The Deccan beyond the plume hypothesis

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The Deccan beyond the plume hypothesis Hetu
Sheth, Indian Institute of Technology (IIT) Bombay
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Structure of India
India-Seychelles break-up
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Basic geology
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The Dediapada dyke swarm
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Bouguer anomaly map
16-24-km-thick igneous layer under the region
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Heat flow, thermal springs, high gravity anomalies
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Normal crustal thickness 35 to 41 km
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P-wave velocity anomaly
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Hotspot tracks
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India and Indian Ocean

• Must consider
• Palaeolatitudes
• Vishnu FZ
• 60-61 Ma volcanism
• Age data questioned
• Deccan plume died at 30 Ma?

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Late-stage Deccan volcanism
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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

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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

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Physio-graphy of India
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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

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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

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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

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The basement Archaean crystallines, the great
Vindhyan Basin (Mid-Late Proterozoic), the
Gondwana basin (Permo-Carboniferous to Upper
Jurassic)
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• Pachmarhi planation surfaces on Mid-Triassic
  Gondwana sandstone, covered by Deccan lavas
• Spectacular post-Deccan uplift

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Jabalpur
Patalkot
Pachmarhi
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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