Abstract: Testing Models for Blind Faults and Wide Folds, Southern California

SORLIEN, C. C., Institute for Crustal Studies, Univ. California, Santa Barbara, CA, 93106, ([email protected]); andL. SEEBER, Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY ([email protected]);M. J. KAMERLING, Institute for Crustal Studies, ([email protected]); andN. PINTER, Geology Dept., Southern Illinois University, Carbondale, III, 62901, ([email protected])

Post-Miocene oblique convergence across the now southern margin of the western Transverse Ranges resulted in a continuous 220 km-long, 30+ km-wide anticline, cut by known or presumed left-lateral strife-slip faults at the base of the forelimb and/or at its crest. The Santa Monica Mountains and northern Channel Islands (SMMCI) are the topographic expression of this anticline. A 7-15 km-wide S-dipping forelimb, and 15-30 km-wide backlimb are traced along the entire length of the structure. Backlimb progressive down-to-the-north tilting interpreted offshore is modelled as horizontal axis rotation of a rigid block above a giant, N- dipping, concaveup fault, the proposed SMMCI thrust. The width of the backlimb is related to the size of the listric fault and limb dip is related to slip. Wide, very gently dipping backlimbs can be explained with much less slip than is predicted by ramp-flat models, and kink-axial surfaces need not be created The wide forelimbs can be explained by rigid-rotation above shallow convex-up portions of faults, and/or more fully explained by internal deformation. While the deep fault is slipping and the shallow fault is locked during initial thrust reactivation, slip is absorbed by a wide, progressively tilting forelimb.

The ~6 km wavelength folding of the Mid Channel (Blue Bottle) trend beneath Santa Barbara Channel provides a clear example to compare models. The amplitude of the fold has increased at 2 mm/yr along much of its length. The backlimb is the same width, 4 km, for strata of different age, but the dip increases with increasing age for the post-160,000-year reflections dated at ODP site 893. It does not make any sense here for the slip to be greater than or equal to the backlimb width, as is required for synthrust strata by the fault-bend fold and fault-propagation fold models. If this were true, slip since deposition of a 110,000 (+/- 10%) horizon would be 36 mm/yr, and would be 80 mm/yr since 50,000 years. In contrast, a listric thrust model predicts that slip is proportional to limb dip, and that the observed fold can be created with I or 2 orders of magnitude slower slip.

AAPG Search and Discovery Article #90935©1998 AAPG Pacific Section Meeting, Ventura, California