PROVENANCE AND TECTONIC SETTING OF A COARSE GRAINED FLYSCH: CASE STUDY FROM ANDAMAN FLYSCH AT TRENCH SLOPE BREAK, NORTH ANDAMAN ISLANDS

Overriding controls of tectonics over sedimentation in a steeply dipping shelf –slope environment, is reflected from exposures of coarse-grained flysch in an uplifted subduction complex in North Andaman Islands, India. Gravity driven , coarse-grained, deep marine channel sandstones are exposed in patches over a 20 kilometers stretch across several islands with measured thickness , in excess of 250m. Series of fining-upward, 1-2 m cycles of coarse-grained channel sandstones with channel lag conglomerates, independent clast supported polymictic conglomerate beds , thin sand-shale intercalations and rare pelagic shale make up the broad facies association. The sandstones exhibit penecontemporaneous deformation features including dish and pillar structures, convolute laminae and load casts. Majority of the sand-rich system has been attributed to sandy debris flow, comingled with turbidites and classical turbidites. The sandstones are essentially Lithic wacke with abundant clay clasts of pebble to boulder sizes, ophiolite basement derived clasts of serpentine, ortho pyroxene ,along with fragments of chert, acidic igneous rock fragments, jaspers, older sandstone pieces and some opaque’s enmeshed in a rich clay matrix. Square-shaped monocrystalline quartz, polycrystalline quartz, zoned plagioclases along with the extraneous rock fragments suggests a multiple provenance for these deep water deposits.

Geochemical investigation involving major and minor element analysis of conglomerates and sandstones reflect derivation from island arc to active continental margin. Based on modal composition of sandstones, a magmatic arc is indicated. Rare earth element study supports a felsic magmatic origin for the clastics.

The data from outcrop studies supported by petrography and geochemical analysis, suggest that the nearest cratonic platform, the Myanmar shelf on the north ,provided the detritus . The rising subduction complex in the form of Andaman islands on the west contributed the magmatic basement derived fragments . Acidic igneous fragments are suggested to have been incorporated from the volcanic arc in the east. The magmatic and acidic igneous fragments supplemented the main clastic supply from the north.