(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).
Geological Society of America, pp. 808–809.
Sheth misreads key aspects of my earlier comment. For the record: I consider ‘uplift’ an issue central to the plume debate. I reiterate; uplift ‘remains a polemic issue’, (i.e. worthy of discussion). Word limits preclude the desired detailed dialogue, but some important issues are raised below.
I entirely agree that concerted field work is necessary, but both this paper and Sheth (2005) offer little new field data, and instead rely largely upon an interpretation of previous authors’ information. Such retrospectives do not permit the reader to evaluate the relative merits of plume versus non-plume models, and so cannot materially progress debate.
The argument regarding southward (not eastward, as suggested by Sheth) younging of the main Deccan edifice remains robust. Three independent lines of geological evidence support this interpretation: Two independent lines are usually deemed sufficient to indicate a scientific ‘truth’.
Sheth concludes, logically, that Western Ghats is the product of post-Deccan denudational processes. This particular interpretation has long been available (e.g. Widdowson and Cox, 1996; Widdowson, 1997; Gunnell and Fleitout, 1998; Widdowson and Mitchell, 1999). Given this issue is not in dispute, why raise it here?
Sheth asserts that Cox’s (1989) plume-head drainage idea is problematic. Perhaps, but the fact that radial drainage patterns do occur in key CFBPs remains a valid, if inexplicable (?) observation. Cox’s idea was superseded by arguments provided in Widdowson and Cox (1996), Widdowson (1997; see fig. 14), and AFTA data (Gunnell et al., 2003), and so becomes irrelevant for contending pre-eruptive uplift.
Sheth argues, correctly, that the nature of the pre-Deccan palaeosurface holds important clues regarding pre-eruptive uplift in the DVP (Jerram and Widdowson, 2005). Much of this surface remains buried by the Deccan lavas, and is both inaccessible and unknowable. It only becomes exposed around the northern and eastern periphery of the main lava pile. Such peripheral localities, including many of those described by Sheth, were hundreds of kilometres from the Deccan eruptive loci. If any uplift did occur here, it would have been minimal at such large distances from the focus of putative plume head uplift, and thus consistent with that affecting the Dongargaon basin, for example (Tandon, 2002; Samant and Mohabey 2005).
The pre-eruptive palaeosurface has been significantly modified by the crustal loading of the Deccan edifice, and in its western extensions suppressed far below datum. Thus, the gross form and elevation of this basement – basalt contact is largely an artefact of post-eruptive flexural adjustment. Nevertheless, Sheth argues that this highly modified surface reveals a ‘peneplain’, and that its preservation as such precludes significant fluvial incision. Possibly: But peneplains are the consequence of erosion, and the classical, albeit obsolete, Davisian model requires regional uplift as a trigger for peneplanation to proceed. Etchplanation is more appropriate to the development of the pre-Deccan surface (e.g. Büdel, 1982). Here, thick alteration mantles accumulate through tropical weathering of surfaces exposed during prolonged periods of tectonic stability. If, as Sheth requires, such conditions had characterised the pre-Deccan land surface, then the widespread absence of deep weathering mantle preserved beneath the lava units may instead indicate that this landscape had been thoroughly stripped prior to DVP eruptions. Etchplain stripping may be achieved through widespread fluvial erosion induced by regional uplift (Borger and Widdowson, 2001).
Offshore sedimentary records in the Krishna, Godavari, and Narmada-Tapti basins, all reveal significant increases in Late Cretaceous depositional flux (Halkett et al. 2001): these data are consistent with pre-eruptive regional erosion of peninsular India – starting with the stripping of an easily erodable weathering mantle perhaps?
If pre-eruptive (plume-driven?) uplift had occurred in pre-Deccan peninsular India, what might then be recorded in the erosional and sedimentary chronologies of the DVP peripheral regions? Removal of any easily erodable weathering mantle, perhaps; minimal changes in elevation, possibly; development of shallow basins receiving fine clastic input from the plume-uplift effects hundreds of kilometres away - may be. This interpretation of the available infra- and intra-trappean sedimentary (i.e. Lameta Beds) data is equally plausible using the same compendium of field evidence provided by Sheth. Accordingly, I offer a modified, précis version of Sheth’s own summary:
‘Any original flatness and elevation of the pre-Deccan landscape has been significantly modified by syn- and post-eruptive isostatic adjustment deriving, initially, from the loading of the DVP edifice, and subsequently by denudational unloading. The occurrence of a stripped, pre-eruptive etchplain, together with associated offshore sedimentological data, are consistent with those phenomena predicted had a large plume head upwelled beneath India during the Late Cretaceous.
Post-Deccan uplift has elevated both the pre-Deccan, and post-Deccan surfaces. This uplift of the Western Ghats is not related to a putative Deccan plume: it is not domal, occurs beyond the limits off the Deccan lava cover, and represents a later, denudationally-driven, uplift (Widdowson, 1997). Thus, the easterly drainage of the Indian peninsula is not plume-related dome flank drainage, and is largely antecedent to denudational uplift effects’.
To summarise, of those observations described by Sheth, most, if not all, can equally and adequately be explained by the passage of India over a static, spatially restricted, mantle melting anomaly during the Late Cretaceous: For want of a better term, and until consensus offers me a better alternative, I will continue to call this anomaly, sensu lato, a ‘mantle plume’. I end by reiterating the rationale to my initial comment: The challenge to Sheth remains to deliver us an alternative, ‘non plume’, model that can better explain the Deccan CFBP.
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