The origin and evolution of complex transfer zones (graben shifts) in conjugate fault systems around the Funan Field, Pattani Basin, Gulf of Thailand

Abstract

Changes in dip province associated with conjugate fault sets produce complex transfer zones (locally known as graben shifts) between sets of convergent conjugate faults in the Pattani Basin. 3D seismic data over a well-developed graben shift geometry in the Funan Field area of the Pattani Basin has revealed details of the transfer zone development. In map view, the graben shift is seen as west dipping faults forming a wedge-shaped incursion into a zone of east-dipping faults. The wedge boundaries trend NE–SW and NW–SE at a high angle to the N–S striking normal faults. Within the transfer zones, faults only slightly overlap with their neighbours, with individual faults tending to widen upwards. The narrow regions of overlap indicate displacement transfer must occur between more widely separated faults, not neighbouring faults. The initial Oligocene–Early Miocene syn-rift fault pattern appears to have been strongly influenced by pre-existing basement trends as indicated by: (1) the restriction of west dipping secondary faults to the present day area of the graben shift, (2) the line of the NE–SW boundary to the graben shift coinciding with fault segment linkage geometries in two major syn-rift faults, and (3) curvature of minor fault tips into the transfer zone. The subsequent conjugate fault system developed during thermal subsidence shows that the dip asymmetry and map-boundaries of the fault dip panels are strongly influenced by the syn-rift fault geometry. There is no indication of active strike-slip faulting affecting the graben shift geometry. Fault displacement diagrams commonly show linkage between deeper and shallower displacement maxima, indicating early (Late Oligocene–Early Miocene), minor, west-dipping normal faults influenced the location and width of higher Middle–Late Miocene conjugate faults. Hence, two stages of structural inheritance led to the Middle–Late Miocene graben shift geometry.