Seismic and density heterogeneities of lithosphere beneath Siberia: Evidence from the Craton long-range seismic profile

Abstract

The estimate of seismic lithosphere thickness in Siberia remains controversial in spite of long-range controlledesource data available from peaceful nuclear explosions (PNE). Published models of layered upper mantle based on this evidence fail to unambiguously constrain the asthenospheric depth. The observed velocity changes may be due either to vertical layering or to lateral heterogeneity, which are difficult to discriminate because of large (1000 km) PNE spacing. Among the upper mantle models, obtained with reference to Moho velocities derived from higher-resolution chemical explosion data, we focus especially on lateral density heterogeneity. The model reveals three velocity layers, with velocities 8.0e8.5 km/s in Layer 1, 8.6e8.7 km/s in Layer 2, and~8.5 km/s in Layer 3. Layers 2, which varies strongly in thickness, may consist of dense eclogite, judging by the high velocities. Its base may correspond to the base of the lithosphere underlain by the lower-velocity asthenospheric material of Layer 3.The lateral variations in velocity within Layer 1 and in thickness of Layer 2 correlate with major tectonic units: the West Siberian basin, the Tunguska basin with the PermianeTriassic continental flood basalts (the large igneous province of Siberian Traps), as well as the Vilyui basin and the Yakutian kimberlite province. Isostasy in the West Siberian and Vilyui basins results in thick sediments and thin crust, while the large depths of the basement and the intraecrustal discontinuity in the Tunguska basin isostatically compensate the elevated surface topography due to voluminous lavas. The magmatism left its imprint in the mantle as an attenuated "eclogitic layer" beneath the Tunguska basin. However, the available data are still insufficient to understand the exact causes of this attenuation, because mantle conditions may have changed during the elapsed 250 m.y. since then.

Water in Omphacite and Garnet From Pristine Xenolithic Eclogite: T‐X‐fO2 Controls, Retentivity, and Implications for Electrical Conductivity and Deep H2O Recycling

“…In contrast, laboratory measurements on peridotite and eclogite xenoliths have yielded maximum Vp of 8.6 km/s, which could be higher assuming high modes and, in peridotite, favorable orientation of olivine (Bascou et al, 2011;Kobussen et al, 2006). Melnik et al (2015) found three velocity layers, suspecting that eclogite contributes to the middle one with the highest Vp. The Vp estimate for eclogite xenoliths in this study is relatively high at 8.4-8.8 km/s (Table 1), but given the estimated eclogite volume in the regional lithospheric mantle (4% based on its abundance in the xenolith suite from Udachnaya; Sobolev, 1990), the bulk of the velocity signal likely comes from the dominantly peridotitic mantle.…”

“…They range from a few volume percent based on petrologic and heat flow constraints (e.g., McLean et al., 2007; Russell et al., 2001; Schulze, 1989) to ∼20% based on joint density and gravity constraints and the observation that cratonic shear‐wave velocities are higher than those that can be generated by peridotite alone (Garber et al., 2018). The upper mantle structure beneath the Siberian craton is not well constrained due to a dearth of seismic stations and events (Cherepanova & Artemieva, 2015), but anomalous V p of up to 8.9 km/s has been determined (Melnik et al., 2015). In contrast, laboratory measurements on peridotite and eclogite xenoliths have yielded maximum V p of 8.6 km/s, which could be higher assuming high modes and, in peridotite, favorable orientation of olivine (Bascou et al., 2011; Kobussen et al., 2006).…”

“…Melnik et al. (2015) found three velocity layers, suspecting that eclogite contributes to the middle one with the highest V p. The V p estimate for eclogite xenoliths in this study is relatively high at 8.4–8.8 km/s (Table 1), but given the estimated eclogite volume in the regional lithospheric mantle (4% based on its abundance in the xenolith suite from Udachnaya; Sobolev, 1990), the bulk of the velocity signal likely comes from the dominantly peridotitic mantle.…”

Seismological Studies on the Deep Interiors of the Earth Viewed from the Polar Region

“…By using active source dataset applied to the long-distance seismic profiles by the Peaceful Nuclear Explosions (PNEs), a wide area of Siberia has been investigated, and thickness of the lithosphere was derived [22]. However, variations in the thickness of the asthenosphere underneath the lithosphere could not be determined by the surveys.…”

“…The derived velocity variations in the upper mantle were supposed to be caused by a vertical-layered structure or otherwise horizontal heterogeneity; however, it is difficult to distinguish the candidate exactly because of the sparse distribution of the PNE shot interval about 1000 km. Among the upper mantle models on the basis of seismic velocity at the Moho discontinuity derived from the high-resolution PNE data, the horizontal heterogeneity of the density structure was precisely examined [22]. The derived velocity model was characterized by three layers: the first layer is of 8.0-8.5 km/s, the second of 8.6-8.7 km/s, and the third of ~8.5 km/s.…”