Structural-metamorphic evolution of the tia complex, new england fold belt; thermal overprint of an accretion-subduction complex in a compressional back-arc setting

Abstract

Rocks in the Tia Complex underwent a simple, clockwise P- T- t-deformation path, moving from mid-crustal levels at blueschist facies conditions ( D 1, D 2; 6 kbar, 200°C) to upper-crustal levels at high- T amphibolite facies conditions ( D 4, D 5; 2.5 kbar, 600°C), in a relatively short time (∼ 15 Ma). The rocks remained under these conditions for approximately 40 Ma ( D 6) before they cooled and were brought to the surface ( D 7, D 8). During D 6,Permian rift basins were opening as thermal gradients in the Tia Complex reached a maximum. D 1–2 deformation was associated with accretion-subduction processes and was followed by progressively less intense, mainly E–W oriented compression and uplift during D 3–5, bringing the rocks to within 2 kbars of the surface. D 6 is characterized by limited extensional deformation, and final uplift took place during D 7–8, along shear zones that have orientations similar to D 3–5 thrusts. The shift from subduction-dominated processes to thermally-dominated processes occurred during D 3–4 uplift of ∼4 kbar, around 310-300 Ma. This coincided with a change in tectonic setting of the Tia Complex from an accretionary prism to a back-arc position, as the subduction zone stepped eastward. An increased heat flow, associated with back-arc processes, gradually caused thermal weakening of the lithosphere, which, due to external, subduction-related, compressive forces, resulted in uplift ( D 3). With continued heating of the crust, thermal doming resulted and uplift stopped ( D 4–5), whilst thermal weakening led to limited crustal collapse as Permian basins opened ( D 6). The extremely high heat flow that was maintained for ∼40 Ma. suggests a heat source external to the crust, possibly associated with thermal relaxation, dehydration and detachment of the remnant stab left beneath the New England Orogen after the subduction zone shifted east. A stable crustal, thermal structure was re-established by 260 Ma, as the crust strengthened and could further support subduction-induced, compressional structures ( D 7–8).