Structure of the ponga unit: evidence for secondary oroclinal buckling in the foreland fold and thrust belt of the Variscan orogen, Cantabrian orocline, northern Spain
Date
2016-12-02
Authors
Del Greco, Kassandra
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Abstract
The origin of the Cantabrian orocline of the Variscan orogen in NW Iberia remains a topic of debate. We present a structural study of the Ponga Unit, a Cambrian to Carboniferous tectonostratigraphic package within the Variscan foreland fold and thrust belt that lies within the core region of the Cantabrian orocline. Our primary goal is to determine if the structure of the Ponga Unit is attributable to secondary orocline formation or if west-plunging regional folds in the area reflect lateral ramps in underlying Variscan thrust sheets.
Our mapping and structural analysis within the Ponga Unit focuses on the Laviana, Rioseco and Campo de Caso thrust sheets, and associated bounding thrusts. More than 800 structural orientation measurements were collected across the study area during a four-week field campaign. These data, coupled with data compiled from regional geological maps, allow for analysis of the crustal structure. West-plunging folds of the Laviana, Rioseco and Campo de Caso thrust sheets form km-scale anticline-syncline pairs, producing a complex fold interference pattern that is characteristic of the Ponga Unit. Our analysis shows that: 1) the geometry of the west-plunging folds is inconsistent with a lateral-ramp related interpretation; 2) the map pattern resembles a mushroom fold interference pattern that is the result of two deformation phases including secondary, orocline-related N-S shortening immediately after the cessation of E-W Variscan shortening; and 3) paleomagnetic data, notably a ‘B’ remanence magnetism, in the Ponga Unit likely overlaps in time with the cessation of Variscan deformation and records post-Variscan deformation associated with the onset of oroclinal buckling. Our results indicate that early N-S trending folds, which resulted from Variscan orogenesis, were refolded by a N-S oriented compressive stress that is attributable to the secondary buckling of the Cantabrian orocline.
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Keywords
Geology, Tectonics, Structural Geology, Pangea