Topography and Gravity Reveal Denser Cratonic Lithospheric Mantle Than Previously Thought

Abstract

Although the density structure of the cratonic lithospheric mantle (CLM) is critical for understanding the evolution of continents, there is little consensus on this important geodynamic property. The traditional model of strong isopycnicity (Jordan, 1978) assumes that the compositional buoyancy balances the thermal effect at any given depth within the mantle lithosphere. This proposition is supported by the xenolith data showing ancient subcontinental lithosphere could be more depleted than recent lithosphere (e.g., Griffin et al., 2009). However, the isopycnicity hypothesis faces challenges from recent observations, such as the stratified lithospheric anisotropy implying different ages of lithospheric layers (Yuan & Romanowicz, 2010), variations in mantle xenolith composition suggesting lithospheric alteration (Lee et al., 2011), large vertical motions of cratons due to lithospheric delamination (DeLucia et al., 2018;Hu et al., 2018), cratonic lithosphere destruction (Zhu et al., 2011), and cratonic basin subsidence resulting from thermal-chemical alteration within the cratonic lithosphere (Liu et al., 2019). These debates necessitate an updated view on the density structure of the CLM.Geodynamics studies provided contrasting views on the density of the CLM. While early geophysical calculations based on seismic data argued for a potentially dense CLM (

Geoid Reveals the Density Structure of Cratonic Lithosphere

“…In all these cases, the best‐fitting geoid profiles contrast sharply with those based on the isopycnicity hypothesis (black curves in Figure 10) where both the amplitude and gradient of craton geoid are too large. Thus inferred negative buoyancy of the SCLM is close to that from two previous studies (Kaban et al., 2003; Wang et al., 2022), although the latter did not constrain the depth profile of density distribution.…”

“…Here we consider both crustal isostasy and lithospheric isostasy (Figure 3), with the former corresponding to the traditional isopycnicity hypothesis. For both cases, we adopt a globally uniform reference frame for the topography of continents and oceans (Wang et al., 2022). In the case of crustal isostasy, the observed cratonic topography (∼0.5 km) is fully compensated by the crust.…”

“…The geoid output is a combined effect of internal density anomalies and the topographies at the surface and CMB (core‐mantle boundary) (Zhong et al., 2008). The model topography is converted from the sub‐surface normal stress using the following equation: $$$h=\frac{\tau }{{\Delta}\rho g}$$$ where $$h$$ is surface topography relative to the MOR, a globally uniform reference topography that avoids inconsistency in evaluating the buoyancy of cratonic versus oceanic lithosphere (Wang et al., 2022); $${\Delta}\rho $$ is the density contrast between mantle and water in oceans and that between mantle and air on continents; $$\tau $$ is the sub‐surface normal stress. Since the solution of the sub‐surface vertical stress already includes the effect of input density anomalies like that of the crustal layer, the modeled surface topography represents the vertical deflection of the sub‐surface mantle.…”

“…Accordingly, results based on this assumption would naturally favor the isopycnic SCLM that has little topographic contribution. In contrast, calculations based more on observed lithospheric seismic properties suggest a large negative topography originated from the mantle, possibly due to the downwelling of the convective mantle (Forte et al., 1993), or a dense SCLM (Kaban et al., 2003), or a combination of both (Wang et al., 2022). Most of these studies did not utilize the geoid constraint.…”

“…Magenta contours outline major cratons following Wang et al. (2022). (b) Observed geoid from EGM2008 (Balmino et al., 2012).…”