Crustal Structure Across Coast Shear Zoone in SE Alaska and Western British Columbia: Extension of ACCRETE Wide-Angle Results to 3-D

The accreted terranes and the continental arc in southeastern Alaska and western British Columbia represent one of the best areas to study crustal structures and processes for the growth of continental crust. In particular, the structure and formation of the Coast Shear Zone (CSZ) and the Coast Mountains Batholith (CMB) is critical for understanding these processes. In 1994, the multidisciplinary ACCRETE project was carried out in this area targeting the structural contrasts across the CSZ. Seismic profiles consist of closely spaced marine air gun shot lines into 3C recording stations on land yielding densely spaced receiver gathers. Present research extends the previous ACCRETE interpretation of the crustal structure across the CSZ into 3D using two additional wide-angle lines. Line 1255, which is perpendicular to the CSZ, extends for ~80 km to the west of the CSZ, and line 1256 extends for ~200 km on the west of the CSZ, approximately parallel to the CSZ. With the basically 2-D data from line 1255 and 3-D data from line 1256 with 4 consecutive stations, a 1-D velocity model is determined to the west of the CSZ. Thereafter, several groups of 1-D models are derived to constrain the variations of the upper crustal structure across and along the strike of CSZ. A 2-D model is then obtained from Moho reflections to constrain the variations of Moho depth across the CSZ. To the west of the CSZ, the crust is modeled as consisting of 4 layers with velocities increasing from ~6.0 km/s in the upper crust to 6.8 km/s near the Moho depth of ~27 km. A shallow Tertiary graben is identified by ~300 ms advances in both the first arrivals and the reflections at the west end of shot line 1255 and the south end of shot line 1256. The observed variations of the Moho depth are in agreement with those obtained previously along the main ACCRETE transect (~27 km to the west and ~32 km to the east of the CSZ), and show that the Moho ramp is characteristic of the CSZ. The upper mantle velocity is ~7.7-7.8 km/s indicating high mantle temperature.