A Framework of Microtectonic Studies
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Overprinting
Lineation
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Abstract The Late Triassic - Early Jurassic non marine clastic sediments of the Molteno, Elliot and Clarens Formations were studied to deduce their mineralogy and tectonic provenance. The study is based on road-cut exposures of the formations in the Eastern Cape Province of South Africa. Petrographic studies based on quantitative analysis of the detrital minerals shows that the clastic sediments (mostly sandstones) are predominantly made up of quartz, feldspars, and metamorphic and igneous rock fragments. Among the main detrital framework grains, quartz constitutes about 62-91%, feldspar 6-24% and 3-19% of lithic fragments. The sandstones can be classified as both sublitharenite and subarkose. Although, most of the sandstones (> 70 %) plotted in the sub-litharenite field. Petrographic and XRD analyses revealed that the sandstones originated from granitic and metamorphic rock sources. The QFL (Quartz-feldspar-lithic fragments) ternary diagrams indicate that the sandstones were derived from recycled or quartzose source rocks reflecting a craton interior or transitional continental setting which probably came from the Cape Fold Belt. This possibly revealed that most of the sandstones might have been derived as a result of weathering and erosion of igneous and metamorphic rocks in the Cape Supergroup. The study has revealed the depositional environments, and provide a basis for the description and interpretation of the sedimentology of the Molteno, Elliot and Clarens Formations.
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The median antiformal axis of the Otago Schist, New Zealand, is marked by a zone of relatively high‐grade (up to garnet‐biotite‐albite) greenschist facies rocks. 40Ar/39Ar geochronology has been carried out in conjunction with structural analysis in regions distant from the effects of the Alpine Fault to determine the origin of this metamorphic welt. We have determined that the metamorphic welt is bounded on its northern and southern sides by multistage ductile shear zone(s) marked by intensely developed fabrics, and/or low‐angle normal faults. These structures extend over a strike length of >∼200 km and on the southern side of the metamorphic welt they mark the boundary between the Caples and the Torlesse terranes. The oldest such shear zone formed between 122 and 118 Ma. The metamorphic welt was exhumed beneath low‐angle normal faults and ductile shear zones that formed from 112 to 109 Ma. The shear zones form the carapace to elongate domal culminations in the central Otago Schist. These geomorphological features are Cretaceous metamorphic core complexes dissected by younger Quaternary faults. Exhumation of the shear zones occurred shortly before volcanogenic sediments began to deposit on a Cretaceous unconformity. We propose that both the Cretaceous unconformity and the underlying ductile shear zones and low‐angle faults are a direct result of extensional tectonism. The ductile shear zones display both 'cross‐belt' and 'belt‐parallel' stretching lineations. Cross‐belt stretching may have been caused by rollback of the subducting Pacific slab. Belt parallel extension is interpreted to have taken place during extension associated with rifting between Australia and Antarctica, prior to breakup.
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Greenschist
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Detachment fault
Mylonite
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The Solli Hills pluton (250 km 2 ) in northern Nigeria is an example of a Pan‐African granite emplaced circa 580 Ma ago in the Jos ‐ Adamawa block. Its micro structures and internal fabrics have been studied in order to help constrain the kinematics of the Pan‐African orogeny in this block. We used field observations and measurements, petrographic examinations, and measurements of the magnetic susceptibility and anisotropy of magnetic susceptibility of 124 regularly spaced stations. Most samples are ferromagnetic, i.e., belong to the magnetite‐series granites, and present predominantly magmatic microstructures. Particularly remarkable are the asymmetrical zoning of the petrographic types, the susceptibility magnitudes, and the planar ‐ linear magnetic fabrics. The magnetic foliations and lineations are very consistent in the pluton, around N 20°E, 60°W and N 205°E, 10°, respectively. Field and microscopic observations reveal that deformation occurred everywhere in the pluton and immediate country‐rock envelope during a dextral strike‐slip synplutonic episode. It is suggested that the Solli Hills pluton was emplaced in a late Pan‐African dextral tear‐fault system. The striking fabric asymmetry of the pluton strongly suggests a shear zone termination emplacement model.
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Abstract Curvilinear steep shear zones originate in different tectonic environments. In the Chottanagpur Gneiss Complex (CGC), the steeply dipping, left-lateral and transpressive Early Neoproterozoic Hundru Falls Shear Zone (HFSZ) with predominantly north-down kinematics comprises two domains, e.g., an arcuate NW-striking (in the west) to W-striking (in the east) domain with gently plunging stretching lineation that curves into a W-striking straight-walled domain with down-dip lineation. The basement-piercing HFSZ truncates a carapace of flat-lying amphibolite facies paraschist and granitoid mylonites, and recumbently folded anatectic gneisses. The carapace—inferred to be a midcrustal regional-scale low-angle detachment zone—structurally overlies an older basement of Early Mesoproterozoic anatectic gneisses intruded by Mid-Mesoproterozoic/Early Neoproterozoic granitoids unaffected by the Early Neoproterozoic extensional tectonics. The mean kinematic vorticity values in the steep HFSZ-hosted granitoids computed using the porphyroclast aspect ratio method are 0.74–0.83 and 0.51–0.65 in domains with shallow and steep lineations, respectively. The granitoid mylonites show a chessboard subgrain microstructure, but lack evidence for suprasolidus deformation. The timing relationship between the two domains is unclear. If the two HFSZ domains were contemporaneous, the domain of steep lineations with greater coaxial strain relative to the curvilinear domain formed due to strain partitioning induced by variations in mineralogy and/or temperature of the cooling granitoid plutons. Alternately, the domain of gently plunging lineations in the HFSZ was a distinct shear zone that curved into a subsequent straight-walled shear zone with steeply plunging lineation due to a northward shift in the convergence direction during deformation contemporaneous with the Early Neoproterozoic accretion of the CGC and the Singhbhum Craton.
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The Cloncurry Fault Zone is a north-northwest-trending zone of complex deformation over 100 km long and up to 7 km wide near the eastern edge of the Mt Isa Inlier. The zone includes mylonites in an anastomosing shear-zone system with variably plunging mineral lineations within a north-northwest subvertical girdle that formed synchronously with north-northwest- and south-southeast-plunging folds. No clear overprinting of lineations in different orientations is observed, and the complexity of the penetrative fabrics can be attributed to strain partitioning during east-northeast contraction, rather than requiring a more complex history of overprinting relations. The mylonites formed at temperatures of 350–500°C, below the ∼650°C metamorphic peak that occurred during the regional D2 event, and they are superimposed on Maramungee-aged granites (1555–1545 Ma), implying that the majority of fabrics formed during D3. D3 was followed by the development of a D4 sinistral Riedel strike-slip fault system involving east-southeast contraction, which was coincident with massive Na–Ca alteration and brecciation within the zone. Reactivation with a normal component of movement occurred some time after the Jurassic. The Cloncurry Fault Zone is a crustal-scale feature of the Mt Isa inlier that records strain partitioning and a deformation history lasting over 1.5 Ga.
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A major tectono-metamorphic event affected the southern half of the Northern Marginal Zone of the Limpopo Belt at 2.0 Ga. Structural analysis along a 43 km-long profile perpendicular to strike shows extension inclined toward the northeast, extension parallel to axes of folds that uplift the southeast of the section (i.e., the Central Zone of the Limpopo Belt), and dextral shear. These deformation patterns developed coevally. The lineations form distinct, tightly clustered groups that occur together in the profile: there is no evidence for superimposed deformation or deformed lineations, nor are lineations observed cross-cutting each other. This single episode of deformation can therefore be called transpression. Petrographic and thermobarometric evidence support this kinematic picture, in that decreases of both pressure (2.5 kb) and temperature (150°C) are documented from southeast to northwest across strike. $$^{39}Ar-^{40}Ar$$ step-heating of metamorphic hornblende from amphibolites has yielded ages of 2.00 Ga in the northwest; based on a redefinition of closure temperatures in amphiboles, we propose that this is the age of the peak conditions (6.5 kb, 600-650°C) of the transpressional episode. This age is in agreement with existing Pb/Pb and Sm/Nd garnet ages for the dextral shear in the southeast. In contrast, biotite in the northwest yields an Rb/Sr age of 1.86 Ga and coupled with existing garnet and sphene data, this suggests a post-transpressional cooling rate of $$1.0 \pm 0.5 K/m.y$$. We obtained five near-plateau ages between 2.00 and 1.94 Ga in chemically diverse hornblendes from adjacent amphibolites. Because the hornblendes show negligible evidence for mineralogical and microstructural recrystallization, this age spread probably reflects chemically dependent diffusivity differences upon uniform slow cooling; it can be quantitatively modeled using the concept of "ionic porosity," Z. In our samples, Z ranges from 36.7% to 37.8%; the extreme values correspond to a $$41 \pm 9 K$$ difference in the model closure temperature. This translates into a model cooling rate of $$0.7\pm_{0.3}^{0.9} K/m.y$$. Within error this agrees with the independent cooling rate from garnet, sphene, and biotite. Biotites in the southeastern block yield an Rb/Sr age of 1.97 Ga, suggesting faster cooling than in the northeast. Contrasting cooling histories north and south of the studied Transition Zone following a 2.00 Ga tectonometamorphism entirely independent of and superimposed on the late Archean event negates the hypothesis of slow cooling after a supposed 2.65 Ga Tibetan-style orogeny. Vital parts of the evidence for such late Archean continental collision models for the Limpopo Belt can now be shown to be early Proterozoic in age.
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Sediments of Ouachita facies extend subsurface from the Ouachita Mountains of Oklahoma and Arkansas to the Marathon and Solitario uplifts of extreme southwest Texas. Although these sediments have been subjected to strong dynamic metamorphism, the metamorphic grade attained is uniformly low. Sediments of Ouachita facies are essentially unaltered in some areas; elsewhere they range in degree of metamorphism as high as the biotite zone. Mineralogical change and cataclastic effects are key criteria for discerning the stages of progressive metamorphism in sediments of Ouachita facies. Metamorphic changes are most easily observed in shales and argillaceous sandstones--limestones and cherts react less conspicuously to metamorphic stress. Mineralogical change resulting from metamorphism reduces the porosity and non-fracture permeability of sandstones to such an extent that they are not good petroleum reservoirs. The petrography of the so-called schists of Luling field, Caldwell County, Texas, and of some other deep wells along the Luling-Mexia-Talco fault system is discussed briefly. It is shown that they are generally comparable in mineralogy and degree of metamorphism with sediments exposed in the Ouachita Mountains of McCurtain County, Oklahoma. End_of_Article - Last_Page 2626------------
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