THREE STAGE HISTORY OF THE THREE PAGODAS FAULT, WESTERN THAILAND

The Three Pagodas Fault Zone (TPFZ) encompasses a 50 km wide zone defined by numerous fault traces and strike ridges of Paleozoic limestone. The alignment of folds and the offset of a linear zone of Cretaceous to early Tertiary granitic rocks, indicate approximately 300 km of left-lateral slip along the combined TPFZ and parallel Mae Ping Fault. This slip either accommodated part of the mid-Tertiary extrusion of Indochina, or formed as a zone of late-Cretaceous to Paleocene transpression related to oblique convergence between the Indian and Shan Thai plates prior to and during collision. Recent remote sensing, field work and paleoseismic trenching, have focused on the younger right-lateral reactivation of the TPFZ. The TPFZ’s proximity to two large dams makes this part of the fault’s history important for seismic hazard evaluation.

Two Tertiary basins lie within the TPFZ. Both of these basins contain locally derived conglomerate and sandstone strata that accumulated as alluvial fans. The fanglomerates in both of these basins have been significantly dissected and dip up to 30°. The formation of these basins suggests a significant amount of right-lateral transtensional slip along segments of the fault. By correlation with similar fanglomerates in the rift-basins of northern Thailand, these fanglomerates may be early-mid Miocene, consist with an onset of right-lateral motion about 25 – 20 Ma.

Remote sensing and field investigation suggest that at least two strands of the fault are active with right-lateral slip. The most obvious of these is the Baan Song Kalia segment near Three Pagodas Pass, which dextrally offsets several drainages by a few to tens of meters. Trenching along the fault identified a west-dipping fault that cuts soil horizons with up to 10 cm of reverse-slip. A terrane of Triassic sandstone just west of this strand contains incised meanders indicating young uplift. Furthermore, focal mechanisms of two recent, nearby earthquakes suggest modern north-south compression – consistent with transpression across the TPFZ. These observations combined with the inversion of the mid-Tertiary basins suggest that modern slip involves right-lateral transpression. Thus, the TPFZ appears to have had three distinct episodes of slip: a early Tertiary sinistral period, followed by mid-late Tertiary dextral transtension and late-Tertiary to Holocene dextral transpression.