The striking change in the trend of the Appalachian mountain range—from south-southwesterly to easterly through central Pennsylvania—reflects a bend in the direction of large folds in the upper crust, which elevate and expose old resistant rocks that support many of the mountains there. Similar bends are common in other mountain ranges, and their origin and timing during continent-continent collision have been enigmatic. Ong et al. looked at twinning in calcite crystals in limestone across Pennsylvania. The orientation of twins reflects stress acting during the initial deformation of the rocks before most of the folding began (strain hardening then locks the twins in). The data suggest that, initially, compression was to the northeast, orthogonal to the southwesterly direction of the southern Appalachians; the early-formed twins in the north were then rotated clockwise. These data, and other studies of later strain, imply that the folds and thrust faults formed after this initial deformation on top of the bend and were not later rotated themselves. A rigid salient in the North American crust that became increasingly important as a barrier during collision with what is now western Africa ~300 million years ago may have caused these dynamics. — BH
Geol. Soc. Am. Bull. 119, 796 (2007).