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Jarl Øvstedal - One of the best experts on this subject based on the ideXlab platform.
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Cirque glaciers as morphological evidence for a thin Younger Dryas ice sheet in east‐central southern Norway
Boreas, 2008Co-Authors: Svein Olaf Dahl, Atle Nesje, Jarl ØvstedalAbstract:Three localities with marginal moraines deposited by former Cirque glaciers are investigated in east-central southern Norway. The wet-based (erosive) Cirque glaciers with aspects towards S-SW and N-NE are mapped at altitudes above 1100 m, and have a mean equilibrium-line altitude of 1275 m. With a suggested mean annual winter precipitation close to the average for the modern accumulation season (1 October-30 April) when the Cirque glaciers existed, the mean air-temperature depression during the ablation season (1 May-30 September) is calculated to be 6–7°C lower than at present. The high-altitude Cirques of central Rondane were still covered by ice when the low-altitude Cirque glaciers developed in distal position for this massif in eastern Rondane and on isolated mountains. Hence, the Cirque glaciers are suggested to have existed during the deglaciation after the Late Weichselian maximum, and most likely during the Younger Dryas (11000–10000 BP). The Cirque glaciers indicate a downwasting ice-sheet surface well below an altitude of 1100 m prior to the Younger Dryas, and this supports a limited (small) vertical extent for the Late Weichselian ice sheet in this region. With the contemporaneous level for instantaneous glacierization (glaciation threshold) just below the highest elevated peaks in east-central southern Norway, this fits with the idea of a continuous downwasting of the Late Weichselian ice sheet since the ‘first’ nunataks appeared. The occurrence of the Cirque glaciers indicates a multidomed Scandinavian ice-sheet geometry during the Late Weichselian.
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Cirque glaciers as morphological evidence for a thin younger dryas ice sheet in east central southern norway
Boreas, 2008Co-Authors: Svein Olaf Dahl, Atle Nesje, Jarl ØvstedalAbstract:Three localities with marginal moraines deposited by former Cirque glaciers are investigated in east-central southern Norway. The wet-based (erosive) Cirque glaciers with aspects towards S-SW and N-NE are mapped at altitudes above 1100 m, and have a mean equilibrium-line altitude of 1275 m. With a suggested mean annual winter precipitation close to the average for the modern accumulation season (1 October-30 April) when the Cirque glaciers existed, the mean air-temperature depression during the ablation season (1 May-30 September) is calculated to be 6–7°C lower than at present. The high-altitude Cirques of central Rondane were still covered by ice when the low-altitude Cirque glaciers developed in distal position for this massif in eastern Rondane and on isolated mountains. Hence, the Cirque glaciers are suggested to have existed during the deglaciation after the Late Weichselian maximum, and most likely during the Younger Dryas (11000–10000 BP). The Cirque glaciers indicate a downwasting ice-sheet surface well below an altitude of 1100 m prior to the Younger Dryas, and this supports a limited (small) vertical extent for the Late Weichselian ice sheet in this region. With the contemporaneous level for instantaneous glacierization (glaciation threshold) just below the highest elevated peaks in east-central southern Norway, this fits with the idea of a continuous downwasting of the Late Weichselian ice sheet since the ‘first’ nunataks appeared. The occurrence of the Cirque glaciers indicates a multidomed Scandinavian ice-sheet geometry during the Late Weichselian.
Svein Olaf Dahl - One of the best experts on this subject based on the ideXlab platform.
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Cirque glaciers as morphological evidence for a thin Younger Dryas ice sheet in east‐central southern Norway
Boreas, 2008Co-Authors: Svein Olaf Dahl, Atle Nesje, Jarl ØvstedalAbstract:Three localities with marginal moraines deposited by former Cirque glaciers are investigated in east-central southern Norway. The wet-based (erosive) Cirque glaciers with aspects towards S-SW and N-NE are mapped at altitudes above 1100 m, and have a mean equilibrium-line altitude of 1275 m. With a suggested mean annual winter precipitation close to the average for the modern accumulation season (1 October-30 April) when the Cirque glaciers existed, the mean air-temperature depression during the ablation season (1 May-30 September) is calculated to be 6–7°C lower than at present. The high-altitude Cirques of central Rondane were still covered by ice when the low-altitude Cirque glaciers developed in distal position for this massif in eastern Rondane and on isolated mountains. Hence, the Cirque glaciers are suggested to have existed during the deglaciation after the Late Weichselian maximum, and most likely during the Younger Dryas (11000–10000 BP). The Cirque glaciers indicate a downwasting ice-sheet surface well below an altitude of 1100 m prior to the Younger Dryas, and this supports a limited (small) vertical extent for the Late Weichselian ice sheet in this region. With the contemporaneous level for instantaneous glacierization (glaciation threshold) just below the highest elevated peaks in east-central southern Norway, this fits with the idea of a continuous downwasting of the Late Weichselian ice sheet since the ‘first’ nunataks appeared. The occurrence of the Cirque glaciers indicates a multidomed Scandinavian ice-sheet geometry during the Late Weichselian.
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Cirque glaciers as morphological evidence for a thin younger dryas ice sheet in east central southern norway
Boreas, 2008Co-Authors: Svein Olaf Dahl, Atle Nesje, Jarl ØvstedalAbstract:Three localities with marginal moraines deposited by former Cirque glaciers are investigated in east-central southern Norway. The wet-based (erosive) Cirque glaciers with aspects towards S-SW and N-NE are mapped at altitudes above 1100 m, and have a mean equilibrium-line altitude of 1275 m. With a suggested mean annual winter precipitation close to the average for the modern accumulation season (1 October-30 April) when the Cirque glaciers existed, the mean air-temperature depression during the ablation season (1 May-30 September) is calculated to be 6–7°C lower than at present. The high-altitude Cirques of central Rondane were still covered by ice when the low-altitude Cirque glaciers developed in distal position for this massif in eastern Rondane and on isolated mountains. Hence, the Cirque glaciers are suggested to have existed during the deglaciation after the Late Weichselian maximum, and most likely during the Younger Dryas (11000–10000 BP). The Cirque glaciers indicate a downwasting ice-sheet surface well below an altitude of 1100 m prior to the Younger Dryas, and this supports a limited (small) vertical extent for the Late Weichselian ice sheet in this region. With the contemporaneous level for instantaneous glacierization (glaciation threshold) just below the highest elevated peaks in east-central southern Norway, this fits with the idea of a continuous downwasting of the Late Weichselian ice sheet since the ‘first’ nunataks appeared. The occurrence of the Cirque glaciers indicates a multidomed Scandinavian ice-sheet geometry during the Late Weichselian.
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Cirque glacier activity in arctic Norway during the last deglaciation
Quaternary Research, 2007Co-Authors: Øyvind Paasche, Svein Olaf Dahl, Jostein Bakke, Reidar Løvlie, Atle NesjeAbstract:Abstract Numerous Cirques of the Lofoten–Vesteralen archipelago in northern Norway have distinct moraine sequences that previously have been assigned to the Allerod-Younger Dryas (∼ 13,400 to 11,700 yr BP) interval, constraining the regional distribution of the equilibrium-line altitude (ELA) of Cirque and valley glaciers. Here we present evidence from a once glacier-fed lake on southern Andoya that contests this view. Analyses of radiocarbon dated lacustrine sediments including rock magnetic parameters, grain size, organic matter, dry bulk density and visual interpretation suggest that no glacier was present in the low-lying Cirque during the Younger Dryas-Allerod. The initiation of the glacial retreat commenced with the onset of the Bolling warming (∼ 14,700 yr BP) and was completed by the onset of Allerod Interstade (∼ 13,400 yr BP). The reconstructed glacier stages of the investigated Cirque coincide with a cool and dry period from ∼ 17,500 to 14,700 yr BP and a somewhat larger Last Glacial Maximum (LGM) advance possibly occurring between ∼ 21,050 and 19,100 yr BP.
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Cirque Glacier Activity in Arctic Norway During the Last Deglaciation
2004Co-Authors: Øyvind Paasche, Svein Olaf Dahl, Jostein Bakke, Reidar Løvlie, Atle NesjeAbstract:Numerous Cirques of the Lofoten–Vesteralen archipelago in northern Norway have distinct moraine sequences that previously have been assigned to the Allerod-Younger Dryas (∼13,400 to 11,700 yr BP) interval, constraining the regional distribution of the equilibrium-line altitude (ELA) of Cirque and valley glaciers. Here we present evidence from a once glacier-fed lake on southern Andoya that contests this view. Analyses of radiocarbon dated lacustrine sediments including rock magnetic parameters, grain size, organic matter, dry bulk density and visual interpretation suggest that no glacier was present in the low-lying Cirque during the Younger Dryas-Allerod. The initiation of the glacial retreat commenced with the onset of the Bolling warming (∼14,700 yr BP) and was completed by the onset of Allerod Interstade (∼13,400 yr BP). The reconstructed glacier stages of the investigated Cirque coincide with a cool and dry period from ∼17,500 to 14,700 yr BP and a somewhat larger Last Glacial Maximum (LGM) advance possibly occurring between ∼21,050 and 19,100 yr BP. © 2007 University of Washington. All rights reserved.
Ian S. Evans - One of the best experts on this subject based on the ideXlab platform.
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The dynamics of mountain erosion: Cirque growth slows as landscapes age
Earth Surface Processes and Landforms, 2019Co-Authors: Iestyn D. Barr, Ian S. Evans, Jeremy C. Ely, Matteo Spagnolo, Matt D. TomkinsAbstract:Glacial Cirques are widely used palaeoenvironmental indicators, and are key to understanding the role of glaciers in shaping mountain topography. However, notable uncertainty persists regarding the rate and timing of Cirque erosion. In order to address this uncertainty, we analyse the dimensions of 2208 Cirques in Britain and Ireland and model ice accumulation to investigate the degree of coupling between glacier occupation times and Cirque growth. Results indicate that during the last ~120 ka, Cirques were glacier‐free for an average of 52.0 ± 21.2 ka (43 ± 18%); occupied by small (largely Cirque‐confined) glaciers for 16.2 ± 9.9 ka (14 ± 8%); and occupied by large glaciers, including ice sheets, for 51.8 ± 18.6 ka (43 ± 16%). Over the entire Quaternary (i.e., 2.6 Ma), we estimate that Cirques were glacier‐free for 1.1 ± 0.5 Ma; occupied by small glaciers for 0.3 ± 0.2 Ma; and occupied by large glaciers for 1.1 ± 0.4 Ma. Comparing occupation times to Cirque depths, and calculating required erosion rates reveals that continuous Cirque growth during glacier occupation is unlikely. Instead, we propose that Cirques attained much of their size during the first occupation of a non‐glacially sculpted landscape (perhaps during the timeframe of a single glacial cycle). During subsequent glacier occupations, Cirque growth may have slowed considerably, with the highest rates of subglacial erosion focused during periods of marginal (small glacier) glaciation. We propose comparatively slow rates of growth following initial Cirque development because a ‘least resistance’ shape is formed, and as Cirques deepen, sediment becomes trapped subglacially, partly protecting the bedrock from subsequent erosion. In support of the idea of rapid Cirque growth, we present evidence from northern British Columbia, where Cirques of comparable size to those in Britain and Ireland developed in less than 140 ka.
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Effects of glaciation on the clinometry and hypsometry of the Romanian Carpathians
2018Co-Authors: Niculita Mihai, Ian S. EvansAbstract:SRTM1 Digital Elevation Model for many mountain ranges in Romania were processed to provide slope gradient distributions for each 50 m band of altitude. The effects of Cirques are seen in an increase in standard deviation of gradient, and a spread in all percentiles, especially an increase in the 95th, at relevant altitudes. Otherwise, variations in median gradient with altitude differ between ranges, and do not show the general increase found in the Alps below Cirque floor altitudes by several authors.
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climate patterns during former periods of mountain glaciation in britain and ireland inferences from the Cirque record
Palaeogeography Palaeoclimatology Palaeoecology, 2017Co-Authors: Iestyn D. Barr, Ian S. Evans, Jeremy C. Ely, Matteo Spagnolo, Chris D Clark, Xavier M Pellicer, Ramon Pellitero, Brice R. ReaAbstract:We map glacial Cirques, and analyse spatial variability in their altitude and aspect to derive a long-term, time-integrated, perspective on climate patterns during former periods of mountain glaciation (likely spanning multiple Quaternary glaciations) in Britain and Ireland. The data reveal that, although air temperatures were important, exposure to moisture-bearing air masses was the key factor in regulating sites of former mountain glacier formation, and indicate that during such periods, moisture supply was largely controlled by North Atlantic westerlies, with notable inland precipitation gradients (precipitation decreasing inland), similar to present day. In places, trends in Cirque altitude may also reflect regional differences in the extent of Cirque deepening, controlled by the dimensions and dynamics of the glaciers that came to occupy them. Specifically, comparatively deep Cirques in coastal locations may reflect the former presence of dynamic (fed by moisture from the North Atlantic), but comparatively small, glaciers (largely confined to their Cirques). By contrast, decreasing Cirque depth further inland, may reflect the former presence of larger and/or less dynamic ice masses, occupying comparatively continental climatic conditions.
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Glacial Cirques in the Romanian Carpathians and Their Climatic Implications
Springer Geography, 2016Co-Authors: Marcel Mîndrescu, Ian S. EvansAbstract:This section summarizes a morphometric analysis of glacial Cirques from the Romanian Carpathians, further inferring climatic information from spatial patterns of morphometric traits. Results derived from the detailed statistical analysis of a comprehensive database of glacial Cirques are presented briefly. The distribution of Cirques by altitude, aspect, size, classification (Cirque grade), and geology is presented and related to controlling factors. New contributions concerning the palaeoglaciation level during Late Pleistocene in the Romanian Carpathians, and the direction of prevailing winds during glaciation are provided. Statistical distributions of Cirque size and shape are outlined and illustrated. A comprehensive data base of all glacial Cirques in the Romanian Carpathians is now available to guide or substantiate further comparative or/and interdisciplinary studies. It is also possible to rank the mountain ranges by the degree of glacial modification.
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Cirque form and development in Romania: Allometry and the buzzsaw hypothesis
Geomorphology, 2014Co-Authors: Marcel Mîndrescu, Ian S. EvansAbstract:Abstract A complete inventory of the 631 glacial Cirques in Romania (and adjacent Ukraine) shows that they are scale-specific and develop allometrically, extending in length more rapidly than they deepen: shape changes with size. Horizontal dimensions are around 700 m, vertical around 300 m. On each quantitative measure they form a single, unimodal population with limited variation in size (on a logarithmic scale). Cirque axial gradients (15–33°) are appropriate to occupation by rotationally-flowing glaciers. Lake basins are more common (26%) on granitic rocks, compared with 10.6% of all Cirques. Geology also affects vertical dimensions and gradients. The relation between subjective (five grades) and objective measures of Cirque development is shown by an r 2 of 62% when Grade is predicted from maximum gradient, minimum gradient, and plan closure. Cirques larger in horizontal dimensions have better grades. Cirque floors cover around 28% of Cirque map area. Larger Cirques, and nested inner and outer Cirques, are common in ranges that rose well above the snowline, and are associated with more extensive, more symmetrical glaciation and more cols. In size and shape Romanian Cirques are similar to those in England and Wales. They are similar in some respects to those in part of the British Columbia Coast Mountains, except for smaller vertical dimensions. They may have developed only in the last few glaciations: mountain ranges can be ranked by degree of glacial modification and symmetry of glaciations. The ‘buzzsaw hypothesis’ is applicable only in the cores of the higher ranges: elsewhere, summit surfaces that are essentially preglacial survive.
Atle Nesje - One of the best experts on this subject based on the ideXlab platform.
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Cirque glaciers as morphological evidence for a thin Younger Dryas ice sheet in east‐central southern Norway
Boreas, 2008Co-Authors: Svein Olaf Dahl, Atle Nesje, Jarl ØvstedalAbstract:Three localities with marginal moraines deposited by former Cirque glaciers are investigated in east-central southern Norway. The wet-based (erosive) Cirque glaciers with aspects towards S-SW and N-NE are mapped at altitudes above 1100 m, and have a mean equilibrium-line altitude of 1275 m. With a suggested mean annual winter precipitation close to the average for the modern accumulation season (1 October-30 April) when the Cirque glaciers existed, the mean air-temperature depression during the ablation season (1 May-30 September) is calculated to be 6–7°C lower than at present. The high-altitude Cirques of central Rondane were still covered by ice when the low-altitude Cirque glaciers developed in distal position for this massif in eastern Rondane and on isolated mountains. Hence, the Cirque glaciers are suggested to have existed during the deglaciation after the Late Weichselian maximum, and most likely during the Younger Dryas (11000–10000 BP). The Cirque glaciers indicate a downwasting ice-sheet surface well below an altitude of 1100 m prior to the Younger Dryas, and this supports a limited (small) vertical extent for the Late Weichselian ice sheet in this region. With the contemporaneous level for instantaneous glacierization (glaciation threshold) just below the highest elevated peaks in east-central southern Norway, this fits with the idea of a continuous downwasting of the Late Weichselian ice sheet since the ‘first’ nunataks appeared. The occurrence of the Cirque glaciers indicates a multidomed Scandinavian ice-sheet geometry during the Late Weichselian.
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Cirque glaciers as morphological evidence for a thin younger dryas ice sheet in east central southern norway
Boreas, 2008Co-Authors: Svein Olaf Dahl, Atle Nesje, Jarl ØvstedalAbstract:Three localities with marginal moraines deposited by former Cirque glaciers are investigated in east-central southern Norway. The wet-based (erosive) Cirque glaciers with aspects towards S-SW and N-NE are mapped at altitudes above 1100 m, and have a mean equilibrium-line altitude of 1275 m. With a suggested mean annual winter precipitation close to the average for the modern accumulation season (1 October-30 April) when the Cirque glaciers existed, the mean air-temperature depression during the ablation season (1 May-30 September) is calculated to be 6–7°C lower than at present. The high-altitude Cirques of central Rondane were still covered by ice when the low-altitude Cirque glaciers developed in distal position for this massif in eastern Rondane and on isolated mountains. Hence, the Cirque glaciers are suggested to have existed during the deglaciation after the Late Weichselian maximum, and most likely during the Younger Dryas (11000–10000 BP). The Cirque glaciers indicate a downwasting ice-sheet surface well below an altitude of 1100 m prior to the Younger Dryas, and this supports a limited (small) vertical extent for the Late Weichselian ice sheet in this region. With the contemporaneous level for instantaneous glacierization (glaciation threshold) just below the highest elevated peaks in east-central southern Norway, this fits with the idea of a continuous downwasting of the Late Weichselian ice sheet since the ‘first’ nunataks appeared. The occurrence of the Cirque glaciers indicates a multidomed Scandinavian ice-sheet geometry during the Late Weichselian.
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Cirque glacier activity in arctic Norway during the last deglaciation
Quaternary Research, 2007Co-Authors: Øyvind Paasche, Svein Olaf Dahl, Jostein Bakke, Reidar Løvlie, Atle NesjeAbstract:Abstract Numerous Cirques of the Lofoten–Vesteralen archipelago in northern Norway have distinct moraine sequences that previously have been assigned to the Allerod-Younger Dryas (∼ 13,400 to 11,700 yr BP) interval, constraining the regional distribution of the equilibrium-line altitude (ELA) of Cirque and valley glaciers. Here we present evidence from a once glacier-fed lake on southern Andoya that contests this view. Analyses of radiocarbon dated lacustrine sediments including rock magnetic parameters, grain size, organic matter, dry bulk density and visual interpretation suggest that no glacier was present in the low-lying Cirque during the Younger Dryas-Allerod. The initiation of the glacial retreat commenced with the onset of the Bolling warming (∼ 14,700 yr BP) and was completed by the onset of Allerod Interstade (∼ 13,400 yr BP). The reconstructed glacier stages of the investigated Cirque coincide with a cool and dry period from ∼ 17,500 to 14,700 yr BP and a somewhat larger Last Glacial Maximum (LGM) advance possibly occurring between ∼ 21,050 and 19,100 yr BP.
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Cirque Glacier Activity in Arctic Norway During the Last Deglaciation
2004Co-Authors: Øyvind Paasche, Svein Olaf Dahl, Jostein Bakke, Reidar Løvlie, Atle NesjeAbstract:Numerous Cirques of the Lofoten–Vesteralen archipelago in northern Norway have distinct moraine sequences that previously have been assigned to the Allerod-Younger Dryas (∼13,400 to 11,700 yr BP) interval, constraining the regional distribution of the equilibrium-line altitude (ELA) of Cirque and valley glaciers. Here we present evidence from a once glacier-fed lake on southern Andoya that contests this view. Analyses of radiocarbon dated lacustrine sediments including rock magnetic parameters, grain size, organic matter, dry bulk density and visual interpretation suggest that no glacier was present in the low-lying Cirque during the Younger Dryas-Allerod. The initiation of the glacial retreat commenced with the onset of the Bolling warming (∼14,700 yr BP) and was completed by the onset of Allerod Interstade (∼13,400 yr BP). The reconstructed glacier stages of the investigated Cirque coincide with a cool and dry period from ∼17,500 to 14,700 yr BP and a somewhat larger Last Glacial Maximum (LGM) advance possibly occurring between ∼21,050 and 19,100 yr BP. © 2007 University of Washington. All rights reserved.
Thibault Catry - One of the best experts on this subject based on the ideXlab platform.
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Large-scale destabilization of a basaltic shield volcano (Piton des Neiges volcano, La Réunion Island)
2010Co-Authors: Marie Chaput, Vincent Famin, Laurent Michon, Thibault CatryAbstract:Piton des Neiges is a highly extinct shield volcano eroded by three deep depressions (Cirques) surrounding the summit of the edifice. Within the Cirques are exposed old basic products from the volcano shield building stage (>2,1Ma – 430 ka). We carried out a structural investigation in the basic deposits of the Cirque of Cilaos (south of Piton des Neiges summit) to constrain the destabilization processes that affected the edifice during its early aerial phase of construction. The internal structure of the Cirque of Cilaos is made of piled basic lava flows and breccias, capped by lavas from the differentiated stage (< 350 ka). The breccias mainly consist in debris avalanche deposits and debris flows containing only basic elements. Two kinds of debris avalanche breccias can be distinguished: the " lower " breccia, with a consolidated green-dark matrix, is widely zeolitised, chloritized and serpentinized and is crosscut by many intrusions (dikes and sills). The " upper " breccia presents a light brown zeolitized matrix, poorly consolidated and poorly intruded. Both avalanches display jigsaw-cracked blocks, up to a few tens of meters in diameter. Several evidences of brittle deformation (faults and/or intrusions) were recorded within the breccias and the basic lava flows. Paleo-stresses reconstructed from deformation data allow the recognition of two successive episodes of deformation (Figure 1):-‐ The first, older step of deformation is extensional with a minimum principal stress σ 3 oriented N-S. It is recorded within the " lower " breccia and the underlying zeolitized lava flows which are assumed to be the oldest formations of the Cirque.-‐ The second, later step of deformation is extensional with σ 3 oriented NW-SE. It is widely expressed in all basaltic formations (lavas and breccias) of the Cirque. It is coeval with strike-slip faulting whose minimum principal stress is also oriented NW-SE. Shear structures and beddings dips within the breccia suggest a runout toward the NW. Intrusions from the basic stage, striking N20-N40 and dipping NW, are also consistent with a flank destabilization toward the NW. These results in Cilaos show that the dismantling of Piton des Neiges volcano is the consequence of both slow and rapid destabilizations that occurred in at least two episodes: (1) a N-S destabilization during the initial eruptive history of the volcano, whose related deposits, zeolitized and chloritized, are still visible at the bottom of the Cirque, in riverbeds. (2) a NW rapid collapse producing debris avalanche deposits (that currently cover the main part of the Cirque of Cilaos), accompanied by slow internal deformation that affected the whole stratigraphic sequence. The temporal evolution of the stress fields recorded in Cilaos can also be observed north of Piton des Neiges, in the Cirque of Salazie. A recent study within debris avalanche deposits attests that the northern flank of the edifice was affected by an extensional deformation (rapid failure combined with slow deformation) in a N-S direction (Famin and Michon, 2010). New deformation data, collected in overlying pahoehoe lava flows (in the northern scarp of the Cirque of Salazie), evidence that this flank undergone a later stage of slow internal deformation in a NW-SE direction. The combined study of Cirques of Cilaos and Salazie reveals that (1) dismantling events on Piton des Neiges volcano are characterized by the combination of episodic catastrophic flank collapses and slow deformation, (2) the stress field evolved from a N-S-to a NW-SE-oriented extension during the shield building stage, perhaps as a consequence of the nearby growth of the Alizés or and/or Piton de la Fournaise volcanoes, and (3) this evolution was a large-scale process that affected the whole edifice.
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large scale destabilization of a basaltic shield volcano piton des neiges volcano la reunion island
AGU, 2010Co-Authors: Marie Chaput, Vincent Famin, Laurent Michon, Thibault CatryAbstract:Piton des Neiges is a highly extinct shield volcano eroded by three deep depressions (Cirques) surrounding the summit of the edifice. Within the Cirques are exposed old basic products from the volcano shield building stage (>2,1Ma – 430 ka). We carried out a structural investigation in the basic deposits of the Cirque of Cilaos (south of Piton des Neiges summit) to constrain the destabilization processes that affected the edifice during its early aerial phase of construction. The internal structure of the Cirque of Cilaos is made of piled basic lava flows and breccias, capped by lavas from the differentiated stage (< 350 ka). The breccias mainly consist in debris avalanche deposits and debris flows containing only basic elements. Two kinds of debris avalanche breccias can be distinguished: the " lower " breccia, with a consolidated green-dark matrix, is widely zeolitised, chloritized and serpentinized and is crosscut by many intrusions (dikes and sills). The " upper " breccia presents a light brown zeolitized matrix, poorly consolidated and poorly intruded. Both avalanches display jigsaw-cracked blocks, up to a few tens of meters in diameter. Several evidences of brittle deformation (faults and/or intrusions) were recorded within the breccias and the basic lava flows. Paleo-stresses reconstructed from deformation data allow the recognition of two successive episodes of deformation (Figure 1):-‐ The first, older step of deformation is extensional with a minimum principal stress σ 3 oriented N-S. It is recorded within the " lower " breccia and the underlying zeolitized lava flows which are assumed to be the oldest formations of the Cirque.-‐ The second, later step of deformation is extensional with σ 3 oriented NW-SE. It is widely expressed in all basaltic formations (lavas and breccias) of the Cirque. It is coeval with strike-slip faulting whose minimum principal stress is also oriented NW-SE. Shear structures and beddings dips within the breccia suggest a runout toward the NW. Intrusions from the basic stage, striking N20-N40 and dipping NW, are also consistent with a flank destabilization toward the NW. These results in Cilaos show that the dismantling of Piton des Neiges volcano is the consequence of both slow and rapid destabilizations that occurred in at least two episodes: (1) a N-S destabilization during the initial eruptive history of the volcano, whose related deposits, zeolitized and chloritized, are still visible at the bottom of the Cirque, in riverbeds. (2) a NW rapid collapse producing debris avalanche deposits (that currently cover the main part of the Cirque of Cilaos), accompanied by slow internal deformation that affected the whole stratigraphic sequence. The temporal evolution of the stress fields recorded in Cilaos can also be observed north of Piton des Neiges, in the Cirque of Salazie. A recent study within debris avalanche deposits attests that the northern flank of the edifice was affected by an extensional deformation (rapid failure combined with slow deformation) in a N-S direction (Famin and Michon, 2010). New deformation data, collected in overlying pahoehoe lava flows (in the northern scarp of the Cirque of Salazie), evidence that this flank undergone a later stage of slow internal deformation in a NW-SE direction. The combined study of Cirques of Cilaos and Salazie reveals that (1) dismantling events on Piton des Neiges volcano are characterized by the combination of episodic catastrophic flank collapses and slow deformation, (2) the stress field evolved from a N-S-to a NW-SE-oriented extension during the shield building stage, perhaps as a consequence of the nearby growth of the Alizes or and/or Piton de la Fournaise volcanoes, and (3) this evolution was a large-scale process that affected the whole edifice.