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A Shale Detachment in Thailand: Evidence of Brittle Deformation

Abstract

A detachment can be defined as a horizon or zone, centimeters to kilometers in thickness, which mechanically decouples deforming rocks or sediments from underlying, non-deforming sequences. Shale detachments have been previously described as largely ductile in their mechanism of deformation, however, increasing resolution of seismic imaging and understanding of these zones suggest brittle, fault-dominated deformation may have a significant role in their behavior and the deformation of overlying fold and thrust belts. Dependence on seismic imaging and other indirect and low-resolution study methods has resulted from the lack of outcropping shale detachment zones for detailed study. Recent investigation details the structural style of an exceptionally well-exposed upper-level shale detachment zone in the Sap Bon Formation of the Khao Khwang Fold and Thrust Belt, Central Thailand. We use detailed structural analysis to investigate the deformational mechanisms of this exhumed detachment zone, as an analogue to active modern-day examples. We present detailed structural cross-sections through the detachment zone and use a multi-analysis approach to constrain geochemistry and deformational temperature given the potential influence on the rheology of the detachment zone when active. Carbon and oxygen stable isotope geochemistry is used to investigate the temperature of syn-deformational fluid flow while illite crystallinity analysis is used to estimate the maximum deformational temperature. Deformational complexity and intensity increases with proximity to the basal thrust of the detachment zone and is associated with increasing temperature of syn-deformational fluids and higher TOC values. Centimeter-scale ‘shale shear zones’ within the Sap Bon Formation accommodate significant shortening in complex, three-dimensional geometries and are associated with higher degree of illite crystallinity values. These shear zones exhibit higher strain than surrounding rock, and a quantitative relationship between finite strain (RS) and illite crystallinity is observed. These shear zones and associated structural features occur at a scale well beyond the resolution of seismic imaging. Finally, we present a suite new data and structural interpretation from other known shale detachments in the Chrystalls Beach Complex in southern Otago, New Zealand, and the Osen Røa thrust sheet in the Norwegian Caledonides, for comparison with results from the Khao Khwang FTB.