Anatomy of the Mondoñedo Nappe basal shear zone

Journal of Structural Geology, Vol. 15, No. lZ, pp. 1405 to 1419, 1993 Printed in Great Britain 0191­­s141193 $06.00+0.00 © 1993 Pergamon Press Ltd Anatomy of the Mondofiedo Nappe basal shear zone (NW Spain) J. ALLER and F. BASTIDA Departamento de Geologia, Universidad de Oviedo, Spain (First received 17 March 1992; accepted in revised form 14 January 1993) Abstract­This paper analyses a major shear zone from the Iberian Hercynian belt which forms the basal thrust of the Mondoiiedo Nappe. The shear zone developed by ductile deformation under amphibolite facies metamorphic conditions and later by brittle­ductile deformation ·in greenschists facies. Folds in the shear zone are asymmetric, very tight, Ie or similar class and frequently developing sheath geometries. The sheath folds originated by non­coaxial flow superimposed on earlier irregularities. The fabric of quartzitic rocks in the shear zone changes from bottom to top from ultramylonites through blastomylonitic rocks to non­mylonitic tectonites. c­axis fabrics vary across the shear zone, but show a dominant monoclinic symmetry. The blastomylonitic rocks include the fabrics representing the highest temperatures. The main foliation ofthe schists results from flattening of an earlier foliation, recording occasional microfolds. The use of different kinematic criteria has allowed an analysis of their validity as well as an assessment of movement direction towards the foreland of the orogen. INTRODUCTION THE Mondoiiedo Nappe basal shear zone is well suited for the study of problems arising in this type of structural association, and has already been the subject of some previous papers (Bastida & Pulgar 1978, Pulgar 1980, Martinez­Catalan 1985, Bastida et at. 1986). The shear zone, 3.4 km wide, is located in the northern part of the Iberian Hercynian belt (Fig. 1), presenting a continuous outcrop with strong ductile deformation all along the Cantabrian coast. It is bound at its base by a major thrust developed in brittle­ductile conditions during the last stage of the shear zone deformation history. The shear zone outcrops again, further to the east, in an area closer to the front of the Mondoiiedo Nappe (Fig. 1). Here the shear zone is characterized by a brittle­ductile deformation zone not wider than 100 m, above the basal thrust. GEOLOGICAL SETTING The shear zone is the lower boundary of a large thrust sheet, the Mondoiiedo Nappe, situated in the western part of the Westasturian­Leonese zone, a major tectonic zone in the hinterland of the Iberian Hercynian belt. The Mondoiiedo Nappe unit is made up of a stack of recumbent, almost isoclinal, large amplitude E­facing D 1 folds (Fig. 1) with an associated tectonic foliation (Sl)' These folds are affected by the nappe basal thrust and the associated shear zone. This shear zone is important in the western part of the nappe, where a large number of shear related E­facing minor folds (D z) are developed. In this part, Sl has been flattened (Sl+z), crenulated or replaced by a new mylonitic foliation (Sz). Finally, the whole nappe has been folded into a gentle synform in the east and an antiform in the west (D 3 ), both with a subvertical axial plane and a large wavelength. D 3 minor folds and S., crenulation cleaval!e occur locallv. D., folds homoaxially overprint D 1 folds producing a Type 3 interference pattern (Ramsay 1967) that can be seen both on the map and in cross­section (Fig. 1). Uplift due to the D 3 major antiform has resulted in exposure of the thrust and the basal shear zone around the Xistral semi­window (Fig. 1) where amphibolite facies metamorphism was reached (Martinez­Catalan 1979, 1980). It is in this western outcrop of the shear zone, in the section between the coastal localities of Burela and Foz (Fig. 1), that most of this work was focused. GEOLOGY OF THE CROSS­SECTION The rocks affected by the shear zone as well as those of the Xistral semi­window, belong mainly to the Cindana Group (Lower Cambrian) (Walter 1966, 1968). Precambrian rocks, consisting of micaschist with common sandstone intercalations and porphyroids outcrop just at the base of the nappe. The Candana Group consists of four units, only two of which are affected by the shear zone: the lower Candana formed by coarse­grained feldspathic sandstones and quartzite with intercalated schists, and the middle Candana formed by micaschist, commonly with some sandstone, and minor black slate, carbonatic and amphibolitic layers. The macroscopic structure shown in Fig. 2 consists of several very tight D 1 folds, downward­facing due to their location in the western limb of a large gentle D 3 synform. The basal thrust, also in a tilted position, can be seen in the western part of the limb. The ductile deformation of the shear zone is shown by an association of minor structures that developed in the nappe footwall (shaded area in profiles of Fig. 1). The general features of metamorphism and its chronological relationship with the deformation in the Mondoiiedo Nappe have been pointed by out Bastida & Pullmr (1978). Martinez­Catalan (19R5) and Ba!'\tida et 1406 J. ALLER and F. BASTIDA HERCYNIAN BELT IN THE IBERIAN PENINSULA 43' 30' B '- I A­A' o 10 20Km s=s=iI:::::=::::::E'==s=iI:::::=:::ll 10' NW ｾＮｭｯ NW SIL. ORO. r:==:::1 Luarca, Agueiro and Gorganta Fms. ia"""i Cabos Series a) Upper quartzite t:::::::::J ｾ a P£ Vegadeo limestone 1:::::::[ Condano [[ill]] Villalba Group Series I::'..':.:::«>j >: Tertiary and Quater nor y Cover I: ｾＺ Hercynian Granitoids ­­­­ ­ BlOT ITE _._._.­ ALMANDINE ｾ Basal Q Shear Zone Central Iberian Zone _ •• ­ •• _ •• ANDALUSITE "'­"'­"'SILLIMANITE Fig. 1. Geological map and cross-sections of the Mondoiiedo area, according to Bastida et ai. (1986). BT, basal thrust. al. (1986). The metamorphic zones that occur on the coastal region of the Mondofiedo Nappe unit are shown in Fig. 1. The shear zone sector is situated within the andalusite and almandine zones (Fig. 2). The andalusite zone oartlv overorints the staurolite zone with relict staurolite occurring locally as residuals inside andalusite porphyroblasts. Three metamorphic episodes can be differentiated in the area of Fig. 2 (Bastida et al. 1986): M}> with a oaral!enesis containinl! staurolite and I!amet (medium The Mondofiedo Nappe basal shear zone pressure); M z, with andalusite (rise in temperature and perhaps a pressure drop); and M 3, presenting retrograde metamorphism to greenschists facies. M I is pre-D z. The rise in temperature during M z is coeval with ductile deformation in the shear zone (M z early syn-D z). M3 is late- to post-D z (end of ductile deformation in the shear zone and development of the basal thrust) and is related to the drop in pressure and temperature during and after uplift of the nappe rocks. 1407 that will be considered together (Punta Morago). All three zones are situated along the normal limbs of major D I folds. This position is likely to have favoured the development of D z folds, since the reverse limbs were likely situated in a stretching position. The lithology of the folded zones consists of quartzite, feldspar sandstone and micaschist. The quartzite and the sandstone occur in thin layers. In Punta das Cabras and Morago these rocks are more abundant than the schists, whereas in Punta Riomar the amount of quartzite and sandstone is often equivalent to the amount of schist. These lithologies may have favoured the development of D z folds, contrasting with the massive quartzite that occurs in the lower Candana, and the Precambrian or middle Candana schistose zones. The folds are E­facing and commonly occur as strongly asymmetric anticline­syncline couples. Small fold trains also occur locally. Folds show varying limb thickness. The D z folds are not associated with major folds and preserve the same asymmetry regardless of their location within the D 1 folds. The length of the short limbs ranges in most cases between 20 and 50 em, measured in the fold profile along the layer between two adjacent hinges. Fold hinges are SE­ to SW­plunging (Fig. 2), with a increased dispersion of plunge directions towards the STRUCTURES IN THE BASAL SHEAR ZONE The strong ductile deformation associated with the shear zone gave rise to a large number of small-scale D z structures, and pre-existing structures (D I ) were modified. D z structures consist of folds, foliations and lineations as well as microfabrics, which will be discussed separately. Folds Folds are very abundant in the western part of the section, where they are mainly concentrated in four narrow discrete zones (Fig. 2): Punta das Cabras, Punta Riomar and two others with very similar characteristics, IV V (f) w Z C9 I ｾ a:: a:: ｾ C9 ｾ ｾ W U U) l.L I >- l.L. 0 ｻＺｾ ?ft 30 CD "­ 20 c!l ｾ la ｾＢ ＱＺＭ ｲ ＿ ＭＺｬ ＢｸＬＫｯ Z :::J c:::r 0 4a 2fl l cr= 13.1 40 ?fl30 30 "- "­20 W 0 20 Ｔ ｾ 10 ｉｾ a'+­1r­+­"'P­,o 20 4060 80 a 6080 100 " 5a N o 252 N"134 cr= 11.3 o o a ...J 0 lJ... (7 '" =7 0-::6.35 ::.. 40 ® ｾ 7al055 5a 60 70 3a 20 M ｾ 10 \iJ II 'X o ' o 2040 a 2040 a SE NW 11 IV III I V I i [ VI z (f)(f) <t ｾ a:l ::Ii <t U a: lIJ Lower Condono quartzite ｾ 0 ..J 00 ­ '" Q. ｾ Za Q...J f­O ulJ... w...J ＨＺ＼ｾ au ...JZ + v1 IV III .. + + <:(0 Villalba Series ­­""' [:oom UU CLlJ... N"37 <:(0 Fig. 2. Cross­section of the Mondoiiedo Nappe basal shear zone, showing orientation of D z folds, histograms of interlimb angle frequency, number of folds and apical directions of conical folds along the section. Contours: 1, 2, 4 and 8%. Abbreviations: PC, Punta das Cabras; PR, Punta Riomar; PM, Punta Morago; RO, Ria d'Ouro; RF, Ria de Foz; BT, basal thnH1.t J. ALLER and F. BASTIDA 1408 . o • Q..N IC-3 IC-2 1.6 '( 1.8 IC-3 III 16 3­2 fC-2 • 12 1.2 ｾ I 08 '" 0.6 . • • 04 .. '" . . • o 02 . • o '" 04 N: 106 N: 106 COMPETENT INCOMPETENT MAXIMUM DIP 18 02 50 50 50 40 40 40 40 30 30 30 % 30 30 30 20 10 10 10 10 10 10 SHAPE 5.5 AMPLITUDE PUNTA DAS CABRAS 0+­....­­,­,­+ 50 40 20 4.5 % 40 20 3.5 60 50 20 2.5 1 ­­- N: 61 60 20 o­+­,­­­­,­+­­ ｾＮ INTERVALS ( DEGREES) 20 8 C 0 E F 0.8 LAYER % 50 O+­i­T...­r­t­ 06 LAYER 60 % 04 N:60 60 % P; ­­- a 3­ 18 IC- 18 1 N: 138 60 % • 3­18 08 0.6 • 51­60 61­70° 71_80° 81­90° 60 I '" 3 -IC 02 IC-18 ｾＧＭ ｾ o •• • • :: . 04 . 18 1 • 06 . 3­ 18 0.8 0.6 • IC 02 IC-18 0.2 .o .... o 0.8 • 04 18 1.2 - I 0.6 04 02 . , "," . ........... '" . . .... . • III 3-IC • 14- 0 .... . \..> .. 3­2 IC-2 1.6 .. tI 0 '" .. • IC III IC-3 ｾ . 14 • o 08 ... .., ｾＭＸＮＱ ... 2 • • 3­2 . .... & 2- 0..'" 3 o 1.4 i B C 0 E F 2.5 4.5 1.5 35 5.5 SHAPE AMPLITUDE PUNTA RIOMAR O+­ro...­rf­ O­+­r­r­l,­- B C 0 E F 35 55 2.5 . 4.5 . SHAPE AMPLITUDE PUNTA DEL MORAGO Fig. 3. Classification of fold morphologies on Bastida (1993) diagrams (above) and histograms for Hudleston's (1973) parameters (below) in the three main D z fold localities of the shear zone. base of the shear zone. Curved hinges are common and give rise to conical or sheath folds and to eye­shaped structures. The interlimb angle distribution shows that D z folds are very tight to isoclinal (Fig. 2). The fold geometry has been analysed using Hudleston's (1973) classification (Fig. 3). Amplitudes show a higher mode in Punta das Cabras and Punta Morago than in Punta Riomar. There is a great diversity in fold shape. The analysis of fold layers has also been carried out using Ramsay's method (1967), modified by Bastida (1993) (Fig. 3). This analysis shows that in Punta das Cabras the large majority of folds are subsimilar, without geometrical differences between adjacent folded layers of different lithology. Class lC folds are most common in the other two localities, with alternation of class 3 and lC folds that imply a difference in mechanical behaviour between layers. Several mesoscopic Type 3 fold interference patterns (Ramsay 1967), pre­dating D 3 , have been recorded in Punta Riomar. At this locality, two superimposed generations of microfolds also deform the previous 51 foliation. This suggests that progressive superimposed folding took place during the evolution of the shear zone. Folding mechanisms The geometry of the folds, together with the type of deformation that affected the shear zone suggest that a significant part of the fold evolution resulted from passive amplification of pre­existing folds or irregularities in non­coaxial flow. Evidence for a passive mechanism operating during at least the later stages of D z folding is provided by the broad similarity between quartz c­axis The Mondofiedo Nappe basal shear zone orientation patterns in limbs and hinges of folded quartzite layers. An example is shown in Fig. 4. The presence of sheath folds provides further evidence of non-coaxial progressive deformation (Quinquis et at. 1978, Cobbold & Quinquis 1980, Ramsay 1980, Skjernaa 1989). The initiation of D 2 folding would have been enhanced by the orientation of So in the shortening field of deformation, due to obliquity between So and the shear plane (Ramsay 1980, Martinez-Catalan 1985). The origin of pre-existing irregularities necessary for fold amplification in shear zones has been a matter of discussion (Berthe & Brun 1980, Cobbold & Quinquis 1980, Lister & Williams 1983, Platt 1983, Ghosh & Sengupta 1984). The lack of periodicity in the case of anticline-syncline couples (mainly present in Punta das Cabras) suggests an initial mechanical instability which does not necessarily derive from a buckling mechanism. Rather, it could derive from an instability in the flow, producing local increases in the shear deformation rate (Lister & Williams 1983, Platt 1983). If buckling had taken place it would have stopped operating at a very early stage since its evolution, always implying propagation by wave lateral aggregation, should have involved a periodicity (Biot et at. 1961, Biot 1965, Cobbold 1975). In those cases where there is a periodicity and where the behaviour of competent and incompetent rocks is different (mainly Punta Riomar), buckling seems to be the fold initiation mechanism. However, the geometry of the folded layers suggests a remarkable kinematic amplification. Therefore, it seems unlikely for buckling to have given rise, in this zone, to very tight or isoclinal folds as described by Ghosh & Sengupta (1984) in their 1409 study of folds of the Kolar Gold Field (India). These observations suggest slightly different folding evolution patterns for the different localities of the shear zone. Passive evolution seems to have played an important role in Punta das Cabras, while in Punta Riomar.buckling must have been more important. Punta Morago appears to be intermediate between the two. The differences in evolution may be conditioned by lithological differences and, perhaps, by small differences in pressure and temperature, which are higher towards the base of the nappe and decrease as the deformation continues. As regards fold hinge orientations, the asymmetric character of most projections (Fig. 2) with respect to the direction of shear suggests a slight initial obliquity between So and the shear plane (Martinez-Catalan 1985). The gradual approach of the projection maxima in sectors IV, III and I (Fig. 2) to the shear direction and the increase in dispersion suggest stronger deformation of the plane containing the hinges toward the basal part of the shear zone (Sanderson 1973, Williams 1978). Lineations A well developed mineral lineation (L m ) commonly occurs on the mylonitic foliation along most of the shear zone, mainly in its lower part (Fig. 5). The lineation is defined by the dimensional preferred orientation of micas, especially biotite, and quartz aggregates. The lineation plunges gently towards the east, and is usually folded by later D 2 structures. This folding is especially evident in Punta Riomar. In the upper part of the shear zone, the most frequent lineation is an intersection NORMAL LIMB ­­­­­.Lm N=209 N = 212 Fig. 4. Quartz c-axis fabrics for limbs and hinge of a minor D z fold. Locality 162 (Fig. 6). 1410 J. ALLER and F. + + .. BASTIDA ＫｾＮ ..... rJ + ... ... . + . . + SE . .. + . .- Fig. 5. Distribution of L m (above) and shear band poles (below) along the shear zone. Contours: 1, 2, 4 and 8%. Abbreviations and ornamentations as in Fig. 2. lineation between the regional foliation S2 and So, which is parallel to D 2 fold hinges. Quartzite microstructures With quartz microstructures it is possible to distinguish several areas of mylonitic development along the shear zone. There is also a marked change in microstructure development upwards, away from the basal thrust. In quartzitic layers near to the basal thrust (Fig. 6) the foliation is defined by small grains (Fig. 7a) with straight grain boundaries and a strong shape preferred orientation. Intracrystalline deformation microstructures are scarce. These features seem to indicate a total and fast dynamic recrystallization resulting in an overall decrease in grain size. The microstructure of these rocks is that of an ultramylonite. A mylonitic foliation is also developed in Punta Morago. Here, small equidimensional recrystallized grains ｣ｯｮｳｴｩ ｵｾ･ a dominant matrix in which elongate old grains with a strong preferred shape orientation define the foliation. To the west of Punta Morago, the transition to a mylonitic foliation can be seen as weakly oriented old grains become strongly oriented. Quartz ribbons are the cores of the old grains that remain unrecrystallized. Above the ultramylonites of the basal part, intracrystalline deformation features such as deformation bands and subgrains are well developed. On the whole, the grain size is larger than in the more basal quartzite and the foliation is less well developed (Fig. 7b). The larger grain size together with the less well developed preferred grain shape orientation seem to indicate a lower stress, at least during the final stages of microstructural development, than in quartzite near the basal thrust. Further to the east (higher in the shear zone), quartz grains show a weak to absent preferred grain shape orientation. The main foliation is defined by a small amount of long mica crystals with a grain shape orientation (Fig. 7c). Usually, these samples are predominantly coarse grained with irregular grain boundaries, but not strongly lobate or serrated. Grain size decreases towards the eastern part of the section. Subgrains and deformation bands are abundant and a minority of small recrystallized grains with very few deformation microstructures also occurs. The quartzite commonly exhibits a weak foliation oblique to the main one, which formed by local dynamic recrystallization (Fig. 7c). Similar microstructures, although finer­grained, have been presented by Lister & Snoke (1984, figs. 4e and 10d), who interpreted them as recrystallized type II S-C mylonites. Well­oriented biotite ribbons (long and thin mica fish in Lister & Snoke 1984) suggest intense deformation. All these rocks on top of the basal ultramylonites can be considered as blastomylonites, in the sense defined by Sibson (1977). Non­mylonitic tectonites occur predominantly in the eastern half, and also above the shear zone (Fig. 6). The boundary with the preceding group is at the andalusite isograd. Non­mylonitic tectonites also appear in the footwall. The quartz preferred grain shape orientation is IG IG ; (1l ｾ o N=203 PC e 20 NW ｾ PR Ｚ ＾ ＼ Ｚｾ Ｚ ＼ｾ＾ Ｚ ｾＺ＾Ｂ＼ ｾｯＺ＾ ｡Ｚｯｾ 43 41/47 125 "" --i 11 1 I 010 0 0 ｾＧＢ I 'C- '­­ｾ ::l ___ "­­ '­­ PM --:::'" ----___ "----- ＬＧＭｾＷＶ ::­1­­­­­­­ Ｍ Ｌ ＧｩｾＢ ｾＭｬ 142 108 '1 "-- '- '-.. i ­ ­ ­­­ｾ 5 ,­­­­­­­ '­··.[­­­t­­­­­­___ SE RF RO 95 ｾＮ ｾ ｾ ＭＺｩｾ Ｎ 164 162/ ­ ­ ｾ ++: >:' I _" ... .................. >;:-.::"t 7:" b . ro::o71L C' d r:-'l M' C· f":":iI • ｾＹｳ Almandine 0 l.L.Ll recam nan ｾ ower an ana L:..;J Iddle andana IL.....:.IUpper Candana _ .._ ..­ A d I t ' n a USI e IT7lp Quartzite ｇｉｾ ｾ ｟ｾ ＬＢＭ Ｑ Ｏ ＮＭｌ Ｏ ｟ ｌ ＮＺＯ ｦ ｌＭＧｾ IG SG CG SG SG ｾ Schists Ｐ＾＿ｾ Ｔ］ SG ｾ Blostomylonites ｾ Non­ Mylonitized ｾｑｵ｡ｲｴｺｩ ･ , ｾ I Z (1l cr" ｾ < '"'' I:ll '"e:.- SG rg. ｾｊｇＺ CG go (1l e: ｾ N o ::l S1 Cleavage MAIN FOLIATION IN SCHISTS _ _Mylonites ｾ 1 ::ll (1l (1l ｾ QUARTZITE ｾ go Phyllonltlc ['E:>j Schistosity with ｾ microfolds Schistosity DMOInly schists N=205 N=215 N=208 Fig. 6. Quartz c-axis fabrics (lower-hemisphere, equal-area projections; contours: 0.5, 1,2,4 and 8%), showing locality and fabric type (foliation is the vertical E­W plane and L m is horizontal), isograds, quartzite tectonite types and schist foliation types along the shear zone. Abbreviations as in Fig. 2. N=219 ,... ,... ,... .j:>. 1412 J. ALLER and F. BASTIDA weak and crystal-plastic deformation features in the grains vary, though in most cases deformation bands and undulose extinction are common (Fig. 7d). These rocks differ from the blastomylonitic rocks in that they do not show the microstructural features of S-C mylonites. Main foliation (Sl +z or Sz) in micaschists Micaschists in the shear zone have a high quartz content which usually gives a heterogeneous domainal character to the foliation. It is possible to distinguish two microstructural types: phyllonitic foliation and schistosity. Phyllonitic foliation (Sz) occurs at the base of the shear zone (Fig. 6) and is formed by very fine-grained minerals; biotite has a strong grain-shape preferred orientation and quartz domains have a mylonitic character. Schistosity presents scarce microfolds mainly in two zones (Fig. 6). In Punta Riomar, the main foliation associated with D z microfolds is crenulated by late D z microfolds. Shear band foliation Small extensional shear bands within the schists are common in the shear zone (Fig. 5). Their spacing ranges from several decimetres to less than 1 em. Locally they appear as very short cleavage domains associated with gentle folding. They also occur as mesoscopic bands, which locally become small-scale normal faults that can also be interpreted as riedels. The angle between bedding and shear bands ranges in most cases between 20° and 40°. Conjugate sets of shear bands are locally present. In such cases, one of the sets is clearly dominant. Shear bands commonly dip east or southeastwards (Fig. 5). However, there are also cases where the shear bands dip westwards, presenting a reverse sense of movement. Field observations suggest a good correlation between the shear band orientation and their position on different limbs of the major D 1 folds. In the normal limb the bands dip east or southeastwards, while in the reverse limbs they dip westwards. This means that the orientation of the anisotropy controls the shear band orientation. If shear bands are formed in some geometric relationship with the shear direction as several authors have proposed (Platt 1984, fig. 1, Weijermars & Rondeel 1984), the orientation of the shear bands can be explained by deformation partitioning (Lister & Williams 1983) (Fig. 8). In reverse limbs, the anisotropy represented by bedding is oblique to the shear direction and the shear bands can be related to slip or flow parallel to the anisotropy due to strain partitioning. If this is so, major D 1 folds would have gone on tightening during the late stages of D z owing to a flexural-flow or flexural-slip component generated at the reverse limbs. At the same time, the reverse limbs would have become thinner because of a coaxial component of flow. Shear bands give a lenticular or sigmoidal microstructural aspect to the main foliation. During shear band development biotite and chlorite, and less commonly quartz, crystallized along the shear plane. The former have a well-developed preferred grain-shape orientation. Quartz commonly forms fine-grained aggregates that define very narrow bands parallel to the shear bands, enhancing the mylonitic character of the rock. The presence of chlorite implies that the shear bands developed during retrograde metamorphism that can be correlated to the M 3 episode. These shear bands also affect D z folds and late D z biotite porphyroblasts. According to this, shear band formation represents a very late episode in the formation of the Mondoiiedo Nappe basal shear zone. Quartz c-axis fabrics The quartz c-axis orientation in quartzite samples from the shear zone and from adjacent zones has been analysed using a universal stage. Figures 4, 6 and 10 show the diagrams obtained for the shear zone and for the rocks that immediately overlie it, while Fig. 9 illustrates the results for samples from the footwall. According to the position of maxima and the symmetry relative to the foliation (Lister 1977, Schmid & Casey 1986), the following types of fabrics can be observed in these figures. (1) A random fabric (R) (sample 301). (2) Fabrics with orthorhombic symmetry (0). One of the samples (302) presents a maximum subperpendicular to the foliation and two other samples (B-8 and B-9) present maxima that are symmetrical with respect to the vertical N-S plane. (3) Monoclinic small-circle girdle fabrics (SG) (samples 8, 43, 108, 125, 142 and B-2B). The best developed maximum (or pair of maxima) forms a smaller angle with the foliation than that of the other maxima. (4) Monoclinic type I crossed girdle fabrics (CG). The line linking the inclined maxima can be orthogonal to the foliation (samples 20, 76, 95, 162 and B-4) or oblique (samples 11,300 and B-2). In some cases, one of the branches of the crossed girdle is poorly developed. (5) Monoclinic incomplete single girdle fabrics (IG) (samples 25, 41, 47 and 74). These have a sigmoidal trend with two inclined maxima, except for sample 74 which has a single centred maximum. There is a complete transition between this fabric and type CG. Quartz c-axis fabrics along the shear zone display a variable degree of c-axes dispersion, with Rand 0 fabrics having the largest amount of dispersion and IG the least. The amount of dispersion increases away from the core of the shear zone (Figs. 6 and 9) and corresponds to a gradation from higher to lower strain, and higher to lower metamorphic grade. Some of the quartz c-axis fabrics from outside the shear zone (R and 0 types), and therefore not affected by D z , show a symmetry consistent with a coaxial deformation (cf. Raleigh 1965, Tullis et al. 1973, Etchecopar 1977, Lister & Hobbs 1980). c-axis fabrics from inside the shear zone have a monoclinic symmetry (SG, CG and IG types) consistent with rotational strain due to the ....., ::r' (1) 3:: o ::l 0- o ::l. (1) 0- o Z t» '0 '0 (1) crt» t» C/O C/O ::r' (1) .., t» N o :::l (1) Fig. 7. Different types of quartzitic tectonites. (a) Ultramylonites in the basal part. (b) BlaslOmylonites in Punta das Cabras. (c) Blastomylonites (recrystallized S-C mylonites) from the lower half of the shear zone; note the thin, long and preferentially oriented micas and the weak oblique foliation in quartz. (d) Non-mylonitic quartzite from the upper part of the shear zone. All sections normal to the foliation and parallel to L m . Scale bar for photographs = 0.5 mm. The Mondonedo Nappe basal shear zone w E Fig. 8. ｧｮｩｯｴｩｲ｡ｾ of simple shear deformation component in the reverse limb of D 1 folds to form shear bands (s.b.) with the opposite movement sense to the ones in the main shear zone (see text). simple shear component (cf. Bouchez & Pecher 1976, 1981, Etchecopar 1977, Lister & Hobbs 1980, Simpson & Schmid 1983). However, there are some samples outside the shear zone that also have a monoclinic symmetry (SG and CG types). Samples B2B and B4 present fabrics very similar to those in the shear zone and can thus be interpreted as local manifestations of D z· Peripheral maxima in the position found in our samples are indicative of low-temperature basal slip, ｾｨ ｲ･｡ｳ peripheral and inclined or centred maxima may ｉｮ､ ｾ｡ｴ･ basal and rhombohedral or prismatic slip at medIUm temperature, all along an <a>-axis (Nicolas & Poirier 1976, Vernon 1976, Bouchez 1977, Nicolas et ai. 1977, Bouchez & Pecher 1981, Hobbs 1985). Inclined or centred maxima suggest higher temperature <a> slip on rhombohedral or prismatic planes (Bouchez & Pecher 1981, Hobbs 1985). The different slip systems active in quartz across the shear zone agree with the temperature changes recorded by the metamorphic assemblages (Fig. 6). The distribution of c-axis maxima in CG fabrics, in the context of a retrograde metamorphism, suggests that these fabrics may be associated to a drop in temperature. This agrees with c-axis orientations of old and new grains in a mylonite from Punta Morago (Fig. 10). New recrystallized grains have an increased dispersion of c­axes, associated with the activation of basal slip. Although there is no one­to­one correspondence between microstructural and c­axis orientation types, a correlation of the two features of the rock fabric has been established (Fig. 6). Rand 0 fabric always correspond to non­mylonitic tectonites. SG fabric commonly occurs in non­mylonitic tectonites, although it is also found in basal ultramylonites and, rarely, in blastomylon.ites..CG ｦ｡｢ｲｩｾ presents a wider microstructural variety SInce It occurs In all the microstructural types described here; this probably means that it has a transitional character between SG and IG types. Finally, IG type is recorded only in blastomylonites. SHEAR ZONE MOVEMENT DIRECTION Two types of criteria have been used: those that do not provide information on the movement directional sense 1415 like the mineral lineation, shear band foliation fold hinges and conical fold apical directions, and ｴｨｯｳｾ that do supply data about the movement directional sense like fold asymmetry and facing, oblique foliation by dynamic recrystallization in quartzite and quartz c­axis distributions. In order to visualize the movement direction, the direction of the first eigenvector of Bingham's distribution (Whitten 1966, Cheeney 1983) has been projected in Fig. 11 for several criteria in different sectors along the shear zone. Movement direction indicators The mineral lineation (L m ) is folded in some cases by D z folds. Hence, it does not record all the D z deformation. In addition, the folding of L m causes its dispersion, which can be mainly observed in Punta Riomar (Fig. 5, localities 40­47). However, quartz c­axis fabrics show symmetries that generally agree with the position of this lineation, indicating that L m agrees with the movement direction in late stages of the deformation. The first eigenvector of L m distributions has a direction between N75°E and N115°E (Fig. 11), that is, with a dominant eastward component. The relationship between the position of the shear bands and the type of major fold limbs (D I) on which they develop implies that it is difficult to use the shear bands as a shear­sense indicator. However, shear band formation is related to shear zone deformation and therefore provides information about its movement direction during the late stage at which shear bands formed. Since So approximately coincides with the shear plane, the direction contained in So which is perpendicular to the intersection between So and the shear bands was used as a direction movement indicator. The first eigenvectors of the corresponding distributions have a fan­like distribution whose directions range between N65°E and N1400E (Fig. 11). Fold hinge directions cannot be considered here as a good criterion to determine the shear direction, because the corresponding eigenvectors do not agree with those of the rest of the criteria, except for sector 1 (Punta das Cabras) (Fig. 11). The cause of the hinge directions dispersion has been discussed above. Geometric and experimental models indicate that apical directions of sheath folds can be used to deduce shear direction regardless of means of fold nucleation (Cobbold & Quinquis 1980, Ramsay 1980). The directions of the first eigenvector of conical folds apical directions for different domains of the shear zone define a fan of directions between N700E and N115°E (Fig. 11). Shear-sense indicators .The strongly asymmetric D z folds always have a facing WIth an eastward component. This provides strong evidence for eastward shear sense for conical folds apical directions. Oblique foliations by quartzite dynamic recrystallization are common in our samples (Fig. 7c). This criterion (Means 1981) mostly indicates an eastward 1416 J. ALLER and F. BASTIDA Fig. 9. Quartz c-axis fabrics (lower-hemisphere, equal-area projections; contours: 0.5, 1 and 2%) in the footwall of the Mondonedo Nappe basal shear zone. Numbers refer to locality and distance in metres below the basal thrust. movement directional sense. Asymmetry of the main girdle is the most suitable criterion in quartz c-axis distribution diagrams in this shear zone (criterion 2 in Simpson & Schmid 1983). In most cases, the asymmetry indicates a shear sense with a dominant eastward motion of the hanging wall. Other criteria, such as those using the fabric skeleton or the leading edge (criterion 3 in Simpson & Schmid 1983) usually indicate the same shear direction. The kinematic criteria are, in general, in good agreement with a movement direction between N75°E and N115°E for the shear zone. Figure 11 shows that only two points corresponding to folds from two sectors are deviant. However, the distribution of kinematic criteria ALL GRAINS L--_-----. L m OLD ｭｌｾＭ ｾ N=151 m ｾＧＭｌ N= 210 Fig. 10. Quartz c­axis fabric for a quartzite sample from Punta Morago (locality 76, Fig. 6). Fabrics for old and new grains are shown separately for the same sample. Contours: 0.5, 1,2,4 and 8%. 1417 The Mondonedo Nappe basal shear zone 2 + • Lm , • Fold hinges, + Shear bands foliation • Apical directions of conical folds NW SE PC PM PR . . . ＱＳ ｾ RF 5 4 Fig. 11. First eigenvector pattern of the movement direction distributions deduced from different criteria along the shear zone. Abbreviations as in Fig. 2. in Fig. 11 does not allow deduction of temporal or spatial systematic changes in the shear direction. SHEAR ZONE STRUCTURAL MODEL The Mondonedo Nappe basal shear zone shows evidence for strong ductile deformation which is mainly due to a simple shear component approximately parallel to the normal limbs of earlier major D} folds. Evidence for this deformation is given by the development of several types of structures that occur at meso- and microscopic scales and that imply a general movement towards the foreland (eastwards). On the whole, these structures display some spatial variation that suggests a decrease in D 2 strain towards the upper part of the shear zone. However, this decrease presents some remarkable anomalies due to the existence of bands with a large number of folds (Fig. 12). Apart from ductile deformation, it is worth mentioning that the shear zone is cut at the base by a major brittle thrust nearly parallel to the shear plane. The shear zone developed in an area with a preexisting metamorphic gradient (M}). During an eaFly period of ductile deformation in the shear zone, an M 2 episode developed. The characteristics of the D 2 fabric are influenced by the intensity of this metamorphism. The M 2 isograds are later cut by the basal thrust during retrograde metamorphism (M 3) associated with uplift of the nappe. The distribution of structures within the shear zone allows a division into three parts: basal, middle and upper. The basal part (Fig. 12) is characterized by the presence of phyllonites and some ultramylonites with SG type quartz c-axis fabrics. This implies strong ductile deformation in a late stage at low temperature near to the basal thrust related to retrograde metamorphism. The middle part, whose eastern boundary coincides with the andalusite isograd (Fig. 12), is characterized by the presence of blastomylonites and dominant IG and CG quartz c-axis fabrics, which suggest a deformation in amphibolite facies. Another notable feature is the pres- METAMORPHISM ANDALUSITE ALMANDINE I I BIOTITE MAIN FOLIATION IN SCHISTS , ｉｍｃｒｏｆｌｄｾ SCHISTOSITY S2 S2 ­ 1.1 SCHISTOSITY PHYLLONITIC (S2) S, CLEAVAGE MICROFOLDS QUARTZITES MYLq] MAINLY BLASTOMYLONITES 1 NON ­ MYLONITISED QUARTZITE LOCALl'MYLONITES QUARTZ C AXIS FABRICS SGI MAINLY CG and IG 1 SG and CG I 0 f' LOCAL SG MAIN O2 FOLDS AREAS I ｾｈｒｕｓｔ ASAL ｾＭ･ｳ ｬ J ｾ PC • PR II ｾｒ PM MIDDLE _I__­­­­­'U'­'­P­'­­P=­:ER­'­­­­ +! Fig. 12. Scheme of the Mondoiiedo Nappe basal shear zone showing position of its basal, middle and upper parts, according to variations in metamorphic grade, foliation type, quartz c-axis fabrics and distribution of D z folds. Abbreviations as in Fig. 2. _ 1418 J. ALLER and F. ence of three bands with peculiar structural characteristics. The first band is located in Punta das Cabras, where a large amount of subsimilar folds appear. The second band (Punta Riomar) contains a large'number of D z folds with variable geometries (Fig. 4) and interference patterns of Type 3; L m is frequently folded and quartz c-axis fabrics are SG here. The deformation of this band appears to have a long history that includes late movement during retrograde metamorphism. The third band (Punta Morago) contains a large number of folds and mylonites with quartz ribbons; mylonitization here took place during retrograde metamorphism although earlier than in the previous band. The upper part of the shear zone (Fig. 12) is featured by the presence of nonmylonitic quartzite and by SG and CG c-axis fabrics, indicating deformation at a temperature that was on the whole lower than in the middle part. This is consistent with data on the metamorphism. There is no clear upper boundary to the shear zone, but a gradual transition into rocks that do not display shear zone textures. The boundary has been located at Ria de Foz (Fig. 12). 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