Distinguishing the Effect of Diapir Growth on Magnetic Fabrics of Syn-Diapiric
1 Distinguishing the effect of diapir growth on magnetic fabrics of syn-diapiric
2 overburden rocks: Basque-Cantabrian basin, Northern Spain
3 4 Ruth Soto1*, Elisabet Beamud2,3, Eduard Roca3, Eloi Carola3, Ylenia Almar4 5 6 1 IGME. C/ Manuel Lasala, 44, 9B, 50006 Zaragoza, Spain. 7 2 Laboratori de Paleomagnetisme CCiTUB-ICTJA CSIC. ICT “Jaume Almera”, Solé i Sabarís, s/n, 08028 Barcelona, Spain. 8 3 Institut GEOMODELS-Group Geodynamics and Basin Analysis, Universitat de Barcelona, 08028 Barcelona, Spain 9 4 ICT “Jaume Almera”, CSIC, Solé i Sabarís, s/n, 08028 Barcelona, Spain 10
11 *Corresponding author. Tel.: +34 976 555 153; Fax: +34 976 553 358. E-mail address: [email protected] 12
13 Abstract
14 An analysis of Anisotropy of Magnetic Susceptibility was done on Aptian-
15 Albian sediments from the Basque-Cantabrian basin. Thirty-nine sites were collected
16 from the halokinetic sequences of the Bakio, Bermeo, Guernica and Mungia diapirs; 28
17 sites were sampled close to diapirs and 11 sites far from the diapir edges. The magnetic
18 foliation is parallel to bedding, suggesting it reflects depositional and compaction
19 processes, whereas the orientation of magnetic lineation varies. Far from the diapir
20 edges, the magnetic lineation is interpreted as being related to the regional Pyrenean
21 compression. Close to diapir edges, the observed behaviour shows that diapirs,
22 predominantly formed by rigid ophites, have acted as buttress forming shadow areas at
23 their northern faces protected from the Pyrenean compression. The high sensitivity of
24 AMS allows considering it a very useful tool to distinguish deformation in halokinetic
25 sequences related to diapir growth and/or subsequent compression.
26
27 Keywords: magnetic fabrics, salt tectonics, diapir, halokinetic sequences, Bakio diapir
28
29 Introduction
30
1 31 The full characterisation of strata adjacent to salt structures is fundamental in
32 exploration and exploitation of geologic reservoirs, despite they often appear hidden in
33 seismic lines and good outcrop examples are scarce. Deformation studies in these strata
34 have been mostly based on the analysis of mesoscale structures from outcrop examples
35 (e.g. Rowan et al., 1999, 2003; Giles and Rowan, 2012; Hearon et al., 2015; Poprawski
36 et al., 2014; Alsop et al., 2015, 2016). In this work, we propose the use of the
37 Anisotropy of Magnetic Susceptibility (AMS) to analyse the deformation of salt-related
38 synkinematic strata. This use is important since it can give information even in absence
39 of strain markers and/or poorly developed mesoscale brittle structures. It can be also
40 applied to subsurface diapirs as AMS data can be reoriented to geographic coordinates
41 using paleomagnetic data.
42 AMS represents a powerful tool for geologists, as it gives information related to
43 the petrofabric of rocks. In structural studies, it represents a recognised indicator of
44 deformation (e.g. Hrouda, 1982), even in very subtle deformed rocks which lack strain
45 markers (e.g. Kissel et al., 1986). Applied to salt tectonics, AMS data obtained from
46 rocks outcropping in the interior of salt structures can give information in relation to
47 diapiric flow or internal deformation (Smíd et al., 2001; Soto et al., 2014; Santolaria et
48 al., 2015). We have selected several diapirs, in the Basque-Cantabrian basin, which
49 display well-exposed halokinetic sequences and suitable rocks for AMS analysis, to
50 study the power of this approach in such geological settings.
51
52 Geological setting
53
54 The study area is located in the northern margin of the Basque-Cantabrian basin,
55 nowadays part of the southern Eurasian plate (Fig. 1). The Basque-Cantabrian basin was
2 56 developed during the Mesozoic Pyrenean rift associated with the opening of the North
57 Atlantic Ocean and Bay of Biscay (García-Mondéjar, 1996). From the Late Cretaceous,
58 the African plate began to drift northwards conditioning the convergence between Iberia
59 and Europe and the inversion of the Basque-Cantabrian basin in the context of the
60 Pyrenean orogeny (Gómez et al., 2002) (Fig. 1).
61 The study area is characterised by Triassic to Cenomanian rocks deformed by a
62 large WNW-ESE fold locally pierced by several salt diapirs (Bakio, Bermeo, Guernica
63 and Mungia diapirs) (Cuevas and Tubia, 1985) (Figs. 1 and 2). These diapirs are
64 composed of Triassic evaporites, red clays and basic subvolcanic rocks (ophites) and
65 flanked by Jurassic to Cretaceous materials. The ophites constitute their caprock and
66 due to their high resistance to erosion dominate the outcrops (Fig. 2). They are flanked
67 by Aptian-Albian syn-diapiric rocks organised in sequences limited by angular
68 unconformities becoming conformable as distance to the diapir edges increases. These
69 sequences are characterised by lateral facies variations and mass-transported deposits
70 created at the diapir roofs, typical of halokinetic hooks and wedges triggered by diapir
71 growth (Ferrer et al., 2014; Poprawski et al., 2016; Roca et al., 2016). The geometry of
72 these halokinetic sequences was not modified during the subsequent Pyrenean
73 compression with the exception of the NNW-SSE folds located to the South of the
74 Bakio diapir and a slight E-W folding to the West of the Bermeo diapir (Fig. 2). The
75 Pyrenean compression inverted the northern part of the Basque-Cantabrian basin by
76 means of north-directed thrusts that propagated from South to North and the
77 development of a cleavage mostly oriented E-W to ENE-WSW in the study area (e.g.
78 Gómez et al., 2002) (for example Fig. 3, site BK01). Locally, as in site BK03, cleavage
79 together with faults and tension gashes are associated to syn-diapiric layer-parallel slip
3 80 of a thick bed of breccias with irregular base above marls occurred during syn-diapiric
81 drape folding (Fig. 3).
82
83 Sampling and laboratory analysis
84
85 Thirty-nine sites (6 to 12 cores per site) of Aptian-Albian marls, marly limestones,
86 fine sandstones and lutites were analysed by means of low-field AMS measured at room
87 temperature. All sites were collected from halokinetic sequences related to the Bakio,
88 Bermeo, Guernica and Mungia diapirs (Fig. 2). Twenty-eight sites were sampled close
89 to diapir edges (sites located less than 1 km from the diapir walls except sites BK15 and
90 BK59 situated between two diapirs and further from their walls, and considered related
91 to Bermeo diapir) and 11 far from that (Table 1). The AMS analysis was done using a
92 KLY3 from Zaragoza’s University. Data were processed using Anisoft 4.2 (Chadima
93 and Jelinek, 2009) to obtain the directional and tensor data (where Kmax, Kint and Kmin
94 are the maximum, intermediate and minimum principal axes of the magnetic ellipsoid,
95 respectively) and the parameters defined by Jelinek (1981), the corrected anisotropy
96 degree Pj and the shape parameter T, ranging from -1 (prolate ellipsoid) to +1 (oblate
97 ellipsoid).
98 Low-temperature AMS (LT-AMS) of 5 representative sites (6 samples per site)
99 was measured to analyse the contributions from paramagnetic and ferromagnetic (s.l.)
100 minerals to the total AMS and assess the significance of the low-field AMS. This was
101 measured following the method proposed by Parés and van der Pluijm (2002).
102 Additionally, three types of experiments were performed to characterise the
103 ferromagnetic (s.l.) minerals: (1) thermal demagnetization of the natural remanent
104 magnetization (NRM) of all samples using the thermal demagnetisers TSD-1
4 105 (Schonstedt) and MMTD-80 (Magnetic Measurements) and a superconducting rock
106 magnetometer SRM 755R (2G), (2) isothermal remanent magnetisation (IRM)
107 acquisition up to 1 T and three-axis IRM (in fields of 1.2, 0.3 and 0.1 T) thermal
108 demagnetisation as in Lowrie (1990) using an IM10-30 pulse magnetiser (ASC
109 Scientific), a TSD-1 thermal demagnetiser and a magnetometer JR6A (Agico), all
110 measured in the Paleomagnetic Laboratory of Barcelona (CCiTUB-CSIC), and (3) K–T
111 curves of selected samples using a KLY3.
112
113 Results
114
115 Magnetic properties and ferromagnetic (s.l.) mineralogy
116
117 The bulk magnetic susceptibility (Km) of the studied rocks ranges from 50 to
118 412 x 10-6 SI (Table 1). Most magnetic ellipsoids are oblate and the corrected
119 anisotropy degree Pj is low (Pj≤1.1), typical of weakly deformed sediments. A
120 significant correlation between Pj and lithology is observed, showing variable Pj values
121 in a wider range between 1 and 1.1 in marls and fine sandstones, and values between 1
122 and 1.03 in marly limestones (Fig. 4). Km, Pj and T parameters do not show any
123 significant variation related to distance of sites to diapir edges (Fig. 4).
124 K–T curves display a concave-hyperbolic shape in its initial part indicating a
125 paramagnetic behaviour up to 300-400ºC (Fig. 5). Thermal demagnetisation of three-
126 axis IRM shows the predominance of low coercivity minerals (< 0.1-0.3 T) and the
127 complete demagnetisation below 590 °C in all samples (Fig. 5). Maximum unblocking
128 temperatures of the NRM demagnetisation range between 480 and 550ºC (Fig. 5).
129 Altogether it points to the occurrence of magnetite as the main ferromagnetic (s.l.)
5 130 phase. Although the formation of new magnetic phases upon heating obscures some of
131 the thermomagnetic experiments, the main decrease in magnetic susceptibility below
132 590 °C also supports the occurrence of magnetite. Thermal demagnetisation of three-
133 axis IRM reveals an additional and progressive IRM drop below 350 °C (Fig. 5) that
134 might be attributed to the occurrence of either pyrrhotite and greigite (Larrasoaña et al.,
135 2007) or maghemite (Liu et al., 2005). The increase in bulk susceptibility at low
136 temperature with respect to its value at room temperature is similar in all samples, being
137 the LT/RT ratio between 1.7 and 3.1 (Fig. 6), regardless of lithology or distance of sites
138 to diapir edges. These LT/RT ratios indicate the predominance of paramagnetic
139 minerals controlling the total AMS, which represent good markers of rock petrofabric
140 (e.g. Oliva-Urcia et al., 2009).
141 142 Magnetic fabric
143
144 The good correspondence between axes of LT and RT-AMS magnetic ellipsoids
145 corroborates the dominance of paramagnetic minerals to the total AMS (Fig. 7), as LT-
146 AMS amplifies the contribution of paramagnetic minerals (Parés and van der Pluijm,
147 2002). Most magnetic ellipsoids show a well-defined magnetic foliation parallel to
148 bedding with Kmin grouped and perpendicular to bedding. The magnetic lineation,
149 defined by Kmax, is contained in the bedding plane in most sites (Table 1), but five sites
150 do not have a well-defined magnetic lineation (sites BK01, BK10, BK20, BK54 and
151 BK55; where e12>45º). Site BK03 presents a prolate magnetic ellipsoid and a magnetic
152 foliation that does not coincide neither with bedding nor cleavage (see Fig. 3) and has
153 been discarded for further structural interpretations.
154 In sites located far from the diapir edges, the magnetic lineation shows a
155 dominant WSW-ENE to E-W orientation. Close to diapir edges, however, the
6 156 orientation of the magnetic lineation varies strongly depending on site location (Figs. 8
157 and 9). In the southeastern edge of the Bakio diapir, the magnetic lineation is oriented
158 parallel to the diapir walls (Fig. 9); it shows a preferred NE-SW orientation in sites
159 located in the northern sector of its eastern edge and an ENE-WSW orientation in sites
160 located in the southern sector of the same edge. Close to the Bermeo diapir, site BK22
161 shows its magnetic lineation parallel to the E-W orientation of this structure and sites
162 BK19 and BK16, BK57 and BK58, located around this diapir, show a WNW-ESE and
163 ENE-WSW trend, respectively. Sites BK15 and BK59, considered related to Bermeo
164 diapir and located between the Bermeo and Guernica diapirs, show a roughly N-S trend
165 for the magnetic lineation. And the magnetic lineation orientation of site BK61 is
166 parallel to the Guernica diapir wall (Fig. 9). A remarkable feature is that sites located at
167 the northern edges of diapirs have magnetic lineation oriented perpendicular or highly
168 oblique to diapir walls. These orientations are A) roughly N-S (sites BK51 and BK62 in
169 Bakio diapir and, BK15 and BK59 in Bermeo diapir), B) NE-SW (site BK28 in Bermeo
170 diapir) and C) NW-SE (site BK27 in Bakio diapir) contrasting with the orientation
171 shown by sites located at thesouthern edges of the diapirs. All sites without a defined
172 magnetic lineation (BK01, BK10, BK20, BK54 and BK55) are also located on the
173 northern sides and close to diapirs (Fig. 9).
174
175 Discussion
176
177 The magnetic foliation of all sites, except for site BK03, is parallel to bedding
178 and has been interpreted related to depositional and compaction processes. On the
179 contrary, the orientation of the magnetic lineation varies through the studied area and
180 has been interpreted as controlled by tectonic processes. Far from the diapir edges, the
7 181 magnetic lineation shows a WSW-ENE to E-W trend (Fig. 8). We interpret it as related
182 to the N-S Pyrenean compression. This interpretation is justified as a cleavage
183 associated with the Pyrenean orogeny is observed in the studied area. Formation of
184 cleavage and/or incipient cleavage is able to reorient a previous magnetic fabric (Soto et
185 al., 2007; Oliva-Urcia et al., 2013). Sedimentary processes triggering the magnetic
186 lineation acquisition can be discarded, as its orientation does not coincide neither with
187 paleocurrents (turbidites were sourced in the North, but they were driven by the diapir
188 relief) nor with slumping (triggered by the diapir growth) directions detected in the
189 Bakio diapir by Poprawski et al. (2014) (Fig. 2).
190 Close to the diapir edges, two different types of behaviour are observed (Fig. 9).
191 Sites located on the southern sides of diapirs show a magnetic lineation parallel to the
192 diapir walls. We interpret the magnetic lineation observed at the southern walls
193 associated to the Pyrenean compression stresses deviated around the diapirs. These
194 diapirs are mainly composed of hard subvolcanic rocks (ophites) that act as a buttress
195 hinding the northward propagation of deformation and producing stress perturbations
196 able to reorient the magnetic lineation parallel to the diapir walls (Fig. 9). On the
197 northern sides of diapirs, however, the magnetic lineation is either perpendicular/highly
198 oblique to the diapir walls or could not be defined. In this case, we interpret the
199 magnetic lineation associated to the outer-arc extension occurred during salt rise (e.g.
200 Giles and Rowan, 2012) (see Fig. 10). Magnetic lineation in extensional scenarios
201 coincides with the stretching direction (e.g. Mattei et al., 1997), therefore, it is expected
202 that outer-arc extension related to salt rise also orients the magnetic lineation parallel to
203 the extensional direction which would be perpendicular to the salt wall ridge (Fig. 10).
204 The occurrence of sites without defined magnetic lineation and with magnetic lineation
205 acquired during Mesozoic diapir growth points to the existence of areas (“shadow
8 206 area”) protected from the subsequent Cenozoic Pyrenean compression at the northern
207 edges of the diapirs due to the presence of rigid ophites (Fig. 9). This work highlights
208 the potential of AMS studies applied to halokinetic sequences to characterise their
209 outer-arc deformation and so, identifying the trend of the diapir edges. It also indicates
210 that caution is required in interpreting magnetic lineations from halokinetic sequences if
211 subsequent tectonic events are present.
212
213 Conclusion
214
215 The application of AMS to syn-diapiric overburden rocks highlights its potential
216 to study deformation in halokinetic sequences related to passive salt rise. Aptian-Albian
217 turbiditic series from the Basque-Cantabrian basin have been analysed. Paramagnetic
218 minerals dominate the total AMS validating AMS results in terms of reflecting the
219 petrofabric of the studied rocks. The observed magnetic foliation is parallel to bedding
220 and the orientation of the magnetic lineation variable and related to different
221 deformation processes. Far from the diapir edges, magnetic lineation is related to the
222 Cenozoic Pyrenean compression which propagated from South to North. Close to the
223 diapirs, it shows the effect of diapirs filled with ophites as rigid bodies deflecting
224 Pyrenean compression at their southern faces and protecting Mesozoic syn-diapiric
225 deformation at their shadow areas located to the North.
226
227 Acknowledgements
228
229 Funding came from projects CGL2014-54118-C2-1-R and CGL2014-54118-C2-2-R
230 (Spanish Ministry). Thanks to the University of Zaragoza and the Paleomagnetic
9 231 Laboratory (CCiTUB-ICTJA CSIC) where the AMSand rock-mag analyses were
232 carried out. We are grateful to Frederic Escosa and to Mark Dekkers for his constructive
233 comments.
234
235
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330
14 330 Figure captions
331
332 -Figure 1. Location of the study area in the frame of the Basque-Cantabrian basin,
333 northern Spain, and cross-section across the Basque-Cantabrian basin (modified from
334 Pedreira et al., 2007).
335 -Figure 2. Geological map of the study area showing bedding plane data from field
336 work and from EVE (1991, 1992, 1993a, 1993b), location of sites and paleocurrent
337 directions from Poprawski et al. (2014).
338 -Figure 3. In situ magnetic ellipsoids of sites BK01 and BK03 showing their
339 relationship with bedding and cleavage planes. Lower-hemisphere equal area
340 stereoplots.
341 -Figure 4. Pj-T graphs in function of different lithologies indicating sites sampled close
342 or far from the diapir edges (circle and square symbols, respectively).
343 -Figure 5. Representative examples of rock magnetic experiment results. (a-d)
344 Thermomagnetic curves in argon atmosphere. Heating and cooling curves are in red and
345 blue, respectively. Insets show enlarged heating curves. (e, f) Thermal progressive
346 demagnetisation of the natural remanent magnetisation (NRM). (g, h, i) Three-axes
347 IRM demagnetisation as in Lowrie (1990).
348 -Figure 6. Ratio between the magnetic susceptibility (Km) at low and room temperature
349 (LT/RT) where LT/RT=3.8 corresponds to perfect paramagnetic behaviour (Lüneburg
350 et al., 1999).
351 -Figure 7. Stereoplots of the RT-AMS (left), LT-AMS (middle) and T–Pj diagrams
352 (right) differentiating the RT- and LT-AMS values for each site. Confidence ellipses for
353 AMS principal axes are shown. Lower-hemisphere equal-area stereoplots after bedding
354 tilt correction.
15 355 -Figure 8. Stereoplot showing Kmax (magnetic lineation), density plot and rose diagram
356 after bedding tilt correction for sites located far from the diapir edges and for sites
357 located close to Bakio and Bermeo diapirs. Lower-hemisphere equal area stereoplot.
358 -Figure 9. Geological map of the study area showing the magnetic lineation (Kmax) after
359 bedding tilt correction and magnetic lineation trajectories. Magnetic lineation of sites
360 located close to diapir edges is represented in red, whereas black lines represent
361 magnetic lineation of sites located far from the diapir edges. Sites BK01, BK10, BK20,
362 BK54 and BK55 do not show defined magnetic lineation and site BK03 has been
363 discarded for further structural interpretations (see text for further explanation).
364 Magnetic fabric acquired during or shortly after the deposition syn-diapiric rocks is only
365 observed at the shadow areas located on the northern faces of diapirs (see text for
366 further explanation).
367 -Fig. 10. Active/inactive outer-arc deformation model related to salt rise in halokinetic
368 sequences. The main stretching direction at the active stretching area is perpendicular to
369 the salt wall ridges. The analysis of inactive stretched areas of rocks previously placed
370 in the arching salt wall roof reveals that magnetic lineation would be also oriented
371 perpendicular to the salt wall ridge in protected areas (i.e. where subsequent Pyrenean
372 compression is not able to reorient the magnetic fabric).
373
374 Table captions
375
376 -Table 1. Site means of magnetic parameters measured at room temperature.
377
378
16 3ºW 2ºW Bay of Biscay 1. Bilbao anticlinorium STUDY AREA 2. Biscay synclinorium N 3. North-Biscay ant. 3 A’ 2
1
43ºN Basque-Cantabrian basin
Tertiary Cenom.-Maastric.Cenom.-Maastrichtian Aptian-Albian Oxfor.-BarremianOxfordian-Barremian Pyrenees Low.-Mid.Lower-Middle Jurassic Jurassic Iberian Triassic Peninsula A 25 km Paleozoic Ebro basin
A Basque-Cantabrian basin A’ SSW Coastline NNE 0
IBERIAN PLATE EURASIAN PLATE 25
Mesozoic-Cenozoic 50 km Upper-middle crust Lower crust 2º50’W 2º45’W BAY OF BISCAY 40 1. BAKIO SALT DIAPIR San Juan de 40 2. BERMEO SALT DIAPIR Gaztelugatxe 28 25 27 33 3. GUERNICA SALT DIAPIR BK04 32 4. MUNGIA SALT DIAPIR 15 N 48 10 35 25 50 15 15 25 85 28 20 45 BK51 61 30 BK01 BK17 15 BK02 BK62 BK50 50 30 40 BK03 20 44 15 15 50 BK57 15 10 BAKIO BK05 75 10 BK58 45 BK28 70 35 49 25 30 40 40 57 76 30 30 BK55 15 BK07 BK26 34 BK06 15 30 BK12 20 24 86 50 40 60 10 58 40 30 45 BERMEO 50 76 40 BK27 20 50 1 BK14 BK23 20 20 46 60 45 22 25 62 BK15 Sites for 60 80 40 BK22 42 BK08 BK19 22 57 AMS 23 BK13 35 2 40 40 43º25’N BK54 20 80 35 53 BK59 60 75 BK56 60 50 79 60 paleocurrents 30 30 25 56 BK11 70 75 40 BK16 40 62 5 25 5 55 BK09 70 40 20 60 70 50 30 40 75 BK18 30 60 60 25 5 MUNDAKA 16 40 10 20 40 15 50 45 30 20 40 BK29 20 45 20 45 54 12 20 30 55 BK24 40 32 25 25 65 40 BK25 15 15 33 30 80 10 BK21 39 20 42 80 35 60 20 35 35 30 20 20 40 40 55 65 20 25 30 45 28 20 20 10 20 25 36 34 30 40 54 30 80 40 29 82 40 10 70 49 45 30 50 85 30 30 20 50 45 20 73 60 20 BK10 25 10 40 55 10 10 BK20 82 49 40 60 40 20 40 20 60 70 60 40 30 40 80 30 15 50 20 50 50 5 70 30 10 20 40 55 20 25 76 10 20 40 40 80 80 60 20 30 5 40 30 30 45 80 70 10 40 3 40 80 35 40 BK61 40 4 15 50 30 70 20
QUATERNARY (i) Volcanic rocks (pillow lavas) 0 2 km i Cenomanian f (h) Mudstones (deep sea fan environment) LATE g (g) Turbiditic sandtones and mudstones (deep sea fan environment) h (f) Turbiditic sandstones, conglomerates and mudstones (deep sea fan environment) e Albian (e) Carbonatic turbidites and marls (deep sea fan environment)
c d (d) Sandstones and marls (talus environment) b b (c) Breccias, marls and sandstones (talus environment) EARLY Aptian a c (b) Reefal to calcarenitic limestones (platform environment) CRETACEOUS (a) Marls and marly limestones (talus to outer platform environment) Neocomian- latest Jurassic Limestones, marls and marly limestones JURASSIC Malm Limestones and marly limestones Dogger Marls and marly limestones TRIASSIC Keuper Facies Evaporites and mudstones / Basic sub-volcanic rocks BK01 BK03 N=6 N=10
Kmax Kint S0 Kmin Cleavage 1 1 1 Close to the diapir edges Far from the diapir edges
T0 0 0 1 1.25 1 1.25 1 1.25 Pj Pj Pj
MARLS FINE SANDSTONES MARLY LIMESTONES -1 -1 -1 400 BK15-3b 12 up/W up/W heating run 300 8 N K (E-6) 480 550 N 200 cooling run 4
K (E-6) 460 NRM 250 400 390
100 0 0 200 400 600 800 370 100 heating run T (°C) 0 310 0 100 200 300 400 500 600 700 800 T(°C) a) 280 150
500 NRM BK19-10b 16 400 12 BK22-5B BK27-13A K ATC 300 8 heating run ATC e) f) cooling run (E-6) K (E-6) 200 4
0 100 0 200 400 600 800 T (°C) heating run g) 0 1 0 100 200 300 400 500 600 700 800 b) BK03-8c Mx 0.1 T T(°C) My 0.3 T Mz 1.2 T 300
BK26-6a M/Mmax 8
200 6 0 heating run K 0 100 200 300 400 500 600 700 4 T(°C) cooling run (E-6)
K (E-6) 2 h) 100 1 0 0 200 400 600 800 BK12-6a T (°C) heating run 0 0 100 200 300 400 500 600 700 800 c) M/Mmax T(°C)
300 0 0 100 200 300 400 500 600 700 BK27-7a 40 T(°C) K (E-6) 30 200 i) 1 20 BK27-6b cooling run heating run
10 K (E-6) 100
0 M/Mmax 0 200 400 600 800 heating run T (°C) 0 0 0 100 200 300 400 500 600 700 800 d) 0 100 200 300 400 500 600 700 T(°C) T(°C) 1600
1400
1200
Perfect Paramagnetic
1000
800
BK05 Km (10-6) LT 600 BK09
BK15
400 BK18
BK28 200 3.8 times RT 1.5 times RT 2 times RT 0 0 200 400 600 800 1000 Km (10-6) RT RT-AMS LT-AMS Pj-T SITE N=9 N=6 1 BK05
SITE BK05 T 1 1.15
RT AMS LT AMS Kmax -1 Kint Pj Kmin 1 SITE N=10 N=6 BK09 SITE BK09 T 1 1.15
RT AMS LT AMS -1 Pj
1 SITE N=11 N=6 BK15 SITE BK15 T 1 1.15
RT AMS LT AMS -1 Pj
1 SITE N=8 N=6 BK18 SITE BK18 T 1 1.15
RT AMS LT AMS -1 Pj
1 SITE N=11 N=6 BK28 SITE BK28 T 1.15
RT AMS LT AMS -1 Pj N=102 N=167 N=76 SITES CLOSE SITES CLOSE SITES FAR TO BAKIO TO BERMEO FROM DIAPIRS DIAPIR DIAPIR
circle=15% circle=13% circle=13% 2º50’W 2º45’W BAY OF BISCAY San Juan de Gaztelugatxe BK04
N BK01 BK51 BK62 BK03 BK17 BK02 BK05 BAKIO BK57 Pyrenean regional BK06 BK50 BK26 BK07 BK55 BK28 shortening direction BK12 BK23 BK58 BK27 BK19 BERMEO Sites for BK14 43º25’N BK08 BK22 BK15 AMS BK13 BK54 Kmax (after bedding BK16 70 BK56 BK59 tilt correction) in sites BK09 BK11 MUNDAKA showing a well- BK24 BK29 defined magnetic BK18 lineation: Kmax in sites BK25 far from diapir BK21 edges BK10 BK20 Kmax in sites close to diapir edges
Shadow BK61 areas 0 2 km DEFORMATION RELATED TO DIAPIR GROWTH:
Main streching direction (perpendicular to the salt wall ridge) Active stretching area (arching salt wall roof)
Inflating salt wall
Inactive stretched areas of rocks previously placed in the arching salt wall roof