Lower‐Crust Ductile Flow and its Mechanical Relation with Syn‐Collision Crustal Extension in Orogenic Belt

Abstract

AbstractIn attempt to reveal the mechanical relation between lower‐crust ductile flow and upper‐crust extension in collisional orogenic belt, the present paper makes numerical simulation using the Finite Element Method (FEM), which is based on stress‐strain constitution of incompressional Non‐Newtonian rheology. Results show that, under the tectonic compression and the gravitational loading conditions, the crust in mountain‐building process undergoes complex ductile flow. Kinematics of ductile flow is time‐dependent, and influenced by the shape of the crust transition zone (CTZ) between orogenic belt and frontal stable plate. Under the mechanic balance of tectonic compression and gravitational loading, the ductile flow is facilitated first in mountain root. However after several Maxwell times, it will be localized narrowly to the CTZ, where the flow pattern is featured by a downward flow near the mountain side and an upward flow near stable‐plate side. This flow pattern causes stress partition and displacement differentiation spatially in orogenic belt. In details, the minimum principle stress in upper crust near the mountain center will change from horizontal compression to horizontal tension. The magnitude of tensile stress is mainly related with the height of mountain belt above reference surface. It indicates that crustal extension in syn‐collision orogenic belt has a mechanic relation with gravitational collapse, but the extension is probably intensified further by lower‐crust ductile flow. For this consideration, it is concluded that mechanics of compression‐extension coupling in orogenic belt is derived from the lower‐crust ductile flow that is initiated by the gravitational loading and horizontal tectonic compression. Using model result, the north‐south crustal extension in the southern Qinghai‐Xizang Plateau has been discussed.