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