Brady P. Rhodes

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

Abstract The northern Western Ranges of Thailand contain mylonitic gneisses of the Doi Suthep Metamorphic Complex (DSMC). Near Chiang Mai, mylonitic foliations outline a southeast-trending dome, and stretching lineations trend N80°W. Low-grade metasedimentary rocks and Tertiary fanglomerates flank the 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 early Miocene, with detachment faulting, mylonitization, and uplift during Oligocene 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 that 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°–50°, and several outcrop-scale fold hinges suggest that the Mae Rim Formation is deformed into a set of non-cylindrical folds. Numerous, variably-striking, high-angle normal faults suggest either multiple deformations or deformation in a three-dimensional strain field. The discovery of an intraformational angular unconformity confirms that the Mae Rim Formation accumulated during folding.

Palaeocurrents and provenance of the Mae Rim Formation, Northern Thailand: implications for tectonic evolution of the Chiang Mai basin

The Chiang Mai basin is the largest of a zone of rift basins that define the Northern Thailand Basin and Range Province. This province lies within the Shan Thai block of NW Indochina that extruded out of the India–Asia collision. The Mae Rim Formation crops out along the western side of the Chiang Mai basin. Detachment faults border the Mae Rim Formation on the west and north and juxtapose it against the high-grade Western Ranges metamorphic complex and its low-grade cover of Palaeozoic Shan-Thai rocks. Understanding the age and tectonic relationships between rifting, detachment faulting, ductile shear, uplift and exhumation of the Western Ranges metamorphic complex is critical to understanding the underlying deformational mechanisms that accompanied Indochinas extrusion. To better understand the palaeogeography of the Chiang Mai basin and its relationship to the Western Ranges metamorphic complex, several partial sections were measured, and palaeocurrent and clast-composition data from the Mae Rim Formation were collected. Poorly organized, immature conglomerates of the Mae Rim Formation probably represent alternating debris flows, sheet-flood, and braided channel deposits. The Mae Rim Formation probably accumulated in range-front alluvial fans, and graded rapidly into lacustrine deposits within the buried central part of the Chiang Mai basin. Except for the youngest strata, where gneissic clasts predominate, virtually all clasts in the Mae Rim Formation came from the low-grade metasedimentary rocks that form the cover of the Western Ranges metamorphic complex. Imbricated clasts indicate eastward palaeocurrents away from the metamorphic complex. Most of the Mae Rim Formation accumulated during the uplift of the Western Ranges but before erosion had breached the detachment fault. These new data suggest a tectonic history for the Chiang Mai basin that involved distinct phases of extension from late Oligocene to Quaternary, with at least two intervening periods of basin inversion.

Evidence for coseismic subsidence events in a southern California coastal saltmarsh

Paleoenvironmental records from a southern California coastal saltmarsh reveal evidence for repeated late Holocene coseismic subsidence events. Field analysis of sediment gouge cores established discrete lithostratigraphic units extend across the wetland. Detailed sediment analyses reveal abrupt changes in lithology, percent total organic matter, grain size, and magnetic susceptibility. Microfossil analyses indicate that predominantly freshwater deposits bury relic intertidal deposits at three distinct depths. Radiocarbon dating indicates that the three burial events occurred in the last 2000 calendar years. Two of the three events are contemporaneous with large-magnitude paleoearthquakes along the Newport-Inglewood/Rose Canyon fault system. From these data, we infer that during large magnitude earthquakes a step-over along the fault zone results in the vertical displacement of an approximately 5-km2 area that is consistent with the footprint of an estuary identified in pre-development maps. These findings provide insight on the evolution of the saltmarsh, coseismic deformation and earthquake recurrence in a wide area of southern California, and sensitive habitat already threatened by eustatic sea level rise.