Structural analysis of the accretionary complex in Sirikit Dam area, Uttaradit, Northern Thailand

Abstract

The structure, strain history, and metamorphism of a multiple deformed sequence of metagreywacke and phyllite in the Sirikit Dam area have been investigated. Tectonically enclosed in this sequence is the complex zone of mafic and ultramafic rocks known as the “Nan River Ophiolite Belt” which has been interpreted as the suture between Shan-Thai and Indo-China terranes. The whole sequence is in thrust contact with less deformed massive volcanics in the eastern part of the study area. The dominant structure in the sequence consists of strongly developed foliation (S d, slaty cleavage in phyllite and spaced cleavage in metagreywacke) which is axial planar to tight to isoclinal folds and dips shallowly towards the northwest. Transposition of the bedding is so strong that folds are rarely observed. The stretching lineation on the foliation surface (S d) plunges down-dip towards the west-northwest. S-C fabrics, asymmetric porphyroclasts and asymmetric pressure shadows developed in narrow zones of higher strain consistently indicate reverse sense of shear. The slaty cleavage (S d) is invariably crenulated and superimposed by a set of asymmetric kinks. The last ductile deformation style is a series of upright open folds trending north-northeast. Brittle deformation occurred later and produced northeast striking normal faults which are better developed in massive volcanics and are locally reactivated as strike-slip faults. The lithology, style of deformation, and metamorphism of the Sirikit Dam metagreywackes bear strong similarity to those observed in ancient and modern accretionary complexes in many parts of the world. The tight-isoclinal folds, S d, stretching lineation, and crenulations are interpreted as the direct result of non-coaxial progressive deformation due to east-directed accretionary thrusting and kinks are probably related to late accretionary thrusting. We correlate the upright open folds in the study area with the upright folds in the Triassic turbiditic greywacke sequence exposed along Lampang-Phrae and Phrae-Uttaradit highways. This late deformation is possibly the result of the collision between Shan-Thai and Indo-China terranes. Normal and strike-slip faults are due to Tertiary tectonic events related to the northward movement of the Indian plate.