SOUTHEAST-DIRECTED BACKTHRUSTING AND CRUSTAL THICKENING IN THE MASSANUTTEN SYNCLINORIUM, ROCKINGHAM AND PAGE COUNTIES, VIRGINIA

The Massanutten synclinorium is the dominant topographic and geologic structure of the Shenandoah Valley of Virginia, extending for approximately 160 km along a northeast strike. The synclinorium consists of open to tight folds that are typically upright or inclined to the northwest. However, previous work and more recent mapping have identified regionally extensive southeast-verging structures. Brent (1960) mapped a southeast-verging, overturned anticline east of Laird's Knob and a syncline with two overturned limbs near Runkles Gap. Recent 1:24,000-scale geologic mapping in the Elkton West 7.5-minute quadrangle, supported by the STATEMAP program, confirms and extends Brent's observations. New data from north of Runkles Gap indicate that the central Massanutten syncline is overturned and verges to the southeast. In the Fridley Gap area, farther to the northwest, Rader and Gathright (2001) show a northwest-dipping reverse fault on their 1:100,000-scale geologic map of the Charlottesville 30- x 60-minute quadrangle. Recent mapping in this area has identified structural repetition of the Massanutten and Bloomsburg Formations along a southeast-directed imbricate thrust system with at least two splays. Apparent dip-slip displacement along the major splay appears to be at least 1 km. Observed minor structures indicate additional displacement along northwest- and southeast-dipping bedding surfaces, suggesting that shortening was, in part, accommodated by flexural slip. The southwest termination of the major thrust splay occurs in the vicinity of Laird's Knob, which coincides with the change in vergence, from southeast to northwest, of the central Massanutten syncline. Farther to the northeast, preliminary mapping across the Massanutten synclinorium in the Tenth Legion quadrangle suggests similar localized, southeast fold vergence related to southeast-directed backthrusts. Thus, southeast-directed backthrusting likely produced local modification of the predominantly northwest-verging structure of the Massanutten synclinorium and suggests crustal thickening along the axis of the Massanutten Mountain complex. This may explain, in part, the present-day high relief of these ridges relative to the rest of the Shenandoah Valley.