The "Large" in Large Igneous Provinces: Using Digital Geological Maps to Determine the Area, Magma Flux, and Potential Environmental Impact of the Wrangellia Flood Basalts

Large igneous provinces (LIPs), such as continental flood basalts and oceanic plateaus, are formed by relatively short duration, massive outpourings of basalt in intraplate settings. Their emplacement has been associated with global climatic and biotic change (e.g., end-Permian Siberian LIP). The magmatic products of a LIP typically cover an area >1 Mkm2, however erosion and exhumation may substantially reduce the original area and volume of a LIP, especially oceanic plateaus that have been tectonically dispersed during accretion (e.g., Caribbean, Wrangellia). The availability of digital geologic maps from government geologic surveys now allows for measuring the precise areal distribution of remnant LIP-products, which is essential information for estimating total volumes and ultimately potential environmental effects. The Wrangellia flood basalts represent one of the best-exposed accreted oceanic plateaus on Earth. This Triassic LIP is exposed in numerous fault-bound blocks in a belt extending discontinuously for 2300 km in the Pacific Northwest of North America. It contains exposures of submarine and subaerial volcanic rocks representing composite stratigraphic thicknesses of 3.5-6 km. From recently compiled digital geologic maps (British Columbia, Yukon, Alaska), the mapped exposures of the Wrangellia flood basalts are relatively small (25,256 km2 with 75% from Vancouver Island), which leads to minimum calculated erupted volumes of up to 1.4 x 105 km3 and an estimated magma flux of 0.03 km3/yr. The original areal distribution was substantially greater, perhaps by an order of magnitude or more, as the outcrop extent does not include regions covered by younger strata and surficial deposits nor does it account for the volcanic component of the terrane that may have been subducted. However, even this minimum volumetric output rate is comparable to recent estimates of long-term volumetric eruption rates for ocean islands such as Iceland (0.02-0.04 km3/yr) and Hawaii (0.02-0.08 km3/yr) [1]. The Wrangellia flood basalts were emplaced during a single phase of tholeiitic volcanism at ca. 230 Ma, possibly within as few as 2 Myr, onto preexisting submerged arc crust in equatorial latitudes in the eastern Panthalassic Ocean. This age corresponds to the Carnian-Norian boundary of the Upper Triassic [2], a time of global-scale climatic and biotic crisis, which was followed by strong radiation of the dinosaurs. A combination of precise U-Pb geochronology of the Wrangellia basalts, area/magma flux estimates (including integration of geophysical seismic reflection profiles to account for plutonic components) and quantification of the associated volatile production will be required to fully evaluate this potential link between volcanic activity and global environmental impacts. [1] White, S.M. et al. (2006) Geochemistry Geophysics Geosystems, 7, Q03010, doi: 10.1029/2005GC001002. [2] Furin, S. et al. (2006) Geology 34, 1009-1012, doi: 10.1130/G22967A.1.