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Climatic forcing on channel profiles in the eastern cordillera of the Coroico Region, Bolivia
(2011)
Orographic precipitation has a large impact on channel morphology and rock uplift via a positive feedback to erosion. We show that in the Eastern Cordillera of Bolivia, channel concavities reach their highest values where annual precipitation increases in the downstream direction, exceeding 3000 mm. The steepest channels are upstream of this zone of high concavity, where precipitation rates are <1000 mm yr(-1). Channels exhibit graded forms both upstream and downstream of this transient reach. We conclude that the prolonged effect of orographic erosion and related tectonic uplift is the preservation of channels with extreme concavities in the Eastern Cordillera.
In this study, we investigate how immersive 3D geovisualization can be used in higher education. Based on MacEachren and Kraak's geovisualization cube, we examine the usage of immersive 3D geovisualization and its usefulness in a research-based learning module on flood risk, called GEOSimulator. Results of a survey among participating students reveal benefits, such as better orientation in the study area, higher interactivity with the data, improved discourse among students and enhanced motivation through immersive 3D geovisualization. This suggests that immersive 3D visualization can effectively be used in higher education and that 3D CAVE settings enhance interactive learning between students.
Thick sedimentary fills in intermontane valleys are common in formerly glaciated mountain ranges but difficult to quantify. Yet knowledge of the fill thickness distribution could help to estimate sediment budgets of mountain belts and to decipher the role of stored material in modulating sediment flux from the orogen to the foreland. Here we present a new approach to estimate valley fill thickness and bedrock topography based on the geometric properties of a landscape using artificial neural networks. We test the potential of this approach following a four-tiered procedure. First, experiments with synthetic, idealized landscapes show that increasing variability in surface slopes requires successively more complex network configurations. Second, in experiments with artificially filled natural landscapes, we find that fill volumes can be estimated with an error below 20%. Third, in natural examples with valley fill surfaces that have steeply inclined slopes, such as the Unteraar and the Rhone Glaciers in the Swiss Alps, for example, the average deviation of cross-sectional area between the measured and the modeled valley fill is 26% and 27%, respectively. Finally, application of the method to the Rhone Valley, an overdeepened glacial valley in the Swiss Alps, yields a total estimated sediment volume of 9711km(3) and an average deviation of cross-sectional area between measurements and model estimates of 21.5%. Our new method allows for rapid assessment of sediment volumes in intermontane valleys while eliminating most of the subjectivity that is typically inherent in other methods where bedrock reconstructions are based on digital elevation models.
Natural catchments are likely to show the existence of knickpoints in their river networks. The origin and genesis of the knickpoints can be manifold, considering that the present morphology is the result of the interactions of different factors such as tectonic movements, quaternary glaciations, river captures, variable lithology, and base-level changes. We analyzed the longitudinal profiles of the river channels in the Stura di Demonte Valley (Maritime Alps) to identify the knickpoints of such an alpine setting and to characterize their origins. The distribution and the geometry of stream profiles were used to identify the possible causes of the changes in stream gradients and to define zones with genetically linked knickpoints. Knickpoints are key geomorphological features for reconstructing the evolution of fluvial dissected basins, when the different perturbing factors affecting the ideally graded fluvial system have been detected. This study shows that even in a regionally small area, perturbations of river profiles are caused by multiple factors. Thus, attributing (automatically)-extracted knickpoints solely to one factor, can potentially lead to incomplete interpretations of catchment evolution.
The topographic signature of a mountain belt depends on the interplay of tectonic, climatic and erosional processes, whose relative importance changes over times, while quantifying these processes and their rates at specific times remains a challenge. The eastern Andes of central Bolivia offer a natural laboratory in which such interplay has been debated. Here, we investigate the Rio Grande catchment which crosses orthogonally the eastern Andes orogen from the Eastern Cordillera into the Subandean Zone, exhibiting a catchment relief of up to 5000 m. Despite an enhanced tectonic activity in the Subandes, local relief, mean and modal slopes and channel steepness indices are largely similar compared to the Eastern Cordillera and the intervening Interandean Zone. Nevertheless, a dataset of 57 new cosmogenic 10Be and 26AI catchment wide denudation rates from the Rio Grande catchment reveals up to one order of magnitude higher denudation rates in the Subandean Zone (mean 0.8 mm/yr) compared to the upstream physiographic regions. We infer that tectonic activity in the thrusting dominated Subandean belt causes higher denudation rates based on cumulative rock uplift investigations and due to the absence of a pronounced climate gradient. Furthermore, the lower rock strength of the Subandean sedimentary units correlates with mean slopes similar to the ones of the Eastern Cordillera and Interandean Zone, highlighting the fact, that lithology and rock strength can control high denudation rates at low slopes.
Low denudation rates measured at the outlet of the Rio Grande catchment (Abapo) are interpreted to be a result of a biased cosmogenic nuclide mixing that is dominated by headwater signals from the Eastern Cordillera and the Interandean zone and limited catchment sediment connectivity in the lower river reaches. Therefore, comparisons of short- (i.e., sediment yield) and millennial denudation rates require caution when postulating tectonic and/or climatic forcing without detailed studies. (C) 2015 The Authors. Published by Elsevier B.V.
Denudation processes and sediment transfer are investigated in a high-elevation, low-relief environment (eastern Altiplano, Bolivia) using Be-10, Al-26, and in situ C-14 analysis in fluvial sediments. Concentrations of the long-lived nuclides Be-10 and Al-26 yield consistently low catchment-wide denudation rates of similar to 3-29 mm ky(-1) (integrating over 21-194 ky), which reflect the low geomorphic gradients and the discontinuity of fluvial transport along the eastern Altiplano margin. No significant correlation is recorded between denudation rates of individual catchments and morphological basin parameters (slope, area, elevation). This is attributed to the overall little variability in morphology. The agreement between the denudation rates and published modern sediment discharge data suggests steady landscape evolution of the eastern Altiplano from the latest Pleistocene until today. While Be-10 and Al-26 provide long-term estimates on sediment production, in situ cosmogenic C-14 is used to trace short-term sediment storage. In situ C-14 concentrations are comparatively low indicating that C-14 decayed during alluvial storage over at least the past similar to 11-20 ky. We assume storage at shallow depth (2 m) and consider the influence of soil-mantled hillslopes on the in situ C-14 concentration. Our results illustrate the importance of sediment storage even over short distances and demonstrate the potential of in situ C-14 to study sediment routing and transfer times within drainage systems. However, this study also demonstrates that the long-lived Be-10 and Al-26 nuclides can provide adequate estimates on long-term denudation rates even if sediment transport is not fast but interrupted by several thousands of years of storage.
We surveyed fluvial terraces to decipher the Quaternary increment of crustal shortening and shortening rate in the on-shore Makran Accretionary Wedge. We focused on three major catchment basins and associated fold systems. Terrace profiles reconstructed from differential GPS measurements combined with DEM revealed two regional dominant wavelengths, about 5 km in the northern part of the study area and about 15 km to the south. These two wavelengths suggest the existence of two active decollement layers at two rooting depths. The average shortening rate due to folding is estimated at 0.8-1.2 mm/a over the last 130 ka. This accounts for 10-15% of the shortening rate (similar to 8 mm/a) given by kinematic GPS measurements between Chabahar and Bazman and 3% of the convergence between Arabia and Eurasia, across the Makran subduction zone. Despite active deformation and a relatively high shortening rate, the geophysical record shows nearly absent seismic activity in Makran. We propose that strain accumulated in folds over intermediate decollement levels within a thick, incompletely lithified sedimentary cover explains the essentially aseismic, recent tectonics in this region. The importance of folds points to imperfect Coulomb behaviour of the wedge. (C) 2012 Elsevier B.V. All rights reserved.
The Kohat and Potwar fold thrust belts (KP-FTB) in Pakistan exhibit structural variations over 250 km along strike within the Himalayan fold and thrust system. Our 3D deformation model shows that Kohat surface structures evolved above an active roof thrust in Eocene evaporites. The ramp-forming duplexes in the Kohat were stacked and passively transported toward the foreland above new ramps, resulting in up to 5 km of thickening between the two decollements. Ramps from the Kohat extend into the Potwar as thrust tips of fault propagation folds. The basement slope changes from flat (beta < 1 degrees) below the northern part to north-dipping (beta > 1 degrees) below the southern part, corresponding to the change in structural style and complexity of the KP-FTB. The Kalabagh Fault Zone, linking the two belts, is interpreted as a zone of complex dextral strike-slip rotational faulting. Salt expulsed from the hanging walls of normal faults and under synclines in the Kalabagh Fault Zone moved toward the footwall of normal faults, accumulated in the cores of anticlines, and formed lobe structures at the deformation front. The fundamental reasons for the variable structural styles are changes in decollement strength, basement slope, preexisting normal faulting, presence of a secondary decollement and spatially-variable salt mobility and accumulation.
The Salt Range in Pakistan exposes Precambrian to Pleistocene strata outcropping along the Salt Range Thrust (SRT). To better understand the in-situ Cambrian and Pliocene tectonic evolution of the Pakistan Subhimalaya, we have conducted low-temperature thermochronological analysis using apatite (U-Th-Sm)/He and fission track dating. We combine cooling ages from different samples located along the thrust front of the SRT into a thermal model that shows two major cooling events associated with rifting and regional erosion in the Late Palaeozoic and SRT activity since the Pliocene. Our results suggest that the SRT maintained a long-term average shortening rate of similar to 5-6 mm/yr and a high exhumation rate above the SRT ramp since similar to 4 Ma.
The Kohat fold and thrust belt in Pakistan shows a significantly different structural style due to the structural evolution on the double décollement compared to the rest of the Subhimalaya. In order to better understand the spatio-temporal structural evolution of the Kohat fold and thrust belt, we combine balanced cross sections with apatite (U?Th-Sm)/He (AHe) and apatite fission track (AFT) dating. The AHe and AFT ages appear to be totally reset, allowing us to date exhumation above structural ramps. The results suggest that deformation began on the frontal Surghar thrust at-15 Ma, predating or coeval with the development of the Main Boundary thrust at-12 Ma. Deformation propagated southward from the Main Boundary thrust on double de?collements between 10 Ma and 2 Ma, resulting in a disharmonic structural style inside the Kohat fold and thrust belt. Thermal modeling of the thermochronologic data suggest that samples inside Kohat fold and thrust belt experienced cooling due to formation of the duplexes; this deformation facilitated tectonic thickening of the wedge and erosion of the Miocene to Pliocene foreland strata. The spatial distribution of AHe and AFT ages in combination with the structural forward model suggest that, in the Kohat fold and thrust belt, the wedge deformed in-sequence as a supercritical wedge (-15-12 Ma), then readjusted by out-sequence deformation (-12-0 Ma) within the Kohat fold and thrust belt into a sub-critical wedge.