Stratigraphyand 40Ar/39Ar geochronologyof the Santa Rosabasin,Baja California:Dynamic

The Santa Rosa basin of northeastern Baja California is one of several transtensional basins that formed during Neogene oblique opening of the Gulf of California. The basin comprises Late Mio-cene to Pleistocene sedimentary and volcanic strata that define an asymmetric half-graben above the Santa Rosa detachment, a low-angle normal fault with ca. 4–5 km of SE-directed displacement. Stratigraphic analysis reveals systematic basin-scale facies variations both parallel and across the basin. The basin-fill exhibits an overall fining-upward cycle, from conglomerate and breccia at the base to alternating sandstone-mudstone in the depocentre, which interfingers with the fault-scarp facies of the detachment. Sediment dispersal was transverse-dominated and occurred through coa-lescing alluvial fans from the immediate hanging wall and/or footwall of the detachment. Different stratigraphic sections reveal important lateral facies variations that correlate with major corrugations of the detachment fault. The latter represent extension-parallel folds that formed largely in response to the ca. N-S constrictional strain regime of the transtensional plate boundary. The upward vertical deflection associated with antiformal folding dampened subsidence in the northeastern Santa Rosa basin, and resulted in steep topographic gradients with a high influx of coarse conglomerate here. B