Abstract: Shallow Structure of Northern Santa Rosa-Cortes Ridge, Southern California Continental Borderland

Michael E. Field

The northern Santa Rosa-Cortes Ridge, a large north to northwest-trending anticlinal structure composed of broadly and gently folded preorogenic rocks, was studied by high-resolution seismic-reflection profiling for evidence of recent fault and slump activity. Faults traceable for tens of kilometers and having tens of meters of vertical offset are present on both flanks of the ridge and on the slope southeast of the Channel Islands. Faults near the surface of the sea floor are numerous and parallel fold axes; most are pre-Holocene and relatively minor (apparent vertical offset ~ 3 to 15 m). The latter characteristic, as well as the apparent en echelon fault pattern, may be due to dominant strike-slip faulting. The greatest density of faults (1 to 2 faults per kilometer of profiling) is on the ridge top which is a relatively flat, wave-beveled surface formed by truncated middle Tertiary rocks and veneered with thin (<1 to 4 m) and discontinuous Quaternary sands. Some faults appear to offset the sea floor, a probable indication of Holocene faulting, but the total number is small. Their presence, coupled with the abundance of earthquake epicenters recorded in the region in the past 43 years, indicates that the northern Santa Rosa-Cortes Ridge is an area of active tectonic stress. Winnowing of the ridge-top sediments is taking place; however, the major mode of sediment transport is by gravity. The upper flanks of the ridge are areas of active slumping and sediment creep. Zones of slumped sediment are up to 9 km wide, and incipient slumps (over-steepened and thick deposits of unconsolidated sediment) are common.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA