Downloaded from geology.gsapubs.org on March 1, 2010 Flow of Canary material through a subcontinental lithospheric corridor beneath Africa to the Mediterranean

S. Duggen1,2*, K.A. Hoernle1, F. Hauff1, A. Klügel3, M. Bouabdellah4, M.F. Thirlwall2 1IFM-GEOMAR, Leibniz Institute of Marine Sciences, Research Division Dynamics of the Ocean Floor, Wischhofstrasse 1-3, 24148 Kiel, Germany 2Department of Earth Sciences, Royal Holloway University of London, Egham, Surrey TW20 OEX, UK 3Universität Bremen, Fachbereich 5, Geowissenschaften, Postfach 33 04 40, 28334 Bremen, Germany 4Department of Geology, Faculty of Sciences, B.P. 524, 60000 Oujda, Morocco

ABSTRACT the passive continental margin near the Canary We present geochemical data of lavas from northwest Africa, allowing us for the fi rst time Islands to at least the Middle Atlas (Fig. 1). to carry out large-scale “mapping” of sublithospheric mantle fl ow beneath the northwest Afri- The geodynamic evolution of the western- can plate. Our study indicates that Canary mantle plume material traveled laterally along a most Mediterranean is complex, but Miocene subcontinental lithospheric corridor (i.e., at depths that are usually occupied by continental subduction rollback of old Tethys oceanic litho- lithospheric mantle) more than 1500 km to the western Mediterranean, marking its route sphere, causing progressive delamination (peel- over the last 15 m.y. through a trail of . A three-dimensional geodynamic ing off) of subcontinental lithospheric mantle reconstruction, integrating results from geophysical studies, illustrates that long-distance ca. 8–10 Ma (continental-edge delamination), lateral fl ow of mantle material into and through a subcontinental lithospheric corridor can affected both the lithospheric thickness and be caused by a combination of (1) defl ection of upwelling plume material along the base of mantle fl ow beneath northern Morocco (Duggen the lithosphere, (2) delamination of subcontinental mantle lithosphere beneath northwest et al., 2003, 2005; Gutscher et al., 2002). Africa, and (3) subduction suction related to the rollback of the subducting oceanic plate in The geochemical composition of lavas the western Mediterranean. Although the fl ow of plume material beneath oceanic lithosphere from the northwest can provide to mid-ocean ridges or along the base of continental rifts has been previously shown, this study important information about their sources and demonstrates that plume material can also fl ow large lateral distances through subcontinental processes in the upper mantle. We present new corridors from suboceanic to nonrifting subcontinental settings, generating continental intra- major and trace element and Sr-Nd-Pb-isotopic plate volcanism without the need for a plume to be located directly beneath the continent. data from mafi c Middle Atlas lavas (Tables DR1 and DR2 and details about analytical methods INTRODUCTION In northwest Africa, mafi c Neogene intraplate are found in the GSA Data Repository1) and The oceanic and continental parts of the volcanism is found in the Atlas system and along integrate these with published geochemical northwest African plate host areas with Neo- the Mediterranean coast (volcanic fi elds Siroua, and geophysical results from other (volcanic) gene igneous activity (Lustrino and Wilson, Saghro, Middle Atlas, Guilliz, Gourougou , and areas on the northwest African plate. This study 2007), linked to different geodynamic Oujda-Oranie; Fig. 1A) (Lustrino and Wilson, allows the northwest African upper mantle com- processes . In the eastern Atlantic Ocean, 2007). The Atlas Mountains extend from the position to be mapped for the fi rst time and pro- volcanism is associated with passive Atlantic-African continental margin vides new constraints for mantle fl ow and the well-resolved low P- and S-wave seismic adjacent to the Canary Islands to the Mediter- origin of continental intraplate volcanism. tomographic velocity anomalies, extending to ranean coast and represent an inverted, failed the core-mantle boundary (see Montelli et al., rift system, formed during the opening of the RESULTS AND DISCUSSION 2006, including their summary of other seis- central Atlantic in the early Mesozoic. Over Composition of the Upper Mantle Beneath mic tomographic models). The Canary Islands the past 45 m.y., the former rift was deformed the Northwest African Plate and seamounts to the northeast have intraplate due to compression during the collision of the Mafi c lavas with intraplate geochemical sig- geochemical compositions and form a volcanic African and European plates (Gomez et al., natures on the northwest African plate are inter- chain with a northeast to southwest age pro- 2000). Removal (delamination) of subcontinental preted to be derived from sublithospheric mantle gression over the last 70 m.y. (Fig. 1A) (Geld- mantle lithosphere is proposed to result from the sources by low-degree partial melting (Duggen macher et al., 2005; Hoernle and Schmincke, collision of these major plates and to be respon- et al., 2005; El Azzouzi et al., 1999; Geldmacher 1993; Hoernle et al., 1991; Lundstrom et al., sible for intermediate-depth earthquakes and et al., 2005; Hoernle and Schmincke, 1993; 2003). The age progression can be explained volcanism in the Atlas system (Ramdani, 1998). Hoernle et al., 1991, 1995; Lundstrom et al., by movement of the lithosphere above a deeper Elevated topography, high heat fl ow, and 2003; Lustrino and Wilson, 2007). Mafi c igne- upwelling using the same Euler pole and angu- gravity and geoid anomalies point to the exis- ous rocks from the Middle Atlas have trace ele- lar plate velocity as other volcanic chains in the tence of an abnormally shallow lithosphere- ment patterns similar to those from the Canary Atlantic Ocean. Although other models have asthenosphere boundary beneath the western Islands (Fig. 2) and show a similar range of ele- been proposed to explain the Canary Islands part of the Atlas system (60–80 km, compared vated Dy/Yb and Zr/Hf ratios and high FeO but volcanism (e.g., see summaries in Anguita to 130–160 km for normal northwest Afri- relatively low SiO2. For the Middle Atlas and the and Hernan, 2000, and Lustrino and Wilson, can lithospheric thickness), as illustrated on Canary Islands, respectively, Dy/Yb = 2.8–3.4 2007), the plume hypothesis can best explain six lithospheric profi les across northwest Africa the geophysical, geochemical, and geological (Urchulutegui et al., 2006; Missenard et al., 1GSA Data Repository item 2009071, Tables DR1 constraints (Geldmacher et al., 2005; Hoernle 2006; Teixell et al., 2005) (Fig. 1). The region of (major and trace element data) and DR2 (Sr-Nd-Pb- and Schmincke, 1993; Hoernle et al., 1991; abnormally thin lithosphere forms a northeast- isotope data), and details about analytical methods, is available online at www.geosociety.org/pubs/ft2009. Lundstrom et al., 2003). striking subcontinental lithospheric corridor htm, or on request from [email protected] or beneath northwest Africa that is ~80–120 km Documents Secretary, GSA, P.O. Box 9140, Boulder, *E-mail: [email protected]. high and ~200–300 km wide and extends from CO 80301, USA.

© 2009 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, March March 2009; 2009 v. 37; no. 3; p. 283–286; doi: 10.1130/G25426A.1; 3 fi gures; Data Repository item 2009071. 283 Downloaded from geology.gsapubs.org on March 1, 2010 A 5° W 0.705 NW African lithosphere 10° W W. Mediterranean 35° N A Gourougou Oujda-Oranie 15° W Guilliz Canary track 250 km 30° N 68 Ma 55 Ma Atlas Middle system Atlas F 47 Ma E 0.704 30 Ma >17 Ma Saghro Agadir Siroua Gou-

D Sr Canary Islands Lanzarote C W. Mediterranean >15 Ma B rougou Oujda-Oranie 4 Ma 12 Ma A 86 n subcont. Gourougou NW Africa tion) Mid 11 Ma 15 Ma 25 Ma r (projec F 1 Ma lithospheric corrido Atlasdle Guilliz Guilliz HIMU E Sr/ Middle

D 87 C Sag 0.703 Atlas 100 km B S hro Current Changing A iroua oceanic lithosph B Su Atlantic Canary plume center lithospheric thickness Islands 50 km 400 km bducted slab MORB

150150 km km 0.702 dor . corri 18 19 20 21 hosph ont. lit 206 204 an subc NW Afric Pb/ Pb HIMU

ere of 15.7 NW B African lithos- Oujda-Oranie Canary Delaminated Slab rollback and phere Atlas subcontinental continental-edge delamination Pb Gou- mantle plume Middle

lithosphere causing subduction suction 204 rougou 15.6 Atlas (70–0 Ma) (since 45–25 Ma) (since ca. 10–8 Ma)

Pb/ Guilliz

Figure 1. Map of the northwest African plate (A) and fl ow of Canary mantle plume material 207 Atlantic Canary MORB Islands under northwest Africa through a subcontinental lithospheric corridor in a three-dimensional model (B). A: The orange area displays the Canary hotspot track on the oceanic side of the northwest African plate with ages of the oldest lavas from each island (red areas) or sea- 15.5 NHRL mount (gray circles), indicating a southwest-directed age progression and the location of 18 19 20 21 206Pb/204Pb the current plume center beneath the western Canary Islands (Geldmacher et al., 2005). Also 15 shown are the Atlas Mountains (gray fi eld), location of the northwest African subcontinen- NW African lithosphere C tal lithospheric corridor in green, inferred from profi les (A–F) based on geophysical data Lower Shoshonites (Urchulutegui et al., 2006; Missenard et al., 2006; Teixell et al., 2005), and northwest African 10 crust Neogene continental intraplate volcanic fi elds. B: The three-dimensional model illustrates how Canary mantle plume material fl ows along the base of the oceanic lithosphere that Gourougou thins to the east (Neumann et al., 1995) and into the subcontinental lithospheric corridor be- 7/4 Pb 5 Δ neath the Atlas system, reaching the western Mediterranean. Plume push, eastward-thinning Guilliz Oujda-Oranie lithosphere, delamination of northwest African subcontinental lithosphere, and subduction 0 NHRL suction related to rollback of the subducting slab in the Mediterranean are proposed to be Canary the main mechanisms for causing Canary plume material to fl ow ≥1500 km to the northeast. Islands –5 0 1020304050 and 2.8–3.6, Zr/Hf = 37–56 and 34–49, 1000 La/Yb FeO = 11.1%–12.1% and 10.9%–14.2%, Figure 3. Geochemical diagrams with radio- and SiO2 = 38.4%–45.5% and 36.9%–49.8% genic isotope and trace element ratios, (Hoernle and Schmincke, 1993; Lundstrom 100 illustrating variable mixing between sub- et al., 2003) (Table DR1). Combined with petro- lithospheric melts with northwest African continental lithospheric material. Mafic logical melting experiments, these data argue 10 intraplate lavas from the Middle Atlas vol- for the presence of garnet-peridotite and/or canic fi eld (this study) and from the Guilliz Basanites and alkali basalts -pyroxenite in their mantle source(s) and melting Sample/primitive mantle Middle Atlas and Gourougou volcanic fields (Duggen depths on the order of 60–120 km (Hirschmann Canary Islands et al., 2005) point to a sublithospheric mantle 1 source similar to that of the Canary Islands and Stolper, 1996; Kogiso et al., 1998; Lund- Cs Rb Th Nb K Ce Pr P Sm Hf Ti Tb Li Ho Tm Lu Tl Ba U Ta La Pb Sr Nd Zr Eu Gd Dy Y Er Yb volcanism (Canary mantle plume material). strom et al., 2003), i.e., at depths that are nor- Oujda-Oranie intraplate lavas indicate a mally occupied by subcontinental lithosphere. Figure 2. Incompatible trace element dia- similar African lithospheric contaminant gram showing the similarities in incompat- but were derived from a different sublitho- Some northwest African sublithospheric melts ible trace element concentrations of mafi c have interacted with the overlying lithosphere spheric mantle source and don’t appear to lavas from the Middle Atlas (this study) and involve Canary plume material. The isotopic (Duggen et al., 2005; El Azzouzi et al., 1999; the Canary Islands (fi eld defi ned by n = 36 composition of the mafi c lavas are age- Lundstrom et al., 2003). Isotope (Sr-Nd-Pb) and basanites to alkali basalts with MgO > 8 wt% corrected values (e.g., 12 Ma for three lavas trace element ratios (e.g., La/Yb and K/La) of [Lundstrom et al., 2003] normalized to from the southwestern Middle Atlas vol- primitive mantle composition [Sun and canic fi eld and 1 Ma for lavas for the remain- the northwest African intraplate lavas form cor- McDonough, 1989]). 87 86 Δ der according to Harmand and Cantagrel , relations converging at elevated Sr/ Sr, 7/4Pb, 1984, otherwise as found in the cited lit- 208 204 Pb/ Pb, and La/Yb ratios (e.g., Figs. 3A–3C). erature). Other data sources are intraplate These systematics can be explained by inter action mantle source from each volcanic area on the mafi c lavas from the Canary Islands (Lund- of sublithospheric melts, characterized by rela- northwest African plate. The least-contaminated strom et al., 2003; Widom et al., 1999) and tively high 206Pb/204Pb but low 87Sr/86Sr, Δ7/4Pb lavas (low 87Sr/86Sr, low Δ7/4Pb) of the Middle the continental volcanic fi elds Gourougou, Guilliz, and Oujda-Oranie (Duggen et al., and La/Yb ratios, with northwest African litho- Atlas, Guilliz, and Gourougou volcanic fi elds 2005); Moroccan (Gourougou) shoshonites sphere, including metasomatized lithospheric overlap with the fi eld of Canary Islands lavas. (Duggen et al., 2005); Middle Atlas lower mantle or melts therefrom (e.g., shoshonitic com- The similarity in trace element and radiogenic crustal granulites (La/Yb from Moukadiri ponents) and the lower crust (e.g., Atlas lower isotopic composition of the least-contaminated and Pin, 1998, Δ7/4Pb estimated); Δ7/4 is the vertical deviation from the Northern crustal granulites) (Fig. 3). lavas from the northwest African continent Hemisphere Reference Line (NHRL) (Hart, The mixing trends also provide important con- (Figs. 2 and 3) suggests that these lavas share 1984); high time-integrated 238U/204Pb (HIMU) straints on the composition of the sublithospheric a common sublithospheric mantle source with (Zindler and Hart, 1986).

284 GEOLOGY, March 2009 Downloaded from geology.gsapubs.org on March 1, 2010 lavas from the oceanic Canary Islands. Mafi c (Lanzarote) is thinner than beneath the more from the Middle Atlas area toward the western- lavas from the Oujda-Oranie volcanic fi eld due western islands based on petrological data from most Mediterranean (Fig. 1B). Horizontal fl ow east of Guilliz and Gourougou, however, defi ne mantle xenoliths (Neumann et al., 1995). It has of the Canary plume material beneath northwest an independent mixing trend. The mixing trend also been proposed that Canary plume material Africa is likely to have terminated beneath the indicates that the lithospheric contaminant is the fl ows toward the northwest African continen- Gourougou volcanic fi eld adjacent to the Medi- same but that a different sublithospheric mantle tal margin along the base of the lithosphere terranean coast, since Miocene volcanism in the source with a more radiogenic Pb-isotopic following the hotspot track, possibly within a westernmost Mediterranean basin has a geo- composition (Fig. 3) underlies Oujda-Oranie, corridor formed by thermal erosion of the base chemical signature typical of subduction zone constraining the eastern boundary of the Canary- of the oceanic lithosphere as it moved over the lavas, and intraplate volcanism in this area is type mantle. plume (Geldmacher et al., 2005; Hoernle and absent (Duggen et al., 2008). As noted above, Schmincke, 1993; Hoernle et al., 1991; Lund- mafi c lavas from the nearby Oujda-Oranie vol- Evidence for Flow of Canary strom et al., 2003). Plume push in combination canic fi eld, which do not lie above the subconti- Mantle Plume Material Through a with eastward thinning of the oceanic lithosphere nental lithospheric corridor, do not show a geo- Subcontinental Lithospheric Corridor can explain the fl ow of mantle material from the chemical affi nity to the Canary lavas (Fig. 3), Beneath Northwest Africa plume center, located under the western Canary indicating that they are not derived from Canary Although the geochemical data indicate a Islands, to beneath the eastern Canary Islands plume material. common sublithospheric mantle source for the and into the southwestern end of the subconti- It has been shown that fl ow of mantle plume Neogene lavas from the Canary Islands, Middle nental lithospheric corridor (Fig. 1). material can take place beneath oceanic litho- Atlas, Guilliz, and Gourougou areas, the continen- Second, delamination of subcontinental litho- sphere to mid-ocean ridges (Schilling, 1973) tal northwest African volcanic fi elds are not part sphere beneath the northwest African Atlas sys- or along the base of continental rifts (Ebinger of the Canary hotspot track. The age-progressive tem (Ramdani, 1998) is likely to be responsible and Sleep, 1998). Our study demonstrates that hotspot track is restricted to the oceanic part of for the existence of the subcontinental litho- plume material can fl ow large lateral distances the northwest African plate and is associated spheric corridor (Fig. 1B). Delamination could from suboceanic to nonrifting subcontinental with a major mantle upwelling, currently located have been caused by the inferred shortening of settings through subcontinental corridors also, beneath the western Canary Islands at the south- 17%–45% and thickening of a lithospheric root, causing continental intraplate volcanism with- western end of the age-progressive hotspot track resulting from the African-European collision out the need for a plume to be located directly (Fig. 1). Although older seismic tomographic (since the early Miocene and most likely since beneath the continent. studies suggested that mantle upwelling beneath ca. 45 Ma) (Gomez et al., 2000). Since the Atlas the easternmost central Atlantic may also extend Mountains comprise an inverted failed rift sys- Anomalously Voluminous Volcanic Events beneath the western Mediterranean and Europe tem (Gomez et al., 2000), we propose that Meso- on the Northwest African Plate and Other (Hoernle et al., 1995), newer studies show that zoic rifting may have weakened the Atlas mantle Surface Expressions of the Mantle Processes well-resolved positive P- and S-wave seismic litho sphere, making it more susceptible to delami- The Middle Atlas hosts the most voluminous anomalies beneath the Canary Islands (≤400 km nation during the subsequent African-European Neogene volcanic fi eld in northwest Africa in diameter) do not extend beneath northwest collision. Removal of subcontinental lithospheric (Fig. 1A). Based on geophysical data, the subcon- Africa at depths of 300 and 600 km, ruling out mantle and formation of a lithospheric corridor tinental lithospheric corridor is widest and shal- an exceptionally wide Canary plume and sug- could have drawn Canary plume material into lowest under the Middle Atlas region (Fig. 1B, gesting that there is not deep upwelling directly the lithospheric channel under the Atlas system, profi le F). Infl ow of Canary mantle material into beneath northwest Africa (Montelli et al., eventually widened by thermal erosion related to the widening and shallowing subcontinental litho- 2006). Volcanic rocks with Canary Islands– the fl ow of Canary mantle through the corridor spheric corridor can explain why the most volu- type geochemical compositions are furthermore (Fig. 1B). The process can also explain why sub- minous igneous episode of the northwest African restricted to areas above the lithospheric chan- lithospheric melts were generated at 60–120 km continent in the past million years took place in nel in Morocco (Middle Atlas, Guilliz, and melting depths (as inferred above), although nor- the Middle Atlas: A greater vertical component Gourougou), whereas those not above the sub- mal northwest African lithospheric thickness is of fl ow combined with greater amounts of mate- lithospheric channel (e.g., Oujda-Oranie) have on the order of 130–160 km. rial at wider and shallower parts of the corridor distinct isotopic compositions, indicating a rela- Third, it has been proposed that rollback sub- will result in greater amounts of mantle material tionship between Canary-like geochemistry and duction of Tethys oceanic lithosphere beneath undergoing decompression partial melting, pro- the subcontinental lithospheric corridor (Fig. 3). the westernmost Mediterranean triggered ducing larger volumes. The geochemical and geophysical constraints delamination of subcontinental lithosphere Geophysical data suggest that ~50% of the now available suggest that Canary mantle plume beneath the northern edge of Morocco since present thickening and elevation of the Atlas material only fl owed laterally beneath northwest 8–10 Ma (Duggen et al., 2005). The infl ow Mountains results from buoyant sublithospheric Africa through a lithospheric corridor, causing of sublithospheric mantle may have gener- material (Teixell et al., 2005), indicating that the continental intraplate volcanism above it. ated intraplate volcanism in the Guilliz and inflow of Canary mantle plume material into An interplay of several mechanisms may be Gourougou areas. Recently trench-parallel/ the lithospheric corridor is an ongoing process responsible for the >1500 km lateral fl ow of arc-parallel mantle fl ow in the mantle wedge has and may be directly contributing to the high plume material from the Canary hotspot to the been demonstrated with both geochemical and elevation of the Middle Atlas. The infl ow of Mediterranean. First, active mantle upwelling geophysical techniques and has been shown to Canary mantle, causing uplift of the overly- can cause plume material to spread out laterally be associated with trench retreat/rollback (and ing lithosphere, may thus partly be responsible along the base of the lithosphere through plume advance), such that the retreating plate causes for the devastating 1960 earthquake in Agadir push. Thinning of the lithosphere will fur- lateral fl ow of mantle into the area from which (Fig. 1A) that killed about a third of the city’s ther enhance the fl ow of plume material along it has retreated (Faccenna et al., 2005; Hoernle population and left most of the remainder its base to areas of thinner lithosphere (Sleep, et al., 2008; Long and Silver, 2008). Combined homeless (Meghraoui et al., 1998). Finally, the 2008). It has been proposed that the litho- slab rollback and continental-edge delamination mechanism also provides an explanation for sphere beneath the easternmost Canary island could therefore have caused suction of material the long-standing riddle as to why the second-

GEOLOGY, March 2009 285 Downloaded from geology.gsapubs.org on March 1, 2010 largest recorded tholeiitic eruption occurred on Geldmacher, J., Hoernle, K., Bogaard, P.v.d., Duggen, tive Zone, in Steward, J.H., and Vita-Finzi, C., 40 39 one of the oldest Canary islands (A.D. 1730– S., and Werner, R., 2005, New Ar/ Ar age and eds., Coastal tectonics: The Geological Society geochemical data from seamounts in the Canary of London Special Publication 146, p. 239–253. 1736 Lanzarote Timanfaya fi ssure eruption) and Madeira Volcanic Provinces: A contribution Missenard, Y., Zeyen, H., de Lamotte, D.F., Leturmy, (Carracedo et al., 1992). Without fl ow of Canary to the “Great Plume Debate”: Earth and Plan- P., Petit, C., Sébrier, M., and Saddiqi, O., 2006, mantle plume material into the northwest Afri- etary Science Letters, v. 237, p. 85–101. Crustal versus asthenospheric origin of relief can subcontinental lithospheric corridor, vol- Gomez, F., Beauchamp, W., and Barazangi, M., of the Atlas Mountains of Morocco: Journal 2000, Role of the Atlas Mountains (northwest of Geophysical Research, v. 111, p. 1–13, doi: canism on the easternmost Canary Islands is Africa) within the African-- 10.1029/2005JB003708. likely to have been extinct long ago or at most to boundary zone: Geology, v. 28, p. 775–778, doi: Montelli, R., Nolet, G., Dahlen, F.A., and Masters, have generated only minor volumes of lava. 10.1130/0091-7613(2000)28<775:ROTAMN> G., 2006, A catalogue of deep mantle plumes: 2.0.CO;2. New results from fi nite-frequency tomography: Gutscher, M.-A., Malod, J., Rehault, J.-P., Contrucci, Geochemistry, Geophysics, Geosystems, v. 7, ACKNOWLEDGMENTS I., Klingelhoefer, F., Mendes-Victor, L., and Q11007, doi: 10.1029/2006GC001248. We are grateful to A. Moukadiri, P.v.d. Bogaard, Spakman, W., 2002, Evidence for active sub- Moukadiri, A., and Pin, C., 1998, Géochimie (élé- and the Centre Regional Geologique de Meknes for duction beneath Gibraltar: Geology, v. 30, ments majeurs et terres rares) des granulites assisting with fi eldwork and to H. Anders, I. Marte- p. 1071–1074, doi: 10.1130/0091-7613(2002) méta-sédimentaires en xénolithes dans les lock, D. Rau, and C. Roberts for analytical assistance. 030<1071:EFASBG>2.0.CO;2. basaltes alcalins quaternaires du Moyen Atlas Comments and suggestions from C. Lundstrom, Harmand, C., and Cantagrel, J.M., 1984, Le vol- (Maroc): Arguments en faveur de la nature M. Lustrino, and an anonymous reviewer as well as canisme alcalin tertiaire et quarternaire du pour partie restitique de la croûte inférieure: the editor (A. Barth) are greatly appreciated. The Moyen Atlas (Maroc): Chronologie K/Ar Comptes Rendue Académie des Sciences Paris project was supported by the Deutsche Forschungs- et cadre géodynamique: Journal of African de la terre et des planètes, v. 327, p. 589–595. gemeinschaft (HO1833/5, 16, and 18; DU426/1 Earth Sciences, v. 2, p. 51–55, doi: 10.1016/ Neumann, E.R., Wulff, P.E., Johnsen, K., Andersen, and 3) and a UK Royal Society–Leverhulme Trust 0899-5362(84)90019-8. T., and Krogh, E., 1995, Petrogenesis of spinel Senior Research Fellowship to Thirlwall, allowing Hart, S.R., 1984, A large-scale isotope anomaly in harzburgite and dunite suite xenoliths from Duggen to stay more than a year at Royal Holloway the Southern Hemisphere mantle: Nature, Lanzarote, eastern Canary Islands: Implications University of London. This publication is contribution v. 309, p. 753–757, doi: 10.1038/309753a0. for the upper mantle: Lithos, v. 35, p. 83–107, no. 164 of the Sonderforschungsbereich 574 “Volatiles Hirschmann, M.M., and Stolper, E.M., 1996, A pos- doi: 10.1016/0024-4937(95)91153-Z. and Fluids in Subduction Zones” at Kiel University. sible role for garnet pyroxenite in the origin Ramdani, F., 1998, Geodynamic implications of of the “garnet signature” in MORB: Contri- intermediate-depth earthquakes and volcanism REFERENCES CITED butions to Mineralogy and Petrology, v. 124, in the intraplate Atlas Mountains (Morocco): Anguita, F., and Hernan, F., 2000, The Canary Is- p. 185–208, doi: 10.1007/s004100050184. Physics of the Earth and Planetary Interiors, lands origin: A unifying model: Journal of Hoernle, K., and Schmincke, H.U., 1993, The role v. 108, p. 245–260, doi: 10.1016/S0031-9201 Volcanology and Geothermal Research, v. 103, of partial melting in the 15-Ma geochemical (98)00106-X. p. 1–26, doi: 10.1016/S0377-0273(00)00195-5. evolution of Gran Canaria: A blob model for Schilling, J.-G., 1973, Iceland mantle plume: Geo- Carracedo, J.C., Rodríguez-Badiola, E., and Soler, V., the Canary Hotspot: Journal of Petrology, v. 34, chemical study of Reykjanes Ridge: Nature, 1992, The 1730–1736 eruption of Lanzarote, p. 599–626. v. 242, p. 565–571, doi: 10.1038/242565a0. Canary Islands: A long, high-magnitude basal- Hoernle, K., Tilton, G., and Schmincke, H.U., 1991, Sleep, N.H., 2008, Channeling at the base of the tic fi ssure eruption: Journal of Volcanology and Sr-Nd-Pb isotopic evolution of Gran Canaria : litho sphere during the lateral fl ow of plume Geothermal Research, v. 53, p. 239–250, doi: Evidence for shallow enriched mantle be- material beneath fl ow line hot spots: Geochem- 10.1016/0377-0273(92)90084-Q. neath the Canary Islands: Earth and Plan- istry, Geophysics, Geosystems, v. 9, p. 1–35. Duggen, S., Hoernle, K., Bogaard, P.v.d., Rüpke, L., etary Science Letters, v. 106, p. 44–63, doi: Sun, S.-s., and McDonough, W.F., 1989, Chemical and Phipps Morgan, J., 2003, Deep roots of 10.1016/0012-821X(91)90062-M. and isotopic systematics of oceanic basalts: the Messinian salinity crisis: Nature, v. 422, Hoernle, K., Zhang, Y.S., and Graham, D., 1995, Implications for mantle composition and p. 602–606. Seismic and geochemical evidence for large- processes , in Saunders, A.D., and Norry, M.J., Duggen, S., Hoernle, K., Bogaard, P.v.d., and Garbe- scale mantle upwelling beneath the eastern At- eds., Magmatism in the ocean basins: The Geo- Schönberg, D., 2005, Post-collisional transition lantic and western and central Europe: Nature, logical Society of London Special Publication from subduction- to intraplate-type magmatism v. 374, p. 34–39, doi: 10.1038/374034a0. 42, p. 313–345. in the westernmost Mediterranean: Evidence Hoernle, K., Abt, D.L., Fischer, K.M., Nichols, H., Teixell, A., Ayarza, P., Zeyen, H., Fernàndez, M., for continental-edge delamination of subconti- Hauff, F., Abers, G., Bogaard, P.v.d., Heydolph, and Arboleya, M.-L., 2005, Effects of mantle nental lithosphere: Journal of Petrology, v. 46, K., Alvarado, G., Protti, J.M., and Strauch, W., upwelling in a compressional setting: The p. 1155–1201, doi: 10.1093/petrology/egi013. 2008, Arc-parallel fl ow in the mantle wedge Atlas Mountains of Morocco: Terra Nova, Duggen, S., Hoernle, K., Klügel, A., Geldmacher, J., beneath Costa Rica and Nicaragua: Nature, v. 17, p. 456–461, doi: 10.1111/j.1365-3121. Thirlwall, M.F., Hauff, F., Lowry, D., and Oates, v. 451, p. 1094–1097. 2005.00633.x. N., 2008, Geochemical zonation of the Mio- Kogiso, T., Hirose, K., and Takahashi, E., 1998, Urchulutegui, J.F., Fernàndez, M., and Zeyen, H., cene Alborán Basin volcanism (westernmost Melting experiments on homogenous mixtures 2006, Lithospheric structure in the Atlantic- Mediterranean): Geodynamic implications: of peridotite and basalt: Application to the gen- Mediterranean transition zone (southern Spain, Contributions to Mineralogy and Petrology, esis of oceanic island basalts: Earth and Plan- northern Morocco): A simple approach from v. 156, p. 577–593. etary Science Letters, v. 162, p. 45–61, doi: regional elevation and geoid data: Comptes Ebinger, C.J., and Sleep, N.H., 1998, Cenozoic 10.1016/S0012-821X(98)00156-3. Rendus Geoscience, v. 338, p. 140–151, doi: magmatism throughout east Africa resulting Long, M.D., and Silver, P.G., 2008, The subduction 10.1016/j.crte.2005.11.004. from impact of a single plume: Nature, v. 395, zone fl ow fi eld from seismic anisotropy: A Widom, E., Hoernle, K., Shirey, S.B., and Schmincke, p. 788–791, doi: 10.1038/27417. global view: Science, v. 319, p. 315–319, doi: H.-U., 1999, Os isotope systematics in the El Azzouzi, M., Bernard-Griffi ths, J., Bellon, H., 10.1126/science.1150809. Canary Islands and Madeira: Lithospheric Maury, R.C., Piqué, A., Fourcade, S., Cotten, Lundstrom, C.C., Hoernle, K., and Gill, J., 2003, contamination and mantle plume signatures: J., and Hernandez, J., 1999, Évolution des U-series disequilibria in volcanic rocks from Journal of Petrology, v. 40, p. 279–296, doi: sources du volcanisme marocain au cours du the Canary Islands: Plume versus lithospheric 10.1093/petrology/40.2.279. Néogène (Evolution of the sources of Moroc- melting: Geochimica et Cosmochimica Acta, Zindler, A., and Hart, S., 1986, Chemical geodynam- can volcanism during the Neogene): Comptes v. 67, p. 4153–4177, doi: 10.1016/S0016-7037 ics: Annual Review of Earth and Planetary Sci- Rendue Académie des Sciences Paris, Sciences (03)00308-9. ences, v. 14, p. 493–571, doi: 10.1146/annurev. de la terre et des planètes, v. 329, p. 95–102. Lustrino, M., and Wilson, M., 2007, The circum- ea.14.050186.002425. Faccenna, C., Civetta, L., D’Antonio, M., Funi ciello, Mediterranean anorogenic Cenozoic igne- F., Margheriti, L., and Piromallo, C., 2005, ous province: Earth-Science Reviews, v. 81, Manuscript received 11 August 2008 Constraints on mantle circulation around the p. 1–65, doi: 10.1016/j.earscirev.2006.09.002. Revised manuscript received 5 November 2008 deforming Calabrian slab: Geophysical Re- Meghraoui, M., Outtani, F., Choukri, A., and Manuscript accepted 7 November 2008 search Letters, v. 32, L021874, doi: 10.1029/ de Lamotte, D.F., 1998, Coastal tectonics across 2004GL021874. the South Atlas Thrust Front and the Agadir Ac- Printed in USA

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