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Comparison Between Inverted Oblique Rift Sandbox Models and Phitsanulok Basin Structural Styles, Onshore Thailand


The Phitsanulok basin is the largest onshore rift basin in Thailand and has been a major hydrocarbon producer for over 30 years. The basin has a complex evolutionary history that includes transtensional rifting, followed by transtensional to transpressional reactivation, post-rift thermal subsidence, and late inversion. The effect of tectonic stress regime variations on structural style and basin architecture is still ambiguous. To reveal the influence of tectonic boundary condition changes on the Phitsanulok basin, a 30° underlapping, dextral pull-apart basin sandbox model configuration was used to generate a complex set of oblique-slip faults within the basin stepover. We focused our analysis on the stepover region only and subjected the model basin to i) transtension (reference model), ii) transpression and iii) inversion (transtension followed by transpression). The resultant fault geometries and basin evolution was quantitatively analyzed from top surface and internal section photographs. Models were reconstructed as 3D seismic volumes using Petrel and Cegal seismic interpretation software. The reference model developed 45°-75° dipping faults that had azimuths primarily aligned 0° to 35° clockwise of the master fault orientation. The transpressional model showed shallower 40°-50° fault dips and had azimuths closely aligned to the master faults. The inverted model developed curved inverted oblique-slip normal faults that had 50°-75° dips at depth and shallower 40°-50° dips; fault azimuths showed two populations of 0°-5° and 35°-40° clockwise from the master faults. The northern Phitsanulok basin shows regular extensional fault arrays that dip at >45° and compare most closely to the transtensional reference model. In contrast, the southern Phitsanulok basin faults shows shallower dipping faults, erosional unconformities and perturbed fault arrays that compare more closely to the inverted oblique rift sandbox model. Fault dip angle patterns may indicate that late transpression affected the southern Phitsanulok basin margin south of 16°N latitudes.