Abstract

The Pliocene Loreto basin is a transtensional basin, exposed just north of Loreto, Baja California Sur, that consists of nonmarine to marine sedimentary rocks and interbedded tuffs. On the basis of stratigraphic study and 40Ar/39Ar dating of tuffs, the southern Loreto basin began to form at ∼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 deposition 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 ∼2.36 to ∼2.0 Ma, when volcanism increased within the northern part of the basin. The Pacific-North America boundary was first fully located within the Gulf of California beginning at ∼3.5 Ma, at which time the zone of transform-related deformation 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 faulting in southern California on the Elsinore and San Jacinto faults. These widespread events may indicate a minor change in the plate boundary at ∼2.5 Ma.

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