Messinian Structures

The Messinian unit is characterized by distinctive sets of structures, identified as low amplitude, weakly asymmetric folds and thrust faults (Figure 4.2a and 4.3), following the interpretation of Bertoni and Cartwright (2006). These structures are most prominent within the upper units of the Messinian (M6 - M3) and they define deformation in the Messinian evaporite which does not wholly conform to that of the Plio-Quaternary overburden (Gradmann et al., 2005, Bertoni and Cartwright, 2006, Netzeband et al., 2006b).

These sets of Messinian thrust related structures are characterized by an imbricate geometry arrangement. They strike at similar orientation to the post-Messinian thrust faults and folds (140^o-170^o Figure 4.2a). They do not occur uniformly throughout the study area, but are extensively developed within the northern region of the data set.

This study uses a single horizon reflection (M6) and 3 selected vertical seismic sections to describe examples of thrust faults in the upper Messinian intervals located within the northern region of the Gal C survey (Figure 4.19). These case study structures are collectively referred to as Structure M and are representative of the structural style of the Messinian thrust faults

The structures in Gal C

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because they are almost isolated from the region where the post-Messinian detached (Figure 4.2).

Structure M consists of several thrust faults number F1-F15. These faults are curvi-linear in map view, with a change in separation distance along their widths and having the same strike orientation to the post-Messinian ramping faults (NW-SE), with only a few minor faults striking NNW-SSE. Individual thrust faults within the echelon system have an along strike length ranging from 0.5km to 9km and they tend to soft-link or transfer shortening to each other along strike, common for imbricate faults located in a compressional setting (Dahlstrom 1970; Morley et al. 1990).

Seismic sections along Structure M show that the faults are characterized by hanging wall anticlines, and footwalls which are synclinal (Figure 4.21). The hanging walls vary from symmetric to asymmetric generally verging northeast. This vergence direction is opposite to the regional compression expected from downslope gravity gliding of the Levant margin (Figure 4.1). The faults dip between 07^o to 15^oSW, and they ramp through M45 and M50, with variable detachment above and below Horizon M45 (Figure 4.20). Individual thrust faults within the imbricate array have throws ranging from a few ms to 100 ms (TWT) as maximum values. The hangingwalls and footwalls of some of the thrust faults are observed to terminate onto Horizon M as described by Bertoni and Cartwright (2006), while others have their upper tip terminating onto the hangingwall of other faults in succession. Some appear to have their hangingwalls almost horizontally transposed above the footwall, similar to fault bend folds (Rich 1934), while others show absence of roof detachment, and resembling fault propagation folds (Suppe, 1983, 1985).

4-65 Figure 4.19: Time dip horizon (M6) map of the northern segment of the Gal C survey showing the case study thrust faults (Structure M). SF represents Strike-slip fault, TF-Thrust fault in the post-Messinian, MF-Thrust fault in the Messinian.

The structures in Gal C

4-66 Figure 4.20: Map of Horizon M5 in TWT (ms). (a) Time dip map (b) Time dip structural map. Traces of Messinian thrust faults (F1-F15) in echelon arrangement. Map location is shown in Figure 4.19..

Line a0- a2 show locations of the 3 sections in Figure 4.21. Blue and green colours represent structural high. Red and yellow colours structural low. Contour interval: 25ms.

4-67 Figure 4.21: Selected seismic sections perpendicular to structure B illustrating along strike changes in structural arrangement along strike. Locations of seismic lines a0-a2 (a-c) in Figure 4.20. Sections have an approximate vertical exaggeration of 2. Horizons M5, M6 and IPM1 are regional stratigraphic horizons.

The structures in Gal C

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The thrust faults within Structure M, show changes in structural style and arrangement from independent ramp anticlines to overlapping hybrid duplexes (Mitra 1986), along strike (Figure 4.20 and 4.21). The simplest form is observed towards the north western end of Structure M (Figure 4.20 and 4.21a). In this zone, the thrust faults (F1 to F6) along Horizon M6, tend to have almost equal spacing distance, ranging between 800 -1000 m with exception of F4 and F5 which have a spacing distance less than 400 m (Figure 4.21 a). The apparent dip slip displacement of each of the thrust faults (F1-F6) at the north-western zone is between c.

100 m and c. 250 m (Figure 4.21 a). F1 to F6 in pairs closely resemble those of the independent ramp anticlines (Mitra 1986), because their relative spacing distances are greater than their displacements. Most of the faults (F1-F6) at this zone are observed to detach above Horizon M5 and they tend to overlie other minor faults similar to the duplex structures described by Dahlstrom (1970) (Figure 4.21 a).

The mid to south eastern region of Structure M shows a more complex fault imbrication compared to the north western end (Figure 4.21 b and c). The relative spacing distances between the faults (F1-F3, F7-F15) in these zones are generally lower than those in Figure 4.21 a. The relationship between F2, F8-F10 in the mid zone (Figure 4.22 b), and F1, F7,F11-F12 close to the south eastern end of Structure M (Figure 4.22 c), closely approaches the partial overlap to complete overlap duplex type (Mitra 1986). F1 and F11 are characterized to have their upper tip terminate onto Horizon M at the central zone (Figure 4.21 b), this may indicate Horizon M is an erosional surface as suggested by (Bertoni and Cartwright 2006).

The profile in the south eastern end of Structure M (Figure 4.21 c), shows that F1 and F7 tend to decrease in dip towards their upper tips such as they appear to have an upper detachment zone. F11 at this zone, has its upper tip unto Horizon M and it‘s partially overlapped by F1.

F12 on the other hand has its tip within the unit between Horizon M and M6 and it is

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overlapped by F11. Majority of the faults are observed to detach below M5, with only a few that detach above 1t (Figure 4.21 c).