INHERITED LITHOSPHERIC CONTRAST BETWEEN THE NORTHERN AND SOUTHERN APPALACHIANS

Modern understanding of the tectonic evolution of the Appalachian orogen now allows for recognition of most of the first-order events and features of the entire mountain belt. These events and features indicate that from the Mesoproterozoic to at least the Permian, the northern and southern segments of the Appalachians record periods of first-order tectonic evolutionary divergence; these significant contrasts appear to spatially coincide with the New York promontory.

Contrasts between segments of the orogen existed from the outset of the Appalachian cycle. It has been recognized that Mesoproterozoic basement rock types south of approximately Pennsylvania are different than those to north and more recently it has been shown that basement rocks in each region display a difference in Nd and Pb isotopic signatures. During the Neoproterozoic, an early stage of rifting at ca. 770-680 Ma is recorded in the southern Appalachians, but not present in the northern part of the orogen. In the middle Paleozoic, the Acadian orogeny, attributed to the accretion of Avalonia, is a northern Appalachian event with no counterpart in the southern orogen. The volume of magmatism in both the Devonian and Carboniferous is distinctly different in the two segments; Devonian magmatism is prevalent through much of the northern Appalachians whereas Carboniferous magmatism is mainly concentrated in the southern Appalachians. Finally, late Paleozoic, Alleghanian tectonic styles contrast sharply between the segments, with substantially more shortening in the southern Appalachians, whereas strike-slip tectonics prevailed in the northern orogen.

These first-order contrasts all occur in the vicinity of the New York promontory, suggesting that a fundamental orogenic boundary persisted there from the Mesoproterozoic to the Mesozoic. The enduring contrasts between the northern and southern segments of the orogen indicate that this boundary was not an ephemeral feature, such as a plate triple junction. These contrasts may reflect, in part, variations in plate tectonic geometry, but we suggest that a difference in the lithospheric substrate from the outset of the Appalachian cycle, as reflected by contrasts in the Mesoproterozoic basement in each segment, could be the root cause of the significant contrasts outlined above.