Sinistral transpressional and extensional tectonics in Dronning Maud Land, East Antarctica, including the Sør Rondane Mountains
Tsuyoshi ToyoshimaYasuhito OsanaiSotaro BabaTomokazu HokadaNobuhiko NakanoTatsuro AdachiMakoto OtsuboMasahiro IshikawaYoshifumi Nogi
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The opening of the Japan Sea backarc basin accompanied dextral, transtensional deformations in the NE Japan arc. To understand the intra-arc deformation we have studied paleomagnetism in the Uetsu area, NE Japan. Among the samples collected at 70 sites for paleomagnetic measurement reliable directions were obtained at 38 ones. About half of the sites yielded clockwise and the others show opposite declinations. Our data indicate that the northern Uetsu area rotated counterclockwise as a single block, whereas the southern Uetsu area was broken into a number of blocks that was rotated clockwise by the dextral transtension along the Nihonkoku-Miomote Line. Not only the Uetsu area but the entire eastern margin of the Japan Sea experienced such transtensional deformations in the Early to early Middle Miocene. The crust under NE Japan was broken into blocks but they rotated coherently with dextral transtension.
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The Periadriatic Line (PAL) is a remarkable, several hundred kilometer long fault system of the Alpine orogen. Its dextral character was documented by several authors using diverse criteria, but detailed kinematics and timing of movements had not been investigated along its whole length. Structural and paleomagnetic measurements, mapping, and stratigraphic and sedimentological studies have helped to unravel the Miocene‐Pliocene evolution of the Slovenian segment of the PAL. Brittle deformation was characterized by NW‐SE to N‐S compression and perpendicular tension. Deformation has resulted in dextral strike‐slip faulting, folding, and tilting of beds. The first transpressional event corresponds to the first phase of lateral extrusion of the East Alpine‐Western Carpathian‐Northern Pannonian block in the early Miocene (24–17.5 Ma). After a short period of transtension during the Karpatian (17.5–16.5 Ma), dextral transpression reoccurred during the middle Miocene to Pliocene and lasted up to the Quaternary. Middle Miocene dextral slip can be connected to the second phase of extrusion. The highly deformed rocks within the dextral shear zones show variable clockwise, sometimes counterclockwise, rotations. The mechanism of rotation seems to be complex, ranging from regional rotation to local folding due to pure or simple shear (domino‐type rotation).
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geodynamics
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SHEARING ZONES IN THE IZERA METAMORPHIC TERRAIN, SUDETEN (SW POLAND)
Summary
First structural data on the kinematics of deformations of Izera Metamorphic Terrain (IMT) in West Sudeten (SW Poland) are presented in the paper. New structural study of IMT supports the concept of existence of numerous largescale penetrative shear zones. These zones are defined by mylonitic Izera gneisses that deformed Rumburk granites to a different degree. Mylonitic are also three zones of mica schists running evenly with a parallel of latitude. Only lensoidal domains of Rumburk granites are devoid of shearing processes. Lineations in IMT are extensional (Lx) and not of Ly type (i.e. parallel to axis Y of ellipsoid of final deformation) as was previously thought. The kinematic analysis of IMT indicates a complex pattern of displacements with a distinct domination of sinistral transptession. The latter is characteristic for northern and north-eastern parts of IMT. However, in the southern part of IMT dominated conditions of sinistral transpression. Most probably during the sinistral transpression (D1) domains were thrusted towards the south west and the middle crust was tectonic ally thickened. Next during the sinistral transtension in the northern slope of elevated core of dome the crust became extented. In the core of dome the emplacement of Karkonosze granitoids took place. The youngest, rare and non-penetrative dextral shear zones (D3) are related to the later extension during the Stephanian and Early Permian.
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We discuss the opening mechanism of the Japan Sea in Miocene time using (1) tectonic and published paleomagnetic data along the eastern margin from the north of Hokkaido Island to Sado Island, (2) a mechanical model which is tested by small‐scale physical modeling, and (3) crustal structure and bathymetric features in the Japan Sea which constrain our kinematic model and preopening reconstructions. Our main conclusions are the following. The eastern margin of the Japan Sea was, as a whole, a dextral shear zone about 100 km wide. This conclusion is supported by the existence of a ductile dextral shear zone in Central Hokkaido (Hidaka Mountains) and associated brittle deformation in western Hokkaido and northeastern Honshu. The stress field during the opening (which ended about 12 Ma ago at the end of the middle Miocene) changes from right‐lateral transpression in the north to right‐lateral transtension in the south. The western margin, along the Korean peninsula, during the same period, also was an active dextral shear zone. Paleomagnetic results indicate that clockwise rotations occurred in the south during the opening and counterclockwise rotations in the north. We propose a model of right‐lateral pull‐apart deformation with clockwise rotations of rigid blocks in the southern transtensional domain and counterclockwise rotations in the transpressional one. Small‐scale physical models show that the clockwise rotation in transtension is possible provided that the eastern boundary (Pacific side) is free of stress. The opening stopped and compression subsequently began about 12 Ma ago. Finally, we show that the dextral shear, which is distributed over the whole Japan Sea area, is accommodated by N‐S trending right‐lateral faults and rotation of blocks located between these right‐lateral faults.
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Abstract Transected F 1 fold structures in eastern Ireland are associated with subhorizontal stretching in the S 1 , cleavage whereas axial planar cleavage contains a vertical elongation direction. This suggests that the non‐axial planar cleavage was influenced by a distributed strike‐slip ductile shear. A major NE‐SW trending F 1 syncline is described in which the minor F 1 folds show systematic variations in cleavage transection parameters. On the steep limb of the major syncline the cleavage transects the minor F 1 folds in a consistently clockwise sense, whereas on the normal limb anticlockwise transected folds are seen. Axial planar cleavage occurs at the core of the major syncline. Fold profile analysis indicates that the buckling of the layers began before the initiation of the cleavage. Open, parallel folds at the major synclinal hinge zone are progressively ‘flattened’ on the steep limb towards a major D 1 sinistral transcurrent fault. The angular transection, A, attains a maximum of 15° clockwise which diminishes to <5° at higher strains adjacent to the major fault. Incremental fibre growth in pressure shadows show a two‐stage tectonic strain superposition of vertical pure shear followed by sinistral transcurrent simple shear during the development of the clockwise transecting cleavage. Anticlockwise transected folds were influenced by local dextral strike‐slip on the southern margins of a rigid terrane. As a regional feature, the clockwise transection is explained by a sinistral transpressive deformation of end‐Silurian age.
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Oblique continent—continent collision between Iranian microcontinent and Arabian plate is the main cause of transpression and transtension regimes in this area. Zagros orogeny in this area resulted in formation of various structures including thrust fault-related folds, dextral and sinistral, strike slip faults, normal fault related to dextral strike slip fault. Thrust faults within the area under study are duplex with general trend of NW-SE and dip toward the north-east with right slip component. Strike slip fault regime acts as tear fault and it is active yet. Therefore, simultaneous presence of thrusts and strike slip faults illustrates convergent dextral transpressional tectonic regime while this transpressional regime accompanied with transtension as well; since normal faults are also seen in alluvium around depression of Sirjan which can be as a result of extension stresses due to strike slip faults activity of the district. The results achieved from geometry and kinematic analysis of west of Sirjan structures indicate that structures of the area have characteristics of internal part of Zagros orogeny.
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The late Silurian to mid- or late Devonian interval in the Caledonides was a period dominated, sequentially, by sinistral transpression, strike-slip and transtension during the development of mainly non-marine ‘red-bed’ basins following the Ludlow–PrÍdolÍ transition from marine to terrestrial sedimentation. The tectonic event that led to and generated the sinistral Devonian basins was the highly oblique sinistral closure of the Iapetus Ocean between Laurentia and Baltica and between Laurentia and Avalonia. We examine the diachronous closure of Iapetus, the contrasting tectonic modes arising from that closure, and the nature and origin of subsequent Devonian deformation north and south of the Iapetus Suture in the context of progressively changing, sinistrally dominated relative plate motion between Laurentia and Avalonia–Baltica. We suggest that, from about 435 to 395 Ma, there was about 1200 km of sinistral strike-slip relative motion between Laurentia and Baltica. Our lower and upper estimates of Silurian–Devonian relative plate motion rates of 30 mm a −1 and 67 mm a −1 based upon geological data, are similar to present rates.
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