P.W. Geoff Tanner

Strain partitioning in transpression zones

Abstract Transprcssional strain acting upon structurally anisotropic rocks can be partitioned into separate deformational domains of pure shear and simple shear. This contrasts with homogeneous transpression in which both the pure shear and the simple shear strain components are uniformly distributed across the zone of deformation. Structural weaknesses capable of partially or fully accommodating one component of deformation include lithological contacts, rheological heterogeneities, and faults or shear zones situated within the deformation zone or lying along its boundaries. Partitioning of transprcssional strain can occur when stress is applied oblique to pre-existing structural weaknesses, or can occur during later stages of progressive strain, when the early deformation of isotropic rocks imparts sufficient anisotropy to allow subsequent strain to be partitioned. Partitioning of transpressional strain into domains lying parallel to the deformation zone boundaries can be distinguished from ‘fault-stepped’ transpression, in which strain is partitioned along the length of a segmented fault zone. Mesofracture analysis of rocks affected by mid-Devonian deformation on both sides of the Highland Boundary Fault Zone (HBFZ) in central Scotland shows that strain was not homogeneous. The mesofracture data suggest that regional north-south compression, orientated oblique to the pre-existing NE-SW-trending HBFZ, was partitioned into separate deformational domains. The HBFZ accommodated most of the simple shear component, whilst the rocks flanking the zone were deformed predominantly by pure shear. A contemporary example is the San Andreas Fault in central California, where analyses of neo-tectonic stresses show that the direction of principal compression is perpendicular to the fault zone. Many examples of the partitioning of transpressional strain have also been recognised at destructive plate margins where the direction of plate motion is oblique to the edge of the over-riding plate.

The flexural-slip mechanism

Abstract Detailed field studies of turbiditic sequences from South Georgia (South Atlantic), North Devon (England) and Cardigan Bay (Wales) show that flexural slip occurs on discrete movement horizons between rock packets in which the beds have welded contacts. Stair-stepping displacements of sedimentary dykes and of early quartz veins show that the movement horizons generally have a decimetre to metre spacing and are marked by bedding-parallel quartz veins. These veins are from 1 mm to several cm thick and can be used to identify movement horizons in the absence of displaced markers; they consist of several sheets of quartz fibres which each carry a slickenfibre lineation and which together preserve a record of the displacements on an individual surface. Complex slickenfibre patterns, and departures from ‘ideal’ behaviour, in which slip occurs orthogonal to the fold hinge, probably result from changes in slip vector on the limbs of growing non-cylindrical folds. Movement horizons show many of the features associated with large-scale thrusting, such as ramps, duplexes and imbricate structures, and the shear sense given by fibre steps on these surfaces and by duplexes within stratigraphically-restricted packages changes across fold hinges. Chevron folds are thought to develop mainly by flexural flow in the early stages, with flexural slip becoming dominant later as the beds become lithified; new slip surfaces are generated as the dip of the fold limbs increases.

The Highland Border Complex, Scotland: a paradox resolved

A previously inexplicable difference between the Highland Border Complex, Scotland, and its correlative in Ireland, the Clew Bay Complex, is that rocks of Caradoc–Ashgill age occur only in the former. We reject evidence from supposed chitinozoa for this dichotomy: no sedimentary rocks of proven age younger than Arenig occur in the Highland Border Complex. Consequentially, the stratigraphy must be totally recast, and the ‘exotic terrane model’ replaced by one in which a largely autochthonous Highland Border Complex in stratigraphical continuity with the Dalradian (Grampian terrane), was overridden by the Highland Border ophiolite (Midland Valley terrane). Tectonic models for SE Laurentia are thereby considerably simplified.

Morphology and geometry of duplexes formed during flexural-slip folding

Abstract Bedding-parallel slip on the limbs of chevron-style folds during flexural-slip folding results occasionally in the formation of small-scale duplexes. The duplexes described here from Upper Carboniferous turbidites in SW England generally range in thickness from 0.3 to 30 cm, and are up to 2.4 m long. They have smooth nearly flat roofs, and both the internal thrusts and the slickenfibres on them are commonly oblique to the mean transport direction as indicated by slickenfibres on the floor and roof thrusts. The morphology of the duplexes suggests that they developed between active floor and roof thrusts and they show a characteristic lack of hangingwall anticlines on the link thrusts. Their shear sense always corresponds to that required by flexural slip, and the total bedding-parallel displacement across each is commensurate with that predicted by the flexural-slip model. All thrust surfaces in the duplexes are marked by quartz fibre veins (a feature which distinguishes them from soft-sediment and other pre-folding duplexes) and carry slickenfibres whose mean orientation and complex variations in slip direction on different fibre sheets on the same slip surface are identical to those on flexural-slip movement horizons from the same fold limb. The duplexes form either as a result of resistance to thrust propagation by local facies or bed thickness changes, or develop as transfer structures between the tips of movement horizons propagating along adjacent slip surfaces. Late-stage duplexes develop from imbricated fibre veins and also form on slip surfaces oblique to the axial planes of major folds.

Testing for the presence of a terrane boundary within Neoproterozoic (Dalradian) to Cambrian siliceous turbidites at Callander, Perthshire, Scotland

The Southern Highland Group (Dalradian) and Keltie Water Grit Formation, which includes the Lower Cambrian Leny Limestone, form an inverted, 1.4 km thick, largely arenaceous, sequence at Callander. The grits have the same detrital mineralogy throughout, mainly quartz, plagioclase (An1–3), muscovite, and biotite. Chlorite formed from detrital biotite during low-grade regional metamorphism (T<270°C). There are some vertical changes in major element (but not trace element) chemistry of the grits, and detrital muscovites have a wide, but comparable, range in composition throughout, apart from an influx of Na-rich micas in the Keltie Water Grits. 40Ar/39Ar laser fusion dating of detrital muscovites yields an age spectrum with a peak at 1600–1800 Ma in the Dalradian rocks; similar old ages occur in the Keltie Water Grits but are diluted by ages of 507–886 Ma. We interpret these new data as showing that the rocks were most likely deposited as a single sequence, possibly with a disconformity, in Neoproterozoic to Early Cambrian times, before the onset of Grampian orogenesis in the Early Palaeozoic. No major structural or stratigraphical breaks have been identified and there is no direct evidence for the presence of two separate terranes.

Tectonic position of the Dalradian rocks of Connemara and its bearing on the evolution of the Midland Valley of Scotland

The Dalradian terrane of Connemara was thrust southsoutheastwards about 460 Ma ago (Rb–Sr and K–Ar ages). It rides on a major thrust of post-D age over mylonitised acidic volcanic rocks of putative lower Ordovician age and contains a number of thrusts of similar age. Several major S- to SE-directed thrusts also limit the southeastern margin of the Dalradian rocks in Mayo and Tyrone. It is suggested by analogy with Ireland that during mid-Ordovician times the Highland Boundary fault in Scotland could have been a thrust zone which carried the Scottish Dalradian rocks over a lower Ordovician basement now represented only as fragments in the Highland Border Complex.

A late Vendian age for the Kinlochlaggan Boulder Bed (Dalradian)

The Kinlochlaggan Boulder Bed occurs in Neoproterozoic Dalradian rocks of the Grampian Mountains of Scotland. It is currently thought to represent a glacial event (c. 800 Ma) unique in the North Atlantic region, and be part of a sequence correlated by different workers with either the Grampian or Appin groups. We correlate the Boulder Bed with the Port Askaig Tillite of Vendian (c. 650 Ma) age, and deduce that the sequence in which it occurs belongs to the younger Argyll Group. The latter is preserved within the D2 Kinlochlaggan Synform, and a previously unrecognized discontinuity of regional importance may separate inverted Argyll Group rocks from the underlying Grampian Group.

The giant quartz-breccia veins of the Tyndrum–Dalmally area, Grampian Highlands, Scotland: their geometry, origin and relationship to the Cononish gold–silver deposit

The area lies within a ∼15 km-wide compartment of polyphase-deformed Dalradian (Neoproterozoic) rocks, bounded by the NE-trending Tyndrum and Ericht–Laidon transcurrent faults. Sinistral movement on these faults caused a periclinal structure, the Orchy Dome, to develop from flat-lying Dalradian rocks. This dome controlled the spatial distribution of lamprophyre intrusions and explosion breccia pipes, before being cross-cut by a network of near-vertical faults. Some of these faults are host to giant, segmented, quartz-breccia veins up to 5 km long and 19 m thick, formed by cyclic injection of over-pressured Si-rich fluid into newly-formed faults. The quartz-breccia bodies consist of a plexus of quartz veins with cockade and vuggy textures, indicative of open-space, high-level crystallisation. The faults comprise a NE-trending set of mineralised veins, including the Cononish Au–Ag deposit, and two pairs of conjugate [NW- and NE-trending] and [NNW- and NNE-trending], generally non-mineralised, faults. Their geometry is that predicted by the Coulomb model for Riedel R and R′ shear fractures, modified by variations in pore fluid pressure. They were active c. 430–425 Ma ago, coincident with emplacement of the Lochaber Batholith, whose buried extension, together with the mantle, probably provided the bulk of the fluid needed to form the veins.

Structural controls and origin of gold–silver mineralization in the Grampian Terrane of Scotland and Ireland

Gold-bearing mineral deposits occur over a strike distance of >300 km within the Grampian Terrane of Scotland and Ireland. This terrane consists of Neoproterozoic–Lower Ordovician rocks of the Dalradian Supergroup that were polyphase deformed and metamorphosed during the c . 470 Ma Grampian Orogeny. Sulphide-rich Au–Ag deposits occur in Scotland at Calliachar–Urlar Burn, Tombuie, Tyndrum and Cononish, and in Ireland at Curraghinalt (Omagh), Cavanacaw, Croagh Patrick, Cregganbaun and Bohaun. They are hosted by 0.1–6 m thick quartz veins and have a similar overall mineralogy, including native gold, As, Cu, Fe, Pb and Sn sulphides, with hessite, tetrahedrite and electrum present in the first six localities above. The mineralized quartz veins, which are characterized by open-space textures, crystallized at c . 3–5 km depth in the crust. All of the deposits were structurally controlled and, apart from Curraghinalt, occur within second-order Riedel R, R′ and T fractures resulting from a regional N–S-trending maximum principal stress. These deposits are of Upper Silurian to Lower Devonian (post-Scandian) age, and are inferred to have crystallized from hot, silica-rich metamorphic fluids derived from dehydration reactions at the greenschist/amphibolite-facies boundary. Curraghinalt is an older, Grampian, thrust-related deposit. Plutonic igneous rocks (mainly granitoid) contributed in part to the fluids, which were channelled into major orogen-parallel, strike-slip faults, to be injected by fault-valve pumping into the damage zones and fault breccias of newly formed Riedel fractures. Any residual fluid probably percolated to the ground surface to form Rhynie chert-type hot-springs.