Structural geology related with fracture development and fluid evolution at Khao Tham Phedan, Nakhon Sawan Province, Central Thailand

Abstract

Owing to the complexity of structural geology and diagenetic history across Permian carbonates at western margin of Indochina block in Nakhon Sawan province, in the northern part of Central Thailand, there are still many poorly understood aspects. The overprint from several tectonic stages and multiphase fluid crossflows, means an integration of the study of structural evolution and characterisitc of isotopic signatures is required to better understanding the carbonate diagenetic and tectonic history across the region. The study area preserves two structural styles,  less and more intensely deformed zone along with two main fracture orientations, NE-SW and NNE-SSW. Carbonates textures show the rocks responed to deformation by forming cross-cutting veins, reactivated fractures, fractures exposed to multiple phases of fluid flow, and an abundance of microfractures in matrix portions of the rocks. 

Using stable carbon-oxygen isotope plots, the area’s diagenetic history is categorized into two stages. Firstly, deformation with two main relative timings: earlier deformation and reactivated deformation. The early deformation indicates a fluid history active during mid-mesogenesis (using the fluid classification of Warren et al., 2014). Secondly, NW-SE fracture reactivation led calcite precipitation from hotter fluids in re-opened fractures during reactivated deformation stage in the late mesogenetic also seen in some samples collected from deformed intervals with calcitic bed-parallel slippage. Subsequently, a new set of opened fractures were established during uplift when meteoric water entry enhanced dissolution and calcite precipitation as fracture fills in NW-SE and WSW-ENE, slickenside coats and speleothems. Additionally, there was an organic matter influence on bicarbonate ions in burial water, which is captured by isotope signatures in samples plotting in the “transitional to more negative carbon isotope” field (indicating possible catagenic fluids). Regionally, oxygen isotope signatures in the study area indicate higher fluid temperatures when compared to other areas within Indochina block. An early mesogenetic characterisitc is absent when compared with isotopic signatures from same age carbonates in the Saraburi and Sin Phu Horm areas.