Plate tectonics is a fundamental theory that describes our planet, telling us how the continents and oceans move on the Earth’s surface. These plate movements also control a wide range of other natural phenomena, as eloquently summarized by Welsh indie band, The Royston Club, in their hit song “Believe it or not”. The first line of the song goes “we move like tectonic plates without the consequences of tsunamis and earthquakes, famine and heartbreak”. Tsunamis and earthquakes are some of the obvious consequences of plate tectonics, caused by a sudden release of elastic strain energy that builds up as plates slide past each other. Famine is a less obvious consequence, but still an important one as climate and tectonics are closely related. Continents colliding together or rifting apart can drastically change ocean circulation and the Earth’s climate – for example, the opening of the Drake passage between South America and Antarctica is thought to have led to the build-up of the Antarctic ice cap – so understanding these boundary conditions is vital when comparing past climate conditions to present-day climate change. Finally, there has been a lot of heartbreak during my research in plate tectonics, whether it’s troubleshooting code bugs or encountering writing block, but I am very pleased to present it to you here in Scientific Reports.
In our paper “Assessing plate reconstruction models using plate driving force consistency tests”, we present an algorithm that calculates the plate driving force balance for any GPlates plate reconstruction (a model which shows plate motion over hundreds of millions of years). These models are not based on physics – they instead are constrained by geological data – so it is possible for plate motions to be inconsistent with the forces in some regions, which has consequences for the thousands of studies that use plate reconstructions models. Our code can find these regions or times where the forces are not balanced, and so could help identify potential errors in the models.
We calculated a simple force balance, where plates are driven by the weight of plates as they sink into the mantle (“slab pull”) and the difference in gravitational potential energy between mid ocean ridges and deep ocean floor (“GPE force” or “ridge push”), and resisted by drag on the base of plates (“mantle drag”). Some parameters in these calculations are poorly known, so we calibrated these parameters to give the best balance for present-day forces. Our simple force calculation gave a good match at present-day: the residual force, or missing component required to balance forces, was less than 1% of the total force balance. However, the match in the past was poorer, with the residual force reaching 50% of the force balance in all five models.
These large residual forces indicate that something is missing in the force balance. This could be either a missing force, or a missing element (or error) in the plate reconstruction. We used the example of the Pacific plate 60 million years ago to investigate these two options for the poor force balance. Analysis of the force vectors show that the driving slab pull force and resisting mantle drag are in similar directions, instead of being in opposite directions as expected. This is due to a large slab pull force towards the southwest-dipping subduction zone between the Pacific and Australia plates. However, a review of regional tectonic models shows a number of alternative scenarios, including a transform boundary or an east-dipping subduction zone. Together with the unbalanced forces, this suggests that the current implementation in the plate models could be an error.
With this example, we demonstrated how plate driving forces can be used to identify uncertain regions in plate reconstructions. This is important not only to the plate modelling communities, but also to the number of fields that use plate models as constraints, from climate modelling to natural resource prospecting. We freely share our code, together with a user guide, and hope that this could be used by plate modellers to improve plate reconstruction models. With this method, we can hopefully model how tectonic plates move without any unwanted consequences.