SYNTECTONIC SEDIMENTATION IN THE CHIANG MAI BASIN, NORTHERN THAILAND BASIN AND RANGE PROVINCE

Chiang Mai Basin (CMB) is the largest of the intermontaine basins of the Northern Thailand Basin and Range Province. Most of the Tertiary basin fill is buried beneath modern floodplain deposits; however interbedded conglomerate, sandstone and minor mudstone of the Mae Rim Formation (MRF) crop out in the low hills that border the western side of the basin. In order to understand the provenance and paleogeography of the basin during deposition of the MRF, we measured several partial sections, and collected paleocurrent and clast-compostion data throughout the basin.

Low-angle normal faults border the MRF on the west and north and juxtapose it against the Tertiary high-grade Doi Suthep metamorphic core complex (DSMC) and its low grade Paleozoic cover. Poorly organized, generally lenticular, variably clast- and matrix-supported, texturally immature conglomerates in the MRF probably represent alternating matrix-rich and clast-rich debris flows and fluvial channel-fills. The generally disorganized, texturally immature, but quartz-rich sandstones may largely represent clast-poor debris flows. Numerous outcrop-scale normal faults cut the generally tilted MRF.

Except for a small region in the west-central part of the basin, where gneissic clasts predominate, virtually all clasts came from the low-grade metasedimentary rocks that form the cover of the DSMC. Paleocurrents likewise show an eastward transport from the core complex. However, erosion has removed most of the cover rocks. The Mae Rim Formations most likely accumulated during the uplift of the core complex at a time when the cover had been breached only locally. However, age control on the MRF is poor. It underlies dissected and faulted alluvial fans with Mid-Miocene vertebrate fossils; but its base is nowhere exposed.

We suggest that the Mae Rim Formation marks a distinct pulse of uplift in the DSMC to the west. During reconnaissance study of a small basin to the east of the CMB, we discovered deposits similar to the MRF, suggesting that the same event affected a wide region of northern Thailand.