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.
The Lizard Complex of SW England includes thrusted units of peridotites that were initially exhumed from upper mantle ( c. 52 km) to lower crustal ( c. 24 km) depths during a period of Early Devonian rifting and break-up. This basin closed during the Late Devonian, when the Lizard Complex was thrust towards the NNW along a major low-angle detachment and became incorporated within a series of Variscan thrust nappes. In the peridotites, a primary high- T and high- P spinel lherzolite mineral assemblage ( c . 1119°C and c. 15.7 kbar) was progressively exhumed and re-equilibrated to conditions of lower T and P ( c . 991–1010°C and c . 7.5 kbar) during the development of kilometre-scale mylonitic plagioclase- and amphibole-bearing mantle shear zones. These fabrics demonstrably pre-date emplacement related structures. The new structural and geochemical evidence from the peridotites also strongly suggests that the Lizard Complex formed in a rifted, non-volcanic continental margin setting, possibly in a pull-apart basin, rather than at a mid-ocean ridge. The P–T and textural evolution of the Lizard peridotites supports growing evidence that shear zones in the lithospheric upper mantle may to some extent accommodate large-scale displacements associated with crustal extension and continental breakup.
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.
Like many rift basins worldwide, the Inner Moray Firth Basin (IMFB) is bounded by major reactivated fault zones, including the Helmsdale Fault and the Great Glen Fault (GGF). The Jurassic successions exposed onshore close to these faults at Helmsdale and Shandwick preserve folding, calcite veining and minor faulting consistent with sinistral (Helmsdale Fault) and dextral (GGF) transtensional movements. This deformation has been widely attributed to Cenozoic post-rift fault reactivation. Onshore fieldwork and U–Pb calcite geochronology of five vein samples associated with transtensional movements along the Helmsdale Fault and a splay of the GGF show that faulting occurred during the Early Cretaceous ( c. 128–115 Ma, Barremian–Aptian), while the Helmsdale Fault preserves evidence for earlier Late Jurassic sinistral movements ( c. 159 Ma, Oxfordian). This demonstrates that both basin-bounding faults were substantially reactivated during the episodic NW–SE-directed Mesozoic rifting that formed the IMFB. Although there is good evidence for Cenozoic reactivation of the GGF offshore, the extent of such deformation along the north coast of the IMFB remains uncertain. Our findings illustrate the importance of oblique-slip reactivation processes in shaping the evolution of continental rift basins given that this deformation style may not be immediately obvious in interpretations of offshore seismic reflection data. Supplementary Material: Appendix A – orthomosaic model obtained from unmanned aerial vehicle (UAV) photography of the Helmsdale locality (GeoTiff format); Appendix B – orthomosaic model obtained from UAV photography of the Shandwick locality (GeoTiff format); Appendix C – geochronology data; and Appendix D – additional thin section microphotographs of sample HD1 showing repeated cycles of syntaxial grain growth are available at https://doi.org/10.6084/m9.figshare.c.6708518
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.
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.
Research Article| May 01, 1991 Interlinked system of ductile strike slip and thrusting formed by Caledonian sinistral transpression in northeastern Greenland Robert E. Holdsworth; Robert E. Holdsworth 1Department of Geological Sciences, University of Durham, Durham DH1 3LE, England Search for other works by this author on: GSW Google Scholar Robin A. Strachan Robin A. Strachan 2Department of Geology, Oxford Polytechnic, Oxford OX3 3BP, England Search for other works by this author on: GSW Google Scholar Author and Article Information Robert E. Holdsworth 1Department of Geological Sciences, University of Durham, Durham DH1 3LE, England Robin A. Strachan 2Department of Geology, Oxford Polytechnic, Oxford OX3 3BP, England Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1991) 19 (5): 510–513. https://doi.org/10.1130/0091-7613(1991)019<0510:ISODSS>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Robert E. Holdsworth, Robin A. Strachan; Interlinked system of ductile strike slip and thrusting formed by Caledonian sinistral transpression in northeastern Greenland. Geology 1991;; 19 (5): 510–513. doi: https://doi.org/10.1130/0091-7613(1991)019<0510:ISODSS>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The geometry and kinematic evolution of oblique convergence zones are poorly understood, especially in the more deeply eroded ductile roots of ancient orogenic belts. As a consequence, evidence for major lateral displacements may remain undetected. In Dronning Louise Land, northeastern Greenland, a major partitioned system of strike-slip and ductile thrust shear zones has formed in response to hitherto unrecognized Caledonian sinistral transpression. This system formed at mid-crustal depths (amphibolite facies), possibly due to oblique collision between Baltica and Laurentia during Ordovician to Early Devonian time. An early phase of low-angle strike slip is superseded by synchronous compressional thrusting and high-angle sinistral displacements. These are partitioned into shear zones arranged in a fashion similar to the fault patterns observed in the hanging walls of modern-day oblique convergent margins. Left-lateral displacements in the eastern Greenland Caledonides are likely to be tens to hundreds of kilometres. A direct correspondence between stretching lineations and Caledonian plate motion vectors is unlikely, although the strike-slip shear zone is probably parallel to the Laurentian paleo-plate margin. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
This chapter contains sections titled: Introduction Basin History of the Dalradian Supergroup Cambrian–Ordovician Shelf Sedimentation in North-West Scotland Summary References
Abstract Field characteristics of crustal extrusion zones include: high-grade metamorphism flanked by lower-grade rocks; broadly coeval flanking shear zones with opposing senses of shear; early ductile fabrics successively overprinted by semi-brittle and brittle structures; and localization of strain to give a more extensive deformation history within the extrusion zone relative to the flanking regions. Crustal extrusion, involving a combination of pure and simple shear, is a regular consequence of bulk orogenic thickening and contraction during continental collision. Extrusion can occur in response to different tectonic settings, and need not necessarily imply a driving force linked to mid-crustal channel flow. In most situations, field criteria alone are unlikely to be sufficient to determine the driving causes of extrusion. This is illustrated with examples from the Nanga Parbat-Haramosh Massif in the Pakistan Himalaya, and the Wing Pond Shear Zone in Newfoundland.
Strong tidal currents in the Pentland Firth separating NE Scotland and Orkney have stripped recent seafloor sediments revealing geological structures in parts of the Devonian Orcadian Basin that are otherwise limited to narrow coastal outcrops. Here we interpret and analyse a 220 km2 high-resolution bathymetric dataset and combine these findings with onshore aerial imagery, field observations, and photogrammetric Virtual Outcrop Models created from coastal outcrops. This approach allows a reappraisal of the structure, stratigraphy and tectonic evolution of the Orcadian Basin, and gives new insights into fold and fault structures developed at sub-seismic to reservoir scales. A major basin-scale Devonian structure – the Brough-Brims-Risa Fault – can be traced from Caithness to Hoy and has partitioned reactivation-related deformation in the Orcadian Basin during later deformation episodes. Carboniferous inversion-related folds formed during E-W shortening are mapped and correlated between Caithness and Orkney. These structures are cross-cut by widespread highly connected fracture networks formed by steeply-dipping ENE-WSW and subordinate WNW-ESE trending structures developed during Permian NW-SE transtensional rifting. The offshore dataset demonstrates the continuity and topology of structures related to superimposed rifting and basin inversion events across a much greater scale range than was hitherto possible. [end].
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