The density and evolution of sub-cratonic lithospheric mantle constrained by geological and geophysical observations

Wang, Yaoyi

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https://hdl.handle.net/2142/115565 Copy

Description

Title

The density and evolution of sub-cratonic lithospheric mantle constrained by geological and geophysical observations

Author(s)

Wang, Yaoyi

Issue Date

2022-04-20

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

Liu, Lijun

Doctoral Committee Chair(s)

Liu, Lijun

Committee Member(s)

• Lundstrom, Craig C
• Marshak, Stephen
• Guenthner, William

Department of Study

Geology

Discipline

Geology

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Sub-cratonic mantle lithosphere
• density of mantle lithosphere
• lithospheric evolution
• craton conditional stability
• topography
• gravity
• geoid
• data-assimilation models

Abstract

The cratonic crust is the most long-lived tectonic unit on Earth. Its longevity is generally attributed to the underlying lithospheric keels that we thought to be neutrally buoyant and mechanically strong. However, this reasoning violates recent observations revealing cratonic layering, deformation and alteration, raising questions about both the dynamic properties (density and viscosity) and long-term stability of cratonic lithosphere. The density structure of the sub-cratonic lithospheric mantle (SCLM) remains debated. We suggest that one important reason for which many geodynamic studies favor neutrally buoyant SCLM is that they adopted separate reference frames when estimating the isostatic effects of continental and oceanic lithosphere, while instead a globally consistent one should be used. This reflects a misconception that continental crust perfectly balances the surrounding oceanic lithosphere. Using a unified global reference frame with recent constraints on crustal properties and residual topography, we show that assuming neutrally buoyant SCLM leads to prominent negative residual topography (~ -1.3 km) and positive residual gravity (~354 mGal) within cratons relative to oceans, neither of which can be explained by the effects of the convecting mantle. This requires the SCLM, especially that with thick keels, to be less compositionally buoyant and denser than previously thought, a conclusion supporting recent observations on SCLM deformation. While topography and gravity provide useful information on the net buoyancy of SCLM, we suggest that the geoid can help constrain both the net buoyancy and the depth-dependent density distribution of the SCLM. The geoid is a key observable for understanding the dynamics of the deep Earth, but has been considered largely transparent to long-wavelength (>300 km) shallow density structures, especially those of the SCLM. Here, we demonstrate that the observed flat craton-ocean geoid pattern, traditionally interpreted as reflecting neutrally buoyant cratonic keels, constrains the depth-dependent density distribution of SCLM. Using both simple theoretical calculations and sophisticated numerical models, we show that the recent seismic data on lithospheric structure require the existence of a dense SCLM to explain the observed topography and the geoid. In practice, topography reveals the net buoyancy of the SCLM, while the geoid further delineates the depth-dependence of excess density. We find that the mantle lithosphere below large cratons should bear net negative buoyancy close to that of a pure-thermal lithosphere, with most of the excess density distributed within the lower half of the lithosphere. Density profiles of small cratons, due to strong edge effects from surrounding orogenic belts, are harder to constrain, except that their mantle lithosphere is also negatively buoyant. Besides the density structure, the time evolution of SCLM can also be inferred from geophysical observations, such as observed topography, crustal thickness and lithosphere- asthenosphere boundary (LAB). We find that cratonic residual topography strongly depends on lithospheric thickness, implying overall denser-than-ambient lithospheric mantle, with potential density stratification across either the mid-lithospheric discontinuity or ~190 km depth. We further show that the thickness of craton crust correlates linearly with that of the mantle lithosphere, with highest values from large stable cratons. This reveals the coupled crust-mantle evolution where lithospheric thinning causes surface uplift and crustal erosion. Tectonic and geologic data support repeated post-Rodinia removals of lower cratonic lithosphere during supercontinent separation, especially when cratons overrode the large low shear wave velocity provinces. A strong correlation of depth-dependent North American lithospheric anisotropy with its Phanerozoic plate motions further implies repeated lower-lithosphere restoration after removal. Geodynamic models properly reproduce this deformation style with lithospheric delamination and relamination generating surface uplift while subsequent lithospheric stabilization causing subsidence. We conclude that the cratonic lithosphere has been following this cyclic yoyo-style dynamic evolution since the Neoproterozoic. To quantitatively study the time-dependent behaviors of cratonic lithosphere, we attempt to simulate the delamination process of dense SCLM in global data-assimilation models, which incorporate the realistic subduction and upwelling plumes, as well as the large low shear velocity provinces (LLSVP). Our preliminary results suggest that SCLM delamination and relamination could be affected by other mantle processes. For example, subducting slabs can entrain delaminated cratonic lithosphere into the lower mantle, while mantle upwelling can help facilitate the relamination process. These results provide useful insights on future research involving additional geodynamic calculations and relevant observations in order to further constrain the temporal evolution of the SCLM.

Graduation Semester

2022-05

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Yaoyi Wang

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