How to dig a submarine canyon with no net erosion

Submarine canyons are deep valleys through which sediments eroded from the continents are delivered to the deep ocean, across the continental margins and slopes. One prominent example is the Monterrey Canyon (pdf poster).

They are often regarded as the result of erosion produced by the submarine flow, but this is what we just found in seismic data from the Ebro delta (published last week in Geology, link to abstract):

Survival of a submarine canyon during long-term outbuilding of a continental margin
David Amblas et al., Universitat de Barcelona, E-08028 Barcelona, Spain. Geology, doi: 10.1130/G33178.1 [GSA link] [pdf]
ResearchBlogging.org The resemblance between subaerial and submarine canyons has led to the long-standing view of submarine canyons as purely erosive landforms. Yet submarine canyons forming at continental slopes that grow from long-term accumulation of sediment are observed both at the present sea floor and further beneath, buried under sediment. This suggests that the canyon is prograding together with the margin. David Amblas and colleagues document the Pleistocene coevolution of a submarine canyon and adjacent slope along the Ebro Margin (NW Mediterranean) using a 3-D seismic image of the seafloor and subsurface. Seismic reflectors beneath the present-day canyon and adjacent slope show that net accumulation has occurred in both areas over the last 500,000 years. Seismic mapping reveals a mid-Pleistocene canyon beneath the modern canyon that is morphologically similar in planform but exhibits a different long profile shape. An explanation for the change in long-profile shape is proposed in terms of the dominant sedimentation processes in the canyon. This study aims to broaden thinking about canyon evolution and the processes that govern it during outbuilding of a continental margin. From this perspective, canyons are submarine landscape features that prograde at rates comparable to those of the margin, mostly undergoing net deposition while they serve as the outlet channel for the continental sediment supply towards the abissal plains.
Pleistocene and present submarine canyon, and
conceptual model of the depositional control of
its evolution

Amblas, D., Gerber, T., De Mol, B., Urgeles, R., Garcia-Castellanos, D., Canals, M., Pratson, L., Robb, N., & Canning, J. (2012). Survival of a submarine canyon during long-term outbuilding of a continental margin Geology DOI: 10.1130/G33178.1