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Birth of the Red Sea

Peter Betts standing on exposed coral reef on the Farasan Islands

Peter Betts standing on exposed coral reef on the Farasan Islands in the southern Red Sea. The coral has been uplifted by upwelling of a salt diapir beneath the reef.

By Peter Betts

New evidence about the creation of the Red Sea has fundamentally changed how geologists understand the birth of oceans.

The thin, solid crust on which we live represents only about 1% of the distance to the Earth’s centre: 5–10 km thick under the oceans and 25 –70 km thick under the continents. Yet its evolution and movement, and its interaction with the underlying mantle, give rise to almost all the geological events of significance to us, including earthquakes, volcanic eruptions and the formation of mineral deposits, oil and gas reserves, underground lakes, mountains and oceans.

The crust and upper mantle is broken into a jigsaw of moving tectonic plates. Their movement crushes them together, pulls them apart and makes them slide past each other. Some are pushed down under neighbouring plates in a process known as subduction.

While we have learned a lot about the movement of continents and what happens when they crash into one another, the formation of ocean crust – a major force that drives plate tectonics and regenerates the plates – remains one of the least understood processes related to plate tectonics.

However, recent research into the geology and geophysics of the Red Sea has overturned the conventional view that ocean crust formation is a continuous process whereby a rift opens in the ocean floor like a zipper and magma bubbles up from the underlying mantle to push it apart. Instead, seafloor formation actually happens in bursts separated in geography and time.

Ten years ago I was keen to become a leading authority on the evolution of the Australian plate. My research had focused on understanding the architecture and geological evolution of Australia’s ancient crust, which is more than a billion years old.

So how is it that today one of my major research interests is to understand the evolution of the Red Sea, the one place on the planet where continental crust is transitioning from a continental rift to a juvenile ocean basin?

It all began with a knock on my door. Khalid Almalki, a student with PhD sponsorship from Saudi Arabia, was looking for a supervisor and wanted to continue studying the Farasan Islands in the southern Red Sea with the intention of mapping out the salt domes that often form the walls of oil reservoirs. It wasn’t my area of expertise, and I remember thinking that I would probably not see Khalid again.

But Khalid liked my group’s approach of combining structural geology and geophysical interpretation and modelling to explore the geometry of the Earth’s crustal elements and show how rocks deform under stresses. He joined our research group and began researching salt dome tectonics in the Red Sea.

The Red Sea has a special place in plate tectonics because it represents the only example where a continental rift is transitioning to a juvenile ocean basin (Fig. 1). The southern part of the Sea is undergoing active seafloor spreading, driving Africa and Arabia apart. But while there is a continental rift in the northern part of the Red Sea, Africa and Arabia remain firmly attached – a single plate yet to be split apart.

This offers a unique opportunity to understand how continental plates split apart to make separate plates, and provides hints about the geological processes involved. It is an active tectonic setting where low-level earthquakes and tremors continually occur in response to separation.

The continental margin of Saudi Arabia and the Afar Depression in East Africa record 30 million years of volcanic and igneous activity, and there are large topographic escarpments and small mountain ranges that have formed as the continents continue to split apart.

Young geoscientists are introduced to the Red Sea in textbooks because it represents one arm of a classic triple junction where the African, Arabian and Indian plates all meet. This junction is centred in Ethiopia above the Afar Plume, a buoyant upwelling from the deep mantle that interacts with the base of the Earth’s rocky crust and upper mantle.

Until now, the opening of the Red Sea and the separation of the Arabian and African plates has been interpreted as a relatively simple process driven by the upward pressure of this plume. This is what I also believed when I first met Khalid. As it turns out, I couldn’t have been more wrong.

Our research started in the Farasan Islands, an archipelago of Pleistocene to Pliocene (< 5 million years) islands and limestone reefs lifted above sea level by the pressure of buoyant salt domes beneath. The story became really interesting when Khalid started presenting us with magnetic and gravity data collected over the Farasan Islands and the adjacent Miocene sediments (5–23 million years old) between the islands and the Saudi Arabian coastal plains.

High resolution airborne magnetic data showed the presence of magnetic stripes in the ocean crust beneath the sediments. These stripes form as magnetic minerals align with the Earth’s magnetic field as new ocean crust is formed. Each stripe thus represents a change in the polarity of the Earth’s magnetic field flips in geological time.

The stripes suggested that the Miocene sediments had been deposited on ocean crust that was more than 20 million years old. This was counter to conventional interpretations of the Red Sea, which implied that sea-floor spreading and ocean crust formation began only 5 million years ago.

We then applied the latest geophysical processing methods which revealed the presence of eight stripes that could be correlated to a global database. This suggested that the ocean crust formed 22–26 million years ago during the Oligocene era.

The implication of this was that ocean crust formation in the Red Sea occurred in two discrete spreading events: the present-day seafloor spreading was superimposed on an older segment of ocean crust. The two events were separated by 17 million years of relative tectonic calm during which 4 km of sediment was laid down.

So what stalled the seafloor spreading? The answer was not to be found in the Red Sea itself but further to the north in the Zagros Mountains of Iran. These mountains record the closure of another ancient ocean called Tethys.

Seafloor spreading in the Red Sea was likely initiated as the hot mantle of the Afar Plume thermally weakened the African–Arabian crust. It began pushing the African and Arabian plates apart, but stalled when the leading edge of the Arabian plate collided with Iran, slowing the northward migration of Arabia and creating the Zagros Mountains. That was 22 million years ago.

Fast forward to the present day. The geological community has now collected 50 years of geological and geophysical data in and around the Red Sea. Perhaps the most important dataset is a combined satellite and marine magnetic, gravity and sea floor depth data that provides high resolution images of the entire Red Sea for the very first time. The patterns in the magnetic and gravity data reveal the extent of present-day seafloor spreading, the distribution of the ocean crust, and the extent of the older spreading segment.

We applied geophysical filtering techniques used for mineral exploration to this data, and the results were intriguing. They tell us, for instance, that the conventional thinking about oceans opening by continuously unzipping is not applicable to the Red Sea. Rather, the process appears to happen in segments.

The pattern of ocean crust formation shows many similarities to the Woodlark Basin, a small ocean basin formed at the northern edge of the Australian tectonic plate to the immediate east of Papua New Guinea. This poses compelling questions around why such very different tectonic systems would produce almost identical patterns of spreading.

We are now collaborating with the University of Adelaide to map how magma pushes up from the mantle so that we can understand the link between magmatism and ocean crust formation. And at Monash University we are working with Prof Sandy Cruden to create an innovative modelling experiment to replicate the patterns of Red Sea ocean formation. We hope this will lead to a deeper understanding of the dynamics of this intriguing tectonic system.

Khalid’s knock at my door opened up new and exciting research and collaboration opportunities, took me out of my comfort zone and opened my eyes to another world.

Peter Betts is a Professor in the School of Earth, Atmosphere and Environment and Associate Dean of Graduate Research at Monash University's Faculty of Science.