[AMERICAN JOURNAL OF SCIENCE, VOL. 299, JANUARY, 1999, P ]

[AMERICAN JOURNAL OF SCIENCE, VOL. 299, JANUARY, 1999, P. 69–89] DUCTILE AND BRITTLE EXTENSION IN THE SOUTHERN LOFOTEN ARCHIPELAGO, NORTH NORWAY: I...
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[AMERICAN JOURNAL

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SCIENCE, VOL. 299, JANUARY, 1999, P. 69–89]

DUCTILE AND BRITTLE EXTENSION IN THE SOUTHERN LOFOTEN ARCHIPELAGO, NORTH NORWAY: IMPLICATIONS FOR DIFFERENCES IN TECTONIC STYLE ALONG AN ANCIENT COLLISIONAL MARGIN ANDRE C. KLEIN*‡, MARK G. STELTENPOHL*, WILLIS E. HAMES*, and ARILD ANDRESEN** ABSTRACT. The Lofoten archipelago, north Norway, occupies the most internal position of the Caledonian belt in northern Scandinavia, and rocks and structures exposed there are crucial to understanding processes of how the Baltic basement and its cover allochthons responded to continental lithospheric subduction and subsequent continental separation. Relatively little is published about the structural and metamorphic development and especially the timing of these events; consequently, it is unknown how features exposed on these spatially isolated islands relate to those of the adjacent mainland. Rocks in Lofoten were affected by Caledonian regional metamorphism, and structures record tops-east contraction and later extension related to late- to post-Caledonian basement exhumation. Tops-west extension is preferentially developed in meter to kmscale ductile shear zones containing west-dipping extensional shear bands, westverging rootless folds, and asymmetric feldspar porphyroclasts. West-plunging, sinistral-oblique elongation lineations in the mylonitic foliation are interpreted to indicate the line of transport. Mesoscopic backfolds are locally developed within the tops-west shear zones and further document west-directed transport of structurally higher rocks. The ductile extensional shear zones locally are cross cut by low-angle, tops-west cataclastic normal faults, reflecting progressive unroofing of the shear zones to shallower crustal levels during late- and post-Caledonian extensional events. Northeast-striking, high-angle, brittle normal faults truncate all other fabrics and structures and juxtapose structurally deep undeformed Precambrian basement with the Caledonian nappe sequence. 40Ar/39Ar data indicate that ductile extension in Lofoten likely initiated soon after the Caledonian metamorphic peak (at ⬃430 Ma) and continued episodically until at least Permian time. Rocks passed through the brittle/ductile transition rapidly at ⬃275 Ma. Recent paleomagnetic data also point to later phases of brittle faulting in Lofoten, one in the Jurassic/Cretaceous and one related to Tertiary opening of the Norwegian Sea. The magnitude and style of late- to post-Caledonian extension and basement exhumation in Lofoten differs markedly from the formation of phenomenal Devonian basins and exposures of Caledonian ultra-high pressure assemblages in the Western Gneiss Region (WGR) of southern Norway. Differences in the styles of extension between northern and southern Norway are interpreted to reflect inherited differences in crustal architecture that may have been enhanced during the Caledonian orogeny. INTRODUCTION

Extensional structures and fabrics in the Western Gneiss Region (WGR) of southwestern Norway expose deep sections of continental lithosphere exhumed following continent-continent collision (Norton, 1986; Se´ranne and Se´guret, 1987; Andersen and Jamtveit, 1990; Andersen and others, 1991). Comparable crustal-scale extension has been reported in the Caledonides of East Greenland (Hartz and Andresen, 1995). Over the past decades there has been little record of syn- to post-Caledonian extension reported in the basement of northwestern Norway, although it has recently been recognized in the Lofoten archipelago (Holloman and others, 1995, 1996; Hames and * Department of Geology, Auburn University, Auburn, Alabama 36849 Present address: Department of Geology and Geophysics, Rice University, MS-126, 6100 Main Street, Houston, Texas 77005-1892 ** Department of Geology, University of Oslo, P.O. Box 1047, Blindern, 0316 Oslo, Norway ‡

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Andresen, 1996a; Holloman, ms; Klein, ms; Mooney, ms; Waltman, ms). The evolutionary history of the northern WGR (Lofoten block) differs strikingly from its southern WGR counterparts, especially in terms of metamorphism, deformation, pressures attained during subduction, and residence time in the footwall of a major fault zone. For example, the WGR of southern Norway was penetratively-deformed and subjected to ultra-high pressure (UHP) metamorphic conditions during the Caledonian orogeny (Bryhni and Sturt, 1985). Many basement exposures in the Lofoten region of northern Norway, however, appear to have experienced little or no Caledonian deformation (Hakkinen, ms; Griffin and others, 1978; Tull, 1978; Bartley, 1981, 1982). The differences between the WGR and Lofoten block are enigmatic given that the two areas have a similar structural setting. Explanations for these differences are examined herein based on the results of new structural studies aimed at characterizing the Caledonian and later tectonic evolution of the southern Lofoten islands (Holloman and others, 1995, 1996; Holloman, ms; Klein, ms; Mooney, ms; Klein and Steltenpohl, 1999). This paper focuses on the islands of Vestvågøy, Værøy, and Røst (fig. 1), because they are the only islands in southern Lofoten with exposures of aluminous metasedimentary rocks, making them ideal for an integrated structural, geochronologic, and thermobarometric study. Synchronous contraction and extension have been documented from several orogenic belts around the world (the Higher Himalaya, Burchfiel and Royden, 1985; southwestern Norway, Norton, 1986; the Variscan belt in Europe, Steltenpohl and others, 1993), and these examples have added greatly to our understanding of the stresses present in an evolving orogen. Modern subduction zones, like the one along the western coast of North America, contain a great deal of along-strike variability. Such variability in ancient subduction zones is commonly masked by later deformation or unfavorable levels of erosion, but the Norwegian Caledonides, with excellent exposure of subequal amounts of basement and cover, provide a paramount opportunity to study an ancient subducted margin. The ancient A-type subduction zone boundary (Hodges and others, 1982) is exposed along the west coast of Norway and has been the subject of numerous reports. Most studies have focused on the southern WGR, where large-scale extension driven by post-Caledonian gravitational collapse of the orogen apparently led to the formation of the Devonian molasse basins and juxtaposed them against basement eclogitized during Caledonian subduction (Norton, 1986; Se´ranne and Se´guret, 1987; Andersen and Jamtveit, 1990; Andersen and others, 1991). It must be noted here that the final closure of Iapetus during the Silurian involved both contractional and extensional stresses at various levels of the Caledonian nappe stack at various times (Andersen and others, 1991; Northrup, 1996). Therefore, the distinction between Caledonian and post-Caledonian, as used in this paper, is not related to an absolute change in stress regimes. The boundary is drawn, instead, at the end of prograde regional metamorphism of the Baltic basement and its overlying cover at ⬃430 Ma (Steltenpohl and Bartley, 1988; Hames and Andresen, 1996a; Northrup, 1996; Dunlap and Fossen, 1998). Recent detailed geologic mapping of the southern Lofoten archipelago has revealed that post-Caledonian extension significantly modified the earlier contractional edifice (Holloman, ms; Klein, ms; Mooney, ms; Waltman, ms; Klein and Steltenpohl, 1999). Of singular interest is how the continental margin at the present-day latitude of Lofoten (68°N) responded to the waning stages of continent-continent collision and subduction to depths approaching 40 km. In contrast to the spectacular examples of deep crustal exhumation reported from southern Norway, mentioned above, a master extensional decollement (Nordfjord-Sogn Detachment Zone of Andersen and Jamtveit, 1990) has not been reported. In addition, Lofoten does not expose UHP assemblages of Caledonian age, such as the coesite-eclogite facies basement of the WGR of southwest Norway (Griffin and Brueckner, 1980, 1985; Gebauer and others, 1985). Given that the rocks

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Fig. 1. Location of the Lofoten archipelago and study area. Lithologies exposed in Lofoten are primarily Precambrian basement gneiss of the Lofoten block (light gray) and allochthonous metasedimentary rocks of the Leknes group (black). The Ofoten region of north Norway is indicated.

exposed in these two terranes are lithologically similar and occupy a comparable structural setting, why did they behave so differently in response to subduction and subsequent eduction? This paper addresses this question in light of new structural, petrologic, and geochronologic data from the southern Lofoten archipelago.

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The Lofoten archipelago (68°N) is a basement ridge in the north Norwegian shelf, associated with gravity, magnetic, and topographic highs. Exposures in Lofoten represent the most distal component of the Caledonides in the north Norwegian margin (fig. 1). The Caledonides are composed of a variety of vertically stacked nappes, thrust from west to east on to the Baltoscandian platform during partial subduction beneath Laurentia in early to middle Paleozoic time (Gee, 1975; Roberts and Gee, 1985). The relationship of the Lofoten region to the Caledonides on the Norwegian mainland has been debated since detailed mapping of the area was begun in the early 1970’s (Griffin and Taylor, 1978; Tull, ms and 1978), although it is now generally accepted that Baltic basement exposed on the mainland is continuous with Lofoten basement (Olesen and others, 1996). The Lofoten block is composed of primarily Precambrian basement gneiss (Foslie, 1941; Vogt, 1942; Andresen and Tull, 1986). Griffin and others (1978) report areally extensive mangeritic and charnokitic intrusions in Lofoten dated at ⬃1700 to 1800 Ma. The so-called ‘‘mangerite suite’’ was emplaced under granulite-facies (clinopyroxene-zone) conditions. The Lofoten basement was locally retrograded to amphibolitefacies in shear zones accompanying allochthonous Caledonian rocks, although, as a whole, the basement was little-affected by Caledonian reheating (Griffin and others, 1978). In central Lofoten, allochthonous cover rocks, informally known as the ‘‘Leknes group’’ (Tull, ms and 1978; Sigmond and others, 1984), lie in thrust contact with Precambrian gneisses and granitic plutons typical of the northern WGR exposed in the Lofoten islands (Griffin and others, 1978). Based on poorly-constrained Rb-Sr and K-Ar dates from the Leknes group, Tull (ms and 1978) and Griffin and others (1978) suggested that the Leknes group was emplaced onto the basement during an amphibolite-facies metamorphic event that occurred between 1200 to 1100 Ma. Tull (ms) indicated that the metamorphism (his M2) of the Leknes group is unrelated to the Caledonian metamorphism described by Bartley (ms) on Hinnøy (⬃100 km northeast of Vestvågøy). New isotopic data, however, indicate development of amphibolite-facies porphyroblasts in schists and amphibolites of the Leknes group occurred during Caledonian metamorphism (Hames and Andresen, 1996a, b). Records of this event are variably overprinted by subsequent retrogressive events (Klein and Steltenpohl, 1999). The type locality of the Leknes group on Vestvågøy is composed of aluminous schist, calc-silicate gneiss, quartzite, marble, amphibolite, and voluminous quartzofeldspathic mica schist (fig. 2). Two lithologically distinct sequences are found within the Leknes group on Vestvågøy (Klein, 1997). The ‘‘southern units’’ (fig. 3) are in thrust contact with the mangeritic basement and contain the bulk of the quartzofeldspathic mica schist and amphibolite, as well as thin (⬍5 m) layers of quartzite, marble, and calc-silicate gneiss. The ‘‘northern units’’ have been faulted over the southern units and contain predominantly amphibole-rich schist, two-mica schist, and quartz-rich amphibolite. STRUCTURAL DEVELOPMENT

The structural development of the southern Lofoten archipelago is outlined in table 1 and covers mainly the Caledonian and later history of the region. Due to the local intensity of Caledonian deformation and metamorphism, evidence relating to the pre-Caledonian geologic history has been largely obscured near Caledonian shear zones. With depth below Caledonian thrusts or across post-Caledonian normal faults with large displacements, Caledonian deformation generally is absent, and Proterozoic structures remain intact. In general, the pre-Caledonian geologic history is dominated by development of the Baltic Shield and the northern WGR by a series of orogenic and

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Fig. 2. Lithotectonic map of the study area on Vestvågøy and diagrammatic cross section A-A⬘ (same horizontal scale as map). In general, the Leknes group is a complexly interlayered and anastomosing sequence of variably felsic and mafic schists and gneisses with minor amounts of quartzite, marble, and amphibolite. See figure 3 for a measured section of part of the Leknes group.

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Fig. 3. Diagrammatic illustration of the tectonostratigraphic architecture of the Leknes area (no scale). Small arrows are Caledonian thrust faults. Excellent exposure in the southern units allowed for the development of a detailed tectonostratigraphic column. Lithologies are summarized from Klein (ms).

intrusive events that are difficult to evaluate in rocks of the study area due to the high metamorphic grade and localized migmatization of many of the rocks. Caledonian orogeny.—The onset of Caledonian orogeny in the Lofoten region is characterized by east-directed thrusting of Leknes group assemblages onto the Baltic craton. Thrusting was accompanied by local retrogression of basement rocks from granulite-facies to amphibolite-facies metamorphic assemblages. Metamorphic grade of allochthonous cover rocks increased from east to west, with schists exposed in the Røst district having experienced kyanite-orthoclase zone conditions and local migmatization (Holloman, ms; Mooney, ms; Waltman, ms). Thermobarometric data reflect this west-

TABLE 1

Structural development of the southern Lofoten archipelago

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ward increase, with conditions on Vestvågøy averaging 600°C and 6 kb, increasing to 750°C and 8.2 kb on Røst (Mooney, ms; Waltman, ms). Similarly, 40Ar/39Ar ages of muscovite porphyroblasts indicate that rocks of the Røst district cooled through their respective blocking temperatures ⬃100 my later than rocks exposed on Vestvågøy (Hames and Andresen, 1996a). These data are not too surprising considering the location of these rocks in the footwall of a decollement. Rocks exposed on Vestvågøy and Røst were most likely farther apart during the onset of Caledonian orogeny than they are today. The Caledonian orogeny was a major contractional event (D2 of Klein, ms) at the latitude of Lofoten, producing numerous thrust faults, large-scale basement-cored folds (F2), and pervasive metamorphic and/or mylonitic foliations (S2) (Bartley, 1982; Steltenpohl, 1987; Holloman, ms; Klein, ms; Mooney, ms). The Caledonian foliation disappears with depths of less than 0.5 km into the basement below the basal Caledonian thrust. This phenomenon was recognized by early workers throughout Lofoten-Ofoten (Tull, ms; Griffin and others, 1978) and initially was attributed to the Lofoten terrane having occupied a high crustal level during Caledonian orogenesis (e.g. Hakkinen, ms; Griffin and others, 1978). It is now generally accepted, however, that the Lofoten block was tectonically buried by the composite Caledonian allochthon, and existing pressuretemperature data suggest that Lofoten was subducted to depths of 25 to 35 km (Mooney, ms; Waltman, ms). Late- to post-Caledonian.—Following the peak of Caledonian metamorphism (at ⬃430 Ma), rocks of the Lofoten archipelago underwent two kinematically distinct folding events (F3 and F4 of Klein, ms; F3a and F3b of Waltman, ms) related to regionally diachronous deformation (D3 and D4 on Røst, Holloman, ms; D3/D4 on Vestvågøy, Klein, ms; D3 of Waltman, ms). F3 folds on Vestvågøy have a dominant west-northwest trend, parallel to the Caledonian (and post-Caledonian, see below) transport direction (fig. 4A). These folds are termed crossfolds and appear to represent shortening directed at a small angle to the trend of the orogen (Steltenpohl and Bartley, 1988). F4 fold axes, on the other hand, parallel the orogenic trend (north-northeast) but verge to the west in the opposite sense from that of nappe transport (fig. 4B) and thus are considered backfolds (Steltenpohl and Bartley, 1988). Interestingly, the crossfolds are concentrated at the highest exposed structural levels on Vestvågøy, whereas the backfolds only occur proximal to the basement-allochthon contact. As a result, fold interference patterns between F3 and F4 were never observed. D4 was a major extensional episode throughout the entire Lofoten archipelago (Løseth and Tveten, 1996; Holloman, ms; Mooney, ms; Klein, ms). Late-phase, topswest, ductile shear zones that affect the basement and cover units on Vestvågøy are mainly localized within 1 km of the basement-allochthon contact (fig. 5). Rare tops-west shear zones occur higher in the nappe stack, where they were observed to truncate F3 folds; hence the late-phase shear zones are interpreted as D4 structures. The east-northeasttrending, northwest-dipping shear zones are defined by an S4 extensional crenulation cleavage (Platt and Vissers, 1980) that is subparallel to S2 but generally has a steeper dip (fig. 6A). These relationships lead to the development of local S-C composite fabrics (Lister and Snoke, 1984) in the basement and S-NSC (Dennis and Secor, 1987) composite fabrics in the cover units (fig. 6B). Local high-strain mylonite zones are defined by stretched quartz and feldspar and may represent competence contrasts between different lithologies. A mineral elongation lineation (L4) in the S4 mylonitic foliation plunges west northwest and is interpreted to indicate the line of transport (Klein, ms; Klein and Steltenpohl, 1999). These relations suggest that the D4 slip line was largely coaxial with D2, and that post-Caledonian transport of structurally-higher rocks was in the opposite direction from that of Caledonian nappe emplacement. The west-northwest plunge of L4 in the north-dipping D4 shear zones indicates a component of left-slip and

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Fig. 4(A) Field photograph of an F3 crossfold within mafic gneiss of the Leknes group, ⬃2 km west of Leknes. Surface being folded is S2 gneissosity developed during Caledonian thrusting. Hammer is ⬃30 cm long. (B) Outcrop photo of a mesoscopic F4 backfold in basement mangerite orthogneiss, looking north and down plunge, ⬃4 km southwest of Leknes. Note west vergence of axial surface and lack of axial planar fabric. Lens cap is ⬃5 cm in diameter.

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Fig. 5. Map of D4 ductile shear zones within the Leknes group and subjacent basement mangerite (outlined in bold lines, gradient pattern). The two shear zones may be connected but are not shown to do so due to lack of exposure. Lower-hemisphere equal-area projections are of L4 mineral elongation lineation data from each shear zone. Contours are 2, 6, and 8 percent.

characterizes the D4 extensional event. Local S-NSC relations also indicate a strong sinistral component to D4 (Klein and Steltenpohl, 1999). Argon isotopic analysis of metamorphic minerals recrystallized during D4 provides clues as to the absolute timing of this late-phase shearing event. Metamorphic conditions during D4 were in the greenschist facies, with chlorite commonly overgrowing biotite within D4 shear zones. The shear zones also commonly contain fine-grained, recrystallized muscovite that clearly is younger than the coarse mica flakes formed during D2 (that is, the coarse-grained muscovite is often preserved in boudins within S4, whereas the fine-grained mica defines S4). Recrystallized micas in S4 fabrics on Vestvågøy yield ages of ⬃365 Ma (Hames and Andresen, 1996a). Similar fabrics along basement-cover contacts on Værøy and Røst yield lepidoblastic muscovite with ages of ⬃275 Ma. These relationships suggest a diachronous or otherwise time-transgressive evolution of D4 structures from northeast to southwest Lofoten. D5 brittle faulting.—Brittle faults truncate earlier ductile structures and fabrics and can generally be divided into two main types: fabric forming and non-fabric forming faults. Fabric forming faults range in orientation from low-angle to high-angle. The low-angle faults occur only in the cover rocks, and their orientation may have been influenced by

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Fig. 6(A) Interlayered quartzite and quartz-rich mica schist exposed on the beach, ⬃3.25 km west of Leknes. View is looking north and down the dip of the dominant schistosity. The schistosity in the rock is offset in a top-down-to-the-west sense of shear. Hammer head is ⬃10 cm across. (B) Oriented hand sample of sheared basement mangerite from quarry ⬃1.5 km southwest of Leknes. Sample was cut perpendicular to the mylonitic foliation and parallel to the elongation lineation. Note subhorizontal S2 fabric offset along S4 extensional shear bands dipping moderately to the right (west).

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the strong fabric already present; they were never observed to juxtapose differing lithologies. High-angle faults were easier to recognize in the field. A subvertical, northwest-striking D5 fault on Værøy juxtaposes undeformed deep crustal basement rocks with the strongly Caledonized Leknes group. Brittle fractures associated with D5 faults locally contain celadonitic muscovite with 40Ar/39Ar ages of ⬃265 Ma (Hames and Andresen, 1996a). The fact that this fault contains a fabric defined by metamorphic minerals and strained quartz (Mooney, 1997) suggests that faulting occurred near the brittle-ductile transition, where the rocks were hot enough to deform semi-plastically. On Vestvågøy, a subvertical northeast-striking D5 breccia zone truncates the basementallochthon contact and is interpreted to be a late-phase, down-to-the-east normal fault. Cataclastic rocks related to this fault zone are exposed also in the northwestern part of the study area and on the northeastern tip of neighboring Flakstadøy. The breccia zones lack any planar fabric and have a similar orientation to the late-stage faults described on Hinnøy (Bartley, 1981), on Grytøya (Van Winkle, ms; Van Winkle and others, 1996), and in Skånland (Steltenpohl, 1987) on the adjacent mainland. The breccia zones may also be related to the Vestfjorden-Vanna fault complex (Andresen and Forslund, 1987; Olesen and others, 1997) that trends northeast-southwest throughout Lofoten-Vesterålen, and to which Olesen and co-workers assigned a Permian age based on paleomagnetic studies. Other late-phase D5 brittle faults recognized in the area deform units of the overlying allochthon. Summary.—The Caledonian orogeny at the latitude of Lofoten was characterized by east-southeast-directed, basement involved thrusting and prograde amphibolite facies metamorphism of the allochthonous cover rocks. On Vestvågøy, D2 was the dominant deformational episode, creating an S2 schistosity throughout the Leknes group and to depths of generally less than 0.5 km into the basement. Following the metamorphic peak, the basement-allochthon contact was reactivated and again locally sheared as the cover allochthons moved to the west-northwest. This event, D3/D4, created two kinematically distinct fold sets at different levels within the nappes. Due to their lack of fold interference and similar metamorphic conditions of formation, F3 and F4 may have formed at roughly the same time (Steltenpohl and Bartley, 1988). The later stages of fold development were accompanied by ductile shearing and retrograde metamorphism. The ductile extensional shear zones locally are cross cut by low-angle, tops-west cataclastic normal faults, reflecting progressive unroofing of the shear zones to shallower crustal levels during late- and post-Caledonian extensional events. Northeast-trending, high-angle, brittle normal faults truncate all fabrics and structures and juxtapose structurally-deep, undeformed Precambrian basement with the Caledonian nappe sequence. DISCUSSION

Kinematic significance of late-stage crossfolds and backfolds.—Crossfolds and backfolds locally developed within the Leknes group give map-scale evidence for sinistral shearing and extension. Cross-strike structures such as crossfolds have been reported throughout western Ofoten (Steltenpohl and Bartley, 1988; Van Winkle, ms; Van Winkle and others, 1996) and can be related to the finite strain ellipse (fig. 7) of a large-scale sinistral shear zone (as in Wilcox and others, 1973). The crossfolds lie within the compressional field of the strain ellipse documenting orogen-parallel movements. F3 crossfolds on Vestvågøy are geometrically, kinematically, and temporally related to post-metamorphic D4 shear zones occurring in the lower part of the nappe stack implying that D3 and D4 may have been contemporaneous. This is consistent with microstructural observations that indicate similar deformational conditions for D3 and D4. The fact that crossfolds and backfolds formed at different structural levels within the Leknes group may reflect extensional strain partitioning in a transtensional event.

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Fig. 7. Map-scale features reflecting orogen-parallel and extensional motion along the basement-cover contact of west-central Vestvågøy. Gray scale map image is of thrusts and the shoreline as illustrated in figure 2. See text.

The east-northeast-trending, D4 ductile shear zones and their associated backfolds can be related to an extensional kinematic plan (Osmundsen and Andersen, 1994). Backfolds have axes that are parallel to the trend of the orogen but verge in the opposite sense from that of nappe transport (Steltenpohl and Bartley, 1988). D4 shear zones trend

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Fig. 8. Cartoon diagram depicting progressive fold axial plane rotation in the direction of extension if the WGR of northern Norway occupied a position in the footwall of a major west-dipping normal fault (Coker and others, 1995). The figure represents D4 evolution of Vestvågøy at ⬃365 Ma. Folds have been dragged in the direction of extension and become progressively more overturned as the fault is approached. Short dashed lines represent ductile conditions. Topmost diagram is modified from Model III of Brun and Choukroune (1983).

east-northeast, and L4 stretching lineations trend oblique to the dip direction, predominantly west, imparting a component of sinistral shear to the extension. The relationship of the crossfolds and backfolds to the late, D4 ductile shear zones suggests that the folds formed in response to late- or post-Caledonian extension with a sinistral strike-slip component. The west-vergent, late-stage backfolds have upright hinge planes in Ofoten (Steltenpohl and Bartley, 1988) but progressively become recumbent in Lofoten (fig. 8). The fact that backfolds in Lofoten are restricted to discrete, late-stage shear zones suggests that extension may have been accommodated along a system of west-dipping normal faults rather than along a single detachment. Alternatively, D4 extensional shearing and backfold development may have been restricted to discrete zones of the crust by the relative paucity of water available to promote ductile deformation (see below). This alternative is consistent with Bartley’s (1982) interpretation to account for the downward disappearance of Caledonian thrust-related fabrics on the neighboring island of Hinnøy. Thus, a model invoking the importance of a single master extensional decollement in unroofing the Lofoten block is equally attractive. The diachronous nature of extension across Lofoten, borne out in the mineral cooling ages reported by Hames and Andresen (1996a), is consistent with either interpretation. Comparison of extensional development throughout the southern Lofoten Islands.—Detailed field mapping throughout southern Lofoten (fig. 1), that is, Vestvågøy (Klein, ms), Værøy

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(Mooney, ms; Waltman, ms), and Røst (Holloman, ms; Waltman, ms), has revealed ductile and brittle extensional structures and fabrics previously undocumented in this region. Geologic mapping on west-central Vestvågøy documents thrust-related structures and fabrics (Tull, 1978) that were later overprinted by discrete extensional shear zones (Klein, ms). The shear zones contain both meso- and microscopic extensional shear bands (fig. 6), that cross cut the Caledonian fabric and indicate tops-west movement. Asymmetrical feldspar and mica porphyroclasts that record a tops-west sense of shear were observed within the lowest 1 km of structural thickness in the Leknes group and topmost 0.5 km of underlying basement, suggesting that extensional movements on Vestvågøy were concentrated along the basement-cover contact. The S4 mylonitic foliation developed during extension dips moderately to the north, and a mineral elongation lineation (L4) developed in the plane of S4 plunges to the west-northwest, indicating a sinistral strike-slip component to the extension. 40Ar/39Ar ages of ⬃365 Ma from recrystallized muscovite in the Leknes group mylonites on Vestvågøy are interpreted to reflect the timing of mylonite development during extension (Hames and Andresen, 1996a). The island of Værøy (fig. 1) exposes west-dipping Caledonian shear zones with east-vergent fabrics and structures that have been locally overprinted by late(?)- to post-Caledonian, tops-west, mylonitic and cataclastic structures and fabrics (Mooney, ms; Waltman, ms). West-dipping zones of mylonite-ultramylonite contain asymmetrical feldspar and mica porphyroclasts with fine-grained, recrystallized tails indicating a tops-west sense of shear when viewed on a surface parallel to the mineral elongation lineation and perpendicular to the mylonitic foliation (Mooney, ms). Muscovite from mylonitic rocks on Værøy yields ages of 270 Ma (Hames and Andresen, 1996a). A large-scale brittle normal fault that cuts the island in half dips steeply to the southwest and offsets the Caledonian fabric in a top-down-to-the-west sense of shear. Hydrothermal muscovite from fractures associated with this normal fault yields 40Ar/39Ar ages of about 265 to 255 Ma (Hames and Andresen, 1996a), interpreted to record the timing of extensional movements across the fault. The islands of the Røst district, at the southernmost terminus of the Lofoten archipelago, expose imbricated Caledonian (?) thrusts that juxtapose rocks lithologically similar to the Leknes group (Tull, ms) with Precambrian basement (Holloman, ms). The rocks of the Røst district presently have the geometry of a northeast-southwest doublyplunging antiform (Holloman, ms). Thrust-related fabrics and structures have locally been overprinted by tops-east and tops-west, mylonitic extensional fabrics and structures (Holloman, ms). 40Ar/39Ar ages of muscovite porphyroblasts and recrystallized muscovite are also Permian in Røst (⬃275 Ma) and are interpreted to reflect an episode of unroofing and extension (Hames and Andresen, 1996a). In summary, the findings of this study are consistent with abundant evidence documenting post-orogenic extensional shearing throughout various segments of the Caledonian belt (Andersen and Jamtveit, 1990; Andresen and Forslund, 1987; Bukovics and Ziegler, 1985; Coker and others, 1995; Norton, 1986; Olesen and others, 1997). 40Ar/39Ar cooling dates indicate that brittle and ductile extensional shearing in Lofoten occurred over a protracted span, lasting nearly 150 my and broken up into pulses separated by 30 to 50 my intervals of quiescence (Hames and Andresen, 1996a). The same evidence indicates that extensional shearing did not last the full 150 my at any one place but was diachronous throughout Lofoten, younging to the southwest. Olesen and others (1997) provide temporal constraints on episodes of brittle faulting in the LofotenLopphavet region using paleomagnetic methods. They conclude that two phases of faulting and brecciation affected rocks in this region, one Permian in age and the other recent/Tertiary, and that there may have been a westward shift of fault activity from Carboniferous-Permian to Late Jurassic-Early Cretaceous time. The westward younging

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pattern of metamorphic cooling ages (Hames and Andresen, 1996a) and brittle fault propagation is consistent with the Lofoten terrane occupying a position in the footwall of a west-dipping, crustal-scale extensional fault located somewhere west of the present Norwegian coast (Coker and others, 1995; Hames and Andresen, 1996a). Differences in tectonic styles between the WGR and the Lofoten block.—The structural and metamorphic styles in Lofoten appear to differ substantially from those of the WGR of southwestern Norway. Brittle and ductile extensional structures and fabrics are locally well-developed in southern Lofoten, but they are not as regionally pervasive as the extensional fabrics found in the WGR (Andersen and Jamtveit, 1990; Andersen and others, 1991). Large areas of the southwest Norway WGR have yet to be explored, however, and undeformed Precambrian granites have been discovered structurally beneath the Devonian mylonites (J. Bartley, personal communication, 1998). Even so, the scale difference in the amount of Caledonian and post-Caledonian sheared rocks between the two regions is remarkable. Furthermore, the presence of Caledonian eclogite bodies in the WGR contrasts sharply with the lack of such ultra-high pressure (UHP) rocks of the same age reported in Lofoten. Finally, the WGR of the NordfjordSognefjord region is perhaps best known for its large-scale basins filled with several (?) km of Devonian sediments deposited on the hanging wall of a crustal-scale extensional detachment (Norton, 1986; Se´ ranne and Se´ guret, 1987; Andersen and Jamtveit, 1990). No Devonian deposits have been reported in Lofoten. It is important to note here, however, that basement exposures in southwest Norway reach as much as 75 to 100 km farther into the hinterland of the orogen than the exposures in Lofoten, relative to the basal Caledonian thrust (fig. 1). If any Caledonian UHP rocks exist at the latitude of Lofoten, they may be farther west and submerged in the continental shelf or removed by later normal faulting. The structural and petrologic nature of the Baltic basement in the WGR and in the Lofoten block also provide clues on the differing tectonic styles that resulted in the above mentioned variations. A long-standing problem connected with rocks in Lofoten is that vast regions of Precambrian basement lack Caledonian deformational fabrics and structures; undeformed rocks occur to within ⬃0.5 km structurally beneath the thrust fault along the base of the main Caledonian allochthon (Tull, 1978; Griffin and others, 1978; Bartley, 1982; Hodges and others, 1982; Van Winkle and others, 1996; Klein, ms; Klein and Steltenpohl, 1999). This observation indicates that deformation decreased structurally downwards during Caledonian orogenesis, in sharp contrast to the classic model of penetrative flow below the brittle-ductile transition proposed by Armstrong and Dick (1974). Rocks of the WGR, on the other hand, were penetratively tectonized at most presently exposed levels during the Caledonian orogeny, with relatively undeformed rocks commonly occurring as lenses or pods within a highly strained matrix. Basement rocks of the WGR are mainly polydeformed, kyanite-zone gneisses with strong gneissic foliations or schistosities and numerous ductile shear zones that locally encapsulate coesite-eclogite lenses (Andersen and Jamtveit, 1990). Associated with these eclogite lenses are granulite-facies basement gneisses that have been retrograded to amphibolite-facies schists and gneisses. The protolith lithologies (for example, Proterozoic granitic rocks) in both southern and northern Norway are similar, however, requiring an explanation to account for these along-strike differences. One possible explanation for the differences in metamorphism and deformational style is the residence time of the Baltic basement in the footwall of the Caledonian A-type subduction zone (Hodges and others, 1982). The response of rocks to changes in pressure is nearly instantaneous whereas temperature adjustments are much slower due to the low thermal diffusivity of silicate minerals. In addition, a depressed geotherm in a subduction zone will also cause rocks to experience temperatures that are lower than normal for a given depth. It follows from this reasoning that the WGR of southwest

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Norway would have resided at depth for a longer period of time than the rocks of the Lofoten block, allowing the temperature to equilibrate with the pressure and causing the rocks to deform in a more uniformly plastic manner. Geochronologic data indicate that the WGR underwent rapid exhumation prior to molasse deposition in the Devonian basins (⬃390 Ma) (Norton, 1986; Fossen and Dunlap, 1998; Dunlap and Fossen, 1998). Ages reported for initial exhumation of the Lofoten block, based on 40Ar/39Ar analysis of hornblende from the Leknes group (Hames and Andresen, 1996b), suggest that the Lofoten block maintained high temperatures (⬎500°C) for a length of time comparable to that of southwest Norway (⬃30 my). Prograde dynamothermal metamorphism of the footwall block had effectively ceased in both areas by 390 Ma. The response of the Lofoten block to subsequent exhumation was different from that of the WGR. Recent thermochronologic data from the WGR of southern Norway suggest a period of relative tectonic and thermal stability during the interval from 380 to 330 Ma (Dunlap and Fossen, 1998); most of the exhumation and cooling through ⬃350°C in the allochthonous nappes (Dunlap and Fossen, 1998, p. 607) took place prior to ⬃390 Ma (Fossen and Dunlap, 1998). In contrast, allochthonous rocks in Lofoten unroofed through metamorphic temperatures in several stages over ⬃150 m.y. (Hames and Andresen, 1996a) (fig. 9). D4 ductile extension on Vestvågøy occurred at ⬃365 Ma and was accompanied by fabric development and metamorphic mineral recrystallization (Klein and Steltenpohl, 1999). A similar D4 event on Røst occured ⬃90 my later, at 275 Ma (Hames and Andresen, 1996a; Holloman, ms; Waltman, ms). Interestingly, ductile shearing and cooling through 350°C on Røst (Hames and Andresen, 1996a) was concurrent with brittle faulting and cooling through ⬃200°C in southwest Norway (Torsvik and others, 1992; Dunlap and Fossen, 1998). Not only was exhumation of the

Fig. 9. Comparison of the thermal history of the Lofoten block (Hames and Andresen, 1996a) with that of southern Norway (adapted from Dunlap and Fossen, 1998). The thermal path of southern Norway was derived using K-feldspar 40Ar/39Ar data, whereas the Lofoten paths were derived using muscovite 40Ar/39Ar data. As a result, overall temperatures are different. It is important to note, however, the discordance in the shapes of the paths, which indicate relative thermal stability for southern Norway between 380 to 330 Ma (Dunlap and Fossen, 1998) and thermal instability of Lofoten in the same interval. Thermal instability in Lofoten is related to discrete stages of tectonic unroofing (Hames and Andresen, 1996a). See text.

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Lofoten block less ‘‘instantaneous’’ than the WGR, but unroofing was also diachronous from northeast to southwest across Lofoten. Existing geothermobarometric data reveal that pressure and temperature differences between the two regions during Caledonian orogenesis may have played important roles in determining later extensional styles. Rocks of the Leknes group in Lofoten experienced temperatures higher than those reported for allochthonous nappes in southwest Norway (550-750°C for Lofoten, 350-450°C for SW Norway) (Hames and others, 1992; Hames and Andresen, 1996a; Dunlap and Fossen, 1998). Basement rocks of the WGR suffered high temperatures (550-750°C, Griffin and Carswell, 1985), whereas the Lofoten basement stayed relatively cool (⬍300°C, Griffin and others, 1978). Rocks from both areas experienced vastly different pressures, as well. Quantitative geobarometry involving compositions in the allochthonous kyanite-bearing migmatites of the Røst District give peak metamorphic pressures of 9.2 kb, corresponding to crustal depths of ⬃30 km (Mooney, ms; Waltman, ms). Estimates of Lofoten basement pressures based on fluid inclusion studies are even lower (Griffin and others, 1978). Numerous published reports, however, indicate that the WGR basement likely reached much higher pressures (⬎16 kb, Andersen and Jamtveit 1990; ⬃17-21 kb, Griffin and Carswell, 1985; crustal depths approaching 100 km for the southern WGR, Smith and Lappin, 1989). Such vast pressure differences likely correspond to subduction depth along a single subduction zone, although the cause for such variability is unresolved. The subduction angle in southern Norway may have been greater than that for northern Norway, allowing the WGR basement to attain greater crustal depths and generate the UHP assemblages. Alternatively, variations in the thickness of the overriding plate may have played a role in subduction depth. Contrasts in deformational style, temperatures, and pressures attained during subduction may have set the stage for subsequent responses to crustal extension and gravitational collapse along the subducted western margin of Baltica. In fact, differences in crustal response to Caledonian contraction may also have been influenced by earlier crustal formation and deformational events. In the long and varied history of the Scandinavian Caledonides, some event produced differences between the Lofoten block and the WGR such that their deformational and thermobarometric paths diverged. The event in question is considered responsible for dewatering the Lofoten basement to such a degree that deformation was only possible in narrow shear zones where fluids were present (Bartley, 1982). This is in sharp contrast to the WGR, where deformation is more pervasive and the extensional shear zones are major, large-displacement features (Dunlap and Fossen, 1998), suggesting that metamorphic fluids had more wholesale effects on the evolution of the WGR of southern Norway. SUMMARY

Recent detailed field mapping in southern Lofoten has led to the recognition of sinistral-oblique-slip, top-west movement along moderately north-northwest dipping shear zones concentrated near basement-allochthon thrusts. There are many structural and kinematic similarities between the Lofoten shear zones and the extensional shear zones in southern Norway. In both regions, ductile extension was accommodated by exploiting earlier fabrics. Furthermore, both areas have well-developed sets of backfolds (Steltenpohl and Bartley, 1988; Osmundsen and Andersen, 1994) and abundant shear sense indicators that document west-directed movement of structurally higher rocks. An important difference is that the shear zones in southern Norway record displacements on the order of several kilometers to many tens of kilometers (Dunlap and Fossen, 1998). Ductile shears in Lofoten, on the other hand, do not juxtapose different lithologies or metamorphic grades and thus are interpreted to have much smaller displacements.

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We suggest that the extensional style of the Lofoten block is different from its southern counterpart only in terms of timing and scale. It is likely that these differences stem from the influence of basement dewatering resulting from along-strike variations in subduction depth during Caledonian orogeny. Devonian extension in Lofoten is interpreted to have initiated during the waning stages of Caledonian orogenesis and continued episodically through middle Carboniferous time at progressively higher crustal levels, based on microstructural analysis of extensional mylonites from Vestvågøy (Klein, ms). Late Carboniferous to Permian 40Ar/39Ar muscovite dates document that the Lofoten block remained at elevated temperatures much later than its southwest Norway counterpart (Hames and Andresen, 1996a). Furthermore, the tectonic and thermal instability of Lofoten during the interval from 380 to 330 Ma is in sharp contrast to the relative stability of the WGR during that time (Dunlap and Fossen, 1998). The deformational history of the two regions became coincident again in the Permian, when brittle faulting in the WGR (Torsvik and others, 1992; Dunlap and Fossen, 1998) was contemporaneous with the onset of brittle crustal extension in Lofoten (Hames and Andresen, 1996a). The mechanism that caused the geologic path of the Lofoten block to diverge from that of the WGR prior to or during the Caledonian orogeny is unresolved, but the most important result of that event was the relative dewatering of the Lofoten basement with respect to the WGR of southern Norway. The paucity of metamorphic fluids in the basement limited Caledonian deformation (Bartley, 1982) and, as a result, post-Caledonian extension in Lofoten was mainly accommodated by reactivating Caledonian thrusts and deforming hydrous basement enclaves. Because these features are limited in lateral extent in Lofoten, the ductile shear zones that exploited them are not crustal-scale detachments like their WGR counterparts. ACKNOWLEDGMENTS

Portions of this work constitute part of the first author’s M.S. thesis at Auburn University. This work was supported by the NSF through grant EAR 9506698 to W. Hames and M. Steltenpohl, a Norwegian Marshall Foundation grant to M. Steltenpohl, and a Geological Society of America Grant-in-Aid to A. Klein. Support for travel to various meetings to present preliminary findings was provided by the College of Science and Mathematics, Auburn University, Auburn, Alabama to A. Klein. Careful reviews by J. Bartley, C.J. Northrup, and an anonymous reviewer greatly improved the content of the manuscript. REFERENCES Andersen, T. B., and Jamtveit, B., 1990, Uplift of deep crust during orogenic extensional collapse: a model based on field studies in the Sogn-Sunnfjord region of western Norway: Tectonics, v. 9, p. 1097–1111. Andersen, T. B., Jamtveit, B., Dewey, J. F., and Swensson, E., 1991, Subduction and education of continental crust: Major mechanisms during continent-continent collision and orogenic extensional collapse, a model based on the south Norwegian Caledonides: Terra Nova, v. 3, p. 303–310. Andresen, A., and Forslund, T., 1987, Post-Caledonian brittle faults in Troms: geometry, age and tectonic significance: The Caledonian and Related Geology of Scandinavia (Cardiff, Sept. 22-23, 1989) (Conference abstract). Andresen, A., and Tull, J.F., 1986, Age and tectonic setting of the Tysfjord gneiss granite, Efjord, north Norway: Norsk Geologisk Tidsskrift, v. 66, p. 69–80. Armstrong, R. L., and Dick, H. J. B., 1974, A model for the development of thin overthrust sheets of crystalline rock: Geology, v. 3, p. 35–40. Bartley, J. M., ms, 1980, Structural geology, metamorphism, and Rb/Sr geochronology of east Hinnøy, north Norway: Ph.D. thesis, Massachusetts Institute of Technology, Cambridge, Massachusetts, 263 p. ————— 1981, Field relations, metamorphism, and age of the Middagstind quartz syenite: Norsk Geologisk Tidsskrift, v. 61, p. 237–248. ————— 1982, Limited basement involvement in Caledonian deformation, Hinnøy, north Norway, and tectonic implications: Tectonophysics, v. 83, p. 185–203. Brun, J.-P., and Choukroune, p., 1983, Normal faulting, block tilting and de´ collement in a stretched crust: Tectonics, v. 2, p. 345–356. Bryhni, I., and Sturt, B. A., 1985, Caledonides of southwestern Norway, In Gee, D. G. and Sturt, B. A., editors, The Caledonide Orogen—Scandinavia and related areas: Chichester, Wiley & Sons, p. 89–107.

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