Mid-crustal partitioning and attachment during oblique convergence in an arc system, Palaeoproterozoic Ketilidian orogen, southern Greenland

Abstract

Abstract: The Ketilidian orogen in SE Greenland contains a continental magmatic arc (Julianehab batholith) and forearc which formed in the overriding plate of a Palaeoproterozoic oblique subduction boundary between c. 1854 and 1795 Ma. During uplift and deformation of the forearc between c. 1795 and 1780 Ma, its sedimentary sequences were invaded by arc-type magmas and metamorphosed at HT–LP upper amphibolite facies conditions which led to widespread anatexis and generation of abundant S-type granites. Transpression during early deformation (D 1 ) was partitioned into arc-parallel, sinistral strike-slip in the magmatic arc rocks of the batholith and arc-normal contraction in the sedimentary and volcaniclastic sequences of the proximal forearc (Psammite Zone). Subsequently, D 1 fabrics in the Psammite Zone were overprinted and transposed in an intense, flat-lying, D 2 ductile shear zone with arc-parallel stretching lineations and top-NE kinematic indicators. The kinematic significance of the D 2 shear zone is that it accommodated vertical differences in strain and displacement and permitted deformation to be partitioned differently at different levels in the crust. As such, it is an attachment or coupling zone that provided kinematic linkage between deformation at different crustal levels during NE transport of a forearc sliver. Overprinting of the upper crustal domain of D 1 strike-slip partitioned transpression in the forearc during D 2 is attributed to migration of the attachment zone to a higher crustal level. This migration was a consequence of an enhanced geothermal gradient in the proximal part of the forearc due to the emplacement of batholith-related plutonic suites during D 2 .