GRAVITY ANOMALIES AND SUBSURFACE MODELS OF THE READING PRONG IN EASTERN PENNSYLVANIA

The objective of this project is to define the structural relationship between the crystalline Precambrian rocks of the Reading Prong and the surrounding Paleozoic sedimentary rocks in eastern Pennsylvania through the use of sub-surface modeling based on gravity data.

The Reading Prong constitutes a complex nappe system in which crystalline basement rocks and their sedimentary cover are involved in large, thrust faulted, recumbent folds formed during the Taconic Orogeny that were subsequently deformed during the Alleghenian Orogeny (Drake, 1999). Two tectonic models have been developed to explain the structure of the Reading Prong. Drake (1969) proposed that the Reading Prong structure is the result of shallow low-angle thrust faults that were subsequently deformed. Herman and Monteverde (1989) proposed a tectonic model based upon a master decollement fault, suggesting a much deeper depth to the basement rocks as well as steeper faulting. We are testing the validity of these hypotheses by developing subsurface models based on the modeling of gravity data.

High-resolution gravity data have been collected along transects perpendicular to the strike of the Reading Prong rocks. The data were used to construct gravity maps and two-dimensional gravity profiles across the Reading Prong exposures. Regional trends were removed in order to highlight variations associated with the crystalline rocks. The anomalies across these rocks range in amplitude from +2.8 to 4.4 mgals with wavelengths between 4 and 7 km.

Subsurface density models based on the observed gravity anomalies have been produced. A density contrast of approx. +.1 g/cm³ was used for the density models based on average densities of 2.8 g/cm³ for Reading Prong crystalline rocks versus 2.7 g/cm³ for the Paleozoic sedimentary cover. The preliminary results suggest calculated anomalies based on Drake’s shallow thrusts are significantly lower than the observed gravity anomalies even when the density contrast is maximized at 0.2 g/cm³. Herman and Monteverde’s decollement model produces calculated gravity anomaly values comparable with the observed values. The models suggest that the Reading Prong rocks dip steeply towards the southeast along listric reverse faults, possibly verging into a master decollement fault at depths approaching 3-5 kilometers.