Abstract Recent detrital zircon results in both the central Appalachians and New England demonstrate that middle Ordovician, ‘Taconic’ island arcs, long considered to be peri-Laurentian, are built upon or associated with rock of Gondwanan affinity. This trip will visit granulite-facies orthogneiss of the Wilmington Complex, a 475–480 Ma magmatic arc, and the adjacent Wissahickon Formation. The Wissahickon Formation is intruded by and interlayered with meta-igneous rocks with arc affinity and contains detrital zircon populations characteristic of both Gondwanan and Laurentian sources. The Chester Park Gneiss, now known to have detrital zircon age spectra which match the Gondwana-derived Moretown Terrane in New England, is also featured. The trip will examine contact relationships between arc and Laurentian rocks and a newly discovered location where metapelitic rock contains garnet with crystallographically oriented rutile inclusions, possibly indicative of ultrahigh-temperature or ultrahigh-pressure metamorphism. We will discuss similarities between rocks of the central and northern Appalachians and evaluate a new model wherein the central Appalachian rocks were originally part of the Taconic arc in New England and were translated by strike-slip deformation to their present position in the orogen.

Abstract This field trip examines two distinct lithotectonic groups of the Pennsylvania Piedmont, separated by the Martic Line, each consisting of massifs of Mesoproterozoic gneiss overlain unconformably by Paleozoic metasediments. To the north of the Martic Line, the Mesoproterozoic gneisses are lithologically similar to rocks of the Adirondack anorthosite-mangerite-charnockite-granite (AMCG) association, and also include amphibolite-facies gneisses of felsic to mafic bulk composition. The overlying Paleozoic quartzite and carbonate ± semipelite succession records only greenschist-facies metamorphism, with on-going debate as to the extent of Taconian, Acadian, and Alleghanian contribution to the low-grade metamorphism and pervasive deformation. South of the Martic Line, AMCG lithologies are absent from the Mesoproterozoic gneisses. Paleozoic rocks of the Wissahickon Formation record low-pressure, high-temperature (0.3–0.4 GPa, 600–700 °C; andalusite-sillimanite) Silurian metamorphism, and Devonian moderate-pressure, moderate-temperature (0.6–0.8 GPa, 500–600 °C; kyanitesillimanite) metamorphism. Additionally, the Wissahickon Formation east of the Rosemont Shear Zone records Ordovician magmatic activity and limited contact metamorphism associated with emplacement of the Wilmington Complex. The Wissahickon Formation (sensu lato) is informally subdivided into three units: Glenarm Wissahickon (overlying Baltimore Gneiss and Glenarm Group, between the Embreeville and Street Road Faults), Mount Cuba Wissahickon (south and east of the Street Road fault, including a strip immediately east of the Wilmington Complex), and Wissahickon Formation (sensu stricto) (east of the Wilmington Complex). Day 1 treats the rocks north of the Martic Line. Day 2 addresses rocks south of the Martic Line, and within and around the Wilmington Complex.

Abstract This field trip provides an overview of both the geology underlying the city of Philadelphia and the hydrology of the Schuylkill River watershed. Philadelphia is located at the contact between two major East Coast physiographic provinces, the New Jersey Coastal Plain and the Piedmont. The Piedmont rocks are examined during this field trip. In this area, the Piedmont exhibits low to moderate relief and is underlain by folded and faulted sedimentary and metasedimentary rocks of early Paleozoic age. These rocks are interpreted to represent the collision of a magmatic arc with a continental landmass and adjacent forearc basin sediments during the early Paleozoic; they are intensely deformed and metamorphosed to the amphibolite and granulite facies. The Piedmont rocks appear in the type section of the Wissahickon Formation, which will be the geologic focus of this field trip. The hydrology of the Schuylkill River and one of its major tributaries, Wissahickon Creek, will also be discussed during the trip.

Geologic mapping, structural analysis, gravity modeling, and magnetics indicate that the Phoenix, Texas, Chattolanee, and Towson gneiss anticlines near Baltimore, Maryland are part of a large refolded crystalline nappe system rooted beneath the Towson anticline. Map patterns and limited structural and geophysical data from the Woodstock, Mayfield, and Clarksville anticlines suggest they form a similar nappe system that has no clearly evident root zone. Grenville-age (1,000–1,200 m.y.) Baltimore Gneiss comprises the cores of the nappes; Lower Cambrian-Precambrian to Ordovician metasedimentary rocks (Glenarm Supergroup) compose the cover. Three periods of deformation affected the Baltimore Gneiss (D 1 , D 2 , D 3 ). D 1 (Grenvillian) is evidenced by radiometric age data; however, except for a transposed compositional layering in certain felsic gneiss, all D 1 structures were obliterated or completely obscured by later tectonism. D 2 (Taconic-Acadian) involved three phases of folding (F 2a , F 2b , F 2c ), amphibolite facies metamorphism, local migmatization, and the development of pervasive structural elements. D 3 (Alleghanian-Palisades) resulted in predominantly brittle faulting and open folding (F 3 ). The causes of D 1 are enigmatic. D 2 was the result of early Paleozoic collision and suturing of an ocean floor/island arc terrane (Baltimore Complex) with a continental margin or fringing microcontinent (Baltimore Gneiss-Glenarm terrane). D 3 was the manifestation of early Mesozoic continental rifting, and possibly latest Paleozoic transcurrent plate motions. The tectonic evolution of the Baltimore Gneiss-Glenarm terrane near Baltimore involved a complex sequence of compressional and extensional ductile strain followed by brittle-ductile to brittle displacements. These deformations were manifested structurally in the emplacement, multiple refolding, and subsequent faulting of a large crystalline nappe system.