# Quantifying deformation processes in the SE Pyrenees using U-Pb dating of fracture-filling calcites

D. Cruset, J. Vergés, R. Albert, A. Gerdes, A. Benedicto, I. Cantarero, A.

Travé

► To cite this version:

D. Cruset, J. Vergés, R. Albert, A. Gerdes, A. Benedicto, et al. Quantifying deformation processes in the SE Pyrenees using U-Pb dating of fracture-filling calcites. Journal of the Geological Society, 2020, pp.jgs2020-014 (IF 1,81). 10.1144/jgs2020-014. hal-02950990

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# Quantifying deformation processes in the SE Pyrenees using U-Pb dating of fracture-filling calcites

Article in Journal of the Geological Society - August 2020

DOI: 10.1144/jgs2020-014

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# Quantifying deformation processes in the SE Pyrenees

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## using U-Pb dating of fracture-filling calcites

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D. Cruset<fnref n="1" />,<fnref n="2" />, J. Vergés<fnref n="2" />, R. Albert<fnref n="3" />, <fnref n="4" />, A. Gerdes<fnref n="3" />, <fnref n="4" />, A. Benedicto<fnref n="5" />, I. Cantarero<fnref n="1" />, A.

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### Travé<fnref n="1" />

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i cantarero@ub.edu, atrave@ub.edu

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dcruset@ictja.csic.es, jverges@ictja.csic.es

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gerdes@em.uni-frankfurt.de, albertroper@em.uni-frankfurt.de

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antonio.benedicto@u-psud.fr

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## ABSTRACT

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A significant challenge in brittle fold and thrust belts analysis is to quantify the timing of the deformation processes, since suitable minerals for dating and well-preserved growth strata sediments are scarce or absent. Here, we quantify the duration of thrust sheet emplacement and shortening rates in the SE Pyrenean thrust sequence using U-Pb dating of fracture-filling calcites. The obtained U-Pb dates reveal a minimum duration for the emplacement of each thrust unit (18.7 Ma for the Bóixols-Upper Pedraforca, 11.6 Ma for the Lower Pedraforca and 14.3 Ma for the Cadí), and that piggy-back thrusting was accompanied by post-emplacement deformation of upper thrust sheets above lower ones during their south-directed tectonic transport. Shortening rates of 0.6 mm/yr, 3.1 mm/yr and 1.1 mm/yr from older to younger emplaced thrust sheets were calculated. The results also reveal the formation of local normal faults during the late

<footnote n="1">¹ Departament de Mineralogia, Petrologia i Geologia Aplicada, Facultat de Ciències de la Terra, Universitat de Barcelona (UB), Martí i Franquès s/n, 08028, Barcelona, Spain.</footnote>

<footnote n="2">² Group of Dynamics of the Lithosphere (GDL), Institut de Ciències de la Terra Jaume Almera, ICTJA-CSIC, Lluís Solé i Sabaris s/n, 08028 Barcelona, Spain.</footnote>

<footnote n="3">³ Department of Geosciences, Goethe University Frankfurt, 60438 Frankfurt am Main, Germany.</footnote>

<footnote n="4">⁴ Frankfurt Isotope and Element Research Center (FIERCE), Goethe University Frankfurt, Frankfurt am Main, Germany.</footnote>

<footnote n="5">⁵ UMR Geops, Université Paris Sud, 91405 Orsay, France.</footnote>
31 Oligocene as a result of the late stages of compression and exhumation in the SE
32 Pyrenees. Finally, we observed that temperatures higher than 110 °C could be a limiting
33 factor when applying the U-Pb dating method.

34 Keywords: U-Pb calcite geochronology, deformation, SE Pyrenean fold and thrust belt.

35 **1. INTRODUCTION**

36 In the upper crust, compressional deformation leads to the development of brittle
37 fracturing. Fracture sets show complex crosscutting relationships and document
38 polyphase deformation and long-lasting thrusting (and folding) accompanied with fluid
39 flow and precipitation of fracture-filling cement (e.g., Zhang et al., 2004; Roure et al.
40 2005; Travé et al., 2007; Cosgrove, 2015). One of the most common fracture-filling
41 minerals is calcite, which very often shows syn-kinematic textures and therefore, a
42 direct relationship with tectonic deformation (e.g., Travé et al., 1998; Bons et al., 2012).

43 In large orogenic belts, the timing and duration of ductile thrusting has been constrained
44 using U-Pb dating of metamorphic minerals (e.g., Mottram et al., 2015). This approach
45 has been a challenge to apply to brittle fold and thrust belts due to the lack of suitable
46 minerals for dating or well-preserved growth strata sediments. However, recently, U-Pb
47 dating of calcite has been used to constrain the absolute timing of brittle deformation in
48 compressional settings (Beaudoin et al., 2018; Hansman et al., 2018; Parrish et al.,
49 2018). The development of this dating method has opened the possibility to quantify
50 thrusting duration and shortening rates in the brittle realm.

51 The southern Pyrenees is a fold and thrust belt in which the sequence of thrusting is
52 well-known due to the exceptional preservation of the thrust sheets and growth strata
53 sediments deposited in the thrust front (e.g., Meigs et al., 1996; Vergés et al., 2002;
54 Beamud et al., 2010; Carrigan et al., 2016). The abundance of calcite veins observed in
the south Pyrenean thrust sheets has allowed for several fluid flow studies in this fold
and thrust belt in the last decades (Travé et al., 1997, 2007; Beaudoin et al., 2015;
Cruset et al., 2016, 2018; Crognier et al., 2018; Cruset, 2019; Nardini et al., 2019). This
abundance of fracture-filling calcites throughout the entire pile of thrust sheets reveals
the southern Pyrenees as the perfect site for proving the quantification of deformation
processes using U-Pb calcite dating during the complete evolution of a fold and thrust
belt.

In this study we present a new data set of 46 U-Pb ages obtained from 66 fracture-filling calcites sampled in the piggy-back thrust sequence of SE Pyrenean fold and thrust belt.

The results enable us to provide more precise information regarding deformation processes in the SE Pyrenees, such as the minimum duration of thrust sheet emplacement and shortening rates. Furthermore, the obtained U-Pb dates allow us to constrain in time the changes of the main stress orientations and the shift from compressional to extensional deformation, as well as to prove the coexistence between both tectonic regimes. Finally, we comment on the possible influence of temperature on the suitability of calcite for U-Pb geochronology. To do this, we have considered previously published clumped isotopes data from Cruset et al. (2016; 2018; 2019; 2020), Cruset (2019) and in Nardini et al. (2019) of several of the dated fracture-filling calcites used in this study.

## 2. GEOLOGICAL SETTING

The Pyrenees formed within the framework of the Alpine orogeny and consist of a doubly verging orogenic belt generated during the continental collision between Iberia and Eurasia plates (Fig. 1A), from Late Cretaceous to Miocene (Muñoz, 2002; Vergés et al., 2002). This collision resulted from the partial subduction of the Iberian plate
79 beneath the Eurasian plate (e.g., Choukroune, et al., 1989; Roure et al., 1989; Muñoz, 80 1992; Vergés et al., 2002; Chevrot et al., 2018; Grool et al., 2018; Teixell et al., 2018).

81 The SE Pyrenees consist of a south-directed piggy-back thrust sequence emplaced from 82 Late Cretaceous to Oligocene. This sequence consists of the stacking of three thrust 83 sheets and a deformed foreland basin (Fig. 1B and C). From top-and-older to bottom- 84 and-younger, these are the Bóixols-Upper Pedraforca, the Lower Pedraforca and the 85 Cadí thrust sheets, and the Ebro foreland basin (Vergés et al., 2002).

86 The Bóixols-Upper Pedraforca thrust sheet consists of an extensional basin affected by 87 diapirism and incorporated into the Pyrenean orogen during the Late Cretaceous- 88 Palaeocene compression (Puigdefàbregas and Souquet, 1986; Saura et al., 2015). The 89 Upper Pedraforca thrust sheet emplaces an up to 1.6 km thick succession of Upper 90 Triassic, Jurassic and Cretaceous pre-compressive sedimentary rocks over Upper 91 Cretaceous to Palaeocene syn-orogenic sediments of the south Pyrenean foreland basin 92 (Vergés et al., 2002).

93 The Lower Pedraforca thrust sheet consists of an allochthonous klippe detached in the 94 Upper Triassic Keuper facies and emplaced from Lower to Middle Eocene 95 (Puigdefàbregas et al., 1986; Burbank et al., 1992). This thrust unit places Upper 96 Triassic and Jurassic pre-compressive sedimentary successions and upper Cretaceous to 97 Paleocene syn-orogenic deposits over Eocene foredeep marine sediments of the Cadí 98 thrust sheet. The rocks forming the Lower Pedraforca thrust sheet have a thickness of up 99 to 2400 m in its northern imbricate and less than 500 m in the southernmost thrust 100 nappes. The emplacement of this thrust unit was under marine conditions, as attested by 101 the syn-orogenic fan delta conglomerates deposited at the thrust front (Vergés, 1993).
102 The lowermost Cadí thrust sheet is linked to the Axial Zone antiformal stack formed by
103 the Orri and Rialp basement thrusted units (Muñoz, 1992) (Fig. 1C), which are
104 unconformably overlain by thin Mesozoic rocks and thick Paleogene rocks. This thrust
105 sheet has a thickness of up to 3 km in its northern sector and up to 1 km in the south.
106 The southern boundary of the Cadí thrust sheet is the Vallfogona thrust, which places
107 middle Eocene syn-orogenic marine deposits over Eocene-Oligocene continental
108 deposits of the Foreland Ebro basin (Burbank et al., 1992; Vergés et al., 2002). The
109 activity of this thrust fault started in the middle Eocene under marine conditions and
110 finished during the lower Oligocene under continental conditions (Burbank et al., 1992).

111 The Ebro foreland basin represents the non-marine stage of the South Pyrenean foreland
112 basin (Vergés et al, 2002), which developed from the middle Priabonian (Costa et al.,
113 2010). The fold system deforming the eastern region of the Ebro foreland basin was
114 detached above the Cardona evaporites (Sans et al., 1996).

115 3. METHODOLOGY

116 Field work consisted of structural data acquisition and sampling of veins in damage
117 zones of main reverse, strike-slip and normal faults in both pre-shortening rocks as well
118 as in syn-orogenic deposits (Figs. 1 and S1 and Table S1). Previous fracture data from
119 Cruset et al. (2016; 2018; 2019; 2020), Cruset (2019) and Nardini et al. (2019) have
120 been integrated in this study. In these previous works, fracture types were divided
121 considering crosscutting relationships, the direction and sense of motion on individual
122 fault planes determined by striae sets, and orientations of stress axes deduced from
123 conjugate sets of veins.
Fracture-filling cement was characterized by means of optical and cathodoluminescence microscope to determine their mineralogy and texture. From 66 samples, 35 mounts were prepared for U-Pb dating.

The U-Pb dating method is similar to that previously described by Ring and Gerdes (2016) and Burisch et al. (2017). U-Pb dates were acquired using laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) at FIERCE (Frankfurt Isotope and Element Research Center, Goethe Universität), applying a modified method of Gerdes and Zeh (2006, 2009). A ThermoScientific Element XR sector field ICPMS was coupled to a RESOlution 193nm ArF excimer laser (COMpexPro 102) equipped with a two-volume ablation cell (Laurin Technic S155). Samples were ablated in a helium atmosphere (300 mL/min) and mixed in the ablation funnel with 1100 mL/min argon and 5 mL/min nitrogen. Signal strength at the ICP-MS was tuned for maximum sensitivity whereas keeping the oxide formation (monitored as ²⁴⁸ThO/²³²Th) below 0.2% and no fractionation of the Th/U ratio. Static ablation used a spot size of 213 µm and a fluency of about 2 J/cm at 12 Hz. For NIST SRM-614 this yielded a depth penetration of ~0.6 µm/s and an average sensitivity of 380,000 cps/µg for ²³⁸U. The detection limit for ²⁰⁶Pb and ²³⁸U was ~0.2 and 0.03 ppb, respectively.

Data were acquired in fully automated mode overnight in three sequences of 598 analyses each. Each analysis consisted of 18 s of background acquisition followed by 18 s of sample ablation and 25 s of washout. During 36 s of data acquisition, the signal of ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb, ²³²Th, and ²³⁸U was detected by peak jumping in pulse-counting and analogue mode with a total integration time of ~0.1s, resulting in 370 mass scans. Before analysis, each spot was pre-ablated with 8 laser pulses to remove surface contamination. Soda-lime glass NIST SRM-614 was used as primary reference material
(RM) together with three carbonate RMs, which were bracketed in between the analysis of samples (see Table S2 for validation results).

Raw data were corrected offline using an in-house VBA spreadsheet program (Gerdes and Zeh, 2006, 2009). Following background correction, outliers (±2σ) were rejected based on the time-resolved ²⁰⁷Pb/²⁰⁶Pb, ²⁰⁸Pb/²⁰⁶Pb, ²⁰⁶Pb/²³⁸U, and ²³²Th/²³⁸U ratios. These ratios were corrected for mass biases and drift over time, using NIST SRM-614. Due to the carbonate matrix, an additional correction was applied (sequence 1: 4.8%, sequence 2: 10.6%, sequence 3: 11.8%), which was determined using WC-1 carbonate RM (Roberts et al., 2017). The ²⁰⁶Pb/²³⁸U downhole-fractionation during 20s depth profiling was estimated to be 3%, based on the common Pb corrected WC-1 analyses, and was applied as an external correction to all carbonate analyses. Uncertainties for each isotopic ratio are the quadratic addition of the within run precision, counting statistic uncertainties of each isotope, and the excess of variance (Horstwood et al., 2016) calculated from the SRM-614 and the WC-1 after drift correction. In case of the 208Pbcom/206Pb ratio the uncertainty from the radiogenic 208Pb correction was also added. To account for the long-term reproducibility of the method we added by quadratic addition an expanded uncertainty of 1.5% to the final age of all analysed carbonates. This was deducted from repeated analyses (n = 7) of ASH-15D between 2017 and 2019. A summary report of the U-Pb dating procedure is presented in Table S2.

Reference material ASH-15D (2.936 ± 0.140 and 2.908 ± 0.122 Ma; Vaks et al., 2013) was measured in sequences 2 and 3 for quality control. In addition, a stromatolitic limestone from the Cambrian-Precambrian boundary in South-Namibia (here called NAMA) was analysed during sequences 1 and 3, as an in-house RM for quality control
(obtained ages agree with the U/Pb zircon age of 540.1 ± 0.1 Ma from the directly overlying ash layer, Spitskopf formation; Linnemann et al. 2018). Results on the secondary RM imply an accuracy and repeatability of the method of about 1.5 to 2%. The analytical results are presented in Tables S2 and S3. Data were displayed in Tera-Wasserburg plots (Table S2) and ages were calculated as lower concordia-curve intercepts using the same algorithms as Isoplot 4.14 (Ludwig, 2012). In addition, the data was plotted in the modified ²⁰⁸Pb/²⁰⁶Pb vs. ²³⁸U/²⁰⁶Pb Tera-Wasserburg diagrams after correction for radiogenic ²⁰⁸Pb and ages were calculated from the X-axis intercepts (Parrish et al. 2018). In all cases, the ages overlapped within their uncertainties and thus showed a 1 to 1 correlation between the ²⁰⁸Pb/²⁰⁶Pb- and ²⁰⁷Pb/²⁰⁶Pb-based Tera-Wasserburg age. The ages discussed in this paper are the Tera-Wasserburg intercept ages. All uncertainties are reported at the 2σ level.

# 4. RESULTS

## 4.1. Fracture analysis

The studied samples consisted of calcite cement precipitated in four fracture sets related to compressional tectonics and in one extensional set, all of them already described in Cruset et al., (2016; 2018; 2019; 2020), Cruset (2019) and Nardini et al. (2019) (Figs. 2, 3 and S1). The compressional sets include: 1) N-S, NNW-SSE and NNE-SSW trending en-échelon vein arrays (Fig. 2A); 2) E-W trending folding-related veins (Fig. 2B); 3) E-W trending reverse faults Fig. 2C); and 4) NW-SE and NE-SW trending strike-slip faults (Fig. 2D). The extensional fracture set corresponds to NNW-SSE and NW-SE trending normal faults (Fig. 2E).

N-S, NNW-SSE and NNE-SSW veins are related to layer-parallel shortening, whereas E-W folding-related veins and reverse and strike-slip faults are related to subsequent
folding and thrusting (Cruset, 2019). N-S, NNW-SSE and NNE-SSW veins and E-W
folding-related veins are bed-perpendicular, and cut the hangingwall of thrust sheets,
whereas reverse and strike-slip faults are mainly located along the basal thrust zones
(Fig 3A). Normal faults are observed in the crest of the Puig-reig anticline, in the Ebro
basin, and cutting the northern domain of the SE Pyrenean thrust system (Fig. 3A). The
stereographic projection of measured fracture sets and the calculated orientation of main
stresses indicate a N-S compression for compressional fracture sets and an NE-SW
extension for the extensional set (Figs. 3B and S1).

## 4.2. Calcite cement petrography

The five different fracture sets contain bladed, blocky, and elongated calcite cement.
Karstic breccias in Jurassic rocks of the Bóixols-Upper Pedraforca thrust sheet and a
fault breccia in Eocene-Oligocene rocks from the Ebro foreland basin also have been
studied for U-Pb calcite dating. These breccias only contain blocky and bladed calcite
crystals.

Bladed crystals are up to 2 mm long (Fig. 4A and B) and have been observed mainly in
N-S, NNW-SSE and NNE-SSW veins. The disposition of these crystals indicates that
their growth occurred from both fracture walls to the vein centre or from one vein wall
to another. Blocky crystals have a size ranging between 100 µm and 3 mm (Fig. 4C and
D). They have been observed in N-S, NNW-SSE and NNE-SSW and E-W folding-
related veins, as well as in rhomb-shaped veinlets formed by crack-seal mechanism in
reverse and strike-slip faults (Fig. 4E and F). In these faults, blocky calcites are also
arranged in stepped slickensides, thus indicating the sense of shear. In N-S, NNW-SSE
and NNE-SSW veins, blocky crystals occasionally contain trails of host-rock arranged
219 parallel to fracture walls and perpendicular to the opening direction, also indicating a
220 crack-seal mechanism of vein formation (Fig. 4G).

221 Elongated calcite crystals are arranged parallel, oblique, or perpendicular to fracture
222 walls and have lengths ranging between 400 µm and 3 mm (Fig 5A to F). Elongated
223 calcite crystals arranged perpendicular to fracture walls are observed in N-S, NNW-SSE
224 and NNE-SSW trending veins. Elongated crystals arranged parallel or oblique to
225 fracture walls are observed in reverse faults, strike-slip faults and in normal faults. The
226 disposition of the oblique elongated crystals with respect to fracture planes and stepped
227 slickensides containing elongated calcite parallel to fracture walls indicate the sense of
228 shear (Fig. 5E and F).

229 **4.3. U-Pb calcite dating**

230 Using U-Pb geochronology, we document 46 dates, ranging from 70.6 ± 0.9 Ma to 2.8 ±
231 1.8 Ma, obtained from 43 fracture-filling calcites and three calcites precipitated between
232 karstic and fault breccia clasts (Figs. 6 and Tables S2 and S3). In the Bóixols-Upper
233 Pedraforca thrust sheet, dates range from 70.6 ± 0.9 Ma to 12.7 ± 2.9 Ma (Fig. 6). For
234 the Lower Pedraforca thrust sheet, dates range from 48.3 ± 1.3 Ma to 13.4 ± 1.5 Ma and
235 for the Cadí unit from 36.7 ± 1.5 Ma to 4.6 ± 4.6 Ma (Fig. 6). This last young date
236 (sample C12), as well as the one obtained in cement within fault breccia clasts from the
237 Ebro foreland basin (sample GPR401; 2.8 ± 1.8 Ma; Fig. 6), have high uncertainties and
238 high proportions of common lead, resulting in sub-horizontal arrays (Table. S2) and in
239 consequence, they are not considered in the discussion. Two U-Pb dates measured in
240 calcite cement within karstic breccias in Jurassic rocks of the Bóixols-Upper Pedraforca
241 thrust sheet yield ages between 37.1 ± 0.7 Ma and 34.4 ± 0.9 Ma (samples P9a and
242 P9b).
243

# 5. DISCUSSION

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On the other hand, blocky and bladed textures indicate precipitation after vein opening or at lower rates than vein opening. Despite this, the presence of some of the studied blocky textures in veins formed by crack-seal mechanism and in stepped slickensides also indicates syn-kinematic growth. Furthermore, clumped isotopes thermometry applied to several of these blocky and bladed calcites, presented previously in Cruset et al. (2016; 2018; 2019; 2020) and Cruset (2019), indicate that most of them precipitated from fluids in thermal disequilibrium with their adjacent host rocks. This disequilibrium indicates rapid fluid flow and precipitation just after fracture opening (Bons et al., 2012; Beaudoin et al., 2014), thus suggesting that the studied blocky and bladed calcites dated in this study are also syn-kinematic.

The results obtained in this study evidence certain limitations of the U-Pb method in the southern Pyrenees. Firstly, the U-Pb dating failed in calcite cement Cc2 from the Vallfogona thrust and northern Cadí thrust sheet (with clumped temperatures of 154 ± 2 °C and 113 ± 9 °C, respectively), calcite cement Cc1 in the Puig-reig anticline (129 ± 8 °C), and in dolomite cement Dc1 (149.4 ± 12.5 °C) and Dc2 (124.7 ± 6.2 °C) in the Upper Pedraforca thrust sheet (temperatures are from Cruset et al., 2016; 2018; 2019; 2020 and Cruset, 2019; Table 1). All these samples have temperatures above 110 °C and too high initial Pb/U contents, suggesting that temperature could be a limiting parameter of the U-Pb dating method. Secondly, U-Pb dating also failed in fracture-filling calcites hosted in siliciclastic deposits, which derived from igneous and metamorphic basement sources (e.g., GDV13, 309B; 311A; Table S1). In these samples, the lead content is
268 high (from 2.9 to 29.0 ppm and a mean value of 13.8 ppm; Table S2), whereas the U
269 content is within the magnitude of some successful samples (between 2 and 659 ppb
270 and a mean value of 85 ppb; Table S2). Such high lead content indicates the influence
271 of the host rock composition on successful U-Pb dating.

272 Despite these limitations, the obtained U-Pb dataset fits with the timing of deformation
273 within the SE Pyrenean piggy-back thrust system, already constrained by a large
274 number of growth strata units dated by both biostratigraphic analysis for marine
275 deposits, and magnetostratigraphy and apatite fission-track thermochronology for non-
276 marine deposits (e.g., Meigs et al., 1996; Beamud et al., 2010; Rahl et al., 2011;
277 Rushlow et al., 2013; Carrigan et al., 2016; Fig. 6). Furthermore, the results provide
278 broader information regarding the duration of thrusting propagation and pervasive
279 deformation of the upper SE Pyrenean thrust sheets. Dates obtained in the Bóixols-
280 Upper Pedraforca thrust sheet ranging from 70.6 ± 0.9 Ma to 55.3 ± 0.5 Ma are older
281 than Palaeocene syn-tectonic sediments fossilizing this structure (Tavani et al., 2017).

282 Therefore, they have been interpreted to be related to the emplacement of this thrust unit
283 and record a minimum period of 15.3 Ma (Fig. 6). Thus, younger U-Pb dates obtained
284 in the Bóixols-Upper Pedraforca from 48.9 ± 3.1 to 25.7 ± 1.9 Ma are thus related to its
285 post-emplacement (Figs. 6 and 7). In this line, dates ranging from 48.9 ± 3.1 Ma to 44.7
286 ± 4.1 Ma are coeval with the emplacement of the Lower Pedraforca thrust sheet (Vergés
287 et al., 2002; Grool et al., 2018). U-Pb dates obtained in this thrust unit from fracture-
288 filling calcites in both pre-compression rocks and Lutetian growth strata lying along the
289 thrust front range from 48.3 ± 1.3 Ma to 41.5 ± 0.7 Ma. The two U-Pb dates of ~41 Ma
290 fit with the initial emplacement of the Cadí thrust sheet but also with the development
291 of the Lower Pedraforca imbricates (Burbank et al., 1992). Thus, the U-Pb dates
292 measured in this thrust unit document a minimum duration of emplacement of 7.3 Ma
and suggest that it was ~5.4 Ma younger than previous estimates (Vergés et al. 2002; Fig. 6). The uppermost Bóixols-Upper Pedraforca also registers dates ranging from 39.5 ± 1.2 Ma to 25 ± 17 Ma, which are consistent with the U-Pb dates measured in the Cadí unit ranging from 38.8 ± 1.1 to 25.7 ± 1.9 Ma (Fig. 6). Considering these Bartonian-Chattian U-Pb dates in both thrust units, we estimate a minimum duration of emplacement of 12.9 Ma for the Cadí thrust sheet. Post-Oligocene dates have not been used in these calculations since compressional deformation in the SE Pyrenees finished in the Oligocene (Vergés et al., 2002; Grool et al., 2018). The estimated durations for the emplacement of the SE Pyrenean thrust sheets are calculated from U-Pb dates without considering time gaps between each unit (Fig. 6, dotted grey lines). In a scenario of continuous thrust sheet propagation as suggested by Vergés et al. (2002) and Grool et al. (2018), and considering the time gaps, maximum emplacement durations of 18.8 Ma, 11.6 Ma and 14.3 Ma are obtained for the Bóixols-Upper Pedraforca, Lower Pedraforca and Cadí thrust sheets, respectively.

The spatial distribution of the U-Pb dates reflects the thrust propagation history of the SE Pyrenees. In this sense, dates from the Bóixols-Upper Pedraforca register its emplacement from Late Cretaceous to Palaeocene and its post-emplacement deformation above the Lower Pedraforca and Cadí from early Eocene to late Oligocene (Fig. 7). Such post-emplacement deformation is also observed in the Lower Pedraforca, where the early Oligocene date of 30 ± 1.4 Ma records its deformation above the Cadí unit. In the same line, late Oligocene U-Pb dates of 29.3 ± 1.8 Ma and 25.9 ± 8.9 Ma obtained in the Cadí unit are coeval with dated growth strata within foreland (Meigs et al., 1996; Carrigan et al., 2016; Figs. 6 and 7).
Another important information provided by the U-Pb results is related to the rates of thrust sheet emplacement and their variations through time in fold and thrust belts. To calculate these rates, we used the minimum shortening already established by Vergés et al. (2002) and Grool et al. (2018) for the Bóixols-Upper Pedraforca (11 km), Lower Pedraforca (36.2 km) and Cadí (16.4 km) thrust sheets and the durations of emplacement estimated from U-Pb dating measured in the studied fracture-filling calcites. Shortening values were obtained from the restoration of both cover and basement thrust sheets represented in the balanced cross-section J3 from Vergés (1993), which is the most representative of the studied area. Thus, maximum rates of 0.7 mm/yr for the Bóixols-Upper Pedraforca (11 km/15.3 Ma), 4.9 mm/yr for the Lower Pedraforca (36.2 km/7.3 Ma) and 1.3 mm/yr for the Cadí (16.4 km/12.9 Ma) thrust sheets are obtained. On the other hand, considering the time gaps between each thrust sheet, rates of 0.6 mm/yr, 3.1 mm/yr and 1.1 mm/yr are calculated (Fig. 6). In this last scenario, the calculated rates are within the same range of previous estimates calculated for the southern Pyrenees from balanced and restored cross-sections combined with stratigraphic constraints (between 0.4 and less than 4 mm/yr; Grool et al., 2018).

Finally, from 70.6 ± 0.9 Ma to 25.9 ± 8.9 Ma, the dominant stress regime shows a N-S compression that agrees with the mean Pyrenean tectonic transport direction (compressional fracture sets; Fig. 7), whereas local normal faults showing NE-SW extension formed from 30 ± 1.4 Ma to 12.7 ± 2.9 Ma (extensional fracture set; Fig. 7). The orientations of these analysed normal faults do not fit with the main E-W trend observed in the SE Pyrenean folds and thrusts, except for those formed by crestal collapse in the Puig-reig anticline (Fig. S1). Therefore, Oligocene normal faults could have formed during the exhumation of the central and south eastern Pyrenees related to the compression and stacking of its hinterland according to thermochronological data
341 (e.g., Fitzgerald et al., 1999, Gibson et al., 2007; Fillon and van der Beek, 2012; 342 Rushlow et al., 2013). The areal restriction of NNW-SSE normal faults at the northern 343 domain of the Bóixols-Upper Pedraforca, Lower Pedraforca and Cadí thrust sheets and 344 their coexistence with compressional veins showing U-Pb ages of 28.4 ± 0.8 Ma, 25.9 ± 345 8.9 Ma and 25.7 ± 1.9 Ma supports this exhumation scenario (samples B32, B46 and 346 P12a, respectively; Figs. 6 and 7). Contrarily, Miocene normal faults and reactivated 347 reverse faults probably formed during the post-orogenic exhumation and collapse of the 348 SE Pyrenees during upstream incision of the Ebro basin before its capture by the 349 Mediterranean Sea at 12 to 8.5 Ma (Garcia-Castellanos et al., 2003; Fillon and van der 350 Beek, 2012; Rushlow et al., 2013; Pérez-Rivarés et al., 2018). According to these 351 authors, the Ebro basin capture resulted in the lowering of the regional base level and 352 the excavation of the buried SE Pyrenean fold and thrust belt. 353

## 6. CONCLUSIONS

354 Duration of thrust sheet emplacement and shortening rates in the SE Pyrenean piggy- 355 back thrust system were quantified applying U-Pb geochronology to fracture-filling 356 calcites. The results reveal emplacement durations of 18.8 Ma for the Bóixols-Upper 357 Pedraforca, 11.6 Ma for the Lower Pedraforca and 14.3 Ma for the Cadí unit, with 358 shortening rates of 0.6, 3.1 and 1.1 mm/yr, respectively. U-Pb dates also indicate that 359 during progressive south-directed Pyrenean thrust sheet stacking, emplacement of lower 360 thrust sheets deformed the previously emplaced upper ones. Furthermore, local normal 361 faults with Late Oligocene ages from 30 ± 1.4 to 25.7 ± 1.9 Ma are consequence of the 362 Pyrenean hinterland exhumation during the late compressional stages. Finally, U-Pb 363 dates indicate that in the SE Pyrenees, temperatures higher than 110 °C influence 364 negatively in the suitability of calcite and dolomite for dating. 365
365 ACKNOWLEDGEMENTS

366 This research was carried out within the framework of DGICYT Spanish Project
367 PGC2018-093903-B-C22 Ministerio de Ciencia, Innovación y Universidades/Agencia
368 Estatal de Investigación/Fondo Europeo de Desarrollo Regional, Unión Europea, and
369 Grups Consolidats de Recerca "Geologia Sedimentària" (2017SGR-824) and
370 "Modelització Geodinàmica de la Litosfera" (2017SGR-847). This is FIERCE
371 contribution No. XX. We thank Nick Roberts, Guilhem Hoareau and the Editor Randall
372 Parrish for their constructive comments that improved the quality of the manuscript.
373 Also, we would like to thank Katherine Mottram, Elena Druget and Chris Clark for
374 geological discussions of a previous version of the manuscript. Finally, we thank
375 Frances Luttikhuizen for the linguistic revision of this draft of the manuscript.

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<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

573 Fig. 1. A) Simplified map of Iberia reporting the location of the Pyrenees and the studied area.

574 B) Structural sketch of the studied area with the location of the studied structures and suitable

575 samples for dating. The red line shows the location of the cross-section in C. For locations of

576 samples where U-Pb dating failed see Table S1. C) Geological cross-section of the SE Pyrenees

577 from Vergés (1993).
<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

579 Fig. 2. Representative images of the five fracture sets observed in the study area. A) En-échelon

580 vein arrays, Cadí thrust sheet; B) Folding related veins, Cadí thrust sheet; C) Reverse faults,

581 Lower Pedraforca thrust sheet; D) Strike-slip faults, Bóixols-Upper Pedraforca thrust sheet; E)

582 Normal faults, Lower Pedraforca thrust sheet.
<figure><img src="image_15.png" /></figure>

B Time

<figure><img src="image_16.png" /><figcaption>Fig. 3. A) Sketch representing the five fracture sets identified in this study: 1) En-échelon vein arrays; 2) Folding related veins; 3) Reverse faults; 4) Strike-slip faults; 5) Normal faults. For complementary details see Cruset et al. (2016, 2018 and 2020), Cruset (2019) and Nardini et al. (2019). B) Stereographic projections of the mean orientation of fractures 1 to 5 calculated from fracture data presented in Fig. S1.</figcaption></figure>
<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /></figure>

<figure><img src="image_21.png" /></figure>

<figure><img src="image_22.png" /></figure>

<figure><img src="image_23.png" /></figure>
Fig. 4. Optical microscope images (parallel and crossed Nicols) of the main features of the

Bladed and blocky calcite crystals. A-B) Bladed calcite. C-D) Blocky crystals within N-S vein

arrays. E-F) Blocky calcite crystals precipitated in rhomb-shaped veinlets formed by crack-seal

mechanism in a strike-slip fault. G) N-S vein formed by crack-seal mechanism and filled with

blocky crystals. Note how host-rock trials arranged parallel to fracture walls and perpendicular

to the opening direction are included within the calcite cement.
<figure><img src="image_24.png" /><figcaption>597 Fig. 5 Optical microscope images (parallel and crossed Nicols) of the main features of the</figcaption></figure>

<caption>598 elongated crystals. A-B) Elongated crystals perpendicular to the walls of a N-S vein. B-C)</caption>

<caption>599 Elongated crystals parallel to the walls of a reverse fault. C-D) Elongated crystals oblique to</caption>
600 fracture walls of a reverse faults. Note how the disposition of the crystals indicate the sense of

601 shear.
<figure><img src="image_25.png" /></figure>

Fig. 6 U-Pb dates from SE Pyrenean fold and thrust belt and calculated shortening rates. Colour bars on the right indicate previous age estimations of the Pyrenean compression and NE Iberian extension based on Vergés et al. (2002). Ages for the Cadí unit and for the tectonic activity of the foreland Ebro basin are modified according to Rushlow et al. (2013), Meigs et al. (1996) and Carrigan et al. (2016). The timing of the development of the endorheic to exorheic foreland Ebro basin is based on Garcia-Castellanos et al. (2003) and Costa et al. (2010).
<figure><img src="image_26.png" /></figure>

<figure><img src="image_27.png" /></figure>

<figure><img src="image_28.png" /></figure>

<figure><img src="image_29.png" /></figure>

609

610 Fig. 7. Simplified sketch of the southern Pyrenees based on U-Pb ages showing the sequence of

611 deformation and its propagation to the foreland. Symbols represent the type of fracture as shown

612 in Fig. 6. Error of ages are reported in Fig. 6.
<table><thead><tr><th>Sample</th><th>Cement Type</th><th>n</th><th>Δ<sub>47</sub></th><th>T °C</th><th>Measured U-Pb age?</th><th>Reference</th></tr></thead><tbody><tr><td colspan="7"><strong>Bóixols-Upper Pedraforca thrust sheet</strong></td></tr><tr><td>CN33</td><td>Cc1</td><td>2</td><td>0.54 ± 0.006</td><td>88.4 ± 3.8</td><td>No</td><td>Nardini et al. (2019)</td></tr><tr><td>CN38</td><td>Cc2</td><td>1</td><td>0.646</td><td>40.7 ± 16.1</td><td>Yes</td><td>Nardini et al. (2019)</td></tr><tr><td>CN20</td><td>Cc3</td><td>2</td><td>0.55 ± 0.007</td><td>85.3 ± 4</td><td>No</td><td>Nardini et al. (2019)</td></tr><tr><td>CN15</td><td>Cc4</td><td>2</td><td>0.54 ± 0.001</td><td>87.6 ± 0.6</td><td>No</td><td>Nardini et al. (2019)</td></tr><tr><td>P24</td><td>Dc1</td><td>4</td><td>0.46 ± 0.014</td><td>149.4 ± 12.5</td><td>No</td><td>Cruset (2019)</td></tr><tr><td>P32/P75</td><td>Dc2</td><td>4</td><td>0.49 ± 0.008</td><td>124.7 ± 6.2</td><td>No</td><td>Cruset (2019)</td></tr><tr><td>P4</td><td>Cc4</td><td>1</td><td>0.556</td><td>81 ± 21.7</td><td>Yes</td><td>Cruset (2019)</td></tr><tr><td>P2</td><td>Cc5</td><td>3</td><td>0.56 ± 0.022</td><td>79.7 ± 12</td><td>Yes</td><td>Cruset (2019)</td></tr><tr><td>P7/P9</td><td>Cc10</td><td>1</td><td>0.675</td><td>30.3 ± 13.6</td><td>Yes</td><td>Cruset (2019)</td></tr><tr><td>P14/P15</td><td>Cc7</td><td>3</td><td>0.52 ± 0.007</td><td>100.78 ± 4.2</td><td>Yes</td><td>Cruset (2019)</td></tr><tr><td>P6/P8</td><td>Cc8</td><td>4</td><td>0.52 ± 0.008</td><td>101.4 ± 5</td><td>Yes</td><td>Cruset (2019)</td></tr><tr><td>P44</td><td>Cc14</td><td>4</td><td>0.58 ± 0.009</td><td>67.27 ± 4.8</td><td>Yes</td><td>Cruset (2019)</td></tr><tr><td colspan="7"><strong>Lower Pedraforca thrust sheet</strong></td></tr><tr><td>Q2/Q27/Q3-1</td><td>Cc3</td><td>4</td><td>0.58 ± 0.011</td><td>69.08 ± 5.34</td><td>Yes</td><td>Cruset et al. (2020)</td></tr><tr><td>Q24</td><td>Cc6</td><td>3</td><td>0.57 ± 0.008</td><td>74.16 ± 4</td><td>Yes</td><td>Cruset et al. (2020)</td></tr><tr><td colspan="7"><strong>Cadí thrust sheet</strong></td></tr><tr><td>GDV30/GDV26</td><td>Cc2</td><td>3</td><td>0.46 ± 0.002</td><td>154 ± 2</td><td>No</td><td>Cruset et al. (2018)</td></tr><tr><td>GDV13</td><td>Cc5</td><td>3</td><td>0.53 ± 0.023</td><td>105 ± 14</td><td>No</td><td>Cruset et al. (2018)</td></tr><tr><td>B34/B2</td><td>Cc2</td><td>4</td><td>0.50 ± 0.028</td><td>113 ± 10</td><td>No</td><td>Cruset et al. (2019)</td></tr><tr><td>B22</td><td>Cc3</td><td>4</td><td>0.55 ± 0.026</td><td>86 ± 7.2</td><td>Yes</td><td>Cruset et al. (2019)</td></tr><tr><td colspan="7"><strong>Ebro foreland basin</strong></td></tr><tr><td>309B</td><td>Cc1</td><td>3</td><td>0.548 ± 0.009</td><td>92 ± 5</td><td>No</td><td>Cruset et al. (2016)</td></tr><tr><td>317</td><td>Cc1</td><td>3</td><td>0.49 ± 0.010</td><td>129 ± 8</td><td>No</td><td>Cruset et al. (2016)</td></tr><tr><td>311D</td><td>Cc2</td><td>3</td><td>0.551 ± 0.004</td><td>90 ± 3</td><td>No</td><td>Cruset et al. (2016)</td></tr><tr><td>311A</td><td>Cc2</td><td>3</td><td>0.574 ± 0.010</td><td>77 ± 5</td><td>No</td><td>Cruset et al. (2016)</td></tr></tbody></table>

613

614 Table 1. Available Δ<sub>47</sub> and clumped temperatures measured in the dated calcites and dolomites

615 in the SE Pyrenees. Data from Cruset et al. (2016; 2018; 2019; 2020), Cruset (2019) and

616 Nardini et al. (2019).