Tectonophysics 359 (2002) 255 – 270 www.elsevier.com/locate/tecto
The Late-Variscan fault network in central–northern Portugal (NW Iberia): a re-evaluation F.O. Marques a,*, A. Mateus b, C. Tassinari c a
CGUL Departamento e Geologia, Faculdade Cieˆncias, Universidade de Lisboa, Edifı´cio C8, Piso 6, 1749-016, Lisboa, Portugal b CREMINER, Dep. de Geologia, FCUL, Edifı´cio C2, Piso 5, 1749-016, Lisboa, Portugal c Instituto de Geocieˆncias, Universidade de Sa˜o Paulo, Sa˜o Paulo, Brazil Received 20 August 2001; accepted 12 August 2002
Abstract The fault network in central – northern Portugal, especially the fault system with mean strike N25j, has been used to help deduce the late Palaeozoic dynamics of Western Europe. On the other hand, the N80j strike – slip fault system was recognized in previous works as Late-Variscan and left lateral, but was scarcely mapped and its importance neglected. This study shows that the N25j faults were dextral in the late stages of the Variscan Orogeny and sinistral during the Alpine Cycle due to pervasive reactivation. Fault rocks and intrusions are clearly different, according to age: mostly high temperature quartz infillings, but locally muscovite, tourmaline, and aplite dykes in Late-Variscan times, and low temperature cataclasites, fault gouges and Mesozoic mafic dykes in younger Alpine times. We dated dextral N45j segments of the N25j fault system because they are less prone to reactivation by the NNW – SSE Alpine compression, and can thus preserve the Variscan record. We used K – Ar in muscovite concentrates and obtained a minimum age of the analysed faults of ca. 312 Ma, which sets a lower limit to the so-called Late-Variscan wrench-faulting period. The present study shows that the N80j fault set is pervasive and sinistral in central – northern Portugal, and therefore, that it only admits one brittle dextral conjugate, the N25j fault system. Both were generated by a maximum compressive stress bearing between N50j and N55j in azimuth. We did not find evidence of a Variscan sinistral strike – slip movement in the N25j fault system, and therefore, this kinematics is believed to represent only the displacements accommodated during the Alpine Cycle. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Late-Variscan faulting; Alpine re-activation; Fault rocks; Kinematics; Isotope chronology; NW Iberia
1. Introduction Ribeiro (1974) defined three main deformation phases (Dn) that characterize the evolution of the * Corresponding author. Tel.: +351-217500000; fax: +351217500064. E-mail address:
[email protected] (F.O. Marques).
Variscan fold belt in Portugal. D3 was dated by Dallmeyer et al. (1997) in NW Iberia at ca. 315 Ma. For N Portugal, Ribeiro (1974) defined Late-Variscan as the time period after D3, and thus younger than ca. 315 Ma. Arthaud and Matte (1975) defined LateVariscan as the time period between 310 and 270 Ma in SW Europe. According to Ribeiro (1974) and Arthaud and Matte (1975), this should be the period
0040-1951/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 1 9 5 1 ( 0 2 ) 0 0 5 1 4 - 0
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of a widespread episode of fracturing in the Variscan belt of SW Europe. However, the upper age limit of the strike –slip event should be younger than 270 Ma, because all post-kinematic granitoids (dated between 295 and 270 Ma; Dallmeyer et al., 1997 and references therein) are affected by the widespread LateVariscan wrench faulting. In fact, Arthaud and Matte (op. cit.) suggested that the fracturing episode in the Iberian Peninsula should be older than the Triassic and younger than the Variscan granitic magmatism (340 – 280 Ma) or the two-mica granites dated at ca. 300 Ma. This is in apparent contradiction with the conclusions of Gonza´lez-Casado et al. (1996) and Tornos et al. (2000), who established the onset of the Alpine Cycle between 280 and 270 Ma ago in Central Iberia; therefore, Late-Variscan must be older than this age. For Gonza´lez-Casado et al. (1996), the Late-Variscan wrench faulting is restricted to a short period of ca. 10 Ma (between 300 and 290 Ma ago). Arthaud and Matte (1975) recognized three main Late-Variscan strike – slip fault systems in Iberia: dominant NE – NNE (always sinistral), and subordinate NW – NNW (Late-Variscan dextral) and E –ENE (sinistral). From this geometry and kinematics, they concluded that a N –S maximum compressive stress was responsible for the development of the whole of the fracture network in Iberia. Ribeiro (1974), Ribeiro et al. (1979), and Pereira et al. (1984, 1993) defined two main episodes of faulting in N Portugal: (1) Variscan faulting—two conjugate systems, bearing in average N80j (sinistral) and N25j (dextral), contemporaneous with D3 (the last Variscan folding phase with axial planes striking approximately N135j). (2) Late-Variscan strike – slip faulting—a dextral N135j system conjugate to a sinistral N25j family, Permian in age. The argument used for the relative age of both systems was that they cut the D3 structures and Late-Variscan granites (300 – 270 Ma). However, on this basis, they can even be Alpine, because an upper age limit is not presented. By definition, the conjugate character of fault systems must rely on evidence that support their contemporary development. Compatible geometry and kinematics is not enough. The above studies were purely geometric, because no data were presented as to the main kinematic markers—slip lines—or to the fault rocks and infillings. The study by Arthaud and Matte (1975) did not include important faults, already
recognized by Ribeiro (1974), striking N25j and dextral in Late-Variscan times. It also did not account for the N – S faults present in all studied areas of northern – central Portugal. Previous work on Late-Variscan fracturing (Ribeiro, 1974; Arthaud and Matte, 1975; Ribeiro et al., 1979; Pereira et al., 1984, 1993) did not describe or characterise the fault rocks or infillings. However, this can be a key for separating Late-Variscan strike – slip faulting from younger, Alpine, re-activation. The reason is that the Variscan structures we presently observe at the earth’s surface (e.g., post-kinematic granitoids) formed some 10 – 15 km deep. Mateus (1995) showed that Late-Variscan faults formed during crustal uplift, most probably related with the isostatic rebound of the Variscan orogen (Late-Carboniferous extensional collapse of Gonza´lez-Casado et al., 1996). The Variscan brittle crust cannot presently be observed in the central domain of the belt because it was removed by post-collisional Variscan denudation. On the contrary, the Alpine compressive reactivation of the Late-Variscan fracture network now observed at the surface, caught these fractures at a higher, colder crustal level, and thus fault rocks reflect these conditions (mostly breccias and fault gouges). Then, we sought for ‘‘hot’’ features related with faults to characterize the Variscan fracture network. This methodology has been used by Marques and Mateus (1998, 2001), and is in agreement with data recently presented by Tornos et al. (2000). The study we carried out covered most of northern –central Portugal (Fig. 1) and is representative of the Late-Variscan brittle deformation in W Iberia. It comprises areas dominated by granites (areas 2, 3 and 8 in Fig. 2), areas dominated by metasediments (areas 1 and 6 in Fig. 2), one area of high-grade metamorphic rocks (area 5 in Fig. 2) and areas with an identical proportion of granites and metasediments (areas 4 and 7 in Fig. 2). To our knowledge, the importance of the N80j fracture system, as one of the most penetrative in northern– central Portugal and as the Variscan sinistral conjugate to the dextral N25j system, has never received due attention in the published literature. In the present work, we face, then, the problem of bracketing the age interval of formation of the LateVariscan fracturing, and characterize the faults as to geometry, kinematics and infillings. The only fault
F.O. Marques et al. / Tectonophysics 359 (2002) 255–270 Fig. 1. Sketch map with Late-Variscan folds, main faults and exhumed granitic core of the mountain belt in NE Portugal. q—represents quartz infillings. Faults marked 1 and 2 have the same geometry but opposite kinematics; fault 2 has been strongly reactivated by Alpine sinistral movement, while fault 1 preserves the Late-Variscan dextral kinematics.
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Fig. 2. Location of the study areas with respective contoured maps of poles to planes. Dating of faults was performed in the area marked 8.
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system worth dating isotopically is the N25j, because: (1) the N155j and N110j faults have a complex history, which includes early Variscan deformation in ductile fashion (e.g., Ribeiro, 1974; Arthaud and Matte, 1975) and are, thus, not Late-Variscan; (2) the N80j fault system must be Late-Variscan because it cuts all previous Variscan structures and could not be generated by the Alpine NNW –SSE compression; and (3) it has been used to help deduce the late Paleozoic dynamics of Western Europe. Examination of the published literature shows that all the chronology of Late-Variscan fracturing has been done by correlation (relative chronology). No one else before the present work has put an isotope age on a Late-Variscan brittle fault. This results from the fact that: (1) the faults of the N25j system have been pervasively reactivated by the Alpine NNW – SSE compression (more than 90% in the present study); (2) early Alpine fluid circulation has significantly altered earlier fault rocks (Tornos et al., 2000; Gonza´lez-Casado et al., 1996); (3) the older record is, therefore, scarce, and, yet more, comprising most of the times mere quartz precipitates; and (4) very few authors (e.g., Tornos et al., 2000; Gonza´lezCasado et al., 1996) worried about the fault rocks and their meaning in terms of age. What is then the alternative? To look carefully at the 20– 30% of fault segments in which we can still recognize the older (Late-Variscan?) record. Even there, it is very difficult to find datable rocks because fault infillings are, most of the times, mere quartz precipitates. We looked especially for the N45j segments of major N25j faults because they are less prone to reactivation by the Alpine NNW – SSE compression. In such segments, inside granitic rocks, we could still find well-preserved mineral infillings (muscovite and tourmaline) and aplitic dykes. Because this is a rare situation, we had to use independent data to try and establish the age of fault kinematics. In the present work, we used mineral parageneses of fault infillings, pressure and temperature estimates and type of fault rocks to characterize the Late-Variscan fracture network and separate it from the effects of its Alpine reactivation. The age of some faults was estimated by isotope dating. We used K/Ar to date muscovite concentrates from fault surfaces and from an aplite dyke intruded into a N45j dextral strike – slip fault. The age of the colder, younger reactivation was constrained only by geomorphology and tectonics.
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2. Fault data 2.1. Structural 2.1.1. Geometry The location of the mapped fault networks is presented in Fig. 2 and the geometrical characterization summarized in the stereographic plots inserted in that figure. The contoured projections of partial (Fig. 2) and total of faults (Fig. 3A) reveal the existence of four sub-vertical fracture systems, in agreement with all published literature to present, with azimuths: N80 F 20j, N25 F 25j, N155 F 15j and N110 F 10j systems. The present study reveals that the N80j fault system, similarly to the N25j fault system, is pervasive in northern – central Portugal. Its importance was neglected in all previous works, and hence its exclusion as the Late-Variscan conjugate of the N25j system in all previous literature (Fig. 4A). We will come back to this question in the discussion. The N155j family is even practically absent, or poorly represented, in some of the studied regions of Portugal (e.g., areas 4 and 6 in Fig. 2), and this will also be discussed below. All fault systems now presented were already known and referred to in previous works (e.g., Ribeiro, 1974; Arthaud and Matte, 1975). We now call N25j, N80j, N155j and N110j to the fault sets known in previous works, respectively, as NNE – SSW (Ribeiro, 1974) or NNE –NE (Arthaud and Matte, 1975, p. 148), as E – W to ENE – WSW (Arthaud and Matte, 1975, p. 151), as NW –SE to NNW –SSE (Arthaud and Matte, 1975, p. 149), and as ESE – WNW (Ribeiro, 1974). Slip data represented in the stereographic plots of Fig. 3B show two distinct distributions: a steeply dipping cluster corresponding to cold striations (mostly fault gouges) and a gently dipping cluster of hot fibres (mostly quartz). 2.1.2. Kinematics The older kinematics of faults can be observed at different scales: 2.1.2.1. Map scale. Quartz veins in fault bends and jogs striking N45j (q in Fig. 1) indicate a dextral strike – slip movement in the N25j fault system (Fig. 4B). In places, the N45j segments of the N25j faults
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Faults marked 1 and 2 in Fig. 1 are parallel, but with opposite senses of shear: fault marked 1 is dextral (the Gimonde– Santa Comba de Rossas dextral strike – slip fault of Ribeiro, 1974) and fault marked 2 is left lateral. The latter was extensively re-activated as left lateral during the Meso-Cenozoic (Cabral, 1989) and, thus, it is difficult to evaluate its Variscan kinematics. On the contrary, the fault marked 1 in Fig. 1 has not been reactivated by the NNW – SSE Alpine maximum compressive stress
Fig. 3. (A) Statistics of the total of faults studied. Contoured map of poles to fault planes. In the stereoplots of Fig. 2, all sets are visible, but the predominance of each set is different from place to place. It is also clear from all plots of Fig. 2 that the N25j and N80j families prevail. Then, when we put all data together, the dominance of those two fault systems is much greater and the other systems become less visible (not absent). They are there but, obviously, in a much smaller percentage—see the tails at < 4% to >2% striking approximately ENE – WSW (poles to planes striking NNW – SSE—our N155j) and NNE – SSW (poles to planes striking ESE – WNW—our N110j). (B) Contoured map of striations in fault surfaces.
are filled with aplite dykes like the ones dated in the present work, attesting to the dextral character of the N25j fault system (Fig. 4B).
Fig. 4. (A) Sketch to summarize geometry and kinematics of the main fault systems. r1 is the maximum compressive stress deduced from brittle Late-Variscan fault sets (N25j and N80j). (B) Sketch to illustrate map-scale shear sense criteria and typical location of datable fault rocks not reactivated by Alpine compression.
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because it still records a dextral kinematics. Then, it must be dextral in Variscan times, as already recognized by Ribeiro (1974). Using the same criterion as Ribeiro (op. cit.), the Gimonde – Santa Comba de Rossas fault must be Late-Variscan because it cuts axial planes of D3 folds. Ellipsoidal, en e´chelon quartz veins are often found striking N25j, although individually they bear approximately N60E. This geometry and arrangement indicates a dextral transcurrent movement in the N25j fracture system. 2.1.2.2. Mesoscopic scale. Our detailed study of the fracture network in central –northern Portugal showed that more than 90% of the faults belonging to the N25j system have been extensively reactivated by the Alpine compression and did act as conduits for intense hydrothermal circulation, with almost complete obliteration of the Variscan record. In only 20– 30% of the studied N25j faults were we able to locally observe the older mesoscopic shear sense criteria (mostly quartz fibres), attributable to the Late-Variscan stress field. The best place to find the older record is in the N45j segments of major N25j faults, because that bearing is less prone to reactivation by the NNW –SSE Alpine compression. We have used classic shear criteria to evaluate fault kinematics (Fig. 5A and B). In the older mineral precipitates and fault rocks they indicate a dextral strike – slip movement in the N25j system, and a sinistral strike –slip movement in the N80j system. Shear criteria preserved in cold fault gouges are variable, but most indicate an important incorporation of vertical movements, responsible for the generation of Alpine grabens and pop-ups (e.g., Ribeiro et al., 1990; Cabral, 1989; Marques et al., 2002). 2.1.3. Alpine reactivation Post-Miocene Alpine maximum compression (r1) in W Iberia was deduced from folds and faults and is approximately NNW– SSE (e.g., Ribeiro et al., 1990). Alpine rejuvenation of the Late-Variscan fracture network is characterized by cold fault rocks, like breccias (which include late fracturing and brecciation of the earlier Variscan quartz infillings) and fault gouges, and by intrusion of Mesozoic dykes. The N155j fault system was reactivated still as dextral, the N25j faults variably reactivated from
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strike – slip (left-lateral) to dip slip, and the N80j fault family reactivated as thrusts only when not vertical (otherwise no reactivation can take place because faults are perpendicular to the Alpine maximum compression). 2.2. Fault rocks and mineral infillings Usually, the main segments of the major fault zones in NW Iberia comprise thick siliceous polyphase infillings that are discontinuous in strike and dip. A close examination of these hydrothermal precipitates reveals also that they often contain fragments of several fault rocks developed under different P – T conditions, including (proto-) mylonites, ultracataclasites and various quartz breccias (that can be grouped into different types according to the mineralogical nature of their matrix and to the characteristics of the fragments incorporated—e.g. Mateus, 1995). The evolution of these fault-controlled hydrothermal systems, in terms of P – T conditions and fluid chemistry, can be presently evaluated on the basis of isotopic data, detailed characterization of fluid inclusions trapped in successive quartz generations, and mineral equilibria studies. Details on the evolution experienced by many of these geochemical systems (particularly those that have implications in ore-forming processes) can be found in three recent works (Boiron et al., 1996; Noronha et al., 2000; Tornos et al., 2000) and references therein. The earliest, most common mineral precipitates identified along many segments of major fault zones cutting Variscan granites (Fig. 6) comprise aggregates of greyish milky quartz, sometimes coupled by notable amounts of muscovite and tourmaline and/or accessory contents of apatite and arsenopyrite. Usually, the quartz grains of these aggregates display strong wavy extinction, deformation bands and serrated inter-granular boundaries; evidence of interand/or intra-granular sub-granulation can also be found in many samples. When significantly rich in muscovite and tourmaline, these mineral precipitates show another type of quartz that generally occurs as elongated fibres, recording coeval strike – slip fault movements (see above). In these cases, muscovite sheets show only incipient bend gliding and typically define a conspicuous fabric that, regularly, is reinforced by the alignment (following the c axis direc-
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Fig. 5. (A) Photo of vertical NE – SW fault with muscovite and tourmaline infilling; the unusually long crystals of tourmaline are the kinematical marker. (B) Dextral shear criteria in vertical NE – SW fault. NE is to the right in both images.
tion) of prismatic, locally brittle deformed, crystals of tourmaline. The mineral paragenesis quartz + muscovite + tourmaline F apatite F arsenopyrite documents the circulation of late-magmatic, silica saturated and
boron enriched aqueous fluids of variable salinity under minimum temperatures of about 400 F 50 jC and pressures ranging from 3 to 5 kbar (Neiva et al., 1990; Gomes, 1994; Mateus, 1995; Mateus and Noro-
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Fig. 6. Location of the main areas for which there are data concerning fault rocks. We used as background the fracture network reported by Pereira et al. (1993). Circles represent areas where the hydrothermal, fault mineral precipitates were examined in detail by different authors, reporting the evolution of P – T conditions by means of fluid inclusion studies and/or mineral equilibria: 1—Gomes (1994, 1995); 2—Neiva et al. (1990) and Neiva (1994); 3—Do´ria (1999), Noronha et al. (2000) and Guedes (2001); 4—Mateus (1995), Mateus et al. (1995), Mateus and Barriga (1995) and Barriga et al. (1995); 5—Almeida and Noronha (1988); 6—Sousa and Ramos (1991) and Boiron et al. (1996); 7—Couto et al. (1990); 8—Goncßalves et al. (1995a,b) and Costa (work in progress); 9—Lourencßo (1997) and Lourencßo et al. (in press); 10—Mateus et al. (2001); 11—Gomes (1996); 12—Nogueira (1997); 13—Do´ria (1999); Noronha et al. (2000); 14—Guedes (2001); 15—Mateus and Dias (1997) and Costa (work in progress). Cross signs indicate sites where fault rocks were investigated by A. Mateus (present study and unpublished data).
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nha, 2001). It represents, probably, the waning stages of the igneous activity nearly contemporaneous with the emplacement of aplite dykes along the major N45j fractures. The P – T estimation for this first evolving stage is compatible with the P – T conditions required for the establishment of the prevailing microstructures preserved either in the deformed granite or in the mineral infillings (Mateus, 1995; Mateus et al., 1995), and is consistent with ranges obtained by other authors for the development of synchronous shear zones in other domains of the Variscan Orogen (e.g., Burg and Laurent, 1978; Berthe´ et al., 1979; Igle´sias and Choukroune, 1980). Massive, and usually strongly fractured, milky quartz infillings record the second hydrothermal stage, with regional meaning (Mateus, 1995; Noronha et al., 2000). According to the available data (Mateus, 1995; Mateus et al., 1995; Marques and Mateus, 1998; Nogueira, 1997; Do´ria, 1999), the development of these quartz aggregates is mostly restricted to releasing bends or jogs of the major strike – slip fault zones, or to the widespread ENE –WSW to NE –SW veins with metric length. They are mainly composed of hardened, coarse quartz grains that in general show evident intra-, inter- and trans-granular fracturing; occasionally, quartz deposition is followed by the precipitation of Mg- or Fe-bearing carbonates, or fine-grained muscovite aggregates. Given the structural control of these hydrothermal mineral infillings and their later Alpine, strong brittle deformation, primary kinematic criteria are rarely preserved; being close to fault surfaces, they show strong crushing, and the observed slikenlines are due to younger displacements. The second hydrothermal stage involves mostly the circulation of low salinity aqueous-carbonic fluids of moderate to low salinity ( < 10 wt.% eq NaCl) under P –T conditions ranging from 450 to 325 F 25 jC and from 3 to 3.5 kbar, respectively (Mateus, 1995; Boiron et al., 1996, 2001; Noronha et al., 2000; Mateus and Noronha, 2001; Guedes, 2001; Guedes et al., 2001). Their origin is believed to be initially derived from mixing of the residual magmatic fluids with those related to the metamorphic degassing and further equilibration with C-bearing metasediments (in order to become variably enriched in CO2 F CH4 F N2 F H2S). Fluid focusing towards the fault segments in a third stage enabled the precipitation of milky
quartz + chlorite F sulphides (mostly pyrite) and, consequently, induced partial replacement of the earlier muscovite into illite. Grains of this new quartz generation are usually free of optical effects ascribable to plastic yielding, although they may record significant inter- and trans-granular fracturing. Chlorite F pyrite deposition seems to occur later, sealing preferentially small fault surface depressions adjoining the jutting-outs carved in quartz aggregates. In many places, the overall arrangement of the crystallographic oriented chlorite sheets preserve, therefore, the dextral strike – slip fault movements synchronous to their development (see above). This third hydrothermal stage is very well represented in many N25j fault segments (see Fig. 6), being crucial to the development of the earlier mineralising events recorded by several long-lived ore-forming systems intimately related to dilatancy processes and to subsequent fluid flow towards the active segments of the fault zones. During the third hydrothermal stage, the fluids significantly changed their composition with the progressive cooling of initial circulation temperatures close to 300 jC to around 150– 120 jC, under pressure conditions ranging from 2 to 0.5 kbar (e.g. Mateus, 1995; Boiron et al., 1996; Nogueira, 1997; Do´ria, 1999; Noronha et al., 2000; Guedes, 2001). The first evolving steps are believed to take in aqueous-carbonic fluids very similar to those that are involved in the second hydrothermal stage, although much more depleted in carbon volatiles and showing lower salinity (usually < 5 wt.% eq NaCl). Gradual and variable mixing of these fluids with shallow, modified meteoric waters, determines the chemical composition of the circulating hydrothermal solutions during the subsequent evolving steps, which can also be decisively influenced by depressurisation processes (as shown, e.g., by Cathelineau et al., 1993; Mateus, 1995; Barriga et al., 1995; Mateus and Noronha, 2001—see also the theoretical approach of the problem adressed by Bowers, 1991). The later fluids of this stage are thus predominantly aqueous, meteoric in origin, and display very low salinities (often < 2 wt.% eq NaCl). Changes in fluid composition during the third hydrothermal stage have a regional meaning and should mark an important modification of the geological conditions, possibly recording the transition between the Late-Variscan isostatic rebound and the Early
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Alpine extension, as suggested by Tornos et al. (2000).
3. Geochronology The only fault system worth dating isotopically is the N25j, because: (1) the N155j and N110j faults have a complex history, which includes early Variscan deformation in ductile fashion (e.g., Ribeiro, 1974; Arthaud and Matte, 1975), and are therefore not LateVariscan; and (2) the N80j fault system must be LateVariscan because it cuts all previous Variscan structures and could not be generated by the Alpine NNW –SSE compression. When trying to isotopically date the N25j faults, we faced a major difficulty already experienced by earlier investigators: to find well-preserved and isotopically datable fault rocks clearly related with the Late-Variscan fracturing episode. They are very scarce for the reasons mentioned in the introductory section. The well-preserved mineral association was found in a N45j set of dextral strike –slip faults, because this bearing is less prone to reactivation by the Alpine maximum compressive stress when compared with faults of the same system with azimuth closer to N –S. We have found two particular rocks to try and date the Late-Variscan N25j: an aplite dyke intruding a N45j dextral fault, and muscovite associated with tourmaline that fills those faults (see Fig. 2 for location). The granite affected by brittle faulting is biotitic granite and, therefore, muscovite and tourmaline are only related with faulting and fluid circulation. The K –Ar age determinations were conducted at the Geochronological Research Center of the University of Sa˜o Paulo using the techniques described by Amaral et al. (1966) with modifications. The K analysis was performed by flame photometry with a Micronal B-262 apparatus using a lithium internal
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standard. The Ar extraction was made in a high vacuum system with pressure usually less than 10 7 mm Hg. Isotopic analysis of the purified argon was made in a MS-1 Nuclide mass spectrometer, now fully upgraded. All ages were calculated with the decay constants recommended by Steiger and Jager (1977) and are given with standard error (1r) estimates. The constants used in the calculations were: kb ¼ 4:962 1010 year1 ; kj ¼ 0:581 1010 year1 ; ð40 Ar=36 ArÞatm ¼ 295:5; 40
K ¼ 0:01167%Ktotal
Analytical data are presented in Table 1. K – Ar analysis performed on two muscovite samples from fault surface and from aplite dyke intruded in fault yield ages of 311 F 10 and 312 F 7 Ma, respectively. Both results are in good agreement within analytical uncertainties, and can be interpreted as cooling ages of the dated minerals.
4. Discussion 4.1. Geometry The scattering of data around the mean value of strike of fault systems may seem exaggerated at first sight. However, many are the factors that justify the observed scattering: (1) faults rotate during deformation of crustal blocks (which also rotate with earlier faults inside) (e.g., Cobbold et al., 1989; Marques, 2001); (2) the fault trace can be strongly sinuous (due, for instance, to the way they interact and propagate) (e.g., Marques, 2001); (3) the studied fault network affects rocks as different as granites (isotropic in most cases), various types of metasedimentary rocks with a
Table 1 Analytical data of K/Ar dating SPK
Field reference
Analysed material
Rock
%K
40
ArRad CcSTP/g ( 10 6)
40
ArAtm (%)
Age (Ma)
Maximum error (Ma)
7768 7767
H9 A94 H9-Ap A94
Muscovite Muscovite
Fault surface Aplite
7.3952 5.7240
97.54 75.81
5.43 5.54
311 312
10 7
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strong original planar anisotropy, and mafic/ultramafic high-grade metamorphic rocks (mostly foliated and folded); and (4) we mapped the fault networks at a much more detailed scale (hectometers to kilometers) than previous work (kilometers to tens and hundreds of kilometers). 4.2. Kinematics Taking into account the age, the geometry and the kinematics of the studied fault pattern, we can conclude that the N25j system is the Late-Variscan dextral conjugate of the sinistral N80j fault system. On one hand, all previous literature on the LateVariscan fracturing in northern – central Portugal recognized the existence of a left-lateral, strike – slip, N80j fault system (e.g., the E – W system of Arthaud and Matte, 1975), although not penetrative as the present study shows. On the other hand, the same authors (e.g., Ribeiro, 1974; Arthaud and Matte, 1975) suggest a N – S maximum compressive stress for Late-Variscan times, which is not compatible with a pervasive N80j, sinistral strike –slip fault system. The correct evaluation of the importance of the N80j fault system and of the dextral Late-Variscan kinematics of the N25j fault system is essential for the deduction of the orientation of the maximum compressive stress in Late-Variscan times. The N – S r1 is also not compatible with the N – S faults present in all studied areas of central – northern Portugal (cf. Fig. 3A). 4.3. Dynamics It is known since the works of Ribeiro (1974) and Arthaud and Matte (1975) that the N155j and N110j fault systems are older in the Variscan record and show a complex evolution. Arthaud and Matte (1975) even suggest that the N155j (their NNW – SSE system) faults started as normal faults. Therefore, we have chosen to estimate the orientation of the maximum compressive stress of Late-Variscan times from faults generated only at this stage, which are the N25j and N80j systems. What has been the evolution of the stress field since the late stages of the Variscan Orogeny? MesoCenozoic N –S to NNW– SSE compression in W Iberia is the result of collision between the African
and the Eurasian plates. It has re-activated most of the pre-existing Late-Variscan fracture network and so it must be taken into consideration before coming to any conclusions as to the older (Variscan) kinematics and dynamics. During the time span 312– 270 Ma (Late-Variscan) what information can we gather about the stress field, from structures other than faults? The stress regime proposed by, e.g., Dias and Ribeiro (1991) for the Variscan structures in W Iberia, from folds and thrusts, is left lateral transpression with maximum compressive stress striking between N45j and N60j. Compression with identical strike was also responsible for the NW – SE to NNW – SSE D3 folds, dated at ca. 315 Ma by Dallmeyer et al. (1997). D3 is followed by uplift, mostly in the core of the mountain belt, and thus information about the compressive stress is not clear. However, in the more external zones of the Variscan Orogen in SW Portugal (South Portuguese Zone), the deformation can be followed and must have taken place some time after the Late Westphalian ( < ca. 290 Ma), because this is the age of the youngest sediments affected by D1 (Pereira, 1999). D1 and D2 of SW Portugal are still the result of NE –SW compression, with vergence of folds and thrusts to the SW. The latest Variscan deformation is, at least in SW and W Portugal, the result of a WNW – ESE to E – W maximum compressive stress (D3 of Ribeiro and Silva, 1983, and Ribeiro, 1983), which follows a deformation phase (D2) responsible for the generation of dominant sinistral ENE – WSW ‘‘brittle shears’’ and subordinate dextral N –S ‘‘brittle shears’’ (Ribeiro, 1983). The E – W compressive event was dated, by correlation (Ribeiro et al., 1980), between the Autunien and the Lower Triassic (280 –220 Ma). None of these stress fields contemporaneous with the development of the Late-Variscan fracture network is compatible with a left-lateral N25j fault system. The E –W compression would still reactivate this system as dextral. The NE – SW compression that generates the NE vergence in the N and the SW vergence in the S seems to be continuous since, at least, the Namurian (ca. 320 – 315 Ma, the age of NW – SE D3 folds—Dallmeyer et al., 1997) till the Stephanian (290 –280 Ma, the maximum age of SW verging folds and major thrusts in SW Portugal—Pereira, 1999) or even the Lower Permian (Autunien—280 –260 Ma). This fold/thrust
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event is followed by N – S folding and crenulations in the Lower Permian. 4.4. Age We used the estimated temperature conditions of formation of fault mineral infillings and fault rocks development to establish the relative chronology of fault movements. The metamorphic record of the Variscan belt in its late stages is characterised by a significant rise of the geotherms in the first 20– 30 Ma of the post-collisional isostatic rebound, from ca. 300 to 270 Ma, which is the age of intrusion of postkinematic granitoids. According to Gonza´lez-Casado et al. (1996), the episyenites presently observed at the surface on the eastern Iberian Central System formed ca. 277 Ma ago at about 6.5 km depth and at temperatures between 350 and 650 jC. This episode marks the onset of the Alpine Cycle and is followed by an important cooling of the upper/intermediate part of the crust. Given this thermal evolution, one concludes that fault segments with preserved high temperature mineral infillings are older than those with ‘‘cold’’ fault rocks. Because Alpine compressive events affected an isostatically rebounded crust (‘‘hot’’ rocks were faster brought to shallow crustal levels), one finds, at the present day, ‘‘hot’’, older fault mineral infillings reactivated by ‘‘cold’’, cataclastic faulting, in the form of clay-gouges. Alpine compressive ‘‘hot’’ fracturing
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seats deep in the crust and cannot be directly observed. Therefore, the earlier fault mineral infillings must be Variscan in age, being the development of the clay-gouges the result of the Alpine compressive phases. Thick clay fault gouges are found in the ENE– WSW thrusts that limit the Alpine pop-up of Serra da Estrela (late Miocene in age) and in reactivated Variscan faults that bound Plio-Quaternary grabens. The age of ca. 311 Ma obtained by K/Ar in muscovite concentrates probably dates the time of muscovite formation and is thus considered as a minimum age boundary for the faulting episode. The age of the fractured granite has been determined by Simo˜es et al. (1999) using U – Pb systematics; monazites yield a 318 F 2 Ma age and zircons a 314 F 4 Ma lower intercept age, which indicates that the granite formed by partial melting of older rocks. Some faults of the N25j system are filled with chlorite + sulphides (like the one on Fig. 5B), and quartz still shows dextral strike – slip; the same kinematics can be deduced from the crystallographic oriented chlorite sheets. If comparison of this mineral association with similar parageneses of event 2 of Tornos et al. (2000) is valid, then dextral kinematics was still active between 280 and 270 Ma ago. As shown in the introductory section, there is still controversy as to the age of Late-Variscan fault formation. The age here presented establishes a lower
Fig. 7. Schematic chronogram showing the main Late-Variscan and Early-Alpine tectonic events in central/western Iberia. Ages of the Alpine extensional regime are from Gonza´lez-Casado et al. (1996); ages of D3 and post-kinematic granitoids in the Central Iberian Zone are from Dallmeyer et al. (1997) and references therein; ages of the Late-Variscan wrench faulting are from the present work; Geological Time Scale is from Harland et al. (1990).
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limit to the Late-Variscan wrench-faulting episode (Fig. 7). Therefore, it cannot be as limited in time as suggested by Gonza´lez-Casado et al. (1996) (from ca. 300 to 290 Ma). The upper limit cannot be the Triassic as suggested by Arthaud and Matte (1975) for Iberia, because then the faulting episode would in great part be early Alpine as defined by Gonza´lez-Casado et al. (1996). Because the youngest Variscan granitoids in Iberia crystallized ca. 270 Ma ago and are affected by the Late-Variscan fracturing, the age of the upper limit of this event must be sometime younger than 270 Ma (Fig. 7). The short interval between the age of the granite and the isotope age of faulting suggests that the earlier fracturing took place during granite emplacement and, thus, at relatively high temperatures. The stress field that generated the D3 folds (ca. 315 Ma) is compatible with the N25j (dextral) and N80j (sinistral) conjugates. Marques (2001) showed that plastic materials, like clay in the lab or hot granite in nature, can deform by simultaneous flattening and fracturing. Therefore, there is no incompatibility in having simultaneous folding and fracturing in the model or in nature, when we say that Variscan fracturing can be as old as ca. 312 Ma, the age of the latest Variscan folding (D3).
5. Conclusions The age and kinematics of the N80j fault system is consensual: it is Late-Variscan and sinistral. Conversely to previous published work, the present study shows that the N80j fault system is pervasive in central –northern Portugal. Therefore, this system only admits one dextral conjugate, the N25j fault system. We did not find evidence for a sinistral kinematics of the N25j fault system during Variscan times. A lower limit to the age of the Late-Variscan wrench-faulting episode is now established by K/Ar isotope dating of fault rocks; this age is ca. 312 Ma. In W Iberia, the upper limit of the age of the LateVariscan strike – slip faulting must be somewhat younger than 270 Ma, because this is the youngest age of post-kinematic granites cut by Late-Variscan faulting. However, this upper limit can vary along strike of the Variscan Chain in Iberia if we take into account data from Gonza´lez-Casado et al. (1996).
Acknowledgements This paper is a contribution to projects DIWASTE (PRAXIS/P/CTE/11028/98) and GEOMODELS (PCTI/CTA/32742/99 and PCTI/CTA/38695/2001), who supported fieldwork, and ICCTI (Portugal)CAPES (Brasil) who supported cooperation and geochronological analysis. Manuscript quality was improved by the comments of reviewers Martinez Catalan and G. Gleizes.
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