Folding of layered cover due to dip-slip basement faulting
File(s)
Author(s)
Ameen, Mohammed Sulaiman
Type
Thesis
Abstract
This study deals with forced folding in a layered cover due to dip slip basement faulting. It is aimed at determining the possibility of deducing the type of dip slip basement fault from surface exploration in the layered cover. Many experiments were carried out to investigate the effect of variation in geometry, angle of dip and displacement of basement faults on the strain pattern andmodes of failure in the cover rocks. In the experiments thecover rocks were simulated by a wax multilayer and the faulted basement by pre-cut wooden blocks. The results showed a significant difference in strain pattern and modes of failure and geometry of major structures (forced folds) in the cover rocks arising from different types of basement faulting. It is concluded that forced folding can developover high angle normal and reverse faults (dip ) 450) where the cover accommodates itself to the faulting by rigid body rotation and internal deformation. The deformation is highlyconcentrated in the vicinity of the basement fault and dies out away from these areas. The extensional strain pattern inthe cover rocks over planar normal fault is found to be dominated by later parallel extension in a direction approximately perpendicular to the strike of the basement fault. However local layer parallel contraction develops inthe inner arc of syriclinal folds which form over curviplanar normal faults. Extensional strain pattern in models of forced folds resulting from reverse basement faulting is heterogeneous in both space and time. Forced folding oververtical dip slip fault is a transitional case between normal and reverse faults. Angular shear and layer parallel slip also plays an important role in the deformation of the cover rocks. Layer parallel slip plays a more significant role over reverse basement faults than over normal and vertical faults. The potential patterns of macrofaulting in the cover rocks have been deduced from the strain pattern and found to vary with the type of basement fault. The effect of the type of basement fault on strain pattern and modes of failure in the cover rocks analogue is further confirmed by comparing the experimental results with those obtained from real geological situations. Mesoscopic and/or microscopic structures in real forced folds resultingfrom reverse, vertical and normal faults have been used for this purpose. For example field studies carried out on the Purbeck -Isle of Wight Monocline both on the Dorset Coast and on the Isle of Wight indicate that the fold did not develop as an "independent buckle fold" as was suggested by earlier workers. The monocline, which affects the Upper Cretaceous and Tertiary rocks occurred as a forced "drape" fold as a result of reverse movement on a pre-existing basement fault. The cover rocks deformed mainly in a ductile manner on a macroscopic scale (i.e. folding). This took place by mesoscopic and microscopic cataclastic deformation and pressure solution. During folding the pre-existing fractures were rejuvenated by pressure solution and/or faulting, a fact that has been overlooked by previous workers. Layer parallel slip played an important role in the folding of the cover. Macro faulting of the Upper Cretaceous and Tertiary rocks does not occur on the Isle of Wight and is restricted to the Dorset Coast part of the monocline where the ratio of basement fault displacement/cover thickness is considerably larger. The strain pattern in the Upper Cretaceous and Tertiary rocks is heterogeneous in both time and space. This can be clearly seen when the mesoscopic and macroscopic structures are examined. The results of the present study indicate that it is possible to predict the type of basement faults from detailed strain and geometrical analysis in the cover rocks. The accuracy with which the basement fault type can be determined depends very much upon the amount of out crop of the cover rocks.
Date Issued
1988-01
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Cosgrove, John
Sponsor
Arabian Gulf University
Publisher Institution
Imperial College London (University of London)
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)