STRUCTURAL METAMORPHIC EVOLUTION OF THE TIA COMPLEX, NEW-ENGLAND FOLD BELT - THERMAL OVERPRINT OF AN ACCRETION SUBDUCTION COMPLEX IN A COMPRESSIONAL BACK-ARC SETTING

Rocks in the Tia Complex underwent a simple, clockwise P-T-t-deformation path, moving from mid-crustal levels at blueschist facies conditions (D1, D2; 6 kbar, 200-degrees-C) to upper-Crustal levels at high-T amphibolite facies conditions (D4, D5; 2.5 kbar, 600-degrees-C), in a relatively short time (approximately 15 Ma). The rocks remained under these conditions for approximately 40 Ma (D6) before they cooled and were brought to the surface (D7, D8). During D6, Permian rift basins were opening as thermal gradients in the Tia Complex reached a maximum. D1-2 deformation was associated with accretion-subduction processes and was followed by progressively less intense, mainly E-W oriented compression and uplift during D3-5, bringing the rocks to within 2 kbars of the surface. D6 is characterized by limited extensional deformation, and final uplift took place during D7-8 along shear zones that have orientations similar to D3-5 thrusts. The shift from subduction-dominated processes to thermally-dominated processes occurred during D3-4 uplift of approximately 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 (D3). With continued heating of the crust, thermal doming resulted and uplift stopped (D4-5), whilst thermal weakening led to limited crustal collapse as Permian basins opened (D6). The extremely high heat flow that was maintained for approximately 40 Ma, suggests a heat source external to the crust, possibly associated with thermal relaxation, dehydration and detachment of the remnant slab left beneath the New England Orogen after the subduction zone shifted east. A stable crustal, thermal structure was reestablished by 260 Ma, as the crust strengthened and could further support subduction-induced, compressional structures (D7-8).