Sub-surface Models of Long- and Short-wavelength Gravity Anomalies in Pennsylvania

Over the past several years we have been collecting and compiling gravity data in various areas in Pennsylvania to complement existing data previously compiled by the National Image and Mapping Agency and GeoNet. Supported by the Pennsylvania Geological Survey, the aim of this project is to generate a gravity map for the state. This has involved the collection of approximately 4000 new observations and identification of previously acquired data from other sources that had not been included in the above listed data bases. While we are still in the process of cleaning up the data set, it is now possible to use the data to model subsurface density changes for both short and long-wavelength anomalies. An intriguing feature of the gravity map of Pennsylvania is the long-wavelength NE-SW-trending positive and negative anomalies that have little direct correlation with the observed surface geology. The negative anomalies range in amplitude from -12 to - 40 mgals, with wavelengths from 80 to 150 km, while the positive anomalies have amplitudes from 11 to 54 mgals and wavelengths between 100 and 135 km. We have modeled several of these using both wavelength analysis and simple two-dimensional modeling. The results suggest that, unlike previous interpretations that suggested shallow basins or intrusions, part of the cause of these anomalies may be as deep as topographic variations at the crust-mantle boundary. With well-constrained regional trends we have also been able to use these data to isolate and model short- wavelength anomalies. Within the Newark Basin in southeastern Pennsylvania one focus has been on the diabase intrusions. The gravity data demonstrate a remarkable special coincidence of 5 to 10 mgal positive anomalies with the known outcrop pattern of the sills, however there are also some areas where the sill is observed to outcrop, but where the gravity signature is minimal or does not exist. The density models of the sills range in thickness from .3 km to almost 1 km and generally increase in structural thickness from east to west, suggesting a possible source towards the west-central portion of the basin, or conversely greater removal of material (and uplift) towards the east.