Timing of rapid cooling and erosional decay of two volcanic islands of the Canary Archipelago: implications from low-temperature thermochronology

The Canary Archipelago comprises seven volcanic islands formed by the activity of the Canary mantle anomaly that might have been caused by an ascending plume at the NW-African passive margin. The “Basal Complex (BC)”, which contains the islands pre-shield rock formations, is exposed in the northwest and central Fuerteventura and NW-La Gomera and preserves the archive of giant landslides that caused the removal of most of the shield-stage volcanic rocks. Tools, like low-temperature thermochronology (LTT) are sensitive to rapid cooling activities that accompany landslides. In addition, integrating LTT data with time–temperature (t–T) numerical modelling are a powerful tool for reconstructing the thermo-tectonic evolution as well as defining and quantifying long-term landscape evolution in a variety of geological settings. To unravel part of the long-term landscape evolution of Fuerteventura and La Gomera, zircon and apatite fission-track, and (U–Th)/He data combined with t–T numerical modelling were applied to 39 samples representing the main rock units of the BCs and younger magmatic rocks on both islands. In Fuerteventura, the Northwest and Central Basal Complexes reveal rapid cooling/exhumation of more than 200 °C at ~ 20 Ma. The quantification of the thickness of the rock column using the t–T cooling path would need the knowledge of the palaeo-heat flow. The published thickness of the moved rock column in Fuerteventura and La Gomera does not point to an extreme high heat flow. Therefore, the formation of a giant landslide leads to the removal of ~ 2.0 (± 0.5) km of the volcano rock column. Offshore, such a landslide has led to part of the Puerto Rosario large debris avalanche. The “Central Basal Complex” revealed two more rapid cooling/exhumation events at ~ 16 Ma and ~ 14 Ma that might also be related to landslides. The three landslides might be responsible for the formation of the nowadays Puerto Rosario Debris Avalanche Unit offshore. What might have caused the landslides in Fuerteventura. Age data published provide evidence for magmatic and tectonic activity that occur at the time of the formation of the giant landslides. In addition, the Miocene climate significant changes lead to changes in precipitation, and such changes might also provide a destabilisation of pyroclastic units. Therefore, the causes of the giant landslides might be related to more than only one process. The La Gomera BC has experienced two rapid cooling/exhumation events: the first at ~ 9 Ma, which might have caused ~ 2.0 (± 0.2) km of erosion forming the offshore Tazo avalanche, also known as the Tazo landslide. The second rapid cooling at ~ 8.0 Ma is located at the northwest of the Island and might have been caused by the Garajonay caldera collapse and followed by landslides. The landslides are assumed to have formed the Segments I, II, III, and VIII of the submarine debris avalanches offshore. Like Fuerteventura, both landslides might have been triggered by tectonic and magmatic activities as well as due to variation in precipitation caused by climate variation.


Introduction
Giant landslides often cause the denudation of volcanic edifice on volcanic islands by rapid erosion (Moore 1964;Lipman et al. 1988;Inokuchi 1988;Moore et al. 1989Moore et al. , 1994Iverson 1991Iverson , 1995McGuire 1996;Krastel et al. 2001; 1 3 Oehler et al. 2005;Boulesteix et al. 2013;Marques et al. 2019). The rapid mass movement can be triggered by earthquakes, new volcanic eruptions, tectonic activities, instabilities of the volcanic rock pile or a significant increase in precipitation due to climate change.
By applying luminescence dating, the deposition age of a landslide mass can be dated up to an average of 100 ka. Electron spin resonance dating of landslide masses would extent the dating possibility to about 2 Ma. Elder mass movements still seek for dating techniques. In volcanic areas, the surface of landslide remnants was covered again by volcanic flows. Dating those younger volcanic flows by applying K-Ar, 40 Ar/ 39 Ar or U/Pb techniques allowed receiving a minimum age for the mass movement (Marques et al. 2019).
In the cases of giant mass movements, resulting in the abrupt denudation of more than 2000 m of vertical material, initial rock temperatures of the suddenly exposed basal rocks drop instantaneously from high temperatures to temperatures expected on or in proximity to the Island surface. The initial rock temperature varies in dependency of thermal gradient. In active volcanic islands, a thermal gradient of more than 80 °C/km can be reached (IGME 1991a;Santamarta and Expósito 2014;Carlino 2018).
Such instantaneous drops in rock temperature can be recognised by applying low temperatures thermochronological (LTT) dating techniques, such as apatite and zircon fission-track and (U-Th)/He analysis. Helium diffusion in apatite is sensitive in the T-range of 45 °C/1 Ma to 75 °C/1 Ma; Apatite fission-tracks anneal in the T-range of 60 °C/10 Ma-110 °C/10 Ma; helium diffusion in zircon is sensitive to 50 °C/1 Ma to 185 °C/1 Ma depending on the amount of amorphisation, and Zircon fission-tracks anneal in the T-range 190 °C/10 Ma to 330 °C/10 Ma depending on the amount of amorphisation as well. Annealing and diffusion means that the revealed ages are getting younger as longer apatite and zircon are geologically kept in the specific temperature range. A sudden drop in temperature below the lower temperature of the temperature range, therefore, will freeze the system and the age received will date the sudden drop in temperature. Applying the four thermochronometers in one sample will provide the temperature decrease over more than 200 °C. To reveal the related time-temperature history of the apatite and zircon minerals and, therefore, to the rocks they have been taken from, the LTT ages are combined with numerical modelling of the time (t)-temperature (T)-evolution. In the case of giant landslides on volcanic islands, such LTT data sets derived from nowadays exposed basal volcanic, intrusive, and sedimentary units below the former > 2000 m high volcanic edifices which provide the ability to reconstruct a possible rapid cooling event from formation, or rather deposition to surface exposure.
To test the low-temperature thermochronological tools as an approach to date the rapid drop in rock temperature on volcanic islands, we chose the volcanic islands Fuerteventura and La Gomera of the Canary Archipelago. In the northern and central part of Fuerteventura Island, the nowadays exposed "Basal Complex" rock units are discussed as unravelled by giant landslides from more than 2000-3000 m high volcanic edifices (Stillman 1999;Fig. 1) in Miocene time. The Puerto Rosario debris avalanches have been formed by  Acosta et al. (2003); slightly changed). Yellow lines encircle submarine debris avalanches that are important for this study. The submarine debris avalanche VIII and I might have partly been caused by the Tazo landslide and the submarine debris avalanche II and III might have partly been caused by the Garajonay caldera collapse and followed landslides landslides since 20 Ma (Acosta et al. 2003;Casillas et al. 2011).
The northern part of La Gomera Island also exposes its "Basal Complex" because of the abrupt denudation of at least 1,300 m to 1900 m vertical rock volume of its volcanic edifice (Cendrero 1970). The exhumation age of the "Basal Complex" is assumed to have occurred several times between 12.1 and 6.4 Ma leading to the formation of the onshore and offshore Tazo, San Marcos, and other avalanches (Acosta et al. 2003;Ancochea et al. 2006;Casillas et al. 2008bCasillas et al. , 2010Fernández et al. 2015).
In the following, we present evidence for dating of rapid mass movements using LTT-dating techniques combined with numerical modelling. We integrate published formation ages, LTT data, and provide viable thermal histories of the "Basal Complex" of Fuerteventura and La Gomera.

Geologic setting
The Canary Archipelago is located 100-700 km west of Morocco (28.1°N latitude) in front of the Eastern Central Atlantic passive margin (Fig. 1). The seven Islands are commonly separated into (1) an internal group comprising Fuerteventura and Lanzarote, which exhibit a sub-parallel NNE-SSW alignment to the African coast, and (2) an external group, comprising the Islands El Hierro, La Gomera, La Palma, Tenerife, and Gran Canaria, which exhibit an E-W alignment. The internal group called "Eastern Canary Ridge" also comprises the submarine Conception Bank to the north and the Amanay and Banquete edifices to the south (Uchupi et al. 1976;Anchochea et al. 1996). The geological evolution of the Islands has been changed over time.
(2) The differences in regional distribution of the Islands have been taken as an argument against the old hot spot theory. Anguita and Hernán (1975), Araña and Ortis (1991) and Ancochea et al. (1996) assume an alternative formation model of the volcanic rocks as a "fracture-induced decompression melting of the asthenosphere". Their model connects to Alpine tectonics and is proposed to have propagated from the adjacent Moroccan Atlas Mountains towards the west.
(3) The latest theory on the origin of the Islands reassessed the plume related theory by Carracedo et al. (1998), Hoernle et al (2002), Fullea et al. (2015), Miller et al. (2015), Sagan (2018), Sagan et al. (2020) and Carnevale et al. (2021). They explain the differences to "normal hotspot-related volcanic zones" as caused by smaller volumes of magma and eruption rates and slow motion of the related plate. Stable and radiogenic isotope data support this theory (Hoernle and Tilton 1991;Demény et al. 1998Demény et al. , 2004Hoernle et al. 2002;Abu El-Rus et al. 2006).
Surface lithology on the Islands exposes volcanic and sedimentary rocks that are partly covered by sediments of Cenozoic to recent age (Fúster et al. 1968;Coello et al. 1992;Ancochea et al. 1996Ancochea et al. , 2006. The exposure of a Basal Complex (BC) on Fuerteventura and La Gomera exhibits a speciality and allows a view into the early formation stage of both Islands.
(4) Recently, the existence of seamounts in the Canary Islands environment (van den Bogaard 2013) of various ages, since the Cretaceous, have questioned the theory of the hot spot and point (Sagan et al. 2020) towards edge-driven convection (King and Anderson 1998) as the origin of Mesozoic and Cenozoic magmatism in the eastern Atlantic Ocean.

Fuerteventura Island
The geological evolution of Fuerteventura is divided in four main stages ( Fig. 2; Table 1): (A) a "Mesozoic oceanic crust with sedimentary rocks"; (B) the "Eocene-Oligocene Submarine and Transitional Volcanic Group and Intrusions"; (C) a "Miocene "Subaerial Volcanic Complexes and Intrusions" comprised in series I of Fúster et al. (1968); and (D) a "Pliocene-Quaternary sedimentary and volcanic complex" comprised in series II-IV of Fúster et al. (1968). Stage A, B, and part of C also comprise the so called "Basal Complex", a term used for all exposed Mesozoic sedimentary rocks, submarine volcanic rocks, intrusive rocks, and magmatic dikes. Stage B is often separated into the "Submarine Volcanic Group" and the "Transitional Volcanic Group". The classification in Series I-IV is given by Fúster et al. (1968). The classification of A1-A4 is provided by Balogh et al. (1999). The latest classification in EM1-EM4 is published by Muñoz et al. (2003).
Stage A, B, and part of C: the "Basal Complex" (Early Jurassic to Miocene) The "Basal Complex" is exposed in the Northwest ("Northwest Basal Complex": NW-BC) and Central part ("Central Basal Complex": C-BC) of Fuerteventura. In the northwest of the Island, the Basal Complex is partly overlain by the volcanic rocks of the Northern Volcanic Complex (NVC, Series I) and sediments and sedimentary rocks of the "Pliocene to Quaternary Sedimentary and Volcanic Complex". The "Central Basal Complex" is partly overlain by volcanic rocks of the Central Volcanic Complex (CVC, Series I) and volcanic and sediments and sedimentary rocks of the "Pliocene to Quaternary Sedimentary and Volcanic Complex" (Series II-IV). We separated the "Central Basal Complex" into a "West-and East-Central Basal Complex": The "West-Central Basal Complex" extends from the Atlantic Ocean to Vega de Rio Palmas. The "East-Central Basal Complex" extends from Vega de Rio Palmas to the Betancuria. The Mesozoic oceanic crust of the "Basal Complex" consists of tholeiitic N-type mid-ocean-ridge basalts of Lower Jurassic age (Toarcium, Steier et al. 1998) overlain by a thick deep-water sedimentary sequence of Lower Jurassic to Upper Cretaceous age. The formation age of the sedimentary sequence is well constrained by paleontological evidence (Renz et al. 1992;Steiner et al. 1998). Oligocene pillow lavas, hyaloclastites, basaltic volcanic breccias of the "Submarine Volcanic Group" (SVG), the submarine growth stage of Fuerteventura, unconformable overlie the Mesozoic sedimentary sequence. The Upper Oligocene basaltic submarine-subaerial lava flows of the "Transitional Volcanic Group" (TVG), which corresponds to the emergence stage of the island follow the "Submarine Volcanic Group". Numerous small plutonic and hypabyssal intrusions occur within the Mesozoic to Cenozoic bedded succession. Crosscutting relationships led to recognise four intrusive episodes for the "Basal Complex" [if not stated otherwise all ages of the rock groups A1-A3 are 40 Ar/ 39 Ar mineral ages or whole rock ages (wr)], AFT = apatite fission-track age (Ignacio de et al. 2002 Féraud et al. (1985), Le Bas et al. (1986), Ibarrola et al. (1989a), Cantagrel et al. (1993), Sagredo et al. 1996), Balogh et al. (1999), Ignacio de et al. (2002, and Gutiérrez et al. (2006). 40 Ar/ 39 Ar age data are published by Féraud et al. (1985), Balogh et al. (1999), and Gutiérrez et al. (2006). U-Pb ages of zircon are taken from an internal report of Casillas (2022).

Stage C: Miocene volcanic complexes and intrusions (series I)
The volcanic rocks (lava flows and pyroclastic rocks: basalts, trachybasalts) of the initial subaerial volcanic eruptions are summarised in the Miocene Subaerial Volcanic Group (SAVG) as the first subaerial eruptions around 24-22 Ma. Coeval to the SAVG is the Miocene N-S trending basic dike swarm (23-17 Ma) that occur in the "Basal Complex". The initial volcanic activity is followed by eruptions building the three Volcanic Complexes on Fuerteventura Island, the southern, central, and northern volcanic edifice. The Volcanic Complexes that reach a maximum elevation of about 3000 m display a heterogeneous series of basalts, trachybasalts, debris avalanche breccias, and debris flow breccias that are crosscut by dikes and plutonic bodies of unknown age. All ages are either K-Ar ages or 40 Ar/ 39 Ar ages of whole rock or minerals.
In the core of the Northern and Central Volcanic Complexes, the hypabyssal root of successive growth occurs as a series of plutonic rocks (pyroxenites, gabbro's, syenites, basaltic to trachybasaltic dykes; equivalent to the intrusive periods three to four of the "Basal Complex"). K/Ar ages are based on Abdel Monen et al. (1971), Féraud et al. (1985), Coello et al. (1992), and Ancochea et al. (1996). 40 Ar/ 39 Ar age data are published by Féraud et al. (1985).

Stage D: Pliocene-Quaternary sedimentary and volcanic complex (series II-IV)
The post-Miocene volcanism on Fuerteventura occurred only in the Central and Northern regions of the island, producing eruptive cycles with K/Ar ages of ~ 5 Ma, 2.9-2.4 Ma, 1.8-1.7 Ma, 0.8-0.4 Ma, and < 0.1 Ma. Renewed activity formed small basaltic volcanoes and associated lava fields during the Pliocene that continued until the prehistoric. Series II comprises three periods of volcanic activity during the Lower Pliocene, Upper Pliocene, and at the Pliocene-Pleistocene boundary; series III crops out in a few localities of Pleistocene age; and series IV ranges from Upper Pleistocene to Holocene. Littoral and shallow-water marine deposits, aeolian and alluvial complexes and paleosol deposits are formed during the Pliocene-Quaternary period (Abdel Monen et al. 1971;Ibarrola et al. 1989a, b;Coello et al. 1992;Ancochea et al. 1996).
Miocene submarine volcanic stage and intrusions (12.1 ± 0.1-9.0 ± 0.5 Ma) As in Fuerteventura, in La Gomera, the term "Basal Complex" is used for a suite of different sedimentary, volcanic, and intrusive rocks. The "Basal Complex", which exposed at the NW part of the island, is formed by siliciclastic and carbonatic sedimentary rocks (older than 20 Myr), Miocene submarine volcanic rocks (pillow lavas, trachytic breccias), and ultramafic to mafic intrusions of Miocene age. The P1 suite consists of kaersutite pyroxenite, kaersutite, and amphibole-gabbro intrusions. All BC rocks are cut by an extremely dense network of basic dikes and are markedly deformed. The published K/Ar and 40 Ar/ 39 Ar age data suppose a formation age between 11.5 ± 0.7 and 8.9 ± 0.1 Ma for the rocks of the "Basal Complex". K/Ar ages are based on Abdel Monen et al. (1971), Cantagrel et al. (1984), Cubas et al. (1994) and Anchochea et al. (2003Anchochea et al. ( , 2006. 40 Ar/ 39 Ar age data are published by Herrera et al. (2008). consists of alkaline gabbro, monzodiorite, and syenite intrusions. Famous is the Tamargada monzonite-syenite complex (9.1 ± 0.3 Ma, 8.9 ± 0.1 Ma, respectively). The upper most unit, the Upper Old Edifice-2 of basaltic and trachybasaltic flows and dikes cover the age range between 7.5 ± 0.4 Ma and 6.5 ± 0.3 Ma. The Upper Old Edifice reached a final height of about 2200 m. Associated with the Old Edifice is a cortege of felsic intrusive rocks, the Vallehermoso trachyticphonolitic complex with a K/Ar whole rock age range from 8.6 ± 0.4 to 6.4 ± 0.5 Ma. The Tamargada syenite revealed Island showing the distribution of the main rock units, the locations of the dated samples, and the thermochronological ages. The inset shows the area covered by the map from La Gomera (modified after Casillas et al. 2010). The outline of the possible Garajonay landslide was taken from Paris et al. (2005). Samples with yellow background were numerically modelled. Ta.C. Tamargada Complex a K/Ar whole rock age of 9.1 ± 0.3 Ma and a whole rock 40 Ar/ 39 Ar age of 8.5 ± 0.2 Ma. The Old Edifice rock units are characterised by intense interaction of crosscutting dykes (1 every 10 m or even less) of basaltic composition (10.5 ± 0.2-8.1 ± 0.5 Ma) and less frequent felsic (trachytic to phonolitic) ones (8.0 ± 0.4 Ma). K/Ar ages are based on Abdel Monen et al. (1971), Cantagrel et al. 1984, Féraud et al. (1985 and Anchochea et al. (2006Anchochea et al. ( , 2008. 40 Ar/ 39 Ar age data are published by Herrera et al. (2008).

Thermochronology
Thermochronology is based on the accumulation and thermally controlled retention of isotopic daughter products and linear crystal defects produced during the radioactive decay of the parents. Due to the temperature sensitivity of the thermochronometers, ages provide information about the cooling history of the rock. If temperatures decrease rapidly such as it is attributed to the cooling of volcanic flows, thermochronological ages represent the formation age of the flow.
Apatite and zircon fission-track (AFT and ZFT, respectively) dating techniques were performed on 21 samples located on Fuerteventura Island (Table 3 Sample preparation and analyses followed the same protocol as, e.g. Karl et al. (2013) with the exception of the apatite etching conditions. 5.5 N HNO 3 for 20 (± 1) s at 21 (± 1) °C that was applied to all apatite mounts. We extracted suitable zircon grains for ZFT analyses from 4 new samples (Figs. 2, 3; Tables 5, 6) and suitable apatite grains for AFT dating from 21 new samples (Figs. 2, 3; Tables 5, 6). As the spontaneous track densities were generally low with values ranging from ~ 0.1 to ~ 0.7 insufficient number of confined   fission-tracks (CTs) were etched. To reveal more CTs by artificial etchant conduits, a second set of apatite mounts was prepared from samples of Fuerteventura and La Gomera. The artificial etchant conduits are formed by the irradiation with accelerated heavy ions (Jonckheere et al. 2007), like the irradiation with fission fragments from a 252 Cf source (Donelick and Miller 1991). The apatite mounts were irradiated by 197 Au ion at the universal linear accelerator (UNI-LAC) facility, GSI Helmholtzzentrum für Schwerionenforschung GmbH in Darmstadt, Germany with an energy of 11.1 MeV/nucleon, a fluence of 1 × 10 6 ions/cm 2 , and an angle of 15° with the vertical. Irradiation followed the protocol published by Jonckheere et al. (2007). After irradiation, the mounts were etched with 5.5 N HNO 3 for 20 (± 1) s at 21 (± 1) °C. After etching CTs were located within the apatite grains (Table 7). The published apatite and zircon fission-track ages and the apatite (U-Th)/He (AHe) and zircon (U-Th)/He (ZHe) data were re-examinate and used to numerically model the time-temperature-evolution for the first time (Table 8). Reexaminate of the fission-track data means: We remeasured the confined fission-track length in old apatite grains (Wipf et al. 2010) and determined the angle of the CT to the c-axes of the apatite crystal. We chose the crystal size and form as the selection criterion for zircon and apatite grains for the (U-Th-Sm)/He system as it is used in such data Beucher et al. 2013;Green and Duddy 2018), as well as homogeneity in grain size and chemical content [U, Th, Sm, low radiation damage (eU-value)] of used apatite and zircon grains. This was possible as the old already dated [(U-Th-Sm)/He dating] apatite and zircon grains were documented with photos. Therefore, we only present clear grains for apatite and clear, light coloured grains for zircon, respectively, when the requirement for a full morphology was fulfilled. The influence of zircon colour as a selecting criterion in fission-track dating is well described by Garver and Kamp (2002). This criterion can also be applied to ZHedating. In addition, since the last publication in 2010 the Hediffusion and fission-track annealing models and the knowledge on the influence of grain shape have been advanced significantly. Therefore, a re-examination and new numerical modelling of the already published data was necessary.

Numerical modelling of the time-temperature evolution
Numerical modelling of thermochronological data allows reconstructing the cooling history of crustal segments. A general law for numerical modelling of the t-T history of rocks is the increase in reliability with increasing amount of thermochronometers considered. The software code HeFTy ® (v.1.9.3.) (Ketcham 2005(Ketcham , 2017Ketcham et al. 2007aKetcham et al. , b, 2009) was used to test potential time-temperature (t-T) paths against the thermochronological data set considering the published geological evolutions. Our aim was to test the hypotheses of Stillman (1999) that giant landslides have degenerate the first shield-stage volcanoes (initial Miocene Subaerial Volcanic Group) using our thermochronological data set. Relicts of this shield-stage volcanoes are the intrusive rocks of the North Basal Complex and the intrusive and sedimentary rocks of the West-Central Basal Complex. If the hypotheses are true, we expect to see a fast cooling event before the formation of the Stage C Miocene Subaerial Volcanic Complexes (Series I of Fúster et al. 1968).
The software HeFTy ® (v.1.9.3.) uses diffusion kinetics for the He-diffusion in apatite and zircon and annealing kinetics for the annealing of fission-tracks in apatite. The published geological evolution was transferred into t-T-constraints that are defined by geological events including a range of implied uncertainty (Table S1). For example: Sample Lag 1 was taken from a pillow basalt layer. Therefore, the initial start of the numerical modelling was at surface temperature. If, however, samples of intrusions were modelled the initial start temperature was a high as magma temperature provided by literature for such petrography. Furthermore, we try to provide less constrains as possible. The ZFT data are integrated as the second constrain and the t-T -box is kept very broad (Table S1). The third constrain is provided by the published geological evolution that provides evidence of near surface conditions for the intrusive rocks of the "Basal Complexes". The fourth constrain tested a possible thermal influence by volcanic rocks on the AFT-and AHe data. The software code runs t-T paths through the t-T-constrains areas to find possible solutions for a viable t-T history that fit the thermochronological input data. Statistical comparison (G.O.F.: goodness of fit) of the thermochronological data set generate by the possible t-T paths with the real thermochronological data set leads to the three categories: a best-fit t-T path (black line in the graphs), G: good t-T path, and A: acceptable t-T paths. The numerical models run until 1000 good t-T path have been found. Within the diagrams, the P describes the amount of t-T path runs to receive 1000 good t-T path. When possible, all available thermochronometers were combined and jointly modelled (Table S1; Table 9). The thermochronological data sets used for the numerical modelling are:

Zircon fission-track data
Four new samples revealed enough zircon grains for fissiontrack dating (Figs. 2, 4; Tables 3, 5). For better understanding, we also discuss and integrate the 6 already published ZFT ages of Wipf et al. (2010). All zircon grains of the magmatic rocks were etched with the same etching time and showed the same medium white colour, indicating a similar annealing temperature. Zircon fission-track ages of all  (Galbraith 1981). Nearly all ZFT ages are younger than the related intrusion or sedimentation age indicating a post-intrusion cooling history.

Apatite fission-track data
21 new samples revealed enough apatite grains for fissiontrack dating (Figs. 2, 4; Tables 3, 5). For better understanding, we also discuss and integrate the 5 already published AFT ages of Wipf et al. (2010). AFT ages range from 20.5 ± 2.4 to 2.7 ± 0.6 Ma. All samples passed the c 2 -test indicating a homogeneous distribution with respect to 1σ-error of the single grain ages (Galbraith 1981). One apatite fission-track ages is interpreted as a formation age. The other AFT ages are younger than the formation age of the 14.0 ± 2.5 n.a.

Formation ages
The Tindaya trachyte (#FU-12-08) has a formation age of 18.7 ± 0.3 Ma, and an AFT age of 17.8 ± 5.0 Ma. Within error, the AFT age is the same as the formation age. Therefore, the AFT age might indicate a fast cooling of the Tindaya trachyte and no heating above 60 °C afterwards.
Cooling ages Even that the AFT ages have a large range, three cluster can be seen including the error in the data: Cluster I: 20.5 ± 2.4 Ma-17.5 ± 2.0 Ma; av. 18.7 ± 2.4 Ma.
Cluster III: 4.7 ± 0.8 Ma-2.7 ± 0.6 Two NE-SW-trending fine-grained trachytic dikes of unknown formation age that cut Lower Cretaceous sedimentary rocks in the "West-Central Basal Complex" revealed AFT ages of 2.7 ± 0.6 Ma and 4.7 ± 0.8 Ma. These dikes are located close (tenth of metre) to a large Pliocene basaltic flow of the Series II. They might have been thermally influenced by the basaltic flow. Therefore, it cannot be excluded that the age might represent a reheating and, thereafter, fast cooling caused by the Pliocene basaltic flow.  Guenther et al. (2013) The average etch pit size, called D par , range from 2.9 ± 0.5 to 1.5 ± 0.1 µm. Apatite grains of one coarse-grained syenite show the larges etch-pits. Within error, all apatites of the carbonatites and syenites are characterised by D par values above 2.0 µm. No relation between AFT ages and D par has been encountered. Generally, AFT ages do not show any trends when compared to their elevation or spatial distribution.
Only six samples, five of the "Northwest Basal Complex" and one of the "West-Central Basal Complex", revealed more than 50 length measurements of confined fission-track (CT) to perform the numerical modelling of the t-T evolution (CT's: 106-51; Table 7). Eight samples provided length measurements between 1 and 23 confined fission-tracks. The mean confined fission-track length values range from 11.2 ± 2.4 to 14.0 ± 1.5 µm. The skewness of 12 CT-data is negative. One showed a positive skewness. The c-axes correction of the confined track lengths led to a corrected confined fission-track length (Lc) distribution between 12.1 ± 2.0 and 14.9 ± 1.8 µm. The long Lc distribution indicates a fast cooling history for the analysed samples. The etch pit size D par of each apatite grain was determined for all samples used for length measurement. The mean D par values range from 2.5 ± 0.3 to 1.5 ± 0.1 µm indicating a domination of fluorine-rich apatite grains.

Re-interpretation of published zircon (ZHe) and apatite (AHe) (U-Th)/He data
Three Lower Cretaceous sandstones and one lower Miocene magmatic dike of the "West-Central Basal complex" and one syenite and one gabbro of the "East-Central Basal Complex" samples revealed well-shaped zircon (3-1) and apatite grains (3 or 2) for (U-Th)/He dating (Table 8; Figs. 2, 4). The He age variation of single zircon grains within one sample are within error. No correlation between the eU-value and the determined ZHe and AHe ages exists. The ZHe single grain ages vary from 21.1 ± 1.7 to 13.3 ± 1.1 Ma and are younger than the formation age indicating cooling ages. The zircon (U-Th)/He ages of the gabbro (#FU-16-07*) and syenite (#FU-17-07*) are in average younger (av. 14.3 ± 1.2 Ma) than those of the sandstones (#FU-02-07*, #FU-06-07*, #FU-08-07*: av. 19.5 ± 1.6 Ma). Considering the diffusion behaviour of He in zircon grains a temperature below 185 °C was reached much earlier for the "West-Central Basal complex" than for the "East-Central Basal Complex". One basaltic dike (#FU-14-07*) of the sheeted dike complex ("West-Central Basal Complex") revealed an average ZHe cooling age of 14.9 ± 1.2 Ma, which is within error the same as the ZHe cooling age of the "East-Central Basal Complex".

Time-temperature (t-T) numerical modelling
The numerical modelling of the t-T path for the samples with either more than 50 confined fission-track length or (U-Th)/He data revealed an area of acceptable (green colour), good (orange colour), and one best-fit t-T path (black). In general, the goodness of fit (G.O.F.) is more than 0.87. The best constrained t-T path would have a G.O.F. of 1.0. In some cases, such as the AHe data of the Lower Cretaceous sandstone (#FU-02-07*), the G.O.F. is lower in value and, therefore, part of the t-T path is less well constrained. The reasons for the t-T boxes are described in "Numerical modelling of the time-temperature evolution".
A moderate cooling from ~ 500 to 280 °C within 7 Myr since the formation (27.3 ± 0.6 Ma) is followed by a decrease in T reaching low temperatures (40-20 °C) at ~ 19 Ma in less than 1 Myr. In all five cases, the best-fit t-T path is nearly vertical. Reheating is excepted by the thermochronological data and lead to a maximum T of ~ 80 °C at ~ 14 Ma for a short time. The amount of reheating is different in the samples analysed. Thereafter, the temperature decreases to recent surface temperature.
Summarising the possible t-T evolution of the best-fit path, the t-T evolution is characterised by four major pattern:

Sandstones and basaltic dikes
The t-T evolution of sandstone #FU-02-07* and #FU-08-07* are well constrained as up to three zircon grains and up to two apatite grains have been used for the numerical modelling. The third sandstone (#FU-06-07*) is less constrained. The three sandstones revealed a similar t-T evolution from formation to the Lower Miocene (20 Ma). A Gradual increase of temperature from surface temperature at sedimentation age (open box between 137 and 112 Ma) can be observed reaching ~ 300 °C at about 55 Ma, which is coherent with the described metamorphic grade by Steiner et al. (1998), and the ZFT ages. A moderate decrease in temperature reached ~ 250 °C at 20 Ma. Thereafter, the t-T path indicates that the sandstones cooled rapidly to a temperature between 40 and 20 °C at ~ 20 Ma. The two sandstones #FU-02-07* and #FU-08-07* which are located close to each other are kept at surface temperature until today. The same constrains for the numerical modelling of the sandstone #FU-06-07* indicate a t-T path that is quite different. This sandstone might have been reheated to ~ 80 °C at ~ 14 Ma and cooled, thereafter, to recent surface temperatures.
Carbonatite dike (#FU-01-08, Form. age: 26.2 ± 0.2 Ma) The t-T path indicates a fast cooling from 450 to 50 °C at ~ 22 Ma. Between ~ 22 and ~ 18 Ma, the dike cooled to surface temperature. The following increase in temperature reached a maximum of ~ 80 °C at about ~ 14 Ma. Decrease in temperature reached surface temperature at recent time again.
Summarising the t-T evolution of the "West-Central Basal Complex" leads to the following: • at ~ 55 Ma the sandstones reached a maximum temperature of ~ 300 °C °C in average and kept the temperature until ~ 21 Ma. • at ~ 21 Ma rapid cooling to near surface temperature happened to the sandstones and the basic dikes of the sheeted dike swarm complex. • at ~ 14 Ma reaching a new individual maximum T of ~ 80 to ~ 115 °C. • at ~ 14 Ma rapid decrease in T to surface temperature of nearly all analysed samples (cooling by 60-95 °C). The surface temperature is kept until recent.

East-Central Basal Complex
One syenite (#FU-16-07) and one gabbro (#FU-17-07) of the Vega de Rio Palmas Ring complex revealed zircon and/or apatite (U-Th)/He single grain ages that have been used to model the t-T evolution (Tables 3, 5, 8; Fig. 8). The t-T evolution of syenite #FU-16-07* is well constrained as two zircon grains and three apatite grains have been used for the numerical modelling. In addition, the regional geological evolution indicated that the intrusive rocks were covered with volcanic rocks at about 15 Ma. A moderate cooling ~ 550 °C to ~ 250 °C occurred between ~ 18 and ~ 16 Ma. Thereafter, the temperature dropped nearly instantaneously at ~ 15 Ma to ~ 30 °C. Temperature increase reached ~ 80 °C at ~ 14 Ma and is followed by a rapid decrease of T to surface temperature. The surface temperature was kept until recent time.
Summarising the t-T evolution four major pattern is seen: • moderate cooling ~ 550 to ~ 250 °C occurred between ~ 18 Ma and ~ 15 Ma. • rapid cooling at ~ 15 Ma from intrusion temperature to near surface temperature. • reheating to a maximum T of 80 °C at ~ 14 Ma. • rapid decrease of T to surface temperature at ~ 14 Ma.
Surface T was kept until recent. Fig. 4 Formation ages and thermochronological ages of the four areas in Fuerteventura. Temperature according to the known final diffusion and annealing temperature at a cooling rate of 10 °C/Ma. As part of the "Basal Complexes" are overlain by subaerial volcanic rocks younger than 20 Ma, intrusive and sediment rocks of the "Basal Complex" must have been at the surface before the subaerial volcanic rocks were deposited. If not otherwise mentioned, the dashed lines represent possible heating and cooling path between the thermochronological ages of the area. NW-BC Northwest Basal Complex, CBC Central Basal Complex, WC-BC West-Central Basal Complex, EC-BC East-Central Basal Complex. The age of the Garajonay landslide is taken from Paris et al. (2005) ◂ Fig. 5 Thermal history models of the samples from the "Northwest Basal Complex" modelled using the software code HeFTy (Ketcham et al. 2007a(Ketcham et al. , b, 2009. The left window displays the t-T paths, the right column displays the c-axes corrected confined fission-track lengths distribution overlain by a calculated probability density function for the best-fit t-T solution. All constrains are provided in Table S1. Resulting t-T curves show three different path envelopes; green path envelope: acceptable fit (all t-T paths with a merit function value of at least 0.05), orange path envelope: good fit (all t-T paths with a merit function value of at least 0.5), and black line: bestfit path of all accepted and good paths (Ketcham et al. 1999, Ketcham et al. 2007a, b, 2009. P amount of t-T path runs necessary to receive 1000 good t-T path, Ac acceptable fit models, G good fit models, A D determined FT age with 1−σ error, A M modelled FT age, L D determined central c-axes corrected confined fission-track length with 1−σ error, L M modelled confined fission-track length with 1−σ error, G.O.F. goodness of fit, Ng number of single grains, Nt number of single confined fission-tracks

Zircon fission-track (ZFT) data
Six samples from the suite P1, P2, and P3 of the "La Gomera Basal Complex" revealed enough zircon grains for fissiontrack dating (Tables 4,6;Figs. 3,9). ZFT central ages range between 9.5 ± 0.6 Ma and 8.3 ± 0.7 Ma (Figs. 3, 9; Table 6). All samples passed the c 2 -test indicating a homogeneous distribution with respect to 1σ-error of the single grain ages (Galbraith 1981). With the exceptions of #Lag 3 (9.5 ± 0.6 Ma) and #Lag 4 (8.3 ± 0.7 Ma) sample (Tamargada monzonite and syenite) the ZFT ages are slightly younger than the related extrusion or intrusion age. The samples #Lag 5 (8.6 ± 0.5 Ma), #Lag 7 (9.0 ± 0.5 Ma), #Lag 10 (9.5 ± 0.5 Ma), and #Lag 11 (9.2 ± 0.6 Ma) are located within a narrow area in the NW of the Island. The ZFT cooling ages are within error the same with an average cooling age of 9.1 ± 0.5 Ma, which is similar to the proposed intrusion age of the Tamargada syenite (9.1 ± 0.3 Ma). The Tamargada monzonite sample (#Lag 3) revealed a ZFT age of 9.5 ± 0.6 Ma, which correlates with the proposed intrusion age of 8.9 ± 0.1 Ma within error. The Tamargada syenite sample (#Lag 4) revealed an age of 8.3 ± 0.7 Ma, which also shows the same age within error as the proposed intrusion age of 9.1 ± 0.3 Ma. Within error, both ZFT ages correlate with the proposed average formation age of 9.0 ± 0.2 Ma of the Tamargada complex. Both ages indicate a fast cooling from magma intrusion temperature to the zircon fission-track annealing temperature. As the zircon grains were clear white with a long etching time, we assume that the ZFT age represent a total annealing temperature of about 300 °C. Summarising the ZFT ages and considering the error only two age groups can be recognised: • av. 8.9 ± 0.7 Ma: ZFT formation age of the Tamargada monzonite and syenite and fast cooling to below 300 °C. • av. 9.1 ± 0.5 Ma: ZFT cooling ages of samples (#Lag 5, 7, 10, 11) located close to each other. Extreme interesting is that within error the ZFT age is the same as the intrusion age of the Tamargada ring complex.

Apatite fission-track (AFT) data
Eight samples (submarine volcanic rocks, P1, P2, P3 suites, a trachyte breccia) of the "La Gomera Basal Complex" revealed enough apatite grains for fission-track dating (Tables 4,6;Figs. 3,9). The AFT ages range between 12.9 ± 3.1 Ma and 4.8 ± 0.7 Ma. All samples passed the c 2 -test indicating a homogenic population in each sample set. Except for #Lag 8 all AFT ages are younger than the related extrusion or intrusion age indicating a post-extrusion and intrusion cooling history. The oldest AFT age (12.9 ± 3.1 Ma) of a pegmatitic gabbro (#Lag 8, P2 suite) is within error the same as the proposed intrusion age (10.8 ± 0.1 Ma). Considering the D par of 2.3 ± 0.4 µm a very fast cooling to a temperature of 110 °C or even below might have occurred. The AFT age (10.7 ± 0.9 Ma) of the pillow basalt (#Lag 1, submarine volcanic rocks) correlates with the proposed extrusion age (11.5 ± 0.7 Ma) within error. The AFT ages (7.6 ± 1.6 Ma) of a syenite dike (#Lag 5, P2 suite,) and the AFT age (6.9 ± 1.8 Ma) of an amphibole gabbro (#Lag 6, P3 suite) with the same intrusion age (10.6 ± 0.1 Ma) are of the same age. Similarly, ages are revealed by #Lag 3 (6.5 ± 1.1 Ma; Tamargada monzonite, P3 suite), and #Lag 4 (5.6 ± 1.9 Ma; Tamargada syenite, P3 suite). Therefore, an average AFT age of 6.7 ± 1.6 Ma is calculated using the AFT ages of #Lag 3, 4, 5, and 6. A submarine hyaloclastite (#Lag 2) of 11.0 ± 0.2 Ma showed an AFT age of 8.9 ± 1.2 Ma. This age also correlates with the average age 6.7 ± 1.6 Ma of #Lag 3, 4, 5, and 6 within error. A trachyte dike (#Lag 9) with a formation age of 10.4 ± 0.2 Ma provided the youngest AFT age of 4.8 ± 0.7 Ma. The average D par size, range from 3.0 ± 0.4 to 1.8 ± 0.2 µm. Apatite grains of the trachyte breccia and the basic pillow lava show the larges etch-pits. Within error, all apatite grains of the amphibole gabbro, the syenite, the pegmatitic gabbro, and Tamargada monzonite are characterised by D par values above 2.0 µm. No relation between AFT ages and D par has been encountered. The elevation of the samples range between 430 m.a.s.l. and 3 m.a.s.l. Comparing all ages with the elevation did not provide any indication for an elevation dependency of the AFT ages.
Only two samples (#Lag 1, #Lag 3) revealed enough confined spontaneous fission-track length (CT, > 50) and etch pit size (D par ® ) data to perform numerical modelling of the t-T evolution showing D par ® values between 1.9 ± 0.2 and 2.9 ± 0.4 µm (average: 1.2 µm). Mean track lengths vary between 13.6 ± 1.7 µm (#Lag 3) and 14.4 ± 1.4 µm (#Lag 1). Lag 4 only revealed eight confined fission-tracks with an average fission-track length distribution of 12.8 ± 2.4 µm. In general, the skewness of the three CT-data is negative. The c-axes correction of the confined track lengths (Lc) led to a length distribution between 13.6 ± 2.3 and 15.2 ± 1.2 µm. Generally, Lc distributions showing longer lengths indicate a fast exhumation history. Even that these three samples provide a positive correlation between the average D par -value and the AFT central Age, including the other samples no correlation is in the data. According to Burtner et al. (1994), Donelick et al. (2005) and Barbarand et al. (2003), the large D par indicate Cl-rich apatite grains and short D par F-rich apatite grains.

Thermal history (t-T) modelling
Two samples, the pillow basalt (#Lag 1) and the Tamargada monzonite (#Lag 3) revealed enough confined fission-track length to perform a numerical modelling of the t-T evolution (Fig. 10). Pillow basalt: from a marine extrusion temperature at 11.5 ± 0.7 Ma the temperature increased to ~ 75 °C at 10.0 Ma. Thereafter, the temperature dropped rapidly reaching ~ 20 °C at ~ 9 Ma. Interesting enough the T increased again and reached a max. T of ~ 50 °C at 7 Ma. A second rapid decrease reaching surface T (20 °C) at about 6 Ma. The temperature kept at about 20 °C until recent.
Summarising the t-T evolution five major pattern is seen: • at ~ 10 Ma reaching a maximum temperature of ~ 75 °C. • at ~ 9 Ma rapid cooling to ~ 20 °C. • at ~ 7 Ma reaching a new maximum T of ~ 50 °C. • at ~ 6 Ma rapid decrease in T to surface temperature.
Tamargada monzonite: Following a fast cooling from more than 500-220 °C at ~ 9 Ma the t-T evolution gradual decrease reaching surface temperature recently.
Summarising the t-T evolution two major pattern are seen: • at ~ 9 Ma rapid cooling from ~ 500 to 220 °C. • gradual decrease of T to recent surface temperature.

Northwest Basal Complex
The "Northwest Basal Complex" comprises Upper Oligocene carbonatites, pyroxenites, Ijolites, and syenites, as well as layered and coarse-grained gabbros. The formation ages of carbonatites, pyroxenites, Ijolites, and Syenites range between 28.3 ± 0.2 Ma and 27.2 ± 0.4 Ma. Zircon and apatite fission-track ages are partly from the same samples displayed in a time-temperature diagram show a cooling path between formation age and ~ 19 Ma (Fig. 4). Thereafter, the complex must have been reheated to temperatures between 60 and 110 °C depending on the length of the heating period. The information that the Northwest Basal Complex has been at or near the surface at about 19 Ma was given by literature and the recent occurrence of relict volcanic rocks younger than 19 Ma partly overlying the complex. More detailed information is provided by the numerical modelling.
The modelled t-T evolution of the samples revealed a moderate cooling from ~ 500 to ~ 280 °C between the formation age and ~ 20 Ma (Fig. 5). A significant cooling occurred instantaneous around 20 Ma and rock temperature decreased from 280 to ~ 20 °C. Stillman (1999) proposed a giant landslide earlier than 18.3 Ma. Acosta et al. (2003) attributed the Puerto Rosario debris Avalanche offshore to have been caused by giant landslides older than 17.5 Ma (Fig. 1). Therefore, we assume that the rapid cooling path most likely reflects the rapid cooling caused by the instantaneous denudation of the large northern volcanic edifice forming part of the Puerto Rosario debris Avalanche offshore.
Following the rapid cooling at 20 Ma, the t-T evolution indicates a reheating from surface temperature to ~ 100 °C. Such an increase in temperature possibly implies the thermal influence of a newly formed volcanic edifice (the Northern Volcanic Complex) that superimposed the earlier rock formations. New volcanic activity might also increase the geothermal gradient. Therefore, the height of the new volcanic edifice cannot be calculated. Considering the published formation ages of the deposited volcanic rocks of the Northern Volcanic Complex (NCV), the NVC I with ages between 18.7 ± 0.3 and 15.3 ± 1.3 Ma and the volcanic rocks of the NCV-II with an av. age of 13.6 ± 0.8 Ma, would explain the increase in temperature. The apatite fission-track ages of av. 18.7 ± 2.4 Ma and 14.5 ± 2.9 Ma fall within this time interval, in general. Also, the AFT age (17.8 ± 5.0 Ma) of the Tindaya trachyte dome, which are within error the same as the published formation age falls within the time of reheating of the intrusive rocks. From ~ 14 Myrs on a slow cooling to 20 °C indicate a low erosion or/and decrease of the geothermal gradient leading to the recent surface exposure of the "Northwest Basal Complex".

West-Central Basal Complex
Lower Cretaceous sandstones, Upper Oligocene to Lower Miocene intrusions, basic dikes of a sheeted dike swarm, and various dikes represent the lithological units of the "West-Central Basal Complex". The formation of the sandstones range between 137 and 112 Ma and of the magmatic rocks Fig. 6 Thermal history models of Lower Cretaceous sandstone samples from the "West-Central Basal Complex" modelled using the software code HeFTy (Ketcham et al. 2007a(Ketcham et al. , b, 2009. The left column displays the t-T paths, the right column displays the helium diffusion profile. For further information, please see Fig. 4 ◂ 1 3 Fig. 7 Thermal history models of samples from the "West-Central Basal Complex" modelled using the software code HeFTy (Ketcham et al. 2007a(Ketcham et al. , b, 2009. The left column displays the t-T paths, the right column displays the helium diffusion profile. For further information, please see Fig. 4 range between 26.2 ± 0.2 and 17 Ma (Fig. 4). Zircon and apatite fission-track and (U-Th)/He ages partly from single magmatic samples displayed in a time-temperature diagram show a cooling path between formation age and ~ 19 Ma (Fig. 4). Similarly, after a subsidence period reaching around 300 °C the sandstones cooled slowly to 200 °C at ~ 19 Ma and, thereafter, instantaneously to near surface temperatures. Towards younger time, the complex must have been reheated to temperatures between 60 and 110 °C depending on the length of the heating period. The information that the West-Central Basal Complex has been at or near the surface at about 19 Ma was given by literature and the recent occurrence of relict volcanic rocks younger than 19 Ma partly overlying the complex. More detailed information is provided by the numerical modelling.
The temperature evolution of the sandstones reached ~ 300 °C or more at ~ 55 Ma (Fig. 6). Such high temperature agrees with the published metamorphic grade of the Lower Cretaceous sedimentary rock series (Steiner et al. 1998). They termed that the sedimentary rock sequence was affected by thermal low greenschist grade to intermediate greenschist grade, and interpreted the thermal metamorphism as caused by the Oligocene to Miocene various intrusions and the sheeted dike swarm. Therefore, reaching the Fig. 8 Thermal history models of samples from the "East-Central Basal Complex" modelled using the software code HeFTy (Ketcham et al. 2007a(Ketcham et al. , b, 2009. The left column displays the t-T paths, the right column displays the helium diffusion profile. For further information, please see Fig. 4 1 3 high temperature at ~ 55 Ma might be related to the start of the submarine growth stage of Fuerteventura with increased magmatic activity. The temperature decreased slowly reaching ~ 250 °C at 20 Ma. Thereafter, the temperature instantaneously decreased to a near surface temperature. Similarly, the t-T history of the carbonatite and the two basic dikes indicate a rapid cooling from formation temperature to near surface temperature at ~ 20 Ma (Fig. 7). Such a rapid temperature decrease occurred at a similar time in the "Northwest Basal Complex". It seems to be likely, that the causes are similar. The rapid decrease in temperature might have been caused by the movement of a giant landslide leading to an instantaneous denudation of the volcanic edifice on top of the "Basal Complexes" and, therefore, to an instantaneous drop in temperature. We cannot exclude but also, we cannot assume that the giant landslide causing instantaneous erosion in the "Northwest Basal Complex" area and the giant landslide causing instantaneous erosion in the "West-Central Basal Complex" has occurred at the same time. It could also be within error of the dating and numerical modelling that the two giant landslide events occur separately. Part of the samples slowly cooled to recent surface temperatures. Whereas other samples were heated to a new temperature maximum between ~ 80 and ~ 115 °C at ~ 14 Ma and cooled rapidly to near surface temperature, thereafter. The T-increase is assumed to be related to the next generation (CVC I-III) of massive volcanic activity causing the formation of a new volcanic edifice (Central Volcanic Complex). The rapid cooling at ~ 14 Ma might also be related to a new formation of a landslide. The Pliocene evolution is documented in two NE-SW-trending trachytic dikes crossing the Upper Jurassic to Cretaceous sedimentary rocks of the "West-Central Basal Complex". However, as Fig. 10 Thermal history models of samples from La Gomera modelled using the software code HeFTy (Ketcham et al. 2007a(Ketcham et al. , b, 2009. The left column displays the t-T paths, the right column displays the helium diffusion profile. Lag 1 are a sample of pillow lavas extruded to the ocean. For further information, please see Fig. 4 1 3 noted above, these dikes are located close (tenth of metre) to a large Pliocene basaltic flow of the Series II. They might have been thermally influenced by the basaltic flow. Therefore, it cannot be excluded that the age might represent a reheating and, thereafter, fast cooling caused by the Pliocene basaltic flow.

East-Central Basal Complex
Miocene syenite and gabbro intrusions (("Vega de Rio Palmas Ring Complex"; Betancuria Complex) and a trachyte represent the analysed lithological units of the "East-Central Basal Complex". The formation age of the one syenite and one gabbro intrusion ("Vega de Rio Palmas Ring Complex") is 18.7 ± 0.8 Ma and 18.4 ± 0.3 Ma, respectively. Zircon and apatite fissiontrack and (U-Th)/He ages partly from single magmatic samples displayed in a time-temperature diagram show a rapid cooling path between formation age and ~ 18 Ma (Fig. 4). This decrease in temperature might be related to the crystallisation and cooling of the magma. Up to a temperature of about 75 °C the t-T path cools gradually and is followed by a slow cooling to surface temperature. More detailed information is provided by the numerical modelling.
The numerically modelled t-T path is constrained by the formation age, the thermochronological ages and the request for a near surface occurrence at ~ 16 Ma. Gabbros and syenites of the "Vega de Rio Palmas Ring Complex" are overlain by volcanic rocks of younger age. Both t-T path shows cooling from intrusion temperature to near surface temperatures between 18 and 16-15 Ma. In both cases, an increase in temperature is possible before the t-T path reached surface temperature at ~ 10-9 Ma.
Summarising, the giant landslide at ~ 20 to ~ 19 Ma is attributed to have led to the deposition of the offshore Puerto Rosario debris Avalanche that covers an area of about 3500 km 2 (Stillman 1999;Acosta et al. 2003; Fig. 1). The formation age of the Puerto Rosario debris Avalanche was provided by Acosta et al. (2003) with older than 17.5 Ma. Nevertheless, it remains highly uncertain whether only one giant landslide resulted in the formation of the debris avalanche unit, or it reflects a product of several repeatedly deposited debris flows (Acosta et al. 2003). The one of the "East-Central Basal Complex" at ~ 16 Ma to ~ 15 Ma might be of local origin. If the mass movement did not reach the ocean, it could be that the increase in temperature at that time in the "West-Central Basal complex" is related to the deposition of the landslide mass on top of the volcanic rocks of the "West-Central Basal Complex".
What might have caused the initiation of the two landslides? The movement at ~ 20 Ma to ~ 19 Ma and at ~ 16 Ma to ~ 15 Ma might have been triggered by the new volcanic activity forming the North Volcanic and the Central Volcanic Complex (Coello et al. 1992;Ancochea et al. 1996). Therefore, the landslide might have been triggered by new volcanic activity on the Island. In addition, the tectonic activity that affected the Miocene Volcanic Complexes of Fuerteventura (Gutiérrez et al. 2006;Fernández et al. 2006) could have co-helped the formation of these large gravitational slides. A third possibility might be the change in climate leading to increase in seasonal rain. As within the Miocene two climate optimums (Zachos et al. 2001(Zachos et al. , 2008 exist, we cannot rule out the influence of climate change on the formation of the landslides. According to Hendriks et al. (2020) and Steinthorsdottir et al. (2021), the sea surface temperature in the region of the Canary Islands dropped rapidly at about 19 Ma. Therefore, we do not exclude an influence by climate change on the formation of the giant landslides.

La Gomera Island
The Basal Complex in La Gomera Island comprises Middle to Upper Miocene pillow basalts, submarine basaltic and trachytic hyaloclastite, amphibole gabbros, trachyte, amphibole pyroxenites, syenites, monzonites, and pegmatitic gabbros. The formation ages range between 12.1 ± 0.1 and 8.9 ± 0.1 Ma. As the formation age (11.5 ± 0.7 Ma) and the AFT age (10.7 ± 0.9 Ma) of the pillow basalt (#Lag 1) are the same within error, it is assumed that the AFT age does not represent a reheating event but displays the formation of the pillow basalt (Fig. 9). Nevertheless, we did used the numerical modelling to test if the difference of ages could have been caused by a reheating event (Fig. 10). The results indicate that the data set could be numerically modelled with a reheating event. The numerical modelling presented the result of a possible two reheating event. To decide between the two possibilities, age data with less error are needed. The sample representing the submarine trachytic breccia (#Lag 2) has an AFT age (8.9 ± 1.2 Ma) that is younger than the age range covered by the error of the formation age (10.98 ± 0.08 Ma). Therefore, it might be possible that the rock suite was thermally altered after the deposition either by an increase in heat flow, by overlain rocks or by both processes. The sample was taken from a unit that is located close to the Tazo landslide plane. According to Anchochea et al. (2006) and Casillas et al. (2010Casillas et al. ( , 2011, the Tazo giant landslides have moved large masses of volcanic material in short time in the north-western sector of La Gomera. The movement of the Tazo landslide (Figs. 1, 2) has been dated at ~ 9.4 Ma, which is between the formation age of the submarine trachytic breccia, and the AFT age. Therefore, it could be possible that not all landslide material was moved into the ocean (Fig. 1) but was partly deposited above the submarine trachytic breccia, which would explain the younger apatite fission-track age.
All samples of P1 suite, P2 suite and P3 suite are in the area where the morphology has significant changed at ~ 8.0 Ma by the occurrence of the Garajonay caldera collapse followed by a fast removal of rocks (Paris et al. 2005;Rodriguez-Losadaa and Martinez-Frias 2004). According to Paris et al. (2005), we assumed that the Garajonay landslide caused the unravel of the rocks that were analysed in this study by moving a large volume of volcanic rocks (Fig. 9). Unfortunately, the ZFT-and AFT ages of the magmatic rocks taken from the "Basal complex" P1 suite are not from the same sample. The age difference between the formation age of the amphibole gabbro (#Lag 7, 10.6 ± 0.1 Ma), the amphibole pyroxenite (#Lag 10, 12.1 ± 0.1 Ma), and the trachytic dike (#Lag 11, 10.7 ± 0.1 Ma) and the ZFT ages (#Lag 7, 9.0 ± 0.5 Ma; #Lag 10, 9.5 ± 0.5 Ma; #Lag 11, 9.2 ± 0.6 Ma) indicate a differentiated fast cooling of the rocks from about 500 °C to about 300 °C. Thereafter, the rocks might have moved directly to the surface at ~ 8.0 Ma. One amphibole pyroxenite (#Lag 6; Form. age: 10.6 ± 0.1 Ma) and one trachytic dike (#Lag 9; Form. age: 10.4 ± 0.2 Ma) revealed only AFT ages (6.9 ± 1.8 Ma, 4.8 ± 0.7 Ma, respectively) but have within error similar formation ages as the samples #Lag 7 and #Lag 11.
The Lower Old Edifice P2 suite is represented by two samples a syenite dike (#Lag 5), and a pegmatitic gabbro (#Lag 8). The sample #Lag 8 is located close to the Garajonay landslide, whereas the sample #Lag 5 is further away. The temperature of sample #Lag 5 decreased gradually from 500 °C at 10.6 ± 0.1 Ma to 300 °C at 8.6 ± 0.5 Ma, and 110 °C at 7.6 ± 1.6 Ma (Fig. 9). We cannot exclude that the AFT age represents a reheating by post-Garajonay landslide intrusions or volcanic activity. In Fig. 9, we assumed a steady cooling path from high temperature to surface in a very short time. The thermochronological data of #Lag 8 revealed indicate a fast cooling from formation temperature (500 °C) at 10.8 ± 0.1 Ma to 110 °C at 12.9 ± 3.1 Ma. Within error, the formation age and the thermochronological age is the same. The Upper Old Edifice is represented by one monzonite sample (#Lag 3) and one syenite sample (#Lag 4) of the Tamargada Plutonic Complex. The formation age of the Tamargada monzonite (9.1 ± 0.3 Ma) and the Tamargada syenite (8.9 ± 0.1 Ma) are within error the same. Considering the thermochronological age distribution the monzonite and the syenite cooled fast from 500 °C at formation temperature to 300 °C at 9.5 ± 0.6 Ma and 8.3 ± 0.7 Ma and, thereafter, to surface temperatures at ~ 8.0 Ma (Fig. 9). The apatite fission-track ages (~ 110 °C) are much younger (6.5 ± 1.1 Ma; 5.6 ± 1.9 Ma) but within error are the same for the monzonite and the syenite indicating a new temperature increase of the intrusive rocks. More detailed information is gained by the numerical modelling (Fig. 10). We performed a numerical modelling with and without the occurrence of the Garajonay landslide. The results indicate that both t-T evolution paths are possible. The rapid cooling of the monzonite extends from ~ 500 to ~ 200 °C at ~ 9.0 Ma. Thereafter, the t-T path changed to an intermediate degree of cooling with a slight break at about 6 Ma.
Summarising the former results: the increase in temperature after formation of the pillow basalt might be related to the deposition of about 1.000 m of volcanic rocks above the pillow basalts. The calculation of the height of the volcanic edifice depends on the assumption of general geothermal gradient at about 10 Ma. Therefore, the thickness of the volcanic pile could have been less or more than 1000 m. In addition, Demény et al. (2010) showed that the green schist facies metamorphism of the Basal Complex of La Gomera was induced by the interaction with meteoric water. The inferred isotopic compositions of the meteoric water indicate that the water infiltrated the rocky building at an elevation of approximately 1500 m above sea level, suggesting the existence of a subaerial volcano that was formed during the intrusive activity and that has been denudated or remains buried by subsequent volcanic events and landslides. Therefore, the thickness of the volcanic pile could have been even more than 1000 m.
As mentioned by Anchochea et al. (2006) and Casillas et al. (2010Casillas et al. ( , 2011, giant landslides such as the Tazo landslide in the north-western sector of La Gomera have moved large masses of volcanic material in short time, which was followed by longer times of stagnation. The movement of the Tazo landslide has been dated at ~ 9.4 Ma. This timing of mass movement falls together with the rapid cooling of the pillow basalt in the NW, and the intrusion of the Tamargada monzonite (8.9 ± 0.1 Ma) and syenite (9.1 ± 0.3 Ma). Casillas et al. (2010Casillas et al. ( , 2011 and Fernández et al. (2015) relate the triggering of the Tazo landslide with the movement of the Guillama and Montaña de Alcalá faults, under the Lower Old edifice (LOE). However, it appears that the intrusion of monzonitic or syngenetic magma could also have co-assisted the occurrence of the Tazo landslide. The Tazo landslide possibly caused the offshore occurrence of the submarine debris avalanche (Segments I and VIII; Fig. 1). According to Anchochea et al. (2006), the Lower Old Edifice (LOE) formed during that time indicating a major phase of volcanic activity and, therefore, a high heat flow resulting in a high geothermal gradient. A height of up to 1900 m for the Lower Old Edifice volcano with a location in the general area of the Tamargada intrusive complex was discussed. Considering the published data, we assume a geothermal gradient of about 100 °C for rocks surrounding the monzonite. As motioned before, we cannot exclude the influence of climate change on the formation of the Tazo landslide. Böhme et al. (2003Böhme et al. ( , 2008Böhme et al. ( , 2011 and Henderiks et al. (2020) discuss a significant increase in humidity at about 9.0 Ma. It might be that the tectonic activity described by Casillas et al. (2010) and Fernández et al. (2015) at the Guillama and Montaña de Alcalá faults combined with the intrusion of the Tamargada monzonite/syenite, and a higher precipitation have triggered the development of the Tazo landslide. Considering the occurrence of the Garajonay landslide at ~ 8.0 Ma, the samples of the P2 suite cooled to near surface temperatures. The t-T path between 8.0 Ma and recent is not constrained by thermochronological data. In contrast, the samples of the P1 suite and P3 suite indicate a temperature increase after the landslide occurred. This temperature increase could have been caused by new volcanic activity. Our data do not provide any evidence on what kind of process triggered the occurrence of the Garajonay landslide. If the movement of the Garajonay landslide occurred earlier, it might be related to the intrusion of the Tamargada monzonite and syenite.

Conclusion
The decay of volcanos on Volcanic Islands such as La Gomera and Fuerteventura is often related to the movement of giant landslides causing the formation of submarine debris avalanches, and eventually Tsunamis. Dating the movement of such landslides would increase the understanding of the possible causes that trigger the movement, and furthermore might provide a hint to Tsunami formation over time. Younger formation of landslides can be dated by luminescence or electron spin resonance dating. If organic particles are involved even 14 C-dating might be a technique to reveal the age of the movement. However, all this dating techniques can only be applied to an age range between recent and about 2 Ma (electron spin resonance dating). If the sliding plan has been heated to a temperature that generated a melt and K-minerals are crystallised out of the melt on the plane surface K-Ar or even 40 Ar/ 39 Ar-dating could be applied. Low-temperature thermochronological dating techniques such as fissiontrack dating and (U-Th)/He dating have the advantage to reveal a t-T cooling/exhumation path of magmatic rocks that were below a certain pile of volcanic rocks before the instantaneous movement of the overlain rock pile. Therefore, this study tested the possibility of dating landslides by thermochronological tools using the cooling/exhumation history of the magmatic rocks. Our research could provide evidence that the proposed giant landslide (Stillman 1999) leading to destruction of the Northern Volcanic Complex and the central Volcanic complex occurred at ~ 20 Ma on Fuerteventura. Furthermore, we identified a possible second movement of volcanic rocks at about 16 Ma in the "East-Central Basal Complex" of Fuerteventura. Two landslides are described for La Gomera. The Tazo landslide formed at ~ 9.4 Ma (Casillas et al. , 2011 and the Garajonay landslide formed at ~ 8.0 Ma (Paris et al. 2005;Rodriguez-Losadaa and Martinez-Frias 2004). The thermochronological data presented in this paper provide evidence for the movement of both landslides. Summarising the research, it is possible to date landslides on volcanic Islands with thermochronological data. Furthermore, if the palaeo geothermal gradient is known, a numerically gained cooling path can be transferred into the thickness of the rock mass moved by the landslide.