foreland-propagating thrust architecture of the East Greenland Caledonides 72°-75°N, The

Journal of the Geological Society, Nov 2004 by Higgins, A K, Elvevold, S, Escher, J C, Frederiksen, K S, Et al

Abstract:

Systematic geological mapping of the East Greenland Caledonides demonstrates that the orogen is built up of WNW-directed thrust sheets displaced across foreland windows. The foreland windows in the southern half of the orogen are characterized by a thin (220-400 m) Neoproterozoic to Lower Palaeozoic succession, structurally overlain by two major Caledonian thrust sheets (Niggli Spids and Hagar Bjerg Thrust Sheets). The metasediments of the upper-level Hagar Bjerg Thrust Sheet host 940-910Ma granites and migmatites formed during an early Neoproterozoic thermal or orogenic event, as well as Caledonian 435425 Ma granites and migmatites. The uppermost unit of the thrust pile, the Franz Joseph Allochthon, comprises a very thick (18.5 km) Neoproterozoic to lower Palaeozoic sedimentary succession (Eleonore Bay Supergroup, Tillite Group, Kong Oscar Fjord Group). Total westward displacement of the thrust sheets was about 200-400 km, with shortening estimated at 40-60%. Major extensional faults post-date thrusting. Restoration of the thrust sheets indicates that the sequence of Caledonian orogenic events now preserved in East Greenland was initiated several hundred kilometres ESE of present-day East Greenland, as Baltica and its marginal assemblage of Early Palaeozoic accretions began to impinge on the Laurentian margin.

Keywords: Caledonian orogeny, Greenland, Laurentia, thrusts.

The Early Palaeozoic closure of the Iapetus Ocean and the consequent collision of Laurentia, Baltica and Avalonia resulted in the formation of the Himalayan-scale Caledonian mountain belt. Of the various parts of the Caledonides that are exposed along the border of the present-day North Atlantic Ocean, the remote and relatively inaccessible East Greenland sector has to date remained the most poorly known. The entire 1300 km length of the East Greenland Caledonides (Fig. 1) has now been remapped over a period of 30 years, as part of a regional 1:500000 mapping project, initiated by the then Geological Survey of Greenland, and continued by the present Geological Survey of Denmark and Greenland. A geological map of the entire orogen at a scale of 1:1000 000 has recently been published (Henriksen 2003).

The 1997-1998 remapping of the Kong Oscar Fjord region (72°-75°N, Fig. 2; Henriksen 1998, 1999; Escher 2001) was the first systematic Survey mapping of this classic region since the pioneer activities of Lauge Koch's long series of expeditions between 1926 and 1958 (Haller 1971; Koch & Haller 1971). The major lithotectonic divisions established in the Kong Oscar Fjord region can be traced throughout the southern half of the orogen and document regional large-scale thrusting (Higgins & Leslie 2000; Higgins et al. 2001). Isotopic dating has defined the Archaean-Palaeoprotcrozoic protolith ages of the basement gneiss complexes (Thrane 2002; F. Kalsbeek, unpubl. data), clarified the nature of an early Neoproterozoic (c. 930 Ma) tectonothermal event (Kalsbeek et al. 2000; Leslie & Nutman 2000, 2003; Watt et al. 2000; Watt & Thrane 2001), and constrained the timing of Caledonian orogenic events (Watt et al. 2000; Kalsbeek et al. 2001e).

An independent project to study compressional and extensional processes in the East Greenland Caledonides was initiated in 1995 by a group at the University of Oslo, led by Arild Andresen. This project has focused on key areas, with particular emphasis on the timing and significance of synorogenic extension (Hartz & Andresen 1995; Andresen et al. 1998; Hartz et al. 2000), and has been backed up by structural and isotopic studies (e.g. White & Hodges 2002; White et al. 2002). This group contributed field mapping results that have been incorporated into the Survey's Kong Oscar Fjord map sheet (Escher 2001). Their regional interpretations of the orogcn differ in some details from those presented in this paper (see also below),

The new tectonic architecture for the region 72°-75°N presented here (Figs 2-4) comprises foreland windows at the lowest structural level that are overridden by several allochthonous rock assemblages. This structural scenario was originally outlined in a restricted circulation Survey report (Elvevold et al. 2000). The present paper revises and substantiates the conclusions of Elvevold et al. (2000), summarizes the salient features of the newly established Iithotectonic divisions of the Kong Oscar Fjord region, and outlines the implications of thrust restoration for reconstruction of the Laurentian margin prior to Caledonian orogenesis.

This paper does not give details of the northern half of the East Greenland Caledonides (north of 76°N; Fig. 1). This northern region is composed of a sequence of west-directed thrust sheets derived from the Laurentian margin (Higgins et al. 2001; Rasmussen & Smith 2001), that progressively increase in age and metamorphic grade from west to east. The highest thrust sheet preserved is a Precambrian orthogneiss complex containing widespread cclogites that are witness Io a complex history of high-pressure and ultrahigh-prcssure Caledonian metamorphism (Gilotti 1993, 1994; Gilotti & Ravna 2002).