THE EFFECT OF SYNTECTONIC LOADING ON THE DEVELOPMENT AND STRUCTURAL ARCHITECTURE OF A FOLD-AND-THRUST BELT: AN EXAMPLE FROM THE CENTRAL APPALACHIANS

Numerous analog and numerical models predict that syntectonic load is a primary control on the development and structural architecture of a growing thrust wedge. In this study, the effect is documented in the Late Paleozoic Appalachian fold-and-thrust belt of the Pennsylvania salient. Fluid inclusion microthermometric data of CH₄±CO₂ and aqueous fluid inclusions in syntectonic vein minerals is used to estimate maximum paleo-overburden thickness from 265 locations. If the known stratigraphic thickness from the sample stratigraphic level through the Mississippian is known from published values, any excess thickness is assumed to be Pennsylvanian-Permian syntectonic molasse. The syntectonic loads were not instantly emplaced across the fold-and-thrust belt, but instead were time-transgressive with thrust wedge growth, with erosion of structurally elevated topography toward the hinterland resulting in forelandward deposition. This load transfer contributed to the increase of pore-fluid pressure at depth and slip along a detachment.

Restored post-Mississippian syntectonic overburdens are typically low (<1.5 to 3.0 km) in the central part of the salient, the ‘Juniata Culmination,’ which is dominated by 7-8 imbricate horses of Cambro-Ordovician carbonates that culminates in an antiformal stack of two to three carbonate thrust sheets comprising the Nittany anticline. In contrast, syntectonic loads are significantly larger above the two major synclinoria which were major depositional centers. In the southern part of the salient, the ~25 km wide Broadtop synclinorium had 4.3 to 7.2 km of syntectonic load and exhibits little to no imbrication of the Cambro-Ordovician unit and was likely a piggy-back basin during thrust wedge deformation. Similarly, the anthracite belt had 5.0 to > 10.0 km of syntectonic load, and the Cambro-Ordovician unites exhibits only low-amplitude thrust-folds.

The two depositional centers are related to the emplacement of the Blue Ridge anticlinorium in the southern part of the salient and the emplacement of crystalline massifs associated with the Reading Prong, Honey Brook Upland and similar structures to the east. The low depositional loads are between these two thrust systems. This provides important clues as to the tectonic origin of the salient.