Exhumation of UHP Terranes: A Case Study from the Dabie Mountains,Eastern China |
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| Abstract: | Two processes are suggested to explain how UHP rocks are exhumed from mantle depths. One is removal of the overburden either by erosion or by extension, whereas the other involves the uplifting of the UHP rocks through the overburden. Application of either of these mechanisms to the Dabie Mountains, however, is fraught with difficulty. When combined with previously published data, new studies on metamorphic P-T paths, regional structures, and deep upper-mantle architecture revealed by seismic tomography lend support to a multi-stage exhumation process that operated in the Dabie Mountains. The first stage (230 to 200 Ma) is characterized by ductile deformation, produced during eclogite-facies recrystallization under a geothermal gradient as low as 10°C/km, implying a synsubduction exhumation. Some of the UHP rocks evidently were exhumed to a depth of ~60 km, as indicated by petrological study of the Shuanghe eclogite. The second stage (200 to 170 Ma) attended ductile deformation and amphibolite-facies retrograde metamorphism. Subduction of the Yangtze block was halted by slab breakoff at a depth of ～200 km. The resultant geothermal gradient recovered to ～20″C/km. Slab breakoff permitted buoyancy-driven ascent of the UHP low-density melange to shallow crustal levels in a diapir structure. When the UHP portion of the mountain root rose, the shallow portion was heated to a temperature higher than that of the peak metamorphic pressure. The third stage (170 to 120 Ma) is characterized by extension and thermal uplift, as well as erosion. Sedimentary basins and volcanic rocks developed on both sides of the Dabie Mountains. Gab-bro-pyroxenite intruded the hanging wall of the UHP terrane, and granite, as well as migmatite, developed in that stage. Exhumation mechanisms might include corner flow for the first stage, buoyancy-driven squeezing-up for the second stage, and crustal extension, as well as erosion, for the third. Rupture and loss of the subducted lithospheric plate generated the gravity instability that resulted in exhumation of the subducted UHP section. |
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