Research On The Structural Thermochronology And Deformation Modeling For Post-orogenic Dome Structures Of The North Dabie

The Dabie orogenic belt (DOB), located in eastern China, was developed due to continent-continent collision between the South China block and North China block. After post-orogenic doming, the tectonic framework of the northern DOB formed.The North Dabie Dome (NDD) mainly consisted of migmatitic gneiss and the Early Cretaceous plutons. The NDD is bounded by the Xiaotian-Mozitan shear zone (XMSZ) to the north and the Wuhe-Shuihou shear zone (WSSZ) to the south. Based on field observation, the XMSZ, the northern boundary of NDD, strikes NWW-SEE and dips at 20-40°toward NNE or NE. The lineation in mylonite plunges NWW. The macrostructures (the S-C structure andσ-type feldspar porphyroclast) and microstructures (the S-C structure,σ-type feldspar porphyroclast and"mica fish"structure) and quartz C-axis fabrics all show top-to-NWW shear sense. The east segment of the WSSZ strikes NWW-SEE and dips at about 60°toward SE or ESE. The lineation in mylonite plunges about 60°toward S. The field structures, microstructures and quartz C-axis fabrics in the WSSZ all show top-to-NWW shear sense. The attitude of the foliation and lineation gradually varies from mylonite in the two shear zones to gneiss in the NDD and the interior of the NDD between the two shear zones also exhibit widespread, top-to-WNW ductile deformation dated as the Early Cretaceous. Across the NDD from north to south, Gneiss foliation in the northern part of the dome dips at 20-40°toward NE or NNE whereas that in the southern part dips at 50-60°towards SE or SSE. Foliation in the middle part of the dome shows variable strikes and dip angles while the lineation consistently plunges WNW or ESE. The S-C structure,σ-type feldspar porphyroclast and quartz C-axis fabrics all show top-to-NWW shear sense. All of these demonstrate that the two shear zones experienced the same flow deformation as the interior of the NDD. The gradually varying gneissositic foliation inside of the NDD indicates that the XMSZ and WSSZ were originally one flat-lying detachment shear zone with uniform top-to-WNW shear sense before doming of the NDD. The migmatitic gneiss exposes more often in the central part of the NDD than in the south and north sides and the NDD is more widely in the west than in the east, which indicated the maximum uplift happened in the central NDD and the west part of the NDD uplifted more intensively than the east part.By microscopic observation of oriented thin-sections the quartzes in mylonite all dynamic recrystallized, mostly by grain boundary migration recrystallization and some with grain boundary reduction recrystallization under high temperature condition. Feldspar in the mylonites shows widespread dynamic recrystallisation mainly by both bulging recrystallisation and subgrain rotation recrystallisation. All of these indicate the deformation temperature of 600~650°C. Using a geothermal gradient of >30°C/km normally for post-orogenic extension setting, the XMSZ and WSSZ formed at a middle curst level. Calculating by Al-in-hornblende geobarometer, the average pressure of 5.21 kbar was acquired, indicating depth of 18.3 km.The mylonite in the XMSZ was covered by the Early Cretaceous volcanic rocks north to the NDD, which suggests that the magmas erupted after mylonite in the XMSZ uplifted to surface. The zircons from the undeformed plutons along the XMSZ gave U-Pb age of 130 Ma, which shows that the movement of XMSZ ended at least before 130 Ma. A number of biotite and hornblende from mylonite in the XMSZ were collected for 40Ar/39Ar dating and a series of plateau ages from 120 Ma to 142 Ma were acquired, which were all explained as cooling age. The obtained 40Ar-39Ar ages from the western segment are significantly younger than those of the same minerals from the eastern one, denoting the west segment of the XMSZ experienced slower uplift. The middle segment of the XMSZ gives the oldest 40Ar/39Ar cooling age of 142 Ma (hornblende). On the basis of the cooling age distribution along the XMSZ, it is inferred that along an EW transverse the middle part of the NDD uplifted first during the doming. The deformed plutons in the NDD gave ages of older than 132 Ma while the undeformed ones along XMSZ gave U-Pb age of 130 Ma (this work), which shows that the middle-lower crustal ductile flow ended at about 132 Ma. Hornblende and biotite ages from the same samples of XM24-3 and XM52-1 in the XMSZ only show slight difference, indicating the NDD experienced fast cooling from 132 Ma to 127 Ma. The 40Ar/39Ar ages of the XMSZ are consistent with the 40Ar/39Ar ages of 126 Ma (biotite) and 130 Ma (muscovite) from the WSSZ, demonstrating the XMSZ and WSSZ were originally one shear zone. The lower crustal ductile flow and sub-horizontally detachment in NDD had lasted from about 145 Ma to 132 Ma and the fast uplift had been occurred between 132 Ma and 127 Ma in the NDD.To explore the geometric and kinematic pattern, a flat-lying shear zone theoretic model was used to quantitatively model the process that the XMSZ and WSSZ had been rebuilt by the dome. By modeling, the top-to-NWW (280°) shear sense and the kinematic vorticity number (0.95) for the original flat-lying shear zone were acquired. After rotation around a certain axis, the attitude of foliations and lineations by theoretic calculation can be respectively consistent with the natural attitude varying in the Xiaotian cross-section in east segment, the Mozitan cross-section in middle segment, the Dingpu cross-section in west segment of XMSZ and the Changpu section of WSSZ.The results from modeling not only prove that the XMSZ and WSSZ were originally the same flat-lying shear zone, but also indicates that the geometrical differences of the whole XMSZ resulted from asymmetrical rebuilt during doming. The south of the theoretic model must be rotated up around an east-west axis to correspond with the sections of XMSZ, and the north of theoretic model must be rotated up around the same axis to correspond with the section of WSSZ. These progresses indicated that the interior of the NDD had been uplifted higher than the north and south sides and the NDD exhibits an arch-shape as a whole. The rotation angle in the theoretic model of the Dingpu cross-section is larger than the Xiaotian cross-section and an additional rotation around a south-north horizontal axis was done in the Mozitan cross-section. These information verified that the west part of the NDD had more uplift than the east part.The kinematics of the NDD shows lower crustal flow. During the content-content collision and exhumation of ultra-high pressure metamorphic rocks, the crust of the NDD had been thickened and therefore a huge gravity potential energy was stored in the NDD crust relative to the adjacent South China block. During the continental-scale, extensional regime in East China, the lower crust was heated and softened and hence gravitational collapse would take place in lower crust with increasing plasticity of lower crustal rocks. When the lower crust flowed towards SEE, the flat-lying ductile shear zone with top-to-NWW shear sense came to being. Just for the lower crustal flow, the lithosphere isostatic rebound became one of the most important factors resulting in development of arch-shape of the NDD.