Rapid Variation of Crustal Thickness From an Ancient Craton to a Young Orogenic Belt

The thickness of the Earth\u27s crust varies significantly, ranging from a few km along mid-ocean ridges to about 80 km along convergent boundaries. The accurate determination of the depth to the Moho, which is the boundary between the crust and the mantle, provides important information about the geological processes that led to the variations. Stacking of about 1100 source-normalized seismograms (or receiver functions) recorded at about 30 sites on the Siberian Craton and the Mongolian foldbelt reveals a dramatic change in Moho depth, from about 37 km beneath the Siberian Craton to about 45 km beneath the foldbelt. The change takes place over a distance of less than 20 km. The measurements suggest that the Baikal rift zone was probably formed along this zone of sudden change in Moho depth. We use the following stacking procedure to estimate the spatial variation of the Moho depths. In order to find the optimal depth beneath a recording site, we apply a series of candidate depths in the range of 15 to 55 km at an interval of 1 km. For each candidate depth, we compress or stretch a two-layer crustal model and calculate the moveout and the ray parameters using ray-tracing based on the modified crustal model. The amplitudes on the receiver functions corresponding to the moveout are stacked. The optimal Moho depth is the one that gives the maximum stacking amplitude