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

Abstract

Archean cratons are the most stable tectonic units and their lithospheric mantle is chemically depleted and buoyant relative to the underlying mantle. The chemical depletion leads to high viscosity that maintains the long-term stability of cratons. However, the eastern part of the North China Craton ($1200 km in horizontal length scale) had been extensively reactivated and modified over a time scale of $100 Myr in the Mesozoic and Cenozoic. While the causes for the weakening of the North China Craton, a necessary condition for its reactivation, are still in debate, we investigate gravitational instability of compositionally buoyant lithosphere, by computing 2-D thermochemical convection models with different buoyancy number, lithospheric viscosity, and rheology. We find that the gravitational instability of cratonic lithosphere can happen over a larger range of buoyancy numbers with non-Newtonian rheology, but lithospheric instability with Newtonian rheology only happens with relatively small buoyancy numbers. For cratonic lithosphere with non-Newtonian rheology and relatively weak temperature-dependent viscosity, the instability starts in the cold, shallow part of the lithosphere and has small horizontal length scale (<300 km), leading to efficient thermal and chemical mixing with the underlying mantle. For cratonic lithosphere such as the eastern North China Craton, the instability process is episodic and consists of multiple instability events that may last for $100 Myr. The instability process revealed from our study explains the observations of episodic magmatism/volcanism events, geochemical mixing, and time scales associated with the reactivation of the North China Craton.

Continental crust formation at arcs, the arclogite “delamination” cycle, and one origin for fertile melting anomalies in the mantle

“…We note that the viscosity of the lithospheric mantle may also play a role as in the case of destabilization of cratonic mantle [76][77][78][79][80][81][82]. In the case of cratonic lithosphere, the lithospheric mantle could be *200 km thick, whereas the lower crust is less than 20 km thick.…”

Heat flux and topography constraints on thermochemical structure below North China Craton regions and implications for evolution of cratonic lithosphere

“…We first present a case with B = 0.4 and Ra l = 2930 (case B43 in Table ) as a reference case. The gravitational instability process for the cratonic lithosphere in our models here is similar to that in Wang et al []. Figure shows several snapshots of temperature and composition, as well as surface topography and heat flux for this case.…”

“…The viscosity employed in this study is compositional‐, temperature‐, depth‐ and strain rate‐dependent, and the effective composite viscosity is defined as [e.g., Wang et al , ] where ε is the second invariant of the strain rate tensor and τ T and n are the transition stress and the stress exponent, respectively [ Hirth and Kohlstedt , ], η TC represents the depth‐, composition‐ and temperature‐dependent viscosity that is expressed as where η c and η h reflect the composition‐ and depth‐dependence, respectively, and E * = E /( R Δ T ) is the dimensionless activation energy with R as the gas constant and E as the activation energy. Although it has been suggested that the viscosity contrast between the cratonic lithosphere and the asthenosphere may not exceed a factor of 100 [e.g., O'Neill et al , ], with the consideration of stress‐dependent viscosity, we set η c to be 1000 for cratonic lithosphere and 1 for the normal mantle, respectively, following Wang et al []. The depth‐dependence parameter, η h , is set to be 1 for upper mantle (i.e., above a depth of 410 km), 5 for the transition zone (i.e., between depths of 410 km and 660 km), and 30 for lower mantle (i.e., below a depth of 660 km) (Figure c).…”

“…The effective composite viscosity (equation ) for the left half of the cratonic lithosphere is set to be 10 times higher than that of the right half, and the left and right sides of the model lithosphere are used to simulate the stable western and unstable eastern NCC, respectively (Figure c). Previous studies have shown that the stability of the buoyant cratonic lithosphere is controlled by the lithospheric viscosity (or Rayleigh number) [ Jaupart et al , ] and for non‐Newtonian viscosity by the lithospheric viscosity just before the onset of instability [ Wang et al , ]. Therefore, similar to Wang et al [], a local Rayleigh number associated with the cratonic lithosphere is computed for each case: where δ and η l are the thickness and the averaged viscosity of the cratonic lithosphere, respectively.…”

“…Previous studies have shown that the stability of the buoyant cratonic lithosphere is controlled by the lithospheric viscosity (or Rayleigh number) [ Jaupart et al , ] and for non‐Newtonian viscosity by the lithospheric viscosity just before the onset of instability [ Wang et al , ]. Therefore, similar to Wang et al [], a local Rayleigh number associated with the cratonic lithosphere is computed for each case: where δ and η l are the thickness and the averaged viscosity of the cratonic lithosphere, respectively. The averaged viscosity η l is determined from the lithospheric viscosity weighted by strain rate just before the onset of instability in the cratonic lithosphere, and the onset time is defined as the time when the time‐dependent averaged flow velocity of the cratonic lithosphere first surpasses 1% of its maximum [ Wang et al , ].…”