Ophiolitic rocks of the Indo-Myanmar Ranges, NE India: relicts of an inverted and tectonically imbricated hyper-extended continental margin basin? I. SOIBAM 1 *, M. CH. KHUMAN 2 & S. S. SUBHAMENON 1 1 Department of Earth Sciences, Manipur University, Imphal-795003, Manipur, India 2 Department of Geology, Nagaland University, Kohima Campus, Meriema, Kohima-797001, Nagaland, India *Corresponding author (e-mail: ibotombi2002@yahoo.co.uk) Abstract: The Indo-Myanmar Ranges (IMR) of NE India are host to various ophiolitic rocks, including metamorphosed Alpine-type harzburgite and lherzolite. Compared to abyssal peridotites of normal oceanic lithosphere, these ultramafic rocks are enriched in trace and rare earth elements. Spilitic pillow lavas along with mafic dykes and sills locally intruded into the serpentinized ultra- mafic rocks and associated pelagic sediments exhibit alkaline compositional affinities. Ophiolite formation and emplacement were by a process analogous to that described for mantle exhumation in hyper-extended continental margin settings and ophiolites in parts of the European Alps, involv- ing very slow passive continental margin rifting accompanied by slow upwelling or extensional unroofing of the subcontinental upper mantle up to the seafloor. Preliminary palaeomagnetic measurements conducted on ultramafic rocks within the IMR ophiolite belt give a virtual geomag- netic pole (VGP) at 478 N, 0458 E for thermal demagnetization (TDM) measurements and 338 N, 0138 E for the alternating field demagnetization (AfD) measurements, requiring an anticlockwise rotation of the ultramafic bodies by 148 during the subduction process. The original trend of the spreading axis of the ophiolites was probably NE – SW, with spreading directed NW – SE. Compu- tation of palaeolatitude of the ultramafic rocks gives an average value of 24.678. Comparison between the palaeolatitude and the present latitude of the sample sites provides a mere latitu- dinal shift of less than 18. Field studies, combined with an analysis of structural and tectonic features in the IMR, suggest a generalized WNW – ESE (east – west) compression and NNE – SSW (north – south) extension con- tradictory to the NNE – SSW contraction indicated by seismic data. Area balancing techniques employed along sections orientated perpendicular to regional tectonic strike in the IMR reveal sys- tematic variations in the amount of crustal shortening, with a maximum of approximately 60% recorded in the Nagaland–Manipur segment along 25.6448 N, 93.8268 E–25.0768 N, 95.8978 E. The amount of shortening gradually decreases away from the axis of maximum shortening and on both sides. Calculations of relative plate motion based on rotation vectors given by different workers for various plate pairs represented in the region reveal that the interaction between the Indian and Myanmar plates can ideally produce the structural and tectonic features of this range. Dextral shear coupled to oblique subduction of the Indian Plate below the Myanmar Plate can best explain all of the structural and tectonic features present in the IMR. The geological, structural and deformational featu- res of a region are strongly influenced by proximity to plate margins and the pre-existing tectonic history. Systematic study and analysis of the struc- tural and tectonic features imprinted in deformed rocks, along with petrological and geochemical studies, will therefore prove useful in unravelling the geological history and plate tectonic setting of a region. Continental margin basins, which are subse- quently transformed into orogenic belts and uplifted to become mountain ranges, have widely vari- able geomorphic, geological and tectonic settings. The Indo-Myanmar Ranges (IMR; formerly the Indo-Burman Ranges) of NE India (Fig. 1) encom- pass one such orogen, serving as an important key- stone in the tectonic development of SE Asia in that this orogen exhibits evidence of both collision and island arc subduction as a result of the convergence between the Indian and Myanmar (Burma) plates. The region above 268 N latitude is thought to be pre- dominantly characterized by collision mechanisms, whereas the southern region is dominated by sub- duction processes (Kayal 1995). The arcuate nature of the IMR, with its maximum convexity towards the west at around 248 N latitude, suggests active subduction in this region. Transcurrent motion and oblique subduction between the Indian and From:Gibson, G. M., Roure, F. & Manatschal, G. (eds) Sedimentary Basins and Crustal Processes at Continental Margins: From Modern Hyper-extended Margins to Deformed Ancient Analogues. Geological Society, London, Special Publications, 413, http://doi.org/10.1144/SP413.12 # 2015 The Author(s). Published by The Geological Society of London. All rights reserved. For permissions: http://www.geolsoc.org.uk/permissions. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics by guest on November 2, 2015 http://sp.lyellcollection.org/ Downloaded from