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Deformation of the North China Craton: Insights into Earth’s Ancient Crust

The configuration of Earth’s continents has undergone significant transformations throughout its extensive geological history, impacting their geographical locations and topographical features as the crust alternately expands and contracts. Among the regions of continental crust, cratons have exhibited remarkable stability over vast time scales, remaining relatively undisturbed by tectonic upheavals or mantle dynamics.

A recent study published in Nature Geoscience delves into the processes responsible for the deformation of these ancient cratons, a phenomenon referred to as decratonization. Despite theories suggesting that subduction—the sinking of a denser tectonic plate beneath another—and deep mantle plumes—the rise of hot material from the mantle—could contribute to this process, the precise mechanisms behind the alteration and eventual breakdown of cratons remain partially understood.

Professor Shaofeng Liu from the China University of Geosciences and his research team have focused their investigation on the North China Craton (NCC) over the last 200 million years. By employing four-dimensional mantle flow models that integrate data on surface topography, lithospheric deformation, and seismic tomography, the team has mapped the NCC’s evolution since the middle Mesozoic era, approximately 168 million years ago.

The findings reveal two critical phases of deformation that the NCC has experienced. The first stage involved the subduction of the Izanagi plate at a shallow angle, known as flat-slab subduction, which provoked crust thickening in the overriding Eurasian plate. This tectonic activity led to the creation of significant topographic features, including the Taihang Mountain range. This compression was the result of the eastward movement of the Eurasian plate, which effectively shortened the land.

In a subsequent phase, the retreat of the subducting plate initiated flat-slab rollback, which resulted in a significant 26% reduction in lithospheric thickness. This transition marked a change in the NCC’s movement from an eastward to a southward direction, which consequently reduced the convergence rate between the two tectonic plates.

Throughout this protracted transformation, the researchers noted a series of geological activities spanning millions of years. This began with the formation of thrust and transpressional faults during the Jurassic and early Cretaceous periods and continued with episodes of crustal extension interspersed with compressional instances from the late Cretaceous period (93–80 million years ago). The cumulative effects ultimately led to the fragmentation of the craton.

For validation, the team constructed three flow models reflecting the region’s tectonic history, cross-referencing predictions of contemporary structures with seismic tomography data. Their flat-slab rollback model successfully reconstructed a large mantle transition zone characterized by a slab measuring 4,000 km wide and extending up to 660 km deep, which contributed to the development of a substantial mantle wedge.

This mantle wedge is evidenced by the volcanic rock recorded in the present day, showcasing the transformation of subducted carbonates into carbonated peridotite within the upper mantle. Over the course of tens of millions of years, this feature diminished as slab rollback continued.

The decratonization phenomenon observed in the NCC may not be unique to this region. Professor Liu posits that similar processes might have affected other cratons across the globe, albeit with regional variations, and this topic remains a key area of ongoing research.

“Cratons in other parts of the world, such as North America, South America, and the Yangtze region of China, may have undergone comparable deformation. For instance, the Yangtze craton displayed intense rollback subduction, while the North American craton shifted into trench retreat without significant slab rollback,” he noted.

This investigation underscores that cratons situated in continental interiors generally face a lower risk of destabilization compared to those positioned near tectonic plate boundaries, which can be more vulnerable to subduction and rollback dynamics over geological timeframes.

“The ancient lithosphere is susceptible to fragmentation, with this disintegration stemming from particular types of subduction near oceanic plates, offering insights into the evolution of continents throughout Earth’s history,” concludes Professor Liu.

More information:
Shaofeng Liu et al, Craton deformation from flat-slab subduction and rollback, Nature Geoscience (2024). DOI: 10.1038/s41561-024-01513-2

Source
phys.org