RESEARCH ARTICLE Seismic Moments of Intermediate‐Depth Earthquakes 10.1029/2018TC005336 Beneath the Hindu Kush: Active Stretching of a Blob Special Section: of Sinking Thickened Mantle Lithosphere? Fifty Years of Plate Tectonics: then, now and beyond Peter Molnar1 and Rebecca Bendick2 1Department of Geological Sciences and Cooperative Institute for Research in Environmental Sciences, University of Key Points: Colorado Boulder, Boulder, Colorado, USA, 2Department of Geosciences, University of Montana, Missoula, Montana, • Intermediate‐depth seismicity beneath the Hindu Kush suggests USA rapid stretching and sinking of mantle lithosphere • This region shows removal of Abstract Intermediate‐depth seismicity beneath the Hindu Kush may reveal the Earth's best example of mantle lithosphere in action, better a stretching blob of mantle lithosphere sinking through the asthenosphere. Seismically, this region is the than anywhere Earth's most active at intermediate depths (70–300 km). Fault plane solutions show nearly vertical stretching of the region in which the earthquakes occur, and a summation of seismic moment tensors implies that on average material at a depth of 300 km moves downward at ~40 mm/year relative to the Correspondence to: overlying crust. As shown by Kufner et al. (2017, https://doi.org/10.1016/j.epsl.2016.12.043) and Zhan and P. Molnar, [email protected] Kanamori (2016, https://doi.org/10.1002/2016GL069603), the central part of the zone stretches so rapidly that this rate there is ~100 mm/year, much faster than the present‐day convergence rate of ~15 mm/year at the surface and across the Hindu Kush. The pattern of stretching, with maximum strain rates in the Citation: Molnar, P., & Bendick, R. (2019). middle depths of the intermediate‐depth range, resembles that beneath the southeastern Carpathians, where Seismic moments of Lorinczi and Houseman (2009, https://doi.org/10.1016/j.tecto.2008.05.024) have shown that seismicity is ‐ intermediate depth earthquakes consistent with a blob of mantle lithosphere stretching rapidly and sinking into the asthenosphere, though beneath the Hindu Kush: Active stretching of a blob of sinking more slowly than occurs beneath the Hindu Kush. thickened mantle lithosphere? Tectonics, 38, 1651–1665. https://doi. org/10.1029/2018TC005336 1. Introduction Received 20 SEP 2018 Although many have come to believe that mantle lithosphere beneath some continental regions has been Accepted 9 APR 2019 removed, controversy continues to surround the mechanisms by which this occurs, with some doubting that Accepted article online 17 APR 2019 Published online 15 MAY 2019 the process occurs at all (e.g., McKenzie & Priestley, 2008). Bird (1979) proposed that mantle lithosphere could delaminate from the overlying crust by peeling away from it, and as a result, the overlying crust would rapidly become exposed to the hotter underlying asthenosphere. Houseman et al. (1981) suggested alternatively that the enhanced gravitational instability of the mantle lithosphere, when thickened along with the overlying crust during horizontal shortening, could grow rapidly so that much if not most of the mantle lithosphere would des- cend into the underlying asthenosphere and be removed. Numerous studies exploited these possibilities to explain surface uplift supplemental to isostatic compensation of thickened crust (e.g., Bird, 1979; England & Houseman, 1989), compositions of major elements of volcanic rock in convergent belts like the Andes (e.g., Jull & Kelemen, 2001; Kay & Kay, 1993; Kay & Mahlburg‐Kay, 1991), and trace elements and isotopic ratios of volcanic rock in Tibet and elsewhere (e.g., Turner et al., 1993; Turner, Arnaud, et al., 1996; Turner, Kelley, et al., 1996; Turner et al., 1999). Some of these cases may have involved removal of lower crust along with mantle lithosphere. Although a few tomographic images suggest that mantle lithosphere is descending into the asthenosphere—for instance beneath the Sierra Nevada of California (e.g., Jones et al., 2014), the Alpine belts of Europe (e.g., Lippitsch et al., 2003; Ren et al., 2012), and perhaps Tibet (e.g., Chen et al., 2017; Ren & Shen, 2008)—demonstrations of ongoing removal of continental mantle lithosphere are few. Among the evidence of oceanic lithosphere sinking into the asthenosphere, inclined zones of earthquakes beneath island arc structures are perhaps the most convincing (e.g., Isacks et al., 1968), and the case for negative buoyancy of old oceanic lithosphere is well established (e.g., Isacks & Molnar, 1969, 1971). Lorinczi and Houseman (2009) extended that logic by exploiting the nearly vertical zone of earthquakes beneath the southeastern Carpathians to estimate the rate at which a blob mantle lithosphere stretches as it sinks into the asthenosphere. We exploit similar analyses by Kufner et al. (2017), Lister et al. (2008), ©2019. American Geophysical Union. and Zhan and Kanamori (2016) to the Hindu Kush region to address how a blob of mantle lithosphere All Rights Reserved. may be sinking beneath that region. MOLNAR AND BENDICK 1651 Tectonics 10.1029/2018TC005336 When plate tectonics was first recognized, Isacks et al. (1968) deduced that intermediate‐ and deep‐focus earthquakes occurred in descending slabs of stiff, cold, negatively buoyant oceanic lithosphere. In general, these descending slabs could be linked through the shallow part of subduction zones to lithosphere at the surface, either with a continuous zone of earthquakes or through mechanical arguments related to stress transfer in strong material (e.g., Elsasser, 1969). Then, in a synthesis of fault plane solutions for intermedi- ate and deep earthquakes around the globe, Isacks and Molnar (1969, 1971) showed that for deep earth- quakes, P axes, orientations of maximum compressional strain, consistently plunge down the dips of seismic zones at depths >300 km; for intermediate‐depth earthquakes, however, T axes, orientations of maximum extensional strain, plunge downdip, in many, though not all, regions. Thus, slabs seem to be stretching as gravity acting on excess mass in the slabs pulls them down, like dangling springs hanging from and attached to lithosphere above. The intermediate and deep zones of seismicity are also spatially asso- ciated with low attenuation of seismic waves (e.g., Oliver & Isacks, 1967; Utsu, 1966) and high seismic wave speeds (e.g., Mitronovas & Isacks, 1971; Utsu, 1971), both of which are signatures of cold lithospheric mate- rial at depth. When plate tectonics was recognized, also obvious was a difference between continental and oceanic litho- sphere: thick continental crust makes continental lithosphere buoyant (e.g., McKenzie, 1969). With such buoyancy, subduction of continental lithosphere should be either rare or different from that of oceanic litho- sphere. Among 29 zones of intermediate and/or deep earthquakes that Isacks and Molnar (1971) considered, four lie within continental regions, and three of them cannot yet be reconciled with subducting slabs of ocea- nic lithosphere that can be traced to the surface. The dipping zone of the intermediate‐depth earthquakes beneath the Indo‐Burman Ranges, just east of India, clearly does reflect subduction of lithosphere beneath the Ranges (e.g., Chen & Molnar, 1990; Hurukawa et al., 2012; Le Dain et al., 1984; Ni et al., 1989). The rela- tionship of the 1954 deep Spanish earthquake to subduction remains obscure. The remaining two, the Vrancea zone beneath the southeastern Carpathians and the Pamir‐Hindu Kush in central Asia, lie in regions of recent if not active convergence, but each is complicated in its own way. Lorinczi and Houseman (2009) suggested that the intermediate‐depth earthquakes of the Vrancea zone occur not in a subducted slab of lithosphere, but rather in a sinking blob of mantle lithosphere drawn into it from the surrounding regions, as would occur as a convective instability grew. Convective instabilities should lead to high internal strain rates in the sinking material, higher than where nearly rigid, plate‐like lithosphere sinks at subduction zones. Indeed, Lorinczi and Houseman (2009) reported unusually high strain rates at intermediate depths. We extend Lorinczi and Houseman's logic and similar analyses taken by Kufner et al. (2017), Lister et al. (2008), and Zhan and Kanamori (2016) to the Hindu Kush region, the Earth's most active region of intermediate‐depth seismicity, to explore whether a blob of mantle lithosphere might similarly be sinking beneath that region. 2. Carpathians The Carpathian mountain belt wraps around the Pannonian Basin (Figure 1), which underwent stretching and thinning of its lithosphere in Miocene time (e.g., Horváth, 1993; Horváth et al., 2006; Royden, Horvath, Nagymarosy, et al., 1983; Royden, Horvath, & Rumpler, 1983) and is underlain by thinned continental crust (e.g., Środa et al., 2006). Seismic tomography reveals low P and S wave speeds in the uppermost mantle beneath the basin and hence corroborates the inference of lithospheric thinning (Dando et al., 2011; Mitterbauer et al., 2011; Ren et al., 2012). Starting before and continuing during the stretching and thinning of the Pannonian crust, the Carpathian mountain belt developed by crustal shortening and thickening (e.g., Behrmann et al., 2000; Burchfiel & Nakov, 2015; Castelluccio et al., 2016; Faccenna et al., 2014; Knapp et al., 2005). Although some argue that subduction of oceanic lithosphere occurred beneath the Carpathians, Knapp et al. (2005) inferred that subduction of oceanic lithosphere