Large Igneous Provinces

Large igneous provinces (LIPs) represent relatively short periods of voluminous basaltic magmatism. This magmatism is present throughout the geologic record and shares a temporal coincidence with many of Earth's mass extinctions. However, the link between volcanism and extinction remains enigmatic. One proposed link is that volatiles released during LIP magmatism drive climate change. This hypothesis requires extremely high eruption rates coincident with and immediately preceding the mass extinction event. Over the past few years, I have worked to quantify eruption rates in two LIPs: the Deccan traps, which were emplaced coeval with the end-Cretaceous extinction and the accreted Siletzia oceanic plateau, which is not associated with paleoclimate change nor an extinction event. The eruption rates in these two provinces are markedly different, providing one possible explanation for this difference.

Deccan Traps

The Deccan Traps is a LIP in central India that contains >1.3 million km³ of erupted basalt. It was emplaced during the latest Cretaceous and earliest Paleocene and has long been considered a potential factor in driving the end-Cretaceous mass extinction, along with the Chicxulub impact. Over the past few years I have been working with Blair Schoene and Kyle Samperton (Princeton University) to constrain eruption rates in this LIP. We are using high-precision U-Pb zircon geochronology, which provides temporal resolution of 50-30 kyr for rocks of these age. However, zircon rarely crystallizes in basaltic melts. So, we have adopted a sampling strategy in which we collect sedimentary and paleosol material (red boles) from interflow horizons. These horizons represent surfaces that were exposed during breaks in basalt eruption and may have collected small amounts of zircon-bearing volcanic ash during silicic eruptions. We have found that ca. 20-30% of these horizons contain a single population of Cretaceous or Paleocene zircons, and we have interpreted these dates as depositional. These results showed that most of the Deccan traps erupted over a short period of time starting 250 kyr before the mass extinction (Schoene et al., 2015). Of critical importance for evaluating the role of these eruptions in driving the end-Cretaceous extinction is the eruption rate in the last 100 kyr prior to the extinction event. We are currently finishing an additional dataset that addresses this issue.

Siletzia

I have also been working to define the volcanic stratigraphy and assess eruption rates in a LIP exposed in western Washington, western Oregon, and on southern Vancouver Island with Ken Clark (University of Puget Sound) and Michael Polenz (Washington DNR). This LIP is named the Siletzia terrane and is composed of >1.7-2.6 million km³ of basalt that was erupted during the Eocene. Several lines of evidence suggests that it represents an accreted oceanic plateau, including a short period of regional shortening during the Eocene and geophysical evidence that shows that it is currently connected to a hanging slab of subducted oceanic crust. Nevertheless, the origin of this terrane has remained controversial due to uncertainty over the terrane's stratigraphy. Therefore, much of my research has been focused on clarifying this stratigraphy using U-Pb geochronology. Like the Deccan Traps, there are few rocks to date via U-Pb zircon geochronology within Siletzia and I've been focused on locating zircons in interflow silicic ash beds, gabbros, and detrital zircons in interbedded sedimentary rocks. The dates have shown that a large sedimentary section previously thought to floor the terrane is actually thrust under Siletzia, thereby reducing the need for the basalts to have erupted on the continental margin and supporting their origin as an oceanic plateau (Eddy et al., 2017). The data also show that the terrane was built over a minimum duration of several million years, much slower than the LIPs related to mass extinctions.