Chronology and evolution of the northern Iceland Plateau

New aeromagnetic data, K-Ar age determinations of dredged marine igneous rocks, as well as other geophysical evidence have shed light on the chronology, nature and evolution of the northern Iceland Plateau. Correspondence between seismic refraction profiles taken on the Jan Mayen Ridge and westward through Jan Mayen Island, suppressed aeromagnetic anomalies, earthquake surface wave studies, and ages of dredged igneous rocks suggest these strata may form an extended region of thickened crust, possibly of Caledonian age, extending westward toward the Kolbeinsey Ridge and northwest to the south wall of the Jan Mayen Fracture Zone.     

Thermobarometry and Pressure-Temperature Paths in the Grenville Province of Ontario

To constrain the tectonic and metamorphic history of the GrenvilleProvince of southern Ontario we have quantitatively evaluatedchanges in peak metamorphic pressures and temperatures in theregion. Pressures increase northwest from the Frontenac Axistowards the Grenville Front, and they increase from 4–6kb near Madoc to 10–11 kb south of North Bay. Furtherto the north pressures decrease to 8–9 kb in the GrenvilleFront Tectonic Zone north of the French and Mattawa Rivers.Temperatures form a broad high, reaching 800?C northeast ofParry Sound, and decreasing to 400–500?C in the HastingsLow near Madoc, 600–650?C east towards the Ottawa River,and 650–700?C near Sudbury. This regional P-T distributionis in good agreement with constraints available from the distributionof aluminosilicate polymorphs. Comparison of thermobarometric results with regional tectonicfeatures shows a sharp discontinuity across the Mattawa andFrench Rivers, with a 1–2-kb pressure drop to the north.This implies that the major movement along this zone since theGrenville event was ‘south-side-up’ rather than‘north-side-up’ as suggested by Lumbers (1971).Large P-T discontinuities are not apparent across the domainboundaries mapped by Davidson and co-workers east of Parry Sound,but small discontinuities may exist. Sparse data may indicatethat the Central Metasedimentary Belt equilibrated at 1–2kb lower pressures than the Central Gneiss Terrane. Zoning profiles in garnet-pyroxene pairs have been used to placeconstraints on the metamorphic pressure-temperature-time pathin the Parry Sound, Port Severn, Bancroft, and Mattawa areasof the Grenville Province, Ontario. A nonlinear fitting routinewas used to obtain best-fit core and rim analyses for garnetsand pyroxenes. These results were combined with plagioclasecore/rim analyses to obtain estimates of peak and retrogradeconditions. The resultant retrograde P-Tpath has a slope of7 ? 10 b/?C, and involves pressure changes of 0?6–2?1kb for temperature changes of 60–130?C. Present address: Department of Geosciences, University of Arizona, Tucson, Arizona 85721     

Late- and postglacial history of the Great Belt, Denmark

On the basis of shallow seismic records, vibrocoring, macrofossil analyses and AMS radiocarbon-dating, five stratigraphical units have been distinguished from the deepest parts of the central Great Belt (Storebælt) in southern Scandinavia. Widespread glacial deposits are followed by two lateglacial units confined to deeply incised channels and separated by an erosional boundary. Lateglacial Unit I dates from the time interval from the last deglaciation to the Allerød; lateglacial Unit II is of Younger Dryas age. Early Holocene deposits show a development from river deposits and lake-shore deposits to large lake deposits, corresponding to a rising shore level. Lake deposits are found up to 20 m below the sea floor, and the lake extended over some 200–300 km2. The early Holocene freshwater deposits are dated to the time interval c. 10900 to c. 8800 cal. yr BP and the oldest shells of marine molluscs from the Great Belt are dated to c. 8100 cal. yr BP.     

ESTIMATING RATES OF DENUDATION USING COSMOGENIC ISOTOPE ABUNDANCES IN SEDIMENT

We propose, as a testable hypothesis, a basin-scale approach for interpreting the abundance of in situ produced cosmogenic isotopes, an approach which considers explicitly both the isotope and sediment flux through a drainage basin. Unlike most existing models, which are appropriate for evaluating in situproduced cosmogenic isotope abundance at discrete points on Earth's surface, our model is designed for interpreting isotope abundance in sediment. Because sediment is a mixture of materials, in favourable cases derived from throughout a drainage basin, we suggest that measured isotope abundances may reflect spatially averaged rates of erosion. We investigate the assumptions and behaviour of our model and conclude that it could provide geomorphologists with a relatively simple means by which to constrain the rate of landscape evolution if a basin is in isotopic steady state and if sampled sediments are well mixed.     

Little Ice Age advance and retreat sediment budgets for an outlet glacier in western Norway

Burki, V., Hansen, L., Fredin, O., Andersen, T. A., Beylich, A. A., Jaboyedoff, M., Larsen, E. & Tønnesen, J.‐ F. 2009: Little Ice Age advance and retreat sediment budgets for an outlet glacier in western Norway. Boreas, Vol. 39, pp. 551–566. 10.1111/j.1502‐3885.2009.00133.x. ISSN 0300‐9483 Bødalsbreen is an outlet glacier of the Jostedalsbreen Ice Field in western Norway. Nine moraine ridges formed during and after the maximum extent of the Little Ice Age (LIA). The stratigraphy of proglacial sediments in the Bødalen basin inside the LIA moraines is examined, and corresponding sediment volumes are calculated based on georadar surveys and seismic profiling. The total erosion rates (e_tot) by the glacier are determined for the periods AD 1650–1930 and AD 1930–2005 as 0.8 ± 0.4 mm/yr and 0.7 ± 0.3 mm/yr, respectively. These rates are based on the total amount of sediment delivered to the glacier margin. The values are almost one order of magnitude higher than total erosion rates previously calculated for Norwegian glaciers. This is explained by the large amount of pre‐existing sediment that was recycled by Bødalsbreen. Thus, the total erosion rate must be considered as a composite of eroded bedrock and of removed pre‐existing sediments. The total erosion rate is likely to vary with time owing to a decreasing volume of easily erodible, unconsolidated sediment and till under the glacier. A slight increase in the subglacial bedrock erosion is expected owing to the gradually increasing bedrock surface area exposed to subglacial erosion.     

Holocene moraine and paleosol stratigraphy, Bugaboo Glacier, British Columbia

Sequences of tills, buried paleosols, wood and tephra in lateral moraines provide a record of Holocene advances and retreats of the Bugaboo Glacier in British Columbia. The oldest paleosol is tentatively classified as a Spodosol (Cryorthod). It incorporates Mazama tephra (6,800 B.P.) and charcoal and humus dated at 3,390 and 4.400B.P., respectively, and records early and middle Holocene warming and/or drying. This paleosol overliesa latest Pleistocene or early Holocene till associated with a nearby end moraine and assigned to the regionally known 'Crowfoot Advance'. Less-developed paleosols (Cryumbrepts) are formed on Neoglacial tills deposited shortly before 3,000 B. P., between c. 2,500 to 1,900 B. P., and between c. 900 B.P. and the 19th century. The paleosols and surface soils form microcatenas with morphological variations due to differences in original topography and vegetation. The chronology derived from these paleosols and tills generally agrees with, but increases the resolution of, what is known of Holocene glacier histories in the Canadian Cordillera.