Ductile transpression and localization of deformation along tectonic boundaries in the Caledonides of northwestern Ireland

Abstract

Orogenesis is increasingly interpreted in terms of strain focusing, localization and partitioning processes. Such heterogeneous deformation is considered a consequence of the tectonic framework, with pre-existing structural and stratigraphic variability providing inherent zones of crustal weakness. Detailed structural investigation of Neoproterozoic Dalradian metasediments in the Glencolumbkille region, northwest Ireland, enables patterns of reworking and strain localization to be assessed in terms of four overprinting ductile deformation episodes. A well-defined and intricate Dalradian stratigraphy provides readily distinguishable markers which not only focus deformation along marked rheological boundaries, but also aid in the definition and identification of resultant geometries. Overall structural and stratigraphic relationships show that whilst D1 was not associated with major structures, D2 is related to north–northeast directed folding and ductile thrusting resulting in a major phase of crustal thickening and almandine-amphibolite facies metamorphism. Structures generated during D2 deformation subsequently became the locus of intense D3 strain and were reactivated in an oblique sense associated with south or southwest directed translations. Local overprinting relationships clearly demonstrate S2 fabrics being transposed by S3 resulting in a composite foliation over large areas. Similarly, the L2 mineral lineation is abruptly transposed by L3 over relatively small distances indicating high D3 strain gradients and the susceptibility of lineations to reworking. The final stage of ductile deformation (D4) which was increasingly localized and focused into earlier (D2−D3) high strain zones, is marked by a pronounced phase of sinistral transpression associated with clockwise cleavage and minor fold transection of northwest verging upright folds. Sinistral shear is strongly partitioned in to the steep limbs of mesoscopic F4 folds. The detailed investigation of structures generated within such multiply deformed and reworked zones provides evidence of both the kinematic and tectonic evolution of regional deformation systems.

Tectonic and kinematic evolution within mid-crustal orogenic root zones: a case study from the Caledonides of northwestern Ireland

“…Metadolerites appear to be deformed by both the S2 and S3 regional fabrics which are themselves locally folded by crenulations which may be traced into the regional S4 cleavage of the host rock. These relationships appear to indicate a pre-D2 age of metadolerite emplacement similar to that observed in Glencolumbkille immediately to the north (Alsop, Bryson & Hutton, 1998), but different to that observed elsewhere in Donegal at these stratigraphic levels (Hutton & Alsop, 1995). Peak metamorphic conditions in the area are associated with the growth of (subsequently chloritized) garnet porphyroblasts.…”

“…Peak metamorphic conditions in the area are associated with the growth of (subsequently chloritized) garnet porphyroblasts. These contain relict internal S2 cleavage alignment and are partially augened by S3, indicating a post-D2, pre-D3 (MP2) phase of metamorphism identical to that observed in Glencolumbkille (Alsop, Bryson & Hutton, 1998). Growth of 'late-stage' biotite porphyroblasts which overprint ductile (D4) fabrics is observed throughout the Malin and Glencolumbkille region (e.g.…”

“…2), Howarth, Kilburn & Leake, (1966) described a relatively complete Dalradian succession from the Glencolumbkille Schist Formation through to the Slieve Tooey Quartzite Formation. Recent mapping, however, suggests that the Glencolumbkille Schist Formation is representative of the younger Easdale Subgroup and may be further subdivided into graphitic pelite and psammitic members (Alsop, Bryson & Hutton, 1998), whilst the Malin Schist Formation crops out over a larger area than previously envisaged (see next section). Thus, a relatively complete but condensed (~400 m thickness) stratigraphic sequence is observed from the Malin Schist Formation, Skelpoonagh Bay Limestone Formation, Glen Head Schist Formation, Glencolumbkille Dolomite Formation, Port Askaig Tillite Formation to the overlying Slieve Tooey Quartzite Formation (Figs 2, 4; Table 1).…”

“…2) were previously interpreted as representing Appin Group (Howarth, Kilburn & Leake, 1966;Howarth, 1971;Pitcher & Berger, 1972;Johnston, 1997) with Anderson (1954) recognizing the lithological similarity and correlating the pelites on either side of the fault. The Glencolumbkille Schist Formation is separated from the rest of the sequence by tectonic sliding (see Section 4) and is now considered to represent Easdale Subgroup of the Argyll Group (Alsop, Bryson & Hutton, 1998;Long, McConnell & Alsop, 1999). The formation may be subdivided into two interbanded members (Alsop, Bryson & Hutton, 1998), which are described in detail below.…”

“…The Glencolumbkille Schist Formation is separated from the rest of the sequence by tectonic sliding (see Section 4) and is now considered to represent Easdale Subgroup of the Argyll Group (Alsop, Bryson & Hutton, 1998;Long, McConnell & Alsop, 1999). The formation may be subdivided into two interbanded members (Alsop, Bryson & Hutton, 1998), which are described in detail below.…”