(2007). Discussion: Sheth, H.C. Plume-related regional pre-volcanic uplift in the Deccan Traps: Absence of evidence, evidence of absence.
In: Foulger, G. R. and Jurdy, D. M. eds.
Plates, Plumes, and Planetary Processes.
Special Paper (430).
Boulder, Colorado, USA: The Geological Society of America, pp. 805–806.
Sheth offers an interesting, albeit controversial, interpretation to some of the geological and geomorphological data from the Deccan Volcanic Province (DVP). He continues an essentially ‘anti-plume’ theme adopted in previous papers (e.g. Sheth, 1999; 2005). Here, he challenges the ‘plume model’ by attempting to demonstrate a ‘lack of regional uplift’ prior to the Deccan eruptions. There are some excellent background sections, but it would be wrong to conclude that this paper offers either a comprehensive or a balanced review of the available geophysical, geochemical, and geomorphological data. Moreover, the issues regarding plume-related uplift, and associated geophysical data concerning evidence of a Deccan plume, are actually far more complex than admitted (e.g. Kennet and Widyantoro, 1999; Burov et al. 2005).
Sheth correctly focuses upon the volcano-sedimentary evolution of the DVP in seeking evidence for, or against, pre-eruptive doming of the region, but provides only those interpretations consistent with a particular viewpoint. Alternative interpretations do exist, and these are more widely accepted. Of more immediate concern is that this paper fails to present us with a viable alternative to the currently preferred ‘plume-based’ view for the DVP; this is a serious omission. Sheth’s assertion, that the Deccan Volcanic Province (DVP) is not the result of a Late Cretaceous mantle plume, might be made more credible if it were tested and explored through application of the available, alternative models (e.g. edge-driven convection, EDC): Instead, only a critique of previous work is offered.
It is apparent that Sheth favours the EDC family of models (e.g. Sheth, 1999; 2005), but fails to consider that these might present even greater inconsistencies if adopted as an alternate explanation of the DVP and the wider pre- syn- and post-rift evolution of the Western Indian margin. For any model to be credible for the DVP, it must adequately explain that, (a); independent of global eustatic changes, offshore sedimentation rates increase significantly around peninsula India during the Late Cretaceous, and then wane again during early Palaegene (most scholars would conclude that this indicates regional uplift of the continental hinterland), and (b); the fact that rift-related dyking, the Seychelles rifting event, and associated sea-floor spreading clearly post-date the bulk of the DVP tholeiitic eruptions (e.g. Hooper, 1990; Devey and Stephens, 1991; Widdowson et al., 2000). Another well-documented observation is the fact that (c); the major DVP eruptive units young (both stratigraphically and chronologically), southwards (Cox 1983; Devey and Lightfoot, 1986; Mitchell and Widdowson, 1991; Vandamme et al. 1993). Most contend that this DVP structural asymmetry is consistent with the rapid drift of peninsular India over a mantle melting anomaly. Without resorting to the petrogenetic arguments rehearsed elsewhere, it remains an effective argument that these stratigraphical, volcanological, geochronological, and palaeomagnetic data are more consistent with the plume model of DVP evolution, than any of the available alternatives (Jerram and Widdowson, 2005).
Like many geoscientists, I am not particularly committed to any model of CFBP evolution. Even so, most would agree that any geological/geophysical model is simply a conceptual framework which aids an understanding of a wide range of observations; typically, it is usually a simplification of the natural world, and hence often imperfect. However, we should at least demand equal testing of all available models; and that this testing should first require determining the model outcome, and then its comparison with actual observational data. A vast body of recent geological and geomorphological data now exists for the DVP, including examples by Sheth (2001a; 2001b), which might allow the first attempts at this approach. However, much of the observation presented here by Sheth regarding uplift, or lack thereof, is either equivocal, or else can equally be explained in a manner consistent with the very plume-rift interaction model he seeks to contend (e.g. Cox, 1978; White et al., 1987; Ernst and Buchan, 2003; Courtillot et al., 1999). Specific objections to Sheth’s interpretations are given in a subsequent discussion (see below).
Another serious omission is the lack of reference to Courtillot et al. (2003) (or indeed, to any of the Deccan research published by Courtillot and colleagues). These authors conclude, reasonably, that in order to demonstrate the presence of a plume, five specific and stringent observational criteria must be met. Unsurprisingly, most of the ‘mantle plume’ sites around the globe fail these tests. However, of those examples that do apparently fulfil all the criteria of plume-related volcanism, the Deccan is given as a prime example. Interestingly, Courtillot et al. (2003) do not consider pre-DVP uplift as being a critical factor, and whether such uplift can actually prove that mantle plumes exist, or not, still remains a polemic issue (see response by Anderson, 12th January).
To the credit of the instigators of this debate, we can now accept that the wider plume model needs to be scrutinised, and perhaps used with a more circumspection. However, it is neither logical nor scientific to conclude that simply because not every fact or interpretation fits a particular geological model, the model then becomes redundant or disproved. For any (CFBP) model to be acceptable, it must be capable of explaining most of the collected observations, and also be viable under most of those comparable geological situations found elsewhere (Jerram and Widdowson, 2005). Accordingly, we should now focus on which model best fits the available DVP data, and thereby seek to develop and refine a wider understanding of the geologically complex phenomena that gave rise to this CFBP.
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