Why do continental margins subside faster than expected? The process accelerating the cooling of the Earth's crust
The flow of water through porous basalt rocks caused accelerated heat removal from the Earth's depths, cooling the crust and causing the seabed to subside, upon which layers of sediment about 8 km thick accumulated. This process changes the way scientists understand the development of continental margins, the assessment of oil and gas resources in oceanic basins, and the reconstruction of ancient sea-level changes.
The Earth's crust off the coast of the eastern USA, in the space between the American continent and the Atlantic Ocean, cooled at a rate up to 1.6 times faster than the rate assumed by accepted geological models. This discovery was revealed in a study conducted at the University of Haifa and published in the journal Communications Earth & Environment.
The authors of the study found that following the accelerated cooling, the rocks became denser, the area subsided faster, and space was created for the accumulation of thick layers of sediment. "The finding changes the way we understand the development of continental margins. If the crust cools faster than we thought, then the subsidence of the seabed and the accumulation of sediment layers above it also occur faster," said Dr. Guy Lang of the Leon Charney School of Marine Sciences at the University of Haifa and the Geological Survey of Israel, one of the study's authors. "Therefore, processes that we attributed for years only to passive cooling now require an additional explanation."
When continents break apart and move away from each other, the Earth's crust stretches and becomes thinner. As the process progresses, hot magmatic material rises from the Earth's depths and cools to form new oceanic crust. In the space between the continents and the new ocean, vast areas remain, known as "passive continental margins." Over time, these areas cool through upward heat conduction. The cooling leads to an increase in crust density, its subsidence, and its coverage by thick layers of sediment. The thickness of the sediment layers is dictated by the crust's cooling rate: the faster the cooling, the thicker the layers that will subside in a shorter time.
In about half of these areas, huge amounts of molten rock (magma) rose from the Earth's depths during the continental breakup. Part of that molten rock penetrated the crust, and part erupted and covered it with thick layers made of basalt. Because of this, these areas are called "magma-rich continental margins." For decades, geological models struggled to explain why these areas subsided much faster and why layers of sediment accumulated above them that were much thicker than expected by models assuming passive heat conduction.
In the current study, Dr. Lang, Prof. Itzik Makovsky, and Prof. Uri ten Brink, from the Dr. Moses Strauss Department of Marine Geosciences at the University of Haifa, the Geological Survey of Israel, and the US Geological Survey, sought to examine which processes could explain the rapid subsidence of a vast area located off the Atlantic coast of the USA. This area is considered a clear example of a magma-rich continental margin.
To this end, the researchers developed a new mathematical model designed to reconstruct how the continental margins cooled and subsided after the continental breakup. The model combined three central processes: the stretching of the Earth's crust and the layers beneath it, the addition of volcanic rocks to the crust, and changes in the rate of heat transfer through the rocks. The researchers compared the model's predictions to subsurface data from the study area, which were based on seismic measurements and allowed for estimating the thickness of the crust and the thickness of the sediment layers that accumulated above it during the first 26 million years after the continental breakup.
To test the stability of the results, the researchers performed 30,000 repeated adjustments of the model to different samples from the database, and then compared its predictions also to the subsidence history reconstructed from a deep research borehole in the area. The results of the study show that during the first 26 million years after the continental breakup, sediments up to about 8 km thick accumulated in the study area, while accepted geological models predicted a maximum accumulation of about 3 km. Even after the researchers took into account the uneven stretching of the crust and the volcanic rocks added to it, the models still could not explain the full subsidence. Only when a faster cooling rate was included in the model did it manage to align with the measured data.
The researchers believe that the accelerated cooling was made possible by water that flowed through the porous basalt rocks, heated up at depth, and carried the heat upward. As the heat was removed faster, the crust became denser and the area subsided at a faster rate. Similar processes are observed today in Iceland and East Africa.
"The ability to more accurately reconstruct the cooling and subsidence rate of continental margins is important far beyond understanding the development of the area studied. It can change the way we interpret the thickness of sediment layers, reconstruct ancient sea-level changes, and assess the thermal history of basins where oil and gas systems developed," the researchers concluded.
