the diffuse flow spring removing more during the growing season. At the stream sites, most denudation occurred during the winter and spring seasons, and least during the summer.Fourier analysis showed that variations in denudation occur on deterministic (long‐wave) as well as stochastic (shortwave) time scales. As contributing variables, discharge varied in short‐wave and long‐wave cycles, whereas soil pCO2 showed only a long‐wave cycle. The 12 month deterministic cycles were the most important, with changes in discharge taking precedence over soil pCO2. Time series regression explains up to 69 per cent of changes in denudation through rain and soil pCO2. Time cycles in available water are the key controlling factor of denudation, and amounts of available soil CO2 may not be as important in the temporal patterns of karst downwearing as has been believed previously.

Reply. Denudational isostatic rebound of intraplate highlands: The Lachlan river valley, Australia

AbstractTwo main points are raised in replying to Fabel et al. (1993) in their discussion of the paper by Bishop and Brown (1992). It is argued that local variations in denudation are insufficient to invalidate the key conclusions, especially as the amount of sediment deposited in the Murray Basin is a key (and known) measure of the amount of denudation. The total evidence for rebound is now known to be regional in extent, supporting the relevance of a regional isostatic model. We note that isostatic rebound need not to be continuous, so that evidence of episodic rebound does not conflict with it, and that a lack of isostatic rebound would require a strongly opposing active tectonic regime. That is, a key general conclusion is that denudational rebound should always be assessed as an explanation for uplift before active tectonism is invoked.