Two seismic reflection lines recently shot in the western part of the Worcester Basin suggest that its western boundary is defined by major syn-depositional normal faults, downthrowing to the east and active throughout Permo-Triassic times. Farther east, other faults can be seen within the basin. These moved at various times during the Permian and Triassic and profoundly affected patterns of sedimentation. The subsidence history of the basin can be explained by two phases of lithospheric stretching, one in the Permian and the second in early Triassic times, giving a cumulative stretching factor of about 1.2. This is consistent with the regional pattern of normal faulting provided that stretching was either one-dimensional, oblique to the N–S and E–W directions, or two-dimensional.