A METASEDIMENTARY ORIGIN FOR MOJAVE-SONORAN DERIVED DETRITUS IN THE NANAIMO BASIN (BC) AND IMPLICATIONS FOR KULA-FARALLON SPREADING

Detrital zircon from the Nanaimo Basin, British Columbia, Canada provide a critical constraint on the Late Cretaceous paleogeography of the North American Cordillera. Recent dating demonstrated that zircon populations in the basin are similar to those in coeval forearc basins and the Pelona-Orocopia-Rand (POR) schists of southern California. An expanded conventional detrital zircon dataset (N=47, n=8826) and new zircon depth-profiling of a subset of samples (N=17, n=3782) from the basin provide new insights into the basin’s provenance. Two detrital zircon facies are present: facies “A” dominated by Mesozoic grains (modes ~150 and 90 Ma), and facies “B” containing Mesozoic grains (modes ~84 and 77 Ma) and Proterozoic grains (modes ~1700 and 1380 Ma) with thin rims. The transition from facies A to facies B is diachronous across the basin. In the south, rocks with zircon maximum depositional ages younger than ~84 Ma contain facies B zircon populations whereas ~130 km north, this transition occurs at ~72 Ma. Depth-profiling of facies B rocks has revealed metamorphic and magmatic rim growth between 110 and 64 Ma (mode 84 Ma). Virtually all (>99%) of Proterozoic grains show evidence of rim growth.

The timing of zircon rim growth in facies B rocks is coincident with the timing of metamorphism and partial melting of the POR schists and widespread aluminous magmatism in the Mojave-Sonoran region (MSR). We suggest that zircon grains with rims derive from a POR-like metasediment that was exhumed from the base of the crust during excision of the Baja-BC block, and that this event was coincident with the formation of the Kula Plate. The transition to facies B detrital zircon signatures in the southern Nanaimo Basin corresponds to the earliest possible onset of Kula-Farallon spreading at ~84 Ma; in the north it is coincident with basin-wide gravel deposition and increased exhumation in the Cascade Crystalline Core at ~72 Ma. Our results suggest drainages that supplied the Nanaimo Basin with sediment never reached the MSR but instead sampled metasedimentary rocks exhumed along the margin of the nascent Baja-BC block. This interpretation removes the requirement that the block was adjacent to the MSR until the end of Nanaimo Group deposition (~62 Ma) and is consistent with an earlier onset of northward translation at ~72 Ma.