Constraints from Low-Temperature Thermochronology Exhumation Of

Geological Society, London, Special Publications

Exhumation of the Sierra de Cameros (Iberian Range, Spain): constraints from low-temperature thermochronology

P. Del Río, L. Barbero and F. M. Stuart

Geological Society, London, Special Publications 2009; v. 324; p. 153-166 doi:10.1144/SP324.12

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P. DEL RI´O1*, L. BARBERO1 & F. M. STUART2 1Departamento de Geologı´a, Universidad de Ca´diz, 11510 Puerto Real, Ca´diz, Spain 2Isotope Geosciences Unit, Scottish Universities Environmental Research Centre, Rankine Avenue, Technology Park, East Kilbride G75 0QF, UK *Corresponding author (e-mail: [email protected])

Abstract: We present new fission-track and (U–Th)/He data from apatite and zircon in order to reconstruct the exhumation of the Sierra de Cameros, in the northwestern part of Iberian Range, Spain. Zircon fission-track ages from samples from the depocentre of the basin were reset during the metamorphic peak at approximately 100 Ma. Detrital apatites from the uppermost sediments retain fission-track age information that is older than the sediment deposition age, indicating that these rocks have not exceeded 110 8C. Apatites from deeper in the stratigraphic sequence of the central part of the basin have fission-track ages of around 40 Ma, significantly younger than the stratigraphic age, recording the time of cooling after peak metamorphic conditions. Apatite (U–Th)/He ages in samples from these sediments are 31–40 Ma and record the last period of cooling during Alpine compression. The modelled thermal history derived from the uppermost sediments indicates that the thermal pulse associated with peak metamorphism was rapid, and that the region has cooled continuously to the present. The estimated palaeogeothermal gradient is around 86 8Ckm21 and supports a tectonic model with a thick sedimentary fill (c. 8 km) and explains the origin of the low-grade metamorphism observed in the oldest sediments.

Convergence of the Eurasian and Iberian plates In the first model Guiraud (1983) and Guiraud & during the Palaeogene–early Miocene led to the Se´guret (1984) proposed an east–west-striking, formation of the Pyrenees in the northern Iberian south-dipping normal fault in the basement as the margin. The Iberian Range, located to the south structure responsible for the formation of a syncline of the Pyrenees, is an intraplate mountain chain basin. The sedimentary infilling of this basin pro- that formed as a result of migration of compres- gressively onlapped the structure as the depocentre sional deformation towards the inner part of the migrated northwards (Fig. 2a). In this model the Iberian microplate during Alpine collision. The total thickness of the sedimentary fill would not Cameros Basin, located in the northwestern exceed 5 km. During the Tertiary the basin was most part of the Iberian Chain (Fig. 1), represents inverted, with the normal faults being reactivated a synrift sequence formed prior to the Alpine as reverse faults. convergence during the Upper Jurassic–Mid In a second model Mas et al. (1993) and Guimera` Cretaceous extensional period. It was inverted et al. (1995) (Fig. 2b) interpreted the Cameros during the subsequent Alpine shortening, either Basin as a sag basin that formed on a ramp of an by the reactivation of the synrift normal faults or extensional, subhorizontal fault located several on newly formed faults and thrust (Casas-Sainz & kilometres deep in the Variscan basement. The Gil-Imaz 1998). sedimentary units that fill the basin decrease in The stratigraphy, mineralogy, metamorphism thickness towards the southern border and onlap to and structure of the Sierra de Cameros have the north on the Jurassic, prerift deposits. The depo- been studied for several decades (e.g. Tischer centres are located on the normal fault ramp and 1966; Mas et al. 1993; Casas-Sainz & Gil-Imaz migrated progressively towards the north. Again, 1998; Mata et al. 2001). However, the timing and the maximum vertical thickness reached by the sedi- mechanism of the basin inversion, the origin of the mentary fill would be about 5 km. Tectonic inver- greenschist-facies metamorphism of sediments in sion of the basin was caused by a newly formed the deepest part of the basin and the geometry of thrust developed along the northern border, and a the sedimentary filling remain unclear. Three thrust system to the south. In these two models tectonic models have been proposed to explain the the origin of low-grade metamorphism remains formation of the Cameros Basin and the geometry unclear and some authors claim for a hydrothermal of the synrift deposits. origin (e.g. Casquet et al. 1992).

From:LISKER, F., VENTURA,B.&GLASMACHER, U. A. (eds) Thermochronological Methods: From Palaeotemperature Constraints to Landscape Evolution Models. Geological Society, London, Special Publications, 324, 153–166. DOI: 10.1144/SP324.12 0305-8719/09/$15.00 # Geological Society of London 2009. 154 P. DEL RI´O ET AL.

Fig. 1. Schematic geological map of the Sierra de Cameros showing the location of the study area within the context of the Iberian plate and the locations of cross-sections and samples for this study. EXHUMATION OF THE SIERRA DE CAMEROS 155

2000), and fluid inclusions in quartz and calcite filling extensional cracks (Mata et al. 2001) also support this hypothesis. The Sierra de Cameros shows several features that make it an interesting area for the study of the evolution of intraplate rifting process and far-field effect of the plate boundaries. These include: (1) a well-preserved, thick synrift sedimentary sequence; (2) a unique low-grade metamorphic event and several folding stages that did not appear in other parts of the Iberian Chain; and (3) the presence of thick, well-preserved foreland basin deposits (Ebro Basin) in which it is possible to study the evolution of the eroded part of the Sierra de Cameros. The application of low-temperature thermo- chronometers such as fission track and (U–Th)/ He to detrital minerals provides a unique tool to better understand the thermal history of the Sierra de Cameros and, consequently, its geological evolution. This is facilitated by the presence of detrital apatite and zircon crystals in the detrital rocks of the sedimentary sequence of the Cameros Basin. The combination of two low-temperature thermochronological methods allows for new constraints on the palaeogeothermal gradients to be placed, which in turn can be compared to previous data from other independent techniques.

Fig. 2. Summary of the tectonic models proposed to Geological framework explain the Cameros Basin evolution (modified after Gil-Imaz et al. 2002). (a) Synsedimentary synclinal The Sierra de Cameros forms the main part of the model proposed by Guiraud (1983) and Guiraud & northwestern sector of the Iberian Range. It is bor- Se´guret (1984), with a maximum vertical thickness of dered by two continental basins, the Ebro and about 5 km. (b) Sag-basin model after Mas et al. (1993) Duero basins to the north and south respectively, and Guimera´ et al. (1995) in which the units onlap onto and by two Paleozoic massifs, the Sierra de la the prerift sequence; a maximum vertical thickness of Demanda to the west and the Moncayo Massif to about 5 km is reached in this model. (c) Synsedimentary the east (Fig. 1). Sediments of the Sierra de synclinal model proposed by Casas-Sainz & Gil-Imaz Cameros were deposited in a Mesozoic basin that (1998), with vertical stacking of the units reaching a maximum vertical thickness of about 8 km. formed as a consequence of northward propagation of rifting related to the progressive opening of the North Atlantic Ocean and the Bay of Biscay. Trans- tensional rifting of the Bay of Biscay resulted in the Casas-Sainz & Gil-Imaz (1998) proposed an progressive destruction of the Middle–Late Jurassic alternative model in which the basin exhibits a half- carbonate platforms and the development of a new graben geometry controlled by a listric fault devel- extensional system in the Iberian Range (Mas et al. oped along previous Variscan structures. This fault 2002). The basin exhibited a half-graben-like geo- does not extend beyond the limits of the basin, and metry that was controlled by a system of extensional the units stack vertically and decrease in thickness listric faults with NW–SE strikes that were devel- towards the basin margins (Fig. 2c). The oped on earlier Variscan structures. The synrift maximum vertical thickness reached by the sedi- stratigraphic sequence spans from Upper Jurassic mentary fill according to this model was 8 km. (Tithonian–Berriasian) to the upper–lower Cretac- During the Tertiary basin inversion the Upper Trias- eous boundary (Albian–Cenomanian) (Mas et al. sic sediments (Keuper facies) were the detachment 1994; Munõz et al. 1997). Depositional sequences level for thrusts involving the Mesozoic cover. are classically divided into five lithostratigraphic This model can satisfactorily explain the origin of groups: the Tera, Oncala, Urbioń, Enciso and low-grade metamorphism in the deep basin mainly Olivań groups (Tischer 1966). Sediments are as a consequence of burial. The interpretation of largely fluvial sandstones, lutites and conglomerates the magnetic fabric of rocks (ASM: Gil-Imaz et al. (Tera, Urbioń and Olivań groups), and lacustrine 156 P. DEL RIÓ ET AL. carbonates with siliciclastic influence (Oncala and populations: 60.9 + 2.9 Ma (P1), 80.0 + 4.1 Ma Enciso groups). The clastic sequences are charac- (P2) and 122.9 + 15.5 Ma (P3). Mean track length terized by compositional immaturity (arkoses are associated with the P1 and P2 age populations abundant) characteristic of proximal facies. The are 12.8 + 2.2 and 11.7 + 2.4 mm, respectively, geochemistry of the detrital sediments reveals an showing in both cases a negatively skewed distri- acid-igneous or metamorphic source rock (Mata bution (Fig. 4). Dpar of the P1 and P2 populat- et al. 2000). The sediments may have been sourced ions have average values of 1.38 + 0.14 and in areas close to the Spanish Central System, which 1.38 + 0.13 mm, respectively. Fluorine (1.32– is dominated by acid granites and orthogneisses. 1.49 apfu (atoms per formula unit)) and chlorine This is consistent with the palaeo-current directions (0.02–0.03 apfu) contents show a negative corre- (Tischer 1966) and the compositional immaturity lation with the Dpar values (Fig. 5). Sample of the sediments. Depositional ages used in the MAO-4 (lower part of the Olivań Group) shows present work are based on Charophyceae biozones two age populations of 54.3 + 2.4 Ma (P1) and (Schudack 1987), pollen analysis (Martıń i Closas 69.8 + 5.6 Ma (P2) that are younger than its 1989) and correlations made by Mas et al. (1993), stratigraphic age, and a third population of which give depositional times for the Cameros 127.3 + 24.7 Ma (P3) that is older than its strati- Basin as follows: lower Albian for the Olivań graphic age. P1 and P2 age populations have mean group; Aptian for the Enciso group; Barremian– track lengths of 11.3 + 2.3 and 11.6 + 2.2 mm, Valanginian for the Urbioń group; Berriasian for respectively, showing in both cases a negatively the Oncala group; and Tithonian for the Tera group. skewed distribution (Fig. 4). Average Dpar measure- The presence of chloritoid in the deepest sediments are 2.2 + 0.3 mm for P1, 2.1 + 0.3 mm for ments cropping out at present indicates that the P2 and 2.1 + 0.3 mm for P3. Fluorine and chlorine region underwent a low-grade metamorphism that contents are 1.26–1.41 and 0.02–0.03 apfu, peaked of approximately at 326–350 8C and 1 kb respectively, and show a negative correlation with (Casquet et al. 1992; Mata et al. 2001). Ar–Ar the measured Dpar (Fig. 5). Finally, sample POC-9 and K–Ar ages for peak temperatures are approxi- exhibits a single AFT age of approximately mately 100 Ma (Goldberg et al. 1988; Casquet 48.3 + 4.1 Ma. et al. 1992). During the Tertiary the basin underwent AFT data from nine sandstone samples along the tectonic inversion related to Alpine compression profile B–B0 are shown in Figure 1. Most samples that produced folding and exhumation of the have several age populations, most of them whole Mesozoic basin The Cameros Basin sedi- younger than the depositional age (see radial plots ments were thrusted 25–30 km north along the in Fig. 6). Sample JS-6 (Olivań) has an age popu- Cameros thrust over the Tertiary molasse of the lation of 123.2 + 14.9 Ma, which is older than the Ebro Basin (Guimera` et al. 1995; Casas-Sainz & depositional age (Lower Albian), but the other two Gil-Imaz 1998). During the Palaeocene and components are younger. Apatites from samples Eocene, the Sierra de Cameros was the source area JS-9 and JS-11, both from the middle part of the for the detrital sediments that were deposited in Olivań group, yield a single age population of the Ebro Basin, in particular for its westernmost 55.6 + 3.2 and 34.5 + 1.9 Ma, respectively. The part (the Rioja Trough) where they reach a number of confined tracks in samples from the maximum thickness of 5 km (Munõz-Jimeńez & B–B0 profile is far too low to allow a reasonable Casas-Sainz 1997). statistical evaluation of the data. Mean track length in samples JS-9 and JS-11 are 10.61 + 2.53 Results (n ¼ 19) and 11.32 + 2.34 (n ¼ 9) mm, respectively. Dpar values for these samples are in the Fission-track results are shown in Table 1. Radial range of 2.4 and 6.0 mm. plots of zircon fission-track (ZFT) ages of the two (U–Th)/He ages (AHe) have been determined samples from profile A–A0 are displayed in on apatites from two sandstone samples from Figure 3. Zircons from sample MAO-3 from the profile B–B0 (see Fig. 6 and Table 2). Sample Olivań group have two age populations that JS-21 (Tera) yields AHe age of 31.1 + 1.5 Ma, are considerably older than the sediment strati- and two splits of apatites from JS-12 (Olivań) of graphic age (Lower Albian). Zircons from POC-9 39.8 + 2.8 and 37.6 + 1.8 Ma. (Urbioń group) yield a single fission-track age population of about 85 Ma. This is considerably younger Thermal modelling than the sediment deposition age (Valanginian– Barremian). Thermal modelling has been performed on samples Apatite fission-track (AFT) data have been MAO-3 and MAO-4 from the Olivań group in which obtained in three samples along the profile A–A0 track lengths associated with the two youngest age (Fig. 3). Sample MAO-3 (Olivań) shows three age populations have been clearly separated. From the Table 1. Apatite and zircon fission-track age data from the Sierra de Cameros

† 6 6 6 Sample No. Stratigraphic No. rD (10 rs (10 ri (10 Central age Dpar (mm)/MTL Age populations (Ma + 1s), Nf, W (%) Fit (mineral age of tr cm22) tr cm22) tr cm22) + 1s (Ma) (mm) + 1s statistics 2 type)* Group grains (ND) (Ns) (Ni) P(x ) P1 P2 P3

MAO-3 Lower Albian 42 1.132 1.955 5.269 71.7 + 3.0 P1 ¼ 1.4 + 0.1/12.8 + 2.2 60.9 + 2.9 80.0 + 4.1 122.9 + 15.5 x 2 ¼ 39 (Ap) Oliva´n (5249)(4995)(13462) 0.00 P2 ¼ 1.4 + 0.1/11.7 + 2.4 Nf ¼ 20.5 Nf ¼ 19.2 Nf ¼ 4.1 l ¼ 37

P3 ¼ 1.3 + 0.4 W ¼ 13 W ¼ 12 157 W ¼ 18 P(F) ¼ 0% CAMEROS DE SIERRA THE OF EXHUMATION MAO-3 Lower Albian 24 0.394 13.524 1.7256 211.4 + 18.7 184.9 + 13.4 419.6 + 82.7 x 2 ¼ 24 (Zr) Olivań (1826)(3511)(448) 0.04 Nf ¼ 19.4 Nf ¼ 4.6 l ¼ 21 W ¼ 27 W ¼ 39 P(F) ¼ 0% JS-6 Lower Albian 74 1.007 1.547 4.696 58.0 + 3.4 P1 ¼ 3.8 + 1 45.2 + 2.8 57.8 + 6.0 123.2 + 14.9 x 2 ¼ 74 (Ap) Olivań (4669)(5583)(16948) 0.00 P2 ¼ 3.1 + 1 Nf ¼ 45.1 Nf ¼ 24.4 Nf ¼ 3.5 l ¼ 67 P3 ¼ 3.4 + 2 W ¼ 15 W ¼ 15 W ¼ 23 P(F) ¼ 0% JS-8 Lower Albian 20 1.124 1.499 5.053 55.7 + 3.4 P1 ¼ 4.5 +3 49.2 + 3.4 70.7 + 7.0 x 2 ¼ 23 (Ap) Olivań (5212)(1041)(3509) 0.07 P2 ¼ 2.6 + 1 Nf ¼ 13.0 Nf ¼ 7.0 l ¼ 17 W ¼ 18 W ¼ 19 P(F) ¼ 0% MAO-4 Lower Albian 46 1.141 1.503 4.943 59.1 + 2.4 P1 ¼ 2.2 + 0.3/11.3 + 2.3 54.3 + 2.4 69.8 + 5.6 127.3 + 24.7 x 2 ¼ 42 (Ap) Olivań (5289)(3848)(12655) 0.00 P2 ¼ 2.1 + 0.3/11.6 + 2.2 Nf ¼ 32.5 Nf ¼ 12.5 Nf ¼ 1.0 l ¼ 41 P3 ¼ 2.1 + 0.3 W ¼ 15 W ¼ 15 W ¼ 20 P(F) ¼ 0% JS-9 Lower Albian 15 0.929 1.452 3.973 55.6 + 3.2 P1 ¼ 2.4 + 0.4 (Ap) Olivań (4307)(878)(2402) 2.55 JB-17 Lower Albian 23 1.066 1.220 3.532 64.4 + 6.2 21.4 + 5.1 60.0 + 4.1 105.6 + 12.6 x 2 ¼ 25 (Ap) Olivań (4942)(663)(1919) 0.00 Nf ¼ 2.4 Nf ¼ 15.1 Nf ¼ 5.5 l ¼ 18 W ¼ 36 W ¼ 24 W ¼ 26 P(F) ¼ 0% JS-11 Lower Albian 15 0.883 0.722 2.830 34.5 + 1.9 P1 ¼ 6.0 + 1.4 (Ap) Olivań (4093)(556)(2179) 90.44 JS-12 Lower Albian 61 1.065 1.163 4.193 48.5 + 2.3 P1 ¼ 2.3 + 0.7 41.9 + 1.6 62.7 + 3.6 x 2 ¼ 56 (Ap) Olivań (4941)(4598)(16578) 0.00 P2 ¼ 2.2 + 0.5 Nf ¼ 43.3 Nf ¼ 16.7 l ¼ 56 W ¼ 15 W ¼ 15 P(F) ¼ 0% JS-13 Apitan 35 0.831 1.065 3.226 46.5 + 2.7 P1 ¼ 4.8 + 1.0 40.8 + 1.7 88.6 + 12.7 x 2 ¼ 32 (Ap) Enciso (3855)(1985)(6015) 0.00 P2 ¼ 4.3 + 1.1 Nf ¼ 30.6 Nf ¼ 4.4 l ¼ 32 W ¼ 18 W ¼ 20 P(F) ¼ 0% JS-15 Hauterivian– 22 1.101 0.931 3.486 50.6 + 4.3 P1 ¼ 4.7 + 1.1 39.0 + 3.8 58.2 + 8.2 100.7 + 19.0 x 2 ¼ 21 (Ap)Barremian (5102)(747)(2796) 0.00 P2 ¼ 4.7 + 1.5 Nf ¼ 11.8 Nf ¼ 8.0 Nf ¼ 2.2 l ¼ 17 Urbioń P3 ¼ 4.0 + 0.1 W ¼ 24 W ¼ 21 W ¼ 23 P(F) ¼ 2% (Continued) 158

Table 1. Continued

POC-9 Hauterivian– 25 1.168 0.270 1.099 48.3 + 4.1 Barremian

(Ap) (5417)(202)(822) 82.61 RI DEL P. Urbio´n POC-9 Hauterivian– 20 0.3854 9.3645 2.9108 84.7 + 5.5 (Zr)Barremian (1787)(1615)(502) 65.73 ´

Urbio´n O 2

JS-21 Tithonian 21 0.863 1.322 6.139 28.7 + 1.5 P1 ¼ 3.2 + 0.3 25.5 + 1.8 34.6 + 2.9 x ¼ 19 AL ET (Ap) Tera (4001)(1347)(6256) 0.13 P2 ¼ 3.2 + 0.3 Nf ¼ 13.1 Nf ¼ 7.9 l ¼ 18 W ¼ 17 W ¼ 15 P(F) ¼ 0% .

*Ap, apatite; Zr, zircon. †CN-5 for apatites and CN-2 for zircons. Samples MAO-3, MAO-4 and POC-9 measured by Luis Barbero z (CN-5) ¼ 337.8 + 5.2 and z (CN-2) ¼ 137.5 + 4.5. All samples JS measured by Pedro del Rio z (CN-5) ¼ 339.2 + 8.2. rD, glass monitor track density; rs, spontaneous track density; ri, induced track density; ND,s,i , number of tracks counted to determinate the reported track density. 2 Nf, number of grains in age population; W, relative standard deviation; x , goodness of ﬁt parameter; l, degrees of fredoom; P(F), probability that the random variation alone could produce the observed F value for peak 2 or 3. MTL, mean track length. EXHUMATION OF THE SIERRA DE CAMEROS 159

Fig. 3. Cross-section along A–A0 (see Fig. 1 for the location and legend) modiﬁed after Gil-Imaz (2001), showing the location of the samples and AFT and ZFT radial plots.

analysis of the age population distribution in these Discussion two samples, it is clear that both have undergone partial resetting after deposition. Both populations The single ZFT age component that is younger retain an inherited thermal signal from the source than the depositional age in sample POC-9 (top areas. Constraints used for modelling include: (1) of the Urbioń group) indicates that, at least in the the depositional age of the sediment (Lower eastern part of the basin (profile A–A0), tempera- Albian); (2) the fact that the AFT system is not tures above 300–350 8C were reached (Yamada fully reset indicates that maximum tempera- et al. 1995; Tagami et al. 1998), probably during tures reached after deposition should be less than the low-grade metamorphic event (108–86 Ma: approximately 120 8C (in agreement with the Casquet et al. 1992). However, the two old age stratigraphic position of these two samples from populations of samples MAO-3 and MAO-4 indi- the Olivań group); and (3) for the predepositional cate that the Olivań group sediments were only history a free temperature constraint has been partially reset. The vertical distance between used for times in the 280–120 Ma interval. The samples POC-9 and MAO-3, according to the ver- chlorine content of these two samples is similar tical stacking model of Casas-Sainz & Gil-Imaz to the Durango apatite, therefore the Laslett et al. (1998), can be easily deduced by adding the sedi- (1987) annealing model has been used. Modelling mentary thickness for the Olivań and Enciso was performed using the AFTSolve program groups, and is about 2.8 km. The difference in (Ketcham et al. 2000). Results from modelling palaeotemperature between these two samples, on indicate that: (1) the post-depositional history is the basis of the apatite and zircon fission-track similar in both samples, showing that a fast data (ZFT closure temperature of 350 8C: Tagami heating period soon after deposition was followed et al. 1998) and an AFT closure temperature of by continuous cooling from maximum palaeo- 110 8C (Green et al. 1989), is estimated to be temperatures of 110 8C at 108–80 Ma to the around 240 8C (ZFT ages are clearly reset in present day (which is consistent with the timing sample POC-9 but not in sample MAO-3). The of the low-grade metamorphic event of the area); palaeogeothermal gradient calculated with these and (2) a pre-Albian inherited history is poorly data is close to 86 8Ckm21. Based on low-grade constrained by modelling. metamorphic parageneses, Mata et al. (2001) 160 .DLRI DEL P. ´ O TAL ET .

Fig. 4. Thermal models and mean track-length distribution for samples MAO-3 (left) and MAO-4 (right) for the AFT age populations P1 (up) and P2 (down). The dashed lines correspond to partial annealing zone boundaries for AFT. The T–t path envelopes are plotted. The light grey area includes the acceptable paths, and the dark grey area includes the good paths. The constraints are shown as boxes (explanation is in the text). EXHUMATION OF THE SIERRA DE CAMEROS 161

Fig. 5. Fluorine v. AFT ages and Dpar values for samples (a) MAO-3 (upper part of the Oliva´n group) and (b) MAO-4 (lower part of the Oliva´n group).

Fig. 6. Cross-section along B0 –B (see Fig. 2 for the location and legend) (structural interpretation from Casas-Sainz & Gil-Imaz 1998, p. 812, ﬁg. 8A) showing the location of the samples and the radial plots for AFT and AHe ages. 162 P. DEL RI´O ET AL.

s calculated a similarly high maximum palaeo- 2 geothermal gradient of approximately + 70 8Ckm21 at the end of the extensional stage. (Ma) Temperatures obtained in samples located within Error the Urbio´n group can be attained in the basin depocentre if we consider a tectonic model of vertical superposition of the sedimentary units with a high geothermal gradient. When considering the alternative model of lateral stacking of sedimen-

(Ma) tary units (Mas et al. 2002), the deepest part of the basin cropping out at present (the base of the Corrected age Urbio´n group) would only be covered by a maximum sedimentary thickness of 2 km (Mata et al. 2001). This implies that a geothermal gradi-

m) 21

FT ent of more than 150 8Ckm is necessary to m ( explain the high temperatures reached in the Urbio´n group. Therefore, the sag basin model of Mas et al. (1993) and Guimera` et al. (1995) requires a geothermal gradient of at least 21

(Ma) 150 8Ckm that, on the basis of the FT and Raw age ﬂuid inclusion data and in the absence of magma- tism or a hydrothermal ﬂuid activity, seems unrea- listic (Casquet et al. 1992). Furthermore, other

m) arguments against this hypothesis come from the 353040 24.4 22.8 0.61 24.5 0.53 0.70 39.8 31.1 35.2 2.8 1.5 1.7 m ( Mean

radius source–sediment balance, because if we compare the volume of sediments deposited in the Ebro foreland basin eroded from the Sierra de Cameros it is possible to estimate that the 10 10 10 2 2 2

) amount of removed material from the Cameros 3 10 10 10 Basin cannot be balanced with a tectonic model 4He (cm which proposes a maximum thickness of eroded sediments of 2 km (see e.g. Munõz-Jimeńez & Casas-Sainz 1997). AFT data from all samples from profile A–A0 and B–B0 (except JS-6) show cooling ages Th (ng) younger than their depositional age and record the time of cooling from more than 110 8C. However, AFT age populations range from 21 to 124 Ma. –B profile in the Sierra de Cameros 0 The presence of several age components in U (ng) samples with evidence of total resetting of the fission tracks in apatite (for example, the presence of chloritoid in the mineral paragenesis) could be explained by differences in chemical composition 2 0.246 0.106 8.1 2 0.034 0.122 1.8 2 0.104 0.035 3.3 of the apatites, which produces different kinetic grains No. of behaviour during annealing (Green et al. 1986; Ketcham et al. 1999). A study of Dpar and the chemical composition of apatites in these samples is underway to shed more light on the origin of the dispersion of the results. n n age The youngest age components of the AFT data ´ ´ from the B–B0 profile (Fig. 7a) record a first Group He age results in apatites from the B Oliva Tera Oliva / cooling event at 45–55 Ma in the northern part of the Cameros Basin, close to the present-day thrust front (as seen in samples JS-6, JS-8 and JS-9). The (U–Th) second, more important, cooling event is recorded by samples JS-11–JS-15 (see Fig. 7) at 30– 40 Ma. This coincides with an important period of JS-12-IIIJS-21-III Lower Albian Tithonian JS-12-I Lower Albian Table 2. FT, alpha-ejection correction factor. Sample No. Stratigraphic tectonic activity, with a horizontal displacement EXHUMATION OF THE SIERRA DE CAMEROS 163

Fig. 7. (a) AFT age populations and AHe cooling ages v. the estimated depth of the samples at the time of deposition from samples along the B0 –B profile. (b) The main thermal events that occurred in the Cameros Basin (Casas-Sainz & Gil-Imaz 1998, p. 814, fig. 9B). velocity of the thrusts of 0.8 and 1.25 mm year21 for these samples (Fig. 7a), consistent with rapid the central and eastern part of the basin, respectively cooling through the apatite partial annealing zone (Munõz-Jimeńez & Casas-Sainz 1997), and so we (PAZ; from 110 to 60 8C) to the AHe closure temp- consider that this age can constrain the age of the erature (60–70 8C: Farley 2000). main pulse of tectonic inversion in the area. The The cooling rate can be determined by combin- AHe ages of JS-12 and JS-21 are similar to the ing the AFT and ZFT data. For sample POC-9, in youngest apatite fission-track age component in the eastern part of the Urbion group, is around 164 P. DEL RIÓ ET AL.

5.4–8.9 8CMa21. These rates indicate that the Appendix basin cooled rapidly between 85 and 48 Ma. The modelled thermal histories for samples MAO-3 Analytical methods and MAO-4 from the upper and lower parts of Thirteen samples of sandstones were collected along two the Olivan group show a fast heating during the cross-sections (north–south and NE–SW) perpendicular metamorphic event, up to 100 8C, of about 5– to the strike of regional structures. Mineral separations 8 8CMa21. A continuous regional cooling of 21 were carried out using standard methods. In this work we around 0.6–1.6 8CMa is observed from the use the z approach for fission-track analyses of individual thermal peak (c. 100 Ma) to present. This is consist- apatite and zircon crystals (Hurford & Green 1983). Apa- ent with the progressive exhumation of the Cameros tites and zircons were mounted in U-free epoxy resin and Basin during the Tertiary that resulted in the in a piece of Teflon, respectively, and polished with 1 accumulation of approximately 5 km of Tertiary and 3 mm diamond and 0.25 mm aluminium oxide. To sediments in the western part of Ebro Basin, the reveal the spontaneous tracks, the apatite crystals were Rioja Trough, which represents the foreland basin etched in 5 M HNO3 for 20 + 1 s. The zircons were pre- of the Sierra de Cameros (Munõz-Jimeńez & pared according to the procedure described by Naeser Casas Sainz 1997). et al. (1987). Three replicates for each sample were etched with a low-melting-point binary eutectic of NaOH and KOH at 239 8C for a period of between 6 and 100 h Conclusions to ensure all of the zircon was adequately etched. We applied the external detector method using thin The Sierra de Cameros was an intracratonic sedi- U-free muscovite sheets as detectors. We used two dosi- mentary basin filled by continental synrift sediments meters at the top and bottom of the irradiation capsule to during the lower Cretaceous. During the extensional determine the flux of thermal neutrons’ irradiation; CN-5 stage, the sedimentary fill attained temperatures of and CN-2 for apatite and zircon, respectively. The more than 110 8C, as demonstrated by the young irradiation was performed at HIFAR (Australia). The AFT ages of all samples except those of the upper- neutron fluence was 9 1015 and 1 1015 ncm22 for most Olivan group sediments. Temperatures above apatites and zircons, respectively. All of the irradiated 300 8C were reached in the Urbioń group, as mica sheets were etched in 5 M HF at room temperature revealed by the young ZFT age of sample POC-9. for 40 min. For track counting and length measurements, This is consistent with the low-grade metamorphism a magnification of 1250 was used. observed in the central part of the basin. From an The best-fit binomial peak-fitting routine of Galbraith independent estimate of the palaeogeothermal gra- & Green (1990) was used to deconvolve the fission-track dient (86 8Ckm21), a sediment thickness of 8 km grain-age spectra in samples containing more than one is necessary to explain these temperatures and the age population. Calculations were performed using the sediment volume found in the Ebro foreland basin. Windows version of the BINOMFIT program of Brandon These data suggest that a model of vertical sediment (1992, 1996). The procedures were based on the accumulation is more feasible than the lateral stack- maximum-likelihood method, where the best-fit solution ing model. Alpine compression produced the tec- is determined by comparing the age distribution of the tonic inversion of the basin during the Tertiary. data to a predicted mixed binomial distribution. Combining ZFT and AFT data, we calculated that Apatite grains for (U–Th)/He age determinations between 85 and 48 Ma rocks from the Urbioń were hand-picked using a stereographic binocular micro- group were cooled at a rate of 5.4–8.9 8CMa21. scope (220 magnification). Only grains with good An increase in the cooling rate is identified by the crystal shape and no mineral inclusions were selected. coincidence of the youngest age component of Two crystals per sample were packed in Pt foil tubes and AFT results and the AHe cooling ages, at about loaded in batches of 40 into an ultra-high vacuum extrac- 40 Ma. This represents the time of maximum tion cell. Helium was extracted by heating the foil with uplift and cooling, which produced the exhumation an 808 nm diode laser to approximately 1000 8C. After a of the main part of the basin related to the short clean-up step, helium was measured using a Hiden Alpine orogenesis. HAL3F gas source quadrupole mass spectrometer follow- We thank B. Ventura and two anonymous reviewers for the ing the procedures of Foeken et al. (2006). Samples were 230 helpful comments made to a previous version of this manu- then diluted in 5% HNO3 and spiked with Th and script. This work has been supported by MCYT project 235U, and the U and Th concentrations measured using a BTE2002-04168-C03-02 and by the Consolider-Ingenio VG PQ2.5 inductively coupled plasma mass spectrometer 2010 Programme under project CSD2006-0041 ‘Topo- (Balestrieri et al. 2005). Ages were corrected for Iberia’. This research is part of the objectives of the alpha-recoil using the procedure of Farley et al. (1996). Junta de Andalucıá PAI group RNM-160. We thank D. Vilbert and J. Foeken for assistance at SUERC. The Apatite compositions were measured by electron SUERC He laboratory is supported by the Scottish microprobe (Jeol Superprobe JXA-8900-M) using operat- Universities. ing conditions of 15 kV, 20 nA and a beam diameter of EXHUMATION OF THE SIERRA DE CAMEROS 165

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