STRIKE SLIP TECTONICS AND SEDIMENTATION IN THE LATE JURASSIC ARC OF SOUTHERN ARIZONA RELATED TO NORTHWARD DRIFT OF NORTH AMERICA

Late Jurassic sinistral strike-slip intra-arc basins formed along the axis of earlier Jurassic extensional intra-arc basins in Arizona. 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. Strike-slip basins in the Late Jurassic arc formed in response to changing plate motions that induced northward drift of North America relative to the paleoPacific. Drift out of the northern horse latitudes into northern temperate latitudes brought about wetter climatic conditions, with eolianites 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.

We recognize two types of strike slip basins in the Late Jurassic arc: structurally simple stepover basins dominated by silicic calderas formed from large-volume eruptions, and structurally complex releasing-restraining bend basins. Releasing-restraining bend basins have two distinguishing characteristics: (1) they contain small polygenetic, multivent volcanic complexes, which form where strands of the fault frequently plumb small batches of magma to the surface at releasing bends, and (2) they contain huge unconformities created by partial basin inversion at restraining bends. Sequence stratigraphic analysis in the Santa Rita Mountains demonstrates the relationship of eight unconformity-bounded sequences to high-angle intrabasinal faults, which alternated between normal-slip separation and reverse-slip separation. Five of the eight unconformities show extreme vertical relief (900 – 1800 m) and very high paleo-slope gradients (20° - 71°), with pronounced asymmetry facing away from the master fault toward the basin; these unconformities are interpreted to represent paleo-slide scars produced during basin inversion events. The other three unconformities are more symmetrical, with vertical relief of 300 – 400 m and paleo-slope gradients of 20° to 40°. These probably represent paleocanyons cut during basin inversion events. The name "releasing-restraining bend basin" emphasizes the fact that both processes mold the basin as it slips along the strike-slip fault.