Hervé Guillou

Bio: Hervé Guillou is an academic researcher from Centre national de la recherche scientifique. The author has contributed to research in topics: Volcano & Lava. The author has an hindex of 48, co-authored 190 publications receiving 6643 citations.

Hotspot volcanism close to a passive continental margin; the Canary Islands

Abstract: The Canarian Archipelago is a group of volcanic islands on a slow-moving oceanic plate, close to a continental margin. The origins of the archipelago are controversial: a hotspot or mantle plume, a zone of lithospheric deformation, a region of compressional block-faulting or a rupture propagating westwards from the active Atlas Mountains fold belt have been proposed by different authors. However, comparison of the Canarian Archipelago with the prototypical hotspot-related island group, the Hawaiian Archipelago, reveals that the differences between the two are not as great as had previously been supposed on the basis of older data. Quaternary igneous activity in the Canaries is concentrated at the western end of the archipelago, close to the present-day location of the inferred hotspot. This is the same relationship as seen in the Hawaiian and Cape Verde islands. The latter archipelago, associated with a well-defined but slow-moving mantle plume, shows anomalies in a plot of island age against distance which are comparable to those seen in the Canary Islands: these anomalies cannot therefore be used to argue against a hotspot origin for the Canaries. Individual islands in both archipelagoes are characterized by initial rapid growth (the ‘shield-building’ stages of activity), followed by a period of quiescence and deep erosion (erosion gap) which in turn is followed by a ‘post-erosional’ stage of activity. The absence of post-shield stage subsidence in the Canaries is in marked contrast with the major subsidence experienced by the Hawaiian Islands, but is comparable with the lack of subsidence evident in other island groups at slow-moving hotspots, such as the Cape Verdes. Comparison of the structure and structural evolution of the Canary Islands with other oceanic islands such as Hawaii and Reunion reveals many similarities. These include the development of triple (‘Mercedes Star’) rift zones and the occurrence of giant lateral collapses on the flanks of these rift zones. The apparent absence of these features in the post-erosional islands may in part be a result of their greater age and deeper erosion, which has removed much of the evidence for their early volcanic architecture. We conclude that the many similarities between the Canary Islands and island groups whose hotspot origins are undisputed show that the Canaries have been produced in the same way.

Geology and volcanology of La Palma and El Hierro, Western Canaries

Abstract: The western Canaries, relatively little studied until a few years ago from the geological point of view, have however provided decisive data for understanding many of the most important geological problems of the Archipelago, which would probably have been dilucidated earlier, had the study begun with the most recent islands, as occurs in similar chains of oceanic volcanic islands in other parts of the world. To summarize the main geological features and evolutionary characteristics of both islands we emphasize the following stages of development: During the Pliocene, a submarine volcanic edifice or seamount formed in the island of La Palma, made up of pillow lavas, pillow breccias and hyaloclastites, intruded by trachytic domes, plugs of gabbros, and a highly dense dyke swarm. The intense magmatic and dyke intrusion uplifted the searnount up to 1,500 m, tilting it 45-50" to the SW. This intrusive phase was followed by a period of quiescence and erosion of the emerged submarine edifice. The definitive consolidation and progression of the construction of the island continued from at least 1.77 ma in angular and erosive discordance over the submarine basement. The subaerial volcanic reactivation, in which explosive volcanism predominated during the initial stages, producing abundant volcanoclastic and phreatomagmatic materials at the base of the subaerial edifice, persisted in a highly continuous manner until at least 0.41 ma. This initial subaerial stage shaped the northern volcanic shield, formed by the accumulation of several superimposed volcanoes, approximately concentric in relation to one another and the submarine basement. The initial stage of the northern volcanic shield lasted between 1.77 and 1.20 ma, during which period the Garafia volcano was built to a height of 2,500-3,000 m, with steeply sloping flanks, formed predominantly by alkaline basalts with abundant pahoehoe lavas. The rapid growth and progressive instability of the Garafia volcano culminated some 1.20 ma ago in a gravitational landslide of the south flank of the volcanic edifice. The eruptive activity that followed the collapse built the Taburiente volcano, that rests upon a clear angular discordance caused by the landslide. The landslide depression was filled completely some 0.89 ma ago, as shown by the age of the first lavas to overflow the collapse embayment. The filling-in of the depression by the Taburiente volcano lavas finally formed a sequence of horizontal lavas, predominantly alkaline basalts, that ponded against the headwall of the landslide scarp fonning a plateau in the centre of the volcanic shield. Coinciding approximately with the Matuyama-Brunhes boundary (0.78 ma) an important reorganisation of the Taburiente volcano took place, the dispersed emission centres of which progressively concentrated in three increasingly defined rifts (NW, NE and N-S) and subsequently in a central edifice situated at the geometrical centre of the volcanic shield. The abundant emissions of this final stage covered the earlier formations with sequences of lava flows up to 1,000 m thick in places, with the exception of a part of the alignments of cones of the rifts. The basaltic lavas evolved towards more differentiated phonolitic and trachytic terms at the terminal phases of construction of the volcano. One of these rifts, the southern or Cumbre Nueva rift, developed more than the others, possibly because the volcanism already began to migrate southwards, forming a N-S trending dorsal ridge over 2,500 m high. The progressive instability of the Cumbre Nueva rift, due to overgrowth, triggered the gravitational landslide of the western flank, in a process that took place about 560 ka ago, involving the detachment of some 180-200 km3 and the formation of a wide depression (the Valle de Aridane) and the beginning of the formation, by incision and retrogressive erosion, of the Caldera de Taburiente. The activity subsequent to the collapse in the northern shield was preferentially concentrated in the interior of the new collapse basin, quickly building the Bejenado stratovolcano. This activity was coetaneous with that of other residual centres dispersed over the flanks of the shield. The initially basanite lavas of Bejenado volcano evolved to mafic tephrites in differentiated lateral and terminal vents. The activity of the volcanic shield ceased definitively some 0.4 ma ago. After a transition period with a certain degree of activity associated with Bejenado late peripheral vents, volcanism was definitively located until the present in the new Cumbre Vieja volcano, at the south of the island. The oldest Cumbre Vieja lavas have been dated in 123 ka, although the first eruptions of the volcano may be considerably older. During this last stage of volcanism in La Palma a N-S trending rift has been formed, with predominantly basanitic, tephritic and tephri-phonolitic lavas, and intrusions of domes of tephri-phonolites and phonolites, frequently associated with eruptive vents. Numerous submarine eruptive vents, severa1 of which are apparently very recent, have recently been observed and sampled at the prolongation of the Cumbre Vieja rift southwards in the ocean. The foreseeable geological evolution of this rift is similar to that of its Cumbre Nueva predecesor, towards a progressive development and increasing instability, although changes may take place that may modify it towards more stable configurations, fundamentally the submarine progression of the southern tip of the rift, that could redistribute the volume of emitted materials, reduce the aspect ratio of the volcano and, consequently, its instability. The en echelon faults generated during the 1949 eruption have been interpreted as a possible detachment of the western flank of the volcano, although a more favourable hypothesis would be that such faults are surficial and contribute to accommodating the volcano by reducing its instability. A noteworthy aspect is the important role played by the mobility of the general feeding system of the volcanism in shaping the form and structure of the island. If the volcanism had not continually migrated southward since the final stages of construction of the northern shield, the island of La Palma would probably have taken on a similar configuration to that of the islands of El Hierro or Tenerife, in the shape of a triangular pyramid, with triple-armed rifts and landslide lobes between the rifts. The southward migration of volcanism in La Palma left the northern shield extinct, the rifts incomplete and finally configured an island lengthened in a N-S direction. Another point of interest is that the islands of La Palma and El Hierro are the first of the Canaries to form simultaneously, with possibly alternating eruptive activity, at least in the most recent period. This separation in a «dual line» of islands and the greater depth of its oceanic basement account for the long time they have required to emerge since the formation of the prior island of La Gomera. The island of El Hierro is geologically somewhat younger than La Palma and, because it formed over a stationary source of magma, it presents, in comparison, a perfect, concentric development, with superimposed volcanoes and a regular three-armed rift geometry. The activity of the subaerial volcanism began in El Hierro with the development of Tinor volcano on the NE flank of the island (approximately from 1.12 to 0.88 ma), with the emission of massive typical basalts. The volcano developed quickly, with different stages of growth, the eruption of Ventejis volcano being the terminal explosive stage, and probably the precursor of the collapse of the NW flank of the edifice some 882 ka ago. The emissions of the new volcano -El Golfo, approximately 545 to 176.000 ka- totally filled the depression of the lateral collapse of Tinor volcano, the lava flows of which then spilled over the flanks of the earlier volcano. The beginning of the construction of the El Golfo volcano seems to have taken place after a relatively long period of activity, probably coinciding with the maximum development of the Cumbre Nueva rift on La Palma. The initial subaerial activity at El Golfo was characterised by basaltic lavas that evolved to trachybasalts and trachytes, and finally towards more differentiated eruptive episodes indicative of the terminal state of the volcanic activity of the El Golfo volcano. The excessive growth of this volcano triggered the failure of its north flank, generating the spectacular scarp and present El Golfo depression. Subsequent volcanism, from emission vents arranged in a three-armed rift system (rift volcanism, with ages ranging from 145 ka to 2,500 years, with probably prehistoric eruptions), implies the much more moderate continuation of the earlier predominantly basanitic-tephritic volcanic activity. This period may correspond to that of maximum development of the Cumbre Vieja rift, in the island of La Palma.

Eruptive and structural history of Teide Volcano and rift zones of Tenerife, Canary Islands

Abstract: The Teide and Pico Viejo stratocones and the Northwest and Northeast Rifts are products of the latest eruptive phase of the island of Tenerife, initiated with the lateral collapse of its northern flank that formed the Las Canadas Caldera and the Icod–La Guancha Valley ca. 200 ka. The eruptive and structural evolution of this volcanic complex has been reconstructed after detailed geological mapping and radioisotopic dating of the significant eruptive events. A set of 54 new 14 C and K/Ar ages provides precise age control of the recent eruptive history of Tenerife, particularly Teide Volcano, the third-highest volcanic feature on Earth (3718 m above sea level, >7 km high), and unique in terms of its intraplate setting. The development of the Teide–Pico Viejo Volcanoes may be related to the activity of the Northwest and Northeast Rifts. Volcanic and intrusive activity along both rift zones may have played an important role in activating the gravitational landslide and in the subsequent growth, nested within the collapse embayment, of an increasingly higher central volcano with progressively differentiated magmas. The coeval growth of the central volcano with sustained activity along the rifts led to a clear bimodal distribution in composition of eruptive products, with the basaltic eruptions in the distal part of the rifts and phonolitic and more explosive eruptions in the central area, where the differentiated stratocones developed. Current volcanic hazard in Tenerife is considered to be moderate, because eruptive frequency is low, explosivity is modest, and the eruptive activity of the Teide stratocone seems to have declined over the past 30 k.y., with only one eruption in this period (1150 yr B.P.).

40Ar/39Ar and K-Ar chronology of Pleistocene glaciations in Patagonia

Abstract: During the Pleistocene, east of Lago Buenos Aires, Argentina, at 46.5 °S, at least 19 terminal moraines were deposited as piedmont glaciers from the Patagonian ice cap advanced onto the semi-arid high plains adjacent to the southern Andes. Exceptional preservation of these deposits offers a rare opportunity to document ice-cap fluctuations during the last 1.2 m.y. 4 0 Ar/ 3 9 Ar incremental-heating and unspiked K-Ar experiments on four basaltic lava flows interbedded with the moraines provide a chronologic framework for the entire glacial sequence. The 4 0 Ar/ 3 9 Ar isochron ages of three lavas that overlie till 90 km east of the Cordillera at Lago Buenos Aires, and another 120 km from the Andes along Rio Gallegos at 51.8 °S that underlies till, strongly suggest that the ice cap reached its greatest eastward extent ca. 1100 ka. At least six moraines were deposited within the 256 k.y. period bracketed by basaltic eruptions at 1016 ′ 10 ka and 760 ′ 14 ka. Similarly, six younger, more proximal moraines were deposited during an ∼651 k.y. period bracketed by an underlying 760 ′ 14 ka basalt and the 109 ′ 3 ka Cerro Volcan basalt flow that buried all six moraines. Coupled with in situ cosmogenic surface exposure ages of moraine boulders, the 109 ka age of Cerro Volcan implies that moraines deposited during the penultimate local glaciation correspond to marine oxygen isotope stage 6. Further westward toward Lago Buenos Aires, six additional moraines younger than the Cerro Volcan basalt flow occur. Surface exposure dating of boulders on these moraines, combined with the 1 4 C age of overlying varved lacustrine sediment, indicates deposition during the Last Glacial Maximum (LGM, 23-16 ka). Although Antarctic dust records signal an important Patagonian glaciation at 60-40 ka, moraines corresponding to marine oxygen isotope stage 4 are not preserved at Lago Buenos Aires; apparently, these were overrun and obliterated by the younger ice advance at 23 ka. Notwithstanding, the overall pattern of glaciation in Patagonia is one of diminishing eastward extent of ice during successive glacial advances over the past 1 m.y. We hypothesize that tectonically driven uplift of the Patagonian Andes, which began in the Pliocene, yet continued into the Quaternary, in part due to subduction of the Chile rise spreading center during the past 2 m.y., maximized the ice accumulation area and ice extent by 1.1 Ma. Subsequent deep glacial erosion has reduced the accumulation area, resulting in less extensive glaciers over time.

IntCal13 and Marine13 radiocarbon age calibration curves 0-50,000 years cal BP

Abstract: Additional co-authors: TJ Heaton, AG Hogg, KA Hughen, KF Kaiser, B Kromer, SW Manning, RW Reimer, DA Richards, JR Southon, S Talamo, CSM Turney, J van der Plicht, CE Weyhenmeyer

The IntCal20 Northern Hemisphere Radiocarbon Age Calibration Curve (0-55 cal kBP)

Abstract: Radiocarbon (14C) ages cannot provide absolutely dated chronologies for archaeological or paleoenvironmental studies directly but must be converted to calendar age equivalents using a calibration curve compensating for fluctuations in atmospheric 14C concentration. Although calibration curves are constructed from independently dated archives, they invariably require revision as new data become available and our understanding of the Earth system improves. In this volume the international 14C calibration curves for both the Northern and Southern Hemispheres, as well as for the ocean surface layer, have been updated to include a wealth of new data and extended to 55,000 cal BP. Based on tree rings, IntCal20 now extends as a fully atmospheric record to ca. 13,900 cal BP. For the older part of the timescale, IntCal20 comprises statistically integrated evidence from floating tree-ring chronologies, lacustrine and marine sediments, speleothems, and corals. We utilized improved evaluation of the timescales and location variable 14C offsets from the atmosphere (reservoir age, dead carbon fraction) for each dataset. New statistical methods have refined the structure of the calibration curves while maintaining a robust treatment of uncertainties in the 14C ages, the calendar ages and other corrections. The inclusion of modeled marine reservoir ages derived from a three-dimensional ocean circulation model has allowed us to apply more appropriate reservoir corrections to the marine 14C data rather than the previous use of constant regional offsets from the atmosphere. Here we provide an overview of the new and revised datasets and the associated methods used for the construction of the IntCal20 curve and explore potential regional offsets for tree-ring data. We discuss the main differences with respect to the previous calibration curve, IntCal13, and some of the implications for archaeology and geosciences ranging from the recent past to the time of the extinction of the Neanderthals.

The Neogene Period

Abstract: An Astronomically Tuned Neogene Time Scale (ATNTS2012) is presented, as an update of ATNTS2004 in GTS2004. The new scale is not fundamentally different from its predecessor and the numerical ages are identical or almost so. Astronomical tuning has in principle the potential of generating a stable Neogene time scale as a function of the accuracy of the La2004 astronomical solution used for both scales. Minor problems remain in the tuning of the Lower Miocene. In GTS2012 we will summarize what has been modified or added since the publication of ATNTS2004 for incorporation in its successor, ATNTS2012. Mammal biostratigraphy and its chronology are elaborated, and the regional Neogene stages of the Paratethys and New Zealand are briefy discussed. To keep changes to ATNTS2004 transparent we maintain its subdivision into headings as much as possible.