Effets de la charge des édifices volcaniques sur la propagation de structures régionales compressives : exemples naturels et modèles expérimentaux

Nous présentons ici des exemples naturels d'édifices volcaniques coniques reposant sur un substratum fragile, soumis à une compression régionale ainsi que des résultats expérimentaux. Nous montrons que la charge de l'édifice induit une perturbation de la déformation régionale se traduisant par une déflexion et une horizontalisation des structures compressives régionales. Le contrôle tectonique est de nature topographique. Nous discutons ensuite certaines conséquences, en particulier concernant l'étalement gravitaire des volcans. We present natural examples and experimental models of volcanic cones located above brittle substratum undergoing regional compressive deformation. The volcanic loading induces a strain partitioning involving deflection and flattening of regional compressive structures. The main control is the topographic load. Anticlinal thrust ridges, observed around many volcanoes, have generally been interpreted as being due to gravitational spreading; however, this study shows that this is not necessarily the case, as they can also be a symptom of regional compression

Field evidence for summit subsidence, flank instability and basal spreading at Mt Cameroon volcano, West Africa

Mt Cameroon is a steep lava-dominated volcano located on the coast of the Gulf of Guinea. This 1400 km3 edifice is one of two active centres in the Cameroon Volcanic Line. Despite recent lava eruptions along its rift zones in 1999 and 2000, little geological or monitoring data are available to understand the structure of this large volcanic system. Here we report results from a field campaign dedicated to mapping geological structures in the summit area and at the SE base of Mount Cameroon. Eruptive fissures and open fractures’ orientation, vents’ location and alignment above 3500 m a.s.l were systematically surveyed. In addition to the tectonically-controlled N40°E orientation of eruptive fissures along the rift zones, other dominant orientations were identified such as N60°E (summit vents alignment), N20°E and N90° (extension related structures). These were attributed to local instability around the summit, stress field re-orientation around the head of a deep valley cutting through the NW flank and radial pattern around the summit. Inward-dipping structures were also observed to border the relatively flat upper part of the rift zones. Geological profiles were also measured along rivers cutting through a topographic bulge at the SE base of Mt Cameroon. This topographic step was seen to be associated with deformed Miocene sediments from the Douala basin overlain by volcanic products.Weak sediments of this area are deformed by N50- 60°E trending asymmetrical folds verging toward the SE and by N10-30°E trending symmetrical folds and thrusts. Initial NE-SW trending structures formed following the sliding of sediments on the flank of a NE-SW elongated uplift dome. Later, the same area has been deformed by NNE-SSW trending compressive structures linked to the spreading of Mt Cameroon southern flank toward the SE. Combined with the interpretation of a 30 m Digital Elevation Models and multispectral satellite data, the field observations suggest that Mt Cameroon is affected by major instabilities. Both slow spreading movements and catastrophic collapses of the steep flanks are interpreted to result from complex interactions between the growing edifice, repeated dyke intrusions, the weak sedimentary substratum and tectonic structures

Granular fingering as a mechanism for ridge formation in debris avalanche deposits: Laboratory experiments and implications for Tutupaca volcano, Peru

The origin of subparallel, regularly-spaced longitudinal ridges often observed at the surface of volcanic and other rock avalanche deposits remains unclear. We addressed this issue through analogue laboratory experiments on flows of bi-disperse granular mixtures, because this type of flow is known to exhibit granular fingering that causes elongated structures resembling the ridges observed in nature. We considered four different mixtures of fine (300–400 µm) glass beads and coarse (600–710 µm to 900–1000 µm) angular crushed fruit stones, with particle size ratios of 1.9–2.7 and mass fractions of the coarse component of 5–50 wt%. The coarse particles segregated at the flow surface and accumulated at the front where flow instabilities with a well-defined wavelength grew. These formed granular fingers made of coarse-rich static margins delimiting fines-rich central channels. Coalescence of adjacent finger margins created regular spaced longitudinal ridges, which became topographic highs as finger channels drained at final emplacement stages. Three distinct deposit morphologies were observed: 1) Joined fingers with ridges were formed at low (= 1.9) size ratio and moderate (10–20 wt%) coarse fraction whereas 2) separate fingers or 3) poorly developed fingers, forming series of frontal lobes, were created at larger size ratios and/or higher coarse contents. Similar ridges and lobes are observed at the debris avalanche deposits of Tutupaca volcano, Peru, suggesting that the processes operating in the experiments can also occur in nature. This implies that volcanic (and non-volcanic) debris avalanches can behave as granular flows, which has important implications for interpretation of deposits and for modeling. Such behaviour may be acquired as the collapsing material disaggregates and forms a granular mixture composed by a right grain size distribution in which particle segregation can occur. Limited fragmentation and block sliding, or grain size distributions inappropriate for promoting granular fingering can explain why ridges are absent in many deposits

Contrasting catastrophic eruptions predicted by different intrusion and collapse scenarios

Catastrophic volcanic eruptions triggered by landslide collapses can jet upwards or blast sideways. Magma intrusion is related to both landslide-triggered eruptive scenarios (lateral or vertical), but it is not clear how such different responses are produced, nor if any precursor can be used for forecasting them. We approach this problem with physical analogue modelling enhanced with X-ray Multiple Detector Computed Tomography scanning, used to track evolution of internal intrusion, and its related faulting and surface deformation. We find that intrusions produce three different volcano deformation patterns, one of them involving asymmetric intrusion and deformation, with the early development of a listric slump fault producing pronounced slippage of one sector. This previously undescribed early deep potential slip surface provides a unified explanation for the two different eruptive scenarios (lateral vs. vertical). Lateral blast only occurs in flank collapse when the intrusion has risen into the sliding block. Otherwise, vertical rather than lateral expansion of magma is promoted by summit dilatation and flank buttressing. The distinctive surface deformation evolution detected opens the possibility to forecast the possible eruptive scenarios: laterally directed blast should only be expected when surface deformation begins to develop oblique to the first major fault

Determinación preliminar de parámetros morfométricos de los Domos Potrero - Complejo Volcánico Chachani

Los flujos de lava son uno de los productos volcánicos más representativos de las erupciones efusivas, y se forman cuando el magma extruye efusivamente a través de un cráter o una fisura. Son muchos los factores que determinan el emplazamiento de los flujos de lava, incluida la tasa de efusión, el volumen, la topografía y la reología, que a su vez depende de la composición del magma y del proceso de enfriamiento en la superficie (Castruccio et al., 2013; Harris & Rowland, 2015; Miyamoto & Sasaki, 1997; Vallejo, 2017). Los flujos de lava son una mezcla de roca fundida (fase líquida), cristales (fase sólida), burbujas (fase gaseosa). Macdonald (1953) describió las características superficiales y secciones transversales de tres tipos de lava: Pāhoehoe, 'a'ā y lava en bloques. Los flujos de lava emitidos por los volcanes del sur del Perú son principalmente de tipo lava en bloques y presentan una alta viscosidad debido a su composición andesítica a riolítica. Siendo uno de los ejemplos más emblemáticos, los flujos de lava emitidos por los distintos edificios que componen el Complejo Volcánico Chachani (CVC) (Aguilar et al., 2022). El CVC se encuentra a 22 km al noroeste del centro de la ciudad de Arequipa y es catalogado como un volcán potencialmente activo (Fig. 1). Con un área de ~ 600 km2 y un volumen estimado de 290 – 350 km3, el Chachani es considerado como uno de los complejos volcánicos más extensos y voluminosos de la Zona Volcánica Central de los Andes (Aguilar et al., 2022). El CVC está compuesto por al menos 12 edificios volcánicos agrupados espacial y temporalmente en: (1) Grupo de Edificios Antiguos, y (2) Grupo de Edificios Jóvenes. El Grupo de Edificios Antiguos estuvo en actividad durante el Pleistoceno Temprano - Medio ( 400 - 56 ka), formado por los edificios El Ángel, Domos Potrero, La Horqueta, El Rodado, flujos de lava Uyupampa, Chachani, Cabrerías y el domo Volcancillo (Aguilar et al., 2022). Este segundo grupo estuvo caracterizado por el emplazamiento de flujos de lava, domos y domos-colada. Si bien, se conoce la historia eruptiva del CVC, los trabajos no se han enfocado en la caracterización morfométrica de los flujos de lava y de los domo-colada, por lo cual, el presente estudio tiene por objetivo calcular los parámetros morfométricos y así, entender el dinamismo eruptivo de los flujos de lava

Succesive desetabilization of a dome complex constructed on an extinct, hydrothermally altered volcano: The Tutupaca Volcano case study (Southern Perú)

The Tutupaca volcanic complex (17°01' S, 70°21' W) is located to the south of Peru, and belongs to the Central volcanic Zone of the Andes. Tutupaca is composed of an old, hydrothermally altered and highly eroded basal edifice, and two younger twin peaks, located to the northern part of the complex (the Western and Eastern Tutupaca; Samaniego et al., 2015). The youngest Eastern edifice of Tutupaca is composed by at least 7 coalescing lava domes (named Dome I to VII by Manrique, 2013) and its associated deposits, among which are block-and-ash flow and debris avalanche deposits. We identifiedtwo debris avalanche deposits associated with this edifice. An older deposit (Azufre debris avalanche) was channelized in the valleys located to the E and SE of the volcano, reaching up to 3.5 km from its source region. This DAD occurred soon after the emplacement of the first Eastern Tutupaca domes (I, II,III) and its age was recently estimated by exposure dating at 6-8 ka BP. The younger deposit (Paipatja debris avalanche) outcrops immediately to the NE of the amphitheater and was associated with a large PDC deposits that was radiocarbon dated at 218 ± 14 a BP (Samaniego et al., 2015; Valderrama et al., 2016). Both debris avalanche deposits have two different sub-units: (1) the main subunit, hereafter called hydrothermal-altered blocks-rich debris avalanche deposit (HA-DAD) that is a whitish-yellow volcanic breccia with heterolithological and heterometric blocks, and (2) dome-rich debris avalanche (DR-DAD) sub unit, composed by non-altered dome blocks. In proximal areas, the DR-DAD overlaps the HA-DAD; whereas, in distal areas, these two units are mixed forming a hummocky and/or ridged topografphy. In addition to the similar facies of these DAD, we propose that the triggering mechanism for these debris avalanches was similar in both cases. The ascent of a dacitic magma, coupled with the fact that the Tutupaca dome complex was constructed on top of an older, hydrothermally-altered volcanic edifice, induced the destabilisation of the edifices, producing the debris avalanche and its related pyroclastic density currents

Ruta del Sillar: Quebrada de Añashuayco, Arequipa – Perú

Arequipa es la segunda ciudad más poblada del Perú, y en los últimos años ha experimentado un crecimiento poblacional acelerado, sin considerar un plan urbano, lo que ha conllevado al asentamiento de ~25% del millón de habitantes en zonas de alto y moderado peligro por su cercanía a los volcanes Misti y Chachani. El centro histórico de la ciudad (casco antiguo), reconocida por la UNESCO como Patrimonio Cultural de la Humanidad, está construido con rocas volcánicas (sillar) que le dan a la ciudad una peculiaridad arquitectónica. Estas rocas provienen de los depósitos de ignimbritas producto de erupción esexplosivas voluminosas (Ignimbrita Aeropuerto de Arequipa). Con la finalidad de promover espacios con interés geológico que se conviertan en herramientas para la educación, difusión y comunicación de los peligros relacionados a la actividad volcánica, el Proyecto GA17F “Estudiar y Evaluar los peligros asociados a los volcanes Chachani y Casiri” del INGEMMET y el proyecto IGCP 692 “Geopatrimonio para la resiliencia ante peligros naturales” propusieron 6 geositios pilotos cercanos a la ciudad: 1) las canteras de sillar, donde afloran las Ignimbritas Aeropuerto de Arequipa, que han jugado un rol importante en el desarrollo de la ciudad y se han convertido en un atractivo turístico en potencia. 2) Valle del río Chili, donde se observan afloramientos de la Ignimbrita Río Chili y depósitos volcanoclásticos del Misti y Chachani. 3) Mirador de los volcanes Misti y Chachani, localizado en el borde norte del cañón del río Chili, desde el cual, se observa la base de ambos volcanes. 4) Volcán monogenético Nicholson. 5) Campo Monogenético de Yura, localizados en pueblos tradicionales del distrito de Yura. 6) Domo Volcancillo, donde se aprecian los depósitos más antiguos y más jóvenes del Complejo Volcánico Chachani. De estos 6 geositios se ha realizado un trabajo más extenso y detallado en la “Canteras de Añashuayco”, donde se explota el sillar. El presente resumen muestras los estudios realizados en las canteras donde se desarrollaron reuniones de comunicación con la “Asociación Turística de Cortadores y Artesanos Ruta del Sillar Cantera Añashuayco”, evaluación de peligros volcánicos, análisis de la percepción del peligro volcánico y valoración de la zona como geositio

Monogenetic Strombolian Activity at Aguelmane Sidi Ali Volcano (Middle Atlas Volcanic Province, Morocco)

The Quaternary Middle Atlas Volcanic Province is the largest and youngest volcanic field in Morocco. It hosts a hundred of well-preserved strombolian cones and maars which emitted numerous mafic pyroclastic deposits and lava flows covering a surface of ca. 960 km2 . The 2.27 Ma Aguelmane Sidi Ali monogenetic scoria cone was emplaced in a tectonic basin of the same name on the southern edge of the Bou-Anguer-Aïn-Nokra syncline in the eastern folded part of the Middle Atlas. Aguelmane Sidi Ali scoria cone was formed by a short-lived eruption of days to weeks. The eruptive dynamic recorded in the volcanic deposits suggested that the eruption started with an initial explosive activity that produced pyroclastic deposits of Strombolian type. Following this initial phase and as the cone grew, the eruption changed to a Hawaiian lava-fountain style. Volcanic activity emplaced agglutinate lava spatter near the vent in addition to lava flows with abundant inflation features such as tumuli and pressure-ridges. The continuous flow of significant lava volume provided significant pressure on the southern flank that breached the scoria cone. Subsequently, the eruption became more stable, and changed to a second regular Strombolian activity, rebuilding a new cone and leading to a volcano of 0.02 km3 . Aguelmane Sidi Ali history illustrates volcanological setting where tectonics and volcanism are intimately interrelated. Understanding this interaction is necessary for recognizing the relationship between tectonics, deformation processes and magma transport through the lithosphere