Application of general shear theory to the study of formation mechanism for the metamorphic core complex: A case of Xiaoqinling in central China

The kinematic vorticity number and strain of the mylonitic zone related to the detachment fault increase from ESE to WNW along the moving direction of the upper;plate of the Xiaoqinling metamorphic core complex (XMCC) and the geometry of quartz c-axis fabrics changes progressively from crossed girdl es to single girdles in the same direction. Therefore, pure shear is domina nt in the ESE part of the XMCC while simple shear becomes increasingly impo rtant towards WNW. However, the shear type does not change with the strain across the shear zone, thus the variation of shear type is of significance in indicating the formation mechanism. The granitic plutons within the XMCC came from the deep source and their emplacement was an active and forceful upwelling prior to the detachment faulting. The PTt path demonstrates that magmatism is an important cause for the formation of the XMCC. The formati on mechanism of the XMCC is supposed to be active plutonism and passive det achment. Crustal thickening and magmatic doming caused necking extension wi th pure shear,and magmatic heating and doming resulted in detachment extens ion with simple shear and formed the XMCC.