EGU21-10797 Similarities between the and Canary Hotspots Revealed by Seismic Anisotropy from Teleseismic and Local Shear-Wave Splitting with the SIGHT Project

David Schlaphorst ([email protected]) with Graça Silveira, João Mata, Frank Krüger, To r s t e n Dahm, Ana Ferreira Motivation

§ Islands of Madeira and Canaries: two examples of hotspot surface expressions. § Hotspot tracks have been reconstructed to past locations close to south-western part of Iberian Peninsula and north- western . § Due to their proximity, interconnected origin of these two hotspots has been proposed but details remain unclear. § Better understanding of the crust and upper mantle structure beneath these islands is needed to investigate this potential connection.

Geldmacher et al. (2005) Madeira and Anisotropy with SIGHT | EGU21-10797 2 Motivation

§ Subsurface structure has influence on stress field. § Can be investigated studying seismic anisotropy patterns of the region. local S § In crust: orientation in the direction of maximum stress is observed à parallel to alignment of fractures or cracks. Melt and fluids can enhance strength. § Upper mantle: orientation influenced by mantle flow and general plate motion. § Anisotropy can also be “frozen in” the lithosphere, retaining orientations source side S § Common method: shear-wave splitting observations of data from teleseismic events. di Leo et al. (2012) § Multiple anisotropic layers possible. (Example of multiple § Include local events to distinguish crustal from anisotropic regions at upper mantle influences. teleseismic SKS a subduction zone.)

Madeira and Canary Islands Anisotropy with SIGHT | EGU21-10797 3 What is shear-wave splitting? At the surface, using a broadband N Surface Φ 3-component seismic station, we can measure the orientation (φ) and time delay (δt).

δt After leaving the anisotropic region, the waves retain their polarisation and time difference.

One wave travels faster than the other.

AnisotropicSplits in two shear-waves with regionorthogonal polarisation (that may both be different to the initial polarisation) in anisotropic medium.

Shear-wave with certain polarisation. Madeira and Canary Islands Anisotropy with SIGHT | EGU21-10797 4 −17.2° −16.8° −16.4° 33.2° 33.2° Data: Seismic Stations A) Madeira Archipelago C) EuAs Madeira Porto Santo NAm 32.8° 32.8° A B Af

Event search criteria for SKS/S: 140 135 SAm Desertas Islands 32.4° 32.4° § Minimum Magnitude: 5.5 (SKS), 2.0 (S) 17.95 mm/yr § Distance: 85˚ – 135˚ (SKS) −17.2° −16.8° −16.4° § Maximum incidence angle: 30˚ (S) −18° −17° −16° −15° −14° −13° 30° 30° § Signal/noise ratio: 3 B) Canary Islands Elevation (km) § Difference between source polarisation and backazimuth: 30˚ (SKS) −4000 −2000 0 2000 150 155 29° 160 29° § Date search interval: 165 170 § 175 15/04/2011 – 22/09/2012 (DOCTAR temporary network, Madeira) § 21/07/2010 – 16/10/2019 (permanent 28° 28° stations, Madeira) § 21/07/2010 – 16/10/2019 (Canaries) El Hierro

17.90 mm/yr 27° 27° −18° −17° −16° −15° −14° −13°

Madeira and Canary Islands Anisotropy with SIGHT | EGU21-10797 5 −18° −17° −16° −15° −14° −13° Data: Events 30° 30° A) Local S (Canary Islands) 0 0 20 20

40 40 Lanzarote

29° La Palma Depth (km) 29° 60 60 Tenerife 0 50 100 500 Number of results no. events Good+fair [SKS null] results | no. total results: El Hierro 28° 28° La Gomera Fuerteventura Gran Canaria M § S (only possible for Canaries; see figure A, B) 2 3 § Canaries: 241 | 584 4 27° 27°

28° § SKS (see figure C) B) Local S (El Hierro) C) Teleseismic SKS § Madeira: 128 [53] | ~6000 § Canaries: 202 [32] | ~20000

27.8°

27.6° M 2 3 4

−18.2° −18°

Madeira and Canary Islands Anisotropy with SIGHT | EGU21-10797 6 Method We use the SHEBA code, based on Teanby et al. (2004). horizontal Radial and (E–W) Clear onset transverse Before correction. 3 components horizontal of SKS components. (N–S) phase. After correction. vertical (Z) (Notice how energy is almost absent from second component.) Fast (solid) and slow |spol – baz|<30˚ (hashed) shear wave before and after δt correction. Cluster analysis. (Notice how they before after after Error surface, (Notice how both δt and φ almost lie completely normalised normalised φ are stable over different on top of each other.) blue cross start (A) and end (F) times marks best of the observation window. Also, the error surface Particle motion before solution. shows one clear result.) and after correction. (Notice how the circular motion becomes almost before after completely straight.)

Madeira and Canary Islands Anisotropy with SIGHT | EGU21-10797 7 Delay Time (s) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Results: Delay times 0 Local S Canary Islands

20 § Stacked SKS delay times larger than S.

El Hierro § Stacked SKS delay times show slight Depth (km) 40 La Palma La Gomera Tenerife decrease from west to east (much more Gran Canaria Fuerteventura Lanzarote pronounced in the much larger Canary 60 western archipelago). islands

eastern SKS Canary Islands islands western islands

eastern PS DG SKS Madeira islands 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Number of stacked events

1 2 5 10 20 25 Madeira and Canary Islands Anisotropy with SIGHT | EGU21-10797 8 Results: Western vs eastern Canaries S delay times

Delay Time (s) Delay Time (s) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0 0 Local S Local S Moho (Gorbatikov et al., 2013) % anisotropy % anisotropy 20 20 02468 02468

El Hierro Fuerteventura

Depth (km) 40 Depth (km) 40 Lanzarote

60 60 Carracedo (1999) SKS SKS 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 § Western islands have much stronger shallow

1 2 5 10 20 25 1 2 5 10 20 25 anisotropy (and are more seismically active). Number of stacked events Number of stacked events § The most of that shallow anisotropy is found close beneath the crust (e.g., El Hierro Moho based on Gorbatikov et al., 2013).

Madeira and Canary Islands Anisotropy with SIGHT | EGU21-10797 9 −17.5° −17° −16.5° A) Madeira Archipelago 33° Porto Santo

Madeira

32.5° Desertas Islands 17.95 mm/yr

−17.5° −17° −16.5° −18° −17° −16° −15° −14° −13° 30° 30° Results: stacked SKS B) Canary Islands Lanzarote La Palma −17.5° −17° −16.5° 29° 29° Tenerife Fuerteventura A) Madeira Archipelago Gran Canaria 33° 28° El Hierro 28° Porto Santo La Gomera

Madeira 17.90 mm/yr 27° 27° −18° −17° −16° −15° −14° −13° 32.5° Desertas Islands La Palma Tenerife 17.95 mm/yr C) E)

−17.5° −17° −16.5° −18° −17° −16° −15° −14° −13° § 30°In general: large variation over short distances. 30° § However:B) Canary stacked Islands results can introduce bias by assuming one anisotropic layer. LanzaroteD) El Hierro § VariationLa Palma in individual results suggests more 29° 29° Number of stacked events complex subsurface that cannot be explained by Tenerife Fuerteventura 1 2 5 10 20 25 simple one, two or inclined layer cases. δt (s) Gran Canaria 1 2 3 Madeira and Canary Islands Anisotropy with SIGHT | EGU21-10797 10 28° El Hierro 28° La Gomera

17.90 mm/yr 27° 27° −18° −17° −16° −15° −14° −13°

C) La Palma E) Tenerife

D) El Hierro Number of stacked events

1 2 5 10 20 25 δt (s) 1 2 3 Results: stack different segments over multiple stations

§ Proximity of stations enable combining results. § Fresnel Zone (FZ) estimation introduce further anisotrpoy depth constraint (see figure). § Azimuthal variation within one station introduces minimum depth limit (at which FZs do not overlap anymore). § Different patterns at same azimuth at different stations introduces maximum depth limit. § (This is a work in progress – first results Alsina & Snieder (1995) from Madeira on the following slides.)

Madeira and Canary Islands Anisotropy with SIGHT | EGU21-10797 11 Results: stack different segments over multiple stations

−17.2˚ −17˚ −16.8˚ −16.6˚ −16.4˚ −16.2˚ −17.2˚ −17˚ −16.8˚ −16.6˚ −16.4˚ −16.2˚ −17.2˚ −17˚ −16.8˚ −16.6˚ −16.4˚ −16.2˚ Most results from north show All results from south show All results from southwest 33˚ general N-S or E-W alignment. 33˚ 33˚ general alignment. 33˚ 33˚ show general alignment. 33˚

32.8˚ 32.8˚ 32.8˚ 32.8˚ 32.8˚ 32.8˚

32.6˚ 32.6˚ 32.6˚ 32.6˚ 32.6˚ 32.6˚

32.4˚ 32.4˚ 32.4˚ 32.4˚ 32.4˚ 32.4˚

−17.2˚ −17˚ −16.8˚ −16.6˚ −16.4˚ −16.2˚ −17.2˚ −17˚ −16.8˚ −16.6˚ −16.4˚ −16.2˚ −17.2˚ −17˚ −16.8˚ −16.6˚ −16.4˚ −16.2˚ N-S E-W

(individual and stacked results shown in the location relative to station; circles represent 85˚ and 135˚ distance; crosses are null results)

Madeira and Canary Islands Anisotropy with SIGHT | EGU21-10797 12 Results: stack different segments over multiple stations

−17.2˚ −17˚ −16.8˚ −16.6˚ −16.4˚ −16.2˚ Some stations have uniform alignment 33˚ in NE-SW or NW-SE direction. 33˚

32.8˚ 32.8˚

32.6˚ 32.6˚

32.4˚ 32.4˚

−17.2˚ −17˚ −16.8˚ −16.6˚ −16.4˚ −16.2˚

(individual and stacked results shown in the location relative to station; circles represent 85˚ and 135˚ distance; crosses are null results)

Madeira and Canary Islands Anisotropy with SIGHT | EGU21-10797 13 Results: stack different segments over multiple stations

§ Events with baz from west. § Unfortunately azimuthal gap for teleseismic events. § Events with baz from north and south. § Often uniform patterns. § FZs from those directions overlap north and south of Madeira. § Events with baz from east. § More complex patterns. § FZs can overlap beneath the island. § Some central islands have uniform orientations.

Madeira and Canary Islands Anisotropy with SIGHT | EGU21-10797 14 Results: SKS – Alignment in the east strong strong −17.5° −17° −16.5° § Interaction with more dominant part

A) Madeira Archipelago weak of “frozen in” anisotropy. 33° Porto Santo ? PMPST § Slow African plate motion between Madeira 60 Ma and today (likely not enough ~21.85 mm/yr

32.5° to reorient large-scale orientations). Desertas Islands 17.95 mm/yr

−17.5° −17° −16.5° −18° −17° −16° −15° −14° −13° 30° 30° B) Canary Islands Lanzarote La Palma 29° 29° Tenerife Fuerteventura 60 Ma today Gran Canaria 28° El Hierro ~21.85 mm/yr 28° La Gomera from Gplates (Müller et al., 2018)

17.90 mm/yr 27° 27° −18° −17° −16° −15° −14° −13°

Madeira and Canary Islands Anisotropy with SIGHT | EGU21-10797 15 C) La Palma E) Tenerife

D) El Hierro Number of stacked events

1 2 5 10 20 25 δt (s) 1 2 3 Results: local S

−18° −17° −16° −15° −14° −13° 30° 30° A) Canary Islands

29° 29° La Palma Tenerife Lanzarote

El Hierro 28° 28° La Gomera Fuerteventura Gran Canaria

27° 27° −18° −17° −16° −15° −14° −13° −18.2° −18° −17.8° 0 3 El Hierro B) 10 Rift zones – Carracedo (1999)

20 27.8°1 27.8° § Aligned along rift zones (crustal 30 features). 40 Event Depth (km)

2 50 27.6° 27.6° § Radial pattern on El Hierro. δt (s) 0.5 1.0 −18.2° −18° −17.8° Madeira and Canary Islands Anisotropy with SIGHT | EGU21-10797 16 Discussion

§ Stacked SKS delay times larger than S. § SKS (stacked or individual): à Anisotropy mostly in the mantle. § Large variation over short distances. § SKS delay times show slight decrease from à Mantle flow diversion due to upwelling dominant. west to east (much more pronounced in à Maybe subvertical structures in parts of upwelling. the much larger Canary archipelago). § Stronger alignment in the east. à Uniform pattern likely as combination of “frozen à Could be related to location of mantle in” anisotropy and present-day mantle flow. upwelling. à African plate motion change. § Western islands have much stronger § Local S: shallow anisotropy. § Aligned along rift zones (crustal features). àYounger Islands. Melt? § Radial pattern on El Hierro. § The most of that shallow anisotropy is à Predominantly beneath crust-mantle boundary. found right beneath the crust. à Island uplift most likely due to magmatic à Potentially crustal underplating. underplating.

Madeira and Canary Islands Anisotropy with SIGHT | EGU21-10797 17 Broader context and impact on global models −20° − 15° −10° −5° 0°

§ Fast shear-wave orientations show large 40° variations over short distances. 40° § Shown on the right are stacked results for better visibility but the general picture would be similar using individual results.

§ In contrast to broad alignment with results 35° from NW Africa and the Iberian peninsular 35° (taken from Wüstefeld et al., 2009). § Global mantle flow model (Conrad & Behn, 2010) predict rapid change around 30° Canary archipelago. 30° § Likewise, future models should include a 1 s SKS splitting similar feature beneath Madeira. this study previous studies ISA −20° −15° −10° −5° 0° Madeira and Canary Islands Anisotropy with SIGHT | EGU21-10797 18 References

§ Alsina, D., & Snieder, R. (1995). Small-scale sublithospheric continental mantle deformation: constraints from SKS splitting observations. Geophysical Journal International, 123(2), 431-448. DOI: 10.1111/j.1365-246X.1995.tb06864.x § Carracedo, J.C . (1999). Growth, structure, instability and collapse of Canarian volcanoes and comparisons with Hawaiian volcanoes. Journal of Volcanology and Geothermal Research, 94(1-4), pp.1-19. DOI: 10.1016/S0377-0273(99)00095-5 § Conrad, C . P., & Behn, M. D. (2010). Constraints on lithosphere net rotation and asthenospheric viscosity from global mantle flow models and seismic anisotropy. Geochemistry, Geophysics, Geosystems, 11(5). DOI: 10.1029/2009GC002970 § Di Leo, J.F.,Wookey, J., Hammond, J.O.S., Kendall, J.M., Kaneshima, S., Inoue, H., Yamashina, T., & Harjadi, P. (2012). Deformation and mantle flow beneath the Sangihe subduction zone from seismic anisotropy. Phys. Earth Planet. Inter. 194:38–54. DOI: 10.1016/j.pepi.2012.01.008. § Geldmacher, J., Hoernle, K., Bogaard, P., Duggen, S., & Werner, R. (2005). New 40Ar/39Ar age and geochemical data from seamounts in the Canary and Madeira volcanic provinces: support for the mantle plume hypothesis. Earth and Planetary Science Letters, 237(1-2), 85-101. DOI: 10.1016/j.epsl.2005.04.037 § Müller, R.D., Zahirovic, S., Williams, S.E., Cannon, J., Seton, M., Bower, D.J., Tetley, M.G., Heine, C ., Le Breton, E., Liu, S., & Russell, S.H. (2019). A global plate model including lithospheric deformation along major rifts and orogens since the Triassic. Tectonics, 38(6), pp.1884-1907. DOI: 10.1029/2018TC005462 § Wüstefeld, A., Bokelmann, G., Barruol, G., & Montagner, J. P. (2009). Identifying global seismic anisotropy patterns by correlating shear-wave splitting and surface-wave data. Physics of the Earth and Planetary Interiors, 176(3-4), 198-212. DOI: 10.1016/j.pepi.2009.05.006

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