Effects of a Weak Lower Crust on the Flexure of Continental Lithosphere

Abstract

The rheology of continental lithosphere controls seismicity, orogeny, basin-formation in continents, and is partially responsible for the bimodal recycling of Earth's surface wherein continental lithosphere may be older than several Ga while oceanic lithosphere is generally younger than 200 Ma. Because of the sensitivity of lithospheric rheology to temperature and composition, increased crustal thickness may give rise to an intermediate weak layer (i.e., weak lower crust) (Bird, 1991;Chen & Molnar, 1983), which may have a significant impact on the tectonics of orogenic regions such as Tibet (Clark & Royden, 2000;Royden et al., 1997). Lithospheric rheology is frequently investigated in mineral physics experiments (Goetze & Evans, 1979) and inferred from field observations of radial seismic anisotropy (e.g., Shapiro et al., 2004) and surface wave tomography (Shen et al., 2013) which show that crustal rocks (e.g., quartz, diabase) may become extremely weak for conditions under which adjacent lithospheric mantle rocks remain much stronger (e.g., olivine). Lithospheric rheology is also frequently constrained by observations of flexure in response to surface loads such as mountains ranges, plateaus, basins, and glacial isostatic adjustment (Watts, 2001). However, it is not necessarily clear what effects an intermediate weak layer may have on lithospheric flexure and what

Seismic Azimuthal Anisotropy of Northeastern Tibetan Plateau From Ambient Noise Double Beamforming Tomography: Implications for Crustal Deformation

“…We suggest that the lattice‐preferred orientation (LPO) of anisotropic minerals such as mica and amphibole in the mid‐to lower crust produces the observed strong anisotropy. Furthermore, the anisotropy and velocity pattern transition depths (Figure 9) also correlate well with the estimated elastic lithosphere thickness ( T e ) (Chen et al., 2015; Li et al., 2013) and the termination of seismicity, suggesting a warm and weak lower crust that is mechanically decoupled from the rigid upper crust (Bellas & Zhong, 2021).…”