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Geology.Gsapubs.Org on September 13, 2012 Downloaded from geology.gsapubs.org on September 13, 2012 Geology Large-scale active slump of the southeastern flank of Pico Island, Azores A. Hildenbrand, F.O. Marques, J. Catalão, C.M.S. Catita and A.C.G. Costa Geology 2012;40;939-942 doi: 10.1130/G33303.1 Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/ to subscribe to Geology Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. 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Notes © 2012 Geological Society of America Downloaded from geology.gsapubs.org on September 13, 2012 Large-scale active slump of the southeastern fl ank of Pico Island, Azores A. Hildenbrand1,2,*, F.O. Marques3, J. Catalão4, C.M.S. Catita4, and A.C.G. Costa1,4 1Univ Paris-Sud, Laboratoire IDES, UMR8148, Orsay, F-91405, France 2CNRS, Orsay, F-91405, France 3Universidade Lisboa, 1749-016 Lisboa, Portugal 4Universidade Lisboa and IDL, 1749-016 Lisboa, Portugal ABSTRACT 40°N Corvo We report evidence for ongoing lateral slump of part of the EU southeastern fl ank of the Pico volcanic ridge in the Azores. Data Graciosa from a high-resolution digital elevation model, fi eld work, GPS, Flores MAR São Jorge Terceira and radar interferometry show that: (1) the slumping sector is NA Faial several cubic kilometers in size; (2) the structure involves several TR Pico curved scars with normal fault kinematics; (3) the central part is 38°N São Miguel undergoing little horizontal displacement toward the southeast (1.6 Madalena 5 km N ± 1.3 mm/yr), but signifi cant downward movement (5–12 mm/yr); AF Santa Maria and (4) the outer part of the southeastern fl ank of Pico is subsid- ing faster than the inner parts; this likely refl ects recent individu- São Caetano Gloria FZ alization of a steep seaward-dipping fault in the moving mass. The Lajes R East Azores FZ slump shares similarities with active slumps recognized elsewhere, 30°W 28°W 26°W although the studied area may represent only the proximal part of a Figure 1. Location of Azores volcanic archipelago near triple junc- much larger complex potentially affecting the deep submarine base tion between North American (NA), Eurasian (EU), and African (AF) of the island. Displacement of the subaerial part of the southeastern plates. Bold lines show Mid-Atlantic Ridge (MAR) and Terceira Rift fl ank of Pico seems to be accommodated by the movement and rota- (TR). FZ—fracture zone. Inset: Shaded relief map showing main mor- phology of Pico Island. Dotted lines highlight axis of main N110° rift tion of large blocks along listric normal faults. zone. Two main landslide failures affect southern fl ank of island. Rectangle shows geographical extension of studied southeastern INTRODUCTION collapse area (Fig. 2). R is Ribeiras town. The growth of volcanic islands is generally punctuated by large lateral fl ank failures, which can trigger destructive tsunamis (Keating and McGuire, 2000; McMurtry et al., 2004). Giant sector collapses have been recognized around numerous islands worldwide, e.g., in Hawaii including fast-growing phases and multiple destruction events, such as (Lipman et al., 1988; Moore et al., 1994; Morgan et al., 2000), French vertical caldera collapse, lateral landslides, and rock fall of various sizes Polynesia (Clouard and Bonneville, 2004; Hildenbrand et al., 2006), (Mitchell, 2003; Calvert et al., 2006; Hildenbrand et al., 2008; Silva et the Canary Islands (Krastel et al., 2001; Walter and Schmincke, 2002; al., 2012). Boulesteix et al., 2012), Reunion Island (Duffi eld et al., 1982; Gillot et Pico Island is a narrow and steep volcanic ridge, formed by magma al., 1994), and the Caribbean arc (Le Friant et al., 2003; Samper et al., concentration along the N110° trend (Fig. 1). The oldest volcanic units 2007). A number of triggering factors have been proposed over the past are exposed in the central and eastern parts of the island, whereas more 20 years (e.g., McGuire, 1996; Elsworth and Day, 1999; McMurtry et al., recent volcanic activity produced the Pico volcano, which occupies the 1999, 2004; Klügel et al., 2005; Quidelleur et al., 2008), including con- western third of the island (Woodhall, 1974; Nunes, 1999). centration of dikes along rift zones and associated fl uid pressurization Due to its steep topography, Pico Island is particularly sensitive to by heating and/or compression of groundwater trapped between dikes, fl ank instability. The southern fl ank of the ridge shows several curved ground shaking by large regional earthquakes, gravitational spreading structures that are concave toward the ocean (Fig. 1), previously inter- of the volcanoes along weak geological layers, or sea-level variations preted as refl ecting early caldera development, faulting, or ancient lat- associated with climatic changes. eral collapse (Woodhall, 1974; Nunes, 1999; Mitchell, 2003; Nunes et Contrasting types of sector collapse have been distinguished (Moore al., 2006). Knowledge of the geometry, volume, and kinematics of these et al., 1994): (1) slow-moving rotational landslides along a deep-sea structures, and understanding of their recent evolution and susceptibil- detachment fault, often called slumps; and (2) catastrophic landslides ity to further movement, are critical for risk and hazard assessment, but produced by the rapid detachment of the island fl ank, which may release remain poorly constrained. Our study focuses on the unstable southeast- fast-moving debris avalanches. These two kinds of processes are not ern part of the ridge; data from a high-resolution digital elevation model mutually exclusive; part of a slump may suddenly collapse and trigger a (DEM), fi eld work, GPS, and radar interferometry show that part of the tsunami (e.g., Tilling et al., 1976; Moore et al., 1994). fl ank is currently being displaced toward the ocean, accommodated by The Azores archipelago, in the Atlantic Ocean, comprises nine the motion of large blocks that may eventually detach, with catastrophic active volcanic islands and numerous linear submarine ridges developed consequences. close to the triple junction between the North American, Eurasian, and African lithospheric plates (Fig. 1), in an area characterized by impor- SLUMP GEOMETRY AND KINEMATICS tant regional deformation and recurrent high-magnitude earthquakes Using cartographic data and maps generated in 2005 by the Portu- (Borges et al., 2007). The islands are marked by a multistage evolution, guese Army Institute (scale 1:25,000), we developed a DEM with 10 m spatial resolution and a vertical accuracy of 2 m. It was used to produce a *E-mail: [email protected]. shaded relief map (Fig. 2A), a slope map, and topographic cross sections GEOLOGY, October 2012; v. 40; no. 10; p. 939–942; Data Repository item 2012264 | doi:10.1130/G33303.1 | Published online 9 August 2012 GEOLOGY© 2012 Geological | October Society 2012 of America.| www.gsapubs.org For permission to copy, contact Copyright Permissions, GSA, or [email protected]. 939 Downloaded from geology.gsapubs.org on September 13, 2012 A Main failure WSW ENE Pico Island axis Curved Stable fault S1 old volcanic scarp succession S2 Infilling lava flows S3 π Td Td Main failure Infilling lava flows Ribeiras π 150 m Stable old volcanic succession Displaced block ? 1 km 20 m Fig. 3 and Item DR2 Main failure B Figure 3. Photograph taken from sea showing geometric relation- ships between main geological units exposed along southern end of main collapse rim at shore level. Td—talus deposits; pi symbols— levels of Strombolian fallout deposits; blue outline and symbol show point of view from which photo exposed in Item DR2 (see footnote 1) S1 was taken (upward). S2 S3 east, the slope is smoothed by recent lava fl ows erupted from volcanic cones located along the main rift zone, and from parasitic cones developed Main failure Within slump Very recent lava flows within the collapse sector. The S3 scarp has been previously interpreted as Recent lava flows Large volcanic cones (recent) fossil coastal cliffs (Nunes, 1999); however, it defi nes a clear embayment Intermediate lava filling Early lava filling Other cones partly covered by recent lava fl ows, which cascaded down to the sea and Talus deposits (Td) (recent and old) formed a lava delta. Outside slump Dip > 25° Main Pico ridge Fig. 3 and Item DR2 Old volcano Dip < 25° The architecture and kinematics of the slump are further constrained by fi eld data (Fig. 3; Item DR2). The in situ old volcanic sequence is Figure 2. A: Main morphostructural units within southeastern slump exposed along coastal cliffs as a pile of thin lava fl ows, including red lev- area. Main scars here recognized are highlighted by white dashed els of Strombolian fallout deposits (π symbol in Fig. 3). This succession lines. Black line indicates inferred continuation of main structure ex- is interrupted by a main scarp making up the western rim of the slump tending to southern part of axial rift zone.
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