Silurian Deformation Research Articles

The Rheic riddle: ocean closed, but no orogen

Abstract Although the Rheic Ocean was the widest to be closed during the assembly of Pangaea, its suture zone is dominated by a younger (Rhenohercynian) Wilson Cycle. The present paper briefly summarizes the Rhenohercynian and reviews the relicts of the Rheic. The absence of a Rheic orogen is mainly due to bipolar subduction of the Rheic Ocean, which created two magmatic arcs and largely destroyed the passive margin prisms, thus depriving the collision zone of building material. Also, displacement of Variscan zones in England, France and Germany along a curvature of the dextral Bristol/Bray/Bohemia shear zone has effected their anticlockwise rotation. Younger, ENE-trending reverse faults have truncated the NE-trending structural boundaries and excised pre-Devonian, Rheic-related rocks west of the River Rhine, while the northern Rheic Arc and Cambrian–Silurian deposits of the Rheic Ocean have been preserved further east. Opening of the Rhenohercynian successor ocean to the Rheic put a brake on plate convergence and hindered formation of a large collisional belt. Scarce relicts of the southern Rheic Arc and rocks with Silurian deformation exist in the (largely eroded) upper plate of the Rhenohercynian collisional belt, mostly in the form of clasts and detrital zircons in Upper Devonian/lower Carboniferous flysch and in tectonic mélange at the base of the allochthon.

Ordovician–Silurian deformation of the Neoproterozoic upper gneiss unit in the northern Seve Nappe Complex: implications for subduction of the Baltican margin

The upper gneiss unit is exposed in the northernmost Seve Nappe Complex (SNC) in the Scandinavian Caledonides. To investigate the Caledonian tectonic history of the unit, in situ white mica and biotite 40 Ar/ 39 Ar geochronology was applied to a leucogranite and two paragneisses. The leucogranite exhibits low-strain traits. Biotite porphyroblasts yielded a cooling age of 459 ± 2 Ma. White mica that replace biotite and plagioclase provided a crystallization age of 436 ± 5 Ma. White mica in both paragneisses exhibit high-strain characteristics associated with top-to-the-east sense of shear. One paragneiss provided dispersed late Cambrian to Late Ordovician dates with the youngest approximating deformation at 459 ± 2 Ma. The older dates are interpreted to reflect 40 Ar inherited from a previous metamorphic event. The second paragneiss yielded a deformation age of 434 ± 2 Ma. The collective dataset is interpreted to record exhumation of the upper gneiss unit at c. 459 Ma and deformation and fluid infiltration at c. 434 Ma during continental collision. The events closely resemble the deformation histories of other northern SNC terranes. Synthesizing these results with those for other northern SNC terranes suggests that the Baltican margin underwent oblique, south-to-north subduction during late Cambrian time. Supplementary material: Tables providing mineral chemistry of white mica and biotite and 40 Ar/ 39 Ar geochemistry results are available at https://doi.org/10.6084/m9.figshare.c.6639993 Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/topic/collections/the-caledonian-wilson-cycle

Petrology and geochronology of the San Martín de los Andes batholith: Insights into the Devonian magmatism of the North Patagonian Andes

Evidence is growing that a belt of Devonian plutonism is present in the Northern Patagonian segment of the present-day Andes. This plutonism formed along the paleo-Pacific margin of the Gondwana supercontinent. New whole rock chemistry and SHRIMP U–Pb zircon data for plutonic rocks of the San Martín de los Andes batholith (southwestern Argentina) provide insights into the magmatic and tectonic evolution of the southwestern Gondwana margin during the middle Palaeozoic. The San Martín de los Andes batholith was formed between 390 and 374 Ma and intruded into a Lower Paleozoic metamorphic basement that had been previously affected by Late Ordovician-middle Silurian deformation. Most of the batholith is composed of metaluminous, calc-alkaline tonalites and granodiorites, with subordinate volume of peraluminous garnet-bearing granites. Discrete and disperse gabbroic and dioritic rocks hosted in tonalites and granodiorites occur as either microgranular enclaves or syn-magmatic dykes. Tonalitic, granodioritic and granitic rocks develop transitional contacts among them revealing coetaneous building up throughout the plutonic suite, but when interpreting their chemical variability, they are not strictly comagmatic rocks. Collectively, field, petrographic, and geochemical data of the San Martín de los Andes batholith are interpreted to reflect concurrence of crystal fractionation, crustal assimilation and magma mixing in driving petrological diversification. Yet, gabbroic and dioritic rocks in the San Martín de los Andes batholith display geochemical characteristics of calc-alkaline primitive basalts that concur with what is being hypothesized to be a magmatic arc zone for the Devonian period. This study discusses how to interpret the evolution of the San Martín de Los Andes batholith together with other geodynamic models such as the Chaitenia magmatic arc.

Patterns of Silurian deformation and magmatism during sinistral oblique convergence, northern Scottish Caledonides

Regional ductile thrusting and syn-kinematic granitic magmatism within the Caledonides of northern Scotland occurred within a sinistrally oblique convergent tectonic setting during the Silurian closure of the Iapetus Ocean. The highest thrust nappes are dominated by structures of probable Grampian (Ordovician) age, and Scandian (Silurian) deformation dominates the underlying thrust nappes. Deformation was overall foreland-propagating but the nappe stack was modified by out-of-sequence thrusting and probable synchronous development of thrusts at different structural levels. Localized dextrally transpressive deformation is related to an inferred lateral ramp located offshore. New U–Pb zircon ages from syn-tectonic granites indicate that the internal Naver Thrust was active between c. 432 and c. 426 Ma. This is consistent with other data sets that indicate that contractional deformation and high-grade metamorphism, and by implication displacements in the Moine Thrust Zone, may have lasted until c. 420–415 Ma. The synchroneity of thrusting and strike-slip movements along the Great Glen Fault implies that partitioning of transpressional strain occurred above a regional basal decollement. The short duration of the Scandian orogen in Scotland ( c. 437–415 Ma?) is consistent with only moderate crustal thickening and a location on the periphery of the main Laurentia–Baltica collision further north. Supplementary material: Details of analytical procedures, complete U-Pb isotopic data and methods of U-Pb age calculation and error reporting are available at https://doi.org/10.6084/m9.figshare.c.4962251

Late Silurian deformation in the Łysogóry Region of the Holy Cross Mountains revisited: restoration of a progressive Caledonian unconformity in the Klonów Anticline and its implications for the kinematics of the Holy Cross Fault (Central Poland)

The Klonów Anticline is located in the western outskirts of the Łysogóry Region of the Holy Cross Mountains. A fresh look onto the cartographic evidence suggests that this structure shows the key characteristics of a progressive unconformity. This becomes evident when compared to the classical example of the Sant Llorenç de Morunys Anticline (eastern Pyrenees, Spain). The geometry of the unconformity requires a phase of thrusting or reverse faulting along the Holy Cross Fault in the latest Silurian–earliest Devonian (Late Caledonian deformation) followed by Variscan thin-skinned thrusting. Thick- and thin-skinned kinematic scenarios for the latest Silurian-earliest Devonian deformation of the Łysogóry Region are explored

The Connemara Antiform and its possible Silurian history

The Connemara Antiform has iconic status in the geology of western Ireland. With its eastto-west trend and eastward plunge and the disposition of Dalradian stratigraphy about its axis, it forms the dominant structure in the present-day landscape and is a focal feature of geological maps of the area. Its genesis has long been ascribed to D4, the last main major deformation phase of the Ordovician Grampian Orogeny that affected the Dalradian rocks of Connemara. But does the structure have such a straightforward and simple origin? This paper investigates the possible role of middle late Silurian deformation in the origin of the Connemara Antiform. Overview of the geology of Connemara Connemara was part of the south-eastern Laurentian margin Dalradian sequence that experienced a major short-lived early Ordovician orogenic episode, the Grampian Orogeny, caused by collision with, and obduction of, an arc that developed above oceanward-directed subduction (Chew 2009). The Connemara Terrane became detached from the Laurentian margin and now lies to the south of the obducted arc and the South Mayo Trough (Fig. 1). The translation of the Connemara Terrane from the Laurentian margin to its current position is indicated by an influx of coarse clastic rocks of Connemara provenance into the c. 464Ma Bunnacunneen Member of the Mweelrea Formation of the South Mayo Trough (McConnell et al. 2009). Dewey (2005) provides a succinct account of the local plate-tectonic, metamorphic and structural history of the Connemara Terrane and the South Mayo Trough during the Grampian Orogeny, and Graham et al. (1989), Leake and Tanner (1994), McConnell et al. (2002) and Long et al. (2005) provide a comprehensive overview of the detailed geology of the area. The Grampian Orogeny began with the subduction of rifted Laurentian margin sediments beneath a continent-facing oceanic arc c. 478Ma (Dewey 2005). The Connemara Dalradian sediments were buried in a thick nappe pile of D2 and D3 folds and experienced high-pressure, lowtemperature Barrovian metamorphism (Leake and Tanner 1994). Palaeomagnetic studies indicate that the D3 folds originally dipped c. 208 south (Robertson 1988). Between 474Ma and 470Ma (Friedrich et al. 1999) a suite of metagabbros and orthogneisses (the Metagabbro Orthogneiss Complex) were injected into the deforming nappe pile during D2 D3 in an east west belt that extends for more than 80km along the southern margin of the terrane (Fig. 1). The high-temperature metagabbros and orthogneisses drove sillimanite-grade, hightemperature, low-pressure Buchan metamorphism. Migmatites resulting from the metagabbro heat pulse formed at c. 6kbar but crystallised at much lower pressures of c. 3.5kbar, indicating rapid exhumation (Barber and Yardley 1985). Rapid exhumation may have been facilitated by extensional collapse. The Renvyle Bofin Slide, a lowangle post-D3 extensional detachment, has, on the Irish Journal of Earth Sciences 32 (2014), 71 78 doi: 10.3318/IJES.2014.32.71 # 2014 Royal Irish Academy

The Connemara Antiform and its possible Silurian history

The Connemara Antiform has iconic status in the geology of western Ireland. With its east-to-west trend and eastward plunge and the disposition of Dalradian stratigraphy about its axis, it forms the dominant structure in the present-day landscape and is a focal feature of geological maps of the area. Its genesis has long been ascribed to D4, the last main major deformation phase of the Ordovician Grampian Orogeny that affected the Dalradian rocks of Connemara. But does the structure have such a straightforward and simple origin? This paper investigates the possible role of middle–late Silurian deformation in the origin of the Connemara Antiform.

Timing of fault reactivation in the upper crust of the St. Lawrence rift system, Canada, by K–Ar dating of illite-rich fault rocks1GEOTOP Contribution 2012-0002.

Formed during late Proterozoic – early Paleozoic, the St. Lawrence rift system of eastern North America hosts an active seismic zone, also displaying evidence of subsequent tectonic reactivation in Paleozoic and Mesozoic times. However, lack of a detailed reconstruction of the basement geometry and limited geochronological constraints limit our understanding of its structural evolution. Late Silurian – Early Devonian fault reactivation is demonstrated here in typical fault sites of the St. Lawrence rift system by combining structural observations of basement structures and a mineralogical, morphological, and K–Ar isotopic study of clay-rich fault material (<0.4 µm et 1–2 µm). The K–Ar data of clay-rich size fractions from gouges of varied rock types from Saint-Laurent and Montmorency faults define two isochron ages at 436 ± 45 and 406 ± 22 Ma that are within analytical uncertainty and give an average at 421 ± 15 Ma. However, these two faulting episodes could also picture a single long-lasting phase of foreland subsidence of the Appalachian orogen starting during the Late Ordovician – Early Silurian deformation in the external Humber zone and continuing during Late Silurian – Early Devonian subsidence related to a normal faulting. This interpretation is based on a different mineral composition of the younger size fractions that consist of smectite-enriched clay minerals and could, therefore, correspond to the end of a reactivation event that was episodically active between 436 ± 45 and 406 ± 22 Ma. The faults were selectively reactivated depending on their location relative to pre-existing metamorphic and tectonic fabrics of the Grenvillian basement. The identified Late Silurian – Early Devonian reactivation of the Saint-Laurent and Montmorency faults was contemporaneous with the subsidence of the adjacent Appalachian foreland, resulting in SE-dipping faults in the crystalline basement of the St. Lawrence rift system and NW-dipping faults in the Appalachian cover.

Silurian deformation and metamorphism of Ordovician arc rocks of the Casco Bay Group, south-central Maine

The Casco Bay Group in south-central Maine consists of a sequence of Late Cambrian to Early Ordovician interlayered quartzofeldspathic granofels and pelite (Cape Elizabeth Formation) overlain by Early to Late Ordovician back-arc volcanic (Spring Point Formation) and volcanogenic sedimentary rocks (Diamond Island and Scarboro formations). These rocks were tightly folded and subjected to low-pressure amphibolite-facies metamorphism in the Late Silurian. This phase of deformation and metamorphism was followed by the development of a variety of structures consistent with a period of dextral transpression in Middle Devonian – Early Carboniferous time. Previously dated plutons within the sequence range in age from 422–389 Ma and record a period of prolonged intrusive activity in the region. Similarities in age, volcanic rock geochemistry, and lithologic characteristics argue strongly for a correlation between rocks of the Casco Bay Group and those in the Miramichi belt of eastern Maine and northern New Brunswick. The Cape Elizabeth Formation correlates with Late Cambrian to Early Ordovician sediments of the Miramichi Group (Gander Zone) and the Spring Point through Scarboro formations correlate with Early to Late Ordovician back-arc basin volcanics and volcanogenic sediments of the Bathurst Supergroup. The folding and low-pressure metamorphism of the Casco Bay Group is attributed to Late Silurian to Early Devonian terrane convergence and possible lithospheric delamination that would have resulted in a prolonged period of intrusive activity and elevated temperatures at low pressures. Continued convergence and likely plate reconfigurations in the Middle Devonian to Carboniferous led to widespread dextral transpression in the region.

Proterozoic – Early Palaeozoic rocks and the Tyennan Orogeny in central Victoria: The Selwyn Block and its tectonic implications

Aeromagnetic and field data suggest that meta‐igneous rocks exposed on the south coast of central Victoria at Waratah Bay, Phillip Island, Barrabool Hills and inland near Licola, are continuous—beneath Bass Strait—with Proterozoic/Cambrian igneous rocks in King Island and Tasmania. This correlation is supported by a pre‐Early Ordovician unconformity above gabbro protomylonite at Waratah Bay, age equivalent to the Tasmanian Tyennan unconformity. Cambrian volcanics at Licola and unusual features of the Melbourne Zone sequence indicate that Tyennan continental crust extends north as basement to the central Victorian portion of the Lachlan Fold Belt. In contrast, adjacent parts of the Lachlan Fold Belt in Victoria contain conformable sea‐floor sequences that span the Early Cambrian to Late Ordovician, with no evidence of either Cambrian deformation or underlying continental basement. The block of Tyennan continental crust beneath central Victoria—the Selwyn Block—is fundamentally different, and has influenced temporal and spatial patterns of sedimentation, deformation, metamorphism and plutonism. Palaeogeographical reconstructions suggest that the block was a submarine plateau that lay outboard of the Australian craton, upon which a condensed Ordovician sequence was deposited. The sequence above the Selwyn Block unconformity at Waratah Bay is similar to widespread post‐Tyennan sediments in western Tasmania. During Late Ordovician and Early Silurian deformation, the Selwyn Block protected much of the overlying sedimentary sequence. Instead, shortening was focused into the Stawell and Bendigo Zones to the west. These zones were sandwiched between the Selwyn Block and the Australian craton in a ‘vice’ scenario reminiscent of some Appalachian orogenic events. The region above the Selwyn Block was downwarped adjacent to the overthrust Bendigo Zone as a foreland deep, into which a conformable clastic wedge of sediment was deposited in Late Ordovician to Devonian time, prior to final Middle Devonian deformation. The Selwyn Block includes the Cambrian calc‐alkaline Licola and Jamieson Volcanics that are correlated with the Tasmanian Mt Read Volcanics. In Victoria, these form a basement high controlling the unusual down‐cutting thrusts in the overlying Melbourne Zone and explaining the major structural vergence reversal between the Melbourne and Tabberabbera Zones. The Selwyn Block has exerted some control on the timing, chemistry and distribution of post‐orogenic granites, and on central Victorian gold mineralisation. Reactivated faults in the block influenced deposition, and continue to control the deformation of the portions of the Otway and Gippsland Basins that lie above it.

Taking the roof off a suture zone: basin setting and provenance of conglomerates in the ORS Dingle Basin of SW Ireland

Abstract The Late Silurian to Mid-Devonian Dingle Basin occupies a central position within the Iapetus Suture Zone of SW Ireland. The basin is believed to have formed from late Silurian times onwards as a product of sinistral transpression along several major faults within this suture zone. Conglomeratic sediments were deposited by moderately large gravelly fans shed into the basin from the NW (Glashabeg Formation) and the SE (Trabeg Conglomerate Formation). The systems fed into a large apparently through-going sandy, axial river (Slea Head Formation) that flowed towards the NE. The lateral, basin-margin systems were sourced from two disparate source terranes. To the NW lay a basic volcanic hinterland with some intermediate volcanic rocks and limestones, mudstones, sandstones and chert. During the Early Devonian time the SE drainage basin was underlain by sandstones, quartzites, phyllites and limestones probably intruded by a granite. Some of these lithologies can be found in outcrop in the pre-Dingle Group of the peninsula. Others need to be correlated with rocks of the oceanic terranes in the northern part of the suture zone. The southerly derived clasts have corollaries in the rocks of the Avalonian Leinster Terrane south of the suture. Following partial inversion of the Dingle Basin, the southerly hinterland was apparently further unroofed during mid-Devonian time, when the Inch Conglomerate Formation was deposited by alluvial fans shed northwards from a source area formed along the Dingle Bay Lineament. The Inch conglomerates are characterized by distinctive clasts of schist, gneiss, mylonite, tourmalinite and granite. The general picture of the Early Devonian deformation, intrusion by granite and unroofing of terranes currently partially exposed in central and southern Ireland within the Iapetus Suture Zone is largely consistent with clast lithotypes. However, some exploratory isotopic data indicate at least two possible vagaries in the interpretation. First, model ND T DM ages of Trabeg sedimentary clasts yield several results older than typical southern Iapetean or Avalonian crustal material. This suggests a complex history of sedimentary mixing of material across the developing Iapetus Ocean. Second, two of three Rb-Sr muscovite-whole-rock dates of Inch metamorphic clasts indicate Silurian ages. These data similar to Rb-Sr dates derived from the Carnsore and Saltees granites in the Rosslare Terrane, perhaps extending the geographical spread of this Silurian deformation.

Fault reactivation within Avalonia: plate margin to continental interior deformation

Lithotectonic terranes commonly have faults with movement histories that reflect their original tectonic setting and their subsequent re-activation during terrane accretion and post-accretionary dispersal. Since later movements tend to overprint evidence of earlier motions, documenting fault reactivation can be a difficult task. Avalonia, the largest terrane within the Canadian Appalachians preserves evidence for repeated episodes of movement along NE-trending fault zones in a variety of tectonic settings between the late Neoproterozoic and late Paleozoic. Evidence of Neoproterozoic motion is preserved in pre-final crystallization deformation fabrics in arc-related igneous complexes which intrude shear zones. These record strike-slip motion related to oblique subduction along the continental margin of Gondwana. In the Paleozoic, Avalonia migrated from its original peri-Gondwanan setting. Mid-Ordovician to earliest Silurian deformation and magmatism is attributed to the sinistral accretion of Avalonia to Laurentia. This reactivated Neoproterozoic shear zones and resulted in basin inversion in mainland Nova Scotia and the formation of mylonites and injection of dikes into brittle fractures in southern New Brunswick. Sinistral motion was accompanied by terrane dispersal and was followed in the Late Silurian and Early Devonian by further reactivation in the form of dextral strike-slip that reflects convergence between Laurentia and Gondwana. Following the accretion and dispersal of Avalonia, the fault zones became stranded within the continental interior. Subsequently, the fault zones accommodated local stresses which were far-field responses to collisional tectonics associated with the mid-Late Devonian Acadian and the Late Carboniferous Alleghanian orogenies. Hence, the generation of these fault systems along plate margins in the late Neoproterozoic and early Paleozoic profoundly influenced the tectonic evolution of late Paleozoic intra-continental deformation. Late Carboniferous deformation is recorded in (a) the mylonitic and cataclastic fabrics of the shear zones themselves, (b) the orientations of spatially related fold structures, (c) local controls on basin formation and sedimentary facies and (d) offsets in stratigraphy. This motion was predominantly dextral and records continued convergence between Laurentia and Gondwana during the amalgamation of Pangea.

Ordovician and Silurian metamorphic cooling ages along the Laurentian margin of the Quebec Appalachians: Bridging the gap between New England and Newfoundland

Geochronological studies on the timing of deformation and metamorphism along the Laurentian margin have shown that the ages of metamorphism events change along the strike of the Newfoundland–New England segment of the Appalachian orogen. The Quebec Appalachians represent a critical transition zone, where new single-grain laser 40 Ar/ 39 Ar plateau ages show that both Middle Ordovician and Silurian deformation and metamorphism are recorded. Parts of the analyzed samples give 40 Ar/ 39 Ar muscovite ages ranging from 468 to 461 Ma. These samples were taken within structural windows of the eastern internal Humber zone that are overlain by ophiolitic and melange rocks of the Dunnage zone. Other analyzed samples yielded Silurian 40 Ar/ 39 Ar muscovite ages clustering from 425 to 421 Ma. These samples were taken across the Notre Dame Mountains anticlinorium, which represents a structural and metamorphic dome within the internal Humber zone. The Middle Ordovician ages of southern Quebec are coeval with data from western New England and are interpreted as representing cooling shortly after the peak Taconian regional metamorphism. The Silurian ages, which have never been previously documented in the Quebec Appalachians, are comparable to those described in Newfoundland; however, they are not attributed to tectonism that shows characteristics of an orogen-scale continent-continent collision (Salinian orogeny), but rather those of late Taconian backthrusting and normal faulting or a period of crustal extension that followed the main Taconian crustal thickening.

Silurian deformation in eastern Notre Dame Bay, Newfoundland: Discussion

I southeast thrusting and ocean closure across Exploits Subzone resembles that of Pickering et al. (1988), but Lafrance and Williams assume deposition in a single marine basin on Gondwanan side of Iapetus and postdepositional thrusting I rather than deposition in several basins on Laurentian side I of Iapetus and syndepositional thrusting (Arnott et al. 1985; Pickering et al. 1988). We wish to comment on terrane analysis offered by them and to point out a number of errors I 1 in their presentation. Of five defined in paper, two (Twillingate and Chanceport) are known Ordovician suspect terranes. The New World Island and Port Albert terranes, while suspect to North America during Early and Middle Ordovician, share same Late Ordovician to Early Silurian rocks and stratigraphy. Virtually all previous workers have also recognized Late Ordovician Early Silurian sedimentologic links between New World Island Terrane (Exploits Subzone) and Twillingate Chanceport (Notre Dame Subzone), indicating Middle Ordovician accretion to North America (Kay 1967; 1969; Helwig and Sarpi 1969; Nelson and Casey 1979; Nelson 1981; Arnott 1983a; 19836: Arnott et al. 1985; Pickering et al. 1988; Williams et al. 1988; and others). These data imply that any Silurian basin(s) in this region must have been on Laurentian margin of Iapetus rather than Gondwanan margin. The authors' Dildo Run Terrane, coextensive with Dunnage MClange, is interpreted as being faulted against New World Island Terrane, following Kay (1970) and van der Pluijm (1986). However, this contact has also been interpreted as stratigraphic (Home 1969; Hibbard and Williams 1979). The authors present no evidence that any of their terranes were distinctive in Silurian time period of chief interest to their paper. According to authors, Twillingate and Dunnage were affected by the earlier event. However, they conclude that these evolved on opposite sides of an Ordovician ocean. Hence there is no reason to suppose that pre-Silurian events can be correlated in any meaningful sense

Silurian deformation in eastern Notre Dame Bay, Newfoundland: Reply

Silurian deformation in eastern Notre Dame Bay, Newfoundland: Reply

Silurian deformation in eastern Notre Dame Bay, Newfoundland

Eastern Notre Dame Bay, Newfoundland, is divided into five fault-bounded terranes. They are, from north to south, the Twillingate Terrane, the Chanceport Terrane, the New World Island Terrane, the Dildo Run Terrane, and the Port Albert Terrane. The New World Island Terrane is characterized by fault-repeated sequences of Middle Ordovician to Early Silurian turbiditic sandstones (Sansom Formation) and conglomerates (Goldson Formation). The Chanceport Terrane has a lower volcanic unit and an upper sedimentary unit consisting of red and green siltstones–shales overlain by turdiditic sandstones. This sequence is structurally overlain by a mafic and felsic volcanic unit.The clastic sedimentary rocks of the Chanceport, New World Island, and Port Albert terranes best record the Silurian deformation in the area. Silurian deformation is divided into two deformation events: an Early Silurian D1 thrusting event and a Late Silurian D2 dextral ductile faulting event. The Early Silurian Joey's Cove Mélange constrains the age of D1 thrusting. Few small-scale fault ramps and intrafolial F1 folds are associated with D1 thrusting. Most penetrative deformation structures in eastern Notre Dame Bay formed during D2. Three fold generations (F2, F3, F4), the regional cleavage (S3), and tectonic mélanges are associated with D2 dextral ductile faulting. D2 structures overprint Early Silurian Goldson conglomerates, and are overprinted by Late Silurian to Early Devonian Loon Bay Suite intrusions. Devonian to Mesozoic brittle D3 faults cut across the ductile regional structures.Silurian deformation in eastern Notre Dame Bay began during the closure of the Iapetus Ocean when the Chanceport, New World Island, and Port Albert terranes, and possibly the Twillingate and Dildo Run terranes, were thrust towards the south over the Gander Zone. D2 dextral ductile faults formed to accommodate the nonorthogonal final closure of the Iapetus Ocean. The closure of the Iapetus Ocean in eastern Notre Dame Bay was oblique with a dextral horizontal component.

The Paleozoic tectonic evolution of the northern edge of North America: A structural study of Northern Ellesmere Island, Canadian Arctic Archipelago

On Ellesmere Island, the Clements Markham fold belt of the Franklinian mobile belt is flanked to the northwest by a Caledonian terrane, Pearya. On the basis of the Proterozoic to mid‐Ordovician stratigraphic and structural similarities with Spitsbergen and on the mid to late Silurian relationship between Pearya and Ellesmere Island, some strike‐slip displacement must have been involved in the accretion of Pearya to its present position. However, the structural style of the Clements Markham fold belt on Ellesmere Island is characterized by chevron folds with axial planar cleavage (flexural folding) and shallow plunging fold axes. The present terrane boundary faults show steeply plunging lineations indicating a vertical component of motion strongly predominating the strike‐slip component. Thus the first detailed structural study of the area has produced no positive structural evidence for late Silurian transcurrent shear or transpression. Therefore decoupling of a strike‐slip component of plate motion from a distributed compressive component is suggested for northern Ellesmere Island. Accretion of Pearya most likely occurred along one of the major fault zones to the southeast of the thrust faults marking the present terrane boundary. However, indications of strike‐slip displacement have not yet been observed along these faults possibly due to later thrust faulting. The Silurian Imina and Lands Lokk formations form an accretionary wedge of orogen‐derived turbidites which were deposited on oceanic or highly attenuated continental crust. Thus the time interval spanning the deposition and deformation of the turbidite formations dates the period of accretion and of predominantely orthogonal continental convergence. In the light of current ideas concerning the episodic and diachronous development of orogenic belts, the Silurian deformation in northwestern Ellesmere Island may be interpreted as the beginning of the orogenic cycle that culminated in the Late Devonian ‐ early Carboniferous Ellesmerian event.

Cleaved microgranite dykes of the Shap swarm in the Silurian of NW England

AbstractFelsite‐microgranite dykes, chemically comparable with the Shap swarm and associated with the Shap intrusion, are present in Silurian sediments of the southern Lake District. They were emplaced after the country rocks had undergone Acadian (late Caledonian) folding and cleavage‐formation but are themselves weakly cleaved. This confirms a ‘flattened buckle’ model for the Acadian deformation in NW England, which in turn establishes the link between sinistral transpression in this region and the clockwise transection of the folds by cleavage. The evidence also shows that the Acadian cleavage developed episodically, and that the Shap‐Skiddaw magmatism occurred during one or more stress‐relief episodes. The emplacement age of these intrusions thus constrains the age of the Acadian orogeny in NW England which was late in the Lower Devonian (according to currently available isotopic evidence), significantly later than the Silurian deformation of the Southern Uplands.

The emplacement and fault history of the Coolac Serpentinite, Lachlan Fold Belt, southeastern Australia

The Tumut Synclinorial Zone, located in the Lachlan Fold Belt, southeastern Australia, forms a fault-bounded belt of early Silurian volcanics and flyschoid metasediments of the Tumut Trough. The Mooney Mooney Fault System, containing an extensive ultramafic belt known as the Coolac Serpentinite, forms the eastern margin of the trough. The ultramafics together with mafic volcanics and intrusive grabbroic rocks have been previously interpreted as early Palaeozoic oceanic crust, dismembered and obducted during late Silurian deformation of the Silurian trough sequence. Recent work, however, has unravelled a more complex history involving several periods of movement. The ultramafics and early Silurian volcanics are intruded by early Silurian gabbro and syn-kinematic late Silurian granodiorite. These intrusive relationships indicate that the ultramafic rocks were present in approximately their present structural position prior to deformation of the trough sequence. The ultramafics therefore cannot represent early Silurian oceanic crust, obducted during the Late Silurian deformation. They probably represent either early Silurian or Cambrian-Ordovician mantle-derived material emplaced within a strike-slip fault zone during early Silurian oblique extension.

The significance of primary structures in the Ordovician pillow lava sequence of Western Norway in an understanding of major fold pattern

SummaryThe present paper deals with the Lower Ordovician–Silurian rocks between Fördefjord and Sognefjord on the western coast of Norway. During the Caledonian orogeny this sequence was subjected to several phases of Silurian deformation, the first of which resulted in the formation of a large-scale anticline (the Stavfjord Anticline), which now occupies part of an overthrust unit. On the basis of ‘way-up’ criteria in the partly well-preserved Lower Ordovician pillow lavas it has been possible to determine with some confidence the geometry of the Stavfjord Anticline, its probable regional extension and the position of the axial plane trace. Arcuate deflections in the trend of this axial plane trace are a result of large-scale refolding, the later fold axes having WNW–ESE trends.