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Landscapes are sculpted by a complex response of surface processes to external forcings, such as climate and tectonics. Several major river captures have been documented in the Hengduan Mountains, leading to the hypothesis that the region experiences exceptionally high rates of drainage reorganization driven by horizontal shortening and propagating uplift. Here we determine the prevalence, intensity, and spatial patterns of ongoing drainage reorganization in the Hengduan Mountains and evaluate the relative timescales of this transience by comparing drainage divide asymmetry for four geomorphic metrics that operate at different spatial and temporal scales. Specifically, we calculate the migration direction and the divide asymmetry index (DAI) for drainage divides using catchment-restricted topographic relief (CRR), hillslope gradient (HSG), normalized channel steepness (ksn), and normalized channel distance (χ). ksn and χ are both precipitation-corrected to account for the strong precipitation gradient across the region. The different spatial scales of these geomorphic metrics allow us to establish the relative timescales of observed landscape transience in the Hengduan Mountains, where local-scale metrics measure short-term change and integral quantities measure long-term disequilibrium. We find a high incidence of strongly asymmetric divides in all metrics across the Hengduan Mountain region. Although the magnitude of asymmetry varies significantly between metrics, possibly due to a combination of metric-specific thresholds and varying proxy relationships with erosion rate, a majority of divides agree on divide migration direction. Agreement in divide migration direction indicates an actively responding landscape when asymmetry is high and a state of quasi-equilibrium when asymmetry is low. Disagreements between the integral quantity, χ , and the other geomorphic metrics can be explained by different timescales of the underlying geomorphic processes, with χ reflecting a long-term response and CRR, HSG, and ksn capturing short-term perturbations to catchment structure. These perturbations include various transient mechanisms, such as differential tectonic uplift or erodibility, glacial alteration, and river captures. Our work confirms the high incidence of drainage reorganization across the Hengduan Mountains and highlights both transient and stable areas in the landscape with high resolution. We also offer valuable insights into the application of geomorphic metrics that can be generalized and applied to the study of landscape transience and drainage divide asymmetry in other settings. [ABSTRACT FROM AUTHOR] more...

Landscapes are sculpted by a complex response of surface processes to external forcings, such as climate and tectonics. Several major river captures have been documented in the Hengduan Mountains, leading to the hypothesis that the region experiences exceptionally high rates of drainage reorganization driven by horizontal shortening and propagating uplift. Here we determine the prevalence, intensity, and spatial patterns of ongoing drainage reorganization in the Hengduan Mountains and evaluate the relative time scales of this transience by comparing drainage divide asymmetry for four geomorphic metrics that operate at different spatial and temporal scales. Specifically, we calculate the migration direction and the divide asymmetry index (DAI) drainage divide asymmetry in catchment-restricted topographic relief (CRR), hillslope gradient (HSG), normalized channel steepness (ksn), and normalized channel distance (χ). ksn and χ are both precipitation-corrected to account for the strong precipitation gradient across the region. The different spatial scales of these geomorphic metrics allow us to establish the relative timescales of observed landscape transience in the Hengduan Mountains, where local scale metrics measure short-term change and integral quantities measure long-term disequilibrium. We find a high incidence of strongly asymmetric divides in all metrics across the Hengduan Mountain region. Although the magnitude of asymmetry varies significantly between metrics, possibly due to a combination of metric-specific thresholds and varying proxy relationships with erosion rate, a majority of divides agree on divide migration direction. Agreement in divide migration direction indicates active landscape response when asymmetry is high and a state of quasi-equilibrium when asymmetry is low. Disagreements between the integral quantity, χ, and the other geomorphic metrics can be explained by different timescales of the underlying geomorphic processes, with χ reflecting a long-term response and CRR, HSG, and ksn capturing short-term perturbations to catchment structure. These perturbations include various transient mechanisms, such as differential tectonic uplift or erodibility, glacial alteration, and river captures. Our work confirms the high incidence of drainage reorganization across the Hengduan Mountains and highlights both transient and stable areas in the landscape with high resolution. We also offer valuable insights on the application of geomorphic metrics that can be generalized and applied to the study of landscape transience and drainage divide asymmetry in other settings. [ABSTRACT FROM AUTHOR] more...

Author(s): Rong Yang [sup.1] , Sean D. Willett [sup.1] , Liran Goren [sup.2] Author Affiliations: (1) Geological Institute, Swiss Federal Institute of Technology, Zurich, Switzerland (2) Geological and Environmental Sciences, [...], The relict landscapes of southeast Tibet are being formed in situ as a result of river drainage reorganization that renders rivers unable to balance tectonic uplift, so these landscapes may not provide an unaltered record of past geomorphic conditions. Origin of low-relief landscapes High elevation, low relief surfaces or 'relict landscapes' are generally thought to be uplifted, old landscapes that preserve past tectonic and environmental conditions. Rong Yang et al. have tested current models of landscape evolution using digital topographic analysis of so-called relict landscapes in southeast Tibet and Yunnan province, China -- dramatic scenery boasting up to 3 km of relief encompassing the gorges of the Yangtze, Mekong and Salween rivers. They find that the region may not provide an unaltered record of past geomorphic conditions. Rather, these landscapes form in situ as a result of river drainage reorganization that renders rivers unable to balance tectonic uplift. Landscapes on Earth retain a record of the tectonic, environmental and climatic history under which they formed. Landscapes tend towards an equilibrium in which rivers attain a stable grade that balances the tectonic production of elevation and with hillslopes that attain a gradient steep enough to transport material to river channels. Equilibrium low-relief surfaces are typically found at low elevations, graded to sea level. However, there are many examples of high-elevation, low-relief surfaces, often referred to as relict landscapes.sup.1,2, or as elevated peneplains.sup.3. These do not grade to sea level and are typically interpreted as uplifted old landscapes, preserving former, more moderate tectonic conditions.sup.4. Here we test this model of landscape evolution through digital topographic analysis of a set of purportedly relict landscapes on the southeastern margin of the Tibetan Plateau, one of the most geographically complex, climatically varied and biologically diverse regions of the world. We find that, in contrast to theory, the purported surfaces are not consistent with progressive establishment of a new, steeper, river grade, and therefore they cannot necessarily be interpreted as a remnant of an old, low relief surface. We propose an alternative model, supported by numerical experiments, in which tectonic deformation has disrupted the regional river network, leaving remnants of it isolated and starved of drainage area and thus unable to balance tectonic uplift. The implication is that the state of low relief with low erosion rate is developing in situ, rather than preserving past erosional conditions. more...

Structural control on drainage patterns has long been suggested to play an important role in the development of fluvial systems, including their map pattern. Along the western Taiwan orogenic wedge, a prominent drainage pattern change is present, from predominantly orogen parallel drainages in the south to river systems that flow perpendicular to the long axis of the mountain belt in the north. Such change likely reflects the temporal evolutionary process as the Taiwan orogen propagates southward. We propose that the flow direction change of river systems is produced by the development of the fold‐and‐thrust belt on the pro‐wedge side of the mountain belt and is facilitated by drainage reorganization that includes abrupt drainage captures and more gradual divide movements, permitting drainage basins to attain a geometrically stable configuration with time. Our observations in Taiwan demonstrate not only how tectonic uplift is effective at modifying river patterns in the early stages of orogenic development but also how time and increased stream power encourage river networks to adjust to more effective patterns in mature mountain belts. [ABSTRACT FROM AUTHOR] more...

Author(s): Frédéric Herman [sup.1] [sup.2] , Diane Seward [sup.3] , Pierre G. Valla [sup.1] [sup.2] , Andrew Carter [sup.4] , Barry Kohn [sup.5] , Sean D. Willett [sup.2] , Todd [...], To establish what effect the Late Cenozoic cooling climate shift might have had on global erosion, inverse modelling of thermochronometric ages is used to show that erosion rates are increased by cooling, especially in glaciated mountain ranges. A cooling climate moves mountains Climatic and tectonic changes are thought to influence topography and erosion rates. A prime example of a major climate shift is the Late Cenozoic cooling, but its impact on global erosion remains uncertain. This paper quantifies erosion rates on the basis of inverse modelling and thermochronometric data from around the world and finds an increase in erosion rates at all latitudes, coinciding with enhanced cooling during the Late Cenozoic. In particular, mountain erosion rates have increased in the past 6 million years and most rapidly in the past 2 million years. The increase in erosion is most pronounced in glaciated mountain ranges, suggesting an important influence of glacial processes on erosion. Climate influences the erosion processes acting at the Earth's surface. However, the effect of cooling during the Late Cenozoic era, including the onset of Pliocene-Pleistocene Northern Hemisphere glaciation (about two to three million years ago), on global erosion rates remains unclear.sup.1,2,3,4. The uncertainty arises mainly from a lack of consensus on the use of the sedimentary record as a proxy for erosion.sup.3,4 and the difficulty of isolating the respective contributions of tectonics and climate to erosion.sup.5,6,7. Here we compile 18,000 bedrock thermochronometric ages from around the world and use a formal inversion procedure.sup.8 to estimate temporal and spatial variations in erosion rates. This allows for the quantification of erosion for the source areas that ultimately produce the sediment record on a timescale of millions of years. We find that mountain erosion rates have increased since about six million years ago and most rapidly since two million years ago. The increase of erosion rates is observed at all latitudes, but is most pronounced in glaciated mountain ranges, indicating that glacial processes played an important part. Because mountains represent a considerable fraction of the global production of sediments.sup.9, our results imply an increase in sediment flux at a global scale that coincides closely with enhanced cooling during the Pliocene and Pleistocene epochs.sup.10,11. more...

At the end of the Miocene, the European Alps ceased outward expansion, and tectonic uplift and exhumation shifted into the orogen interior. This shift is consistent with a change from orogenic construction to orogenic destruction, reflecting an increase in the ratio of erosional flux to accretionary flux. The coincidence of this change with an increase in sediment yield from the Alps suggests a climate-driven increase in erosional flux. The timing of deformation and sediment release from the southern Alps indicates that the tectonic change occurred synchronous with the last phase of the Messinian salinity crisis. We attribute the increase in erosional flux to a climatic shift to wetter conditions throughout Europe, likely augmented by the base-level fall that occurred during the Mediterranean dessication. This climate change is represented in the stratigraphic record by the Lago Mare deposits of the Mediterranean salinity crisis. Keywords: Mediterranean salinity crisis, European Alps, Miocene, climate, erosional flux, accretionary flux. more...

Recent observations of an association between forearc basins and slip during subduction thrust earthquakes suggest a link between processes controlling upper plate structure and seismic coupling on the subduction-zone thrust fault. We present a mechanism for the formation of these basins where sedimentation occurs on landward-dipping segments of the subduction wedge, which itself is actively growing through the accretion of material from the subducting plate. Our numerical simulations demonstrate that sedimentation stabilizes the underlying wedge, preventing internal deformation beneath the basin. Maximum slip during great-thrust earthquakes tends to occur where sedimentary basins stabilize the overlaying wedge. The lack of deformation in these stable regions increases the likelihood of thermal pressurization of the subduction thrust, allows the fault to load faster, and allows greater healing of the fault between rupture events. These effects link deformation of the subduction wedge to the seismic coupling of the subduction thrust. Keywords: forearc basins, subduction zone, critical wedge, numerical modeling, subduction wedge, subduction thrust earthquakes. more...