Interacción Litosfera Oceánica-Punto Caliente. Aplicación al Volcanismo Intraplaca (Archipiélagos de Canarias y Sociedad) y Dorsal Medioceánica (Galápagos)

   The most characteristic features of ridge morphology, and its crustal and lithospheric structure constitute valuable constraints in the evolution and the degree of interaction between hotspots and oceanic lithosphere, providing new knowledge on mantle dynamics, associated volcanism and crustal growth. Most of oceanic lithosphere accretion process is generated from decompression of magma chambers at mid-ocean ridges producing the upwelling of mantle like material to the ocean floor. Other processes as hotspots may add extra material to the oceanic lithosphere. The interaction between mantle plumes and the oceanic lithosphere may occur at on-ridge, increasing the magma supply, or off-ridge, generating intraplate volcanic islands and seamounts. A multidisciplinary study (wide-angle, gravity, bathymetry) is carried out to unravel the hotspot-oceanic lithosphere interaction in two intraplate volcanic chains (Canary Archipelago, East-Central Atlantic, and Society Islands, South Pacific), and a mid-ocean ridge (Galápagos Spreading Centre, East Equatorial Pacific), and finally to develop a lithospheric scenario assuming an elastic plate model.

   Wide-angle seismic data along three radial profiles constrain the internal architecture of the volcanic apron north of Gran Canaria (Canary Is). The wedge apron extends 35-60 km off the volcanic edifice, and its P-wave velocity is 3.60-4.65 km/s, and is correlated to volcanoclastic material. Cretaceous and Upper Jurassic pre-volcanic sediments are present below the apron, and have been covered by extruded material since Middle Miocene. The thickness of this sedimentary layer is 3 km, and the mean velocity is 3.30 km/s (lower than the upper volcanic apron, sampled as a low velocity layer in the wide-angle record sections). Two velocity gradients within the igneous crust differentiate between upper and lower layers. The upper crustal velocity increases with depth from 4.20-4.60 km/s up to 5.20-5.60 km/s, and the velocity in the lower crust varies from 6.60-6.80 km/s to 7.30 km/s. The total thickness of the igneous crust is ~7-8 km at distances greater than 30 km from the island, indicating that the oceanic crust off the volcanic edifice is normal type. The base of the crust (Moho) is detected ~15 km beneath the oceanic basin north of Gran Canaria and deepening toward the island 18-19 km below the volcanic edifice. At distances larger than 10 km off the islands normal upper mantle velocities of 7.80 to 8.00 km/s are estimated. Anomalous low mantle velocities (7.6 km/s) and densities of 3.2 g/cm³determined by modelling of gravity anomalies support the existence of underplated material at the base of the crust beneath the volcanic edifice reaching depth of 25 km, presently interpreted as the result of fractionation of silicate liquids. This plutonic complex extends toward Tenerife, at least up to the Southeast of Anaga Massif. A prominent high amplitude wide-angle reflection (observed in the Güimar station CD-82 Line 12) suggests a low velocity zone within the upper mantle beneath the Anaga Massif plutonic complex which may be associated to partial melting.
   The spectral analysis of admittance and coherence from gravity and bathymetry resolves the equivalent elastic thickness (Te) of the lithosphere in the Canary Islands. Admittance, once corrected from the influence of the continental margin, gives a Te=35 km. The coherence analysis, which take into account the bathymetric swell around the islands, gives a more accurate value of 32 km, implying that thermal rejuvenation does not occur in the Canaries. The swell is partially supported by thermal expansion within the lower lithosphere, at depths greater than 50 km, associated with an excess temperature of -360 °C and a negative density contrast of ~37 kg/m³. Convection within a low viscosity layer involving the lowermost lithosphere, and presumably the uppermost asthenosphere is probably the mechanism which maintains the thermal anomaly.

   A detailed seismic and gravity crustal and upper mantle study has been carried out in the Society Islands, along several profiles sampling the crust around the islands of Tahiti (~1.7 Ma) and of Raiatea (~.2.5 Ma). Two distinct layers over the igneous crust have been identified. The uppermost, 0.7-1.5 km thick and velocity <4 km/s, is interpreted as oceanic and pelagic sediments. The lowermost, 1.6-2.4 km thick and mean velocities of 4.40-4.90 km/s, is probably composed by volcanoclastic material and lava flows from the volcanic edifices, and evidences the huge amount of material extruded by the Society volcanism. This interpretation is constrained by the lack of this volcanic layer along the profile east of Tahiti, which samples the crust over further areas, where scarce volcanic material is supplied from the islands. Normal igneous oceanic crust, with a total thickness of 6.8-7.4 km, is observed at distances larger than 30 km away from the islands. It is characterised by an oceanic layer 2 (1.6-2.4 km thick) with high average velocities of 6.40-6.70 km/s (except east of Tahiti, where it is 5.45 km/s), and an oceanic layer 3 (4.4-5.4 km thick) and mean velocity of 7.1 km/s. The Moho off and beneath Raiatea is 12-13 km deep. At 60 km off Tahiti, the Moho is 11 km deep, and reaches 14 km beneath the volcanic edifice. Standard mantle velocities larger than 7.8 km/s indicate the lack of underplated material below the Society Islands. One dimensional modelling of the gravity anomaly profiles assuming an elastic plate model supports an increase of Te from 9-14 km beneath the western islands to 16-22 km beneath the eastern islands. This suggests that the stress relaxation is still active beneath the younger edifices of the chain. The low Te for the age of the lithosphere at the time of loading found below the older islands indicates 45-60 Ma of thermal rejuvenation.

   The structural differences between Canary and Society Islands (the presence of plutonic complex and elastic thickness) can be interpreted as differences in the strength of the mantle plume causing the volcanism. A weak plume beneath the Canaries is not able to modify the thermal structure of the lithosphere, and affects only the deep lithosphere. While a strong plume beneath the Society is responsible of the thermal rejuvenation, and produces hotter and less dense material which is not able to pond at the base of the crust (underplating). Comparison of the Society Islands with other Pacific intraplate hotspots (Hawaii and Marquesas) suggests that magma composition plays an important role in the underplating process, such as the predominance of tholeiitic basalts favour the creation of plutonic complex, while alkali basalts not.

   High resolution bathymetry along the intermediate-spreading rate, Cocos-Nazca ridge, known as the Galápagos Spreading Centre (GSC), is marked by systematic, along-axis changes in axial morphology between the Inca FZ at 85.5°W and the 95.5°W propagator. Within ~350 km of the Galápagos hotspot (currently located beneath Isla Fernandina 170 km south of the GSC) the ridge axis is associated with an EPR-like axial high. At increasing distance from the hotspot the axial high broadens and deepens forming a distinctive transitional morphology that is neither an axial high or a rift valley. The axis in this transitional region is typically a broad zone, up to 20 km wide, consisting of very rough volcanic and fault-generated topography. West of 95°W this transitional morphology evolves into a 20-40 km wide, 400-1500 m deep axial valley typical of the slow spreading MAR. Axial depth and morphology are related along the GSC with axial highs restricted to depths shallower than 2100 m, a transitional morphology for depths between 2100 m and 2750 m, and an axial valley for depths greater than 2900 m. There is not an abrupt change from axial high to rift valley along the GSC, but a distinct transitional morphology which occurs over a distance of ~200-300 km along-axis, and is accompanied by gravity-estimated crustal thickness variations of >1-2 km. The boundary between axial high and this transitional morphology is quite abrupt, and occurs along a segment less than 9 km long. These systematic changes in axial morphology are primarily caused by variations in magma supply along the GSC due to the entrainment and dispersal of hot, plume mantle from the nearby Galápagos hotspot. However, the changes in morphology are not symmetric about the Galápagos hotspot or the Galápagos FZ at 91 °W. The axial high topography found along the GSC nearest the hotspot extends further east of the 91°W FZ than to the west, and the rift valley which develops west of 94°W is not found at comparable distances along the GSC east of the hotspot. This asymmetry is attributed to a full spreading rate increase along the GSC from 48 mm/yr at 97°W to 66 mm/yr at 85°W. Axial depth variations are also asymmetric across the 91°W FZ, and can be explained by a spreading rate control, or as the effect of a mantle plume being swept eastward by the absolute motion of the Nazca plate. The absence of any asymmetry in the off-axis depth variations favours a spreading rate control rather than the second hypothesis.

CAPITULO PRIMERO: INTRODUCCIÓN 1
1.1 – Objetivos y estructura del trabajo 3
1.2 – Origen de la litosfera oceánica: dorsales mesoceánicas 7
1.3 – Propiedades y evolución de la litosfera oceánica 11

CAPITULO SEGUNDO: ÉL ARCHIPIÉLAGO CANARIO: VOLCANISMO INTRAPLACA Y EXPANSION LENTA (ATLÁNTICO) 17
2.1 – Introducción 19
2.2 – Contexto tectónico 21
2.2.1 – Cinemática del Atlántico Central 21
2.2.2 – El Archipiélago Canario 24
2.3 – Experimentos geofísicos 29
2.3.1 – Gran Canaria: M24 29
2.3.1.1 – Objetivos 29
2.3.1.2 – Sísmica de gran ángulo 30
2.3.1.3 – Gravimetría 62
2.3.1.4 – Resultados 67
2.3.2 – Tenerife: CD82 69
2.3.2.1 – Objetivos 69
2.3.2.2 – Resultados actuales 70
2.3.2.3 – Sísmica de gran ángulo: Perfil 12 72
2.3.2.4 – Resultados 77
2.4 – Estructura litosférica 79
2.4.1 – Estimación de las propiedades reológicas de la litosfera: técnicas espectrales 79
2.4.1.1 – Admitancia 81
2.4.1.1 -Coherencia 83
2.4.2 – Aplicación de las técnicas espectrales al estudio de la litosfera oceánica en el entorno del Archipiélago Canario 88
2.4.2.1 – Estimación del espesor elástico utilizando la función de admitancia 89
2.4.2.2 – Estudio del swell de Canarias a partir de la coherencia entre gravimetría y batimetría 94
2.4.3 – Discusión 102
2.5 – Discusión y conclusiones 107

CAPITULO TERCERO: EL ARCHIPIÉLAGO DE SOCIEDAD: VOLCANISMO INTRAPLACA Y EXPANSION RÁPIDA (PACIFICO) 117
3.1 – Introducción 119
3.2 – Contexto tectónico 121
3.2.1 – Cinemática del Pacífico Sur 121
3.2.2 – El Superswell del Pacífico Sur 124
3.2.3 – El Archipiélago de Sociedad 126
3.3 – El experimento geofísico PASO-94 129
3.4 – Sísmica de gran ángulo 133
3.4.1 – Perfil T8 134
3.4.2 – Perfil T3-T6 145
3.4.3 – Perfil B1-B2 161
3.5 – Gravimetría 171
3.6 – Estructura litosférica 175
3.6.1 – Perfil T3-T6 176
3.6.2 – Perfil B1-B2 179
3.6.3 – Perfil B4-T1-T42-T8 179
3.6.4 – Discusión y conclusiones 183
3.7 – Discusión y conclusiones 189

CAPITULO CUARTO: VARIATIONS IN AXIAL MORPHOLOGY ALONG THE GALAPAGOS SPREADING CENTER AND THE INFLUENCE ON THE GALAPAGOS HOTSPOT 205
4.1 – Introduction 207
4.2 – Tectonic setting 211
4.3 – Data 215
4.4 – Results 217
4.5 – Discussion 227
4.6 – Conclusions 233

CAPITULO QUINTO: DISCUSIÓN FINAL Y CONCLUSIONES 235
5.1 – Canarias y Sociedad: interacción litosfera oceánica-punto caliente 237
5.2 – Galápagos: interacción dorsal oceánica-punto caliente 243
5.3 – Conclusiones 245
5.4 – Perspectiva de futuro 247

REFERENCIAS 249

APENDICE A: TABLAS DE POSICIÓN DE LOS RECEPTORES DE GRAN ANGULO Al

APÉNDICE B: REGISTROS SÍSMICOS DE GRAN ANGULO (M24 Y CD-82) 131
Gran Canaria M24 B3
Perfil P l B3
OBH 1 B3
0131 12 B4
OBH 3 B5
Estación 1 B6
Estación 2 B7
Estación 5 B8
Perfil P2 B9
OBH 5 B l O
OBH 6 1311
Estación 1 B12
Estación 2 B13
Estación 6 B14
Perfil P3 B15
OBH 7 B16
Estación 1 B17
Estación 7 B18
Estación 8 B19
Tenerife CD-82 L12 Estación Güimar B20

APÉNDICE C: REGISTROS SÍSMICOS DE GRAN ANGULO DE PASO-94 C 1
Perfil T8 C3
OBH 1 C3
OBH 2 C4
Estación 1 C5
Estación 5 C6
Estación 8 C7
Perfil T3 C8
OBH 3 C9
OBBl0 C9
OBVI 11 CIO
Estación 5 C11
Estación 6 C12
Perfil T6 C13
OBB3 C13
OBBl2 C14
Estación 2- C15
Estación 5 C16
Estación 6 C17
Perfil B2 C18
OBB6 C18
OBB7 C19
Estación 3 C20
Perfil B1 C20
OBB7 C21

1. La corteza oceánica fuera de los edificios volcánicos (>30 km), tanto en Canarias como en Sociedad, tiene una estructura similar a la observada en otras regiones de la misma edad no afectadas por plumas mantélicas. Las principales diferencias ocurren siempre en la corteza superior, probablemente como consecuencia del aporte de material volcanoclástico y flujos de lava desde las islas y montes submarinos más próximos, y/o inyecciones magmáticas a través de fracturas corticales.
2. El manto superior bajo el sector norte de Gran Canaria y sudeste del macizo de Anaga (Tenerife) tiene una estructura anómala, propia de complejos plutónicos subcorticales (espesor de ~6-10 km y velocidades entre 7.60-8.00 km/s) creados a partir de la cristalización de magmas en la base de la corteza.
3. Existen indicios de que el proceso de creación de dicho complejo plutónico bajo el Macizo de Anaga sigue activo, en forma de una zona de baja velocidad sísmica inmersa en el manto superior.
4. No se han encontrado evidencias de la existencia de complejos plutónicos bajo ninguno de los edificios volcánicos estudiados en el Archipiélago de Sociedad. Esta diferencia respecto a otros Archipiélagos del Pacífico puede estar debida, en parte, a la mayor alcalinidad de los magmas extruídos, y/o a una pluma mantélica más potente que genere magmas más calientes y de menor densidad.
5. A nivel litosférico, se confirma el rejuvenecimiento térmico bajo el Archipiélago de Sociedad a partir de los bajos valores del espesor elástico en las islas más antiguas (~9-14 km), provocado por la acción de una pluma mantélica fuerte.
6. La anomalía térmica asociada al volcanismo canario es más débil que la de Sociedad, y no ha producido rejuvenecimiento térmico (espesor elástico de 32-35 km). La litosfera presenta un exceso de temperatura de 360 °C a profundidades superiores a los 50 km. Esta perturbación es la causante de la elevación batimétrica que rodea el Archipiélago, y que probablemente es sostenida en parte por procesos convectivos en la litosfera profunda y astenosfera superior.
7. La Dorsal de Galápagos está caracterizada por cambios sistemáticos en la morfología axial, ocasionados por el exceso de aporte magmático desde el punto caliente de Galápagos.
8. La variación desde una morfología de valle a una de alto axial está acompañada por un engrosamiento cortical (calculado mediante gravimetría) >1-2 km.
9. La asimetría, tanto en la profundidad axial como en la morfología, respecto a la zona de fractura por donde la pluma mantélica inyecta material a la dorsal parece estar principalmente controlada por la velocidad de expansión de la dorsal.