Geometry of Subducted Slabs Related to San Andreas Transform

Development of the San Andreas transform by rise-trench encounter in coastal California influenced the structural evolution of a large region within the adjacent continent. Continuation of arc magmatism and tectonism depends upon the presence of a subducted slab of lithosphere at depth beneath an arc-trench system. The lack of subduction at the transform plate boundary along the California continental margin led to the growth of a slab-free region beneath the part of the continental block adjacent to the San Andreas transform. Geometric analysis based on ideal assumptions predicts that generation of a lengthening transform by rise-trench encounter will also generate an expanding triangular hole or window in the slab of lithosphere subducted beneath the continent. One leg of the slab-window is the adjacent transform, but the orientations and lengths of the other two legs depend upon the relative motions of the three plates involved. By inference, arc volcanism and tectonism cannot persist across the no-slab area at the surface above the slab-window. The actual configuration of the slab-free region adjacent to the transform will depart from ideal predictions where adjustments to the conditions of rise-trench encounter involve changes in the motions of surface plates or subterranean ruptures in subducted slabs. The extent of the expanding slab-free region adjacent to the San Andreas transform can be reconstructed through time from detailed knowledge of oceanic plate boundaries and motions offshore. The progressive switchoff of Neogene arc volcanism conformed to expected patterns in time and space when the age of oceanic lithosphere being consumed near the coast is taken into account. The extent of the growing no-slab area at the surface above the widening slab-window at depth has been largely coextensive with the gradually expanding Basin and Range province of extensional tectonism and bimodal volcanism. Diapiric upwelling of asthenosphere through the evolving slab-window in the subducted lithosphere probably influenced magma genesis and geodynamic behavior within the slab-free region. Bulk uplift of the adjacent Sierra Nevada and the nearby Colorado Plateau, as well as opening of the Rio Grande Rift, were possibly related to the same mantle processes.

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