Synrift magmatism in a Cenozoic rift basin from 3D seismic data, Wichianburi Sub-basin, Petchabun Basin, Thailand: part 2. How rift structure and stratigraphy modify intrusion morphology

Abstract

The Latest Oligocene–Pleistocene basic sheet intrusions in the Late Oligocene–Pliocene Wichianburi Sub-basin (onshore Thailand) and Eocene intrusions of the Ceduna Sub-basin (offshore southern Australia) provide examples of igneous intrusion architecture in rift and post-rift basin settings respectively that have been imaged by 3D seismic reflection data. These examples indicate that rift-related intrusions are overall smaller, have a higher aspect ratio, and tend to be more planar or transgressive in a single direction than the saucer-shaped sills that dominate in the post-rift setting of the Ceduna Sub-basin. The long-axis trend and dip direction of most sills in the rift setting investigated tend to conform with rift structure (i.e. fault strike directions and bedding dip and strike directions), and give a strong orientation bias, whereas orientations in the post-rift basin are more varied. The few saucer-shaped sills that formed in the Wichianburi Sub-basin are asymmetric and incompletely developed. A third area, the Kora region of the Taranaki Basin, is a region where intrusions are related to a volcanic centre. This gives the intrusions a mixture of circumferential and radial intrusions related to magmatic processes (e.g. loading by the volcanic edifice, magma chamber pressure) and rift-related influences. Rift-related sills commonly form stacked arrays. Fundamental factors that affect the morphology of sills include magma composition, volume, reservoir pressure and the mechanical stratigraphy of the host rocks. In rifts extra complexities affecting some sills, or parts of sills, include rift structure (faults, rotated, folded bedding), basin size and morphology, stress distribution and sedimentary architecture. Supplementary material: Thumbnail time–structure maps of 107 sills from the Wichianburi Sub-basin mapped from 3D seismic reflection data are available at https://doi.org/10.6084/m9.figshare.c.4620101