Evidence for Small-Scale Mantle Convection in the Upper Mantle beneath the Baikal Rift Zone

Inversion of teleseismic P wave travel time residuals collected along a 1280-km-long profile traversing the Baikal rift zone (BRZ) reveals the existence of an upwarped lithosphere/asthenosphere interface, which causes a travel time delay of about 1 s at the rift axis ( central high ). An area with early arrivals relative to the stable Siberian platform of up to 0.5 s is observed on each side of the rift, about 200 km from the rift axis ( flank lows ). While the location of the central high is approximately fixed in the vicinity of the rift axis, those of the flank lows vary as much as 200 km with the azimuth of the arriving rays. We use three techniques to invert the travel time residuals for velocity anomalies beneath the profile. Two of the techniques assume an isotropic velocity structure, and one of them considers a transversely isotropic velocity model with a vertical axis of symmetry. We use independent geophysical observations such as gravity, active source seismic exploration, and crustal thickness measurements to compare the applicability of the models. Other types of geophysical measurements suggest that the model involving transverse isotropy is a plausible one, which suggests that the central high and flank lows are caused by the combined effects of an upwarped asthenosphere with a 2.5% lateral velocity reduction, and a velocity increase due to transverse isotropy with a vertical axis of symmetry. We consider the anisotropy to be the result of the vertical component of a lithosphere/asthenosphere small-scale mantle convection system that is associated with the rifting

S K S Splitting beneath Continental Rift Zones

We present measurements of S K S splitting at 28 digital seismic stations and 35 analog stations in the Baikal rift zone, Siberia, and adjacent areas, and at 17 stations in the East African Rift in Kenya and compare them with previous measurements from the Rio Grande Rift of North America. Fast directions in the inner region of the Baikal rift zone are distributed in two orthogonal directions, NE and NW, approximately parallel and perpendicular to the NE strike of the rift. In the adjacent Siberian platform and northern Mongolian fold belt, only the rift-orthogonal fast direction is observed. In southcentral Mongolia, the dominant fast direction changes to rift-parallel again, although a small number of measurements are still rift-orthogonal. For the axial zones of the East African and Rio Grande Rifts, fast directions are oriented on average NNE, that is, rotated clockwise from the N-S trending rift. All three rifts are underlain by low-velocity upper mantle as determined from teleseismic tomography. Rift-related mantle flow provides a plausible interpretation for the rift-orthogonal fast directions. The rift-parallel fast directions near the rift axes can be interpreted by oriented magmatic cracks in the mantle or small-scale mantle convection with rift-parallel flow. The agreement between stress estimates and corresponding crack orientations lends some weight to the suggestion that the rift-parallel fast directions are caused by oriented magmatic cracks

Reply [to “Comment on “SKS Splitting beneath Continental Rifts Zones” by Gao et al.”]

Vauchez et al. [this issue] (hereinafter refered to as VBN) interpret the petrologic, tomographic, and anisotropy data from continental rifts to support a model of continental rifting [Nicolas, 1993; Nicolas et al., 1994] in which the lithosphere splits along the rift axis and asthenosphere flows in from the sides to fill the resulting gap. We suggest here that the data can also be described by a model in which the lower lithosphere is modified or eroded by active mantle upwelling over a region of significantly greater dimensions than the rift graben and that partial melt developing in the upwelling region can account for the widespread volcanism, as well as the seismic properties. Nicolas [1993] argued that rift-aligned anisotropy could be explained by rift-parallel mantle flow. We thank VBN for bringing this relevant paper to our attention. Volcanism about the East African Rift and the Rio Grande is not confined to the rifts but extends hundreds of kilometers from the rift axes (Mount Kilimanjaro, Mount Elgon, Mount Kenya in East Africa, The Jemez Lineament on the Rio Grande) in regions uplifted relative to their surroundings. The low-velocity tomographic anomalies also extend beneath the uplifted regions and are thought to be related to the uplift possibly supporting it by thermostatic buoyancy. The size of the P and S velocity contrasts and attenuation of high frequencies have led to the suggestion that large regions of the anomalous bodies have temperatures at or above the solidus [Achauer et al, 1994; Slack et al., 1994, 1996]. The wide extent of the anomalous regions is not explicable as resulting from an abyssal lithospheric dike beneath the rift intruded by asthenosphere. The extension of the East African, Baikal, and Rio Grande rift grabens has been estimated to be about 10 km [Baker et al., 1972; Baldridge et al., 1984; Morgan and Golombek, 1984; Logatchev and Florensov, 1978]. Passive influx of asthenosphere into a 10 km lithospheric dike is insufficient to explain the tomographic anomalies [Davis, 1991]. In addition, the amount of finite strain from lithospheric diking is insufficient to explain the anisotropy anomalies. Active replacement or modification of lower lithosphere either prior to, or contemporaneous with, rifting could generate tomographic anomalies of this magnitude

Thickness of the Crust along the Irkutsk-Uian-Bator-Undershill Profile from Spectral Ratios of Body Seismic Waves

The thickness of the crust is estimated from the frequency intervals between the maxima of spectral ratios of body waves. This method was applied to processing data obtained at 15 observation sites by using digital seismic stations located along the Irkutsk-Ulan-Bator-Undurshill profile. The results obtained are consistent with the previous estimates from correlation of deep seismic sounding (DSS) data for southeastern Siberia with effective heights of topographic relief

Upper mantle flow beneath and around the Hangay Dome, central Mongolia

International audienceMongolia represents the northernmost area affected by the India–Asia collision, and it is actively deformed along transpressive belts closely associated with large-scale strike-slip faults. The active and past mantle flow beneath this region is, however, poorly known. In order to investigate deep mantle deformation beneath central Mongolia and its relation with the surrounding major structures such as the Siberian craton, the Gobi–Altay belt and the Baikal rift, a NS-trending profile of broadband seismic stations has been deployed in the summer 2003 from the southern Siberian craton to the Gobi–Altay range, crossing the entire Hangay dome. Mantle flow is deduced from the splitting of teleseismic shear waves such as SKS phases. In eastern Mongolia, the permanent station ULN in Ulaanbaatar reveals the presence of two anisotropic layers, the upper one being oriented NE–SW, close to the trend of lithospheric structures and the lower one NW–SE, close to the trend of Eurasia absolute plate motion. Along the NS profile in central Mongolia, seismic anisotropy deduced from SKS splitting reveals a homogeneous NW–SE trending structure, fully consistent with the observations made in the Altay–Sayan in western Mongolia. The observed delay times of 1.5 to more than 2.0 s favor consistent mantle flow over large mantle thicknesses. Since the lithosphere is less than 100 km thick beneath central Mongolia and since the observed fast directions are parallel to the trend of the lithospheric structures but also close to the trend of the absolute plate motion, we propose that both the lithosphere and the asthenosphere may join their anisotropic effects beneath central Mongolia to explain the large delay times. Although GPS vectors represent the instantaneous displacement of the Earth's surface and SKS splitting the time and vertical integration of finite strain at depth, we use the opportunity of the dense geodetic measurements available in this region to discuss the anisotropy pattern in term of present-day deformation. In the Eurasia-fixed reference frame, GPS and SKS both depict a similar trend beneath central Mongolia, suggesting a lithospheric block “escaping” toward the east that could orient olivine a-axes in the upper mantle, within a transpressive tectonic regime. A different behaviour is observed in western Mongolia: the GPS vectors trend NS, close to the regional compression direction, whereas the fast SKS directions trend EW, suggesting a tectonic regime close to a mode of axial shortening, generating the development of an EW-trending foliation at depth. We therefore propose that Mongolia is a place where active and frozen lithospheric deformation may add their effects, together with the sublithospheric flow. In the three sources of anisotropy inferred, a fundamental role is played by the Siberian craton that acted as an undeformable core of the continent through time: the frozen Paleozoic and Mesozoic structures wrap around the craton, building up the fast anisotropic direction pattern; the present-day sublithospheric flow induced by the plate motion is likely deflecting around its deep roots; finally, the present-day tectonic regime appears to be controlled by the presence of the craton to the north

Low Seismic Velocity Layers in the Earth\u27s Crust of Eastern Siberia (Russia) and Mongolia: Receiver Function Data and Geological Implication

Analysis of teleseismic receiver functions at digital stations along the Bratsk-Irkutsk-Ulanbaatar-Undurshil profile suggests that low-velocity layers in the Earth\u27s crust exist not only beneath the Baikal rift zone, where such a layer was found earlier by Deep Seismic Sounding (DSS), but also beneath Early Paleozoic Sayan-Baikal, Paleozoic Mongolian, Early Mesozoic Mongolia-Okhotsk fold areas, and beneath the Siberian platform. The reliability of detection of the low-velocity layers by receiver function analysis has been checked by numerical modeling. The results of this modeling demonstrate that receiver functions can reveal the low- velocity layers in the crust if the initial model (starting approximation) is close to real velocity distribution, and if the model medium is divided into thin layers. Averaged DSS velocity model without low-velocity layers was used as starting approximation for the inversion of observed receiver functions. The low-velocity layers are interpreted to reflect inhomogeneities of the Earth\u27s crust formed during its evolution. Most of these layers are presumed to correspond to thick mylonite zones related to large pre-Cenozoic thrusts. The mylonites possess a great seismic anisotropy caused by the mineral orientation formed by the ductile flow in large thrust zones. They can result in low-velocity layers only for seismic waves whose rays are oriented perpendicular to the mylonite foliation, i.e., in the direction of the minimum velocity; the velocities along the foliation direction can be rather high. Therefore, the low-angle mylonite zones can be distinguished by the receiver function method, which uses the waves from the teleseismic events with nearly vertically oriented rays. The suggestion that the low-velocity layers mark low-angle thrusts is in agreement with gravity and geological data. The amount of overthrusting is estimated to be as large as several hundred kilometers. Multichannel seismic profiling can be used to verify the existence and the deep geometry of the presumed thrusts

Seismic Anisotropy and Mantle Flow beneath the Baikal Rift Zone

SEISMIC studies have shown that continental rifts such as Lake Baikal and the Great Rift Valley of East Africa are like mid-ocean rifts in that they lie above broad regions of asthenospheric upwarp of much greater extent than the surface expression of rifting1-4. The direction of mantle flow in such regions can be investigated using the seismic anisotropy created by flow-induced orientation of mantle olivine crystals5-8. Seismic studies of the Mid-Atlantic Ridge have revealed upwelling mantle flow beneath the ridge and flow normal to the ridge axis on either side8-10. Here we present results from an array of seismic stations across the Baikal rift zone in southern Siberia. The splitting in arrival times of SKS seismic waves indicates that the upper mantle beneath the rift zone is anisotropic, with the fast direction (which reflects the direction of mantle flow) being horizontal and normal to the rift axis. This suggests that the broad upwarp associated with this continental rift is caused by similar mantle flow to that at mid-ocean rifts. This may help to elucidate the processes involved in continental rifting