32 Latest Research Advances in Structural Geology and Tectonics (Posters)

This session addresses deformation across spatial and temporal scales through analytical techniques, computational advances, and/or novel interdisciplinary methods. We encourage submissions that address the five Grand Challenges for tectonics research.

Sunday, 19 October 2025
8:00 AM - 5:30 PM
HBGCC, Hall 1

Deformation in the Pulo do Lobo Formation affected by the Southern Iberian Shear Zone: Field Observations and Magnetic Fabrics

Paper number: 32-7
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

The Southern Iberian Shear Zone (SISZ) is a 50 km long transpressional shear zone (thrust, left-lateral, and flattening kinematics), which forms a Variscan suture between the Ossa-Morena and South Portuguese zones. The SISZ juxtaposed the Beja-Acebuches Metabasites (BAM) in the hanging wall over the Pulo de Lobo metasedimentary rocks (PdL) in the footwall. The PdL underwent an early D1 deformation and low temperature metamorphism followed by a localized prograde metamorphism during SISZ movement as the hotter BAM, which experienced an early high temperature metamorphism, thrust over the PdL. A sharp metamorphic gradient within the PdL is displayed in metapelite layers which range from garnet-bearing mica schists near the BAM contact to low temperature slates 250 meters away. The purpose of this project is to determine the extent of SISZ deformation within the PdL and its effects on structures formed by the D1 event, with a focus on well-exposed transects near Almonaster La Real, Spain. Macroscopic structures in the PdL attributed to the SISZ occur within 250 m of the boundary. They include foliations, scarce lineations, quartzite boudins, and an anastomosing array of cross-cutting thrust/reverse/oblique shear bands, all of which intensify towards the boundary. In addition, Anisotropy of Magnetic Susceptibility (AMS) of 467 samples within 300 m of the BAM boundary significantly differ from 26 samples > 1400 m away from this boundary. The former are mostly oblate with AMS shape parameter T>0.6, and their magnetic fabric orientations are consistent with macroscopic foliations and lineations, where found. The K1-K2 foliation plane is oriented roughly 310/75N and K1, like sporadic macroscopic lineations, displays a wide range of orientations within the foliation plane, mostly plunging obliquely toward the SE. AMS features do not systematically vary within 0-300 m across the shear zone. However, the AMS fabrics become less clustered between 200-300 m, suggesting that >200-250 m from the BAM boundary, the only evidence for SISZ activity in these rocks is AMS fabric. Farther away from the BAM boundary, the AMS shape parameter is more variable and not strongly oblate (e.g., 35% are prolate with T<0). The fabric orientation is also different, with K1-K2 defining a foliation roughly clustered at 080/20S and K1 orientations tightly clustered at 090/0, which is consistent with D1 structures.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-412091

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