Figure 3 - uploaded by David Peacock
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Photograph of an east-west striking normal fault in Liassic limestones and shales at East Quantoxhead, Somerset. A shale-filled pull-apart has been created by refraction of the fault through the more brittle limestone (Peacock & Sanderson, 1992). This structure is related to Mesozoic extension of the Bristol Channel Basin.
Source publication
Southern England is in the foreland of the Tertiary Alpine deformation, being similar to 700 km from the main orogenic zone. The main Alpine deformation in southern England involved maximum compressive stress (sigma(1)) orientated approximately N-S and with the least compressive stress (sigma(3)) being vertical. This generally caused gentle folding...
Citations
... Evidence of intraplate faulting and folding, attributed to the far-field influence of Alpine collision during the Cenozoic, has been well documented across Britain and Ireland (Dewey and Windley, 1988;Rosenbaum et al., 2002). The impact of shortening is readily observed in southern England, where inversion-related folds and outcrop-scale conjugate strike-slip faults are common Peacock, 2009). More regionally, Cenozoic strike-slip faulting in Britain and Ireland has been recognised to occur along two major fault orientations; NNW-SSE striking dextral structures and NE-SW sinistral structures ( Fig. 1a) (Turner, 1997;Cooper et al., 2012). ...
A paucity of displacement markers can make constraining the kinematics and interaction of regional-scale strike-slip faults difficult. In this study, a high-resolution aeromagnetic survey allows quantitative analysis of kilometre-scale Paleogene faulting in the north of Ireland. Mapping offset dykes and igneous centres reveals the presence of two orientations of Cenozoic strike-slip faults attributed to broadly N-S Alpine shortening arising from the convergence of Africa and Europe. Reactivated NE-SW striking faults accommodated up to 2.3 km of sinistral displacement mainly during the Paleocene. Following tectonic quiescence, up to 2.5 km of Oligocene-aged dextral movement was accommodated on newly-formed NNW-SSE striking faults. Displacement analysis suggests that the faults were kinematically linked and experienced a switch in polarity of the predominantly active fault orientation during the Paleogene, forming an apparent conjugate system with asymmetric displacement distributions. The switch in fault polarity is attributed to a rotation in the regional maximum horizontal stress orientation and the locus of deformation moving from pre-existing reactivated structures to newly formed faults. The analysis also illustrates that, despite low strain rates, the interaction of the principal components of the fault system controlled the formation of sedimentary basins and defined the present-day geography of the region, far from an active plate boundary.
We present the first detailed stratigraphic and structural geological map of offshore North Cornwall and Devon, SW England, based on freely-available bathymetric data. Although the bathymetry is often spectacular, revealing fold and fault structures exposed on the seabed at a range of scales and with high resolution, interpretation is not as straightforward as it might appear and depends critically upon both accuracy and knowledge of the onshore geology. Unfortunately, onshore stratigraphic controls are limited and restricted to several thin ‘named shales’ whose coastal outcrops are not always well-constrained. In addition, the structure is markedly non-cylindrical on local to regional scales making seaward projection problematic, whilst the impact of early thrusting on the stratigraphy has often been previously neglected. We therefore developed a workflow to handle the problems we encountered including: recognition of vertical to horizontal, and 3D to 2D projections; variations in bathymetric data characteristics; prediction of expected seabed outcrop geometries based on coastal structures; incorporating non-cylindrical effects; and problems with quantitative GIS terrain profiles and structural measurements that result in an absence of such measurements. The geological map produced should therefore be viewed as a step forward, but as forming a base for further detailed bathymetric mapping.
Changing stress regimes control fracture network geometry and influence porosity and permeability in carbonate reservoirs. We investigate the impact of stress-regime change on fracture network permeability utilizing outcrop data analysis and a displacement-based linear elastic finite element method. The model is based on fracture networks, specifically, fracture sub-structures. The Latemar, predominantly affected by subsidence deformation and Alpine compression, is taken as an outcrop analogue for isolated (Mesozoic) carbonate buildups with fracture-dominated permeability. We apply a novel strategy involving two compressive boundary loading conditions, constrained by the NW-SE and N-S stress directions in the study area. Stress-dependent heterogeneous apertures and effective permeability were computed by: (i) using the local stress state within the fracture sub-structure and (ii) running a single-phase flow analysis considering the fracture apertures in each fracture sub-structure. Our results show that the impact of the modelled far-field stresses at: (i) subsidence deformation from the NW-SE, and (ii) Alpine deformation from N-S, increased the overall fracture aperture and permeability. In each case, increasing permeability is associated with open fractures parallel to the orientation of the loading stages and with fracture densities. The anisotropy of permeability is increased by the density and connectedness of the fracture network and affected by of shear dilation. The two far-field stresses simultaneously acting within the selected fracture sub-structure at a different magnitude and orientation do not necessarily cancel out each other in the mechanical deformation modelling. These stresses effect the overall aperture and permeability distributions. These effects, which may be ignored in simpler stress-dependent permeability, can result in significant inaccuracies in permeability estimation.
Members of the Society assembled at the car park at Kilve beach [ST 1445 4425], on a wet and windy January morning. The purpose of the field trip was to determine the timing and nature of structures along the southern margin of the Bristol Channel Basin, examine evidence for the role of elevated fluid pressure in the structural development of the basin (including looking at the early Jurassic mud volcanoes) and further examine stratigraphic and structural evidence for basin inversion. The trip included visits to three principal locations where a range of sedimentary strata and structures are exposed.
Palaeogene fluvial deposits in southern Britain have four distinct and interesting characteristics. First, the river systems were initiated on a ‘blank canvas’ of Cretaceous Chalk left exposed and deeply eroded because of a relative fall in sea-level. Second, river hydrology was controlled by the abnormally warm, wet subtropical climate of the Eocene which includes several short-lived but intense global warming events. Third, the river systems occupied a lowland coastal belt at the southern margin of the North Sea Basin and nearly all of the fluvial deposits contain evidence for marine influence. Fourth, tectonic compression related to Alpine mountain building may (or may not) have been important in creating an undulating landscape of growth folds and basins which diverted river courses and controlled areas of deposition. This paper describes the distribution and morphology of Palaeogene fluvial deposits, focussing primarily on the Hampshire Basin where the two main fluvial intervals are represented by the Late Palaeocene to Early Eocene Reading Formation and the Eocene Poole Formation. The characteristics of the two contrasting mud-rich and sand-rich fluvial intervals are considered together with an assessment of the relative importance of inherited topography, inversion tectonics, climate change and source area in shaping the river systems and their deposits. A new model for the Poole Formation is introduced which considers the possibility that these rivers bypassed the eastern half of the present Hampshire Basin to feed sandy tidal deltas of the correlative Bagshot Sands in the present London Basin.
