How plate tectonics pulled open a sea from across the Pacific

Tug of war across the sea

Between 6 and 8 million years ago, a massive tear ripped open the Gulf of California in North America. Experts long wondered what triggered this fast split. The answer might lie thousands of miles away on the far side of the sea.

Earth has a hard outer shell of crust and upper mantle called the lithosphere. This outer shell is broken into big slabs that shift through plate tectonics. Because a slab forms one continuous piece of rock, what happens on one end can echo across the entire ocean floor.

Testing the crust with code

To see how this works, scientists used geodynamic models, which are digital tests that mimic how planets move. They examined a moment when double subduction started in the Western Pacific, where two separate rock slabs sank down at the same time. This kind of digital test matters because we cannot travel back in time to measure ancient forces directly.

As these two heavy slabs sank, their weight dragged on the Pacific plate. The digital test shows that this downward pull sent an extensional stretch straight across the rock. It passed between 5 and 15 megapascals of stress, which is a standard scientific unit used to measure pressure and mechanical pull.

This transmitted force reached all the way across the sea. There, it loaded the southern San Andreas fault with extra stress. It also pulled apart the Gulf of California oblique rift, a fracture zone where the ground stretches open while sliding sideways at an angle.

What we still do not know

This big pull worked because the local crust was already preconditioned, meaning it was warm, thin, and primed to break. Without that weak spot, the distant tug might not have torn the land open. Scientists now want to learn if distant slab sinks have sparked continental rifting events around the world, or if this ocean was a special case.