2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 10
Presentation Time: 4:05 PM


DEWEY, John F., Geology, UC Davis, One Shields Avenue, Davis, CA 95616, dewey@geology.ucdavis.edu

Lithospheric-scale transtension generates bulk regional constriction in both the upper crustal brittle, and lower crustal ductile fields. In thin attachment zones, below the brittle-ductile transition, more complex strain fields,including flattening, can be generated. Crustal-scale constriction generates a regionally sub-horizontal crenulated and folded foliation with LS fabrics in the ductile field where the transport direction is at greater than 20° to the deformation zone boundary and a vertical crenulated and folded foliation with LS fabrics where the transport direction is at less than 20° to the deformation zone boundary. Transtension involves horizontal shortening and crustal thinning, and is commonly accompanied by low-angle extensional detachments. The horizontal shortening is expressed by folding of low-angle extensional detachments and sub-horizontal foliations, constrictional,LS, fabrics, and vertical foliations. In the Western Gneiss Region of Norway and in Bushmanland/Namaqualand in South Africa, the horizontal shortening that accompanied transtension has been mistaken for evidence of regional crustal shortening/thickening (Solundian Orogeny in Norway). In the Coso/China Lake region of southern California, a transtensional zone was, until recently, considered to be a transpressional zone These may be called spoof orogenies. Transtensional shortening events are characterized by low-grade/younger over high-grade/older across gently-dipping faults ( extensional detachments, not thrusts), down-pressure/hot metamorphic assemblages (crustal thinning, not thickening), and constrictional LS,not flattening SL fabrics and structures. Plate boundary zone orogenies involve regional horizontal shortening, crustal thickening, prograde metamorphism, foreland basins with clastic wedges, well-defined edges, and, commonly, terminating unconformities.