Remagnetization of the Tonoloway Formation and the Helderberg Group in the Central Appalachians: testing the origin of syntilting magnetizations

Abstract

SUMMARY Palaeomagnetic, rock magnetic, geochemical and strain studies of the Helderberg Group and the Tonoloway Formation in the Valley and Ridge province of West Virginia were performed to test the origin of syntilting magnetizations and to test the mechanisms of chemical remanent magnetization (CRM) acquisition. The test for a connection between strain and remagnetization was performed by comparing the types and levels of strain with the magnetic properties between the generally coarse-grained, thickly bedded Helderberg and the thinly bedded and finer-grained Tonoloway. Standard tilt tests as well as optimal differential untilting from two transects on the Wills Mountain anticline indicate that the Helderberg contains a pervasive late syntilting (∼30 per cent untilting) characteristic remanent magnetization (ChRM) whereas the Tonoloway contains a well defined early syntilting ChRM-1 (∼76 per cent untilting) as well as a ChRM-2, which is similar to the ChRM in the Helderberg. The ChRMs reside in magnetite. The rock magnetic results from the two units are similar although the Helderberg samples have a finer apparent grain size, perhaps as a result of magnetic hardening. Strain data reveal an apparent correlation between the syntilting magnetizations and types/degrees of pressure solution strain. Pressure solution strain is higher in the Helderberg and lower in the Tonoloway. Strain may have caused rotation of magnetic minerals, but it not clear that the differences in strain between the units were high enough to cause the rotations that are needed to explain the tilt test results. Several other viable hypotheses were also investigated. One hypothesis is a strain-enhanced chemical process that caused dissolution and precipitation of new magnetite in solution structures during folding. A preliminary test of this hypothesis produced inconclusive results. Another hypothesis, consistent with some of the characteristics of the magnetizations in the Helderberg and Tonoloway units, is that the rocks were partially to completely remagnetized by a piezoremanent remagnetization process. Geochemical results show that origin of the remanence carrying grains cannot be attributed to orogenic type fluids but were probably caused by a burial diagenetic process.