Gneiss domes and stratified middle to lower crustal flow in continental mobile belt

Abstract

The mechanisms of the continental activation and regeneration after cratonization and orogeny are important subjects for studies of plate tectonics in continental interiors. Such studies provide important constraints on our understanding the continental crustal active belts that are widely developed in continental margins and intracontinental environment. Dome structures with middle and lower crustal high- grade metamorphic rocks in the cores and middle crustal low- grade metamorphic rocks in the mantle are among the most typical characteristics of continent crustal mobile belts. Based on structural analysis and synthesis of dome structures from Paleoproterozoic - Cenozoic typical continental crustal mobile belts, it is shown that they have many common characteristics: ① Development of thick- skinned structure. Evidence of involvement of lower crustal rocks in the deformation due to strong thermal anomalies and high strains is well preserved. ② The cores of the domes are usually composed of high- grade metamorphic rocks (generally with emplacement of granites or formation of migmatites). Metamorphism grades vary from lower amphibolite facies to upper amphibolite facies, or locally granulite facies in the cores to greenschist facies or lower greenschist facies in the mantles. ③ Stratified flow is an important form of deformation in active crustal zones. ④ The rocks at different structural (crustal) levels and stratigraphic horizons were subjected to strong shearing. Deformation structures in rocks from the cores and mantles at different levels are consistent in geometry, kinematics and dynamics. Stretching lineations and A- type folds are widely developed. Regionally, A- type or B- type domes are formed. ⑤ The mantles and cores are kinematically coupled and rheologically decoupled, which results in the occurrence of shear discontinuities (or tectonic discontinuity contact- TDC) of different scales between the two units and within them. From the above characteristics, it is proposed that subhorizontal (tangential) shearing and stratified flow is the common flow pattern in the middle and lower crust in the continental crustal mobile belt. Tangential shearing and subhorizontal flow, combined with vertical movement and superimposed deformation during progressive shearing, resulted in the occurrence of the dome structures widely developed in continental crustal mobile belts. They gradually evolve into linear A- type or B- type domes, and sometimes into metamorphic core complexes.