Steep Axial Planes Research Articles

Integrative structural study of the Kibuye-Gitarama-Gatumba area (West Rwanda): A contribution to reconstruct the Meso- and Neoproterozoic tectonic framework of the Karagwe-Ankole Belt

The geodynamic evolution of the Mesoproterozoic Karagwe-Ankole Belt, situated in Central Africa, is largely unconstrained and topic of discussion. To overcome this knowledge gap, the tectonic history of the Western Domain has been studied, focusing on bedding, tectonic foliations and lineations, and quartz vein generations from the Kibuye-Gitarama-Gatumba area (West Rwanda). The results of our study reveal two fold generations, that are interpreted as the consequence of two separate compressional deformation events. The first fold generation (F1) consists of regional-scale symmetrical folds, with a N30W to NS axial plane strike and variable fold-cylindricity. The second generation (F2) is expressed by local small-scale folds with a generally steep axial plane, superposed on the regional fold structure. The strike of the F2 axial planes varies over the study area (N60W vs N20W), possibly linked to a buttressing effect from the rigid Gitarama batholith. Three quartz vein categories were determined: early quartz veins, strongly shortened or emplaced during F1 (A-category), bedding-parallel, boudinaged quartz veins, deformed pre- to syn-F2 (B-category) and undeformed, syn- to post-F2 quartz veins (C-category). Based on geochronological data from literature, the first major compressional event can be connected to the Late Mesoproterozoic Rodinia assembly during the Stenian-Tonian, while the youngest tectono-metamorphic event may represent the influence of the Late Neoproterozoic Gondwana assembly in the study area. This study furthermore demonstrates that the tectonic history of the Western Domain differs significantly from that of the Eastern Domain.

Assetto strutturale della catena a pieghe e sovrascorrimenti Umbro-Sabina (Italia Centrale) derivato dal rilevamento dei fogli 366 “Palombara Sabina” e 375 “Tivoli” (Progetto CARG).

Structural Scheme of the Umbria-Sabina fold-and-thrust belt (Central Italy) derived from the geological mapping of 366 Palombara Sabina and 375 Tivoli sheets (CARG Project).The geological mapping of the sheets 366 Palombara Sabina and 375 Tivoli (1: 50.000 scale; CARG Project; in preparation) allowed the revision of the structural setting of the westernmost portion of this part of the Neogene fold-and-thrust belt of the Central Apennines.The area comprises the Cornicolani, Lucretili, eastern Sabini, Tiburtini, Ruffi and Prenestini Mts (Latium region). Most of the area (Cornicolani, Lucretili and Tiburtini Mts) is made of the pre-orogenic deposits (Upper Triassic-Serravallian) referred to the calcareous-marly-siliceous Sabina Succession, that were accumulated in the southern part of the Umbria-Marche-Sabina Pelagic Basin (Sabina Domain), proximal to the Latium-Abruzzi Carbonate Platform. In general, in the eastern areas, more proximal to the platform, coarser lithofacies crop out. Moreover, in the Cornicolani and western Lucretili Mts (Castelvecchio Mt) areally restricted outcrops of condensed pelagic carbonates (Bugarone Group and calcari dolomitici di S. Angelo Romano) document the occurrence of Jurassic intrabasinal structural highs evolved into Pelagic Carbonate Platforms. These structural highs were generated after the Early Jurassic rifting, that broke-up the pre-existing carbonate platform (Calcare Massiccio).In the mapped area, the Latium - Abruzzi Platform is represented in the hangingwall of the Olevano-Antrodoco line only in the southern Prenestini Mts (Rocca di Cave), by upper Cretaceous shelf carbonates. Whereas in the footwall of the Olevano-Antrodoco line it crops out along the Aniene River valley. Both Sabina and Latium - Abruzzi successions are overlain by upper Tortonian - lower Messinian marls and siliciclastic syn-orogenic deposits, best observed in the easternmost areas and inbetween the Ruffi and Prenestini Mts.In their turn, the syn-orogenic deposits are unconformably overlain by post-orogenic Plio-Pleistocene marine clastic deposits and Quaternary volcanic and alluvial sediments. From a structural point of view, the Sabina Domain is arranged into several thrust sheets verging toward the east, developed in middle-late Miocene-early Pliocene times. The early Pliocene Olevano-Antrodoco out-of-sequence thrust marks the eastern tectonic boundary of this structural domain where the pelagic and slope sediments overthrust the Latium - Abruzzi Carbonate Platform and its associated syn-orogenic deposits. In late Miocene-early Pliocene extensional tectonics down-threw the western border of this imbricated structure enhancing the marine ingression along the peri-Tyrrhenian margin. The western border of the region was also reactivated by N-S right-lateral strike-slip fault dissecting the earlier structures in Pleistocene times.From top of the orogenic edifice three main tectonic units have been identified:- Unit 1 (Morra Mt) made up of Upper Triassic to Lower Jurassic dolomite limestones (Dolomia Principale) and Calcare Massiccio, overthrust onto unit 2.- Unit 2 (comprising Cornicolani Mts, western Lucretili Mts) made up of Dolomia Principale, Calcare Massiccio, Corniola detritica and, in the Cornicolani and western Lucretili Mts, Bugarone Group. Long wavelength upright folds mainly characterize this unit and faults with high angle cuts-offs prevail, and Jurassic structural highs occur (Cornicolani Mts, Castelvecchio Mt).- Unit 3 (comprising Elci Mt, eastern Lucretili Mts and Tiburtini Mts) made up of most of the Sabina basin stratigraphic succession - from Corniola detritica Fm. to the Guadagnolo Member (marly lithofacies); the unit is characterized by NW to NE verging overturned to recumbent folds.- Unit 4 (comprising Prenestini Mts, Ruffi Mts and eastern Sabini Mts) made up of the upper portion of the basin succession between the Marne a Fucoidi and Upper Miocene siliciclastics and the Rocca di Cave carbonate platform succession.This unit is made up by two sub-units, based on the deformation-style and stratigraphy: the sub-unit 4A (Granaro Mt, Colle Serviano and Arzillo Mt in the western Prenestini Mts), is mainly characterized by east-verging packed asymmetrical folds with sub-vertical axial-planes, and N-S striking dextral-transpressive faults. The sub-unit 4B (Castel Madama area, eastern and central Prenestini Mts, Ruffi Mts and eastern Sabini Mts) is characterized by a wide symmetrical anticline in the western portion, and by folds with very steep axial planes, and N-S to NNW-striking dextral transpressional faults.The units are separated by low-angle thrusts or by high-angle transpressive faults: the stratigraphic successions characterizing each unit rejuvenate eastward. The innermost unit 1 and unit 2, where massive or poorly stratified deposits are exposed, are mainly deformed by high angle faults. Whereas the more external unit 3 and unit 4 are composed by alternating carbonate, marls and siliciclastics, and are mostly characterized by east-verging slightly to strongly asymmetrical and overturned folds.A new geometric and kinematic scenario for the Neogene evolution of the units preserved in the hangingwall of the Olevano-Antrodoco thrust is here outlined.The revision of the stratigraphy and structural data put in evidence the presence of paleo-structures of the Tethyan passive margin not evidenced in the past cartography, their influence on Neogene compressive structures and the coexistence of non-coaxial structural trends in areas of limited extension.

Interferência entre duas faixas móveis neoproterozóicas: o caso das faixas Brasília e Ribeira no Sudeste do Brasil

Neoproterozoic metasedimentary successions of the Andrelândia Sequence, gneisses of the Guaxupe Massif and Paleoproterozoic ortogneisses, with igneous zircon of 2.088_26 Ma, constitute the Precambrian units in the Tres Coracoes region, southern Minas Gerais State. Deformational structures and metamorphic associations indicate two collisional events related to the Neoproterozoic orogeny, that originated the Brasilia and Ribeira fold belts.Two continuous deformational phases (DB1 and DB2) and a relatively high pressure metamorphic event (MB) were related to the Brasilia fold belt. Superposed and related to the Ribeira fold belt, two deformational phases (DR1 and DR2) and a lower pressure metamorphic event (MR) are recognized. DB1 and DB2 produced nappes with movement to ESE, E and ENE. MB metamorphism started during DB1, as indicated by metamorphic isograds that are dislocated by DB1 and DB2 structures. K-feldspar-in/muscovite-out and sillimanite-in isograds related to the granulite facies are concordant with DB2 thrust faults, showing that the MB peak was sin-DB2. Folds with steep axial planes and axes plunging toWSWwere produced during DR1 that folded the nappes lateraly. The fact that metamorphic isograds related to theMRmetamorphic event cutMBisogrades and pre-existing structures shows the superimposition of this event on the Brasilia fold belt evolution. Finally, the last deformational phase DR2 produced dextral transcurrent ductile-brittle shear zones with SW-NE strike, that cut structures and metamorphic isograds of both fold belts.

Transient inversion during the opening stageof the Wilson cycle “Sardic phase” in the Iberian Variscides –Stratigraphic and tectonic record

The Variscides of Iberia have a bilateral symmetry with east vergence in the eastern branch and west vergence in the western, on both sides of a Centro-Iberian Zone (CIZ), with predominant steep axial planes. All the structures curve around the Ibero-Armorican Arc (IAA). Unconformities in the sedimentary sequences of Cambrian to Early Ordovician age were ascribed to “Sardic phase” by correlation with similar tectonosedimentary events in Sardinia. Recent studies showed diachronism between these events in Sardinia and Iberia but migration of major geodynamic regime in time may be due to regional variation of major events at plate tectonic scale. We studied in detail two critical areas in the CIZ, the Marão anticline in the NE and the Amêndoa-Carvoeiro synform in the SW. Two unconformities can be put in evidence, as elsewhere in CIZ. A stronger lower unconformity of a Volcano-Sedimentary Complex of Lower Arenig (and Tremadocian?) age on top of a Cambrian clastic sequence with flysch characteristics; and a milder upper unconformity of Armorican Quartzite of Arenig age on both the Volcano-Sedimentary Complex and the Cambrian sequences. The lithostratigraphy of the studied areas is described and correlated with other areas in Iberia. The Volcano-Sedimentary Complex and coeval magmatic bodies with bimodal composition are briefly described. The Sardic event corresponds to folds with steep axial planes at high angles to Variscan structures that produce the penetrative cleavage that cut across the unconformity surfaces. Sardic thrusts are also present and can be explained by thin-skinned compressive tectonics. Sardic folds and thrusts suggest a brief period of transient inversion between a major extensional regime from Cambrian to Devonian. The obliquity of Sardic structures to Variscan compression suggests a component of transpression during the Sardic tectonic event, corresponding to a tectonically enhanced unconformity near the Cambro-Ordovician boundary. The transient Sardic inversion is interpreted in terms of a break-up unconformity related to the migration of an intracratonic rift; in the Ordovician this rift moves into the SW of Ossa Morena Zone (OMZ) and since then become the SW Iberia suture during the Variscan Wilson cycle. This migration induced transient compression and dextral strike-slip in the major boundary between CIZ and OMZ due to presence of incipient primary curvature in this segment of IAA.

Origin of a large-scale fold nappe in the Montagne Noire, Variscan belt, France

Detailed structural analysis of a large recumbent fold in low-grade Palaeozoic sediments of the Montagne Noire, and the previously established tectonometamorphic history of its crystalline substratum, indicate a two-stage development history. First, relatively homogeneous crustal thickening produced folds with steep axial planes associated with a regional cleavage ( S 1). Subsequently, these structures became rotated and amplified in a subhorizontal flow with pure shear and simple shear components related to low-angle thrusting. The development of an associated horizontal crenulation cleavage ( S 2) with a component of vertical shortening of S 1, plus evidence for a synchronous decompression of autochthonous units allows this flow regime to be placed in the context of a thrust wedge undergoing gravitational spreading above a continuously contracting footwall. Decoupling of an extending thrust wedge from a contracting footwall by a basal thrust with low shear resistance, explains bulk shortening, uplift and exhumation of mid- to lower-crustal autochthonous basement during thrusting and recumbent folding at higher levels. Structural relationships in the southern Montagne Noire demonstrate that traditional tectonic units recognized in this area, the so called ‘nappes’, are in fact unrelated to the recumbent folding as inferred previously. These ‘nappes’ postdate not only the recumbent folding, but also a later phase of refolding ( D 3) associated with the formation of a gneiss dome. The traditional nappe division can be simplified to a hanging wall and footwall domain of a major décollement that formed during a second syn-collisional collapse ( D 4) associated with the formation of intermontane Stephanian basins. The late timing of the ‘nappes’ and simplified tectonic structure of the southern Montagne Noire imply a thinner D 2 recumbent fold structure with only one major anticline–syncline pair, and is in better agreement with the low metamorphic grade of the area.

Cambrian‐Silurian oceanic rocks, upper Howqua River, eastern Victoria: Tectonic implications

A Cambrian‐Silurian succession, with basal mafic volcanic rocks and chert along with much more widespread upper units of quartz‐rich turbidites and black siliceous shale, is exposed in the upper Howqua River area of eastern Victoria and forms much of the basement of the Tabberabbera Zone. This succession is typical of the stratigraphy of upper oceanic successions that are widely preserved in circum‐Pacific orogenic belts and, along with similar sections on the south coast of New South Wales and in central Victoria, provides evidence that much of the Lachlan Fold Belt developed on oceanic basement. The Early Ordovician turbidites indicate that the ocean floor lay adjacent to a continent (eastern Gondwana). Silurian turbidites were derived from a western source reflecting Early Silurian reactivation and eastward growth of the former Delamerian mountain chain in western Victoria. The Cambrian‐Silurian succession contains one major phase of folds, F1, which are northwest trending and open to close with steep axial planes; fold vergence indicates tectonic transport towards the southwest, which is characteristic of the Tabberabbera Zone. A major fault zone occurs along the southwestern contact of the Cambrian volcanic rocks and highly deformed Ordovician‐?Silurian rocks to the southwest are part of the Mt Useful Slate Belt. The deformation occurred in a Silurian compressional backarc setting distant from a convergent margin to the east (New England Fold Belt) and was a consequence of crustal weakening over time as the proportion of weak strata in the total crustal column increased and the region was subjected to thermal softening.

Geometry and evolution of superposed folding in the Zawar lead-zinc mineralised belt, Rajasthan

The lead-zinc bearing Proterozoic rocks of Zawar, Rajasthan, show classic development of small-scale structures resulting from superposed folding and ductile shearing. The most penetrative deformation structure noted in the rocks is a schistosity (S1) axial planar to a phase of isoclinal folding (F1). The lineations which parallel the hinges ofF1 folds are deformed by a set of folds (F2) having vertical or very steep axial planes. At many places a crenulation cleavage (S2) has developed subparallel to the axial planes ofF2 folds, particularly in the psammopelitic rocks. The plunge and trend ofF2 folds vary widely over the area.

Structure and metamorphism of the Calliope Volcanic Assemblage: Implications for middle to Late Devonian orogeny in the northern New England Fold Belt

The Late Silurian to Middle Devonian Calliope Volcanic Assemblage in the Rockhampton region is deformed into a set of northwest‐trending gently plunging folds with steep axial plane cleavage. Folds become tighter and cleavage intensifies towards the bounding Yarrol Fault to the east. These folds and associated cleavage also deformed Carboniferous and Permian rocks, and the age of this deformation is Middle to Late Permian (Hunter‐Bowen Orogeny). In the Stanage Bay area, both the Calliope Volcanic Assemblage and younger strata generally have one cleavage, although here it strikes north to northeast. This cleavage is also considered to be of Hunter‐Bowen age. Metamorphic grade in the Calliope Volcanic Assemblage ranges from prehnite‐pumpellyite to greenschist facies, with higher grades in the more strongly cleaved rocks. In the Rockhampton region the Calliope Volcanic Assemblage is part of a west‐vergent fold and thrust belt, the Yarrol Fault representing a major thrust within this system. A Late Devonian u...

A layered basic intrusion, deformed and metamorphosed in granulite faciès of the Sri Lanka basement

A layered basic intrusion has been found in the Central Granulite Belt of the Sri Lanka continental basement. It intruded parallel to bedding, before all or early during deformation of neighbouring metasediments. Deformation, affecting metasediments and the intrusion alike, includes flattening to c. 1/20 of the original thickness and NNW-stretching to c. 20 times the original length. The intrusion is now 170–300 m thick. Most of the deformation was acquired under granulite facies metamorphism. The intrusion was then folded, still at high T, by a large F4-synform with an axis parallel to str1 and a steep axial plane. A steep axial plane cleavage and minor folds are related to this big fold. Stretching continued along its axis. Late during formation of this fold a granite intruded, mainly following S4 cleavage planes. The intrusion shows a homogeneous gabbroic series at the bottom, followed upwards by a differentiated and layered series. A thin sequence of ultramafic rocks occurs near the middle. This indicates multiple melt-injection. More homogeneous partly biotite-bearing amphibolites form the top of the succession. Magmatic layering is well preserved, but no magmatic minerals or grain fabrics have escaped deformation or metamorphism. Static annealing under granulite facies conditions outlasted all deformation and was accompanied and followed by the beginning of cooling. Hornblende-Plagioclase coronas formed round garnets at this stage. Geochemical work, carried out by STOSCH (1991) on our samples, confirms the cumulate nature of the rocks.

Structural geology of the Lys-Caillaouas massif, Central Pyrenees. Evidence for a large scale recumbent fold of late Variscan age

A review of data on the Lys-Caillaouas massif leads to a re-interpretation of its Variscan structural evolution. During a first phase of N-S shortening upright folds with steep axial plane foliations were formed. Subsequent regional metamorphism was followed by porphyroblast rotation and formation of gently dipping crenulation cleavages. During this event a N-closing recumbent fold at the scale of the massif developed, with relatively undeformed first phase foliations in the upper limb (suprastrucfure) and highly deformed and transposed foliations in the lower limb (infrastructure). This recumbent fold is argued to result from temperature induced gravity collapse of a crustal block previously having subvertical planes of anisotropy.

The Sveconorwegian orogenic belt, a case of late-proterozoic plate-collision

A geotetctonic model is proposed for the Sveconorwegian orogeny (1200-850 Ma), based on compilation of lithological and structural evidence. This analysis suggests a N—S trending, asymmetrical orogenic belt with longitudinal zonation. In the western-most core zone, synkinematic intrusions dominate, while the central zone consists primarily of reworked basement with subordinate intrusions and metasediments. In the easternmost marginal zone, series of pre-Sveconorwegian mega-units are separated by thrust zones and finally bordered by the Sveconorwegian frontal thrust towards the east. Graben formation in the Telemark area preserves the deposits of previous epicratonic stages and marks the initiation of Sveconorwegian activities at about 1190 Ma. The subsequent orogenic development, associated with lithospheric subduction, shows tight folding about N—S axes with steep axial planes in the west, and more open NW—SE folding with gentle westward dipping axial planes towards the east. Regional metamorphism of low-P, high-T conditions imply steep geothermal gradients and suggests hercynotype crustal evolution, in agreement with a Cordilleran-type plate collision. In the Bamble district, important structural alignment forms a zone of crustal anisotropy which was repeatedly reactivated and ultimately became the site for Permian rifting forming the Oslo Graben. The lithological and structural characters as well as the crustal evolution for the Sveconorwegian orogeny show strong similarities with the Grenville development in southeastern Canada. Structural vergence and orogenic zonation suggest that the Sveconorwegian front and the Grenville front are equivalent and form different parts of a single orogenic belt, permitting a tentative reconstruction of the Late-Proterozoic, North-Atlantic Shield.

Geology of the Torlesse Supergroup in the northern Ben Ohau Range, Canterbury

Abstract Torlesse rocks in the northern Ben Ohau Range form steeply plunging north-facing structures with steep axial planes trending mostly NNE and NE. In a central belt of the range textural zone 2 (and some zone 3) schists have been developed, probably after the folding, by a mechanism including flattening across the schistosity plane. Fold axes have variable attitudes and plunge north in the non-schistose rocks. In the schists the axes are more strongly aligned and plunge south to give overturned structures and well developed lineations. Decollements with dextral vergence com plicate the pattern of the main structure, the Bush Stream anticline, which is exposed along the crest of the range; they also affect the Whale Stream syncline to the east. Local south-facing folds in the non-schistose rocks of the eastern foothills may indicate earlier stages of structural development. Movement along the Great Groove Fault on the eastern edge of the schist has been complex, possibly changing from a system with a...

DISCUSSION OF BOTH PAPERS BY FRANCIS COWARD

D R . R. G. P ARK congratulated both authors on their interesting accounts of the very complex geology of the southern Outer Hebrides. Lacking first-hand knowledge of that area, he was tempted, despite his sceptical approach to structure correlation in general, to try to match the structural sequence proposed by the authors with the sequence obtained by the speaker and others in their recent work on the southern Laxfordian belt of the Scottish mainland, but found considerable difficulty in doing so. A superficial similarity existed between the f 3 folds of the authors and the large-scale second Laxfordian folds on the mainland with a NW–SE trend and steep axial planes. The distribution of these structures showed a marked heterogeneity of deformation with narrow intensely deformed synformal belts (e.g. the Gairloch schist belt) separated by wide, relatively little-deformed zones. A characteristic feature of the Scourie dyke margins in the highly deformed zones was the presence of tight small-scale folds with relatively sharp antiformal and synformal hinges suggesting fairly ductile behaviour of dykes and gneiss, whereas no small-scale folding of dyke margins has been observed in the antiformal zones. In contrast, during the earlier Laxfordian deformation, small-scale folding of dyke margins is apparently absent throughout the region investigated and the dykes have developed a foliation only which is absent from the enclosing gneisses. In the light of these observations, could Dr. Coward comment on the role of small-scale f 1 and f 2 folding of the dyke margins in his area and on the physical conditions

Significance of Variable Plunge and Trend of Small-Scale Upright Folds in the Type Aravalli Rocks around Udaipur, Rajasthan (Western India)

Research Article| May 01, 1972 Significance of Variable Plunge and Trend of Small-Scale Upright Folds in the Type Aravalli Rocks around Udaipur, Rajasthan (Western India) ASHIT BARAN ROY ASHIT BARAN ROY Department of Geology, University of Rajasthan, Udaipur, India Search for other works by this author on: GSW Google Scholar Author and Article Information ASHIT BARAN ROY Department of Geology, University of Rajasthan, Udaipur, India Publisher: Geological Society of America Received: 01 Nov 1971 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1972, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1972) 83 (5): 1553–1556. https://doi.org/10.1130/0016-7606(1972)83[1553:SOVPAT]2.0.CO;2 Article history Received: 01 Nov 1971 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation ASHIT BARAN ROY; Significance of Variable Plunge and Trend of Small-Scale Upright Folds in the Type Aravalli Rocks around Udaipur, Rajasthan (Western India). GSA Bulletin 1972;; 83 (5): 1553–1556. doi: https://doi.org/10.1130/0016-7606(1972)83[1553:SOVPAT]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The variable axial trend and plunge in the folds of the Aravalli rocks of early Precambrian age (2,500 to 2,000 m.y.) around Udaipur (lat 24° 35′ N.; long 73° 41′ E.) in southern Rajasthan, has been interpreted as being the result of interference of two periods of folding. The later folds with approximately north-south, steep axial planes are thought to have developed over an early set of overturned folds. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Multiple Deformation in Archean Rocks of the Vermilion District, Northeastern Minnesota

The structure of the Precambrian rocks of the Vermilion district is critically examined. It is demonstrated that two significant deformations (F1 and F2) have affected the area in addition to a later set of faults, joints, and kink-bands (F3). The F1 folds are tight to isoclinal with gently plunging axes and vertical axial planes trending west–northwest and containing iron formation and greenstones in anticlinal cores. The second deformation forms a series of open to close asymmetric folds (F2) with steep axes and vertical axial planes trending east–west. Of these, F1 has the more significant effect on the pattern of rock outcrop and it is suggested that it is directly related to the diapiric rise of the batholiths lying immediately to the north and south. The F2 folds may be accounted for by further compression between the two granite bodies.

The structure of the Palaeozoic sediments of the Mossman and Cooktown areas, North Queensland

The Palaeozoic geosynclinal sediments of the Mossman and Cooktown areas consist of shallow‐water shelf sediments in the west, subdivided into two formations, overlain by a thick pile of deep‐water turbidites occupying the remainder of the Palaeozoic outcrop and constituting one formation. The sediments have been strongly folded in four phases, and are now characterised by widespread steep dips and steeply plunging folds.The first folds (B1) were acute folds with steeply dipping axial planes striking north or northnorthwest, and with almost horizontal axes. Broad open B2 folds, with steep axial planes of various trends, were superimposed on the steeply dipping beds of the B1 folds. Few minor structures are associated with either of these phases of folding. The third phase of folding (B3) resulted in tight, almost isoclinal, folds with steep axial planes trending between north and northwest, and with steep plunges. These folds were accompanied by a strongly developed axial plane slaty cleavage which, in many areas, led to a progressive transposition of the original sedimentary bedding into a new lamination. The final period of folding (B4) was a minor one, affecting only a limited area in which the slaty cleavage of the B3 folds is the dominant planar element. The cleavage is folded into sharp‐angled chevron folds with almost horizontal axes and gently dipping axial planes.

STRUCTURAL GEOLOGY OF PRECAMBRIAN ROCKS SOUTH OF BANCROFT, ONTARIO

Structural analysis of Precambrian metamorphic rocks south of Bancroft, Ontario, has disclosed fold structures in lithologic layering, which are examined on the working hypothesis that two distinct generations are present. Folds of the first system, in the southern part of the mapped area, are fairly open, nearly recumbent, and plunge gently eastward, whereas folds of the second system, found in the northern part of the area, are tighter, with steep southeast-dipping axial planes and variably oriented axes. Uniformly steep stratiform foliation and linear mineral segregations, characterizing an extensive terrain between the so-called Hastings basin and the Haliburton–Hastings highlands, seem to have evolved by progressive tightening of the second folds and transposition of the layering parallel to the axial planes. In the area studied, the distinction between the highlands and the basin is basically one of predominant rock type, with progressive changes in structural character evident between the two. As formerly reported, the distinction involved strongly contrasting structural characteristics, with intervening zones of faulting.This part of the Grenville province is structurally very complex, and considerably more data and analysis are needed to unravel the tectonic history.

Structures in the Lower Paleozoic Meguma Group, Nova Scotia

At least three generations of folds and faults affect slates, schists, and quartzites of the Meguma Group and other Lower Paleozoic rocks over a large part of mainland Nova Scotia. The main folds ( F 1 ) are upright and low-plunging and trend northeast to east closely parallel to the strike of a steep axial-plane foliation ( S 1 ). Mid-Devonian granite batholiths intersect the F 1 folds with little disruption of the trends. Cross-folds ( F 2 and F 3 ), although widespread, are less pervasive than the F 1 folds; most, but not all, are small-scale structures restricted to the foliated argillites. The cross-folds deform the S 1 foliation and are therefore younger than the F 1 folds. The F 2 folds are typically interfolial or interstratal, angular, and Z -shaped in profile; nearly vertical F 2 axial planes strike northward, and the axes of the F 2 folds in S 1 are steep. In metamorphic aureoles of granites, quartz-feldspar veins and andalusite porphyroblasts cross F 2 folds and crinkles, indicating that the granites are younger than the F 2 folds. Nearly all F 3 folds are open S -shaped kinks with steep axial planes striking northwest. The kinks deform porphyroblasts in the granite aureoles and pass into small sinistral faults which cut the granites. However, large sinistral faults parallel to the kinks offset the folded Meguma rocks, but not the granites. Thus the sinistral faults, and possibly the kinks, were formed before and after the emplacement of the granites. The Z -shape of the pregranite F 2 folds and the occurrence of postgranite dextral offsets along and across the S 1 foliation suggest that northeast- to easterly directed dextral movements may also have been repeated. Possibly the maximum compression was originally oriented northwest to north perpendicular to the F 1 axes and changed to an east-west direction to form the northerly trending F 2 folds; a continuation of this compression, both before and after granites were intruded, led to the F 3 S -kinks and sinistral faults striking northwestward, and to dextral faults striking northeast to eastward.

The structural history of the Lewisian rocks of Gairloch, Wester Ross, Scotland

The Lewisian rocks of the Gairloch area comprise a small outcrop of older granulite-facies gneisses (the Ialltaig gneisses), the deformation and metamorphism of which is provisionally assigned to the Scourian period, and a much more extensive group of younger rocks (the Gairloch schists and gneisses) considered to represent a series of sediments (deposited unconformably on the older gneisses) and basic igneous rocks, both extrusive and intrusive. The deformation and metamorphism of the younger group, provisionally assigned to the Laxfordian period, fall into three principal phases designated early, main, and late. During the early phase, a foliation was induced during metamorphism, probably of low grade, and large-scale folds were formed that were completely re-oriented or obliterated during the succeeding phase. The main phase was characterized by major folds trending nw-se with steep axial planes and later by smaller-scale nw-se folds and steep axial-plane foliation slightly oblique to the major folds. Several dolerite dykes were intruded at South Sithean Mor after the main-phase folding. Epidote-amphibolite-facies metamorphism accompanied the formation of the main-phase foliation and continued in the late phase, which commenced with paracrystalline folds and concluded with brittle structures produced during retrogressive metamorphism. The formation of the gneisses is assigned to the initial stages of the late phase.

Joint-drags in Mid-Argyllshire

Joint-drags are angular fold-structures, with steep axial planes, which frequently appear to have been developed during faulting. In Mid-Argyll, the fractures which form the axial planes to the joint-drags trend consistently NW.-SE. and dip steeply south-westwards; there is a dextral sense of movement along these fractures. It is suggested that the joint-drags were formed as a series of slip structures complementary to the sinistral NNE.-SSW. tear-fault system of the SW. Highlands of Scotland.