VVOL.OL. 115,5, NNO.O. 2 A PPUBLICATIONUBLICATION OFOF TTHEHE GGEOLOGICALEOLOGICAL SSOCIETYOCIETY OOFF AAMERICAMERICA FFEBRUARYEBRUARY 22005005 Subduction zone backarcs, mobile belts, and orogenic heat Inside: Subduction zone backarcs, mobile belts, and orogenic heat, ROY D. HYNDMAN, CLAIRE A. CURRIE, AND STEPHANE P. M AZZOTTI, p. 4 2003–2004 Division Officers and Past Chairs, p. 11 GeoVentures™ 2005, p. 27 Volume 15, Number 2 FEBRUARY 2005 Cover: The North America Cordillera mobile GSA TODAY publishes news and information for more than belt illustrating the high elevation and complex 18,000 GSA members and subscribing libraries. GSA Today tectonics characteristic of hot, thin backarc lead science articles should present the results of exciting new lithospheres. Backarcs remain weak enough to be research or summarize and synthesize important problems deformed by plate margin forces for long geological or issues, and they must be understandable to all in the earth periods. 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Sajban 32 Announcements ADVERTISING: Classifieds & Display: Ann Crawford, 1-800-472-1988, ext. 1053, (303) 357-1053, Fax 303-357-1070; [email protected] 34 Journal Highlights GSA ONLINE: www.geosociety.org 35 Classified Advertising Printed in the USA using pure soy inks. 38 GeoMart Geoscience Directory 50% Total Recovered Fiber 10% Postconsumer Subduction zone backarcs, mobile belts, and orogenic heat Roy D. Hyndman, Pacific Geoscience Centre, Geological Survey of Canada, P.O. Box 6000 Sidney, British Columbia V8L4B2, Canada, and School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia V8W3P6, Canada, [email protected]; Claire A. Currie, School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia V8W3P6, Canada; and Stephane P. Mazzotti, Pacific Geoscience Centre, Geological Survey of Canada, P.O. Box 6000 Sidney, British Columbia V8L4B2, Canada ABSTRACT hot backarc lithosphere, not from the Two important problems of continental orogenic deformation process itself. tectonics may be resolved by recogniz- INTRODUCTION ing that most subduction zone backarcs The model of plate tectonics with have hot, thin, and weak lithospheres narrow plate boundaries provides an Figure 1. The North America Cordillera mobile over considerable widths. These are (1) excellent first order description of global the origin of long-lived active mobile belt. The high elevation and complex current tectonics. Plate tectonics also provides tectonics illustrate the hot, weak, backarc belts contrasted to the stability of cra- an elegant explanation for orogenic lithosphere deformed by variable margin forces. tons and platforms, (2) the origin of crustal shortening and thickening in the heat of continental collision orog- terms of continental or terrane colli- eny. At many continental margin plate sion. However, a number of large-scale deformation over long geological peri- boundaries, there are broad belts with tectonic problems are not explained by ods. They have some characteristic that a long history of distributed deforma- the simple rigid plate and continental allows them to maintain an especially tion. These regions are mobile because collision models. In this article, we pres- thick strong lithosphere, such as a more the lithosphere is sufficiently weak to ent explanations for two such tectonic refractory mantle composition (Jordan, be deformed by the forces developed problems: the origin of long-lived active 1978; Forte and Perry, 2000). The rea- at plate boundaries. We conclude that “mobile belts” that lie along a number son for the long histories of tectonic mobile belts are weak because they are of continental margin plate boundaries, activity in the mobile belts has not been hot, and they are hot as a consequence contrasted to the stability of cratons and clear. Also, most continental mobile of their position in present or recent platforms, and the origin of the heat of belts have high elevation; they are backarcs. Most continental backarcs continental collision orogeny. Current mountain belts. Surprisingly, however, have thin and hot lithospheres, not just mobile belts are up to 1000 km wide there is commonly no crustal thicken- those with active extension or rifting. and cover nearly a quarter of the conti- ing. In this article, we emphasize the Moho temperatures are 800–900 °C and nents (e.g., Stein and Freymueller, 2002; important distinction between long-lived lithosphere thicknesses are 50–60 km, Thatcher, 2003). For example, the North mobile mountain belts where the eleva- compared to 400–500 °C and 150–300 American Cordillera has been tectoni- tion is mainly thermally-supported and km for cratons and other regions with a cally active for >180 m.y., with evidence short-lived continent or terrane collision thermotectonic age greater than ca. 300 for older tectonic events extending this orogenies where there is major crustal Ma. The temperature differences result history to >350 m.y. (e.g., Burchfiel et shortening and thickening. in backarc lithospheres that are weaker al., 1992) (Fig. 1). Rates of deformation Plate tectonics provides an elegant than cratons by more than a factor of in current mobile belts are commonly explanation