17:15 〜 19:15
[SCG55-P20] Characteristics and Evolution Process of the Tectonic Regions in and around the Antarctic Sea as based on Marine Geophysical Data (Final Report)
キーワード：南極プレート、南極海、大洋中央海嶺
The Antarctic Plate including the continental region is known to be classified into 11 tectonic regions. And also, the area south of 30°S including the Antarctic Plate and its adjacent area is classified into more than 20 tectonic regions (Hayes, et al., 2009). Each tectonic region has experienced proper tectonic history and evolution process after the break-up of the Gondowanaland: cooling after the accretion of the oceanic crust at the mid-ocean ridges, subsidence of the seafloor due to the cooling and the oceanic sedimentation loading, and transition of the oceanic plate motion by the fluctuation of the Euler pole location for past 200 million years. The palaeo water depth of each era in each tectonic region during this period has been re-constructed by Hayes, et al. (2009).
The current study is proposed in order to clarify the uniqueness of each tectonic region in the Antarctic sea, the possible interaction with adjacent tectonic regions and its transition, and finally to estimate the driving force that led the evolution of each region. This study is based on analysis of marine geophysical data (seafloor topography, gravity, geomagnetic anomaly), seafloor age, total sediment thickness, etc., which are available at NCEI (National Centres for Environmental Information, NOAA) database.
It is of useful to estimate the difference in status of the thermal contraction processes along the mid-oceanic ridges bordering the Antarctic plate. In this study, GDH1 Model (Stein and Stein, 1994, Nature), was used as the standard model of the plate thermal contraction. This model was used as the standard to calculate the depth anomaly in order to estimate the difference in thermal processes along the mid-oceanic ridges bordering the Antarctic plate.
NCEI provides two types of the global one-arc-minute-gridded topography/bathymetry data for both latitude and longitude: height/depth to the bedrock from the mean sea surface and those to the true surface including the glacier from the mean sea surface, as “ETOPO1-Bed” and “Etopo1-Ice”, respectively.
However, since the seafloor is covered by sediment and then most of the seafloor is deformed by the isostatic equilibrium due to the sediment loading, the bathymetry data in NCEI database do not show the true crustal depth. In order to calculate the true crustal depth as the result of the thermal contraction, the reduction due to the isostatic effect is to be requested before calculation of the depth anomaly.
NCEI also provides the database of the total sediment thickness "Globsed Ver.2." This database was used to obtain the isostacy-reduced water depth as mentioned above.
The Pacific-Antarctic ridge and its adjacent area is characterised by relatively positive depth anomaly except for large-scale fracture zones and transform faults. The ridge which is the southern extension of the East Pacific Rise is known as the fastest spreading ridge with large volume of magma supply. Therefore, the (corrected) water depth might be shallower compared with that expected from the GDH1 model. Heat loss and the subsequent cooling of the seafloor are expected along the fracture zones and transform faults. On the other hand, the relatively slow spreading ridges in the Atlantic and Indian ridges including AAD are characterised by relatively negative depth anomaly due to poor magma supply and cooling of the seafloor. Especially, the depth along AAD is about 500m deeper than that expected from the GDH1 model.
The total sediment thickness database can be used in order to examine not only the characteristics of its sediment thickness derived from the supply from the adjacent continent, but also to calculate the mean sedimentation rate together with the seafloor age data (Müller et al., 2020). Mean sedimentation rate is calculated as total sediment thickness divided by seafloor age, unit in m/Myr or mm/kyr.
The sedimentation rate map (sediment thickness over seafloor age) shows variations around the Antarctica. The sedimentation rate shows about 50 m/million years of the coast of Antarctic Peninsula and off the coast of the East Antarctica although quite narrow continental shelf is formed around Antarctica. However, in the Weddel Sea, just east of the Antarctic Peninsula, the sedimentation rate is drastically low, totally 30 m/million years. The area north of the Weddel Sea near the South Sanwich Islands shows several local high sedimentation rate spots, more than 80 m/million years. The variation of these sedimentation rate may due to the supply of the deposits from the land nearby and the status of the glaciers at these continents.
The current study is proposed in order to clarify the uniqueness of each tectonic region in the Antarctic sea, the possible interaction with adjacent tectonic regions and its transition, and finally to estimate the driving force that led the evolution of each region. This study is based on analysis of marine geophysical data (seafloor topography, gravity, geomagnetic anomaly), seafloor age, total sediment thickness, etc., which are available at NCEI (National Centres for Environmental Information, NOAA) database.
It is of useful to estimate the difference in status of the thermal contraction processes along the mid-oceanic ridges bordering the Antarctic plate. In this study, GDH1 Model (Stein and Stein, 1994, Nature), was used as the standard model of the plate thermal contraction. This model was used as the standard to calculate the depth anomaly in order to estimate the difference in thermal processes along the mid-oceanic ridges bordering the Antarctic plate.
NCEI provides two types of the global one-arc-minute-gridded topography/bathymetry data for both latitude and longitude: height/depth to the bedrock from the mean sea surface and those to the true surface including the glacier from the mean sea surface, as “ETOPO1-Bed” and “Etopo1-Ice”, respectively.
However, since the seafloor is covered by sediment and then most of the seafloor is deformed by the isostatic equilibrium due to the sediment loading, the bathymetry data in NCEI database do not show the true crustal depth. In order to calculate the true crustal depth as the result of the thermal contraction, the reduction due to the isostatic effect is to be requested before calculation of the depth anomaly.
NCEI also provides the database of the total sediment thickness "Globsed Ver.2." This database was used to obtain the isostacy-reduced water depth as mentioned above.
The Pacific-Antarctic ridge and its adjacent area is characterised by relatively positive depth anomaly except for large-scale fracture zones and transform faults. The ridge which is the southern extension of the East Pacific Rise is known as the fastest spreading ridge with large volume of magma supply. Therefore, the (corrected) water depth might be shallower compared with that expected from the GDH1 model. Heat loss and the subsequent cooling of the seafloor are expected along the fracture zones and transform faults. On the other hand, the relatively slow spreading ridges in the Atlantic and Indian ridges including AAD are characterised by relatively negative depth anomaly due to poor magma supply and cooling of the seafloor. Especially, the depth along AAD is about 500m deeper than that expected from the GDH1 model.
The total sediment thickness database can be used in order to examine not only the characteristics of its sediment thickness derived from the supply from the adjacent continent, but also to calculate the mean sedimentation rate together with the seafloor age data (Müller et al., 2020). Mean sedimentation rate is calculated as total sediment thickness divided by seafloor age, unit in m/Myr or mm/kyr.
The sedimentation rate map (sediment thickness over seafloor age) shows variations around the Antarctica. The sedimentation rate shows about 50 m/million years of the coast of Antarctic Peninsula and off the coast of the East Antarctica although quite narrow continental shelf is formed around Antarctica. However, in the Weddel Sea, just east of the Antarctic Peninsula, the sedimentation rate is drastically low, totally 30 m/million years. The area north of the Weddel Sea near the South Sanwich Islands shows several local high sedimentation rate spots, more than 80 m/million years. The variation of these sedimentation rate may due to the supply of the deposits from the land nearby and the status of the glaciers at these continents.