Jianjun Wu

Date of Award


Degree Type


Degree Name

Doctor of Philosophy


As one of the first targets of the LITHOPROBE project, the Kapuskasing uplift (KU), located in the central Canadian shield, has been subjected to extensive geological and geophysical studies for the last decade.;To better delineate the crustal structure across the KU, we have reprocessed some seismic reflection data. By carefully applying standard seismic processing techniques such as first arrival muting, F-K filtering, refraction and residual static corrections, we have enhanced seismic images significantly. Our tests indicate that the full data set should be used without stacking two adjacent shot gathers into one in order to preserve dipping reflections and to keep high fold coverage.;For the first time, the shallow structure of the Ivanhoe Lake fault zone (ILFZ) is clearly imaged on a seismic section as a series of prominent northwest-dipping reflections with listric geometry. It appears that the ILFZ is a steep fault ({dollar}\sim{dollar}50{dollar}\sp\circ{dollar}) at the surface but quickly flattens out to {dollar}\sim{dollar}20{dollar}\sp\circ{dollar} at shallow depths. 2-D tomographic inversion of multiple coverage first arrivals, picked from the seismic reflection data, was also performed to reconstruct P-wave velocity structure of the ILFZ near the surface. The most striking aspect of the inversion is that it reveals a northwest-dipping high-velocity zone, in excellent agreement with the seismic reflection images. It shows that the west-dipping reflectors mark the boundary juxtaposing high-velocity against low-velocity rocks, providing additional and firm support for the interpretation of the seismic reflection images. Direct correlation with geological observations indicates that the high reflectivity associated with the fault zone most likely originates from mylonites.;The least expected yet most important finding of the reprocessing is a pronounced northwest-dipping midcrustal reflector, originating from the Abitibi belt and plunging under the KU. The existence of such a reflector is independently confirmed by wide-angle reflection data from a cross-profile. This reflector is also detected by two other reflection profiles crossing the ILFZ about 80 km to the southwest. Its concave-down shape and broad lateral extent, in conjunction with the present geological and other geophysical data, suggest that it represents underthrusting of the Abitibi greenstone rocks beneath the KU. With the overthrust (KU) and underthrust (Abitibi) defined, it becomes evident that the KU is a product of intra-plate collision. The large-scale underthrusting of the Abitibi rocks may not only be primarily responsible for the emplacement of the KU, but also act as a means to balance the overloaded Kapuskasing dense rocks in the upper crust. Neither the seismic refraction nor reflection data show conclusive evidence for a thick crustal root under the KU. (Abstract shortened by UMI.)



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