Abstract
AbstractMore than half of the world's large rivers flow towards the ocean crossing passive continental margins. Here using an analytical solution and numerical models, we demonstrate that on passive margins, river basins may be integrated by major margin‐parallel channels, which form as a flexural isostatic response of the lithosphere to mechanical/erosional unloading along the margin. We analyzed the downstream courses of large rivers flowing across the passive margins and find that the majority of them (31 of 36) have major margin‐parallel channels. Occurrences of these channels are generally consistent with the model predictions, although the exact locations and geometry of these rivers may also be controlled/changed by other factors. Our results suggest that the lithosphere strength has an important control on the geometry of large river systems on passive margins, linking the evolution and routing of the Earth's freshwater systems to its deep interior dynamics.
Highlights
The geometry of river networks determines how water runoff is routed across continents and has a direct influence on freshwater input into the ocean, carbon cycling, and the pattern of freshwater ecosystems
Using an analytical solution and numerical models, we demonstrate that on passive margins, river basins may be integrated by major margin-parallel channels, which form as a flexural isostatic response of the lithosphere to mechanical/erosional unloading along the margin
We suggest that the scattering is in part due to the non-deterministic nature of surface processes, but more importantly, the developments of axial rivers may be controlled by other factors besides the isostatic response to the erosional unloading, such as active tectonics and sediment deposition offshore
Summary
The geometry of river networks determines how water runoff is routed across continents and has a direct influence on freshwater input into the ocean, carbon cycling, and the pattern of freshwater ecosystems. Drainage networks along passive margins are typically characterized by a dichotomy of river basin sizes, with small coastal rivers that drain directly to the ocean separated from large river basins that drain the continental interior surrounded by a rim of high topography This topographic bulge is thought to form during continental breakup as a flexural isostatic response to continental stretching (Braun & Beaumont, 1989) and/or progressive erosion of an escarpment or topographic step (Gilchrist & Summerfield, 1990) that separates the coastal plain from the relatively flat, elevated continental interior (e.g., Figures 1a and 1b). Flexural isostasy which works to create and maintain the escarpment results in low-amplitude subsidence of the continental area behind it (Figure 1c) (Sacek et al, 2012) We postulate that this topographic low leads to the formation of a valley that parallels the passive margin and truncates the inland river network, diverting flow to a large, margin-parallel river. The results support our hypothesis and indicate that the strength of the lithosphere is a fundamental component to the drainage network geometry on passive margins, the detailed patterns are controlled by other parameters
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