The Tertiary geology of northern Thailand is dominated by sedimentary basins which have been previously interpreted as extensional, strike-slip and compressional features. A new regional interpretation of Landsat data, combined with field observations, shows an intimate relationship between these basins and regional strike slip faults. Most basins show half graben geometries in cross section, but in map view some occur as fault bend basins at the tips of NE-SW trending sinistral strike-slip faults. Others form a linked system of basins that define a 'lazy S' pull-apart geometry between offset sinistral fault segments. The relationship between basin geometry and the strike-slip faults is consistent with present day displacements, including offsets in the course of the Mekong River. This shows that there was no reversal in the sense of movement on these faults since basin formation in the Late Oligocene - Early Miocene.
The NE-SW trending sinistral faults mark the northern limit of basin formation. The southern limit of this style of basin formation is marked by the NW-SE trending dextral Mae Ping fault. To the south Tertiary basins extend into the Gulf of Thailand but are offset from the northern Thailand basins and have a distinctly different structural character and subsidence history, controlled by oblique extension. The confinement of the northern Thailand basins to a zone between NE-SW trending sinistral faults and the NW-SE trending dextral strike slip faults suggests that they formed in a laterally extruded crustal wedge, consistent with N-S compression associated with Indo-Eurasia collision.