Separation of Diamagnetic and Paramagnetic Fabrics Reveals Strain Directions in Carbonate Rocks

Abstract

We present a new procedure for separating magnetic fabrics in coccolith‐bearing chalk samples, demonstrated in the case studies of three sites located within the Dead Sea Fault (DSF) plate boundary. The separation is achieved by combining measurements of room temperature and low‐temperature anisotropy of magnetic susceptibility (RT‐AMS and LT‐AMS, respectively) with anisotropy of anhysteretic remanence magnetization (AARM). The LT‐AMS, measured at ~77 K, enhances the fabric of paramagnetic clay minerals. The AARM represents the fabric of ferromagnetic Fe oxides. By subtracting the paramagnetic and ferromagnetic fabrics from the RT‐AMS, the diamagnetic fabric is separated. In the studied samples, we found that the ferromagnetic contribution to the bulk magnetic fabric is negligible and could be excluded from the subtraction procedure. Our analysis indicates that in chalks with a negligible ferromagnetic contribution, diamagnetic fabric predominates the rock bulk magnetic fabric, if the mean susceptibility is <−6 × 10−6 SI, whereas with a mean susceptibility >11 × 10−6 SI, paramagnetic fabric predominates. In the studied rocks, the paramagnetic clay minerals preserve the original depositional fabric, whereas the diamagnetic minerals show a tectonic fabric. We propose a mechanism by which coccolith rotation under tectonic strain contributes to the development of the diamagnetic fabric parallel to the shortening direction. We infer that the diamagnetic fabrics of the studied rocks indicate strain regime of approximately N‐S horizontal shortening near strands of the DSF system. This suggests a deflection of the regional principal strain axes near the DSF. The diamagnetic fabric is more sensitive to tectonic strain than paramagnetic fabric in chalks and provides a valuable strain indicator near major faults.

Anisotropy of Full and Partial Anhysteretic Remanence Across Different Rock Types: 2—Coercivity Dependence of Remanence Anisotropy

“…AARMs have been successfully measured on apparently undeformed to weakly deformed limestones with weak magnetizations (Jackson, Craddock, et al, 1989;Raposo et al, 2006), speleothems (Ponte et al, 2017), and coccolith calcite rocks (Issachar et al, 2018). The degree of AARM can record gradual changes in finite strain, even when the AMS does not (Housen & van der Pluijm, 1991).…”

Strain Field Associated With a Component of Divergent Motion Along the Southern Dead Sea Fault: Insights From Magnetic Fabrics

“…The NW–SE SH max direction is perpendicular to the Baraq Fault, and therefore can be associated with its early reverse motion. Moreover, the point distribution of the paramagnetic subfabrics close to the Baraq Fault (Figure ) supports this conclusion, because the paramagnetic subfabrics do not preserve the depositional fabric, as they do away from faults (Issachar et al, ).…”

“…Figure shows the k m of chalk samples at low temperature ( k m LT ) versus k m at room temperature ( k m RT ). The positive and high linear correlations of southern Golan and Jordan Valley samples (Figure a) and central Arava Valley samples (Figure b) indicate that increase in susceptibility is related to increase in paramagnetic contents, whereas diamagnetic and ferromagnetic contents in the samples are almost constant (Issachar et al, ). Moreover, the linear regression indicates that the ferromagnetic contribution in the samples is negligible since the point k m LT = k m RT is close to the susceptibility of calcite (Figure ).…”

“…However, when measured at ~77°K, the LT‐AMS reflects the paramagnetic subfabric. The diamagnetic subfabric is analytically separable by subtracting the paramagnetic subfabric from the RT‐AMS (Issachar et al, ).…”

“…The magnetic fabrics of the chalk are dominated by diamagnetic subfabric of the calcite and paramagnetic subfabric of the clays. By combining AMS measurements at different temperatures, it is possible to separate these subfabrics (Issachar et al, ). It was demonstrated that the diamagnetic subfabrics of the chalk reflect preferred crystallographic orientation of calcite and are of tectonic origin, whereas the paramagnetic subfabrics reflect the preferred orientation of clays and tend to preserve the depositional fabric (Issachar et al, ).…”