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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 (D1, D2, D3). D1 (Grenvillian) is evidenced by radiometric age data; however, except for a transposed compositional layering in certain felsic gneiss, all D1 structures were obliterated or completely obscured by later tectonism. D2 (Taconic-Acadian) involved three phases of folding (F2a, F2b, F2c), amphibolite facies metamorphism, local migmatization, and the development of pervasive structural elements. D3 (Alleghanian-Palisades) resulted in predominantly brittle faulting and open folding (F3).

The causes of D1 are enigmatic. D2 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). D3 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.

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