Gravity Modeling Across The Huntingdon Valley-Cream Valley Shear Zone System In Southeastern Pennsylvania

Abstract

The Huntingdon Valley-Cream Valley shear zone system (HV-CV) is a major structure in the Pennsylvania Piedmont that separates rocks with contrasting metamorphic and deformational histories. A steep gradient in Bouguer gravity anomalies, up to 3 mgal/km, coincides with the shear zone system. The gradient, a regional anomaly on both the Bouguer gravity anomaly map and the isostatic residual anomaly map, goes through the entire study area from southwest to northeast. The calculation of Bouguer horizontal derivative anomalies narrows the anomalous source of the gradient to the HV-CV shear zone, which is several kilometers south of the traditional Martic Line. The lack of linear anomalies along the HV-CV shear zone system on the Bouguer vertical second derivative anomaly map suggests that the major density contrast, which produces the gradient, is in the lower crust or upper mantle. Further modeling of Bouguer gravity profiles across the HV-CV shear zone shows that the best model is one with a density of 0.35 glee, and a steplike offset of about 8 km with a steep dip angle at depth of 25 km to 33 km, which could be an offset in the Moho surface. Therefore, the HV-CV shear zone system is interpreted to be a lower crustal structure producing a large offset in the Moho surface. The gravity modeling supports the interpretation that the HV-CV shear zone system is a steeply-dipping Paleozoic terrane boundary in the southeastern Pennsylvania Piedmont.