Upper-mantle velocity structure beneath the Siberian platform

Abstract

S U M M A R YWe present a new velocity model for the continental upper mantle beneath central Siberia based on observations of the 1982 'RIFT' Deep Seismic Sounding (DSS) profile. Three Peaceful Nuclear Explosions (PNE) were detonated to provide energy for the 2600 km long profile that extends from the Yamal Peninsula to the Mongolian border SE of Lake Baikal. In this paper, we model seismic recordings from the northernmost explosion since data from that shot shows unambiguous arrivals from the mantle-transition-zone discontinuities. The analysis combines forward-traveltime modelling and waveform matching using reflectivity synthetic seismograms.Our model for the lithosphere has velocities of 8.25-8.20 km s-' from the Moho to 117km depth. Between 117 and 123km depth, a strong velocity gradient (8.30 -8.53 km s-') is required while a moderate gradient (8.53-8.55 km s-l) exists between 123 and 136km depth. A low-velocity zone from 136 to 210km depth terminates this phase arrival branch. The gradient again rises between 210 and 233 km and depth, culminating in a high-gradient zone (8.63-8.80 km s-') between 233 and 235 km depth. Below the high-gradient zone, more moderate gradient (8.80-8.85 km s-') is required from 235 to 253 km depth, terminating in a zone of lower velocity (8.62 -8.64 km s-') from 253 to 400 km depth. The upper-mantle transition zone consists of two high-gradient zones separated by a more moderate gradient. The upper zone is best modelled as a 35 km thick velocity gradient (8.64-9.45 km s-') from 400 to 435 km depth. The existence of the velocity gradient is based on the observation that arrivals from this feature can be identified starting at 1580 km range and rapidly become prominent with increasing distance. A model with a first-order discontinuity predicts significant arrivals at ranges closer than 1580 km. Our observations contain no compelling evidence for a 520 km discontinuity, although a small discontinuity cannot be ruled out. The lower-transitionzone discontinuity is modelled as a 4 km thick gradient zone (10.25-10.62 km s-I) from 655 to 659 km depth.

Seismic inhomogeneities in the upper mantle beneath the Siberian craton (Meteorite profile)

“…The conditional homogeneity of the layer is due to the paucity of data on its properties. Figure 3 shows the traveltime curves of the first arrivals and subsequent phases of reflected waves along the Meteorite profile, supplemented after (Egorkin, 1999;Pavlenkova et al, 1996;Priestley et al, 1994). The greatest attention is attracted by the apparent velocities of first-arrival waves increased dramatically to 8.7-8.9 km/s at different offset distances (shaded in Fig.…”

Density heterogeneity of the cratonic lithosphere: A case study of the Siberian Craton

“…700 km) show that the upper mantle of the Siberian Craton is strongly heterogeneous, with the presence of a series of thin high-velocity and low-velocity layers beneath some parts of Siberia (Fuchs and Wenzel, 1997). Velocity anomalies associated with these multiple LVZs may sometimes be interpreted as the LAB (Priestley et al, 1994), although many profiles do not show any velocity change at ca. 200-250 km depth Suvorov et al, 2010).…”

Fine structure of the 410‐km discontinuity

“…Because the wavelength of body phases typically used to study the discontinuities is of the order of the length scale of the discontinuity itself, any conclusive seismic estimation of the fine structure of the 410 is made difficult by the apparent transition impedance and thickness potentially being as much a function of source spectral content as actual structure [Burdick and Helmberger, 1978; Hell-/rich and . Reported thicknesses vary by up to a factor of 10, from 35 km Under the central Eurasian craton [Priestley et al, 1994] to 2-4 km under oceanic tiple ScS bounces, whereas more recent studies have found slightly positive to no correlation, as inferred from P to SV conversions and long-period SS precursors [Shearer, 1993;Flanagan and Shearer, 1998]. From laboratory experiments and thermodynamic calculations, it is expected on the basis of Clapeyron slope magnitudes that the 410-km discontinuity should have more topography than the 670-km discontinuity ], but seismically the opposite has been found [Shearer, 1991;Flanagan and Shearer, 1998].…”