Fold and cleavage development within cambrian metasediments of the vale of ffestiniog, North Wales
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Abstract New road cuttings have exposed a large part of the Merioneth Series. The rocks are low‐grade thinly layered metasediments which have undergone extensive buckling resulting in tectonic ripples, mullions, and crenulations. It is suggested that the periclinal form of these structures may be the result of buckling layered sequences with longitudinal and transverse zones of constraint (heterogeneities). The process is analogous to ‘quilting’ of thin metal sheets and the heterogeneities are perhaps due to layer thickness variation. There are two foliations; which may form a progressive sequence of cleavage development during the evolution of the fold style.Keywords:
Cleavage (geology)
Crenulation
Crenulation
Cleavage (geology)
Muscovite
Pressure solution
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Inclined northeast‐vergent asymmetric regional F1 folds cut by thrust faults characterize a fold‐to‐fault transition into the Mt Wellington Fault Zone on the eastern margin of the Melbourne Zone of central Victoria. In the most southerly exposed part of the Mt Wellington Fault Zone at Lake Glenmaggie, polydeformed interbedded sandstone and mudstone of Silurian(?) age are unconformably overlain by weakly deformed Late Devonian conglomerate and volcanic rocks. Three deformations pre‐date the Late Devonian succession, which has itself been warped into a series of open folds. Thrusting (D1) along southwest dipping reverse faults and thrusts produced imbrication and juxtaposition of different lithological groups. Fault slices of strongly cleaved, interbedded sandstone and mudstone abut against a weakly deformed mudstone‐siltstone sequence, thereby suggesting significant displacement on some of these faults. Thrust sheets consisting of large panels of overturned homoclinally dipping strata represent the faulted out lower limbs of inclined to overturned regional F1 folds. Northwest‐trending polydeformed high strain zones (slides), varying from 50 to 800 m in width, cut across the regional F1 folds. These slides are dominated by strong D2 deformation, typified by refolded F1 folds and subvertical northwest‐trending crenulation cleavage (S2). The D3 deformation is more regionally extensive and characterized by a subvertical north‐trending crenulation cleavage (S3) which locally overprints the D1 and D2 structures within the high strain zones. In slide zones where D2^D3<30°, D3 structures reorient the D2 structures rather than producing F3 fold interference structures. The marked increase in intensity of deformation from west to east across the Melbourne Zone, coupled with a development of fold asymmetry, is typical of shear strain gradients from shallow to deeper levels within thrust belts. Both the direction of movement on faults and the sense of asymmetry of the F1 folds indicate tectonic transport from the southwest to the northeast. This suggests that the Melbourne Zone is allochthonous and has been involved in eastward‐transport along a major mid‐crustal detachment whose surface expression is considered to be the Mt Wellington Fault Zone.
Crenulation
Imbrication
Devonian
Conglomerate
Thrust fault
Echelon formation
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Abstract Detailed 3‐D analysis of inclusion trails in garnet porphyroblasts and matrix foliations preserved around a hand‐sample scale, tight, upright fold has revealed a complex deformation history. The fold, dominated by interlayered quartz–mica schist and quartz‐rich veins, preserves a crenulation cleavage that has a synthetic bulk shear sense to that of the macroscopic fold and transects the axis in mica‐rich layers. Garnet porphyroblasts with asymmetric inclusion trails occur on both limbs of the fold and display two stages of growth shown by textural discontinuities. Garnet porphyroblast cores and rims pre‐date the macroscopic fold and preserve successive foliation inflection/intersection axes (FIAs), which have the same trend but opposing plunges on each limb of the fold, and trend NNE–SSW and NE–SW, respectively. The FIAs are oblique to the main fold, which plunges gently to the WSW. Inclusion trail surfaces in the cores of idioblastic porphyroblasts within mica‐rich layers define an apparent fold with an axis oblique to the macroscopic fold axis by 32°, whereas equivalent surfaces in tabular garnet adjacent to quartz‐rich layers define a tighter apparent fold with an axis oblique to the main fold axis by 17°. This potentially could be explained by garnet porphyroblasts that grew over a pre‐existing gentle fold and did not rotate during fold formation, but is more easily explained by rotation of the porphyroblasts during folding. Tabular porphyroblasts adjacent to quartz‐rich layers rotated more relative to the fold axis than those within mica‐rich layers due to less effective deformation partitioning around the porphyroblasts and through quartz‐rich layers. This work highlights the importance of 3‐D geometry and relative timing relationships in studies of inclusion trails in porphyroblasts and microstructures in the matrix.
Crenulation
Foliation (geology)
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Crenulation
Lineation
Shearing (physics)
Cleavage (geology)
Syncline
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Abstract Low grade metasediments and metavolcanics of the Hill End Synclinorial Zone within the Rockley district, NSW have experienced two phases of macroscopic folding (D1 and D2), both of which are post‐latest Silurian in age. No hiatus is evident between D1 and D2. D1 produced large Fi folds (λ/2 usually > 2 km) lacking mesoscopic elements and having variable axial trends. D2 was associated with the development of regional slaty cleavage (S2) and mesoscopic folds which are parasitic on plunging macroscopic F2 folds (λ/2=0.4–2 km). D2 strain is variable, being most intense in the north of the district where slaty cleavage and tight mesoscopic F2 folds are well developed, and weakest in the south where mesoscopic folds are absent or usually gentle and cleavage is often feebly developed even in mica‐rich rocks, which are stratigraphic equivalents to slates and schists in the north. The F1 fold mechanism may involve multiple folding, simultaneous folding in more than one direction, or complex buckling of layers of variable thickness. D1 and D2 are tentatively correlated with folding events elsewhere in the Hill End Synclinorial Zone. Key Words: Lachlan Fold Belt tectonicslayer thickness variationsRockley districtsuperposed folds Notes Present address: Department of Geology, Bendigo College of Advanced Education, Bendigo, Vic. 3550, Australia.
Crenulation
Mesoscopic physics
Cleavage (geology)
SLATES
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Abstract Published descriptions and new field observations in Ireland and Canada confirm that strain-slip cleavage should not be classified with fracture cleavage. The term crenulation cleavage is proposed to replace the generic one of strain-slip cleavage. Crenulation cleavage differs essentially from slaty and fracture cleavage in that it only develops in laminated rocks and is consequently nearly always a secondary structure. Intense metamorphism converts crenulation cleavage into a new schistosity or cleavage with characteristics similar to slaty cleavage. Two fundamentally different types are recognized and it is suggested that crenulation cleavage is a valuable indicator of metamorphic conditions during the various phases of deformation.
Crenulation
Cleavage (geology)
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Crenulation
Cleavage (geology)
Bedding
Flysch
SLATES
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Crenulation
Overprinting
Anticline
Bedding
Foliation (geology)
Cleavage (geology)
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Crenulation
Cleavage (geology)
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Citations (18)