Hosgri fault zone, offshore Santa Maria Basin, California

Abstract

The Hosgri fault zone (HFZ) Is the name given to the southern section of the major coastal fault in central California. The Hosgri separates Transverse Range structure from offshore Santa Maria Basin structure and is a key element for any tectonic model that includes this economically significant region. Previous published maps have not adequately defined the southern termination of the HFZ, the style of faulting on the HFZ, and the relation of the HFZ to surrounding structures. Using more than 1,500 mi of processed seismic reflection data, we have mapped upper Miocene and Pliocene structure in the region of the HFZ offshore from Point Sal in the north, to Point Conception in the south where the HFZ ends against east-west structures in the westernmost Santa Barbara Channel. In the same area, eastwest-trending structures in the western Transverse Ranges north of the channel abut against the HFZ.The HFZ is an oblique right-slip fault along most of its length, but significant changes in the style of faulting are associated with variations in fault trend. North of Point Arguello, the HFZ appears to dip at a high angle in the upper 2,000 m of section and is distinguishable from thrust and reverse faults developed to its west. Between Point Arguello and Point Conception it may be a northeast-dipping thrust. Along its mapped length, east-side-up vertical separation is typical and may be more than 400 m on a Pliocene unconformity. Older horizons show more separation; the lower Miocene is up on the east by almost 1 km off Purisima Point. However, individual en echelon segments of the fault show west-side-up vertical separation where expected in an oblique right-slip fault system. No piercing points were found to define strike separation. Pliocene drag folds indicate dextral slip in Pliocene and later time.

Influence of fault trend, bends, and convergence on shallow structure and geomorphology of the Hosgri strike-slip fault, offshore central California

“…1;Hanson et al, 2004;Lettis et al, 2004;Dickinson et al, 2005). At its south end, the Hosgri fault initiates at the northern boundary of the western Transverse Range province offshore Point Arguello (Steritz and Luyendyk, 1994;Sorlien et al, 1999). As it extends north, this mainly offshore fault system intersects the Big Sur coast at Piedras Blancas and at Point Sur where strands are referred to as the San Simeon and Sur faults.…”

“…Where the faults overlap, the transfer of slip (i.e., stepover) results in the development of a rhomboidal pullapart basin (DiSilvestro et al, 1990;Hanson et al, 2004). The width of the stepover (~5 km) is considered too large for a transfer of slip during a large earthquake, so the maximum rupture length for an earthquake along the Hosgri fault zone is the distance between a Hosgri endpoint northwest of Point Estero and the southern termination of the fault offshore Point Arguello (Steritz and Luyendyk, 1994;Lettis et al, 2004), ~110 km.…”

Salinia revisited: a crystalline nappe sequence lying above the Nacimiento fault and dispersed along the San Andreas fault system, central California

“…Offset along the H-SS segment of the H-SS-SG fault zone that is less than the offset to the north of SS (see other segments discussed below) and south of Point Arguello (PA, Figure 1) is either owing to: (i) strike-slip being taken up in thrust faulting and folding at the southern end of the Hosgri segment (Steritz and Luyendyk 1994;Sorlien et al 1999aSorlien et al , 1999b, (ii) crustal shortening within the rotating Transverse Ranges (Dickinson et al 2005), (iii) slip along yet-to-be-identified faults in the offshore or Santa Maria basin, also ruling out crustal shortening Langenheim et al 2010), (iv) ∼15 km (9 mi) of slip inland from near Point Buchon and the Diablo Canyon nuclear reactor along the onshore OWHF zoneOWHF , or (v) 23 km (14 mi) of dextral slip on the OWHF zone ) (OWHF, Figure 1). …”

Offset of Latest Pleistocene Shoreface Reveals Slip Rate on the Hosgri Strike-Slip Fault, Offshore Central California

“…The Hosgri-San Gregorio fault system, the most extensive of these western structures, extends about 400 km along the central California coast. At its south end, the Hosgri fault initiates at the northern boundary of the western Transverse Range Province offshore Point Arguello and Point Conception (Steritz and Luyendyk, 1994;Sorlien et al, 1999). As it extends north, this mainly offshore fault system intersects the Big Sur coast at Piedras Blancas and at Point Sur, where strands have been referred to as the San Simeon and Sur faults (Dickinson et al, 2005).…”