Integrated geological and geophysical studies in the SG4 borehole area, Tagil Volcanic Arc, Middle Urals: Location of seismic reflectors and source of the reflectivity

Near-vertical incidence reflection seismic data acquired in the Tagil Volca nic Are (Middle Urals) show the upper crust to be highly reflective. Two in tersecting seismic lines located near the ongoing similar to 5400 m deep SG 4 borehole show that the main reflectivity strikes approximately N-S and di ps similar to 35 degrees-55 degrees to the east. Prominent reflections inte rcept the borehole at similar to 1000, similar to 1500, 2800-2900, similar to 3400, and between similar to 4000 and 5400 m, which correspond to interv als of low velocity/low density/low resistivity. The surface projections of these reflections lie parallel to the strike of magnetic anomaly trends. M ultioffset vertical seismic profile (VSP) data acquired in the SG4 borehole show a seismic response dominated by P to S reflected converted waves from the moderately east dipping reflectivity and from a set of very steep east dipping reflectors not imaged by the surface data Modeling of the VSP data constrains the depth at which reflectors intercept the borehole and sugges ts that the P to S conversions are best explained by low-velocity porous in tervals rather than higher-velocity mafic material. The most prominent east dipping reflection on the surface seismic data is only imaged on VSP shots that sample the crust closer to the E-W seismic line. This discrepancy bet ween the VSP and the surface seismic data is attributed to rapid lateral ch anges in the physical properties of the reflector. Surface and borehole dat a suggest that the low-velocity/low-density/low-resistivity intervals are t he most important source of reflectivity in the SG4 borehole area, although lithological contrasts may also play a role. Drill cores from the these zo nes contain hydrothermal alteration minerals indicating interaction with fl uids. Tectonic criteria suggest that they might represent imbricated fractu re zones often bounding different lithologies and/or intrusions. Some of th em might also represent high-porosity lava flows or pyroclastic units, comm on in island are environments.