The Origin And Implication Of The Steep Gravity Gradient In The Vicinity Of The Martic Zone, Southeastern Pennsylvania

Abstract

A long, narrow, straight belt of closely spaced Bouguer gravity contours and lithologic boundaries coincides with the Martic shear zone, southeastern Pennsylvania. The most likely cause of the steep gravity gradient is sought by checking the influence of the density of surface lithologies, mass distribution associated with the topography, and the isostacy model. 15-20 km difference in crustal thickness across the Martic Zone is calculated using Sharma's data and maximum depth equation. Based on a shear zone geometry, gravity anomaly parallelism, and crustal thickness contrast, the Martic Zone is proposed to be the western boundary of the Piedmont terrane. The Martic Zone is compared with other areas such as the San Andreas Fault and the Alpine Fault to evaluate possible plate boundary features. This tectonic interpretation of the Martic Zone may contribute to a new view on the central Appalachian orogenic belt.