Sliding Friction and Overthrust Faulting

Abstract

The puzzle of tectonic sliding is illustrated by means of the Roberts Mountains overthrust in north-central Nevada. A model of the thrust, consisting of a wedge-shaped sheet gliding into a subsiding and migrating basin of deposition, is described, and calculations are made for the requisite coefficient of sliding friction assuming only normal pore-water pressure (answer, ) and for the requisite pore-water pressure under the whole thrust sheet, using a coefficient of internal friction for typical rocks, according to the Hubbert-Rubey hypothesis (answer, ). Neither result is attractive, and attention is turned to the premises of friction. Recent observations and hypotheses on mechanisms of sliding for metals and at least some non-metals emphasize the interdependent role of stresses normal and tangential to the sliding plane in creating and rupturing adhesions or junctions between the surfaces. These junctions alone support the load and make up the true area of contact which commonly is a small fraction of the apparent area. Weak materials at the interface inhibit junction growth but only if their critical shear strength is exceeded by tangential stresses necessary for that growth. This implies a means for estimating coefficients of friction from strengths of materials. Since strengths of materials are known to be sensitive to temperature, confining pressure, and, most notably, strain rate, the classical laws of friction expressed in the relation Frictional force = coefficient X load should not be expected to hold through the whole range of environmental conditions in nature. Coefficients ordinarily obtained cannot be applied safely to tectonic sliding. Coefficients of internal friction and sliding friction, though analogous, are distinct. Crude calculations suggest that an interface of saturated clay between limestone blocks could inhibit junction growth in the manner indicated and yield a coefficient of sliding friction compatible with gravity gliding of a thrust sheet on a slope of 2 or 3 degrees. But in the virtual absence of detailed laboratory studies or adequate data, application of the adhesion hypothesis to rocks, though intriguing, must remain tentative.