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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS017 Oral Presentation 2371
Thinning of hot indian lithosphere : causes and consequences
Dr. O.P. Pandey
Theoritical Geophysics National Geophysical Research Institute (NGRI) IASPEI
Indian shield is unique and differs considerably from other stable areas of the earth. It has remained
dynamic since at least late Archeans due to a number of long sustained tectonothermal events which
repeatedly rejuvenated and remobilized the entire lithosphere. The continued reactivation over such a
long period of time, which continues even now is not evident elsewhere. About two decades back, a
preliminary attempt was made to study the thermal regime of the Indian lithosphere using the then
available meager heat flow data set, which indicated the possibility of a quite thin and warm lithosphere
beneath Indian shield. Since then, there has been a dramatic increase in heat flow numbers as well as
new geological and geophysical informations. Consequently, the crustal structure is now better
understood warranting a fresh look over the Indian shields thermal regime. Present study validates the
presence of a thin lithosphere (average ~ 105 km compared to 250-300 km in similar terrains
elsewhere) and a quite warm Moho with an average temperature close to 600oC beneath Indian
cratonic shield. Thinning of the lithosphere has now been confirmed by recent multimode surface
waveform tomographic and other geophysical studies. Beneath many of the late Archean segments,
heat flow from the mantle exceeds 30 mW/m2. An interesting inference which emerges from this study
is that beneath certain ancient cratons, where gravity bias is distinctly positive, Moho temperatures and
mantle heat flow reaches as high as 800oC and 45 mW/m2 respectively with depth to the
asthenosphere at only 65 km. It is felt that the long sustained reactivation may have led to the
enrichment of lithophilic/radioactive elements at shallow mantle levels, which made it buoyant and
fertile, thereby allowing for a thinner and weaker lithosphere. Such an order of thinning would result
into the underlying lithosphere being subjected to a much higher insitu temperatures leading to
sustained regional uplifting and exhumation of deeper crustal layers, which now appears to be so
prevalent in the Indian shield. It may not be just a coincidence that almost all the
catastrophic/damaging earthquakes which occurred since 1618 lie either on uplifted blocks or on their
escarpment edges.
Keywords: lithospheric thinning, mantle heatflow, moho temperatures