Tectonic segmented characteristics of passive continental margins on both sides of the South Atlantic and their causes
The passive continental margin basin is the most oil- and gas-rich basin type in the world, mainly distributed in the three ocean periphrasites of the Atlantic, Indian and Arctic Oceans and the southeastern edge of the Mediterranean Sea. Among them, the passive continental margin basin of the South Atlantic Ocean is also a hot spot for oil and gas exploration in the world today. The passive continental margin basins of the South Atlantic include the Niger Delta, Gabon, Bas-Congo and Kwanza basins, as well as the Santos and Campos basins on the east coast of South America, all of which have large oil and gas fields. The South Atlantic Ocean is divided from north to south by the equatorial section, the middle section, the southern section and the Falkland section. The northern end of the equatorial section is separated from the Central Atlantic Ocean by the Marathon Fault Zone, while the middle section is located south of the Romansh Fault Zone, from the Benue Rift Valley to the southern end of the Rio Grande Fault Zone. The southern section is bounded by the Rio Grande Ridge and the Whale Ridge to the north, and the Falkland section to the south to the Bouvet Triad and is the hub of the Atlantic Ridge and the Southwest Indian Ocean Ridge.
During the Late Jurassic period, South America and the African continent were both part of the West Gondwana continent and were in the pre-rift craton phase. By the Early Cretaceous period, under the influence of Tristan mantle hot column activity, the southern tip of the South Atlantic Ocean began to crack along the paleopan-African orogenic belt, and developed from south to north, forming a continental rift valley system. During the fracture process, the South American plate rotates clockwise, the lithosphere of the large African and South American continents is completely separated, and the two shores of the South Atlantic enter the passive continental margin evolution stage.
The total length of the transition basin in the equatorial section of the South Atlantic Ocean is about 2900 km, which is controlled by the two main conversion fault zones of São Paulo and Romansh. Near the outer side of the transition ridge is generally an oceanic-continental boundary, and there is a narrow thinning zone during the transition from the crust to the oceanic crust. The basin is limited by the transition ridge, and the sedimentary cover is dominated by Cretaceous and Cenozoic clastic rock deposits. The Fosdo Amazon Basin is located in northeast Brazil, the basin base is Precambrian Guyana-Brazilian craton, sedimentary strata are distributed from the Lower Cretaceous to the Quaternary Series, mainly submarine fan sedimentation. Affected by the rapid deposition of strata since the Cenozoic era, the basin has developed a typical gravity slide structure above the Upper Cretaceous marine mudstone layer. The crust of the Kuanzha Basin is thinned slowly, and a large number of deep grabens are developed in the basement of the basin, mainly positive faults inclined to the sea, and the original sedimentary thickness can reach thousands of meters of evaporite formation. The sedimentary thickness of the existing salt rocks increases from the continental margin to the deep sea, which has obvious zonality, which can be divided into different salt structure sub-belts of extension, transition and extrusion type, and finally disappears in the outer high and low zones and the ocean-land boundary.
The opening of the South Atlantic Ocean rift spreads from south to north, and the basin group generally undergoes three stages: rift period, transition period and drift period. In the north-south direction, the tectonic, sedimentary, and thermal evolution processes have gradual or segmented characteristics, which are manifested as basin width, sedimentary construction composition, structure, and structural style. The southern section is characterized by magma-type edges, the amount of settlement after rupture is small, the mantle upwells in the rift period, the decompression melting is enhanced, and a large number of igneous rocks intrude under the earth's crust, and the reflective wedge body tilting to the sea often appears above the transition crust, which is the product of the strong accumulation of magma activity in the early rift valley. As the rift valley begins to settle thermally, under the influence of the tilting of the positive fault inclination to the land, the reflex wedge structure inclined to the sea and the volcanic strata tilted towards the expansion center. After the late Cretaceous, the volcanic strata were covered by post-rift sedimentation and entered the thermal deposition stage, and the basins on both sides were characterized by the development of gravity slip structures, and marine siliceous clastic deposition was the mainstay.
The middle continental margin is weak relative to the magmatic activity of the southern section, which is an extremely stretched magma poor passive continental margin, and the southern part is semi-closed to the open sea, and a large number of salt rocks develop in the basin. Its distribution range is affected by the initial fracture location in the South Atlantic, and the thickness of the existing salt rock sedimentary increases from the continental margin to the deep sea. Evaporite sedimentation does not develop in the Niger Delta basin in the northern part of the central section, and the Acata Formation mud shale controls its tectonic pattern as a slip layer, and the development of a large number of mud bottoms can be compared with the salt basin. Among them, the transition tectonic characteristics of the equatorial section are obvious, the basin basement is mainly controlled by the transition fault system, the regional tectonic stress characteristics change from tensile shear to compressive torsion, and the sediments in the transition fault subsidence zone are deformed and uplifted, forming a large-scale conversion ridge distributed nearly parallel to the transition fault zone. At present, the basins with a high degree of exploration in the periphery of the South Atlantic Ocean are mainly the Niger Delta Basin, the Angolan Coastal Basin Group and the Greater Campos Basin Group. They are in a similar tectonic evolution background, and the exploration strata can be expanded horizontally between basin groups. In the east-west direction, along the transition fault, the sequence of the rift valley-drift period in the basin can be compared through conjugate studies, and then the favorable zone can be determined.
The width of the shelf determines the extent of the passive continental margin basin, and the slope tilt puts the entire South Atlantic basin group in the background of gravity, and its slope determines the degree of gravity slide structure and salt structure development in the basin. According to the comparison of the tectonic profiles of the basin group, the segmental differences in the tectonic characteristics of the basin group on both sides of the South Atlantic are obvious. The southern section is a magma-type basin with intense magmatic activity, accompanied by high-velocity zones below the ocean-land transition crust, and a sea-tilted reflective wedge can be seen above the transition crust. The middle section is a saline mud tectonic basin, the magmatic activity is weaker than the southern section, the crustal stretching is stronger, the Apt order salt rock is developed, and the salt structure has obvious zonality from land to sea, and the tip is extinct in the outer high and low zone and the ocean-land boundary. The comparative analysis of the development law of basin groups is conducive to exploring the segmented similarities and differences of the north-south structure of the South Atlantic, the tectonic relationship between the conjugate basins of the east and west coasts, the relationship between the basin group and the plate boundary, and the relationship between the basin group and the deep structure, which is of great significance to human oil and gas exploration.
Bibliography:
Wang Weiguang and Cheng Kenan: "Comparison of Petroleum Geological Characteristics of Key Passive Continental Margin Basin in the South Atlantic"
Liu Zuodong: "Tectonic Sedimentation and Hydrocarbon Accumulation in Global Passive Continental Basins: A Case Study of the South Atlantic Peripheral Basin"
