Remanence rotation due to rock strain during folding and the stepwise application of the fold test

Abstract

Numerical modeling was conducted to determine the effects of simple models of folding strain on prefolding Fisherian distributions of magnetization. I found that the constant volume, simple shear associated with flexural slip/flexural flow folding can rotate a prefolding magnetization to have a maximum precision parameter K value at partial unfolding and thus appear to be synfolding in age. If the magnetization is treated as a passive line marker, this behavior only occurs for steep initial magnetizations. The initial homogeneity of the Fisherian precision parameters from each limb is degraded by this type of deformation and may be used to detect a strain modified remanence. If the magnetization is carried by rigid, mechanically active ellipsoidal or spherical particles, a maximum K value at partial unfolding will occur for any initial inclination, with the special case that rigid spheres will be rotated to cause a maximum K value at 50% unfolding for any initial inclination. Homogeneity of limb precisions is not affected in this case. A discontinuous model of strain during folding, in which pressure solution removes material at an axial‐planar, spaced cleavage and causes limb dips steeper than bedding dips, could also produce a maximum K value at partial unfolding. Strain and remanence studies of flexural slip folding in the Allentown Formation dolomite and the Bloomsburg Formation red sandstones and mudstones suggest that (1) the rigid grain model may be a more realistic model of magnetic particle behavior during shear strain and (2) about one quarter to one third of the theoretically predicted, simple shear strain occurs in these rocks. This amount of strain cannot explain the synfolding behavior observed in the Allentown Formation but could explain the maximum K values at 80%–90% unfolding seen in the Bloomsburg Formation and, perhaps, other Appalachian clastic rocks.