Kinematics and Timing of Deformation in the Renvyle-Bofin Slide, northern Connemara, Ireland
D.G.R. Powell & A.R. Allen
Department of Geology, University College Cork, Ireland
The Renvyle-Bofin Slide (RBS) is a major tectonic discontinuity that runs across the whole of the northern part of the Connemara terrane. The slide divides the polyphase (D1-D4) Dalradian metasediments of Connemara into two groups: the rocks of the Cornamona Marble, Ben Levy and Lough Kilbride Formations to the north, and the stratigraphically older rocks up to the Ballynakill Schist Formation to the south of the slide. The RBS transgresses the stratigraphy along its length but is always located at the base of the Ben Levy Formation. A large syn-orogenic mafic complex, the Dawros-Currywongaun-Doughruagh Complex (DCDC) intrudes the Dalradian to the north of the slide.
The slide itself consists of a ~50m wide zone of mylonite. Two rock types dominate: quartz-biotite mylonites and quartz-muscovite-chlorite mylonites. Both rock types contain strong mylonitic foliations defined by the parallel alignment of micas and elongate bands of ribbon quartz. Large (<50mm) porphyroclasts of vein quartz are common.
Field and textural overprinting relationships have identified three distinct groups of structural features: Early, higher temperature (Bt-grade) structures indicative of sinistral movement (D1rbs), retrograde (Musc-Chl-grade) structures resulting from extension (top to the north movement; D2rbs), and late brittle and semi-brittle features which cross-cut all earlier structures (D3rbs). Evidence for sinistral displacement includes a well-developed mineral lineation parallel to strike, mantled vein-quartz porphyroclasts, stair stepping, shear folds and S-C fabrics. Structures interpreted as forming due to extension include a dip-parallel lineation, sheath folds, shear-band cleavage and a ubiquitous S-C fabric.
The identification of structures indicating both strike-slip and dip-slip movements suggest a two-stage deformational history. Extension is interpreted as a later phase of movement, due to the overprinting of biotite-grade structures by ones of chlorite grade. Also, no syn-regional D3 contact metamorphic rocks associated with the DCDC occur to the south of the slide, a fact supported by the obliteration of early sinistral mylonites by the heating effects of the DCDC. It is suggested that structures formed by sinistral movement on the RBS (D1rbs) have been obliterated by contact heating by the DCDC during the regional D3 event, and subsequent extensional movement produced the current foliation in the rocks adjacent to the slide. Therefore, D1rbs must have occurred prior to the intrusion of the DCDC, i.e. pre-D3, probably syn-D2, and extension (D2rbs) took place after intrusion, post-D3 and is possibly related to the uplift of the terrane during D4, with orogenic collapse exploiting a pre-existing structure.