Structural development of the Mid-Tertiary Doi Suthep Metamorphic Complex and Western Chiang Mai Basin, Northern Thailand

The northern Western Ranges of Thailand contain mylonitic gneisses of the D oi Suthep Metamorphic Complex (DSMC). Near Chiang Mai, mylonitic foliations outline a southeast-trending dome, and stretching lineations trend N80 deg rees W. Low-grade metasedimentary rocks and Tertiary fanglomerates flank th e dome on the east, and widespread chloritic breccia marks the intervening Doi Suthep detachment fault. We interpret that the metamorphic complex developed between Triassic and ea rly Miocene, with detachment faulting, mylonitization, and uplift during Ol igocene to Miocene. Within the DSMC, dikes of mutually cross-cutting, hence coeval, granitic pegmatite and aplite cut mylonitic orthogneiss. Although the contacts of the dikes-are sharp, the dikes contain a mylonitic fabric t hat parallels the fabric in the surrounding mylonitic orthogneiss; thus, we infer that intrusion occurred during mylonitization. East of Doi Suthep, Tertiary fanglomerates of the Mae Rim Formation overlie the Doi Suthep detachment fault. Variable strikes, with dips of 10 degrees -50 degrees, and several outcrop-scale fold hinges suggest that the Mae Rim Formation is deformed into a set of non-cylindrical folds. Numerous, varia bly-striking, high-angle normal faults suggest either multiple deformations or deformation in a three-dimensional strain field. The discovery of an in traformational angular unconformity confirms that the Mae Rim Formation acc umulated during folding. (C) 1999 Elsevier Science Ltd. All rights reserved .