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Abstract

Lineaments in marine gravity anomalies, bathymetry, and magnetic anomalies, along with previously mapped onshore faults, have been used to construct a Seismo-Tectonic Map of the region extending from Lázaro Cardenas in Michoacan, Maxico, to north of Point Conception in Alta California. Earthquake epicenters taken from the NOAA/NGDC Hypocenter Data File for all events with Mb>4.2 occurring between 1963 and 1985 are also shown on the map. We have tried to identify the principal faults and lineaments of the province. The structural pattern that emerges for the Gulf of California, the California Borderland, and the Pacific margin of Baja California is similar to that seen onshore in southern Alta California and northern Baja California. Present-day shear between the Pacific and North American plates is distributed across a fairly wide zone of subparallel crustal slices. In Mexico, where this zone includes most of the Gulf crust, more than one extensional axis commonly occurs at the same plate rotational latitude. Earlier faulting along the Pacific side of the peninsula and in the outer Borderland appears to have occurred in a similar style. In the southernmost Gulf the shear is presently less widely distributed; in this area several horsts of continental crust, which are relicts of early rifting, are now fixed to the North American plate. Gravity anomalies show that the Tosco-Abreojos Fault Zone may have been much wider and more discontinuous than indicated by shallow seismic-reflection data. The eastern part of the Rivera Fracture Zone and the East Pacific Rise north of lat. 17.5°N are reorienting in a small but very complicated area where the boundaries of five plates and crustal blocks nearly coincide. We identify the seaward extensions of the Colima Graben and the Tepic-Chapala Fault Zone, which bound the Jalisco tectonic block.

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