Morphologic modelling of continental slopes Neil C Mitchell

Tributaries converge smoothly in elevation at confluences (Playfair’s Law) - implies erosion by many

Channels are concave upwards. Inverse relation between channel gradient and contributing area, with similar

Slope-heading and shelf-heading canyons have similar concavity -> During low-stand conditions, all canyons supplied

Knickpoints in tectonically active slopes anticline channel Knickpoint migration (Alaska). Detachmentlimited erosion? Knickpoint diffusion

Diffusive-like terrain: ? ∂H/∂t = K ∂2 H/∂x 2 Threshold Diffusive USA Atlantic Mitchell

Origin of diffusion? ∂H/∂t = K ∂2 H/∂x 2 Derived by combining Sediment transport

Upper slope - a sandy clinoform rollover. Predictable from how currents decline with depth

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Morphologic modelling of continental slopes Neil C. Mitchell Cardiff University USA Atlantic coast • Canyon erosion • Diffusive topography Southern Santa Cruz Mountains, CA

Tributaries converge smoothly in elevation at confluences (Playfair’s Law) - implies erosion by many erosive flows on adjacent channels. Mitchell, Am. J. Science 04.

Channels are concave upwards. Inverse relation between channel gradient and contributing area, with similar graph slopes to some bedrock-eroding rivers. In bedrock-eroding rivers, concavity arises from discharge compensated by gradient. How does concavity arise in submarine canyons? Taiwanese rivers (Whipple) Slope-confined linear canyons Mitchell, Marine Geol 05.

Interpretation a Plan Profile Flows originate from slope failures in canyon walls. Frequency & size of sedimentary flows increases downslope, so time-averaged erosion rate increases with canyon area. Mitchell, Am. J. Science 04.

Slope-heading and shelf-heading canyons have similar concavity -> During low-stand conditions, all canyons supplied by hemipelagic material. (Otherwise, shelfedge supply only would have eroded back shelf-heading canyons differently. )

Knickpoints in tectonically active slopes anticline channel Knickpoint migration (Alaska). Detachmentlimited erosion? Knickpoint diffusion (S. Barbados prism). Transportlimited erosion? Mitchell, Bull GSA, 2006.

Diffusive-like terrain: ? ∂H/∂t = K ∂2 H/∂x 2 Threshold Diffusive USA Atlantic Mitchell & Huthnance, in review.

Origin of diffusion? ∂H/∂t = K ∂2 H/∂x 2 Derived by combining Sediment transport flux proportional to gradient Q = -K ∂H/∂x with ∂H/∂t = -∂Q/∂x (continuity) Flume experiments Effect of gravity on bedload produces down-slope flux proportional to gradient. K~<u>2 to K~<u>3 Mitchell & Huthnance, in review.

Upper slope - a sandy clinoform rollover. Predictable from how currents decline with depth and bedload models. Atlantic USA Forward simulation Gradient Mitchell & Huthnance, in review.