Rheological and inherited structural effects on kinematic partitioning in the Southern Iberian shear zone in the Pulo do Lobo metasedimentary rocks

Dyanna M. Czeck, University of Wisconsin-Milwaukee
Manuel Díaz-Azpiroz, Universidad Pablo de Olavide
Carlos Fernández, Universidad Complutense de Madrid
Nancy Carman, University of Wisconsin-Milwaukee
J Schneider, University of Wisconsin-Milwaukee
Steph Fones, University of Wisconsin-Milwaukee

Abstract

The Southern Iberian Shear Zone (SISZ) forms a 50 km Variscan suture in the Iberian Massif. It is oriented 295/50NE and deforms the lower half of the relatively homogeneous Beja-Acebuches Metabasites (BAM) to the north and the upper part of the complex and heterogeneous Pulo do Lobo formation (PdL) to the south. Previous studies in the BAM indicate that the SISZ deformed by left-lateral transpression with variable extrusion directions. We focus on unraveling the PdL structural history related to the SISZ, which contains interbedded quartzites and metapelites. The PdL displays a sharp metamorphic gradient at its northern boundary, presumably from contact with the BAM that experienced an early high temperature metamorphism. The lithologic consequence of this metamorphic gradient is that the metapelite layers range from garnet-bearing mica schists near the BAM contact to low temperature slates 250 meters away. In the slate region >250 meters from the BAM contact, dominant structures include gently to moderately northeast dipping foliation, meter to decameter scale folds that range from open and upright to tight and south-verging inclined, and north-dipping thrust faults. A transitional zone between 250-150 m away from the BAM contact displays infrequent structures found closer to the shear zone, suggesting that the SISZ influences 250 m of PdL. Within 150 m of the BAM contact, the PdL contains moderately-steeply northeast dipping foliations and scarce variably oriented lineations. The contrasting layer rheologies resulted in flattened quartzite boudins. Tight intrafolial folds within 50 m of the BAM contact mark the most intense SISZ PdL deformation. The main foliations and boudins are cut by an anastomosing network of north-dipping thrust/reverse shear bands with variable amounts of lateral displacement. Boudins and shear bands increase in frequency toward the BAM contact. The coplanar nature of PdL structures outside and inside the influence of the SISZ suggest that it exploited previous structures. In particular, the prior foliations were utilized and likely steepened during flattening. The anastomosing shear bands may have exploited the ramp-flat geometries of earlier thrust faults; alternatively, the shear bands and thrust faults may be coeval brittle and ductile expressions of the same kinematics. One intriguing observation is the extreme kinematic partitioning across the SISZ; the BAM accommodated more left-lateral and the PdL accommodated more thrust deformation. We hypothesize that this strain partitioning was influenced by the preexisting north-dipping foliations and possibly thrust faults that could easily accommodate the thrust component of SISZ transpression.

Session

Convergent and transpressional orogens