DO TOPOGRAPHIC BARRIERS PREVENT HINTERLAND DETRITUS FROM REACHING FORELAND BASINS? TWO EXAMPLES FROM THE TACONIC AND ACADIAN OROGENIES IN THE NORTHERN APPALACHIANS

Detrital zircon studies provide constraints on the depositional age and provenance of Early Paleozoic foreland and hinterland basins in the northern Appalachians. Collision of Gondwanan-derived terranes with Laurentia occurred during both the Ordovician Taconic and Devonian Acadian orogenies. Detrital zircon populations from Early Ordovician to Early Devonian units in hinterland basins in New England indicate that both Laurentia and Gondwanan-derived terranes contributed sediment. In contrast, Early Ordovician to Late Devonian rocks deposited in foreland basins on the Laurentian margin contain only Laurentian-derived detritus, with the exception of some Late Devonian units from the Catskill basin, which contain only a minor component of Gondwanan-derived sediment. Previous studies have noted that foreland basin deposits lack detritus derived from accreted terranes in the hinterland, but commonly explain the absence by invoking tectonic burial of Gondwanan-derived terranes. The ubiquitous presence of mixed Laurentian and Gondwanan-derived detrital zircons in hinterland basins indicates that tectonic burial cannot explain the absence of detrital zircons derived from accreted terranes in foreland basins. Ordovician and Devonian airfall tephras preserved in Laurentian foreland basins record explosive hinterland arc and slab break-off volcanism. Ordovician slates from the Giddings Brook thrust sheet in the Taconic range record a shift in εNd from -12 to -8, toward more juvenile values, at approximately 466 Ma, which is interpreted to reflect the weathering of Ordovician arc rocks proximal to the Laurentian margin. This suggests that fine-grained mud was transported from the hinterland to foreland even though coarse-grained detritus was not. We suggest that foreland-directed thrusting of the rifted margin of Laurentian created topographic barriers that prevented coarse-grained hinterland detritus from reaching the foreland basins and that erosion of the thrust sheets recycled Laurentian detritus into the foreland basins. The intensity of crustal shortening in the hinterland may still have affected the rate of sedimentation in the foreland basin, but it was modulated by the reactivation of thrusts along the Laurentian margin whenever a critical taper was reestablished.