2023 Author: Bryan Walter | [email protected]. Last modified: 2023-05-21 22:24
Scientists have reconstructed the tectonic history of the lithosphere in the South Atlantic Scotia Sea region. Based on a set of geophysical and geological data, the researchers modeled the formation of the Scotia lithospheric plate between the Antarctic and South American plates. The separation of South America from Antarctica and the expansion of the ocean floor in the Drake Passage area was the last stage in the formation of the cold Antarctic circumpolar current - one of the factors of the global cooling that began about 50 million years ago. This is reported by an article in the journal Earth-Science Reviews.
The Scotia Sea is located in the southern Atlantic Ocean and is bounded by the South Georgia Archipelago, the South Sandwich and South Orkney Islands. The western part of the Scotia Sea communicates with the Pacific Ocean through the Drake Passage. The basin of the sea is formed by a Scotia plate and two micro-plates - Sandwich and Shetland. In the north and east, this basin is bordered by the South American Plate, in the south and west - on the Antarctic.
Geography of the Scotia Sea. CSS - Scotia Central Sea; DoB - Dove basin; ESS - East Scotia Sea; FT - Falkland Trench; FPB - Falkland Plateau Basin; JaB - Jane's pool; PoB - Powell Pool; PrB - Pool Protector; ScB - Scan pool; WSS = West Scotia Sea. BaP - Barker plateau; BB - Bruce's bank; BuB - Birdwood Bank; CD - Darwin's Cordillera; DaB - Davis Bank; DB - Discovery Bank; JB - Jane's bank; FKI - Falkland Islands; MBE - Maurice Ewing Bank; MFTB - Magallanes fold belt; PB - Piri bank; RVB - Rocas Verdes Basin; SFZ - Shackleton Fault Zone; SSA - South Sandwich Arc; SST - South Sandwich Trench; TR - Uplifting Terror
The Scotia Plate was formed as a result of the interaction of Africa, South America and Antarctica - the remnants of the Gondwana mega-continent, the disintegration of which began at the end of the Early Jurassic era, 190-180 million years ago. At present, South America, the Scotia Plate and Antarctica are moving in approximately the same direction, but at different speeds, therefore, from the north and from the south, the Scotia Plate is limited by transform faults (North and South Scotia ridges). Its western boundary, passing under the Drake Passage, is distinguished by a complex structure with a transition from the subduction zone in the northern part to the Shackleton fault zone and the young Shetland microplate at the tip of the Antarctic Peninsula. The eastern part of the sea basin is formed by the spreading zone between the Scotia Plate and the Sandwich Microplate, on which the subduction of the South American Plate formed an arc of the South Sandwich Islands.
Tectonic map of the Scotia Sea region. Active slab boundaries are indicated by red lines. The former plate boundaries of the Scotia Sea region are highlighted in gray. Black arrows represent plate movements relative to the mantle over the past 5 Ma. ESR - East Scotia Ridge; MBL - Mary Bird Land; MFFZ - Magallanes-Fagnano fault zone; NSR - North Scotia Ridge; SG - South Georgia; SSP - South multilayer slab; SSR - South Scotia Ridge; SSSZ - South Sandwich Subduction Zone; TdF - Tierra del Fuego
A group of researchers from the University of Utrecht led by Suzanna H. A. van de Lagemaat has reconstructed the kinematics of lithospheric plates in the Scotia Sea region over the past 182 million years, that is, since the time when the first signs of the collapse of the Gondwana megacontinent were recorded. Scientists processed the results of geophysical and geological studies of the Antarctic Peninsula, Tierra del Fuego, islands bordering the Scotia Sea basin, and rocks of the ocean floor.
Map of lines of marine magnetic anomalies of the Scotia and Weddell seas. Isochrones are colored according to age
The kinematic model is based on paleomagnetic measurements, which make it possible to establish the position of individual blocks of the continental lithosphere at different times, as well as to determine the age and direction of movement of the oceanic crust based on strip magnetic anomalies. To refine the reconstruction, the scientists used the data of gravimetric measurements, seismic sounding, studying the morphology of the bottom and geochemical analysis of rocks in cores taken in various parts of the Scotia Sea.
Reconstructions of the tectonic setting 182, 154, 140 and 110 million years ago in the fixed frame of reference of East Antarctica. Dark green areas are continents within the current coastline, dark gray areas are stretched and transitional continental crust. Red lines - active plate boundaries, red arrows - movements at active plate boundaries, dark blue lines - marine magnetic anomalies
Reconstructions of the tectonic setting 80, 66, 50 and 36 million years ago in the fixed frame of reference of East Antarctica. Dark green areas are continents within the current coastline, dark gray areas are stretched and transitional continental crust. Red lines - active plate boundaries, red arrows - movements at active plate boundaries, dark blue lines - marine magnetic anomalies
Reconstructions of the tectonic setting 26 and 15 million years ago in the fixed frame of reference of East Antarctica, as well as the modern setting. Dark green areas are continents within the current coastline, dark gray areas are stretched and transitional continental crust. Red lines - active plate boundaries, red arrows - movements at active plate boundaries, dark blue lines - marine magnetic anomalies
Reconstruction of tectonic processes in the region shows how about 182 million years ago, Eastern Gondwana (Antarctica, Australia, India) began to separate from Western Gondwana (South America, Africa). Over the next 20 million years, separation processes also began in Western Gondwana. First, the ocean basin between Antarctica and Africa opened, then, about 140 million years ago, between Antarctica and South America. This basin (Rocas Verdes) existed for 30 million years and reached a width of 250 kilometers, but disappeared when the continents temporarily reunited.
Evolution of the Scotia plate 85-59 million years ago. Dark shades - continental crust, light - oceanic crust: yellow - South American Plate (SAM); blue - Antarctic Plate (AP); green - Scotia slab; gray - Pacific plates. Horizontal shading: continental crust originating from the South American Plate; vertical shading: continental crust originating from the Antarctic plate. White and black arrows - absolute movement of the South American and Antarctic plates, respectively
Evolution of the Scotia plate from 59 million years ago to the present. Dark shades - continental crust, light - oceanic crust: yellow - South American Plate (SAM); blue - Antarctic Plate (AP); green - Scotia slab; gray - Pacific plates. Horizontal shading: continental crust originating from the South American Plate; vertical shading: continental crust originating from the Antarctic plate. White and black arrows - absolute movement of the South American and Antarctic plates, respectively
A new phase of separation began about 80 million years ago, when the South American Plate began to move westward relative to the Antarctic. At the same time, in the area of the continental bridge between Antarctica and South America, a subduction zone opened and a system of faults arose, forming a new plate, which included part of the South American lithosphere. This was due to the detachment of the continental crust of South America and its transfer to the Scotia plate. Remnants of this crustal area are the present-day Piri, Bruce and Discovery uplifts in the Scotia Sea, the North Ridge and South Georgia Islands.
Stratification and transfer of a part of the South American lithosphere to the Scotia plate
About 50 million years ago, the Drake Passage opened, and around the same time between Antarctica and Australia, the Tasmanian Passage formed, and the southern mainland was isolated from the rest of the continental blocks. This created conditions for the formation of a closed flow of cold ocean waters around Antarctica. Chronologically, the complete separation of Antarctica coincides with the beginning of a stable global cooling. It is believed that it was the result of a number of factors that caused a general decrease in the level of carbon dioxide in the atmosphere (changes in the weathering regime, the rate of accumulation of marine carbonate sediments). Major paleogeographic changes that influenced the nature of circulation in the ocean should also have played a role in the development of the climatic situation.
Kinematic reconstruction of the Scotia Sea region from 80 to 34 million years ago in the mantle frame of reference. A – H: major plate boundaries, modern coastlines and continental blocks, and A'– H '- the position of Tierra del Fuego and the South Sandwich Trench relative to the mantle
Kinematic reconstruction of the Scotia Sea region from 26 million years ago to the present time in the mantle frame of reference. A – H: major plate boundaries, modern coastlines and continental blocks, and A'– H '- the position of Tierra del Fuego and the South Sandwich Trench relative to the mantle
However, the connection of the Atlantic with the Pacific Ocean for a long time (at least 15 million years) remained unstable due to the narrowness of the Drake Passage and the presence of islands in its area, which impeded the development of the cold current. This affected the rate of cooling in the Antarctic region. Only after the expansion of the Scotia plate about 30–25 million years ago, the strait zone became wide enough to establish a powerful circumpolar current that isolated Antarctica from the warm waters of low latitudes, and an ice sheet began to grow on the mainland. Now in the meridional direction, the Scotia plate extends for about 800 kilometers.
Previously, scientists presented a global tectonic model covering the history of plate movement over the last billion years, and how the climate of the southern continent changed in previous geological eras, you can learn from our material "Green Antarctica".
