*3.3. Cretaceous Collapse Grabens*

On the Base Cretaceous map (Figure 6), a characteristic polygonal pattern of grabens between the salt structures defines the flexure between polygonal sag basins or "pods" around the salt structures. The collapse grabens all appear in the Cretaceous above the "salt-saddles" or supra salt [19], and this may be related to a Cretaceous phase of regional tilting and inversion (Figure 7), as suggested by Reference [3] and others. Figure 4 shows a change from extension to compression in Early Cretaceous.

The WNW-ESE striking grabens seem to be the best developed, indicating a tensional stress direction of SSW-NNE. If this stress was compressive, the relative high saddle-ridges acting as salt feeders between the sag basins would be compressed and experience longitudinal extensional faulting in the upper part due to bending. Concurrently, subsidence would be increased in the sag-basins ("pods") and sedimentation would resume (Figure 7). This inversion phase would thus lead to increased salt movement and basin subsidence. Furthermore, this may have resulted in salt withdrawal from the pod-defining ridges when salt moved into the major salt diapirs and later caused graben collapse in these areas. Longitudinal narrow horsts are observed on top of some of the major salt structures (Figure 8b). These may have developed as normal faults simultaneously with the graben development, but later developed into horsts due to upward salt movement. As salt moves into the highest structure, salt is withdrawing from the lower connected saddle ridges. As a consequence, the extensional faults on these saddles developed into grabens due to net salt withdrawal here.

**Figure 6.** Base Cretaceous (BCU) time map provided by Lotos showing Cretaceous collapse grabens North of the Ula Fault. The Triassic sag-basins or pods are the polygonal flat areas between positive salt-structures and supra-salt collapse grabens. The arrows indicate the direction of salt movement away from the collapsing grabens accumulating into the growing salt diapirs. Red colors define the shallowest areas, and purple defines the deepest areas. Red line shows the Northern part of Model 2.

**Figure 7.** Principle sketch showing how a compressional stress regime would affect basins, salt saddle and diapir area. Extensional stresses above the saddle area act as a hinge between subsiding basins. Extensional faults will follow the length axis of the saddle areas and could collapse when compression ceases and renewed subsidence occurs. This could also explain the existence of crestal horsts seen on the major salt structures (Figure 8).

**Figure 8.** (**a**) Effect of Early Cretaceous compression from SW on an unfaulted surface. Blue arrows indicate areas with increased subsidence, red arrows show areas with increased uplift. In the saddle areas between basins and over the highest salt structures extensional faults will occur, as indicated by the stippled lines and marked by E. The background figure is based on the BCU surface and is manipulated in a photo editor to mimic a smoother pre-faulting surface. (**b**) Effect of renewed subsidence. Collapse grabens (most prominent along the WNW-ESE trend) have developed along the salt saddles. Horsts are developed on the highest salt structures. Background figure is an unedited BCU surface interpreted by LOTOS.
