Melt equilibration depths as sensors of lithospheric thickness during Eurasia-Arabia collision and the uplift of the Anatolian Plateau

A co-investigation of mantle melting conditions and seismic structure revealed an evolutionary record of mantle dynamics accompanying the transition from subduction to collision along the Africa-Eurasia margin and the >1 km uplift of the Anatolian Plateau. New 40Ar/39Ar dates of volcanic rocks from the Eastern Taurides (southeast Turkey) considerably expand the known spatial extent of Miocene-aged mafic volcanism following a magmatic lull over much of Anatolia that ended at ca. 20 Ma. Mantle equilibration depths for these chemically diverse basalts are interpreted to indicate that early to middle Miocene lithospheric thickness in the region varied from ∼50 km or less near the Bitlis suture zone to ∼80 km near the Inner Tauride suture zone. This southward-tapering lithospheric base could be a vestige of the former interface between the subducted (and now detached) portion of the Arabian plate and the overriding Eurasian plate, and/or a reflection of mantle weakening associated with greater mantle hydration trenchward prior to collision. Asthenospheric upwelling driven by slab tearing and foundering along this former interface, possibly accompanied by convective removal of the lithosphere, could have led to renewed volcanic activity after 20 Ma. Melt equilibration depths for late Miocene and Pliocene basalts together with seismic imaging of the present lithosphere indicate that relatively invariant lithospheric thicknesses of 60–70 km have persisted since the middle Miocene. Thus, no evidence is found for large-scale (tens of kilometers) Miocene delamination of the lower lithosphere from the overriding plate, which has been proposed elsewhere to account for late Miocene and younger uplift of Anatolia. INTRODUCTION Hallmarks of continental collision include lithospheric thickening and shortening, uplift and exhumation of crust, slab breakoff, diffuse volcanism, and possible delamination (van Hunen and Miller, 2015). In southeastern Anatolia, subduction of oceanic lithosphere transitioned to that of buoyant attenuated continental lithosphere in the middle to late Eocene (“soft” collision), with arrival of more typical Arabian lithosphere at the trench in the early Miocene (Ballato et al., 2011; Darin et al., 2018, and references therein). A magmatic lull was terminated by a flare-up ca. 20 Ma (Schleiffarth et al., 2018), possibly triggered by upwelling asthenosphere associated with rollback and breakoff of the subducted portion of the Arabian plate (e.g., Keskin, 2003) and/or asthenosphere heating of delaminated or otherwise thinned lithospheric mantle beneath the overriding Eurasian plate (e.g., Pearce et al., 1990). Upwelling asthenosphere could be dynamically supporting the Anatolian Plateau at >1 km elevation, since its thin lithosphere (≤60 km; Angus et al., 2006; Delph et al., 2017) should result in lower elevations if the lithosphere is in isostatic balance (Boschi et al., 2010; Uluocak et al., 2016). However, uplift of the Anatolian Plateau interior began no earlier than ca. 11 Ma (Meijers et al., 2018a, and references therein). These events should have been contemporaneous, since slab rollback and breakoff and/or delamination of mantle lithosphere are anticipated consequences of continental collision (van Hunen and Miller, 2015), and associated asthenospheric upwelling due to mantle reorganization could lead to mantle melting and isostatic—and possibly dynamic—support of a relatively high plateau (Keskin, 2003; Şengör et al., 2003; McNab et al., 2018). Melting conditions for mafic volcanic rocks in the Eastern Tauride region (Fig. 1) combined with seismic imaging (Fig. 2) provide windows into mantle evolution in response to collision and uplift of the Anatolian Plateau. Here, we considered mantle depths of melt equilibration for relatively Mg-rich lavas as paleoproxies for the lithosphere-asthenosphere boundary (LAB) and compared them to seismic images of modern-day upper-mantle architecture, leading us to propose that relatively thin lithosphere has characterized much of this region from the early Miocene to present. Slab breaks/tears and/or small-scale convective removal of the Anatolian lithosphere likely accompanied melting but do not appear to explain late Miocene uplift of the