Wrench Faulting in Andaman Forearc Basin, India

Abstract

ABSTRACT The study is based on the landsat, surface geologic, multifold CDP seismic and subsurface data obtained from drilling. Broad tectonic framework had been known from earlier work. The present study has concluded that under thrusting of the Indian plate is the west of the Andaman-Nicobar Islands, India, has produced conjugate shear fractures which are expressed as wrench faults. Movement along these wrench fault in turn, has generated en echelon folds in the fore arc basin that are prospective exploration targets, specially, the late Cretaceous to Oligoceme sequence. INTRODUCTION The paper deals with the wrench faulting is the Andaman forearc basin and the resulting structural features, and analyses the structural pattern derived from the regional primary compression caused by the eastward oblique convergence of the Indian plate against the Eurasian plate. It also attempts to assess the hydrocarbon potentiality of the en echelon anticlinal structures that have formed due to wrench movements. The area studied covers 20,000 sq. kms of the offshore region to the east of the Middle and South Andaman islands including the Ritchie. Archipelago and the shallow shoal area know as Invisible Bank, occurring to the east of the Rutland group of islands(Fig. 1). The term "wrench fault" is Meant here in the same sense as used by Anderson (1951) and Moody (1956), without any genetic connotation. This refers to the faults with dominant relative horizontal movement of blocks for teas of kilometers or more and have more or less vertical fault plane, and is synonamous with strike slip and transcurrent fault. The types "right lateral" and "left lateral" wrench are those where the relative movements of blocks are clockwise and anticlockwise respectively. Phenomenon of regional wrench faulting and its effect have great importance is petroleum exploration. Among the important Investigations, Moody (1956) studied the wrench fault tectonics and analyzed the successive order of wrenching and fold that could develop due to regional compressive force or by couple. Wilcox et.al (1973) studied laboratory clay models and worked out the pattern of development of en echelon folds, antithetic and synthetic faults faults by wrench movements. It was pointed out that in case of non parallel movement of crustal blocks, the convergent and divergent wrenching takes place. Convergent wrenching tends to enhance compressive wrench structures namely, folds, conjugate strike-slip faults and strong convergence produces reverse faults and thrusts while diverging wrenching causes normal faults and tensional structures. He also concluded that the structures and structural traps in a wrench terrain, generally can be predicted with a high degree of accuracy. Moody (1973) discussed application of basin wrench tectonics in petroleum geology and is of the opinion that any sedimentary basin which is traversed or bounded by a wrench fault system would be a good prospective petroleum province, provided that the sedimentary fill has the requisite vertical and lateral faces variability. Harding et.al (1983) and Harding (1985) have described negative and positive flower structures in seismic sections and related them to specific modes of formation. Their distinct characteristics have been used to identify certain structural styles.