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, or as elevated peneplains. These do not grade to sea level and are typically interpreted as uplifted old landscapes, preserving former, more moderate tectonic conditions. 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.
We present a new algorithm to solve the basic stream power equation, which governs channel incision and landscape evolution in many geomorphic settings. The algorithm is highly efficient because ...computation time increases linearly with the number of points used to discretize the landscape and is ideally suited to parallelization. It is also unconditionally stable because it uses an implicit scheme for the time integration of the landscape evolution equation, which means that large time steps can be used without sacrificing accuracy. In this paper we describe the algorithm and present results that demonstrate its efficiency and accuracy.
► We propose a new, very efficient method to solve the stream power law. ► The method is order N (number of nodes). ► It is fully implicit. ► It is parallel. ► It can be used to solve very large problems (10^8 nodes).
Dynamic Reorganization of River Basins Willett, Sean D.; McCoy, Scott W.; Perron, J. Taylor ...
Science (American Association for the Advancement of Science),
03/2014, Volume:
343, Issue:
6175
Journal Article
Peer reviewed
River networks evolve as migrating drainage divides reshape river basins and change network topology by capture of river channels. We demonstrate that a characteristic metric of river network ...geometry gauges the horizontal motion of drainage divides. Assessing this metric throughout a landscape maps the dynamic states of entire river networks, revealing diverse conditions: Drainage divides in the Loess Plateau of China appear stationary; the young topography of Taiwan has migrating divides driving adjustment of major basins; and rivers draining the ancient landscape of the southeastern United States are reorganizing in response to escarpment retreat and coastal advance. The ability to measure the dynamic reorganization of river basins presents opportunities to examine landscape-scale interactions among tectonics, erosion, and ecology.
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. The uncertainty arises mainly from a lack of consensus on the use of the sedimentary record as a proxy for erosion and the difficulty of isolating the respective contributions of tectonics and climate to erosion. Here we compile 18,000 bedrock thermochronometric ages from around the world and use a formal inversion procedure 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, our results imply an increase in sediment flux at a global scale that coincides closely with enhanced cooling during the Pliocene and Pleistocene epochs.
A numerical model of the coupled processes of tectonic deformation and surface erosion in convergent orogens is developed to investigate the nature of the interaction between these processes. Crustal ...deformation is calculated by a two‐dimensional finite element model of deformation in response to subduction and accretion of continental crust. Erosion operates on the uplifted surface of this model through fluvial incision which is taken to be proportional to stream power. The relative importance of the tectonic and erosion processes is given by a dimensionless “erosion number” relating convergence velocity, rock erodibility, and precipitation rate. This number determines the time required for a system to reach steady state and the final topographic shape and size of a mountain belt. Fundamental characteristics of the model orogens include asymmetric topography with shallower slopes facing the subducting plate and an asymmetric pattern of exhumation with the deepest levels of exhumation opposite to subduction. These characteristics are modified when the regional climate exhibits a dominant wind direction and orographically enhanced precipitation on one side of the mountain belt. The two possible cases are dominant wind in the direction of motion of the subducting plate and dominant wind direction in the opposite direction of the subducting plate velocity. Models of the former case predict a broad zone of exhumation with maximum exhumation in the orogen interior. Models of the latter case predict a focused zone of exhumation at the margin of the orogen and, at high erosion number, a reversal in the topographic asymmetry. Natural examples of these two cases are presented. The Southern Alps of New Zealand exhibits the climate and exhumation asymmetry characteristic of wind in the direction opposite to motion of the subducting plate. The asymmetry of topography suggests that erosion is not efficient enough to have reversed the topographic asymmetry. The contrasting example of dominant wind in the direction of subduction motion is provided by the Olympic Mountains of Washington State. In this case, exhumation of deep levels of the Cascadia accretionary wedge shows a broad domal pattern consistent with the observed orographic precipitation.
Ecosystem diversity and human activity in dry climates depend not just on the magnitude of rainfall, but also on the landscape's ability to retain water. This is illustrated dramatically in the High ...Plains of North America, where despite the semi-arid modern and past climate, the hydrologic conditions are diverse. Large rivers sourced in the Rocky Mountains cut through elevated plains that exhibit limited river drainage but widespread surface water in the form of ephemeral (seasonal) playa lakes
, as well as extensive groundwater hosted in the High Plains aquifer of the Ogallala formations
. Here we present a model, with supporting evidence, which shows that the High Plains landscape is currently in a transient state, in which the landscape has bifurcated into an older region with an inefficient river network and a younger, more efficient, river channel network that is progressively cannibalizing the older region. The older landscape represents the remnants of the Ogallala sediments that once covered the entirety of the High Plains, forming depositional fans that buried the pre-existing river network and effectively 'repaved' the High Plains
. Today we are witnessing the establishment of a new river network that is dissecting the landscape, capturing channels and eroding these sediment fans. Through quantitative analysis of the geometry of the river network, we show how network reorganization has resulted in a distinctive pattern of erosion, whereby the largest rivers have incised the older surface, removed fan heads near the Rocky Mountains and eroded the fan toes, but left portions of the central fan surface and the Ogallala sediments largely intact. These preserved fan surfaces have poor surface water drainage, and thus retain ephemeral water for wetlands and groundwater recharge. Our findings suggest that the surface hydrology and associated ecosystems are transient features on million-year timescales, and reflect the mode of landscape evolution.
Madagascar exhibits high endemic biodiversity that has evolved with sustained and stable rates of speciation over the past several tens of millions of years. The topography of Madagascar is dominated ...by a mountainous continental rift escarpment, with the highest plant diversity and rarity found along the steep, eastern side of this geographic feature. Using a process-explicit model, we show that precipitation-driven erosion and landward retreat of this high-relief topography creates transient habitat organization through multiple mechanisms, including catchment expansion, isolation of highland remnants, and formation of topographic barriers. Habitat isolation and reconnection on a million-year timescale serves as an allopatric speciation pump creating the observed biodiversity.
The eastern margin of Madagascar has a prominent relief change from the flat coastal plain to the low‐relief high plateau, characterizing a typical great escarpment topography at a passive margin. A ...quantification of the spatial distribution of erosion rates is necessary to understand the rate of landscape evolution. We present catchment‐averaged erosion rates from detrital cosmogenic 10Be concentrations, systematically covering distinct morphological zones of the escarpment. Erosion rates are differentiated across the escarpment, where the high plateau and the coastal plain are slowly eroding with an average rate of 9.7 m/Ma, and the escarpment basins are eroding faster with an average rate of 16.6 m/Ma. The Alaotra‐Ankay Graben related basins have the highest erosion rate with an average rate of 27 m/Ma. The spatial pattern of erosion rates indicates a retreating escarpment landscape. Retreat rates calculated from the 10Be concentrations are from 182 to 1,886 m/Ma. The rates of escarpment retreat on Madagascar are consistent with a model of a steady retreat from the coastline since the time of rifting, similar to the Western Ghats escarpment on its conjugate margin of the India Peninsula.
Plain Language Summary
Eastern Madagascar is characterized by a distinct escarpment with high relief, normally indicative of high erosion rates. We investigate this by measuring erosion rates using cosmogenic isotope concentrations in sediment derived from throughout Madagascar. We calculated erosion rates and related these to distinct geomorphic zones. The pattern of erosion rates is consistent with inland retreat of the escarpment at about 1 km/Ma. This rate is consistent with geomorphic evidence and is comparable to the conjugate margin of western India.
Key Points
Presentation of new detrital cosmogenic 10Be data that systematically covers the great escarpment of eastern Madagascar
Erosion rates inferred from 10Be concentrations are 9.7 to 27 m/Ma, systematically vary among distinct morphological zones
Escarpment retreat rates from 10Be concentrations are 182 to 1,886 m/Ma, consistent with evidences for captures and divide migration
Convergence and collision between India and Eurasia have produced the Tibetan Plateau, which stands 5 km high over a region of 3 million km2. Its southeastern margin lies in the restraining bend ...between the Sichuan basin and the Eastern Himalayan Syntaxis. In this region three parallel rivers, the Salween, the Mekong and the Yangtze, carve gorges up to 3 km deep. Along the longitudinal profiles, large-scale knickzones, defined by very high steepness, correspond to the gorges of the Salween and Mekong. The Yangtze, instead, has a nearly linear profile upstream from its first big bend. New low-temperature thermochronometric data reveal a complex pattern of erosion in the Three Rivers Region. From the Salween in the west to the Yangtze in the east the magnitude and rate of erosion decrease. From south-to-north erosion rates exhibit variable gradients in space and time. Along the Salween and the Mekong a northward increase of erosion rate is followed by a decrease with additional distance to the north. Variations of erosion rate in time are characterized by a deceleration along the Salween and a general deceleration with local acceleration along the Mekong. This pattern, together with river profile analysis, is best explained by active coupling between tectonics and river incision related to the indentation and northward migration of the corner of the Indian continent.
•54 new thermochronometric ages from southeastern Tibet are presented.•Inferred erosion rates decrease from west to east.•Inferred erosion rates show spatial and temporal variations from south-to-north.•Locus of fast erosion migrates northwards along the Salween and Mekong.•An active coupling between river incision and tectonics is inferred.
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