Major Recumbent Folds Research Articles

Lateral constraint rather than escape along a terrane boundary near Skelleftehamn

Mylonitised greywackes with superposed pseudotachylites (fossil earthquakes) mark the eastern end of a W-E trending terrane boundary between the Skellefte supracrustal group of Svecokarelia to the north and the Bothnian gneisses of Svecofennia to the south. This region has been claimed to provide the clearest evidence that early Proterozoic plate tectonics (>1.85 Ga) involved the same processes as Phanerozoic continental suturing and convergence. Comparisons between the tectonic histories of greywackes in, and on either side, of this boundary demonstrate that the southern terrane already had major recumbent folds at upper amphibolite metamorphic facies before it met and converged on the northern terrane and eventually rose relative to it. These nappes were refolded by upright folds that are circumferential to a syn-Svecofennian granite south of the boundary but the first folds visible in the lower grade rocks to the north. The youngest Svecofennian structures in the region were kink bands that developed in the boundary while it was a steep seismic zone. These kink bands record lateral tectonic constraint rather than lateral escape. West of the study area, mylonites of the terrane boundary are intruded by sheets of post Svecofennian Skellefte granites and plutons of 1.75 Ga Revsund granites.

Exhumation process of the Nago subduction-related metamorphic rocks, Okinawa, Ryukyu island arc

The Kunigami zone in Okinawa is an extension of the Shimanto zone, Japan. The rocks make up the main part of the Nago metamorphic rocks, and such metamorphic rocks are exceptional in the Shimanto zone. The Anne complex, in the older Motobu zone, is also metamorphosed. The reason for why and how this kind of the metamorphism occurred, and especially why and how the metamorphic rocks were exhumed, is yet uncertain and unresolved. To understand the metamorphic and exhumation process in Okinawa, a structural study is undertaken, and its relation to the Eocene ridge subduction is discussed. We believe exhumation was performed by formation of a D2 extrusion wedge, made up of the Nago metamorphic rocks. The base for this wedge is a subduction thrust, and the roof is a detachment fault. Internally, there exists another Kijoka detachment fault, which is a brittle low-angle fault with top to the northwest shear sense, and the D2 major recumbent folds and thrusts show top to the southeast opposite shear sense in the Kunigami zone. This is the first report that finds detachment faults from the typical and ancient accretionary complex. M2 is mostly retrograde related to exhumation, but its medium P/T-type prograde metamorphism, abnormal at subduction zones, represents a high thermal gradient during ridge subduction. As a result, this ridge subduction is responsible for exhumation. At the time of accretion of the Kunigami zone, D1 ductile contraction and constriction exhibited top to the southeast shear sense, but an opposite and extensional shear sense is recognized in the proto-wedge. During D1, the wedge had already been active and begun to exhume. M1 of the Miyagi complex is accretion related and also of medium P/T-type metamorphism, and is a consequence of Cretaceous ridge subduction without any ability to cause much exhumation.

Geology and models of salt extrusion at Qum Kuh, central Iran

Profiles through the summit of a small nearly axisymmetric extrusion of Oligocene and Miocene salt, and simple analogue models of it, simulate the profiles of piles of ductile nappes extruded from convergent orogens. The salt extrudes from a reactive diapir along a major strike-slip fault at about 82 mm a −1 and rises 315 m above the central plateau of Iran. The salt has the distinctive smooth profile of a viscous fountain in which an asymmetric apron of allochthonous salt gravity-spreads over its surroundings from a summit dome. Curtain folds developed in the source layer extrude from the diapir and are refolded by major recumbent folds with circumferential axes that simulate nappes. Minor flow folds with circumferential axes refold major folds in the top 10-50 m of surficial salt. Master joints >100 m long indicate brittle failure of dilated salt by regional stress fields. Tuned to the dimensions of Qum Kuh, analytical and analogue models of viscous extrusions constrain the dynamic salt budget and a time of extrusion of at least 42 000 years. New analogue models suggest that the number, amplitude and spacing of major recumbent folds within the extruded salt (and ductile nappe piles) record the number, amount and relative timing of fluctuations in the driving forces.

Damaran basement-cored fold nappes incorporating pre-collisional basins, Kaoko Belt, Namibia, and controls on Mesozoic supercontinental break-up

Deformation styles due to the collisional convention of Gondwana that ended ~500 Ma are reassessed in the Kaoko fold-thrust belt of the NE Damaran orogen. Downward facing sequences on the overturned limb of a major recumbent fold nappe are exposed in the Hoanib valley beneath pre-Damara basement, initially identified in a D2/D3 antiformal refold core, constituting the Obias River Window. Basement paragneisses, psammitic and pelitic protoliths, were infolded into the core of a major recumbent anticlinal Di fold nappe structure (dimensions 55 km across × 150 km+ along tectonic strike), comparable in dimensions to other major fold nappes (e.g. the Loch Tay nappe of the Scottish Highlands). D2/D 3 refolding produced large-scale polyphase fold traces later affected by a D4 phase of open cross folding. The nappes detached along flat lying sole thrusts (Eastern Zone), soled by the Sesfontein master thrust, and are backed by a structurally steep root zone (Central Zone). Farther west ramped-up syn-orogenic granitoids intruded into the Damara sediments (Western Zone). Some thrust zones mark major fades changes and imply a pre-compressional identity, with most complete stratigraphic sequences immediately to their western side. The latter Damaran (750 to 600 Ma) sequences progressively on-lap westwards onto basement, to exclude successively lowest stratigraphic units. Prior to collision half-graben sub-basin fills were accommodated by extensional fault systems. Subsequent Damaran compression reversed extensional faults as thrust zones. Intra-Pangean extension and break-up later reactivated Damaran thrusts, expressed as rift basins that developed during the latest Palaeozoic and culminated during the Early Cretaceous with the opening of the South Atlantic. Karoo to post-Karoo sediments and volcanics were accommodated in the resulting half-grabens. Indeed the Damaran continental suture probably inverted as the southern South Atlantic breakaway zone between Africa and South America. Atlantic shelf sediments now cover the suture apart from in southernmost Namibia, where it is marked by the Damaran thrust-floored Oranjemund Complex of the Gariep Belt. The Oranjemund Complex incorporates an exotic Adamastor Oceanic volcanic prominence thrust onto the proto-African foreland.

Extensional geometries as a result of regional scale thrusting: tectonic slides of the Dunlewy-NW Donegal area, Ireland

The synmetamorphic ductile dislocations, known in the British Caledonian literature as ‘Tectonic Slides’, pose a classical structural problem. That is, despite being associated with synchronous contractional folds and cleavages the low angle dislocations have the effect, in many celebrated cases, of juxtaposing younger over older rocks: a geometry normally associated with extensional rather than contractional deformation. Recent models have attempted to demonstrate that this is the result of thrust reactivation of original, sedimentary, extensional growth faults. The Appin Group Dalradian metasediments of the complex and small Dunlewy area of NW Donegal, Ireland, contain the following geometric elements: (a) an early strike-swing-related stratigraphic facies change; (b) a major inter-deformational dolerite sheet; (c) major regional recumbent folds and slides; (d) major structures related to the 400 Ma sinistral Main Donegal Granite shear zone. This solution to the structural geometry reveals that the early mid-crustal (~11 km depth) D 2 Ardsbeg-Dunlewy Slide is a thrust to the northwest. Its hangingwall contains rocks two-thirds of which are younger than the rocks of the footwall, together with major recumbent folds, coeval with the underlying thrust, which face downwards into the thrust in the direction of transport. Rather than thrust reactivation of an original extensional growth fault, we find that both stratigraphic and structural constraints are satisfied by a double thrusting model, with fault-bend folding onto an upper ramp of an earlier formed but penecontemporaneous and kinematically linked major fold pair. This solution to the geology also allows us to recognize that the regional (pre-granite) structure of the Dalradian of NW Donegal is a series of major D 2 synmetamorphic thrust bounded nappes possibly involving up to 250 km of northwesterly overthrusting.

Superimposed upper proterozoic collision-controlled orogenies in the Mozambique Orogenic Belt of Kenya

The geological history of the northernmost part of the Mozambique Orogenic Belt, exposed in north-central Kenya, is described. A basement of ∼ 1200-Ma (Kibaran) migmatites is overlain by altered sediments with lateral and vertical facies changes defining eight lithostratigraphic groups. There is no evidence for any Archaean-early Proterozoic crust within the mapped part of the orogenic belt. A tectonothermal (Samburuan-Sabachian) event in the amphibolitegranulite facies at ∼ 820 Ma produced major recumbent folds with ductile thrusting to interleave the basement, metasedimentary cover and slices of oceanic metavolcanic complexes. There was syntectonic emplacement of crustal melt granites and metabasic dykes. We interpret this tectonothermal event to be related to plate collision, oblique to the north-south orogenic strike, between the Archaean Tanzanian craton in the west and an eastern Kibaran craton. A one-dimensional manifestation of the suture between these cratonic areas is possibly exposed at West Pokot in western Kenya. Post-collisional greenschist-amphibolite facies (Baragoian-Barsaloian) deformation at between 620 and 570 Ma produced regional upright folds and vertical ductile strike-slip shear zones striking subparallel to the orogenic strike. The metamorphism culminated during the intrusion of syntectonic granites. High-level open folding and brittle shears record the final events of the orogeny. Uplift and cooling, which extended beyond the confines of the orogen, are dated by ubiquitous mineral ages of ∼ 500-480 Ma. Whereas the roughly contemporaneous (Pan-African) Nubian Shield to the north preserves large segments of island arc/ophiolitic material, in the Mozambique Orogenic Belt this volcanic material was mostly consumed during the Upper Proterozoic oblique continent-continent collision. The roots of the collision zone are now exposed in the highgrade Mozambique Orogenic Belt and contrast with the higher exposure levels seen in the Nubian Shield.

The Dalradian rocks of the Lecht, NE Scotland: stratigraphy, faulting, geochemistry and mineralisation

ABSTRACTAir photo interpretation and geochemistry have been used to supplement sparse field data in compiling a reconnaissance geological map of the area around the Lecht. Upper Appin and lower Argyll Group rocks are exposed, the best marker horizons being correlatives of the Appin and Jura Quartzites. Scatter plots, Q mode factor analysis and Pearson correlation coefficients of geochemical data were successfully used in the correlation of the pelite and limestone outcrops. The presence of major recumbent folds is inferred. The area is cut by numerous minor faults which are thought to be related to a major transcurrent fault, the probable north-eastern continuation of the Loch Tay fault, which was active in Lower Old Red Sandstone times.The Lecht manganiferous ironstone is a seepage-bog ore type deposit derived by weathering of the local Dalradian sequence. Despite intensive leaching, this sequence still shows anomalously high levels of several elements, particularly Mn, Ba and Zn. In addition, stratiform sphalerite and manganiferous garnet are present in Blair Atholl Subgroup limestone and pelite horizons, respectively. Mineralogical and geochemical evidence suggests the presence of Zn–Pb–(Ag)–(?Au) exhalative mineralisation within the upper Appin Group Dalradian of the region, of which the Lecht represents the distal manganiferous expression.

The Gillbrea Nappe in the Skiddaw Group, Cockermouth, Cumbria, England

SummaryNew observations of sedimentary and tectonic structures confirm ‘pre-cleavage’ folding of the Arenig Loweswater Flags and Kirk Stile Slates. A major recumbent fold is thought to have developed by gravity gliding during Arenig-Llanvirnian times.

Upper Proterozoic stratigraphy and depositional environments of the Tossåsfjället Group, Särv Nappe, southern Swedish Caledonides

Abstract A thick upper Proterozoic sedimentary sequence (4.5–6 km) has been identified in the Särv Nappe of the southern Swedish Caledonides. The sequence, defined as the Tossåsfjället Group, is divisible into five formations named in ascending order as the Lunndörrsfjällen Formation (alluvial), Kråkhammaren Formation (alluvial-littoral), Storån Formation (littoral), Lillfjället Formation (glacial-periglacial-aquatic), and Lövan Formation (alluvial). Palaeocurrents throughout the whole of the group suggest a general sediment transport from the Fennoscandian Shield towards the northwest. The group is cut at high angles by a tholeiitic dyke swarm. During the Caledonian Orogeny the Särv Nappe was displaced east-southeastwards over a distance exceeding 250 km. In its main outcrop area in Jämtland the nappe occurs as a major recumbent fold; its lower limb is highly attenuated, mylonitized and largely cut out by the basal thrust plane.

Discussion of the North Ballachulish Dalradian

D r . A. H. H ickman writes: Dr J. L. Roberts (1976, this Journal 132 ,139–54) admits that he has not established the existence of the Ballachulish Syncline ( Bailey 1934 ). Roberts accepts Bailey’s stratigraphy, confirms the outcrop pattern and the D3 Stob Ban Synform but finds no evidence that the central outcrop of the Ballachulish Limestone Formation occupies the refolded inverted limb of a major recumbent fold. Bailey (1910) first postulated the Ballachulish Syncline to explain a vertical repetition of limestone, slate and schist which did not correspond to his concept of the area’s stratigraphic succession. However, it is now known that the Ballachulish Limestone Formation contains thick intercalations of black slate and, less commonly, phyllite ( Hickman 1975 , p. 126) so that no structural repetition need be envisaged at Callert. Bailey’s sections through the Ballachulish Syncline (e.g. Bailey & Maufe 1960, fig. 7) show that over large areas its core consists of a remarkably uniform thickness (about 100 m) of Ballachulish Limestone Formation underlain and overlain by Leven Schist Formation. From Glencoe to Coire Mhorair (2185 7585), Coire an Easain (2250 7495) and the ‘Windows of Etive’ the length: thickness ratio of this core is about 100:1. Bailey (Bailey & Maufe 1960, p. 83) suggests that the core may even extend farther SE to Beinn Doirean (but see Thomas & Treagus 1968) and NE to Kinlochlaggan. The writer considers that these various isolated outcrops of limestone are probably local intercalations within the Lochaber Group. Dr Roberts does not mention that to the

Discussion of structures at the southern margin of the Culm synclinorium

D rs E. G. F reshney , S. M c C ourt & E. B. S elwood write: The key to the undoubtedly complex structures of the south-west England region lies in a full understanding of the stratigraphic sequences, which are only now being resolved. Unfortunately Hobson & Sanderson's interpretation (1975 this Journal 131, 337–52) is based on stratigraphy which is not soundly conceived. Their proposition that there are major recumbent folds in the Boscastle area, while almost certainly true in principle, is nonetheless erroneous in the case of the folds actually described. A considerable part of the evidence for the folding is based on a misinterpretation of the stratigraphy, which is summarized in Table 1 (details of the geological evidence are given in Supplementary Publication No. SUP 18015 (9 pages), deposited with the British Library at Boston Spa, Yorkshire, U.K. and The Geological Society library). The fold reconstructions of Hobson & Sanderson depend on the equation of the Barras Nose Formation and the Trambley Cove Formation, but the strong stratigraphical and palaeontological evidence that these are separate and distinct formations indicates that the reconstruction is untenable. The acquisition of consistent data on fold facing and sense of overturning in rocks below the Crackington Formation is very difficult, owing to the paucity of reliable sedimentary structures and the intense dismemberment of the folds. Thus any attempt using such data to construct folds which do not agree with fairly reliable stratigraphical evidence must be regarded with caution. Hobson & Sanderson's structural synthesis (1975 fig. 9) indicates major

Major structures in Morar and Knoydart, NW Scotland

The Moine and Lewisian rocks of Morar and Knoydart on the west coast of Scotland have been involved in at least three major deformation phases. The earliest folds, in which Lewisian rocks form the cores of anticlines, and the youngest Moine formation the cores of synclines, are perfectly isoclinal. The second phase has produced a major recumbent fold, essentially a westward closing antiform, which refolds the previous structural succession. This recumbent fold, which is termed the Knoydart Fold, is underlain by a clearly recognized slide of approximately the same age which is called the Knoydart Slide. It is proposed that the axial plane of the Knoydart Fold is cut at a small angle by the slide, so that the fold and its closure have two main outcrops, one below the slide in the west and another above the slide in the east. The whole assemblage is finally refolded into the more open Morar Antiform in the west and the Ben Sgriol Synform in the east. The proposed structure implies that the Lewisian outcrops around the Morar Antiform are on the lower limb of the Knoydart Fold and root eastwards. These rocks may be traced round the hinge of Knoydart Fold and are represented on the upper limb by the Lewisian in east Morar, which consequently roots westwards.

Caledonian Geology of Scoresby Sund Region, Central East Greenland: ABSTRACT

The Geological Survey of Greenland has completed the first 3 years of a 5-year mapping campaign in the Scoresby Sund region which includes the southernmost Caledonian fold belt of East Greenland. The preliminary results show that it is possible to distinguish 4 main geologic units in the Caledonian fold belt. (1) A Precambrian crystalline basement with a cover of Precambrian metasediments and metavolcanics. These rocks are reworked by Caledonian folding to a varying degree. (2) A metamorphosed but nonmigmatized supracrustal complex composed of psammitic and pelitic rocks with occasional bands of calcareous rocks and totaling several kilometers in thickness. These rocks represent Caledonian geosynclinal deposits of miogeosynclinal aspect and are probably of very late Precambrian age. (3) A Caledonian infracrustal complex of mainly gneisses, migmatites and synkinematic granites. This unit is formed by migmatization of Caledonian geosynclinal deposits. (4) Late- to post-Caledonia intrusions, mainly of granitic type. Westward directed thrust-sheets with a displacement of several tens of kilometers are present along the western rim of the fold belt. They comprise largely Caledonian supracrustal rocks, but in places basement rocks are incorporated in the thrust sheets. The central part of the N-S directed fold belt is characterized by infracrustal rocks which exhibit simple low-dipping macroscopic structures, which in parts of the area can be shown to be the limbs of major recumbent folds. The Caledonian supracrustal rocks in the western part of the region were metamorphosed under high-pressure facies conditions and are characterized by kyanite-bearing rocks, whereas the central migmatitic part of the region was formed under low-pressure conditions and is characterized by cordierite-bearing assemblages. End_of_Article - Last_Page 2485------------

Metamorphic Structures in Part of the Eastern Otago Schists

Structures in part of the eastern Otago schists have been separated into several descriptive types and mapped accordingly. Structures have also been identified as products of four deformational phases and mapped on this basis. Phase II and III structures predominate. Mesoscopic folds, lineations, and s-surfaces developed during the phase II deformation in conjunction with formation of macroscopic folds. Phase III structures, not represented by macroscopic folds, include most of the mesoscopic types of the preceding deformation, but are considerably more variable in style and orientation. The only first-phase structure recognised is a schistosity; the only fourth-phase structure is a fine crenulation of micas.The orientation of large stratigraphic units has been determined in numerous localities by tracing contacts between units of contrasting lithology and by measuring the orientation of tabular bodies defined by schist tor ridges. These data along with observations of mesoscopic structures are used to construct a tectonic profile showing the macroscopic structure along a narrow belt extending about 25 miles in the Deep Stream — Nenthorn Stream area. The structure is dominated by one major recumbent fold closing to the south-west and having subsidiary macroscopic folds in the hinge area and on the upper limb.

The relationship of the Moine Schists and Lewisian Gneisses near Mallaigmore, Inverness-shire

A new map of the Mallaigmore area defines the area of the Lewisian gneiss more accurately than maps hitherto available, and shows the extent of transitional rock types produced by mechanical and metasomatic processes, occurring between the Lewisian and the Moine metasediments. Analyses of rocks from both major divisions show systematic differences in major elements and their ratios, but do not reveal any striking anomalies in minor element distribution. The criteria for recognising the Lewisian and the transitional rocks are given, and a major recumbent fold is postulated to account for the presence of the Lewisian.

TECTONICS OF THE COLDINGHAM BAY AREA OF BERWICKSHIRE, IN THE SOUTHERN UPLANDS OF SCOTLAND

Summary Structures are described from two sequences of Lower Palaeozoic rocks exposed in the Coldingham Bay area. In the north of the region the Coldingham Beds consist of alternating greywackes, shales and distinctive calcareous siltstones; thin greywackes and shales of the Linkim Beds occupy the shore to the south. The minor folds in the Coldingham Beds are much more complex than those found in the Linkim Beds. In the Coldingham Beds early folds were refolded to form a series of cross-folds; these, subsequently compressed into isoclines, were in turn refolded into a series of open concentric structures. In contrast, the minor folds within the Linkim Beds consist only of the late open structures. The major structure of the area is considered to be a southward-facing recumbent anticline involving both the Coldingham Beds and the Linkim Beds. The Lower Old Red Sandstone rocks which succeed the Linkim Beds to the south are folded into two asymmetrical anticlines, the southern having broadly arched the brow of the underlying recumbent structure. It is postulated that one phase of deformation affected the Coldingham Beds before the Linkim Beds were deposited and that, if so, the Coldingham Beds may be early Ordovician or older, rather than Silurian as previously thought. The major recumbent fold, the superimposed open folds and the refolded isoclines represent fold styles and structures which have not been recognized before in the Southern Uplands.

The Ben Lui Recumbent Syncline (S.W. Highlands)

AbstractStructural evidence from the Ben Lui district supports Bailey's view that the hinge of a major recumbent fold can be demonstrated there. Way up evidence indicates that all the recumbent structures from the Ben Udlaidh fold upwards face towards the south-east and south-south-east. The Ben Udlaidh and Ben Lui folds are therefore synclines. The attitude and character of the folds suggest gravitational flow.

The Geometry of the Ben Lui Fold (S.W. Highlands)

AbstractIt has been claimed that the hinge of one of the two major recumbent folds of the S.W. Highlands is exposed near Ben Lui. In this paper it is claimed that the form of the outcrops permits the existence of this recumbent fold only if the fold-axis plunges towards the south-west. Evidence is given that the plunge is in fact towards the west, and on this basis a profile has been constructed to illustrate the observed geometrical relationships of the rocks.

The Glen Orchy Anticline (Argyllshire)

I. Introduction. In a previous paper [1, p. 586] an explanation has been offered of the tectonics of the schists occupying a district of the West Highlands between Loch Tulla and Appin, in the south, and the River Spean, beyond Ben Nevis, in the north (fig. 1). In the present paper the south-eastern continuation of this district is dealt with, from the head of Loch Awe to Beinn Achallader, one of the summits of the Beinn Doirean range (Pl. X). In the district originally described lying to the west and north of Loch Tulla the following stratigraphical sequence was established; but, whether it should be read upwards or downwards, is still unascertained:— 9. Eilde Flags. 8. Glen Coe Quartzite (fine-grained). 7. Leven Schists (grey phyllites, with ‘Banded Series’ next the Glen Coe Quartzite). 6. Ballachulish Limestone (dark grey, with a thick cream-coloured margin next the Leven Schists). 5. Ballachulish Slates (black). 4. Appin Quartzite (pebbly). 3. Appin Limestone (cream, pink, or very pale blue). 2. Appin Phyllites (with a large proportion of flaggy quartzite). 1. Cuil Bay Slates (black). These rock-groups, as previously shown, are disposed in two major recumbent folds, known as the Ballachulish and Appin Folds respectively, to which may be added the subordinate Aonach Beag Fold, recognized as yet in the neighbourhood of Ben Nevis only. For convenience of description, the cores of these three folds have been arbitrarily defined as those portions which are made up of rocks included in Groups 1-6 of the foregoing stratigraphical table.