TECTONICS, VOL. 23, TC2001, doi: /2002tc001401, 2004

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1 TECTONICS, VOL. 23,, doi: /2002tc001401, 2004 Geometry of eclogite-facies structural features: Implications for production and exhumation of ultrahigh-pressure and high-pressure rocks, Western Gneiss Region, Norway Michael P. Terry 1 and Peter Robinson 2 Department of Geosciences, University of Massachusetts, Amherst, Massachusetts, USA Received 22 April 2002; revised 25 August 2003; accepted 13 November 2003; published 4 March [1] Well-preserved eclogite-facies structural features in metamorphosed gabbro and diorite gneiss in Baltica crust, formed at a minimum depth of km during the Scandian orogeny, allow direct inferences to be made regarding the geometry of folds, kinematics, and original structural orientations related to production and exhumation of high-pressure rocks. Folds associated with eclogite-facies fabrics are isoclinal to tubular with axes parallel to the trend of a stretching lineation. Strain estimates and L > S or L S fabrics indicate that these structures were formed in a constrictional strain field. Locally, they are well preserved in eclogite-facies mylonite zones at least 40 m thick that cut Proterozoic gabbro and adjacent gneiss. Where steeply dipping, they show a north-sideup shear sense across a younger, northeast trending, shallowly plunging, amphibolite-facies anticlinorium formed in a constrictional, noncoaxial strain field. Where original structural facing direction can be inferred, small-circle rotation of the eclogite-facies lineation about the anticline axis indicates that the relative motion vector of Baltica with respect to overlying imbricated crust and Laurentia was oriented 320 in a present-day reference frame. This result is identical to the orientation of the relative motion vector estimated from paleomagnetic plate reconstructions and consistent with Late Silurian to Early Devonian oblique sinistral convergence between Baltica and Laurentia. Results of this study indicate that coherent Baltica crust at km depth experienced subvertical and horizontal shortening while extending toward the foreland during tops-southeast thrusting parallel to plate motion. High-pressure rocks were progressively imbricated and stacked along eclogitefacies mylonite zones, with the deforming zone separating coherent Baltica crust from imbricated basement above. The constrictional strains under eclogite-facies conditions are interpreted to have 1 Now at Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth, Germany. 2 Now at Geological Survey of Norway, Trondheim, Norway. Copyright 2004 by the American Geophysical Union /04/2002TC resulted from stretching associated with sinking of the cool dense mantle lithosphere. INDEX TERMS: 8102 Tectonophysics: Continental contractional orogenic belts; 8015 Structural Geology: Local crustal structure; 8110 Tectonophysics: Continental tectonics general (0905); 8025 Structural Geology: Mesoscopic fabrics; 8030 Structural Geology: Microstructures; KEYWORDS: exhumation, eclogite-facies, Western Gneiss Region. Citation: Terry, M. P., and P. Robinson (2004), Geometry of eclogite-facies structural features: Implications for production and exhumation of ultrahigh-pressure and high-pressure rocks, Western Gneiss Region, Norway, Tectonics, 23,, doi: /2002tc Introduction [2] One of the most difficult problems faced in deciphering thermal and mechanical processes in the lower part of a thickened crust during continental collision is tectono-metamorphic overprinting of both mineral assemblages and early structural features during syn- and post-collisional exhumation. Even in well-exposed parts of subducted continental crust, where high-pressure and ultrahigh-pressure mineral assemblages are preserved, structures that reflect their production and earliest phases of exhumation tend to be preserved only in isolated eclogite boudins. Knowledge of the geometry, kinematics, timing, metamorphic conditions and original orientation of eclogite-facies structural features is crucial for placing constraints on mechanisms related to the subduction and exhumation of continental crust during continental collision. The purpose of this paper is to present results of detailed mapping and structural analysis of Proterozoic basement and associated tectonic cover that were deeply subducted during the Silurian-Devonian Scandian orogeny in locations on Nordøyane where strain partitioning has allowed preservation of eclogite-facies features both in boudins and the enclosing gneiss. In this paper, we provide a brief summary of the younger features that overprint those formed at a minimum depth of km, provide specific examples where geometry, kinematics, and original orientation can be inferred from relationships with later features, and discuss models that may explain these observations. 2. Tectonic Framework and Geologic Setting [3] The Scandian orogeny, a result of the collision between Baltica and Laurentia, included emplacement of 1of23

2 Figure 1. (a) Generalized geologic map of the Molde-Alesund region showing narrow refolded synclines of tectonic cover in Baltica crust and location of the study area. (b) Generalized tectonostratigraphic map modified from Gee et al. [1985]. Large arrows show the orientation of a time averaged relative motion vector for Baltica with respect to Laurentia approximated from reconstructions of Torsvik [1998]. thrust nappes followed by and synchronous with subduction of Baltica beneath Laurentia, which produced high-pressure (HP) and ultrahigh-pressure (UHP) rocks and extensional orogenic collapse. Paleomagnetic and paleogeographic data indicate that Baltica collided with Laurentia about 425 Ma with a latitudinal velocity of 8 10 cm yr 1 that resulted in oblique collision [Torsvik, 1998]. Eclogite-facies metamorphism continued at least until 402 ± 2 Ma [R. D. Tucker data from Lutro et al., 1997; Carswell et al., 2003]. This was accompanied by extensional exhumation that is divided into two phases that occurred between ca. 400 and 390 Ma [Terry and Robinson, 2003]. The first phase formed ductile detachment faults in the hinterland and was active during continued convergence [Lutro et al., 1997] until 395 Ma. The second phase was associated with development of lowangle detachments including brittle faults that brought Devonian sedimentary rocks and underlying nappes into contact with high-pressure Baltica basement [Andersen and Jamtveit, 1990; Andersen et al., 1991, 1994]. These phases of extension are interpreted to have resulted from the gravitational potential produced by a thick pile of nappes and a reduction of the orogen normal component of the relative motion vector of Baltica with respect to Laurentia. The oldest preserved Devonian basins with a minimum age of Ma [Allen, 1976; Tucker et al., 1997, 1998] indicate that sedimentation overlapped both phases of extension. Here we focus on development of structural features in deep crust interpreted to have been contemporaneous with the first phase of exhumation. Detailed mapping by Robinson [1995, 1997], and Terry and Robinson [1998, 2003] indicates that numerous tectonic cover sequences have been tightly folded into Baltica basement and that basement is imbricated (Figure 1). These observations are consistent with petrographic and geothermobarometric data of Wain [1997] and Wain et al. [2000] that showed dramatic differences in pressure, up to 10 kb, in crustal eclogites from granitoid gneisses that define zones of high-pressure and ultrahigh-pressure segments of Baltica crust. 3. Geologic Map of Nordøyane [4] The tectono-stratigraphic units mapped on Nordøyane ( the North Islands ) correlate with some of the units 2of23

3 Figure 2. Generalized geologic map of Nordøyane showing the major lithotectonic units, with keys to areas and locations of special study in this paper, and locations of cross sections. mapped by Robinson [1995] in the Moldefjord syncline (Figure 1) including allochthonous Baltica basement and the Sætra and Blåhø Nappes of the middle and upper allochthons, respectively. The nappes, now tightly folded into Baltica basement, are distinctive and correlate with similar units mapped outside Nordøyane and across the orogen. [5] Mapping on Nordøyane allows Baltica basement to be divided into three major lithotectonic units, Southern, Central and Northern, that appear to have different metamorphic histories (Figure 2). The Southern Unit is composed of granitic gneiss, hornblende gneiss, and biotite-hornblende-gneiss with coarse-grained plagioclase porphyroclasts. It contains amphibole eclogites, indicating eclogite-facies conditions, however, the eclogite grain size is generally finer than in the Northern Unit. The Central Unit contains granitic and intermediate gneisses with abundant amphibolite boudins, interpreted as deformed mafic dikes, but lacking eclogite-facies assemblages. Apparently all or part of the Central Unit didn t reach eclogite-facies conditions during the Scandian. The Northern Unit consists of three major rock types, Ulla Gneiss, migmatitic gneiss, and augen orthogneiss (Figure 2). The Ulla Gneiss (Figure 3a) is a heterogeneous mixture of coarse-grained eclogite boudins and hornblende ± clinopyroxene - garnet - biotite - plagioclase - quartz gneiss, preserving eclogitefacies structural features that are the focus of this paper. The augen-orthogneiss and migmatitic-biotite-plagioclase-quartz ± hornblende-gneiss (Figures 3b, 3c, and 3d) dominate structurally higher parts of the Northern Unit. The augen orthogneiss (Figures 3c and 3d), containing K-feldspar, plagioclase, biotite, and quartz, is interpreted to crosscut both migmatitic gneiss and Ulla Gneiss. It has not been dated on Nordøyane, but a U-Pb zircon age of 1508 Ma from augen orthogneiss on the Molde peninsula suggests a minimum age for Ulla Gneiss [Tucker et al., 1991]. The augen orthogneiss, Ulla Gneiss, and migmatitic gneiss are crosscut by the Flem and Haram garnet-corona gabbros (Figure 2). These gave Sm-Nd ages interpreted as igneous of 1289 ± 48 Ma and 926 ± 70 Ma, respectively [Mørk and Mearns, 1986], but new U-Pb igneous zircon ages by T. Krogh are 1252 ±4 Ma and 1466 ± 2 Ma [Robinson et al., 2003]. [6] The southern, central and northern segments of Nordøyane contain tectonic cover correlated with the Sætra and Blåhø Nappes elsewhere. The rock type correlated with the Sætra Nappe, here only in the central segment, is quartzite with interlayered amphibolite that is interpreted as deformed mafic dikes [Robinson, 1995]. The major rock types correlated with the Blåhø Nappe include garnet-biotite 3of23

4 Figure 3. Photographs showing the major rock types in Baltica basement in the northern unit. (a) Typical exposure of Ulla Gneiss on the northwest coast of Haramsoy with abundant crustal eclogites that illustrates the heterogeneity of this unit. (b) Migmatitic gneiss with leucosomes that are interpreted as partial melts from the surrounding gneiss. Leucosome are modified by sheath folds in the center of photo and truncated by later leucosomes parallel to the axial surface of the major fold. (c) Crustal eclogites in augen orthogneiss exposed on the northwest coast of Flemsoy/Skuløy. (d) Photograph of a horizontal surface showing augen orthogneiss. K-feldspar augen define a subhorizontal stretching lineation formed during late top-west extensional shearing folded during the development of a later vertically dipping crenulation. ± kyanite ± sillimanite schist or gneiss, garnet amphibolite, marble, calc-silicate, and felsic garnet gneiss. 4. Structural Geology of Nordøyane [7] The combined structural, petrographic, and field data show a progression of fabrics and structures that are related to production and exhumation of high-pressure rocks. We divide them into early features formed or interpreted to have formed at eclogite-facies conditions and late features that formed at granulite to low-amphibolite-facies conditions. The late structures [see Terry and Robinson, 2003], related to extension, dominate the map pattern (Figure 2). These are associated with top-southwest or left-lateral shear interpreted to have developed in a constrictional strain field associated with a sub-horizontal ENE-WSW stretching lineation and vertical and NNW-ESE shortening. The geometry of late folds and other features are shown in a cross section through the southwestern part of the area in Figure 4. Here tectonic cover nappes provide excellent markers. These structures resulted from the two phases of extension that contributed to exhumation of eclogite-facies rocks. [8] The earliest structure associated with the first phase of extension is the contact between the central and southern segments (Figure 4). This is tentatively interpreted as an early extensional-fault juxtaposing eclogite-facies rocks of the southern segment against lower grade rocks of the central segment that lack evidence for eclogite-facies metamorphism. This fault and adjacent rocks were then subjected to complex progressive deformation involving folds ranging from isoclinal recumbent and sheath-like to open and upright. This folding is associated with sub-horizontal stretching lineations defined by the alignment of hornblende, quartz and feldspar rods, and re-oriented early microfolds (Figures 5a, 5b, and 5c). The second phase of extension is represented by the low-amphibolite-facies Åkre 4of23

5 Figure 4. Composite cross section D-D 0 through the southern and central segments of Figure 2 showing the geometry of late folds and interpreted tectonometamorphic relationships between the major lithotectonic units. Mylonite, defining the contact between the central and northern segments and crosscutting earlier amphibolitefacies structures. This mylonite zone generally dips steeply, however on central Haramsøya (Figure 5d) the foliation has moderate dips resulting from progressive deformation within the zone. Lineations within the mylonite are sub-horizontal, trend ENE, and are associated with left-lateral shear. The northern segment shows the best preservation of eclogite-facies structural features and fabrics that are the focus of this paper. Their preservation is attributed to greater strength of the Northern Unit during later overprinting at granulite- to low-amphibolite-facies conditions [Terry and Robinson, 2003]. This different behavior is illustrated by the broad scatter of poles-to foliation and lineation from the Figure 5. Representative equal-area projections of foliations and lineations associated with amphibolite-facies structures from the (a) southern (b) central, and (c) northern segments and (d) the Åkre mylonite. 5of23

6 Figure 6. Generalized geologic map of the Haram Gabbro with equal area projections of poles to foliations and lineations in different areas. (a c) Data on foliation (dots) and lineation (crosses) collected in gabbro mylonite zones in areas a, b, and c, separated by vertical dashed lines, from west to east across the intrusive complex. (d) Mylonite derived from granitoid gneiss within the eclogite-facies shear zone on south contact of gabbro where early lineations have been statically replaced by amphibolite-facies assemblages. (e) Ulla Gneiss exposed at the northeast contact of the gabbro where early lineations have undergone in-plane rotation in response to later amphibolite-facies deformation. (f) Ulla Gneiss exposed between eastern contact of the gabbro and Åkre Mylonite. Note north dipping foliation and gently east plunging lineation. (g) Exposure of the low amphibolite-facies Åkre Mylonite that truncates earlier amphibolite-facies structural features. Note steep foliation and gently east plunging lineation. (h) Southern and central units located south of the interpreted contact of the Åkre Mylonite. northern segment (Figure 5c) as compared to narrow scatter in the central and southern segments (Figures 5a and 5b). 5. Eclogite-Facies Structural Features and Fabrics [9] Here we give and discuss the best examples of eclogite-facies structural features and fabrics, including those in Haram Gabbro, in Ulla Gneiss and near Flem Gabbro on northwest Flemsøya, in microdiamond-bearing kyanite gneiss on Fjørtoft, and in Ulla Gneiss on northwest Haramsøya (Figure 2). Relationships in these places allow direct inferences to be made regarding the orientation, kinematics and geometry of folds and shear zones associated with processes operating at minimum depths of km during continental collision Haram Gabbro [10] The Haram Gabbro (Figures 2 and 6) is a metamorphosed Proterozoic intrusive complex with an exposed length of 1.3 km and width of 0.3 km. It is subdivided into three major primary igneous units, coarse-grained maficgabbro (Figure 7a), anorthositic gabbro (Figure 7a), and cumulate-layered gabbro (Figure 7b). It shows wellpreserved primary igneous textures (Figures 7b, 7c, 7d), illustrating the strain partitioning in the northern segment. Eclogite-facies structural features are represented by steeply dipping anastomosing zones of mylonite and mylonitic 6of23

7 Figure 7. Photographs showing major units in the Haram Gabbro and primary structures. (a) View looking northeast from northwest corner of Haram complex showing anorthositic gabbro in the foreground and mafic gabbro in the background. (b) Cumulate-layered gabbro. Pyroxene-rich layers grade into felsic tops indicating original tops-south (right). (c) Coarse-grained pegmatite composed of clinopyroxene, orthopyroxene, titanomagnetite and plagioclase within massive anorthositic gabbro. Coarse plagioclase growth on walls with interstitial pyroxene grades into graphic intergrowth texture in core. (d) Close-up of gabbro pegmatite. See color version of this figure in the HTML. gneiss that cut the gabbro and adjacent Ulla Gneiss (Figure 6). These shear zones, ranging in thickness from centimeters to tens of meters, show steeply plunging lineations (Figures 6a, 6b, 6c, and 6d). Figure 6e shows in-plane rotation of early lineation interpreted to have resulted from localized flattening at the northeast contact of the gabbro. The surrounding Ulla Gneiss, Åkre Mylonite, and the gneiss south of the mylonite show steeply dipping foliation with a younger sub-horizontal lineation (Figures 6f, 6g, and 6h) Primary Structural Features [11] Primary structural and textural features are well preserved outside of mapped shear zones in the Gabbro. They include graded cumulate layering, undeformed pegmatites, and ophitic textures. Cumulate layering marked by alternating orthopyroxene/clinopyroxene-rich and plagioclase-rich layers ranges from 2 to 30 cm thick (Figure 7b). In the southwest cumulate layers grade from mafic bases to felsic tops with tops consistently to the south. Coarse-grained pegmatites composed of plagioclase, magnetite, clinopyroxene, and orthopyroxene show no evidence of deformation (Figures 7c and 7d). Ophitic textures have been observed on outcrops of the coarse-grained mafic gabbro, and in thin-section, where primary clinopyroxene or orthopyroxene are present. This has been described also by Mørk [1985] in the Flem Gabbro Shear Zones [12] Eclogite-facies shear zones within Haram Gabbro show grain-size reduction accomplished during shearing and an increase in modal garnet (Figures 8a and 8b). Foliation is steep and associated with a steeply plunging stretching lineation (Figures 8c and 8d). Lineations are defined by deformed ilmenite, porphyroclasts of pyroxene with symmetric to asymmetric aggregates of garnet and omphacite associated with development of coronas, and preferred orientation of plagioclase, kyanite and biotite. Equal area projections of poles to foliation and lineation in shear zones are shown for the western, west-central, and eastern parts of the gabbro in Figures 6a, 6b, and 6c, and for the shear zone along the south contact in Figure 6d. The shear zones locally occur at contacts between different compositions of gabbro, indicating that they may have developed along pre-existing weaknesses. They also cross- 7of23

8 Figure 8. (a and b) Photographs show an eclogite-facies shear zone crosscutting the coarse-grained mafic gabbro. View looking down lineation. Box shows the location of Figure 8b. The light-colored compositional layer cut by the shear zone is a late stage igneous pegmatite composed of plagioclase, clinopyroxene and magnetite. (c and d) Photographs show the lineation on a foliation surface in a shear zone near the northwest margin. See color version of this figure in the HTML. cut more homogeneous compositions of gabbro and are responsible for juxtaposing units of different composition. Locally, cumulate gabbro is juxtaposed against coarsegrained mafic gabbro (Figure 9) where cumulate layering is rotated into parallelism with a steeply dipping shear indicating a north-side-up sense of shear, consistent with sigma structures developed on porphyroclasts and microstructures in oriented samples. [13] A few narrow south-dipping (50 ) shear zones show well-developed sigma structures indicating north-sidedown. Generally, these shear zones are discontinuous and show no evidence for large-scale displacements. Locally, they join with vertically dipping shear zones and may be reasonably interpreted as conjugate shear zones. Mineralogical associations in these zones are identical to nearvertical shear zones with major north-side-up shear sense and show no evidence of being different in age. We interpret these particular zones to have developed in response to flattening or internal heterogeneity within the gabbro in a strain field where north-side-up shear was predominant. [14] Both the north and south contacts of the Haram Gabbro have well-developed shear zones. The shear zone at Figure 9. Generalized sketch of a shear zone that juxtaposes cumulate gabbro against coarse-grained mafic gabbro where cumulate layering is rotated into parallelism with a steeply dipping shear. 8of23

9 the south contact is at least 40 m wide and contains mylonite derived both from gabbro and from granitic to intermediate gneiss. Mylonitized gneiss dominates, with gabbro occurring as isolated fragments. The fragments have well-developed mylonite at their contacts and locally show mesoscale folding of layering with axes parallel to the early lineation. The folding is interpreted as a product of progressive deformation within the mylonite. In the granitic gneiss, eclogite-facies lineation has undergone static replacement by hornblende and is well preserved as a relic lineation (Figure 6d) in the same orientation as lineation in rocks with well-preserved eclogite-facies assemblages (Figures 6a, 6b, and 6c). This major shear zone appears to be in contact to the south with the Åkre mylonite that is near vertical and has well-developed subhorizontal lineation (Figure 6g). The shear zone on the northern margin of the gabbro is at least 20 m wide. It shows a similar alternation of granitoid gneiss and gabbro mylonite, but is dominated by gabbro mylonite. The most important observation within the contact shear zones is that there is no significant difference in orientation of the stretching lineation or dip of foliation compared to shear zones within the gabbro. This indicates that these shear zones were generated in the same strain field and metamorphic conditions, and both gneiss and gabbro experienced the same thermal and structural evolution Metamorphism and Microstructures [15] All samples of gabbro examined in thin section, including those that appear to show no deformation in the field, show evidence for eclogitization of the primary mineral assemblage. Evidence includes development of two major types of corona textures between mafic minerals and plagioclase, including the following phases from core to rim: (1) omphacite-garnet-plagioclase, or orthopyroxene or clinopyroxene-omphacite-garnet-plagioclase and (2) rutilegarnet-plagioclase. The type 1 corona texture in Figure 10a occurs in medium-grained gabbro that appears to be undeformed in outcrop and where the original mafic phases appear to have been replaced at eclogite-facies conditions. Clinopyroxene is zoned from core to rim from Jd = 27 to Jd = 6 with the other grains in the thin section showing compositions up to Jd = 32 [Terry et al., 2000b]. The occurrence of quartz inclusions in omphacite is similar to occurrences for coesite described by Smith [1984] but no coesite or associated textures have been identified in the Haram Gabbro. Figures 10c and 10d show photomicrographs from an oriented sample of mylonitized gabbro from a narrow near-vertical shear zone at the contact between the cumulate and anorthositic gabbros in Figure 7a. The coarsegrained porphyroclast of igneous orthopyroxene shows a type 1 corona texture where fine-grained omphacite has partially replaced original orthopyroxene. Prismatic garnet inclusions inside the orthopyroxene are interpreted to have been plagioclase laths that were replaced by garnet and are interpreted to be a relict igneous ophitic texture. The growth of omphacite in the strain shadow suggests that these fabrics were produced during prograde metamorphism at the time of deformation. This asymmetric replacement of the orthopyroxene in strain shadows is consistent with a north-sideup sense of shear. Plagioclase-rich zones define an asymmetric shear fabric that indicates the same sense of shear as the porphyroclast. This shear sense is consistent with observations of mesoscale shear zones responsible for major juxtaposition of different rock types (Figure 9). The presence of retrograde symplectites of orthopyroxene and plagioclase shown in Figure 10e suggest decompression into the granulite-facies. This occurred before hydration associated with amphibolite-facies overprinting represented by static replacement by hornblende and plagioclase of earlier assemblages in the eclogite-facies shear zones near gabbro contacts Northwest Flemsøya [16] Northwest Flemsøya shows complex folding involving typical granitic gneisses of the structurally higher part of the Northern Unit (Figures 2 and 11) [Mørk, 1985]. These include biotite-hornblende migmatite gneiss and augen gneiss that are crosscut by variably eclogitized gabbro. The contact zone between the granitic rocks and Ulla Gneiss to the south is associated with boudins of peridotite that locally contain garnet, orthopyroxene, Crdiopside, Cr-spinel and olivine with mineralogical and compositional features that are consist with a mantle origin [Carswell, 1968, 1973, 1986; Brueckner, 1977; Medaris, 1980, 1984; Mørk, 1985]. This spatial association may indicate a tectonic contact between these two major units of Baltica basement. The complex fold pattern in the extreme northwestern part of Flemsøya (Figure 11) is interpreted to be the result of interference between earlier eclogite-facies structures and later progressive folding associated with top-west shear. Equal area projections in Figures 11a 11f summarize the orientation of structural elements associated with both early and late structures, recognized by amphibolite-facies assemblages that overprint eclogite-facies structures, extensional kinematic indicators, and crosscutting shear zones, along the coast. As observed in Haram Gabbro, early lineations (Figures 11b, Figure 10. (a) Photomicrograph of sample 1296a shows relatively coarse-grained omphacite in undeformed metamorphosed gabbro. (b) Photomicrograph of sample 419 shows a type 1 corona in moderately deformed gabbro with igneous clinopyroxene and orthopyroxene surrounded by euhedral garnet. (c and d) Photomicrographs of oriented sample 453 in plane polarized and crossed polarized light from mylonitized gabbro with a porphyroclast of orthopyroxene viewed in a plane normal to foliation and parallel to lineation. Light layers represent plagioclase-rich zones where plagioclase has a preferred grain-shape orientation in an oblique orientation relative to foliation that corresponds to a northside-up sense of shear consistent with mesoscale structures shown in Figure 9. (e) Electron back-scattered image of part of sample 453 showing plagioclase-orthopyroxene symplectite on omphacite suggesting decompression into the granulitefacies field. 9of23

10 11c, and 11f) are typically at a high angle to later lineations. Poles to foliations and their cylindrical-best-fits shown in Figures 11a, 11c, and 11e indicate that late folds change trend from WNW to ENE along the coast and have a variable plunge. This change, that begins east of the Flem Gabbro, is attributed to gentle warping around the more competent Ulla Gneiss. However, it is possible that this change records a change from initially WNW to ENE Figure of 23

11 Figure 11. Detailed map of northwest coast of Flemsøya with equal area projections (a f) of poles to foliations and lineations. The box is the location of Figure 12. trending structures that occurred during exhumation from granulite- to amphiblole-facies conditions [Terry and Robinson, 2003] Geometry of Eclogite-Facies Folds [17] The detailed map in Figure 12a shows a doubly closed fold, most probably anticlinal, interpreted to have formed at eclogite-facies conditions. The interior of the fold is composed of Ulla Gneiss that contains abundant small eclogite boudins and a crudely developed compositional layering that defines the overall structure and internal mesoscale folds. Eclogite layers, concordant with layering in the mafic part of the Ulla Gneiss, define the eastern closure of this fold. This eastern closure is shown in Figure 12b. It has a tube-like geometry and is interpreted to be parasitic to the larger fold. The map pattern and available structural data allow two alternative interpretations either as a syncline or an anticline. The favored anticline interpretation shown in section in Figure 12c is most consistent with known stratigraphic relationships and kinematics of eclogite-facies structural features discussed in more detail below. Equal area projections in Figures 12d, 12e, 12f, and 12g show poles of foliations, lineations and fold axes in a sequence from west to east across the early fold structure (locations in Figure 12a). Figure 12g shows the orientation of garnet-omphacite layers and omphacite lineations within the area of Figure 12b. The omphacite lineations are parallel to the plunge indicating that the fold was formed at eclogite-facies conditions. Relict lineations now defined by rods composed of aggregates of hornblende, clinopyroxene, and plagioclase measured along a N-S traverse through the central part of the fold in Figure 12f agree with meso-scale folds and are parallel to the omphacite Figure 12. (a) Detailed map in the area near a well-preserved eclogite-facies fold on the northwest coast of Flemsøya showing location of Figure 12b. (b). Photograph of the eastern closure of the early fold where dark layers represent mafic compositions that locally preserve eclogite-facies assemblages. The form lines in Figure 12a on the eastern closure show the map view of this structure. (c) Cross section C-C 0 showing the geometry of the eclogite-facies fold being folded by late amphibolite-facies structures. (d g) Equal-area projections of poles to foliations and lineations from west to east across the early fold structure with locations shown in Figure 12a. See text for discussion. 11 of 23

12 lineations plotted in Figure 12g. The shallowly plunging lineation reflects modification of the early lineation related to late top-west shearing at the contact with more ductile rocks. A more extreme example of modification of the early lineation occurs at the contact with more ductile units of the early fold, where the steepest lineation is at the contact between Ulla Gneiss and the more granitic units which are dominated by shallowly plunging lineation Figure of 23

13 (Figure 12e). At the western closure of the early fold, eclogites are retrogressed and the hinge shows a moderate plunge (Figure 12d). The lineations measured outward from the Ulla Gneiss in the core of the eastern fold closure in Figure 12b show progressive rotation from moderate northeast plunge to a shallow northeast plunge. [18] Structural data and mapping in northwest Flemsøya are consistent with early development of sheath and tubular folds at eclogite-facies. The early anticline (?) is exposed in the limb of an asymmetric, amphibolite-facies fold that has a north-dipping axial surface and occurs near the contact between Ulla Gneiss and the granitic gneisses interpreted to lie structurally above. Late modification of this fold involved rotation of early lineations and fold axes in a direction consistent with top-west shearing. The metamorphic conditions during late non-coaxial strain are estimated to have begun at 780 C, 13 kb, and continued into the amphibolite-facies [Terry and Robinson, 2003] Flem Gabbro [19] Variably eclogitized Flem Gabbro (Figure 11) studied by Mørk [1985] and Mørk and Mearns [1986] contains northeast- and northwest trending shear zones (Figure 11d top) associated with steeply plunging lineation formed under eclogite-facies conditions (Figure 11d bottom). The western contact is tectonic and shows truncation of late subhorizontal structures against weakly eclogitized gabbro. Late granitic pegmatite veins crosscut the migmatitic gneiss country rock and the gabbro and are present on the contact. Near pegmatite veins in gabbro containing original pyroxene + olivine assemblages and in eclogitized parts of the gabbro, the assemblages are retrogressed to hornblende and plagioclase. The original intrusive contact of the gabbro is preserved at the shore on the southeast side. Here, the gabbro is much finer-grained and fine-grained mafic dikes crosscut the augen orthogneiss. These relationships are consistent with the nearest date for the igneous protolith of the augen orthogneiss at 1508 ma [Tucker et al., 1991] and the recent date by T. Krogh on a gabbro pegmatite at 1252 Ma [Robinson et al., 2003]. The orientations of the lineation within the augen orthogneiss, the gabbro and the mafic dikes are consistent with eclogite-facies lineation in the same outcrop. Thus only minor modification of the lineation in the augen orthogneiss, defined by deformed phenocrysts of K-feldspar and plagioclase, took place since its formation under eclogite-facies conditions Metamorphism of Flem Gabbro [20] Detailed petrologic research by Mørk [1985] and Mørk and Mearns [1986] of variably eclogitized gabbro and isolated eclogite boudins on northwest Flemsøya indicates temperatures and pressures of approximately 750 ± 60 C and approximately kb. These conditions are interpreted to have been synchronous with the development of eclogite-facies lineations [Mørk, 1985]. Upper limits on pressures were inferred from the absence of jadeite in granitic compositions. Migmatitic gneisses enclosing eclogites and gabbros, that are strongly overprinted by top-west shear fabrics, show well-developed granulite- to amphibolite-facies assemblages [Mørk, 1985]. This indicates that northwest Flemsøya had a similar temperature and pressure history to the area around the Haram Gabbro, including decompression into the granulite facies and later amphibolite-facies overprinting Fjørtoft [21] Augen orthogneiss and migmatitic gneiss dominate exposures on Fjørtoft with lesser Ulla Gneiss and rocks assigned to the Blåhø Nappe (Figure 2). The map pattern in Figure 2 is similar to the interference pattern on northwestern Flemsøya. North dipping rocks of the Blåhø Nappe on the northeastern part of the island (Figure 13), show well-preserved early structural features within a very heterogeneous sequence of garnet-biotite gneiss, kyanite-garnet-biotite gneiss, calcsilicate, and marble. One important difference from typical Blåhø observed during mapping by Mørk [1989] and L. F. Dobrzhinetskaya (Diamantførende gneiser på Fjørtoft I den vestlige gneisregionene, Møre og Romsdal, unpublished Norges geologiske undersøkelse report, series number , 10 pages, 1993) is the presence of abundant eclogites and localized occurrences of ultramafic rocks. The eclogites of Fjørtoft are divided into three types: (1) very-coarse-grained garnet-omphacite eclogites typical of the northern unit, (2) kyanite-omphacitegarnet eclogite, and (3) compositionally layered eclogite probably derived from volcanics within the stratified Blåhø Nappe sequence. Ultramafic rocks occur both in the Nappe and near contacts with basement (Figure 13). The easternmost peridotite contains garnet, orthopyroxene, clinopyroxene, olivine, and Cr-spinel [Jamtveit et al., 1991]. Peridotite that occurs within the Nappe to the west is heavily serpentinized and retains no primary minerals. The association of peridotite and kyanite eclogite in or near contacts of the Blåhø Nappe is a special feature of the northern segment of Nordøyane. This association occurs at Nogva, Flemsøya, where garnet-biotite schist or gneiss containing calcsilicate boudins is in contact with kyanite eclogite and Ulla Gneiss containing peridotite. This allows correlation of the Nogva and Fjortøft areas of Blåhø rocks. [22] The southern contact of Blåhø with basement on Fjørtoft is not exposed. Basement to the south includes augen orthogneiss, phyllonite, garnet-corona gabbro, and locally calcsilicate and kyanite-eclogite. The contact area is strongly overprinted by linear and planar fabrics related to late top-west shearing reflecting strain partitioning around eclogite-rich rocks of the Blåhø. Details of relationships on this contact are poorly known, but strong intermixing of rock types and features of metamorphic petrology suggest that the contact is a major fault zone. In the NW of Figure 13, Blåhø is underlain by Ulla Gneiss containing strongly deformed eclogite mylonite that may have an early HP origin Geometric Relationships Between Early and Late Structural Features [23] Geometric relationships between early and late structural features on Fjørtoft are similar to other areas preserving eclogite-facies structures. Foliations parallel to variably developed compositional layering have an E-W to WNW strike and moderate to steep north dips (Figure 14a). Rods 13 of 23

14 Figure 13. Detailed geologic map of the northeastern part of Fjørtoft. Location is shown in Figure 2. of mineral aggregates, early folds, and preferred orientation of kyanite define early lineations in garnet-biotite gneiss, garnet-kyanite gneiss, and metamorphosed igneous rocks. These show a broad scatter with trends that range from northwest to northeast (Figure 14c). Where both are present, early and late lineations (Figure 14b) lie in the same foliation plane. Late lineations show the broad scatter that is typical for them in the northern segment [Terry and Robinson, 2003]. Scatter of early lineations is interpreted to reflect varying degrees of rotation during late tops-west or left-lateral shearing, similar to rotations observed on northwest Haramsøya. Mylonitic zones at the extreme northwest exposure of the Blåhø Nappe locally truncate an earlier WNW striking foliation. Early isoclinal fold axes in Figure 14d are parallel to early lineations. This relationship is confirmed in Figures 14e and 14f. Figure 14e shows 95% confidence cones for early folds and for lineations. The cylindrical best fit for poles to foliations in Figure 14f allows calculation of a 95% confidence cone for the associated fold axis, which is indistinguishable from the cones in Figure 14e. We think the cone in Figure 14f is due to early tight to isoclinal folding, though it could be due to gentle warping of the foliation. Our interpretation is consistent with observations of mesoscale early folds, distribution of mapped contacts in the Blåhø Nappe, and relationships described elsewhere in the northern segment. [24] Kinematic indicators associated with the early lineation, including porphyroclasts with well-developed asymmetry and shear bands, are exposed in a quarry in the northeastern part of the Blåhø Nappe (Figure 15). They indicate a north-side-up shear sense in agreement with observations in eclogite-facies mylonite zones in Haram Gabbro Metamorphism [25] Discovery of microdiamonds by chemical dissolution of the kyanite-garnet gneiss indicates that the Blåhø Nappe at Fjørtoft has experienced pressures of >35 kbar [Dobrzhinetskaya et al., 1995; Larsen et al., 1998]. This is consistent with associated kyanite-eclogites containing polycrystalline quartz after coesite and assemblages of garnet + omphacite + kyanite ± zoisite ± phengite. Application of garnet-omphacite-kyanite-coesite and garnetomphacite-phengite-coesite barometers and the garnet-clinopyroxene thermometer yield conditions of 820 C and kbar (125 km depth) compatible with the presence of microdiamond [Terry et al., 2000b; E. Ravna and M. Terry, Geothermobarometry of UHP and HP eclogites and schists: An evaluation of equilibria between garnetclinopyroxene-kyanite-phengite-coesite/quartz, submitted to Journal of Metamorphic Geology, 2003]. Petrologic work and temperature and pressure estimates by Larsen et al. [1998] show two retrograde events in three different bulk 14 of 23

15 Figure 14. Equal-area projections of major structural features within the Blåhø Nappe on the northeast end of Fjørtoft. (a) Poles to foliations and layering indicating that planar fabrics dip dominantly north and strike ENE to WNW. (b) Equal-area projection showing sub-horizontal amphibolite-facies lineations. The broad scatter in the trend is typical of that observed in the northern unit of Nordøyane (see Figure 5c) and is interpreted to reflect a different greater strength relative to the central and southern segments (Figures 5a and 5b). (c) Early lineations interpreted to have formed at eclogite-facies conditions. Broad scatter in these reflects variations in the strike and dip of foliation/layering. (d) Axes of intrafolial folds related to the early eclogite-facies fabrics. (e) Mean vectors and 95% confidence cones for early lineations (solid lines) and foldaxes (dashed). (f) Cylindrical-best-fit for poles to foliations in Figure 14a. Figures 14e and 14f combined indicate that steeply plunging folds, lineations and orientations of foliations are primarily a result early eclogite-facies deformation with relatively weak over-printing by amphibolite-facies deformation. granulite- to amphibolite-facies lineation that is commonly at a high angle to the eclogite-facies lineation is the geometrical relationship used to identify the retrogressed eclogite-facies lineation where it has not been rotated during top-west shearing (Figure 16). Figure 16a is a sketch of a vertical foliation surface where both early and late lineations lie in the same plane. Late lineations in the east part of the exposure are parallel to the axes of late open folds (Figure 16b). Early lineations (Figure 16c) are parallel to axes of steeply plunging isoclinal or tubular folds (Figure 16d). The orientations of structural features are shown in Figure 16e. On a regional scale, these relationships are like those observed near the Haram Gabbro, where early lineations in gabbro lie in the plane of the steeply dipping foliation as well as in the compositional layering of the surrounding Ulla Gneiss that also shows well-developed late lineations (Figure 6f). These relationships are consistent throughout the Ulla Gneiss wherever the early lineation is present, indicating that the layer-parallel foliation was compositions: (1) C with minimum pressures of kbar and (2) C at kbar for late granulite-facies overprinting. The higher-pressure estimates agree with temperature and pressure estimates for the garnet peridotite exposed at the south contact of the Blåhø Nappe at 711 to 780 C, 23 to 27 kbar [Jamtveit et al., 1991]. These data indicate that rocks in or near the Blåhø Nappe on Fjørtoft decompressed from eclogite-facies through the granulite-facies, consistent with observations at northwest Flemsøya and in Haram Gabbro Northwest Haramsøya [26] Within Ulla Gneiss exposed on Northwest Haramsøya it is possible to identify eclogite-facies linear structural features based on their angular relationship to late features. In the areas described in previous sections, it was possible to trace eclogite-facies linear structural features into adjacent retrogressed gneiss where the eclogite-facies fabric had undergone static retrogression and is present as a relict lineation defined by aggregates of mafic minerals and folds. In these examples, fabrics were related directly to eclogitefacies conditions by their relationship to later fabrics. The Figure 15. (a and b) Photograph and sketch showing a weathered porphyroclast in garnet-biotite-hornblendeclinopyroxene schist with an asymmetry that indicates a north-side-up sense of shear. (c and d) Photograph and generalized sketch showing a shear band with the same shear sense. Location is shown on Figure of 23

16 Figure 16. (a) Sketch map of a vertical foliation surface showing early and late lineations in the Ulla Gneiss. Boxes show the location of photographs. (b) Photograph of late lineations defined by microfolds and preferred grain shape orientation. (c) Photograph of the early lineation defined by rods of mineral aggregates that are parallel to early mesoscale folds. (d) Down-plunge view of early fold. (e) Equal-area projections of poles to foliations and lineations show detailed geometric relationships between early and late structures. formed under eclogite-facies conditions and has been variably transposed during late top-west shearing Finite Strain Estimates and Related Observations [27] Finite strain estimates were made in augen orthogneiss in two locations near the east contact of the Flem Gabbro (see Figure 11), one from an outcrop near the contact, and one from a boulder nearby. The outcrop exhibits L > S fabrics and measurements were made using K-feldspar augen. The boulder contains the same retrogressed eclogitized mafic dikes as the outcrop. This boulder exhibits L S fabrics and separate measurements were made on plagioclase and K-feldspar augen. Axial ratios were measured in XZ, YZ and XY planes of strain where possible, or the third ratio was calculated from two ratios when all three planes could not be reached. This approach assumes that there was no original preferred orientation. Results show that the finite strain ellipsoids produced under eclogite-facies conditions plot in the constrictional field (Figure 17). The axial ratios of K-feldspar are essentially indistinguishable from undeformed orthoclase which is consistent with the weak deformation observed in the field. Differences between plagioclase and K-feldspar in the boulder are attributed to the high modal abundance of plagioclase and to strain partitioning around less deformed and less ductile K-feldspar phenocrysts. Finite strains estimated from K-feldspar near the contact show a larger flattening component than do Figure 17. Flinn diagram summarizing finite strain estimates in K-feldspar and plagioclase augen gneisses at the contact of the Flem Gabbro. 16 of 23

17 accurate ratios could be obtained from these exposures, the observations are consistent with development in a constrictional strain field. Quantitative estimates in favorable rock types can thus be used in combination with other observations to identify the type of finite strain associated with eclogite-facies structural features. Naturally the estimates are not intended as quantitative estimates of bulk strain in the entire subducting Baltica crustal plate. Figure 18. (a) Sketch map from field notes shows the gabbro near Ulla Fyr. Solid dots show locations of structural data in Figure 18b. (b) Equal-area diagram of structural features. (c) Photograph of contact between mylonitized gabbro on the left and Ulla Gneiss on the right. (d) Generalized sketch shows mafic layer adjacent to hammer with a strong asymmetry indicating a north-sideup sense of shear. Location is shown on Figure 2. strains estimated from plagioclase in the boulder. This may be explained by flattening of the augen orthogneiss closer to the contact of the main mass of gabbro. [28] Similar relationships are observed near Ulla Fyr (Figure 2) where a small garnet corona gabbro crosscuts Ulla Gneiss and augen orthogneiss (Figure 18a). Retrogressed mafic dikes, interpreted to be related to the gabbro, crosscut the augen orthogneiss. Early lineations in eclogitized gabbro, identical to those in Haram Gabbro, can be traced directly into the adjacent augen orthogneiss and into Ulla Gneiss (Figures 18a and 18b). This small gabbro body occurs on the hinge of a major early fold and the contact between the Ulla Gneiss and gabbro shown in Figure 18a is strongly sheared and shows static replacement of mafic phases by hornblende at the contact and within Ulla Gneiss. Compositional layers in Ulla Gneiss locally show asymmetries consistent with north-side-up sense of shear, indicating that development of constrictional strain under eclogitefacies conditions is associated with noncoaxial strain (Figures 18c and 18d). The augen orthogneiss shows L S fabrics, with the lineation defined by deformed K-feldspars that have similar YZ axial ratios to deformed plagioclase from the boulder described in Figure 17. Although no 5.6. Original Orientation of Eclogite-Facies Structural Features [29] Determination of the original orientation of early linear structural features requires accurate knowledge of the history of the surfaces that contain them. In Nordøyane, this is further complicated by fact that late folding is interpreted to have resulted from noncoaxial constrictional strain [Terry and Robinson, 1998; Terry, 2000; Terry and Robinson, 2003]. This problem is eliminated locally in rocks where the simple shear component of the late strain is partitioned away from planes containing the early linear structure. In such rocks late folds would form in a coaxial strain field and any rotation of the plane containing the early fabric would occur by rotation about the axes of later folds. Thus the early fabric orientation could be inferred by small circle rotation about the late fold axes, assuming knowledge of the appropriate direction for rotation. [30] Another difficulty is that late folds were produced during progressive non-coaxial deformation under amphibolite-facies conditions. Late fold axes show a broad scatter from WNW to ENE, interpreted to represent progressive deformation during top-west or left-lateral shearing [Robinson and Terry, 1998; Terry and Robinson, 1998; Terry, 2000]. To try to overcome this problem, a cylindrical best fit to poles of foliations in west Haramsøya near Haram Gabbro was used to define the rotation axis. If the assumption of local small circle rotation during late folding is correct, the pole to the plane containing the early lineation and the late rotation axis must lie along the best-fit great circle of poles to foliations that defines the late fold axis (Figure 19a). [31] This approach was attempted in several other areas where early linear structural features are well preserved and the original way-up can be inferred from stratigraphic relationships or vergence in early folds. In these other areas including Fjørtoft, northwest Haramsoy and Ulla Fyr, the pole to the plane containing the rotation axis and the orientation of the early lineation does not lie on the bestfit great circle for poles to foliation as it does in Figure 19a. Results from these other areas are thus compatible with inplane rotation of the early lineation resulting from noncoaxial deformation. However, the correct relationship is shown when linear and planar data from the Haram Gabbro is combined with foliation data from the west coast of the Haramsøy in Figure 19a. [32] The Haram Gabbro offers the best preservation of early lineations which are contoured in Figure 19a. These correct relationships allow simple small circle rotation of the plane containing the maximum concentration of early lineations about the late fold axis, thus permitting original trends of eclogite-facies structural features to be inferred. 17 of 23