Understanding how delamination and relamination shape continental long-term evolution: a more active role of continents in earth’s history

Peng, Lihang

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https://hdl.handle.net/2142/130034 Copy

Description

Title

Understanding how delamination and relamination shape continental long-term evolution: a more active role of continents in earth’s history

Author(s)

Peng, Lihang

Issue Date

2025-07-13

Director of Research (if dissertation) or Advisor (if thesis)

Liu, Lijun Liu

Doctoral Committee Chair(s)

Lundstrom, Craig Campbell

Committee Member(s)

• Guenthner, William
• Maguire, Ross
• Stewart, Michael

Department of Study

Earth Sci & Environmental Chng

Discipline

Geology

Degree Granting Institution

University of Illinois Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Craton
• Sub-Cratonic Lithospheric Mantle
• Long-term stability
• Delamination
• Relamination
• Rifting

Abstract

Continents, with a buoyant felsic crust, are traditionally thought to play a passive role in plate tectonics, floating on the soft asthenosphere, drifted by subducting oceanic plates. Cratons, as the continental core, are characterized by old crust (> 2.5 Ga) and thick lithosphere (~200 km thick). They are believed to be stable with the minimum deformation since their formation, thanks to the buoyancy and rigidity of the Sub-Cratonic Lithosphere Mantle (SCLM). However, increasing evidence shows that many cratons have experienced multiple stages of vertical movements, such as uplift and subsidence, during geological history. For example, many cratonic basins around the globe started to develop right after the Great Unconformity, followed by almost synchronous uplift in Mesozoic during the separation stage of the Pangaea Supercontinent. Such geological observations suggest that cratons are not always boringly stable but could experience significant vertical movement along with super-continental cycles, likely driven by density changes of the SCLM, such as refertilization of the depleted SCLM and delamination of the dense SCLM. Furthermore, a study on the breakup of South America and Africa reveals widespread delamination of the lower SCLM happened under South America and Southern Africa during their breakup, likely caused by perturbation of mantle plumes and the rifting process. However, although some geodynamic studies showing the feasibility of delamination, the exact cause and consequence of delamination, and the fate of the delaminated SCLM have not been thoroughly explored. Here, we first investigate SCLM’s delamination and show that delamination of the lower SCLM occurs due to the perturbation of mantle plumes and/or in extensional tectonic settings. For the former one, together with the seismological evidence under the Illinois and Michigan Basins, we develop a geodynamic model and show that basin termination and inversion in the Mesozoic is likely due to a regional scale mantle-plume-induced delamination event. Specifically, the hot mantle plume could melt some pre-existing vertical channels connecting weak and wet Mid-Lithospheric Discontinuities (MLDs) to the asthenosphere, causing melting and weakening of the MLDs, resulting in delamination of the lower dense portion of the SCLM. For the latter case, we develop a model showing that delamination could also happen when a craton is under an extensional environment. With a geological example of the West Yangtze Craton (WYC), we suggest that the WYC has undergone modification and delamination during late Cenozoic when the WYC transitioned to transtension during the India-Tibet collision. In this case, thanks to the gradual extensional deformation, the weak and hydrous MLDs could be exposed to the hot asthenospheric mantle as extensions create pathways from the asthenosphere to the MLDs, such that delamination could happen more naturally without the help of mantle plumes. After delamination, a natural question arises: where will those delaminated SCLM go? We develop geodynamic models to show that delaminated SCLM would sink to the Mantle Transition Zone (MTZ) first. During their stay in MTZ, their buoyancy will gradually increase due to their increase in temperature. Moreover, we find that, if SCLM consists of dense basaltic layers after gradual refertilization, these dense basaltic layers could be peeled off from the rest of chunk of delaminated SCLM which is more depleted. This peeling-off process is due to density contrast between basaltic composition and peridotitic composition at around 660-750 km depth, and it could be facilitated by a low viscosity channel at around 600 km depth (due to grain size reduction). Therefore, given long enough time (approximately 80-120 Myr), the delaminated SCLM could gain its buoyancy by increasing its temperature and/or losing the dense basaltic composition. As a result, the delaminated SCLM would finally rise up and relaminate back to the base of the lithosphere. Both delamination and relamination could have significant impact on the surface and the upper mantle environment. Our model suggests delamination corresponds to kilometer scale of rapid uplift, consistent with the observed present-day high topography in Southern Africa and Brazilian Plateau, as well as the inferred scale of uplift during the basin version of the Illinois and Michigan Basins. The relamination would cause hundred-meter scale of initial uplift, followed by long-lived kilometer scale of subsidence in cratonic basins due to subsequent cooling of the relaminated lithosphere. Such delamination, relamination, subsequent cooling, and refertilization could form a cycle during super-continental cycles, well explaining the temporal evolution of cratons as they tend to cycle from subsidence to uplift multiple times in geological history. As for the upper mantle, our model shows the delaminated cold SCLM could lower down the neighboring mantle temperature by 50-100 degree C, providing an explanation why the Atlantic and Indian Oceans have lower hotspot and Mid-Ocean Ridge temperature compared to those in the Pacific, as seismic tomography suggest wide-spread delamination had happened during the opening of the Atlantic and Indian Ocean. Furthermore, delamination and relamination could contribute to upper mantle heterogeneities as those delaminated depleted and/or enrich basaltic pieces are mixed into the upper mantle, explaining the presence of continental lithosphere geochemical signatures in basalts found in Atlantic and Indian Ocean. Lastly, we investigate how SCLM’s density affects craton’s long-term evolution by various geodynamic models. We first compare the horizontal deformation and stress of cratons with a positively buoyant, neutrally buoyant, and negatively buoyant SCLM. Our results show that all cases exhibit compressional stress in the upper crust, contrary to conventional thought: only negatively buoyant SCLM will result in compressional stress. As for deformation, cratons with either positively or neutrally buoyant SCLM will have extensional deformation after one billion years of evolution, while the craton with negatively buoyant SCLM mostly maintain its original length. Furthermore, we find when the SCLM is undergoing density change, such as refertilization (density increase) and delamination (density decrease), combined with weak suture zones, the craton could undergo 1) extensional deformation when the SCLM is positively buoyant, forming grabens and rifts along the sutures, 2) and compressional deformation when the SCLM is changing to negatively buoyant, resulting in rift inversion. This framework provides an alternative model of horizontal deformation within continents as it can be dictated by internal density changes within SCLM, and the conventional wisdom is that continent’s horizontal deformation is always controlled by external forces (i.e. mantle plumes and/or far-field stresses from plate tectonics). This alternative framework could especially explain the formation and final inversion of the Midcontinental Rift (MCR), where the initial rifting is caused by an incomplete delamination, and the final inversion is due to the following relamination of the delaminated SCLM, subsequent cooling, and refertilization. Further analysis is needed to explore the detailed processes of how a continent could break when delamination happens. Quantitative analysis on how delamination-relamination and plate-tectonics far-field stresses contribute to MCR’s formation is needed. Overall, we provide a novel framework of continental evolution where continents are not just passive objects, but a more active role in earth’s long-term evolution, affecting aspects of earth geodynamic processes, geochemistry, surface processes, and biological evolutions.

Graduation Semester

2025-08

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/130034

Copyright and License Information

Copyright 2025 by Lihang Peng

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