Regional-scale Folds Research Articles

Köli nappes in the north-central Swedish Caledonides – new views on stratigraphy and structural evolution

The Köli Nappes of the Scandinavian Caledonides are composed of oceanic and island-arc terranes of Cambro–Silurian age and represent the upper part of the major Caledonian Upper Allochthon. Field, structural, microscopic, geophysical and geochemical data, together with a review of published information show new aspects on stratigraphy, structural evolution, nappe geometry and late Ordovician greywacke provenance in the Köli Nappes of southwestern Västerbotten and northern Jämtland counties, Sweden. It is proposed here to divide the lower part of the Köli succession of this area, i.e. the correlative of the Seima Formation, into three formations, the Kvemoen Mica Schist, the Ankarede Volcanite, and the Gilliks Formations. Trace element geochemistry of Ordovician Gilliks Formation greywackes indicates a provenance from a continental magmatic arc. New data also show that the overlying quartzite and limestone succession interpreted as the Vojtja and Slätdal Formations here is representing the top of the Stikke Nappe succession, which thus now reaches up into the highest Ordovician. The early repetition and inversion of the stratigraphy is an effect of D1-D2 deformation. The subsequent D3 episode comprises thrusting of, for example, the Middle Köli Nappe over the Lower Köli Nappe. A new aspect is that fault bends in D3 thrusts may give rise to fault-bend folds in overlying units, which are also D3 structures. The D3 episode also includes movement of the Köli units relative to the underlying Seve Nappes with top-to-the-west-directed shearing at the Seve-Köli boundary. It is succeeded by D4 regional-scale folds, related to nappe stacking in the underlying allochthons. The east-dipping limbs of these folds are thus locked for later, top-to-the-west shear deformation. Therefore, at least these parts of the Seve-Köli boundary cannot be regarded as re-activated during late (D5) and post-Caledonian gravitational deformation. Further work will have to show, whether the observed latest stage deformation represents late Caledonian tectonics or is related to subsequent extensional tectonics.

Emplacement of ellipsoid-shaped (diapiric?) granite: Structural and gravimetric analysis of the Oulmès granite (Variscan Meseta, Morocco)

The Oulmès granite is a NE–SW elongated stock intruded in Ordovician metasedimentary rocks which crop out at the core of a regional anticlinorium of the Moroccan Variscan Meseta. The stock, dated at around 300 Ma, is made up of peraluminous two-mica granite, with subordinate amounts of muscovite leucogranite. Mineral composition and textural features of both the granite and the thermal aureole enable us to constrain the P–T conditions during magma intrusion in ≈250–300 MPa and ≈600 °C at the contact between the granite and its host rock. Emplacement of the Oulmès granite postdates the main regional structures and is coeval with the late stages of Variscan compression. Prior to granite intrusion, the host rocks were affected by a main deformation phase responsible for the development of a penetrative slaty cleavage and regional-scale folds. Granite emplacement gave way to a 2 km-thick strain aureole, which includes both the uppermost part of the stock and the surrounding host rocks. The strain aureole is characterized by a gently-dipping planar-linear fabric with a generally weak N–S oriented stretching lineation. In the granite, the fabric of the strain aureole consists of a solid-state, locally mylonitic, foliation. In the host rocks, a secondary foliation develops which crenulates the older regional foliation. Magmatic structures, mainly consisting of steeply dipping N–S oriented magmatic foliation, are preserved in the central part of the pluton. The 3D shape of the granite has been modelled from gravity data and, together with the observed magmatic flow along subvertical planes and the strain recorded in the aureole, leads us to envisage a diapiric-dominated mechanism for the emplacement of the Oulmès granite. Drawing on the petrogenesis of peraluminous granites, the diapiric ascent of the granite probably has not exceeded 10 km.

Gold-bearing fault zones related to Late Archean orogenic folding of upper and middle crust in the Abitibi granite-greenstone belt, Ontario

This work investigates the structures of the Abitibi granite-greenstone belt, Ontario, with a focus on regional-scale folds and on regional-scale auriferous fault zones. The folds and the fault zones resulted from Late Archean orogenesis. The analysis is based on studies of surface geology, and on modelling of the gravity field. Amphibolite-grade orthogneiss was involved in the regional folding that also deformed the overlying greenstones. Folding resulted in uplift of the orthogneiss that preserves the metamorphic mineral assemblage ferropargasite + garnet + biotite + andesine + quartz. Geothermobarometric calculations indicate recrystallization of a metatonalite sample under peak paleopressures of 7.0 ± 1.2 to 7.9 ± 1.1 kb, providing evidence that folding involved crust to depths of (at least) 18–25 km. The regional Bouger anomaly map reveals previously unrecognized first-order folds. Interpretation of the Bouger anomaly map also confirms that granitic batholiths are hosted by anticlines. A wavelet-based inversion method was used to determine the locations, orientations and continuations at depth of the maxima of density contrasts. The maximum density contrasts correspond to steeply dipping geological contacts on the limbs of the open to tight regional-scale folds. Regional-scale fault zones only have prominent gravity signatures where they correspond to sheared rocks on the limbs of folds. Modelled gravity profiles were used to produce profiles that visualize the shapes of batholiths and greenstone bodies within the present-day upper crust. The granitic batholiths have tabular shapes which are locally folded, with preserved main body thicknesses of 1–3 km. The greenstones form keels, corresponding to synclines, some of which reach 5–6 km depth. The results of this study confirm that the principal deformational response of the Abitibi granite-greenstone belt to Late Archean collisional tectonics was large half-wavelength (10–40 km) folding of the upper and middle crust. Our results and interpretations also strongly suggest the regional-scale fault zones, which are loci for orogenic gold deposits, are fold-related structures. They represent quasi-continuous lineaments of shearing and faulting on the limbs of regional folds, and locally in the cores of detachment anticlines as shown in a previous paper.

Centrifuge modelling of folding in high-grade rocks during rifting

Centrifuge modelling is used to simulate the progressive development of structures in a simplified crustal profile during rifting. This paper focuses on folding of ductile layers representing the middle to lower crust. Displacement along a pre-existing cut in the layer representing mantle lithosphere creates a broad shear zone in overlying ductile layers and an asymmetric rift in upper layers. The footwall of the ‘mantle lithosphere’ layer and overlying shear zone are rotated to sub-horizontal during rise of the basal ductile layer (representing asthenosphere) as an isostatic response to thinning in the extended model. Synthetic and antithetic faults develop in models with thicker ductile ‘crustal’ layers. Although upper, semi-brittle layers, representing upper crust constitute a simple fault-bounded or sag ‘basin’, the underlying ductile layers are complexly folded. Open, upright folds developed over the crest of the footwall ‘mantle lithosphere’ layer are tightened and increased in amplitude as they progressively rotate to a recumbent attitude. Refolding occurs during rise of the basal ductile material representing asthenosphere and from boudinage of overlying competent layers. This study suggests that regional-scale folds in some high-grade terrains previously interpreted as evolving in a convergent margin tectonic setting may instead be produced as a result of flow of the ductile crust during rifting.

Paleomagnetic polarity reversals in Marinoan (ca. 600 Ma) glacial deposits of Australia: Implications for the duration of low-latitude glaciation in Neoproterozoic time

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Upper and lower plate juxtaposition, deformation and metamorphism during crustal convergence, Trans-Hudson Orogen (Quebec–Baffin segment), Canada

The lower plate of the Quebec–Baffin segment of Paleoproterozoic Trans-Hudson Orogen comprises parautochthonous Archean crystalline basement of the Superior Province (3.22–2.74 Ga) and imbricated Paleoproterozoic rift-margin strata (2.04–1.92 Ga). The collisional upper plate contains (1) allochthonous ophiolitic units (2.00 Ga), (2) a continental arc terrane (Narsajuaq arc; 1.86–1.82 Ga), (3) shelf sequence strata (ca. <1.93 Ga and >1.86 Ga) and basement orthogneisses (ca. 1.95 Ga), (4) foreland basin deposits (<1.93 Ga and >1.86 Ga), and (5) a batholith which intrudes the shelf and foreland basin rocks (Cumberland batholith; 1.86–1.85 Ga). Lower and upper plate assemblages were juxtaposed during a 1.82–1.79 Ga accretion–collision event, and deformed into regional-scale folds during two post-collisional folding episodes (ca. 1.76–1.74 Ga). As a result of the structural relief generated by the post-collisional folding and subsequent exhumation, all orogenic elements can be directly examined along a 400 km, across-strike transect. In the external zone (northern Quebec), lower plate rift-margin units were internally imbricated (Cape Smith Belt) and translated southward across the Superior Province basement. Paleoproterozoic amphibolite facies mineral growth is syn to post-thrusting. Tectonic imbrication has resulted in a structural inversion of the accreted (i.e. upper plate) ophiolite stratigraphy. North of the Cape Smith Belt, mid-crustal plutonic units of the Narsajuaq arc form an upper plate crystalline wedge whose sole thrust is localized near the thrust belt's basal décollement. Within Narsajuaq arc, syn-magmatic granulite facies assemblages are retrogressed to amphibolite facies assemblages in proximity to major accretion–collision faults. In the internal zone (southern Baffin Island), a tripartite orogen-scale tectonostratigraphy is exposed on the flanks of the post-collisional fold structures. At the lowest structural level, lower plate Superior Province orthogneisses are imbricated with Paleoproterozoic rift-margin units, and mineral growth is syn-thrusting. At intermediate structural level, upper plate orthogneisses are correlated with the Narsajuaq arc in northern Quebec. At the highest structural level, a north-facing shelf sequence is structurally overlain by an assemblage of clastic rocks interpreted as a foreland basin sequence. Dominantly-monzogranite plutons of the Cumberland batholith are restricted to the upper structural level and indicate pre-emplacement imbrication of tectonostratigraphic units. Within both upper and lower plates, granulite facies assemblages are retrogressed to upper amphibolite assemblages in proximity to 1.82–1.79 Ga accretion–collision structures. Exposures of Superior Province basement across the orogen document a progressive increase in the extent of reworking during accretion of upper plate units, from a narrow shear zone in the external zone to km-scale imbrication and more pervasive shear deformation in the internal zone. A minimum of 160 km of basement underthrusting is constrained by the structural overlap of upper plate allochthons on lower plate units. Basement involvement during post-collision shortening is manifest as thick-skinned folding, indicating coupling across the collisional basal décollement.

An Overview of Paleozoic-Mesozoic Structures Developed in the Central White-Inyo Range, Eastern California

The central White-Inyo Range of eastern California is composed of a 7.5 km thick succession of Neoproterozoic to Upper Cambrian sedimentary rocks that have been deformed into a series of NE and N to NW-trending folds. The map distribution of the sedimentary rocks is primarily controlled by the younger N to NW-trending folds that define the White Mountain-Inyo anticlinorium. Smaller-scale folds and associated cleavage, which strongly fans in a divergent pattern about the fold hinge surfaces, are associated with both periods of fold development. Older NE-trending folds and associated cleavage are developed in a NW-trending belt that crosses the range in a structural saddle between the White Mountain and Inyo anticlines. These NE-trending folds are of smaller wavelength than the older N to NW-trending regional-scale folds, although both fold generations are developed on the cm-km scale. The intensity of both fold and cleavage development generally increases toward the west. Interference between the NE and the N to NW-trending folds has led to the development of a series of structural basins. We correlate formation of the NE-trending structures to the Antler orogeny, and formation of the younger N-to NW-trending structures with translation on the Last Chance thrust, which is exposed in the Inyo Range and presumably underlies the entire central White-Inyo Range. In the eastern and central portions of the range, it is clear that both periods of folding pre-date the emplacement of Jurassic and Cretaceous plutons that are correlatives to the intrusive suites exposed in the Sierra Nevada batholith to the west. On the western margin of the range another set of N to NW-trending structures, which are coaxial with the earlier N to NW-trending set, is probably associated with the East Sierran thrust belt; this thrust belt is better recorded in the southern Inyo Mountains, and developed in the Jurassic period.

Symmagmatic folding of the base of the Bergell pluton, Central Alps

Evidence for magmatic, submagmatic and solid-state deformation in tonalite, granodiorite and country rocks found at the deep-seated floor (22–26 km) of the Bergell pluton demonstrates that final emplacement and crystallization occurred during regional deformation of the pluton and the underlying country rocks. After northward emplacement over the country rocks, but before complete crystallization, the floor of the pluton was folded during simultaneous N-S shortening and E-W stretching. This is evidenced by synmagmatic folds with E-W striking, nearly vertical axial planes, and by regional east-plunging stretching lineations in the country rocks which are parallel to the regional-scale fold axes and the magmatic mineral lineations in the pluton. Opposite senses of shear from the well-foliated, occasionally mylonitic contact suggest that deformation was mostly accomplished by pure shear. Synmagmatic deformation is related to late-stage N-S shortening of the Alpine orogen and shows that the still partially molten pluton responded to low differential stress very much like the country rocks deformed in the solid-state at high temperatures. Post-emplacement tilting associated with backthrusting along the Insubric mylonites led to the exposure of the pluton's floor at its present-day western margin.

Implications of overprinting deformations and fold interference patterns in the Melbourne Zone, Lachlan Fold Belt

Upper crustal shortening associated with development of a northeast‐directed thrust‐belt during dextral strike‐slip ‘docking’ of the Melbourne Zone with the Tabberabbera Zone has led to complex deformation patterns and regional scale fold interference in the Nagambie‐Rushworth area of central Victoria. Mutually interfering, contemporaneous and diachronous north‐south and northeast‐southwest shortening deformations have locally produced ‘dome and basin’ fold patterns due to interference of east‐west and northwest‐southeast fold sets. In the northern part of the zone first generation folds are east‐west‐trending, cut by north‐dipping thrust faults and have a weak, east‐west trending, steeply dipping S, slaty cleavage. To the south, first generation folds are northwest‐southeast trending and show curvilinear axial surface traces and overprinting cleavages in an area of overlapping deformation fronts. The major control is the northeast transport of the Melbourne Zone along a major mid‐crustal detachment whose...

Actively evolving microplate formation by oblique collision and sideways motion along strike-slip faults: An example from the northeastern Caribbean plate margin

The pattern of folding, faulting, and late Quaternary coral-reef uplift rates in western and central Hispaniola (Haiti and Dominican Republic) suggest that the elongate Gonave microplate, a 190,000-km 2 area of the northeastern Caribbean plate, is in the process of shearing off the Caribbean plate and accreting to the North American plate. Late Cenozoic transpression between the southeastern Bahama Platform and the Caribbean plate in Hispaniola has inhibited the eastward motion of the northeastern corner of the plate. Transpression is manifested in western and central Hispaniola by the formation of regional scale folds that correspond to present-day, anticlinal topographic mountain chains continuous with offshore anticlinal ridges. Areas of most rapid Quaternary uplift determined from onland coral reefs 125 ka and younger, coincide with the axial traces of these folds. Offshore data suggest recent folding and faulting of the seafloor. Onshore reef data do not conclusively require late Quaternary folding, but demonstrate that tectonic uplift rates of the axial areas of the anticlines decrease from the Northwest Peninsula of Haiti (0.37 mm/yr) to to the central part of the coast of western Haiti (0.19 mm/yr) to the south-central part of western Haiti (0 mm/yr). Formation of the 1200-km-long Enriquillo-Plantain Garden-Walton fault zone as a ‘bypass’ strike-slip fault has isolated the southern edge of the Gonave microplate and is allowing continued, unimpeded eastward motion of a smaller Caribbean plate past the zone of late Neogene convergence and Quaternary uplift of coral reefs in Hispaniola. Offshore seismic reflection data from the Jamaica Passage, the marine strait separating Jamaica and Haiti, show that the Enriquillo-Plantain Garden fault zone forms a narrow but deep, active fault-bounded trough beneath the passage. The active fault is continuous with active faults mapped onshore in western Haiti and eastern Jamaica; the bathymetric deep is present because the Jamaica Passage fault segment represents a 50-km-wide, transtensional left-step of the fault trace between Haiti and Jamaica. Onshore satellite imagery and field observations suggest that the Enriquillo-Plantain Garden fault forms a continuous trace extending from central Hispaniola east of Lake Enriquillo, Dominican Republic, to the westernmost end of the southern peninsula of Haiti. The regional lineament corresponds to a recent fault scarp in Quaternary alluvium of the Clonard pull-apart basin in the central part of the southern peninsula of Haiti and suggests that at least this part of the lineament has undergone recent slip. Calmus (1983) has suggested a total offset of 30–50 km of the Enriquillo-Plantain Garden fault zone in the southern peninsula of Haiti using the apparent offset of lithologic units. Seismic reflection data from Lake Enriquillo document recent deformation of Quaternary lake sediments where the lineament crosses the lake. Leveling of the crest of a late Holocene coral reef and associated algal tufa around Lake Enriquillo demonstrate late Holocene vertical movement and tilting in a 1500-m-wide zone parallel to the fault trend. Lateral offset is difficult to show in the Enriquillo Valley area because of rapid recent sedimentation into the valley. The pattern of inactive strike-slip faults and fold belts of Cenozoic age in Cuba and the Yucatan basin suggest that two elongate microplates were sheared off the proto-Caribbean plate and accreted to the North American plate by a similar process in Paleocene and Eocene times. Age of terminal deformation in western, central and eastern Cuba is consistent with southeastward younging and migration of arc collision. The similarity of the size and sequence of events in Cuba suggests that the process of oblique collision and sideways motion of the plate along a new strike-slip fault towards a free face may be an important process of microplate formation and interplate transfer in other areas.

Deformation paths in the shallow levels of an accretionary prism: The Eocene Shimanto belt of southwest Japan

Detailed structural analysis of coherent turbidites in a part of the Eocene Shimanto belt on the Muroto Peninsula, Shikoku Island, Japan, reveals a structural evolution that is typical of the shallow levels of accretionary prisms in style and sequence of deformational fabrics, but anomalous in that it includes both a marked rotation in shortening direction and a major out-of-sequence thrust. Deformation throughout a history of progressive lithification and burial was dominated by subhorizontal shortening and seaward-vergent shear. Structures evolved from predominantly mesoscale thrusts and folds to larger, more brittle faults, and finally to kilometer-scale, amplified fault-bend folds with an associated pressure-solution cleavage. This structural progression is consistent with increasing strength of the rocks, decreasing fluid pressures, and increasing temperature and pressure as the sediments were dewatered, lithified, and accreted to the prism. In contrast to this straightforward structural evolution, the prism-scale deformation of the Eocene Shimanto belt is difficult to reconcile with simple models of prism developmentthat assume orthogonal subduction and seaward-stepping imbrication. A 50° counterclockwise rotation in the shortening direction and a major out-of-sequence (or intraprism) thrust are recorded by two distinct structural domains. In both, the earliest structures are clastic dikes and faults indicative of north-south shortening. One domain is characterized by east-striking, regional-scale folds and cleavage that are transected by an east-northeast-trending cleavage; however, the other is characterized by northeast-striking folds and cleavage that are transected by a slightly more north-striking cleavage. The different deformation histories of the two do-mains require that they be separated by a major intraprism fault. This fault carries tectonic melange containing blocks of pillow basalt and chert, indicating that it rooted in the decollement zone beneath the prism. The tectonostratigraphy and deformation histories of the two domains suggest that both before and after the out-of-sequence fault developed, higher structural levels of the prism experienced significant subhorizontal shortening, whereas lower structural levels bypassed this zone of deformation.

Grenville-Age, Polyphase Deformation of Mid-Proterozoic Basement, NW Van Horn Mountains, Trans-Pecos, Texas

Psammitic, pelitic, and mafic schists in a basement horst in the NW Van Horn Mountains of west Texas preserve evidence of a multiphase, synmetamorphic, Grenville-age ductile deformation. Three phases of progressive isoclinal folding followed by two later, less intense, nearly coaxial folding phases have been identified. The second phase of deformation produced the dominant foliation and foliatipn-intersection lineation observed throughout the area. This deformation is inferred to have resulted from a regional-scale folding, possibly associated with nappe formation, at or near peak metamorphic conditions. Mineral assemblages indicate amphibolite facies, and Fe-Mg exchange thermometry provide maximum temperatures of corresponding to mid-amphibolite facies conditions. Three subsequent fold generations () formed under progressively lower metamorphic conditions; produced a weakly developed foliation ( ) whereas neither of the two later folding phases formed a penetrative fabric. Fluids or heat associated with the widespread intrusion of late syn- to post-orogenic pegmatites facilitated a static recrystallization event which altered previously formed deformational fabrics. Other basement exposures in the Van Horn area record a later phase of Grenville-age brittle deformation that has wholly or partly obscured the earlier-formed ductile features. The Grenville-age, progressive deformational history recorded in the basement rocks of the Van Horn area is similar to that noted in portions of the Llano Uplift of central Texas and in the Adirondack Mountains of New York and suggests that the Grenville orogeny was characterized by oblique convergence along the length of the North American craton.

Geochemistry of Dalradian pelites from Connemara, Ireland: new constraints on kyanite genesis and conditions of metamorphism

Whole-rock and mineral analyses of Dalradian pelites from W Connemara show that kyanite pelites are not unusually Mg-rich as previously supposed. Their chemistry is transitional, especially in Ca content, between overlying Fe-rich staurolite pelites and underlying Ca-rich rocks. The flysch basin in which they were deposited was established relatively early in the west, its sediment fill including carbonate-rich influxes derived from an adjacent swell to the east which later subsided to form an enlarged basin. Kyanite has a very restricted spatial distribution because its development was controlled by Fe deficiency associated with the carbonate-rich influxes, which in turn led to relatively high X Mg during metamorphism. P–T estimates for the metamorphic peak are 650° ± 25 °C and ~8kbar, about 50 °C and 3kbar greater than for similar rocks in the east. The difference can be related to structural level with respect to regional-scale folds, which may have developed as a huge backfolded complex during the emplacement of the Connemara allochthon.

Tectonic evolution of the easternmost Piedmont, North Carolina

The northeastern North Carolina Piedmont is made up of a basement sequence of unknown age (possibly a southern extension of the Grenville-age Goochland terrane of eastern Virginia), structurally overlain by a volcanogenic sequence of probable late Precambrian to early Paleozoic age (an eastward continuation of lithologies occurring in the Carolina slate belt). A model is proposed in which the basement and volcanogenic sequences are separated by a pre-Alleghanian [Taconic(?)] decollement. Northwestward and westward vergence of associated folds of the upper plate suggests that the volcanogenic sequence was thrust from the southeast or east over the basement terrane. This deformation occurred during a greenschist-grade (M 1 ) metamorphic event. Alleghanian granites (313 to 285 Ma) intrude both the basement and volcanogenic sequences, clearly cutting the inferred decollement in some areas. This Alleghanian plutonism was accompanied by regional heating to produce the upper greenschist- to amphibolite-grade (M 2 ) assemblages of the Raleigh belt. The Raleigh belt remained hot until late Alleghanian compression produced regional-scale (F 3 ) folds and D 3 mylonite zones with the resultant uplift and cooling of this belt by 250–240 Ma.

Alleghanian deformation and metamorphism in the eastern North Carolina Piedmont

Rb-Sr isotopic data document Alleghanian ages for the metamorphic event which formed the upper greenschist- to amphibolite-grade Raleigh belt and for the D 3 deformational event which formed regional-scale folds and mylonite zones in the eastern North Carolina Piedmont. The Butterwood Creek granite, which straddles the eastern boundary between the Raleigh belt and the Eastern slate belt, has a whole-rock isochron age of 292 ± 31 Ma. Biotite ages indicate that the eastern, undeformed part of the pluton cooled rapidly after intrusion, whereas a younger biotite age of 251 Ma indicates slow cooling for the western, foliated portion of the granite which lies in the Raleigh belt. Other biotite ages within the Raleigh belt range from 247–238 Ma. The thermal maximum for the Raleigh belt probably accompanied the intrusion of numerous granitic plutons during the time 315 to 285 Ma. The biotite ages indicate that the Raleigh belt remained at elevated temperatures and did not cool to the blocking temperature of biotite until 250–240 Ma. Rapid cooling of the eastern part of the Butterwood Creek helps define the eastern boundary of the Alleghanian thermal event, which corresponds to the eastern limit of the high-grade rocks of the Raleigh belt. The S 3 tectonic biotite foliation in the western part of the Butterwood Creek and in other Alleghanian plutons of the Raleigh belt, associated regional F 3 folds, and the D 3 Hollister mylonite zone which cuts the Butterwood Creek pluton formed between the time of intrusion of the Butterwood Creek at 292 Ma and regional cooling by 250–240 Ma. This study further defines the narrow Alleghanian thermotectonic belt along the eastern Piedmont which is known as the Raleigh belt in Virginia and North Carolina and reappears as the Kiokee belt in South Carolina and Georgia.