Composite ferromagnetic fabrics (magnetite, greigite) measured * by AMS and partial AARM in weakly strained sandstones from western Makran, Iran

We present examples of composite magnetic fabrics (AMS and partial AARM) in five sites from the western Makran accretionnary prism. These rocks are weakly strained sandstones of Mio-Pliocene age. Rock magnetic investigations show a large occurrence of magnetite and also some greigite at two sites. Partial anhysteretic remanent magnetization coercivity spectra reveal that grain size is not homogeneous suggesting several grain fractions. AMS foliations are close to bedding, except at one greigite-bearing site where AMS foliation is almost perpendicular to bedding. Soft AARM (0–50 mT) foliations are slightly oblique both to bedding and AMS foliation. Surprisingly, hard AARM (50–100 mT) foliations are perpendicular to bedding. Comparison of two limbs of one fold suggests that AMS foliations are pre-tilting while hard-AARM foliations are post-tilting. Corrected degree of anisotropy parameters display similar trends for all sites (P′AMS≈ 1.04 < P′softAARM≈ 1.11 < P′hardAARM≈ 1.19). We suggest that AMS, soft AARM and hard AARM reflect distinct magnetic grain populations. However, a possible relationship between hard AARM and a component of natural remanent magnetization is possible. Our study shows that two orthogonal fabrics exist in these weakly strained rocks; coarse grains are parallel to bedding, while fine grains are aligned perpendicular to bedding.