Geometry, classification and kinematics of S-C and S-C′ fabrics in the Mushandike area, Zimbabwe

A Variscan continental collision of the West Sudetes and the Brunovistulian terrane: a contribution from structural and metamorphic record of the Stronie Formation, the Orlica-Ś nie_ znik Dome, SW Poland Abstract Structural, petrological and geochronological data from marbles and mica schists combined with those from highly disputable gneisses and eclogites of the Orlica-Ś nie_ znik Dome (OSD), gave new insights into Variscan evolution of eastern borderland of the West Sudetes. It is shown that the Variscan tectonometamorphic evolution of the OSD began with E-W oriented subhorizontal shortening (D1 stage) related to the collision of the West Sudetes terranes and the Brunovistulian terrane. The shortening led to generally upright folding, which resulted in formation of the steep N-S trending metamorphic planar fabric S1, thickening of the lithosphere and burial of the Stronie Formation under greenschist facies conditions. As a consequence of subsequent, near-coaxial gravity-controlled vertical shortening (D2), the S1 foliation was deformed in tight recumbent folds F2. The flattening strain was associated with the progression to amphibolite-facies conditions (from ca. 510 upto ca. 620°C) and uplift from depths corresponding to 9-10 kbar to depths corresponding to 7-8 kbar. On microscale, the flattening strain is documented by rotation of the mineral fabric overgrown by syn-D2 prograde garnet porphyroblasts. The Sm-Nd Grt-WR isochron age, correlated with the D2 event yields 346.5 ± 4.4 Ma. Further deformation and progressive metamorphism led to development of the subhorizontally disposed S2 axial plane schistosity, which terminated at the metamorphic temperature peak. During retrogression but still under ductile conditions of deformation, the S2 planes were reactivated during successive top-to-the-N shear movement of the OSD (D3 stage). Due to the mutual interaction of the OSD with the adjacent terranes the shear deformations were localized within marginal parts of these units. Finally, as a result of the NE-SW and the NW-SE oriented shortenings (D4 stage and D5 stage, respectively), both the structural surfaces and metamorphic isograds were regionally folded with W(NW)-ward plunges. A synthesis of new and existing data shows an overall similarity in sequence of Variscan deformations (D1-D2-D3) in the West Sudetes borderland to that observed in the eastern flank of the Moldanubian domain.

Section: D3mentioning

confidence: 99%

A Variscan continental collision of the West Sudetes and the Brunovistulian terrane: a contribution from structural and metamorphic record of the Stronie Formation, the Orlica-Śnieżnik Dome, SW Poland

Jastrzębski

2008

“…C 0 -type shear band cleavage is oblique to the shear zone boundaries and to the older foliation in mylonites (White 1979;Platt and Vissers 1980). As described by Dennis and Secor (1987), Passchier (1991), Blenkinsop and Treloar (1995), the angles between the shear bands and the shear zone margin are 15-35° ( Fig. 3e, h, i-k).…”

Section: Structural Observationsmentioning

confidence: 85%

A Late Jurassic–Early Cretaceous metamorphic core complex, Strandja Massif, NW Turkey

Elmas

Yılmaz

Yi̇ği̇tbaş

et al. 2010

In the eastern part of the Strandja Massif constituting the east end of the Rhodope Massif, the amphibolite facies basement rocks intruded by Permian metagranites are juxtaposed against the greenschist facies cover metasediments of Triassic-Middle Jurassic protolith age. The distinct metamorphic break between the basement and cover rocks requires a missing metamorphic section. The boundary between the two groups of rocks is a ductile to brittle extensional shear zone with kinematic indicators exhibiting a top to the E/NE shear sense. Footwall rocks are cut by weakly metamorphosed and foliated granite bodies which are clearly distinguished from the Permian metagranites by their degree of deformation, cross-cutting relations and syn-tectonic/kinematic character. Also, hangingwall rocks were intruded by unmetamorphosed and weakly foliated leucogranites. 40 Ar/ 39 Ar data indicate that the ductile deformation from 156.5 to 143.2 Ma (Middle Oxfordian-Earliest Berriasian) developed during the syntectonic plutonism in the footwall. Deformation, and gradual/slower cooling-exhumation survived until to 123 Ma (Barremian). The mylonitic and brittle deformation in the detachment zone developed during OxfordianEarliest Berriasian time (155.7-142.6 Ma) and Early Valanginian-Aptian time (136-118.7 Ma), respectively. Our new field mapping and first 40 Ar/ 39 Ar ages demonstrate the existence of an extensional core complex of Late Jurassic-Early Cretaceous age not previously described in the Rhodope/Strandja massifs.

“…Any one of the boundaries of these fish can have curvature less than those of the steep sigmoid fish. These fish resemble morphologically the 'banded type' porphyroclasts of Blenkinsop and Treloar (1995). From Fig.…”

Section: Morphological Classification and Usefulness As Shear Sense Imentioning

confidence: 95%

“…Within this limit of local orientation (a), the aspect ratio (R) ranges from 2.08 to 5.6 (14 measurements). Such mineral fish resemble 'r-Type porphyroclast system' of Simpson (1986), 'r a -Type porphyroclast system' of Passchier and Simpson (1986), 'porphyroclastic type' of Blenkinsop and Treloar (1995) and group-2 mica fish of ten Grotenhuis et al (2003). However, while the sigma structure is defined by tapering pressure shadows of other minerals at a pair of opposite corners of the porphyroblastic minerals (as per fig.…”

Section: Morphological Classification and Usefulness As Shear Sense Imentioning

confidence: 96%

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Mineral fish: their morphological classification, usefulness as shear sense indicators and genesis

Mukherjee

2010

Mineral fish are sheared and commonly asymmetric mineral grains or clusters of grains. This work reports 11 sub-types of mineral fish showing a top-to-SE sense of ductile shearing in the Karakoram Metamorphic Complex (KMC). The mineral fish are of three broad geometries: sigmoid, lenticular and parallelogram. Reliable senses of shear are indicated by the overall asymmetry and inclination of mineral fish. On the other hand, the true shear sense is not always indicated by either the orientations of their cleavage planes or those of the individual grains in composite mineral fish. The ranges of local orientations of single sigmoid mineral fish that include the lower values (\23°) in the KMC indicate their extensive ductile shearing. The studied mineral fish were products of a range of deformation mechanisms including homogeneous deformation, simple shear, intra-granular slip, crystal-plastic deformation, fracturing and synthetic shearing. Additionally, some examples might have undergone duplex slips and a few nucleated and grew either prior to or during the top-to-SE shearing.