ROSENBERG, LEWIS I., Consulting Geologist, Templeton, CA; and JOSEPH C. CLARK, IUP, Indiana, PA

Abstract: Southern San Gregorio fault: Stepover Segmentation vs. Through-Going Tectonics

An important outstanding problem is the southern extent of the San Gregorio fault zone (SGFZ) and its postulated continuity with the Hosgri fault zone to the south. Whereas Graham and Dickinson (1978) postulated that the SGFZ is continuous with the Hosgri fault zone, Greene and others (1973) suggested that the main SGFZ turns inland south of Monterey Bay to join the Palo Colorado fault in the northern Santa Lucia Range.

We identified a zone of late Quaternary faulting north of Point Sur that is as much 2 km wide and trends N. 50-60°W. The two most important faults within this zone are the Garrapata fault, which offsets Holocene colluvium; and the Rocky Creek fault, which offsets and deforms marine terrace deposits.

The Garrapata fault strikes N55°W, intersecting the coast in a 55-m-wide shear zone, and vertically offsets the lowest marine terrace by 1.7 m. Offshore, a 150-m-wide gap in the kelp aligns with this zone; and onshore features suggesting active faulting include aligned drainage offsets, ridge saddles, and springs. A weak air photo lineament trends N52°W across this terrace and the shoreline angle appears right-laterally offset as much as 122 m. Along this trend, the Garrapata fault juxtaposes granite and colluvium along a near-vertical shear zone; and juxtaposes two different colluvial deposits. Radiocarbon dating yields ages of 1,200±-60 and 9,750±-60 ybp for the faulted colluvium.

The Rocky Creek fault offsets and deforms a marine terrace deposit at an elevation of 110 m. The fault offsets Bixby Creek in a right-lateral sense approximately 1.4 km and continues to the south along the linear canyon of Sierra Creek.

Although the continuity of these faults with offshore segments of the San Gregorio fault is still to be ascertained, our results to date strongly suggest that at least some of the dextral shear of the offshore SGF is distributed to en echelon intraSalinian fault segments of the northern Santa Lucia Range.

AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California