Large Recumbent Folds Research Articles

Polyphase Archean to Paleoproterozoic deformation along the northwest margin of the Johannesburg Dome, Kaapvaal Craton

Despite extensive gold exploration, the reasons for the dome shape of the Paleoproterozoic Johannesburg Dome (JD) remain elusive. We investigated the structural record of an outlier of Archean to Paleoproterozoic supracrustal rocks (the Zwartkops Hills Outlier, ZHO) overlying the JD to investigate its tectonic evolution. Steep gneissic fabrics in the basement pre-date the intrusion of ∼ 3225 Ma trondhjemite (new U–Pb zircon age). Two Archean deformation events affected West Rand Group rocks and pre-date Platberg Group deposition. To the south of the ZHO, a newly recognized, large-scale, recumbent fold formed during top-to-the-south shearing. In the central part of the ZHO, steeply W-dipping stratification results from E-W shortening. Two more deformation events are Paleoproterozoic in age and affected all units. A shallowly S-SW-dipping cleavage formed in the biotite zone of the greenschist facies is associated with N–S shortening and top-to-the-north shearing. The cleavage transects the large recumbent fold and is observed down to the upper 30 m of the basement. Finally, a weak N–S shortening event is inferred from kink folds and north-dipping axial planar cleavage. These data are consistent with the existing regional models of (i) south- and east-directed fold-and-thrust belts formed during sedimentation of the Archean Central Rand Group, (ii) north-verging schist belt formed during, or after the Transvaal Supergroup deposition, and (iii) impingement of the Bushveld Complex onto the JD during N–S shortening. These findings are integrated with previous regional sedimentological, seismic, and structural studies to suggest that the JD granitoid basement formed a paleo-high during the deposition of the Central Rand and Platberg groups. Once buried in the upper crust, the paleo-high acted as a rigid granitoid promontory that indented weak surrounding cover rocks during Paleoproterozoic N–S contraction events, creating the dome shape of the Johannesburg Dome.

Deformation within an exposed salt wall: Recumbent folding and extrusion of evaporites in the Dead Sea Basin

Despite the enormous global interest in salt tectonics, which is largely driven by its importance to hydrocarbon exploration, direct field-based studies of salt exposed at the Earth's surface are rare. However, Mount Sedom, located at the western side of the Dead Sea Basin, presents one such opportunity for detailed analysis of salt and the associated sedimentary and structural record of its movement. The Sedom salt wall is a 10 km × 1.5 km N–S trending ridge comprising a range of Late Miocene-Pliocene evaporites and clastics, which have penetrated the overlying Pleistocene clastic sequence. The salt wall displays a moderate-steep west dipping western margin and an overturned (west-dipping) eastern flank. The sedimentary record of passive wall growth includes sedimentary breccia horizons that locally truncate underlying beds and are interpreted to reflect sediments having been shed off the crest of the growing salt wall. Structurally, the overturned eastern flank is marked by upturn within the overburden, extending for >300 m from the salt wall. Deformation within the evaporites is characterised by ductile folding and boudinage, while a 200 m thick clastic unit within the salt wall forms a tight recumbent fold traceable for 5 km along strike and associated with a 500 m wide inverted limb. This overturned gently-dipping limb is marked by NE-directed folding and thrusting, sedimentary injections, and a remarkable attenuation of the underlying salt from ∼380 m to <20 m over just 200 m of strike length. The inverted limb is overlain by an undeformed anhydrite, gypsum and clastics caprock, thought to be the residue from a now dissolved salt sheet that extruded over the top of the fold. Expulsion of salt down the regional slope towards the NE, combined with subsequent dissolution of evaporites, may have resulted in local ‘pinching shut’ of the salt wall, leading to a distinctive hour-glass map pattern. This area also coincides with deposition of a thicker overlying clastic sequence, indicating continued subsidence of this part of the salt wall. Our detailed fieldwork forms the first direct observation and description of large recumbent folds within salt walls, and permits analysis below the limits of seismic resolution. It thereby allows more rigorous testing of salt tectonic models and mechanisms.

Cambrian orogeny in the Ribeira Belt (SE Brazil) and correlations within West Gondwana: ties that bind underwater

Abstract A 530–490 Ma tectono-metamorphic event, the Búzios orogeny, is recognized within the Ribeira Belt, along the coast of SE Brazil. Tectonic evolution started with a Late Neoproterozoic marine basin and volcanic activity at c . 610 Ma. The rocks in this basin were affected by high-grade metamorphism at c . 530 Ma, coeval with deformational phases D 1 –D 2 , which generated compressive low-angle tectonic structures with top-to-NW tectonic transport. Large recumbent folds with NW–SE axes parallel to the main stretching lineation formed during D 3 as the Cabo Frio tectonic domain, the focus of this study, collided with the Oriental terrane to the NW. D 4 sub-vertical shear zones are limited in extent. A new U–Pb age of 501±6 Ma is reported for zircon from an amphibolite-facies shear zone related to either D 3 or D 4 . Post-tectonic 440 Ma pegmatites mark the final stage of tectono-magmatic activity. The Cabo Frio tectonic domain has African affinities and is exotic to the Ribeira Belt. Middle Cambrian deformational and metamorphic ages are also reported from the ‘Angolan’ Pan-African belt, the southern Kaoko and Damara belts in Namibia, and the Cuchilla Dionisio–Punta Del Este terrane in Uruguay. The occurrence of Cambrian metamorphic rocks along the present African and South American coastlines shows that Mesozoic rifting closely follows Palaeozoic sutures of West Gondwana.

A new web-based operative skills assessment tool effectively tracks progression in surgical resident performance

Despite the enormous global interest in salt tectonics, which is largely driven by its importance to hydrocarbon exploration, direct field-based studies of salt exposed at the Earth's surface are rare. However, Mount Sedom, located at the western side of the Dead Sea Basin, presents one such opportunity for detailed analysis of salt and the associated sedimentary and structural record of its movement. The Sedom salt wall is a 10 km × 1.5 km N–S trending ridge comprising a range of Late Miocene-Pliocene evaporites and clastics, which have penetrated the overlying Pleistocene clastic sequence. The salt wall displays a moderate-steep west dipping western margin and an overturned (west-dipping) eastern flank. The sedimentary record of passive wall growth includes sedimentary breccia horizons that locally truncate underlying beds and are interpreted to reflect sediments having been shed off the crest of the growing salt wall. Structurally, the overturned eastern flank is marked by upturn within the overburden, extending for >300 m from the salt wall. Deformation within the evaporites is characterised by ductile folding and boudinage, while a 200 m thick clastic unit within the salt wall forms a tight recumbent fold traceable for 5 km along strike and associated with a 500 m wide inverted limb. This overturned gently-dipping limb is marked by NE-directed folding and thrusting, sedimentary injections, and a remarkable attenuation of the underlying salt from ∼380 m to <20 m over just 200 m of strike length. The inverted limb is overlain by an undeformed anhydrite, gypsum and clastics caprock, thought to be the residue from a now dissolved salt sheet that extruded over the top of the fold. Expulsion of salt down the regional slope towards the NE, combined with subsequent dissolution of evaporites, may have resulted in local ‘pinching shut’ of the salt wall, leading to a distinctive hour-glass map pattern. This area also coincides with deposition of a thicker overlying clastic sequence, indicating continued subsidence of this part of the salt wall. Our detailed fieldwork forms the first direct observation and description of large recumbent folds within salt walls, and permits analysis below the limits of seismic resolution. It thereby allows more rigorous testing of salt tectonic models and mechanisms.

Late amalgamation in the central part of West Gondwana: new geochronological data and the characterization of a Cambrian collisional orogeny in the Ribeira Belt (SE Brazil)

New U–Pb data reveal that during the mid-Cambrian the central part of West Gondwana was still undergoing a high-grade tectonometamorphic event corresponding to collision. The studied area is located in the southeastern part of the Pan-African–Brasiliano Ribeira Belt, in SE Brazil. The area is part of the Cabo Frio Tectonic Domain (CFTD) which is limited to the NW by a major NE–SW striking thrust zone which separates it from the Neoproterozoic “Oriental terrane”, whereas to the SE it is covered by the Atlantic Ocean. The domain comprises a Paleoproterozoic orthogneissic basement tectonically interleaved with younger supracrustal rocks, folded and metamorphosed at upper amphibolite to granulite facies during the mid-Cambrian. The supracrustal rocks are subdivided in two successions: Búzios (Al-metapelites, calcsilicates and amphibolites) and Palmital (quartz–feldspathic metasediments with minor metapelites). These successions were deposited in a deep oceanic environment between ca. 620 and 525 Ma as indicated by SHRIMP U–Pb data for detrital zircons and by T DM model ages. The metamorphic peak, defined by the mineral associations Ky + Kfs in metapelites and Cpx + Grt + Qz in amphibolites, occurred at minimum pressure of 9 kbar and temperature in excess of 780 °C. At this stage migmatites were generated by partial melting in all lithostratigraphic units, including the amphibolites. The metamorphic peak was also contemporaneous with top to the NW thrusting, testified by mineral and stretching lineations related to progressive deformation phases D1 and D2. The metamorphic peak was dated between 525 and 520 Ma, as determined by U–Pb analyses of zircons of leucosomes. During deformation phase D3, large recumbent folds developed with NW–SE axes, parallel to the main direction of movement. The CFTD was juxtaposed at this stage to the “Oriental terrane” by a major NE–SW striking thrust fault. U–Pb dating of monazites from metapelites and of sphenes from amphibolites revealed ages of about 510 Ma for the mineral growth. The sillimanite, aligned as L 3, partially replaced kyanite, indicating a clockwise P– T– t path for the central and eastern areas of the CFTD. After docking into the Ribeira belt, during the late Cambrian, the western limit of CFTD was affected by a transcurrent dextral shear zone that developed a NE–SW stretching lineation related to D4, under amphibolite facies conditions. This is recorded in monazites and zircons within this shear zone with U–Pb ages ranging from 505 to 490 Ma. At this stage, the central and eastern parts of CFTD were already cooling at a rate of 10 °C/Ma. After 480 Ma, the cooling rate diminished to 5 °C/Ma. A 207 Pb/ 206 Pb age in rutile (480±5 Ma) and a U–Pb zircon age in a post-tectonic pegmatite (440±11 Ma) mark the stabilization of the area during the Ordovician–Silurian boundary. This well-constrained Cambrian collisional orogeny is here denominated the Búzios Orogeny and is the youngest tectonometamorphic event recorded in the Brasiliano belts of Brazil. This orogeny is contemporaneous with some other marginal orogenies (e.g. Pampeana; Ross) that probably accompanied the final adjustments of the Precambrian continents to complete the formation of Gondwana in the Ordovician.

A computer program for the simulation of folds of different sizes under the influence of gravity

Folding&g is a computer program, based on the finite element method, developed to simulate the process of natural folding from small to large scales in two dimensions. Written in Pascal code and compiled with Borland Delphi 3.0, the program has a friendly interactive user interface and can be used for research as well as educational purposes. Four main menu options allow the user to import or to build and to save a model data file, select the type of graphic output, introduce and modify several physical parameters and enter the calculation routines. The program employs isoparametric, initially rectangular elements with eight nodes, which can sustain large deformations. The mathematical procedure is based on the elasticity equations, but has been modified to simulate a viscous rheology, either linear or of power-law type. The parameters to be introduced include either the linear viscosity, or, when the viscosity is non-linear, the material constant, activation energy, temperature and power of the differential stress. All the parameters can be set by rows, which simulate layers. A toggle permits gravity to be introduced into the calculations. In this case, the density of the different rows must be specified, and the sizes of the finite elements and of the whole model become meaningful. Viscosity values can also be assigned to blocks of several rows and columns, which permits the modelling of heterogeneities such as rectangular areas of high strength, which can be used to simulate shearing components interfering with the buckling process. The program is applied to several cases of folding, including a single competent bed and multilayers, and its results compared with analytical and experimental results. The influence of gravity is illustrated by the modelling of diapiric structures and of a large recumbent fold.

Large extensional structures developed during emplacement of a crystalline thrust sheet: the Mondoñedo nappe (NW Spain)

The Mondoñedo nappe is a crystalline thrust sheet characterized by large recumbent folds, regional intermediate-pressure metamorphism, synkinematic intrusion of granitoids during nappe emplacement, and an extensional ductile shear zone developed within the nappe during thrusting. A large tectonic window permits the study of the footwall unit, revealing another extensional shear zone contemporaneous with thrusting and a low-pressure metamorphic evolution, in contrast to that of the hanging wall unit. The two main extensional shear zones produced E–W extension parallel to the direction of orogenic shortening and normal to the orogenic structural trend. Furthermore, subordinate N–S longitudinal extension was accommodated by normal faults in the footwall, and some of these faults were used as lateral ramps in late stages of thrusting. The role of the extensional shear zones and faults described is discussed in the context of an evolving orogenic wedge dominated by plate convergence but characterized by large-scale rheological heterogeneities within it. Deep-seated viscous flow, triggered by heat accumulation, seems to account for the horizontal stretching and probable tapering of the orogenic wedge, which was induced by gravitational instabilities due to partial melting and underplating by buoyant continental crust.

Structure and stratigraphy of the south flank of the Kisseynew Domain in the Trans-Hudson Orogen, Manitoba: implications for 1.845-1.77 Ga collision tectonics

On the south flank of the Kisseynew Domain, orthogneisses derived from 1.92-1.85 Ga volcano-plutonic rocks are overlain by paragneisses (Burntwood and Missi groups) derived from 1.855-1.84 Ga marine turbidite and 1.845-1.83 Ga terrestrial clastic and volcanic rocks. The sediments in these groups are interpreted as having been shed into the Kisseynew paleobasin from an active margin bordering the Flin Flon Belt. The sedimentation apparently followed early microcontinental collision and accompanied the last arc magmatism in the Trans-Hudson Orogen. The sedimentary rocks and their basement were deformed into a complexly refolded stack of large recumbent folds. Premetamorphic F1 structures represent a fold and thrust system initiated during the sedimentation. These structures are interpreted as transported toward the Kisseynew Domain in the northeast and the hinterland in the southwest. F2 structures (~1.82 Ga) comprise westerly transported nappes. During 1.82-1.80 Ga high-grade metamorphism, the early structures were overturned, amplified, and refolded. Basement-cored culminations and sheet-like synforms of paragneiss were horizontally attenuated and transported south and southwest. North- and northeast-trending F4 folds and F5 faults formed after 1.79 Ga. The whole cycle of deformation is related to stages of continental collision between the internal (juvenile) zone of the Trans-Hudson Orogen and the three surrounding Archean cratons (Sask, Superior, and Hearne). The F4 upright folds and steep F5 faults are interpreted as the record of intracontinental transpression, strongly controlled by the Superior Craton boundary.

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.

Development of large north-facing folds and their relation to crustal extension in the Alborán domain (Alpujarras region, Betic Cordilleras, Spain)

Detailed research in the eastern part of the Alpujarras region, Betic Cordilleras, has revealed the existence of large recumbent fold structures involving lithological sequences previously considered to belong to different tectonic units. We have reconstructed a large, complex fold structure (which can be followed axially for over 50 km) based on a careful revision of the boundaries between lithological formations, a structural analysis of the carbonate and metapelite sequences, and the use of mineralogical techniques such as measurements of d 001 basal spacing of phengite, paragonite and chlorite, determinations of the illite `crystallinity' index (IC) and b 0 parameters of mica and chlorite. Structural field data described in this paper include gently dipping axial plane cleavage surfaces, common occurrence of gently dipping boudinage and pinch-and-swell structures and associated sub-orthogonal stretching lineation, similarity in orientation of this stretching lineation with the slip direction of the low-angle normal faults, and close association between north-facing recumbent folds and top-to-the-north low-angle normal faults (the slip direction of the faults being broadly perpendicular to the fold axes). These features clearly indicate that extensional processes played a decisive role in the generation of the Alpujarras large recumbent folds. The development of these recumbent folds probably occurred during the Early to Middle Miocene extensional tectonics that affected the Betic Cordilleras. The formation of large-scale folds in association with normal faulting is consistent with the P–T paths and geochronological data established for the Alpujarride region.

Depositional models for moraine formation in East Antarctic coastal oases

AbstractThis paper examines the origin of moraine ridges in East Antarctic coastal oases and derives depositional models appropriate for the reconstruction of Quaternary history. On the basis of morphology, structure and sedimentology, four principal types of ridge may be identified: (1) type A moraines from when the basal debris zone crops out near an ice margin; (2) type B moraines form when large recumbent folds develop in the basal debris zone; (3) type C moraines are ice-contact screes and fans which form when debris accumulates at steep or cliffed ice margins; and (4) type D moraines are thrust-block moraines that form when unconsolidated sediment is entrained by freezing, shearing and thrusting of sediment blocks at the base of the glacier. Simple calculations of the rate of debris accumulation at ice margins suggest that type A, B and C moraines take thousands of years to form and record stable ice margins. Type D moraines are structural features that may form relatively quickly when ice margins override unconsolidated sediment. Constructing models to explain the origin of the moraines is an important part of reconstructing the Quaternary history of Antarctic coastal oases, because the models provide a basis for reconstructing the position and behaviour of the ice sheet during advance and retreat

Depositional models for moraine formation in East Antarctic coastal oases

AbstractThis paper examines the origin of moraine ridges in East Antarctic coastal oases and derives depositional models appropriate for the reconstruction of Quaternary history. On the basis of morphology, structure and sedimentology, four principal types of ridge may be identified: (1) type A moraines from when the basal debris zone crops out near an ice margin; (2) type B moraines form when large recumbent folds develop in the basal debris zone; (3) type C moraines are ice-contact screes and fans which form when debris accumulates at steep or cliffed ice margins; and (4) type D moraines are thrust-block moraines that form when unconsolidated sediment is entrained by freezing, shearing and thrusting of sediment blocks at the base of the glacier. Simple calculations of the rate of debris accumulation at ice margins suggest that type A, B and C moraines take thousands of years to form and record stable ice margins. Type D moraines are structural features that may form relatively quickly when ice margins override unconsolidated sediment. Constructing models to explain the origin of the moraines is an important part of reconstructing the Quaternary history of Antarctic coastal oases, because the models provide a basis for reconstructing the position and behaviour of the ice sheet during advance and retreat

A palaeomagnetic study of recumbently folded and thermally metamorphosed Torridon Group sediments, Eishort anticline, Skye, Scotland

Two geologically significant palaeomagnetic components have been isolated in samples of Torridon Group sandstones from the Kishorn nappe of NW Scotland. The samples were collected from the Eishort anticline, a large recumbent fold of Caledonian age. The softer component (which is demagnetized by low temperatures and weak magnetic fields), when in situ, displays a steep upward direction typical of reversed Tertiary magnetizations. Both polarities of the harder component (which is demagnetized by high temperatures only) are present and, when dip corrected, display either northwesterly upward or southeasterly downward directions characteristic of Torridonian remanences. It is suggested that the Tertiary component is of chemical origin and the result of alteration of magnetite (titanomagnetite) grains. The geographical distribution of these grains is irregular and as a result the distribution of the Tertiary overprint is irregular. The Torridonian palaeomagnetic components from both limbs of the fold can be restored by rotation about the fold axis to internally consistent directions which lie close to previously published data from the Caledonian foreland. No correction is necessary for finite strain which suggests that the Eishort anticline formed by a flexural slip mechanism. However, an anticlockwise rotation of 26° is required to reconcile the Torridonian directions from the Kishorn nappe with those of the Caledonian foreland which suggests that, during emplacement, the thrust sheet rotated on the underlying fault surface.

A reappraisal of the facing confrontation in north Cornwall: fold- or thrust-dominated tectonics?

The problem of conflicting fold or thrust solutions to the interpretation of the structure of north Cornwall is discussed via new detailed mapping of the coastal sections. In addition to facing evidence, shear criteria have been extensively applied in order to sustain new interpretations. The facing confrontation at Polzeath is confirmed but the high strain zone centred upon Tintagel is ascribed to later superimposed northwards directed ductile shearing. Peak metamorphism was synchronous with this event. Subsequent northwards displacement became confined to discrete thrust planes dismembering the south-facing structures which developed during an intermediate phase of backthrusting. The major structures are thrust-dominated, though large recumbent folds were generated during the early ductile phases. The facing confrontation is probably allochthonous having been carried northwards on a blind thrust. It is suggested that the important Rusey Fault Zone is the roof thrust to a triangle zone in which the lower grade rocks of the Culm Basin are uplifted over an underlying antiformal stack.

Tectonic evolution of the west Spitsbergen fold belt

Abstract The west Spitsbergen fold belt has a complex tectonic history which is recorded in a thick, nearly complete upper Proterozoic-Phanerozoic layered sequence. Work since 1977 near Bellsund allows recognition of the main deformational events in that segment of the fold belt. The strata are grouped informally into the metamorphosed Hecla Hoek (HH) sequence (Proterozoic) and the Van Keulenfjorden (VK) sequence (Carboniferous-Cretaceous), separated by a pronounced unconformity. The HH is divided into the Antoniabreen succession, the Chamberlindalen succession, and the Kapp Lyell tillite; the first two consist of diverse clastic and carbonate rocks, along with some volcanic rocks. The VK is a platform sequence of shallow marine and terrestrial sedimentary rocks. These layered rocks are intruded by a few Mesozoic dolerite sills and dikes. Subhorizontal Paleogene sedimentary rocks are preserved in a small graben. Both layered sequences have undergone strong deformation. Structures (mainly Caledonian age) in the HH can be grouped as phase 1 (small isoclinal folds, subhorizontal axial planar foliation, ridge-groove lineation in the foliation, and large recumbent folds), phase 2 (tight to isoclinal folds, axial planar foliation), and younger (weak folds and foliations, kink bands, crinkles). Structures (mainly Tertiary age) in the VK include faults (thrust, reverse, down-to-the-east bedding-plane normal, other normal); folds (symmetric, asymmetric, overturned, recumbent, isoclinal); and foliation (in some tightly folded Triassic shales). Six deformational events can be identified here, each described below in terms of age, intensity, and kinematic pattern: • D1 - Vendian or early Paleozoic; very strong; NNW-SSE shortening?, NNW transport? • D2 - Early Paleozoic (pre-Carboniferous); strong; NE-SW shortening, NE vergence. • D3 - Middle (?) Carboniferous; moderate; unclear (limited exposure). • D4 - Early Cretaceous (?); weak; extension, direction unclear. • D5 - Early (?) Tertiary; very strong locally; uplift in W, ENE-WSW shortening, ENE translation. • D6 - Middle (?) Tertiary; moderate; NE-SW extension.

Structural and sedimentary history of the Quaco Head area, southern New Brunswick

Remapping of the Quaco Head area of southern New Brunswick has revealed a more complex structure than had been previously documented. The structure is dominated by two major thrust faults that dip to the south and southeast at between 35 and 45°. The lower (younger?) thrust emplaces Precambrian basalts and tuffaceous siltstones (Coldbrook Group) over alluvial sediments of the Boss Point and Tynemouth Creek formations (Westphalian A–B). Resting unconformably on the upper surface of the basalt is a thin layer (≤ 10 m) of marine, basal Windsor Group limestone, disconformably overlain by alluvial conglomerates of inferred Hopewell Group age. This sequence is truncated by an older, southeast-dipping thrust fault, to the southeast of which lie Hopewell conglomerates overturned towards the west and northwest. The latter sequence probably forms the lower limb of a large recumbent fold. Folding and thrusting in the Quaco Head area may have begun during Westphalian B times, although major movement was probably post-Westphalian C and occurred contemporaneously with dextral strike-slip movement on the Minas geofracture. Following this Alleghenian–Variscan deformation, the area underwent deep erosion prior to the deposition of a thick sequence of Triassic alluvial sediments. The more southerly reverse fault was reactivated as a normal fault during a phase of late Triassic or Jurassic faulting along the Bay of Fundy that was probably related to early Atlantic rifting.

Additional evidence of pre-Silurian high-pressure metamorphic rocks in Spitsbergen

It has been previously suggested that the high-pressure metamorphic rock complcx of Motalafjclla. central-western Spitsbergen, is older than Lower Silurian. An unconformity has been discovered at the base of a limestone which contains fossils similar to those reported as Lower Silurian from the same massif. This proves that the high-pressure metamorphics represent an older rock complex. The unconformity surfacc is invcrted in the area, and large recumbent folds involving the Silurian Bulltindcn Formation havc been mapped. These folds are considered to be prc-Carboniferous,

Finite strain patterns in some Alpine nappes

The patterns of finite strain in some Alpine nappes and thrust sheets are reviewed, together with their tectonic evolution. Glarus Nappe. The finite strain pattern, approximately that of heterogeneous simple shear, was acquired during Upper Oligocene times when the Helvetic thrust block was deformed together with its infrastructure and basement. The Glarus Thrust was formed during Miocene times and modified by Pliocene to Recent differential uplift. Morcles Nappe. This large recumbent fold nappe overrode the Aiguilles Rouges Massif and its cover towards the northwest. The finite-strain pattern resulted from successive diagenetic compaction, multilayer buckling and heterogeneous simple shear. High strains occur along the inverted limb of the nappe and fold hinges are oriented subperpendicular to the NW stretching lineation. Final emplacement of the nappe, during the Miocene, was perhaps a later event. Chartreuse Massif. 33% E-W shortening was developed across this fold and thrust belt during post-Helvetian times by décollement along the Oxfordian shales, in response to thrusting and uplift in the Belledonne basement. The Oxfordian shales had earlier accommodated a pre-Helvetian (probably Oligocene) deformation in the basement and its Trias-Bajocian cover, when N030° trending, subhorizontal folds, cleavage and a stretching lineation (18% N030° stretching) were developed. Digne Nappe. Detached along the Keuper evaporites, the cover was displaced westwards in two stages: (1) late Oligocene, with folding, cleavage development, formation of the frontal slices and an overall E-W shortening of 20% and (2) late Miocene-Pliocene, after emplacement of the Embrunais Nappes in the east, producing the Digne Thrust. It is concluded that for these Alpine nappes geological criteria must be used to establish their evolution. If the internal deformation can then be assigned to part of this evolution, the pattern of finite strain can be used to quantify and elucidate that part. A maximum of information is gained if the incremental deformation history can also be obtained from study of syntectonic fibres.

Additional evidence of pre-Silurian high-pressure metamorphic rocks in Spitsbergen

It has been previously suggested that the high-pressure metamorphic rock complcx of Motalafjclla. central-western Spitsbergen, is older than Lower Silurian. An unconformity has been discovered at the base of a limestone which contains fossils similar to those reported as Lower Silurian from the same massif. This proves that the high-pressure metamorphics represent an older rock complex. The unconformity surfacc is invcrted in the area, and large recumbent folds involving the Silurian Bulltindcn Formation havc been mapped. These folds are considered to be prc-Carboniferous,

The upper Batinah Complex, Oman: allochthonous sediment sheets above the Semail ophiolite

The upper Batinah Complex consists of allochthonous sedimentary sheets that structurally overlie both the Semail ophiolite and its cover of mélange (lower Batinah Complex). Sequences represent a former sedimentary transition across the outer part of a passive Mesozoic continental margin. From proximal to distal, units include redeposited quartzitic sandstones and shallow-water limestones, large carbonate buildups floored by Triassic mafic lavas, and pelagic radiolarites or hemi-pelagic calcilutites also underlain by Triassic mafic lavas. Sheets of these upper Batinah Complex sequences were emplaced onto the upper surface of the Semail ophiolite in Late Cretaceous time, during or soon after the advance of the Semail Nappe onto the Arabian margin. Emplacement was along subhorizontal fault zones, with sheets of distal facies followed by successively more proximal sheets. Structural style suggests a low overburden during deformation, with ductile flow of argillaceous material but more brittle behavior of other lithologies. Large recumbent folds are present. Fold orientations imply sheet transport from between northeast and southeast.We conclude that the Batinah sheets represent part of the passive margin to a continental mass originally northeast of the main Arabian margin, but which is no longer present. We favour high-level gravity emplacement of Batinah sheets, and argue against trench accretion as an emplacement mechanism for either the sheets or the Semail Nappe itself.