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 pseudo- tachylyte 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. For permission to copy, contact [email protected] 61 © 2006 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/2/1/61/3334709/i1553-040X-2-1-61.pdf by guest on 27 September 2021 M.G. STELTENPOHL et al. (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 inter- nal tectonic position within the northern parts of this restored orogen (Fig. 1). Surprisingly, how- ever, little Caledonian imprint is preserved in the area of Lofoten basement rocks. Caledonian structures Figure 2 and fabrics along the base of the cover alloch- thons 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