P-T-t-deformation framework of an accretionary prism, southern New England Orogen, eastern Australia: implications for blueschist exhumation and metamorphic switching

The Tia Complex is located in the southern New England Orogen of eastern Australia and provides a detailed record of the structural-metamorphic evolution of an ancient accretionary prism. This record is characterized by six stages of deformation that were accompanied by a transition from moderate-pressure–low-temperature to low-pressure–high-temperature metamorphism. The composition of jadeitic pyroxene (subduction), magnesioriebeckite (exhumation), and actinolite (heating) have been used to model P-T conditions during these structural events, which range from P = 6.3–6.7 kbar and T = 320–350°C (subduction) to 1.5–2.0 kbar and 400–420°C (heating). On the basis of new structural and metamorphic data combined with preexisting age data, the evolution of this accretionary prism can be divided into two main stages: (1) blueschist formation and exhumation and (2) elevated heat flow and anatexis. To explain these features, a new model is presented that requires (1) wedge underthrusting and rear wedge extension associated with a stationary subduction hinge and (2) subduction hinge migration resulting in the relocation of the accretionary wedge onto the upper plate and heating during exposure to the mantle wedge. To explain the event chronology preserved in the Tia Complex, both extensional collapse and subduction hinge migration models are required.