A block diagram of the Cambrian-Ordovician lithotectonic unit illustrates three-dimensional structural interrelationships within that sequence along the length of the central Appalachian Valley and Ridge and Plateau provinces. Examination of the block diagram and the sections used to construct it illustrates that the Valley and Ridge portion of the central Appalachians can be divided into three sections based on shortening differences within the Cambrian-Ordovician lithotectonic unit. These differences are measured between the Allegheny structural front and the northwestern edge of the Great Valley. In the southern part of the central Appalachians, the shortening across this segment is approximately 23% of a 61-km undeformed length. To the north, shortening increases to 4 % of a 119-km undeformed length across Shenandoah County, Virginia, and Hardy and Grant Counties, West Virginia. In this central section, shortening increases from that in the southern section and ranges from 39% of a 120-km undeformed length across the Broadtop coal basin to 44% of a 195-km length across the Nittany arch. This central section can be further subdivided on the basis of internal shortening differences. To the north of the Nittany arch, the Lackawanna syncline and the structures bounding it assume a more northerly trend, and the shortening across this northernmost section is only 8% of an 88-km undeformed length. Our discussion is focused on Cambrian-Ordovician fault systems within the the southern part of the middle section, and on their relationship to higher level struct re, northwest in the plateau.

End_of_Article - Last_Page 1451------------