The anatomy of Continental Flood Basalt Provinces: geological constraints on the processes and products of flood volcanism
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.
Section snippets
What are the key geological features of CFBPs?
What is a CFBP? A CFBP can be defined as a series of volcanic outpourings erupted onto areas of continental crust and which are comprised predominantly of great thicknesses of basaltic lava flows. They are a major, but separate, subset of large igneous provinces (LIPs), which may include eruptive and intrusive bodies displaying a wide range of chemistries (i.e., from mafic to silicic) and which may affect either continental (e.g., Parana–Etendeka, Deccan, etc.) or oceanic crust (e.g., Ontong
Duration of flood volcanism
Despite the immense size (i.e., 105–106 km3) of the larger CFBPs, the repeated, high-volume eruptive episodes that generate them occur over a relatively short period of geological time. The typical ‘lifetime’ of a CFBP is <10 Ma but, importantly, the rate of eruption is not uniform throughout this lifetime. In many instances, a peak tholeiitic basalt output is achieved during a <0.5–5 Ma acme in which >70% of the products may be erupted. However, where CFBPs are associated with continental
Pre-, syn- and post-eruption uplift
One of the fundamental tenets of the plume model for CFBP eruption is evidence of ‘pre-eruption uplift’. This is the assumption that immediately prior to the onset of eruptions, the lithosphere above the plume will be heated and domed by the dynamic buoyancy of hotter, less dense material within the rising and decompressing plume head (e.g., White and McKenzie, 1989, Campbell and Griffiths, 1990, Ernst and Buchan, 2003). A number of studies have sought to demonstrate that such doming and
Establishing the environmental impact of CFBP eruptions
Flood basalt lava flows and their associated volcanic effects have been implicated by many studies for their role in mass extinctions and other serious environmental impacts (see reviews by Hallam and Wignall, 1997, Rampino and Self, 1999, Wignall, 2001). For example, initial volatile release estimates from single eruptions, such as the Roza member of the Columbia River Basalt, indicate that prodigious amounts of S, Cl, and F were injected into the upper troposphere and lowermost stratosphere (
Closing remarks
Whether or not CFBPs are the products of plumes or other mechanisms of generating mantle melting anomalies, it is clear that if we are to further understand the processes by which these large volumes of melt are generated in the mantle, we first need to fully determine the temporal, spatial, and volcanological characteristics. In effect, any complex geochemical or geophysical models for the mantle must first address the field observations, volcanology, and, importantly, recognize the salient
Acknowledgements
This work was supported by funding provided to DAJ by Elf GRC and the EU 5th Framework Project SIMBA (CONTRACT N°: ENK6-CT-2000-00075), and to MW by NERC (Grant No. GR3/11474), and the Open University Research Development Fund (RDF). James Day, Jo Garland, Graham Thompson, Richard Single, Henry Emeleus, and Stephen Self are thanked for their help during manuscript preparation. Keith Cox is remembered for his encouragement with helping MW develop Deccan research themes. Helpful review comments
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