Cathy J. Busby
Santa Barbara, CA
Climatic and Tectonic Controls On Jurassic Intra-Arc Basins Related to Northward Drift of North America
We hypothesize that Late Jurassic strike-slip intra-arc basins formed along the axis of earlier Early to Middle Jurassic extensional intra-arc basins in western North America. These two types of intra-arc basins contrast with each other in several ways.
- Subsidence in the Early to Middle Jurassic extensional arc was uniformly fast and continuous, whereas at least parts of the Late Jurassic arc experienced rapidly-alternating uplift and subsidence, producing numerous large-scale unconformities within the basins. Such “porpoising” is produced as locations alternate between subsiding releasing bends and uplifting restraining bends along strike-slip faults.
- Volcanism occurred only in releasing bends in the Late Jurassic arc, producing more episodic and localized eruptions than in the extensional arc, where volcanism was voluminous and widespread;
- Sediment sources in the Late Jurassic strike-slip arc were localized, with restraining bends shedding sediment into nearby releasing bends. The overall fault-controlled pattern of “porpoising” subsidence, episodic volcanism, and localized sediment source would indicate strike-slip rather than continental rift tectonics for the Bisbee basin of southern Arizona.
We hypothesize that strike-slip basins in the Late Jurassic arc formed in response to changing plate motions inducing northward drift of North America relative to the paleoPacific. Drift out of the horse latitudes into temperate latitudes brought about wetter climatic conditions, with eolianites becoming replaced by fluvial, debris flow and lacustrine sediments. “Dry” eruptions of welded ignimbrite were replaced by “wet” eruptions of nonwelded, easily reworked ignimbrite and phreatoplinian fall.
This Late Jurassic transition from hyperarid to more temperate climatic conditions forms a superregional “timeline” that ties the Cordilleran plate margin to events in the interior of the continent. Our stratigraphic work also demonstrates the usefulness of ignimbrites for making regional stratigraphic correlations between ranges scattered hundreds of miles apart, in spite of Jurassic strike slip disruption, Laramide shortening, Basin and Range extension, and Mesozoic through Cenozoic plutonism and alteration.
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