DEEP STRUCTURES ALONG THE BOUNDARY OF A COLLISIONAL BELT - ATTENUATION TOMOGRAPHY OF P-WAVES AND S-WAVES IN THE GREATER CAUCASUS

Seismic attenuation (Q(-1)) of P and S waves shows a major discontinui ty from the Russian platform to the tectonically active Greater Caucas us. 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 a ttenuation from individual spectra, using a non-linear least-squares p rocedure to determine an attenuation parameter (t) simultaneously wit h source parameters at frequencies between 1 and 15 Hz. The t measure ments are then inverted for spatial variations of Q(-1), with parametr izations of varying complexity. Model variance for heterogeneous struc tures improves by more than 30 per cent compared with homogeneous para metrizations. Site corrections also significantly improve the fit. In these inversion results, mountainous regions exhibit Q values 2-3 time s 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 t he Q(-1) measurements, at least beneath the mountains. Hence Q(-1) var iations may give a reasonable proxy for temperature; if so, then tempe rature beneath the mountains exceeds that beneath the shield by 70 deg rees-400 degrees C. These temperature increases may not be high enough to generate widespread partial melting beneath the mountains, but cou ld produce regional metamorphism and could contribute substantially to isostatic compensation of the mountains. Whatever their origin, the b oundary in seismic attenuation is abrupt and large between stable crat on and an adjacent mountain belt, demonstrating that Q(-1) is a sensit ive indicator of tectonic process.