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 (

The rapid and controlled generation of polypeptides with ultrahigh molecular weight (MW) and welldefined chain end functionality has been a great challenge.To tackle this problem, we report here an initiation system based on a S-Sn Lewis pair, trimethylstannyl phenyl sulfide (PhS-SnMe 3 ), for the ring-opening polymerization (ROP) of α-amino acid N-carboxyanhydrides (NCAs). This initiator displays a strong solvent effect, and can yield polypeptides with high MW (>1.0 × 10 5 g•mol −1 ) and low polydispersity index within a few hours. The MWs of the obtained polypeptides are strongly dependent on the THF/DMF ratio. The polymerization follows a typical first-order kinetic character with respect to the monomer concentration in mixed THF and DMF. Moreover, a highly reactive phenyl thioester is in situ generated at the C-terminus of the polypeptides, which is readily accessible for native chemical ligation affording high MW and site-specific protein−polypeptide conjugates. Together, this initiator sheds light on regulating the ROP of NCAs via appropriate Lewis pair and solvent selection, and is particularly useful in preparing ultrahigh MW polypeptides within a short period of time.

Different from normal subduction, flat slabs have small dip angles and lie nearly horizontally beneath an overriding plate. The best examples for ongoing flat subduction are those beneath South America, clearly seen in geophysical images (Gutscher et al., 2000;Hayes et al., 2012) and likely caused by subducting oceanic plateaus and nearby cratonic roots Manea et al., 2012). However, due to sparse observational constraints, the existence and mechanisms of flat slabs during the geological past remain debated, especially in regions with a complex tectonic history. A notable example is the Late Cretaceous-early Cenozoic Farallon flat slab beneath the western

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