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Description
The Laguna Salada fault in the study area is an oblique-dextral fault zone which has been highly active throughout the late-Quaternary. The zone contains several distinct fault strands whose activity and senses of slip vary during the late-Quaternary. The northern section of the fault zone has been highly active throughout the Holocene and consists of a single fault strand exposed at the base of the crystalline rangefront of the Sierra de los Cucapas. This section of the fault has had an oblique-dextral sense of slip during this time. The southeastward continuation of this single fault strand has been largely inactive throughout the Holocene and has had a prior history of largely lateral movement, with a minor vertical component of slip as indicated by the lack of structural relief across it. Southeast of the single rangebounding fault strand, slip has been stepped during the Holocene to the southwest onto two distinct oblique-dextral fault strands which displace alluvial deposits of late-Quaternary age along their lengths. The more easterly of these strands extends to the southeast, parallel to the rangefront, until it intersects the Canon Rojo fault. The northeast striking, normal Canon Rojo fault has been active during the Holocene and has stepped much of the slip from the Laguna Salada fault to the Chupamiertos fault further to the southwest. The geometry of right-stepping subparallel oblique-dextral fault zones is repeated at two other points along the southwestern margin of the Sierra Mayor. The geometry and sense of slip on this system of faults, and the geophysically determined shape of basin fill in Laguna Salada, leads to the suggestion that the Laguna Salada basin has been generated by pull-apart between subparallel oblique dextral fault zones. This has created a series of four, adjacent, northwest trending rhombochasms which define the deeper section of sedimentary fill in Laguna Salada. A cessation in basin development in the northernmost rhombochasm occurred during the late-Pleistocene. This may have occurred by either an extended seismic gap or by purely strike slip along the Laguna Salada fault zone which was not stepped to the Chupamiertos fault via the Canon Rojo fault. The system of right stepping lateral faults exposed along the western margin of the Sierra de los Cucapas and Sierra Mayor is probably the connecting link between the Elsinore fault in southern California and the Cerro Prieto fault in the northern Gulf of California.
