MAGNETIC ANOMALY CORRELATION OF STRUCTURES IN EAST-CENTRAL PENNSYLVANIA

Over the past several years we have been collecting ground-based total-field magnetic data in east-central Pennsylvania for the purpose of interpreting and correlating subsurface structures. The data were collected along traverses as well as on a grid at approximately quarter-mile spacing. The magnetic information has subsequently been compiled and presented in a series of contour maps at various scales and show an excellent spatial correlation with several different structural features in eastern Pennsylvania.

In our study area (Easton, Riegelsville, Bedminster, Quakertown, Allentown and Nazareth Quadrangles), the structures are the result of tectonic events that created and then subsequently rifted Pangaea. Compression during the Paleozoic orogenic events has resulted in exposures of folded and faulted of Pre-Cambrian igneous and metamorphic rocks, while extension in the Mesozoic produced a series of rift basins along the eastern margin of North America. These basins were filled with cyclic, continental lacustrine sediments and intruded by a series of tholeiitic basalt sills.

Our magnetic anomalies clearly identify the transition from the Mesozoic sediments across the Triassic Border Fault into the igneous and metamorphic rocks of the Pre-Cambrian Reading-Prong. There is approximately a 2000 nT (gamma) increase from the basin into the crystalline rocks to the north with short wavelength anomalies and gradients as high as 1000 nT per kilometer in places. Within the basin there is a spatial correlation of the magnetic data with the location of the diabase sills (Coffman Hill and Haycock sheets). While there is an increase in the magnetic field over the sills, in general the anomaly wavelengths are much longer and the amplitudes lower.

Further north of the Basin are discreet outcroppings of the Pre-Cambrian Reading Prong gneisses surrounded by Cambrian and Ordovician dolomites. Here the magnetic values increase over the gneisses, but the amplitude is not as great as that demonstrated over the Pre-Cambrian rocks near the basin.

When these data are combined with gravity data that have also been collected, it is possible to develop models that can help control the subsurface attitude of these different structural elements.