Emrys Phillips

Polyphase deformation at an oscillating ice margin following the Loch Lomond Readvance, central Scotland, UK

The sequence of glacitectonic disturbance of an ice-contact delta during the initial stages of deglaciation following the Loch Lomond Readvance is examined. An ice-marginal, Gilbert-type deltaic sequence exposed in Drumbeg quarry, Drymen (Scotland) displays a polyphase deformation history that is punctuated by periods of erosion and deposition. Progradation of the delta into ice-dammed Lake Blane led to a temporary stabilisation of the Loch Lomond glacier during recession from its Loch Lomond Readvance maximum position. This was followed by a NE-directed readvance into the ice-contact slope of the delta, resulting in proglacial deformation (D1) and the formation of a thrust-block ridge. In the middle to outer parts of the delta complex, sedimentation continued uninterrupted. Subsequent retreat of the ice was accompanied by erosion and deposition as the ice-contact slope was re-established. A second phase of readvance resulted in subglacial, ductile shearing (D2) and deposition of a diamicton during NE-directed over-riding of the delta sediments by the ice. The direction of ice flow subsequently changed towards the SE. This was followed by a further retreat of the ice and re-establishment of the fan-delta complex. The complexity of the glacitectonic sequence at Drumbeg records the impact of an oscillating ice margin responding to either relative deepening of the lake waters in contact with the receding snout, or climatic controls. It provides further evidence that many of the Scottish Highland glaciers were subject to active recession rather than stagnation at the end of the Loch Lomond Readvance.

Detrital Avalonian zircons in the Laurentian Southern Uplands terrane, Scotland

The Silurian–Ordovician Southern Uplands terrane occupies a key position in the Caledonian orogen, yet its genesis is controversial. Marginal-basin, backarc, and forearc tectonic regimes have all been invoked as operative at the Laurentian margin of the Iapetus Ocean. Fresh andesitic detritus within turbidite sandstones has, until now, been assumed to provide evidence for an Ordovician suprasubduction volcanic arc, a central feature of most models. However, high-precision thermal-ionization mass spectrometer U-Pb and laser-ablation data for detrital zircons from the sandstone prove Neoproterozoic volcanism at 557 ± 6 Ma (2σ) and probably also at 613 ± 12 Ma (2σ). The complex crystallization history recorded by the zircons shows assimilation of 1043 ± 7 Ma (2σ) Grenvillian basement into the andesite magma. The fact that no zircons have been found having ages that overlap the Caradocian depositional age of the host sedimentary rocks undermines all extant terrane models. The age profile of the detrital zircons is typical of Gondwana and Avalonia. This finding has important implications for the paleogeography of the Iapetus Ocean during the Ordovician, because the zircon data require the introduction of Avalonian detritus into a sedimentary basin marginal to Laurentia.

Ice stream motion facilitated by a shallow-deforming and accreting bed

Ice streams drain large portions of ice sheets and play a fundamental role in governing their response to atmospheric and oceanic forcing, with implications for sea-level change. The mechanisms that generate ice stream flow remain elusive. Basal sliding and/or bed deformation have been hypothesized, but ice stream beds are largely inaccessible. Here we present a comprehensive, multi-scale study of the internal structure of mega-scale glacial lineations (MSGLs) formed at the bed of a palaeo ice stream. Analyses were undertaken at macro- and microscales, using multiple techniques including X-ray tomography, thin sections and ground penetrating radar (GPR) acquisitions. Results reveal homogeneity in stratigraphy, kinematics, granulometry and petrography. The consistency of the physical and geological properties demonstrates a continuously accreting, shallow-deforming, bed and invariant basal conditions. This implies that ice stream basal motion on soft sediment beds during MSGL formation is accommodated by plastic deformation, facilitated by continuous sediment supply and an inefficient drainage system.

The Moniaive Shear Zone: a major zone of sinistral strike-slip deformation in the Southern Uplands of Scotland

Synopsis The Moniaive Shear Zone is a recently delineated major zone of enhanced ductile deformation (up to 5 km wide) within the Southern Uplands of Scotland. The northern margin of the zone coincides with the Orlock Bridge Fault at the boundary between Ordovician and Silurian grey-wackes. Outwith the shear zone, the greywackes are typically weakly deformed, with the regional S1 fabric being developed within mudstone interbeds. Within the shear zone, Silurian Gala Group greywackes are deformed by locally pervasive linear and planar fabrics which yield a consistent sinistral sense of shear. The shear-zone fabric, in general, dips steeply towards the NW with its strike being sub-parallel to that of the regional S1 cleavage. Detailed petrological and micro-structural studies have demonstrated that the Moniaive Shear Zone comprises several zones of high strain enclosing lenticular domains of relatively low strain. The relationship between the Moniaive Shear Zone and the Cairnsmore of Fleet Granite indicates that the main phase of ductile movement on the zone occurred prior to 392 Ma although the exact age of ductile deformation on the shear zone is uncertain.

Micromorphological evidence for polyphase deformation of glaciolacustrine sediments from Strathspey, Scotland

Abstract A combination of field investigation and micromorphological analysis has been applied to polydeformed Late Devensian rhythmites and glacigenic diamicton, exposed in Strathspey, Scotland. This provided information on the geometry, kinematics and relative ages of ductile and brittle structures, and records a complex subglacial deformation history. The deformation is interpreted as resulting from a single progressive event, associated with over-riding of proglacial lake sediments by wet-based ice. The earliest deformation (‘D1’) resulted from compaction/loading (pure shear) and imposed a bedding-parallel (S1) fabric throughout the rhythmites. S1 was subsequently deformed by kink bands and minor ductile shearing during ‘D2’. A later ‘D3’ event, characterized by soft-sediment deformation and fluidization of matrix-poor sands, was accompanied by an increase in pore water pressure. This lead to hydrofracturing of the rhythmites. The most intense deformation (‘D4’), which resulted from simple shear, was partitioned into the upper part of the sequence. It produced folding, thrusting and brittle microfaulting in response to NNW-directed ice-push. These findings indicate that, in general, subglacial deformation is not homogeneous and can extend to depths of >3 m below the presumed ice-sediment interface.

The Glacial History of the British Isles during the Early and Middle Pleistocene: Implications for the long-term development of the British Ice Sheet

Abstract We review the evidence for Quaternary glaciation in the British Isles and adjoining seas. Attention is given to the types of onshore and offshore evidence and the robustness of these evidence sources. We find evidence for onshore lowland glaciation during Marine Isotope Stages 16, 12, 10, 6 and 2.

Fracturing, block faulting, and moulin development associated with progressive collapse and retreat of a maritime glacier: Falljökull, SE Iceland

Since 2007, Falljokull in southeast Iceland has been undergoing passive downwasting, providing an ideal opportunity to study a range deformation structures developed in response to ice-marginal collapse and retreat. An integrated terrestrial lidar, Ground Penetrating Radar, and glaciological structural study of the clean, debris-free ice at the margin of Falljokull has allowed a detailed model of the surface and subsurface 3D structure to be developed. Collapse of the glacier margin takes the form of a multiple rotational failure controlled by large-scale, down-ice dipping normal faults. As the fault-bound blocks of ice are displaced downslope, they rotate leading to localized compression and the formation of down-faulted graben-like structures. Moulins present within the marginal zone of Falljokull are closely associated with the zones of relatively more intense brittle deformation which crosscut the glacier. A model is proposed where the moulins have formed in response to the progressive collapse of englacial drainage channels located along down-ice dipping normal faults. The preferential development of the moulins and englacial drainage channels along the normal faults weakens the ice along these structures, promoting or even accelerating further collapse of the ice margin. The complex pattern of surface lowering within the marginal zone of Falljokull has also been shown to be directly related to movement on the main faults controlling the collapse of the ice margin. This evidence suggests that structurally controlled collapse may, in some instances, have a profound effect on glacier surface lowering and geodetic mass balance measurements.

Zircon age constraints on the provenance of Llandovery to Wenlock sandstones from the Midland Valley terrane of the Scottish Caledonides

Synopsis Detrital zircon populations within the Llandovery to Wenlock sandstones of the southern Midland Valley of Scotland indicate that the recycled orogenic provenance for these sedimentary rocks was essentially bimodal, comprising a younger Lower Palaeozoic component and an older predominantly Mesoproterozoic component. The Lower Palaeozoic contribution is dominated by Arenig/Llanvirn (c. 475 Ma) zircons interpreted as having been derived from a volcanic-plutonic source located within the Midland Valley terrane. The dominant Mesoproterozoic component within the sandstones is c. 1000 Ma and is thought to represent detritus shed from a Grenvillian (c. 1000–1800Ma) basement to the Midland Valley terrane. The scarcity of Archaean zircons precludes the Grampian metamorphic terrane Dalradian Supergroup as a supplier of sediment to the Ordovician–Silurian basins located along the southern margin of the Midland Valley. The age profiles of detrital zircon populations do not fit with a simple model of unroofing of a volcanic-arc complex. Rather they point to the periodic uplift of fault-bound, dismembered blocks of volcanic and plutonic rocks during a prolonged (Llandovery through to at least early Devonian) period of sinistral strike-slip deformation, and it was this which controlled basin development, sedimentary facies distribution and deformation along the southern side of the Midland Valley terrane. Appendices 1 & 2 can be found at http://www.geolsoc.org.uk/SUP18370

The Orlock Bridge Fault in the Southern Uplands of southwestern Scotland; a terrane boundary?

The Orlock Bridge Fault separates the Ordovician and Silurian turbidite sequences within the Southern Uplands thrust belt. A large biostratigraphical break and the 1 km wide sinistral Slieve Glah Shear Zone associated with the fault in northern Ireland led to previous interpretation as a major regional structure, possibly a terrane boundary. In Scotland, however, the stratigraphical break is much less and an association with inliers of the Moffat Shale Group suggests that the fault is essentially similar to the other tract-bounding faults which originated as syn-D 1 thrusts within the imbricate stack. Localized sinistral deformation apparent along the trace of the Orlock Bridge Fault in southwestern Scotland, associated with post- 1 reactivation, is comparable to that seen at Slieve Glah. Further east, a broad zone (up to 8 km) of sinistral ductile deformation, the Moniaive Shear Zone, is recognized adjacent to the Orlock Bridge Fault over a strike length of about 100 km. However, this zone differs from the Slieve Glah Shear Zone in its width and its location relative to the fault, suggesting that it is not simply related to the fault but represents a more regional deformation. Sinistral reactivation of the Orlock Bridge Fault was possibly initiated in the Wenlock during the peak of sinistral shear at the thrust front, although it may have developed over a long time contemporaneously with, but locally post-dating, the Moniaive Shear Zone. The latter deforms porphyroblasts with the thermal aureole of the c . 392 Ma Cairnsmore of Fleet granite pluton, which was emplaced into and largely post-dates the shear zone, but is deformed by the Orlock Bridge Fault. Major dip-slip reactivation of the fault post-dates the Moniaive Shear Zone and regional metamorphism and probably occurred in the Carboniferous or Permian. There is some evidence for a deep crustal feature coincident with the Orlock Bridge Fault, possibly the boundary between different crustal blocks in the collage of terrane fragments accreted during the final closure of Iapetus, which may explain the unusual extent of the reactivation of the Orlock Bridge Fault within the allochthonous Southern Uplands thrust stack. However, the situation of the fault within the Southern Uplands terrane and, in Scotland, the biostratigraphical evidence of no major stratigraphical break across the fault and the lack of any clear relationship between the Orlock Bridge Fault and the Moniaive Shear Zone indicate that the fault should not be regarded as a terrane boundary.

The Northern Belt 100 years on : a revised model of the Ordovician tracts near Leadhills, Scotland

A new model for the provenance, depositional environment and tectonic setting of the Northern Belt of the Southern Uplands is presented. This turbiditic sandstone dominated sequence was deposited in a sand-rich submarine fan environment, overlying hemipelagic mudstones. The oldest sandstones are rich in juvenile ophiolitic material and record the first clastic input into the Southern Uplands basin. The bulk of the Northern Belt sedimentary sequence, however, is dominated by relatively quartzose sandstones derived from a Proterozoic continental/metamorphic source represented by the Midland Valley terrane of Scotland and Ireland. The quartzose dominated sequence was punctuated by the input of fresh volcanic detritus shed from a oceanic/continental island-arc situated to the W/NW of the Southern Uplands basin, with sediment dispersal turning to the NE along the axis of the basin in Scotland. The tectonic setting of the Southern Upland basin remains uncertain. The complex provenance of the sandstones and recognition of major olistostrome units within the Northern Belt succession suggest that it was tectonically active. The onset of clastic deposition within the Southern Uplands broadly corresponds to ophiolite obduction in both Scotland and Ireland, possibly in response to collision of a Cambrian-early Ordovician island-arc system with the Laurentian continental margin. If this interpretation is correct then the possibility arises that the Southern Uplands-Midland Valley terranes record the dismembering of this island-arc complex within an overall transpressional regime.