Tectonics, fracturing of rock, and erosion

[1] We argue that by fracturing rock, not by raising it relative to base level, tectonics plays its most important role in causing rapid incision of valleys and rapid erosion of hillslopes. Tectonic deformation riddles the upper crust with fractures, which not only provide avenues for water flow and thus promote weathering and further disintegration of rock but also fragment bedrock into debris that is readily extracted and transported by surface processes. Bends in active faults require straining of adjacent rock masses. Aftershocks that occur subsequent to slip on primary faults reflect penetrative brittle deformation of the upper crust. At least some aftershocks must nucleate or lengthen cracks, which contribute to the comminution of these rock masses. Scaling rules suggest that dimensions of ruptures for very small (M < 2) earthquakes can be meters or less. The Gutenberg-Richter recurrence relationship implies that such earthquakes are common, as high-magnification seismographs in low-noise environments confirm. Moreover, large differences among fault plane solutions for aftershocks show that the small faults on which they occur are not parallel to one another; some faults must intersect. Thus the upper crust in tectonically active regions should be fragmented into blocks down to the scale of boulders or smaller. Dismembered rock arrives at the Earth’s surface already prepared to be transported away. As a corollary, both deeply exhumed lower crust and posttectonic igneous rock, never deformed under brittle conditions and not deformed recently, should be less susceptible to detachment and subsequent transport than fractured rock.