The Baltimore Gneiss domes of the Maryland Piedmont

Abstract

Gneiss domes in the Maryland Piedmont near Baltimore contain a wide variety of metamorphic rock types, all metamorphosed to amphibolite grade and collectively named Baltimore Gneiss. Most were derived from interlayered sedimentary and volcanic rocks into which granitic plutons were emplaced in both Precambrian and Paleozoic time. Migmatitic features are common in the Baltimore Gneiss. Most seem to refl ect either original sedimentary layering or original layering that has been enhanced by later metamorphic differentiation, but some involved partial melting, perhaps during the Precambrian metamorphism. We fi nd no evidence for widespread melting in rocks at the present level of exposure during the Paleozoic. Limited fl uid-driven K-metasomatism may well have occurred locally, but we fi nd no evidence for metasomatism on a regional scale. Pressure-temperature conditions for the Baltimore Gneiss during the Paleozoic metamorphism are estimated to be 7-9 kb and 625-675 °C.For half a century, the anticlinal structures exposing basement gneiss at Baltimore have been cited as classic examples of mantled gneiss domes, but more recent fi eld studies and geophysical investigations suggest that they are the result of interference between late upright folds and early nappe structures formed by a combination of isoclinal folding along subhorizontal axial surfaces and thrust faults rooted in a regional décollement.

Magnetic Anomalies in Extensional Detachments: The Xistral Tectonic Window of the Lugo Dome (NW Spain)

“…They may form by constrictional buckling or interference of large buckle folds (Yin, ), with large wavelengths favored by contrasting competence. They can be fault‐bend folds in thrust belts involving cover and basement (Fisher & Olsen, ). Alternatively they can develop from gravity instabilities, commonly in the internal parts of cordilleran and collisional orogens, related with late‐orogenic or postorogenic extensional processes, such as arching of detachment faults due to isostatic rebound (core complexes; Coney & Harms, ; Lister & Davis, ), or flow of partially molten rocks (e.g., Vanderhaeghe, ; Whitney et al, ).…”

Laurentian and Amazonian sediment sources to Neoproterozoic–lower Paleozoic Maryland Piedmont rocks

“…1075 Ma (Aleinikoff et al, 2004). Seven antiformal domes on the northern and western sides of the city of Baltimore expose the Mesoproterozoic rocks in their cores (Mathews, 1907;Hopson, 1964;Fisher and Olsen , 2004). The Setters Formation was deposited nonconformably on these metaigneous rocks and partially or completely rings each dome in map view (Williams, 1891;Knopf and Jonas, 1923;Hopson, 1964).…”

“…The Setters Formation is dominated by metamorphosed feldspathic muddy sandstone, siltstone, and shale (Hopson, 1964;Fisher, 1971) and is depositionally overlain by the Cockeysville Marble (Williams, 1892;Mathews and Grasty, 1909;Choquette, 1960). The Setters Formation and the underlying Mesoproterozoic rocks were intruded by granitic plutons in early to middle Paleozoic time (Fisher and Olsen, 2004). The deposi tional ages of the Setters Formation and Cockeysville Marble are not well known but usually are inferred to be latest Neo protero zoic or Cambrian (Williams, 1891;Higgins, 1972;Fisher and Olsen, 2004).…”

“…The Setters Formation and the underlying Mesoproterozoic rocks were intruded by granitic plutons in early to middle Paleozoic time (Fisher and Olsen, 2004). The deposi tional ages of the Setters Formation and Cockeysville Marble are not well known but usually are inferred to be latest Neo protero zoic or Cambrian (Williams, 1891;Higgins, 1972;Fisher and Olsen, 2004). On the southeastern side the metaigneous rocks and their metasedimentary cara pace were overthrust by a metamorphosed Cambrian mafic-ultramafic complex (Sinha et al, 1997).…”