Images of Early Continental Breakup in and around the Gulf of California and the Role of Basal Shear in Producing Wide Plate Boundaries

Citation

Persaud, Patricia (2004) Images of Early Continental Breakup in and around the Gulf of California and the Role of Basal Shear in Producing Wide Plate Boundaries. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/8K9G-SS73. https://resolver.caltech.edu/CaltechETD:etd-12032003-113513

Abstract

Active faulting in the northern Gulf of California occurs over a broad zone, 70 x 200 km, affecting two-thirds of the width of new crust that has formed there starting at 6 Ma. This is an unusually wide plate boundary zone with a high density of faults and no evidence for the formation of normal oceanic crust. Over 3000 km of high-resolution, multichannel seismic data were used to map out this zone of distributed faulting and identify multiple basins within the broad rift zone. Previously, numerical models have shown that deformation shifts from one place to another by various mechanisms of strengthening of the active rift zone relative to adjacent regions. Models are presented here that for the first time, show the development of multiple active faults across the width of the plate boundary. These models do not rely on strengthening or weakening effects; rather they assume that shear at the base of the brittle crust is distributed and explore the effects of distributed shear on the style of deformation. Addiionally, the effect of obliquity on the style of deformation is studied and the styles of faulting produced in the models represent a wide range of geological structures, ranging from half-grabens to flower structures. The style of faulting in the northern Gulf of California is produced in a model with distributed shear using the published obliquity for this region.

One mechanism for distributing shear at the base of the brittle crust is lower crustal flow. If a significant amount of lower continental crust exists within the Gulf, it may have flowed in the past. A study of the crustal thickness variations in the continental margins of the Gulf is presented here, that shows thinner crust in a ~50 km wide zone close to the Gulf, along the entire eastern Baja California peninsula. This thinned crust is associated with the eastern Peninsular Ranges batholith. In contrast, the western part of the batholith has a fairly uniform thickness of 35-40 km. It is possible that at the time of breakup, the lower crust was still behaving ductilely and that some of the lower continental crust from the margins now exists within the Gulf.

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