Transtension in Arcs and Orogens

Transtension is the oblique divergence between bounding plates or blocks that combines a coaxial orthogonal extension with a deformation zone boundary-parallel, noncoaxial, component, which generates a bulk constrictional strain. The coaxial component determines the rate and amount of crustal/lithospheric thinning and part of the horizontal extension. The non-coaxial component controls the vorticity, the horizontal shortening, and part of the horizontal extension. The instantaneous stretching direction bisects the angle between the direction of divergence (transport direction) and the zone boundary orthogonal. In the brittle regime, normal fault arrays accommodate vertical shortening and horizontal extension whereas simultaneous wrench fault arrays allow horizontal shortening. All faults, except those that are vertical and parallel with the zone boundary, rotate either with or against vorticity, depending on their orientation with respect to the transport direction and the zone boundary. Block rotation controls fault slip-direction and sense. Where the transport direction is at greater than 19.5° to the zone boundary, the coaxial component, vertical shortening, normal faults, and horizontal foliation dominate. At angles less than 19.5°, the non-coaxial component, horizontal shortening, wrench faults, and vertical foliation dominate at small strains and for transport directions close to the zone boundary. For larger strains, the deformation path passes through pure prolate constriction and the two shortening axes (Z and Y) swap so that a horizontal foliation may be superposed on an earlier vertical foliation. In the brittle regime, normal-dominant-over-wrench fault systems are superposed on wrench-dominant-over-normal ones. Thus, polyphase deformation sequences may develop as a result of a continuous strain path. In the Late Cambrian/Early Ordovician oceanic island arc of western Newfoundland, dextral transtension generated a vertical NE-striking schistosity in pillow lavas, with a subhorizontal stretch and NW-striking dikes prior to the intrusion of the Twillingate Granite at 510 Ma. Continued dextral transtension led to the development, from 488 to 455 Ma, of one or more supra-subduction zone ophiolite complexes in dextral pull-apart basins, prior to the mid-Ordovician collision of the arc and obduction onto the Laurentian continental margin. In the Eocene Guam segment of the Marianas Arc, sinistral transtension generated a boninitic dike complex with orthogonal folds. In the Western Gneiss Region of southwest Norway, Late Silurian/Early Devonian sinistral transtension generated constrictional fabrics in gneisses exhumed beneath west-slipping extensional detachments carrying growing extensional basins that were experiencing simultaneous N-S shortening. In the Basin and Range, between the Lake Tahoe-Las Vegas seismic zone and the Garlock Fault, high-strain-rate dextral transtension between the Sierra Nevada and the Colorado Plateau resulted in bulk constriction with folded, "turtleback" detachments and a regionally low average elevation.