The 3-D crustal structure in the Manicouagan region: new seismic and gravity constraints

Abstract

Lithoprobe line 55 represents an oblique seismic crossing of tectonic domains in the Manicouagan region of the Grenville Province in eastern Quebec that has provided a basis for construction of a 3-D model of crustal structure. Reprocessing of the seismic profile presented here has enhanced the imaging of prominent, out-of-the-plane reflection zones that originate from high-pressure metamorphic rocks northwest of the line. We show that after conventional migration the apparent dips of out-of-plane reflections can be compared with regional foliation trends, but the reflection orientations, in themselves, provide only weak constraints on the orientation of out-of-plane of reflectors. We describe and implement several graphical techniques to estimate reflector orientation near large bends in the profile. Using a prestack processing technique, we find poorly resolved but shallow dips and approximately eastward strike directions for reflections from Berthé, Hart Jaune, and Tshenukutish terranes near Relais Gabriel. While many of these seismically derived attitude measurements are broadly consistent with existing 3-D structural models, some discrepancies have yet to be fully resolved. The Bouguer gravity field in this area is dominated by several intense anomalies, including the Grenville Front gravity low and the Manicouagan Plateau gravity high. The residual gravity field, obtained by subtracting the predicted anomalies for these features from the Bouguer gravity field, provides evidence in support of a lateral basement ramp.