Polygonal fault systems are widely developed in fine-grained sedimentary successions and have been recognized in over 50 basins worldwide. They are normal faults with modest throw values (typically 10-100 m), organized with a characteristic plan form pattern that is crudely polygonal, but with considerable variation in specific planform patterns. They have been attributed to four genetic mechanisms: gravity collapse, density inversion, syneresis and compactional loading. Their strain characteristics allow them to be distinguished from tectonic normal faults. The strengths and weaknesses of the four genetic mechanisms are considered in the light of these strain characteristics. It is argued that syneresis offers the likeliest mode of genesis and best explains the local and global features of these extraordinary structures. The detailed physical mechanism driving syneresis remains poorly understood.
Decompaction routines are used in basin modelling packages to calculate sediment thickness and material properties such as thermal conductivity. However, compaction in nature is dependent on initial porosity, composition, and effective stress, and a considerable range of porosity-depth trends exists. Simple thermal modelling demonstrates that significant uncertainties (up to VRE ± 0.5) arise in predicted maturities due to this variation. Furthermore, the validity of using porosity loss as a measure of compaction is questionable because changes in solid volume can occur. Chemical reaction may increase or decrease porosity without changing sediment thickness, although an apparently smooth transition occurs from dominantly mechanical processes of porosity loss (e.g. grain rearrangement) at shallow levels to dominantly chemical processes (e.g. grain dissolution/cementation) at depth. That compaction and porosity loss are processes dependent upon effective stress, time and temperature is illustrated by the observation of overpressuring in the subsurface, comparison of experimental and natural compaction rates and analysis of porosity-depth trends for sediments of different ages. Mechanistic models of the processes involved in compaction (e.g. pressure solution) also indicate time dependency. Time-dependent models of compaction can be constructed, but these are difficult to incorporate into basin models as they cannot be run in a simple backstripping mode.
We present a novel method to reconstruct the pressure conditions responsible for the formation of fluid escape pipes in sedimentary basins. We analyzed the episodic venting of high-pressure fluids from the crests of a large anticlinal structure that formed off the coast of Lebanon in the past 1.7 m.y. In total, 21 fluid escape pipes formed at intervals of 50–100 k.y. and transected over 3 km of claystone and evaporite sealing units to reach the seabed. From fracture criteria obtained from nearby drilling, we calculated that overpressures in excess of 30 MPa were required for their formation, with pressure recharge of up to 2 MPa occurring after each pipe-forming event, resulting in a sawtooth pressure-time evolution. This pressure-time evolution is most easily explained by tectonic overpressuring due to active folding of the main source aquifer while in a confined geometry.
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