Fingerprinting orogenic delamination

Mihai N. Ducea* University of Arizona, Department of Geosciences, Tucson, AZ 85721, USA

A signifi cant portion of the Earth’s lithosphere is recycled into the at the end of the Variscan orogen. This observation is used to postulate deeper mantle, as required by mass balance considerations in orogenic lithospheric delamination under the Iberian Massif. Magmatism formed in environments. The two principal mechanisms for recycling are subduction response to delamination can be either from the upwelling asthenosphere at plate margins and delamination. Subduction is a well-understood pro- or from the downgoing drip (Elkins-Tanton, 2007). Adiabatic upwelling cess that is essential to the plate tectonic engine of planet Earth. Delamina- of asthenospheric mantle has long been the most signifi cant expected geo- tion, on the other hand, requires recycling via convective removal of the logic product in response to delamination (Kay and Kay, 1993; Ducea lower parts of the lithosphere, and is more diffi cult to detect. One chief and Saleeby, 1998). Surprisingly, unless major fl ood basalt provinces are argument for delamination comes from extreme shortening at continen- products of delamination (Bedard, 2006), most areas suspected to have tal convergent margins, which requires far thicker mantle lithospheres undergone recent delamination have only minor associated mafi c mag- than observed (DeCelles et al., 2009). The second argument comes from matism. For example, the Puna region in the central Andes (Kay et al., the intermediate average composition of the continental crust (Rudnick, 1994; Drew et al., 2009) and the southern Sierra Nevada in California 1995), which requires a large ultramafi c complementary residue at the (Ducea and Saleeby, 1996, 1998; Farmer et al., 2002), two areas most bottom of the continental crust; such a reservoir has not been identifi ed likely subject to recent delamination, are characterized by volumetrically over large portions of continental areas. Delamination (Bird, 1979), con- insignifi cant mafi c magmatism at the time of delamination. This obser- vective removal, foundering, and lithospheric dripping are terms used for vation suggests that perhaps the size of drips is small (few kilometers), the process of detachment and sinking of the lower parts of the continental therefore their ability to sink is limited, and the corresponding ascending lithosphere other than those that may have been buried into the mantle via asthenospheric column is short and unlikely to melt extensively (Drew continental subduction. Most researchers using the term “delamination” et al., 2009). Furthermore, smaller convective instabilities develop over refer to a density-driven process of foundering, and do not imply its origi- larger time scales, perhaps in the range of tens of million years, in con- nal “peeling-off” signifi cance as defi ned by Bird (1979), which is closer trast to larger drips that can appear as catastrophic events in the geologic to tectonic underplating in shallow subduction environments. Delamina- record. An even more puzzling observation is that the syn-delamination tion is a form of vertical and spatially localized tectonics often generating “bulls-eye”−shaped volcanism in map view of the Sierra Nevada is ultra- amoeba-like or circular surface effects that are regional results of tectono- potassic and has isotopic signatures expected of the partial melting of magmatic processes at convergent plate margins. the downgoing drip, the lower crustal root of the Sierra Nevada batholith Mantle delamination is the process of foundering of dense, unstable (Manley et al., 2000). Indeed, amphibole or phlogopite-bearing eclogite mantle lithosphere into the asthenosphere until it reaches thermal equilib- facies rocks or peridotites will likely undergo limited dehydration melting rium with the surrounding asthenosphere (Houseman et al., 1981; House- upon adiabatic descent (Elkins-Tanton, 2007), providing an explanation man and Molnar, 1997). Crustal delamination requires that the lowermost for syndrip alkalic lithologies. crust undergoes mineral equilibration under eclogite facies, and becomes Areas that potentially lost their crustal and mantle roots via delami- denser that the underlying mantle (Arndt and Goldstein, 1989). Eclogite nation and continental extension following extreme thickening, such as formation in the lowermost crust can be a solid-state process only, e.g., the Basin and Range (Meissner and Mooney, 1998) and the central part in areas thickened by continental collision (Leech, 2001), or can involve of the Coast Mountains batholith (western Canada) (Calkins et al., 2010) separation of intermediate composition arc-forming melts in Cordilleran appear to have associated mafi c magmatism that transitioned from “older” subduction environments (Ducea, 2002; Lee et al., 2006). Trace element lithospheric mantle (characteristic of the pre-delamination and extension data from most modern Andean arc and back arc volcanoes (Chiaradia et mantle) to “young” asthenospheric mantle over tens of millions of years, al., 2009; Mamani et al., 2010) provide evidence that andesitic and dacitic e.g., 40 m.y. for the Coast Mountains region (Manthei et al., 2010). Specif- ε magmas formed in such tectonic settings are, to a fi rst order, extracted out ically for the Coast Range region, there is a marked change of Nd isotopic of an eclogitic residue. signatures of primitive, non-contaminated basaltic magmas from values of Snapshots of modern delamination are obtained using indirect imag- +3 to +5 in basalts older than 20 Ma to >+7 in basalts younger than 10 Ma ing of the deeper Earth. Tomographic images of dense lithospheric bodies (Manthei et al., 2010). This observation is again consistent with smaller located within the asthenosphere (e.g., Zandt et al. [2004] for the Sierra drips and a rather long-lived process of “cleansing” of the uppermost man- Nevada region in California, and Fillerup et al. [2010] for the eastern Car- tle of the excess lithosphere at the end of the orogenic cycle. pathians) provide compelling documentation of areas undergoing delami- In the case of the Iberian Massif (Guttierez-Alonso et al., 2011), the nation/dripping at the present time. Both examples (and others) provide identity of the two different mantle reservoirs, pre- and post delamina- ε evidence for vertical tectonics at the scale of 50–100 km within the asthe- tion, is also defi ned via the Nd of whole-rock basaltic samples and their nosphere. Proposed geologic consequences of large-scale delamination corresponding model ages (age of extraction from a depleted mantle res- are magmatic and geomorphic (Kay and Kay, 1993). ervoir, assuming a single melting event), prescreened for possible crustal ε Gutierrez-Alonso et al. (2011, p. 155 this issue of Geology), use iso- contamination effects. The transition from lower to higher Nd values took topic tracers to detect an important Paleozoic delamination event in West- place over a period of 20 m.y. (from 305 to 285 Ma), similar to the Cordil- ern Europe. In this new paper, the authors argue that a change from conti- leran cases described above. It is well established that Variscan orogeny nental lithospheric to asthenospheric mantle isotopic signatures took place in Europe culminated with a Himalayan-style continental collision during the early to middle Carboniferous, and was followed by extensional col- *E-mail: [email protected]. lapse (Menard and Molnar, 1988) that led to the formation of the classic

© 2011 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, February February 2011; 2011 v. 39; no. 2; p. 191–192; doi: 10.1130/focus022011.1. 191

Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/39/2/191/3541118/191.pdf by guest on 29 September 2021 European coal-bearing continental basins of the Carboniferous. Litho- Elkins-Tanton, L.T., 2007, Continental magmatism, volatile recycling, and a spheric delamination associated with orogenic collapse is entirely plausi- heterogeneous mantle caused by lithospheric gravitational instabilities: ble, especially given that the crustal thickness of the Variscan orogen may Journal of Geophysical Research-Solid Earth, v.112, B03405, doi:10.1029/ 2005JB004072. have been more than double the normal crustal thickness in some parts of Farmer, G.L., Glazner, A.F., and Manley, C.R., 2002, Did lithospheric delamina- Europe (Medaris et al., 2003). One of the intriguing aspects of Gutierrez- tion trigger late Cenozoic potassic volcanism in the southern Sierra Nevada, Alonso et al.’s paper is that it suggests that delamination took place in a California?: Geological Society of America Bulletin, v. 114, p. 754–768, core region that underwent severe oroclinal bending, similar to the modern doi:10.1130/0016-7606(2002)114<0754:DLDTLC>2.0.CO;2. Fillerup, M.A., Knapp, J.H., Knapp, C.C., and Raileanu, V., 2010, Mantle earth- south and eastern Carpathians and the adjacent Transylvanian basin. quakes in the absence of subduction? Continental delamination in the Ro- A temporal change in Nd isotopes of basaltic magmas is in itself manian Carpathians: Lithosphere, v. 2, p. 333–340, doi:10.1130/L102.1. insuffi cient to fi ngerprint delamination, as extension alone, for example, Garzione, C.N., Hoke, G.D., Libarkin, J.C., Withers, S., MacFadden, B., Eiler, can produce the same result. However, such observations, when taken into J., Ghosh, P., and Mulch, A., 2008, Rise of the Andes: Science, v. 320, account together with other lines of evidence, can be extremely power- p. 1304–1307, doi:10.1126/science.1148615. Gutierrez-Alonso, G., Murphy, J.B., Fernández-Suárez, J., Weil, A.B., Franco, ful in detecting past delamination events, as demonstrated by Gutierrez- M.P., and Gonzalo, J.P., 2011, Lithospheric delamination in the core of Pan- Alonso et al. (2011). gea: Sm-Nd insights from the Iberian mantle: Geology, v. 39, p. 155–158, A regional increase in elevation is predicted by buoyancy consider- doi:10.1130/G31468.1. ations in areas undergoing delamination, because the densest part of the Houseman, G.A., and Molnar, P., 1997, Gravitational (Rayleigh-Taylor) instabil- ity of a layer with non-linear viscosity and convective thinning of conti- lithosphere is replaced. There are numerous areas geologically active in nental lithosphere: Geophysical Journal International, v. 128, p. 125–150, the Cenozoic in which rapid elevation increase is attributed to post-delam- doi:10.1111/j.1365-246X.1997.tb04075.x. ination surface uplift (e.g., Garzione et al., 2008). Geodynamic model- Houseman, G.A., McKenzie, D.P., and Molnar, P., 1981, Convective Instability of ing is another avenue for future research to break new ground and make a Thickened Boundary-Layer and Its Relevance for the Thermal Evolution specifi c geologic predictions in delamination science; the most promising of Continental Convergent Belts: Journal of Geophysical Research, v. 86, p. 6115–6132, doi:10.1029/JB086iB07p06115. geomorphic/magmatic targets for future, modern, or recent delamination Kay, S.M., Coira, B., and Viramonte, J., 1994, Young Mafi c Back Arc Volcanic- are depressions/basins in orogenic plateaus, such as the Altiplano-Puna in Rocks as Indicators of Continental Lithospheric Delamination beneath the the central Andes. Argentine Puna Plateau, Central Andes: Journal of Geophysical Research. Solid Earth, v. 99, p. 24323–24339, doi:10.1029/94JB00896. ACKNOWLEDGMENTS Kay, R.W., and Kay, S.M., 1993, Delamination and Delamination Magmatism: I gratefully acknowledge support by the U.S. National Science Foundation Tectonophysics, v. 219, p. 177–189, doi:10.1016/0040-1951(93)90295-U. under grants EAR-0910941, EAR-0907880, EAR-0606967, and EAR-0309885. Leech, M.L., 2001, Arrested orogenic development: eclogitization, delamination, and tectonic collapse: Earth and Planetary Science Letters, v. 185, p. 149– 159, doi:10.1016/S0012-821X(00)00374-5. REFERENCES CITED Lee, C.T.A., Cheng, X., and Horodyskyj, U., 2006, The development and refi ne- Arndt, N.T., and Goldstein, S.L., 1989, An Open Boundary between Lower Conti- ment of continental arcs by primary basaltic magmatism, garnet pyroxenite nental-Crust and Mantle—Its Role in Crust Formation and Crustal Recycling: accumulation, basaltic recharge and delamination: insights from the Sierra Tectonophysics, v. 161, p. 201–212, doi:10.1016/0040-1951(89)90154-6. Nevada, California: Contributions to Mineralogy and Petrology, v. 151, Bedard, J.H., 2006, A catalytic delamination-driven model for coupled genesis p. 222–242, doi:10.1007/s00410-005-0056-1. of Archaean crust and sub-continental lithospheric mantle: Geochimica et Mamani, M., Worner, G., and Sempere, T., 2010, Geochemical variations in igne- Cosmochimica Acta, v. 70, p. 1188–1214, doi:10.1016/j.gca.2005.11.008. ous rocks of the Central Andean orocline (13°S to 18°S): Tracing crustal Bird, P., 1979, Continental Delamination and the Colorado Plateau: Journal of thickening and magma generation through time and space: Geological Soci- Geophysical Research, v. 84, p. 7561–7571. ety of America Bulletin, v. 122, p. 162–182, doi:10.1130/B26538.1. Calkins, J.A., Zandt, G., Girardi, J., Dueker, K., Gehrels, G.E., and Ducea, M.N., Manley, C.R., Glazner, A.F., and Farmer, G.L., 2000, Timing of volcanism in 2010, Characterization of the crust of the Coast Mountains Batholith, Brit- the Sierra Nevada of California: Evidence for Pliocene delamination of ish Columbia, from P to S converted seismic waves and petrologic model- the batholithic root?: Geology, v. 28, p. 811–814, doi:10.1130/0091-7613 ing: Earth and Planetary Science Letters, v. 289, p. 145–155, doi:10.1016/j (2000)28<811:TOVITS>2.0.CO;2. .epsl.2009.10.037. Manthei, C.D., Ducea, M.N., Girardi, J.D., Patchett, P.J., and Gehrels, G.E., Chiaradia, M., Muntener, O., Beate, B., and Fontignie, D., 2009, Adakite-like 2010, Isotopic and geochemical evidence for a recent transition in mantle volcanism of Ecuador: lower crust magmatic evolution and recycling: Con- chemistry beneath the western Canadian Cordillera: Journal of Geophysi- tributions to Mineralogy and Petrology, v. 158, p. 563–588, doi:10.1007/ cal Research. Solid Earth, v. 115, p. B02204, doi:10.1029/2009JB006562. s00410-009-0397-2. Meissner, R., and Mooney, W., 1998, Weakness of the lower continental crust: DeCelles, P.G., Ducea, M.N., Kapp, P., and Zandt, G., 2009, Cyclicity in Cordil- a condition for delamination, uplift, and escape: Tectonophysics, v. 296, leran orogenic systems: Nature Geoscience, v. 2, p. 251–257, doi:10.1038/ p. 47–60, doi:10.1016/S0040-1951(98)00136-X. ngeo469. Medaris, G., Ducea, M., Ghent, E., and Iancu, V., 2003, Timing of high-pressure Drew, S.T., Ducea, M.N., and Schoenbohm, L.M., 2009, Mafi c volcanism on the metamorphism in the Getic-Supragetic basement nappes of the South-Car- Puna Plateau, NW Argentina: Implications for lithospheric composition and pathian mountains fold-thrust belt: Lithos, v. 70, p. 141–161. evolution with an emphasis on lithospheric foundering: Lithosphere, v. 1, Menard, G., and Molnar, P., 1988, Collapse of a Hercynian Tibetan Plateau into a p. 305–318, doi:10.1130/L54.1. Late Paleozoic European Basin and Range Province: Nature, v. 334, p. 235– Ducea, M.N., 2002, Constraints on the bulk composition and root foundering 237, doi:10.1038/334235a0. rates of continental arcs, A California arc perspective: Journal of Geophysi- Rudnick, R.L., 1995, Making Continental Crust: Nature, v. 378, p. 571–578, cal Research, v. 107, p. 2304, doi:10.1029/2001JB000643. doi:10.1038/378571a0. Ducea, M.N., and Saleeby, J.B., 1998, A case for delamination of the deep batho- Zandt, G., Gilbert, H., Owens, T.J., Ducea, M., Saleeby, J., and Jones, C.H., 2004, lithic crust beneath the Sierra Nevada, California: International Geology Active foundering of a continental arc root beneath the southern Sierra Ne- Review, v. 40, p. 78–93, doi:10.1080/00206819809465199. vada in California: Nature, v. 431, p. 41–46, doi:10.1038/nature02847. Ducea, M.N., and Saleeby, J.B., 1996, Buoyancy sources for a large unrooted mountain range, the Sierra Nevada, California: Evidence from xenolith thermobarometry: Journal of Geophysical Research, v. 101, p. 8229–8244, doi:10.1029/95JB03452. Printed in USA

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