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The geological record of landforms and sediments produced beneath deglaciating ice sheets offers insights into inaccessible glacial processes. Large subglacial valleys formed by meltwater erosion of sediments (tunnel valleys) are widespread in formerly glaciated regions such as the North Sea. Obtaining a better understanding of these features may help with the parameterisation of basal melt rates and the interplay between basal hydrology and ice dynamics in numerical models of past, present, and future ice-sheet configurations. However, the mechanisms and timescales over which tunnel valleys form remain poorly constrained. Here, we present a series of numerical modelling experiments, informed by new observations from high-resolution 3D seismic data (6.25 m bin size, ∼4 m vertical resolution), which test different hypotheses of tunnel valley formation and calculate subglacial water routing, seasonal water discharges, and the rates at which tunnel valleys are eroded beneath deglaciating ice sheets. Networks of smaller or abandoned channels, pervasive slump deposits, and subglacial landforms are imaged inside and at the base of larger tunnel valleys, indicating that these tunnel valleys were carved through the action of migrating smaller channels within tens of kilometres of the ice margin and were later widened by ice-contact erosion. Our model results imply that the drainage of extensive surface meltwater to the ice-sheet bed is the dominant mechanism responsible for tunnel valley formation; this process can drive rapid incision of networks of regularly spaced subglacial tunnel valleys beneath the fringes of retreating ice sheets within hundreds to thousands of years during deglaciation. Combined, our observations and modelling results identify how tunnel valleys form beneath deglaciating mid-latitude ice sheets and have implications for how the subglacial hydrological systems of contemporary ice sheets may respond to sustained climate warming.

Funding information: This work was funded by the Natural Environment Research Council consortium grant BRITICE-CHRONO NE/J009768/1 and by the NERC Radio- carbon Facility and the NERC Cosmogenic Isotope Analysis Facility. The project benefited from the PalGlac team of researchers with funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme to CDC (Grant Agreement No. 787263) and supporting SB and AI. JCE acknowledges support from a NERC independent fellowship award (NE/R014574/1). We thank the PISM team of ice-sheet modellers and Evan Gowan for ICESHEET, and note that the development of PISM was supported by NSF grants PLR- 1603799 and PLR-1644277 and NASA grant NNX17AG65G. DP acknowledges funding from the Italian PNRA project IPY GLAMAR (grant number 2009/A2.15), and from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 656821 (project SEAGAS). The BRITICE‐CHRONO consortium of researchers undertook a dating programme to constrain the timing of advance, maximum extent and retreat of the British–Irish Ice Sheet between 31 000 and 15 000 years before present. The dating campaign across Ireland and Britain and their continental shelves, and across the North Sea included 1500 days of field investigation yielding 18 000 km of marine geophysical data, 377 cores of sea floor sediments, and geomorphological and stratigraphical information at 121 sites on land; generating 690 new geochronometric ages. These findings are reported in 28 publications including synthesis into eight transect reconstructions. Here we build ice sheet‐wide reconstructions consistent with these findings and using retreat patterns and dates for the inter‐transect areas. Two reconstructions are presented, a wholly empirical version and a version that combines modelling with the new empirical evidence. Palaeoglaciological maps of ice extent, thickness, velocity, and flow geometry at thousand‐year timesteps are presented. The maximum ice volume of 1.8 m sea level equivalent occurred at 23 ka. A larger extent than previously defined is found and widespread advance of ice to the continental shelf break is confirmed during the last glacial. Asynchrony occurred in the timing of maximum extent and onset of retreat, ranging from 30 to 22 ka. The tipping point of deglaciation at 22 ka was triggered by ice stream retreat and saddle collapses. Analysis of retreat rates leads us to accept our hypothesis that the marine‐influenced sectors collapsed rapidly. First order controls on ice‐sheet demise were glacio‐isostatic loading triggering retreat of marine sectors, aided by glaciological instabilities and then climate warming finished off the smaller, terrestrial ice sheet. Overprinted on this signal were second order controls arising from variations in trough topographies and with sector‐scale ice geometric readjustments arising from dispositions in the geography of the landscape. These second order controls produced a stepped deglaciation. The retreat of the British–Irish Ice Sheet is now the world’s most well‐constrained and a valuable data‐rich environment for improving ice‐sheet modelling. Publisher PDF

Motivated to help improve the robustness of predictions of sea level rise, the BRITICE-CHRONO project advanced knowledge of the former British–Irish Ice Sheet, from 31 to 15 ka, so that it can be used as a data-rich environment to improve ice sheet modelling. The project comprised over 40 palaeoglaciologists, covering expertise in terrestrial and marine geology and geomorphology, geochronometric dating and the modelling of ice sheets and oceans. A systematic and directed campaign, organised across eight transects from the continental shelf edge to a short distance (10s of kilometres) onshore, was used to collect 914 samples which yielded 639 new ages, tripling the number of dated sites constraining the timing and rates of change of the collapsing ice sheet. This special issue synthesises these findings of ice advancing to the maximum extent and its subsequent retreat for each of the eight transects to produce definitive palaeogeographic reconstructions of ice margin positions across the marine to terrestrial transition. These results are used to understand the controls that drove or modulated ice sheet retreat. A further paper reports on how ice sheet modelling experiments and empirical data can be used in combination, and another probes the glaciological meaning of ice-rafted debris.

Traditional approaches to develop 3D geological models employ a mix of quantitative and qualitative scientific techniques, which do not fully provide quantification of uncertainty in the constructed models and fail to optimally weight geological field observations against constraints from geophysical data. Here, using the Bayesian Obsidian software package, we develop a methodology to fuse lithostratigraphic field observations with aeromagnetic and gravity data to build a 3D model in a small (13.5 ​km ​× ​13.5 ​km) region of the Gascoyne Province, Western Australia. Our approach is validated by comparing 3D model results to independently-constrained geological maps and cross-sections produced by the Geological Survey of Western Australia. By fusing geological field data with aeromagnetic and gravity surveys, we show that 89% of the modelled region has >95% certainty for a particular geological unit for the given model and data. The boundaries between geological units are characterized by narrow regions with

Mechanical twins form by the simple shear of the crystal lattice during deformation. In order to test the potential of narrow twins in monazite to record the timing of their formation, we investigated a ca. 1700 Ma monazite grain (from the Sandmata Complex, Rajasthan, India) deformed at ca. 980 Ma, by electron backscattered diffraction (EBSD), transmission electron microscopy (TEM), and atom probe tomography (APT). APT 208Pb/232Th ages indicate that the twin was entirely reset by radiogenic Pb loss during its formation at conditions far below the monazite closure temperature. The results are consistent with a model where Pb is liberated during rupture of rare earth element–oxygen (REE-O) bonds in the large [REE]O9 polyhedra during twinning. Liberated Pb likely migrated along fast diffusion pathways such as crystal defects. The combination of a quantitative microstructural investigation and nanogeochronology provides a new approach for understanding the history of accessory phases.

The Delamerian Orogen formed at the final stages of assembly of the Gondwana supercontinent. This system marks the initiation of subduction of the Pacific oceanic lithosphere along a prior rifted and extended passive margin. This paper explores the magmatic consequences following the early Cambrian initiation at the palaeo-Pacific margin in South Australia (SA) and western Victoria. Our data reveal a 50 Ma syn- to post-Delamerian tectono-magmatic history. Sampled from drill core from beneath the eastern Murray Basin cover in eastern SA, boninitic high Mg andesite from drill hole KTH12 and 516.1 ± 2 Ma quartz diorite suggest that first subduction established a volcanic arc within easternmost SA. Pacific-ward trench retreat then resulted in arc migration to reach the Mt Stavely Belt and Stawell Zones in western Victoria by ~510 Ma where boninitic arc magmatism continued until ~490 Ma. In the SA foreland of the Delamerian Orogen, early (522 ± 4 Ma) alkali basalt gave way to intrusion and extrusion of MORB-like tholeiites of back-arc basalt character. Through much of the middle and late Cambrian the SA Delamerian was in the back-arc and under extension but with periodic compression resulting from periodic Pacific-Australian plate coupling beneath the forearc in western Victoria. In SA syn-tectonic I- and S-type granites reflect interaction of MORB-like back-arc magmas and their transported heat with continental-derived sediment of the Kanmantoo Group. The termination of the Delamerian orogeny at ~490 Ma was accompanied by buoyancy-controlled, exhumation and erosion. This was driven by delamination of a mafic, crustal underplate, whose re-melting at 1.5 to 2 GPa and 1050 °C generated the unique 495 ± 1 Ma Kinchina/Monarto adakite. Delamination resulted in lithospheric mantle thinning and local convective overturn allowing upwelling of the asthenosphere to drive the post-kinematic magmatic phase of the Delamerian, yielding voluminous 490 Ma–470 Ma A-type granites.

Southern India lies in an area of Gondwana where multiple blocks are juxtaposed along Moho-penetrating structures, the significance of which are not well understood. Adequate geochronological data that can be used to differentiate the various blocks are also lacking. We present a newly acquired SIMS U–Pb, Lu–Hf, O isotopic and trace element geochemical dataset from zircon and garnet from the protoliths of the Nagercoil Block at the very tip of southern India. The data indicate that the magmatic protoliths of the rocks in this block formed at c. 2040 Ma with Lu–Hf, O-isotope and trace element data consistent with formation in a magmatic arc environment. The zircon data from Nagercoil Block are isotopically and temporally distinct from those in all the other blocks in southern India, but remarkably correspond to rocks in East Africa that are exposed on the southern margin of the Tanzania–Bangweulu Block. The new data suggest that the tip of southern India has an African affinity and a major suture zone must lie along its northern margin. All of these blocks were finally brought together during the Ediacaran-Cambrian amalgamation of Gondwana where they underwent high to ultrahigh temperature metamorphism.

Correlation of Rodinian and Gondwanan crustal domains relies on a thorough knowledge of those vestiges preserved today. The Bunger Hills hold a critical place in East Antarctica, recording the Mesoproterozoic assembly of Australo-Antarctica in Rodinia and the Neoproterozoic–Cambrian amalgamation of Indo- and Australo-Antarctica in Gondwana. It is situated in a region of disputed overlap between the different components of Rodinia and Gondwana, where there is little consensus on the location of sutures in this region and thus often speculative geological interpretations. The Bunger Hills therefore provide an opportunity to better understand the tectonic setting and palaeogeography during the assembly of these supercontinents. Recent work has confirmed that the Bunger Hills are one of few rare outcrops in Wilkes Land, East Antarctica that can be directly correlated with the broader Musgrave–Albany–Fraser–Wilkes Orogen (MAFWO). Whilst other constituent terranes of the MAFWO have been intensely studied, our geological knowledge of the Bunger Hills was comparatively limited until recently. In light of recent geological and geophysical developments, this contribution serves as an updated and concise standalone reference for the present state of knowledge of the Neoarchean–Cambrian evolution of the Bunger Hills region.

The last Fennoscandian Ice Sheet provides a valuable scenario for testing and evaluating numerical ice sheet models with a large amassed database of landform, stratigraphic and dated evidence of ice sheet activity. In contrast to the core shield area (Norway, Sweden and Finland) of the ice sheet, fewer investigations beyond the shield (Denmark, Germany, Poland, Lithuania, Latvia, Estonia and Russia) attempt to gather local to regional information into ice sheet wide syntheses of ice margin and lobe dynamics. For example, many detailed investigations across these countries remain disconnected with adjacent areas applying varying methods and naming schemes making it difficult to reconcile at the ice sheet scale.Here we present a systematic and spatially coherent reconstruction of ice margin dynamics for the whole southern and eastern margin, from Denmark to arctic Russia. The landform to reconstruction method allows for a consistent approach to be applied to the 1.2+ million km2 mapping area despite a wide range of glaciological landform and data variability (DEM vary in resolution from 0.4 m-25 m) found in the 1.2+ million km2 study area. We propose this reconstruction as a first-order framework of ice marginal dynamics that can be used to develop second-order and more detailed knowledge of fluctuations when more closely connected to stratigraphic and geochronometric investigations. Rather than a simple concentric retreat pattern often envisaged the landform record and its frequent overprinting forces a solution of complexity with lobe interactions and readvances.

The Laurentide Ice Sheet (LIS) was the largest of the ephemeral Pleistocene ice sheets in the Northern Hemisphere, with a Last Glacial Maximum (LGM) ice volume similar to the modern Antarctic ice sheet. A recent inventory of paleo-ice streams across the LIS shows many similarities with present-day ice streaming in Antarctica, where ice streams account for approx. 90% of mass loss. However, in the Mackenzie Lowlands of the Northwest Territories, Canada, the paleo-ice stream record is enigmatic. Previous work has identified a number of large paleo-ice streams, including the Mackenzie Trough, Anderson, Bear Lake and Fort Simpson ice streams, however, their extent, configuration, temporal relationship to each other and spatial evolution over time remains poorly constrained. Consequently, their impact on the rate and style of deglaciation of the northwestern sector of the LIS is poorly understood.Here we utilise the newly available high resolution Arctic DEM (0.5 m resolution) to re-map glacial landforms across the Mackenzie Lowlands in greater detail (area >800,000 km2). We then use this landform record to reconstruct the ice dynamics in this region following the well-established approaches of flowset mapping and the glacial inversion method. The high resolution data allow us to present a detailed reconstruction of LGM ice flow over the Mackenzie Lowlands and resolve the configuration and evolution of ice streams over time. The landform record suggests that the ice streams operated time-transgressively during deglaciation, switching on and off at different times. While ice contact landforms, such as moraines, lateral and submarginal meltwater channels and ice-contact deltas, show the overall retreat of the LIS towards the Keewatin Dome in the east, in several regions the ice retreat record is complex, suggesting interlobate ice configurations with multiple ice retreat directions.

Glaciers and ice sheets fluctuate in response to climatic change and often record these changes by building ice-marginal (terminal and lateral) moraines. Therefore, glacial landscapes are a potentially valuable archive of terrestrial palaeoclimate change. Typically, a cooling climate causes glaciers to expand and warming causes glaciers to shrink. However, the influence of high-relief mountainous topography on glacier dynamics complicates this behaviour, such that ice-marginal moraines are not always a straightforward palaeoclimate indicator. We used a higher-order ice-flow model to simulate change in glacier erosion, extent, and thickness in the response to climatic change and the resulting formation and preservation of moraines in a synthetic mountain landscape. Our results show that the rate of palaeoclimatic change relative to the glacier’s response time determines the geometry, number and position of ice-marginal moraines. However, glaciers can build distinct moraines in the absence of climate change, and the distance from the glacial maximum may not represent the chronological order of moraine formation. While moraines can be preserved despite erosion during subsequent glaciations, moraine sequences frequently contain gaps that could be misinterpreted as representing more stable palaeoclimates. These results provide theoretical understanding for the interpretation of glacial landforms both in the field and from satellite data (e.g. digital terrain models) to understand Quaternary climate change.

At the Last Glacial Maximum (LGM), Britain and Ireland were covered by the British–Irish Ice Sheet (BIIS): a dynamic ice mass that comprised the southwestern extremity of the much larger European Ice Sheet Complex. However, our understanding of the extent of the BIIS has changed dramatically in recent years from a previously established view of a more restricted ice sheet, located mostly on the present-day landmass, and with significant ice-free enclaves, to an extensive marine-terminating ice sheet. Deciphering the extent and chronology of the BIIS is critical for understanding landscape evolution. It is now clear that the LGM was just one phase of an evolving ice-sheet complex, with evidence for asynchronous maximum extents across the ice-sheet sectors. Based on the lateral extents of the BIIS, the ice mass must have been sufficiently thick to completely submerge the highest terrain in the interior of the ice sheet. The widespread presence of trimlines and strongly weathered mountain summits is now mostly interpreted as evidence of nonerosive cold-based ice, rather than as LGM ice thickness markers in the landscape. Deciphering landscape history using geomorphology alone (in the absence of geochronology) is therefore complicated and can be problematic. Our understanding on the evolution of the ice sheet through time has been dramatically improved in recent years by comprehensive geochronological programmes involving BRITICE-CHRONO and other teams.

Here we present a comprehensive dataset of glacial geomorphological features covering an area of 65 000 km2 in central west Poland, located along the southern sector of the last Scandinavian Ice Sheet, within the limits of the Baltic Ice Stream Complex. The GIS dataset is based on mapping from a 0.4 m high-resolution digital elevation model derived from airborne light detection and ranging data. Ten landform types have been mapped: mega-scale glacial lineations, drumlins, marginal features (moraine chains, abrupt margins, edges of ice-contact fans), ribbed moraines, tunnel valleys, eskers, geometrical ridge networks, and hill–hole pairs. The map comprises 5461 individual landforms or landform parts, which are available as vector layers in GeoPackage format at https://doi.org/10.5281/zenodo.4570570 (Szuman et al., 2021a). These features constitute a valuable data source for reconstructing and modelling the last Scandinavian Ice Sheet extent and dynamics from the Middle Weichselian Scandinavian Ice Sheet advance, 50–30 ka, through the Last Glacial Maximum, 25–21 ka, and Young Baltic advances, 18–15 ka. The presented data are particularly useful for modellers, geomorphologists, and glaciologists.

The Capricorn Orogen, Western Australia, is a complex orogenic zone that records the convergence and collision of the Archaean Yilgarn and Pilbara cratons in forming the West Australian Craton (WAC), then over one billion years of subsequent intracontinental reworking. Granites associated with these tectonothermal events (the Dalgaringa, Bertibubba, Moorarie, Durlacher and Thirty Three supersuites) are exposed in the western part of the Capricorn Orogen. This study integrates radiogenic (U–Pb and Hf) and stable isotope (O) analysis of zircon grains from granitic rocks in the Capricorn Orogen to determine their ages and magmatic sources, including the relative contributions of mantle versus crustal material. Granites from the margin of the Yilgarn Craton record periods of crustal growth and reworking during the Archaean that influenced later Proterozoic magmatic events. Components of the Capricorn Orogen, collectively termed the Glenburgh Terrane, have previously been considered to be exotic to the adjacent Pilbara and Yilgarn cratons. However, new U–Pb zircon geochronology and Lu–Hf isotope compositions of basement rocks in the Glenburgh Terrane (the Halfway Gneiss) have similarities to some terranes of the Yilgarn Craton, and are interpreted to represent a reworked portion of the craton that was re-accreted during the Glenburgh Orogeny. Arc magmatism during the Ma Glenburgh Orogeny resulted in a period of crustal growth, with magmas representing a mixture of 50–90 % mantle-derived magmas and 50–10 % magmas derived from an evolved crustal component with an isotopic composition equivalent to that of the Halfway Gneiss. Following assembly of the WAC, granite magmatism in the Capricorn Orogen records a significant change from one dominated by mantle-derived magmatism to one dominated by crustal melting and an increased contribution from metasedimentary material. This transition reflects a geodynamic evolution from subduction–accretion to collision and intracratonic reworking. The isotopic characteristics of granites from the Moorarie Supersuite indicate three distinct sources: (1) a metasedimentary component; (2) an evolved crustal component, comparable with the Glenburgh Terrane; (3) a mafic juvenile component. Following this, the Hf–O compositions of the Durlacher Supersuite indicate that they were derived from reworking of the Moorarie Supersuite granites, and require no juvenile contribution or any additional sedimentary source. The isotopic compositions of the Thirty Three Supersuite pegmatites indicate that they were largely derived from reworking of the Moorarie and Durlacher supersuites.

Glassy melt inclusions are unique geological repositories that preserve evidence of the formation and evolution of mantle and crustal-derived magmas. However, the mechanisms responsible for their preservation in slowly cooled crustal rocks remain contentious, in some part due to their small size (commonly

Predicting the future response of ice sheets to climate warming and rising global sea level is important but difficult. This is especially so when fast‐flowing glaciers or ice streams, buffered by ice shelves, are grounded on beds below sea level. What happens when these ice shelves are removed? And how do the ice stream and the surrounding ice sheet respond to the abruptly altered boundary conditions? To address these questions and others we present new geological, geomorphological, geophysical and geochronological data from the ice‐stream‐dominated NW sector of the last British–Irish Ice Sheet (BIIS). The study area covers around 45 000 km2 of NW Scotland and the surrounding continental shelf. Alongside seabed geomorphological mapping and Quaternary sediment analysis, we use a suite of over 100 new absolute ages (including cosmogenic‐nuclide exposure ages, optically stimulated luminescence ages and radiocarbon dates) collected from onshore and offshore, to build a sector‐wide ice‐sheet reconstruction combining all available evidence with Bayesian chronosequence modelling. Using this information we present a detailed assessment of ice‐sheet advance/retreat history, and the glaciological connections between different areas of the NW BIIS sector, at different times during the last glacial cycle. The results show a highly dynamic, partly marine, partly terrestrial, ice‐sheet sector undergoing large size variations in response to sub‐millennial‐scale climatic (Dansgaard–Oeschger) cycles over the last 45 000 years. Superimposed on these trends we identify internally driven instabilities, operating at higher frequency, conditioned by local topographic factors, tidewater dynamics and glaciological feedbacks during deglaciation. Specifically, our new evidence indicates extensive marine‐terminating ice‐sheet glaciation of the NW BIIS sector during Greenland Stadials 12 to 9 – prior to the main ‘Late Weichselian’ ice‐sheet glaciation. After a period of restricted glaciation, in Greenland Interstadials 8 to 6, we find good evidence for rapid renewed ice‐sheet build‐up in NW Scotland, with the Minch ice‐stream terminus reaching the continental shelf edge in Greenland Stadial 5, perhaps only briefly. Deglaciation of the NW sector took place in numerous stages. Several grounding‐zone wedges and moraines on the mid‐ and inner continental shelf attest to significant stabilizations of the ice‐sheet grounding line, or ice margin, during overall retreat in Greenland Stadials 3 and 2, and to the development of ice shelves. NW Lewis was the first substantial present‐day land area to deglaciate, in the first half of Greenland Stadial 3 at a time of globally reduced sea‐level c. 26 ka bp, followed by Cape Wrath at c. 24 ka bp. The topographic confinement of the Minch straits probably promoted ice‐shelf development in early Greenland Stadial 2, providing the ice stream with additional support and buffering it somewhat from external drivers. However, c. 20–19 ka bp, as the grounding‐line migrated into shoreward deepening water, coinciding with a marked change in marine geology and bed strength, the ice stream became unstable. We find that, once underway, grounding‐line retreat proceeded in an uninterrupted fashion with the rapid loss of fronting ice shelves – first in the west, then the east troughs – before eventual glacier stabilization at fjord mouths in NW Scotland by ~17 ka bp. Around the same time, ~19–17 ka bp, ice‐sheet lobes readvanced into the East Minch – possibly a glaciological response to the marine‐instability‐triggered loss of adjacent ice stream (and/or ice shelf) support in the Minch trough. An independent ice cap on Lewis also experienced margin oscillations during mid‐Greenland Stadial 2, with an ice‐accumulation centre in West Lewis existing into the latter part of Heinrich Stadial 1. Final ice‐sheet deglaciation of NW mainland Scotland was punctuated by at least one other coherent readvance at c. 15.5 ka bp, before significant ice‐mass losses thereafter. At the glacial termination, c. 14.5 ka bp, glaciers fed outwash sediment to now‐abandoned coastal deltas in NW mainland Scotland around the time of global Meltwater Pulse 1A. Overall, this work on the BIIS NW sector reconstructs a highly dynamic ice‐sheet oscillating in extent and volume for much of the last 45 000 years. Periods of expansive ice‐sheet glaciation dominated by ice‐streaming were interspersed with periods of much more restricted ice‐cap or tidewater/fjordic glaciation. Finally, this work indicates that the role of ice streams in ice‐sheet evolution is complex but mechanistically important throughout the lifetime of an ice sheet – with ice streams contributing to the regulation of ice‐sheet health but also to the acceleration of ice‐sheet demise via marine ice‐sheet instabilities.

This research was funded by the UK Natural Environment Research Council grant NE/J007196/1 ‘Britice‐Chrono’. The work was supported by the NERC Radiocarbon Facility (Allocation No. 1722.0613 and 1878.1014). Understanding the pace and drivers of marine‐based ice‐sheet retreat relies upon the integration of numerical ice‐sheet models with observations from contemporary polar ice sheets and well‐constrained palaeo‐glaciological reconstructions. This paper provides a reconstruction of the retreat of the last British–Irish Ice Sheet (BIIS) from the Atlantic shelf west of Ireland during and following the Last Glacial Maximum (LGM). It uses marine‐geophysical data and sediment cores dated by radiocarbon, combined with terrestrial cosmogenic nuclide and optically stimulated luminescence dating of onshore ice‐marginal landforms, to reconstruct the timing and rate of ice‐sheet retreat from the continental shelf and across the adjoining coastline of Ireland, thus including the switch from a marine‐ to a terrestrially‐based ice‐sheet margin. Seafloor bathymetric data in the form of moraines and grounding‐zone wedges on the continental shelf record an extensive ice sheet west of Ireland during the LGM which advanced to the outer shelf. This interpretation is supported by the presence of dated subglacial tills and overridden glacimarine sediments from across the Porcupine Bank, a westwards extension of the Irish continental shelf. The ice sheet was grounded on the outer shelf at ~26.8 ka cal bp with initial retreat underway by 25.9 ka cal bp. Retreat was not a continuous process but was punctuated by marginal oscillations until ~24.3 ka cal bp. The ice sheet thereafter retreated to the mid‐shelf where it formed a large grounding‐zone complex at ~23.7 ka cal bp. This retreat occurred in a glacimarine environment. The Aran Islands on the inner continental shelf were ice‐free by ~19.5 ka bp and the ice sheet had become largely terrestrially based by 17.3 ka bp. This suggests that the Aran Islands acted to stabilize and slow overall ice‐sheet retreat once the BIIS margin had reached the inner shelf. Our results constrain the timing of initial retreat of the BIIS from the outer shelf west of Ireland to the period of minimum global eustatic sea level. Initial retreat was driven, at least in part, by glacio‐isostatically induced, high relative sea level. Net rates of ice‐sheet retreat across the shelf were slow (62–19 m a−1) and reduced (8 m a−1) as the ice sheet vacated the inner shelf and moved onshore. A picture therefore emerges of an extensive BIIS on the Atlantic shelf west of Ireland, in which early, oscillatory retreat was followed by slow episodic retreat which decelerated further as the ice margin became terrestrially based. More broadly, this demonstrates the importance of localized controls, in particular bed topography, on modulating the retreat of marine‐based sectors of ice sheets. Publisher PDF Publisher PDF

The British and Irish Ice Sheet (BIIS) was highly dynamic during the Late Quaternary, with considerable regional differences in the timing and extent of its change. This was reflected in equally variable offshore ice‐rafted debris (IRD) records. Here we reconcile these two records using the FRUGAL intermediate complexity iceberg–climate model, with varying BIIS catchment‐level iceberg fluxes, to simulate change in IRD origin and magnitude along the western European margin at 1000‐year time steps during the height of the last BIIS glaciation (31–6 ka bp). This modelled IRD variability is compared with existing IRD records from the deep ocean at five cores along this margin. There is general agreement of the temporal and spatial IRD variability between observations and model through this period. The Porcupine Bank off northwestern Ireland was confirmed by the modelling as a major dividing line between sites possessing exclusively northern or southern source regions for offshore IRD. During Heinrich events 1 and 2, the cores show evidence of a proportion of North American IRD, more particularly to the south of the British Isles. Modelling supports this southern bias for likely Heinrich impact, but also suggests North American IRD will only reach the British margin in unusual circumstances.

We report the magnetic anisotropy of films of the magnetostrictive alloy galfenol deposited on a LiNbO3 substrate measured with the magneto-optic Kerr effect (MOKE). This anisotropy is present in 250 nm thick films but is absent in 600 nm thick films, most probably showing a loss in epitaxy with thickness. An anisotropy constant of k u = 3.45kJm−3 is deduced. Magnetic stripe domains (width 30 μm) oriented along the easy axis, parallel to the substrate ( 1 1 ¯ 00 ) axis, could be created and imaged.

This research was funded by the UK Natural Environment Research Council consortium grant NE/J007196/1 BRITICE-CHRONO. The work was supported by the NERC Radiocarbon Facility and NERC Cosmogenic Isotope Facility Analysis. New optically stimulated luminescence dating and Bayesian models integrating all legacy and BRITICE-CHRONO geochronology facilitated exploration of the controls on the deglaciation of two former sectors of the British–Irish Ice Sheet, the Donegal Bay (DBIS) and Malin Sea ice-streams (MSIS). Shelf-edge glaciation occurred ~27 ka, before the global Last Glacial Maximum, and shelf-wide retreat began 26–26.5 ka at a rate of ~18.7–20.7 m a–1. MSIS grounding zone wedges and DBIS recessional moraines show episodic retreat punctuated by prolonged still-stands. By ~23–22 ka the outer shelf (~25 000 km2) was free of grounded ice. After this time, MSIS retreat was faster (~20 m a–1 vs. ~2–6 m a–1 of DBIS). Separation of Irish and Scottish ice sources occurred ~20–19.5 ka, leaving an autonomous Donegal ice dome. Inner Malin shelf deglaciation followed the submarine troughs reaching the Hebridean coast ~19 ka. DBIS retreat formed the extensive complex of moraines in outer Donegal Bay at 20.5–19 ka. DBIS retreated on land by ~17–16 ka. Isolated ice caps in Scotland and Ireland persisted until ~14.5 ka. Early retreat of this marine-terminating margin is best explained by local ice loading increasing water depths and promoting calving ice losses rather than by changes in global temperatures. Topographical controls governed the differences between the ice-stream retreat from mid-shelf to the coast. Publisher PDF

The geometry and composition of deformation-related low-angle boundaries in naturally deformed olivine were characterized by electron backscattered diffraction (EBSD) and atom probe tomography (APT). EBSD data show the presence of discrete low-angle tilt boundaries, which formed by subgrain rotation recrystallisation associated with the (100)[001] slip system during fluid-catalysed metamorphism and deformation. APT analyses of these interfaces show the preferential segregation of olivine-derived trace elements (Ca, Al, Ti, P, Mn, Fe, Na and Co) to the low-angle boundaries. Boundaries with 2°), the interfaces become more ordered and linear enrichment of trace elements coincides with the orientation of dislocations inferred from the EBSD data. These boundaries show a systematic increase of trace element concentration with disorientation angle. Olivine-derived trace elements segregated to the low-angle boundaries are interpreted to be captured and travel with dislocations as they migrate to the subgrain boundary interfaces. However, the presence of exotic trace elements Cl and H, also enriched in the low-angle boundaries, likely reflect the contribution of an external fluid source during the fluid-present deformation. The observed compositional segregation of trace elements has significant implications for the deformation and transformation of olivine at mantle depth, the interpretation of geophysical data and the redistribution of elements deep in the Earth. The observation that similar features are widely recognised in manufactured materials, indicates that the segregation of trace elements to mineral interfaces is likely to be widespread.

The offshore sector around Shetland remains one of the least well‐studied parts of the former British–Irish Ice Sheet with several long‐standing scientific issues unresolved. These key issues include (i) the dominance of a locally sourced ‘Shetland ice cap’ vs an invasive Fennoscandian Ice Sheet; (ii) the flow configuration and style of glaciation at the Last Glacial Maximum (i.e. terrestrial vs marine glaciation); (iii) the nature of confluence between the British–Irish and Fennoscandian Ice Sheets; (iv) the cause, style and rate of ice sheet separation; and (v) the wider implications of ice sheet uncoupling on the tempo of subsequent deglaciation. As part of the Britice‐Chrono project, we present new geological (seabed cores), geomorphological, marine geophysical and geochronological data from the northernmost sector of the last British–Irish Ice Sheet (north of 59.5°N) to address these questions. The study area covers ca. 95 000 km2, an area approximately the size of Ireland, and includes the islands of Shetland and the surrounding continental shelf, some of the continental slope, and the western margin of the Norwegian Channel. We collect and analyse data from onshore in Shetland and along key transects offshore, to establish the most coherent picture, so far, of former ice‐sheet deglaciation in this important sector. Alongside new seabed mapping and Quaternary sediment analysis, we use a multi‐proxy suite of new isotopic age assessments, including 32 cosmogenic‐nuclide exposure ages from glacially transported boulders and 35 radiocarbon dates from deglacial marine sediments, to develop a synoptic sector‐wide reconstruction combining strong onshore and offshore geological evidence with Bayesian chronosequence modelling. The results show widespread and significant spatial fluctuations in size, shape and flow configuration of an ice sheet/ice cap centred on, or to the east of, the Orkney–Shetland Platform, between ~30 and ~15 ka BP. At its maximum extent ca. 26–25 ka BP, this ice sheet was coalescent with the Fennoscandian Ice Sheet to the east. Between ~25 and 23 ka BP the ice sheet in this sector underwent a significant size reduction from ca. 85 000 to

Comprehensive mapping and the Briticechrono geochronology provides a reconstruction of the last advance and retreat of the only land-terminating ice lobe of the western British Irish Ice Sheet. The Irish Sea Glacier was fed by ice from Lake District, Irish Sea and Wales, and extended to maximum limits in the English Midlands. During ice retreat after 27 kyrs, a series of reverse bedrock slopes rendered proglacial lakes endemic in the land-system. Not resembling the more extensive definitions of the classical ‘Glacial Lake Lapworth’, these ice contact lakes were smaller time transgressive moraine- and bedrock-dammed basins that evolved with ice marginal retreat. Combining, for the first time on glacial sediments, OSL bleaching profiles for cobbles with single grain and small aliquot OSL measurements on sands, has produced a coherent chronology from these heterogeneously bleached samples, and constrained for the Irish Sea Glacier a post 30ka ice maximum advance, 26.5±1.8ka maximum extent, and 25.3±1.6 to 20.6±2.2ka retreat vacating the region. With retreat of the Irish Sea Glacier an opportunistic Welsh re-advance 19.7±2.5ka took advantage of the vacated space and rode over Irish Sea Glacier moraines. Our geomorphological chronosequence shows a glacial system forced by climate, but mediated by piracy of ice sources shared with the larger and marine terminating Irish Sea Ice Stream to the west. The Irish Sea Glacier underwent changes flow regime and fronting environments driven by stagnation and decline as the primary impetus to advance was diverted. Ultimately, the glacier of the English Midlands display complex uncoupling and realignment during deglaciation and ice margin retreat towards upland hinterlands ~17.8 kyrs (Lake District and Pennines) and asynchronous behaviour as individual adjacent ice lobes became increasingly important in driving the landform record.

Geochronological data from zircon in Archean tonalite–trondhjemite–granodiorite (TTG) gneisses are commonly difficult to interpret. A notable example is the TTG gneisses from the Lewisian Gneiss Complex, northwest Scotland, which have metamorphic zircon ages that define a more-or-less continuous spread through the Neoarchean, with no clear relationship to zircon textures. These data are generally interpreted to record discrete high-grade events at ca. 2.7 Ga and ca. 2.5 Ga, with intermediate ages reflecting variable Pb loss. Although ancient diffusion of Pb is commonly invoked to explain such protracted age spreads, trace-element data in zircon may permit identification of otherwise cryptic magmatic and metamorphic episodes. Although zircons from the TTG gneiss analyzed here show a characteristic spread of Neoarchean ages, they exhibit subtle but key step changes in trace-element compositions that are difficult to ascribe to diffusive resetting, but that are consistent with emplacement of regionally extensive bodies of mafic magma. These data suggest suprasolidus metamorphic temperatures persisted for 200 m.y. or more during the Neoarchean. Such long-lived high-grade metamorphism is supported by data from zircon grains from a nearby monzogranite sheet. These preserve distinctive trace-element compositions consistent with derivation from a mafic source, and they define a well-constrained U-Pb zircon age of ca. 2.6 Ga that is intermediate between the two previously proposed discrete metamorphic episodes. The persistence of melt-bearing lower crust for hundreds of millions of years was probably the norm during the Archean.

The Southern Irumide Belt (SIB) of Zambia consists of predominantly Mesoproterozoic terranes that record a pervasive tectono-metamorphic overprint from collision between the Congo and Kalahari cratons in the final stages of Gondwana amalgamation. This study applies multi-method thermochronology to samples throughout southern Zambia to constrain the post-collisional, Phanerozoic thermo-tectonic evolution of the region. U-Pb apatite and 40Ar/39Ar muscovite data are used to constrain the cooling history of the region following Congo–Kalahari collision, and reveal ages of c. 550–450 Ma. Variations in the recorded cooling ages are interpreted to relate to localised post-tectonic magmatism and the proximity of analysed samples to the Congo–Kalahari suture. Apatite fission track data are used to constrain the low-temperature thermo-tectonic evolution of the region and identify mean central ages of c. 320–300, 210–200 and 120–110 Ma. Thermal modelling of these samples identifies a number of thermal events occurring in the region throughout the Phanerozoic. Carboniferous to Permian–Triassic heating is suggested to relate to the development of Karoo rift basins found throughout central Africa and constrain the timing of sedimentation in the basin. Permian to Jurassic cooling is identified in a number of samples, reflecting exhumation as a result of the Mauritanian–Variscan and Gondwanide orogenies. Subsequent cooling of the majority of samples occurs from the Cretaceous and persists until present, reflecting exhumation in response to larger scale rifting associated with the break-up of Gondwana. Each model reveals a later phase of enhanced cooling beginning at c. 30 Ma that, if not an artefact of modelling, corresponds to the development of the East African Rift System. The obtained thermochronological data elucidate the previously unconstrained thermal evolution of the SIB, and provides a refined regional framework for constraining the tectonic history of central Africa throughout the Phanerozoic.

The Singhbhum Craton and supracrustal provinces in eastern India are one of a few Precambrian terranes that record protracted sedimentation and magmatism spanning the Paleoarchean to Neoproterozoic. A paucity of robust geochronological and isotope geochemical data and the perpetuation of poor-quality data have hampered petrogenetic interpretations, and regional and global tectonic correlations. Here, we report new zircon U–Pb and Hf isotopic data, and new muscovite 40Ar/39Ar data from key stratigraphic intervals in the Archean nucleus and the North Singhbhum Mobile Belt. Together with critical scrutiny of available geochronological data, it is clear that the Older Metamorphic Tonalite Gneiss (with several Hadean zircon xenocrysts), the Older Metamorphic Group and parts of the Iron Ore Group were contemporaneous at ca. 3510–3375 Ma. Zircon Hf and whole-rock Nd data from these units reveal juvenile but also subchrondritic values that suggest crustal recycling operated in the Hadean and early Archean. Continued juvenile magmatic addition and crustal reworking of older portions of the craton led to the progressive emplacement of the Singhbhum Granite at ca. 3365–3285 Ma. Minor Meso- to Neoarchean granitoid emplacement, mafic dyke swarms and sedimentation completed the building of the Archean nucleus. In the early Proterozoic, supracrustal deposition and magmatism of the poorly temporally-constrained Dhanjori and Chaibasa Formations occurred along the northern margin of the craton, and equivalent units along the southern margin in the Rengali Province. In the North Singhbhum Mobile Belt, a major hiatus then occurred. Deposition and emplacement of the conformable Dhalbhum, Dalma and Chandil Formation followed, with zircon U–Pb data of 1629 ± 4 Ma from rhyolite flows in the lower Chandil Formation providing a maximum age for the underlying formations. Detrital zircon from three samples in the Dhalbhum and Chandil Formations reveal equivalent zircon populations, where >65% are juvenile to moderately evolved ca. 2550–2430 Ma grains, with additional 2800–2600 Ma, 2150–1950 Ma and ca. 1750 Ma subpopulations. Provenance for these samples is probably predominantly from the Dharwar ± Bastar Cratons to the southwest. In a tectonic context, arc-related orogenesis at ca. 1.6 Ga started from the southern tip of India and propagated northwards to the Singhbhum Craton to explain derivation of detritus from the topographically-higher Dharwar ± Bastar Cratons. Slab roll-back induced separation in the Mesoproterozoic before final re-amalgamation at ca. 1.0 Ga is evidenced by widespread Grenvillian-aged magmatism, metamorphism and deformation in Peninsular India, including syenitic rocks in the Chandil Formation, zircon Pb-loss recorded in other samples and a muscovite 40Ar/39Ar age along the Singhbhum Shear Zone of 970 ± 8 Ma. No further tectonothermal or magmatic activity is known in the Singhbhum Craton or supracrustal provinces, attesting to its final stabilization in the earliest Neoproterozoic.

Eleanore Blereau, Chris Clark, Fred Jourdan, Tim E. Johnson, Richard J. M. Taylor, Peter D. Kinny, Martin Danisik, Martin Hand, Ela Eroglu

The Fraser Zone is a major lithotectonic domain of the Albany–Fraser Orogen, Western Australia, which records Proterozoic modification of the margin of the Archean Yilgarn Craton. The Fraser Zone is volumetrically dominated by gabbroic rocks and their metamorphosed equivalents. However, little is known of the pressure–temperature–time (P–T–t) history or the geodynamic setting of these mafic rocks. When considered within the context of existing P–T constraints from spatially-associated metapelitic rocks, modelled phase equilibria suggest that both the unmetamorphosed gabbros and the granulite facies metagabbroic rocks equilibrated at 950–900 °C and ∼7 kbar, interpreted to record the conditions of magmatic crystallisation and peak metamorphism, respectively. These data support the view that mafic magmatism was the thermal driver for high-T, low-P granulite facies metamorphism. The absence of garnet from the metagabbroic rocks, and the lack of evidence for its former presence (i.e., as inclusions), argues that, during metamorphism, the rocks never reached pressures above those they attained at the thermal peak. Coronae of zircon around ilmenite in the magmatic rocks reflect a local supply of Zr as it exsolved from ilmenite, permitting earlier growth of zircon around ilmenite than elsewhere during melt crystallisation. U–Pb dating of coronal zircon (1315 ± 5 Ma) and a discrete magmatic zircon grain isolated from ilmenite (1296 ± 5 Ma) constrain the duration of magmatic crystallisation between ca. 10 and 30 Ma. Zircon in a metamorphosed gabbro constrains the timing of granulite facies metamorphism to 1293 ± 6 Ma, synchronous with final crystallisation of the mafic magmas. Based on the implied metamorphic evolution of these rocks and that of the surrounding supracrustal package, along with existing isotopic and geochemical data, we suggest the Fraser Zone probably formed in a backarc, or perhaps an intracontinental rift setting, and records successive emplacement of gabbroic rocks into a thick, sediment-filled basin. The older gabbroic rocks record hydration and reaction with the devolatilising and/or partially molten metasedimentary rocks into which they were emplaced. Subsequent granulite facies metamorphism of these hydrated rocks was driven by the heat provided by the intrusion of younger mafic magmas.

Brandon L. Alessio, Alan S. Collins, Chris Clark, Stijn Glorie, Pete R. Siegfried, Richard Taylor

In our study we explore the applicability of rutile as a pathfinder for orogenic gold deposits, which are an important source of this metal worldwide. We analysed rutile associated with orogenic Au deposits from three different Precambrian terranes, the Capricorn Orogen, the Barberton Greenstone Belt and the Ashanti Belt, all of which formed under greenschist conditions and share similarities in the style of mineralisation. Microtextural evidence from scanning electron microscopy and electron back-scatter diffraction indicates that rutile formed during the main deformation and alteration stage in these rocks, and is therefore related to mineralisation. We used electron microprobe and laser ablation ICP-MS to investigate the trace element compositions of rutile and we compared our results to other gold deposits. We find that hydrothermal rutile from gold deposits contains certain trace element characteristics, in particular high Sb concentrations (up to ∼1500 ppm in Au deposits of the Capricorn Orogen), that are distinct from rutile from non-mineralised rocks of various petrogenetic origin. Other elements, such as W and Sn, are found to be more enriched in rutile from other rock types, namely felsic magmatic rocks and hydrothermal veins, and are therefore not diagnostic of Au mineralisation in this type of deposits. We also find that the presence of sub-µm-scale inclusions – in particular Zr-(Si, Th)-bearing phases, sulfide minerals and native Au – can severely affect analyses of this type of rutile and compromise the applicability of Zr-in-rutile geothermometry.

Since the introduction of laser-assisted atom probe, analysis of nonconductive materials by atom probe tomography (APT) has become more routine. To obtain high-quality data, a number of acquisition variables needs to be optimized for the material of interest, and for the specific question being addressed. Here, the rutile (TiO2) reference material ‘Windmill Hill Quartzite,’ used for secondary ion mass spectrometry U–Pb dating and laser-ablation inductively coupled plasma mass spectrometry, was analyzed by laser-assisted APT to constrain optimal running conditions. Changes in acquisition parameters such as laser energy and detection rate are evaluated in terms of their effect on background noise, ionization state, hit-multiplicity, and thermal tails. Higher laser energy results in the formation of more complex molecular ions and affects the ionization charge state. At lower energies, background noise and hit-multiplicity increase, but thermal tails shorten. There are also correlations between the acquisition voltage and several of these metrics, which remain to be fully understood. The results observed when varying the acquisition parameters will be discussed in detail in the context of utilizing APT analysis of rutile within geology.

Unravelling the spatio-temporal evolution of orogenic terranes requires a comprehensive understanding of the duration and extent of metamorphic events and hydrothermal alteration. Commonly used minerals such as zircon and monazite may not fully record geological histories in complex tectonic settings because their elemental constituents do not react under many metamorphic and metasomatic conditions. Here, we complement the current geochronological record of the Capricorn Orogen, Western Australia, with titanite U–Pb geochronology and geochemistry of felsic intrusive rocks to draw conclusions about the use of titanite in understanding the evolution of orogenic terranes. Because titanite usually incorporates common-Pb and may be variably reset by multiple metamorphic and hydrothermal events, a workflow is provided here for the systematic and robust interpretation of titanite U-Pb data. The addition of trace element data in titanite is particularly effective for differentiating whether a grain is igneous, recrystallized or metamorphic. We have developed several petrogenetic indices to differentiate these three types of titanite using Zr-in-titanite temperature, Th/U, Th/Pb, Al/(Al + Fe), light to heavy rare earth element ratio, and Eu anomalies. The addition of trace element geochemistry can also highlight anomalously radiogenic 207Pb/206Pb reservoirs. Utilization of our workflow in the Capricorn Orogen reveals that titanite ages from the same samples as published zircon U–Pb data range from coeval to several hundreds of Myr of age difference between the two minerals. Titanite geochronology and trace element geochemistry indicates ~20 Myr of previously unrecognized prolonged cooling for the Capricorn Orogeny to ca. 1750 Ma. The spatial extent of the ca. 1210–1170 Ma part of the Mutherbukin Tectonic Event is also broadened significantly farther north and south than previously recognized. Incorporating titanite geochronology and trace geochemistry with more commonly used techniques (e.g., zircon and monazite petrochronology) extends our ability to resolve the complete history of large-scale orogenic terranes.

Over the three last decades, great efforts have been undertaken by the glaciological community to characterize the behaviour of ice streams and better constrain the dynamics of ice sheets. Studies of modern ice stream beds reveal crucial information on ice-meltwater-till-bedrock interactions, but are restricted to punctual observations limiting the understanding of ice stream dynamics as a whole. Consequently, theoretical ice stream landsystems derived from geomorphological and sedimentological observations were developed to provide wider constraints on those interactions on palaeo-ice stream beds. Within these landsystems, the spatial distribution and formation processes of subglacial periodic bedforms transverse to the ice flow direction – ribbed bedforms – remain unclear. The purpose of this study is (i) to explore the conditions under which these ribbed bedforms develop and (ii) to constrain their spatial organisation along ice stream beds. We performed physical experiments with silicon putty (to simulate the ice), water (to simulate the meltwater) and sand (to simulate a soft sedimentary bed) to model the dynamics of ice streams and produce analog subglacial landsystems. We compare the results of these experiments with the distribution of ribbed bedforms on selected examples of palaeo-ice stream beds of the Laurentide Ice Sheet. Based on this comparison, we can draw several conclusions regarding the significance of ribbed bedforms in ice stream contexts:Ribbed bedforms tend to form where the ice flow undergoes high velocity gradients and the ice-bed interface is unlubricated. Where the ribs initiate, we hypothesize that high driving stresses generate high basal shear stresses, accommodated through bed deformation of the active uppermost part of the bed. Ribbed bedforms can develop subglacially from a flat sediment surface beneath shear margins (i.e., lateral ribbed bedforms) and stagnant lobes (i.e., submarginal ribbed bedforms) of ice streams, while they do not develop beneath surging lobes. The orientation of ribbed bedforms reflects the local stress state along the ice-bed interface, with transverse bedforms formed by compression beneath ice lobes and oblique bedforms formed by transgression below lateral shear margins. The development of ribbed bedforms where the ice-bed interface is unlubricated reveals distinctive types of discontinuous basal drainage systems below shear and lobe margins: linked-cavities and efficient meltwater channels respectively. Ribbed bedforms could thus constitute convenient geomorphic markers for the reconstruction of palaeo-ice stream margins, palaeo-ice flow dynamics and palaeo-meltwater drainage characteristics.