• Hetu C Sheth

      Articles written in Journal of Earth System Science

    • Cones and craters on Mount Pavagadh, Deccan Traps: Rootless cones?

      Hetu C Sheth George Mathew Kanchan Pande Soumen Mallick Balaram Jena

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      Rootless cones, also (erroneously) called pseudocraters, form due to explosions that ensue when a lava flow enters a surface water body, ice, or wet ground. They do not represent primary vents connected by vertical conduits to a subsurface magma source. Rootless cones in Iceland are well studied. Cones on Mars, morphologically very similar to Icelandic rootless cones, have also been suggested to be rootless cones formed by explosive interaction between surface lava flows and ground ice. We report here a group of gentle cones containing nearly circular craters from Mount Pavagadh, Deccan volcanic province, and suggest that they are rootless cones. They are very similar morphologically to the rootless cones of the type locality of Mývatn in northeastern Iceland. A group of three phreatomagmatic craters was reported in 1998 from near Jabalpur in the northeastern Deccan, and these were suggested to be eroded cinder cones. A recent geophysical study of the Jabalpur craters does not support the possibility that they are located over volcanic vents. They could also be rootless cones. Many more probably exist in the Deccan, and volcanological studies of the Deccan are clearly of value in understanding planetary basaltic volcanism.

    • The High Deccan duricrusts of India and their significance for the ‘laterite’ issue

      Cliff D Ollier Hetu C Sheth

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      In the Deccan region of western India ferricrete duricrusts,usually described as laterites,cap some basalt summits east of the Western Ghats escarpment,basalts of the low-lying Konkan Plain to its west,as well as some sizeable isolated basalt plateaus rising from the Plain. The duricrusts are iron-cemented saprolite with vermiform hollows,but apart from that have little in common with the common descriptions of laterite.The classical laterite profile is not present.In particular there are no pisolitic concretions,no or minimal development of con-cretionary crust,and the pallid zone,commonly assumed to be typical of laterites,is absent. A relatively thin,non-indurated saprolite usually lies between the duricrust and fresh basalt.The duricrust resembles the classical laterite of Angadippuram in Kerala (southwestern India),but is much harder.The High Deccan duricrusts capping the basalt summits in the Western Ghats have been interpreted as residuals from a continuous (but now largely destroyed)laterite blanket that represents in situ transformation of the uppermost lavas,and thereby as marking the original top of the lava pile.But the unusual pattern of the duricrusts on the map and other evidence suggest instead that the duricrusts formed along a palaeoriver system,and are now in inverted relief.The two interpretations lead to different tectonic histories.Duricrust formation involved lateral material input besides vertical elemental exchange.We may have reached the stage when the very concepts of laterite and lateritization are hindering progress in regolith research.

    • An Ediacaran–Cambrian thermal imprint in Rajasthan, western India: Evidence from 40Ar-39Ar geochronology of the Sindreth volcanics

      Archisman Sen Kanchan Pande Hetu C Sheth Kamal Kant Sharma Shraboni Sarkar A M Dayal Harish Mistry

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      The Sindreth Group exposed near Sirohi in southern Rajasthan, western India, is a volcanosedimentary sequence. Zircons from Sindreth rhyolite lavas and tuffs have yielded U–Pb crystallization ages of ∼768–761 Ma, suggesting that the Sindreth Group is a part of the Malani magmatic event. Earlier 40Ar-39Ar studies of other Malani volcanic and plutonic rocks yielded disturbed argon release spectra, ascribed to a ∼550 Ma thermal event possibly related to the Pan-African orogeny. To test and confirm this possibility, we dated two whole-rock and three feldspar separate samples of the Sindreth volcanics by the 40Ar-39Ar method. All samples yield disturbed argon release spectra suggesting radiogenic argon loss and with plateau segments at 550 Ma or 490 Ma. We interpret these as events of argon loss at 550–490 Ma related to an Ediacaran–Cambrian thermal event, possibly related to the Malagasy orogeny. The combined older and new 40Ar-39Ar results are significant in showing that whereas Ediacaran–Cambrian magmatic and metamorphic events are well known from many parts of India, they left thermal imprints in much of Trans-Aravalli Rajasthan as well. The overall evidence is consistent with a model of multiphase assembly of Gondwanaland from separate continental landmasses.

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