Forming an ocean basin
This image shows the bathymetry of the South China Sea. Don’t ask me why the southern section is labeled Dangerous Ground, I honestly don’t know; this was just the best shot of the seafloor I could find with a usable license. Anyway, a recent scientific drilling expedition just produced some cool results about this area – it actually started turning into an ocean basin. The South China Sea basin opened over about 10 million years, from the late Oligocene 34 million years ago into the Miocene. Sometimes blocks of continental crust get pulled apart and shifted by larger plate tectonic forces – that’s what happened here. Some of the continent-like crust currently found in the Philippines was literally ripped away from the rest of the continent, leaving behind a basin and forming a series of normal faults that are now buried by sediment coming off of the continent.
In some places, continents are pulled apart without creating true ocean crust. For example, the Iberian Peninsula has substantially rotated compared to the rest of Europe, but it did so slowly enough to avoid creating ocean crust. For a contrast, the Red Sea is opening somewhat slowly, but there is legitimately basaltic ocean crust forming at the bottom.
As part of an expedition by the International Ocean Discovery Program, which runs the scientific drilling ship Joides Resolution, a team of researchers drilled through the sediment layers to sample the rocks in what they thought would be remnants of the continental break up in this area. To their surprise, they found layers of oceanic basalt that had erupted as the continent pulled apart.
This area did not fully pull apart into an ocean, but it started the process. Between 34 and 30 million years ago, this part of the world rifted rapidly, so rapidly in fact that it pulled the crust apart and the mantle began upwelling to fill the space. When hot mantle moves to low pressures, that triggers melting, which lead to the formation of igneous rocks and eventually ocean-like crust. That ocean crust has since been buried, the rifting slowed in the middle Miocene, but it’s still down there making up part of the South China Sea.
-JBB
Image source: http://bit.ly/2BxbUxt
Original paper: https://www.nature.com/articles/s41561-018-0198-1_ _