Deep structures along the boundary of a collisional belt: attenuation tomography of P and S waves in the Greater Caucasus

Seismic attenuation (Q(-1)) of P and S waves shows a major discontinuity from the Russian platform to the tectonically active Greater Caucasus. Broadband records show this boundary over paths less than or equal to 4 degrees long, as revealed by the decay of amplitude spectra from a digital seismic network flanking the Greater Caucasus. We measure attenuation from individual spectra, using a non-linear least-squares procedure to determine an attenuation parameter (t*) simultaneously with source parameters at frequencies between 1 and 15 Hz. The t* measurements are then inverted for spatial variations of Q(-1), with parametrizations of varying complexity. Model variance for heterogeneous structures improves by more than 30 per cent compared with homogeneous parametrizations. Site corrections also significantly improve the fit. In these inversion results, mountainous regions exhibit Q values 2-3 times lower (Q(S) = 775 +/- 75) than do the adjacent shields (Q(S) = 2060 +/- 315), showing that the discontinuity is large. For both regions, Q(P) is roughly equal to Q(S). Comparison of body-wave to coda spectra indicates that intrinsic absorption rather than scattering dominates the Q(-1) measurements, at least beneath the mountains. Hence Q(-1) variations may give a reasonable proxy for temperature; if so, then temperature beneath the mountains exceeds that beneath the shield by 70 degrees-400 degrees C. These temperature increases may not be high enough to generate widespread partial melting beneath the mountains, but could produce regional metamorphism and could contribute substantially to isostatic compensation of the mountains. Whatever their origin, the boundary in seismic attenuation is abrupt and large between stable craton and an adjacent mountain belt, demonstrating that Q(-1) is a sensitive indicator of tectonic process.