Alleghanian transpressional orogenic float in the Baltimore terrane, central Appalachian Piedmont

Alleghanian orogenic-float on the Martic thrust during dextal transpression, central Appalachian Piedmont

“…But, data presented in this paper suggest that the Martic thrust was last active during Alleghanian dextral tranpression. The thrust accommodated orogen-parallel escape similar to the orogenicfloat proposed for other regions of the Piedmont (Gates et al, 1999). The reactivation of the Martic thrust during the Alleghanian orogeny was possibly due to the major variations in the local structural relief along the length of one of the largest transpressional antiforms in the orogen.…”

“…The metamorphic overprint documented by Krol et al (1999), occurs within the segment of the Pleasant Grove shear zone in the western Piedmont of Pennsylvania. The age of this segment of the shear zone has not been directly constrained using geochronology, but the deformation and metamorphism was interpreted to be Alleghanian based on style of deformation and cross-cutting relationships (Valentino et al, 1994;Valentino, 1999;Gates et al, 1999).…”

“…1) has been described as the hinterland of the late Paleozoic dextral transpressional Alleghanian orogeny, that impacted the entire Appalachian mobile belt (Maguire et al, 1999;Gates et al, 1999, Valentino et al, 1994, 1999Valentino, 1999). In the Piedmont, a complex system of anastomosing ductile shear zones occurs between more rigid blocks of Precambrian crystalline basement, specifically various massifs of Grenville (ca.…”

“…These structures comprise thrust faults with oblique displacement rooted in the strike-slip zone, en-echelon domes forming over or between strike-slip zones and the formation of adjacent thrusts. The distribution of strain and the location of faults can be controlled by the occurrence and attitude of pre-existing structures (Dewey and Burke, 1973), or by the juxtaposition of rock bodies with ductility contrast such as decoupling of cover rocks over a deforming crystalline basement block (Gates et al, 1999).…”

The Baltimore Gneiss domes of the Maryland Piedmont

“…Hopson's (1964) magisterial review of Piedmont geology supported and extended Eskola's interpretation. More recent work, however, has revealed new structural complexities in the structure of the Baltimore region, which suggest that the apparent domes are better understood as refolded fold-and-thrust nappes (Bromery, 1968;Fisher, 1989;Gates et al, 1999), and has cast doubt on the extent of partial melting, especially during the Paleozoic (Olsen, 1977(Olsen, , 1999.…”