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.
Summary The rise of mantle plumes to the base of the lithosphere leads to observable surface expressions, which provide important information about the deep mantle structure. However, the process of plume-lithosphere interaction and its surface expressions remain not well understood. In this study, we perform 3-D spherical numerical simulations to investigate the relationship between surface observables induced by plume-lithosphere interaction (including dynamic topography, geoid anomaly and melt production rate) and the physical properties of plume and lithosphere (including plume size, plume excess temperature, plume viscosity, and lithosphere viscosity and thickness). We find that the plume-induced surface expressions have strong spatial and temporal variations. Before reaching the base of the lithosphere, the rise of a plume head in the deep mantle causes positive and rapid increase of dynamic topography and geoid anomaly at the surface but no melt production. The subsequent impinging of a plume head at the base of the lithosphere leads to further increase of dynamic topography and geoid anomaly and causes rapid increase of melt production. After reaching maximum values, these plume-induced observables become relatively stable and are more affected by the plume conduit. In addition, whereas the geoid anomaly and dynamic topography decrease from regions above the plume center to regions above the plume edge, the melt production always concentrates at the center part of the plume. We also find that the surface expressions have different sensitivities to plume and lithosphere properties. The dynamic topography significantly increases with the plume size, plume excess temperature and plume viscosity. The geoid anomaly also increases with the size and excess temperature of the plume but is less sensitive to plume viscosity. Compared to the influence of plume properties, the dynamic topography and geoid anomaly are less affected by lithosphere viscosity and thickness. The melt production significantly increases with plume size, plume excess temperature and plume viscosity, but decreases with lithosphere viscosity and thickness.
The mantle viscosity is a key factor controlling the dynamics of the Earth's interior, but even the Earth's 1-D radial viscosity profile remains under debate. It has been suggested that mantle viscosity generally increases by ∼10-100 times from the upper mantle to the lower mantle, in order to explain the Earth's long-wavelength geoid anomaly (Hager & Richards, 1989; Panasyuk & Hager, 2000). Traditionally, the viscosity structure is modeled as a stepwise increase at 660 km depth where the ringwoodite to bridgmanite + ferropericlase phase transition happens (e.g., Hager & Richards, 1989; Liu & Zhong, 2016; Mitrovica & Forte, 2004). However, some geoid inversion models with many-layered viscosity structure reported an increase in viscosity around 1,000-1,200 km depth accompanied either with or without a viscosity increase at 660 km depth (e.g., Forte et al., 1991; Kido et al., 1998; King & Masters, 1992). More recent long-wavelength geoid analysis reported a viscosity increase at depths of 800-1,200 km (Rudolph et al., 2015), while studies of plume structures from seismology argued for increased mantle viscosity below ∼1,000 km (French & Romanowicz, 2015). Experimental studies suggested that the viscosity in the uppermost part of the lower mantle may gradually increase down to ∼1,000 km depth to reach its maximum due to potential pressure effects (Marquardt & Miyagi, 2015). The possible compositional variations of lower-mantle enrichment in
The eastern North China Craton (NCC) has undergone extensive reactivation during the Mesozoic and Cenozoic, while the western NCC has remained stable throughout its geological history. Geophysical and geochemical observations, including heat flux, surface topography, crustal and lithospheric thicknesses, and volcanism, show significant contrast between the eastern and western NCC. These observations provide constraints on thermochemical structure and reactivation process of the eastern NCC, thus helping understand the dynamic evolution of cratonic lithosphere. In this study, we determined the residual topography for the NCC region by removing crustal contribution to the topography. We found that the residual topography of the eastern NCC region is generally 0.3–0.9 km higher than the western NCC. We computed a large number of two‐dimension thermochemical convection models for gravitational instability of cratonic lithosphere and quantified surface heat flux and topography contrasts between stable and destabilized parts of cratonic lithosphere. These models consider different chemical buoyancy (i.e., buoyancy number B) and viscosity for the cratonic lithosphere. Our models suggest that to explain the difference in heat flux (25–30 mW/m2) and residual topography (0.3–0.9 km) between the eastern and western NCC regions, the buoyancy number B is required to be ~0.3–0.4. This range of B implies that as much as 50% of the original cratonic lithospheric material remains in the present‐day eastern NCC lithosphere and its underlying shallow mantle and that the new lithosphere in the eastern NCC may be a mixture of the relics of old craton materials and the normal mantle.
The slab dynamics of the subducted Izanagi-Pacific plate is still a subject of controversy and its relationship with the tectonic evolution of Eastern Asia remains not well explored. Here, we perform 3-D global convection models to investigate the slab dynamics of the Izanagi-Pacific plate beneath Eastern Asia since the Mesozoic time. We introduce a tracking technique in numerical models to explicitly distinguish the Izanagi slab and the Pacific slab during their subduction processes. We find that all subducted Izanagi slabs have completely fallen into the lower mantle until the late Cenozoic and the stagnant slabs currently observed at the mantle transition zone depth beneath Eastern Asia are entirely from the Pacific plate. We also find that multiple slab stagnation events have occurred during the subduction of the Izanagi plate in the Mesozoic time (∼150–120 Ma, 90–70 Ma) with a timescale of tens of million years. The stagnation of the subducted slabs facilitates the formation of a big mantle wedge beneath the overriding lithosphere and the time periods of the mantle wedge are consistent with the episodes of magmatic activities in Eastern Asia.
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