NO REST FOR THE MOJAVE: TARDY CRETACEOUS MAGMATISM IN THE SOUTHERN CALIFORNIA BATHOLITH

The southern California batholith (SCB), comprised of plutonic rocks in the Mojave, the eastern Transverse Ranges, and Salinia, differs from the Sierra Nevada batholith (SNB) and Peninsular Ranges batholith (PRB) in both its pre-Mesozoic framework and its Late Cretaceous outcomes. In terms of its tectonic framework, the SCB occupies a position at the southeast end of the truncated Cordilleran passive margin, where the craton was juxtaposed most closely with the plate boundary. In terms of its Late Cretaceous outcomes, one of the more surprising results is that the SCB remains magmatically active ca. 5–10 million years longer than either the SNB or the PRB, where magmatism terminates pretty abruptly at ca. 85 or 90 Ma, respectively. In the western Mojave, Upper Cretaceous plutons span 90–77 Ma and dominate ca. 90% of the pre-Cenozoic outcrop. Cretaceous plutons young to the southeast, reaching as young as 74 Ma in the central Mojave, although the Cretaceous pluton density drops to around 50% over the same interval. Geochemical indices of these tardy Cretaceous batholiths of the Mojave reveal only minor differences compared to their pre-85 Ma counterparts, such as a slight trend towards more felsic compositions. Does this slower waning of magmatism require a radically new interpretation of Laramide tectonics, such as that presented by the Hit-and-Run model, does it reflect the initially thicker cratonal lithosphere into which it was emplaced, and/or does it reflect unrecognized complications to the subduction of the flat slab? Possible modifications to the standard flat-slab model include an initially narrow and more northerly slab collision around the location of the modern-day Garlock Fault, and/or a slab tear behind the subducted oceanic plateau causing late asthenosperic upwelling and continued magmatism in the SCB.