GRAVITY AND MAGNETIC MODELING CONSTRAINTS ON THE STRUCTURE OF THE GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA

The Grandfather Mountain window of North Carolina is a central feature in the southern Appalachian Orogen in which Grenville age gneisses and Proterozoic rift related rocks are exposed beneath overlying crystalline thrust sheets. This basic geometry is clearly visible in both gravity and magnetic data and maps. The basic reason for this is that the dominantly Late Precambrian to Ordovician aged overlying thrust sheets are composed of more dense and higher magnetic susceptibility rocks than the Laurentian margin rocks that make up the window itself.

Overall, the Grandfather Mountain window is characterized by an approximately 30 mgal negative Bouguer gravity anomaly, and a negative approximately 200 nT aeromagnetic anomaly. Both gravity and magnetic anomaly boundaries correlate with mapped geologic boundaries and faults, however the basaltic members of the Grandfather Mountain Fm. complicate the signature somewhat, particularly for magnetic data.

Initial modeling results indicate the flanks (NE, SW) of the Grandfather Mountain window are composed of a 4-6 km thick thrust sheet with a 0.15 g/cm³ positive density anomaly compared to the Laurentian rocks within the window. This thrust sheet has a low angle boundary with respect the window (e.g. Linville Falls Fault). Rocks SE of the window are bounded by the Brevard fault and are also higher density, but modeling suggests this boundary is steeper, such that the Brevard fault formed a crustal scale ramp during Alleghenian thrusting and the rocks NE and SW of the window, a flat.

A final question is when and how did the Grandfather Mountain window form? Also, used to constrain the gravity model was crustal thickness data from the CRUST1.0 seismic based data set. Although near the limit of resolution, the CRUST1.0 model indicates thinner crust within the window (41 km thick) compared to its NE and SW flanks (43-44 km). This suggests post-thrusting lower crustal delamination as a viable origin for the current geologic configuration of the Grandfather Mountain window, and that the window itself could be related to younger episodes of uplift within the southern Appalachians.