RESEARCH Age, structural setting, and exhumation of the Liverpool Land eclogite terrane, East Greenland Caledonides Lars Eivind Augland, Arild Andresen, and Fernando Corfu DEPARTMENT OF GEOSCIENCES, UNIVERSITY OF OSLO, P.O. BOX 1047, BLINDERN, 0316 OSLO, NORWAY ABSTRACT The close spatial relationship between Devonian high-pressure rocks (eclogites) and Ordovician–Silurian calc-alkaline plutonic rocks, as observed in Liverpool Land, NE Greenland, is not easily explained by existing tectonic models for the Caledonide orogen. New fi eld stud- ies and isotope dilution–thermal ionization mass spectrometry U-Pb geochronology demonstrate, however, that the association is just coincidental, because the two rock groups are located within distinct terranes separated by a composite structure. The major element is the Gubbedalen shear zone, a N-dipping shear zone dominated by a penetrative top-up-to-the-S ductile fabric. Superimposed brittle-ductile top- down-to-the-N shear zones are typical of the structurally uppermost part of the shear zone. The contact against the hanging wall is the N-dip- ping, brittle Gubbedalen extensional detachment fault. A zircon age of 399.5 ± 0.9 Ma for an eclogite body is interpreted to represent the time of high-pressure metamorphism of the footwall. The host gneiss was migmatized between ca. 388 Ma and ca. 385 Ma, as constrained by the ages of a pegmatite predating migmatization and crosscutting granites. Coeval synkinematic granites intrude along amphibolite-grade, top-to-the-S high-strain zones in the Gubbedalen shear zone. Juxtaposition of the Ordovician–Silurian plutonic terrane (hanging wall) against the Early to mid-Devonian eclogite terrane (footwall) is best explained by a tectonic model involving early mid-Devonian buoyancy-driven exhumation followed by late mid-Devonian syncon- tractional extension related to thrusting on the Gubbedalen shear zone in a dextral strike-slip zone. Subsequent exhumation through the brittle-ductile transition occurred by extension on early semiductile structures and the overprinting Gubbedalen extensional detachment fault, and erosion. LITHOSPHERE; v. 2; no. 4; p. 267–286. doi:10.1130/L75.1 INTRODUCTION 1935; Hansen and Steiger, 1971), the coexistence are: (1) an autochthonous to parautochthonous of which could not be accommodated by simple Archean to Paleoproterozoic basement with a The timing and kinematics of high-pressure tectonic models. We solved the conundrum by variably preserved cover of Neoproterozoic to (HP) metamorphism, exhumation, and crustal discovering the importance of a major structural Silurian sediments; (2) a far-traveled thrust sheet deformation are critical elements for the devel- boundary, investigated through fi eld work and (Niggli-Hagar thrust sheet) composed, from opment of tectonic models for collisional oro- U-Pb isotope dilution–thermal ionization mass the base upward, of Archean and Proterozoic gens like the Caledonides. Other important spectrometry (ID-TIMS) geochronology. In this gneisses, high-grade Mesoproterozoic supra- factors in such reconstructions include the rela- paper, we describe the major N-dipping compos- crustal rocks (Krummedal Sequence), and a tionships between crustal blocks of different ite shear zone and fault separating the Devonian thick package of Neoproterozoic to Ordovician affi nities and origins brought together by strike- eclogite terrane in the footwall from a terrane of sedimentary rocks (Eleonore Bay Supergroup, slip faulting and/or thrusting or extensional fault- predominately Late Ordovician to Silurian plu- Tillite Group, and Kong Oscar Fjord Group); ing. The Caledonides of Scandinavia and North tons in the hanging wall, report U-Pb ages that and (3) late orogenic Devonian continental East Greenland represent the two fl anks of the date the main steps in the evolution of the eclog- deposits in fault-controlled basins (Haller, 1971; Silurian–Devonian collisional orogen between ite terrane and the shear zone, and describe the Larsen and Bengaard, 1991; Higgins et al., 2004; Baltica and Laurentia, where the remains now role of the shear zone in the exhumation of the Andresen et al., 2007). A major unconformity are exposed in a series of nappes and crustal Liverpool Land eclogite terrane. separates the Middle Devonian deposits from blocks on the two Atlantic margins (Haller, 1971; the underlying folded and faulted Neoprotero- Roberts and Gee, 1985). The study of the rela- GEOLOGICAL SETTING zoic to Ordovician sedimentary rocks (Larsen tionships between the various components of the and Bengaard, 1991; Larsen et al., 2008). Silu- Caledonides is never quite straightforward and East Greenland Caledonides rian leucogranites (ca. 435–425 Ma) intrude the over the years has generated many controversies Krummedal Sequence and the lower part of the and debates. In our study, we address a contro- The Late Silurian to Devonian continent- Neoproterozoic sequence (Watt et al., 2000; versial situation in the southern part of Liverpool continent collision between Baltica and Lau- Hartz et al., 2001; Kalsbeek et al., 2001a, 2001b; Land, in the East Greenland Caledonides. The rentia produced the Caledonian orogen, the White et al., 2002; Leslie and Nutman, 2003; fundamental problem at the outset of the study remnants of which presently straddle both sides Andresen et al., 2007). Kalsbeek et al. (2001a, was an apparent paradox between the spatial of the North Atlantic Ocean (Haller, 1971; Rob- 2001b, 2008) argued that most of these leuco- association of Devonian eclogites (Hartz et al., erts and Gee, 1985). The main tectonic elements granites are S-type granites, derived by anatexis 2005) and Silurian plutonic complexes (Kranck, of the East Greenland Caledonides (Fig. 1) of pelitic units within the Krummedal Sequence. LITHOSPHEREFor permission to| Volumecopy, contact 2 | Number [email protected] 4 | www.gsapubs.org | © 2010 Geological Society of America 267 Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/2/4/267/3038294/267.pdf by guest on 25 September 2021 AUGLAND ET AL. All these units are crosscut by two major, late to postorogenic, low-angle, E-dipping, top-to- A Nørreland Window POST-CALEDONIAN the-E extensional faults: the Boyd-Bastionen 78°N 78 Upper Paleozoic and Mesozoic N sedimentary/volcanic rocks detachment and the Fjord Region detachment (Fig. 1), subdividing the region into three Devonian sedimentary rocks crustal-scale fault blocks (Hartz and Andresen, North East 1995; Andresen et al., 1998; Hartz et al., 2000, NIGGLI-HAGAR THRUST SHEET Greenland Neoproterozoic (Eleonore Bay 2001). Major sinistral strike-slip shearing also Dronning Louise Land eclogite province Supergroup and Tillite Group) accompanied continent-continent collision and and early Paleozoic rocks postcollisional extension. This led to regions of 76°N 76 (Kong Oscar Fjord Group) bulk transpression and transtension, and associ- Mesoproterozoic (Krummedal ated extrusion and pull-apart structures (i.e., the Sequence) Western fault zone and the Storstrømmen shear Greenland Storstrømmen shear zone zone; Holdsworth and Strachan, 1991; Larsen Reworked Archean and ice cap Fjord region Proterozoic basement and Bengaard, 1991; Seranne, 1992; Torsvik et detachment al., 1996; Krabbendam and Dewey, 1998; Smith LIVERPOOL LAND ECLOGITE TERRANE et al., 2007; Steltenpohl et al., 2009). 74°N 74 In a slightly modifi ed tectonostratigraphic Inferred present-day thrust front scheme proposed by Higgins et al. (2004), the CALEDONIAN FORELAND Niggli-Hagar thrust sheet is subdivided into A A’ Archean and Paleoproterozoic two tectonic units: the Niggli Spids and Hagar gneisses w/cover Bjerg thrust sheets. The nature of major faults Boyd-Bastionen and shear zones separating different units in the detachment ? ? High-angle extensional fault southern East Greenland Caledonides is debat- 72°N 72 able, as evidenced by confl icting interpretations Thrust of maps and cross sections presented in Hig- Liverpool gins et al. (2008). We follow the view of Haller Ice Land Low-angle detachment Western fault zone (1971) and Andresen et al. (2007), who inter- RenlandB B’ preted structural repetitions within the Niggli- Milne LandScoresby Sun Hagar thrust sheet to be a result of large-scale 100 km recumbent folding. 30°W 25°W d Liverpool Land B PETERMANN BJERG SUESS LAND RØDEBJERG Liverpool Land is an isolated area composed AA’Allochthon of pre-Carboniferous rocks in the southern part of the East Greenland Caledonides, and it is separated from the main outcrop area of Cale- Autochthon Parautochthon donian rocks to the west by a cover of Permian and Mesozoic sediments (Jameson Land Basin; Figs. 1 and 2). The link to the tectonostratig- B B’ raphy established farther west is therefore not Jameson Land Liverpool Land straightforward. Partly migmatized metasedi- basin Hurry Inlet plutonic mentary rocks interpreted as remnants of the terrane Krummedal Sequence (Higgins, 1988; Johnston ? ? Gubbedalen shear zone et al., 2009) have, however, been taken to indi- ? Liverpool Land eclogite terrane cate that the rocks constituting northern Liver- pool Land represent an eastward continuation of Figure 1. (A) Simplifi ed geological map of the main lithotectonic units of the East Greenland Cale- the Niggli-Hagar thrust sheet. donides. (B) Representative profi les. Rectangle indicates the Liverpool Land study area of Figure 2. Previous investigations indicated that most The fi gure is modifi ed from Andresen et al., 2007. of Liverpool Land is dominated by various types of