Axial Plane Schistosity Research Articles

Evolution of the south-central segment of the Archean Abitibi Belt, Quebec. Part II: Tectonic evolution and geomechanical model

In this paper, we describe the relations between the paleogeographic and tectonic evolution of the southwestern part of the Archean Abitibi and Bellecombe belts. Volcanism in the Abitibi Belt created a very thick, anisotropic plate composed of competent volcanic rocks and broken by the Duparquet–Destor break. The depocenters of the upper division of diverse volcanic rocks subsided about 10 km relative to their surroundings, and some central volcanic complexes within this division were consolidated by synvolcanic plutons and their thermal metamorphic aureole. The Cadillac break, a normal fault, separated the Abitibi and Bellecombe belts. The latter consisted of comparatively incompetent sedimentary rocks on top of a basement composed of ultramafic–mafic flows.North–south compression of the volcanic terrain during the Kenoran Orogeny produced a set of flexure folds, F1, that curve around the consolidated cores of central volcanic complexes generally in an easterly direction. Synclinoria nucleated at the deeply subsident depocenters of the upper diverse division. Further north–south flattening and subvertical stretching produced the east-trending F2 folds, their axial-plane schistosity S2, and local superposed schistosities S3 and S4. Southward verging recumbent folds suggest that the Bellecombe Belt simultaneously was pulled northward below the Abitibi Belt. During the orogeny, the Duparquet–Destor and Cadillac breaks were transformed to thrust faults; the Duparquet–Destor break also shows minor (< 3 km) right-lateral strike slip. Diapiric rise of late- to post-kinematic plutons locally distorted earlier schistosities.

Structural geology of the Pelona Schist and Vincent thrust, San Gabriel Mountains, California

The Vincent thrust of the San Gabriel Mountains, southern California, separates an upper plate that contains abundant granitic rocks from a lower plate composed of metagraywackes and metabasites (Pelona Schist). The Vincent thrust is part of a regional thrust system of Late Cretaceous-early Tertiary age (Vincent-Chocolate Mountain thrust). Several earlier workers have concluded that the upper plate was derived from the southwest, implying the former presence of continental rocks separated from North America by a marginal oceanic basin. Other workers consider that regional correlations indicate thrusting of the opposite sense. The Pelona Schist of the San Gabriel Mountains has undergone a complex structural history. The earliest recognized structures are tight, synmetamorphic isoclinal folds (style 1 folds). These folds are present throughout the entire 4 km of presently exposed section and do not become more appressed or more abundant near the thrust. Isoclinally refolded folds and sheath folds were observed at several localities. Textural observations indicate that the present schistosity of the Pelona Schist formed by the extreme flattening of crenulations developed in an earlier schistosity. This is evidence that the isoclinally refolded folds, although only infrequently noticed in the field, are actually widespread. The isoclinal folding was followed by the development of a macroscopic synform (Narrows Synform) and associated minor folds in the 700 m of Pelona Schist closest to the upper plate. These folds (style 2 folds) are more open than the style 1 folds and do not have a well-developed axial-planar schistosity. They are generally parallel in orientation to the style 1 folds. Both style 1 and style 2 folds are in turn parallel to tight folds in mylonitic rocks present at the base of the upper plate. It is believed that the three groups of folds are cogenetic and possibly were produced in a subduction zone. The sheath folds and other evidence of stretching parallel to fold axes might indicate that fold axes were rotated into the direction of thrusting. On the other hand, the parallelism of the early, isoclinal folds and the later, relatively open style 2 folds is evidence against rotation. Thus, the orientation of fold axes relative to the thrusting direction is not clear. Subsequent to metamorphism, the Pelona Schist and the mylonites were folded into a broad arch (style 3 folding). Uncertainty as to the orientation of the axes of style 1 and 2 folds relative to the thrusting direction, the presence of the style 3 folding, and recently described paleomagnetic evidence of large-scale rotation of crustal blocks in southern California make determination of thrusting direction impossible at the present time. Thus, no evidence was found either to support or to invalidate the marginal-basin model for the origin of the Pelona Schist.

Large Scale Basin and Dome Structures in the High Grade Metamorphics, Near Visakhapatnam, South India

Abstract Large scale basin and dome structures with intense buckling in some layers are recorded in the Archaean high grade metamorphic-migmatitc complex of the Eastern Ghats near Visakhapatnam, South India. Two major phases of deformation have been deciphered. The earlier phase was regionally continuous, and resulted in nonplunging, tight and isoclinal (overturned) folds, as well as gently plunging elongate structures. The axial trends of these folds vary from NE-SW to ENE-WSW. Gneissic layering of rocks, development of axial plane schistosity and tight isoclinal angular parasitic minor folds, and large scale shearing and faulting in the axial regions, are correlated to this event. The later phase of deformation was discontinuous and variable in nature. Southerly/southeasterly plunging, open to tight, normal, similar folds belong to this event and they were superposed on the earlier structures. They modified the pattern and plunges of the latter, and also produced intense buckling or gentle warping locally at several places. Development of warp-like open minor folds, and groove and mineral lineations are correlated to this deformation. On the basis of critical synthesis of the available structural data, the evolution of the Balacheruvu basin and Madhuravada dome has been attributed to a single deformation of the earlier phase of folding.

Deformation and metamorphism of the Aileu Formation, north coast, East Timor and its tectonic significance

The large block of metamorphic rocks along the north coast of East Timor is of special interest as it occurs at the boundary between continental and oceanic crust in an island arc-continent collision zone. A detailed study of the structure and metamorphic history of 400 km 2 of this formation showed it has a complex history of penetrative deformation but the structure is coherent. Pelites, psammites and limestones interlayered with dolerites and amphibolites have been metamorphosed in a medium pressure environment. They now form a metamorphic province zoned from low greenschist facies in the southwest to upper amphibolite facies in the east. The earliest recognised deformation phase predated the metamorphism and produced a widespread layer—parallel schistosity but no recognisable folds. The second deformation phase post-dated the metamorphic maximum and micropetrological evidence indicates a gradual cooling during this event. This deformation produced tight folds with an axial plane schistosity and transposed the earlier structures. The progressively weaker third and fourth phases developed crenulation cleavages and related folds, under greenschist facies conditions. Open, fifth phase, macroscopic folds were probably synchronous with strike slip faulting parallel to the north coast. Later dip slip faulting juxtaposed the Aileu Formation with Permian and Mesozoic sediments of very low metamorphic grade. Reconnaissance K/Ar radiometric dating using hornblende and biotite showed the prograde metamorphic maximum occurred before 11 Ma ago and implies that the second, and strongest, deformation phase occurred in the late Miocene. This young age establishes the relationship of the deformation events to the collision between Australia and the Inner Banda Arc. The proposed models for the structure of Timor must be modified to fit the deformation history of the Aileu Formation. If Timor is essentially autochthonous, the Aileu Formation was probably deposited in a Palaeozoic graben and the metamorphic maximum may have occurred in the Jurassic. The overthrusting models must be modified in the light of the close correlation in time between penetrative deformation and emplacement of the proposed thrust sheets. The analogy proposed between Timor and ‘normal’ convergent margins is not supported but it may be possible to draw analogies with the Molucca Sea.

Sequence and style of major post-nappe structures, Simplon—Pennine Alps

In the Upper Pennine nappe complex of the Simplon—Pennine Alps (Switzerland and Italy), at least three phases of major post-nappe folding (in places associated with thrusting) can be distinguished. These are superimposed on an earlier-formed, partly chaotic, complex of tectonic units, including the Bernhard and Monte Rosa continental flakes and the Zermatt—Saas and Antrona ophiolite complexes. The earliest post-nappe folds were essentially isoclinal throughout the whole region and were accompanied by a strong schistosity which is the main foliation in most areas. Later, two successive phases of back-folding led to the present overall structure. Both phases typically show rapid variations in style from open folds lacking axial planar schistosity to very tight structures with complete foliation transposition. This has been demonstrated by systematically mapping the major axial traces over the whole region. Successively removing the major structures in reverse order shows that the ophiolite complexes were originally part of a continuous unit marking an important suture between the Bernhard and Monte Rosa nappes.

Evolution of the sulfide ores of Saladipura, Rajasthan, India

At Saladipura stratiform sulfide ores occur in the Ajabgarh group of the middle Proterozoic Delhi Supergroup. Two coaxial buckle folds and another transverse one have deformed the rocks in the area, but no pronounced fault or shear zone is developed. The rocks evolved through a low-pressure intermediate facies series of metamorphism to a stage where the condition for the coexistence of the triple alumino-silicate polymorphs was closely approached, at about 600 degrees C and 5.5 kb. The origin of the amphibolitic rocks associated with the ores is indeterminate. They may represent a metamorphosed pile of volcanic and avolcanic rocks. Granite in the area, anatectic in origin, was emplaced at a late kinematic stage.Several orebodies occur in the area, of which the largest, defining a plunging synform (syncline), is traced out over a strike about 7 km long. The orebodies have been affected by all the deformation episodes. Different small structures in the ores are discussed. Pyrite and pyrrhotite are the dominant sulfides followed by sphalerite. Coexisting assemblages of ores and their metamorphic fabrics are critically analyzed. The wall rocks of the orebodies are composed of either cordierite + orthoamphibole(s) + quartz + or - scapolite, or muscovite + biotite + quartz + or - andalusite. The following features inter alia suggest the ores to be of sedimentary-diagenetic origin: very thin lamination in the ores, parallelism of the orebodies with the primary laminations in the associated (meta-) sediments, slump structures in ores, buckle-folding in the ore layers with the development of pyritic axial planar schistosity, virtual absence of replacement features between the ores and the silicate, and close association with carbonaceous matter. Sedimentation took place in a near-shore anoxic environment.The ores are metamorphosed. The invariant assemblage, pyrite + pyrrhotite + arsenopyrite (+ (As-S) l + V), and mole percent FeS in sphalerite coexisting with pyrrhotite suggest that the temperature and pressure of ore metamorphism were about 600 degrees C and 5.5 kb, respectively. These data agree well with those obtained from the metamorphosed host rocks. The growing idea that sulfur is internally controlled during metamorphism is upheld and it is concluded that, in ores rich in pyrite, a pyrite [rightharpoons] pyrrhotite (+ S 2 ) break-down should be negligible even in high-grade metamorphism. In the equilibration of Fe-silicates coexistent with sulfides, sulfidation was unimportant. Pegmatoid ores result from the circulation of volatiles emanated during granitic-pegmatitic activity.

Geometry of Superposed Deformations in Hesadi-Tebo Area, Western Singhbhum, Bihar

Abstract The study reveals three generations of deformation which have produced two splanes and a host of linear structures with complex geometric relations. All the three fold systems are, however, not equally developed all over the area and the associated structures also regionally vary in orientation and angular relation; this has resulted in diverse geometric patterns in different subareas. The first deformation produced subhorizontal folds (F1) on bedding (S1) with strongly developed axial plane schistosity (S2). These folds are refolded, along with S2, about a set of steeply plunging axes (F2) at high angles with the F1 axes (60° to 90°). The main structural closures defined by Hesadi antiform and Kamrora synform are caused by the second folds (F2). On these structures are superposed a subhorizontal E-W trending set of folds (F3) with northerly dipping axial planes, resulting in dispersal of all the earlier lineations. The F3 folding was also accompanied by shearing in several tracts producing shear cleavage (S3) with downdip striation and groove lineations. The F3 folds show somewhat regular plunge reversals on macroscopic scale forming highly elongated domes and basins. An attempt has been made to restore the original attitudes of the fold systems.

Distinguishing between inter-kinematic and syn-kinematic porphyroblastesis

Porphyroblasts which include a planar fabric (Si) which can be continuously traced into the matrix schistosity (Se) with or without an angle between Si and Se, and where Se bows around the porphyroblasts, are commonly described in the literature as interkinematic. This paper concludes that these porphyroblasts may very well be synkinematic. It is argued that porphyroblasts commonly grow rapidly relative to the rate of deformation of the rocks in which they lie. A specimen of a tightly folded micaschist containing garnet and biotite porphyroblasts is described in detail. Regional study indicates that the fold developed through a less tightly folded ?phyllite with an axial plane crenulation cleavage. At one stage the porphyroblasts grew rapidly, and some larger ones incorporated helicitic crenulations while others contain planar inclusions. Subsequently further tightening occurred and a new axial plane schistosity developed. Thus, as the fold appears now some porphyroblasts are in accord with the above description of interkinematic porphyroblasts, and yet they are all synkinematic.

Tectonic relationships between the Proterozoic Gawler and Willyama orogenic domains, Australia

Stratotectonic and morphotectonic data from the two principal exposed domains (pre-Adelaidean rocks) of the Gawler sub-province are used to characterize the Proterozoic Olarian orogeny and to distinguish its effects from those of the later Phanerozoic Delamerian orogeny. The principal metasedimentary sequences in the Gawler domain and in the Willama domain are inferred to have been deposited in a single broad zone of early Proterozoic shallow-water sedimentation on older (presumed Archaean) continental crust. The sequence becomes more pelitic upwards and may be interpreted as a transgressive sequence with more distal facies to the east. Three main phases of deformation are recognized, and each phase has similar characteristics and age in both domains. D1 2nd D2 can be dated between 1850 and 1650 Ma, while D3 appears to be about 1650-1540 Ma. In high grade rocks, D1 gave rise to a layer-parallel schistosity, while D2 is characterized by tight folds with a high-grade axial-plane schistosity. The whole subprovince was characterized by high geothermal gradients so that medium- to highgrade metamorphism affected the lower parts of the succession before and during the D1 and D2 deformation episodes. No distinct tectonic zones can be recognized but large-scale stratigraphic inversions (i.e. nappe tectonics) during D1 have been recognized only in the east of the Willyama domain. The higher parts of the stratigraphic succession are generally less deformed and exhibit only low-grade metamorphism. D3 produced relatively open, upright macroscopic folds and was characteristically associated with retrogression, but was demonstrably of pre-Adelaidea n age. The Gawler domain exhibits D3 structures although it lies in the platform west of the Adelaide Geosyncline and was not affected by deformation during Adelaidean sedimentation or by the subsequent Delamerian orogeny. A network of retrograde shear zones is the principal expression of post-Olarian deformation in the Willyama domain which forms part of the basement to the Adelaide Geosyncline. The trends of D2 and D3 folding in the two domains are similar and it is shown therefore that no large-scale rotations of one domain relative to the other has been produced by the Delamerian orogeny. Large-scale translations on discrete faults or on broad zones of simple shear in the basement are not easily ruled out, but if they exist, are probably largely of pre-Adelaidea n age. However, a significant relationship between Olarian structures and variable Adelaidean fold trends has been deduced. The Olarian orogeny may have occurred in close proximity to a continental margin to the east and may thus be related to subduction processes. It differs from linear gneissic belts in Phanerozoic orogenies since it occurs in a more stable stratotectonic environment and over a wider area.

Superposed Folding in the Northern Part of the Gangpur Series, Orissa

Abstract Precambrian metasediments in the northern part of the Gangpur Series, represented by mica schist, phyllite-slate, and schistose quartzite have been investigated over an area of 50 sq km around Lase (22°21' ; 84°45') of Sundargarh district, Orissa. Structural analysis reveals that the earliest folds in bedding (S1) were reclined in nature, but they have been blurred by the profuse development of axial plane schistosity (S2). S2, however, shows upright to slightly overturned folds plunging moderately to E to ESE. The most dominant attitude of S2 nearly normal to the regional fold axis points to the reclined geometry of the folds in S1. Non-penetrative fracture cleavage surfaces (S3) have developed parallel to the axial planes of S2-folds, generally displacing the folded segments. The S3-surfaces, however, show appreciable fanning.

Stratigraphy, structure and metamorphism of Dalradian rocks of the Maumturk Mountains, Connemara, Ireland

Appin (lower) and Argyll (middle) Dalradian metasediments and associated igneous rocks are described and a formal stratigraphy proposed. The deformation history has four main phases, D 1 to D 4 . Major "nappe" structures, formed during D 1 and D 2 , were refolded by two phases of approximately coaxial crossfolding developed during D 3 and D 4 . Two of the major F 4 cross-folds, the Connemara Antiform and Joyce's Synform, superficially dominate the structure of the area. The main penetrative fabric in the rocks, an axial planar schistosity, was developed during D 2 . The deformations were accompanied by metamorphism that reached its peak (in the Maumturks) between D 2 and D 3 (MP 2 ). The rocks were metamorphosed in the amphibolite facies and show an increase in grade from north to south across the area. It is suggested that the prograde MP 2 metamorphism, with coeval growth of staurolite and sillimanite, was partly caused by heat emanating from the complex of basic, ultrabasic and migmatitic rocks located during MP 2 in the upper limb of the F 1 Lissoughter Anticline, structurally above the present day outcrop in the Maumturks and now occurring mainly in south Connemara.

Controls of Tungsten Mineralization at Chhendapathar, District Bankura, West Bengal

Abstract The paper, describes the structure and tectonic environments of the tungsten mineralization at Chhendapathar, India's largest tungsten mining district. The main structural configuration of the district developed as a result of early, cross and later fold movements during the Singhbhum Orogeny (2000-850 m.y.). The early folding generated axial plane schistosity, whereas cross fold movements produced asymmetrical folds, and the later folding gave rise to a broad zone of culmination, that was the site of an intrusion-the source of tungsten-bearing solutions, responsible for the ore deposits. Ore controlling structures like shear-slips, shear-fractures and fault-fractures were primarily formed during the period of cross folding. On mineralization, they developed into vein patterns trending: (i) N70°E, (ii) N50°w, (iii) N70°w, (iv) N50°E, and (v) N10°E. Post-ore faults have dislocated the veins, sometimes rotating their strike by about 25°.

Fabrics and Deformation of Archean Metasedimentary Rocks, Ross Lake – Gordon Lake Area, Slave Province, Northwest Territories

Metagreywackes, which lie stratigraphically above metavolcanics fringing a granitic complex (part basement), exhibit late Archean multiphase fold structures. F1 folds are large linear depressions and anticlinal culminations trending partly concordant with margins of the granitic complex. Open to isoclinal F2 folds vary and curve in trend. Though usually near upright, some F2 folds are overturned away from the complex and others towards F1 depressions. F3 folds are small-scale structures not present in most outcrops, but throughout a large region they are accompanied by a steep S3 axial-plane schistosity striking NW to N.Original orientations of an S2 schistosity, axial planar to F2 folds, are preserved as inclusion trails in biotite porphyroblasts. Angular deflections from the trails show that regional horizontal flattening across the S3 schistosity formed the F3 folds and tightened curved trends of F2 folds. Estimates of strain from the fabrics indicate no increase near the complex.As for folds in other Archean terrains, F1 and F2 folds could have developed mainly as gravity structures during diapiric uprise of a granitic basement. In contrast, the F3 structures may reflect horizontal movements of early crustal plates.

Polymetamorphism of the sulphide ores of Broken Hill, N. S. W., Australia

Two main periods of metamorphism have effected the Broken Hill base metal deposit. The first, at granulite grade, occurred at 1,700 m.y., the second, at lower amphibilite grade, occurred at 500 m.y. The earlier metamorphism correlates with two stages of intense regional folding; the latter occurs as narrow “shears” across the orebody. The prograde metamorphism caused intense brecciation, development of an ore “mush” with ore movement, formation of ore-bearing parapegmatites and boudins and much recrystallization of ore and gangue minerals. The orebody parallels an axial plane schistosity in the wall rocks with numerous ore piercement structures causing local discordancies. Ore in retrograde zones is again brecciated with galena further recrystallizing after destruction of prograde recrystallization. Gangue minerals remain essentially as brecciated fragments without further recrystallization. Secondary hydrothermal veins with rare silver minerals derived from the orebody transect the retrograde zones. Galena is plastically injected into fractures in the retrograde wall rock schists. Prograde ore shows co-recrystallization of various sulphides and gangue minerals yielding characteristic annealed textures. Quartz, garnet, hedenbergite, roepperite and apatite co-recrystallize with galena, sphalerite and chalcopyrite with balanced surface tensions. Retrograde ore shows mainly fragments of gangue and sphalerite set in a matrix of further recrystallized galena or schistose galena with a superimposed sub-grain structure. The significance of the ubiquitous sub-structures within galena is considered in terms of retrograde effects upon high grade metamorphic textures.

Geology of the Gullbridge Copper Deposit, Newfoundland: Volcanogenic Sulfides In Cordierite–Anthophyllite Rocks

The Gullbridge copper deposit occurs in a folded and regionally metamorphosed sequence of Upper Ordovician mafic volcanic and tuffaceous rocks in the Central Mobile Belt of Newfoundland. The ore consists of chalcopyrite, pyrrhotite, and pyrite in a tabular zone of cordierite–andalusite–chlorite schist that is surrounded by cordierite–anthophyilite rock. Regional metamorphism to greenschist or amphibolite fades preceded and accompanied regional de formation that produced tight folds with a strong axial plane schistosity (S1) in the host and country rocks. Contact metamoprhism related to nearby plutons superimposed the cordierite, andalusite, and anthophyllite mineral assemblages upon S1. This was followed by a second deformation and by chloritization of the ore zone. Finally the ore body underwent some minor mineralogical and structural modifications. From various field and laboratory evidence the sulfides, and possibly the associated magnesium alteration, are interpreted as volcanogenic. The Gullbridge deposit is similar to a number of other copper sulfide deposits in cordierite anthophyilite host rocks in North America and Scandinavia.

Superposed Folding in the Aravalli Rocks of the Type Area around Udaipur, Rajasthan

Abstract Around Udaipur, the Precambrian rocks belonging to the Aravalli System are represented by feldspathic greywacke grading to phyllite and mica schist, calc-arenite, and ferruginous and carbonaceous quartzites. The rocks display a high degree of structural complexity resulting from the impress of three distinct periods of folding (F1, F2 and F3). The earliest folding movement (F1) produced tight to isoclinal folds with drawn-out hinges. The F1 folds are accompanied by a set of very well developed axial plane schistosity. These folds are presumed to have been initially recumbent with E-W to NW-SE axial trend. The second fold movement (F2) refolded the F1 folds to upright, open to occasionally isoclinal folds. The F2 folds have variable plunge and an overall northerly axial trend. The third fold movement has resulted in the formation of tight to isoclinal folds (F3) of small and intermediate scales on the limbs of larger F2 folds. The F3 folds are characterized by a set of axial plane crenulation cleavages. The superposition of the three fold systems has resulted in complex interference patterns on different scales.

Origin of ultramafic rocks, their tectonic setting and history: A contribution to the discussion of the paper “the origin of ultramafic and ultrabasic rocks” by P.J. Wyllie

A subdivision of the orogenic peridotites into ophiolitic (or truly Alpine-type) and “root-zone” peridotites is proposed, and their differences in tectonic setting and metamorphic facies are discussed. Some current hypotheses for the origin and mode of emplacement of the orogenic peridotites are briefly reviewed. It is shown next how various microstructural approaches, such as interpretative petrotectonic studies and the less commonly employed comparative petrofabric analysis of peridotites and their country-rocks, may be used to unravel the igneous, metamorphic and tectonic histories of ultramafic rocks in general. Criteria are established by which igneous flow fabrics, tectonic deformation fabrics and piezocrystalline fabrics of olivine may be distinguished. Special attention is paid to the piezocrystalline shape and lattice fabrics of olivine, deduced by Hartman and Den Tex (1964) through application of W.B. Kamb's theory of thermodynamic equilibrium orientation to the orthorhombic crystal lattice of olivine. The petrofabric habit of olivine and its role in shaping foliations or schistosities in metamorphic peridotites is emphasized. In the second part brief summaries are presented of the fabric and metamorphic data concerning three types of peridotite: (1) a “root-zone” peridotite, (2) an ophiolitic peridotite, and (3) some Iherzolitic nodules from alkali-olivine basalts, while their fabric and metamorphic histories are compared and contrasted. It is shown that they are all more of less complex metamorphic tectonites, having suffered several solid-state deformations and metamorphic events that are largely lacking in their host rocks, thus indicating that they were emplaced in the predominantly solid state. The garnetiferous “root-zone” peridotite of Alpe Arami (Swiss Pennine Alps) has a principal γ-olivine fabric perpendicular to the primary layering and first axial plane schistosity. It developed under eclogite- and granulite facies conditions, and is indicative of piezocrystallization in the absence of a liquid capable of dissolving olivine. After its solid emplacement it has also suffered Alpine tectonization and amphibolite facies rétrogradation. The ophiolitic lherzolite of the French Pyrenees has a principal α-olivine fabric perpendicular to the primary layering and first axial plane schistosity. This is also suggestive of piezocrystallization but in the presence of a liquid capable of dissolving olivine. The Iherzolite was then emplaced, aided by explosive gas release but, except for some fracture cleavages, it was not affected by penetrative Alpine deformation or metamorphism. The Iherzolitic nodules from Dreiser Weiher (F.R.Germany) possess a simple fabric, somewhat similar to that of the Pyrenean Iherzolite, whereas those from the Plateau de Velay in France have a much more complex fabric that may be derived from a “Dreiser Weiher fabric” by solid-state deformation and recrystallization of olivine and orthopyroxene crystals. It is concluded that solid derivation from upper mantle material is the most likely origin for the three types of peridotites, and that their impoverishment in eclogitic or basaltic constituents by partial fusion is improbable on account of their fabric and metamorphic history.

Sur la structure du massif du Tribulaun (region de Brenner, Tyrol, Autriche)

Abstract Imbricate structures, phase-two folding (near-horizontal axial planes) with axial-plane schistosity and greenschist-facies metamorphism phase-two refolding (near-vertical axial planes), Jurassic lithostratigraphic units

Deformation and Metamorphism in the Lukmanier Region, Central Switzerland

The Lukmanier region in the central-southern Swiss Alps contains parts of three important structural and stratigraphic units: the Gotthard Massif in the north, the parautochthonous Mesozoic cover of this massif, and the Lucomagno Massif, a frontal part of the Lower Pennine nappe complex to the south. Two successive phases of Alpine folding have been recognized in the Mesozoic cover, both of which occurred after the main northward horizontal transport of the Lower Pennine nappes. The earlier phase, called Phase B, produced three large-scale fold zones in the cover, with a penetrative axial-plane schistosity that strikes east-west and dips steeply north; fold hinge lines are markedly curved in the plane of the schistosity due to differential strain effects. Deformed fossils and ooids indicate the principal axes of Phase B strain and show that the steeply plunging, micaceous mineral lineation, which is found on most schistosity surfaces, is parallel to Phase B maximum strain extension. Regional Phase B folds were modified to various extents by Phase B movements on tectonic slides that lie parallel to the axial-plane schistosity and form the east-west contacts of both massifs with the Mesozoic rocks. The subsequent phase of deformation, called Phase V, produced no large-scale structures, but corrugated bedding and Phase B schistosity and mineral lineation mainly in the southern half of Lukmanier. Fold axial planes strike southeast and dip gently northeast, and fold axes plunge gently southeast. Complex systems of joints and rare, local strike-slip faults developed after Phase V, probably during postorogenic regional uplift. Effects of both phases of Alpine folding are found in the Gotthard and Lucomagno Massifs. Phase B produced the schistosity that strikes east-west and dips steeply north in both massifs, and the shapes of augen, together with deformed schlieren in the Hercynian granodiorite in the Gotthard Massif, indicate that the orientations of Phase B strain axes are similar to those in the Mesozoic cover, with the mineral lineation on the schistosity in both Massifs being parallel to Phase B maximum strain extension. Very intense Phase B deformation in the Gotthard Massif was restricted mainly to mylonitization in narrow east-west zones, and such movements in one zone of ancient paragneisses appear to have controlled the position of the Scopi synclinal zone, the most northerly large-scale Phase B fold zone in the Mesozoic cover. Phase V deformation had little effect in the Gotthard Massif, but Phase V folds are found everywhere in the Lucomagno Massif: their orientation is similar to Phase V folds in the Mesozoic cover. Complex systems of joints in both Massifs probably developed at about the same time as similar joints in the Mesozoic cover. Alpine metamorphism, generally of lower almandine-amphibolite facies, produced a diversity of porphyroblasts in the Mesozoic cover, and they show a variety of textural relations with phases of deformation. These relations change across the area and are due to migration of phases of deformation from south to north relative to stationary metamorphic conditions. Porphyroblast textures also show that shape and predeformational orientation may be important factors controlling amounts of porphyroblast rotation. Regional considerations of Alpine events in Lukmanier and adjacent areas lead to the conclusion that the main northward horizontal transport of the Lower Pennine nappes preceded Phase B deformation in Lukmanier, and in turn Phase B preceded and initiated, by uplift of the Gotthard Massif, the main northward horizontal transport of the Ultrahelvetic and Helvetic nappes.

The Influence of Early Structures on the Orientation of Late-Phase Folds in an Area of Repeated Deformation

Fold structures in the Glen Cannich area of northern Scotland have been studied. The folds occur in the Moine Series, a thick sequence of metamorphosed impure sandstones and shales which have been subjected to repeated folding and metamorphism. Five successive phases of folding have been recognized; major folds of the first three phases account for the over-all structural pattern shown by the lithologic units, while the last two phases occur mostly as minor folds. Three periods of metamorphism are also recognized: progressive metamorphism, a subsequent retrogressive metamorphism, and a second progressive metamorphism. A study of the fourth-phase folds reveals that they range in style from open to tight, and in most of the area are modified concentric folds formed by a combination of deformation by flexure and flattening. Along the northwestern border of the area, however, they have axial-plane schistosity and are essentially similar folds. The change from modified concentric to similar fold geometry is related to an increase in ductility of the rocks as a result of increased metamorphic grade in the area where the similar folds occur. The general strike of the axes and axial planes of the fourth-phase folds is northeast in the western part of the area and in adjacent areas to the north. In the eastern part of the area, however, the axes and axial planes swing to a southeast orientation. The axes of earlier folds in this part of the area also have a southeast orientation, and it is suggested that the southeast trend of the fourth-phase folds has resulted from reactivation of the older folds, such that they controlled the orientation of the later set.