Using Anisotropy of Magnetic Susceptibility (AMS) to Identify a Tectonic Lineation in the Pulo Do Lobo Formation in the Southern Iberian Shear Zone Near Aracena, Spain
Date of Award
Master of Science
Dyanna D Czeck
Julie J Bowles, Manuel M Díaz-Azpiroz
AMS, Pulo do Lobo, Southern Iberian Shear Zone
The Southern Iberian Shear Zone (SISZ) is a Variscan-aged suture representing the closure of a small ocean basin during the Carboniferous. The SISZ is bounded on either side by the well-studied, relatively homogeneous Beja-Acebuches Metabasites (BAM) and the complex and heterogeneous accretionary metasedimentary rocks of the Pulo do Lobo (PdL). The SISZ underwent a combination of shortening and lateral shearing known as transpression. Despite a very well-developed tectonic foliation, kinematic interpretation in the PdL is made difficult by a lack of easily identifiable lineations. Anisotropy of magnetic susceptibility (AMS), a geophysical technique measuring the relative anisotropy of magnetic susceptibility axes of all minerals contained within a rock, has been used successfully to identify magnetic fabrics within deformed rocks. Previous research has established the coaxiality of AMS fabric with a rock’s tectonic fabric in many cases. PdL samples showed low values of magnetic susceptibility (Kmean = 119 SI-6) and distinctly paramagnetic hysteresis, indicating the magnetic fabric in the samples is likely formed by the alignment of iron-bearing phyllosilicates, mostly biotite. AMS data for the PdL showed a strong magnetic foliation matching the orientation of that measured in the field. A number of magnetic lineations were identified along the foliation plane. Roughly half were grouped in a down-dip orientation, consistent with thrust-dominant kinematics. A population of shallow lineation orientations to the geographic SE possibly result from left-lateral shearing experienced within the PdL. Alternatively, variance in lineation orientation could be controlled by variations in the extrusion direction of the pure shear component. Variation in the ratio between the simple and pure shear components across the disparate lithologies of the PdL could also account for the measured oblique lineation orientations. Despite the complexity of their formation, orientation of magnetic lineations calculated using AMS closely matched field lineations, opening possibilities for further use of AMS in the SISZ. AMS parameters were largely controlled by lithology and did not correlate with strain magnitude. The shape of the AMS ellipsoid was oblate to strongly oblate in all samples, consistent with flattening expected in tectonic transpression. Fabric intensity measured in degree of anisotropy ranged from nearly isotropic to moderately anisotropic, with the most anisotropic samples in the quartzite where the paramagnetic fraction of the fabric reached up to 50%. This value is substantially higher than in the phyllites which contained numerous ferromagnetic components that likely decreased the overall degree of anisotropy. A region within 50 meters from the SZ was coincidental with a drop in mean susceptibility. Geochemical data from previous work that studied the same section suggest the influence of syntectonic hydrothermal fluid flow from the MORB-derived BAM side of the SISZ. This is evidenced by the higher bulk susceptibilities closer to the SISZ resulting from the creation of secondary ferromagnetic minerals and Fe-enriched biotite in the proposed hydrothermally affected zone.
Levang, Devin, "Using Anisotropy of Magnetic Susceptibility (AMS) to Identify a Tectonic Lineation in the Pulo Do Lobo Formation in the Southern Iberian Shear Zone Near Aracena, Spain" (2022). Theses and Dissertations. 3108.