Abstract In the Mississippian Moncton Sub-basin, New Brunswick, Canada, 3D seismic data reveal structure within the Mississippian McCully gas field. The asymmetric basin, elongated NE–SW, is bounded to the SE and NW by major dextral fault zones. Curved en-echelon extensional faults strike ESE–WNW. Gentle folds trend NE–SW. Contractional faults show varied orientations. These features are all consistent with deformation of the basin in an environment of dextral transtension. Analogous faults in outcrops, ~13 km to the SW, constrain sub-seismic deformation, and show that deformation occurred soon after deposition, before complete lithification. Strike-slip basins undergo concurrent extension, contraction, and rotation producing complex kinematic history which can be unraveled using the heaves and orientations of fault arrays. Based on measurements from the 3D data set, a horizon in the gas field displays an apparent stretch of ~1.14, and a perpendicular apparent shortening of ~0.976. Faults are oriented, on average, at ~40° to the shear zone boundary. These values show that the basin was deformed in transtension with overall angle of transtension alpha ≈47° to 50°. However, part of this deformation post-dated the overlying Sussex Group; removal of this component suggests alpha was in the range 62–65° during deposition of the Horton Group. Variations across the shear zone indicate that deformation was heterogeneous in space and time; an initial large strike-slip component transitioned to more divergent deformation as the deformation zone widened. Fault curvature likely reflects propagation during development of these heterogeneities. Overall, the deformation reflects an oblique-rift environment with ~N–S extension, that developed between major dextral strike-slip faults. These results show that with 3D seismic data sets it is possible to reconstruct details of strain history in transtensional basins that are potentially useful for understanding basin compartmentalization and fluid flow over time.