Abstract
Moine metasedimentary rocks of northern Scotland are characterized by arcuate map patterns of mineral lineations that swing progressively clockwise from orogen-perpendicular E-trending lineations in greenschist facies mylonites above the Moine thrust on the foreland edge of the Caledonian Orogen, to S-trending lineations at higher structural levels and metamorphic grades in the hinterland. Quartz c-axis fabrics measured on a west to east coast transect demonstrate that the lineations developed parallel to the maximum principal extension direction and therefore track the local tectonic transport direction. Microstructures and c-axis fabrics document a progressive change from top to the N shearing in the hinterland to top to the W shearing on the foreland edge. Field relationships indicate that the domain of top to the N shearing was at least 55 km wide before later horizontal shortening on km-scale W-vergent folds that detach on the underlying Moine thrust. Previously published data from the Moine thrust mylonites demonstrate that top to the W shearing had largely ceased by 430 Ma, while preliminary isotopic age data suggest top to the N shearing occurred at ~470–450 Ma. In addition, data from the east coast end of our transect indicate normal-sense top down-SE shearing at close to peak temperatures at ~420 Ma that may be related to the closing stages of Scandian deformation, metamorphism and cooling/exhumation.
Highlights
Mineral grain shape lineations in metamorphosed and plastically deformed rocks have been used for over a hundred years to infer tectonic transport directions (e.g., Geikie [1]; Peach et al, [2], p. 599; see reviews by Cloos [3]; Anderson [4]; Kvale [5]; Law and Johnson [6])
Beyond the scope of the sampling transect covered in this paper, lineation and maximum principal stretch directions indicated by quartz c-axis fabrics in the uppermost part of the Moine thrust sheet exposed on Meal an t-Sithe at the southwestern corner of Area 1 (Figure 3) display a progressive swing in lineation trend from SE to SSE to S approaching the overlying klippe of the Sgurr Beag thrust sheet, with lineations/principal stretch directions both in the immediate footwall and hanging wall of the Sgurr Beag thrust trending 178◦
We report an integrated microstructural and quartz c-axis fabric study from a 45 km long transect across the Caledonian Orogen in northern Scotland that extends from Loch Broom and Dundonnell (Area 1) in the west, through Loch Glascarnoch, Ben Wyvis and Rogie Falls (Areas 2 and 3), to the Creich Peninsula (Area 4) on the east coast
Summary
Mineral grain shape lineations in metamorphosed and plastically deformed rocks have been used for over a hundred years to infer tectonic transport directions (e.g., Geikie [1]; Peach et al, [2], p. 599; see reviews by Cloos [3]; Anderson [4]; Kvale [5]; Law and Johnson [6]). Traced eastward into the overlying higher grade metasedimentary rocks only very limited data are currently available on: (a) the kinematic significance of these lineations (e.g., relationship to finite principal strain directions and associated shear sense); (b) how the microstructures and crystal fabrics that developed with these grain shape lineations (and their associated foliations) change with increasing deformation temperature/metamorphic grade; and (c) whether these lineations/foliations are everywhere of essentially the same (Scandian) age or if they becomes progressively older (intermediate Scandian-Grampian or Grampian ages) traced eastwards into the overlying higher metamorphic grade thrust sheets, as suggested, for example, by Kelley and Powell [57] in the Fannich Forest area for the adjacent Moine and Sgurr Beag thrust sheets containing E and SE to S plunging lineations, respectively (Figure 1, locations MATK and FK)
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