An interpretation of wide‐angle compressional and shear wave data in southwest Germany: Poisson's ratio and petrological implications

Abstract

Shear wave velocity, especially when used with Compressional wave velocity, is an important constraint on estimates of the composition and physical state of the continental lithosphere. We present shear velocity (νs) models derived from high‐quality S wave data along three profiles in Southwest Germany, which combine with existing P wave velocity (νp) models to give Poisson's ratio (σ) in the crust and uppermost mantle. We then construct a laterally varying petrological model of the crust by comparing our νp and νs models with published laboratory‐measured rock velocities. In general, Poisson's ratio is 0.25 in the uppermost crust, 0.22–0.23 in the mid‐crust (but reaching a low of 0.15 in the Black Forest low‐νp zone), and varies laterally from 0.24 to 0.30 in the lower crust; the average Poisson's ratio of the whole crust is everywhere about 0.25. The laminated signature of the lower crust prominent on reflection and P wave refraction data is absent on the S wave refraction data, implying a strong high/low alternation of Poisson's ratio in the lower crust. This suggests that the lower‐crustal laminae represent mafic/silicic compositional layering (for example, basic sills in an originally more quartz‐rich lower crust) rather than alternating zones of high and low pore pressure. There is no evidence for a shear wave low‐velocity zone (LVZ) in the Black Forest mid‐crust, despite the presence of a strong P wave LVZ; this suggests high quartz content and the presence of pore fluids at low (not high) pore pressure. A clear P‐to‐S converted Moho reflection (Pm S) is visible on several shots, indicating that the Moho is, at least locally, a first‐order discontinuity. No shear wave refractions from the upper mantle are present, even where strong P wave refractions are observed, possibly indicating an increase of Poisson's ratio with depth in the upper mantle. The petrological modeling indicates that the upper and middle crust are probably of granitic composition, that the mid‐crustal low‐νp zones have high quartz content, and that the lower crust probably consists of granulites of laterally varying bulk composition, from granodioritic to gabbroic. The lower crust is more mafic in areas of high reflection density on seismic reflection profiles, suggesting that the laminations are caused by mafic intrusions into the lower crust, either prior to the Variscan orogeny or during (Tertiary?) volcanic activity.