Lateral ramps as an aid to the unroofing of deep‐crustal rocks: Seismic evidence from the Grenville province

Abstract

We suggest that lateral ramps in the basement to orogenic wedges may be important agents in the rapid unroofing of deeply buried continental crust. Our suggestion is based on a study of the distribution of high‐pressure metamorphosed continental rocks in the Mesoproterozoic Grenvillian orogen in eastern Quebec. These rocks were buried to depths of more than 50 km through crustal thickening during a continental collision and were then rapidly unroofed. They are currently exposed in a 100 km × 60 km lobate embayment north of the Manicouagan Reservoir. Using a combination of surface structural data and seismic reflection data from Lithoprobe Line 55, we reconstruct the three‐dimensional form of the high‐pressure rocks. Our reconstruct‐ion methods are based on demonstrable correlations between the attitudes of gneissic layering at the surface and the attitudes of reflectors on the seismic profile. They make extensive use of the fact that much of the seismic reflectivity on Line 55 comes from outside the vertical plane of the seismic line, because of the sinuous nature of the line and its obliquity to tectonic strike. The validity of the reconstructions is demonstrated by the generation of a synthetic seismogram from the interpreted geometry that is very similar to the observed seismogram. Our reconstruction shows that a steep lateral ramp extends to the base of the crust immediately behind the broad lobe of high‐pressure rocks at the surface. The lateral ramp probably provided a favorable pathway for the extrusion of deep‐seated rocks from the deep crust, and their transport onto the foreland.