HR: 09:45h
AN: T31E-08 INVITED     [Abstracts]
TI: The roles of tectonics in erosion: Fracturing and fragmentation are key, rock uplift is not
AU: * Molnar, P
EM: molnar@colorado.edu
AF: Cooperative Institute for Research in Environmental Science (CIRES), University of Colorado at Boulder, Boulder, CO 80309
AU: * Molnar, P
EM: molnar@colorado.edu
AF: Department of Geological Sciences, University of Colorado at Boulder, Boulder, CO 80309
AU: Anderson, R S
EM: andersrs@buffmail.Colorado.EDU
AF: Department of Geological Sciences, University of Colorado at Boulder, Boulder, CO 80309
AU: Anderson, R S
EM: andersrs@buffmail.Colorado.EDU
AF: INSTAAR, University of Colorado at Boulder, Boulder, CO 80309
AU: Anderson, S P
EM: suzanne.anderson@colorado.edu
AF: INSTAAR, University of Colorado at Boulder, Boulder, CO 80309
AU: Anderson, S P
EM: suzanne.anderson@colorado.edu
AF: Department of Geography, University of Colorado at Boulder, Boulder, CO 80309
AB: As Gilbert and Dutton recognized in the 19th century, erosion consists of two processes: "the disintegration of the rocks, reducing them to fragments, pebbles, sand, and clay" [Dutton, 1882] and then their transport. Tectonics contributes to both but more importantly to the first. Although many in the geomorphic community subscribe to "the emerging view that erosion rates adjust to high rates of tectonically driven rock uplift" [Montgomery and Brandon, 2002], numerical models are not needed to see that rather than ``driving" erosion, most "rock uplift" results from erosion via isostatic compensation. Relegation of rock uplift to consequence, not cause, of erosion, however, does not deny tectonics a role in erosion. Tectonics plays its key role by fracturing rock. Fractures not only provide avenues for water flow and thus promote weathering of rock, but also generate erodible fragments that can be extracted and transported on hillslopes or by rivers and glaciers. Tectonics does the first part of erosion (as defined by 19th century geologists): disintegration of massive rocks. Faults are not perfect planes; both local roughness and larger scale bends require straining of the adjacent rock masses upon slip on the fault, as shown well by aftershocks of major earthquakes. Although aftershocks of great earthquakes commonly occur on the faults that rupture in mainshocks, within continents many, if not most, aftershocks occur within the larger volume of rock of adjacent blocks that slipped past one another in mainshocks. Thus, they contribute to the dismemberment of these rock volumes into smaller blocks. Scaling rules for earthquakes suggest that dimensions of ruptures for very small earthquakes, Magnitude < -2, can be meters or less. The Gutenberg-Richter recurrence relationship implies that such earthquakes are common, as recordings by high-magnification seismographs in low-noise environments show. The large differences among fault plane solutions of aftershocks and of microearthquakes in intracontinental settings show that the small faults that rupture in microearthquakes are not parallel to one another and that some faults must intercept others. Thus, it seems likely that the upper crust in tectonically active regions is fractured and fragmented into blocks on the scale of boulders (if not smaller) and ready for the next part of erosion - transportation by slope, river, or glacial processes. A corollary is that deeply exhumed lower crust, which was not recently deformed by brittle fracturing and faulting, will be difficult to erode.
DE: 1815 Erosion
SC: Tectonophysics [T]
MN: Fall Meeting 2005