TECTONICS OF THE PLIOCENE LORETO BASIN, BAJA-CALIFORNIA-SUR, MEXICO, AND EVOLUTION OF THE GULF-OF-CALIFORNIA

The Pliocene Loreto basin is a transtensional basin, exposed just nort h of Loreto, Baja California Sur, that consists of nonmarine to marine sedimentary rocks and interbedded tuffs. On the basis of stratigraphi c study and Ar-40/Ar-39 dating of tuffs, the southern Loreto basin beg an to form at approximately 3.4 Ma as an alluvial basin with moderate rates of subsidence (< 0.4 mm/yr). Between 2.46 and 2.36 Ma, extremely rapid subsidence (5-10 mm/yr) of the basin was accompanied by deposit ion of vertically stacked Gilbert-type fan deltas. During this period, the southernmost basin began to be cut by an array of dextral-normal faults. The basin returned to a moderate rate of subsidence from appro ximately 2.36 to approximately 2.0 Ma, when volcanism increased within the northern part of the basin. The Pacific-North America boundary wa s first fully located within the Gulf of California beginning at appro ximately 3.5 Ma, at which time the zone of transform-related deformati on widened to initiate formation of the Loreto basin. The major change in the Loreto basin at 2.46 Ma may be coeval with the beginning of fa ulting in southern California on the Elsinore and San Jacinto faults. These widespread events may indicate a minor change in the plate bound ary at approximately 2.5 Ma.