The Outer Hebrides Fault Zone is a major reactivated structure cutting amphibolite-grade Lewisian basement gneisses in NW Scotland. During a regionally important phase of sinistral strike-slip movements, the influx of chemically active hydrous fluids along the fault zone was associated with the formation of a network of greenschist-facies phyllonitic shear zones. Later ESE-directed extensional strain was preferentially focused into these pre-existing zones of weakness. The syn-tectonic alteration of a relatively strong, feldspar/hornblende-dominated load-bearing framework microstructure to an interconnected weak layer microstructure of fine-grained, strongly aligned phyllosilicate aggregates leads to the long-term weakening in the fault zone. Comparison with experimental data suggests that this produces a shallowing of the frictional-viscous creep ('brittle-ductile') transition and a substantial reduction in total crustal strength. Similar processes may account for the apparent weakness of many long-lived fault zones.
Abstract. The mechanical interaction of propagating normal faults is known to influence the linkage geometry of first-order faults, and the development of second-order faults and fractures, which transfer displacement within relay zones. Here we use natural examples of growth faults from two active volcanic rift zones (Koa`e, island of Hawai`i, and Krafla, northern Iceland) to illustrate the importance of horizontal-plane extension (heave) gradients, and associated vertical axis rotations, in evolving continental rift systems. Second-order extension and extensional-shear faults within the relay zones variably resolve components of regional extension, and components of extension and/or shortening parallel to the rift zone, to accommodate the inherently three-dimensional (3-D) strains associated with relay zone development and rotation. Such a configuration involves volume increase, which is accommodated at the surface by open fractures; in the subsurface this may be accommodated by veins or dikes oriented obliquely and normal to the rift axis. To consider the scalability of the effects of relay zone rotations, we compare the geometry and kinematics of fault and fracture sets in the Koa`e and Krafla rift zones with data from exhumed contemporaneous fault and dike systems developed within a > 5×104 km2 relay system that developed during formation of the NE Atlantic margins. Based on the findings presented here we propose a new conceptual model for the evolution of segmented continental rift basins on the NE Atlantic margins.
We use two- and three-dimensional seismic data to describe the structural geology of the lateral margin of a deep-water delta lobe within the Niger Delta that has undergone basinward, gravitationally driven translation. We term this region the “lateral strike-slip domain.” Deformation is characterized by a strike-slip fault system that can be followed for a distance of approximately 75 km (47 mi) from the shelf to the slope and toe of slope. On the northwestern side of the fault system, a fold and thrust belt that propagated north to northwest has developed within a large-scale restraining area of 460 km2 (180 mi2). On the southeastern side of the strike-slip fault system, widespread extension has occurred, characterized by several graben and kilometer-scale rollover structures. Lateral margins of gravitational collapses give key information on how they deformed. We estimate a minimum horizontal displacement on the main strike-slip fault of approximately 7 km (4 mi). Structural and kinematic evidence, such as present-day propagating strike-slip faults, for possible future lateral expansions of the lateral strike-slip domain, is described. We expect to observe similar sets of deformation styles at the margins of other preserved gravitational collapse sliding over a detachment whose efficiency in causing downdip slip may vary laterally.
Abstract Rapidly developing methods of digital acquisition, visualization and analysis allow highly detailed outcrop models to be constructed, and used as analogues to provide quantitative information about sedimentological and structural architectures from reservoir to subseismic scales of observation. Terrestrial laser-scanning (lidar) and high precision Real-Time Kinematic GPS are key survey technologies for data acquisition. 3D visualization facilities are used when analysing the outcrop data. Analysis of laser-scan data involves picking of the point-cloud to derive interpolated stratigraphic and structural surfaces. The resultant data can be used as input for object-based models, or can be cellularized and upscaled for use in grid-based reservoir modelling. Outcrop data can also be used to calibrate numerical models of geological processes such as the development and growth of folds, and the initiation and propagation of fractures.
J. R. Mendum, A. J. Barber, R. W. H. Butler, D. Flinn, K. M. Goodenough, M. Krabbendam, R. G. Park & A. D. Stewart 2009. Lewisian, Torridonian and Moine Rocks of Scotland. Geological Convervation Review Series no. 34. xviii + 721 pp. Peterborough: JNCC; distributed in the UK by NHBS Ltd. Price £62.50 (hard covers). ISBN 978 1 86107 483 6. - Volume 147 Issue 4
ABSTRACT Tectonic subsidence in rift basins is often characterised by an initial period of slow subsidence (‘rift initiation’) followed by a period of more rapid subsidence (‘rift climax’). Previous work shows that the transition from rift initiation to rift climax can be explained by interactions between the stress fields of growing faults. Despite the prevalence of evaporites throughout the geological record, and the likelihood that the presence of a regionally extensive evaporite layer will introduce an important, sub‐horizontal rheological heterogeneity into the upper crust, there have been few studies that document the impact of salt on the localisation of extensional strain in rift basins. Here, we use well‐calibrated three‐dimensional seismic reflection data to constrain the distribution and timing of fault activity during Early Jurassic–Earliest Cretaceous rifting in the Åsgard area, Halten Terrace, offshore Mid‐Norway. Permo‐Triassic basement rocks are overlain by a thick sequence of interbedded halite, anhydrite and mudstone. Our results show that rift initiation during the Early Jurassic was characterised by distributed deformation along blind faults within the basement, and by localised deformation along the major Smørbukk and Trestakk faults within the cover. Rift climax and the end of rifting showed continued deformation along the Smørbukk and Trestakk faults, together with initiation of new extensional faults oblique to the main basement trends. We propose that these new faults developed in response to salt movement and/or gravity sliding on the evaporite layer above the tilted basement fault blocks. Rapid strain localisation within the post‐salt cover sequence at the onset of rifting is consistent with previous experimental studies that show strain localisation is favoured by the presence of a weak viscous substrate beneath a brittle overburden.
Abstract Intrusive magmatism is an integral and understudied component in both volcanic and nonvolcanic passive margins. Here, we investigate the thermal effects of widespread ( ca . 20 000 km 2 ) intrusive magmatism on the thermal evolution of organic‐rich sedimentary rocks on the nonvolcanic Newfoundland passive margin. ODP 210‐1276 (45.41°N, 44.79°W) intersects two sills: an older, upper sill and a younger, lower sill that are believed to correspond to the high amplitude ‘U‐reflector’ observed across the Newfoundland Basin. A compilation of previous work collectively provides; (1) emplacement depth constraints, (2) vitrinite reflectance data and (3) 40 Ar/ 39 Ar dates. Collectively, these data sets provide a unique opportunity to model the conductive cooling of the sills and how they affect thermal maturity of the sedimentary sequence. A finite differences method was used to model the cooling of the sills, with the model outputs then being entered into the EASY %R o vitrinite reflectance model. The modelled maturation profile for ODP 210‐1276 shows a significant but localized effect on sediment maturity as a result of the intrusions. Our results suggest that even on nonvolcanic margins, intrusive magmatism can significantly influence the thermal evolution in the vicinity of igneous intrusions. In addition, the presence of widespread sills on nonvolcanic passive margins such as offshore Newfoundland may be indicative of regional‐scale thermal perturbations that should be considered in source rock maturation studies.
New digital methods for data capture can now provide photorealistic, spatially precise, and geometrically accurate three‐dimensional (3‐D) models of rocks exposed at the Earth's surface [ Xu et al. , 2000; Pringle et al. , 2001; Clegg et al. , 2005]. These “virtual outcrops” have the potential to create a new form of laboratory‐based teaching aids for geoscience students, to help address accessibility issues in fieldwork, and generally to improve public awareness of the spectacular nature of geologic exposures from remote locations worldwide. This article addresses how virtual outcrops can provide calibration, or a quantitative “reality check,” for a new generation of high‐resolution predictive models for the Earth's subsurface.
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