Abstract The Outer Continental Borderland offshore southern California consists of an arkose-bearing accretionary prism or truncated forearc basin with two subsequent discrete periods of calcalkaline volcanism and a third prolonged period of alkaline seamount volcanism. The volcanics exhibit progressively less continental influence/contamination with time. The earliest period consists of calcalkaline lavas dated at 30 million years ago (Ma) that erupted prior to the Pacific–Farallon spreading ridge reaching the California margin. These are moderately contaminated by continental rocks. The second more widespread period of calcalkaline activity, from 16 to 18 Ma, consists of lavas ranging from basaltic andesite to rhyolite that generally experienced less continental contamination than the 30 Ma volcanics. The 16 to 18 Ma lavas apparently formed as a triple junction migrated along the margin, although the offshore lavas are not midocean ridge basalt–like, as expected if generated within a slab window. The third, even younger period of volcanic activity produced Rodriguez Seamount on the Patton Escarpment; Northeast Bank within the Outer California Borderland; and San Juan, San Marcos, Little Joe, Davidson, Guide, Pioneer, and Gumdrop Seamounts on the Pacific Plate. These youngest lavas, previously described in the literature, have little to no signature of continental contamination, although lavas that built Rodriguez and Northeast Bank migrated through continental crustal materials. These results are largely based on samples either dredged or collected via remotely operated vehicle from the seafloor. The widespread occurrence of erratics in our California Borderland sample set indicates that their presence must be weighed when reconstructing the geology of the region.

Abstract The Arguello submarine canyon/channel system extends over 300 km from the continental shelf off Point Arguello and Point Conception in southern California westward onto the oceanic crust of the Pacific plate. In the northernmost reaches where the canyon system originates, all stages in the evolution of seafloor morphologic fluid flow features—from pockmarks to gullies to converging rills—are observed, similar to what has been described for the Ascension slope, north of Monterey Bay. These features appear to be active today and are linked to fluid leakage from the underlying hydrocarbon basin. The channel dissects a continental slope that exhibits features consistent with large-scale mass wasting. Upslope scarps may be the source of the morphological feature at the base of the slope previously referred to as the “Arguello submarine fan,” with topographic expressions (e.g., large channel meanders, ridges) that are more consistent with mass transport deposits than with deep-sea fan depositional lobes. The modern canyon crosscuts these deposits and parallels an older, meandering channel/canyon to the west. Modern seismicity along the shelf and slope may have, and potentially still can, trigger landslides on the slope. Seismicity associated with seamount volcanism, past subduction, and Borderland transrotational and extensional processes most likely played a role in stimulating mass wasting. The presence of abundant nearby petroleum suggests that gas venting and hydrate dissociation cannot be ruled out as a triggering mechanism for the slope destabilization occurring today. The canyon/channel continues due south on a path possibly determined by the structural grain of north–south-aligned abyssal hills underlying oceanic basement. At latitude 33°18′N, the channel makes a 90° turn (bend) to the west at the E–W-striking Arguello transform fault wall and develops into a meandering channel system that crosses over abyssal hill crustal fabric. The system ultimately straightens as it continues west before veering north, curving around a thickened crustal bulge at a corner offset in the Arguello fracture zone in complex basement structure, and then finally empties into an 800-m-deep basin depocenter.