Crustal structure of northeastern North America: Results from the Ontario-New York-New England seismic refraction/wide-angle reflection experiment.

The Ontario-New York-New England seismic refraction/wide-angle reflection profile was acquired to investigate the deep structural interrelationships between the southeastern Grenville province and the western New England Appalachians. The Grenville province is characterized by 45 km thick crust, with an average crustal seismic velocity of 6.6 km/s and a Poisson's ratio of 0.28+0.01. In the mid-crust a laminated dome-like body is inferred to be composed of mafic cumulate sills on the basis of its high velocity (7.1 km/s) and Poisson's ratio (0.27). The lower crust is characterized by a velocity of 7.0 km/s which suggests a strongly mafic composition such as garnet pyroxene granulite. The Moho is a variable feature, characterized by en-echelon reflections suggestive of compositional interlayering. An anomalous mantle layer with a velocity of 8.6 km/s is proposed to represent an eclogized basaltic layer added to the lithosphere during Grenvillian orogenesis. The boundary between the Grenvillian craton and the western New England Appalachians is marked by an eastward dipping ramp structure which penetrates to a depth of 25 km where it soles out above a transitional mid-lower crustal interface. The New England Appalachians are characterized by an average crustal velocity of 6.4 km/s and a sharply reflective Moho delineating crustal thinning from 41 km to 37 km towards the Atlantic margin. The lower crustal velocity is 6.8 km/s, with a Poisson's ratio of 0.26+0.01. In contrast to the Grenvillian craton the seismic properties of the Appalachian lower crust are consistent with an intermediate composition interlaced with mafic sills related to extensional underplating and intrusion during the rifting of the Atlantic Ocean.