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Emeishan vs. Deccan: Field evidence for and against regional pre-volcanic uplift, and implications for tectonic models

Hetu C. Sheth
Department of Earth Sciences, Indian Institute of Technology (IIT) Bombay, Powai,
Mumbai 400 076,

The mantle plume model predicts one to few kilometres of regional (~1000 km across), domal lithospheric uplift 5-10 million years before flood basalt volcanism. The uplift is due to heat conduction out of and dynamic support by the hot, buoyant, rising plume head. Field evidence for such uplift would comprise sedimentary sequences that reflect progressive basin shallowing before volcanism, or widespread conglomerates derived from the basement rocks and underlying the first lavas (indicative of differential tectonic uplift along faults). Local uplifts and subsidences cannot be used to prove or disprove plume-derived uplift. In the Permo-Triassic Emeishan flood basalt province of China, domal, kilometre-scale regional pre-volcanic uplift has been identified based on the palaeogeographic shallowing trends recorded in sediments, and conglomerate horizons underlying the initial basalt lavas. This is consistent with the plume model. Such uplift has been claimed for the Deccan flood basalt province of India as well, but field geological data from the Deccan, and India, paint a very different picture of crustal-lithospheric uplift than has been presented in plume models. Over large areas of the Deccan province, the base of the lava pile is in the subsurface. Basalt-basement contacts are observed along the periphery of the province and in central India. In central India (Satpura and Vindhya ranges), long, flat-lying Deccan basalt flows directly overlie extensive planation surfaces cut on various older rocks with different internal structures. Thin, patchy Late Cretaceous clays and limestones (Lameta Formation) separate the basalts and basement locally, but these sediments are known to have been locally derived from the nearby basalts themselves. Thus, (1) the eruption and flowage of the earliest Deccan basalt lava flows onto extensive, flat planation surfaces, (2) the usual absence of basement-derived conglomerates under the first lavas throughout the province, and (3) the usual absence of sedimentary sequences that show shallowing-upward palaeogeography, are all important lines of evidence against pre-volcanic lithospheric uplift and thereby the plume head model for the Deccan Traps. On the other hand, there is much evidence for major (1-2 km) post-volcanic uplift of the Indian peninsula, including the Satpura region, and this is not domal. The easterly drainage of the peninsula, speculated to be dome-flank drainage produced by the plume head, is antecedent to plateau uplift. Evidence for pre-volcanic, regional, domal uplift from several flood basalt provinces of the world (such as Siberia, Emeishan, Ontong Java, Deccan, Columbia River) is variable, even mutually contradictory. Flood basalts of the world are clearly different from each other in more than one way, often significantly so. Obviously, no single model, whether plume-based or not, should be considered alone, at the exclusion of alternative models.
Presentation type: Oral
Session: 3