Relative and absolute plate motion changes in the southeast Pacific: Cause and effect.

摘要

The purpose of this work is to expand upon our understanding of the dynamics of absolute plate motions and the interactions between plates using geological, geophysical, and modeling techniques.; Using fracture zone morphology and magnetics the following relationships are hypothesized: the Pacific-Farallon ridge reorganization began around 35 Ma, the reorganization took around 10 m.y., the greatest changes in ridge geometry took place adjacent to the Nazca portion of the Farallon plate, the Nazca plate underwent an absolute motion change around 26 Ma (O'Connor et al., 1996), South America was the only other plate in the region to under a change in absolute motion at this time (35 Ma), and thus the timing indicates that South America is the cause of Nazca's motion change. Two- and three-dimensional finite element models of the region establish that it is viable that the westward acceleration of South America could have transmitted a compressional stress to the Nazca portion of the Farallon plate, and that the east-west compressional stress generated by South America was transmitted to the ridge where it was concentrated across the transforms. The misalignment of the east-west compressional stress with the slip along the transforms may have impeded spreading and thus caused the ridge to reorganize. Shear stress in the slab at the northern boundary of South America due to South America's acceleration may also be related to the creation of a tear in the Pacific-Farallon slab at 30 Ma and the subsequent change in the absolute motion of the Nazca portion of the Farallon plate. These results lead to the conclusion that the westward acceleration of South America changed the force-balance of the Nazca portion of the Farallon plate, ulimately resulting in the reorganization of the Pacific-Farallon ridge and the 26 Ma absolute motion change of the Nazca plate.; This work was augmented by finite element studies of anomalous normal faulting earthquakes along the present day Eltanin transform system.