Reconciling the geology and geophysics of ocean subduction
The seafloor offers valuable information about the Earth’s internal engine, where mantle convection converts heat into motion. Material from the mantle is dislodged by plate tectonics, then returned to it by subduction, when one plate at the Earth’s crust is forced under another. “The seafloor has been constantly renewed. While we know how the continents formed after separating from the super landmass, Pangaea, around 250 million years ago, the two thirds of the Earth’s surface that were oceanic are blank slates,” explains DEEP TIME project coordinator Karin Sigloch from the French National Centre for Scientific Research, the project host. The problem is that subduction has erased the seafloor surface records. DEEP TIME, funded by the European Research Council, has helped reconstruct the seafloor to its paleo-era surface, revealing how the continents and plate boundaries evolved by tackling a controversial hypothesis that the mantle behaves uniformly enough to be predictable, so allowing ‘hindcast’ modelling. Key to this was analysis of ‘slabs’ – former ocean plates recycled back into the mantle, deforming as they sink towards the Earth’s core. These reflect the environments in which they were ‘sculpted’, offering clues about the influence of ocean trenches, plate movements and mantle dynamics. Generating sharper tomography images of the mantle’s lower half, DEEP TIME improved the inventory of subduction remnants, finding flaws in interpretations of previous blurrier images.
Introducing ‘tomotectonics’
DEEP TIME revisited a repudiated hypothesis which stated that slabs had sunk into ocean trenches and remained stationary over tens of millions of years. Consensus had grown that both the mantle and subduction-related geology must be more complicated and varied. “We posited a simpler and more uniform planet, linking surface and subsurface, using an approach we call ‘tomotectonics’,” adds Sigloch. Reassessing global subduction seismic tomography records down to a depth of around 2 000 km – back 200 million years –, the team couldn’t reject the hypothesis that massive, thickened slabs had simply sunk into place, remaining stable in the mantle. “This is satisfying, because it was what geophysics predicted before more recent, complicated interpretations of the observations,” notes Sigloch. The team also suggests that subduction zones originate mostly within ocean basins, remaining stationary until they pull in a drifting continent. Such collisions add trench materials to the continental margin, either destroying the subduction zone or converting it to a margin-hugging trench. “It’s gratifying to offer insights into the dynamics of continents actually growing,” says Sigloch. “Most subduction zones studied by geologists are on continental shorelines. It is generally underappreciated that previously these would have been trenches offshore.” The team also found that slabs hold a longer ‘memory’ than was previously thought, perhaps going back over 300 million years.
No longer oceans apart
A tantalising result of DEEP TIME’s reassessment of the palaeogeography record is the suggestion of an ‘extra ocean’ west of the Americas, during the dinosaur era. This was previously hypothesised but largely dismissed because its western boundaries did not comprise a major continent but rather trenches hosting microcontinents. “This is a significant revision. This ocean basin would have shaped climate, resulted in species diversification, not to mention generating the natural resources that gave rise to the early American economy, with the gold rush for example!” says Sigloch. Ultimately, DEEP TIME’s work has shifted questions about complexity from the deep subsurface to the shallower surface. “While the mantle’s behaviour is relatively simple, surface trench geometries do become complex, making details about the past difficult to infer from the surface alone. But deep history is clearly preserved in the mantle. Linking both, our work can help in hindcasting,” concludes Sigloch.
Keywords
DEEP TIME, subduction, Pangaea, seafloor, continent, ocean, mantle, plate tectonics, tomography, hindcasting