Distinguishing the effect of diapir growth on magnetic fabrics of syn‐diapiric overburden rocks: Basque–Cantabrian basin, Northern Spain

An analysis of Anisotropy of Magnetic Susceptibility was done on Aptian–Albian sediments from the Basque–Cantabrian basin. Samples were collected from 39 sites in the halokinetic sequences of the Bakio, Bermeo, Guernica and Mungia diapirs; 28 sites were sampled close to diapirs, and 11 sites were far from the diapir edges. The magnetic foliation is parallel to bedding, suggesting it reflects depositional and compaction processes, whereas the orientation of magnetic lineation varies. Far from the diapir edges, the magnetic lineation is interpreted as being related to the regional Pyrenean compression. Close to diapir edges, the observed behaviour shows that diapirs, predominantly formed by rigid ophites, have acted as buttresses, with shadow areas at their northern faces being protected from the Pyrenean compression. The high sensitivity of AMS makes it a very useful tool to distinguish deformation in halokinetic sequences related to diapir growth from that related to subsequent compression.


| INTRODUCTION
The full characterisation of strata adjacent to salt structures is fundamental in the exploration and exploitation of geologic reservoirs; however, they often appear hidden in seismic lines, and good outcrop examples are scarce. Deformation studies in these strata have been mostly based on the analysis of mesoscale structures from outcrop examples (e.g., Alsop, Weinberger, Levi, & Marco, 2015, 2016Giles & Rowan, 2012;Hearon et al., 2015;Poprawski et al., 2014;Rowan, Jackson, & Trudgill, 1999;Rowan, Lawton, Giles, & Ratliff, 2003). In this work, we propose the use of Anisotropy of Magnetic Susceptibility (AMS) to analyse the deformation of salt-related synkinematic strata. This use is important because it can give information even in the absence of strain markers and/or poorly developed mesoscale brittle structures. It can also be applied to subsurface diapirs, as AMS data can be reoriented to geographical coordinates using palaeomagnetic data.
Anisotropy of Magnetic Susceptibility represents a powerful tool for geologists, as it gives information related to the petrofabric of rocks. In structural studies, it is a recognised indicator of deformation (e.g., Hrouda, 1982), even in very subtly deformed rocks that lack strain markers (e.g., Kissel, Barrier, Laj, & Lee, 1986). When applied to salt tectonics, AMS data obtained from rocks outcropping in the interior of salt structures can give information on diapiric flow or internal deformation (Santolaria, Casas, & Soto, 2015;Sm ıd, Schulmann, & Hrouda, 2001;Soto et al., 2014). We have selected several diapirs in the Basque-Cantabrian basin, which display well-exposed halokinetic sequences and suitable rocks for AMS analysis, to study the power of this approach in such geological settings.

| GE OLOGICAL SETTING
The study area is located in the northern margin of the Basque-Cantabrian basin, nowadays part of the southern Eurasian plate ( The study area is characterised by Triassic to Cenomanian rocks deformed by a large WNW-ESE fold locally pierced by several salt diapirs (Bakio, Bermeo, Guernica and Mungia diapirs) (Cuevas & Tub ıa, 1985; Figures 1 and 2). These diapirs are composed of Triassic evaporites, red clays and basic subvolcanic rocks (ophites) and flanked by Jurassic to Cretaceous materials. The ophites comprise their caprock and, due to their high resistance to erosion, dominate the outcrops ( Figure 2). They are flanked by Aptian-Albian syn-diapiric rocks organised in sequences limited by angular unconformities, becoming conformable as distance to the diapir edges increases. These sequences are characterised by lateral facies variations and masstransported deposits created at the diapir roofs, typical of halokinetic hooks and wedges triggered by diapir growth (Ferrer et al., 2014;Poprawski, Basile, Jaillard, Gaudin, & L opez, 2016;Roca et al., 2016).
The geometry of these halokinetic sequences was not modified during the subsequent Pyrenean compression, with the exception of the NNW-SSE folds located to the south of the Bakio diapir and a slight E-W folding to the west of the Bermeo diapir ( Figure 2). The Pyrenean compression inverted the northern part of the Basque-Cantabrian basin by means of north-directed thrusts that propagated from south to north and the development of a cleavage mostly oriented E-W to ENE-WSW in the study area (e.g., G omez et al., 2002; for example Figure 3, site BK01). Locally, as in site BK03, cleavage, faults and tension gashes are associated with syn-diapiric layer-parallel slip of a thick bed of breccias with an irregular base above marls, which occurred during syn-diapiric drape folding ( Figure 3).

ANALYSIS
Samples from 39 sites (6-12 cores per site) of Aptian-Albian marls, marly limestones, fine sandstones and lutites were analysed by means of low-field AMS measured at room temperature. All samples were collected from halokinetic sequences related to the Bakio, Bermeo, Guernica and Mungia diapirs ( Figure 2). Twenty-eight sites were close to diapir edges (sites located less than 1 km from the diapir walls, except sites BK15 and BK59, which were situated between two diapirs, were further from their walls, and were considered to be related to the Bermeo diapir), and 11 were far from diapirs ( degree Pj and the shape parameter T, ranging from À1 (prolate ellipsoid) to +1 (oblate ellipsoid).

| Magnetic fabric
The good correspondence between the axes of the LT-and RT-AMS magnetic ellipsoids corroborates the dominance of paramagnetic minerals to the total AMS (Figure 7), as LT-AMS amplifies the contribution of paramagnetic minerals (Par es & van der Pluijm, 2002).
Most magnetic ellipsoids show a well-defined magnetic foliation parallel to bedding, with K min grouped and perpendicular to bedding.
The magnetic lineation, defined by K max , is contained within the bedding plane at most sites (Table 1), but five sites do not have a welldefined magnetic lineation (sites BK01, BK10, BK20, BK54 and BK55; where e 12 > 45°). Site BK03 has a prolate magnetic ellipsoid and a magnetic foliation that does not coincide with either bedding or cleavage (see Figure 3) and has been discarded from further structural interpretations.
In sites located far from the diapir edges, the magnetic lineation shows a dominant WSW-ENE to E-W orientation. Close to the diapir edges, however, the orientation of the magnetic lineation varies strongly depending on the site location (Figures 8   and 9). At the southeastern edge of the Bakio diapir, the magnetic lineation is oriented parallel to the diapir walls ( Figure 9); it shows a preferred NE-SW orientation at sites located in the northern sector of its eastern edge and an ENE-WSW orientation at sites located in the southern sector of the same edge. Close to the Bermeo diapir, site BK22 shows a magnetic lineation parallel to the E-W orientation of this structure, and sites BK19 and BK16, BK57 and BK58, located around this diapir, show WNW-ESE and ENE-WSW trends, respectively. Sites BK15 and BK59, considered to be related to the Bermeo diapir and located between the Bermeo and Guernica diapirs, show a roughly N-S trend for the magnetic lineation. And the magnetic lineation orientation of site BK61 is parallel to the Guernica diapir wall (Figure 9). A remarkable feature is that sites located at the northern edges of the diapirs have magnetic lineations oriented perpendicular or highly oblique to the diapir walls. These orientations are (1) roughly N-S (sites BK51 and BK62 in the Bakio diapir and BK15 and BK59 in the Bermeo diapir), (2) NE-SW (site BK28 in the Bermeo diapir) and (3) NW-SE (site BK27 in the Bakio diapir), contrasting with the orientations found at sites located at the southern edges of the diapirs. All sites without a defined magnetic lineation (BK01, BK10, BK20, BK54 and BK55) are also located on the northern sides of and close to diapirs (Figure 9).

| DISCUSSION
The magnetic foliation at all sites, except for site BK03, is parallel to bedding and has been interpreted as being related to depositional and compaction processes. However, the orientation of the magnetic lineation varies throughout the studied area and has been interpreted as being controlled by tectonic processes. Far from the diapir edges, the magnetic lineation shows a WSW-ENE to E-W trend (Figure 8). We interpret it as being related to the N-S Pyrenean compression. This interpretation is justified because a cleavage associated with the Pyrenean orogeny is observed in the studied area. Formation of cleavage and/or incipient cleavage can reorient a previous magnetic fabric (Oliva-Urcia et al., 2013;Soto, Casas-Sainz, Villala ın, & Oliva-Urcia, 2007). Sedimentary processes triggering the magnetic lineation acquisition can be discarded, as its orientation does not coincide with either palaeocurrents (turbidites were sourced in the north, but they were driven by the diapir relief) or slumping (triggered by the dia- are able to reorient the magnetic lineation parallel to the diapir walls ( Figure 9). On the northern sides of diapirs, however, the magnetic lineation is either perpendicular/highly oblique to the diapir walls or could not be defined. In this case, we interpret the magnetic lineation to be associated with the outer-arc extension that occurred during salt rise (e.g., Giles & Rowan, 2012;see Figure 10). Magnetic lineation in extensional scenarios coincides with the stretching direction (e.g. Mattei, Sagnotti, Faccenna, & Funicello, 1997); therefore, it is expected that outer-arc extension related to salt rise will also orient the magnetic lineation parallel to the extension direction, which would be perpendicular to the salt wall ridge (Figure 10). The occurrence of sites without defined magnetic lineation and with magnetic lineations acquired during Mesozoic diapir growth points to the existence of areas ("shadow areas") protected from the subsequent Cenozoic Pyrenean compression at the northern edges of the diapirs due to the presence of rigid ophites (Figure 9). This work highlights the potential of AMS studies applied to halokinetic sequences to characterise their outer-arc deformation and thereby identify the trend of the diapir edges. It also indicates that caution is required in interpreting mag- BK28 BK57 Pyrenean regional Pyrenean regional shortening direction shortening direction Pyrenean regional shortening direction F I G U R E 9 Geological map of the study area showing the magnetic lineation (K max ) after bedding tilt correction and magnetic lineation trajectories. Magnetic lineations of sites located close to diapir edges are represented in red, whereas black lines represent magnetic lineations of sites located far from the diapir edges. Sites BK01, BK10, BK20, BK54 and BK55 do not show a defined magnetic lineation, and site BK03 was discarded from further structural interpretations (see text for further explanation). A magnetic fabric acquired during or shortly after deposition in syn-diapiric rocks is only observed in the shadow areas on the northern faces of diapirs (see text for further explanation) [Colour figure can be viewed at wileyonlinelibrary.com]

Inflating salt wall
Active stretching area (arching salt wall roof) Inactive stretched areas of rocks previously placed in the arching salt wall roof Main streching direction (perpendicular to the salt wall ridge)

DEFORMATION RELATED TO DIAPIR GROWTH:
F I G U R E 1 0 Active/inactive outer-arc deformation model related to salt rise in halokinetic sequences. The main stretching direction in the active stretching area is perpendicular to the salt wall ridges. The analysis of inactive stretched areas of rocks previously placed in the arching salt wall roof reveals that magnetic lineation would also be oriented perpendicular to the salt wall ridge in protected areas (i.e. where subsequent Pyrenean compression was not able to reorient the magnetic fabric) [Colour figure can be viewed at wileyonlinelibrary.com]