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MALIBU'S OFFSET STREAMS
E.D. Michael
June 17, 2009

Streams commonly are classified according to a number of schemes such as "stage of development," "genesis," "stream pattern," and "relationship to geologic structures" (Johnson, 1932), Each such scheme has its own nomenclature, and this can lead to confusion, because all streams can be classified according to more than one such scheme. This is the sort of nicety geologists love because it provides an opportunity to demonstrate we know more than others who have never heard of what the devil hell they are talking about. In generic terms then, the southern slopes of the Santa Monica Mountains are almost everywhere deeply incised by streams which, because they have originated in the initial mountain slopes are referred to as "consequent." Those in Malibu all flow southward to drain directly into Santa Monica Bay.

On the other hand, in terms of a scheme base on stream pattern, they have developed rather like those of a tree, i.e., by progressing upslope in the process commonly referred to as "headward erosion." However, the consequent and dendritic characteristics of some Malibu streams have been altered in that their lowermost reaches as a result of being offset. Apparently, this is due to movements along the Malibu Coast fault or one of its splays. Upper reaches of the main channels have been shifted for distances up to several hundred feet west so that their lowermost reaches and mouths are southeast of them. This is consistent with the known west-oblique transcurrent character of the Malibu Coast fault. As to a classification, because of its relationship to the off-setting fault, some geologists would refer to such reaches as "subsequent streams." So much for stream morphology and nomenclature.

The off-set condition comes about by fault movement that ruptures the rocks transverse to the stream channel axis. In Malibu, this initially caused streams so affected to turn from their previous southward flow to one slightly south-southeast. Such movements, periodic and incremental, have proceeded slowly enough that the channels continue to carry flow and thereby maintain themselves while gradually lengthening downstream from the point where the offsets initially occurred. Necessarily, this lengthening develops along the fault trace which is oriented more or less east-west. Technically them, the shifted reach becomes subsequent rather than consequent, because it is controlled by the fault. Both upstream and downstream from this subsequent reach, the stream retains its consequent character. Streams in Malibu that have been offset in this way include that of Piedra Gorda Canyon in the Topanga quadrangle, those of Puerco and Solstice canyons in the Malibu Beach quadrangle, those of Latigo, Escondido, Steep Hill, and Encinal canyons in the Point Dume quadrangle, and, less obviously, those of La Chusa, Los Alisos, and Little Sycamore canyons and, arguably, the unnamed western branch of Deer Canyon all in the Triunfo Pass quadrangle.

Stream offsets record a long-term dynamic tectonic stress condition along the base of the Santa Monica Mountains in Malibu. There is no reason to believe this stress condition is not presently operative.

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