Climatic and tectonic controls on Jurassic intra-arc basins related to northward drift of North America

Upper Jurassic strike-slip intra-arc basins formed along the axis of earlier Lower to Middle Jurassic extensional intra-arc basins in Arizona. These strike-slip basins developed along the Sawmill Canyon fault zone, which may represent an inboard strand of the Mojave-Sonora megashear system that did not necessarily produce large-scale translations. Subsidence in the Lower to Middle Jurassic extensional arc was uniformly fast and continuous, whereas at least parts of the Upper Jurassic arc experienced rapidly alternating uplift and subsidence, producing numerous large-scale intrabasinal unconformities. Volcanism occurred only at releasing bends or stepovers in the Upper Jurassic arc, producing more episodic and localized eruptions than in the earlier extensional arc. Sediment sources in the Upper Jurassic strike-slip arc were also more localized, with restraining bends shedding sediment into nearby releasing bends. Normal fault scarps were rapidly buried by voluminous pyroclastic debris in the Lower to Middle Jurassic extensional arc, so epiclastic sedimentary deposits are rare, whereas pop-up structures in the Upper Jurassic strike-slip arc shed abundant epiclastic sediment into the basins. Three Upper Jurassic calderas formed along the Sawmill Canyon fault zone where strands of the fault progressively stepped westward in a releasing geometry relative to paleo-Pacifi c–North America plate motion. We hypothesize that strike-slip basins in the Upper Jurassic arc formed in response to changing plate motions that induced northward drift of North America, causing sinistral deformation of the paleo-Pacifi c margin. Drift out of the northern horse latitudes into northern temperate latitudes brought about wetter climatic conditions, with eolianites replaced by fl uvial, debris-fl ow, and lacustrine sediments. “Dry” eruptions Busby, C.J., Bassett, K., Steiner, M.B., and Riggs, N., 2005, Climatic and tectonic controls on Jurassic intra-arc basins related to northward drift of North America, in Anderson, T.H., Nourse, J.A., McKee, J.W., and Steiner, M.B., eds., The Mojave-Sonora megashear hypothesis: Development, assessment, and alternatives: Geological Society of America Special Paper 393, p. 359–376. doi: 10.1130/2005.2393(13). For permission to copy, contact [email protected]. ©2005 Geological Society of America. *E-mails: [email protected]; [email protected]; Steiner—[email protected]; [email protected] 360 C.J. Busby et al. spe393-13 page 360