The evolution of the Hornelen Basin detachment system, western Norway: Implications for the style of late orogenic extension in the southern Scandinavian Caledonides
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Greenschist
Crenulation
Metamorphic core complex
Mylonite
Abstract The relationships between brittle detachment faulting and ductile shear zones in metamorphic core complexes are often ambiguous. Although it is commonly assumed that these two structures are kinematically linked and genetically related, direct observations of this coupling are rare. Here, we conducted a detailed field investigation to probe the connection between a detachment fault and mylonitic shear zone in the Ruby Mountain–East Humboldt Range metamorphic core complex, northeast Nevada. Field observations, along with new and published geochronology, demonstrate that Oligocene top-to-the-west mylonitic shear zones are crosscut by ca. 17 Ma subvertical basalt dikes, and these dikes are in turn truncated by middle Miocene detachment faults. The detachment faults appear to focus in preexisting weak zones in shaley strata and Mesozoic thrust faults. We interpret that the Oligocene mylonitic shear zones were generated in response to domal upwelling during voluminous plutonism and partial melting, which significantly predated the middle Miocene onset of regional extension and detachment slip. Our model simplifies mechanical issues with low-angle detachment faulting because there was an initial dip to the weak zones exploited by the future detachment-fault zone. This mechanism may be important for many apparent low-angle normal faults in the eastern Great Basin. We suggest that the temporal decoupling of mylonitic shearing and detachment faulting may be significant and underappreciated for many of the metamorphic core complexes in the North American Cordillera. In this case, earlier Eocene–Oligocene buoyant doming may have preconditioned the crust to be reactivated by Miocene extension, thus explaining the spatial relationship between structures.
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Abstract: The Yunmeng Shan metamorphic core complex (MCC) is composed of the lower plate, the upper plate and the detachment zone. The detachment zone consists of ductile shear zone (mylonite zone), chloritized microbreccias zone and the brittle fault plane. The ductile shear zone contains mylonitic rocks, protomylonites, and mylonites. Finite strain measurements of feldspar porphyroclasts from those rocks using the R f /φ method show that the strain intensities increase from mylonitic rocks ( Es =0.66–0.72) to protomylonites ( Es =0.66–0.83), and to mylonites ( Es =0.71–1.2). The strain type is close to flatten strain. Kinematic vorticity estimated by Polar Mohr diagrams suggest that foliations and lineation of mylonite (0.47< W k <0.85) record a bulk simple‐dominated general shearing at the initial evolution stage of the Yunmeng Shan MCC's detachment zone; and the extensional crenulation cleavage(ecc) (0.34<W k <0.77) recorded a bulk pure‐dominated general shearing at the later stage of the evolution. Kinematic vorticity measurements also show that the Yunmeng Shan MCC detachment zone is a result of a combination of simple‐dominated general shearing caused by crustal extension at the early stage and pure‐dominated general shearing caused by MCC uplifting at the late stage. The ductile thinning estimated by finite strain measurements and estimation of Kinematic vorticity ranges from 52% to 82%, which is the minimum thining estimation. Our studies provide new evidence for mechanisms of the Yunmeng Shan MCC detachement zone.
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Mylonites in the Dabie orogen are fault and shear zone related rocks formed under metamorphic conditions.They can be divided into greenschist,amphibolite,granulite,and eclogite facies mylonites.The main criterion for identifying the mylonites in the Dabie Mountain is the ductile(plastic)deformation of index minerals of the constituent metamorphic facies.This is clearly different from the former definition and description of mylonite.Formation epoch of the mylonites in the Dabie Mountain was approximately the same as(or a little later than)that of peak metamorphism of their host rocks.From older to younger,the series follows the order eclo-gite facies mylonite→granulite facies mylonite →amphibolite facies mylonite→greenschist facies mylonite.According to lithology and texture,the mylonites can be further divided into second and third subclasses if necessary.Former mylonites of eclogite and granulite facies are always superimposed by the later facies and show the appearance of amphibolite or even greenschist facies mylonites;the earlier formed mylonites are thus preserved as relics in their retrograded products.Mylonites in the low-grade metamorphic rocks in southern and northern Dabie Mountain only experienced the metamorphic history of greenschist and amphibolite facies mylonitization during the Paleozoic-Triassic period.
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Abstract The article describes the characteristics of the Yagan metamorphic core complex, especially the associated detachment fault and various extensional structures in its footwall. The age of the complex is discussed in some detail as well. The basic features of the Yagan metamorphic complex (Jurassic in age) are similar to those of the metamorphic core complex (Tertiary in age) in the Cordilleran area; they are as follows: (a) mylonitic gneisses in the footwall, (b) chloritized sheared mylonitic rocks, (c) pseudotachylites and flinty cataclasites or microbreccias, (d) unmetamorphosed or epimetamorphic rocks in the hanging wall with a layer of fault gouges or incohesive fault breccia next to the detachment fault. In contrast to its Cordilleran counterpart, however, there are many extensional faults with different styles (from dactile low‐angle normal faults through brittle — ductile to brittle high — angle normal faults) in the footwall.
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Late Jurassic to Early Cretaceous extensional structures are widespread in Northeast China and the eastern part of the Asian continent. The Yiwulüshan ductile detachment shear zone, the master ductile detachment zone of the Yiwulüshan metamorphic core complex (MCC), occurs in the northern North China Craton. This shear zone dips to the northwest on the western flank and to the southeast on the eastern flank of the MCC and consists of brittle faults, breccia, chloritized microbreccia, and ductile (mylonite) zones. The well-developed mylonitic foliation and stretching lineation are characterized by oriented minerals (quartz and feldspar) that exhibit ductile deformation, including undulose extinction, deformation bands, subgrain formations, and recrystallized grains. The shear indicators, such as asymmetric porphyroclasts, S-C fabrics, and shear bands, suggest consistent top-to-the-NW shear on both the western and eastern flanks of the MCC. Strain measurements using feldspar porphyroclasts show that the strain intensity (Es), Lode's parameter (υ), and Flinn parameter (k) range from 0.46 to 1.03, −0.23 to 0.62, and 0.23 to 1.6, respectively. Kinematic vorticity (Wk) values, estimated by the Polar Mohr diagram and the C′ methods, range from 0.61 to 0.96, with an average of 0.80, for the mylonitic deformation, and from 0.24 to 0.53, with an average of 0.37, for extensional crenulation cleavage (ecc) domains. The kinematic vorticity values, together with the regional structural background, suggest the Yiwulüshan ductile detachment zone experienced simple-shear-dominated general shearing that formed the mylonitic fabric in the early stages of extension. Later in the deformation history, pure-shear-dominated general shearing dominated when ecc developed. Strain measurements and kinematic vorticity estimates indicate that the Yiwulüshan ductile detachment zone is a lengthening-thinning shear zone. It is proposed that simple-shear-dominated general shear developed during crustal extension and that pure-shear-dominated general shear is related to the upward flow of Precambrian basement and Jurassic plutons.
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The Tatla Lake Metamorphic Complex (TLMC), which lies on the southwest side of the Intermontane Belt (IMB) in British Columbia, has characteristics typical of a metamorphic core complex: anticlinorial amphibolite‐grade gneissic and migmatitic core underlying a 1− to 2.5+‐km‐thick zone of mylonite and ductilely sheared metamorphic rocks which is in fault contact beneath an upper plate of low‐metamorphic‐grade cover rocks of the 1MB. Ductile shearing in the TLMC involved tonalitic to granodioritic orthogneiss and structurally overlying amphibolite‐grade metasedimentary rocks and greenschist‐grade chlorite‐actinolite‐albite schist. Structures observed throughout the ductilely sheared rocks include a gently dipping mylonitic foliation (Ss), containing a mineral lineation (Ls) which trends toward 280° (100°) ± 20°. Minor folds of variable trend (Fs), almost exclusively confined to metasedimentary rocks, are interpreted as synductile shear. Vergence of these folds defines a movement sense and direction of top toward 290° ± 20°. Kinematic indicators from rocks not deformed by synductile shear folds indicate a tops‐to‐the‐west sense of shear, while those within metasedimentary rocks (deformed by Fs folds) yield conflicting results, with a tops‐to‐the‐west sense predominating. Calculated a directions from Fs folds which deform Ls lineations indicate nearly horizontal Ds movement, subparallel to 290°–110°. The entire metamorphic core of the TLMC has been deformed by upright, west to west‐northwest trending, shallowly plunging map‐scale folds (F3). The steeply dipping, northwest trending Yalakom fault cuts all units and forms the southwestern margin of the TLMC. U‐Pb zircon geochronology has documented the existence of Cretaceous (107–79 Ma, in the core) and Eocene 55–47 Ma, (in the mylonitic zone) deformation and metamorphism in the TLMC. K‐Ar dates for biotite and hornblende of 53.4–45.6 Ma record the uplift and cooling of the TLMC. During early and middle Eocene time (55–47 Ma) metamorphic rocks of the TLMC were carried to higher crustal levels along the footwall of the TLMC normal ductile shear zone. Final uplift and development of F3 folds (post‐47 Ma) are possibly related to dextral motion along the Yalakom fault The TLMC has structural style and timing of deformation similar to other metamorphic core complexes in southeastern British Columbia. Local and regional evidence is consistent with the formation of the TLMC in a regional extensional setting within a vigorous magmatic arc. Similar interlayered gneisses with near‐horizontal layering and foliation may underlie the entire southern IMB.
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Petrology and structure of La Chilca Shear Zone, Catamarca. Field, petrological and structural studies allowed to define and and to characterize a ductile shear zone on the metamorphic basement of the northwestern Sierras Pampeanas. La Chilca Shear Zone has a NNW trend for about 24 km and a width of 1-4 km in the southern border of the Sierra de Aconquija and northern part of the Sierra de Ambato. It is composed by mylonites, protomylonites and blastomylonites that are the product of the ductile deformation that affected the metamorphic basement originally composed by migmatites and gneisses. The metamorphic conditions during the deformation reached greenschist facies to amphibolite facies, with temperatures around 350 to 500 oC. The dominant mylonitic foliation C is generally trending N 330°/42° ENE. On this plane there is a stretching mineral lineation of N68°/42°. Reverse displacements toward the southwest (N 248°) were determined for this shear zone. Internal local features of direct displacements suggest some complexities in the deformation kinematics.
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<p>Details of U-Pb zircon dating, complete datatables of U-Pb zircon dating, and a compilation of muscovite and biotite Argon dates.</p>
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Muscovite
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