Part I- Basic Principles in Salt Tectonics

7- Growth Faults, Roll-overs, Grabens and Fracture Zones

Contents:

7.1- Growth Faults and Roll-overs

7.2- Growth Faults and Compensatory Grabens

7.3- Growth Faults, Roll-overs and Fault Welds

7.4- Fracture Zones

7.1- Growth Faults and Roll-overs

When extension is associated with halokinesis (salt tectonics), growth faults, roll-overs, compensatory grabens and other extensional structures are quite common on the seismic lines. You are reminded of the following examples:

(i) The hade of the listric faults ;

(ii) The roll-over geometry ;

(iii) The weld formation ;

(iv) The magnitude of extension and

(v) The cartography of fault planes.

Fig. 204- On this tentative interpretation of a seismic line of the Angola offshore, the rotation of the down-thrown blocks (roll-overs), as well as, the thickening of the sedimentary packages toward the faults looking to the right (seaward) are coherent with the decreasing hade of the fault planes. Such a rotation creates a tectonic disharmony at the top of the salt (when the salt is present), which is characterized by downlap seismic surface. This geometry implies an arcuate cartography of the fault plane. A smaller fault weld can recognized at the toe of the first growth-fault. Along the other growth-fault (on the right) several accommodation faults were developed to solve the volume problem created, partially, by the extension and, partially, by the salt flowage. According to the Goguel’s law, during deformation the volume of the sediments must be constant. In spite that this law is an approximate law (lost of volume by compaction of the sediments and salt dissolution), it can be considered as an application to Geology of the first law of thermodynamics or law of conservation of energy. The pull-ups and pull-downs at the top of the infra-salt strata emphasize lateral velocity variations on the cover (salt + overburden).

Fig. 205- On this tentative interpretation (Angola offshore), due to a down to basin salt flowage (leftward) and an extensional tectonic regime, growth-faults were developed on the western flank of salt domes. The associated compensatory subsidence is, easily, deducted from the thickening of the sedimentary intervals against the growth-fault planes. The rotation of the sediments created a potential void, which is filled by normal faulting.

Fig. 206- Roll-overs are antiform structures developed in the down-thrown blocks of growths faults, as illustrated on this tentative interpretation of a GOM seismic line. They are induced by salt flowage, in association with an extensional tectonic regime. Generally, in a sandy-shale sedimentary realm, they are potential hydrocarbon morphological traps by juxtaposition. Salt relics of the pristine salt layers can be found in the up-thrown faulted blocks either in continuity or separated by fault welds.

7.2- Growth-Faults & Compensation Grabens

Fig. 207- On this tentative interpretation of a seismic line of the conventional Angola offshore (water depth less than 200 meters), the salt, completely flew away. It developed a large fault weld between the two growth fault planes and up-dip. Such a flowage created a potential void obliging the sediments to lengthen in order to fill it. The normal-faults extending the overburden created a compensatory graben and thereby a tectonic hiatus at the weld disconformity level.

7.3- Growth-Faults, Roll-overs & Fault Welds

Fig. 208- As on the previous tentative interpretation, a compensatory graben, located in the down-thrown block of the major growth fault, induced by a basinward flowage of the salt, is recognized, easily, on western part of this tentative interpretation of a Congo offshore seismic line, just above a large fault weld.

7.4- Fracture Zones

Fracture zones are zones of fragility of the continental crust of the super-continents, which can favour the breakup of the lithosphere. In offshore, and particularly in Atlantic-type divergent margins, old fracture zones are, often, remobilized directly or indirectly, in associated with transform faults and changes in the rate of sea floor spreading. South Atlantic major fracture zones are well known not only nearby the Mid-Atlantic ridge, but below the margin sediments as well (fig. 209 to fig. 212).

Fig. 209- The major fracture zones between Gabon and Angola trend roughly NE-SW. The principal rift-type basins trend North-South. Between the Angola FZ and Mijuca FZ, for confidential reasons, the fracture zones are not indicated. Several geoscientists hypothesized that fracture zones pre-date and ,partially, control the sea floor spreading and the salt tectonics of the area, as depicted in fig. 210.

Fig. 210- In the 70’s, Petrangol and Total‘s geologists understood the influence of the major fracture zones on salt tectonics in the Kwanza geographic basin (Angola onshore & conventional offshore). ¨The fracture zones created different geological provinces characterized by particular salt tectonics and petroleum systems. In fact, a glance at the cartography of the salt walls (in purple) and the Miocene depocenters, strongly, emphasize the importance of the fracture zones (see next tentative interpretations).

Fig. 211- On this tentative interpretation, the bottom of the autochthonous salt is, mainly, represented by a salt weld. Nevertheless, on the right side of the tentative interpretation, a sharp step, with a magnitude of more or less 1.7 km highlights a major fracture zone. On the left side, the step of the bottom of the salt is apparent. In reality, it seems to correspond to the pull-up created by the allochthonous salt. Two translation onlap surfaces (see fig. 201 and 202) suggest a significant seaward translation of the overburden.

Fig. 212- The Cegonha structure, located offshore Luanda, seems to be induced by the reactivation of a major fracture zone (Hotspur fracture zone), in which different branches can be recognized as illustrated on this tentative interpretation. The original salt domes seem to have been shortened by a compressional strike slip movement of the fracture zone, which created small, but significant steps on the bottom of the salt.

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