Magnetic anisotropy and paleomagnetic inclination shallowing in red beds: Evidence from the Mississippian Mauch Chunk Formation, Pennsylvania

Abstract

Red, hematite‐bearing sedimentary rocks are an important source of paleomagnetic data, particularly for continental apparent polar wander paths during the Paleozoic. This study presents magnetic anisotropy data from the Mississippian Mauch Chunk Formation of eastern Pennsylvania, indicating that these red beds have suffered from a significant amount of paleomagnetic inclination shallowing. Fourteen oriented block samples were collected from normal and reversed polarity strata identified in a previous study. Thermal demagnetization isolated the characteristic remanent magnetization at seven normal polarity horizons at unblocking temperatures greater than 670°C (mean direction, D = 354.0°, I = −18.4°, α95 = 10.2°). Anisotropy of remanence measurements (anisotropy of isothermal remanent magnetization and thermal remanent magnetization) indicate a bedding parallel, foliated magnetic fabric with foliations ranging from 1.1 to 1.35. Thermal demagnetization at 670°C of the isothermal remanent magnetization isolates the magnetic fabric of the characteristic remanence‐carrying grains and indicates stronger foliations (1.15–1.48). Anisotropy of magnetic susceptibility (AMS) also indicates bedding parallel magnetic foliations of 1.01–1.04, typical of red beds. Chemical leaching isolates the AMS of the characteristic remanence‐carrying grains to range from 1.02 to 1.07. This AMS fabric was used to correct the characteristic remanence inclination isolated by thermal demagnetization. The critical parameter needed for an accurate inclination correction, the individual particle anisotropy aχ, was determined from the correlation between the normalized principal axes of remanence and susceptibility anisotropy to be 1.06. The corrected Mauch Chunk direction, D = 354.5°, I = −56.4° results in a corrected Mauch Chunk Formation paleopole (12°N, 108°E) that is consistent with a European igneous paleopole.