Timing of Alpine fault gouges

Earth and Planetary Science Letters 223 (2004) 415 – 425 www.elsevier.com/locate/epsl

Timing of Alpine fault gouges Horst Zwingmann a,*, Neil Mancktelow b a

CSIRO-Division of Petroleum Resources and John deLaeter Centre of Mass Spectrometry, School of Applied Geology, Curtin University, Bentley WA 6102, Australia b Department of Earth Sciences, ETH-Zentrum, CH-8092 Zu¨rich, Switzerland Received 2 December 2003; received in revised form 16 April 2004; accepted 29 April 2004 Available online

Abstract K – Ar ages from clay-rich fault gouges in the European Alps are consistent internally, with established field constraints and with fission track ages, demonstrating the applicability of this method for direct dating of brittle deformation. Illite grown by retrograde hydration of granitic and/or high-grade metamorphic protoliths is the major K-bearing mineral in the fractions that have been separated ( < 0.1 to 6 – 10 Am). The ages obtained are bracketed by apatite and zircon fission ages from adjacent localities. This, together with the interpreted cooling curve for one well-constrained sample from the Simplon Fault, indicates that illite grew (or the K – Ar isotopic system in illite closed) at temperatures in the range 120 – 150 jC, consistent with the stability of illite alone rather than illite – smectite. Average ages obtained for fractions finer than 2 Am were: (1) 5.2 F 2.6 Ma (2r) for 11 analyses from the detachment and footwall of the Simplon Fault; (2) 8.8 F 0.6 Ma for four analyses from the Centovalli Fault at Trontano; (3) 19.9 F 2.6 Ma for two analyses from a major Riedel fault related to the Periadriatic Fault, offsetting the Tertiary Bergell tonalite at Gesero; (4) 21.5 F 5.5 Ma for two analyses from a zone parallel to the Periadriatic Fault in the adjacent Southern Alps in Val Morobbia; and (5) 14.5 F 3.9 Ma for two analyses from a gouge zone on the northern rim of the Tertiary Mauls tonalite in the Eastern Alps. The ages from the Simplon, Centovalli and Periadriatic Faults are not identical and do not lend support to models linking these three faults as a single, coeval structure. Crown Copyright D 2004 Published by Elsevier B.V. All rights reserved. Keywords: geochronology; K – Ar dating; brittle fault; clay gouge; neotectonics; European Alps

1. Introduction Near-surface deformation related to neotectonics is accommodated by brittle faults. Displacement on these discrete fault planes often results in the development of fault gouge, composed of crushed rock fragments and authigenic clay minerals, in particular illite, formed by retrograde hydration reactions. If * Corresponding author. E-mail addresses: [email protected] (H. Zwingmann), [email protected] (N. Mancktelow).

the protolith consists of high grade metamorphic or magmatic rocks, a clear distinction is possible between these newly grown clay minerals and the precursor assemblage. Early studies [1] highlighted the potential for determining the absolute timing of brittle fault history using isotopic dating techniques and this has been confirmed and applied in subsequent studies using K –Ar (e.g. [2– 4]), 40Ar – 39Ar [5] and Rb– Sr [4]. In this study, we attempt to determine the displacement history of young faults that are related to the regional-scale Periadriatic Fault (e.g. [6– 9]) in the

0012-821X/$ - see front matter. Crown Copyright D 2004 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2004.04.041

416

H. Zwingmann, N. Mancktelow / Earth and Planetary Science Letters 223 (2004) 415–425

European Alps, based on dating neocrystalline clay minerals in the fault gouges. Several studies have dated illite in order to establish the timing of prograde incipient metamorphism in the Alps [10 – 12]. However, there have been few studies specifically on the localized retrograde development of clay in discrete fault zones, although such brittle faults are widespread (e.g. [13 – 16]). Only one clay fault gouge age has been published from the Alps [4], for a brittle fault developed in garnet –mica schist from the Kreuzeck Group in the Eastern Alps. Since this pioneering publication in 1987, no further results have been reported, despite the importance of the Alps as a model for late brittle tectonics (e.g. [15,17 – 19]) and the general availability of good field and regional thermochronological constraints (e.g. [20 – 22]). These independent constraints provide an excellent opportunity to test the reliability of the K –Ar illite method for dating brittle fault activity. Dating fault gouges also has important implications for more general studies. Fault gouge zones are not only the most visible record of near surface brittle deformation, they also exert an important control on fluid flow and fluid –rock interaction. As discussed by Knipe [23], such zones can either focus fluid flow or act as permeability barriers, restricting fluid flow and acting as a seal. The understanding of fault processes and the timing and extent of clay-rich fault gouge formation is important in: (1) hydrocarbon exploration, as faults may act as either a conduit zone or a seal for fluids and/or hydrocarbons; (2) civil engineering and the evaluation of earthquake hazards and (3) assessing the suitability of sites for waste storage including nuclear waste.

2. Sample location and description The Alps have a long deformation history that has continued to the present-day and is reflected in a strong interrelationship between topography, erosion and neotectonics. Good surface exposures, together with numerous tunnels, make the region attractive for the study of fault gouges, which are otherwise poorly preserved. In this study, 10 fresh fault gouge samples from discrete brittle faults exposed in surface and tunnel sections have been investigated (Fig. 1, Table 2). All the sampled faults are potentially related to

movement on the Periadriatic (or Insubric) fault system, which is the most important late tectonic fault system in the Alps (e.g. [6– 9]). Location 1 is within the discrete brittle detachment of the Simplon Fault Zone in the Zwischbergen Valley [24,25]. This is a major low-angle normal fault, interpreted from thermal modelling of isotopic mineral ages to have been most active between ca. 18 and 15 Ma, but with continued if diminished relative movement down to at least 5 Ma [26]. Total relative displacement parallel to the fault, as interpreted from the thermal modelling, is ca. 36 km, with 6 km attributed to the discrete brittle detachment in the last 15 Ma. The brittle detachment is marked in the Zwischbergen locality by a 3 – 5 m wide foliated clay gouge zone, in which cm-scale intrafolial folds, delta clasts of relict rock fragments, shear bands in more ductile clay layers, and discrete Riedel fractures all demonstrate continued down-dip normal movement during gouge formation (Fig. 2). Location 2 is from a hydroelectric tunnel in the footwall of the Simplon Fault Zone between Gondo and Varzo, south of the Swiss–Italian border. The three samples were taken at distances of ca. 185 m (2a), 500 m (2b) and 1148 m (2c) from the southern entrance and are from 10–20 cm thick gouge zones (fault strike EW, dip ca. 70jS) developed in the Antigorio granitic gneiss. The direction and sense of movement on these faults could not be unequivocally determined. Location 3 is on the EW-striking, generally steeply dipping, Centovalli Fault near Trontano. Three samples (3a –c) were collected from thin (1 –2 cm) clayrich gouges marking individual fault strands within the broad zone (>50 m) of variable cataclastic overprint. No lineation or kinematic indicators were directly observed in the sampled fault gouges. The lack of any jump in apatite or zircon fission track ages across the Centovalli Fault [22] indicates that there is no significant vertical offset. Dextral strike-slip movement would be consistent with the regional kinematics of the Periadriatic Fault system [7,8]. Location 4 relates to the near vertical to steeply north-dipping Periadriatic Fault east of Bellinzona, where discrete synthetic Riedel faults offset the elongate tail of the 31.9 F 0.1 Ma Bergell tonalite [27]. The Riedel faults are largely truncated by the late brittle component of the Periadriatic Fault, which involves dextral plus N-side up oblique slip. Sample

H. Zwingmann, N. Mancktelow / Earth and Planetary Science Letters 223 (2004) 415–425

417

Fig. 1. Simplified map of the study area with the sample locations and corresponding K – Ar ages [Ma] for < 2 Am illite fractions from the fault gouges.

4a is from one of the larger Riedel faults, which dips 65j toward 044j. At this location, it separates the Bergell tonalite from upper amphibolite grade paragneiss, amphibolite and orthogneiss. A 2-cm-thick band of black, coherent, very fine grained material, possibly pseudotachylite, marks the fault zone adja-

cent to the tonalite, whereas a 2 –5 cm zone of clayrich gouge (sample 4a) and crushed rock fragments is developed against the gneiss. Sample 4b is from a small 10-cm-wide fault in the Southern Alps, south of and nearly parallel to the Periadriatic Fault. The fault dips 83j toward 168j, with striations pitching 85jE.

418

H. Zwingmann, N. Mancktelow / Earth and Planetary Science Letters 223 (2004) 415–425

Fig. 2. Field photograph of rotated clasts in foliated clay-rich fault gouge from the Simplon Fault (location 1). View looking WSW, shear sense normal (dextral for this view).

The sense of movement is not well established but appears to be normal (i.e. S side down). Location 5 is from ca. 200 km further east, in the arc of a regional bend in the Periadriatic Fault south of the Brenner pass. The sample was collected from a 1– 2 cm thick fault gouge zone at the northern contact of the Mauls tonalite, which is probably of a similar age to the mid-Oligocene Bergell tonalite [28]. Cataclasite zones are developed on both northern and southern sides of this pluton [28], with the southern zone marking the Periadriatic Fault itself.

3. Methods Each sample consisted of ca. 250 g of fresh material. Larger pieces were first crushed into chips < 10 mm in size. Sample chips were then gently disaggregated by using a repetitive freezing and thawing technique to avoid artificial reduction of rock components and contamination of finer size fractions with relict K-bearing minerals (e.g. Kfeldspar) [29]. Grain size fractions of < 2, 2 –6 and 6– 10 Am were separated in distilled water (Stoke’s law). Fractions < 0.1 and < 0.4 Am were obtained using a high speed centrifuge. The various fractions separated from the samples were sedimented onto glass slides. Diffractograms were obtained from airdried and glycolated slides, analysed with a Philips automated EPD 1700 X-ray diffractometer using CuKa radiation and 40 kV/30 mA. Samples were scanned over the range 2h = 2 –35j at 0.015j2h/s.

Illite crystallinity [30] determinations were carried out as recommended by Kisch [31]. The percentage of 2M1 muscovite relative to 1Md illite was determined from the ratio I2.80A˚/I2.58A˚ [32]. XRD limits are around 3% and contamination phases cannot be detected below this value. Freshly broken surfaces of sample chips were carbon-coated and examined in secondary and backscattered electron mode using a Philips 300 SEM equipped with an energy dispersive system X-ray analyzer (EDS). A JEOL 2011 200 kV TEM was used for detailed grain-by-grain morphological characterization of the < 0.1 and < 0.4 Am clay fractions and for control of grain-size distribution within the fractions. Samples were prepared by placing one drop of clay solution on a micro carbon grid film and drying under air. The composition of individual particles was investigated by an attached EDS system. The K – Ar dating technique follows standard methods described in detail elsewhere [33,34]. K content was determined by atomic absorption. The pooled error of duplicate K determinations on several samples and standards is better than 2.0%. Ar isotopic determinations were performed using a procedure similar to that described by Bonhomme et al. [35]. Samples were pre-heated under vacuum at 80 jC for several hours to reduce the amount of atmospheric Ar adsorbed onto the mineral surfaces during sample preparation. Ar was extracted from the mineral fractions by fusing samples using a low blank resistance furnace within a vacuum line serviced by an on-line 38Ar spike pipette. The 38Ar spike was calibrated against GA1550 biotite [36]. The isotopic composition of the spiked Ar was measured with an on-line VG3600 mass spectrometer via Faraday cup. The released gases were subjected to a two-stage purification procedure via CuO and Ti getters. Blanks for the extraction line and mass spectrometer were systematically determined and the mass discrimination factor was determined by airshots. About 25 mg of sample material was required for Ar analyses. During the course of the study, the international standards GL-O, HD-B1 and LP6 were measured several times (Table 1, cf. [37,38]). The error for Ar analyses is below 1.00% (Table 1) and the 40Ar/36Ar value for airshots averaged 293.26 F 0.43 (n = 27). The K –Ar ages were

H. Zwingmann, N. Mancktelow / Earth and Planetary Science Letters 223 (2004) 415–425 Table 1 K – Ar standard results Standard no.

Rad 40Ar (mol/g)

GLO-26 GLO-32 GLO-33 GLO-35 GLO-36 HD-B1-38 HD-B1-39 LP6-41 LP6-44 LP6-45 LP6-46

1.1173e 1.1185e 1.1096e 1.1207e 1.1158e 3.3754e 3.3356e 1.9306e 1.9406e 1.9113e 1.9080e

Rad (%) 09 09 09 09 09 10 10 09 09 09 09

40

Ar

94.24 92.50 93.26 92.75 93.26 93.47 89.73 97.00 97.94 97.39 97.12

Age (Ma)

Error Error to (Ma) reference (%)

95.76 95.86 95.12 96.04 95.64 24.30 24.01 128.31 128.95 127.07 126.86

1.44 1.44 1.43 1.44 1.44 0.48 0.35 1.83 1.85 1.81 1.81

0.77 0.87 0.09 1.06 0.64 0.37 0.83 0.32 0.82 0.65 0.81

International standard references: GLO, LP 6 [37], HD-B1[38].

calculated using 40K abundance and decay constants recommended by Steiger and Ja¨ger [39].

4. Results XRD analyses of the samples establish that illite and chlorite are the major mineral phases in the various fractions, with minor traces of kaolinite and quartz (Fig. 3). Glycolated XRD analyses were carried out to investigate the potential occurrence of expandable mixed-layer illite/smectite but no smectite could be identified. The coarser 2 – 6 and 6 –10 Am fractions

419

contain traces of K-feldspar but none was observed in the finer < 2 to < 0.1 Am fractions, which was confirmed by TEM investigations. Illite crystallinity [30] values correspond to diagenetic to anchizonal grade. The analyzed illite fractions are composed mainly of the 1Md and 2M1 polytype suggesting temperatures around 100 – 200 jC during mineral formation [40]. SEM investigations reveal that the illite crystallites in the gouge zone have a strong preferred orientation and regular, euhedral boundaries (Fig. 4A). The hexagonal and prismatic morphologies of the illite laths or plates suggest in-situ neocrystallisation. Euhedral particle outlines are typical of an authigenic or diagenetic origin, in contrast to the more irregular or diffuse outlines characteristic of a detrital origin [41]. The shape of the illite is in keeping with the development of the fault gouges by growth of new grains during retrograde hydration of a high-grade metamorphic or granitic protolith, either coeval with or subsequent to fault activity. TEM observations of the separated clay fractions document the occurrence of two distinct groups of particles: (1) idiomorphic illite fibres with elongated, well crystallized grain edges and (2) idiomorphic platy illite flakes, together with hexagonal idiomorphic chlorite and kaolinite with clear crystallized edges (Fig. 4B). Thirty-three K –Ar illite ages, covering size fractions from < 0.1 to 6 –10 Am, were obtained during this study (Table 2). Three samples were run in

Fig. 3. Air-dried XRD results of selected size fractions of sample 1 – 121.

420

H. Zwingmann, N. Mancktelow / Earth and Planetary Science Letters 223 (2004) 415–425

5. Discussion 5.1. Assumptions and limitations

Fig. 4. Particle morphology of sample 1 – 121. (A) Secondary electron SEM image, (B) TEM image of < 0.4 Am fraction showing idiomorphic illite particles.

duplicate to test the reproducibility of the ages. Two of these duplicates (210 and 220) yield ages within error, the third (223) was slightly outside the error range (8.90 F 0.21 vs. 8.32 F 0.24 Ma, Table 2). The K – Ar ages range from 3.31 F 0.28 Ma (Lower Pliocene –Piacenzian) to 28.82 F 0.58 Ma (Lower Oligocene– Chattian). Radiogenic 40Ar content ranges from 20.1% to 91.3% indicating reliable analytical conditions for all analyses. K contents range from a low 1.11% for sample 294 < 0.4 Am to 7.43% for sample 121 < 2 Am. The relatively high K content of most illite fractions is consistent with an authigenic origin. Lower K concentration in some size fractions is caused by contamination with other mineral phases, such as quartz and chlorite, which is supported by XRD data.

The correlation of K –Ar ages from illite separates with geologically meaningful events requires careful consideration of the assumptions underlying the method. Clauer and Chaudhuri [41] and Hamilton et al. [42] discuss in detail the validity and importance of the assumptions involved in K – Ar dating of authigenic illite (e.g. contamination, closed system behaviour, excess Ar). Firstly, the most important assumption is that there has been no loss or gain of either 40K or 40Ar after illite formation, i.e. closed system behaviour. Ar is the most likely component to be lost, especially as the result of thermal diffusion but also by exchange with hydrothermal fluids [43]. Secondly, it is crucial that only one illite generation is present as the K-bearing phase in the analysed fraction, i.e. that is no contamination by other K-bearing phases. This can be a major obstacle to the acquisition of meaningful K – Ar ages, particularly in samples containing either small amounts of K-bearing detritus or a mixture of illites formed at different times. For neocrystallized illite, the finest separated particle size is derived from the ends of filamentous grains and should represent the most recently grown illite in sedimentary rocks. Conversely, coarser size fractions formed earlier during the illite formation process should yield older ages. However, there is also evidence that illite can recrystallize and coarsen by Ostwald ripening in some hydrothermal systems under conditions appropriate to the fault gouge zones considered here [44,45]. In reality, grain-size fractions of new-grown illite are mixtures of illite particles formed at different times during growth and this growth history is usually investigated by dating a range of different grain-size fractions. Illite from all the fault gouge locations is formed in a low-temperature environment where mineral reactions are governed by kinetics rather than equilibrium thermodynamics [2]. 5.2. Existing age constraints and new results This geochronological study of illite formed in clay-rich fault gouges from the Alps has two funda-

H. Zwingmann, N. Mancktelow / Earth and Planetary Science Letters 223 (2004) 415–425

421

Table 2 K – Ar results and sample locations Location no.

Location description

Sample ID (Am)

K (%)

Rad. 40Ar (mol/g)

1

Simplon fault, Zwischbergen Swiss Coords 652460/112600

2a

Varzo hydroelectric tunnel 180 – 190 m from south entrance Swiss Coords 655500/116200

2b

Varzo hydroelectric tunnel 490 – 520 m from south entrance

2c

Varzo hydroelectric tunnel 1148 m from south entrance

3a

Trontano 01/2, Centovalli fault Swiss Coords 668451/108103 Trontano 01/3, Centovalli fault Swiss Coords 668330/108103 Trontano 01/4, Centovalli fault Swiss Coords 668330/108103

121 121 121 218 218 218 218 218 219 219 219 219 219 219 220 220 220 221 221 222 222 223 223 223 291 291 291 289 289 289 288 288 288

6.54 7.43 6.91 4.60 6.01 6.30 6.34 5.93 3.90 4.36 3.43 3.43 1.89 2.52 5.89 5.89 3.27 4.71 4.52 6.62 6.65 4.56 4.56 4.66 1.06 1.27 1.11 3.18 4.79 4.45 1.44 1.74 1.87

5.6927e 9.2640e 3.4818e 5.0439e 4.3077e 4.3353e 5.3344e 5.4352e 4.5124e 3.5772e 2.0829e 2.2774e 1.0856e 1.8672e 6.1458e 6.4089e 2.6007e 7.1423e 8.0828e 1.0444e 1.3511e 7.0574e 6.5946e 7.1939e 3.8484e 4.2022e 3.4637e 1.0850e 1.9621e 2.1924e 3.2990e 4.8166e 4.5711e

3b 3c

4a

SJ 01/3, Riedel fault, Gesero Swiss Coords 730807/115509

4b

Mo 01/9, Val Morobbia Swiss Coords 114294/724778

5

Mauls 01/1, northern contact to Mauls tonalite east of Mauls

< 0.4 <2 2–6 < 0.1 < 0.4 <2 2–6 6 – 10 < 0.1 < 0.4 <2 <2 D 2–6 6 – 10 <2 <2 D 2–6 <2 2–6 <2 2–6 <2 <2 D 2–6 < 0.4 <2 2–6 < 0.4 <2 2–6 < 0.4 <2 2–6

Rad. (%) 11 11 10 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 10 10 11 11 11 11 11 11 10 10 10 11 11 11

40

31.55 61.14 89.28 34.92 34.29 33.57 52.58 58.45 61.95 27.19 21.06 27.06 25.49 38.51 42.53 43.34 5.49 52.07 67.70 62.04 79.75 52.36 52.37 66.78 44.49 50.85 64.28 66.28 84.69 87.56 56.34 67.09 72.20

Ar

Age (Ma)

Error (Ma)

5.01 7.17 28.82 6.31 4.13 3.96 4.84 5.28 6.66 4.72 3.50 3.82 3.31 4.27 6.01 6.26 4.58 8.72 10.28 9.07 11.68 8.90 8.32 8.88 20.81 18.98 17.90 19.56 23.46 28.18 13.16 15.89 14.04

0.14 0.18 0.58 0.31 0.20 0.15 0.13 0.15 0.24 0.31 0.35 0.21 0.28 0.21 0.18 0.13 0.27 0.26 0.25 0.21 0.24 0.21 0.24 0.23 0.59 0.47 0.39 0.41 0.47 0.57 0.32 0.33 0.30

D indicates sample duplicate.

mental advantages. Firstly, the illite must be primary, because there was no illite in the pre-fault protolith. This distinguishes the current work from earlier studies dating clay-rich gouge in sedimentary rocks that already contained clay minerals prior to faulting (e.g. [2,5]). Secondly, for most samples there are very good constraints on the possible age range for fault gouge formation, allowing independent control on the validity of the results. In particular, brittle faulting and authigenic illite growth occurs under conditions bracketed by the partial annealing zones of apatite and zircon, in the range 60 –120 jC for apatite [46 – 48] and 170 – 390 jC for zircon [49]. We would therefore expect the illite fault gouge ages to lie

between the fission track ages of apatite (AFTA) and zircon (ZFTA), with a tendency to be closer to that of apatite. Fission track ages from the immediate area of most of the samples are available from the literature (nearby ZFTA are only lacking for location 4). These FT ages, with errors, are indicated as bands in Fig. 5, from which it can be seen that the fault gouge results fit very well with the independent FT ages. A detailed comparison on a sample by sample basis, together with a discussion of other constraints more specific to each locality, follows below. The K –Ar ages from the two fine grained fractions ( < 0.4, < 2 Am) of sample 121 (location 1) are 5.0 F 0.1 and 7.2 F 0.2 Ma, respectively. We interpret

422

H. Zwingmann, N. Mancktelow / Earth and Planetary Science Letters 223 (2004) 415–425

Fig. 5. Summary of K – Ar age results showing the variation in age with size fraction and the good correspondence between the ages obtained and the upper and lower age constraints imposed by zircon and apatite fission track ages (ZFTA and AFTA, respectively). Nearby published ZFTA ages are not available for locations 4a (sample 291) and b (sample 289) (Fig. 1). For sample 291, the nearby biotite K – Ar age from the Bergell tonalite [56] is given as an upper constraint. The sample ID’s on the x axis consist of the location number followed by the sample number.

these ages to represent illite growth under hydrous conditions during continued Simplon Fault movement, as indicated by rotated rock clasts, intrafolial folds and shear bands in the foliated gouge, suggesting that clay development was syntectonic (Fig. 2). These K – Ar ages are bracketed by the zircon (11.6 F 1.0 Ma) and apatite (3.6 F 0.6 Ma) FT ages from adjacent locations in the footwall [22]. The 28.8 F 0.6 Ma K – Ar age for the 2 –6 Am fraction is interpreted to be due to Kfeldspar contamination, which is present in trace amounts in the coarser fractions (see XRD above). From field observation, the clay gouge is demonstrably younger than quartz + ankerite F muscovite ( F gold) veins that are common in both the hanging and footwall of the Simplon Fault. Those in the footwall crosscut the mylonitic fabric and have been dated by 40Ar – 39Ar at 11.59 F 0.10 Ma [50]. This establishes that the latest movement on the clay-gouge zone must be younger than 11.6 Ma, consistent with our results. The age span of 5 – 7 Ma would correspond to a temperature range of

ca. 120 – 160 jC on the interpreted cooling curve of Grasemann and Mancktelow [26] for the immediate footwall of the detachment. The occurrence of illite rather than smectite is consistent with studies indicating that the thermally driven dehydration of smectite to illite is already complete at temperatures of 120 – 150 jC (e.g. [40,51]). The K – Ar ages obtained from samples 218– 220 (location 2) range from 3.3 Ma to 6.7 Ma and are dependent on grain size (Fig. 5). The age range is again bracketed by nearby apatite (2.7 F 0.6 Ma) and zircon (9.4 F 1 Ma) FT ages [22]. The observed increase in K – Ar age with increasing grain size for the coarser fractions < 2 to 6 –10 Am (Table 2, Fig. 5) is consistent with increased minor contamination by K-feldspar in the coarse fractions relative to the finer fractions, which in turn implies that the most reliable isotopic ages for authigenic illite are those from the finest size fractions ( < 0.1, < 0.4 Am). XRD and TEM analyses confirm that the highest

H. Zwingmann, N. Mancktelow / Earth and Planetary Science Letters 223 (2004) 415–425

purity illite samples correspond to the finest size fractions, which should give the most reliable K –Ar dates. However, in both samples 218 and 219 there is a consistent reverse trend of increasing ages with decreasing grain size in the finest fractions ( < 0.1, < 0.4 Am, Table 2, Fig. 5). This inverse age trend could be due to enrichment in the finest fraction of very fine illite particles representing nuclei for subsequent illite neocrystallization and growth [52,53]. In a history of continuous overgrowth and grain coarsening following nucleation (i.e. Ostwald ripening [44,45]), coarser grains would therefore give a younger average age. The trend is similar to that reported by Clauer et al. [52], who observed that the smallest fundamental particles have ages the same as or older than larger particles. Whitney and Velde [54] also found that small detrital illite grains may act as nuclei for overgrowths of neoformed illite layers suggesting that an inverse age pattern for illite fractions, as obtained here, is not unusual. However, only two of our samples show this inverse trend of older ages in finer fractions and it is not the general case. The true cause is still an open question. The variation in age with grain size in the finest fractions is still not large and is not significant for regional tectonic correlation. If all the < 2 Am size fraction samples from the Simplon Fault in Zwischbergen and from the footwall in the Varzo tunnel are taken together, an average age for the 11 analyses of 5.2 F 2.6 Ma (2r) is obtained. Considering that thermal modelling indicates the initial period of important relative normal fault motion was approximately 18 –15 Ma [26], the rather young ages for the fault gouge samples confirm that the Simplon Fault has been a long-lived structure, active for at least 10 Ma, as already previously proposed [26]. Samples 221– 223 (location 3) gave K – Ar ages in the range 8.3 to 11.7 Ma, with the < 2 Am fractions giving a tighter age range of 8.3 to 9.1 Ma. The nearby FT age for apatite is 4.7 F 0.8 Ma and for zircon is 10.4 F 1 Ma [22]. There is no jump in FT ages across the Centovalli Fault for a NS profile through Trontano, indicating little vertical component to the brittle fault displacement. The average age for the 4 analyses with size fractions < 2 Am is 8.8 F 0.6 Ma (2r), which suggests that fault gouge formation on the Centovalli Fault is significantly older than on the Simplon Fault. Samples from location 4 are more directly related to the regionally important Periadriatic Fault (Fig. 1).

423

Location 4a is from one of the brittle Riedel faults associated with the late dextral Periadriatic Fault movement. Sample 291 gave a K – Ar age of 19.0 F 0.5 Ma for the < 2 Am fraction and 20.8 F 0.6 Ma for the < 0.4 Am fraction (average 19.9 F 2.6 Ma, 2r). Possible reasons for such a reversed age trend have already been discussed above for samples 218 and 219. The ages are bracketed by apatite FT ages in the range 11 – 15 Ma from the steep zone of the Bergell area immediately north of the Periadriatic Fault (see summary in [55]) and a 21.0 F 0.6 Ma biotite K –Ar age from the tonalite at Gesero [56]. The fault gouge age lies within the 16 –22 Ma range for regional pseudotachylyte ages related to late, brittle dextral movement on the Periadriatic Fault [57]. It is older than the 16.4 F 0.8 Ma age reported for a pseudotachylyte from Arcegno further west [57], which is currently the only direct date from the late, discrete EW surface that regionally appears to truncate most of the movement on the Riedel faults (Fig. 1). Clay-rich fault gouge was sampled from a fault approximately parallel to and 50 m south of the Periadriatic Fault in Val Morobbia (location 4b), within the very low metamorphic grade zone of the Southern Alps. Sample 289 gave K –Ar ages ranging from 19.6 to 28.2 Ma, yielding younger ages with decreasing grain size (Fig. 5). The average age of the < 2 and < 0.4 Am fractions is 21.5 F 5.5 Ma (2r). This normal fault in the Southern Alps is therefore slightly older than the dextral Riedel fault in the Central Alps north of the Periadriatic Fault. Since the Central Alps are uplifted relative to the Southern Alps across the Periadriatic Fault in this region, a younger transition to brittle faulting in the northern block is to be expected. Overall, the ages of the brittle fault gouges directly associated with the Periadriatic Fault (locations 4a and b) are significantly older than those of the Centovalli and Simplon Faults, and a direct connection with synchronous activity, as proposed in several tectonic models for the Central and Western Alps (e.g. [9]), is not supported by the current age results. Sample 288 is from a narrow (1 –2 cm) gouge zone on the northern contact to the Oligocene Mauls tonalite, whose southern boundary is the Periadriatic Fault itself. The K – Ar ages are in the range from 13 to 16 Ma (average age of 14.5 F 3.9 Ma, 2r, for the < 2 and < 0.4 Am fractions), which again is bracketed by apatite (13 F 1 Ma) and zircon (24 F 1 Ma) FT ages from the tonalite [28,58,59].

424

H. Zwingmann, N. Mancktelow / Earth and Planetary Science Letters 223 (2004) 415–425

6. Conclusions The ages from K – Ar dating of illite in fault gouges from surface and subsurface samples are consistent both internally and with independent constraints from field relationships and existing zircon and apatite fission track data. This demonstrates the potential of the method for providing absolute time constraints on the youngest, retrograde, neotectonic part of the Alpine orogeny. There is a general progression to older ages from the Simplon (ca. 5 Ma) to the Centovalli (8– 9 Ma) to the Periadriatic Fault and its associated Riedel structures (ca. 15 – 20 Ma). Although younger reactivation of separate segments could still be invoked, the ages do not lend direct support to the existence of a single, continuous and coeval brittle fault system made up of the Periadriatic, Centovalli and Simplon Faults.

[9]

[10]

[11]

[12]

[13]

[14] [15]

Acknowledgements [16]

A. Todd, CSIRO Petroleum is thanked for technical assistance. M. Sapigni, ENELPOWER is thanked for providing samples from the Varzo hydroelectric tunnel and field support. R. Gaupp and R. Offler are thanked for constructive and helpful reviews. [BOYLE]

[17]

[18]

References [1] J.B. Lyons, J. Snellenberg, Dating faults, Geol. Soc. Amer. Bull. 82 (1971) 1749 – 1752. [2] P. Vrolijk, B.A. van der Pluijm, Clay gouge, J. Struct. Geol. 21 (1999) 1039 – 1048. [3] C.O. Choo, T.W. Chang, Characteristics of clay minerals in gouges of the Dongrae fault, Southeastern Korea, and implications for fault activity, Clays Clay Miner. 48 (2000) 204 – 212. [4] M. Kralik, K. Klima, G. Riedmu¨ller, Dating fault gouges, Nature 327 (6120) (1987) 315 – 317. [5] B.A. van der Pluijm, C.M. Hall, P.J. Vrolijk, D.R. Pevear, M.C. Covey, The dating of shallow faults in the Earth’s crust, Nature 412 (2001) 172 – 175. [6] A. Gansser, The Insubric line, a major geotectonic problem, Schweiz. Mineral. Petrogr. Mitt. 48 (1968) 123 – 143. [7] S.M. Schmid, A. Zingg, M. Handy, The kinematics of movements along the Insubric Line and the emplacement of the Ivrea Zone, Tectonophysics 135 (1987) 47 – 66. [8] S.M. Schmid, H.-R. Aebli, F. Heller, A. Zingg, The role of the Periadriatic Line in the tectonic evolution of the Alps, in: M.P. Coward, D. Dietrich, R.G. Park (Eds.), Alpine Tectonics, Spe-

[19]

[20]

[21]

[22]

[23]

cial Publication - Geological Society vol. 45,Geological Society, London, 1989, pp. 153 – 171. S.M. Schmid, E. Kissling, The arc of the Western Alps in the light of geophysical data on deep crustal structure, Tectonics 19 (2000) 62 – 85. J.C. Hunziker, The evolution of illite to muscovite: an example of the behaviour of isotopes in low-grade metamorphic terrains, Chem. Geol. 57 (1986) 31 – 40. M. Jaboyedoff, M.A. Cosca, Dating incipient metamorphism using 40Ar/39Ar geochronology and XRD modeling: a case study from the Swiss Alps, Contrib. Mineral. Petrol. 135 (1999) 93 – 113. D.L. Kirscher, M.A. Cosca, H. Masson, J.C. Hunziker, Staircase 40Ar/39Ar spectra of fine-grained white mica: timing and duration of deformation and empirical constraints on argon diffusion, Geology 24 (1996) 747 – 750. A. Streckeisen, Junge Bruchsysteme im no¨rdlichen Simplongebiet (Wallis, Schwiez), Eclogae Geol. Helv. 58 (1965) 407 – 416. A. Steck, Junge Bruchsysteme in den Zentralalpen, Eclogae Geol. Helv. 61 (1968) 387 – 393. A. Bistacchi, M. Massironi, Post-nappe brittle tectonics and kinematic evolution of the north-western Alps: an integrated approach, Tectonophysics 327 (2000) 267 – 292. S. Laws, E. Eberhardt, S. Lo¨w, F. Descoeudres, Geomechanical properties of shear zones in the eastern Aar massif, Switzerland and their implication on tunnelling, Rock Mech. Rock Eng. 36 (2003) 271 – 303. H.-R. Maurer, M. Burkhard, N. Deichmann, A.G. Green, Active tectonism in the Central Alps: contrasting stress regimes north and south of the Rhone Valley, Terra Nova 9 (1997) 91 – 94. C. Sue, J. Martinod, P. Tricart, F. Thouvenot, J.-F. Gamond, J. Frechet, D. Marinier, J.-P. Glot, J.-R. Grasso, Active deformation in the inner Western Alps inferred from comparison between 1972-classical and 1996-GPS geodetic surveys, Tectonophysics 320 (2000) 17 – 29. C. Sue, P. Tricart, Widespread post-nappe normal faulting in the internal Western Alps: a new constraint on arc dynamics, J. Geol. Soc. (Lond.) 159 (2002) 61 – 70. G.A. Wagner, G.M. Reimer, E. Ja¨ger, Cooling ages derived by apatite fission-track, mica Rb – Sr and K – Ar dating: the uplift and cooling history of the Central Alps, Mem. Ist. Geol. Mineral. Univ. Padova 30 (1977) (27 pp.). A.J. Hurford, M. Flisch, E. Ja¨ger, Unravelling the thermotectonic evolution of the Alps: a contribution from fission track analysis and mica dating, in: M.P. Coward, D. Dietrich, R.G. Park (Eds.), Alpine Tectonics, Special Publication Geological Society, vol. 45, Geological Society, London, 1989, pp. 369 – 398. M.A. Soom, Abku¨hlungs- und Hebungsgeschichte der Externmassive und der penninischen Decken beidseits der SimplonRhone-Linie seit dem Oligoza¨n: Spaltspurdatierungen an Apatit/Zirkon und K – Ar-Datierungen an Biotit/Muskowit (Westliche Zentralalpen), PhD thesis, Univ. Bern, 1990, 120 pp. R. Knipe, The influence of fault zone processes and diagenesis on fluid flow, in: A.D. Horbury, A.G. Robinson (Eds.), Dia-

H. Zwingmann, N. Mancktelow / Earth and Planetary Science Letters 223 (2004) 415–425

[24]

[25] [26]

[27]

[28] [29]

[30]

[31]

[32]

[33]

[34] [35]

[36] [37]

[38]

[39]

[40] [41] [42]

genesis and Basin Development, AAPG Studies in Geology vol. 36, (1993) 135 – 154. N.S. Mancktelow, The Simplon line: a major displacement zone in the western Lepontine Alps, Eclogae Geol. Helv. 78 (1985) 73 – 96. N.S. Mancktelow, The Simplon Fault Zone, Beitr. Geol. Kt. Schweiz NF 163 (1990) (74 pp.). B. Grasemann, N.S. Mancktelow, Two-dimensional thermal modelling of normal faulting: the Simplon Fault Zone, Central Alps, Switzerland, Tectonophysics 225 (1983) 155 – 165. F. von Blanckenburg, Combined high-precision chronometry and geochemical tracing using accessory minerals: applied to the Central-Alpine Bergell intrusion, Chem. Geol. 100 (1992) 19 – 40. D.F. Sto¨ckli, Tectonics SW of the Tauern Window (Mauls area, Su¨dtirol), Diploma thesis, ETH Zu¨rich, 1995, 270 pp. N. Liewig, N. Clauer, F. Sommer, Rb – Sr and K – Ar dating of clay diagenesis in Jurassic sandstone oil reservoirs, North Sea, Am. Assoc. Pet. Geol. Bull. 71 (1987) 1467 – 1474. B. Ku¨bler, Evaluation quantitative du me´tamorphisme par la cristallinite´ de l’illite, Bull. Cent. Rech. Pau-SNPA 2 (1968) 385 – 397. H.J. Kisch, Illite crystallinity: recommendations on sample preparation, X-ray diffraction settings and interlaboratory standards, J. Metamorph. Geol. 6 (1991) 665 – 675. D.T. Maxwell, J. Hower, High-grade diagenesis and low-grade metamorphism of illite in the Precambrian Belt Series, Am. Mineral. 52 (1967) 843 – 857. G.B. Dalrymple, M.A. Lanphere, Potassium – Argon Dating: Principles, Techniques and Applications to Geochronology, Freeman, San Francisco, 1969, 258 pp. G. Faure, Principles of Isotope Geology, Wiley, New York, 1986, 589 pp. M.G. Bonhomme, R. Thuizat, Y. Pinault, N. Clauer, R. Wendling, R. Winkler, Me´thode de Datation Potassium – Argon, Appareillage et Technique, Strasbourg, 1975, 53 pp. I. McDougall, Z. Roksandic, Total fusion 40Ar/39Ar ages using HIFAR reactor, J. Geol. Soc. Aust. 21 (1974) 81 – 89. G.S. Odin, et al., Interlaboratory standards for dating purposes, in: G.S. Odin (Ed.), Numerical Dating in Stratigraphy, Part 1, Wiley, Chichester, 1982, pp. 123 – 148. J.C. Hess, H.J. Lippolt, Compilation of K/Ar measurements on HD-B1 standard biotite; 1994 status report, in: G.S. Odin (Ed.), Phanerozoic Time Scale, Bulletin de Liaison et d’information, IUGS Subcommission on Geochronology, Paris, 1994, pp. 19 – 23. R.H. Steiger, E. Ja¨ger, Subcommission on Geochronology: convention on the use of decay constants in geo- and cosmochronology, Earth Planet. Sci. Lett. 36 (1977) 359 – 362. R.J. Merriman, S.J. Kemp, A basin maturity chart for clay minerals, Mineral. Soc. Bull. 111 (1996) 7 – 8. N. Clauer, S. Chaudhuri, Clays in Crustal Environments: Isotope Dating and Tracing, Springer-Verlag, Berlin, 1995, 359 pp. P.J. Hamilton, M.R. Giles, P. Ainsworth, K – Ar dating of illites in Brent Group reservoirs: a regional perspective, in:

[43] [44]

[45]

[46]

[47]

[48]

[49]

[50]

[51]

[52]

[53] [54]

[55]

[56]

[57]

[58]

[59]

425

A.C. Morton, R.S. Haszeldine, M.R. Giles, S. Brown (Eds.), Geology of the Brent Group, Special Publication - Geological Society vol. 61, (1992) 377 – 400. I.M. Villa, Isotopic closure, Terra Nova 10 (1998) 42 – 47. D.D. Eberl, J. S´rodon´, Ostwald ripening and interparticle-diffraction effects for illite crystals, Am. Mineral. 73 (1988) 1335 – 1345. D.D. Eberl, J. S´rodon´, M. Kralik, B.E. Taylor, Z.E. Peterman, Ostwald ripening of clays and metamorphic minerals, Science 248 (1990) 474 – 477. G.M. Laslett, P.F. Green, I.R. Duddy, A.J.W. Gleadow, Thermal annealing of fission tracks in apatite: 2. A quantitative analysis, Chem. Geol. 65 (1987) 1 – 13. P.F. Green, I.R. Duddy, Some comments on paleotemperature estimation from apatite fission track analysis, J. Pet. Geol. 12 (1989) 111 – 114. J.D. Corrigan, Apatite fission-track analysis of Oligocene strata in South Texas, U.S.A.—testing annealing models, Chem. Geol. 104 (1993) 227 – 249. R. Yamada, T. Tagami, S. Nishimura, H. Ito, Annealing kinetics of fission tracks in zircon: an experimental study, Chem. Geol. 122 (1995) 249 – 258. T. Pettke, L.W. Diamond, I.M. Villa, Mesothermal gold veins and metamorphic devolatilization in the northwestern Alps: the temporal link, Geology 27 (1999) 641 – 644. A.M. Pytte, R.C. Reynolds, The thermal transformation of smectite to illite, in: T.H. McCulloh, N.D. Naeser (Eds.), Thermal Histories of Sedimentary Basins, Springer-Verlag, New York, 1988, pp. 133 – 140. N. Clauer, J. S´rodon´, J. Francu, V. Sucha, K – Ar dating of illite fundamental particles separated from illite/smectite, Clay Miner. 32 (1997) 181 – 196. P.H. Nadeau, Fundamental particles: an informal history, Clay Miner. 34 (1999) 185 – 191. G. Whitney, B. Velde, Changes in particle morphology during illitization: an experimental study, Clays Clay Miner. 41 (1993) 209 – 218. W. Hansmann, Age determinations on the Teriary MasinoBregaglia (Bergell) intrusives (Italy, Switzerland): a review, Schweiz. Mineral. Petrogr. Mitt. 76 (1996) 421 – 451. F.C. von Blanckenburg, Isotope geochemical and geochronological case studies of Alpine magmatism and metamorphism: the Bergell intrusion and Tauern window, PhD thesis, ETH Zu¨rich, 1989, 186 pp. W. Mu¨ller, G. Prosser, N.S. Mancktelow, I.M. Villa, S.P. Kelley, G. Viola, F. Oberli, Geochronological constraints on the evolution of the Periadriatic Fault System (Alps), Int. J. Earth Sci. 90 (2001) 623 – 653. B. Fu¨genschuh, Thermal and kinematic history of the Brenner area (Eastern Alps, Tyrol), PhD thesis, ETH Zu¨rich, 1995, 226 pp. N.S. Mancktelow, D.F. Sto¨ckli, B. Grollimund, W. Mu¨ller, B. Fu¨genschuh, G. Viola, D. Seward, I. Villa, The DAV and Periadriatic fault systems in the Eastern Alps south of the Tauern Window, Int. J. Earth Sci. 90 (2001) 593 – 622.