CSDMS 2016 annual meeting poster Jean-ArthurOlive
Feedbacks between mass wasting and abyssal hill growth on seismic cycle and geological time scales
Samuel Howell, U. Hawaii , United States.
Luca Malatesta, Caltech , United States.
Xiaochuan Tian, Lamont-Doherty Earth Observatory / Columbia University , United States.
Roger Buck, Lamont-Doherty Earth Observatory / Columbia University , United States.
Garrett Ito, U. Hawaii , United States.
Javier Escartin, CNRS , France.
Abstract:
Feedbacks between surface processes and tectonics are well established in a variety of subaerial geodynamic settings, yet they remain largely unexplored in submarine contexts. Here we investigate potential feedbacks between mass wasting and the growth of abyssal hills, the most common landform on the Earth's surface. Abyssal hills form as lithosphere-scale normal faults uplift newly accreted magmatic terrains near the axis of mid-ocean ridges. In order to assess the effect of mass wasting on fault-induced topography, we analyze the morphology of ~2000 normal fault scarps identified in cross-axis multibeam bathymetry profiles from the intermediate-spreading Chile Ridge. This large population samples the entire life span of a hill-bounding fault, from initiation to abandonment.
We first calculate a running median of the scarp dip population, and find that it spans a range (0–30º) that is clearly distinct from the expected dip of active normal faults (45–60º). Further, median scarp dips tend to increase with increasing scarp throw, until they plateau around 25º for scarp throws exceeding 500 m. This trend is best explained by a model where abyssal hill uplift competes with mass wasting parameterized as a non-linear diffusion process. Specifically, we assume that the local transport of degraded scarp material is a non-linear function of slope that is essentially infinite at a critical value of 25º, and quasi-linear for slopes shallower than ~20º. This model captures the time-integrated effects of rockslides triggered by failure on supercritical slopes (≥25º). Our best-fitting model predicts that ~0.6 km2 of basaltic material gets degraded per km along-axis during the growth of a 1-km high abyssal hill (100 kyr).
We then assess potential links between mass wasting events and seismogenic slip on hill-bounding faults. We first evaluate the frequency-magnitude distribution of earthquakes on the Chile Ridge using the ANSS teleseismic earthquake catalog (USGS). We then attempt to relate earthquake recurrence and cumulated volumes of degraded materials. In our simplest scenario, the measured long-term scarp degradation rate can be accounted for if a rockslide displacing ~104 m3 of debris is triggered whenever an earthquake of magnitude ≥3 occurs on the ridge. Such rockslides have been documented along the walls of the Mid-Atlantic Ridge axial valley. In more complex models, we assume that rockslide volumes (V) follow a Gutenberg-Richter distribution of the form: log10(N≥V) = a - b log10(V). Such a distribution of mass wasting events can produce cumulated volumes of degraded materials that match the diffusion model when using a b-value of ~2/3.
Lastly, we evaluate the effect of mass wasting on the mechanical state of hill-bounding faults and their long-term evolution. We carry out numerical simulations of mid-ocean ridge normal faulting that incorporate magmatic emplacement at the axis and topography diffusion at the seafloor. Our models suggest that scarp degradation at rates documented on the Chile Ridge enhance the life span, spacing and throw of abyssal hills by ~10% relative to a scenario where fault-induced topography remains intact. We thus argue for a subtle, yet quantifiable effect of mass wasting on mid-ocean ridge tectonics that may be exacerbated in ultraslow-spreading settings.
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