Tectonic Evolution of the Dabie-Sulu UHP and HP Metamorphic Belts, East-Central China: Structural Record in UHP Rocks

Abstract

The complex structural history of the Dabie-Sulu terrane is deduced from various scales of structural features studied in UHP metamorphic units combined with metamorphic and thermal data. Excluding pre-UHP events, the following sequence of distinct tectonometamorphic stages is suggested: (1) The first deformation, D1, produced weak foliation and lineation in massive eclogite in the P-T stability field of coesite/diamond under low differential stress. (2) The D2 event is mainly inferred from a dominant coesite eclogite-facies texture characterized by a stretching mineral lineation, mesoscale sheath-like folds, and a network of ductile shear zones. (3) D3 structures and fabrics developed shortly after the formation of granulite/amphibolite—facies symplectites. These structures are characterized by a regional, steeply dipping foliation and heterogeneous compositional layering, eclogite boudinages of various dimensions, intrafolial folds, and ductile shear zones forming an anastomosing pattern, leading to tectonic juxtaposition or nappe-like structures defining shear zone-bounded crustal slices. The D3 deformation event was associated with decompressional partial melting and intense retrogressive metamorphism. (4) Post-collisional ductile crustal thinning and extension affected the D3 foliation and compositional layering, producing a regional, gently dipping D4 main amphibolite-facies foliation and stretching lineation, dome- and arc-shaped structures, and low-angle and extensional detachments typified by different stretching directions. All of these structures formed prior to intrusion of Late Mesozoic plutons, faulting, basin development, and tectonic unroofing at shallow crustal levels (D5). The first two stages of ductile structures (D1 and D2) were related to subduction and collision between the Sino-Korean and Yangtze cratons at UHP (>27 kbar) and HP metamorphic conditions at 230-250 Ma. In contrast, D3 and D4 stages of Late Triassic to Jurassic (~230-170 Ma) ductile deformation accompanied exhumation of UHP and HP metamorphic rocks to crustal levels, initially driven by compression and later by extension. D4 structures dominate the map pattern of most UHP and HP metamorphic rocks in the orogen. The final D5 deformation largely controls the present-day geomorphology of the Dabie-Sulu region. A modified tectonic evolution model for the UHP and HP metamorphic belts is proposed. It involves continental subduction and collision between the Yangtze and Sino-Korean cratons, and subsequent polyphase exhumation of the UHP and HP metamorphic rocks.