Geology & Geophysics Research Spotlight

An Ancient Sea Once Separated the Pacific and Indian Oceans

Seafloor under the hypothesized East Asian Sea vanished 10 million years ago as surrounding plates swallowed it up, according to new reconstructions of plate tectonics in the Philippine Sea region.

Source: Journal of Geophysical Research: Solid Earth

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Underlying the Philippine Sea is seafloor formed by the Philippine Sea tectonic plate. For decades, geologists have disagreed on the details of the plate’s 55-million-year history. Now Wu et al. have developed new tools to reconstruct the plate’s past, unearthing evidence of an ancient sea that existed in the region shortly after dinosaurs went extinct.

The Philippine Sea plate occupies a region where three major tectonic plates converge: the Pacific, Indo-Australian, and Eurasian/Sundaland plates. Smaller plates also border it—along the Philippine Sea plate’s eastern border, for example, both the large Pacific and small Caroline plates subduct beneath it.

Because almost all of the Philippine Sea plate’s boundaries are convergent and because these convergent boundaries are steadily eating away at it and its neighboring plates, its history has been difficult to discern. Scientists agree that the plate formed about 55 million years ago near the equator but disagree on its subsequent size, orientation, and path as it migrated north.

To bring more clarity to the controversy, the team reconstructed the movements of 28 huge slabs of subducted oceanic crust, sunk 500 to 1300 kilometers deep in the region’s mantle. They developed new techniques that rely on seismic tomography imaging to map the slabs in three dimensions, “unfold” their historical movements, and determine their positions on the ocean floor before they sank.

The technique allowed the scientists to find at least 70 million square kilometers’ worth of sunken slabs under East Asia—about one seventh of the total Earth surface area. The authors discovered that more than 15 million square kilometers of this lost material once formed the floor of an ancient sea between the Pacific and Indian oceans. This “East Asian Sea” would have existed 52 million years ago, after dinosaurs had disappeared and new life forms were flourishing worldwide.

Plate tectonic reconstruction showing the approximate location of the East Asian Sea
Plate tectonic reconstruction of East Asia and the surrounding area 52 million years ago showing the historical positions of unfolded sunken slabs in green, brown, orange, and blue. During this time period the East Asian Sea (shown in orange) was an ancient sea that separated the Pacific and Indian-Australian oceans. The East Asian Sea plate was subducted by subsequent northward motion of the Philippine Sea plate and by the convergence of Indo-Australian and Pacific plates toward Eurasia, finally vanishing by 10 million years ago. Credit: Jonny Wu

The ancient sea gradually shrank as the Philippine Sea plate grew and migrated northwest, subducting the northern portion of the East Asian Sea plate. Meanwhile, the Indo-Australian, Pacific, and Caroline plates subducted the southern portion of the East Asian Sea plate. The sea vanished by 10 million years ago.

Assumptions about the history of the Philippine Sea plate underpin models of geological activity that affect populations across the world. The new findings could improve the accuracy of these models. Meanwhile, the new slab unfolding technique could help geologists reconstruct elusive tectonic histories for Earth’s other oceanic regions. (Journal of Geophysical Research: Solid Earth, doi:10.1002/2016JB012923, 2016)

—Sarah Stanley, Freelance Writer

Citation: Stanley, S. (2016), An ancient sea once separated the Pacific and Indian Oceans, Eos, 97, doi:10.1029/2016EO056609. Published on 03 August 2016.
© 2016. The authors. CC BY-NC-ND 3.0