Synrift magmatism in a Cenozoic rift basin, from 3D seismic data, Wichianburi Sub-basin, Phetchabun Basin, Thailand: part 1. Distribution and timing of intrusions and forced folds

Abstract

The Late Oligocene–Pliocene Wichianburi Sub-basin, Phetchabun Rift, Thailand has undergone three phases of development: Late Oligocene–Early Miocene (c. 16 Ma) east–west extension on predominantly NNW–SSE- to north–south-striking faults; 16 Ma–c. 3 Ma NE extension on NNW–SSE-, north–south- and NE–SW-trending faults; and 3 Ma to Present post-rift subsidence. The basin was also affected by intrusive and extrusive igneous activity from 24 to 2 Ma. The relationships among the structure, rift basin stratigraphy and sheet intrusions are unusually well documented as a result of extensive hydrocarbon exploration drilling and 3D seismic data, together with the production of hydrocarbons from igneous sills. The oldest igneous activity (c. 24–13 Ma) lies SW of the Wichianburi Sub-basin, where extrusive rocks exposed at the surface have been dated by K–Ar and 40Ar/39Ar methods. The overall igneous activity youngs from the south of the basin to the north (11–9 Ma). Numerous domal folds occur across the basin; the sill and laccolith dimensions, the rate of fold development, the variable timing of fold formation and the fold geometry all indicate that the folds are related to magma emplacement, not tectonic inversion. Fold formation occurred at c. 16–15 and 12–11 Ma in single or multiple stages. The timing of folding, coupled with the radiometric dating of some sills (K–Ar, 39Ar/40Ar), indicates that the greatest amount of intrusive activity occurred between 16 and 11 Ma. The 16 Ma date coincides with the switch from east–west to NW–SE extension, which promoted dilatancy within the upper crust along a range of synrift fault orientations and some pre-rift trends. The burial history of the main depocentre shows that Miocene sill emplacement occurred at depths