Retrograded Eclogite-Facies Pseudotachylytes As Deep-Crustal Paleoseismic Faults Within Continental Basement of Lofoten, North Norway

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Retrograded eclogite-facies pseudotachylytes as deep-crustal paleoseismic faults within continental basement of Lofoten, north Norway

Mark G. Steltenpohl Gabriel Kassos Department of Geology and Geography, Auburn University, Auburn, Alabama 36849, USA Arild Andresen Department of Geosciences, University of Oslo, Box 1047, Blindern, Oslo 3, Norway

ABSTRACT bole, and dolomite occur as inclusions in gar- consumed, affording the potential for exhuma- net. The Flakstadøy pseudotachylytes indi- tion and exposure of the actual rock products of Field observations and electron micro- cate that the rocks exposed in Lofoten were deep-crustal seismic faulting (e.g., the Himala- probe analyses indicate that pseudotachy- rigid and resilient parts of the lower crust of yas; Kayal et al., 1993; Jackson et al., 2004). lytes discovered on the Lofoten island of an ancient continent from ca. 1.8 Ga until the Thus, eclogitized continental basement exposed Flakstadøy, north Norway, represent rare Middle Ordovician. Subduction to deeper- in the cores of ancient eroded mountain belts are examples of deep-crustal paleoseismic crustal levels (depths >~45 km) caused the fertile grounds for exploring such rock products. faults. The pseudotachylyte occurrences are stiff, nonreacted granulite to accommodate Vast exposures of high- and ultrahigh-pressure restricted to the margins of eclogite-facies aseismic, steady-state fl ow in fl uid-mediated, eclogitized continental basement of the West- shear zones that sharply cut pristine granu- eclogite shear zones by concomitant, brittle, ern Gneiss Region of western Norway make it lite-facies continental basement rocks. Gen- seismogenic failure and pseudotachylyte a prime area in which to explore for deep-focus erally, pseudotachylyte veins are sharply formation. Later in the Middle Ordovician, paleoseismic faults. The island of Holsenøy truncated by the eclogite shears, but some these deep-crustal rocks were exhumed to in the Bergen Arcs (Austrheim and Boundy, have been sheared and folded into them, middle-crustal levels, where they were retro- 1994) and Ålesund (Lund and Austrheim, 2003) documenting prekinematic to synkinematic graded under amphibolite-facies conditions. contain, to the best of our knowledge, Earth’s injection. Textures preserved in the pseudo- Our results help to explain how deep-crustal only known examples of high-pressure pseudotachylyte matrix document crystallization earthquakes form in modern continent-con- tachylytes (Fig. 1). directly from the frictional melt; for exam- tinent collisional zones like the Himalayas. We report the discovery of pseudotachy- ple, dendritic garnets, similar in appearance, lyte veins associated with eclogite-facies shear size, and composition to those from eclogite Keywords: eclogites, Caledonian, deep crust, zones within continental basement in the Lofo- pseudotachylytes of the Bergen Arcs and Åle- paleoseismicity, Lofoten, Norway. ten archipelago, north Norway (Fig. 1), which sund (Austrheim and Boundy, 1994; Lund also appear to be deep-crustal paleoseismic and Austrheim, 2003), refl ect rapid (likely INTRODUCTION faults. Pseudotachylytes are largely accepted as in terms of tens of seconds) crystallization, frictional melts derived from coseismic fault- and distinct fi ning of grains toward the mar- One of the more visual and elucidating tenets ing and are the only known recorders of the gins of the pseudotachylyte veins indicates of plate-tectonic theory is the lateral and vertical process preserved in exhumed rocks (Shand, quenching textures. Electron microprobe progression of earthquake foci and their correla- 1916; Philpotts, 1964). The particular group of analysis and backscattered-electron imaging tion to depth along subduction zones (Wilson, pseudotachylytes described herein is restricted document that the pseudotachylyte matrix 1963). Deep-foci earthquakes (between 300 only to the immediate shoulders of eclogite-

is composed of microlites of garnet (Gr25–30, and 680 km deep) require deep subduction of facies shear zones, a ﬁ eld relation that in itself

Py15–19, and Al54–58), orthopyroxene (En61–64), oceanic lithosphere (e.g., the Philippines; Huru- seems to require a cogenetic, deep-crustal origin

low-Na clinopyroxene (Jd6), amphibole (fer- kawa and Imoto, 1993). Since the ultimate fate (Steltenpohl et al., 2003; Kassos et al., 2003, roan pargasite), with or without K-feldspar, of the deeply subducted oceanic lithosphere is 2004). We present petrographic observations, quartz, biotite, various Fe opaques and Fe- its consumption and recycling within the man- electron microprobe analyses, and backscat- Ti opaques, kyanite, dolomite, and calcite. tle, no surface exposures containing evidence of tered-electron images (BSEs) documenting that The cogenetic eclogite-facies shear zones and deep-focus paleoseismic faulting of this material original eclogite-facies assemblages and tex- pseudotachylytes were variably retrograded are known. Hence, our understanding of such tures were quenched within the pseudotachylyte during Caledonian amphibolite-facies meta- phenomena is fragmentary and based on infer- matrix prior to being strongly retrograded under morphism. Omphacite is replaced by clusters ences from indirect geophysical, mostly seis- amphibolite-facies conditions. Available timing

or symplectites of low-Na clinopyroxene (Jd6) mic, information. On the other hand, subducted constraints suggest that the eclogite-facies

and oligoclase/andesine (An20–36); kyanite, continental crust, being less dense and conse- pseudotachylytes and shear zones, and their orthopyroxene, Na-Ca clinopyroxene, amphi- quently more buoyant, is less likely to be wholly subsequent amphibolite-facies retrogression,

Geosphere; February 2006; v. 2; no. 1; p. 61–72; doi: 10.1130/GES00035.1; 8 ﬁ gures, 6 tables.

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(Griffi n et al., 1978). Structurally isolated bodies Early Carboniferous Paleogeographic Reconstruction of amphibolite-facies metasedimentary rocks, Highly allochthonous the Leknes Group (Fig. 3), are interpreted as Post-Devonian rocks early Caledonian (Ordovician-Silurian) klippen preserved in down-folded and faulted structures Baltic cover (Tull, 1977; Corfu, 2004; Steltenpohl et al., Precambrian basement 2004). In Carboniferous plate reconstructions, east Greenland is welded to the Norwegian mar- Ice gin, and Lofoten clearly occupied the most internal tectonic position within the northern parts of this restored orogen (Fig. 1). Surprisingly, however, little Caledonian imprint is preserved in the area of Lofoten basement rocks. Caledonian structures Figure 2 and fabrics along the base of the cover allochthons gradually disappear structurally downward into the basement over a distance of ~250 m (Tull, 1977), leaving earlier workers to suggest that Lofoten had “completely escaped Caledo- nian metamorphism and deformation” (Griffi n et al., 1978). Two hypotheses have been suggested Alesund to explain these observations. First, the Caledo- nian allochthons passed over Lofoten, and the Western Gneiss downward disappearance of Caledonian fabrics Region and structures may be attributed to the limited availability of fl uids in the anhydrous, granulite- Bergen Arcs facies basement (Bartley, 1982; Steltenpohl et den al., 2004). Second, Lofoten might be a beached

Norway Swe 0 400 km microcontinent (Tull, 1977; Corfu, 2004). Most workers favor the fi rst interpretation (see Hodges Eclogite locality et al., 1982, and Steltenpohl et al., 2004). How- ever, our present understanding of the timing and Figure 1. Early Carboniferous reconstruction of the northern Caledonides (after Roberts structural evolution of Lofoten and its contact and Gee, 1985; Ziegler, 1988) illustrating Lofoten’s internal position within the orogen, with Baltic crust (that is, the Gullesfjorden shear eclogite localities (see text for references), and place names used in text. zone and/or the Austerfjord thrust in Fig. 2) is fragmentary, leaving the problem unresolved (see Hakkinen, 1977; Tull, 1977; Corfu, 2004). Rare eclogite-facies shear zones that sharply cut the granulite-facies gneisses occur on the occurred during the early stages of the Caledo- external Rombak window, to an upper amphibo- islands of Austvagøy and Flakstadøy (Figs. 2 nian orogeny (Steltenpohl et al., 2003; Kassos lite–facies mylonite zone against the Western and 3) and appear to be expressions of early et al., 2003, 2004). Our fi ndings, thus, bear on Gneiss terrane, across the amphibolite-granulite Caledonian deformation (Steltenpohl et al., how earthquakes, like those beneath the active isograd (i.e., the Conrad discontinuity; Olesen et 2003; Kassos et al., 2004; Rehnström et al., Himalayas, are generated in the deep levels of al., 1991), and fi nally into eclogite-facies rocks 2005). Kullerud (1992, 1996) and Markl and the continental crust, where high temperatures in the westernmost Lofoten Islands (Fig. 2). The Bucher (1997) performed detailed petrologic and pressures prevail and plastic, aseismic fail- eastern half of this transect is well characterized and mineral chemical studies on the Lofoten ure might be expected. and accepted as the middle- to upper-crustal, eclogites, and reported that they are variably Caledonian (Silurian), continental-continent to completely replaced by amphibolite-facies GEOLOGIC SETTING subduction zone boundary, where western Bal- assemblages. The same authors surmised that tica was partially subducted beneath Laurentia the eclogites had formed due to fl uids having A transect across the Caledonian orogen (Hodges et al., 1982; Tull et al., 1985). The accessed fractures into the anhydrous, granu- along latitude 68.5°N, Norway (A–A′ in Fig. 2), Lofoten block, on the other hand, is a long-held lite-facies basement units. This unusual style exposes subequal proportions of Baltic Pre- enigmatic terrane with uncertain relations to of occurrence is remarkably similar to eclogites cambrian continental basement and its cover, both Baltica and the Caledonides (Griffi n et al., found in the Bergen Arcs (Fig. 1; Austrheim, and may represent a continuously exposed 1978; Tull, 1977; Tull et al., 1985; Olesen et al., 1987; Austrheim and Griffi n, 1985; Boundy et column through nearly the entire Caledonian 1997; Klein and Steltenpohl, 1999). al., 1992). Shear-zone eclogites in the Bergen lithosphere. This transect progressively traces Lofoten is composed of Archean (2.7 Ga) Arcs and Lofoten thus are important evidence the basement-cover contact, from east to west, rocks migmatized at ca. 2.3 Ga, supracrustals of fl uid fl ow in the deep crust (Austrheim, from the unmetamorphosed and undeformed deposited ca. 2.1 Ga, and extensive mangeritic 1987; Austrheim and Griffi n, 1985; Boundy nonconformity in the Swedish foreland, to a and charnockitic plutons emplaced under granu- et al., 1992; Kullerud, 1996, 2000; Markl and greenschist-facies mylonite zone framing the lite-facies conditions between 1.8 and 1.7 Ga Bucher, 1998; Markl et al., 1997, 1998a, 1998b;

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Amph 18oE Granulite

Allochthonous Caledonian cover ibolite Baltic basement 14oE Lofoten basement e Eclogite locality 69oN H A' Lofoten-Ofoten transect A-A' AT ord Figure 2. Geologic relations Ofotfj Rombak along the Ofoten-Lofoten tran- GSZ window sect (after Tull et al., 1985). See area of Baltic Figure 1 for location of the map Figure 3 e foreland area. The upper cross section e Austvagøy e e shows general geologic features e Flakstadøy along the transect, whereas the Lofoten Islands 68oN lower one depicts metamorphic conditions within the basement Vaeroy complex. Abbreviations are: um—unmetamorphosed; chl— Rost 0 100 km chlorite zone; bt—biotite zone; gt—garnet zone; ky—kyanite A Western A' A Gneiss zone; gran—granulite facies. Baltic Flakstadøy terrane Ofoten Rombak window SL—sea level. See text for Vaeroy GSZ Foreland Rost Leknes Vestvagøy AT synform details. GSZ—Gullesfjorden SL shear zone; AT—Austerfjord ? ? thrust. ?

Caledonian allochthons - variable metamorphic grade

e e e e e gran um e e ky gt bt chl Basement metamorphic conditions

Bjørnerud et al., 2002). The Lofoten eclogites to perform structural investigations (i.e., geo- even within individual outcrops. Nonfoliated further resemble those of the Bergen Arcs in that: metric, kinematic, and microstructural) on the granulitic host rock commonly is progressively (1) they formed prior to retrograde amphibolite- Lofoten eclogite shear zones (Kassos et al., foliated toward the margins of the shear zones facies metamorphism before ca. 433 Ma and, 2003, 2004, 2005; Mager et al., 2004). It was (Fig. 4), and this foliation commonly is asymp- thus, appear to be early Caledonian (Mørk et al., during our fi eld investigations of the shear zones totically swept into them. Displacements along 1988; Steltenpohl et al., 2003) rather than Scan- that we stumbled upon the associated pseudo- individual shear zones, based on displaced dian (425–400 Ma) eclogites like those of the tachylyte veins that are the focus of the present markers and the observation that both termina- classic Western Gneiss Region (Fig. 1; Griffi n report. The structural evolution of the eclogite tions of some shear zones occur within a single and Brueckner, 1980; Hacker et al., 2003; Terry shear zones is complex and beyond the scope outcrop, typically are small, ranging from neg- and Robinson, 2003); (2) both cut Archean-Pro- of the current report. To summarize pertinent ligible to a few centimeters. An “eclogitization terozoic orthogneisses and associated granulite- observations, the eclogites are concentrated on front” may extend for tens of centimeters out- facies gabbroic and anorthositic rocks; and (3) the island of Flakstadøy (Fig. 3), where they side the shear-zone margins (Fig. 4). The shear pressures of eclogitization in these two areas are occur in relatively small, localized areas that zones commonly branch and merge or crosscut much less (Lofoten ~1.4–1.5 gPa; Bergen Arcs range from 40 m2 (Nusfjord) to 1.6 km2 (Ska- one another (Fig. 4). ~1.7 gPa) than the ≤4 gPa estimated for the min- gen). Individual eclogite occurrences do not At the time this report was written, the tim- imum pressures of the Western Gneiss Region connect with one another, and there does not ing of eclogite-facies shear-zone formation (see Austrheim, 1987; Markl and Bucher, 1997; appear to be any particular tectonostratigraphic in Lofoten was only loosely constrained but Steltenpohl et al., 2003). Compared to the Ber- level or zone to which they belong. The shear likely resulted from early Caledonian (Middle gen Arcs, however, the Lofoten eclogites were zones may occur individually, ranging in thick- Ordovician–Early Silurian) orogenesis. Corfu much more intensely retrograded during Cale- ness from a millimeter to <10 m, or as anas- (2004) reported U-Pb mineral dates on zircon donian amphibolite-facies metamorphism, leav- tomosing networks up to 100 m in aggregate and titanite from meta-igneous rocks within ing us with a relatively fragmented understand- thickness. Strikes are highly variable, encom- the Leknes Group (Figs. 2 and 3) that are inter- ing of eclogitization and eclogite shear-zone passing almost all directions, and dips range preted to bracket the time of the amphibolite- development in Lofoten. from vertical to subhorizontal. Most are simple facies event between 461 and 469 Ma. We inter- Despite their obvious signifi cance for defor- shear zones with clear kinematic indicators (e.g., pret this to be the same amphibolite-facies event mation in the deep-continental crust, to our S-C fabrics and asymmetric porphyroclasts) that that retrograded the Lofoten eclogites. This is knowledge, we and our co-workers are the fi rst display highly variable movement directions, consistent with a ca. 433 Ma 40Ar/39Ar cooling

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Myrland

U D Napp

Skagen

Vestvågøy

Storvatnet N

Nusfjord Figure 4. Termination of an eclogite shear zone (diagonally from upper right to lower 0 5 km left) with central pseudotachylyte vein (hand lens is 2.5 cm in length). Gabbronorite is mangerite & leucogabbro Leknes Group charnockite & -norite massive and nonfoliated 5 cm from the paragneisses troctolite eclogite locality center of the shear zone. The teal discolor- orthogneisses anorthosite retro-eclogite locality ation of the gabbronorite corresponds to the leucogneisses gabbro Quaternary “eclogitization front.” A thin shear passes amphibolite suspect eclogite locality deposits from left-lower center to just below the hand lens. At the upper right, the pseudotachylyte Figure 3. Geologic map of Flakstadøy and part of adjacent Vestvågøy illustrating eclogite displays a chilled margin (grayish-tan cen- shear-zone localities (modiﬁ ed from Markl and Bucher, 1997; Klein et al., 1999). ter and dark gray toward margins).

date on hornblende separated from a sample of tabular veins along the shoulders of <2-m-thick (Figs. 5A, 5B, and 6). Inclusions of gabbronorite the retrograded eclogite at Nusfjord (Steltenpohl eclogite shear zones that cut gabbronorite of the host rock are common in the veins (Figs. 5B and et al., 2003). Kassos et al. (2004) reported a 478 basement complex (Figs. 4 and 5A–C). Gab- 6). Where we were able to observe their inter- ± 41 Ma lower-intercept age from U-Pb analy- bronorite host rock is composed of plagioclase action, most pseudotachylyte veins are abruptly

sis of zircons separated from a pre-eclogite- (An50–65), orthopyroxene, clinopyroxene (sub- truncated by the eclogite shears (Fig. 5A). facies felsic dike from the eclogite shear zone at calcic augite), magnetite, ilmenite, and apatite, Importantly, there is no corresponding “other the Myrland locality (Fig. 3). This date carries a with grain size ranging from 0.5 to 1 cm (Kul- half” of the vein on the opposite block, even large error but is compatible with eclogitization lerud, 1992, 1996). Although the mineralogy where displacement along the shear zone is just before amphibolite-facies metamorphism of of the pseudotachylyte matrix, described in the demonstrably negligible. This latter observa- the Leknes Group at ca. 469 Ma. following, contrasts with that of its granulite- tion seemingly requires a cogenetic relation facies host rock, the mineral chemistries and since the veins clearly sourced or fed from the FIELD RELATIONS OF their estimated volume percentages are consis- shears and did not simply behave as passive PSEUDOTACHYLYTES tent with the two rock types being of essentially markers that were crosscut by the shears. Many the same chemical composition. Combined with pseudotachylyte veins parallel the boundaries of Pseudotachylyte veins are associated with their aphanitic character, clear fi eld association the eclogite shear zones (Fig. 4). Thin (<10 cm some eclogite-facies shear zones at the Nusfjord restricted to the margins of shear zones, and thick), small-displacement (<10 cm) shear and Skagen localities on Flakstadøy (Fig. 3). lack of fi eld evidence to the contrary, the veins zones may have pseudotachylyte veins in their The pseudotachylytes do not occur outside of clearly are pseudotachylytes derived from fric- centers (Fig. 4), clearly indicating that they had the immediate area along the contacts of the tional melting of the gabbronorite. nucleated along them. Rarely, pseudotachylyte eclogite shear zones. We only fi nd pseudotachy- The pseudotachylytes are dense, dark green- veins have been sheared, folded, and dragged lytes where there are eclogite shear zones, but, ish gray to black, microcrystalline rocks that into the eclogite shears (Figs. 6 and 7). These conversely, most of the eclogite shear zones do generally occur as thicker (<3 cm) tabular veins sheared pseudotachylytes were only observed in not have associated pseudotachylytes. The pseu- with smaller, thinner (only millimeters thick) the marginal areas of some thicker (3–4 m thick) dotachylytes occur as thin (≤3 cm thick), mostly wedge-shaped veins branching off of them eclogite shears. The veins progressively lose

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A B

C D

1 cm 100 µm

E F

500 µm 500 µm

Figure 5. Collage of fi eld and petrographic images of the Flakstadøy pseudotachylytes. A: Field photo of tabular pseudotachylyte veins (black, below fi ngertip) within gabbronorite abruptly truncated by a thin eclogite-facies shear zone (margin parallels mechanical pencil). B: Close-up view of pseudotachylyte veinlets with wedge-shaped terminations branching out from a thicker, more tabular vein (trending almost vertically in the upper left-hand side of photo). Note numerous brecciated fragments of host gabbronorite in the veins and veinlets. Coin is 2.5 cm in diameter. C: Photograph of thin section of pseudotachylyte margin (tilted toward the right); gabbronorite (light) above and pseudotachylyte (dark) below. Light-colored, <3 mm masses in the pseudotachylyte are wall-rock fragments. Faintly lighter-colored zone (~1.5 mm thick) within the pseudotachylyte paralleling the margin refl ects fi ning of matrix minerals toward the contact. D: Close-up view illustrating minerals and textures that typify the pseudotachylyte matrix of sample NFA-20. Large, inclusion-fi lled, embayed garnet (left center, light gray) contrasts with small, euhedral type at bottom left. E: Dendritic and caulifl ower-shaped garnets (lightest gray) in sample NFA-18. Note the linear traces that intersect center left. Bright spots concentrated in the centers of the linear dendrites are Fe opaques. F: Elliptical clusters (darker ellipses roughly 100 µm wide) of mixed plagioclase and low-Na clinopyroxene are interpreted as replaced omphacite grains.

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defi nition toward the more highly strained cen- MINERALOGY OF PSEUDOTACHYLYTES The matrix of the pseudotachylyte is an ultra- ters of the shear zone. Figure 6 illustrates a spec- fi ne-grained mosaic of (in decreasing volumetric tacular example where pseudotachylyte veinlets Microlites within the pseudotachylyte matrix abundance based on visual estimations) plagio- branching off of a thicker vein, which parallels are generally too fi ne grained to confi dently clase, amphibole, garnet, orthopyroxene, clinopy- the shear zone (in the C-plane orientation), have resolve using an ordinary petrographic micro- roxene, Fe oxides and Fe-Ti oxides, quartz, biotite, been only slightly sheared into parallelism with scope. Microprobe analyses (Tables 1–6), there- pyrite, kyanite, and calcite. There is a distinct fi ning the S-plane orientation of the shear-zone system. fore, were performed using facilities at the Uni- of grains from the vein center, where grains aver- Taken together, fi eld relations clearly indicate versity of Alabama. We probed three different age ~10 µm, toward the contact with the wall rock that the pseudotachylytes and the eclogite shear thin sections from three separate pseudo tachylyte (~7 µm), which probably refl ects chilling along zones formed cogenetically, and that the former veins at the Nusfjord locality (NFA-11, NFA- the margin (Figs. 4 and 5C). Wall-rock fragments slightly predated or temporally overlapped with 18, and NFA-20). All three samples show high within the matrix (Fig. 5C) generally are angular development of the latter. degrees of amphibolite-facies retrogression. and fl attened and range from 3 mm to 10 µm.

A B shear-zone eclogite

granulite Figure 6. Cogenetic sheared and unsheared pseudotachylyte. A: Photo of margin to a left-slip eclogite shear zone. The thick, tabular vein (horizontal, lower center) parallels the shear zone boundary. Note the single, small veinlet branching from the lower margin of the thick tabular vein (lower-left center of photo, vertically beneath the eraser on the pencil) into undeformed granulite. B: Features in A highlighted. Green dashed line is main boundary between sheared eclogitized granulite (above) and pristine granulite host (below). Red dotted traces dipping toward right mimic the S-plane orientation of the shear-zone system, whereas those that are horizontal appear to mark C-planes. Note that two minor, narrow (<0.5 cm), left-slip shears cut the granulite directly beneath the main shear-zone boundary to which they parallel. Note, also, the brecciated wall-rock fragments in the pseudotachylyte veins and veinlets.

A B granulite

shear-zone eclogite

Figure 7. Sheared and folded pseudotachylyte. A: Photo of margin to eclogite shear zone. B: Features in A highlighted. Green dashed line is boundary between sheared and eclogitized granulite (below) and pristine granulite host (above). Red dotted lines trace segments of pseudotachylyte veins and veinlets.

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Garnets of the pseudotachylyte matrix occur in three habits: smaller (averaging 10 μm), euhedral ones; larger (up to 100 μm), severely embayed to near-sieve-textured ones; and dendrite- and cauliﬂ ower-shaped ones (Figs. 5D 0.01 and 5E). Embayed garnets have sieve-like textures with bleb-shaped inclusions that include low-Na clinopyroxene, amphibole, kyanite, quartz, ilmenite (Fe-Ti oxide), and calcite. The smaller garnets have fewer inclusions than the larger ones. Dendritic garnets may be hundreds of microns in length and commonly follow linear traces (Fig. 5E). One dendrite trace was observed to terminate at a high angle upon inter- secting another trace (Fig. 5E). Other dendrites are more bulbous with cauliﬂ ower shapes. Mea-

sured garnet compositional ranges are Gr25–30, Py , and Al , but this represents only nine 15–19 54–58 000000 spot analyses. Garnet generally is not present in the gabbronorite host rock except for centimeter-thick zones that parallel the margin of the pseudotachylyte vein (Fig. 5C). In these zones, garnet, and associated biotite, clinopyroxene, and amphibole, occur only where hypersthene 0.07 0.07 0 0 0.04 0.02 0 0.1 grains are cut by the vein. As is characteristic of the eclogite-facies pseudotachylytes of the Ber- gen Arcs (Austrheim and Boundy, 1994), ﬂ uids attending eclogitization do not appear to have

penetrated more than a centimeter or two into 0 0 0 0.01 the dry granulite host. Plagioclase grains typically range from 10 to

30 µm and show no preferred size or shape. As 0000000000 with other minerals, grain boundaries are often an irregular, polygonal shape, but rounded edges are present as well. Quartz inclusions are common and typically are angular instead of the rounded “bleb” shape of other inclusions. Plagioclase in

wall-rock fragments is mostly labradorite (An50–

65), whereas plagioclase in the pseudotachylyte

matrix is oligoclase/andesine (An20–36). Clinopyroxene ranges in size from ~1 to 10 µm and has no preferred shape. It usually is found in contact with plagioclase, amphibole, orthopyroxene, garnet, and opaque grains. Distinct elliptical clusters (roughly 100 µm wide) of mixed plagio-

clase and low-Na clinopyroxene are interpreted PSEUDOTACHYLYTES FROM OF CLINOPYROXENE ANALYSES MICROPROBE REPRESENTATIVE 1. TABLE as replaced omphacite grains (Fig. 5F). Composi- tions of the clinopyroxenes are mainly diopsidic

ranging upward with Na content to Jd6. Amphibole of the matrix ranges in size from

5 to 15 µm and has grain shapes that vary over 0000000000 a wide range. Many grains show cleavage intersections at roughly 60° and 120°, have an 1.697.35 0.130.04 4.16 0.07 7.73 0.37 8.14 0.27 2.75 0.44 4.68 0.21 6.13 0.18 3.23 0.21 14.930.12 0.19 2.99 0.040.22 0.23 2.84 0 0.68 0.26 5.07 0.62 1.43 0 2.05 0.67 0.16 2.69 0.18 0.06 0.22 3.17 0.61 0.11 0.11 1.94 0.69 0 0.19 2.73 0.28 0.38 0.08 3.06 0.17 0.24 0 2.97 0.2 0 0.14 0.19 0.05 0.2 0.08 0.19 0.05 0.19 0.08 0.21 0.06 0.19 0.06 0.19 0.08 0.1 0.1 0.14

elongate shape, and are anhedral. Inclusions of 47.45 51.16 53.06 50.77 52.81 49.4 52.4 50.6 54.62 52.63 52.81 50.1 51.95 52.22 52.53 53.66 52.88 51.14 52.32 amphibole in other minerals display no preferred NFA-11 NFA-11 NFA-11 NFA-18 NFA-18 NFA-18 NFA-18 NFA-18 NFA-18 NFA-18 NFA-18 NFA-18 NFA-18 NFA-20 NFA-20 NFA-20 NFA-20 NFA-20 NFA-20 3 3 2 2 shape. Amphibole occurs in contact with all O 0.73 0.01 0.51 0.83 0.72 0.18 0.65 0.79 2.8 0.72 0.76 0.55 0.62 0.93 1.24 0.88 1.09 0.96 1.23 O 2 O 3+ 2+ 2 O 0.02 0 0.09 0.41 0.01 0.45 0.15 0.16 0.14 0 0 0.01 0.17 0 0 0.06 0.02 0.02 0 2 2 Weight % oxide Weight SiO K Total 6 oxygens) unit (4 cations, Atoms per formula 100.54 101.27Si 102AlTi 102.75Fe 1.76 101.17 0.32 100.53 1.89 0.05 102.71 0.18 98.83 1.95 0 102.99 0.33 1.86 99.45 0.35 100.29 0 1.93 99.5 0.12 1.84 100.61 0.01 0.21 100.34 1.92 100.31 0.01 0.26 101.68 1.91 0.01 101.44 0.14 1.94 99.56 0.01 0.63 101.74 1.95 0.01 0.13 1.94 0.01 0.12 1.86 0.01 0.22 1.92 0.01 0.09 1.93 0.01 0.12 1.94 0.04 0.14 1.96 0 0.08 1.93 0.12 0.01 1.91 0.13 0 1.9 0.13 0.01 0.01 0.01 TiO Al Cr MgFe Mn 0.69 1.16 0.01 0.93 0.02 0.88 0.02 0.76 0.02 1.06 0.01 0.97 0.02 0.71 0.02 0.42 0.01 0.75 0.79 0.77 0.78 0.72 0.7 0.73 0.71 0.71 0.72 Sum4444444444444444444 other minerals present. Grain boundaries are FeOMnOMgO 10.78CaO 0.28 23.46Na 12.42 20.14 19.77 0.65 21.18 22.04 16.96 0.51 0.6 7.85 16.06 2.84 0.77 23.29 13.94 3.35 22.69 18.98 0.23 22.59 11.4 17.71 0.69 12.55 2.42 5.61 0.65 7.96 2.11Ca 6.74 0.24 13.63 19.67Na 6.19K 14.37 0.13 16.59 0.79 13.82 22.5 8.78 0.05 0.05 14.24 0.02 23.03 8.08 0 0.05 0 13.1 20.85 0.11 8.73 0 12.72 21.87 0.13 0 0.04 8.11 13.5 22.45 0.88 0.12 0.06 22.26 8.7 13.04 0 0.1 22.68 0.05 0.05 12.76 9 22.43 13.24 0.06 0.01 0.02 0.08 21.95 0.11 9.19 0.05 0 0.8 23.26 0.04 8.16 0.06 0.63 0.02 0.1 0.19 0.89 0.01 0.05 0.23 0.9 0.01 0.05 0.83 0.01 0.04 0.86 0.04 0.89 0.07 0.88 0.09 0.89 0.06 0.88 0.08 0.88 0.07 0.91 0.09 usually straight or are a series of short, straight segments deﬁ ning a curve, but curved segments are also observed. Minerals were identiﬁ ed as

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TABLE 2. REPRESENTATIVE MICROPROBE ANALYSES OF CLINOPYROXENE amphibole based on shape, BSE color intensity, FROM SHEAR ZONES and presence of K. Amphibole compositions are SK-1 SK-1 SK-1 SK-1 SK-1 SK-1 SK-1 SK-1 SK-5 mostly ferroan pargasite. Fe oxides and Fe-Ti oxides occur as small Weight % oxide (<10 μm), rounded grains in the groundmass, as SiO2 53.92 54.86 54.06 53.73 54.55 53.09 54.50 53.80 50.64 TiO 0.14 0.14 0.04 0.03 0.09 0.07 0.08 0.03 0.63 inclusions in many phases, and also as veinlets. 2 Typically, they stand out as minute bright dots in Al2O3 9.48 9.90 2.27 1.86 3.64 4.10 1.37 2.89 8.27

Cr2O3 0.00 0.00 0.00 0.00 0.15 0.09 0.00 0.00 0.00 the BSE images (Figs. 5E and 5F). Some grains FeO 5.31 5.63 6.19 6.29 7.39 7.75 11.92 6.21 4.84 display halves of varying Fe and Ti percentages. MnO 0.08 0.00 0.13 0.08 0.00 0.11 0.03 0.03 0.09 Hematite occurs in thin (<3 μm wide), <50-μm- MgO 10.08 9.60 14.84 14.92 21.16 21.06 16.61 14.33 15.09 long veinlets cutting only plagioclase grains. CaO 16.64 15.30 22.53 23.14 10.09 10.46 13.16 16.82 18.48 Na O 5.43 5.56 1.54 1.01 0.81 0.88 0.23 1.57 2.08 The veinlets do not appear to follow any crys- 2 tallographic anisotropies (e.g., cleavage) within K2O 0.01 0.00 0.02 0.00 0.12 0.10 0.06 0.00 0.60 Total 101.09 100.99 101.62 101.07 98.00 97.70 97.96 95.67 100.71 the plagioclase grains. Atoms per formula unit (4 cations, 6 oxygens) Quartz usually is ~5 µm and has no pre- Si 1.91 1.94 1.94 1.95 2.00 1.95 2.07 2.05 1.81 ferred shape. It appears to ﬁ ll voids left by Al 0.40 0.41 0.10 0.08 0.16 0.18 0.06 0.13 0.35 other minerals. Ti 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.02 Biotite is rare and usually occurs in aggregates Fe3+ 0.16 0.07 0.13 0.09 0.00 0.00 0.00 0.00 0.14 of 2–4 grains and in association with amphibole Mg 0.53 0.51 0.79 0.81 1.15 1.15 0.94 0.81 0.80 and plagioclase. Grains range from 10 to 15 µm Fe2+ 0.00 0.10 0.06 0.10 0.23 0.24 0.38 0.20 0.00 in length and have no preferred orientation. Mn 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Ca 0.63 0.58 0.87 0.90 0.40 0.41 0.53 0.69 0.71 Orthopyroxene ranges in size from ~5 to Na 0.37 0.38 0.11 0.07 0.06 0.06 0.02 0.12 0.14 15 µm and has no preferred shape. It usually K 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.03 is found incompletely bounded by amphibole Sum 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 (1/2–2/3 of grain boundary), but not as a core with a typical bull’s-eye pattern of retrogression. These are also usually found in association with plagioclase. Compositions of the orthopyrox-

enes range from En61 to En64. The presence of amphibole and very minor amounts of biotite suggests only minor amounts of ﬂ uids during pseudotachylyte formation and/ TABLE 3. REPRESENTATIVE MICROPROBE ANALYSES OF GARNET or amphibolite-facies retrogression. It is note- FROM PSEUDOTACHYLYTES worthy that hydrated minerals are not present NFA-20 NFA-20 NFA-20 NFA-20 NFA-20 NFA-20 NFA-20 NFA-20 NFA-20 within the granulite-facies gabbronorite, only Weight % oxide several centimeters outside of the margins of

SiO2 38.09 38.130 38.170 38.150 38.310 38.560 38.120 37.910 38.280 the eclogite shear zones and pseudotachylytes, TiO 0.10 0.046 0.068 0.102 0.088 0.102 0.044 0.054 0.120 2 which is consistent with hydrous eclogitization Al O 22.12 21.990 21.950 22.380 22.300 21.740 22.070 22.140 21.980 2 3 of the metastable granulites. Cr2O3 0.00 0.000 0.010 0.025 0.002 0.000 0.010 0.045 0.000 FeO 26.34 26.510 26.780 26.770 26.900 25.730 26.880 26.150 25.300 MnO 0.73 1.018 0.911 0.819 0.898 0.893 0.859 1.088 0.810 INTERPRETATION MgO 4.70 4.000 3.930 4.300 4.340 3.880 3.990 4.080 3.900 CaO 8.90 9.190 9.690 8.840 8.870 9.810 9.020 9.230 10.520 The mineralogy and textures preserved in Na O 0.00 0.000 0.004 0.000 0.000 0.008 0.000 0.000 0.009 2 the pseudotachylytes indicate that the primary K O 0.00 0.044 0.026 0.028 0.008 0.025 0.006 0.018 0.008 2 assemblage that had crystallized directly from Total 100.99 100.930 101.540 101.420 101.710 100.760 101.000 100.710 100.930 the frictional melt was strongly modiﬁ ed by Atoms per formula unit (8 cations, 12 oxygens) amphibolite-facies retrogression. This should Si 5.892 5.924 5.898 5.891 5.899 5.997 5.922 5.897 5.932 be expected since ﬁ eld relations indicate that Al 4.033 4.027 3.997 4.073 4.047 3.985 4.041 4.059 4.015 Ti 0.012 0.005 0.008 0.012 0.010 0.012 0.005 0.006 0.014 the pseudotachylytes formed concomitantly Fe3+ 0.159 0.123 0.197 0.125 0.135 0.007 0.106 0.133 0.098 with the eclogite shear zones, which were also Mg 1.084 0.926 0.905 0.990 0.996 0.900 0.924 0.946 0.901 highly retrograded. Metamorphic development Fe2+ 3.249 3.322 3.263 3.332 3.330 3.339 3.386 3.268 3.181 and retrogression of the shear-zone eclogites Mn 0.096 0.134 0.119 0.107 0.117 0.118 0.113 0.143 0.106 are well documented by the work of Kulle- Cr 0.000 0.000 0.001 0.003 0.000 0.000 0.001 0.005 0.000 rud (1992, 1996, 2000) and Markl and Bucher Ca 1.475 1.530 1.604 1.462 1.464 1.635 1.501 1.538 1.747 Na 0.000 0.000 0.002 0.000 0.000 0.004 0.000 0.000 0.004 (1997). Although the pseudotachylytes formed K 0.000 0.009 0.005 0.005 0.002 0.005 0.001 0.003 0.002 through frictional melting of the same host rock, Sum 16.000 16.000 16.000 16.000 16.000 16.000 16.000 16.000 16.000 eclogite from the shear zones is coarser grained, X 25.398 26.476 27.790 25.283 25.279 27.830 25.834 26.740 29.969 Grs preserves relic eclogite-facies minerals (e.g., X 18.661 16.034 15.682 17.111 17.210 15.315 15.900 16.446 15.458 Py omphacite and garnet), and is more variably X 55.941 57.490 56.528 57.606 57.511 56.855 58.265 56.814 54.572 Alm retrograded. Mineral compositional ranges for

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our pseudotachylyte samples (garnet: Gr25–30, TABLE 4. REPRESENTATIVE MICROPROBE ANALYSES OF AMPHIBOLE FROM PSEUDOTACHYLYTES Py15–19, Al55–58; plagioclase: An20–36; clinopyrox- NFA-20 NFA-20 NFA-20 NFA-20 NFA-20 NFA-20 NFA-20 NFA-20 NFA-20 NFA-20 ene: Jd6) overlap with those reported for the Nusfjord shear-zone eclogites (garnet: Gr , 10–38 Weight % oxide Py10–34, Al50–68; plagioclase: An8–27; clinopyrox- SiO2 40.36 41.25 41.26 40.66 41.19 41 41.1 41.37 41.27 41.49 ene: Jd6–38: from Kullerud, 1992; Markl and TiO 2.0745 2.2604 1.7101 1.9983 2.1902 1.9217 2.0236 2.2605 1.8697 2.0698 Bucher, 1997). Figure 8 plots Kullerud’s (1992, 2 Al2O3 13.63 13.15 13.66 14.11 13.63 13.69 13.66 13.13 13.91 13.35

1996, 2000) and Markl and Bucher’s (1997) Cr2O3 0.0547 0.0248 0.0071 0 0.0428 0.0237 0.0278 0.0007 0.042 0.0117 mineral chemical analyses for omphacite and FeO 15.27 15.5 15.23 15.85 15.19 15.22 15.74 15.16 15.24 15.57 retrograded omphacite, effectively tracking MnO 0.0304 0 0.0675 0.037 0.0331 0.0259 0.0381 0.0328 0.0614 0.0272 the eclogite- to amphibolite-facies retrogres- MgO 10.46 10.64 10.86 10.15 10.46 10.39 10.58 10.64 10.37 10.51 CaO 11.74 11.4 11.72 11.48 11.32 11.6 11.73 11.5 11.75 11.62 sion of the Nusfjord shear-zone eclogites. Our Na O 1.6557 1.6161 1.596 1.7299 1.6369 1.5798 1.6786 1.4522 1.5023 1.6226 clinopyroxene analyses clearly overlap the most 2 K2O 1.9183 1.8848 1.971 2.045 1.9066 1.8645 1.8969 1.9225 1.9045 1.817 highly retrograded shear-zone eclogite samples Total 97.18 97.72 98.07 98.04 97.6 97.3 98.47 97.48 97.91 98.09 (Fig. 8). Similar types of comparisons using Atoms per formula unit (23 oxygens) garnet and amphibole chemistry (not shown) Si 6.06 6.17 6.13 6.05 6.17 6.16 6.1 6.22 6.17 6.17 also indicate that the pseudotachylytes overlap Al 2.41 2.32 2.39 2.48 2.41 2.42 2.39 2.33 2.45 2.45 the compositions of the most highly retrograded Ti 0.23 0.25 0.19 0.22 0.25 0.22 0.23 0.26 0.21 0.21 shear-zone samples. Fe3+ 0.12 0 0.06 0.15 0 0 0.09 0 0 0 Despite the aphanitic nature of the pseudo- Mg 2.34 2.37 2.41 2.25 2.33 2.33 2.34 2.39 2.31 2.31 Fe2+ 1.79 1.94 1.83 1.82 1.9 1.91 1.86 1.91 1.91 1.91 tachylyte matrix, several textural features are Mn 0 0 0.01 000000.01 0.01 reminiscent of those reported for the Lofoten Cr 0.01 0 0 0 0.01 00000 shear-zone eclogites. Most notably, the shear- Ca 1.89 1.83 1.87 1.83 1.82 1.87 1.86 1.85 1.88 1.88 zone eclogites locally preserve omphacite that Na 0.77 0.75 0.73 0.79 0.76 0.73 0.77 0.67 0.69 0.69 has been variably replaced, with textures rang- K 0.37 0.36 0.37 0.39 0.36 0.36 0.36 0.37 0.36 0.36 ing from relic hosts with symplectite rims of Sum 16 16 16 16 16 16 16 16 16 16 low-Na clinopyroxene and albite/andesine (±amphibole) to complete replacement by the same minerals (Kullerud, 1992; Markl and Bucher, 1997; Kassos et al., 2004). Markl and Bucher (1997, p. 20) reported that even where omphacite is not preserved, the mere presence of such symplectites is an “unequivocal indicator that the rock passed through the eclogite stage.” TABLE 5. REPRESENTATIVE MICROPROBE ANALYSES OF Similarly, we interpret distinct clusters of low- ORTHOPYROXENE FROM PSEUDOTACHYLYTES

Na clinopyroxene (Jd6) and oligoclase/andesine NFA-11 NFA-11 NFA-11 NFA-11 NFA-18 NFA-18 NFA-11

(An20–36) in the matrix of the pseudotachylytes to be retro-eclogite indicators (Fig. 5F). The Weight % oxide SiO 50.99 51.16 51.21 51.94 50.95 50.28 51.94 extremely fi ne-grain size (<10 μm) of these 2 TiO 0.10 0.13 0.12 0.16 0.15 0.38 0.16 matrix minerals is an order of magnitude fi ner 2 Al O 3.79 4.16 3.58 2.97 2.71 3.58 2.97 than even the smallest symplectite grains in the 2 3 Cr2O3 0.02 0.00 0.00 0.00 0.04 0.00 0.00 coarser eclogites. Although we did not fi nd relic FeO 23.15 23.46 23.26 23.55 25.38 23.30 23.55 omphacite in our highly retrograded aphanitic MnO 0.66 0.65 0.69 0.65 0.53 0.79 0.65 rocks, we believe future studies likely will. MgO 21.38 21.18 21.64 21.66 20.49 19.36 21.66 Several of our Lofoten pseudotachylyte sam- CaO 0.35 0.51 0.31 0.35 0.26 2.60 0.35 Na O 0.02 0.01 0.00 0.00 0.00 0.22 0.00 ples preserve evidence for crystallization directly 2 K2O 0.00 0.00 0.05 0.03 0.00 0.38 0.03 from the frictional melt. Dendrite garnets from Total 100.46 101.27 100.86 101.31 100.50 100.87 101.31 the Lofoten pseudotachylytes (Fig. 5E) are Atoms per formula unit (4 cations, 6 oxygens) similar to those reported from the Bergen Arcs Si 1.89 1.89 1.89 1.91 1.91 1.87 1.91 and Ålesund (Austrheim et al., 1996; Lund and Al 0.17 0.18 0.16 0.13 0.12 0.16 0.13 Austrheim, 2003) in their appearance, size (both Ti 0.00 0.00 0.00 0.00 0.00 0.01 0.00 ~100 μm, but longer along linear traces), compo- Fe3+ 0.05 0.04 0.05 0.03 0.05 0.13 0.03 Mg 1.18 1.16 1.19 1.19 1.14 1.07 1.19 sition (averaging Gr12, Py29, and Al55), and inclusion relations (e.g., orthopyroxene, Na-Ca clino- Fe2+ 0.67 0.68 0.67 0.69 0.74 0.59 0.69 pyroxene, kyanite, amphibole, and dolomite). Mn 0.02 0.02 0.02 0.02 0.02 0.02 0.02 Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Austrheim et al. (1996) and Lund and Austrheim Ca 0.01 0.02 0.01 0.01 0.01 0.10 0.01 (2003) argued that the dendrites refl ect rapid (in Na 0.00 0.00 0.00 0.00 0.00 0.03 0.00 terms of tens of seconds) crystallization from the K 0.00 0.00 0.00 0.00 0.00 0.02 0.00 melt. The high-pressure inclusions within the Sum 4.00 4.00 4.00 4.00 4.00 4.00 4.00 dendrites are interpreted to refl ect rapid solid-

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TABLE 6. REPRESENTATIVE MICROPROBE ANALYSES OF PLAGIOCLASE FROM state disequilibrium growth following eclog- PSEUDOTACHYLYTES ite-facies seismic failure and pseudotachylyte NFA-20 NFA-20 NFA-20 NFA-20 NFA-20 NFA-20 NFA-20 NFA-20 formation. A lack of equilibrium in our samples is also indicated by the coexistence of several Weight % oxide varieties of pyroxenes (Ca-Na pyroxene and SiO 61.88 61.51 61.88 63.13 61.23 60.74 61.68 63.91 2 hypersthene) and the ranges of compositions of Al2O3 26.11 26.03 26.48 26.72 26.48 25.35 26.7 27.04 FeO 0.47 0.50 0.36 0.27 0.25 0.3466 0.2996 0.2491 various other mineral phases. Austrheim et al. MgO 0.04 0.00 0.00 0.00 0.00 0 0 0 (1996) interpreted similar mineralogical irregu- CaO 5.67 5.69 5.94 5.58 6.11 5.9 6.03 6.24 larities of the Holsenøy pseudotachylytes, which Na O 8.00 8.05 7.32 6.30 7.60 3.69 6.8 4.47 2 are not as strongly retrograded as ours, to refl ect K O 0.26 0.24 0.28 0.24 0.23 0.2877 0.2475 0.191 2 rapid disequilibrium growth from the frictional Total 102.42 102.02 102.25 102.23 101.90 96.34 101.76 102.14 melt. Finally, rapid crystallization from a melt Atoms per formula unit (5 cations) is also indicated by the distinct fi ning of grains Si 2.62 7.39 2.62 2.66 2.61 2.72 2.63 2.70 toward the margins of some of the Lofoten pseu- Al 1.30 3.68 1.32 1.33 1.33 1.34 1.34 1.35 dotachylyte veins (Fig. 5C), a feature also seen Fe 0.02 0.05 0.01 0.01 0.01 0.01 0.01 0.01 Mg 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 in the Holsenøy eclogite-facies pseudotachylytes Ca 0.26 0.73 0.27 0.25 0.28 0.28 0.28 0.28 (Austrheim and Boundy, 1994). Na 1.04 2.95 0.96 0.82 1.00 0.51 0.89 0.58 The high degree of retrogression and disequi- K 0.01 0.04 0.01 0.01 0.01 0.02 0.01 0.01 librium in the pseudotachylyte samples that we probed did not allow us to confi dently assess pressure and temperature conditions of eclogitization. Pressure-temperature estimates deter- mined for eclogitization within the cogenetic 0.6 Jad shear zones are >~1.5 gPa and ~680 °C (mini- mum estimates for Flakstadøy eclogites from Markl and Bucher, 1997). Assuming reasonable bulk-rock densities, this pressure estimate suggests >~45 km depth for shear-zone eclogitization. Frictional melting, which is a very high- Omphacite strain rate phenomenon (>10−1 s−1; McKenzie and Brune, 1972; Sibson, 1975; Spray, 1995), Acm Quad likely occurred over a time frame of less than a few tens of seconds, whereas the plastic shears could have formed over millions of years. The 0.8 pseudotachylytes could have formed at nearly any time during operation of the eclogite shear zones (see below) at crustal levels >~45 km but well below the ~30 km paleodepth estimated for Ca-Mg-Fe the amphibolite-facies retrogression (Hodges et pyroxenes al., 1982; Kullerud, 1992; Mooney, 1997). The Flakstadøy eclogite-facies pseudotachylytes and shear zones are similar enough to Aegerine-Augite those of the Bergen Arcs and Ålesund to suspect a common mechanism for their development. Early interpretations of the Bergen Arcs pseu- 1 dotachylytes were that fl uid-driven eclogitization of the anhydrous granulites resulted in the observed ~10% volume decrease, providing a 0.4 0.2 0 causative link between eclogitization and deep- crustal (<60 km) seismic failure (e.g., Penning- NFA-11 Nusfjord pseudotachylite ton, 1983; Hurukawa and Imoto, 1992, 1993; NFA-18 Nusfjord pseudotachylite Austrheim and Boundy, 1994). Pseudotachylyte NFA-20 Nusfjord pseudotachylite formation, however, requires shearing along frac- SK-1 Skagen shear zone eclogite tures and/or fault planes (i.e., mode II fracturing: SK-5 Skagen shear zone eclogite McKenzie and Brune, 1972; Sibson, 1975). Later Markl & Bucher (1997) shear zone eclogite workers, therefore, stressed the change in rheol- Figure 8. Omphacite compositions of eclogites from Flakstadøy shear zones and pseudo- ogy and the fact that the dry, rigid, nonreacted tachylytes projected on Jad-Acm-Quad ternary diagram. Green and red dotted fi elds are granulites accommodated fl ow in the evolving, omphacite and retrograded omphacite, respectively, from the Nusfjord shear-zone eclogites fl uid-mediated, plastic eclogite shear zones by (from Kullerud et al., 2001). brittle seismogenic failure (Bjørnerud et al.,

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2002; Lund and Austrheim, 2003; Lund et al., would explain the progressive disappearance FUTURE STUDIES 2004). This interpretation found support from of the veins toward the centers of some of the studies on eclogite shears and pseudo tachylytes thicker eclogite shears. Fluid infi ltration would Much more about the rheology and geody- in the Ålesund area, where, in addition, eclog- also be enhanced by brittle fracturing, further namics of the lower crust is yet to be learned ite-facies hydrofractures are reported (Lund and perpetuating operation of the system. from future studies in Lofoten. The granulites Austrheim, 2003; Lund et al., 2004). in Lofoten appear to be only partly reacted Any interpretive model for formation of the CONCLUSIONS to eclogite—even less so than in the Bergen Flakstadøy pseudotachylytes must accommo- Arcs and Ålesund—an important observation date each of the following: crystal-brittle, seis- Available timing, structural, and petrological in itself, not only for the genesis of the pseu- mic failure, and frictional melting of metastable constraints support that rocks presently exposed dotachylytes but also for geodynamic models. granulite; rapid quenching of the melts; crystal- in Lofoten were rigid and resilient parts of the Presently, we are characterizing additional plastic, aseismic fl ow; and all of these happening lower-crustal levels of an ancient continent from localities on Flakstadøy, Røst, Værøy, and together, temporally and spatially, under high- ca. 1.8 Ga until the Middle Ordovician. In the Vestvågøy (Fig. 2), where we have discovered pressure, high-temperature (eclogite-facies) Middle Ordovician, subduction to deeper-crustal additional retro-eclogites, shear-zone eclogites conditions within the lower continental crust. As levels led to cogenetic plastic shearing and local- with associated pseudotachylytes and garnet- has already been established on mineral chemi- ized frictional melting and pseudotachylyte for- coated fractures, kyanite-clinopyroxene plastic cal and petrological grounds (Kullerud, 1992, mation under eclogite-facies conditions (depths shears, and a remarkably voluminous area of 1996; Markl and Bucher, 1997), our fi eld and ~45 km). Stiff, metastable (nonreacted) granu- eclogite (the Skagen locality is greater than structural observations further substantiate that lite accommodated aseismic, steady-state fl ow in ~1.6 km2). Another exciting prospect is that fl uid-mediated eclogitization was responsible cogenetic, fl uid-mediated, eclogite shear zones Lofoten may be a continuous column through for the formation of the Flakstadøy shear zones. by brittle seismogenic failure. Pseudotachylyte the entire Caledonian lithosphere rather than The pseudotachylytes and cataclasites are pre- veins were likely cannibalized as plastic fl ow an allochthonous terrane emplaced upon Bal- served at Nusfjord and Skagen because plastic progressed to consume larger and larger volumes tic basement, as is the case of the Bergen Arcs. shear strains along individual zones appear to be of granulite through time. The process operated Such continuous columns are exceedingly rare small and die out only a few centimeters out- in an on-again and off-again fashion reminiscent and provide our only opportunity to examine side of the shears where eclogitizing fl uids were of the repetitive nature of active and historical the direct products of lower-crustal deforma- able to infi ltrate (e.g., Fig. 4). The mechani- earthquakes. Later in the Middle Ordovician, tion in the context of its spatial and temporal cal conundrum of synchronous crystal-plastic these deep-crustal rocks were exhumed to mid- relation to deformation that had occurred at fl ow and frictional melting (that is, onset of the dle-crustal levels, where they were retrograded shallower levels in the same vertical crustal former should prohibit the latter) in our rocks under amphibolite-facies conditions. section (Axen et al., 1998; Beaumont et al., seems best explained as a spatial phenomenon. Fluid activity in the continental basement rocks 2001; Klepeis et al., 2003). How strain parti- Clearly, fl uids were limited in their ability to of Lofoten was signifi cant in affecting the meta- tions vertically through the entire lithosphere infi ltrate far into the dry, rigid granulites. Once stability and mechanical strength of the roots to and is transmitted laterally is one of the more hydrated, however, the eclogitized volumes the ancient Caledonian mountain belt. During the pressing questions concerning the evolution of rock were substantially weakened as crys- Early Silurian (Scandian phase), synmetamorphic of Earth’s continents (e.g., McKenzie et al., tal-plastic fl ow mechanisms began to operate. emplacement of the Caledonian allochthons at 2000; Abers et al., 2002). Ongoing work along It is reasonable, then, that fl ow in the eclogite mid-crustal levels (~30 km; Hodges et al., 1982; the Gullesfjorden and Austerfjord shear zones shear zones allowed strain to accumulate within Steltenpohl and Bartley, 1987) resulted in dewa- (Fig. 2) is directed toward assessing how the the dry granulitic host until its strength was tering reactions such that fl uids moved downward deep-crustal rocks and structures in Lofoten exceeded, resulting in catastrophic brittle fail- to weaken the uppermost structural levels of the are related spatially and temporally to the over- ure and pseudotachylyte formation. Thus, our granulitic basement complex (Bartley, 1982). lying middle- and upper-crustal sections. observations from Flakstadøy are compatible Our work in Lofoten indicates that during the with the Bergen Arcs model. early Caledonian, fl uids also locally hydrated the ACKNOWLEDGMENTS The rare, plastically deformed pseudo- dry granulitic basement in the deep crust (only 3– tachylyte veins on Flakstadøy evoke a chicken- 4 km beneath the ancient Conrad discontinuity; This research was made possible through a grant before-the-egg argument, since they demon- Fig. 2) and facilitated its brittle (seismic) failure from the Research Council of Norway (to Andresen), and a visiting researcher grant (to Steltenpohl) from and plastic (aseismic) weakening. strate that, at least locally, brittle paleoseismic the Industrial Liason (IL) Fund (Department of Geo- failure preceded plastic yielding. This should To our knowledge, this is only the third local- sciences, University of Oslo). Kassos thanks the Geo- be expected, however, given the on-again/off- ity recognized where deep-crustal paleoseismic logical Society of America for helping to support this again, repetitive nature of active and histori- faults have been exhumed and exposed for direct work. Initial fi eld work for this study was supported by cal seismicity, regardless of focal depth. The observation. Our work demonstrates that despite a a grant from the Norwegian Marshall Fund (to Stelten- Flakstadøy eclogite shears and pseudotachy- strong retrograde overprint, careful fi eld and pet- pohl). Some initial fi eld discoveries were made while supported by the National Science Foundation (NSF lytes should be viewed in the context of such a rological studies on pseudotachylytes preserved grant EAR-9506698 to W.E. Hames and Steltenpohl). cyclical system. Steady-state fl ow in the shear in the exposed deep-crustal roots of ancient col- We thank H. Stowell and C. Zuluaga, who helped zones operated continually, analogous to a strip lisional zones can provide important information with microprobe analyses at the University of Ala- recorder, whereas the pseudotachylytes refl ect on processes controlling the mechanical strength bama, and C. Fleisher, who assisted at the University periodic bursts of seismogenic energy. Pseudo- of the deep lithosphere and the generation of of Georgia. We gratefully thank Håkon Austrheim for many insightful discussions related to this report. We tachylyte veins likely would be cannibalized as deep-foci earthquakes, which has direct applica- also thankfully acknowledge K. Klepeis, R. Wintsch, plastic fl ow progressed, consuming larger and tion for modern continental seismic zones (e.g., and an anonymous reviewer for provocative reviews larger volumes of granulite through time. This Himalayas, see Jackson et al., 2004). that greatly improved this report.

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