THE STRUCTURAL GEOMETRY AND DEVELOPMENT OF THE CENTRAL APPALACHIAN VALLEY & RIDGE PROVINCE, SOUTH-CENTRAL PENNSYLVANIA

A series of 17 regional cross-sections with an along-strike spacing of 5 km have been constructed across the south-central Pennsylvania Valley & Ridge province in order to understand the 3-D structural geometry and kinematic evolution of the fold-and-thrust belt. The sections show that the structural geometry of the fold-and-thrust belt is relatively consistent along much of this ~025°-striking segment. The eastern part of the belt is defined by a series of imbricated Cambro-Ordovician carbonate horses with leading-edge fault-propagation-style folds and highly-faulted, steep to overturned frontal limbs. In the Broadtop synclinorium in the central part of the belt, there is only minor imbrication in the carbonate section. To the west of the synclinorium, the structural style is markedly different to the north and south of the Transylvania fault zone. To the south, two large carbonate horses comprise the Wills Mt. and Friends Cove anticlines that have a similar geometry to those to the east of the synclinorium. To the north, the structure is dominated by an antiformal stack of three carbonate thrust sheets. The total shortening increases from 30% in the south to 44% to the north. A map of the retrodeformed thrusts sheets shows a consistent spacing of major thrust ramps of 13 to 18km. The orientation of the thrust ramps generally parallel present-day fold axes, except for the Path Valley and McConnellsburg thrust ramps that strike ~040°, possibly reflecting an early shortening direction.

The structural style and ramp spacing may be related to the distribution of syn-tectonic loads during thrusting. To address this, paleo-overburden thicknesses were determined using fluid inclusion microthermometry data of CH₄±CO₂ and aqueous fluid inclusions from syn-tectonic veins. On the retrodeformed sections restored overburdens reflect a general pattern of syn-Alleghenian (post-Mississippian) tectonic load toward the hinterland of 2.0 to >4.0 km that may be due to structural overthrusting and/or to sediment shedding off of uplifted mountains (e.g., the Blue Ridge). However, in the Broadtop synclinorium, loads range from 4.3 to 6.1 km reflecting syntectonic sedimentation associated with the uplift and erosion of the thrust belt to the east. Toward the foreland, in the Nittany anticlinorium, loads are generally less than 1.5 km.