PUBLICATIONS Geochemistry, Geophysics, Geosystems RESEARCH ARTICLE The insular shelves of the Faial-Pico Ridge (Azores archipelago): 10.1002/2015GC005733 A morphological record of its evolution Key Points: R. Quartau1,2, J. Madeira2,3, N. C. Mitchell4, F. Tempera5, P. F. Silva2,6, and F. Brandao~ 7 The Pico insular shelf is dominated by volcanic progradation 1Divis~ao de Geologia e Georecursos Marinhos, Instituto Portugues^ do Mar e da Atmosfera I.P., Lisboa, Portugal, 2Instituto Prograding lava deltas that reach the Dom Luiz, Faculdade de Ciencias^ da Universidade de Lisboa, Lisboa, Portugal, 3Departamento de Geologia da Faculdade shelf edge trigger slope instability de Ciencias,^ Universidade de Lisboa, Lisboa, Portugal, 4School of Earth, Atmospheric and Environmental Sciences, Univer- Improved understanding of Faial-Pico 5 ridge geological evolution is sity of Manchester, Manchester, UK, Marine and Environmental Sciences Centre and Institute of Marine Research, Depar- achieved tamento de Oceanografia e Pescas, Universidade dos Ac¸ores, Horta, Portugal, 6Area Departamental de Fısica, Instituto Superior de Engenharia de Lisboa, Lisboa, Portugal, 7Estrutura de Miss~ao para a Extens~ao da Plataforma Continental, Pac¸o Supporting Information: de Arcos, Portugal Supporting Information S1 Shelves surrounding reefless volcanic ocean islands are formed by surf erosion of their slopes Correspondence to: Abstract R. Quartau, during changing sea levels. Posterosional lava flows, if abundant, can cross the coastal cliffs and fill partially [email protected] or completely the accommodation space left by erosion. In this study, multibeam bathymetry, high- resolution seismic reflection profiles, and sediment samples are used to characterize the morphology of the Citation: insular shelves adjacent to Pico Island. The data show offshore fresh lava flow morphologies, as well as an Quartau, R., J. Madeira, N. C. Mitchell, irregular basement beneath shelf sedimentary bodies and reduced shelf width adjacent to older volcanic F. Tempera, P. F. Silva, and F. Brand~ao (2015), The insular shelves of the edifices in Pico. These observations suggest that these shelves have been significantly filled by volcanic pro- Faial-Pico Ridge (Azores archipelago): gradation and can thus be classified as ‘‘rejuvenated.’’ Despite the general volcanic infilling of the shelves A morphological record of its around Pico, most of their edges are below the depth of the Last Glacial Maximum, revealing that at least evolution, Geochem. Geophys. Geosyst., 16, 1401–1420, doi:10.1002/ parts of the island have subsided after the shelves formed by surf erosion. Prograding lava deltas reached 2015GC005733. the shelf edge in some areas triggering small slope failures, locally decreasing the shelf width and depth of their edges. These areas can represent a significant risk for the local population; hence, their identification Received 20 JAN 2015 can be useful for hazard assessment and contribute to wiser land use planning. Shelf and subaerial geomor- Accepted 11 APR 2015 phology, magnetic anomalies and crustal structure data of the two islands were also interpreted to recon- Accepted article online 15 APR 2015 struct the long-term combined onshore and offshore evolution of the Faial-Pico ridge. The subaerial Published online 12 MAY 2015 Corrected 26 JUN 2015 emergence of this ridge is apparently older than previously thought, i.e., before 850 ka. This article was corrected on 26 JUN 2015. See the end of the full text for details. 1. Introduction Volcanic ocean islands grow above sea level by volcanic construction occurring faster than the effects of destructive geological processes. Their present-day morphology results from the action imprinted by vol- canic, erosional, depositional, tectonic, isostatic, eustatic, and mass-wasting processes [Ramalho et al., 2013]. Although surveying island submarine flanks with sonar, seismic, and submersibles has become common, most of the information discerned in the literature about island geological evolution is still based on onshore studies. Furthermore, the majority of the offshore surveys have been focused on their submarine slopes to study mass-wasting deposits and volcanic geomorphology [e.g., Glass et al., 2007; Holcomb and Searle, 1991; Krastel et al., 2001; Le Friant et al., 2011; Llanes et al., 2009; Mitchell et al., 2002; Mitchell, 2003; Moore et al., 1994]. This concentration of effort in the deeper parts of volcanic islands is also a consequence of the difficulty in performing nearshore surveys, because it is dangerous and time consuming to survey shallow-water coastal areas. Consequently, little is known about the shelves surrounding these islands. How- ever, the morphology that results from the imprint of the physical processes over these nearshore features can provide valuable insights on the geological evolution of the islands [Babonneau et al., 2013; Casalbore et al., 2015; Chiocci et al., 2013; Fletcher et al., 2008; Kennedy et al., 2002; Mitchell et al., 2012a, 2008; Quartau and Mitchell, 2013; Quartau et al., 2010, 2012, 2014; Romagnoli, 2013]. The shelves surrounding volcanic ocean islands are formed by a competition between wave erosion, which VC 2015. American Geophysical Union. forms and enlarges them, and volcanic progradation which narrows them [see Quartau et al., 2010, Figures All Rights Reserved. 1 and 2]. In general, the shelves of the Azorean islands dominated by wave erosion tend to be wider and QUARTAU ET AL. THE INSULAR SHELVES OF FAIAL-PICO RIDGE 1401 Geochemistry, Geophysics, Geosystems 10.1002/2015GC005733 present erosional surfaces that are in profile sharply angular with the submarine slopes and are normally covered by widespread sea level highstand sediments [Quartau et al., 2010, 2012, 2014]. The shelves that are dominated by volcanic progradation are narrow, mainly underlain by rocky outcrops of submarine lava flows and are overlain by thinner and more localized sedimentary bodies. The morphology that results from the interplay between these two main processes can be used as a proxy for the relative age of the adjacent subaerial volcanoes, allowing us to reconstruct their extents prior to wave erosion, measure the vertical movements of the island, and estimate the timing of posterosional volcanism. The guidelines on performing this geomorphological analysis have been detailed in Quartau et al. [2014], so only a basic explanation is provided in section 3. In this work, high-resolution seismic reflection, multibeam bathymetry, magnetic, and sediment data are used to characterize the morphology of the shelf surrounding Faial and Pico islands, with a clear focus on Pico. We show how volcanic progradation can be inferred using morphological analysis of the shelves and cliffs. The results suggest that volcanic progradation has been the main process in determining the shelf morphology surrounding Pico Island. A conceptual model of how insular shelves dominated by volcanic progradation evolve is put forward and the term ‘‘rejuvenated shelves’’ is proposed. In addition, we suggest a dimensionless variable as a way to distinguish shelves dominated by wave erosion from those that are rejuvenated. Locally voluminous lava deltas almost entirely fill the erosional shelves and have apparently triggered small mass-wasting events at the shelf edge. Although giant landslides on the flanks of volcanic islands are infrequent, for example, occurring every 100 ka in the Hawaiian archipelago [Garcia et al., 2007] and 125–170 ka in the Canary archipelago [Krastel et al., 2001], smaller failures of the upper slopes are able to generate local but significant tsunamis [Casalbore et al., 2011; Kelfoun et al., 2010]. Small-scale failures caused by lava progradation [Bosman et al., 2014; Chiocci et al., 2008; Poland and Orr, 2014; Sansone and Smith, 2006] probably pose a frequent threat to local populations but one that remains poorly evaluated. Mapping of such areas can be more efficient for hazard assessment than time-consuming techniques such as satellite and ground-based sensors for estimating the subsidence of coastlines. Finally, the combination of the offshore morphology with the onshore geomorphology allowed us to propose a new model for the Faial-Pico Ridge evolution in the Azores archipelago. 2. Regional Setting The Azores archipelago (Figure 1) is located in the central North Atlantic Ocean, on an irregular triangular- shaped plateau limited by the 2000 m bathymetric contour [Lourenc¸o et al., 1998]. This plateau is the result of magmatic activity related to a hot spot close to the triple junction between the Eurasian (Eu), Nubian (Nu), and North American (NA) plates [e.g., Cannat et al., 1999; Gente et al., 2003; Laughton and Whit- marsh, 1974; Schilling, 1975]. The plateau extends beyond the Mid-Atlantic Ridge (MAR) to the NA plate, where the western group of islands (Flores and Corvo) lie. East of the MAR, the plateau is bounded in the north by the Terceira Rift (TR) and in the south by the inactive East Azores Fracture Zone (EAFZ). Linking the MAR to the western tip of the Gloria Fault (GF) lies a complex structure composed of alternating volcanic edifices and tectonic basins that progressively rotates from E-W to WNW-ESE and NW-SE. Over this volcano- tectonic structure which is the expression of the right-transtensional shear zone that constitutes the Eu-Nu boundary [Hipolito et al., 2013; Lourenc¸o et al., 1998; Marques et al., 2013] lie the central (Terceira, Graciosa, S~ao Jorge, Pico, and Faial Islands) and eastern groups