Influence of Sediment Input and Plate-Motion Obliquity on Basin Development Along an Active Oblique-Divergent Plate Boundary: Gulf of California and Salton Trough

Transtensional basins have formed along the Pacific-North America plate boundary in the Gulf of California and Salton Trough region during Late Cenozoic time. Axial basins occupy a 50-60 km wide belt along the main plate boundary, and change from sedimentstarved oceanic spreading centers in the south that are oriented perpendicular to long NW-striking transform faults, to oblique N-trending pull-apart (stepover) basins in the north that contain thick sediments and lack evidence for normal oceanic crust. Marginal basins are found along the flanks of the Gulf-Trough corridor and consist mainly of supradetachment basins (only in the north), transtensional fault-termination basins, and classic orthogonal rift basins. Areview of previous studies suggests that three main parameters govern the structural style, composition, and total thickness of sedimentary basins in this setting: (1) the rift angle (a), defined as the acute angle between the overall trend of the plate boundary and the direction of relative plate motion; (2) proximity to voluminous input of sediment from the Colorado River and other smaller drainages in the north; and (3) the degree of strain partitioning. Detachment faults and supradetachment basins are well documented in the northern Gulf and Salton Trough where α ≥ 30°, whereas no detachment faults are recognized in the central and southern Gulf of California (α < 20°). We suggest that faster extension associated with a higher rift angle is the main factor responsible for creation of supradetachment basins in the northern region. Voluminous input of sediment derived primarily from the Colorado River exerts a first-order control on crustal thickness and composition, lithospheric mechanics, and rift architecture. In the sediment-starved southern Gulf of California, the plate boundary has completed the transition from continental rifts to seafloor spreading centers with normal ocean crust and magnetic lineations. The Guaymas spreading center in the central Gulf has young oceanic crust with an upper layer of sediments and shallow intrusions. In contrast, sediment-filled and overfilled basins in the north are characterized by thick new transitional crust that is formed by input and magmatic modifi- cation of sediment, which fills the new space created by lithospheric rupture and oblique divergence. Thus the rate of sediment input appears to determine whether or not continental rifting progresses to the ultimate formation of a new ocean basin floored by normal basaltic crust.