Paper No. 1
Presentation Time: 1:30 PM-4:15 PM
COMPARISON OF DIFFERENT METHODS OF STRAIN ANALYSIS WITHIN THE TAMAQUA SYNCLINE, CENTRAL PENNSYLVANIA
The Indian Run Member of the Mississippian Mauch Chunk Formation crops out on the north and south limbs of the Tamaqua Syncline on route 309 outside of Tamaqua, Pennsylvania. The sandstone and mudstone redbeds preserve strained mud cracks, raindrop impressions, and mud rip-up clasts, as well as outcrop-scale thrust faults and cleavage, each providing an independent measure of strain direction and /or magnitude. We determined the principle direction and magnitude of strain using different methods to evaluate the consistency of results revealed by using various features and techniques, and to identify discrepancies in shortening direction or strain magnitude that might indicate multiple phases in the Alleghanian Orogeny. The fold axis of the Tamaqua Syncline trends 73° giving a shortening azimuth of 163°. Map-scale thrust faults strike 67°, perpendicular to a shortening direction of 157°, and slickenlines on outcrop-scale, low-displacement (1-3m) thrust faults indicate a shortening direction of 155°. Cleavage formed before significant folding occurred, and its orientation is consistent with a shortening direction of 159°. The direction of shortening derived from elliptical strained raindrop impressions is 161°. Fry center-to-center method on a bedding surface containing hundreds of strained raindrop impressions provided an ellipticity (R) of 1.6. Calculations of ellipticity based on measurements from 40 individual raindrop impressions also yielded an average R of 1.6 with values ranging from 1.2 to 2.0 and a standard deviation of 0.17. Individual raindrops in the Mauch Chunk Formation vary in ellipticity, possibly indicating that some impressions were not initially circular, but the average ellipticity of a large population gives R similar to that derived by the Fry method. Post-deformation clustering of initially distributed mud crack trends yields a shortening direction of 160°. The various methods and deformed features used in this study reveal very similar shortening directions, between 155° to 163°, and R values of 1.6, indicating that only one phase of Alleghanian deformation was responsible for producing these features in the Tamaqua Syncline. Using different methods on different strain markers also verifies that these methods, in this locality, yield compatible results.