Extreme flood events have the potential to cause catastrophic landscape change in short periods of time (10 0 to 10 3 h). However, their impacts are rarely considered in studies of long-term landscape evolution (>10 3 y), because the mechanisms of erosion during such floods are poorly constrained. Here we use topographic analysis and cosmogenic 3 He surface exposure dating of fluvially sculpted surfaces to determine the impact of extreme flood events within the Jökulsárgljúfur canyon (northeast Iceland) and to constrain the mechanisms of bedrock erosion during these events. Surface exposure ages allow identification of three periods of intense canyon cutting about 9 ka ago, 5 ka ago, and 2 ka ago during which multiple large knickpoints retreated large distances (>2 km). During these events, a threshold flow depth was exceeded, leading to the toppling and transportation of basalt lava columns. Despite continuing and comparatively largescale (500 m 3 /s) discharge of sediment-rich glacial meltwater, there is no evidence for a transition to an abrasion-dominated erosion regime since the last erosive event because the vertical knickpoints have not diffused over time. We provide a model for the evolution of the Jökulsárgljúfur canyon through the reconstruction of the river profile and canyon morphology at different stages over the last 9 ka and highlight the dominant role played by extreme flood events in the shaping of this landscape during the Holocene.bedrock erosion | extreme floods | knickpoints | Iceland | cosmogenic 3 He E xtreme floods in both terrestrial and extraterrestrial environments can cause abrupt landscape change that can have longterm consequences (1-5), especially when a geomorphic threshold is exceeded (6). The timescale over which this change is visible is controlled by the ability and efficiency of background processes to reshape the landscape. As a result, progress in understanding both short-term and long-term landscape evolution requires better knowledge of bedrock channel erosion processes and thresholds over the different scales at which geomorphological processes operate (7-10).The majority of research into extreme flood events has focused on the interpretation of deposited sediments (e.g., refs. 11 and 12) and the reconstruction of the hydraulic conditions prevailing during such events (e.g., refs. 13-15). Further work has defined the geomorphic impact of extreme flood events in proglacial areas close to the source of the flood water (e.g., refs. 16 and 17). Studies that examine the processes of bedrock erosion, especially large canyon formation, during extreme flood events can help establish a diagnostic link between formation processes and morphology in canyons in both terrestrial and extraterrestrial settings, but they remain scarce (e.g., refs. 18-20). Here, evidence for bedrock landscape change during extreme floods along the course of the Jökulsá á Fjöllum River (northeast Iceland) is used to test whether the contemporary landscape morphology reflects erosion during rare extreme eve...

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During the Late Carboniferous and Early Permian an extensive magmatic province developed within northern Europe, intimately associated with extensional tectonics, in an area stretching from southern Scandinavia, through the North Sea, into northern Germany. Within this area magmatism was unevenly distributed, concentrated mainly in the Oslo Graben and its offshore continuation in the Skagerrak, Scania in southern Sweden, the island of Bornholm, the North Sea and northern Germany. Available geochemical (major- and trace-element, and Sr-Nd isotope, data) and geophysical data are reviewed to provide a basis for understanding the geodynamic setting of the magmatism in these areas. Peak magmatic activity was concentrated in a narrow time-span from c. 300 to 280 Ma. The magmatic provinces developed within a collage of basement terranes of different ages and lithospheric characteristics (including thicknesses), brought together during the preceding Variscan orogeny. This suggests that the magmatism in this area may represent the local expression of a common tectono-magmatic event with a common causal mechanism. Available geochemical (major and trace element and Sr-Nd isotope data) and geophysical data are reviewed to provide a basis for understanding the geodynamic setting of the magmatism in these areas. The magmatism covers a wide range in rock types both on a regional and a local scale (from highly alkaline to tholeiitic basalts, to trachytes and rhyolites). The most intensive magmatism took place in the Oslo Graben (ca. 120 000 km3) and in the NE German Basin (ca. 48 000 km3). In both these areas a large proportion of the magmatic rocks are highly evolved (trachytes-rhyolites). The dominant mantle source component for the mildly alkali basalts to subalkaline magmatism in the Oslo Graben and Scania (probably also Bornholm and the North Sea) is geochemically similar to the Prevalent Mantle (PREMA) component. Rifting and magmatism in the area is likely to be due to local decompression and thinning of highly asymmetric lithosphere in responses to regional stretching north of the Variscan Front, implying that the PREMA source is located in the lithospheric mantle. However, as PREMA sources are widely accepted to be plume-related, the possibility of a plume located beneath the area cannot be disregarded. Locally, there is also evidence of other sources. The oldest, highly alkaline basaltic lavas in the southernmost part of the Oslo Graben show HIMU trace element affinity, and initial Sr-Nd isotopic compositions different from that of the PREMA-type magmatism. These magmas are interpreted as the results of partial melting of enriched, metasomatised domains within the mantle lithosphere beneath the southern Olso Graben; this source enrichment can be linked to migration of carbonatite magmas in the earliest Paleozoic (ca. 580 Ma). Within northern Germany, mantle lithosphere modified by subduction-related fluids from Variscan subduction systems have provided an important magma source components.

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