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Understanding Continental Subduction: a Work in Progress

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Understanding Continental Subduction: a Work in Progress RESEARCH FOCUS RESEARCH FOCUS: Understanding continental subduction: A work in progress Mihai N Ducea* University of Arizona, Department of Geosciences, Tucson, Arizona 85721, USA, and Universitatea Bucuresti, Facultatea de Geologie Geofizica, Bucharest, Romania A study by Froitzheim et al. (2016, p. 223 in this issue of Geology) adds crust makes it back to the surface of Earth, making UHP rocks extraordi- new constraints to our rapidly evolving ideas and models regarding the pro- narily important. cess of continental subduction. Classic plate tectonics concepts suggested Six distinctive geodynamic mechanisms have been proposed in the lit- that continents do not subduct. Instead, when two continents collide at a erature for continental subduction and development of UHP metamorphic convergent boundary following the consumption of an ocean by subduction, rocks, and also six mechanism exist that explain the subsequent unroofing they accommodate the shortening within the lithosphere, which is thick- of these rocks (Hacker and Gerya, 2013; Gerya, 2011). The exhumation of ened up to twice the normal values. The subducted oceanic slab that brought such rocks is as fascinating a subject as their burial: UHP terrains under- the continents together stalls and eventually breaks off and sinks into the went the most amount of exhumation of any rocks from Earth’s surface. mantle due to its negative buoyancy. In contrast to that view, modern petro- The proposed mechanisms for exhumation are mechanically and geologi- logic, tectonic, and geophysical observations have completely changed this cally plausible but, at this point, also very speculative, since they are so dif- picture still prevalent in many textbooks: continental lithosphere does, in ficult to test. In a very general sense, UHP rocks can move upward within fact, subduct to great depths at major long-lived collisional boundaries, and the crust toward their final resting place at Earth’s surface either as a part of the two colliding plates can be separated by a section of convective upper a large rigid crustal block or, alternatively, via some kind of opportunistic mantle (mantle wedge) similar to the case of oceanic subduction. route that does not require large scale en-masse exhumation (Hacker et There are three important types of observations supporting those as- al., 2013). One of the proposed mechanisms for the first type of unroofing sertions. First, the discovery over three decades ago (Chopin, 1984) of is the eduction mechanism (Andersen et al., 1991) in which continuing ultrahigh-pressure (UHP) metamorphic rocks—crustal rocks in which convergence at a collisional margin takes place synchronous with ejection the stable silica polymorph at peak pressure temperature conditions was of the intervening oceanic crust downward into the mantle. Among the coesite—documented that continental crustal rocks have been buried to mechanisms for selective exhumation, two particularly intriguing ones are >90–100 km in some orogens. After their initial discovery in the Alps, (1) exhumation along a narrow “subduction channel” along which materi- tens of localities of UHP or near-UHP metamorphic rocks have been als can travel both ways (up or down) depending on boundary condi- described globally in a variety of Phanerozoic and older orogenic belts tions, and (2) relamination of subducted sediments by solid-state and/or (Gilotti, 2013). Most of these are unambiguous continental crustal rocks. partially molten-state diapirism through the uppermost mantle (Hacker et Some even contain microdiamonds, indicating that they were buried to as al., 2011). Understanding exhumation of UHP rocks remains very much a much as 150 km (McClelland and Lappen, 2013, and references therein). work in progress and an area in which much is left to be discovered. Fundamentally, all UHP rocks are eclogite facies rocks and the better- Froitzheim et al. (2016) report the discovery of a new near-UHP lo- preserved ones have only limited products of retrogression overprinted cality on Lofoten Island, which is part of the Norwegian Caledonides along their exhumation path. UHP or near-UHP crustal xenoliths found (Kylander-Clark et al., 2009). Variably retrogressed kyanite eclogites that in volcanic rocks from the Pamir Mountains (Central Asia; Hacker et al., have a mafic bulk composition containing omphacite, garnet, and minor 2005; Ducea et al., 2003) also document the process of continental sub- kyanite, phengite, and quartz make up an assemblage that is mafic in bulk duction—in contrast to exhumed UHP rocks, they are crustal fragments chemistry, but also appears continental and crustal. The assemblage was caught in the process of subduction with no evidence for tectonic decom- at a peak metamorphic pressure of as much as 2.8 GPa (equivalent to pression/cooling in their thermobarometric record. Because it is unlikely >90 km depth) and temperatures of 650 °C degrees at ca. 400 Ma, as that continental crust is ever 100–150 km thick anywhere on the planet, determined from garnet whole-rock Lu-Hf geochronology. The peak the implication is that such rocks were subducted to mantle depths before temperature-pressure is indicative of a low thermal gradient that this rock being returned to the surface (Hacker et al., 2013). was subjected to during burial (~7 °C/km), which is typical for materials Second, refined plate-tectonic reconstructions and plate kinematics buried very fast along a plate boundary (subduction complexes around models for the Indo-Asian collision (van Hinsbergen et al., 2012) since the world give similarly low thermal gradients). Whole-rock Hf isotope its beginning, as early as the Paleocene, make very specific predictions re- systematics (extremely low initial 176Hf/177Hf) suggests that the analyzed garding the total amount of shortening along this margin, which is signifi- rock may have an Archean protolith. Together, these results indicate that cantly more than what can be accounted for by crustal shortening in the Hi- the analyzed rock was a lower crustal piece of Baltica, an old lithospheric malayas (DeCelles et al., 2011). More than 1000 km of Indian lithosphere fragment that was subducted beneath Laurentia during the broadly defined are missing and must have been subducted under the Asian continent. Caledonian orogeny as a consequence of the collision and continental sub- Third, seismic images of the ongoing Pamir–Hindu Kush collision sys- duction that followed after the consumption of the Iapetus Ocean. tem show that Indian lithosphere is being subducted to as much as 500 km This is not the first evidence for UHP metamorphism in the Scandina- beneath the surface (Sippl et al., 2013). vian Caledonides (Hacker et al., 2010). In fact, the Lofoten rocks (exposed Between these lines of evidence, a compelling case can be built for the on an island off the northern coast of Norway) are spatially and tectoni- fact that continental lithosphere (crust and mantle) are subductable to great cally correlated to the Western Gneiss Complex (Root et al., 2005), one depths and significant distances away from the collision’s principal suture. of the largest UHP metamorphic terrain known worldwide (Gilotti, 2013). One can further assume that only a small fraction of subducted continental The Caledonian Western Gneiss Complex has similar ages of peak meta- morphism (although, on average, they are older than the age determined *E-mail: [email protected] in this new study of Froitzheim et al.). However, Lofoten rocks are much GEOLOGY, March 2016; v. 44; no. 3; p. 239–240 | doi:10.1130/focus032016.1 GEOLOGY© 2016 Geological | Volume Society 44 | ofNumber America. 3 For| www.gsapubs.org permission to copy, contact [email protected]. 239 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/44/3/239/3549722/239.pdf by guest on 29 September 2021 less deformed than most of the Western Gneiss, suggesting that they may continent collision and orogenic extensional collapse, a model based on the represent legitimate autochthonous Baltica from the hinterland of the col- southern Norwegian Caledonides: Terra Nova, v. 3, p. 303–310, doi:10.1111 lision, and not some potentially allochthonous terranes positioned in be- /j .1365 -3121 .1991 .tb00148.x. Chopin, C., 1984, Coesite and pure pyrope in high-grade blueschists of the West- tween Baltica and Laurentia. ern Alps: a first record and some consequences: Contributions to Mineralogy Froitzheim et al. also deduce a rapid exhumation path following peak and Petrology, v. 86, p. 107–118, doi:10.1007/BF00381838. metamorphism. At >6 mm/yr (and possibly approaching 1 cm/yr), such DeCelles, P.G., Kapp, P., Quade, J., and Gehrels, G.E., 2011, Oligocene-Miocene exhumation rates are really fast compared to most rates of orogenic unroof- Kailas basin, southwestern Tibet: record of postcollisional upper plate exten- sion in the Indus-Yarlung suture zone: Geological Society of America Bul- ing, not quite as high but similar to the rates determined for the Namche letin, v. 123, p. 1337–1362, doi:10.1130/B30258.1. Barwa metamorphic rocks from the eastern Himalayan syntaxis (Ding et Ding, L., Zhong, D., Yin, A., Kapp, P., and Harrison, M.T., 2001, Cenozoic struc- al., 2001). In fact, such exhumation rates are so large that they require tural and metamorphic evolution of the eastern Himalayan syntaxis (Namche some special causative process, whether that is driven by climate condi- Barwa): Earth and Planetary Sciences, v. 192, p. 423–438, doi:10.1016 tions prone to high erosion rates, unusual local geomorphology at the time /S0012 -821X (01) 00463-0. Ducea, M., Hacker, B., Lutkov, V., Minaev, V., Ratschbacher, L., Luffi, P., Schwab, of exhumation, extensional tectonics, or a combination of these. The data M., Gehrels, G., McWilliams, M., Vervoort, J., and Metcalf, J., 2003, Build- in hand for the Lofoten eclogite are not capable of specifically addressing ing the Pamirs: The view from the underside: Geology, v. 31, p. 849–852, the cause of exhumation.
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