Upper-mantle velocity structure beneath the Siberian platform

• Published in: Geophysical Journal International
• Main Authors: Priestley, Keith, Cipar, John, Egorkin, Anatoli, Pavlenkova, Nina
• Format: Text
• Language: English
• Published: Oxford University Press 1994
• Subjects: Articles; Yamal Peninsula; Siberia
• Online Access: http://gji.oxfordjournals.org/cgi/content/short/118/2/369; https://doi.org/10.1111/j.1365-246X.1994.tb03968.x

We 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−1 from the Moho to 117 km depth. Between 117 and 123 km depth, a strong velocity gradient (8.30–8.53 km s−1) is required while a moderate gradient (8.53–8.55 km s−1) exists between 123 and 136 km depth. A low-velocity zone from 136 to 210 km 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−1) between 233 and 235 km depth. Below the high-gradient zone, more moderate gradient (8.80–8.85 km s−1) is required from 235 to 253 km depth, terminating in a zone of lower velocity (8.62–8.64 km s−1) 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−1) 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-transition-zone discontinuity is modelled as a 4 km thick ...