Basement–cover relationships in the East Greenland Caledonides: evidence from the Eleonore Bay Supergroup at Ardencaple Fjord

Abstract

AbstractThe Eleonore Bay Supergroup (EBG) is a 16 km-thick shallow-water sequence of Neoproterozoic age that is preserved within the East Greenland Caledonides in several tracts, surrounded by crystalline gneisses and schistose supracrustal rocks. The apparent downward transition from non-metamorphic EBG into gneiss gave rise to the classic ‘stockwerke’ hypothesis, in which all the metamorphism was regarded as Caledonian, and differences in grade were ascribed to the ascent of a migmatite front to different levels within the orogen. Field and isotopic studies in the 1970s however revealed that the underlying gneisses and schists had undergone orogenic reworking in mid-Proterozoic time; the EBG–basement contact was then interpreted as an approximately bedding-parallel décollement with apparent lag geometry, that is with EBG cover rocks in its hangingwall.Recent work in the northernmost EBG tract, at Ardencaple Fjord, has shed light on the problems posed by the basal relationships of the EBG, and together with regional structural and stratigraphic data leads to the following interpretation. There are two regionally important basement-cover interfaces within the East Greenland Caledonides. The earlier one is between Archaean/early Proterozoic gneisses and early Proterozoic supracrustal rocks, which were pervasively deformed in mid-Proterozoic time and form the basement to the Neoproterozoic Eleonore Bay cover sequence. This was deposited on a vast, continually subsiding shelf that is now preserved in East and NE Greenland and Svalbard, and contains Grenville detritus. EBG deposition was terminated by major extensional faulting of Vendian age; the succeeding Tillite Group is interpreted as a syn-rift sequence, presumably associated with the opening of Iapetus.The EBG–basement contacts that are not late faults are inferred to be extensional shear zones of Vendian age. These were reactivated in compression during the Caledonian orogeny in the Silurian, with metamorphic and fabric convergence, which accounts for the apparent downward transition from sedimentary rocks through schists into gneisses. Caledonian shortening was not large; inversion of the Vendian grabens was incomplete, so that the marginal shear zones retained their lag geometry and large tracts of low grade Eleonore Bay sediments are preserved at the present erosion level, surrounded by Proterozoic basement rocks, within the Caledonian belt of East Greenland.