The crustal structure in central Maine from coherency processed refraction data

Abstract

In 1984, the U.S. Geological Surcvey and the Canadian Department of Energy, Mines and Resources conducted an extensive seismic refraction experiment, the Quebec‐Maine Transect Project. As part of this project, a 180‐km‐long seismic refraction profile was recorded along the axis of the Central Maine synclinorium. High near‐surface velocities of 6.0–6.2 km/s made a conventional refraction analysis (including first arrivals) uninformative below about 10 km. Peak signal frequencies around 30 Hz, recorded at all offsets, and a trace spacing of 800 m led to severe spatial aliasing effects for secondary arrivals on the record sections. To allow a reliable deep structural interpretation, a coherency processing technique was applied to the data to enhance phase coherent secondary arrivals. In addition, interpretation of a coincident wide‐angle reflection study provided constraints on the deep crustal velocities and structure. The crust beneath the Central Maine synclinorium can be described by three major structural units. The upper crust is interpreted as the metasedimentary rocks of the Central Maine synclinorium with velocities that range from 6.2 to 6.3 km/s. Regions of relative lower velocity (6.0–6.1 km/s) within the upper crust are interpreted as granitic intrusions. Low velocity regions, modeled at 2.7 km depth with velocities around 5.4 km/s, are interpreted as seismic evidence for folding and doubling within the upper crustal metasedimentary sequence. At 12–15 km depth, the base of the synclinorium appears alternately as a strong reflector, indicating the presence of layering, and also as a weak reflector, especially NE of the Sebago pluton. A 4 km thick boundary layer at 24 km depth is interpreted as a major intracrustal shear zone in which the velocity increases from 6.4 to 6.8 km/s. The crust thickens from 36 to 40 km going from refraction experiment, the Quebec‐Maine Transect Project. As part of this project, a 180‐km‐long seismic refraction profile was recorded along the axis of the Central Maine synclinorium. High near‐surf ace velocities of 6.0–6.2 km/s made a conventional refraction analysis (including first arrivals) uninformative below about 10 km. Peak signal frequencies around 30 Hz, recorded at all offsets, and a trace spacing of 800 m led to severe spatial aliasing effects for secondary arrivals on the record sections. To allow a reliable deep structural interpretation, a coherency processing technique was applied to the data to enhance phase coherent secondary arrivals. In addition, interpretation of a coincident wide‐angle reflection study provided constraints on the deep crustal velocities and structure. The crust beneath the Central Maine synclinorium can be described by three major structural units. The upper crust is interpreted as the metasedimentary rocks of the Central Maine synclinorium with velocities that range from 6.2 to 6.3 km/s. Regions of relative lower velocity (6.0–6.1 km/s) within the upper crust are interpreted as granitic intrusions. Low velocity regions, modeled at 2.7 km depth with velocities around 5.4 km/s, are interpreted as seismic evidence for folding and doubling within the upper crustal metasedimentary sequence. At 12–15 km depth, the base of the synclinorium appears alternately as a strong reflector, indicating the presence of layering, and also as a weak reflector, especially NE of the Sebago pluton. A 4 km thick boundary layer at 24 km depth is interpreted as a major intracrustal shear zone in which the velocity increases from 6.4 to 6.8 km/s. The crust thickens from 36 to 40 km going from the NE to the SW end of the profile. Surface rocks at the SW end of the profile have a metamorphic depth of burial of at least 15 km, indicating that the crust below the Central Maine synclinorium may have been thickened to at least 55 km during Paleozoic collisions.