The anatomy of Continental Flood Basalt Provinces: geological constraints on the processes and products of flood volcanism

Abstract

The internal architecture of the immense volumes of eruptive products in Continental Flood Basalt Provinces (CFBPs) provides vital clues, through the constraint of a chrono-stratigraphic framework, to the origins of major intraplate melting events. This work presents close examination of the internal facies architecture and structure, duration of volcanism, epeirogenetic uplift associated with CFBPs, and the potential environmental impacts of three intensely studied CFBPs (the Parana-Etendeka, Deccan Traps and North Atlantic Igneous Province). Such a combination of key volcanological, stratigraphic and chronologic observations can reveal how a CFBP is constructed spatially and temporally to provide crucial geological constraints regarding their development.

Using this approach, a typical model can be generated, on the basis of the three selected CFBPs, that describes three main phases of flood basalt volcanism. These phases are recognized in Phanerozoic CFBPs globally. At the inception of CFBP volcanism, relatively low-volume transitional-alkaline eruptions are forcibly erupted into exposed cratonic basement lithologies, sediments, and in some cases, water. Distribution of initial volcanism is strongly controlled by the arrangement of pre-existing topography, the presence of water bodies and local sedimentary systems, but is primarily controlled by existing lithospheric and crustal weaknesses and concurrent regional stress patterns. The main phase of volcanism is typically characterised by a culmination of repeated episodes of large volume tholeiitic flows that predominantly generate large tabular flows and flow fields from a number of spatially restricted eruption sites and fissures. These tabular flows build a thick lava flow stratigraphy in a relatively short period of time (c. 1–5 Ma). With the overall duration of flood volcanism lasting 5–10 Ma (the main phase accounting for less than half the overall eruptive time in each specific case). This main phase or ‘acme’ of volcanism accounts for much of the CFBP eruptive volume, indicating that eruption rates are extremely variable over the whole duration of the CFBP. During the waning phase of flood volcanism, the volume of eruptions rapidly decrease and more widely distributed localised centres of eruption begin to develop. These late-stage eruptions are commonly associated with increasing silica content and highly explosive eruptive products. Posteruptive modification is characterised by continued episodes of regional uplift, associated erosion, and often the persistence of a lower-volume mantle melting anomaly in the offshore parts of those CFBPs at volcanic rifted margins.

Introduction

The products of continental flood basalt (CFB) volcanism represent the largest outpourings of magma in Earth's history. They are important in not only offering an insight into how the planet operates (e.g., Mahoney and Coffin, 1997; and references therein), and with respect to their likely environmental impact (Wignall, 2001; and references therein), but also because they provide analogues for eruptive materials, and styles of eruption observed on the surfaces of the other terrestrial planets and planet-like bodies (Keszthelyi et al., 2000).

Continental flood basalt provinces (CFBPs) are commonly associated with spatially constrained melting anomalies located within the upper mantle (e.g., Ernst and Buchan, 2003). These anomalies are, geologically speaking, long-lived, and during their early stages of activity are capable of extraordinarily high rates of melt production, in the formation of the CFBP at the initiation stages of the anomaly. Once initiated, the melting anomaly is thought to be spatially fixed over time, and is largely uninfluenced by tectonic processes or by movements operating within and affecting the lithosphere lying above. Whether this anomaly is defined as a ‘mantle plume’ sensu stricto, or some other mantle anomaly, remains a polemic issue and is beyond the scope of this paper. Accordingly, debate has largely become polarised between ‘plume’ and ‘nonplume’ hypotheses, with much of the argument conducted using elaborate geophysical and geochemical models. For any model to be demonstrably robust, it must be capable of explaining and predicting readily observed geological phenomena. In other words, it must explain the nature of the eruptive products and their spatial and temporal distribution. Therefore, in order to further constrain and test available models, it becomes crucial to establish the basic observations regarding the nature of CFBPs. These observations include: what are the geological features that characterise the onset and early stages of flood basalt volcanism?; for how long do CFBs continue to erupt, and what is the nature of output variation during their eruptive duration?; how is a CFBP edifice constructed over time, and what characterises their internal structure at the meso- (c. 1–10³ m) and macro- (10³–10⁵ m) scales? In other words, what is the anatomy of a CFBP?

This contribution offers a new overview of the key geological characteristics of CFBPs, presenting examples outlining the construction of the volcanic stratigraphy and architecture, and ultimately to describe the anatomy of CFBPs. The observations presented are based upon detailed volcanological, stratigraphical and geochemical work conducted primarily upon the Paranã–Etendeka, Deccan and North Atlantic Igneous Provinces (NAIP) and, to a lesser extent, other CFBPs including the Columbia River Basalt Province (CRBP) and the Ethiopia–Yemen Province (Fig. 1). In closing, discussion is aimed at the key features of flood volcanism which should help to further resolve and refine geophysical and petrogenetic models for the origin of CFBPs.