Anisotropy of Magnetic Susceptibility (AMS) Fabrics from the Pulo do Lobo formation affected by the Southern Iberian Shear Zone
E. George Meyer, University of Wisconsin-Milwaukee
Devin Levang, University of Wisconsin-Milwaukee
Dyanna Czeck, University of Wisconsin-Milwaukee
Manuel Díaz-Azpiroz, Universidad Pablo de Olavide
Carlos Fernández, Universidad Complutense de Madrid
Abstract
The Southern Iberian Shear Zone (SISZ) is a Variscan transpressional shear zone (with flattening, left-lateral, and thrust kinematics), which forms a suture between the OMZ (Ossa-Morena Zone) and SPZ (South Portuguese Zone) terranes. The shear zone brought two geologic units into juxtaposition, the Beja-Acebuches Metabasites (BAM) and the Pulo de Lobo metasedimentary formation (PdL) comprised of interbedded quartzites and metapelites. The PdL displays heterogeneous strain partitioning between its lithologies demonstrated as differential foliation intensities and quartzite boudins. The PdL also has an anastomosing array of cross-cutting thrust/reverse shear bands with some lateral motion within <250 m of the contact with the BAM. The foliations, boudins, and shear bands become more intense towards the SISZ boundary. The purpose of this project is to study the Anisotropy of Magnetic Susceptibility (AMS) fabrics present in a suite of rocks from the PdL affected by the SISZ located in Almonaster La Real, Spain to aid in defining strain characteristics of the shear zone, and in particular to find a proxy for lineations which are only sporadically identified in outcrop. AMS measurements were carried out on two transects starting at the contact with the BAM and measuring rock fabrics in 31 samples up to 140 meters away. K1 and K2 axes are well-clustered along a plane roughly 130/310 with K3 plotting at the pole to the plane (roughly 15-> 220). Magnetic source components are largely paramagnetic components such as biotite with minor constituents of titanomagnetite or hematite. Magnetic fabrics are consistent with macroscopic foliations and lineations, where found. Like the macroscopic lineations, the magnetic lineations display a wide range of orientations within the foliation plane, with most clustering in an oblique SE orientation. The fabric orientations and degree of anisotropy do not change with this distance across the shear zone. The fabric shape parameters indicate that all rocks underwent a dominant flattening strain. Further magnetic analysis will include measuring samples further away to see if there is a difference given current evidence that the shear zone affects rocks up to ca. 250 meters away. Additionally, future work will include using high-temperature AMS to see if it can highlight different magnetic signatures from higher metamorphic grade rocks closest to the contact with the BAM.
Session
Magnetic fabrics or other records of deformation