ANALOGUE MODELLING OF THE FORMATION OF THE PENNSYLVANIA SALIENT: DO THE APPALACHIANS BEND AROUND AN ANCIENT RIFT?

The Pennsylvania salient is an oroclinal bend in the Central Appalachians at around 40ºN, where Appalachian faults and folds are rotated almost 90º clockwise from roughly north-south orientation observed elsewhere. Geophysical studies indicate that the lower crust beneath the salient has been thickened by a 7–10 km mafic underplate associated with Proterozoic rifting, leading to the proposal that formation of the salient was driven by the contrast between localized convergence against a strong backstop in this region and more distributed deformation to the north and south.

We test the feasibility of this hypothesis using a wide analogue sandbox featuring a mobile baseplate that is pushed underneath a fixed backstop. This allows along-strike variations in long orogenic belts to be successfully modelled. The scaling ratio is ~1x10-6, such that 1 cm ≈ 10 km. A rigid wooden block placed in the bottom of the model prior to the addition of colored fine sand layers is used to represent the strong underplate in the lower crust. A foam wedge attached to the baseplate is used as an analogue for the colliding Gondwanan continental crust. Photographs taken from the sides and top during the deformation of the sand pile are used to record the sequence of deformation, particularly the location of thrusts and back thrusts within the developing orogenic wedge relative to the rigid basement block, and any along-strike rotation of the deforming wedge.

To fully constrain the impact of the rigid basement block, its orientation and position with respect to the rigid baseplate was varied between runs. The effects of modifying the leading edge of the baseplate to more closely match a previously rifted margin were also tested. Based on our results, the basement block has a strong influence on deformation, with the deformation front being localized above the simulated underplate and causing bending of the modelled orogen across its edge. Subsurface motion of the block driven by placing it partially upon the moving baseplate also has a strong effect in the later stages of deformation. These results support the hypothesis that a localized region of stronger lower crust can cause oroclinal bending during tectonic convergence, and generate structures like the Pennsylvania salient.