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

АКАДЕМИК Н.А. ЛОГАЧЕВ И ЕГО НАУЧНАЯ ШКОЛА: ВКЛАД В ИЗУЧЕНИЕ КАЙНОЗОЙСКОГО КОНТИНЕНТАЛЬНОГО РИФТОГЕНЕЗА

N.A. Florensov and N.A. Logatchev pioneered development of fundamental concepts of the structure and evolution of the Baikal system of rift basins. At the turn to the 21st century, in view of the wide availability of scientific research data on the Cenozoic continental rift zones located in Eurasia, Africa and North America, and taking into account the application of new research methods and options to process and analyze huge amounts of geological and geophysical data, a priority was comprehensive modeling of rifting from its origin to the current period of time. This scientific challenge was addressed by the research team under the leadership of N.A. Logachev.Фундаментальные представления о строении и развитии Байкальской системы рифтовых впадин были заложены в трудах Н.А. Флоренсова и Н.А. Логачева. Высокая степень изученности кайнозойских континентальных рифтовых зон Евразии, Африки и Северной Америки, а также новые методики и возможности обработки и анализа больших массивов геологической и геофизической информации выдвинули на рубеже XX и XXI столетий в качестве приоритетной задачи создание комплексной модели развития рифтогенеза с его зарождения до современности. Решение поставленной задачи осуществлялось в рамках работ научной школы под руководством Н.А. Логачева

Efficiency of Finding Muon Track Trigger Primitives in CMS Cathode Strip Chambers

In the CMS Experiment, muon detection in the forward direction is accomplished by cathode strip chambers~(CSC). These detectors identify muons, provide a fast muon trigger, and give a precise measurement of the muon trajectory. There are 468 six-plane CSCs in the system. The efficiency of finding muon trigger primitives (muon track segments) was studied using~36 CMS CSCs and cosmic ray muons during the Magnet Test and Cosmic Challenge~(MTCC) exercise conducted by the~CMS experiment in~2006. In contrast to earlier studies that used muon beams to illuminate a very small chamber area ($< \! 0.01$~m$^2$), results presented in this paper were obtained by many installed CSCs operating {\em in situ} over an area of $\approx \! 23$~m$^2$ as a part of the~CMS experiment. The efficiency of finding 2-dimensional trigger primitives within 6-layer chambers was found to be~$99.93 \pm 0.03\%$. These segments, found by the CSC electronics within $800$~ns after the passing of a muon through the chambers, are the input information for the Level-1 muon trigger and, also, are a necessary condition for chambers to be read out by the Data Acquisition System

Consecutive nucleophilic substitution and aza Diels–Alder reaction—an efficient strategy to functionalized 2,2′-bipyridines

An efficient strategy for the synthesis of functionalized 2,2′-bipyridines is reported. The strategy is based on readily available 3-pyridyl-1,2,4-triazine 4-oxides and uses a reaction sequence of nucleophilic substitution of hydrogen and aza Diels–Alder reaction

ACADEMICIAN N.A. LOGATCHEV AND HIS SCIENTIFIC SCHOOL: CONTRUBITION TO STUDIES OF THE CENOZOIC CONTINENTAL RIFTING

N.A. Florensov and N.A. Logatchev pioneered development of fundamental concepts of the structure and evolution of the Baikal system of rift basins. At the turn to the 21st century, in view of the wide availability of scientific research data on the Cenozoic continental rift zones located in Eurasia, Africa and North America, and taking into account the application of new research methods and options to process and analyze huge amounts of geological and geophysical data, a priority was comprehensive modeling of rifting from its origin to the current period of time. This scientific challenge was addressed by the research team under the leadership of N.A. Logachev

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