Author: Gwendolyn EricksonPublisher:
ISBN:
Category : Basins (Geology)
Languages : en
Pages : 218
Book Description
Associated with the northern strands of the San Andreas fault system in California is a series of small intermontane basins. While it is tempting to ascribe their formation to simple 'pull-apart' tectonics along the dominantly strike-slip fault strands, direct evidence for basin genesis is lacking. In this study, a detailed gravity survey throughout the Little Lake Valley region (Willits, California) provides constraints on mechanisms of basin formation along this young segment of the San Andreas fault system. Interpretation of isostatic gravity anomaly data provides insight into fault geometry, basin structure, and thickness of Quaternary fill in Little Lake Valley, California. Although the active strike-slip Maacama fault zone diagonally trends through the southwest part of the valley, gravity and geologic interpretations indicate the valley conceals an earlier basin and faulting history. Gravity models, mapped geology, and double-difference relocated seismicity data indicate that Little Lake Valley basin geometry records a tectonic history which includes subduction, extension, and presently, dextral strike-slip motion along the Maacama fault. The isostatic gravity anomaly of Little Lake Valley basin is negative (up to 13 mGals) and rhombic in shape. Modeling indicates two splays, less than a km apart, of a reverse up-to-the-east East Valley fault. Cumulative vertical fault displacement along the East Valley fault increases in the southern portion of the valley. Gravity modeling and geology indicate two normal, up-to-the-northwest faults; one in the northwest portion of Little Lake Valley and the other to the south in Redwood Valley. Normal faults are presumed to be temporally related to Clear Lake volcanics (2 Ma to 10ka), located 45 km southwest of Little Lake Valley, and migration of the Mendocino triple junction to the north. While focal mechanisms of double-difference relocated seismicity indicate reverse and normal faults no longer accommodate significant vertical motion, portions of these structures are accommodating strike-slip motion. Consequently, the Maacama fault is occupying an upward-branching flower structure which takes advantage of pre-existing fault structures and may sole into a regional detachment at approximately 10 km depth. Although Little Lake Valley was not initiated as a strike-slip basin, development of the Maacama fault may evolve Little Lake Valley into a strike-slip basin.