Neoarchean decoupling of upper- and mid-crustal tectonothermal domains in the southeast Slave Province: evidence from the Walmsley Lake area

摘要

The Walmsley Lake area of the southeastern Slave Province, Northwest Territories, exposes a crustal transect from greenschist through to upper amphibolite facies. Two tectonothermal crustal domains are proposed based on disparate tectonic histories and rock types. Within the upper tectonothermal domain, D_1 deformation reached lower amphibolite facies prior to 2614 Ma and associated M_1 metamorphic conditions outlasted D_1. Peak M_2 conditions reached middle amphibolite facies prior to 2603 Ma and outlasted D_2 deformation. Deformation style and metamorphic sequencing during these two pre-2600 Ma events are consistent with regional crustal shortening and thickening in the upper tectonothermal domain. At mid-crustal levels in the lower tectonothermal domain, a third tectonothermal event (D_3-M_3) produced uppermost amphibolite-facies peak metamorphic conditions, transposed preexisting fabrics to shallow dips, and produced a subhorizontal foliation and recumbent folds at ca. 2583 Ma. D_3 structures are not present in the upper tectonothermal domain, however, evidence of M_3 heating is locally displayed in rocks at the base of the upper tectonothermal domain. Upper and lower domains were structurally decoupled during the D_3-M_3 event. In rare places where the zone between the two domains outcrops, it comprises a narrow high-strain zone. The metamorphic and structural topology of the Walmsley Lake area is consistent with the topology predicted from thermomechanical modeling and modern observations of gravitational accommodation in structurally thickened crust.