Extensional fold-related sub-seismic fracturing in

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Extensional fold-related sub-seismic fracturing in the Armeña rollover (Pyrenees)
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Tavani S.1*, López B2, Muñoz J.A.3
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1 Dipartimento di Scienze della Terra, dell'Ambiente e delle Risorse, Università di Napoli Federico II,
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Napoli, Italy
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2 CASP, Department of Earth Sciences, University of Cambridge, Cambridge, UK
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3 Geomodels, Departament de Geodínamica i Geofísica, Facultat de Geologia, Universitat de Barcelona,
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Barcelona, Spain
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Abstract
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The Armeña rollover is an outstandingly exposed extensional fault-related fold
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in the southern Pyrenees. It involves more than 3 km of Upper Cretaceous wedge-
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shaped syn-kinematic sediments overlying Jurassic to Lower Cretaceous pre-kinematic
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strata, detached above an Upper Triassic salt layer. Although the rollover was positively
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inverted during the Pyrenean orogeny, its first order extensional architecture and, above
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all, the associated syn-extensional fracture pattern are well preserved, making this
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structure one of the few exposed analogues of reservoirs located in extensional
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rollovers. An integrated macro- and mesostructural study was carried out in the Armeña
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rollover, in order to get insights about the relationships between fold and fracture
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pattern evolution.
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Two sets of bed-perpendicular joints occur in both hanging-wall and foot-wall of
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the NE-dipping Armeña fault, being oriented about parallel (longitudinal) and
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Stefano Tavani
Dipartimento di Scienze della Terra,, dell'Ambiente e delle Risorse.
Università degli Studi di Napoli Federico II,
Largo San Marcellino 10, 80138 Napoli, Italy.
+39 081 2538155
[email protected]
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perpendicular (transverse) to the fault, respectively. The frequency of longitudinal joints
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increases towards the fault. Such an increase is progressive in the central portion of the
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rollover, while at its lateral termination two panels of highly and poorly fractured strata
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are well-recognizable, being divided by a very narrow transitional area. These panels
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coincide with the SW- and NE-dipping limbs of the drag-like syncline that occurs only
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in the lateral termination of the rollover.
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Our study points out that fracture densities varies in the different structural
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positions of the fold, which is expected to influence fluid flow and accumulation. In
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particular, we conclude that widespread longitudinal jointing occurred as part of the
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regional extensional framework and affected the entire rock volume with a ratherr
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constant density. Laterally decreasing fault-propagation rate caused the development of
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the drag-like syncline at the lateral termination of the rollover. There folding of strata
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above the slowly propagating fault's tip was ensured by development of further
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longitudinal joints that subsequently, during the later stage of fault-propagation,
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underwent shearing.

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