Magnetic Fabric as a Strain Indicator in Unconsolidated Sediments of the Chile Triple Junction Area

Abstract

A combined study of anisotropy of magnetic susceptibility (AMS) and Paleomagnetism of sediments from Leg 141 was achieved in order to better constrain directions of deformation in the Chile Triple Junction region. 157 paleomagnetic samples from the upper 20 to 80 mbsf of the sediment sections of each of the five sites investigated during the leg were taken aboard the JOIDES Resolution and measured post-cruise. AMS directions were reoriented in geographic coordinates using the declination of natural remanent magnetization (NRM) cleaned with alternating field (AF). The results revealed that the magnetic fabric and the strain direction obtained from principal susceptibility axes at each site confirm the regional geodynamic context resulting from plate convergence. Below an upper part that generally does not exceed a few meters, the minimum susceptibility axis, K3, is generally reoriented from a direction close to vertical, typical of a normal, primary sedimentary fabric, to a more horizontal and striking in a globally east-west azimuth direction. The direction of tightening and compression inferred from K3 axes direction for each site is related to the location of the site with respect to the ridge and the trench and can be interpreted as a consequence of the overthrust. This study thus confirms the accuracy and the sensitivity of the AMS method as a strain indicator, even in weakly deformed sediments.