Recent thrusting in the Appalachians

Abstract

NUMEROUS orogenic episodes from late Precambrian to early Permian have shaped the Appalachian Mountains. Its last orogeny, the Allegheny orogeny, began before the end of the Lower Carboniferous and ended after the early Permian¹. Most of the Palaeozoic mountain belts of Europe and other continents were worn down long ago. It is still being discussed whether the Appalachians were constantly uplifted in post-Palaeozoic times or whether there was a Cenozoic rejuvenation of uplift^2–4. The composition of the Mesozoic and Cenozoic Atlantic coastal sediments and their gentle dip away from the Appalachians imply that the mountains stood as a high broad arch despite two hundred millions of years of erosion¹. Geological evidence has been found and is reported here of thrusting during the past 12 yr at many locations in the Appalachian Plateau and Valley and Ridge province. Five sites have been selected in Tennessee, West Virginia, and Pennsylvania to show considerable horizontal shortening and associated vertical uplift that actually holds up the mountain ranges. The recent faulting and folding have been quantitatively studied and absolutely dated by offsets of boreholes that have been drilled for blasting operations in road-cuts.