Recent studies hypothesize that some submarine slides fail via pressure‐driven slow‐slip deformation. To test this hypothesis, this study derives pore pressures in failed and adjacent unfailed deep ...marine sediments by integrating rock physics models, physical property measurements on recovered sediment core, and wireline logs. Two drill sites (U1394 and U1399) drilled through interpreted slide debris; a third (U1395) drilled into normal marine sediment. Near‐hydrostatic fluid pressure exists in sediments at site U1395. In contrast, results at both sites U1394 and U1399 indicate elevated pore fluid pressures in some sediment. We suggest that high pore pressure at the base of a submarine slide deposit at site U1394 results from slide shearing. High pore pressure exists throughout much of site U1399, and Mohr circle analysis suggests that only slight changes in the stress regime will trigger motion. Consolidation tests and permeability measurements indicate moderately low (~10−16–10−17 m2) permeability and overconsolidation in fine‐grained slide debris, implying that these sediments act as seals. Three mechanisms, in isolation or in combination, may produce the observed elevated pore fluid pressures at site U1399: (1) rapid sedimentation, (2) lateral fluid flow, and (3) shearing that causes sediments to contract, increasing pore pressure. Our preferred hypothesis is this third mechanism because it explains both elevated fluid pressure and sediment overconsolidation without requiring high sedimentation rates. Our combined analysis of subsurface pore pressures, drilling data, and regional seismic images indicates that slope failure offshore Martinique is perhaps an ongoing, creep‐like process where small stress changes trigger motion.
Key Points
Permeability and pressure measurements were made on submarine slide deposits
Near‐lithostatic fluid pressures exist between low‐permeability clays in many slide deposits
Small stress changes can trigger additional shearing and slow‐slip failure in some slide sediments
Volcanism on Montserrat (Lesser Antilles arc) has migrated southwards since the formation of the Silver Hills ~2.5Ma, and has formed three successively active volcanic centres. The Centre Hills ...volcano was the focus of volcanism from ~1–0.4Ma, before activity commenced at the currently active Soufrière Hills volcano. The history of activity at these two volcanoes provides an opportunity to investigate the pattern of volcano behaviour on an andesitic arc island over the lifetime of individual volcanoes. Here, we describe the pyroclastic stratigraphy of subaerial exposures around central Montserrat; identifying 11 thick (>1m) pumiceous units derived from sustained explosive eruptions of Centre Hills from ~0.8–0.4Ma. Over 10 other, less well- exposed pumiceous units have also been identified. The pumice-rich units are interbedded with andesite lava breccias derived from effusive, dome-forming eruptions of Centre Hills. The stratigraphy indicates that large (up to magnitude 5) explosive eruptions occurred throughout the history of Centre Hills, alongside effusive activity. This behaviour at Centre Hills contrasts with Soufrière Hills, where deposits from sustained explosive eruptions are much less common and restricted to early stages of activity at the volcano, from ~175–130ka. Subsequent eruptions at Soufriere Hills have been dominated by andesitic effusive eruptions. The bulk composition, petrography and mineral chemistry of volcanic rocks from Centre Hills and Soufrière Hills are similar throughout the history of both volcanoes, except for occasional, transient departures to different magma compositions, which mark shifts in vent location or dominant eruption style. For example, the final recorded eruption of Centre Hills, before the initiation of activity at Soufrière Hills, was more silicic than any other identified eruption on Montserrat; and the basaltic South Soufrière Hills episode marked the transition to the current stage of predominantly effusive Soufrière Hills activity. The compositional stability observed throughout the history of Centre Hills and Soufrière Hills suggests that a predominance towards effusive or explosive eruption styles is not driven by major compositional shifts of magma, but may reflect local changes in long-term magma storage conditions that characterise individual episodes (on 105year timescales) of volcanism on Montserrat.
•Presentation of a comprehensive subaerial stratigraphy of Montserrat spanning 1Ma.•Eruptive products from both active centres are compositionally similar.•Activity at the older centre tended towards more sustained explosive eruptions.•Variation may be due to differences in magma ascent, temperature or storage.
The 2008 eruption of Chaitén Volcano was widely cited as the first activity at the volcano for over 9000 years. However, we have identified evidence from proximal pyroclastic deposits for three ...additional explosive eruptions of Chaitén within the past 5000 years. Chaitén has therefore produced at least five explosive eruptions in the Holocene, making it among the most active volcanoes, in terms of explosive output, in the southern part of the Andean Southern Volcanic Zone. All of the five identified Holocene explosive eruptions produced homogeneous high-silica rhyolite, with near identical compositions. Based on our pyroclastic sequence, we suggest that the largest-volume Holocene eruption of Chaitén occurred at ~4.95 ka, and we correlate this with the Mic2 deposit, which was previously thought to originate from the nearby Michinmahuida Volcano.
Huequi, a little-known volcano in the southern part of the Andean southern volcanic zone (SSVZ), shows a regionally unusual eruption style, mineralogy and geochemistry. The volcano comprises multiple ...highly-eroded lava domes. Past eruptions were accompanied by relatively minor explosive activity, most recently from 1890-1920. The rocks erupted by Huequi range from basaltic andesite to dacite, and are highly distinctive when compared to other volcanoes of the SSVZ, being K-poor and Al-rich, and containing euhedral hornblende phenocrysts. Overall compositions suggest a notably water-rich magma source, evolving through high levels of fractionation and subsequent degassing to produce highly porphyritic dome-forming andesites. The ultimate causes of water-rich magmas at this point in the arc remain unclear.
This thesis describes volcanic records from the Andean southern volcanic zone, based on the collection of field data between Calbuco and Puyuhuapi volcanic centres, with a particular focus on the ...Hualaihue peninsula, combined with existing records from the region as a whole. These data, extending the understanding of the volcanic history of southern Chile, are examined for evidence of spatial or temporal variability, which may be used to explore underlying controls on volcanic processes. All three volcanoes on the Hualaihue peninsula have been active in the Holocene. A large mafic scoria unit from Apagado is unusually primitive, providing a potential window into primary magma generation in the arc. Dynamically similar eruptions occurred at Hornopirén and widely along the regional scale Liquiñe-Ofqui fault zone (LOFZ). Although the Hualaihue centres are closely related, petrological evidence indicates a complex magmatic storage system. Effusive activity is predominant at Yate and Hornopirén, and the tephrostratigraphy of the Hualaihue area is dominated by units from Calbuco volcano, to the north. The 2008 eruption of Chaitén provided an analogue for past large explosive eruptions in the region, with tephra deposition reflecting variable eruption intensity in a changing wind field. The regional tectonic setting and the LOFZ influence dyke ascent, volcano morphology and, as demonstrated at Yate, edifice stability, determining the orientation of collapse. Explosive eruption records over the post-glacial period also indicate a limited response of volcanism to deglaciation, suggesting a control on magma storage arising from changing crustal stress regimes, both at the arc front and along the LOFZ. On short timescales, large earthquakes are shown to influence eruption rate across the arc, implying a triggering role for dynamic seismic stresses. This work demonstrates the existence of a range of external forces affecting Chilean arc volcanism, but the degree to which these are quantifiable is strongly constrained by the quality of the available data.
This thesis describes volcanic records from the Andean southern volcanic zone, based on the collection of field data between Calbuco and Puyuhuapi volcanic centres, with a particular focus on the ...Hualaihue peninsula, combined with existing records from the region as a whole. These data, extending the understanding of the volcanic history of southern Chile, are examined for evidence of spatial or temporal variability, which may be used to explore underlying controls on volcanic processes. All three volcanoes on the Hualaihue peninsula have been active in the Holocene. A large mafic scoria unit from Apagado is unusually primitive, providing a potential window into primary magma generation in the arc. Dynamically similar eruptions occurred at Hornopirén and widely along the regional scale Liquiñe-Ofqui fault zone (LOFZ). Although the Hualaihue centres are closely related, petrological evidence indicates a complex magmatic storage system. Effusive activity is predominant at Yate and Hornopirén, and the tephrostratigraphy of the Hualaihue area is dominated by units from Calbuco volcano, to the north. The 2008 eruption of Chaitén provided an analogue for past large explosive eruptions in the region, with tephra deposition reflecting variable eruption intensity in a changing wind field. The regional tectonic setting and the LOFZ influence dyke ascent, volcano morphology and, as demonstrated at Yate, edifice stability, determining the orientation of collapse. Explosive eruption records over the post-glacial period also indicate a limited response of volcanism to deglaciation, suggesting a control on magma storage arising from changing crustal stress regimes, both at the arc front and along the LOFZ. On short timescales, large earthquakes are shown to influence eruption rate across the arc, implying a triggering role for dynamic seismic stresses. This work demonstrates the existence of a range of external forces affecting Chilean arc volcanism, but the degree to which these are quantifiable is strongly constrained by the quality of the available data.
This thesis describes volcanic records from the Andean southern volcanic zone, based on the collection of field data between Calbuco and Puyuhuapi volcanic centres, with a particular focus on the ...Hualaihue peninsula, combined with existing records from the region as a whole. These data, extending the understanding of the volcanic history of southern Chile, are examined for evidence of spatial or temporal variability, which may be used to explore underlying controls on volcanic processes. All three volcanoes on the Hualaihue peninsula have been active in the Holocene. A large mafic scoria unit from Apagado is unusually primitive, providing a potential window into primary magma generation in the arc. Dynamically similar eruptions occurred at Hornopirén and widely along the regional scale Liquiñe-Ofqui fault zone (LOFZ). Although the Hualaihue centres are closely related, petrological evidence indicates a complex magmatic storage system. Effusive activity is predominant at Yate and Hornopirén, and the tephrostratigraphy of the Hualaihue area is dominated by units from Calbuco volcano, to the north. The 2008 eruption of Chaitén provided an analogue for past large explosive eruptions in the region, with tephra deposition reflecting variable eruption intensity in a changing wind field. The regional tectonic setting and the LOFZ influence dyke ascent, volcano morphology and, as demonstrated at Yate, edifice stability, determining the orientation of collapse. Explosive eruption records over the post-glacial period also indicate a limited response of volcanism to deglaciation, suggesting a control on magma storage arising from changing crustal stress regimes, both at the arc front and along the LOFZ. On short timescales, large earthquakes are shown to influence eruption rate across the arc, implying a triggering role for dynamic seismic stresses. This work demonstrates the existence of a range of external forces affecting Chilean arc volcanism, but the degree to which these are quantifiable is strongly constrained by the quality of the available data.
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