Abstract
Abstract. In the Spanish Pyrenees, the Sant Corneli-Bóixols thrust-related anticline displays an outstandingly preserved growth strata sequence. These strata lie on top of a major unconformity exposed at the anticline's forelimb that divides and decouples a lower pre-folding unit from an upper syn-folding one. The former consists of steeply dipping to overturned strata with widespread bedding-parallel slip indicative of folding by flexural slip, whereas the syn-folding strata above define a 200 m amplitude fold. In the inner and outer sectors of the forelimb, both pre- and syn-folding strata are near vertical to overturned and the unconformity angle ranges from 10 to 30°. In the central portion of the forelimb, syn-folding layers are gently dipping, whereas the angular unconformity is about 90° and the unconformity surface displays strong S–C shear structures, which provide a top-to-foreland slip sense. This sheared unconformity is offset by steeply dipping faults, which are at low angles to the underlying layers of the pre-folding unit. Strong shearing along the unconformity surface also occurred in the inner sector of the forelimb, with S–C structures providing an opposite, top-to-hinterland slip sense. Cross-cutting relationships and slip senses along the pre-folding bedding surfaces and the unconformity indicate that regardless of its orientation, layering in the pre- and syn-folding sequences of the Sant Corneli-Bóixols anticline were continuously slipped. This slipping promoted an intense stress deflection, with the maximum component of the stress tensor remaining at low angles to bedding during most of the folding process.
Highlights
Cross-cutting relationships and slip senses along the pre-folding bedding surfaces and the unconformity indicate that regardless of its orientation, layering in the pre- and syn-folding sequences of the Sant CorneliBóixols anticline were continuously slipped. This slipping promoted an intense stress deflection, with the maximum component of the stress tensor remaining at low angles to bedding during most of the folding process
In this work we have focused on the macro- and meso-structures developed within a growth strata wedge and a related major synkinematic unconformity exposed at the forelimb of the Sant Corneli-Bóixols anticline
Bedding-parallel slip occurs along pre- and syn-kinematic strata, which are oriented obliquely to each other, together with mesoscale faults cutting across strata and the unconformity
Summary
Templates used to describe the state of stress of growing regional-scale thrust-related anticlines (e.g. Hancock, 1985; Lisle, 1994; Fischer and Wilkerson, 2000; Belayneh and Cosgrove, 2004; Tavani et al, 2015) typically integrate punctual strain data (e.g. Engelder and Geiser, 1980; Laubach, 1989; Lacombe, 2012; Balsamo et al, 2016) and indirect information provided by the large-scale geometry of the structure, such as curvature or strata thinning or thickening (e.g. Price and Cosgrove, 1990). Widespread documentation of bedding-parallel slip and preservation of layer thickness provides key information for modelling the distribution of stress in actively growing anticlines. These observations indicate flexural-slip folding in the multilayered portions of reservoir-scale thrust-related folds Bedding-parallel slip occurs along pre- and syn-kinematic strata, which are oriented obliquely to each other, together with mesoscale faults cutting across strata and the unconformity This area provides an excellent, almost unique field example to observe, describe and analyse how oblique anisotropies, i.e. layers and unconformity, respond to progressive shortening and related folding in a contractional setting. The studied area allowed us to determine the threshold dip value at which flexural slip is of sufficient magnitude to deflect the maximum principal stress direction from the regional stress field
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