Abstract: Structural Development of the Gualala Block, Northern California

JONES, D. L, C. M. WENTWORTH, and E. E. BRABB, U.S. Geological Survey, Menlo Park, CA

The Gualala sequence comprises a fault-bounded block (terrane) that has been displaced northward along the continental margin from a cryptic site of origin that probably lay south of the Transverse Ranges and outboard of the Peninsula Ranges. Only the Neogene portion of this displacement history can now be deciphered in any detail. New analysis indicates that the eastern bounding fault (San Andreas proper) exhibits ~120 km of total late Cenozoic displacement, of which ~24 km are Quaternary and ~85-95 km are older displacements on the San Gregorio fault. The western bounding fault beneath the ocean is an offset part of the Pilarcitos fault and may have late Cenozoic displacement of ~120 km. The suggested ~150 km offset of Point Reyes northwestward from near the Monterey Peninsula may have involved ~95 km of offset on the San Gregorio fault and ~55 km on a separate, as-yet unspecified fault.

At least three distinct deformations are recorded by structures within the Gualala block. The oldest structures are low-angle normal faults that dropped the strata of Stewarts Point down onto an ophiolitic basement. Younger north-south compression is indicated by the large, east-trending folds of the block and separately by isoclinal folds and imbricate thrust faults (N70E, south dipping) localized near the margin of the Black Point spilite. Subsequent compression across the trend of the San Andreas fault is indicated by numerous north-trending reverse faults in the block and, in the vicinity of Pt Arena, by northeast dipping thrust faults and tightly folded Miocene strata that locally are ovetturned toward the west. Compression normal to the San Andreas fault may continue into late Quaternary time, based on recent study of deformed terraces by Enrico Tavarnelli.

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