- Email: [email protected]
Reply to the comment by Quartau et al. on “Construction and destruction of a volcanic island developed inside an oceanic rift: Graciosa Island, Terceira Rift, Azores”, J. Volcanol. Geotherm. Res. 284, 32–45, by Sibrant et al. (2014)
Journal of Volcanology and Geothermal Research 303 (2015) 193–198
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Reply to the comment by Quartau et al. on “Construction and destruction of a volcanic island developed inside an oceanic rift: Graciosa Island, Terceira Rift, Azores”, J. Volcanol. Geotherm. Res. 284, 32–45, by Sibrant et al. (2014) A.L.R. Sibrant a,⁎,1, F.O. Marques b, A. Hildenbrand a,c a b c
Université de Paris-Sud, Laboratoire GEOPS, UMR8148, Orsay, F-91405, France Univerdade de Lisboa, Lisboa, Portugal CNRS, Orsay F-91405, France
a r t i c l e
i n f o
Article history: Received 4 August 2015 Accepted 12 August 2015 Available online 18 August 2015 Keywords: Volcanic stratigraphy Geochronology Sector collapse Graciosa island Terceira Rift Azores Triple Junction
a b s t r a c t In Sibrant et al. (2014), we reconstructed the evolution of Graciosa Island (Azores). We extensively discussed the nature and the meaning of the destruction episodes, either tectonics or gravitational, and concluded that the island has evolved through major landslides. In their comment, Quartau et al. (2015) conclude that (1) “Sibrant et al. (2014) is based almost solely on subaerial observations,” which is false because we used the bathymetric data available to us (Figs. 3 and 4 in Sibrant et al., 2014). (2) “…the published multibeam sonar data around Graciosa reveals that their proposed successive phases of destruction of the volcanic edifices composing the island by massive landslides is incompatible with the high-resolution bathymetry.” First, saying that the data were published is misleading because only two images are now provided in Quartau et al. (2015). Most of the high-resolution data used by Quartau et al. (2015) are not published, and they still do not release the data for us to analyse and use in this reply. Second, the high-resolution bathymetric maps are not incompatible with our model. For instance, mounds on the eastern submarine slope may actually be landslide blocks, and the platform developed to the S may correspond to flank collapses of the successive volcanoes, blanketed more recently by the young basaltic cover. (3) “The interpretation of collapse structures appears to have originated partly from a misreading of the volcano-stratigraphy and tectonic structures”. We certainly did not “misread” the volcanic stratigraphy and tectonic structures in Graciosa; in great contrast to Quartau et al. (2015), we (a) used major unconformities to establish the volcano-stratigraphy, (b) calibrated this stratigraphy with high precision K–Ar dating, (c) made careful measurement of lava flow attitudes to infer the pre-collapse position of the main edifices, and (d) did not use hypothetical tectonic faults, as Quartau et al. (2015) did, to draw an evolutionary cartoon of Graciosa. The lava flows in the southern cliffs of Graciosa dip inland, as recognised by Gaspar (1996, Fig. 3B), in great contrast to the claims of Quartau et al. (2015), and therefore do not support successive volcanoes coinciding with the centre of the island. (4) From unreleased high-resolution bathymetric data, Quartau et al. (2015) consider that no large landslide(s) occurred in Graciosa, and propose a “new model.” First, their geological model is not new, as it reproduces a cartoon in Gaspar (1996, Fig. 3A). Second, Quartau et al. (2015) have not reported any major faults exposed at the surface, as expected for an island-scale graben, but their summary cartoon of Graciosa evolution is based on major faults. Third, the new model of Quartau et al. (their Fig. 8, and our Fig. 4) misrepresents the initial model of Gaspar (1996, Figs. 4.1 and 4.2), as the whole SW flank of the island has been arbitrarily removed on their drawing (see our Fig. 3). In contrast, Sibrant et al. (2014) propose that the SW flank has been effectively removed, but by landslides. Therefore, their new model for the evolution of Graciosa is based on misleading hypotheses rather than convincing arguments. In conclusion, there is no need to change our proposed new stratigraphy and evolution of the island based on Quartau et al.'s claims because these are not supported by actual geological data—well established and calibrated stratigraphy, geometry of lava flows, and geometry and kinematics of faults. Given the absence of sound geological data to support the evolutionary cartoon proposed by Quartau et al. (2015), we conclude that it is mostly flawed and therefore mere speculation. © 2015 Elsevier B.V. All rights reserved.
⁎ Corresponding author. Tel.: +33 1 69 15 80 89. E-mail address: [email protected] (A.L.R. Sibrant). 1 Now at Laboratoire FAST (CNRS/Univ. P-Sud), Orsay, F-91405, France.
http://dx.doi.org/10.1016/j.jvolgeores.2015.08.008 0377-0273/© 2015 Elsevier B.V. All rights reserved.
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1. Introduction In their comment, Quartau et al. (2015) conclude that (1) our study on Graciosa (Sibrant et al., 2014) is based almost solely on subaerial observations, (2) “…the published multibeam sonar data around Graciosa reveals that their proposed successive phases of destruction of the volcanic edifices composing the island by massive landslides is incompatible with the high-resolution bathymetry,” (3) “the interpretation of collapse structures appears to have originated partly from a misreading of the volcano-stratigraphy and tectonic structures,” and (4) from unreleased high-resolution bathymetric data, no large landslide(s) occurred in Graciosa. Finally, they propose a “new model.” We will focus on these main points of their comment and discuss the lack of reliability of their database, especially regarding the stratigraphic, the morphologic, the tectonics, and the geological aspects. 2. Stratigraphy Sibrant et al. (2014) distinguished six main volcanic complexes in the island, instead of the three reported in Gaspar (1996). Such distinction is not based on our new K–Ar dating, as implicitly mentioned by Quartau et al. (2015). Our work relies on careful fieldwork, including recognition of the main lava sequences, the attitude of the lava flows, the geometry of the units and their mutual relationships, the number and the geometry of the major unconformities, and ultimately the dating of the most critical lava flows (e.g. base and top of sequences above and on top of the unconformities), to calibrate the stratigraphy and constrain the timing of the main steps of evolution. In their comment, Quartau et al. (2015) keep on with the three main complexes following Gaspar (1996). They especially mention that two of the volcanic complexes we distinguished belong to a single volcano, as they define a unique magmatic series in a TAS diagram (their Fig. 3). We wish to remind Quartau et al. (2015) that major elements are by themselves poorly discriminant to distinguish between different edifices, especially in the Azores. The composition of the lavas from Graciosa range from basalt to trachyte according to the total alkalis–silica (TAS) classification (Gaspar, 1996; Almeida, 2001; Beier et al., 2008; Larrea et al., 2014a). As a whole, they define one main alkaline volcanic series (e.g., Larrea et al., 2014a). Following the argument of Quartau et al. (2015), all the island would thus be made of a single volcanic complex. By extension, we remind Quartau et al. (2015) that most of the lavas erupted in the Azores islands during the last 750 kyr are also alkaline in character (e.g., Hildenbrand et al., 2014). Following their argument would mean that all of them constitute a single volcanic complex, which is obviously not acceptable. Quartau et al. (2015) also keep on with the idea that the main volcanic complexes have been erupted on top of each other, from volcanoes mainly located in the centre of the island (their Fig. 8, see our Fig. 4). This clearly cannot be the case, as the units are separated by major unconformities, and the lavas exposed in the southern cliff of the island (Baía do Filipe) show a general dip towards the NE, i.e. opposite to what would be expected from a volcano located in Serra Dormida Hill, as clearly shown in Sibrant et al. (2014). In fact, even Gaspar (1996) showed in his Fig. 4.2 (reproduced in Fig. 3B) that the general dip of all complexes is towards the NE, and therefore the eruptive centre was southwest of the current shore. This indicates that Quartau et al. (2015) not solely misread our former paper but also previous work from Gaspar (1996), and ignored important geometrical characteristics of the actual exposed stratigraphy. 3. Inland morphology and tectonics Based on morphological and calibrated stratigraphic data and the fact that a significant part of several complexes is missing, we discussed extensively the possible genesis of the morphological scarps, by either tectonic processes or lateral flank collapses. From our new data, we
proposed that the island was affected by at least 4 catastrophic flank collapses, separated by reconstruction of nested volcanic edifices in the successive scars, mainly towards the SW, and finally towards the SE. Quartau et al. (2015) raise major criticisms about this interpretation. Following Fig. 3A of Gaspar (1996), they propose instead that the evolution of Graciosa consists of successive volcanic edifices built on top of each other, which have been dismantled in a symmetric way by active faulting following the Terceira Rift. In their comment, Quartau et al. (2015) also extensively mention the role of horizontal marine abrasion, but most of the destruction resulted from vertical erosion (Sibrant et al., 2014), as also drawn by Quartau et al. (2015) in their Fig. 8, despite they state in the main text that vertical erosion is negligible. We here present a gradient map of Graciosa (vertical illumination) built from the digital elevation model of the island, along with similar maps for Faial and Terceira for the sake of comparison (Fig. 1). While Faial and Terceira show conspicuous symmetrical grabens, no similar structure is observed in Graciosa, despite its location in the active Terceira Rift and its relatively old age (Larrea et al., 2014b; Sibrant et al., 2014). Whereas main scarps dipping towards the SW can be identified in Graciosa, no clear scarp dipping to the NE stands out. A few lineaments are visible but certainly cannot be attributed to a major graben at the island scale. At best, a half-graben towards the S may exist, as discussed in Sibrant et al. (2014). On the tectonic map (Figs. 2 and 4 in Quartau et al., and our Figs. 1 and 2), all the major faults referred to by Quartau et al. (2015) are hypothetical, as marked by dashed points. These mostly reproduce the lineaments proposed by Gaspar (1996), except two major faults dipping to the S (Fig. 3B), which are in agreement with our collapse interpretation. If we compare Figs. 2 and 4 of Quartau et al. (2015) based on Gaspar (1996) and the tectonic map published in Gaspar (1996), we can see that the vast majority of the faults are indicated as “falha provável ou encoberta” and “escarpa de falha provável” which mean in English “expected or dissimulate faults” and “expected fault scarps,” respectively. Even some morphological scarps (e.g. South Serra das Fontes) are arbitrarily termed “faults scarps,” without any justification. This is just speculation, and the authors should at least have discussed if such scarps have a tectonic or gravitational origin, as we did in Sibrant et al. (2014). In their comment, Quartau et al. (2015) refer to Hipólito et al. (2013) regarding the tectonic structures, where one will immediately see in Figs. 8 and 9 of Hipólito et al. (2013) that (1) many so-called “faults” are apparently just compaction structures in scoriae deposits (their Fig. 8), and (2) one small fault is shown, in contrast to the many major faults shown in the evolutionary cartoon of Quartau et al. (Fig. 8 of the comment). Moreover, the photograph in Fig. 9 of Hipólito et al. (2013) is parallel to the Terceira Rift (NW-SE), so the fault appears perpendicular to the main graben inferred by Quartau et al. All the above lines of evidence show that no actual measurement in the field supports a major fault associated with the island-scale graben inferred by Quartau et al. (2015) No fault kinematics, so critical to constrain the slip direction and the stress field, has been apparently measured either. 4. Bathymetric data In Sibrant et al. (2014), we used a low-resolution bathymetric grid because it was the only data available to us. Now, Quartau et al. (2015) provide for the first time a couple of high-resolution bathymetric maps, in great part built from unpublished data. Unfortunately, the actual data of the high-resolution bathymetry are still not available to us. Quartau et al. (2015) do not even provide a clean image without any interpretation, so we cannot make a thorough analysis of the data to produce our own interpretation. However, from the low-resolution data and the two images provided Quartau et al. (2015) in their comment, one can easily check that the only large-scale obvious graben is the Terceira Rift (TR), which extends far away from Graciosa. In fact, the island sits inside the TR, not along the main faults of the TR, which
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Fig. 1. Shaded relief maps of Terceira (A), Faial (B), and Graciosa (C) islands (vertical illumination), showing conspicuous grabens in Faial and Terceira, but not in Graciosa. Faults exposed at the surface marked by F. S. stands for morphological scar in Serra das Fontes. (D) Simplified tectonic map of Graciosa, after Hipólito et al. (2013), showing that most faults and fault scarps are inferred from morphological interpretation but do not correspond to observed faults.
Fig. 2. (A) Original tectonic map of Graciosa by Gaspar (1996) indicating very few faults and many “probable faults.” (B) Geological map of Graciosa and tectonic information proposed by Quartau et al. in their comment. Note that several faults scarps marked as “provável” (which means probable in English) in Gaspar (1996) have become true fault scarps in Quartau et al., without any explanation.
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Fig. 3. (A) Original sketches by Gaspar (1996) showing his interpretation of the evolution of Graciosa. Note that the successive volcanoes and the inferred graben appear symmetrical and both located in the centre of Graciosa, but the southwest continuation of the island is not drawn. (B) Geological cross-section by Gaspar (1996). Note that the SW half of the initial volcanoes is missing. Extrapolation of the former shape of the volcanoes from the preserved geological units (conical hypothesis, dashed lines drawn by us) suggest that the first volcano (Serra das Fontes, labelled 1) was culminating much higher than the later volcano (Serra Branca, labelled 2). This means that (1) the two volcanoes were not built conformably on top of each other and (2) that the Serra Branca volcano developed in a depression that carved all the SW half of Serra das Fontes; such depression is consistent with the hypothesis of a large SW-directed landslide between the two volcanic stages (Sibrant et al., 2014). Also note that all the main volcanic units dip towards the NE, and most of the inferred faults dip towards the SW, in great contradiction with the comment by Quartau et al. (2015), but in fairly good agreement with Sibrant et al. (2014).
may explain why there is no major fault visible at the surface of the island, in contrast to Terceira (Navarro et al., 2003; Calvert et al., 2006; Hildenbrand et al., 2014; Marques et al., 2015), and S. Miguel (e.g. Moore, 1990; Carmo, 2013; Sibrant et al., 2015), which sit partially on a master fault of the TR, and also to Faial (Hildenbrand et al., 2012, 2013), which developed on top of a master fault in the diffuse part of the present Eurasia–Nubia plate boundary in the Azores (e.g., Marques et al., 2013, 2014a, 2014b, 2015). One of the main criticisms of Quartau et al.'s comment is based on the recognition of a flat platform around Graciosa on the bathymetric map. They call such platforms insular shelves and attribute them to surf erosion of the volcanic edifices during the Quaternary sea level changes. In their opinion, such platforms rule out our proposed landslides. This is certainly not the case. From their Figs. 6 and 7, it is obvious that the southern shelf is more developed than the northern one, despite the older age of most of the volcanic units exposed in the N. Therefore, these shelves do not rule out our proposal for south-directed landslides. From our study (Sibrant et al., 2014) and from the highresolution bathymetric image, it seems highly plausible that the products of the several proposed slides may have been buttressed on the S wall of the TR, and extensively buried by the young basaltic cover, which can explain the flat and anomalously large shelf developed immediately to the S of the island. In addition, several of the bathymetric mounds exposed SE of the island could represent the distal products of the youngest landslide, which according to Sibrant et al. (2014) occurred towards the SE. 5. The “new” model of evolution of Graciosa by Quartau et al. In their comment, Quartau et al. (2015) propose a new model for the geological evolution of Graciosa Island. However, this model is not new, as it reproduces in great part the model proposed by Gaspar (1996) and shown here in our Fig. 3. In fact, the new model of Quartau et al. (their Fig. 8 and our Fig. 4) misrepresents the initial model of Gaspar (1996, Figs. 4.1 and 4.2), as the whole SW flank of the island has been
arbitrarily removed on their drawing (see our Fig. 3). In contrast, we propose that the SW flank has been effectively removed, but by landslides (Sibrant et al., 2014). In addition, the “new” model presented by Quartau et al. (2015) in their comment shows many inconsistencies, as summarised in our Fig. 4 and listed in the following: Stage A: the initial volcano (Serra dos Fontes) is drawn highly asymmetrical, because the SW flank appears much less developed than the NE flank. This is just an artefact because the topographic surface is deliberately drawn to truncate the lava flows in the SW and so make the island shorter in this direction. Such geometry is unrealistic for a volcanic constructional slope and is in any case not justified from actual data and the drawings of Gaspar (1996) in his Fig. 4.1 reproduced in our Fig. 3A. Stages B and C: the new volcano marked in yellow is, again, drawn asymmetrical. From B to C, the quasi-totality of the edifice (height around 1 km) is removed in less than 24 kyr (between 323 ka and 299 ka), supposedly by vertical erosion, which is considered negligible by Quartau et al. (2015) in the text of their comment. Stage D: in the SW, the “erosional shelf” marked by a dashed line in Quartau et al. erodes a volcanic flank that does not exist. More generally, the dip of the lava flows from the old volcanic edifice (NE sector) increase gradually from ca 8° in Fig. 8A to ~ 30° in Fig. 8D without any justification. Throughout the successive stages, numerous faults are drawn, but they are all hypothetical, as no clear evidence for actual large faults is provided by Quartau et al. 6. References We note that Quartau et al. (2015) avoid citing relevant work our team did in the Azores, but cite all the work of the co-authors (e.g. Quartau 10 self citations, Mitchell 8 self citations) including unpublished MSc and PhD theses, and submitted papers. For instance, when
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Fig. 4. Illustration of the major inconsistencies in Quartau et al.'s “new” model: (A) Initial volcano drawn highly asymmetrical; (B and C) new volcano again asymmetrical, and almost completely removed in ca 24 kyr (between 323 ka and 299 ka); (D) in the SW, “erosional shelf” (dashed line by Quartau et al.) erodes a volcanic flank that does not exist. More generally, the dip of the lava flows from the old volcanic edifice (NE sector) increases gradually from ca 8° in Fig. 8A to ~30° in Fig. 8D. Throughout the successive stages, numerous faults are drawn, but they are mostly hypothetical, as no clear evidence for actual large faults is provided by Quartau et al.
describing the structures in Faial and Terceira, Quartau et al. do not cite relevant recent work by Marques et al. (2014b), which explains the formation of the Faial Graben, and Marques et al. (2015), which reports on the tectonics of Terceira Island. 7. Conclusions We thank Quartau et al. (2015) for their comment, which allowed us to clarify important points and show the reader that the combination of robust stratigraphic, morphological, tectonic, bathymetric, and geochronological data are needed to reconstruct the complex evolution of unstable volcanic islands like Graciosa. We show that (1) Graciosa is not affected by an island-scale graben, as no clear evidence for large normal faults is provided by Quartau et al (2) Construction of the island does not result from successive volcanoes conformably built on top of each other in the centre of the island; instead, it results from nested volcanic edifices built in the successive scars, mainly towards the SW, and ultimately towards the SE. (3) The island was not destroyed by a symmetric graben and wave erosion but was affected instead by lateral destruction towards the SW, in our interpretation by large-scale landslides. (4) We cannot discuss fully the high-resolution bathymetry, as the data have not been released by Quartau et al.; however, the two bathymetric images provided in their comment do not rule out the existence of large landslides. We wish to encourage these authors to publish their highresolution bathymetry acquired during the MARCHE missions (Luis et al., 2007) and EMODnet project as soon as possible, so the bathymetry becomes available and accessible to the scientific community. (5) The
“new” proposed evolution of Graciosa by Quartau et al. shows major inconsistencies with field data, even from Gaspar (1996), and is mostly flawed because it is not supported by geological data. From the above lines of evidence, we argue that the interpretations by Quartau et al. are not supported by geological data, and therefore, it seems obvious to us that there is no need to change the new proposed stratigraphy and evolution of Graciosa as reported in Sibrant et al. (2014). Acknowledgments This is a contribution to Project MEGAHazards, funded by FCT (PTDC/CTE-GIX/108149/2008), Portugal. FO Marques benefits from a Sabbatical fellowship awarded by FCT Portugal. ALR Sibrant benefitted from funding by the French CNRS-INSU and by the French ANR (project RHUM-RUM). This is LGMT contribution 132. Appendix A. Suplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.jvolgeores.2015.08.008. References Almeida, M.H., 2001. A fonte mantélica na região dos Açores: constrangimentos impostos pelas caracteristicas geoquimicas de rochas vulcanicas e de xenolitos ultramaficos (PhD thesis) Azores University, São Miguel.
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Beier, C., Haase, K.M., Abouchami, W., Krienitz, M.S., Hauff, F., 2008. Magma genesis by rifting of oceanic lithosphere above anomalous mantle: Terceira Rift, Azores. Geochem. Geophys. Geosyst. 9, Q12013. Calvert, A.T., Moore, R.B., McGeehin, J.P., Rodrigues da Silva, A.M., 2006. Volcanic history and 40Ar/39Ar and 14C geochronology of Terceira Island, Azores. Portugal. J. Volcanol. Geotherm. Res. 156, 103–115. Carmo, R., 2013. Estudo de neotectonica na ilha de S. Miguel. Uma contribuição para o estudo do risco sismico no arquipélago dos açores (PhD Thesis), Azores university (380 pp.). Gaspar, J.L., 1996. Ilha Graciosa (Açores): Historia Vulcanologica e Avaliaçao do Hazard (Tesis Doctoral), Univ. Dos Açores (361 pp.). Hildenbrand, A., Marques, F.O., Costa, A.C.G., Sibrant, A.L.R., Silva, P.M.F., Henry, B., Miranda, J.M., Madureira, P., 2012. Reconstructing the architectural evolution of volcanic islands from combined K/Ar, morphologic, tectonic, and magnetic data: the Faial Island example (Azores). J. Volcanol. Geotherm. Res. 241–242, 39–48. Hildenbrand, A., Marques, F.O., Costa, A.C.G., Sibrant, A.L.R., Silva, P.F., Henry, B., Miranda, J.M., Madureira, P., 2013. Reply to the comment by Quartau and Mitchell on “Reconstructing the architectural evolution of volcanic islands from combined K/Ar, morphologic, tectonic, and magnetic data: the Faial Island example (Azores)”, J. Volcanol. Geotherm. Res. 241-242, 39-48, by Hildenbrand et al. (2012). J. Volcanol. Geotherm. Res. 255, 127–130. Hildenbrand, A., Weis, D., Madureira, P., Marques, F.O., 2014. Recent plate re-organization at the Azores Triple Junction: evidence from new geochemical and geochronological data on Faial, S. Jorge and Terceira volcanic islands. Lithos 210–211, 27–39. Hipólito, A., Madeira, J., Carmo, R., Gaspar, J.L., 2013. Neotectonics of Graciosa Island (Azores): a contribution to seismic hasard assessment of a volcanic área in a complex geodynamic setting. Ann. Geophys. 56 (6), S0677. Larrea, P., Galé, C., Ubide, T., Widom, E., Lago, M., França, Z., 2014a. Magmatic evolution of Graciosa (Azores, Portugal). J. Petrol. 1–30 http://dx.doi.org/10.1093/petrology/ egu052. Larrea, P., Wijbrans, P.R., Galé, C., Ubide, T., Lago, M., França, Z., Widom, E., 2014b. 40 Ar/39Ar constraints on the temporal evolution of Graciosa Island, Azores (Portugal). Bull. Volcanol. 76, 796. http://dx.doi.org/10.1007/s00445-014-0796-8.
Luis, J., Lourenço, N., Mata, J., Madureira, P., Miranda, J.M., Goslin, J., Perrot, J., Brachet, C., Simão, N., 2007. A highly detailed multibeam bathymetry survey of Azores Triple Junction area. Geophys. Res. Abstr. 9, 08269. Marques, F.O., Catalão, J.C., DeMets, C., Costa, A.C.G., Hildenbrand, A., 2013. GPS and tectonic evidence for a diffuse plate boundary at the Azores Triple Junction. Earth Planet. Sci. Lett. 381, 177–187. Marques, F.O., Catalão, J.C., DeMets, C., Costa, A.C.G., Hildenbrand, A., 2014a. Corrigendum to “GPS and tectonic evidence for a diffuse plate boundary at the Azores Triple Junction”. Earth Planet. Sci. Lett. 381, 2013, 177-187. Earth Planet. Sci. Lett. 387, 1–3. Marques, F.O., Catalão, J., Hildenbrand, A., Costa, A.C., Dias, N.A., 2014b. The 1998 Faial earthquake, Azores: evidence for a transform fault associated with the NubiaEurasia plate boundary? Tectonophysics 633, 115–125. Marques, F.O., Catalão, J., Hildenbrand, A., Madureira, P., 2015. Ground motion and tectonics in the Terceira Island: tectonomagmatic interactions in an oceanic rift (Terceira Rift, Azores Triple Junction). Tectonophysics 651–652, 19–34. Moore, R., 1990. Volcanic geology and eruption frequency, S. Miguel, Azores. Bull. Volcanol. 52, 602–614. Navarro, A., Catalão, J., Miranda, J.M., Fernandes, R.M.S., 2003. Estimation of the Terceira Island (Azores) main strain rates from GPS data. Earth Planets Space 55, 637–642. Quartau, R., Hipolito, A., Mitchell, N.C., Gaspar, J.L., Brandão, F., 2015. Comment on “Construction and destruction of a volcanic Island developed inside an oceanic rift: Graciosa Island, Terceira Rift, Azores” by Sibrant et al. (2014) and proposal of a new model for Graciosa geological evolution. J. Volcanol. Geotherm. Res. 303, 146–156. Sibrant, A.L.R., Marques, F.O., Hildenbrand, A., 2014. Construction and destruction of a volcanic Island developed inside an oceanic rift: Graciosa Island, Terceira Rift, Azores. J. Volcanol. Geotherm. Res. 284, 32–45. Sibrant, A.L.R., Hildenbrand, A., Marques, F.O., Weiss, B., Boulesteix, T., Hübscher, C., Lüdmann, T., Costa, A.C.G., Catalão, J.C., 2015. Morpho-structural evolution of a volcanic Island developed inside an active oceanic rift: S. Miguel Island (Terceira Rift, Azores). J. Volcanol. Geotherm. Res. 301, 90–106.
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Reply to the comment by Quartau et al. on “Construction and destruction of a volcanic island developed inside an oceanic rift: Graciosa Island, Terceira Rift, Azores”, J. Volcanol. Geotherm. Res. 284, 32–45, by Sibrant et al. (2014) A.L.R. Sibrant a,⁎,1, F.O. Marques b, A. Hildenbrand a,c a b c
Université de Paris-Sud, Laboratoire GEOPS, UMR8148, Orsay, F-91405, France Univerdade de Lisboa, Lisboa, Portugal CNRS, Orsay F-91405, France
a r t i c l e
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Article history: Received 4 August 2015 Accepted 12 August 2015 Available online 18 August 2015 Keywords: Volcanic stratigraphy Geochronology Sector collapse Graciosa island Terceira Rift Azores Triple Junction
a b s t r a c t In Sibrant et al. (2014), we reconstructed the evolution of Graciosa Island (Azores). We extensively discussed the nature and the meaning of the destruction episodes, either tectonics or gravitational, and concluded that the island has evolved through major landslides. In their comment, Quartau et al. (2015) conclude that (1) “Sibrant et al. (2014) is based almost solely on subaerial observations,” which is false because we used the bathymetric data available to us (Figs. 3 and 4 in Sibrant et al., 2014). (2) “…the published multibeam sonar data around Graciosa reveals that their proposed successive phases of destruction of the volcanic edifices composing the island by massive landslides is incompatible with the high-resolution bathymetry.” First, saying that the data were published is misleading because only two images are now provided in Quartau et al. (2015). Most of the high-resolution data used by Quartau et al. (2015) are not published, and they still do not release the data for us to analyse and use in this reply. Second, the high-resolution bathymetric maps are not incompatible with our model. For instance, mounds on the eastern submarine slope may actually be landslide blocks, and the platform developed to the S may correspond to flank collapses of the successive volcanoes, blanketed more recently by the young basaltic cover. (3) “The interpretation of collapse structures appears to have originated partly from a misreading of the volcano-stratigraphy and tectonic structures”. We certainly did not “misread” the volcanic stratigraphy and tectonic structures in Graciosa; in great contrast to Quartau et al. (2015), we (a) used major unconformities to establish the volcano-stratigraphy, (b) calibrated this stratigraphy with high precision K–Ar dating, (c) made careful measurement of lava flow attitudes to infer the pre-collapse position of the main edifices, and (d) did not use hypothetical tectonic faults, as Quartau et al. (2015) did, to draw an evolutionary cartoon of Graciosa. The lava flows in the southern cliffs of Graciosa dip inland, as recognised by Gaspar (1996, Fig. 3B), in great contrast to the claims of Quartau et al. (2015), and therefore do not support successive volcanoes coinciding with the centre of the island. (4) From unreleased high-resolution bathymetric data, Quartau et al. (2015) consider that no large landslide(s) occurred in Graciosa, and propose a “new model.” First, their geological model is not new, as it reproduces a cartoon in Gaspar (1996, Fig. 3A). Second, Quartau et al. (2015) have not reported any major faults exposed at the surface, as expected for an island-scale graben, but their summary cartoon of Graciosa evolution is based on major faults. Third, the new model of Quartau et al. (their Fig. 8, and our Fig. 4) misrepresents the initial model of Gaspar (1996, Figs. 4.1 and 4.2), as the whole SW flank of the island has been arbitrarily removed on their drawing (see our Fig. 3). In contrast, Sibrant et al. (2014) propose that the SW flank has been effectively removed, but by landslides. Therefore, their new model for the evolution of Graciosa is based on misleading hypotheses rather than convincing arguments. In conclusion, there is no need to change our proposed new stratigraphy and evolution of the island based on Quartau et al.'s claims because these are not supported by actual geological data—well established and calibrated stratigraphy, geometry of lava flows, and geometry and kinematics of faults. Given the absence of sound geological data to support the evolutionary cartoon proposed by Quartau et al. (2015), we conclude that it is mostly flawed and therefore mere speculation. © 2015 Elsevier B.V. All rights reserved.
⁎ Corresponding author. Tel.: +33 1 69 15 80 89. E-mail address: [email protected] (A.L.R. Sibrant). 1 Now at Laboratoire FAST (CNRS/Univ. P-Sud), Orsay, F-91405, France.
http://dx.doi.org/10.1016/j.jvolgeores.2015.08.008 0377-0273/© 2015 Elsevier B.V. All rights reserved.
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1. Introduction In their comment, Quartau et al. (2015) conclude that (1) our study on Graciosa (Sibrant et al., 2014) is based almost solely on subaerial observations, (2) “…the published multibeam sonar data around Graciosa reveals that their proposed successive phases of destruction of the volcanic edifices composing the island by massive landslides is incompatible with the high-resolution bathymetry,” (3) “the interpretation of collapse structures appears to have originated partly from a misreading of the volcano-stratigraphy and tectonic structures,” and (4) from unreleased high-resolution bathymetric data, no large landslide(s) occurred in Graciosa. Finally, they propose a “new model.” We will focus on these main points of their comment and discuss the lack of reliability of their database, especially regarding the stratigraphic, the morphologic, the tectonics, and the geological aspects. 2. Stratigraphy Sibrant et al. (2014) distinguished six main volcanic complexes in the island, instead of the three reported in Gaspar (1996). Such distinction is not based on our new K–Ar dating, as implicitly mentioned by Quartau et al. (2015). Our work relies on careful fieldwork, including recognition of the main lava sequences, the attitude of the lava flows, the geometry of the units and their mutual relationships, the number and the geometry of the major unconformities, and ultimately the dating of the most critical lava flows (e.g. base and top of sequences above and on top of the unconformities), to calibrate the stratigraphy and constrain the timing of the main steps of evolution. In their comment, Quartau et al. (2015) keep on with the three main complexes following Gaspar (1996). They especially mention that two of the volcanic complexes we distinguished belong to a single volcano, as they define a unique magmatic series in a TAS diagram (their Fig. 3). We wish to remind Quartau et al. (2015) that major elements are by themselves poorly discriminant to distinguish between different edifices, especially in the Azores. The composition of the lavas from Graciosa range from basalt to trachyte according to the total alkalis–silica (TAS) classification (Gaspar, 1996; Almeida, 2001; Beier et al., 2008; Larrea et al., 2014a). As a whole, they define one main alkaline volcanic series (e.g., Larrea et al., 2014a). Following the argument of Quartau et al. (2015), all the island would thus be made of a single volcanic complex. By extension, we remind Quartau et al. (2015) that most of the lavas erupted in the Azores islands during the last 750 kyr are also alkaline in character (e.g., Hildenbrand et al., 2014). Following their argument would mean that all of them constitute a single volcanic complex, which is obviously not acceptable. Quartau et al. (2015) also keep on with the idea that the main volcanic complexes have been erupted on top of each other, from volcanoes mainly located in the centre of the island (their Fig. 8, see our Fig. 4). This clearly cannot be the case, as the units are separated by major unconformities, and the lavas exposed in the southern cliff of the island (Baía do Filipe) show a general dip towards the NE, i.e. opposite to what would be expected from a volcano located in Serra Dormida Hill, as clearly shown in Sibrant et al. (2014). In fact, even Gaspar (1996) showed in his Fig. 4.2 (reproduced in Fig. 3B) that the general dip of all complexes is towards the NE, and therefore the eruptive centre was southwest of the current shore. This indicates that Quartau et al. (2015) not solely misread our former paper but also previous work from Gaspar (1996), and ignored important geometrical characteristics of the actual exposed stratigraphy. 3. Inland morphology and tectonics Based on morphological and calibrated stratigraphic data and the fact that a significant part of several complexes is missing, we discussed extensively the possible genesis of the morphological scarps, by either tectonic processes or lateral flank collapses. From our new data, we
proposed that the island was affected by at least 4 catastrophic flank collapses, separated by reconstruction of nested volcanic edifices in the successive scars, mainly towards the SW, and finally towards the SE. Quartau et al. (2015) raise major criticisms about this interpretation. Following Fig. 3A of Gaspar (1996), they propose instead that the evolution of Graciosa consists of successive volcanic edifices built on top of each other, which have been dismantled in a symmetric way by active faulting following the Terceira Rift. In their comment, Quartau et al. (2015) also extensively mention the role of horizontal marine abrasion, but most of the destruction resulted from vertical erosion (Sibrant et al., 2014), as also drawn by Quartau et al. (2015) in their Fig. 8, despite they state in the main text that vertical erosion is negligible. We here present a gradient map of Graciosa (vertical illumination) built from the digital elevation model of the island, along with similar maps for Faial and Terceira for the sake of comparison (Fig. 1). While Faial and Terceira show conspicuous symmetrical grabens, no similar structure is observed in Graciosa, despite its location in the active Terceira Rift and its relatively old age (Larrea et al., 2014b; Sibrant et al., 2014). Whereas main scarps dipping towards the SW can be identified in Graciosa, no clear scarp dipping to the NE stands out. A few lineaments are visible but certainly cannot be attributed to a major graben at the island scale. At best, a half-graben towards the S may exist, as discussed in Sibrant et al. (2014). On the tectonic map (Figs. 2 and 4 in Quartau et al., and our Figs. 1 and 2), all the major faults referred to by Quartau et al. (2015) are hypothetical, as marked by dashed points. These mostly reproduce the lineaments proposed by Gaspar (1996), except two major faults dipping to the S (Fig. 3B), which are in agreement with our collapse interpretation. If we compare Figs. 2 and 4 of Quartau et al. (2015) based on Gaspar (1996) and the tectonic map published in Gaspar (1996), we can see that the vast majority of the faults are indicated as “falha provável ou encoberta” and “escarpa de falha provável” which mean in English “expected or dissimulate faults” and “expected fault scarps,” respectively. Even some morphological scarps (e.g. South Serra das Fontes) are arbitrarily termed “faults scarps,” without any justification. This is just speculation, and the authors should at least have discussed if such scarps have a tectonic or gravitational origin, as we did in Sibrant et al. (2014). In their comment, Quartau et al. (2015) refer to Hipólito et al. (2013) regarding the tectonic structures, where one will immediately see in Figs. 8 and 9 of Hipólito et al. (2013) that (1) many so-called “faults” are apparently just compaction structures in scoriae deposits (their Fig. 8), and (2) one small fault is shown, in contrast to the many major faults shown in the evolutionary cartoon of Quartau et al. (Fig. 8 of the comment). Moreover, the photograph in Fig. 9 of Hipólito et al. (2013) is parallel to the Terceira Rift (NW-SE), so the fault appears perpendicular to the main graben inferred by Quartau et al. All the above lines of evidence show that no actual measurement in the field supports a major fault associated with the island-scale graben inferred by Quartau et al. (2015) No fault kinematics, so critical to constrain the slip direction and the stress field, has been apparently measured either. 4. Bathymetric data In Sibrant et al. (2014), we used a low-resolution bathymetric grid because it was the only data available to us. Now, Quartau et al. (2015) provide for the first time a couple of high-resolution bathymetric maps, in great part built from unpublished data. Unfortunately, the actual data of the high-resolution bathymetry are still not available to us. Quartau et al. (2015) do not even provide a clean image without any interpretation, so we cannot make a thorough analysis of the data to produce our own interpretation. However, from the low-resolution data and the two images provided Quartau et al. (2015) in their comment, one can easily check that the only large-scale obvious graben is the Terceira Rift (TR), which extends far away from Graciosa. In fact, the island sits inside the TR, not along the main faults of the TR, which
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Fig. 1. Shaded relief maps of Terceira (A), Faial (B), and Graciosa (C) islands (vertical illumination), showing conspicuous grabens in Faial and Terceira, but not in Graciosa. Faults exposed at the surface marked by F. S. stands for morphological scar in Serra das Fontes. (D) Simplified tectonic map of Graciosa, after Hipólito et al. (2013), showing that most faults and fault scarps are inferred from morphological interpretation but do not correspond to observed faults.
Fig. 2. (A) Original tectonic map of Graciosa by Gaspar (1996) indicating very few faults and many “probable faults.” (B) Geological map of Graciosa and tectonic information proposed by Quartau et al. in their comment. Note that several faults scarps marked as “provável” (which means probable in English) in Gaspar (1996) have become true fault scarps in Quartau et al., without any explanation.
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Fig. 3. (A) Original sketches by Gaspar (1996) showing his interpretation of the evolution of Graciosa. Note that the successive volcanoes and the inferred graben appear symmetrical and both located in the centre of Graciosa, but the southwest continuation of the island is not drawn. (B) Geological cross-section by Gaspar (1996). Note that the SW half of the initial volcanoes is missing. Extrapolation of the former shape of the volcanoes from the preserved geological units (conical hypothesis, dashed lines drawn by us) suggest that the first volcano (Serra das Fontes, labelled 1) was culminating much higher than the later volcano (Serra Branca, labelled 2). This means that (1) the two volcanoes were not built conformably on top of each other and (2) that the Serra Branca volcano developed in a depression that carved all the SW half of Serra das Fontes; such depression is consistent with the hypothesis of a large SW-directed landslide between the two volcanic stages (Sibrant et al., 2014). Also note that all the main volcanic units dip towards the NE, and most of the inferred faults dip towards the SW, in great contradiction with the comment by Quartau et al. (2015), but in fairly good agreement with Sibrant et al. (2014).
may explain why there is no major fault visible at the surface of the island, in contrast to Terceira (Navarro et al., 2003; Calvert et al., 2006; Hildenbrand et al., 2014; Marques et al., 2015), and S. Miguel (e.g. Moore, 1990; Carmo, 2013; Sibrant et al., 2015), which sit partially on a master fault of the TR, and also to Faial (Hildenbrand et al., 2012, 2013), which developed on top of a master fault in the diffuse part of the present Eurasia–Nubia plate boundary in the Azores (e.g., Marques et al., 2013, 2014a, 2014b, 2015). One of the main criticisms of Quartau et al.'s comment is based on the recognition of a flat platform around Graciosa on the bathymetric map. They call such platforms insular shelves and attribute them to surf erosion of the volcanic edifices during the Quaternary sea level changes. In their opinion, such platforms rule out our proposed landslides. This is certainly not the case. From their Figs. 6 and 7, it is obvious that the southern shelf is more developed than the northern one, despite the older age of most of the volcanic units exposed in the N. Therefore, these shelves do not rule out our proposal for south-directed landslides. From our study (Sibrant et al., 2014) and from the highresolution bathymetric image, it seems highly plausible that the products of the several proposed slides may have been buttressed on the S wall of the TR, and extensively buried by the young basaltic cover, which can explain the flat and anomalously large shelf developed immediately to the S of the island. In addition, several of the bathymetric mounds exposed SE of the island could represent the distal products of the youngest landslide, which according to Sibrant et al. (2014) occurred towards the SE. 5. The “new” model of evolution of Graciosa by Quartau et al. In their comment, Quartau et al. (2015) propose a new model for the geological evolution of Graciosa Island. However, this model is not new, as it reproduces in great part the model proposed by Gaspar (1996) and shown here in our Fig. 3. In fact, the new model of Quartau et al. (their Fig. 8 and our Fig. 4) misrepresents the initial model of Gaspar (1996, Figs. 4.1 and 4.2), as the whole SW flank of the island has been
arbitrarily removed on their drawing (see our Fig. 3). In contrast, we propose that the SW flank has been effectively removed, but by landslides (Sibrant et al., 2014). In addition, the “new” model presented by Quartau et al. (2015) in their comment shows many inconsistencies, as summarised in our Fig. 4 and listed in the following: Stage A: the initial volcano (Serra dos Fontes) is drawn highly asymmetrical, because the SW flank appears much less developed than the NE flank. This is just an artefact because the topographic surface is deliberately drawn to truncate the lava flows in the SW and so make the island shorter in this direction. Such geometry is unrealistic for a volcanic constructional slope and is in any case not justified from actual data and the drawings of Gaspar (1996) in his Fig. 4.1 reproduced in our Fig. 3A. Stages B and C: the new volcano marked in yellow is, again, drawn asymmetrical. From B to C, the quasi-totality of the edifice (height around 1 km) is removed in less than 24 kyr (between 323 ka and 299 ka), supposedly by vertical erosion, which is considered negligible by Quartau et al. (2015) in the text of their comment. Stage D: in the SW, the “erosional shelf” marked by a dashed line in Quartau et al. erodes a volcanic flank that does not exist. More generally, the dip of the lava flows from the old volcanic edifice (NE sector) increase gradually from ca 8° in Fig. 8A to ~ 30° in Fig. 8D without any justification. Throughout the successive stages, numerous faults are drawn, but they are all hypothetical, as no clear evidence for actual large faults is provided by Quartau et al. 6. References We note that Quartau et al. (2015) avoid citing relevant work our team did in the Azores, but cite all the work of the co-authors (e.g. Quartau 10 self citations, Mitchell 8 self citations) including unpublished MSc and PhD theses, and submitted papers. For instance, when
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Fig. 4. Illustration of the major inconsistencies in Quartau et al.'s “new” model: (A) Initial volcano drawn highly asymmetrical; (B and C) new volcano again asymmetrical, and almost completely removed in ca 24 kyr (between 323 ka and 299 ka); (D) in the SW, “erosional shelf” (dashed line by Quartau et al.) erodes a volcanic flank that does not exist. More generally, the dip of the lava flows from the old volcanic edifice (NE sector) increases gradually from ca 8° in Fig. 8A to ~30° in Fig. 8D. Throughout the successive stages, numerous faults are drawn, but they are mostly hypothetical, as no clear evidence for actual large faults is provided by Quartau et al.
describing the structures in Faial and Terceira, Quartau et al. do not cite relevant recent work by Marques et al. (2014b), which explains the formation of the Faial Graben, and Marques et al. (2015), which reports on the tectonics of Terceira Island. 7. Conclusions We thank Quartau et al. (2015) for their comment, which allowed us to clarify important points and show the reader that the combination of robust stratigraphic, morphological, tectonic, bathymetric, and geochronological data are needed to reconstruct the complex evolution of unstable volcanic islands like Graciosa. We show that (1) Graciosa is not affected by an island-scale graben, as no clear evidence for large normal faults is provided by Quartau et al (2) Construction of the island does not result from successive volcanoes conformably built on top of each other in the centre of the island; instead, it results from nested volcanic edifices built in the successive scars, mainly towards the SW, and ultimately towards the SE. (3) The island was not destroyed by a symmetric graben and wave erosion but was affected instead by lateral destruction towards the SW, in our interpretation by large-scale landslides. (4) We cannot discuss fully the high-resolution bathymetry, as the data have not been released by Quartau et al.; however, the two bathymetric images provided in their comment do not rule out the existence of large landslides. We wish to encourage these authors to publish their highresolution bathymetry acquired during the MARCHE missions (Luis et al., 2007) and EMODnet project as soon as possible, so the bathymetry becomes available and accessible to the scientific community. (5) The
“new” proposed evolution of Graciosa by Quartau et al. shows major inconsistencies with field data, even from Gaspar (1996), and is mostly flawed because it is not supported by geological data. From the above lines of evidence, we argue that the interpretations by Quartau et al. are not supported by geological data, and therefore, it seems obvious to us that there is no need to change the new proposed stratigraphy and evolution of Graciosa as reported in Sibrant et al. (2014). Acknowledgments This is a contribution to Project MEGAHazards, funded by FCT (PTDC/CTE-GIX/108149/2008), Portugal. FO Marques benefits from a Sabbatical fellowship awarded by FCT Portugal. ALR Sibrant benefitted from funding by the French CNRS-INSU and by the French ANR (project RHUM-RUM). This is LGMT contribution 132. Appendix A. Suplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.jvolgeores.2015.08.008. References Almeida, M.H., 2001. A fonte mantélica na região dos Açores: constrangimentos impostos pelas caracteristicas geoquimicas de rochas vulcanicas e de xenolitos ultramaficos (PhD thesis) Azores University, São Miguel.
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