PRECAMBRIAN BASEMENT OF GREENLAND
The Rinkian fold belt in northern West Greenland

Adam A. Garde, in collaboration with Kristine Thrane (Danish Lithosphere Centre), Jim Connelly (Univ. Texas, USA), John Grocott (Kingston Univ., UK), Martin Hand (Univ. Adelaide, Australia), Ken McCaffrey (Univ. Durham, UK), Laurence Page and Ann-Sofie Sidgren (Lund Univ., Sweden).

Rationale. a tectonic and geochronological re-assessment of the Rinkian fold belt
This international project with participants from three different continents is a re-evaluation of the Palaeoproterozoic, 600 km long Rinkian fold belt along the northern coast of West Greenland. The study is sponsored by the Carlsberg Foundation, NERC (U.K.) and NSF (U.S.A.), and was taken up in the light of more recent advances in the understanding of the plate-tectonic setting in the adjacent parts of West Greenland and eastern Canada. The Rinkian fold belt was originally mapped and described between 1962 and 1980, during a period when precise geochronology was not a readily available tool in regional mapping projects, and when it was commonly believed that vertical tectonic movements due to gravitational forces could have a major influence on mountain building. However, the recent advances in the neighbouring regions necessitated the acquisition of new information also from the Rinkian fold belt, in order to obtain a grand overview of the Palaeoproterozoic plate-tectonic assembly of eastern Laurentia.

Activities
The Rinkian project has hitherto comprised three traverses by boat in 2001-2003 in different parts of the belt, with durations between two and four weeks. The last trip included a visit to the high arctic, northern part, which is remote from modern infrastructure and only accessible for geological studies in the months of July and August. Ongoing activities comprise tectonic analysis in part using GIS software, metamorphic petrological studies, and not least U-Pb and Pb-Pb zircon and titanite geochronology (using conventional, ion probe and laser ICP-MS methods), as well as Ar-Ar geochronology of hornblende and muscovite.

Preliminary results
The research group has identified widespread evidence, at outcrop scale and using kinematic indicators, of collisional crustal shortening throughout the Rinkian fold belt. We have also re-interpreted the impressive, previously identified dome-shaped structures in the central part of the belt as late-tectonic buckle folds that were likewise formed during crustal shortening. Preliminary geochronological data from most parts of the belt furthermore indicate that the Rinkian tectono-thermal evolution was largely contemporaneous with that in the Nagssugtoqidian orogen to its south [link til Nagssugtoqider]. Our results therefore indicate that the Rinkian and Nagssugtoqidian belts are the northern and southern parts of one, very large, E-W trending collisional system which continues in the Rae province of eastern Canada. With a length across strike of around 1000 km, the combined orogen is comparable in width to the present-day Himalayan system. With its excellent exposure of deeply eroded rocks it furthermore lends itself to studies of mid-crustal flow in large, collisional orogens. The new plate-tectonic interpretation has also changed the geological scenario of the now exhausted zinc-lead mine at Maarmorilik and may lead to renewed prospection for zinc in the Uummannaq region.

Publications
Connelly, J.N., Thrane, K., Garde, A.A. & Krawiec, A. (submitted): Linking the Palaeoproterozoic Nagssugtoqidian and Rinkian belts through the Disko Bugt region of West Greenland. Submitted to Precambrian Research, August 2004.

Garde, Adam A. 2004: Vestgrønlandsk pladetektonik for 1860 millioner år siden. Carlsbergfondets årsskrift 2004, 24-31.

Sidgren, A-S., Page, L. & Garde, A.A. (submitted): New hornblende and muscovite 40Ar-39Ar cooling ages in the central Rinkian fold belt, West Greenland. Submitted to Geological Survey of Denmark and Greenland Bulletin, June 2004.

Thrane, K., Baker, J., Connelly, J.N. & Nutman, A. P. (submitted): Age, petrogenesis and metamorphism of the syn-collisional Prøven Igneous Complex, West Greenland. Submitted to Contributions of Mineralogy and Petrology, October 2004.