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Jean Braun - One of the best experts on this subject based on the ideXlab platform.
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A review of numerical modeling studies of passive margin Escarpments leading to a new analytical expression for the rate of Escarpment migration velocity
Gondwana Research, 2018Co-Authors: Jean BraunAbstract:Abstract Passive margins are geomorphological features that have historically attracted much attention from the modeling community. In particular, many numerical modeling studies have attempted to explain the longevity of steep Escarpments that formed along continental edges at the transition between low elevation coastal plains and high elevation continental interiors, such as along the coasts of Africa and South America on both sides of the South Atlantic Ocean. In this paper, I review the wide and diverse body of observational constraints gathered to constrain the formation and evolution of passive margins Escarpments, as well as the various mechanisms that have been proposed to explain their anomalously high topography. I then compile and summarize the findings of numerous numerical modeling studies that have been performed in the past twenty years to explain their formation and evolution. I show that many of these studies converged to agree that the longevity of passive margin Escarpments depends on how rapidly they become and remain regional drainage divides and that this is primarily controlled by the flexural isostatic rebound associated with the erosion of the high elevation continental interior. To better quantify these findings, I derive and present a new analytical expression for the migration velocity of an Escarpment once it has become a drainage divide. This expression is validated by a series of numerical experiments using 1D and 2D high resolution landscape evolution models. Interestingly, these models also predict that the rate of erosion at or near Escarpments can be several orders of magnitude smaller than the rate of Escarpment retreat. This may explain the apparent discrepancy between the low estimates of present-day erosion rates obtained mostly from cosmogenic nuclide studies (10m/Myr) and the long-term rates of Escarpment retreat (1km/Myr).
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Evolution of passive margin Escarpments: what can we learn from low-temperature thermochronology?
Journal of Geophysical Research, 2004Co-Authors: Jean Braun, Peter Van Der BeekAbstract:Recent studies integrating geomorphology, thermochronology, cosmogenic erosion rate estimates, and numerical modeling suggest that Escarpment evolution may take place following two dramatically different modes: (1) parallel retreat from the Escarpment's original position at the continent-ocean boundary to its present-day inland position and (2) formation-in-place by progressive downwearing of a plateau initially located between the coast and a preexisting inland drainage divide. Using a three-dimensional finite element model to solve the heat transfer equation, we show that the mode of migration of a passive margin Escarpment can be constrained by low-temperature (apatite (U-Th)/He) thermochronology. We first couple the heat equation solver to a surface processes model that predicts the two different Escarpment evolution modes from only slightly different initial conditions. We predict (U-Th)/He age distributions that are markedly different for the two scenarios. We perform a thorough investigation of the model behavior to determine under which circumstances thermochronological data can be used to constrain passive margin Escarpment dynamics. These conditions include (a) a tall Escarpment, (b) a high geothermal gradient, and/or (c) a low flexural rigidity of the lithosphere. We demonstrate that to determine the rate and mode of Escarpment migration from low-temperature thermochronology, one needs to collect samples along transects perpendicular as well as parallel to the Escarpment. Tightest constraints on Escarpment development are provided by (in ascending order) the minimum (U-Th)/He age encountered seaward of the Escarpment, the location of where the minimum age is found, the slope of the age-distance relationship (in a direction perpendicular to the coast), and the slope of the age-elevation relationship (from a transect parallel to the Escarpment). We finally demonstrate that there are situations where thermochronological data sets do not provide constraints on the mode of Escarpment migration, such as along the Escarpment of southeastern Australia, where migration has possibly been very rapid. Using the Neighborhood Algorithm method, we are, however, able to extract from an existing apatite (U-Th)/He data set very useful constraints on the evolution of the southeastern Australian Escarpment, including the duration of the migration event (
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Evolution of passive margin Escarpments: What can we learn from low‐temperature thermochronology?
Journal of Geophysical Research, 2004Co-Authors: Jean Braun, Peter Van Der BeekAbstract:Recent studies integrating geomorphology, thermochronology, cosmogenic erosion rate estimates, and numerical modeling suggest that Escarpment evolution may take place following two dramatically different modes: (1) parallel retreat from the Escarpment's original position at the continent-ocean boundary to its present-day inland position and (2) formation-in-place by progressive downwearing of a plateau initially located between the coast and a preexisting inland drainage divide. Using a three-dimensional finite element model to solve the heat transfer equation, we show that the mode of migration of a passive margin Escarpment can be constrained by low-temperature (apatite (U-Th)/He) thermochronology. We first couple the heat equation solver to a surface processes model that predicts the two different Escarpment evolution modes from only slightly different initial conditions. We predict (U-Th)/He age distributions that are markedly different for the two scenarios. We perform a thorough investigation of the model behavior to determine under which circumstances thermochronological data can be used to constrain passive margin Escarpment dynamics. These conditions include (a) a tall Escarpment, (b) a high geothermal gradient, and/or (c) a low flexural rigidity of the lithosphere. We demonstrate that to determine the rate and mode of Escarpment migration from low-temperature thermochronology, one needs to collect samples along transects perpendicular as well as parallel to the Escarpment. Tightest constraints on Escarpment development are provided by (in ascending order) the minimum (U-Th)/He age encountered seaward of the Escarpment, the location of where the minimum age is found, the slope of the age-distance relationship (in a direction perpendicular to the coast), and the slope of the age-elevation relationship (from a transect parallel to the Escarpment). We finally demonstrate that there are situations where thermochronological data sets do not provide constraints on the mode of Escarpment migration, such as along the Escarpment of southeastern Australia, where migration has possibly been very rapid. Using the Neighborhood Algorithm method, we are, however, able to extract from an existing apatite (U-Th)/He data set very useful constraints on the evolution of the southeastern Australian Escarpment, including the duration of the migration event (
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modeling postbreakup landscape development and denudational history across the southeast african drakensberg Escarpment margin
Journal of Geophysical Research, 2002Co-Authors: Pieter Van Beek, Jean Braun, M Summerfield, Roderick Brown, Alastair B FlemingAbstract:[1] We employ a numerical surface processes model to study the controls on postbreakup landscape development and denudational history of the southeast African margin. Apatite fission track data, presented in the companion paper, suggest that the Drakensberg Escarpment formed by rapid postbreakup river incision seaward of a preexisting drainage divide, located close to its present position, and subsequently retreated at rates of only ∼100 m m.y.−1. Numerical modeling results support such a scenario and show that the prebreakup topography of the margin has exerted a fundamental control on subsequent margin evolution. The rheology of the lithosphere, lithological variations in the eroding upper crust, and inland base level falls provided secondary controls. A relatively low flexural rigidity of the lithosphere (Te ≈ 10 km) is required to explain the observed pattern of denudation as well as the observed geological structure of the southeast African margin. Lithological variations have contributed to the formation of flat-topped ridges buttressing the main Escarpment, as well as major fluvial knickpoints. Both these features have previously been interpreted as supporting significant Cenozoic uplift of the margin. An inland base level fall, possibly related to back-cutting of the Orange River drainage system and occurring 40–50 m.y. after breakup, explains the observed denudation inland of the Escarpment as well as the development of inland drainage parallel to the Escarpment. Our model results suggest that in contrast to widely accepted inferences from classical geomorphic studies, the southeast African margin has remained tectonically stable since breakup and Escarpment retreat has been minimal (<25 km).
Peter Van Der Beek - One of the best experts on this subject based on the ideXlab platform.
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The influence of rifting on Escarpment migration on high elevation passive continental margins
Journal of Geophysical Research: Solid Earth, 2012Co-Authors: V. Sacek, J Braun, Peter Van Der BeekAbstract:Using numerical models that couple surface processes, flexural isostasy, faulting and the thermal effects of rifting, we show that fault-bounded Escarpments created at rift flanks by mechanical unloading and flexural rebound have little potential to “survive” as retreating Escarpments if the lower crust under the rift flank is substantially stretched. In this configuration, a drainage divide that persists through time appears landward of the initial Escarpment in a position close to a secondary bulge that is created during the rifting event at a distance that depends on the flexural rigidity of the upper crust. Moreover, the migration of the Escarpment to the secondary bulge occurs when the pre-rift topography dips landward, otherwise the evolution of the Escarpment is guided by the pre-existing inland drainage divide. To illustrate this new mechanism for the evolution of passive margins, we study the examples of Southeastern Australia and Southeastern Brazil. We propose that a pre-existing inland drainage divide with rift related flank uplift can produce the double drainage divide observed in Southeastern Australia. On the other hand, we conclude that it is possible that the Serra do Mar Escarpments on the Southeastern Brazilian margin originated as a secondary flexural bulge during rifting that persisted through time. In both cases, the retreating Escarpment scenario is unlikely and the present-day margin morphology can be explained as resulting from rift-related vertical motions alone, without requiring significant post-rift “rejuvenation”.
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Evolution of passive margin Escarpments: What can we learn from low‐temperature thermochronology?
Journal of Geophysical Research, 2004Co-Authors: Jean Braun, Peter Van Der BeekAbstract:Recent studies integrating geomorphology, thermochronology, cosmogenic erosion rate estimates, and numerical modeling suggest that Escarpment evolution may take place following two dramatically different modes: (1) parallel retreat from the Escarpment's original position at the continent-ocean boundary to its present-day inland position and (2) formation-in-place by progressive downwearing of a plateau initially located between the coast and a preexisting inland drainage divide. Using a three-dimensional finite element model to solve the heat transfer equation, we show that the mode of migration of a passive margin Escarpment can be constrained by low-temperature (apatite (U-Th)/He) thermochronology. We first couple the heat equation solver to a surface processes model that predicts the two different Escarpment evolution modes from only slightly different initial conditions. We predict (U-Th)/He age distributions that are markedly different for the two scenarios. We perform a thorough investigation of the model behavior to determine under which circumstances thermochronological data can be used to constrain passive margin Escarpment dynamics. These conditions include (a) a tall Escarpment, (b) a high geothermal gradient, and/or (c) a low flexural rigidity of the lithosphere. We demonstrate that to determine the rate and mode of Escarpment migration from low-temperature thermochronology, one needs to collect samples along transects perpendicular as well as parallel to the Escarpment. Tightest constraints on Escarpment development are provided by (in ascending order) the minimum (U-Th)/He age encountered seaward of the Escarpment, the location of where the minimum age is found, the slope of the age-distance relationship (in a direction perpendicular to the coast), and the slope of the age-elevation relationship (from a transect parallel to the Escarpment). We finally demonstrate that there are situations where thermochronological data sets do not provide constraints on the mode of Escarpment migration, such as along the Escarpment of southeastern Australia, where migration has possibly been very rapid. Using the Neighborhood Algorithm method, we are, however, able to extract from an existing apatite (U-Th)/He data set very useful constraints on the evolution of the southeastern Australian Escarpment, including the duration of the migration event (
Dan N. Barfod - One of the best experts on this subject based on the ideXlab platform.
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apatite u th he age constraints on the development of the great Escarpment on the southeastern australian passive margin
Earth and Planetary Science Letters, 2002Co-Authors: Cristina Persano, Finlay M. Stuart, Paul Bishop, Dan N. BarfodAbstract:The southeast Australian margin, like other high elevation passive margins, is characterised by a steep Escarpment that separates a dissected coastal plain from a low relief inland plateau. Quantitative constraints on the generation of Escarpments can be provided by apatite (U–Th)/He ages. Here we use a coast-perpendicular traverse across the coastal lowlands, Escarpment and plateau to test the three prevailing models of SE Australian Escarpment formation, namely retreat into a downwarped rift shoulder, Escarpment retreat and down-wearing on high elevation rift shoulder with flexural rebound. Apatites from the coastal plain have He ages of between 87 and 112 Ma, suggesting that the coastal lowlands developed very rapidly after rifting and continental break-up at 85–100 Ma. The He age data are inconsistent with the erosion of a downwarped rift margin, and cannot be explained by a constant post-break-up rate of lateral Escarpment retreat across the coastal plain or by constant down-wearing. The data require either rapid Escarpment retreat or rapid in-place excavation of the Escarpment soon after break-up, in response to rifting and the lowering of base levels on the margin of the new continent at break-up, followed by a period of landscape stability and low erosion. Combined with the existing apatite fission track record, the He data are consistent with erosion of 3–4 km within a maximum of 28 Myr of break-up, at a minimum vertical erosion rate of 130 m Myr−1 along the coast. The rapid denudation period across the coastal plain in this region took less than 48 Myr (from the coast to the Escarpment base), which corresponds to an average vertical erosion rate of 45 m Myr−1. This is equivalent to a mean Escarpment retreat rate of 5–10 km Myr−1. Apatite He ages from the plateau (183–247 Ma) indicate that the highlands remained stable throughout continental break-up, experiencing average erosion rates of less than 10 m Myr−1 since the late Palaeozoic/early Mesozoic.
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Apatite (U–Th)/He age constraints on the development of the Great Escarpment on the southeastern Australian passive margin
Earth and Planetary Science Letters, 2002Co-Authors: Cristina Persano, Finlay M. Stuart, Paul Bishop, Dan N. BarfodAbstract:The southeast Australian margin, like other high elevation passive margins, is characterised by a steep Escarpment that separates a dissected coastal plain from a low relief inland plateau. Quantitative constraints on the generation of Escarpments can be provided by apatite (U–Th)/He ages. Here we use a coast-perpendicular traverse across the coastal lowlands, Escarpment and plateau to test the three prevailing models of SE Australian Escarpment formation, namely retreat into a downwarped rift shoulder, Escarpment retreat and down-wearing on high elevation rift shoulder with flexural rebound. Apatites from the coastal plain have He ages of between 87 and 112 Ma, suggesting that the coastal lowlands developed very rapidly after rifting and continental break-up at 85–100 Ma. The He age data are inconsistent with the erosion of a downwarped rift margin, and cannot be explained by a constant post-break-up rate of lateral Escarpment retreat across the coastal plain or by constant down-wearing. The data require either rapid Escarpment retreat or rapid in-place excavation of the Escarpment soon after break-up, in response to rifting and the lowering of base levels on the margin of the new continent at break-up, followed by a period of landscape stability and low erosion. Combined with the existing apatite fission track record, the He data are consistent with erosion of 3–4 km within a maximum of 28 Myr of break-up, at a minimum vertical erosion rate of 130 m Myr−1 along the coast. The rapid denudation period across the coastal plain in this region took less than 48 Myr (from the coast to the Escarpment base), which corresponds to an average vertical erosion rate of 45 m Myr−1. This is equivalent to a mean Escarpment retreat rate of 5–10 km Myr−1. Apatite He ages from the plateau (183–247 Ma) indicate that the highlands remained stable throughout continental break-up, experiencing average erosion rates of less than 10 m Myr−1 since the late Palaeozoic/early Mesozoic.
Peter Van Der Beek - One of the best experts on this subject based on the ideXlab platform.
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Evolution of passive margin Escarpments: what can we learn from low-temperature thermochronology?
Journal of Geophysical Research, 2004Co-Authors: Jean Braun, Peter Van Der BeekAbstract:Recent studies integrating geomorphology, thermochronology, cosmogenic erosion rate estimates, and numerical modeling suggest that Escarpment evolution may take place following two dramatically different modes: (1) parallel retreat from the Escarpment's original position at the continent-ocean boundary to its present-day inland position and (2) formation-in-place by progressive downwearing of a plateau initially located between the coast and a preexisting inland drainage divide. Using a three-dimensional finite element model to solve the heat transfer equation, we show that the mode of migration of a passive margin Escarpment can be constrained by low-temperature (apatite (U-Th)/He) thermochronology. We first couple the heat equation solver to a surface processes model that predicts the two different Escarpment evolution modes from only slightly different initial conditions. We predict (U-Th)/He age distributions that are markedly different for the two scenarios. We perform a thorough investigation of the model behavior to determine under which circumstances thermochronological data can be used to constrain passive margin Escarpment dynamics. These conditions include (a) a tall Escarpment, (b) a high geothermal gradient, and/or (c) a low flexural rigidity of the lithosphere. We demonstrate that to determine the rate and mode of Escarpment migration from low-temperature thermochronology, one needs to collect samples along transects perpendicular as well as parallel to the Escarpment. Tightest constraints on Escarpment development are provided by (in ascending order) the minimum (U-Th)/He age encountered seaward of the Escarpment, the location of where the minimum age is found, the slope of the age-distance relationship (in a direction perpendicular to the coast), and the slope of the age-elevation relationship (from a transect parallel to the Escarpment). We finally demonstrate that there are situations where thermochronological data sets do not provide constraints on the mode of Escarpment migration, such as along the Escarpment of southeastern Australia, where migration has possibly been very rapid. Using the Neighborhood Algorithm method, we are, however, able to extract from an existing apatite (U-Th)/He data set very useful constraints on the evolution of the southeastern Australian Escarpment, including the duration of the migration event (
Yanni Gunnell - One of the best experts on this subject based on the ideXlab platform.
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Butte detachment: how pre‐rift geological structure and drainage integration drive Escarpment evolution at rifted continental margins
Earth Surface Processes and Landforms, 2010Co-Authors: Yanni Gunnell, D. J. HarborAbstract:The erosional pattern of passive margins often follows the fabric of ancient, compressional geological structures exposed by the topographic energy of rifting. As erosion cuts into these belted outcrop systems they impose initial and boundary conditions that steer drainage recession into the plateau edge and control Escarpment-forming conditions. Pattern therefore controls process. Although generic surface process models predict scarp patterns and retreat in settings devoid of geological heterogeneity, they tend to do so only at isolated locations and for periods shorter than the lifespan of the Escarpments. Thus, to focus on relatively narrow strike-perpendicular swaths of passive margin topography misses important aspects of drainage integration, which involves mobile drainage basin boundaries shifting across but also along the strike of inherited geological structures and through continental-scale bioclimatic zones. Space-for-time substitution along three passive margin Escarpments (Blue Ridge, Western Ghats, Eastern Ghats) reveals the significance of Escarpment jumps and the detachment of topographic outliers, here generically termed ‘buttes’, as key processes of Escarpment evolution. The examples show that these continental Escarpments are strongly patterned after pre-rift structural and lithological heterogeneities. As seaward sloping drainages cut into the rift margin, they extend their drainage heads in a non-uniform and unsteady fashion. As a result Escarpments can form, be destroyed, reform, and leave topographic vestiges (buttes) of the retreating Escarpment. Given the pre-rift geological heterogeneities, there are no a priori reasons why Escarpment landscape change should be uniform, steady or self-similar. Copyright © 2010 John Wiley & Sons, Ltd.
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Butte detachment: How pre-rift geological structure and drainage integration drive Escarpment evolution at rifted continental margins
Earth Surface Processes and Landforms, 2010Co-Authors: Yanni Gunnell, D. J. HarborAbstract:The erosional pattern of passive margins often follows the fabric of ancient, compressional geological structures exposed by the topographic energy of rifting. As erosion cuts into these belted outcrop systems they impose initial and boundary conditions that steer drainage recession into the plateau edge and control Escarpment-forming conditions. Pattern therefore controls process. Although generic surface process models predict scarp patterns and retreat in settings devoid of geological heterogeneity, they tend to do so only at isolated locations and for periods shorter than the lifespan of the Escarpments. Thus, to focus on relatively narrow strike-perpendicular swaths of passive margin topography misses important aspects of drainage integration, which involves mobile drainage basin boundaries shifting across but also along the strike of inherited geological structures and through continental-scale bioclimatic zones. Space-for-time substitution along three passive margin Escarpments (Blue Ridge, Western Ghats, Eastern Ghats) reveals the significance of Escarpment jumps and the detachment of topographic outliers, here generically termed ‘buttes’, as key processes of Escarpment evolution. The examples show that these continental Escarpments are strongly patterned after pre-rift structural and lithological heterogeneities. As seaward sloping drainages cut into the rift margin, they extend their drainage heads in a non-uniform and unsteady fashion. As a result Escarpments can form, be destroyed, reform, and leave topographic vestiges (buttes) of the retreating Escarpment. Given the pre-rift geological heterogeneities, there are no a priori reasons why Escarpment landscape change should be uniform, steady or self-similar. Copyright © 2010 John Wiley & Sons, Ltd.
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along strike Escarpment heterogeneity of the western ghats a synthesis of drainage and topography using digital morphometric tools
Journal of The Geological Society of India, 2007Co-Authors: D. J. Harbor, Yanni GunnellAbstract:We present the first synthetic and systematic attempt to fingerprint topographic attributes of the Western Ghats passive margin Escarpment using newly available SRTM digital elevation data. Spanning 12 degrees of latitude, the Escarpment is shown to exhibit contiguous segments where scarp sinuosity and relief, but also drainage basin attributes such as stream orientation, spacing of scarp, coastline and continental divide, basin shape, basin hypsometry and stream longitudinal profile covary in ways that suggest dif ferences in the process of retreat of the Western Ghats as a continuous yet heterogeneous landform. The methodology presented here could serve as an improvable template applicable to other Escarpments around the world for comparative purposes. It can be potentially standardized as a tool designed to construct inferences about the variability of scarp retreat processes under a range of conditions tied to drainage and geological structure. For the Western Ghats, we suggest that site-specific feedbacks between climate, drainage and geologic structure are key to understanding scarp dynamics and the manner in which the evolution of drainage boundaries across strike affect morphology and evolution along strike.
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Tectonic and climatic controls on rift Escarpments: Erosion and flexural rebound of the Dhofar passive margin (Gulf of Aden, Oman)
Journal of Geophysical Research : Solid Earth, 2007Co-Authors: Carole Petit, Marc Fournier, Yanni GunnellAbstract:We investigate the respective roles of climatic parameters and the flexural rigidity of the lithosphere in the erosion history and behavior of two adjacent rift Escarpments along the northern coast of the Gulf of Aden, in Oman. At this 25 Myr old passive margin, we define a type 1 scarp, which is high, sharp-crested and has retreated 25-30 km inland from its master fault, and a type 2 scarp, which exhibits a more rounded profile, lower relief, and still coincides with its mapped normal fault trace. Since about 15 Ma, the margin has been seasonally affected by monsoon precipitation but with contrasting effects at the type 1 and type 2 Escarpments depending on the position of the Intertropical Convergence Zone in the geologic past: during peak monsoon conditions, both scarps experienced heavy rainfall and runoff, whereas during monsoon-starved conditions (such as today), the type 2 scarp experienced a foggy, moist climate while the type 1 scarp remained much drier. In order to assess the relative effects of climate and flexural parameters on the present-day morphology of the Dhofar margin, we present onedimensional numerical models of erosion and flexure along two profiles representative of the type 1 and type 2 scarps. Unlike most surface process models previously published, where present-day topography is the only criterion by which to evaluate the quality of model outputs, model behavior here is additionally constrained by independent estimates of denudation provided by geological cross sections, well-defined fault traces, and other stratigraphic markers. The best fitting models indicate that the type 1 Escarpment formed under relatively arid climatic conditions and was affected by significant erosion, recession and flexural uplift due to a low (7 km) effective elastic thickness. In contrast, the morphology of the type 2 fault scarp was smoothed by a more humid climate, but a high effective elastic thickness ( 15 km) prevented it from uplifting or receding. In addition, we show that the sedimentary load acting at the foot of the Escarpments exerts significant influence on their morphological evolution, though this parameter is often neglected in other scarp evolution models.