Date of Award

1986

Degree Type

Thesis

Degree Name

Master of Science in Oceanography

Department

Oceanography

First Advisor

Roger Li Larson

Abstract

In the first part of this study, magnetic and bathymetric data from an R/V Endeavor survey are combined with earthquake mechanism studies to produce a tectonic model for the Juan Fernandez microplate at the Pacific-Nazca-Antarctic triple junction. Using rate data from the East Ridge of the microplate, a Nazca-Juan Fernandez best fit pole was determined. As is suggested by the fanning of anomalies, this pole is just north of the ridge. The other two poles were determined by closure. Plate motion data for the four-plate system were inverted to define present-day plate motions and rotation poles. The resulting tectonic model predicts that the northern boundary of the Juan Fernandez microplate is a zone of compression and that the West Ridge and southwestern boundary are spreading obliquely. The southeastern boundary is predicted to be obliquely divergent, but present data fail to constrain its geometry. A schematic evolutionary model was also constructed to illustrate the migration and evolution of the triple junction, and relates the formation of the Juan Fernandez microplate to differential spreading rates at the triple junction. Possibilities for the future evolution of the four-plate system are included as part of the model.

In the second part of this study, bathymetric data collected by the Endeavor were used to construct a new contour map of the Chile transform 0 system, a major part of the Nazca-Antarctic boundary, from 100 w to its termination at the East Ridge of the Juan Fernandez microplate at 34°30'S, l09°15'W. Geophysical data from along this boundary had been extremely limited until the Endeavor survey. A generally continuous lineated bathymetric trend can be followed through the entire region, with the transform valley being relatively narrow and well-defined from 109°w to approximately 104°30'W. The fracture zone-parallel topography then widens eastward, with at least two probable en echelon offsets to the south at 104 and 102 W. This bathymetric data, along with new earthquake mechanism data from the transform system and additional data from the Nazca-Antarctic boundary to the east, have been compiled into a new, larger data set for this boundary. Inversion of this data set has produced a new best fit pole for the Nazca-Antarctic plate pair, providing better constraints on the relative motion along this boundary. This new best fit pole matches the data better than the best fit pole calculated from old data, though some discrepancies still remain. Additional data, particularly from the western portion of the boundary, are needed to further refine the pole location.

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