Stratigraphic and structural evolution of the Selenga Delta Accommodation Zone, Lake Baikal Rift, Siberia
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Terrigenous sediment
Half-graben
Continental Margin
The flaggy cherts and argillite in the ophiolitic melange of Bayingou have a SiO2 content of 55.37%-91.19%,a high Al2O3 content between 3.07%-17.66%,and a TiO2 content of 0.12%-0.8%,having a good positive correlation between Al2O3 and TiO2,suggesting that they have a relatively high percentage of terrigenous muddy sediments. Moreover,they have typical NASC-normalized flat REE patterns without any visible Ce negative anomaly; and the Eu has an obvious right-hand dipping pattern. All these indicate a REE distribution pattern similar to that of deposited cherts and argillite found in continental margins. In addition,they have a Ce/Ce* ratio of 1.03-1.45,a (La/Ce)N of 0.62-0.92,a V/Y of 032-8.87,and a Th/U of 2.99-6.45,showing that the cherts and argillite are formed in continental margin settings into which the continental material can be easily transported. Therefore,the Bayingou ophiolite was formed in a transitional environment of continental rift to oceanic rift based on our study.
Terrigenous sediment
Continental Margin
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Terrigenous sediment
Continental Margin
Siliciclastic
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Geochemical characteristics of polymetallic nodules can reflect differences of their growth environments,and can show origins of their dominative factors.In this study,different samples of polymetallic nodules collected from the South China Sea,the Pacific and the Indian Ocean were analyzed using ICP-MS(AES).Their geochemical compositions and characteristics were compared among samples from different regions.According to the data,samples collected from the northwestern continental margin of the South China Sea were rich in Fe,Si,and ∑REE,while poor in Mn,Cu,Co and Ni.Heavy REE(HREE) was depleted relative to light REE(LREE) very obviously,the latter being 19.54 times less than the former.The ratio of Mn/Fe and element correlation of w(Mn)-w(Fe)- w(Cu+Ni) suggested that the origins of samples from the northwestern continental margin of the South China Sea and from a seamount of the Pacific may be of hydrogenic.Samples from sea basins of the Pacific may be diagenetic in origin,and the Indian Ocean samples were both hydrogenic and diagenetic in formation.Different from ocean samples,polymetallic nodules from the northwestern continental margin of the South China Sea have typical characteristics of a marginal sea which were marked by terrigenous sediments including rich elements of Fe,Si,Al and REE.These characteristics showed that the unique terrigenous sedimentary conditions and the changeful paleoceanic environment have an important effect on the growth of nodules in the South China Sea.
Terrigenous sediment
Continental Margin
Seamount
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Abstract In regions experiencing multiple phases of extension, rift‐related strain can vary along and across the basin during and between each phase, and the location of maximum extension can differ between the rift phase. Despite having a general understanding of multiphase rift kinematics, it remains unclear why the rift axis migrates between extension episodes. The role pre‐existing structures play in influencing fault and basin geometries during later rifting events is also poorly understood. We study the Stord Basin, northern North Sea, a location characterised by strain migration between two rift episodes. To reveal and quantify the rift kinematics, we interpreted a dense grid of 2D seismic reflection profiles, produced time‐structure and isochore (thickness) maps, collected quantitative fault kinematic data and calculated the amount of extension ( β ‐factor). Our results show that the locations of basin‐bounding fault systems were controlled by pre‐existing crustal‐scale shear zones. Within the basin, Permo‐Triassic Rift Phase 1 (RP1) faults mainly developed orthogonal to the E‐W extension direction. Rift faults control the locus of syn‐RP1 deposition, whilst during the inter‐rift stage, areas of clastic wedge progradation are more important in controlling sediment thickness trends. The calculated amount of RP1 extension ( β ‐factor) for the Stord Basin is up to β = 1.55 (±10%, 55% extension). During the subsequent Middle Jurassic‐Early Cretaceous Rift Phase 2 (RP2), however, strain localised to the west along the present axis of the South Viking Graben, with the Stord Basin being almost completely abandoned. Rift axis migration during RP2 is interpreted to be related to changes in lithospheric strength profile, possibly related to the ultraslow extension (<1 mm/year during RP1), the long period of tectonic quiescence (ca. 50 myr) between RP1 and RP2 and possible underplating. Our results highlight the very heterogeneous nature of temporal and lateral strain migration during and between extension phases within a single rift basin.
Half-graben
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Half-graben
Echelon formation
Rift zone
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Terrigenous sediment
Continental Margin
Margin (machine learning)
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Cambrian strata wildly spread in southwest Sichuan and are composed of marine terrigenous clastic rocks and carbonate rocks.After analyzing the data of drilling,logging and well logging,as well as facies marks,it is suggested that terrigenous clastic depositional system was developed in Early Cambrian(Qiongzhusi Stage and Canglangpu Stage)and mainly includes terrigenous clastic shore and shallow shelf facies;carbonate platform depositional system was developed in late Cambrian and is mainly composed of carbonate tidal flat,mixing tidal flat,half-restricted to restricted platform facies.Study on depositional characteristics and facies distribution shows that the material source of study area is from Kangdian old land,at that time the water depth progressively increase from northwest to southeast,and the distribution of depositional facies is orderly oriented in the direction from northwest to southwest.
Terrigenous sediment
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Abstract The thickness and distribution of early syn‐rift deposits record the evolution of structures accommodating the earliest phases of continental extension. However, our understanding of the detailed tectono‐sedimentary evolution of these deposits is poor, because in the subsurface, they are often deeply buried and below seismic resolution and sparsely sampled by borehole data. Furthermore, early syn‐rift deposits are typically poorly exposed in the field, being buried beneath thick, late syn‐rift and post‐rift deposits. To improve our understanding of the tectono‐sedimentary development of early syn‐rift strata during the initial stages of rifting, we examined quasi‐3D exposures in the Abura Graben, Suez Rift, Egypt. During the earliest stage of extension, forced folding above blind normal fault segments, rather than half‐graben formation adjacent to surface‐breaking faults, controlled rift physiography, accommodation development and the stratigraphic architecture of non‐marine, early syn‐rift deposits. Fluvial systems incised into underlying pre‐rift deposits and were structurally focused in the axis of the embryonic depocentre, which, at this time, was characterized by a fold‐bound syncline rather than a fault‐bound half graben. During this earliest phase of extension, sediment was sourced from the rift shoulder some 3 km to the NE of the depocentre, rather than from the crests of the flanking, intra‐basin extensional forced folds. Fault‐driven subsidence, perhaps augmented by a eustatic sea‐level rise, resulted in basin deepening and the deposition of a series of fluvial‐dominated mouth bars, which, like the preceding fluvial systems, were structurally pinned within the axis of the growing depocentre, which was still bound by extensional forced folds rather than faults. The extensional forced folds were eventually locally breached by surface‐breaking faults, resulting in the establishment of a half graben, basin deepening and the deposition of shallow marine sandstone and fan‐delta conglomerates. Because growth folding and faulting were coeval along‐strike, syn‐rift stratal units deposited at this time show a highly variable along‐strike stratigraphic architecture, locally thinning towards the growth fold but, only a few kilometres along‐strike, thickening towards the surface‐breaking fault. Despite displaying the classic early syn‐rift stratigraphic motif recording net upward‐deepening, extensional forced folding rather than surface faulting played a key role in controlling basin physiography, accommodation development, and syn‐rift stratal architecture and facies development during the early stages of extension. This structural and stratigraphic observations required to make this interpretation are relatively subtle and may go unrecognized in low‐resolution subsurface data sets.
Half-graben
Syncline
Rift zone
Fault block
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In multirifted regions, rift-related strain varies along and across the basin during and between each extensional event, and the location of maximum extension often differs between rift phases. Despite having a general understanding of multiphase rift kinematics, it remains unclear why some parts of the rift are abandoned, with strain accumulating in previously less deformed areas, and how seismic and sub-seismic scale pre-existing structures influence fault and basin geometries. We study the Stord Basin, northern North Sea, a location characterized by strain migration between two rift episodes. To reveal and quantify the kinematics, we interpreted a dense grid of 2D seismic reflection profiles, produced time-structure and isochore maps, collected quantitative fault kinematic data and calculated the amount of extension (β-factor). Our results show that the locations of basin-bounding fault systems were controlled by pre-existing crustal-scale shear zones. Within the basin, rift faults mainly developed at high angles to the Permo-Triassic Rift Phase 1 (RP1) E-W extension. Rift faults control the locus of syn-RP1 deposition, whilst during the inter-rift stage, sedimentary processes (e.g. areas of clastic wedge progradation) are more important in controlling sediment thickness trends. The calculated amount of RP1 extension (β-factor) for the Stord Basin is up to β=1.55 (±10%, 55% extension). During Middle Jurassic-Early Cretaceous (Rift Phase 2, RP2) however, strain localises to the west along the present axis of the South Viking Graben, with the Stord Basin being almost completely abandoned. Migration of rift axis during RP2 is interpreted to be related to the changes in lithospheric strength profile and possible underplating due to the ultraslow extension (<2mm/yr during RP1) and the long period of tectonic quiescence (ca. 70 myr) between RP1 and RP2. Our results highlight the very heterogeneous nature of temporal and lateral strain migration during and between extension phases within a single rift basin.
Half-graben
Echelon formation
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Terrigenous sediment
Continental Margin
Passive margin
Deposition
Island arc
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