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

¹ Department of Geology and Geography, Auburn University, Auburn, Alabama 36849, USA
² Department of Geosciences, University of Oslo, Box 1047, Blindern, Oslo 3, Norway

View larger version (50K): [in this window] [in a new window]   Figure 1. Early Carboniferous reconstruction of the northern Caledonides (after Roberts and Gee, 1985; Ziegler, 1988) illustrating Lofoten's internal position within the orogen, eclogite localities (see text for references), and place names used in text.

View larger version (52K): [in this window] [in a new window]   Figure 2. Geologic relations along the Ofoten-Lofoten transect (after Tull et al., 1985). See Figure 1 for location of the map area. The upper cross section shows general geologic features along the transect, whereas the lower one depicts metamorphic conditions within the basement complex. Abbreviations are: um—unmetamorphosed; chl—chlorite zone; bt—biotite zone; gt—garnet zone; ky—kyanite zone; gran—granulite facies. SL—sea level. See text for details. GSZ—Gullesfjorden shear zone; AT—Austerfjord thrust.

View larger version (47K): [in this window] [in a new window]   Figure 3. Geologic map of Flakstadøy and part of adjacent Vestvgøy illustrating eclogite shear-zone localities (modified from Markl and Bucher, 1997; Klein et al., 1999).

View larger version (173K): [in this window] [in a new window]   Figure 4. Termination of an eclogite shear zone (diagonally from upper right to lower left) with central pseudotachylyte vein (hand lens is 2.5 cm in length). Gabbronorite is massive and nonfoliated 5 cm from the center of the shear zone. The teal discoloration of the gabbronorite corresponds to the "eclogitization front." A thin shear passes from left-lower center to just below the hand lens. At the upper right, the pseudotachylyte displays a chilled margin (grayish-tan center and dark gray toward margins).

View larger version (160K): [in this window] [in a new window]   Figure 5. Collage of field and petrographic images of the Flakstadøy pseudotachylytes. A: Field photo of tabular pseudotachylyte veins (black, below fingertip) 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 reflects fining 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-filled, embayed garnet (left center, light gray) contrasts with small, euhedral type at bottom left. E: Dendritic and cauliflower-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.

View larger version (77K): [in this window] [in a new window]   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.

View larger version (69K): [in this window] [in a new window]   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.

View larger version (21K): [in this window] [in a new window]   Figure 8. Omphacite compositions of eclogites from Flakstadøy shear zones and pseudotachylytes projected on Jad-Acm-Quad ternary diagram. Green and red dotted fields are omphacite and retrograded omphacite, respectively, from the Nusfjord shear-zone eclogites (from Kullerud et al., 2001).