Buoyancy-driven exhumation deformation: Evidence from the Sulu orogen, eastern China

Whether deformation of exhumed crust in a collisional orogen is driven by buoyancy or tectonic stress remains uncertain. The Sulu orogen in eastern China contains slowly subducted and exhumed high-pressure (HP)−ultrahigh-pressure (UHP) terranes, which provide a good opportunity to understand whether the exhumation deformation was driven by buoyancy. We used field and microscopic observations as well as quartz c-axis fabrics to determine the deformation kinematics, temperatures, and evolution of the exhumed crustal slices. Deformed and undeformed dikes were dated by using the zircon U-Pb laser ablation−inductively coupled plasma−mass spectrometry method to constrain the timing of deformation. Our data demonstrate that each crustal slice was involved in pervasive top-to-the-NW (hinterland) or top-to-the-SE (foreland) ductile deformation during Late Triassic exhumation, and that the base of each slice records deformation that was superimposed during the subsequent exhumation of the underlying slice. The crustal exhumation of the southern Sulu orogen is consistent with the multistage ductile extrusion model. The kinematics of exhumation deformation within each crustal slice conform to an upward asymmetric flow. The flow velocity and corresponding shear sense were affected by temperature variations in each HP slice, whereas they were sensitive to migmatization within the UHP slices. The southern Sulu orogen examples show that crustal flow deformation during exhumation was driven by buoyancy and controlled by viscosity. Unlike the consistent kinematics of tectonics-driven deformation, the kinematics of buoyancy-driven deformation are characterized by variations in the senses of shear within a single crustal slice, and this can therefore be used to distinguish the two types of deformation.