Paper No. 6
Presentation Time: 9:30 AM
PROGRESSIVE, EPISODIC PROCESSES FOLLOWING COLLISION AND CRITICAL COUPLING OF RIDGE SEGMENTS AND CONTINENTS: PLATE MOTION CHANGES, EXTENSION, MANTLE FLOW, UPLIFT, AND INDUCED SUBDUCTION WITHIN CA. 15 MA INTERVALS
Convergence resulting in collision of buoyant oceanic crust with a continental plate leads to coupling followed by abrupt changes in global plate motions. In western North America episodes of critical coupling between the Pacific (PAC) and North America (NA) plates that are recorded by extensional domains distinguished by age and direction of tectonic transport (Eocene [~55-42 Ma], ca. 285o, Oligocene [~35-20 Ma], 240o, Miocene [17-0 Ma], ca. 280o) also are known in Eurasia. The directions record the integration of the independent motions of NA (SW), related to mantle convection, and the PAC (NW) (driven by slab pull). As PAC moves westward dragging NA, the formerly subducting Farallon slab, and Eurasia (EA) with it, brittle extension takes place and core-complexes form within the PAC-NA coupled region as well as shearing and detachment along the Tethys suture. Mantle displaced at the front of NA forms a swell that progresses eastward causing cratonal uplift and exhumation of cover strata. Break-up unconformities (ca. 55, 35, and 17 Ma) and basins record crustal thinning. Intracontinental unconformities record the passage of the mantle swell. During each coupling event as NA and EA move westward strong extensional strain is imposed upon rocks along the southern margin of Eurasia where it detaches from Africa and India. Where mantle flows toward the wake of the moving plates fluid pincers may push Africa and Eurasia closer inducing subduction and adding to constrictional strain. Decoupling may take place after ca. 10 Ma of cooling followed by resumption of subduction. Lulls in subduction-related magmatism, trench “roll-back”, post-orogenic “collapse” recorded by Cordilleran and Pacific “back-arc” basins, which are commonly linked by continental transform faults, global unconformities, and orogenic uplifts are co-genetic and evolve progressively following coupling.