Geologic structures of the low-grade metamorphic terrane, Chon Buri coastal area, Eastern Thailand

Abstract

A low-grade matamorphic terrane along Chon Buri coastal area from Sri Racha to Sattahip, regionally consisting of fine-grained clastic sediments and carbonate rocks of Carboniferous-Permian age, was studied. These rocks were intruded by a Triassic granitic pluton that exposes and marks the eastern limit of the study area. The purpose of this study is to clarify the geologic structure and deformation phases using a structural analysis. The detail study here reveals three deformation episodes and two metamorphism events. The oldest deformation (D[subscript 1]) which accompanies a dynamothermal metamorphism (M[subscript 1]) is recognized by the elongate tight-to-isoclinal folds with fold axial planes being inclined moderately to the west and the axes trended somewhat north-south to northeast-southwest with crossed extensional joints and mormal faults. The microscopic study of c-axis of quartz grains in four orientated thin sections of quartz tectonite also shows a preferred orientation with a point maximum that conform these fold axes. The second deformation (D[subscript 2]) formed the NW-SE cross folds superimposing on the previous folds thus resulted as type-1 interference pattern of Ramsay (1967). This interference folding episode may occur together with, or being shortly followed by a granitic intrusion in Triassic and caused a thermal metamorphism (M[subscript 2]). The last deformation (D[subscript 3]) caused a low-angle reverse faulting which brought the metamorphosed stratified units to overlie the non-metamorphosed younger igneous body. The age of this faulting is inconclusive, though thought to be Middle-to Late Mesozoic or very early Tertiary judging from the unaffected Tertiary alluvial-fan unit lying further to the east.