Active tectonics, landsliding, and the evolution of mountainous topography.

摘要

The topography of the earth's surface is shaped by a complicated interplay of tectonic and climatic forces. Tectonic forces deform the topography in three dimensions, while climatic forces dictate the effectiveness of various geomorphic processes in changing the spatial distribution of mass in the landscape. I describe a series of studies aimed at understanding the roles of particular tectonic and climatic processes in shaping mountainous topography, with examples from the Basin and Range province of the western United States.;To the west, strain in Owens Valley is partitioned between the dextral-slip Owens Valley fault and the normal-slip Independence fault. A geodetic network across Owens Valley will potentially determine the degree of strain partitioning between these faults, and find the loci of strain accommodation within the valley. Geodetic Global Positioning System surveys have so far failed to measure significant relative velocities between network points.;Mountainous topography also evolves through a suite of geomorphic processes, including bedrock landsliding. A granular hillslope model reveals that bedrock landsliding is an episodic process, controlled primarily by the rock mass strength. On cohesive hillslopes, steady baselevel lowering results in a power-law distribution of landslides. Most landslides are small, and remove material only from the toe of the hillslope, creating an inner gorge. Inner gorges are thus due to the stochastic nature of the bedrock landsliding process, and not necessarily to a recent change in baselevel lowering rate. Similar inner gorges are observed in a range of mountainous landscapes.;Experiments with a numerical landscape evolution model indicate that landsliding plays a vital role in the evolution of extensional mountain ranges. Landsliding and bedrock channel incision are the primary erosive processes in the uplifting footwall block, and both are required to produce realistic topography. Ranges rapidly achieve a steady state, wherein the relief is limited by the drainage density and the rock strength. This generates some characteristic landforms associated with these ranges, such as wineglass canyons and triangular facets. The facets are highly reworked by landsliding, and are not planar remnants of the original fault surface.;The southwestern margin of the Basin and Range province is characterized by a number of active, dominantly dextral-slip faults. The Ash Hill fault, which bounds western Panamint Valley, California, shows predominantly dextral slip with a small normal component. The fault warps a late-Pleistocene shoreline, and numerical modeling of that warp indicates 60 m of fault slip since occupation of the shoreline and a short-term minimum slip rate of 0.4-0.5 mm/yr. This slip uplifts Panamint Valley relative to the Argus Range to the west, and is consistent with two contrasting scenarios for how Panamint Valley has evolved.