Denudation of the Coastal Drainages in the Mendocino Triple Junction Region (MTJ), California
Abstract
Changes in Earth's surface topography reflect imbalances between the rates at which rock moves towards Earth's surface (denudation) and the rates at which rock moves upward relative to a stable datum (rock uplift). Where landscapes rapidly respond to changes in uplift rates, denudation rates are assumed to be equal to rock uplift rates regardless of lithologic or climatic conditions. In such an equilibrium case, denudation rates only reflect rock uplift rates, whereas spatial variations in topography reflect the distribution of tectonic rates, climate, rock erodibility, and geomorphic processes. The Mendocino triple junction region (MTJ) in northern California has been considered to be an archetypical example of landscape that has reached such a dynamic equilibrium. Interactions between the North American, Pacific, and Gorda plates have generated an order of magnitude difference in tectonic rates along the coast. Previous studies of uplifted marine terraces have shown that tectonic rock uplift rates from late Pleistocene and Holocene vary by a factor of 8 (0.5-4 mm/yr). This spatial variation in the uplift rates is typically assumed to be equal to fluvial incision rates. Under this assumption of a dynamic equilibrium, the channel morphology and landscape steepness have been studied to understand channel responses with respect to the observed tectonic gradient. Despite the many studies that assume a dynamic equilibrium in this landscape, none have demonstrated this explicitly. We tested this assumption by measuring basin-averaged denudation rates within catchments draining to the coast along the rock uplift gradient in this area. We studied 10 coastal drainages near the MTJ and collected river sand samples to measure the denudation rates using 10Be. We selected drainages whose areas were close to or larger than 10 km2 to provide spatial averaging of discrete mass-transport processes. These drainages encompass the large variation of tectonic rates along the coast—high uplift areas (> 4 mm/yr), low uplift areas (0.5 mm/yr), and the transition zone between these end-members (~ 3 mm/yr). Preliminary results from 3 drainages from low uplift-rate areas and those within the transition zone between low and high rates show that denudation rates were <0.1 - 0.5 mm/yr. The denudation rates were less than 40 % of the rock uplift rate inferred from uplifted marine terraces, suggesting that the landscape may not be in a state of dynamic equilibrium between denudation and uplift rates. To understand the difference between denudation and uplift rates, we compared the denudation with the observed spatial variations in climate and lithology using GIS datasets from the USGS to relate climatic, lithologic, or geomorphic factors to the observed changes in denudation rates. To this end, spatial variations in topographic metrics such as channel steepness for river incision processes and failure index for shallow landslides were compared to denudation rates to determine the erosional processes that may most likely be responsible for the spatial patterns in denudation.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMEP52E..08M
- Keywords:
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- 1815 HYDROLOGY / Erosion;
- 1824 HYDROLOGY / Geomorphology: general;
- 1825 HYDROLOGY / Geomorphology: fluvial;
- 8175 TECTONOPHYSICS / Tectonics and landscape evolution