With progressive Top Salt subsidence, the Z3 Stringer forms large gentle-

gentle-wavelength folds below the subsided areas (Fig. 4.24). Exceptional examples may occur where the stringer could also break below the thinnest zone of the subsided region simultaneously during the development of these regional folding (Fig. 4.24d).

This could be interpreted to be related to local weaknesses within the stringer such as small faults or fractures that help to break apart the stringer at this stage. This suggests that stringer brittle deformation and development of folding can occur simultaneously during the same deformational event (e.g., Zulauf and Zulauf 2005).

Stage 4: During the next stage of the deformation, the Top Salt continues to subside

and the cross-sectional area of the salt section is reduced. The Z3 Stringer is stretched and begins to have low structural relief (Fig. 4.24e). In a similar manner to the previous stage, during flattening and stretching, the stringer may exceed its tensile strength and break laterally. The shallow dips of the stringer below S1 and S5 suggest extreme stretching and flattening prior to the stage of lateral extension. The regional downward displacement by Top Salt and the overburden squeeze the salt section and induce high lateral salt flow, which ultimately reduces the chances of fold formation below the hinge zone.

Stage 5: Continuation of Top Salt subsidence reduces the thickness of the Zechstein salt by lateral salt flow, which causes the stringer to break significantly into single isolated fragments (Fig. 4.24f).

160 Stage 6: The stringer fragments are dragged and carried away from the subsided

regions into the flanks of the synclinal structures. This lateral movement of the stringer fragments is estimated to reach up to 4–5 km in the present study area (Fig.

4.24f).

161

Fig. 4.24: Seismic profiles (a–g) at variable Top Salt subsidence magnitudes were selected to represent the stringer evolution from early salt tectonics to nearly the welding stage. The downward displacement magnitude of Top Salt (subsidence rate) increases from (a) to (g).

162 4.6.7.3 4D evolution of the Z3 Stringer

Areas of stringer deformation below S1 (Fig. 4.12) and S5 (Fig. 4.13) were selected to represent the final 3D configuration of the stringer structural styles at the late stage of Top Salt subsidence. The initial configuration of the Z3 Stringer before any Top Salt displacement is assumed to be flat (Fig. 4.25a).

Early salt tectonics created linear Top Salt synclines and anticlines. In the early stage of subsidence, the stringer formed elongated large-wavelength folds with axes parallel to the regional syncline (Fig. 4.25b). Examples of these folds can be observed below regional synclines of modest subsidence (Fig 4.24c and d). With progressive Top Salt subsidence, the thickness of the salt section is reduced by salt evacuation and therefore higher-velocity salt flow was generated, which consequently stretched and flattened the Z3 Stringer. This resulted in linear to irregular breaks almost parallel to the regional structure (Fig. 4.25c).

This alignment is similar to the formation of subparallel fracture alignments when uniaxial to sub-uniaxial extension is applied to the competent layer during the early stage of deformation, with the x to y strain ratio of 1:0 and 1:0.5 (Abe et al. 2013).

The breaks within the stringer are then connected and linked up to form isolated stringer fragments, which were later carried laterally by salt flow for 4–5 km (Fig.

4.25d).

The final configuration is represented by a large zone of discontinuity where salt only yields smaller stringer fragments. The stringer is almost flat to gently folded in the middle of the subsided area and has a gentle, open inclined fold structure below the flanks of the regional syncline.

163

Fig. 4.25: 3D sketch of the structural evolution of the Z3 Stringer with progressive Top Salt subsidence from (a) to (d).

164 4.7 Conclusion

Internal salt structures were mapped using 3D seismic reflection data from the Silverpit Basin in the Southern North Sea with an aim of evaluating the kinematic evolution of the stringer in areas of salt subsidence related to regional basin buckling. The study concludes with the following:

 The presence of high-acoustic-impedance Z3 Stringer enables tracing the

internal geometry of the salt structure and has significantly helped in improving our understanding of internal salt dynamics within areas of salt subsidence.

 The deformation of the intra-salt stringer is largely influenced by the regional deformational history.

 The structural propagation of the intra-salt Z3 Stringer below areas of Top Salt

subsidence begins with the formation of gentle, long-wavelength folds, followed by lateral stretching, re-flattening, and finally fragmentation and lateral separation of the stringer fragments for distances close to 4–5 km.

 Top Salt structural and salt thickness surfaces are important tools to predict

the internal structural style within the salt.

 In areas where the salt is subjected to high rates of subsidence, extensional

related structures such as boudinage, lateral stretching, and fragmentation of the inclusions are generated. However, contraction-related structures (e.g., folds) are created in areas where Top Salt forms anticlines and the salt section is thick.

 The stringer is fractured and displaced laterally below regions of high subsidence (S1 and S5). However, modest Top Salt subsidence was still not

165 able to fracture the stringer (e.g., stringer below S2, S4), suggesting that the stringer brittle deformations were only initiated during the major salt tectonics in the Cenozoic.

 In areas where the salt section is thick, the stringer is easily folded and more

resistant to brittle deformation; thus steeply inclined fold structures, which in most cases were not seismically imaged, are likely to be continuous or thinned during folding.

Chapter 5: Kinematic evolution of the Z3 Stringer in areas of salt thickening

166

Chapter 5: 3D kinematic evolution of the