The Santa Ynez fault is mappable for 133 km westward from its apparent truncation by the Agua Blanca thrust on the east, nearly to Point Conception on the west. Neogene dip separation is locally large, and substantial left-lateral strike-slip is suspected. Total slip, however, is undefined, and the movement history has been obscure, partly owing to the lack of recognition of any trustworthy piercing point or steep plane.

Some Paleogene and older strata along the fault contain laumontite. This mineral is a distinctive indicator of burial changes in mineralogically immature rocks that have been heated with dilute pore waters at above-average geothermal gradients. Laumontite crystallizes from the surface to more than 7,000 m (controlled by temperature, fluid pressure, and fluid composition); alteration fronts at intermediate depths are clearly defined and locally steep.

Preliminary results of a reconnaissance study suggest that a steeply inclined northeast edge (isograd) of laumontite alteration is offset by the fault 37 km left-laterally. Pervasive alteration south of the fault is conspicuous everywhere in susceptible lithologies of the lower Matilija Sandstone (Eocene) and all older strata west of lat. 118°57^primeW. Alteration north of the fault is conspicuously absent wherever the same units are visible (subsurface and surface) east of the alteration edge near lat. 119°20^primeW. Stratigraphic and structural reasoning suggests that the laumontite crystallized about 22 to 25 m.y.B.P. Santa Ynez fault left-lateral slip necessarily predates the inception of San Gabriel fault right-lateral strike-slip (10 to 13 m.y.?B.P.).

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