Microgeodynamics relates grain-scale deformation microstructures to macroscopic tectonic processes. Here the microgeodynamic approach combines optical and electron microscopy, including electron backscattered diffraction (EBSD), with field geology, geothermobarometry and microphysical modelling to study fault rocks deformed within a major continental strike-slip fault to quantify changes in fault zone structure and rheology with crustal depth. The overall thesis rational therefore is to test existing fault models against an exhumed example of a continental strike-slip fault zone, namely the central Karakoram Fault Zone (KFZ), NW India. This approach establishes changes in deformation processes with depth in the upper- to mid-crust and suggests that a range of fault weakening mechanisms have reduced fault rock shear strengths, typified by friction coefficients of 0.3-0.4. Metamorphic petrology and geothermobarometry are used to place the KFZ in the context of regional tectono-metamorphic evolution. It is shown using diagnostic microstructures and pressure-temperature-time paths that the fault initiated after peak metamorphism (677-736°C, 875-1059 MPa) and subsequent migmatisation (688±44°C, 522±91 MPa) and leucogranite emplacement (448±100 MPa). Retrograde phyllonites formed during later strike-slip deformation are investigated in detail using EBSD, geothermometry and microphysical modelling. The phyllonites formed at 351±34°C and had low shear strength (<30 MPa) during frictional-viscous flow. EBSD is also used to derive a novel strain proxy based on quartz crystal preferred orientation intensity. Application of this method distinguishes deformation distributions in transects across the KFZ. Deformation intensity varies from <0.2 in essentially undeformed domains to 1.6 within shear zone strands formed at 500-550°C and c. 15 km depth. Evaluation of the history of the KFZ suggests that whilst it plays a relatively minor role in accommodating India-Asia collision, it can nevertheless be used as an analogue for major continental strike-slip fault zone structure.

Dyke intrusions and normal faulting play an important role during continental break-up but little is known about how the normal faults develop. Direct evidence of dyke-induced faulting is limited by the lengthy repeat times between individual rifting episodes, the small amount of subaerial rift zones and until recently the technical ability to record small surface changes across large areas. The most recent (2005-2010) rifting episode at the Dabbahu rift segment, Afar, Ethiopia provided a unique opportunity to study dyke-induced fault growth. The combination of new high-resolution topographic LiDAR data and interferometric synthetic aperture radar (InSAR) data provides information of cumulative as well as incremental fault throw. In this thesis I use high-resolution LiDAR data of the Dabbahu rift segment to reveal a dense network of short fault segments (>3400) at various stages of fault linkage set in flood basalt plains. I develop and present a semi-automatic algorithm that extracts throw along surface fault traces from the high-resolution LiDAR DEM. The largest amount of throw (~80 m) is found on faults towards the east of the rift segment. At the central Ado’Ale volcanic edifice predominant bookshelf faulting is evident which might be an indication of a lateral shift of the dykes towards the east. I use the throw data to derive a strain field for the rift. Faults record ~140 m of extension, implying extensive resurfacing. I derived displacement data from two LiDAR surveys and InSAR data, for two separate dyke intrusions. Both data sets show that faults are re-activated in a broad, 3-4 km wide, asymmetric zone parallel to the dyke induced subsidence with the majority of the new throw being accumulated on 1-2 large west-dipping fault structures in the east. The incremental displacement-length, d − L, data presented here is the first quantitative study of accumulation of new fault throw across an entire rift segment. Incremental throw across linkage zones suggest two types of behaviour once fault linkage is complete. 1) Individual fault segments maintain the ability to slip independently. This was previously only observed during analogue modelling. 2) The connected faults act as one throughgoing fault with slip unaffected by the linkage zone. The combination of these two processes might be responsible for the commonly observed small-scale corrugation in d − L data. In contrast to published fault growth models, I present evidence that the remnant fault tip of a linkage zone does not necessarily become inactive once linkage is complete, and that linkage zones do not ‘catch up’ through accelerated throw once linkage is complete.

In September 2005, a 60-km-long dyke intruded the Dabbahu segment of the Nubia-Arabia plate boundary (Afar, Ethiopia), marking the beginning of an ongoing rifting episode. Continued activity has been monitored using Satellite Radar Interferometry, GPS instruments and seismometers deployed around the rift in response to the initial intrusion. These data show that a sequence of 12 new dyke intrusions have reintruded the central and southern section of the Dabbahu segment. Modelling of InSAR data indicates the dykes were between 0.5 and 3 m wide, up to 10 km long and confined to the upper 10 km of crust. Seismicity data imply that the dykes were probably fed from a source near the centre of the segment. The new dykes are concentrated in areas where the 2005 dyke did not produce significant opening, implying that residual tensile tectonic stresses are higher in this location and are focusing the later intrusions. Geodetic data, which quantify the location and extension occurring in each of the events, allows the identification of regions where tensile stress has been increased. Here I demonstrate for the first time the high probability of a link involving stress transfer and dyke intrusions. Since the September 2005 intrusion, background displacement rates are significantly larger than the average secular divergence between Nubia and Arabia. Some of this deformation can be explained using viscoelastic models, which suggest an elastic crustal thickness of 13 km and upper mantle viscosity of 10 ^18.5 Pas. The presence of multiple magmatic sources around the rift zone, however, cause large residuals between the data and model suggesting that viscoelastic relaxation alone cannot account for the observed deformation. It is likely that, with a continued magma supply, dykes will continue to be intruded until the tectonic stress is fully relieved with more eruptions as the rifting episode is concluded.

Himalayan orogenesis is commonly explained with models of channel flow, in which the metamorphic core, referred to as the Greater Himalayan Sequence (GHS), forms a partially molten, rheologically weak mid-crustal flow. Geochronological and thermobarometric studies from the Himalaya provide support for the channel flow model, however, strain-related model predictions are unresolved and the model remains controversial. Additionally, wedge-extrusion, underplating / thrust-stacking and tectonic wedging models are favoured by many as alternative explanations for the formation of the Himalaya. In this thesis, strain-related predictions of the channel flow model for Himalayan orogenesis are tested with field-based structural studies together with laboratory-based microstructural and magnetic fabric analyses. Orogen-perpendicular transects along the Modi Khola and Kali Gandaki valleys in the Annapurna-Dhaulagiri Himalaya of central Nepal were chosen for study. Samples were collected from the GHS and bounding units for crystallographic preferred orientation (CPO) and anisotropy of magnetic susceptibility analyses (AMS). Both techniques were used to quantify deformation fabric strength, which provides a proxy for relative strain magnitude. These data, combined with geochronological and thermobarometric constraints, reveals the kinematic evolution of the GHS in the Annapurna-Dhaulagiri Himalaya. These data can be directly compared to predictions implied by the channel flow model in order to assess its validity. The results support the channel flow model as a viable explanation of the mid-crustal evolution of the GHS. However, lower temperature deformation indicates that exhumation of the GHS was facilitated through wedge-extrusion and thruststacking. The development of the Himalayan orogen in the Annapurna-Dhaulagiri region is best explained by models that allow channel flow, wedge extrusion and thrust-staking to occur in a single orogen. Similar ‘composite models’ for Himalayan orogenesis have been proposed recently by other authors and reflect a growing understanding of how rheological controls on orogenesis can vary both spatially and temporarily.

Polygonal faults are non-tectonic fault systems which are layer-bound (at some vertical scale) and are widely developed in fine-grained sedimentary basins. Although several qualitative mechanisms have been hypothesised to explain the formation of these faults, there is a weak general consensus that they are formed by the coupled deformation and fluid expulsion of the host sediments (consolidation). This thesis presents a numerical framework that can be extended to investigate the role consolidation plays in the development and evolution of these faults. The method is also applicable to reservoir engineering and CO2 storage. An understanding of the coupled mechanical response and fluid flow is critical in determining compaction and subsidence in oil reservoirs and fault-seal integrity during CO2 disposal and storage. The technique uses a fracture mapping approach (FM) and the extended finite element method (XFEM) to modify the single phase FEM consolidation formulation. A key feature of FM-XFEM is its ability to include discontinuities into a model independently of the computational mesh. The fracture mapping approach is used to simulate the flow interaction between the matrix and existing fractures via a transfer function. Since fractures are represented using level set data, the need for complex meshing to describe fractures is not required. The XFEM component of the method simulates the influence of the pore fluid on the mechanical behaviour of the fractured medium. In XFEM, enrichment functions are added to the standard finite element approximation to ensure an accurate approximation of discontinuous fields within the simulation domain. FM-XFEM produces results comparative to the discrete fracture method on relatively coarse meshes. FM-XFEM has also been extended to model the propagation of existing fractures using a mixed-mode criterion applicable to geological media. Stress concentrations at the tips of existing fractures show good agreement with an analytical solution found in literature.

The sedimentary and structural record of the NW Tarim Basin, China, provides an insight into the amalgamation of Central Asia and is an ideal area in which to examine the impact of an inherited tectonostratigraphic framework on the evolution of foreland fold-thrust belts. The NW Tarim Basin contains a thick (3-16 km) sedimentary succession which was deposited from the Late Neoproterozoic onwards, and has been exhumed by a foreland fold-thrust belt system associated with the South Tien Shan mountains during the Middle to Late Cenozoic. The research presented in this thesis combines satellite image interpretation and field investigations in order to examine the tectonostratigraphic framework of the NW Tarim Basin and to ascertain the causes of lateral structural variability and partitioning of the foreland fold-thrust belt system. The Upper Neoproterozoic to Lower Permian sedimentary succession records the progressive evolution of the NW Tarim Basin as a rift, intracratonic and foreland basin. Following a period of subaerial exposure throughout the Mesozoic, tectonic subsidence from the Middle Cenozoic onwards was driven by flexural deflection beneath the Pamirs and Tien Shan orogenic belts. This was coupled with the development of a foreland fold-thrust belt system along the northwest margin of the Tarim Basin. Lateral variations in the structural geometry, architecture and style of the foreland fold-thrust belt system correspond to changes in the thickness of the sedimentary succession and interaction with inherited, basement fault zones. An eastwest transition from the wide, arcuate Keping Shan Thrust Belt into the narrow Kashgar Fold Belt is ascribed to thickening of the Cenozoic (syn-tectonic) foreland basin succession. In contrast, internal variations in the structural architecture of the Keping Shan Thrust Belt are governed by lateral changes in the thickness of the Palaeozoic (pre-tectonic) sedimentary succession. These changes occur abruptly across inherited, Early Permian fault zones that have been reactivated as strike-slip faults in order to accommodate these lateral variations in the structure of the fold-thrust belt.

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This thesis compiles the results of a 3D seismic-based study of the structural evolution of the Danish Central Graben. Detailed interpretation of the geometry, structural and stratigraphic context has been carried out in order to advance our current understanding of the fundamental processes involved in major rift systems as well as to test previous published models, based on 2D seismic data, using recent 3D seismic data. The increased resolution provided by 3D seismic mapping of three key areas has allowed for fault patterns of complex structural elements in the Danish Central Graben to be imaged in three dimensions. The Mid North Sea High is essential for the identification of the basement fault trend and the Arne-Elin Graben is crucial for the determination of the fault trend influence on the Mesozoic deposition in combination with the salt-sediment interaction. Finally the western margin of the Ringkobing-Fyn High is central for the understanding of the origin of the Top Chalk Group depressional structures. Based on the interpretation a new model for the structural evolution for the Danish Central Graben has been proposed. In this model, the basement fault trend consists of a WNW-ESE and N-S rhomboidal fault pattern that formed prior to the Permian rifting of the North Sea. The interpretation of a rhomboidal basement fault trend in combination with salt-sediment interaction suggests less influence from strike-slip movement, which historically has been suggested for the deposition of the Mesozoic strata. The interpretation has revealed that salt-sediment interaction adopt a wide range of geometrical styles, ranging from fallen salt walls/diapirs, rising salt swells and diapirs and detached salt canopy in response to Jurassic rifting and Late Cretaceous/Early Palaeocene inversion of the Danish Central Graben. The regional 3D seismic imaging of Top Chalk Group horizon has revealed a number of enigmatic km-scale sub-circular structures. The spatial resolution of the 3D seismic data in combination with various attributes permitted the interpretation of enigmatic depressions as pockmarks that enlarged through sidewall collapse. The collapsed pockmarks occur on the flanks of major salt and inversion structures and may be of importance for the understanding of reservoir development in the uppermost Chalk Group.

No description available.

No description available.