STRUCTURAL GEOMETRY OF AN EXHUMED UHP TERRANE IN YANGKOU BAY, THE EASTERN SULU OROGEN, CHINA: IMPLICATIONS FOR CONTINENTAL COLLISIONAL PROCESSES

High-precision 1:200-1:1,000 mapping of Yangkou Bay, eastern Sulu orogen, defines the structural geometry and history of one of the world’s most significant UHP (ultrahigh-pressure) belts. At least four stages of folds are recognized in UHP eclogites and associated quartzo-feldspathic gneiss. UHP-eclogite facies rootless F₁ and isoclinal F₂ folds are preserved locally in coesite-eclogite. Mylonitic to ultramylonitic quartzo-feldpsthic and coesite-eclogite shear zones separate small-scale 5-10-meter-thick nappes of ultramafic-mafic UHP rocks from banded quartzo-feldspathic gneiss. These shear zones are folded, and progressively overprinted by amphibolite-greenschist facies shear zones. The prograde to retrograde D₁-D₅ deformation sequence is explained by deep subduction of offscraped thrust slices of lower continental or oceanic crust from the down-going plate, caught between the colliding North and South China cratons in the Mesozoic. After these slices were structurally isolated along the plate interface, they were rolled in the subduction channel during exhumation and structural juxtaposition with quartzo-feldspathic gneisses, forming several generations of folds, sequentially lower-grade foliations and lineations, and intruded by in situ and exotically derived melts. Shear zones formed during different deformation generations are wider with lower grades, suggesting that deep-crustal/upper mantle deformation operates more efficiently, perhaps with more active crystallographic slip systems, than deformation at mid-upper crustal levels.