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Seafloor spreading on the Southeast Indian Ridge near
the Amsterdam-St. Paul hotspot
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Photo: St. Paul Island from the R/V Melville
We spent nearly eight weeks surveying the Southeast Indian
Ridge near the Amsterdam-St. Paul (ASP) hotspot. The hotspot is named
after the two
small islands it created in the Indian Ocean. Here the mid-ocean ridge runs directly through the hotspot plateau. The interaction of the hotspot and the spreading center
makes the tectonic history of the area rich and complex. My research
focus is on the tectonic history as it can be deciphered from the magnetic
anomalies.
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| Below are two maps of the ASP plateau. The first is bathymetry (seafloor depth; scale in km) and the second is the calculated magnetization of the seafloor (from magnetic anomalies) draped over the bathymetry. The spreading center is marked with a dashed line, with each segment labeled (H, I1, etc.) Also marked are the islands, transform faults, a lava flow (lf), and two seamounts: Boomerang Seamount (BS) and St. Pierre (SPr). The Boomerang seamount is an active volcano and is associated with the most strongly magnetized crust in our study area. A lava flow and flank rift volcanism on St. Paul and St. Pierre also show high magnetization. |
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| Seafloor magnetization contains clues to
the area's tectonic history. Magnetization highs follow the propagating
tip of segment J2 and its pseudofaults (wake) created from simultaneously
propagating and spreading. The bands of positively magnetized
crust (green - marked 'a') around segments I1 & I2 are narrower than
predicted by a simple spreading history. The bands are narrower because
until ~600ky ago, the spreading center was located to the northeast of
its present location, creating the 'missing ' positively magnetized crust
there rather than where the spreading center is now.
The plate boundary reorganization likely occurred much like the following cartoon shows. The spreading center jumped from its initial location to its present position on the platform. The jump must reactivate a portion of the inactive fracture zone. However, the fracture zone is harder to reactivate further from the initial spreading center location (longer time to anneal), so it may have been easier to break a new transform on the platform rather than reactivate the strongest part of the inactive Amsterdam fracture zone.
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Also interesting is the Boomerang transform fault (connecting segments I1 & I2). The transform must be at least young as the present-day spreading centers. This means that the transform is less than 600ky old, making it the youngest known transform fault in the world. |
