Fault reactivation and halokinesis: an example from the Penobscot 3D seismic volume, offshore Nova Scotia, Canada

Polyphase deformation on passive margins, including fault reactivation, has been documented globally. These processes form complex structures that can be integral to petroleum systems and can provide essential constraints on the kinematic and structural evolution of rifts and passive margins. In some cases, inversion structures can also be used as global markers for far-field stress changes and help us to understand how plate tectonics operates on Earth. Despite the importance of reactivated faults, their identification, extent mapping, controls on kinematic evolution, and knowledge of interaction within fault populations are often poorly constrained. As such, there is need for detailed investigations of such structures, including their relationship with halokinesis, which can lead to complex and sometimes misleading structural observations.

We present a new structural interpretation of the Penobscot 3D seismic reflection survey, and associated relay ramp, imaged offshore Nova Scotia, Canada, down to ~3.5 s TWT, constrained by two exploration wells. The relay ramp comprises two dominant faults that dip approximately SSE and are associated with smaller antithetic and synthetic faults. The wider fault population is dominated by ~ENE-WSW striking normal faults that dip both NNW and SSE. The two major normal faults display evidence for reverse deformation in their lower portions (below ~2.5 s TWT), which manifests as anticlinal folding and reverse offsets. However, in their upper portions the faults display normal offset. Smaller faults tend to only affect the uppermost strata and do not show evidence of reactivation. Analysis of fault throw demonstrates that movement on the two main faults was coupled during both the reverse and normal deformation intervals. Through our structural analysis and previous regional interpretations of widespread salt kinesis, we determine that the observed style of deformation likely occurred due to normal (extensional) reactivation of reverse faults that had initially formed due to halokinesis of underlying salt. The timing of salt movement broadly corresponds to documented times of kinematic reorganisation on many Atlantic margins, and thus salt kinesis may have been in response to this. The kinematic dichotomy with depth along the two dominant faults is important to document as this style of polyphase reactivation may go unrecognised where seismic data does not image the full depth of a structure. Therefore, reactivation may be more widespread than previously thought if only uppermost parts of structures have been imaged. The interpretation of salt as an important contributor to kinematic reactivation of faults is crucial as it likely provides a mechanism to explain inversion at many other locations globally.