Ancient Earth’s crust breaking down, altering the planet’s geological history
Earth’s oldest continental crust, once believed to be geologically stable, has been gradually eroding over the past 200 million years due to tectonic shifts and mantle processes. Research led by Professor Shaofeng Liu of the China University of Geosciences into the North China Craton suggests that similar tectonic forces may have impacted other cratons worldwide, including those in North and South America. The gradual disintegration of these ancient cratons could alter Earth’s geological stability and potentially affect long-term environmental conditions.
The global plate reconstructions used in this study. Image credit: Liu, S., Zhang, B., Ma, P. et al.
- This discovery challenges the previously accepted notion that the Earth’s earliest crust has remained largely unchanged throughout its history.
- The team used four-dimensional mantle flow models, seismic tomography, and geochemical analysis to track lithospheric deformation and subduction-driven erosion of the NCC.
Scientists discovered evidence that the Earth’s oldest continental crust is progressively eroding. These ancient structures were previously assumed to be geologically stable and resistant to alteration.
Subduction and mantle plumes have been proposed as potential causes for the disintegration of Earth’s ancient crust, though the exact mechanisms remain elusive.
Subduction occurs when a denser tectonic plate is forced beneath another, melting in the mantle, while mantle plumes rise from deeper layers to erode the crust thermally.
A thorough analysis was conducted by the team in the North China Craton (NCC), examining its disintegration over the last 200 million years with advanced four-dimensional mantle flow models.
The researchers investigated how surface topography, lithospheric deformation, and seismic tomography have changed since the middle Mesozoic (around 168 million years ago).
They found two critical stages in the NCC’s distortion. The first entailed the subduction of the oceanic Izanagi plate, which forced the crust of the overriding Eurasian plate to thicken, resulting in topographic highs such as the Taihang Mountains.
The second stage was distinguished by the rollback of the subducting plate, which resulted in lithospheric weakening and, eventually, craton breakup. This rollback allowed for lithospheric extension, which resulted in a 26% thinner crust.
Seismic tomography confirmed the findings, revealing a 4 000 km (2 485 miles) wide slab deep within the mantle transition zone.
This was reinforced further by volcanic rock records that revealed recycled carbonates from the subducted slab, resulting in carbonated peridotite, a vital indicator of mantle activities. Over millions of years, the mantle wedge formed by this rollback dissipated.
“The North American craton, South American craton, and the Yangtze craton in China may have experienced similar deformation. All of these may have experienced early flat-slab subduction. However, intense subsequent rollback subduction might have occurred in the Yangtze craton. In contrast, the North American craton underwent trench retreat following flat-slab subduction but did not exhibit significant slab rollback,” Shaofeng study’s lead author said.
This discovery calls into question the long-held belief that Earth’s earliest crust has stayed relatively intact throughout its history.
This study is significant because the continental crust is essential for supporting life and ensuring stable environmental conditions on Earth.
Its slow collapse could have far-reaching consequences for the planet’s geological stability and long-term evolution. Understanding these processes allows geologists to understand better how the Earth’s surface has evolved and will continue to evolve.
The findings were published on September 6, 2024, in the journal Nature Geoscience, after years of research and analysis.
While the continental crust’s disintegration process has lasted millions of years, the study focuses on proving this continuous breakdown, which is still occurring today.
The study used three global plate reconstruction models along with many other methods. Model 1 (230 Ma to the present) is based on previous rigid plate models, while Model 2 (250 Ma to the present) incorporates a deforming plate model. Model 3 extends back to 410 Ma and introduces the Mongol–Okhotsk Ocean (MOO) closure.
The CitcomS package was used for global mantle flow models, solving thermal convection problems in the mantle. These models incorporated tectonic history, subduction zones, and the evolving thermal structure of the lithosphere. Different cases were tested to explore variations in viscosity layering and subduction angles, providing insights into dynamic topography.
Stratigraphic backstripping was used to identify residual subsidence, a long-wavelength component of the basin’s tectonic subsidence. This was essential in regions like the Ordos Basin and Yanshan area, where tectonic subsidence was influenced by continental thinning and thermal contraction.
The research was led by Shaofeng Liu of the State Key Laboratory of Geological Processes and Mineral Resources, Frontiers Science Center for Deep-time Digital Earth, and School of Geosciences and Resources, China University of Geosciences (Beijing), Beijing.
An international team that included academics from the China University of Geosciences (Beijing), California Institute of Technology, Pasadena, University of Tasmania, Hobart, and the Taiyuan University of Technology, Taiyuan, collaborated in this research.
References:
1 Craton deformation from flat-slab subduction and rollback – Liu, S., Zhang, B., Ma, P. et al. – Nat. Geosci. 17, 936–943 (2024) – September 6, 2024 – https://doi.org/10.1038/s41561-024-01513-2 — OPEN ACCESS
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