Small-scale convection induced by passive rifting: the cause for uplift of rift shoulders
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East African Rift
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Crustal stress pattern provide important information for the understanding of regional tectonics and for the modelling of seismic hazard. Especially for small rifts (e.g. Upper Rhine Graben) and beside larger rift structures (e.g. Baikal Rift, East African Rift System) only limited information on the stress orientations is available. We refine existing stress models by using new focal mechanisms combined with existing solutions to perform a formal stress inversion. We review the first-order stress pattern given by previous models for the Upper Rhine Graben, the Baikal Rift, and the East African Rift System. Due to the new focal mechanisms we resolve second-order features in areas of high data density. The resulting stress orientations show dominant extensional stress regimes along the Baikal and East African Rift but strike-slip regimes in the Upper Rhine Graben and the interior of the Amurian plate.
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Echelon formation
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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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The 1.1 Ga Mid-Continent rift system extends from Kansas through the Lake Superior region and into southern Michigan. The rift is filled with thick sequences of basalt and clastic sedimentary rocks, which are now mostly buried beneath Paleozoic rocks. Rocks of the rift system are exposed only in the Lake Superior region and comprise the Keweenawan supergroup. Seismic reflection surveys by GLIMPCE in 1986 imaged much of the deep structure of the rift beneath the lake in detail. Reflection profiles reveal a deep asymmetrical central graben whose existence and magnitude was not previously documented. Volcanic and sedimentary rocks, in places greater than 30 km thick, fill the central graben, which is bounded by normal growth faults. Thinner volcanic and sedimentary units lie on broad flanks of the rift outside of the graben. Near the rift axis, the pre-rift crust is thinned to about one-fourth of its original thickness, apparently by low-angle extensional faulting and ductile stretching or distributed shear. The sense of asymmetry of the central graben changes along the trend of the rift, documenting the segmented nature of the structure and suggesting the existence of accommodation zones between the segments. The location of the accommodation zones is inferred frommore » abrupt disruptions in the Bouguer gravity anomaly associated with the rift. Late uplift of the central graben transposed graben-bounding normal faults into high-angle reverse faults with throws of 5 km or more.« less
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Tectonic stress field in rift systems a comparison of Rhinegraben, Baikal Rift and East African Rift
Crustal stress pattern provide important information for the understanding of regional tectonics and for the modelling of seismic hazard. Especially for small rifts (e.g. Upper Rhine Graben) and beside larger rift structures (e.g. Baikal Rift, East African Rift System) only limited information on the stress orientations is available. We refine existing stress models by using new focal mechanisms combined with existing solutions to perform a formal stress inversion. We review the first-order stress pattern given by previous models for the Upper Rhine Graben, the Baikal Rift, and the East African Rift System. Due to the new focal mechanisms we resolve second-order features in areas of high data density. The resulting stress orientations show dominant extensional stress regimes along the Baikal and East African Rift but strike-slip regimes in the Upper Rhine Graben and the interior of the Amurian plate.
East African Rift
Half-graben
Rift zone
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The Erlian Basin is a continental rifting basin which is constituted by faulted sags in the styles of graben and/or half-graben controlled by NE-NNE striking normal faults and was filled by the Lower Cretaceous.A lot of faulted-sags concentratedly distributed in three rifting zones extending in different directions(Manite-Wulanchabu rift zone,Wunite rift zone,and Chuanjing-Tenggar rift zone),and a few faulted-sags were scattered over the uplift around the rift zone in the Erlian Basin.The basement of basin was constructed by multicycle tectonism and geological process before the Cretaceous,which was of heterogeneity in crustal nature and had a variety of structure lines.The strong deformation zones were mounted with weak deformation blocks;the former included fold-thrust zone,suture zone and accretionary wedge;the latter included magmatic rock bodies,magmatic arcs or micro continental blocks,etc.The distribution and structural style of faulted-sags were closely related to basement structure.The rift zone is also the strong deformation zone of basement.The various directions in basement strong deformation zone have caused the difference of rift zone in structural style and their arrangement of faulted-sags. Manite-Wulanchabu rift zone which superimposed on the Wuzhumoqin-Erlian curve fold-thrust zone that projected to southeast direction is the orthogonal rifting structure chaining graben and/or half-graben in NNE-NE striking in series.Wunite rift zone which superimposed on the Erlian-Hegenshan suture zone(melange zone)in NEE-striking is of oblique rifting structural feature combining graben and/or half-graben together in right-step echelons.Chuanjing-Tenggar rift zone which superimposed on the Wenduermiao-Xinamulun suture zone and accretionary wedge of north boundary of North China Craton in E-W striking has also the typical oblique rifting structural feature combining graben and/or half-graben together in high-angle echelons or parallel connection.The Early Cretaceous faulted sags within orthogonal rift zone included long narrow but deep graben and/or half-graben.The Early Cretaceous faulted sags within oblique rift zone included short wide but shallow graben and/or halfgraben.Above-mentioned geological features demonstrate that the distribution and structural style of faultedsags in Erlian Early Cretaceous basin was controlled by basement structure.
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East African Rift
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