Shifting tectonic plates gave Alaska’s Aleutian Islands a later-life lift

The first hints that an uplift event had occurred across the Aleutians came in the 1970s when oceanographers took sediment cores from the ocean floor surrounding the islands. The cores contained an anomalous layer of land-derived clay minerals and other sediment deposited during a relatively short period several million years ago. One scenario to explain that sediment is that the islands were suddenly deformed and lifted upward, where stronger winds and heavier rains could carry sediment into the ocean.

But outside the sediment layer, there was no direct evidence for an uplift event, or the timing of this event, in the Aleutians - that is, until this research.

For the study, Cooperdock and Carrera, who is now a postdoctoral researcher at Pacific Northwest National Laboratory, used a technique called apatite thermochronometry to analyze rock samples taken from across the Aleutian archipelago, a span of nearly 1,000 miles. The technique measures the amount of helium gas found within crystals of the mineral apatite.

Apatite contains trace amounts of the radioactive elements uranium and thorium. The decay of those elements produces helium, which escapes the apatite when the crystals are buried deep underground at high temperatures. But when the rocks move from deep underground to near the surface, they cool dramatically, and the helium can no longer escape. By measuring how much helium is trapped in the apatite, and comparing it to how much uranium and thorium remain, the researchers can estimate when the rock cooled, which reveals roughly when it made its way to the surface.

The researchers found that 77% of the rocks they analyzed had cooled down at roughly the same time - a span between around 5 million to 7 million years ago. The cooling ages were consistent regardless of the formation age of the rocks and despite having come from far-separated islands across the arc. The consistent cooling age suggests the rocks came to the surface around the same time, providing the first direct evidence for a massive uplift event that spanned the entire island arc.

The timing of the uplift coincides with a previously known event during which the Pacific plate rotated, causing widespread deformation and uplift along the Ring of Fire. It's now clear, the researchers say, that this plate rotation, driven by the slow churning of Earth's mantle, caused this dramatic uplift of the Aleutians.

"It's really exciting to be able to show that these processes deep within the Earth - many kilometers in depth - are actually driving what we see on the surface," Carrera said. "It's great to have this amazing dataset that's able to demonstrate that link."

The research was supported by the U.S. National Science Foundation (EAR-1949148).