Heterogeneous deformation and quartz crystallographic fabric transitions: natural examples from the moine thrust zone at the stack of glencoul,northern assynt |
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| Institution: | 1. School of Management, Shanghai University, 99 Shangda Road, 200444 Shanghai, China. Shanghai Pulmonary Hospital, 507 Zhengmin Road, 200433 Shanghai, China;2. Shanghai University, 99 Shangda Road, 200444 Shanghai, China, School of Management, 200444 Shanghai, China;3. Shanghai University, School of Management, 99 Shanghda Road, 200444 Shanghai, China |
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| Abstract: | Quartz crystallographic fabric transitions in well-exposed mylonites immediately beneath the Moine Thrust at the Stack of Glencoul (NW Scotland) have been investigated by optical microscopy, X-ray texture goniometry and Orientation Distribution Function analysis. A progressive change is observed from asymmetrical kinked single girdle c-axis fabrics at 0.5 cm beneath the Moine Thrust, through asymmetrical Type I cross-girdle fabrics to symmetrical Type I cross-girdle fabrics at 30 cm beneath the thrust. This c-axis fabric transition is accompanied by a transition from asymmetrical single a-axis maximum fabrics (0.5 cm beneath the thrust) through asymmetrical two maxima fabrics to essentially symmetrical two maxima a-axis fabrics. ODF analysis of these S >L and L - S tectonites indicates that c-axis positions on the ‘leading edge’ of the fabric skeleton are related by a common (a) direction oriented within the XZ plane at a moderate angle to the lineation (X). In contrast, c-axis positions on the peripheral ‘trailing edge’ are related by a positive (r) rhomb pole oriented close to Z; (a) directions lying within this common rhomb plane progressively change through 180° in orientation traced around the c-axis fabric skeleton. Such contrasting ‘single crystal’ rhomb (a) preferred orientations on the ‘leading’ and ‘trailing’ edges of the fabric skeleton are interpreted as indicating localized (grain scale) plane strain and flattening deformation, respectively. They result in tectonites with essentially symmetrical c- and a-axis fabrics which display strongly asymmetrical positive (r) and negative (z) rhomb pole figures. The observed transition in quartz c- and a-axis fabrics is interpreted as indicating an increasing importance of non-coaxial plane-strain deformation as the Moine Thrust is approached. Even immediately (<1 cm) beneath the thrust, however, flow has still significantly departed from bulk simple shear and involved an important (heterogeneous) component of contemporaneous flattening deformation. |
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