Evidence of Quaternary and recent activity along the Kyaukkyan Fault, Myanmar

Abstract

Cenozoic right-lateral shear between the eastern Indian margin and Eurasia is expressed by numerous N-S trending fault systems inboard of the Sunda trench, including the Sagaing Fault. The most easterly of these fault systems is the prominent ∼500 km long Kyaukkyan Fault, on the Shan Plateau. Myanmar’s largest recorded earthquake, Mw 7.7 on 23rd May 1912, focused near Maymyo, has been attributed to the Kyaukkyan Fault, but the area has experienced little significant seismicity since then. Despite its demonstrated seismic potential and remarkable topographic expression, questions remain about the Kyaukkyan Fault’s neotectonic history.

In this study we document robust geologic evidence of fault activity along the Kyaukkyan Fault. Field investigation and interpretation of satellite imagery reveal deformation features developed along a fault system mostly indicative of Quaternary dextral strike-slip faulting. Clearly defined tectonic geomorphology, including fault scarps and linear valleys, are used to trace the northernmost and southernmost part of the fault. The fault’s central section is characterised by a complex pull-apart system, whose normal border faults show signals of relatively slow recent activity.

Dextral transtensional activity along the Kyaukkyan Fault is recorded by geomorphic features such as sag ponds, shutter ridges, offset and beheaded streams, triangular facets and low-sinuosity mountain fronts. The Quaternary age of activity is demonstrated by short-lived geomorphic features such as wind-gaps, offset and deformed alluvial fans, and even offset of man-made structures. In the Inle basin, alluvial fan successions along the easternmost mountain front reveal a vertical transition from faulted and folded alluvial fan sequences adjacent to pre-Cenozoic flanking ranges, to overlying gravels that appear less deformed. Conversely, a locally buried cross-basin fault system has fresh geomorphic expression even within the uppermost parts of the lacustrine/fluvial basin fill. This may indicate Quaternary migration of dominant fault deformation from sidewall faults to a cross-basin fault system, which is indicative of a mature strike-slip fault system and has been observed in other active strike-slip faults around the world and in analogue models.