HR: 1330h
AN: S22A-0436    [PDF]
TI: Mobile Belts, High Lithosphere Temperatures, and Subduction Zone Backarcs
AU: * Hyndman, R D
EM: rhyndman@nrcan.gc.ca
AF: Geological Survey of Canada, Pacific Geoscience Centre, P.O. Box 6000, Sidney, BC V8L4B2 Canada
AB: At many continental plate boundaries, there are mobile orogenic belts or zones of distributed deformation hundreds of kilometres wide. An important characteristic of "mobile belts" is a long geological history of ongoing deformation or of repeated deformation events that suggests long-term lithosphere weakness compared to cratons. Thus, they are "mobile" not just because they are subject to deforming forces, but also because they are weak. In contrast, Precambrian cratons have exhibited very little internal deformation for long geological periods, and are inferred to have long-term strength. We propose that most mobile belts are weak because they are hot, and they are hot because they are in backarcs or geologically recent backarcs. Most continental backarcs are observed to be hot; the temperature at the Moho is ~900C compared to ~400C for cratons, and the lithosphere thicknesses are 50-60 km compared to more than 250 km for cratons. The high temperatures result in thermal expansion and the common high elevations in backarc mobile belts, even where the crust is not thickened. Backarc mobile belt lithospheres are at least a factor of 10 weaker than those of cratons. Backarcs may be hot and have thin, weak lithospheres because of the water driven upward into the backarc mantle wedge from dehydration of the underlying subducting plate. The water reduces the effective asthenosphere viscosity and allows vigorous free convection. High backarc temperatures appear to decay after subduction stops, with a time constant of 50-100 m.y. Many continental mobile belts are hot enough for there to be a weak lower crust detachment so surface tectonics are decoupled from the upper mantle. For example, foreland thrusting at the craton edge of a mobile belt may be driven by continental margin plate boundary forces 100s of km away, with translation of the crust over a lower crust detachment.
DE: 7218 Lithosphere and upper mantle
DE: 8107 Continental neotectonics
DE: 8110 Continental tectonics--general (0905)
DE: 8120 Dynamics of lithosphere and mantle--general
SC: Seismology [S]
MN: 2003 Fall Meeting