| dc.description.abstract | Rifting describes the process of crustal stretching through which the brittle upper crust is subjected to extensional deformation (Falvey, 1974; Bally and Snelson, 1980; Wernicke and Burchfiel, 1982; Steckler, 1985; McKenzie, 1987). It results in the development of basins associated with symmetric or asymmetric grabens or half grabens, bounded by listric or planar normal faults. Well known rift settings have been documented from the North Sea area, Gulf of Suez, East Africa, Basin and Range area etc. In a number of rift settings the normal faults are reactivated pre-existing weakness zones (ancient rifts or orogenic suture zones) within the crust. The crustal scale normal faults usually detach in a shear zone within the igneous/metamorphic basement or at the level of the brittle-ductile shear zone. The structural geometry and the shape of the extensional basins associated with these normal faults are influenced by mechanical stratigraphy, the spatial orientation of the faults with respect to the direction of extension and the interaction between two laterally propagating faults close to each other (Youssef, 1968; Stewart, 1971; Chapman et. al., 1978; Gibbs, 1984; Harding, 1984; Jackson and White, 1989; Bosworth, 1995). This study attempts to understand some of the structural features related to rift settings using analog clay experimental models and structural interpretation of a 3-D seismic dataset from the offshore Mid Norway area.
The analog experimental models focused on the geometries of faults and related fault-propagation (drape) folds, and their interference at different types of transfer zones. Detailed mapping and analysis of the orientations of secondary faults was conducted to investigate the structural controls of the resultant fault geometries and orientations. The map geometries derived from the experimental models were compared with examples of natural structures from East Africa, the Gulf of Suez and the North Sea. The interpretation and detailed analysis of the 3-D seismic dataset from the offshore Mid Norway area helped in understanding the kinematic evolution of a rift related structure where a thick layer of mechanically weak salt is present between the basement and the cover sequence.
This dissertation is divided into three chapters, the first two of which focus on experimental models, whereas the third focuses on a seismic data set. The first chapter discusses experimental clay models to understand the evolution and nature of extensional fault propagation (drape) folds and secondary fault patterns associated with the movement on pre-existing basement faults. A suite of these basements faults (single fault and trapdoor structures) initially oriented at different angles with respect to the direction of extension was considered. A two layered model with stiff clay representing basement and soft clay representing cover sequence was used to take the rheological difference of basement and sedimentary units into consideration. In some setups, the basement fault initially terminated within the stiff clay to study the effect of lateral fault propagation. Relation between the fault density and width of the deformation zone within the drape folds and progressive evolution of the structural geometry was studied in detail. Natural examples from North Sea were considered to compare the results of the study.
The second chapter investigates the geometry, evolution and fault patterns associated with various kinds of basement fault interaction at transfer zones (convergent, divergent and synthetic). Basement faults with initial approaching, laterally offset and overlapping geometries were also modeled. A similar two layered model with stiff clay and wet clay representing basement and cover sequence was used. The interference of fault-propagation folds associated with the faults was also studied. Analysis of fault orientation and 3-D surface modeling of clay was done in detail to study the progressive evolution of the secondary fault pattern and the structural geometry. The results of the study were compared with natural examples from East Africa and Gulf of Suez.
The third chapter involves a 3-D structural analysis of the Smørbukk area of the Halten Terrace, offshore Mid Norway using a 3-D seismic dataset and associated well tops donated by Statoil. The area has a thick sequence of Middle-Late Triassic evaporite interbedded with dolomite and anhydrite rich shale (collectively referred to as ‘salt’) that is stratigraphically located between the Permo-Triassic sedimentary basement and the Jurassic cover. The study includes 3-D seismic interpretation, displayed in both time and depth surfaces and time and depth sections. Restoration of units incorporating sequential restoration and decompaction on a series of depth sections was conducted to understand the evolution of the structures.
The present research involved several innovative approaches and techniques to produce original results that helped to improve the understanding of the broad field of basement involved extensional deformation. Some of these approaches and their advantages are discussed below.
a. Extensional structures involving drape folding on multiple basement-involved normal faults and complex fault transfer zones have been reported from several active rift settings. However experimental modeling of such structures in order to better understand their geometry and evolution was not attempted before. In the present study a novel approach of choosing stiff clay over conventionally used steel ramps to model the basement helped to eliminate some of the mechanical limitations of the existing experimental models. The advantage of this setup was multifold. It helped to study the nature of extensional deformation both in the basement and cover units for single and multiple faults including the trapdoor structures. It also helped to understand the nature of extensional fault propagation folding and its effects on the lateral propagation of the basement faults and their mutual interactions.
b. Insights from previous experimental models were mostly restricted to the understanding of the sectional geometry of the extensional structures (Cloos, 1968; McClay and Ellis, 1987; Withjack et. al., 1990; Miller and Mitra, 2011). A relatively new 3-D laser scanning technology and surface modeling used in the present study helped to study the map geometry of the experimental structures and compare them easily with natural structures.
c. The case study from offshore Mid Norway involved faults detaching at different stratigraphic levels due to the presence of a weak salt layer. Similar structures were reported by previous workers in this area (Withjack et al, 1989; Pascoe et al., 1999; Richardson et al, 2005; Marsh et al., 2010). However decompaction and restoration of these structures were not attempted before. Such structural analyses used in the present study helped to provide insight about the fault evolution, the role of salt in partitioning the extensional deformation, and the conditions for the development of hard-linked or soft linked structures across the salt layer. | en_US |
