3-D Geometry of Inversion Structures in the Mesozoic Fundy Rift Basin, Maritime Canada
Abstract-Geologists have long recognized inverted basins worldwide, mostly within major hydrocarbon producing areas. To develop models for the origin of basin inversion on passive margins, it is necessary to determine the direction of maximum regional shortening during the inversion episode. To determine this shortening direction, it is prerequisite to describe the inversion structures, to be able to distinguish them from extensional features, and to determine how their geometries were influenced by the preexisting extensional fault system. The Mesozoic Fundy rift basin in Maritime Canada is part of the eastern North American rift system and presents a unique opportunity to study inversion structures in both the subsurface and in outcrop. Seismic data show that, in the Fundy rift basin, most inversion-related fold trends are subparallel to the preexisting border-fault strikes and thus are not good indicators of the far-field state of stress. Furthermore, the seismic data show that contractional folds are broader where associated with lower-angle faults and tighter where associated with higher-angle faults. Therefore, fold intensity is a function of preexisting fault dip and is also not a reliable shortening-direction indicator. However, folds that formed either as detachment folds or at fortuitously oriented restraining bends in the border-fault system suggest that, in the Fundy basin, maximum horizontal shortening was oriented NE-SW. Circumstantial field evidence agrees with this shortening direction, as do structural analyses by other workers in the Fundy basin and elsewhere in northeastern North America.