Tectonic evolution of the Baltimore Gneiss anticlines, Maryland

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 1structures 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 1are enigmatic. D 2was 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 3was 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.