CONSTRAINTS ON TIMING OF TERRANE ASSEMBLY IN THE CENTRAL APPALACHIAN PIEDMONT FROM 40AR/39AR THERMOCHRONOLOGY

There are a number of faults in the central Appalachian Piedmont that may have accommodated movement of terranes along the Laurentian margin. The most commonly cited is the Pleasant Grove-Huntingdon Valley shear zone (PGHV), which is the boundary between the Westminster and Potomac terranes of Horton et al. (1991). Other faults include the Rosemont fault in the eastern Piedmont of Pennsylvania and unnamed faults within the Potomac terrane in Maryland and Virginia. ⁴⁰Ar/³⁹Ar thermochronology places constraints on the timing of motion along these faults.

⁴⁰Ar/³⁹Ar white mica ages from rocks representing the Laurentian platform north of the Martic Line and from slope-rise rocks of the Westminster terrane, both in Lancaster Co., PA and along the Susquehanna River, are in the range 398-436 Ma (Early Devonian-Silurian). These can be interpreted as cooling from a Taconic maximum recorded by monazite growth at 450 ± 4 Ma in rocks from the same area (Wise et al., 2007) without subsequent disturbance.

⁴⁰Ar/³⁹Ar data from the eastern PA Piedmont show uniform Late Devonian white mica cooling ages from rocks representing the Laurentian platform across both the PGHV and the Rosemont fault. In the MD-VA Piedmont, initially divergent cooling curves for units on opposite sides of a cryptic fault within the Mather Gorge Formation (Potomac terrane) converge in the Late Devonian, indicating amalgamation at that time (Kunk et al., 2005). Krol et al. (1999) reported similar Late Devonian ⁴⁰Ar/³⁹Ar white mica ages from PGHV in Maryland, with only local early Alleghanian reactivation and white mica growth. Although strike-slip motion can take place without significant changes in crustal level, these data suggest that major displacements through the central Appalachians was completed in the Late Devonian.

Field relationships at the northern edge of the Baltimore Mafic Complex in Pennsylvania indicate that it was thrust over already assembled units of the Potomac terrane. A cluster of Early Mississippian ⁴⁰Ar/³⁹Ar white mica cooling ages from Potomac terrane rocks in the footwall may represent cooling following thrust emplacement of the complex. These data are consistent with Late Devonian assembly of the central Appalachian Piedmont.