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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).
