Thin-skinned thrust-fault complex in the Phu Quoc Basin, SW Vietnam
Authors: Madsen Emil Bach , Boldreel Lars Ole , Pedersen Stig Schack 
Published: May 2016

The Phu Quoc Basin is a Late Jurassic to Early Cretaceous foreland basin located SW of Vietnam in the eastern part of the Gulf of Thailand. A structural model for the basin is provided based on the interpretation of a commercial 2 D digital multi-channel reflection seismic survey in a rectangular open grid with an approximate distance between the lines of 8 km. The survey consists of 29 profiles totalling about 1500 km with a range of 7 sec TWT depth.

The aim of this investigation is to present a structural model for a virgin area, which has not been subjected to exploration drilling. For any future investigation of the potential of this basin it is of crucial importance to know the structural framework, without which all further investigations will be waste of investments.

The deformation of the Phu Quoc Basin took place during the orogenic build-up of the Kampot Fold Belt (Fyhn et al. 2010) in the early to middle Cretaceous, which resulted in the formation of a thin-skinned thrust-fault complex. The main thrust fault elements are flats and ramps. In the distal part of the complex piggyback basins occur with a translation of more than 3 km. The main part of the complex is concealed under Neogene marine deposits. The magnitude of displacement of thrust sheets with an average thickness of 300 m varies from 3 to 8 km. The number of ramps increases towards the proximal part of the complex, and an increasing number of hanging-wall anticlines occur, some of which developed into antiformal stacks. Duplex structures initially lifted the top of the tectonic complex to a higher level, which resulted in a deeper level of erosion into the tectono-stratigraphic units. An example of this is the Hon Mau island in the Nam Du archipelago, where the thrust deformation elevated Permian pyroclastic rhyolites up to a surface-near position.

The thin-skinned thrust-fault model is based on a detailed balanced cross-section analysis. The structural information can be provided for future investigations of the Phu Quoc Basin, either commercial or scientific.

Introduction

A fold belt is generally regarded as a mountain range where rock units have been thrust and folded up into topographic ridges parallel to the structural strike of the thrust faults or directions of the fold axes. The deformation event responsible for such a mountain belt is referred to as an orogeny. However, it is not always that an orogenic event in a present day situation results in a topographic mountain range. In some geological settings the deformation structures may be concealed by post-orogenic deposition. This is seen 1) where a fold belt or part of it is located in an area subjected to post-orogenic subsidence, or 2) if a mountain range was subjected to post-orogenic erosion and thus covered by younger deposits above a post-orogenic unconformity. The structures and architecture created by an orogeny can be described under the heading: a tectonic complex. Three criteria are required to define a tectonic complex: 1) the complex comprises deformation of a substratum of sediments and sedimentary rocks due to compression caused by plate tectonic processes, 2) the structures are characterized by a unique style and architecture, 3) the complex is a structural identity with well defined boundaries in space and time. Furthermore the formation of a tectonic complex is the result of sequential development, which implies the presence of superimposed structures and poly-phase deformation. Finally erosion truncates the tectonically elevated deformed rocks.