The Sörbreen Fm mylonitic metagranites; a result of thrusting, or related to major strike-slip movement along the Billefjorden fault zone
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The Sorbreen Fm mylonitic metagranites; a result of thrusting, or related to major strike-slip movement along the Billefjorden fault zoneKeywords:
Mylonite
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Regional ductile thrusting and syn-kinematic granitic magmatism within the Caledonides of northern Scotland occurred within a sinistrally oblique convergent tectonic setting during the Silurian closure of the Iapetus Ocean. The highest thrust nappes are dominated by structures of probable Grampian (Ordovician) age, and Scandian (Silurian) deformation dominates the underlying thrust nappes. Deformation was overall foreland-propagating but the nappe stack was modified by out-of-sequence thrusting and probable synchronous development of thrusts at different structural levels. Localized dextrally transpressive deformation is related to an inferred lateral ramp located offshore. New U–Pb zircon ages from syn-tectonic granites indicate that the internal Naver Thrust was active between c. 432 and c. 426 Ma. This is consistent with other data sets that indicate that contractional deformation and high-grade metamorphism, and by implication displacements in the Moine Thrust Zone, may have lasted until c. 420–415 Ma. The synchroneity of thrusting and strike-slip movements along the Great Glen Fault implies that partitioning of transpressional strain occurred above a regional basal decollement. The short duration of the Scandian orogen in Scotland ( c. 437–415 Ma?) is consistent with only moderate crustal thickening and a location on the periphery of the main Laurentia–Baltica collision further north. Supplementary material: Details of analytical procedures, complete U-Pb isotopic data and methods of U-Pb age calculation and error reporting are available at https://doi.org/10.6084/m9.figshare.c.4962251
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Abstract The timing of Svalbard's assembly in relation to the mid‐Paleozoic Caledonian collision between Baltica and Laurentia remains contentious. The Svalbard archipelago consists of three basement provinces bounded by N–S‐trending strike–slip faults whose displacement histories are poorly understood. Here, we report microstructural and mineral chemistry data integrated with 40 Ar/ 39 Ar muscovite geochronology from the sinistral Vimsodden‐Kosibapasset Shear Zone (VKSZ, southwest Svalbard) and explore its relationship to adjacent structures and regional deformation within the circum‐Arctic. Our results indicate that strike–slip displacement along the VKSZ occurred in late Silurian–Early Devonian and was contemporaneous with the beginning of the main phase of continental collision in Greenland and Scandinavia and the onset of syn‐orogenic sedimentation in Silurian–Devonian fault‐controlled basins in northern Svalbard. These new‐age constraints highlight possible links between escape tectonics in the Caledonian orogen and mid‐Paleozoic terrane transfer across the northern margin of Laurentia.
Laurentia
Baltica
Devonian
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Thermochronology
Massif
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Abstract The Leannan Fault of north‐west Ireland is a sinistral strike‐slip fault system which juxtaposes Dalradian metasediments of differing structural trends and metamorphic grades. It probably represents a south‐west splay of the Great Glen Fault of Scotland. The recognition and tracing of the Foyle Synform across the fault zone, together with the correlation of regional Dalradian strike swings, lateral sedimentary facies variation and metamorphic grades, suggest a sinistral displacement of 34 km across the fault. Members of the Leannan Fault system displace a Lower Devonian (about 397 Ma) granite, but are overlain by Viséan (about 352 Ma) sandstones, thus constraining major late Caledonian sinistral motions to the Middle to Upper Devonian.
Dalradian
Devonian
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Transpression
Transtension
Greenstone belt
Batholith
Flysch
Forearc
Syncline
Pull apart basin
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Orogeny
Devonian
Detachment fault
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Abstract. In the Late Devonian, Svalbard was affected by a short-lived episode of contraction called the Ellesmerian (Svalbardian) Orogeny, which resulted in top-west thrusting of Proterozoic basement rocks onto Devonian sedimentary strata along the Balliolbreen Fault, a major fault segment of the east-dipping Billefjorden Fault Zone, and juxtaposition of undeformed Mississippian–Permian strata against intensely folded Devonian rocks. The present study of field and seismic data shows that backward-dipping duplexes comprised of phyllitic coal and bedding-parallel décollements and thrusts localized along lithological transitions in thickened uppermost Devonian–Mississippian coals and coaly shales of the Billefjorden Group partially decoupled uppermost Devonian–Permian sedimentary rocks of the Billefjorden and Gipsdalen groups from Devonian rocks during Cenozoic contraction–transpression. In addition, Devonian strata probably experienced syn-depositional, post-Caledonian, extensional, detachment-related folding. Seismic data in Sassenfjorden and Reindalspasset show the presence of Cenozoic duplexes and bedding-parallel décollements within Lower–Middle Devonian, uppermost Devonian–Mississippian and uppermost Pennsylvanian–lowermost Permian sedimentary strata of the Wood Bay and/or Widje Bay and/or Grey Hoek formations, of the Billefjorden Group and of the Wördiekammen Formation respectively, which further decoupled stratigraphic units during Eurekan deformation. Bedding-parallel décollements and thrusts are possibly related to shortcut faulting, a roof décollement of a fault-bend hanging wall (or ramp) anticline, an imbricate fan, antiformal thrust stacks and/or fault-propagation folds over reactivated/overprinted basement-seated faults. Seismic data in Reindalspasset also indicate that Devonian sedimentary rocks might have deposited east of the Billefjorden Fault Zone, thus ruling out Late Devonian reverse movement along the Billefjorden Fault Zone in this area. Based on the present findings, juxtaposition of Proterozoic basement rocks against Lower Devonian sedimentary rocks along the Balliolbreen Fault in central Spitsbergen (e.g., Pyramiden–Odellfjellet) may be explained by down-east Carboniferous normal faulting with associated footwall rotation and exhumation and subsequent top-west Cenozoic thrusting along the Billefjorden Fault Zone. The uncertain relationship of the Balliolbreen Fault with uppermost Devonian–Mississippian sedimentary strata, the poorly constrained nature of the contact (unconformity or bedding-parallel décollements and thrusts?) between Lower Devonian and uppermost Devonian–Mississippian sedimentary strata, and along strike variations in cross-section geometry, offset stratigraphy, and inferred timing and kinematics along the Balliolbreen Fault suggest that this fault consists of several, discrete, unconnected (soft-linked and/or stepping) or, most probably, offset fault segments that were reactivated/overprinted with varying degree during Eurekan deformation due to strain partitioning. Finally, recent evidence for Devonian core complex exhumation and reinterpretation of presumed Ellesmerian structures and of Late Devonian amphibolite facies metamorphism suggest that Ellesmerian contraction is not necessary to explain fault geometries and (differential) deformation within Devonian–Permian sedimentary strata in Spitsbergen.
Devonian
Extensional fault
Orogeny
Anticline
Late Devonian extinction
Basement
Thrust fault
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Thermochronology
Greenschist
Transpression
Devonian
Mylonite
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Abstract The nearly E‐W‐trending Aqqikkudug‐Weiya zone, more than 1000 km long and about 30 km wide, is an important segment in the Central Asian tectonic framework. It is distributed along the northern margin of the Central Tianshan belt in Xinjiang, NW China and is composed of mylonitized Early Palaeozoic greywacke, volcanic rocks, ophiolitic blocks as a mélange complex, HP/LT‐type bleuschist blocks and mylonitized Neoproterozoic schist, gneiss and orthogneiss. Nearly vertical mylonitic foliation and sub‐horizontal stretching lineation define its strike‐slip feature; various kinematic indicators, such as asymmetric folds, non‐coaxial asymmetric macro‐ to micro‐structures and C‐axis fabrics of quartz grains of mylonites, suggest that it is a dextral strike‐slip ductile shear zone oriented in a nearly E‐W direction characterized by “flower” strusture with thrusting or extruding across the zone toward the two sides and upright folds with gently plunging hinges. The Aqqikkudug‐Weiya zone experienced at least two stages of ductile shear tectonic evolution: Early Palaeozoic north vergent thrusting ductile shear and Late Carboniferous‐Early Permian strike‐slip deformation. The strike‐slip ductile shear likely took place during Late Palaeozoic time, dated at 269±5 Ma by the 40 Ar/ 39 Ar analysis on neo‐muscovites. The strike‐slip deformation was followed by the Hercynian violent S‐type granitic magmatism. Geodynamical analysis suggests that the large‐scale dextral strike‐slip ductile shearing is likely the result of intracontinental adjustment deformation after the collision of the Siberian continental plate towards the northern margin of the Tarim continental plate during the Late Carboniferous. The Himalayan tectonism locally deformed the zone, marked by final uplift, brittle layer‐slip and step‐type thrust faults, transcurrent faults and E‐W‐elongated Mesozoic‐Cenozoic basins.
Mylonite
Lineation
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