Santanu Kumar Bhowmik

Abstract

Metamorphic timescales and cooling rates, when combined with the shapes of metamorphic P-T paths and thermobaric ratios (T/P) at metamorphic peak have emerged as powerful tools to fingerprint different tectonic regimes and also help us to establish the changing tectonic style during orogenesis, and thereby the advent of plate tectonics in the Planet Earth. Seen in that context, there is a general consensus that Archaean orogenesis has taken place under hotter mantle conditions (ΔT_P with respect to the present mantle ~100-250 °C) with features of widespread lithospheric peeling, shallow slab break-off, long-lived orogenesis (often ∆t >100 Myrs), involving longer durations of metamorphism and slow cooling (cooling rate at a few10s of °C/Myr) and exhumation history and distributed deformation patterns as compared to deeper slab detachments, short-lived orogenesis (generally ∆t between 40 and 60 Myrs), shorter durations of metamorphism (∆t < 10-20 Myrs), faster cooling (e,g. a few 100s of ^oC/Myr) and exhumation rates and narrow localized deformation patterns since the late Neoproterozoic.

Pulsed heating-cooling cycles during orogenesis, although relatively less studied in the rock record can be used to study the evolution of tectonic patterns during a particular orogenesis. If preserved in the rock record from Archaean to Phanerozoic, the changing patterns of different parameters such as thermobaric ratios, absolute values of T_Max and P at T_Max and duration and retrograde cooling rate of each thermal pulse in pulsating orogenic events together can act as excellent monitors of the secular evolution of the thermal history and tectonic styles in orogens. In this presentation, three such examples of pulsating orogenies in the Indian rock record and in age from the Archaean-Proterozoic boundary, Palaeoproterozoic-Mesoproterozoic transition zone and Mesozoic Era, are discussed to show the temporal evolution of the orogenesis in the Planet Earth.