9:15 AM - 9:30 AM
[SCG55-13] Tectonic evolution of the Mariana Trough
Keywords:Mariana Trough, back-arc basin, magnetic anomaly
The Mariana Trough is a typical active back-arc basin formed by the subduction of the Pacific Plate into the Philippine Sea Plate. Although the age and spreading rate of the Mariana Trough have been partly investigated mainly based on the analysis of geomagnetic anomaly (e.g., Iwamoto et al. 2002; Yamazaki et al. 2003), the central southern region is poorly investigated and there has been no comprehensive study on the formation history. In this study, we compiled all available magnetic and multibeam bathymetry data over the whole basin to examine the history of back-arc spreading of the Mariana Trough. This study is expected to provide new insights into the formation process of back-arc basins and the regional tectonic history.
The bathymetric data used in this study were obtained from 32 research cruises conducted by Japanese research vessels, as well as 11 research cruises by American research vessels. We compiled all available multibeam bathymetry and constructed a 0.001-deg complete grid data covering the basin and the adjacent areas.
Vector geomagnetic field data were collected using shipboard three-component magnetometers (STCM) during 16 Japanese cruises. We use only z-component of vector magnetic anomaly because 1) the Mariana Trough is an almost east-west spreading system located near the magnetic equator, causing small total magnetic anomalies, and 2) the horizontal component of observed raw data is relatively inaccurate due to the insufficient accuracy of gyrocompass in several cruises. The equivalent magnetization intensity of the seafloor was calculated using the fast Fourier transform method (Parker and Huestis, 1974: Macdonald et al., 1980). We also performed forward modeling of magnetic anomalies (Talwani, 1965; Blakely, 1995) along the representative across-axis profile of each segment to find the best-fit rate to explain the observed profile. Based on the forward modeling results for each segment, we finally identify the chrons for all magnetic stripes on our equivalent magnetization distribution map.
The mantle Bouguer anomaly (MBA) and the residual mantle Bouguer anomaly (RMBA) were calculated by combining the satellite-derived free-air gravity anomaly (Sandwell et al., 2014) and our newly obtained bathymetry and seafloor age model. Then the crustal thickness variation is estimated.
Based on the bathymetry and backscatter intensity, we divided the spreading axis into 18 second-order segments. We also recognized 13 OCC-like structures not only in the near-axis but also in the off-axis. Magnetic anomaly data indicate that seafloor spreading was initiated at approximately 6.3 Ma in the central region around 17°30’N, which is the oldest part in the Mariana Trough. We also estimated the age of rifting initiation by assuming the extension rate in the rifting stage is same as the average spreading rate, yielding an age of approximately 8.5 Ma in the central Mariana Trough. Abyssal hill pattern and magnetic anomalies revealed that the spreading occurred in NE-SW direction until 4-3 Ma, and the spreading direction changed perpendicular to the trench. At the same time, rifting of the Okinawa Trough activated, and the Mariana island arc separated from West Mariana Ridge in the southern end. During the initial stage, from 6.3 to 3 Ma, the RMBA values are low (-80 to -50 mGal). After 3Ma, the entire basin spreads at a full rate, ranging from 15 km/Myr in the northern part to 45 km/Myr in the southernmost part. The half-spreading rate is generally faster on the western side of the spreading axis compared to the eastern side across most of the region, indicating asymmetric spreading.
We also estimated the Euler pole for the Mariana Plate after 3 Ma using the result of our magnetic analysis. There is a significant residual velocity in N-S direction between the model and observed velocity, suggesting the existence of arc-parallel extension of the Mariana Plate, implying that the plate is not rigid.
The bathymetric data used in this study were obtained from 32 research cruises conducted by Japanese research vessels, as well as 11 research cruises by American research vessels. We compiled all available multibeam bathymetry and constructed a 0.001-deg complete grid data covering the basin and the adjacent areas.
Vector geomagnetic field data were collected using shipboard three-component magnetometers (STCM) during 16 Japanese cruises. We use only z-component of vector magnetic anomaly because 1) the Mariana Trough is an almost east-west spreading system located near the magnetic equator, causing small total magnetic anomalies, and 2) the horizontal component of observed raw data is relatively inaccurate due to the insufficient accuracy of gyrocompass in several cruises. The equivalent magnetization intensity of the seafloor was calculated using the fast Fourier transform method (Parker and Huestis, 1974: Macdonald et al., 1980). We also performed forward modeling of magnetic anomalies (Talwani, 1965; Blakely, 1995) along the representative across-axis profile of each segment to find the best-fit rate to explain the observed profile. Based on the forward modeling results for each segment, we finally identify the chrons for all magnetic stripes on our equivalent magnetization distribution map.
The mantle Bouguer anomaly (MBA) and the residual mantle Bouguer anomaly (RMBA) were calculated by combining the satellite-derived free-air gravity anomaly (Sandwell et al., 2014) and our newly obtained bathymetry and seafloor age model. Then the crustal thickness variation is estimated.
Based on the bathymetry and backscatter intensity, we divided the spreading axis into 18 second-order segments. We also recognized 13 OCC-like structures not only in the near-axis but also in the off-axis. Magnetic anomaly data indicate that seafloor spreading was initiated at approximately 6.3 Ma in the central region around 17°30’N, which is the oldest part in the Mariana Trough. We also estimated the age of rifting initiation by assuming the extension rate in the rifting stage is same as the average spreading rate, yielding an age of approximately 8.5 Ma in the central Mariana Trough. Abyssal hill pattern and magnetic anomalies revealed that the spreading occurred in NE-SW direction until 4-3 Ma, and the spreading direction changed perpendicular to the trench. At the same time, rifting of the Okinawa Trough activated, and the Mariana island arc separated from West Mariana Ridge in the southern end. During the initial stage, from 6.3 to 3 Ma, the RMBA values are low (-80 to -50 mGal). After 3Ma, the entire basin spreads at a full rate, ranging from 15 km/Myr in the northern part to 45 km/Myr in the southernmost part. The half-spreading rate is generally faster on the western side of the spreading axis compared to the eastern side across most of the region, indicating asymmetric spreading.
We also estimated the Euler pole for the Mariana Plate after 3 Ma using the result of our magnetic analysis. There is a significant residual velocity in N-S direction between the model and observed velocity, suggesting the existence of arc-parallel extension of the Mariana Plate, implying that the plate is not rigid.