Reconstruction and break-out model for the Falkland Islands within Gondwana
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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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New palaeomagnetic data from Lower Carboniferous granitoids of the orogenic root of the eastern Variscan belt (Moldanubian domain) show a polyphase palaeomagnetic record. Comparison of the data with existing palaeomagnetic measurements from Lower Palaeozoic sequences of the Bohemian Massif demonstrates a Carboniferous remagnetization of these latter units. All the data presented suggest that the Saxothuringian basement, the Moldanubian orogenic root system as well as the eastern Neoproterozoic Brunovistulian basement were already assembled during the Early Carboniferous. The whole Variscan belt subsequently rotated in a clockwise direction during Mid–Late Carboniferous times. Structural and geochronological data indicate that this rotation was accompanied by large-scale dextral wrenching along NW–SE-trending lithospheric faults. In a first stage, the blocks limited by wrench-faults rotated anticlockwise in bookshelf manner. The data presented rule out the existing model of oroclinal bending of the Rhenohercynian zone at the eastern termination of the Variscan belt.
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Paleomagnetic and geologic studies were made in the Uetsu area, NE Japan, to clarify the Miocene intra-arc deformation associated with the Japan Sea opening. We obtained paleomagnetic directions from the Early to Middle Miocene pyroclastic rocks and lavas in the area. The average paleomagnetic direction is D = 28.0°, I = 48.7° and α95 = 9.6°. It suggests that the clockwise rotation occurred in this area, which is opposite to the counter-clockwise rotation of NE Japan. A fault was discovered which could be the boundary of this rotating domain. The dextral movement is recognized for this fault, concordant with the clockwise rotation in the area. These paleomagnetic and structural data suggest that the clockwise block rotations occurred in the Uetsu area, accompanied by the dextral deformation.
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Image logs of 13 wells in the Rag Sefid Anticline show two systems of fractures developed under two deformation phase in the Dezful Embayment. This deformation phases are folding resulted from Zagros NE contraction and reactivation of the basement fault rotated the fold axial trace within the Hendijan-Izeh fault zone. Folding phase comprises four sets of fractures, which include axial and cross axial sets that trend parallel and perpendicular to the fold axial trace, respectively and two oblique sets that trend at moderate angles to the axial trace in the eastern part of the fold. Reactivation of the Hendijan-Izeh Fault has caused the restraining bend and dextral shear zone in the western part of the Rag Sefid Anticline. This dextral shear has produced three fracture sets which include two sets of Riedel shear fractures and an extensional set. The mean shortening directions measured from the fold and fault related fracture systems in the eastern and western parts of Rag Sefid Anticline are N022±2° and N064±1°, respectively. The measured NNE and ENE shortening orientations in the well sites is consistent with maximum horizontal stress orientations derived from earthquake focal mechanism solutions.
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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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