Structure, Composition and Evolution of the South Sandwich Island arc: Implications for Rates of arc Magmatic Growth and Subduction Erosion
Abstract
The South Sandwich island arc is a product of westward subduction of the South American plate beneath the small Sandwich plate. The Sandwich plate forms the eastern flank of the world's longest-lived, extant backarc basin, the East Scotia Sea. Magnetic anomalies on the western flank of the backarc indicate continuous spreading since at least 15 Ma. On the eastern flank, however, the present island arc appears to stand on backarc crust formed since 10 Ma. If earlier spreading was symmetric, crust formed at 15 Ma on the eastern flank now lies about 70 km east of the islands, and the arc at that time must have been even further east. As part of the Sandwich Lithospheric and Crustal Experiment (SLICE), wide-angle seismic data were acquired using ocean bottom seismometers (OBS) deployed along two lines crossing the trench, arc and backarc basin. Both OBS data and multichannel seismic reflection (MCS) data were acquired simultaneously using a 5976 cu.in. (98 l) airgun source. The OBS data from the southern profile, which crosses the trench at ~60° S, were modeled using 2-D travel-time inversion and synthetic seismogram methods to obtain a velocity model of the crust. The MCS data were used to constrain the shallow structure in the model. Coincident gravity data were also used to constrain the model. The maximum crustal thickness in the model is 15 km beneath the arc, decreasing to an average of about 10 km beneath the outer forearc. It also includes a 2-km-thick mid-crustal layer with velocity 6.0-6.5 km/s beneath the arc, similar to that observed beneath the Izu-Ogasawara arc, and interpreted as a composite granitic or dioritic intrusion. The crustal basement beneath the arc and inner forearc on this line appears to have formed at the backarc spreading center since 12.5 Ma. The crustal thickness on this part of the model suggests an arc growth rate of ~60 km3/km/Myr, assuming an initial oceanic crustal layer 6 km thick. If forearc slope retreat kept pace with arc migration relative to the Sandwich plate, the rate of subduction erosion may have been up to 47 km3/km/Myr. Alternatively, assuming growth of the arc-trench gap in accordance with one published relation yields a lower subduction erosion rate of 31 km3/km/Myr.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2001
- Bibcode:
- 2001AGUFM.T32D..10L
- Keywords:
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- 3025 Marine seismics (0935);
- 8150 Plate boundary: general (3040);
- 8158 Plate motions: present and recent (3040)