The insular shelves of the Faial‐Pico Ridge (Azores archipelago):...

RESEARCH ARTICLE10.1002/2015GC005733

The insular shelves of the Faial-Pico Ridge (Azores archipelago):A morphological record of its evolutionR. Quartau1,2, J. Madeira2,3, N. C. Mitchell4, F. Tempera5, P. F. Silva2,6, and F. Brand~ao7

1Divis~ao de Geologia e Georecursos Marinhos, Instituto Portugues do Mar e da Atmosfera I.P., Lisboa, Portugal, 2InstitutoDom Luiz, Faculdade de Ciencias da Universidade de Lisboa, Lisboa, Portugal, 3Departamento de Geologia da Faculdadede Ciencias, Universidade de Lisboa, Lisboa, Portugal, 4School of Earth, Atmospheric and Environmental Sciences, Univer-sity of Manchester, Manchester, UK, 5Marine and Environmental Sciences Centre and Institute of Marine Research, Depar-tamento de Oceanografia e Pescas, Universidade dos Acores, Horta, Portugal, 6�Area Departamental de F�ısica, InstitutoSuperior de Engenharia de Lisboa, Lisboa, Portugal, 7Estrutura de Miss~ao para a Extens~ao da Plataforma Continental, Pacode Arcos, Portugal

Abstract Shelves surrounding reefless volcanic ocean islands are formed by surf erosion of their slopesduring changing sea levels. Posterosional lava flows, if abundant, can cross the coastal cliffs and fill partiallyor 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 theinsular shelves adjacent to Pico Island. The data show offshore fresh lava flow morphologies, as well as anirregular basement beneath shelf sedimentary bodies and reduced shelf width adjacent to older volcanicedifices in Pico. These observations suggest that these shelves have been significantly filled by volcanic pro-gradation and can thus be classified as ‘‘rejuvenated.’’ Despite the general volcanic infilling of the shelvesaround Pico, most of their edges are below the depth of the Last Glacial Maximum, revealing that at leastparts of the island have subsided after the shelves formed by surf erosion. Prograding lava deltas reachedthe shelf edge in some areas triggering small slope failures, locally decreasing the shelf width and depth oftheir edges. These areas can represent a significant risk for the local population; hence, their identificationcan be useful for hazard assessment and contribute to wiser land use planning. Shelf and subaerial geomor-phology, magnetic anomalies and crustal structure data of the two islands were also interpreted to recon-struct the long-term combined onshore and offshore evolution of the Faial-Pico ridge. The subaerialemergence of this ridge is apparently older than previously thought, i.e., before �850 ka.

1. Introduction

Volcanic ocean islands grow above sea level by volcanic construction occurring faster than the effects ofdestructive 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 ononshore studies. Furthermore, the majority of the offshore surveys have been focused on their submarineslopes to study mass-wasting deposits and volcanic geomorphology [e.g., Glass et al., 2007; Holcomb andSearle, 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 consequenceof the difficulty in performing nearshore surveys, because it is dangerous and time consuming to surveyshallow-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 featurescan provide valuable insights on the geological evolution of the islands [Babonneau et al., 2013; Casalboreet al., 2015; Chiocci et al., 2013; Fletcher et al., 2008; Kennedy et al., 2002; Mitchell et al., 2012a, 2008; Quartauand Mitchell, 2013; Quartau et al., 2010, 2012, 2014; Romagnoli, 2013].

The shelves surrounding volcanic ocean islands are formed by a competition between wave erosion, whichforms and enlarges them, and volcanic progradation which narrows them [see Quartau et al., 2010, Figures1 and 2]. In general, the shelves of the Azorean islands dominated by wave erosion tend to be wider and

Key Points:� The Pico insular shelf is dominated

by volcanic progradation� Prograding lava deltas that reach the

shelf edge trigger slope instability� Improved understanding of Faial-Pico

ridge geological evolution isachieved

Supporting Information:� Supporting Information S1

Correspondence to:R. Quartau,[email protected]

Citation:Quartau, R., J. Madeira, N. C. Mitchell,F. Tempera, P. F. Silva, and F. Brand~ao(2015), The insular shelves of theFaial-Pico Ridge (Azores archipelago):A morphological record of itsevolution, Geochem. Geophys. Geosyst.,16, 1401–1420, doi:10.1002/2015GC005733.

Received 20 JAN 2015

Accepted 11 APR 2015

Accepted article online 15 APR 2015

Published online 12 MAY 2015

Corrected 26 JUN 2015

This article was corrected on 26 JUN

2015. See the end of the full text for

details.

VC 2015. American Geophysical Union.

All Rights Reserved.

QUARTAU ET AL. THE INSULAR SHELVES OF FAIAL-PICO RIDGE 1401

Geochemistry, Geophysics, Geosystems

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present erosional surfaces that are in profile sharply angular with the submarine slopes and are normallycovered by widespread sea level highstand sediments [Quartau et al., 2010, 2012, 2014]. The shelves thatare dominated by volcanic progradation are narrow, mainly underlain by rocky outcrops of submarine lavaflows and are overlain by thinner and more localized sedimentary bodies. The morphology that results fromthe interplay between these two main processes can be used as a proxy for the relative age of the adjacentsubaerial volcanoes, allowing us to reconstruct their extents prior to wave erosion, measure the verticalmovements of the island, and estimate the timing of posterosional volcanism. The guidelines on performingthis geomorphological analysis have been detailed in Quartau et al. [2014], so only a basic explanation isprovided in section 3.

In this work, high-resolution seismic reflection, multibeam bathymetry, magnetic, and sediment data areused to characterize the morphology of the shelf surrounding Faial and Pico islands, with a clear focus onPico. We show how volcanic progradation can be inferred using morphological analysis of the shelves andcliffs. The results suggest that volcanic progradation has been the main process in determining the shelfmorphology surrounding Pico Island. A conceptual model of how insular shelves dominated by volcanicprogradation evolve is put forward and the term ‘‘rejuvenated shelves’’ is proposed. In addition, we suggesta dimensionless variable as a way to distinguish shelves dominated by wave erosion from those that arerejuvenated. Locally voluminous lava deltas almost entirely fill the erosional shelves and have apparentlytriggered small mass-wasting events at the shelf edge. Although giant landslides on the flanks of volcanicislands 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 ableto generate local but significant tsunamis [Casalbore et al., 2011; Kelfoun et al., 2010]. Small-scale failurescaused by lava progradation [Bosman et al., 2014; Chiocci et al., 2008; Poland and Orr, 2014; Sansone andSmith, 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 suchas satellite and ground-based sensors for estimating the subsidence of coastlines. Finally, the combinationof the offshore morphology with the onshore geomorphology allowed us to propose a new model for theFaial-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 [Lourenco et al., 1998]. This plateau is theresult 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 thenorth by the Terceira Rift (TR) and in the south by the inactive East Azores Fracture Zone (EAFZ). Linking theMAR to the western tip of the Gloria Fault (GF) lies a complex structure composed of alternating volcanicedifices 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-Nuboundary [Hip�olito et al., 2013; Lourenco et al., 1998; Marques et al., 2013] lie the central (Terceira, Graciosa,S~ao Jorge, Pico, and Faial Islands) and eastern groups of islands (S~ao Miguel and Santa Maria Islands).

2.1. Local Geological SettingThe following details of Pico Island’s geology are taken from several studies [Madeira, 1998; Madeira andBrum da Silveira, 2003; Nunes, 1999]. Although there is no consensus about Pico’s volcanic stratigraphy, wewill follow the scheme proposed by Nunes [1999] because it incorporates the most recent and detailedmapping. This volcano-stratigraphy is based on field relations, geomorphological and weathering criteria,and a limited number of radiometric ages.

Pico Island corresponds to the westward subaerial expression of an elongated submarine ridge of around83 km length (Figure 1) [Lourenco et al., 1998; Mitchell et al., 2012b; Stretch et al., 2006]. Above sea level, PicoIsland is 46 km in length, has a maximum width of 15.8 km, and rises to an altitude of 2351 m at Ponta doPico (Figure 2). The subaerial part of the island can be divided into three different physiographic regions[Madeira, 1998]: a tall stratovolcano, Pico Volcano, which dominates the western part of the island; a shield-

Geochemistry, Geophysics, Geosystems 10.1002/2015GC005733


Figure 2. Simplified geologic map of Pico Island (modified from Nunes [1999]) draped over a shaded relief image of the subaerial topogra-phy [from Instituto Geogr�afico do Ex�ercito, 2001a, 2001b, 2001c, 2001d, 2001e, 2001f, 2001g]. Pico Volcano, Topo Volcano, and the AchadaPlateau constitute the three physiographic regions that coincide, respectively, with the Montanha, Topo-Lajes, and S~ao Roque-Piedade Vol-canic Complexes. Straight comb-like black lines represent fault scarps of the graben (see section 2.2 for details): 2 is the Lagoa do Capit~aofault and 3 the Topo fault. Dashed black lines are inferred faults. Delineation of landslide scars in red is from: 4 (S~ao Mateus landslide;adapted from [Madeira, 1998]), 5 (Topo landslide; adapted from Hildenbrand et al. [2012a]), 6 and 7 [adapted from Costa et al., 2014].Names with the dates correspond to historical eruptions. The top right inset represents the Faial-Pico channel and the Faial Island. RVC,CVC, AF, and CapVC correspond, respectively, to Ribeirinha Volcanic Complex, Caldeira Volcanic Complex, Almoxarife Formation, andCapelo Volcanic Complex. Straight comb-like red lines represent fault scarps of the graben structure in the island. Lines offshore represent50, 100, 200, and 500 m bathymetry contours (digitized from Instituto Hidrogr�afico [1999]).

Figure 1. Regional setting of the Azores archipelago within the North American (NA), Eurasian (Eu), and Nubian (Nu) triple junction. Thetop right inset depicts the location of the Azores archipelago in the North Atlantic Ocean. Tectonic structures: MAR, Mid-Atlantic Ridge;NAFZ, North Azores Fracture Zone; PAFZ, Princess Alice Fracture Zone; EAFZ, East Azores Fracture Zone; GF, Gloria Fault; Terceira Rift (TR).Islands: C, Corvo; F, Flores; SJ, S~ao Jorge; Gr, Graciosa; T, Terceira; SM, S~ao Miguel; SMa, Santa Maria. Faial and Pico Islands are highlightedin black. Solid lines correspond to major structures/faults. Bathymetry of the Azores archipelago from Lourenco et al. [1998].



like volcanic structure, Topo volcano, located on the south-central part of the island; and the Achada Pla-teau, a 29 km long, WNW-ESE to NW-SE volcanic ridge. These regions roughly coincide with the three mainvolcanic complexes described by Nunes [1999]: Montanha Volcanic Complex (MVC), Topo-Lajes VolcanicComplex (TLVC), and S~ao Roque-Piedade Volcanic Complex (SRPVC) (Figure 2).

The oldest part of the island is the Topo volcano, which forms a promontory on its southern-central part(Figure 2). It is formed of ankaramitic and basaltic lava flows of the TLVC and the only published K/Ar dateprovided an age of 250 6 40 ka [Chovelon, 1982]. The shield volcano still reaches an elevation of 1020 m,although modified by landslides, displaced by faulting, and covered by younger lavas [Hildenbrand et al.,2012a, 2013; Madeira, 1998; Madeira and Brum da Silveira, 2003; Mitchell et al., 2012a, 2013].

The Achada Plateau (Figure 2) can be divided in two sections: the easternmost section trending WNW-ESE(from the eastern tip of the island to around number 1 in Figure 2) and the westernmost section trendingNW-SE (from around number 1 in Figure 2 to the western end of SRPVC). It is composed of numerousHawaiian/strombolian scoria cones and related lava flows of the SRPVC, located along the WNW-ESE to NW-SE trending axis of the plateau (Figure 2). Lavas issuing from these cones are mostly aa-flows, which cas-caded down the steep northern and southern slopes of the plateau. The oldest subaerial lavas have a K/Arage of 230 ka [Chovelon, 1982] but the present-day morphology is dominated by recent flows. Some ofthese flows have passed over cliffs cut in older units and originated lava deltas, such as the event of 1562–1564 A.D. eruption of Ponta do Mist�erio (Figure 2). Volcanic activity has jumped along the Achada Plateauridge without any well-defined migration pattern [Nunes, 1999; Nunes et al., 2006].

The basaltic stratovolcano of Pico Mountain (Figure 2) rises 2351 m above sea level. MVC volcanism is basal-tic and dominantly Hawaiian-strombolian, with occasional littoral surtseyan eruptions. Besides summit erup-tions, abundant flank eruptions have occurred on Pico Mountain, producing scoria and spatter cones, anderuptive fissures and associated lava flows. The single K/Ar date of this volcanic complex is from Costa et al.[2014] and points out to an age of 53 ka. The very fresh morphology of Pico Mountain and the radiocarbondates indicate intense activity during the Holocene [Madeira, 1998; Nunes, 1999]. Since the settlement in thefifteenth century, there have been two eruptions (Figure 2), in 1718 A.D. (Santa Luzia and S~ao Jo~ao eruptivecenters) and 1720 A.D. (Silveira eruption) and a submarine eruption off the NW coast in 1963 [Madeira,1998, 2005; Nunes, 1999; Weston, 1964; Zbyszewski, 1963].

Pico Island is bordered to the west by a narrow and shallow channel that separates it from Faial Island;hence, it is called the Faial-Pico Channel. This channel (top right inset in Figure 2) is in fact the submarinelink between the two islands. It constitutes an 8 km wide shelf whose depth varies from 200 m at the N andS entrances to an average of 80 m in the middle.

East Faial is formed of volcanic products erupted by the Ribeirinha shield volcano that rose above sea level at�850–800 ka [F�eraud, 1980] and was reactivated 400 ka ago in its NE flank [Hildenbrand et al., 2012b]. Thewest flank of this volcano is overlapped by the Caldeira stratovolcano since 120 ka [Hildenbrand et al., 2012b].Two basaltic fissural volcanic systems developed on SE Faial (Almoxarife Formation at 30 6 20 ka [F�eraudet al., 1980]) and on the west tip of the island (Capelo Peninsula over the last 8 ka [Di Chiara et al., 2014]). Thelater was the site of the only two historical eruptions in 1672–1674 and 1957–1958 [Madeira et al., 1995].

2.2. Tectonic StructureThe island presents two main fault systems [Madeira, 1998; Madeira and Brum da Silveira, 2003]. The WNW-ESE to NW-SE trending normal dextral structures are mostly present on the Achada Plateau and areexpressed by alternating volcanic alignments (scoria cones, scoria ramparts, and crater rows) and short faultscarps. This system forms a graben structure bounded in the north by the Lagoa do Capit~ao and in thesouth by the Topo fault zones (respectively, 2 and 3 in Figure 2). The western portion of the graben is com-pletely covered by Pico stratovolcano while to the east, it is partially filled by lava flows emitted by cones ofthe SRPVC installed along this fault system. The NNW-SSE trending conjugate faults are marked by volcanicalignments and observed at the outcrop scale. Two large gravitational (flank collapse) structures wereobserved; one on the south flank of Pico stratovolcano (4 in Figure 2) (S~ao Mateus landslide [Madeira, 1998;Nunes, 1999]) and another on the south flank of Topo shield volcano (5 in Figure 2) (Arrife landslide [Hilden-brand et al., 2012a, 2013; Madeira, 1998; Madeira and Brum da Silveira, 2003; Mitchell et al., 2012a, 2013]).Two further inferred landslides (6 and 7 in Figure 2) on the north side of the island are largely obscured by



younger volcanism but have been identified in multibeam sonar data [Costa et al., 2014]. GPS and InSARdata covering the entire island from the 2001–2006 period revealed that the average modern subsidence ofthe island is between 5.7 and 7.2 mm/a [Catal~ao et al., 2011].

Eastern Faial Island (top right inset in Figure 2) is dominated by a graben system composed of several nor-mal dextral faults, which form two oppositely verging domino structures [Madeira, 1998; Madeira and Brumda Silveira, 2003; Trippanera et al., 2014].

Figure 3 shows a magnetization map of the Faial-Pico ridge combined with a digital elevation model of theislands and the surrounding seafloor. This map represents a solution from a 3-D inversion of aeromagneticdata from the Azores platform [Luis et al., 1994; Miranda et al., 1991], compiled into a singular grid with0.0058 step (latitude and longitude). Using Mirone software [Luis, 2007], data were reduced to the pole andinverted according the methodology of Parker and Huestis [1974]. The magnetization map results from alayer with constant thickness of 1 km below bathymetric surface. A band-pass filter (1–120 km) was applied

Figure 3. (a) Magnetization intensity (A/m) of the Faial-Pico ridge on a shaded relief map of the islands and seafloor. (b) The light greenpolygon (4) corresponds to the body with high crustal velocities referred in Dias et al. [2007] and the three darker green polygons corre-spond to high-density bodies in Camacho et al. [2007] (1) and Nunes et al. [2006] (2 and 3). Red dashed lines in polygons 1 and 2 areinferred limits of the high-density bodies of Camacho et al. [2007] and Nunes et al. [2006] because the gravimetric surveys were landbased.



in order to ensure convergence. Most of the subaerial part of the islands is dominated by positive magnet-ization values, suggesting construction during the Brunhes Chron (<790 ka) [Quidelleur et al., 2003], whichis consistent with their known recent geological history. However, a significant area, covering the entirechannel and part of east Faial and west Pico, presents negative magnetization values. The negative magnet-ization suggests construction of this area during the Matuyama Chron (>790 ka) [Quidelleur et al., 2003].According to gravity studies, a large high-density body (dark green area numbered 1 in Figure 3), locatedbetween 0.5 and 6 km below sea level, with a wall-like structure develops from the central caldera of Faial[Nunes et al., 2006]. No shipboard gravity data exist in the channel, but this body follows the direction of thegraben system at Faial, which has a morphologic expression in the channel between these two islands[Quartau and Mitchell, 2013; Tempera, 2008], and most likely extends across to Pico Island where anotherhigh-density body can be found [Camacho et al., 2007] (dark green area numbered 2 in Figure 3). Dias et al.[2007] also refer to a body located NNE of Faial Island (light green area numbered 4 in Figure 3) exhibitinghigh values of P-velocity (6.3–7.4 km/s) located deep in the crust, below 6 km. It partially overlaps the north-ern border of the two high-density bodies mentioned by Camacho et al. [2007] and Nunes et al. [2006].These three bodies are probably connected and may correspond to solidified magmatic intrusions relatedwith the feeding structures of the Faial-Pico ridge during the Matuyama period.

2.3. Climate, Wave Regime, and SedimentationPrecipitation on Pico Island varies mainly with altitude. Annual precipitation ranges from 1101 to1845 mm/a in the coastal area [Cruz and Silva, 2001] to more than 3600 mm/a above 800 m in the PicoMountain and Achada Plateau [Agencia Estatal de Meteorolog�ıa and Instituto de Meteorologia de Portugal,2012]. In Pico, the cover by young and very permeable lava flows has resulted in a very low drainage den-sity. The few streams are ephemeral and surface drainage is only present during heavy precipitation epi-sodes, mostly in the rainy season (October–March).

Information regarding oceanographic conditions in the islands of the central group is detailed in Quartauet al. [2012]. Records show that the Azores are struck on average every 7 years by violent storms (extratropi-cal storms or hurricanes [Andrade et al., 2008]). Even outside these stormy events, regular years showremarkable wave energy [Rusu and Guedes Soares, 2012]. The annual prevailing waves [see Quartau et al.,2012, Figure 3] hit the NW (29%) and W (24%) coastlines. They are also the highest, with average significantwave height (Hs) of 2.9 and 3.1 m, respectively. Waves from the N are also important (16%) with Hs of 2.5 m.Waves from the SW also present the highest Hs (3.1 m) but their frequency is much lower (8%). In the spe-cific case of Pico Island, sheltering effects probably occur in the western and north-eastern coasts, causedby the presence of Faial and S~ao Jorge Islands, respectively (Figure 1).

The Azorean islands are very young geomorphologically due to their frequent and recent volcanism. Thereare no wide-open valleys, meandering streams, inundation plains, and all water courses are ephemeral inthe archipelago [Louvat and Allegre, 1998]; thus, sediments on the shelf are mainly sourced from cliff erosion[Quartau et al., 2012]. The residence time of these sediments nearshore is also likely to be small due to thelarge wave energy to which these coasts are exposed. Because of that, the nearshore substrates of theseislands, down to 30–50 m depth, are mostly rocky [Quartau et al., 2012]. Sediments are remobilized fromthe nearshore to the middle and outer shelf by downwelling currents that form during storms [Meireleset al., 2013], forming clinoform bodies [Quartau et al., 2012]. During the storms, these currents are so strongthat fine sediments like silt or fine sand most likely cross the shelf edge and are deposited on the submarineslopes of the island. Measurements of these currents at 20 m water depth showed velocities up to 2 m/sassociated with Hs of 5 m [Youssef, 2005]. Such wave conditions occur on a yearly basis in the Azores andcan easily reach twice this value during stronger storms [Instituto Hidrogr�afico, 2005]. The Azorean shelvesdominated by wave erosion have high coastal retreat rates (0.21 m/yr on average [Borges, 2003]) and con-tain extensive and thick (up to 50 m) aggrading to prograding sedimentary bodies, which are believed tohave been formed in the last 6.5 ka [Quartau et al., 2012].

3. Data Sets and Methods

3.1. Acquisition and Processing of the Data SetHigh-resolution bathymetry of the outer shelf and slope of Faial and Pico islands and in Faial-Pico channelwas acquired in 2003 down to 1600 m (acquisition and processing described in Mitchell et al. [2008]). This



data set was supplemented with further surveying in 2004 in the western inner shelf area of Pico andinner shelf of Faial (acquisition and processing described in Tempera [2008]). Additional multibeambathymetry was acquired in the scope of the Portuguese continental shelf extension program using aSimrad EM710 down to 1760 m (EMEPC, data courtesy). The remaining nearshore gaps around Faial andPico were filled with single-beam bathymetry collected in 2001 (Quartau et al., 2002a) and in 2002 (Quar-tau et al., 2002b) concurrently with the seismic reflection survey. Offshore, the remaining gaps were filledwith single-beam bathymetry from the EMODnet European project. A bathymetric mosaic compilationwas produced from these five data sets with a cell size of 10 m for the multibeam data, 50 m for the near-shore single-beam data, and 500 m for the offshore single-beam data. Figure 4 shows the hydrographicsurveys and Figures 5a and 5b the resulting bathymetry.

High-resolution seismic reflection profiles were collected around Pico Island with a CHIRP sonar system(1.5–10 KHz, Datasonics CAP-6000W) and a boomer (EG&G 230-1) in July 2002, onboard the R/L �AguasVivas and the R/V Arquip�elago [Quartau et al., 2002b]. The seismic tracks (Figure 4) were designed formarine aggregate mapping purposes, so they were not placed optimally for studying the outer shelf ofthe island. The lines were acquired between 10 and 80 m water depth, seldom reaching the shelf edge.Survey lines were mainly normal to the shore separated by �700 m with a cross-line run around theisland at approximately 40 m depth. Acquisition and processing parameters are described in Quartauet al. [2012]. Most of the subbottom analysis was carried out on the boomer data, whereas the CHIRP wasmainly used for seabed classification because of limited penetration over the coarse sandy sedimentaryseafloor [Quartau and Curado, 2002b; Quartau et al., 2003]. Vertical resolution of the boomer data is�1–2 m.

The subaerial digital elevation model of the islands (Figures 5a and 5b) used in this work has a cell size of10 m and was produced by the Portuguese military survey from 1:25000 topographic maps [Instituto Geo-gr�afico do Ex�ercito, 2001a, 2001b, 2001c, 2001d, 2001e, 2001f, 2001g, 2001h, 2001i, 2001j].

Superficial sediment sampling was conducted in November 2003, aboard the R/V Arquip�elago [Quartauet al., 2005]. Fifty-four stations were sampled using a box-corer (Figure 4). Grain size data were obtainedusing a dry sieving technique for material coarser than 21 Ø (2 mm) and a Coulter Counter LS-230 for finer-grained material (<2 mm). The dry sieving was carried out over intervals of 1 Ø (25 Ø, 24 Ø, 23 Ø, 22 Ø,

Figure 4. Location of the data sets used for this study. The light blue and dark green areas offshore represent, respectively, the 2003 and2004 multibeam bathymetric surveys. The dark blue represents the EMEPC multibeam bathymetric survey. The light green area representsthe single-beam bathymetric surveys acquired in 2001-2002 and the orange the single-beam bathymetry from the EMODNET project. Redand blue lines depict, respectively, the CHIRP and boomer seismic profiles. The black dots represent the surface sediment sampling sites.



and 21 Ø sieves). Both data sets were then merged to produce composite grain size distributions. Sedimenttexture analysis shows that most samples are coarse sands according to the Udden-Wentworth scale [Went-worth, 1922], mainly composed of volcaniclastic sand (see Figures S1_a1_i, S1_b1_i, S1_b1_iii, S1_c1_i,S1_c3_i and S1_d3_i in the supporting information). Percentage of carbonate skeletal particles is normallyless than 10%, but can occasionally reach higher values, up to 90%.

Figure 5. (a) Submarine topography: shaded relief imagery derived from the bathymetric compilation. Subaerial topography: shaded reliefbased on the Instituto Geogr�afico do Ex�ercito [2001a, 2001b, 2001c, 2001d, 2001e, 2001f, 2001g, 2001h, 2001i, 2001j] topographic maps; (b)Interpreted submarine topography: alternating black and blue lines represent the shelf edge and corresponding top of the cliffs of theshelf sectors defined around Pico Island. Light green, dark green, magenta, and light blue lines represent gradient breaks in the Faial-Picochannel. Red, black, and dark blue lines represent the shelf edge of the main volcanic edifices in Faial. Letters A–G represent each individ-ual shelf segment or gradient breaks referred to in the text. Brown areas offshore represent submarine lava flow fields and green areas rep-resent rocky outcrops without any interpretable volcanic structure and boulder fields. The black boxes locate the areas represented inFigures S1_a1–S1_f1 of the supporting information. Pink dashed dot line represents the interpreted old south coastline of the westernAchada Plateau. Solid and dashed black lines onshore and offshore are, respectively, faults and inferred faults. Red dashed lines offshorerepresents the limits of the debris avalanches caused by the onshore landslides.



3.2. Geomorphological Analysis of the ShelvesShelves with a sharp seafloor surface (covered or not by sedimentary bodies) are interpreted to haveresulted from surf erosion during the last sea level rise or even from previous sea level cycles. Additionalinformation regarding the island history can be obtained from the depth of the present-day erosional shelfedge if the island has not been uplifting. If the depth of the edge is shallower than 123 m (the maximumdepth reached during the Last Glacial Maximum (LGM) in Bintanja et al. [2005]), these shelves have mostlikely started forming sometime during the rise of sea level after the LGM [Quartau et al., 2014]. Insularshelves in Pico are mostly formed by erosion of rock surfaces, since the predominant volcanism is effusive.Therefore, the depth of the erosional shelf edge should record the water level at that time [Quartau et al.,2010; Trenhaile, 2000, 2001]. Around Pico, the present depth of the shelf edge marks the beginning of theformation of the shelf and a sea level curve might be used to roughly infer the timing of that beginning.That timing marks the end of the main building phase of the volcanic edifice and the start of incision bywave action. If, on the contrary, the depth of the edge is deeper than 123 m depth, these shelf sectors wereformed by erosion during the first lowstand after the main adjacent volcanism took place and have sub-sided since then [Quartau et al., 2014]. A minimum subsidence rate can then be estimated based on theage of the occurrence of the first lowstand after the end of the main volcanism. High cliffs backing theshelves, absence of submarine lava flows and well-developed mass-wasting features on their seawardedges can also be evidence of their old age. Such evidence implies that the volcanic edifice being incisedby wave action was formed before the LGM. We use the term erosional shelves in the following text forthose dominated by wave erosion.

Shelves covered by fresh submarine lava flow morphologies that do not show signs of erosion, with lowadjacent coastlines and commonly irregular in plan view, imply recent volcanic progradation; i.e., after 6.5ka, when sea level reached almost its present-day position in Iberia [Hern�andez-Molina et al., 1994], at thesame latitude as the Azores. Here sea level has not yet intersected these structures so surf has not destroyedtheir primary surface features. The range of depths at which the fresh submarine lava flows occur may con-strain the age of their progradation onto the shelf during the sea-level rise after LGM since their positionsuggests that sea level was already above that depth of emplacement [Quartau et al., 2014]. Erosionalshelves in Faial and Terceira show evidence of volcanic progradation [Quartau et al., 2010, 2012, 2014]; how-ever, these lava flows are often isolated and occupy small areas when compared with the erosional featuresof those shelves. We use the term rejuvenated shelves in the text for those that were formed at the LGM orbefore, but are now dominated by volcanic progradation. Nevertheless, these shelves still preserve some oftheir old evidence of erosion (shelf edges at 2123 m or deeper), although showing widespread volcanicprogradation (low coastlines and the seafloor mostly covered by fresh lava flows). Another sign of shelf reju-venation is the irregular basement of the sedimentary cover (assumed to have been deposited after 6.5 ka,[Quartau et al., 2012]) that can be seen in seismic profiles. Irregular basements suggest that lava flows haveprograded onto the shelf when sea level was at a higher position than the irregularity left by progradationand has not dropped since then. Otherwise, surf erosion would have smoothed these irregular features, cre-ating sharp erosion surfaces (see examples of this interaction in Quartau et al. [2012, Figure 14]).

The bathymetry, seismic profiles, topography, and onshore geology were integrated in a GIS environmentto facilitate interpretation of the nearshore submarine morphology of the islands. The islands were dividedinto different shelf sectors based on their distinctive morphologies, which often have a clear correspon-dence with the onshore geomorphology. Three graphs were produced for each shelf sector showing thevariation of shelf width, shelf break depth, and cliff height. On erosional shelf sectors, there is normally agood correspondence between these three morphologic indicators, i.e., high cliffs are present where thereare wider shelves and deeper shelf edges. On the other hand, rejuvenated shelf sectors also have good cor-respondences, with low or absent cliffs occurring where there are narrow shelves and shallow shelf edges,although still showing localized evidence of their old age (isolated stretches of shelf edges at 2123 m ordeeper and high cliffs). Between these two extremes (erosional and rejuvenated shelves) intermediate com-binations of these characteristics exist. In order to produce the graphs of Figures S1_a1_i, S1_b1_i,S1_b1_iii, S1_c1_i, S1_c3_i, S1_d1_i, S1_d3_i, S1_e1_i, S1_e2_i and S1_f1_i (see supporting information) thebathymetry and topography were resampled to 50 m resolution. The depth of the shelf break and height ofthe cliffs (where present) in each sector was determined at 50 m intervals, respectively, from the bathymet-ric and topographic data. Profiles for each sector were constructed, at 50 m intervals as perpendicular as



possible to the coastline, from the shoreline to the shelf edge, to determine shelf width. Minimum, average,and maximum values of the three morphologic indicators of each sector are presented in Table 1. Thedetailed description and discussion of the different shelf sectors defined in this study can be found at thesupporting information section outside the main article. A brief presentation and discussion of the mainresults is presented in the following section. Readers are invited to skim the supporting information for acomprehensive understanding of the main interpretations achieved in this study.

4. Morphology of the Faial and Pico Island Shelves: Results and Implications

4.1. Conceptual Model of the Development of Rejuvenated ShelvesThe distinct nearshore submarine morphologic features around Pico Island allowed their division into fivemain sectors (Figures 5a and 5b). Their morphologies are directly linked to the evolution of the five mainvolcanic constructions along Pico Island. Thus, from east to the west we have the eastern Achada Plateau(sectors A1 and A2), the Topo volcano (sectors B1 and B2), the western Achada Plateau (sectors C1 and C2),the Pico Volcano (sectors D1 and D2), and the channel Faial-Pico (sectors E1, E2, E3, and E4). From A to D,these shelf sectors have always a north and south counterpart because they result from erosion of more orless centered onshore volcanism. The analysis of the data (Figures S1_a1 to S1_d4 in the supporting infor-mation and Table 1) shows shelves that have been formed by wave erosion and whose accommodationspace has been continually diminished by volcanic progradation, although some subsidence has occurred.The seafloor is either filled by lava flows (brown areas in Figure 5b and Figures S1_a1_i, S1_b1_i, S1_b1_iii,S1_c1_i, S1_c3_i, S1_d1_i, and S1_d3_i) or sedimentary bodies (all areas that are not covered by lava flowsor other unstructured rocky outcrops are sedimentary) underlain by a commonly irregular basement reflec-tion which is typical of shelves dominated by recent volcanic progradation (Figures 6a and 6b and FiguresS1_a2–S1_a5, S1_b2–S1_b4, S1_c2, S1_c4–S1_c5, S1_d2 and S1_d4 in the supporting information). Almostall the shelf sectors have edges at depths below 123 m (Table 1), which suggests that these shelves werenot completely filled by recent volcanic progradation. Their early edges were preserved and reflect an ageof wave incision during the LGM and have subsided quickly, or at an earlier lowstand and subsided moreslowly. GPS subsidence rates (5.7–7.2 mm/a) [Catal~ao et al., 2011] favor the first hypothesis, although asseen by the depth of shelf edges from volcanic edifices of known age in Faial and Terceira, longer-term sub-sidence rates are normally much lower (below 1 mm/a) [Quartau et al., 2010, 2014]. Given also that the agesof the main volcanism of the adjacent edifices are significant older than the LGM, it is more likely that theseshelves started forming at an earlier lowstand although significantly changed by post-LGM volcanicprogradation.

Oceanographic conditions, coastline lithology, and gradient of the shelf surface can be very differentaround ocean island volcanoes worldwide; hence, one should expect a large variety of shelf widening rates.Nevertheless, several studies have shown that long-term (up to several Ma) shelf widening rates can varybetween 0.6 and 9 mm/a [Dickson, 2004; Le Friant et al., 2004; Llanes et al., 2009; Menard, 1983, 1986;

Table 1. Shelf Width, Cliff Height, and Depth of the Shelf Edge of the Sectors Defined for Pico Islanda

E Achada Plateau Topo Sector

Southern Subsector A1 Northern Subsector A2 Southern Subsector B1 Northern Subsector B2

Min Avg Max Min Avg Max Min Avg Max Min Avg Max

Shelf width 138 577 1272 157 822 1283 120 985 2040 773 1293 1851Cliff height 0 66 194 0 84 410 0 30 178 0 34 180Depth of the edge 15 88 163 26 74 127 12 112 199 76 117 198

W Achada Plateau Pico Mountain

Southern Subsector C1 Northern Subsector C2 Southern Subsector D1 Northern Subsector D2

Min Avg Max Min Avg Max Min Avg Max Min Avg Max

Shelf width 79 179 377 234 900 1612 78 250 684 425 1701 2335Cliff height 0 16 42 0 32 157 0 16 32 0 8 21Depth of the edge 5 21 59 16 93 156 6 25 78 22 118 157

aMin, Avg, and Max represent the minimum, average, and maximum values in meters.



Quartau et al., 2010]. However, these rates are underestimated because the effects of shelf-narrowing proc-esses (mainly by volcanic progradation and shelf edge mass wasting), which can also differ greatly betweenislands, were not removed from these calculations. This is probably the reason, why, when looking at datafor shorter periods (less than 100 ka), faster rates are reported. For instance, as in Faial (up to 60 mm/a[Quartau et al., 2010]), Terceira (up to 34 mm/a, calculated from maximum width of sector 4 in Quartau et al.[2014] and the 60 ka volcanism age from Hildenbrand et al. [2014]), or Vulcano islands (up to 34 mm/a, cal-culated from Romagnoli [2013, Figure 3.9]). The likelihood of some of these shelf-narrowing processes hav-ing occurred in such short periods is probably smaller, thus rates at this time-scale tend to approximate tonet erosional values. On the other hand, rates tend to approximate over longer time scales because wave-eroded surfaces trend toward a state of equilibrium, as they become wider and more gently sloping [Quar-tau et al., 2010; Trenhaile, 2000]. Therefore, the best way of determining which process is dominating (waveerosion or volcanic progradation) when comparing different shelf sectors is to use a dimensionless variable.This can be achieved by comparing the maximum shelf width (max_W) against minimum shelf width(min_W) within the same shelf sector (i.e., formed by erosion after the main volcanic phase of the same vol-canic building). If a shelf sector is dominated by wave erosion, the ratio of max_W to min_W should besmall. Shelf sectors that are rejuvenated should present high max_W to min_W ratios, if volcanic prograda-tion is voluminous and heterogeneous, i.e., not constant along the entire coastline. Table 2 shows this analy-sis for Faial and Terceira based on data from Quartau et al. [2012, 2014], where shelves are mostlydominated by wave erosion, and for Pico shelves. The ratio of max_W to min_W in Faial and Terceira is gen-erally between 1 and 2. The values higher than 2 on these islands are related to rejuvenated sectors (sectorsA1 and A2 in Faial and sector 1 in Terceira) or changing wave exposure along the coastline (sector 3 in Ter-ceira). The ratios for Pico’s shelves are much higher (normally above 5 and up to 17). The sectors with lowerratios correspond to coastline segments where volcanic progradation were more or less homogeneousalong-coastline, like sector B2 (ratio of 2). In shelf sectors where volcanic progradation is heterogeneous,such as sector B1 where the progradation of Ribeiras lava delta occurs on its east side, the ratio of max_W

Figure 6. Seismic profiles showing irregular basements beneath sediments, interpreted as having resulted from recent lava progradationto the shelf (uninterpreted (a) and (c) and interpreted (b) and (d)). Vertical exaggeration is approximately 6X. Red solid lines represent theacoustic basement interpreted as the base of the sedimentary bodies. Profile lines are located in Figure S1_a1_i of the supportinginformation.



to min_W is high. Therefore, values higher than 5 appear to be good indicators of rejuvenation. The north-facing shelf sectors are generally wider than the south-facing sectors (Table 1), an observation that is relatedto the greater exposure to wave action (both in frequency and energy, see section 2.3). Nevertheless, theabnormally narrow shelves of subsectors C1 and D1 relatively to their north-facing counterparts is explainedby the greater volcanic progradation to the south. Alternatively, they could also be young erosional shelvesformed by surf erosion after recent and voluminous activity of the western Achada Plateau and Pico Vol-cano. The only exception is the southern Topo subsector B1 that is wider than its northern counterpart (B2);B2 is dominated by lava delta morphology, probably related with infilling of the landslide scar [Costa et al.,2014], which accounts for its smaller shelf width in comparison with B1.

A conceptual model of how these rejuvenated shelves develop is proposed based on the most commonexamples found around Pico (Figure 7). The model is simplified by not including subaerial (explosivevolcanism and stream erosion) and tectonic processes. However, it includes the two main processes(wave erosion and volcanic progradation) and their side effects (sedimentation, shelf width variation andminor slope failure at the shelf edge). The sequence starts with a volcanic island shelf formed by wave ero-sion during one or more glacial-interglacial sea level oscillations at a lowstand sea level position (SL1). Fromthere it can evolve directly into i_a_3 or evolve first to a highstand sea level position with resultant clifferosion and sediment deposition on the shelf (iii) and then on to a two end-member stage (iii_a_2 andiii_b_2):

1. From i to i_a_3. During the rise of sea level from the lowstand SL1 to the midstand SL2, significant vol-canic progradation occurs entirely filling the erosional shelf (i_a_1). The shelf edge is moved landwardand upward (represented by the red star). As sea level continues to rise to the highstand SL3, wave ero-sion of the lava flow occurs and produces a sharp erosional surface above the level of SL2 (representedby the black dot in i_a_2 and i_a_3). When sea level reaches SL3, a sedimentary body starts to grow byerosion of the cliff and nearshore deposits of the lava delta and transport of the eroded material to themid and outer shelf. If volcanic activity continues, the coastline is moved again offshore, further reducingthe shelf width (i_a_3). From i to i_a_3 the shelf edge is moved landward and upward (now representedby the green star).

2. From iii to iii_a_2. During the highstand SL3, volcanic progradation partially fills the shelf (iii_a_1), whichwas already covered by a sedimentary body (iii). Erosion of the coastline lava delta produces a small cliff(iii_a_2). From iii to iii_a_2, the shelf width is reduced but the depositional shelf edge maintains its posi-tion (represented by the green star in iii_a_2).

3. From iii to iii_b_2. During the highstand SL3, a significant volcanic progradation occurs filling the entireshelf, overlapping a previous highstand sedimentary body (iii_b_1). This provokes instability of the shelfedge and upper slope and mass wasting occurs, moving the shelf edge landward and upward. After thisevent, erosion of the coastline lava delta and production of a small cliff occurs (iii_b_2).

This model can be applied to other reefless volcanic island shelves where wave erosion and volcanic pro-gradation are the main development processes, with the latter prevailing over the former.

Table 2. Maximum (Max) and Minimum (Min) Shelf Widths and Their Ratios for the Sectors Defined in Faial [Quartau et al., 2012],Terceira [Quartau et al., 2014], and Pico Island

Faial A1 A2 B C D F G H

Min 440 172 1739 1034 1401 1073 773 1239Max 1174 727 3042 1434 3128 1827 1069 1728Ratio (Max/Min) 3 4 2 1 2 2 1 1

Terceira 1 2 3 4 5 6 NWTR SETR


Pico A1 A2 B1 B2 C1 C2 D1 D2




4.2. Insular Shelf SedimentationMost of the superficial samples collected on the shelves around Pico Island revealed coarse sands to gravelsand no relationship between grain size and depth (see Figures S1_a1_i, S1_b1_i, S1_b1_iii, S1_c1_i, S1_c3_i,S1_d1_i, and S1_d3_i and text in the supporting information). The finer samples were found on more pro-tected bays and downstream of the mouths of small streams. Therefore, the lack of well-developed drain-age, the narrow shelves, and the presence of high wave energy does not allow the development of wave-graded shelves where the grain size of seafloor sediments decreases with distance to the shore as seen inother environments [Dunbar and Barrett, 2005; George and Hill, 2008]. Consequently and similarly to Faialisland’s shelves [Quartau et al., 2012], there is no relationship between depth and grain size on theseshelves.

Figure 7. Conceptual model of the morphologic development of rejuvenated shelves (see section 4.1 for details). Each figure represents schematically a cross-shore profile of the shelfwith very high vertical exaggeration. The model starts at i or iii and develops into the three possible stages i_a_3, iii_a_2, and iii_b_2. Inside the squares, dark gray areas represent thecliff (CLF or CLIFFS), the insular shelf (INSULAR SHELF), and the upper slope (SLP or SLOPE) of a shelf sector of the island, light gray represents erosion within those areas, yellow areasrepresent sandy sedimentary deposits, the orange areas preserved or eroded lava flows, rounded blocks are boulder fields that result from erosion of lava flows, blue lines represent sealevel, black dots represent erosional (shelf) edges, red stars shelf edges formed by volcanic progradation and green stars depositional edges. Between the squares, blue curves representsea level oscillations; blue dots position of sea level inside the curve; blue arrows the direction of sea level variation and equal signal, stable sea level. Black horizontal and vertical arrowsrepresent time evolution.



4.3. The Shelf Morphological Record as Key for Understanding the Evolution of the Faial-Pico RidgeThe shelf edge in the north part of the Faial-Pico channel (E4) has the same trend (NW-SE) as the onshorewestern Achada Plateau and its respective shelf edge (C2). The depth of the edge NW of Cedros volcano(G2) is extremely incised by gullies, but reaches depths of �300 m in the most preserved segments. Thesegullies together with well incised streams at the coastline probably reflect structural weaknesses related tothe graben faults that extend to the west of the island (see dashed lines in Figure S1_f1 of the supportinginformation). The depth of the edges along all these sectors also shows that they have suffered subsidence,implying a relatively old age for their wave incision. The tectonic structures in the channel and the exten-sion of this graben system onto the western Achada Plateau as well as to the west Faial, the depth of theMatuyama body and the three high-density bodies (1, 2, and 4 in Figure 3b) in the channel, all trendingNW-SE, point to an old structure controlling the construction of the proto Faial-Pico ridge. Hildenbrand et al.[2012b] suggested a ridge structure for the Ribeirinha volcano based on the extension of magnetic negativevalues west of Faial. However, the above evidence suggests that instead a subaerial ridge structure devel-oped before the construction of Ribeirinha volcano at 850 ka (a typical circular shield volcano [Madeira,1998]) and extended below the present-day Faial-Pico ridge (see also the supporting information). Subsec-tors A1, A2, and C2 still show this ridge direction, however the depth of their edges is well above that ofsubsector E4. Continuous volcanic progradation has most likely erased the morphological evidence of thedeep shelf edge of this old structure on subsectors A1, A2, and C2.

The convex shelf edges at B1 and B2 sectors and their positions significantly deeper than 2123 m suggeststhat these sectors correspond to the incision of Topo volcano (see also sections S1.2 and S2.2 in the sup-porting information). In addition, the lack of convexity of the SW part of Topo suggests that also this parthas collapsed (red dashed line in Figure 5b represents the likely location of the debris avalanche producedby this collapse). After the three collapses of Topo volcano (in the N, in the SW and in the SE), significant vol-canism from the fissural system of Achada Plateau filled the scars.

In areas where morphological evidence of volcanic progradation exists, it is possible to roughly infer thetiming of volcanic progradation of these shelves with the help of a sea level curve. We are assuming thaterosional surfaces record the water level at that time (see section 3.2):

1. If the seafloor is covered by well-preserved lava flows or the basement under the sediments is irregular,volcanic progradation occurred after the LGM. The presence or not of sharp erosional features and theirdepth range marks the timing of the volcanic progradation. For instance, if sharp erosional features occurabove 240 m, volcanic progradation occurred at �11.15 ka (using the sea level curve of Bintanja et al.[2005]). A typical example of this is i_a_2 of Figure 7 (the black dot marks the depth of the beginning ofwave incision). If no sharp erosional features are present besides a small shore platform and cliffs behind,it means that volcanic progradation occurred after 6.5 ka when sea level approached the present-daylevel. A typical example of this is iii_a_2 of Figure 7.

2. Uncliffed or slightly cliffed subaerial lava deltas also imply that these were emplaced when sea levelapproached the present-day level, �6.5 ka or later. A typical example of this is iii_b_2 of Figure 7, whichmost likely corresponds in Pico to several undated examples, such as Ribeiras (Figure S1_b1_i) or S~aoMateus (Figure S1_c1_i) lava deltas.

A reconstruction of the evolution of the Faial-Pico ridge is therefore proposed based on the informationgiven by the shelf morphological analysis (see supporting information), known volcanic stratigraphy, tecton-ics, gravity data and magnetization values:

1. Development of the Faial-Pico volcanic ridge (Figure 8a) sometime before the emplacement of Ribeirinhavolcano (i.e., before 850 ka).

2. Development of the proto island of Faial with the emplacement of the Ribeirinha volcano reaching maxi-mum elevation at �850 ka [Hildenbrand et al., 2012b] (Figure 8b).

3. Rejuvenation of volcanism in NE Faial around 400–350 ka [Hildenbrand et al., 2012b] (Figure 8c).

4. Development of volcanism in the intersection of the two subridges in Pico, with emersion of Topo shieldvolcano at �250 ka [Chovelon, 1982] (Figure 8d).



Figure 8. Evolutionary stages (a–j) of the Faial-Pico ridge (the positions of the shore line and shelf segments or gradient breaks are schematic) (see section 4.3 for details). Color polygonsrepresent built up of successive volcanic edifices and have the same color legend as in Figure 2. The dashed black line represents the position of the shore line after volcanic construc-tion and/or wave erosion during the time elapsed between each stage. The color lines offshore represent the same shelf edge or gradient break segments as in Figure 5b. Circles andarrows represent, respectively, volcanic cones and direction of the flows issuing from them.



5. Rejuvenation of volcanism in W Faial with the development of the Caldeira volcano at �120 ka [Hilden-brand et al., 2012b] (Figure 8e).

6. Collapses of the northern and southern flanks of the Topo volcano. The minimum age estimation of thenorthern collapse is �70 ka [Costa et al., 2014] (Figure 8f).

7. New volcanic phase with the development of the Pico stratovolcano sometime before 53 ka [Costa et al.,2014]. Infilling of the Topo volcano landslide scars by reactivation of the volcanism of the western andeastern Achada Plateau ridge (Figure 8g).

8. Collapse of the south flank of Pico volcano sometime after 53 ka and eruption of the Almoxarife Forma-tion in Faial at �30 ka [F�eraud et al., 1980] (Figure 8h).

9. During the last sea level rise coastlines receded and insular shelves widened around the volcanic edifices.Simultaneously, volcanic progradation occurred and filled previous erosional spaces on the shelves ofFaial and Pico. Rejuvenation of volcanism in W Faial with the development of the Capelo Peninsulaaround 8 ka [Di Chiara et al., 2014] (Figure 8i).

10. Development of post human-settlement lava deltas in Pico (Ponta do Mist�erio, S~ao Jo~ao, Santa Luzia,and Silveira). In Faial Island eruptions occurred in 1672–1674 and in 1957–1958 A.D [Madeira and Brumda Silveira, 2003] (Figure 8j).

4.4. Implications for Hazard AssessmentIn several places where the lava deltas reached the shelf edge in Pico Island, they have apparently triggeredsmall slope failure and a consequent retreat of the edge (as the example iii_b_2 of Figure 7). This is in accord-ance with reports by other authors of the effects of lava deltas reaching steep submarine slopes, covered byunstable volcanic aprons [Bosman et al., 2014; Chiocci et al., 2008; Poland and Orr, 2014; Sansone and Smith,2006]. Mapping the seabed offshore of these structures may therefore provide an indication of their potentialfor land collapse. Figure 9 shows the areas where lava progradation was voluminous enough to reach theshelf edge, triggering slope failures and provoking its retreat. These areas are potentially unstable and this canbe a simple and time-effective way to assess likely future collapses, perhaps more efficient than using timeseries InSAR or GPS data.

Figure 9. Mapping of the onshore areas that can potentially collapse because lava deltas have reached the shelf break (in red). Legend isthe same as Figures 5a and 5b.



5. Conclusions

Multibeam bathymetry, seismic reflection profiles, and sediment samples from the nearshore submarineareas of Faial and Pico islands were used to map and characterize the morphology of the shelf environ-ments. These insular shelves were formed by wave erosion of the flanks of the volcanic edifices of Faial andPico islands. Pico insular shelves have been significantly modified by later volcanic progradation.Nevertheless, most of Pico’s insular shelves still have edges at depths below 123 m (the LGM sea level) and,given that the adjacent volcanoes are older than the LGM they were probably created before and havesubsided.

The morphology of Pico insular shelves, particularly expressed by the depth of their edges, well-preservedlava flow morphologies and the nature of their basement (sharp or irregular) beneath the sediments sug-gests whether they were last affected by wave erosion or volcanic progradation. This allowed us to recon-struct the history of these shelves and to put forward a conceptual model of the post-LGM morphologicdevelopment of the Pico shelves. They show a dominant role of volcanic progradation in their evolution,prevailing over wave erosion and allow us to classify many of these shelves as rejuvenated. In places wherevolcanic progradation has not been homogeneous along the coastline, a dimensionless variable (the ratiobetween the maximum and minimum shelf width) is proposed to discriminate between mainly erosional ormainly rejuvenated shelves. The ratio shows that Pico shelves are mostly rejuvenated when compared withthe majority of the Faial and Terceira shelves. In addition, the history of these shelves together with the dis-tribution of the gravity data, magnetization values and the onshore geomorphology were used to developan evolutionary model for the Faial-Pico ridge. The interpretation of these data suggests that the first subae-rial volcanism along this ridge is older than has been reported, i.e., before �850 ka.

Lava deltas reaching the shelf break around Pico Island have triggered small mass-wasting events andretreat of the shelf edges. At Ribeiras, this lava delta seems to be unstable and subsiding faster than the sur-rounding areas (see text in supporting information). Collecting multibeam bathymetry can reveal wherethese lava deltas have reached or not the shelf edge, hence providing an effective way, complementary toground deformation monitoring, to assess hazard and produce maps of onshore areas that have a height-ened risk for human occupation.

These results improve the understanding of the processes shaping island shelves at reefless volcanic oceanislands. In addition, the morphological analysis of the shelves provides a significant complement to unveil-ing the geological history of the islands as well as information for risk assessment and coastal land usemanagement.

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AcknowledgmentsSeismic acquisition and sedimentsampling of Pico shelves wereconducted in the scope of the GEMASProject funded by Secretaria Regionaldo Ambiente dos Acores. Thefollowing agencies are gratefullyacknowledged for providing fundingfor the multibeam surveys: the RoyalSociety, the British Council, the HigherEducation Funding Council for Wales,the Regional Directorate for Scienceand Technology of the Azores andPortuguese projects MARINOVA(INTERREG IIIb/MAC/4.2/M11), MAROV,MAYA (AdI/POSI/2003), andMeshAtlantic (AA-10/1218525/BF). Thiswork was also supported by IDLthrough UID/GEO/50019/2013,financed by Fundac~ao para a Ciencia ea Tecnologia (FCT). The authors alsoacknowledge the support of LandmarkGraphics Corporation to IPMA via theLandmark University grant Programand CARIS, Inc., for the CARIS HIPS andSIPS license granted under theacademic agreement withIMAR-Instituto do Mar (Ref.: 2009-02-APP08). Rui Quartau and FernandoTempera acknowledge, respectively,the Ciencia 2008 research contract andthe post-doc grant ref. SFRH/BPD/79801/2011 funded by FCT. We finallywish to express our gratitude for theconstructive reviews by ClaudiaRomagnoli and Christian Huebscherwhich helped to significantly improvethe manuscript. Data can be providedon request to Rui Quartau([email protected]).



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Erratum

In the originally published version of this article, there were some typographical errors that have now been corrected.

In the first sentence in paragraph 2 of Section 2.1, the location has been clarified.

The following references have been updated:

Instituto Geogr�afico do Ex�ercito (2001i), Portuguese military map, Praia do Norte (Faial-Acores), sheet 4, scale 1:25,0000, Lisboa.

Instituto Geogr�afico do Ex�ercito (2001j), Portuguese military map, Feteira (Faial-Acores), sheet 6, scale 1:25,0000, Lisboa.

Quartau, R., F. Curado, T. Cunha, L. Pinheiro, and J. H. Monteiro (2002a), Projecto Gemas-Localizac~ao e distribuicc~ao de areias em redor da

ilha do Faial, Tech. Rep. INGMARDEP 5/2002, Dep. Geol. Mar. IGM, Lisboa.



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