Routing of terrigenous clastics to oceanic basins in the southern Gulf of California, inherited from features of the pre-spreading protogulf

The southern protogulf was a Miocene belt of continental rupture bounded on the northeast by the Sierra Madre Occidental (SMO) volcanic plateau, and on the southeast by the Main Gulf Escarpment of the Baja California (BC) rift shoulder. Since no later than 8-9Ma, before the depression became a deep inlet of the Pacific, the southern protogulf has hosted the principal shear zone between the Pacific and North American plates. As BC gradually acquired Pacific-plate motion, its vector wrt North America rotated and made the shear zone transtensional, leading to the development of several types of pull-apart basins. Some of them, at right steps between en echelon faults, evolved into the oceanic basins that are now the principal Gulf depocenters. Recent geophysical mapping and geologic sampling in the southern Gulf has clarified the location, structures, and composition of the oceanic/continental crustal boundary, and the mechanisms and routes by which terrigenous clastics reach the intracontinental and oceanic basins.Longitudinal (northwest-southeast) thermohaline and tidal currents transport significant volumes of hemipelagic sediment across the floors of gulf basins, but most of the coarser terrigenous clastics arrive there in density flows, primarily turbidity currents that have built channeled deep-sea fans at the mouths of canyons in the continental slopes. Mapped patterns of submarine canyons, channels and fans confirm that turbidity-current transport has primarily been from the northeast margin, which has permanent mainland rivers with large catchments in the SMO. Most of the arid BC peninsula, with ephemeral-stream drainage directed toward the Pacific, contributes little terrigenous input to the deep-water gulf basins, except in the far south: more than a third of the elevated Cabo Block drains into the gulf, building a large fan on the northwest side of Alarcon Basin. The canyons feeding this fan, cut through crystalline rock and probably subaerial in origin, enter the basin at its western corner, in the angle between a northwest-striking sheared margin and a northeast-striking rifted margin. The mirror image of this setting, the eastern corner between mainland sheared and rifted margins, is the site of another turbidite pathway that supplies the eastern side of this basin; an axial ridge along the spreading axis separates turbidites from BC and mainland sources. For the other basins with insignificant BC supply, the principal canyons are systematically at their eastern corners. Flexural uplift of shearing margin segments has limited passage across them of rivers and submarine density flows, except along some well developed Riedel shears, so as at many rhomboidal pull-aparts the corner between shearing and obliquely rifting segments remains the principal locus for entry of terrigenous sediments. Instead of an axial ridge, most oceanic gulf basins have axial troughs, enclosed linear deeps that intercept turbidity currents, cutting off the northwestern part of the basins from the mainland sediment supply. As crustal accretion occurs at these axial troughs they move away from basins' eastern corners, but remain connected to the sediment source by lengthening submarine channels.