MID- TO LATE-MESOZOIC TECTONIC EVOLUTION OF THE GEORGIA BASIN – EVIDENCE FROM DETRITAL ZIRCON ANALYSIS

Georgia Basin is largely a Late-Cretaceous to recent basin that overlies both Wrangellia terrane and Coast Plutonic Complex basement rocks, and beneath that, the subducting Juan de Fuca plate. Biostratigraphic evidence suggests that subsidence of the Georgia Basin primarily occurred during accelerated subduction of the Farallon/Kula Plate beneath North America from Campanian to Maastrichtian time. However, new evidence from detrital zircon indicates that at least two additional, and earlier, Mesozoic episodes of subsidence, deposition and uplift occurred.

Twenty detrital zircon samples comprising 5,057 U/Pb dates were collected from the lower Nanaimo Group. Maximum depositional ages (MDAs) derived from these 20 samples reveal deposition of Jura-Cretaceous (Oxfordian to Berriasian), Albian and Cenomanian strata. Meanwhile, multi-dimensional scaled (MDS) groupings suggest an evolution of source areas over time. During deposition of Jura-Cretaceous strata, Georgia Basin was part of a syn-accretionary arc-basin system, with sediment routing systems mostly separate from those of continental North America. By the Albian, final accretion of the Insular Belt and regional orogenesis associated with the Coast and Cascades thrust belts initiated foreland-style subsidence, and the deposition of terrestrial and shallow-marine strata (molasse). From the Santonian onwards, termination of thrusting and accelerated subduction of oceanic plate beneath Wrangellia initiated forearc-style subsidence, accommodating 4 km of first terrestrial and shallow-marine strata and then deposition of deep-marine strata of the Nanaimo Group.

These results reflect changing tectonic regimes in the Georgia Basin in response to regional events. They also showcase that the history of Georgia Basin is consistent with other Mesozoic sedimentary basins on North America’s western margin (e.g., Great Valley Forearc, California).