Seismic wide-angle study of accreted Proterozoic crust in southeastern Wyoming

Abstract

A seismic wide-angle experiment was conducted in southeastern Wyoming, USA to investigate the seismic character of a postulated Proterozoic magmatic arc south of the suture (Cheyenne Belt) to the Archean Wyoming Province. Recordings from vibrator and dynamite sources with offsets between 34 and 126 km reveal no evidence for Moho reflections. The large-offset recordings contain multicyclic bands of reflective phases from the middle to lower crust. The data were transformed into the intercept time-ray parameter ( τ- p) domain to estimate local depth bounds. A subsequent 1D inversion using high-amplitude τ- p arrivals shows that the reflective part of the crust ranges from the depths of 25 to 40 km. This part of the crust exhibits velocities increasing from about 6.5 to 7.5 km/s. Reflectivity modeling shows that the lower crust might consist of a zone of alternating low- and high-velocity layers with average velocity increasing. The average lower crustal velocity of about 6.9 km/s suggests a predominantly mafic composition with interlayered intermediate to felsic components generating impedance contrasts that cause observable amplitudes from reflections at large offsets but not at clearly pre-critical and near-vertical distances. Our model is consistent with observations of interlayered sequences of gabbroic to ultramafic rocks with more felsic anorthositic and charnockitic rocks in the exposed lower crust of magmatic arc complexes. The lack of wide-angle Moho reflections might be explained by a gradational compositional boundary, or a transitional phase change from granulite to eclogite facies.