THE LEADING EDGE OF THE HUMBER ARM ALLOCHTHON: A RECORD OF FORELAND BASIN SEDIMENTATION AND TERRANE ACCRETION IN THE NEWFOUNDLAND APPALACHIANS

In western Newfoundland, the Humber Arm Allochthon sits structurally above the autochthonous Cambrian to Middle Ordovician carbonate platform and foreland basin of the Laurentian margin. Allochthonous rocks preserve slope and rise equivalents as well as ophiolites of the ancient Iapetus Ocean. The metamorphic sole of the ophiolites constrains ophiolite emplacement to ~470 Ma. The leading edge of the allochthon has been imaged seismically offshore of Port au Port Peninsula in western Newfoundland where it terminates in a tectonic wedge within the Middle Ordovician foreland basin. However, the latest deformation of the allochthon is constrained by Sandbian through Emsian foreland basin successions which have been folded in the roof of the tectonic wedge. Interpretation of offshore seismic data suggests that this later deformation of the allochthon was post-Emsian offshore of Port au Port Peninsula but may have begun earlier (Late Ordovician) farther north. These events were probably associated with westward subduction and accretion of peri-Gondwanan terranes in the hinterland of the orogen.

The Humber Arm Allochthon is traditionally viewed as being assembled prior to emplacement along a single basal thrust. Contrasting lithologies and varying modes of deposition within sedimentary rocks of the Middle Ordovician foreland basin allow better understanding of the timing of allochthon emplacement and its relationship to movement on steep faults. Mapping reveals a lower and upper structural slice of Humber Arm Allochthon. The lower slice, disconformably overlain by Goose Tickle Group, was overridden by the upper slice. This interleaving of Humber Arm Allochthon and Goose Tickle Group may distinguish an earlier Taconian phase of orogenesis from the later Acadian orogenic event that formed the tectonic wedge. Regional mapping and seismic interpretation suggest that steep, deep-seated normal faults that cut both the allochthon and the underlying carbonate platform had a protracted history spanning Neoproterozoic rifting, Middle Ordovician (Taconian) loading of the Laurentian margin, and Early Devonian (Acadian) inversion. They strongly controlled deposition of Middle Ordovician foreland basin strata and may have created topographic irregularities affecting the position of the allochthon’s basal thrust.