Neotectonics and related crustal deformation along Carbyn thrust fault, South Andaman, India: implications of the frontal surface faulting and propagation of tectonic activity towards Andaman trench

Abstract

Paleoseismic investigations have been carried out across an ~ 10-m-high scarp along the Carbyn thrust fault, that marks the structural contact between the overriding Ophiolite suite of the Cretaceous age and under riding Andaman Flysch group (Oligocene) in the outer-arc setting of the Northern segment of Sunda-Andaman subduction zone. The Carbyn thrust fault is one of the important concomitant faults in the accretionary prism of the Andaman subduction zone and the surface trace marks a N-S trending thrust fault with Easterly dip which extends for a length of about 30 km in the Eastern half of South Andaman, starting from Carbyn area up to Chidiyatapu (Southernmost tip of the South Andaman) through Kodiyaghat and Burmanallah areas. The thrust has emplaced dismembered Cretaceous Ophiolite slices over Oligocene siliciclastic turbidites (Andaman Flysch) almost in a N-S trending linear zone of deformation. The 10-m-high scarp expressed at the interface between Ophiolite suite and Andaman Flysch group borders the topographic front of Ophiolite suite and breaks the Quaternary and late Holocene succession almost up to a length of 3.7 km from Carbyn to Burmanallah area, registers the youngest surface deformation along the thrust. The scarp is abrupt and sharp trending in N-S direction shows variations in displacement along the strike and has preserved its initial geometry without erosion. Trench exposure and Georadar survey across the scarp at Carbyn area reveals a very low angle thrust fault that cuts the bedrock surface (Ultramafics of Ophiolite Suite) rocks against the young alluvial surface deposited by Prothrapur Nallah and Carbyn creek at Carbyn area. The thrust fault shows flattening of the fault geometry around 16° at the base of the trench and dips 8–10° at near surface. The expression of the scarp at the frontal position of Ophiolite Suite and verging towards the West displacing late Holocene sediments reflects advancing of thrusting and propagation of tectonic activity towards Andaman trench.