During the mid-1990's I worked in the Laguna Saladaarea in collaboration with Arturo Martin, at CICESE in Ensenada, Mexico. Laguna Salada is located southwest of Mexicali and is bounded on its NE margin by the seismically active Laguna Salada fault, a probable southern continuation of the Elsinore fault in southern California (see map, below). Laguna Salada is a pull-apart basin created by releasing step-overs on the strike-slip Laguna Salada fault system. We are studying the Pleistocene stratigraphic record of subsidence, deposition, and deformation produced by rapid slip on the Laguna Salada fault and related structures. Pleistocene strata are exposed in the footwall of the Canyon Rojo fault, which is a young splay of the Laguna Salada fault along which active, co-seismic slip is presently being accommodated. Because this young fault splay stepped into the basin, stratigraphy that formed in the hangingwall of the Laguna Salada fault is now being uplifted and exposed by the effects of rapid erosion.
This map (adapted from Mueller and Rockwell, 1995) shows the regional tectonic setting of the southern San Andreas fault system, with emphasis on the Laguna Salada fault system. The blue box shows the area of our detailed study. The Canyon Rojo fault is the NNE-trending normal fault within the blue box, and is one of the releasing step-overs which control the kinematics of the Laguna Salada system (green arrows; Mueller and Rockwell, 1995). Detailed graphics of key map relationships in this area are presently in progress and not available in digital format. LS = Laguna Salada.
The Pleistocene section in the Canyon Rojo area is ~1000 meters thick and consists of footwall-derived breccias, conglomerates and sandstones which fine laterally away from the Laguna Salada fault zone. Detailed mapping and stratigraphic analysis reveal a complex history of syn-depositional deformation which occurred in two main phases: (1) folding of the Imperial and Palm Springs Formations during deposition of the lower, well cemented part of the Pleistocene section; and (2) initiation of the Canyon Rojo fault and offset of the upper, uncemented part of the Pleistocene section ("gray gravels" unit). A large-amplitude growth fold produced during stage 1 is cut by an angular unconformity at the base of the gray gravels unit. This unconformity records a reorganization of hanging-wall structures prior to the present phase of uplift in the footwall of the Canyon Rojo fault. Mapping and stratigraphic thickness data indicate that the base of the upper "gray gravels" unit, which is probably less than 1 m.y. old, has experienced about 1000 meters of offset on the Canyon Rojo fault. This suggests a long term slip rate of 1 to 2 mm/yr during Quaternary time, which is consistent with geomorphic evidence for young rapid uplift in the footwall of this fault.
One powerful aspect of this study is our ability to make direct comparisons between Quaternary and Modern depositional systems and the active structures that control them. In particular, we recognize a wide diversity of different types of modern alluvial fans, as reflected in their size, gradient, depositional processes, and sedimentary facies, and we recognize similar variations in the Pleistocene stratigraphy. Notable differences between Pleistocene and modern structures and depositional systems also exist and are important for understanding the evolution of the fault system through time.
Inspired by the Laguna Salada work, I plan to initiate a new study of Plio-Pleistocene stratigraphy associated with active strike-slip faults and related structures in southern California, with emphasis on the Elsinore, San Jacinto, and San Andreas faults. This area is well suited for studies in which the stratigraphy of proximal gravels and breccias derived from active fault blocks can be used to document the history of fault slip and related deformation. Although recent studies have examined Mio-Pliocene stratigraphy in some detail (e.g. Imperial and Palm Springs Formations), very little is known about Plio-Quaternary nonmarine deposits in this area. These strata preserve a record of evolving kinematics and deformation patterns over the past ~2 million years, and should provide important new information about the long-term history of this active fault system.
Mueller, K.J., and Rockwell, T.K., 1995, Late quaternary activity of the Laguna Salada fault in northern Baja California, Mexico. Geol. Soc. America Bulletin, vol 107, p. 8-18.
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