Tectonic Characteristics And Evolution Of The Yagan Metamorphic Core Complex

Detail mapping shows that the Yagan MCC consists of Crystallized metamorphic core (low plate), detachment zone and upper plate .The metamorphic core consists of can be further subdivided into two level parts low-level high metamorphic and upper-level shallow metamorphic structural slice and between them there exists a ductile shear zone. Thus the whole texture of the MCC is characterized by "three layers and tow zones" This reveals a process of extensional uplift of poly -level slices by poly ductile shear zones, a characteristics of poly-level extensional structural systems. The deformational environments change vertically in a sequence of high amphibolite facies. greenschist facies, low green schist facies and brittle shear deformation facies.The attitude of different structural element of the Yagan MCC are keeping coordination. Analysis of kinematics demonstrates the SSE-directed shearing-off of the hanging wall relative to the footwall along the penetrative mylonitic lineation.The formation process is determined based on structural analysis and dating. The first stage was characterized by ductile detachment deformations, and during 228Ma-170Ma took place high temperature flow deformation and in the period of IVOMa- 135Ma middle-low temperature deformations occurred, as shown by deformations feature and history of mylonite granites. The second stage was marked by intrusion of late syn-tectonic granite plutons with 135Ma, the early sub-stage 135Ma-126Ma was characterized by ductile hot uplift and the late 126Ma-90Ma by ductile-brickie uplift. These clearly demonstrate that the MCC formatted by long-history processes with poly stages.By strain analysis, the fabrics deformational style of the Yagan MCC are mainly shearing type, the Yagan MCC is mainly a thinning shear zone with volume loss, which varies from-31%~-58%.Analysis of kinematic vorticity (Wk varies from 0.453-0.809) shows shear type of the shear zone is neither pure shear nor simple shear but general shear. Furthermore, shear type is simple shear-dominated at the early stage by analysis of Wk from the method of Polar mohr and pure shear component increases at the late stage by the methods of ecc.Much synthetic extensional crenulation cleavage ecc is found in mylonitic rocks zone . The average attitude of the synthetic ecc sets (some have propagated and formed low angle normal faults and fault surface may rotate during the formation ) is 167 23 and the same as that of detachment faults which is 151 23 . The propagation of the synthetic ecc set is possibly an effective mechanism for the formation of low angle normal fault.