CORDILLERAN MAGMATIC PATTERNS CONSISTENT WITH FAR-TRAVELED BAJA BC

Magmatism in convergent plate margins is directly related to the depth of the down-going slab. In this geological setting, magmatism is largely a consequence of fluid release associated with heating of the slab and hydration melting of the super adjacent mantle wedge. As a result, magmatic patterns associated with convergent plate boundaries may be used to infer the geometry of subducting slabs. The Laramide magmatic null and subsequent magmatic resurgence of the Western Cordillera has been used to infer flat slab subduction followed by slab foundering of a portion of the Farralon Plate. This hypothesis has been invoked to explain a variety of geologic observations including intra-continental Laramide orogenesis, missing lithosphere beneath the southern Sierra Nevada Batholith and Mojave Desert, and uplift of the Colorado Plateau. Additionally, the flat slab hypothesis has been used as an argument to counter the proposal of significant transcurrent motion of Late Mesozoic Cordilleran terranes (~3,000 km) required for the Baja BC hypothesis. Yet, there is no positive evidence for a flat slab during this time period; the argument stems from the cessation of magmatism. This magmatic gap is consistent with other hypotheses, including Baja BC. Using the pattern of magmatism in the Western Cordillera, with the aim of discriminating between the flat slab and Baja BC hypotheses, we present a series of palinspastically restored maps of the Western Cordillera depicting the transport history of the Baja BC block (8–9 cm/yr) and the development of Cordilleran magmatism after Armstrong and Ward (1993; 1991), between 85 and 45 Ma. Significantly, this restoration places the active Baja BC arc (Insular Belt) outboard of California during the Late Cretaceous/Early Cenozoic Cordilleran magmatic null. This implies that the magmatic null is the result of a passing Baja BC arc rather than a Laramide flat slab. Given the corroborative geological, paleontological, and paleomagnetic evidence for major transcurrent displacements, we argue that the Baja BC hypothesis best explains the observed Cordilleran magmatic patterns during this time.