Comment on Li, J., X. Hu, E. Garzanti, S. Banerjee, and M. BouDagher-Fadel (2019), Late Cretaceous topographic doming caused by initial upwelling of Deccan magmas: Stratigraphic and sedimentological evidence, GSA Bulletin.

Li et al. report that Late Cretaceous sedimentary sequences in the Tethys Himalaya indicate progressive shallowing, and require uplifts of ~100 m. I do not question their field data or the inferred uplifts. However, they relate these observations, as well as some observations from Late Cretaceous sedimentary sequences nearer the Deccan flood basalts, to pre-flood basalt crustal doming by the Deccan plume. While stating, in one sentence, that Sheth (2007) challenged such doming, they make no mention or discussion of any of Sheth’s many specific arguments, including widespread planation of the pre-Deccan landscape in central India and a refutation of the plume head origin of the so-called dome-flank radial drainage of peninsular India (Cox 1989). That drainage, as readers of the  Sheth paper will note, is superimposed on the Deccan Traps and could not have arisen on a pre-flood basalt dome in the basement, based on the simple argument that if that had been the case the Deccan lavas would have flowed along and dammed river canyons and ought to display evidence (such as widespread pillow-palagonite complexes of the Columbia River or South Caucasus flood basalts), but don’t. There are other arguments in the Sheth paper too.

What about the Tethys Himalaya? Li et al. consider this region to mark the passive northern margin of the Indian subcontinent, but if so what about the many Early through Late Cretaceous volcanic belts (e.g., Shyok, Khardung La) and granite-granodiorite batholiths (e.g., Ladakh, Karakoram) known in the Tethys Himalaya, representing ongoing subduction of the oceanic part of the Indian plate under island arcs and the southern margin of Eurasia?  Which means, one has a simple, plate tectonic explanation for the observed uplift and basin shallowing in the Trans-Himalayan region, without appeal to a Deccan plume head 2000 km or more wide.

The part about Deccan-derived spinels and similar basalt detritus entering Himalayan sediments confuses the issue. Sheth (2007) explained these occurrences, and evidence that the Late Cretaceous sediments in central India were derived in part from the basalts themselves. (Curiously, these Late Cretaceous SHALLOW MARINE sediments occur where maximum uplift from the Deccan plume should be expected.) The basalts had already erupted and were being eroded, and this does not require or indicate pre-volcanic uplift of the Deccan basement. The erosion of the basalt may or may not have been accompanied by post-eruptive local uplift.

The paper by Li et al. thus offers new and useful data which are themselves not questionable. But in reaching its broad conclusion about pre-volcanic lithospheric doming by the Deccan plume, this paper does not address many issues covered at length by Sheth (2007) for the Deccan region, ignores ongoing geodynamics and magmatism in the highly active region that was to become the Himalaya and Trans-Himalaya, and mixes up peninsular and extrapeninsular India in a poor synthesis that aims to show an impressive geographic coverage. Their overall conclusion and ambitious title are unsupported by the data.

Hetu Sheth, Mumbai, 11 December 2019

References

• Cox, K. G., 1989, The role of mantle plumes in the development of continental drainage patterns: Nature, v. 342, 873-877.
• Li, J., X. Hu, E. Garzanti, S. Banerjee, and M. BouDagher-Fadel (2019), Late Cretaceous topographic doming caused by initial upwelling of Deccan magmas: Stratigraphic and sedimentological evidence, GSA Bulletin.
• Sheth, H. C. (2007), Plume-related regional pre-volcanic uplift in the Deccan Traps: Absence of evidence, evidence of absence: Discussion, in Plates, Plumes, and Planetary Processes, edited by G. R. Foulger and D. M. Jurdy, pp. 803-813, Geological Society of America, Boulder, CO. Discussion of this paper.