SOUTHEAST END OF THE LATE PALEOZOIC SOUTHWESTERN LAURENTIAN BORDERLAND: ¿QUÉ PASÓ? (Invited Presentation)

The Southwestern Laurentian Borderland (SLaB) tectonic model for the western margin of Laurentia incorporates left slip along the southwestern plate margins with inboard transpressional deformation and explains numerous observations of geologic phenomena during late Paleozoic (Mississippian-middle Permian) time. These phenomena include marginal basin development, widespread unconformities in the upper Paleozoic section, truncation of the early Paleozoic Cordilleran miogeocline, microplate translation, and ongoing deformation in Nevada and southern California between the Antler and Sonoma orogenic events. The separation of the Caborca block (Caborca), now in northern Sonora, from its original position near Death Valley took place by southward propagation of the Slide Mountain ocean/Havallah basin via long left-slip transforms and short spreading centers as Laurentia migrated latitudinally north. The history of Caborca after separation is a critical aspect of the SLaB tectonic development.

What happened in the southern part of the SLaB remains debated, but several lines of evidence shed light on its history. Near the end of early Permian time, the southern flank of Caborca was overthrust by oceanic rocks of the Sonora orogen (Cortés terrane of Mexican literature). Deep-water Ordovician strata of the Sonora orogen contain detrital zircon (DZ) grains characteristic of platformal strata of Caborca, including grains derived from the Peace River arch region of Canada. The Sonora orogen has been interpreted as the western end of the Ouachita-Marathon suture zone; however, deepwater Ordovician rocks farther east notably lack the Canadian DZ. At about the same time, the Sonora orogen was overthrust from the south by El Fuerte terrane, which contains deepwater strata with DZ of Gondwanan provenance. This combination of events, the "Sonoran train wreck," likely arrested the southern translation of Caborca.

The northern flank of Caborca is the site of early Permian plutonism, which overlaps in time with final translation of Caborca. These plutons are the earliest evidence for magmatism along the truncated Cordilleran margin. We suggest that they represent magmatism along a leaky transform, or the remnants of a Scotia-type magmatic arc, rather than inception of the Cordilleran continental-margin arc.