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_Journal of Structural Geology 32 (2010) 362–376_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com locate jsg _Tertiary compressional overprint on Aptian–Albian extensional magnetic fabrics, North-Pyrenean Zone_ _Belen Oliva-Urcia a, Teresa Roman-Berdiel a,*, Antonio M. Casas a, Emilio L. Pueyo b, Cinta Osacara Departamento de Ciencias de la Tierra, Universidad de Zaragoza, 50009 Zaragoza, Spain b Instituto Geologico y Minero de España, Zaragoza, Spain_ _article info_ _Article history: Received 30 July 2009 Received in revised form 21 January 2010 Accepted 24 January 2010 Available online 4 February 2010_ _Keywords: Anisotropy of Magnetic Susceptibility Extension Compression Overprint Pyrenees_ _abstract_ _The Mauleon Basin constitutes part of the North-Pyrenean Mesozoic extensional basins inverted during Pyrenean (Late Cretaceous–Tertiary) orogeny. A structural and magnetic fabric study of the Aptian–Albian black marls of the Mauleon Basin (North-Pyrenean Zone) provides new data indicating that when there is an intense deformation associated with the compressional stage (formation of a pervasive foliation) the magnetic lineation is strongly controlled by the latter tectonic deformation recorded in the basin. However, some areas of the sedimentary basin can still preserve the primary extensional event, depending on its position with respect to basin margin faults and heterogeneous deformation areas. Structural and AMS data define a fabric transition from extensional fabrics indicating an approximate N–S extension in the central (inner) domain of the basin, where compressional deformation was moderate to NE–SW compressional fabrics in boundary domains close to the inverted faults, where compressional deformation was more important (kmax becomes nearly parallel to the main Pyrenean direction NW–SE). Consequently, fabrics in the Mauleon Basin can be interpreted as the result of the overprint of compressional deformation onto a primary extensional fabric, which in turn endures in the central domain of the basin. In spite of its complicated pattern, AMS is revealed useful when considering the long-term history of sedimentary basins undergoing several deformation events._ _© 2010 Elsevier Ltd. All rights reserved._ _1. Introduction_ _A common application of the analysis of Anisotropy of Magnetic Susceptibility (AMS) is the study of deformation in weakly deformed rocks lacking visible markers of deformation (e.g., Sagnotti et al., 1999; Cifelli et al., 2004; Raposo et al., 2006). In sedimentary rocks, the AMS analysis has been largely used in the study of deformation in compressional scenarios, where the magnetic lineation is usually parallel to the fold axes or the strike of thrusts (e.g., Graham, 1966; Kligeld et al., 1983; Pares et al., 1999; Saint-Bezar et al., 2002; Robion et al., 2007). AMS analysis has also been applied in extensional basins, where the magnetic lineation coincides with the stretching direction (e.g., Alfonsi, 1997; Cifelli et al., 2005; Soto et al., 2007, 2008). Nevertheless, most deformed areas record successive deformational events, especially in inverted extensional basins, where an extensional stage is followed by a compressional stage. In these scenarios the AMS can be a useful tool to characterise the deformational history of rocks (see e.g., Gil-Imaz et al., 2000)._ _**T. Roman-Berdiel** Tel.: +34 976 762127; fax: +34 976 761106. E-mail address: mtdjrb@unizar.es (T. Roman-Berdiel)._ _0191-8141 $ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2010.01.009_ _Recent AMS studies in inverted extensional basins, such as the Cabuerna Cretaceous Basin (located within the Basque–Cantabrian basin, Western Pyrenees) in Soto et al. (2007), indicate that AMS records at the earliest stages after deposition and compaction of rocks, with the deformation overprint of the extensional setting. The magnetic lineation in these studies represents the stretching direction of the extensional stage of the basin, and this extensional magnetic fabric is preserved and not modified by subsequent tectonic events, except for sites with compression-related cleavage (Soto et al., 2007). The overprint of a later and compressional tectonic event can produce also internal deformation and the development of compression-related cleavage that modifies the primary magnetic fabric and records the compressional events (Housen and van der Pluijm, 1991; Pares and Van der Pluijm, 2002; Oliva-Urcia et al., 2009)._ _In this study we present new AMS data from the inverted extensional Mauleon Basin (Northern Pyrenees). The main difference with previously studied Mesozoic inverted basins (Soto et al., 2007) is the intense deformation associated with the Cretaceous–Tertiary compressional stage. This deformation gave rise to the formation of a pervasive foliation in the Albian black marls and through most of the Axial Zone (Choukroune and Seguret, 1973)._ _B. Oliva-Urcia et al. Journal of Structural Geology 32 (2010) 362–376_ _363_ _Consequently, this study focuses on aspects related to changes in a primary (sedimentary or extensional) magnetic fabric during foliation development, and provides some clues about the behavior of magnetic fabrics in this tectonic setting. Furthermore it establishes a relationship between magnetic fabric development and Mesozoic extensional or Tertiary compressional stage of deformation depending of their location in the basin and with respect to compressional structures. Reliability of the interpretation of AMS was supported by a rock-magnetic study. Observations in thin sections and X-ray diffraction analyses were done in order to characterise microstructures and mineralogy._ _2. The North-Pyrenean Basin: Structural Setting_ _The Pyrenees formed during convergence between the Iberian and European plates in latest Cretaceous to Oligocene times. Previously, rifting during the Triassic and Jurassic resulted in the formation of a system of extensional basins, with a complex sedimentary evolution changing from terrestrial to lagoonal and finally to shallow marine sediments, and relatively homogeneous facies throughout the Pyrenean realm (Bourrouilh et al., 1995; Biteau et al., 2006). The extensional stage is particularly well recorded in the North-Pyrenean area, the Basque–Cantabrian basin (Western Pyrenees) and the South-Pyrenean Central Unit (SPCU, southern Pyrenees), with similar evolutionary trends. During the continental break-up, opening of the Bay of Biscay and rotation of the Iberian Peninsula (Aptian and Albian times), the basement underwent fundamental changes that influenced the tecto-sedimentary evolution and the structure of the range (Peyberne`s and Souquet, 1984). From Lower Aptian to Lower Albian the formation of deep sedimentary basins (black marls with spicules and ammonites) corresponds to the creation of the European and Iberian margins between the Tethys to the east and the Bay of Biscay to the west (Peyberne`s, 1982a,b). These basins are approximately rhombic-shaped according to the lozenged or rectangular fault pattern in the basement (N 140, N 60, Peyberne`s and Souquet, 1984). Their genesis can be explained by a sinistral transtension connected to the rifting occurring at this time in the Bay of Biscay (Montadert and Winnock, 1971; Deregnaucourt and Boillot, 1982). Subsequent intracontinental oblique convergence generated basin inversion. Compression during the Late Cretaceous and Tertiary Pyrenean orogeny overprinted and partially masked the earlier history. During Alpine deformation south-verging NW–SE thrusts, folds and related cleavage form (Fig. 1). The thrusts affecting the Mesozoic cover initiated in the Triassic materials (Choukroune, 1976; Teixell et al., 2000)._ _We focused our study in the Mauleon Basin, a strongly subsiding basin that preserves most of the Mesozoic stratigraphy. The evolution of this sector of the Pyrenean chain is linked to the strike-slip or transtensional movement of the North-Pyrenean fault and the formation of deep basins (Johnson and Hall, 1989), which are filled with black marls, and interpreted to result by pull-apart mechanisms (Debroas, 1990). The rotation of Iberia during the opening of the Bay of Biscay has been recently constrained to Aptian times (Gong et al., 2008). The Aptian–Albian black marls reach thicknesses of 1500 m in the Mauleon Basin showing clearly the regression of sedimentation at the end of the Albian. At the bottom, these marls change laterally to Aptian marine platform limestones (BRGM, 1969). Overlying the black marls several turbiditic units were deposited in the North-Pyrenean realm during the Late Cretaceous. The depocenters for each turbiditic unit probably do not coincide, and the total sedimentary thickness is about 3000 m. Since the end of the Cretaceous, and during the tectonic inversion stage, the main sedimentation area shifted towards the southern foreland basin, located on the Iberian plate and later on towards the Aquitaine Basin._ _The Mauleon Basin extends 50 km in N105E direction (Fig. 2a). The basin is divided into two sub-basins: the northern part of the basin is separated from the southern part by the Sarrance anticline in the Eastern sector (Fig. 2a and b). The present-day structure is defined by three main thrusts that follow the orientation of the basin (N105E), and from north to south they are Mailh Arrouy, Sarrance and Lakora (Dubos-Salles et al., 2007). The Mailh Arrouy and the Sarrance thrusts can be interpreted as rooted in the Triassic, but they are probably related in depth with reactivated basement normal faults. The Lakora frontal thrust can be interpreted as the southern boundary of the Mesozoic extensional basins (Fig. 2b)._ Ключевые слова: brgm, evolution, stretching direction, grain, low-temperature measurement, formation, tectonophysics, compressional, beddingcleavage plane, analysis, parallel, deformed, bedding, late cretaceous, structural element, size, central, ?eld, liquid nitrogen, geology, hrouda, direction, result, journal structural, limestone, time, neburg, albian, long-term history, increase, pyre, ams, ams data, north-pyrenean fault, phyllosilicates, bedding correction, cleavage formation, hemisphere projection, basin, nonrestored stereogram, bulletin, tecto-sedimentary evolution, orientation, bm, graham, capote, ams result, shape, extensional stage, elsevier, black marl, journal, study, cleavage, zaragoza, parameter, magnetic fabric, maule? basin, shape parameter, magnetic susceptibility, susceptibility, wa, room, mineral, robion, southern, lattard, younger rock, north, iron, stage, zone, ax, peyberne, smith, extensional basin, bourrouilh, room temperature, measured, process, kmin, inverted fault, tectonic, foliation, tectonic correction, mm, general aspect, alfonsi, distribution, extensional, curve, paramagnetic mineral, debroas, teixell, sample, martn-herna ndez, intersection lineation, guret, pyrenean, tension gash, temperature, geological, north-pyrenean zone, deformational history, maule?, earth, kmax, oblate ellipsoid, temperature increase, magnetic lineation, compressional deformation, area, cretaceous, pervasive foliation, structural geology, fault, curie temperature, plane, magnetic ellipsoid, le, bulk susceptibility, doi, interpretation, lineation, degree, site, thrust, measurement, france, correction, rochette, ellipsoid, sedimentary, pyrite, gong, north-pyrenean, pyrenean direction, internal deformation, nw–se, fabric, north-pyrenean basin, anisotropy, hall, oliva-urcia, la, ha, raposo, mesozoic, extension direction, lling, marl, hirt, type, sedimentary rock, biscay, main, sciences, iron oxide, paramagnetic, density diagram, fabric transition, interpreted, cleavage plane, journal structural geology, framboids, data, bulk, compressional stage, inverted, bm bm, rock, black, event, magnetic anisotropy, maximum, n–s, oliva-urcia journal, magnetic laboratory, turbiditic unit, program, choukroune, extension, hysteresis loop, deformation, flinn diagram, intense deformation, pyrenees, structural, geophysical, magnetic, sedimentary basin, pull-apart basin