Receiver-Function Velocity Structure of the Lithosphere beneath the Khibiny and Lovozero Massifs,Northeastern Baltic Shield

Izvestiya, Physics of the Solid Earth - Abstract—The velocity structure of the lithosphere is studied beneath the Khibiny and Lovozero massifs in the northeastern part of the Baltic shield....     

Mantle lithosphere,asthenosphere and transition zone beneath Eastern Anatolia

Journal of Seismology - By using P and S wave receiver functions and P and S wave travel time residuals, we have found velocity models for 16 seismograph stations in Eastern Anatolia. Our study is...     

Deep Structure and Dynamics of the Central Balkan Peninsula from Seismic Data

Izvestiya, Physics of the Solid Earth - Abstract—Analysis of P- and S-receiver functions for 19 seismic stations on the Balkan Peninsula has been performed. Half of the stations are in...     

The Caucasus and the Caspian Basin: Topography of Deep Seismic Boundaries

Izvestiya, Physics of the Solid Earth - Simultaneous inversion of P and S receiver functions and of dispersion curves of Rayleigh waves for 16 seismograph stations provides insight into structure...     

Siberian traps: Hypotheses and seismology data

Siberian traps are the result of huge basalt eruptions which took place about 250 Ma ago over a vast territory of Siberia. The genesis of Siberian traps is attributed to a mantle plume with a center in the region of Iceland or beneath the central Urals in terms of their present coordinates. The eruption mechanism is associated with delamination—replacement of the mantle lithosphere by the deep magma material. The receiver function analysis of the records from the Norilsk seismic station (NRIL) allows comparing these hypotheses with the factual data on the depth structure of the region of Siberian traps. The S-wave velocity section place the seismic lithosphere/asthenosphere boundary (LAB) at a depth of 155–190 km, commensurate with the data for the other cratons. The mantle lithosphere has a high S-wave velocity characteristic of cratons (4.6–4.8 km/s instead of the typical value 4.5 km/s). The seismic boundary, which is located at a depth around 410 km beneath the continents is depressed by ~10 km in the region of the NRIL station. The phase diagram of olivine/wadsleyite transformation accounts for this depression by a 50–100°С increase in temperature. At the depths of 350–400 km, the S-wave velocity drops due to partial melting. A new reduction in the S-wave velocities is observed at a depth of 460 km. The similar anomalies (deepening of the 410-km seismic boundary and low shear wave velocity at depths of 350–400 and 460–500 km, respectively) were previously revealed in the other regions of the Meso-Cenozoic volcanism. In the case of a differently directed drift of the Siberian lithosphere and underlying mantle at depths down to 500 km, these anomalies are barely accountable. In particular, if the mantle at a depth ranging from 200 to 500 km is fixed, the anomalies should be observed at the original locations where they emerged 250 Ma ago, i.e. thousands of km from the Siberian traps. Our seismic data suggest that despite the low viscosity of the asthenosphere, the mantle drift at depths ranging from 200 to 500 km is correlated with the drift of the Siberian lithospheric plate. Furthermore, the position of the mantle plume beneath the Urals is easier to reconcile with the seismic data than its position beneath Iceland because of the Siberian traps being less remote from the Urals.     

Crust and mantle of the Tien Shan from data of the receiver function tomography

A 3-D velocity model of the Tien Shan crust and upper mantle is constructed through the inversion of the receiver functions of P and S waves together with teleseismic traveltime anomalies at nearly 40 local seismic stations. It is found that in the vast central region, where no strong earthquakes have been known over the past century, the S wave velocity at depths of 10–35 km is lower than in adjacent regions by up to 10%. These data are evidence for mechanical weakness of the crust preventing the accumulation of elastic energy. Apparently, the lower velocity and the weakness of the crust are due to the presence of water. The weakness of the crust is one of the possible reasons for the strain localization responsible for the formation of the present Tien Shan but can also be due in part to the young orogenesis. The crustal thickness is largest (about 60 km) in the Tarim-Tien Shan junction zone. The crust-mantle boundary in this region descends by a jump as a result of an increase in the lower crust thickness. This is probably due to the underthrusting of the Tien Shan by the Tarim lithosphere. This causes the mechanically weak lower crust of the Tarim to delaminate and accumulate in nearly the same way as an accretionary prism during the subduction of oceanic lithosphere. In the upper mantle, the analysis has revealed a low velocity anomaly, apparently related to basaltic outflows of the Upper Cretaceous-Early Paleogene. The Cenozoic Bachu uplift in the northern Tarim depression is also associated with the low velocity anomaly. The Naryn depression is characterized by a high velocity in the upper mantle and can be interpreted as a fragment of an ancient platform.     

Seismic waves from large explosions reveal periodical deformations at lithospheric depths

Seismic signals from large explosions recorded by Soviet seismic stations in the distance range from 100 to 2800 km during 12–14 years are analysed. Periodic variations of travel times and amplitudes of P and S waves corresponding to different boundaries in the lithosphere have been discovered. Characteristic periods are close to 2 and 6 years. Since the variations of amplitudes and velocities of seismic waves may be caused by dilatational deformations of rocks, the results obtained reveal a possible existence of slow tectonic stress waves. Tectonic waves were detected not only at seismically active zones but also at aseismic regions of the Russian and Kazakh platforms. At almost all seismic stations these variations increase with the seismic ray depth in the lithosphere. This increase can be explained by the presence of bending deformations and by their attenuation due to the rheological stratification of the lithosphere. Accordingly, the theory of Elsasser's waves is generalized to include bending deformations and the presence of stationary asthenosphere flows.     

Joint inversion of P- and S-receiver functions and dispersion curves of Rayleigh waves: The results for the Central Anatolian Plateau

The P- and S-wave receiver functions and dispersion curves of the fundamental Rayleigh wave are used to study the lithosphere within the Central Anatolian Plateau. The results for eight broadband seismic stations are presented. It is established that within the plateau, the crust with a thickness of about 35 km is underlain by the mantle lid with its bottom at a depth of about 60 km. The velocities of longitudinal (Vp) and shear (Vs) waves in this layer are at most 7.6 and 4.5 km/s, respectively, and the Vp/Vs ratio is close to 1.7 (i.e., by 6% lower than in the standard IASP91 and PREM models). Such a low velocity ratio is characteristic of rocks having high orthopyroxene content. Beneath the high-velocity mantle lid, the S-wave velocity decreases to 4.0–4.2 km/s and the Vp/Vs ratio is close to its standard value (1.8). At most stations, the P-wave receiver functions do not contain seismic phase P410s, which is formed at the global seismic boundary at a depth of 410 km. The seismic boundary at a depth of 410 km is related to the olivine-spinel phase transformation, and its absence can indicate the anomalously low olivine content and high basalt content. This anomaly is probably associated with the subduction of a large amount of oceanic crust during the closure of the Tethys. The results of the study overall indicate the high informativity of the used method.