JURASSIC TECTONICS AND SEDIMENTOLOGIC PROCESSES, SOUTHWESTERN NORTH AMERICA: RELATION TO PLATE BOUNDARY STRESS ORIENTATION

Development of the Jurassic volcanic belt along the southwestern margin of North America records convergence. Easterly-striking extensional structures, such as dikes and faults, that formed after local collisional orogeny (~174 Ma)and throughout the duration of late Jurassic transform faulting (162-148 Ma), track the orientation of the maximum principal stress in the proto-Pacific and North America plates. If extensional faults in the plates are parallel to the maximum principal stress during strike-slip faulting and if Jurassic subduction involved orthogonal convergence, then subduction, rifting and transform faulting are different tectonic manifestations of the same deep process, that is, east-directed asthenospheric flow that carried the proto-Pacific plate toward the North America plate. Diverse middle and late Jurassic tectonic events along the southwestern margin of North America reflect the changing conditions between the converging plates as a function of: 1) facility of subduction (unimpeded, impeded, jammed); and 2) plate properties (e. g. thickness, temperature and resulting viscosity, coherence, i. e. lack of fragmentation). During facile subduction (190-175 Ma) little stress is transmitted from one plate to another across the plate boundary. If the down-going slab does not subduct facilely because it is thick or of low specific gravity, then collision(s) (~174-167 Ma) may occur. Continuing downward flow of the asthenosphere forces local plate reorganization at susceptible points (167-163 Ma). After the plates fully jam, contraction and consequent lithospheric thickening enable transmission of plate-driving stresses across the plate boundary into brittle areas that may fracture into a pattern of systematic extensional faults and complementary lateral faults. Ultimately, in response to fault coalescence, lateral faulting, becomes the most efficient means to accommodate deformation along the plate boundary (162-148 Ma). Intrusions (155-148 Ma) into highly strained and thin areas of the crust may weld transform(s) and “anneal” the lithosphere. The strengthened lithosphere may again transmit stress across the former transform plate boundary (148-123 Ma).