Applications of seismic pattern recognition and gravity inversion techniques to obtain enhanced subsurface images of the Earth's crust under the Central Metasedimentary Belt, Grenville Province, Ontario

Project Lithoprobe's Abitibi-Grenville transect seismic reflection lines 32 and 33 traverse the exposed Central Metasedimentary Belt (CMB) located in the Grenville province of the Precambrian Shield of Canada in southern Onta rio. These seismic lines image a zone with a protracted deformational histo ry spanning more than 300 Myr, Detailed examination of the commercially pro cessed stacked sections reveals a number of significant deficiencies in som e important areas. The image quality in these zones of reduced coherency ne eds to be enhanced to examine specific features and their relation to the s urface geology. Examination of near-vertical seismic data from Lines 32 and 33 revealed that the signal-to-noise ratio was not improved by stacking, d ue to misalignment of signals even after static, normal moveout corrections and residual static corrections. The presumed reason is that reflected sei smic energy following long ray paths in heterogeneous media suffers from re lative advances and delays in its propagation, and hence arrives at slightl y different times at the receivers, tending to be poorly aligned relative t o its theoretical traveltime curves. A pattern recognition (PR) method for signal enhancement followed by energy stacking in moving time windows was u sed in this study to improve the images in spite of misalignments. Reproces sing has refined the geometry of the reflection profiles. The objective of this paper is to use enhanced images of the seismic reflec tion data obtained by using a PR approach together with gravity data, using 2.5-D forward and 3-D inversion routines, to give an improved model of sub surface structure in the vicinity of lines 32 and 33. Line 32 is dominated by southeast-dipping reflectors soling into the lower crust. The listric geometry of the strong reflection packages of the CMB bo undary thrust zone is interpreted to represent a crustal-scale ramp-flat ge ometry that accommodated northwest-directed tectonic transport of the CRIB. This interpretation is also supported by the gravity data.