The Deccan beyond the plume hypothesis Hetu Sheth, Indian Institute of Technology (IIT) Bombay.

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Presentation transcript:

The Deccan beyond the plume hypothesis Hetu Sheth, Indian Institute of Technology (IIT) Bombay

Structure of India India- Seychelles break-up

Basic geology

The Dediapada dyke swarm

Bouguer anomaly map km- thick igneous layer under the region

Heat flow, thermal springs, high gravity anomalies

Normal crustal thickness – 35 to 41 km

P-wave velocity anomaly

Hotspot tracks

India and Indian Ocean Must consider: 1.Palaeolatitudes 2.Vishnu FZ Ma volcanism 4.Age data questioned 5.Deccan plume died at 30 Ma?

Late-stage Deccan volcanism

The pre-volcanic uplift issue: regional vs. local The plume head model predicts broad lithospheric uplift of 1 to few km, a few m.y. before flood volcanism. Small local vertical motions not diagnostic; indicate local tectonics

Local uplift and subsidence Locality - picrites underlain by tilted sediments and conglomerate Basement and basalt clasts in conglomerate Uplift before and also after eruption Bagh beds in western India are marine – subsidence over plume head centre

Physio- graphy of India

Regional pre-volcanic uplift? (1) The 1500-km-long Western Ghats are very youthful, and form the precipitous edge of an elevated, tilted plateau. The plateau has a mature topography (flat, ancient land surface getting dissected again). The highest peaks of the Ghats (up to 2,695 m) are in the charnockite region of southern India

Regional pre-volcanic uplift? (2) Abundant evidence for major post- Deccan uplift of the Western Ghats Prominent easterly drainage not a result of plume-head-caused pre- volcanic doming Easterly drainage is antecedent

Regional pre-volcanic uplift? (3) Base of Deccan lavas is not exposed over much of the province In most cases where base is exposed, no evidence for basin shallowing, no basement-derived conglomerates Flat-lying lavas cover extensive flat erosion surfaces cut across varied rocks (e.g., central India) Thin, local Lameta sediments

The basement: Archaean crystallines, the great Vindhyan Basin (Mid-Late Proterozoic), the Gondwana basin (Permo- Carboniferous to Upper Jurassic)

Pachmarhi: planation surfaces on Mid-Triassic Gondwana sandstone, covered by Deccan lavas Spectacular post-Deccan uplift

Pachmarhi Patalkot Jabalpur

Conclusions Circular outcrop – intersecting rifts Crust and lithosphere unaffected by volcanism; normal thickness 8-9 m.y. total duration of volcanism Pre-volcanic regional doming: absence of evidence, evidence of absence Major (~1-2 km) post-volcanic uplift Continental rifting and break-up is a good and sufficient explanation