The Role of Lower Crustal Rheology in Lithospheric Delamination During Orogeny

Lin Chen

doi:10.3389/feart.2021.755519

Abstract

The continental lower crust is an important composition- and strength-jump layer in the lithosphere. Laboratory studies show its strength varies greatly due to a wide variety of composition. How the lower crust rheology influences the collisional orogeny remains poorly understood. Here I investigate the role of the lower crust rheology in the evolution of an orogen subject to horizontal shortening using 2D numerical models. A range of lower crustal flow laws from laboratory studies are tested to examine their effects on the styles of the accommodation of convergence. Three distinct styles are observed: 1) downwelling and subsequent delamination of orogen lithosphere mantle as a coherent slab; 2) localized thickening of orogen lithosphere; and 3) underthrusting of peripheral strong lithospheres below the orogen. Delamination occurs only if the orogen lower crust rheology is represented by the weak end-member of flow laws. The delamination is followed by partial melting of the lower crust and punctuated surface uplift confined to the orogen central region. For a moderately or extremely strong orogen lower crust, topography highs only develop on both sides of the orogen. In the Tibetan plateau, the crust has been doubly thickened but the underlying mantle lithosphere is highly heterogeneous. I suggest that the subvertical high-velocity mantle structures, as observed in southern and western Tibet, may exemplify localized delamination of the mantle lithosphere due to rheological weakening of the Tibetan lower crust.

Highlights

It is widely thought that the lithosphere mantle thickens in concert with the crust during continental collision (England and Houseman, 1986)
Convergence forces the orogen lithosphere mantle to slide along the weak orogen lower crust and sink downward at the middle of the model (Figure 3B), resulting in a large piece of downwelling similar to the two-sided subduction shown by Gerya et al (2008)
I investigate the role of the lower crust rheology and convergence velocity based on a range of laboratory-derived flow laws in collisional orogeny using 2-D thermomechanical modeling method

Summary

Introduction

It is widely thought that the lithosphere mantle thickens in concert with the crust during continental collision (England and Houseman, 1986). Seismological studies indicate that the India-Asia collision results in the double normal thickness crust of Tibet (e.g., Hirn et al, 1984), but the Tibetan lithosphere mantle does not thicken proportionally. Seismic studies show low velocities and high attenuation in the shallow mantle in the Central Andes, implying the absence of the lithospheric mantle beneath much of the orogen, even though the crust thickness ranges from 50 to 75 km (Yuan et al, 2000; Beck and Zandt, 2002; Schurr et al, 2006).

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