Postrift Deformation on the Passive Margin of Nova Scotia, Canada: Is It Really Passive?

Like the Fundy rift basin of southeastern Canada, the Orpheus basin formed by reactivation of the Cobequid-Chedabucto fault zone during early Mesozoic rifting (i.e., the breakup of Pangea). The Fundy basin has undergone significant post-rift shortening, the timing of which is poorly constrained because post-rift strata are absent. Jurassic to Quaternary post-rift strata, however, overlie the Orpheus basin. We use newly reprocessed seismic-reflection data to better define the geometry and temporal evolution of post-rift deformation in the Orpheus basin. Here, we focus on the post-rift succession between two major angular unconformities, an Oligocene(?) unconformity and the latest Jurassic/early Cretaceous Avalon unconformity.

Strata above the Avalon unconformity are deformed into broad anticlines and synclines, which generally overlie either pre-rift and/or syn-rift salt. Cretaceous strata directly above the Avalon unconformity show minor thinning over the crests of anticlines, suggesting that the rate of sediment aggradation exceeded the rate of anticlinal growth. Higher in the section, strata show greater thickness variations with respect to the anticlines. Most anticlinal growth occurred during the latter half of the Cenozoic and is associated with widespread erosion and, at least locally, uplift. Faults with normal separation cut the post-Avalon strata at the crests of anticlines and elsewhere.

Two hypotheses can explain the post-rift deformation within the Orpheus basin. One hypothesis is that salt movement triggered by gravity-driven processes such as differential loading caused the formation of the post-Avalon structures. The second hypothesis is that post-Avalon structures are related, at least partially, to basement-involved deformation. Results of experimental modeling suggest that the rate of anticlinal growth--if related to gravity-driven processes--would decrease through time as the thickness of the sedimentary cover increased. However, the rate of anticlinal growth in the Orpheus basin has increased through time, favoring some basement-involved deformation.