Crustal structure across the Xisha Trough, northwestern South China Sea
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Structural restoration has been carried out on the northern North Sea (60-62oN), based on the reprocessed, interpreted and depth converted seismic lines NSDP84-1 and 2. Two major rifting events have previously been recognized in the area during the Mesozoic: the Permo-Triassic and Jurassic extension phases. Different structures were formed or, in some cases, the same structures were reactivated during the Permo-Triassic and Jurassic rifting phases. Permo-Triassic rifting affected a 125 km wide area from the Oygarden Fault Zone in the east to the Hutton Fault alignment in the west.. By measuring the length of the profiles before and after faulting, the restorations show that the stretching factors for upper crustal stretching during the Permo-Triassic rifting are 1.11 (11%) for NSDP84-1 and 1.10 (10%) for NSDP84-2 respectively. The Jurassic rifting was confined to a narrower zone mainly in the Viking Graben with the major faults formed on the western side of the graben. Low angle faults are identified in the western flank of Viking Graben in the Tampen Spur area. Low angle supra-basement detachments formed in the late Jurassic are found in Gullfaks area, beneath the Gullfaks Sor block and SE of the Visund fault block. Intra-basement detachments are also found in Tampen Spur area. These detachments are formed by normal faults which flatten in the basement. From the restorations, the stretching factor for the Jurassic rifting is calculated to be 1.12 (12%) for NSDP84-1 and 1.19 (19%) for NSDP84-2. The total extensions for the two rifting phases combined are 1.24 (24%) and for NSDP84-1 and 1.30 (30%) for NSDP84-2. Stretching factors (β) can also be measured by crustal thickness changes, stretching is measured before and after rifting for different area (Horda Platform, Shetland Platform, Viking Graben, and Tampen Spur), and βmean calculate for the Permo-Triassic x rifting phase are calculated 1.25 and 1.16 for NSDP84-1 and 2 respectively. For the Jurassic rifting βmean is calculated as 1.16 for NSDP84-1 and 1.17 for NSDP84-2. These values are similar to previous published results using the same methods in the Northern North Sea and represent the minimum amounts of upper crustal extension on large seismically resolved faults.
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The Central Kerguelen Plateau (South Indian Ocean) is characterized by abundant north‐south striking normal faults, which comprise two prominent north‐south rifts known as the 77°E and 75°E grabens. The 77°E Graben is a well‐defined structure which extends over some 800 km from the eastern margin of the Kerguelen Plateau to about 58.5°S. Over most of its length it is associated with a 10–30 km wide axial rift and with a 100–150 km wide uplift. The 75 °E Graben is less well documented, but the available data suggest that its dimensions and internal structure resemble that of the 77°E Graben. In the better documented 77°E Graben, six rift segments, 50–100 km long, are identified. Faulting is more developed at the northern and southern ends of the 77°E Graben, possibly resulting from the interaction with other rifts. To the north, the 77°E Graben abuts the highly faulted eastern margin of the Kerguelen Plateau and the northern part of an even larger rift zone, the Plate Boundary Rift Zone, which extends along the boundary with the Australian‐Antarctic Basin. To the south, the 77°E Graben adjoins the northwestern end of the Southern Kerguelen Plateau Rift Zone. The 77°E and 75°E grabens, and the other rift zones on the Kerguelen Plateau, appear to have been formed at approximately the same time, between 72 and 60 Ma. They are all part of an important extensional phase which occurred in the region and mark the beginning of the process which led to the development of the Plate Boundary Rift Zone into the Southeast Indian Ridge, between 46 and 43 Ma. The north‐south trend of the 77°E and 75°E grabens is different from that of the other rift zones, which are oriented northwest‐southeast. This geometry suggests that some strike‐slip motion may have occurred along the north‐south trending grabens as a result of extension on the northwest‐southeast trending rifts, particularly the Southern Kerguelen Plateau Rift Zone. However, since near‐surface extension estimates for the Southern Kerguelen Plateau Rift Zone are small, the strike‐slip motion along the 77°E Graben must be equivalently small. Also, the available seismic data from this graben do not show typical seismic characteristics of a strike‐slip environment, such as zones of compression or flower structures. Thus the results of this work are inconsistent with any model for the development of the South Indian Ocean which requires significant amount of transform motion on the 77°E or 75°E grabens. Finally, the seismic data from this area provide a unique opportunity to compare rifting on an oceanic plateau environment with continental rifting. We find great similarities between the two processes, in the segmentation of the rifts, the asymmetric cross section, and the associated shoulder uplifts.
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Rift zone
East African Rift
Rift valley
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The Gulf Coast exhibits a unique association of rift faulting, a tectonic belt, and geosynclinal development. The peripheral graben system of the Gulf Coast is compared with the classic rift systems of Africa and Europe, and is shown to have many characteristic features in common. This rift system is composed of the Balcones-Luling, Mexia-Talco, South Arkansas, and Pickens-Gilbertown rifts. The Mexia-Talco rift is composed of en echelon grabens and half-grabens. These faults are the resultant of post-orogenic taphrogenic movement of the Ouachita mobile belt.
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Half-graben
Seafloor Spreading
Echelon formation
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Abstract Seven back‐arc rifts are recognized in the Izu‐Ogasawara Arc, namely, the Hachijo, the Aogashima, the Myojin, the Sumisu, the Torishima, the Sofu and the Nishinoshima Rifts from north to south. The acoustic stratigraphy is divided into three units (Units A, B and C) based on the seismic reflection profiles crossing the rifts. The structure of the rifts systematically changes from a half‐graben type to a full graben type in the back‐arc rifts from the Hachijo Rift to the Torishima Rift. The Hachijo and the Aogashima Rifts have a structure of half‐graben, and the Myojin Rift has both structural characteristics of a half‐graben and a full graben. The Sumisu and the Torishima Rifts are an asymmetric full graben. The Sofu and the Nishinoshima Rifts have different structural characteristics from the remaining rifts, from the Hachijo Rift to the Torishima Rift. The boundary faults in the back‐arc rifts from the Hachijo to the Torishima Rifts cut to Unit B. Unit B correlates with volcaniclastic sediments during pre‐rift volcanism between 4 and 2 Ma. The pre‐rift volcanism was probably widespread on the northern Izu‐Ogasawara Arc as is the present arc volcanism. These factors suggest that the beginning of rifting is dated at some time after 2 Ma. The developing process of the rift consists of three stages; (i) a sag stage in the crust at the location of the large offset boundary fault; (ii) a stage of half‐graben formation; (iii) a stage of full graben formation. The offset of the boundary faults becomes larger from the Hachijo Rift to the Torishima Rift and the east‐west width of the rifts also widens to the south. This is presumably because the Hachijo Rift is an earlier rifting stage than the Sumisu and the Torishima Rifts. The more primitive structure in the rifting stage from the Torishima Rift to the Hachijo Rift is probably caused by the propagation of rifting from south to north. The structural difference between the rifts in the northern part and the Sofu and the Nishinoshima Rifts seems to be due to structural differences in the crust between the northern and the southern parts from the tectonic gap.
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