Reflections from a mylonitized zone in central Sweden

Abstract

Dipping reflections imaged on a deep seismic reflection profile acquired within the Baltic Shield can be indirectly linked to strain‐induced acoustic impedance contrasts within a regional brittle‐ductile shear zone: the Singö shear zone. Measurements made at various scales and frequencies estimate rock velocity within the shear zone and correlate well with the ductile component of strain previously determined by structural analysis. Velocity measurements include some made on cored field samples at confining pressures of ≤ 0.4 GPa in a laboratory (10 MHz sensor frequencies) and some made in situ using ultrasonic (54 kHz) sensors on exposures within the shear zone. Laboratory measurements show a 10% change in seismic anisotropy and a contrast of >0.5 km s−1 in bulk rock velocity between granitoid protolith and highly strained ultramylonite. The in situ velocity measurements demonstrate that the velocity variations persist over considerable volumes, including zones where strain and associated volume loss of silica remain elevated without mylonitization. Synthetic seismograms generated using a one‐dimensional velocity model of the shear zone show that velocity variations observed in the field were large enough in magnitude and of sufficient scale to produce reflections observable on the deep seismic reflection profile collected using 6–25 Hz frequencies. Ray trace, two‐dimensional forward modeling of a high‐density, wide‐angle reflection section demonstrates that deep dipping reflections could result from strain‐induced acoustic impedance contrasts within the Singö or similar, nearby shear zones.