The mechanism of formation of the Baikal basin

How weak is the subduction zone interface?

“…-10 21 Pa s) [Artyushkov, 1983;Ranalli, 1995]. Such estimates of the sublithospheric mantle viscosity are generally derived from values integrated over depth ranges of up to~1200 km.…”

Is the Proterozoic Ladoga Rift (SE Baltic Shield) a rift?

“…However, recent seismic studies from Cenozoic (Thybo and Nielsen, 2009) and Paleozoic rifts (Lyngsie et al, 2007) have demonstrated that magmatic underplating may result in a flat Moho beneath many (if not most) continental rifts; • G2: in tectonically active rifts, low seismic velocities in the upper mantle caused by high mantle temperatures and a likely presence of partial melts; recent studies demonstrate that absolute seismic velocities may, however, still be high (Achauer and Masson, 2002), G3: in paleorifts, basalteclogite phase transition would increase seismic velocities in the subMoho mantle (Ringwood and Green, 1966), however such eclogitized material will be seismically indistinguishable from the upper mantle; in particular the pattern is not observed in the Midcontinent rift of North America (Shen et al, 2013), G4: in tectonically active rifts, high mantle temperatures and the presence of partial melts should lead to low mantle density and produce negative Bouguer gravity anomalies (Artemjev and Artyushkov, 1971), G5: in case of paleorifts, basalt-eclogite phase transition may, on the contrary, cause a significant density increase in the upper mantle (Artyushkov et al, 1990) with a characteristic gravity high such as in the Midcontinent rift (King and Zietz, 1971); the transformation rate can be significantly increased by an inflow of catalyzing fluid (Ahrens and Schubert, 1975;Austrheim, 1987), G6: high heat flow is expected in case of both active rifting (e.g. caused by mantle convective instability) and passive rifting (due to adiabatic melting associated with lithosphere extensional thinning) (McKenzie and Bickle, 1988), but heat flow anomaly should long have vanished in Precambrian paleorifts, G7: both lithosphere extension and mantle convective instability are expected to produce a significant thinning of the lithosphere (Artyushkov, 1981;Ruppel, 1995), however thermal cooling, particularly in paleorifts, could have led to lithosphere thickening by growth of the conductive boundary layer, G8: Fe-rich basaltic intrusions may produce linear magnetic anomalies along the rift valley because the igneous rocks are often strongly magnetized (Ferré et al, 2014); such anomaly is characteristic of the North American Midcontinent rift (Hinze et al, 1992), similar linear patterns of magnetic anomalies are typical for mid-ocean ridges.…”

Possibility That Continental Lithosphere Is Involved in Subduction in Back-Arc Spreading

“…It has been suggested that it might be related to transformation of gabbro into eclogite [4], to "basification" of continental crust due to emplacement of mafic magmas [5], to subcrustal erosion of continental lithosphere [6], or to rise of a mantle diapir [7,30,32]. The idea of eclogitization as the leading mechanism of formation of back-arc basins is contradicted by the high heat flow in these basins, which would lead to the appearance of pyroxene granulites of moderate density, but never to eclogites.…”