The ACCRETE project is a collaborative endeavor to study terrain accretion in the central segment of the Coast Mountains orogen, on both sides of the border between SE Alaska and British Columbia. The seismic part of the project included 2 experiments: 1)1700 km of 224 channel marine multichannel [MCS] data were shot by R/V EWING along fjords and inland waterways and 2) a wide-angle refraction/reflection study using the same airgun shots recorded by 60 3-component REFTEK instruments deployed at 3-5 km intervals along the Portland Canal fjord, which transects the Coast Mountains.
P- and S- wave Moho reflections are observed on most record sections. Pn is observed at offsets exceeding 120-140 km. P/S converted phases (including those from the Moho) and intracrustal reflections are abundant in some records.
We developed a tomographic P-wave velocity model for the main ACCRETE line along the SW-NE trending Portland Canal fjord. For the starting velocity model we used the model obtained previously by the ray tracing of 5 selected shots. To comply with the limitations of the inversion program, picked travel times were spatially resampled at 1-2 km spacing. To remove the effects of the sediment cover of the bottom of the fjord, we use the velocity model of the sediment layer based on the results of the MCS study. At the first step of the inversion, we used refracted arrivals to obtain the velocities within the upper 20 km of the crust. Moho reflections were employed to constrain the velocities of the lower crust and Moho depth. The resulting tomographic velocity model shows the thickening of the crust in the landward direction, Moho relief possibly related to the accreted terrain boundaries, and a high velocity bulge under the Paleogene batholith. The velocity model obtained in our present inversion will serve as a basis for a more detailed tomographic imaging using P- and S- waves, and also for the prestack migration.
The over 800 km long steep to vertical high temperature ductile Coast shear zone, that coincides with a thermal front between the western side of the high temperature rocks of the Paleogene batholith and the previously cooled crust of the mid-Cretaceous orogen, appears to extend to the Moho, with the Moho on the east side being 5km deeper than on the west side.
The high velocity bulge under the batholith consists of velocities of over 7 km/sec at only 10 km depth. The high velocity bulge dips west from the eastern boundary of the batholith, to about 20 km depth at the Coast shear.
Although conventional processing of the MCS data reveals deep
crustal structures, including Moho, reflectors in the shallow [0 - 10 km]
structure are poorly imaged due to the topographic complexity of the water-sediment
and sediment-crust interfaces. Reflector coherence was improved by applying
a systematic set of time shifts, called "statics" corrections. For the
most part, this two dimensional processing removes a large percentage of