Emergence and evolution of Santa Maria Island (Azores)—The conundrum of uplifted islands revisited

TLDR: In this paper, the authors reconstruct the evolutionary history of Santa Maria with respect to the timing and magnitude of its vertical movements, using detailed field work and 40Ar/39Ar geochronology.

Abstract: The growth and decay of ocean-island volcanoes are intrinsically linked to vertical movements. While the causes for subsidence are better understood, uplift mechanisms remain enigmatic. Santa Maria Island in the Azores Archipelago is an ocean-island volcano resting on top of young lithosphere, barely 480 km away from the Mid-Atlantic Ridge. Like most other Azorean islands, Santa Maria should be experiencing subsidence. Yet, several features indicate an uplift trend instead. In this paper, we reconstruct the evolutionary history of Santa Maria with respect to the timing and magnitude of its vertical movements, using detailed field work and 40Ar/39Ar geochronology. Our investigations revealed a complex evolutionary history spanning ∼6 m.y., with subsidence up to ca. 3.5 Ma followed by uplift extending to the present day. The fact that an island located in young lithosphere experienced a pronounced uplift trend is remarkable and raises important questions concerning possible uplift mechanisms. Localized uplift in response to the tectonic regime affecting the southeastern tip of the Azores Plateau is unlikely, since the area is under transtension. Our analysis shows that the only viable mechanism able to explain the uplift is crustal thickening by basal intrusions, suggesting that intrusive processes play a significant role even on islands standing on young lithosphere, such as in the Azores.

Figures

Figure 4. Photoset 2 of representative palaeo sea-level markers used in this study. (A) Sequence 1162 at Ponta do Norte (looking S). Note the younger lava-fed delta of Pico Alto Volcanic Complex 1163 unconformably overlapping an older lava delta of the same unit, prograding to the ENE. The 1164 passage zone on the younger lava-fed delta occurs at ~110 m in elevation, but the same passage 1165 zone can be seen at ~160 m in elevation across the Ponta do Norte fault, which has a ~50 m of 1166 apparent vertical displacement. (B) Section at Ponta do Morgado/Baía do Cura (looking SSE) 1167 exhibiting a text-book example of a Gilbert-type lava-fed delta, prograding to the east, with the 1168 passage zone at ~130 m in elevation. The vertical continuity of the eastward-dipping foresets of 1169 pillow lavas and hyaloclastites from ~130 m to present sea level shows that volcanic 1170 progradation extended beyond the coeval insular shelf edge. Note also the presence of wave-cut 1171 notches at 18–20 m and 105–110 m in elevation (pointed by the black and white arrows). (C) 1172 Staircase of marine terraces at the northeastern part of the island (looking ENE); dashed lines 1173 mark the inner edge of each terrace, i.e. the position of former shorelines. (D) Pleistocene beach 1174 composed of beach conglomerates (including rounded stranded pumice) and fossiliferous 1175

Table 1. Summary of 40Ar/39Ar geochronology results of palaeo sea-level marker information 1 used in this study. All reported ages are shown with 2σ levels of uncertainty. 2

Figure 5. (A) Plio-Quaternary palaeo-shoreline reconstructions based on uplifted marine terrace 1179 morphology. Lines represent the inner edge of each terrace, i.e. the position of the former shore 1180 angle (solid line = visible/high confidence; dashed line = interpreted/medium confidence; dotted 1181 = interpreted/low confidence). DEM generated from a 1/5,000 scale digital altimetric database 1182 provided by Secretaria Regional do Turismo e Transportes. (B) Cross-shore profiles (p1–p6) 1183 taken along solid black lines represented in (A); the presence of marine terraces is clearly visible 1184 in these profiles (horizontal dotted lines). (C) Photo of marine terrace staircase morphology taken 1185 from point (a) in (A), looking to ENE. Note also the Cabrestantes Fm (Cab) in the foreground. 1186 1187

Figure 8. Sketch representing the main stages in the evolutionary history of Santa Maria Island. 1207 (1a) Seamount stage, deep water substage, during the Late Miocene (?) – onset of island 1208 construction, initially as a largely-effusive submarine volcano (inferred); (1b) Seamount stage, 1209 intermediate water substage, during the Late Miocene – sustained edifice growth by vigorous 1210 submarine effusive volcanism (inferred); (2) Emergent Island stage, at ~6 Ma – first emergence 1211 above sea level by surtseyan volcanism (resulting in a precursory island) and transition to 1212 subaerial volcanism; (3) Shield-building stage, 5.7–5.3 Ma – sustained volcanism leading to the 1213 construction of a subaerial shield volcano, with accelerating subsidence; (4) Erosive stage, 5.3–1214 4.3 Ma – waning volcanism, erosion, and subsidence, leading to the partial or, most probably, 1215 complete truncation of the existing island edifice, and extensive marine sedimentation; the 1216 edifice resembled a wide sandy shoal by the end of this stage; (5) First rejuvenated stage, 4.3–3.5 1217 Ma – renewed vigorous volcanism builds a new island edifice off-centered to the east of the old 1218 edifice, resulting in significant eastwards and westwards coastal advancement by progradation of 1219 lava-fed deltas, under continued subsidence; coastal progradation to east overlapped existing 1220 shelf edge; (6) Second rejuvenated stage, 3.2–2.8 Ma, uplift since ~3.5 Ma – waning volcanism 1221

Figure 1. (A) Map illustrating the bathymetry and geotectonic setting of Santa Maria within the 1120 Azores Triple Junction. Note that Santa Maria is located in the southeastern corner of the Azores 1121 Plateau, wedged in between the Terceira ultra-slow spreading ridge (TR), the dextral transcurrent 1122 Gloria Fault (GF, part of the Azores-Gibraltar fault system), the currently inactive East Azores 1123 Fault Zone (EAFZ), and the early incipient Princess Alice Rift (PAR). White arrows represent 1124 the approximate spreading direction of TR. Upper right inset depicts the regional setting of the 1125 Azores archipelago within the North American (NA), Eurasian (Eu) and Nubian (Nu) triple 1126 junction. (B) Bathymetry/altimetry of Santa Maria Island edifice. Note the extensive insular shelf 1127 to the north of the present-day island. Bathymetry on both subfigures extracted from the 1128 EMODNET web portal (http://portal.emodnet-bathymetry.eu); subaerial topography generated 1129

Figure 2. Geological map of Santa Maria Island (A) after Serralheiro et al. (1987), with a WNW–1133 ESE cross-section (B,) and respective key for both map and section. Approximate sample 1134 locations are plotted in the map. “Pta” and “Mte” correspond to abbreviations of “Ponta” and 1135 “Monte”, respectively. Underlying DEM generated from a 1/5,000 scale digital altimetric 1136 database provided by Secretaria Regional do Turismo e Transportes. 1137 1138

Full text

Dos Santos Ramalho, R., Helffrich, G., Madeira, J., Cosca, M.,
Thomas, C., Quartau, R., Hipólito, A., Rovere, A., Hearty, P. J., &
Ávila, S. P. (2017). Emergence and evolution of Santa Maria Island
(Azores)—The conundrum of uplifted islands revisited.
Geological
Society of America Bulletin
,
129
(3-4), 372-390.
https://doi.org/10.1130/B31538.1
Peer reviewed version
Link to published version (if available):
10.1130/B31538.1
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The emergence and evolution of Santa Maria Island (Azores) – 1
the conundrum of uplifted islands revisited 2
Ricardo S. Ramalho
1,2
, George Helffrich
3
, José Madeira
4,5
, Michael Cosca
6
, Christine 3
Thomas
7
, Rui Quartau
8,5
, Ana Hipólito
9
, Alessio Rovere
10
, Paul J. Hearty
11
, and Sérgio P. 4
Ávila
12,13
5
1
School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, 6
Bristol, BS8 1RJ, UK. 7
2
Lamont-Doherty Earth Observatory at Columbia University, Comer Geochemistry Building, 61 8
Route 9W/ PO box 1000, Palisades, NY-10964-8000, USA. 9
3
Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1-IE-1 Ookayama, Meguro-10
ku, Tokyo, 152-8550, Japan 11
4
Departamento de Geologia, Faculdade de Ciências, Universidade de Lisboa, 1749-016, 12
Lisboa, Portugal. 13
5
Instituto Dom Luiz, Faculdade de Ciências, Universidade de Lisboa, 1749-016, Lisboa, 14
Portugal. 15
6
U.S. Geological Survey, Denver Federal Center, MS 963, Denver, CO 80225, USA. 16
7
Institut für Geophysik, Westfälische Wilhelms-Universität, Corrensstraße 24, 48149 Münster, 17
Germany 18
8
Divisão de Geologia Marinha, Instituto Hidrográfico, Rua das Trinas, 49, 1249-093 Lisboa, 19
Portugal 20

9
Instituto de Investigação em Vulcanologia e Avaliação de Riscos, Universidade dos Açores, 21
Rua da Mãe de Deus, Edifício do Complexo Científico, 3º Andar - Ala Sul, 9500-321 Ponta 22
Delgada, Açores, Portugal. 23
10
MARUM, University of Bremen and ZMT, Leibniz Center for Tropical Marine Ecology, 24
Marum Pavillion 1110, Bremen, Germany 25
11
Department of Environmental Studies, University of North Carolina Wilmington, NC-28403, 26
USA 27
12
Faculdade de Ciências da Universidade do Porto, Rua do Campo Alegre s/n, 4169-007 Porto, 28
Portugal. 29
13
CIBIO, Centro de Investigação em Biodiversidade e Recursos Genéticos, InBIO Laboratório 30
Associado, Pólo dos Açores, Departamento de Biologia da Universidade dos Açores, Campus de 31
Ponta Delgada, Apartado 1422, 9501-801 Ponta Delgada, Açores, Portugal 32
33
ABSTRACT 34
The growth and decay of ocean island volcanoes is intrinsically linked to vertical 35
movements; whilst the causes for subsidence are well understood, uplift mechanisms remain 36
enigmatic. Santa Maria Island in the Azores Archipelago is an ocean island volcano resting on 37
top of young lithosphere, barely 480 km away from the Mid-Atlantic Ridge. Like most other 38
Azorean islands, Santa Maria should be experiencing subsidence. Yet, several features indicate 39
an uplift trend instead. In this paper we reconstruct the evolutionary history of Santa Maria with 40
respect to the timing and magnitude of its vertical movements, using detailed fieldwork and 41
40
Ar/
39
Ar geochronology. Our investigations revealed a complex evolutionary history spanning 42
~6 Ma, with subsidence followed by uplift extending to the present day. The fact that an island 43

located in young lithosphere experienced such a pronounced uplift trend is remarkable and raises 44
important questions concerning possible uplift mechanisms. Localized uplift in response to the 45
tectonic regime affecting the southeastern tip of the Azores Plateau is unlikely since the area is 46
under transtension. Our analysis shows that the only viable mechanism able to explain the uplift 47
is crustal thickening by basal intrusions, suggesting that intrusive processes play a significant 48
role even on islands standing on young lithosphere, such as in the Azores. 49
50
51
INTRODUCTION 52
Ocean island volcanoes are typically subjected to long-term subsidence, as the linear, 53
age-progressive island chains of the Pacific Ocean clearly exemplify. This subsidence trend is 54
essentially driven by mechanisms such as volcanic surface loading (Moore, 1970; Walcott, 1970; 55
Menard, 1983), plate cooling with age (Parsons and Sclater, 1977; Stein and Stein, 1992), and 56
hotspot swell decay (Morgan et al., 1995), all of which are influenced by fast plate movement 57
away from the melting source. All these mechanisms (with perhaps the exception of hotspot 58
swell decay) are reasonably well understood and are consistent within the plate tectonics/isostasy 59
framework. In a similar fashion, within this fast-moving plate scenario, uplift episodes are easily 60
explained by plate bending due to surface loading of younger islands further “upstream” along 61
the chain (Walcott, 1970; Huppert et al., 2015), or by outer trench rise for islands approaching a 62
subduction zone (Schmidt and Schmincke, 2000). A few island systems (e.g. the Cape Verdes, 63
the Canaries, and Madeira Archipelago), however, fall out of pattern and feature numerous 64
volcanic edifices that experienced pronounced uplift trends, vertical stability, or complex 65
uplift/subsidence histories (e.g. Stautigel and Schmincke, 1984; Klügel et al., 1995; Schmidt and 66

Schmincke, 2002; Menendez et al., 2008; Ramalho et al., 2010a,b,c; Madeira et al., 2010; 67
Sepúlveda et al., 2015; Ramalho et al., 2015). These are mostly concentrated in – but not 68
restricted to – the NE Atlantic, where the Nubian plate moves very slowly or is quasi-stationary 69
with respect to the islands’ melting source (Burke and Wilson, 1972; Ramalho et al., 2010b; 70
Ramalho et al., 2015). The mechanisms behind such uplift trends or episodes, however, are still 71
not completely understood and are the subject of contemporaneous debate, being the focus of the 72
present study. 73
Several plausible mechanisms have been put forward to explain ocean island uplift, all of 74
which are likely to contribute in greater or lesser degree to the observed uplift trends/episodes. 75
For uplift acting at broad regional scale, hotspot swell growth by either spreading of melt residue 76
or dynamic topography is regarded as the most plausible mechanism (Morgan et al., 1995; Zhong 77
and Watts, 2002; Ramalho et al., 2010b; Wilson et al., 2010, Ramalho, 2011). At smaller 78
regional scales uplift may be generated by flexural uplift at the forebulge created by surface 79
loading of nearby younger islands/seamounts (McNutt and Menard, 1978; Watts and ten Brink, 80
1989; Grigg and Jones, 1997; Huppert et al., 2015), by flexural uplift induced by subsurface 81
loads (“underplating”)(Watts and ten Brink, 1989; Ali et al., 2003), or by flexural rebound driven 82
by mass wasting or erosion (Menard, 1983; Smith and Wessel, 2000; Menendez et al., 2008). 83
However, these uplift mechanisms still act upon a wide area (which largely depends on plate 84
rheology) and thus cannot be accounted to explain contrasting uplift histories for edifices 85
spatially close together (Ramalho et al., 2010a,b,c). Additionally, surface loading has been 86
shown to only generate uplift in the order of 10’s of meters (unless unrealistically thin elastic 87
thicknesses are considered)(McNutt and Menard, 1978). It also requires younger edifices being 88
loaded at a suitable distance, and fails to explain long-term uplift trends. In a similar fashion, 89