Geodynamics of lithospheric foundering and the evolution of the North China craton

Abstract

Cratons represent the oldest and most stable portions of the continental lithosphere. Compared to normal continental plates, their lithospheric roots may extend three to four times deeper into the mantle. During their formation, cratons expelled heavy metallic elements from their system and became enriched in magnesium, which contributed to their buoyant nature. Despite gradual cooling and increasing density over time, the lithospheric mantle of cratons remains in a delicate balance. Their chemically depleted and thermally cold composition, combined with relatively homogeneous internal structures and their location away from active tectonic zones, have long been recognised as the primary factors contributing to cratonic stability. Nevertheless, certain cratons such as the North China Craton, the Wyoming Craton, and the Brazil Craton stand out as notable exceptions, having undergone substantial modification and partial destruction. Although cratonic regions are typically associated with long-term stability, geological and geophysical observations, including elevated heat flow, reduced lithospheric thickness, widespread magmatism, surface uplift, and alternating extensional and compressional deformation, demonstrate that cratons can experience profound modification or even complete destabilisation. This raises fundamental questions about the mechanisms that control the long-term persistence of cratonic lithosphere. Among all examples, the North China Craton provides one of the most striking cases. Initially formed as a stable Archean craton, its eastern part was dramatically transformed during the Mesozoic into a tectonically active region characterised by extensive magmatism, severe lithospheric thinning, and widespread deformation. Understanding the processes responsible for this transformation is crucial for reconstructing the tectonic evolution of East Asia and for developing a broader understanding of continental lithospheric dynamics. This thesis investigates the geodynamic evolution of the North China Craton, with particular emphasis on the role of eclogite formation in weakening and destabilising the lithosphere. The research combines a global analysis of eclogite-related processes with a regional synthesis of existing models and new numerical simulations that explicitly test the effects of mineral phase transformations under various tectonic regimes. This integrative approach provides a comprehensive understanding of the evolution of the North China Craton and its broader implications for cratonic stability. Chapter 1 examines eclogite-induced lithospheric dripping in tectonically active regions such as continental arcs and collisional orogens. Numerical experiments explore how variations in eclogite thickness, width, and density along with the properties of the surrounding lithospheric mantle govern the initiation and scale of dripping. Three principal styles of lithospheric dripping are identified. Type 1 corresponds to small-scale drips, as inferred beneath the Altiplano-Puna plateau in the Andes, where seismic anomalies suggest localised lithospheric removal. Type 2 represents large-scale drips involving both crustal and mantle components, such as those beneath south-central Tibet and the Sierra Nevada, where eclogite formation explains transient uplift and magmatism. Type 3 describes stable configurations where eclogite remains intact, typical of passive margins and suture zones such as the Barents Sea and Newfoundland Appalachians. The results show that eclogite sinking often occurs episodically, producing pulsating magmatism and surface uplift, consistent with geological observations from orogenic plateaus and active arcs worldwide. This chapter provides the global geodynamic context for understanding how eclogite related processes contribute to lithospheric instability. Chapter 2 focuses on the geological framework and Mesozoic evolution of the North China Craton, critically evaluating the range of existing models proposed for its destruction. Mechanisms such as subduction-driven thermal erosion, magmatic weakening, lithospheric delamination, and crustal extension are systematically reviewed. However, none of these processes alone can fully explain the observed spatial and temporal complexity of the North China Craton deformation. Instead, the data point to an interplay between multiple processes. Delamination accounts for abrupt lithospheric thinning and associated magmatism, as asthenospheric upwelling follows removal of dense lithospheric roots. Flat-slab subduction and the Big Mantle Wedge model explain Jurassic–Cretaceous magmatism, including adakitic and A-type granitic compositions, and the observed landward-to-oceanward migration of igneous activity. Meanwhile, lithospheric extension is supported by Early Cretaceous rift basins, metamorphic core complexes, and elevated heat flow, coinciding with the peak of magmatic activity. A combined scenario of flat-slab subduction, delamination, and back-arc extension best reproduces the observed geological, geochemical, and thermal evolution of the North China Craton. This synthesis demonstrates that cratons can be destabilised when fluids, melts, and thermal perturbations weaken their lithospheric roots. Chapter 3 presents numerical simulations of episodic double lithospheric delamination triggered by crustal shortening and subsequent eclogite formation. The models explore a range of parameters, including shortening rate, the volume and geometry of eclogite, the absence of eclogitisation, and mantle lithosphere viscosity. The simulations reveal that dense eclogite within the lower crust can drive stepwise weakening and removal of the lithosphere, producing two distinct delamination events. The preferred model shows delamination phases at approximately 160–150 Ma and 135–125 Ma, consistent with geological evidence for alternating shortening and extension. These results successfully reproduce key geophysical and geological features, including lithospheric thinning to ~60–100 km, the two-phase extension observed in the central–eastern North China Craton, and the timing and spatial distribution of Mesozoic magmatism. Comparison with seismic tomography and xenolith-derived geotherms supports the plausibility of these removal events. The results indicate that lithospheric delamination is not a single catastrophic process but a sequence of discrete episodes facilitated by inherited lithospheric weaknesses and prior metasomatism linked to subduction. In conclusion, this thesis demonstrates that mineral phase transformations, particularly eclogitisation, play a central role in destabilising continental lithosphere when coupled with favourable tectonic boundary conditions. The North China Craton serves as a natural laboratory illustrating how shortening, magmatism, and eclogite-driven density instabilities can combine to transform a once-stable craton. The findings reveal that although cratons are long-lived, they are not immune to destruction under appropriate geodynamic circumstances. By integrating a global perspective on eclogite-induced instability with a comprehensive regional synthesis and new numerical evidence, this thesis establishes a unified framework for understanding how cratonic lithosphere evolves, weakens, and ultimately collapses.