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

Structure of India

India-Seychelles break-up

Basic geology

The Dediapada dyke swarm

Bouguer anomaly map

16-24-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

3. 60-61 Ma volcanism

4. Age data questioned

5. Deccan plume died at 30 Ma?

Late-stage Deccan

volcanism

The pre-volcanic uplift issue: The pre-volcanic uplift issue: regional vs. localregional 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 subsidenceLocal 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)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)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)Regional pre-volcanic uplift? (3)Base of Deccan lavas is not exposed

over much of the provinceIn 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

ConclusionsConclusionsCircular outcrop – intersecting riftsCrust and lithosphere unaffected by volcanism;

normal thickness8-9 m.y. total duration of volcanismPre-volcanic regional doming: absence of

evidence, evidence of absenceMajor (~1-2 km) post-volcanic upliftContinental rifting and break-up is a good and

sufficient explanation