Craton stability and longevity: The roles of composition-dependent rheology and buoyancy

Abstract

. (2014) 'Craton stability and longevity : the roles of composition-dependent rheology and buoyancy.', Earth and planetary science letters., 391 . pp. 224-233.Further information on publisher's website:http://dx.doi.org/10. 1016/j.epsl.2014.01.038 Publisher's copyright statement: NOTICE: this is the author's version of a work that was accepted for publication in Earth and Planetary Science Letters. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reected in this document. Changes may have been made to this work since it was submitted for publication. A denitive version was subsequently published in Earth and Planetary Science Letters, 391, 2014Letters, 391, , 10.1016Letters, 391, /j.epsl.2014 Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Abstract:Survival of thick cratonic roots in a vigorously convecting mantle system for billions of years has long been studied by the geodynamical community. High strength of the cratonic root is generally considered to be the most important factor, but the role of lithospheric mantle depletion and dehydration in this strengthening is still debated. Geodynamical models often argue for a significant strength or buoyancy contrast between cratonic and non-cratonic mantle lithosphere, induced by mantle depletion and dehydration. But recent laboratory experiments argue for only a modest effect of dehydration strengthening. Can we reconcile laboratory experiments and geodynamical models?We perform and discuss new numerical models to investigate craton stability and longevity with different composition-dependent rheology and buoyancy. Our results show that highly viscous and possibly buoyant cratonic root is essential to stabilise a geometry in which thick cratonic lithosphere and thinner non-cratonic lithosphere coexist for billions of years. Using nonNewtonian rheology, a modest strengthening factor of Δη=3 can protect compositionally buoyant cratonic roots from erosion by mantle convection for over billions of years. A larger strengthening factor (Δη=10) can maintain long term craton stability even with little or no intrinsic buoyancy. Such composition-dependent rheology is comparable to the laboratory experiments. This implies that a strict isopycnic state of cratonic lithosphere may not be necessary for the preservation of a cratonic root, provided a sufficient level of compositional strengthening is present.

Episodic and multistaged gravitational instability of cratonic lithosphere and its implications for reactivation of the North China Craton

“…The instability happens to the shallow part of the lithosphere with relatively short horizontal length scales (Figures and a), which differs significantly from the instability for Newtonian fluids or chemically neutrally buoyant lithosphere that removes mostly the bottom of the lithosphere [e.g., Conrad and Molnar , ; Jaupart et al ., ]. This also differs from convective entrainment in thermochemical convection models that erode and destruct gradually cratonic lithosphere from the bottom of cratonic lithosphere upward [ Beuchert et al ., ; Lenardic and Moresi , ; Shapiro et al ., ; Wang et al ., ]. These distinct features and the increased threshold buoyancy number up to 0.7 can be explained by dynamic interaction between the buoyancy structure and non‐Newtonian rheology.…”

“…For non‐Newtonian models, we use a composite viscosity that incorporates both the Newtonian and non‐Newtonian rheology [e.g., Hirth and Kohlstedt , ], and the effective viscosity is given by [e.g., Podolefsky et al ., ] where ε is the second invariant of strain rate tensor, τ T and n are the transition stress and the stress exponent, respectively, and η c reflects the composition and temperature‐dependent viscosity to be discussed more later. The stress exponent is taken as 3.5 for dislocation creep as reported from laboratory experiments [e.g., Hirth and Kohlstedt , ] and commonly used in geodynamic studies [e.g., Podolefsky et al ., ; Wang et al ., ]. The transition stress could be in the range of 0.1–1 MPa in asthenospheric condition as suggested by Hirth and Kohlstedt [], but we use 0.5 MPa for transition stress here.…”

The helium flux from the continents and ubiquity of low-3He/4He recycled crust and lithosphere

“…There is geodynamic evidence that portions of CLM have been removed beneath some continental areas (Eggler et al, 1988;Molnar et al, 1993;Menzies and Xu, 1998;Lee et al, 2000). Proterozoic non-cratonic CLM is the most probable lithospheric material to be incorporated into the asthenosphere; this is because of its inherent weakness and generally higher density, in contrast with most cratonic CLM (Wang et al, 2014). Proterozoic non-cratonic CLM will generate time-integrated 3 He/(U + Th) variations lower than Archaean cratonic peridotites; and the incorporation of either cratonic or non-cratonic lithospheric mantle incorporated into melts can potentially explain 3 He/ 4 He of CIAV being lower than MORB (Day et al, 2005).…”

The thinning of subcontinental lithosphere: The roles of plume impact and metasomatic weakening

“…Two particle functions are used in our models, with i = 1 and 2 representing the crust and the depleted mantle, respectively. As non‐Newtonian rheology has been found to be important for both the stability and dynamics of the cratonic lithosphere [ Wang et al ., ; Wang et al, ], we use a composite rheology of non‐Newtonian and Newtonian rheology to represent the dislocation creep and diffusion creep, respectively. Thus, the composition‐dependent viscosities are calculated as follows: …”

“…The chemical distinction of highly melt‐depleted cratonic roots is considered to be the most important reason for the survival of Archean lithosphere [ Boyd , ; Carlson et al ., ]. Geodynamical research supports this hypothesis through numerical modeling by using reasonable density structure and mantle rheology [ Doin et al ., ; Lenardic and Moresi , ; O'Neill et al ., ; Wang et al ., ].…”