Determination of flow directions by combining AMS and thin-section analyses: implications for Oligocene volcanism in the Kerguelen Archipelago (southern Indian Ocean)

Abstract

International audienceWe report an anisotropy of magnetic susceptibility (AMS) study carried out on seven Oligocene volcanic sections widely distributed on the surface of the Kerguelen Archipelago. The statistical results at flow scale and at section scale were checked with AMS density diagrams and thin-section analysis. Because of the axis permutations frequently observed for lava flows, two possible flow directions were nicely defined for the majority of the sections. We then used two alternative approaches (imbrication of the minimum axis and symmetry of the AMS distribution) to infer a flow direction for each section. These two methods and thin-section analysis not being decisive for choosing between the two solutions, geological and geomorphological observations were finally used to obtain confident interpretations. The results of this study enable the supposed eruptive centres of the Kerguelen Archipelago to be located more precisely

Magnetic interactions at the origin of abnormal magnetic fabrics in lava flows: a case study from Kerguelen flood basalts

“…We found that the flow direction obtained from the stack of all normal magnetic fabrics is consistent with the geological observations. This supports Henry et al (2003) and Plenier et al (2005)’s conclusions that, at least for the volcanism encountered in Kerguelen, several successive lava flows emplaced under similar conditions have to be analysed altogether. Experimental evidences show that our samples yielding abnormal fabrics seem to be linked to a predominance of SD interacting grains (low‐Ti titanomagnetite) associated in micrometric globules and crystallized in the interstitial glass along the plagioclase laths. Our preferred interpretation to account for abnormal fabrics in these lavas relies on the influence of magnetic interactions. Our model predicts that the degree of anisotropy decreases as the magnetic interactions increase.…”

“…We found that the flow direction obtained from the stack of all normal magnetic fabrics is consistent with the geological observations. This supports Henry et al (2003) and Plenier et al (2005)’s conclusions that, at least for the volcanism encountered in Kerguelen, several successive lava flows emplaced under similar conditions have to be analysed altogether.…”

“…Even though, we were aware that such a sampling was not the best for AMS analysis and may even be unsuitable if a flow scale analysis is required. Nevertheless, reliable mean flow directions have already been obtained from similar samplings on Kerguelen flood basalts by merging AMS results from three to five immediately adjacent flows (Plenier et al 2002; Henry et al 2003; Plenier et al 2005) to decrease the large uncertainties due to the low number of sample per flow.…”

“…An extensive AMS study was carried out on Kerguelen volcanic sequences (Plenier et al 2002; Henry et al 2003; Plenier et al 2005) to decipher if the palaeomagnetic directions have to be tilt‐adjusted before to be used to characterize the past Earth’s magnetic field (Camps et al 2007). All the sequences but one yielded coherent AMS results.…”

Distinguishing the Mélange-Forming Processes in Subduction-Accretion Complexes: Constraints from the Anisotropy of Magnetic Susceptibility (AMS)

“…We used the contour diagram for the principal susceptibility axes distribution for isolating the AMS contribution of subfabrics, if any (e.g., primary sedimentary fabric from tectonic fabric) [101,[103][104][105][107][108][109][110]. As a matter of fact, subfabrics, due to distinct processes or events, may be expressed in different mineral and/or grain-size fractions and are superposed in the conventionally observed AMS [103].…”