Discussion on major southeast-directed Caledonian thrusting and folding in the Dalradian rocks of mid-Ulster: implications for Caledonian tectonics and mid-crustal shear zones
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Dalradian
ABSTRACT Many ductile shear zones are interpreted to operate by simple shear flow but some form under other flow regimes. Lineations and foliations in such shear zones can lie obliquely to those in simple shear zones, which can lead to erroneous tectonic interpretations on the assumption of simple shear flow. This paper describes a gently dipping shear zone system from the N‐central segment of the Palaeoproterozoic Nagssugtoqidian orogen of W. Greenland, which operated with a lateral constriction component. This resulted in the development of upright folds with axes parallel to the transport direction where the constriction component is weak. Where it is strong, a linear fabric and even a subvertical foliation normal to the rotation axis of bulk flow developed. This steep foliation is interpreted as the origin of the Nordre Strømfjord steep belt, previously interpreted as a crustal‐scale sinistral transcurrent shear zone. Shear zones of this type may occur elsewhere and shear zone fabrics should therefore be carefully analysed before the direction of tectonic transpost can be determined
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<p>The area of Cap de Creus, at the eastern termination of the Axial Zone of the Pyrenean Belt, exposes some of the most famous outcrops of ductile shear zones and shear zone networks (Carreras, 2001). Recent studies proposed that the nucleation and growth of such shear zones may have taken place by brittle processes (Fusseis et al., 2006; Fusseis and Handy, 2008).</p><p>The present study investigates the geometrical relationships between fracture systems and some shear zones, the deformation temperature of these shear zones, and the processes leading to the nucleation and growth of shear zones along fracture planes. We selected two areas of the Cap de Creus, the Cala d&#8217;Agulles, and the Punta de Cap de Creus, because they are most intensely dissected by subparallel sets of shear zones and fractures. The orientation of the average shear zone planes is sub-parallel to the orientation of the major set of fractures, and the great extent and close spacing of some shear zones that we characterized by aerial photos from a drone, is similar to the distribution and extent of the fracture planes. These observations, in addition to those of Fusseis et al. (2006) suggest that the shear zones nucleated on previous fracture planes.&#160;</p><p>These fractures are surrounded by haloes of nearly 1 cm thickness affecting the fabric of the country rock, an amphibolite-facies, biotite-andalusite bearing schist. Microscopic observations show that the haloes correspond to the wide-spread presence of thin (less than 2&#181;m thickness) phosphate seams coating the grain boundaries, preferentially those oriented at low angle to the fracture plane, and to the alteration of plagioclase to white mica and sericite, and to the growth of tourmaline, also related to grain boundaries and micro-fractures.</p><p>Deformation temperature in the shear zones is assessed by white mica thermometry and pseudosections. The calculated T of at least 350-400&#176; C is consistent with qualitative observations showing the presence of stable biotite within very fine-grained (<< 10 &#181;m) shear bands and the recrystallization of quartz by rotation of sub-grain boundaries.</p><p>In summary, fractures formed at high temperature, possibly associated with the intrusion of tourmaline-bearing pegmatites and fluids, which predate the ductile mylonitic event (Druguet, 2001; Van Lichtervelde et al., 2017). Fluids altered and weakened a volume of approximately 2 cm thickness all along the fracture planes, whose extent may reach > 100 m. The inferred, relatively high T of ca. &#160;400&#176; C indicates that fracturing is not due to the proximity of the brittle-ductile transition. In addition, no significant micro-fracturing of the mylonites is observed in thin sections. Therefore, fracturing precedes the ductile shear zones, which nucleate on some of the &#8220;inherited&#8221; sets of thin, planar, weakened structures, the large majority of which remains undeformed. These observations raise the question on whether nucleation and propagation of ductile shear zones is mechanically unrelated to brittle fracturing. Their weakening of planar structures would originate from fluid migration along fracture planes, but fracturing would no longer be active during ductile deformation.</p>
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Crustal shear zones are believed to have formed at deeper levels under high ductile, high PT, conditions. Subsequent rise of the shear zone rocks up to the surface, involves a series of deformational and metamorphic processes which leave their effects/imprints in the form of new structures, textures, fabric, and the related petrographic, mineralogical and geochemical features, all of which can be observed and measured. All these elements can be restructured or reorganized to develop quantitative models.Depending upon the type of available data, only one or some specific models for a shear zone can be developed. Amathematical modelcan be developed when the measurable variables (or the controlling factors) of a shear zone can be approximated by some limited range. A mathematical model can be made more realistic if some random components of the shear zone (or the system) - e.g. vertical uplift, rise or fall of temperature, etc-can be considered. This gives rise to astochastic modelfor a shear zone. If the present-day tectonic set-up of a shear zone can be considered in terms of the sum total of a few structural processes (as outlined in the paper) together with one or two random components (outlined in the paper) operative in the system, astochastic processmodel can be developed for the shear zone. Aprocess-response model can be developed when the structural/tectonic factors that constitute the “causes” - i.e. process elements - and those constituting the “effects” - i.e. response elements - for a shear zone are known. A shear zone may take up alinear modelwhen all the associated structural data that have given rise to the development of the shear zone are directly observable and measurable in field sections, such that the observational data yield a certain linear relation of the type y=a+bx.The method of developing a quantitative model for a shear zone has been demonstrated in the paper by taking the actual situation of a shear zone.
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