Acadian Folding Research Articles

Magnetic anisotropy reveals Acadian transpressional fabrics in an Appalachian ophiolite (Thetford Mines, Canada)

SUMMARY Magnetic anisotropy has proved effective in characterizing primary, spreading-related magmatic fabrics in Mesozoic (Tethyan) ophiolites, for example in documenting lower oceanic crustal flow. The potential for preservation of primary magnetic fabrics has not been tested, however, in older Palaeozoic ophiolites, where anisotropy may record regional strain during polyphase deformation. Here, we present anisotropy of magnetic susceptibility results from the Ordovician Thetford Mines ophiolite (Canada) that experienced two major phases of post-accretion deformation, during the Taconian and Acadian orogenic events. Magnetic fabrics consistent with modal layering in gabbros are observed at one locality, suggesting that primary fabrics may survive deformation locally in low strain zones. However, at remaining sites rocks with different magmatic origins have consistent magnetic fabrics, reflecting structurally controlled shape preferred orientations of iron-rich phases. Subhorizontal NW-SE-oriented minimum principal susceptibility axes correlate with poles to cleavage observed in overlying post-obduction, pre-Acadian sedimentary formations, indicating that the magnetic foliation in the ophiolite formed during regional NW-SE Acadian shortening. Maximum principal susceptibility axes plunging steeply to the NE are orthogonal to the orientation of regional Acadian fold axes, and are consistent with subvertical tectonic stretching. This magnetic lineation is parallel to the shape preferred orientation of secondary amphibole crystals and is interpreted to reflect grain growth during Acadian dextral transpression. This structural style has been widely reported along the Appalachian orogen, but the magnetic fabric data presented here provide the first evidence for transpression recorded in an Appalachian ophiolite.

Geochemical Insights Into Provenance of the Middle Devonian Hamilton Group of the Central Appalachian Basin, U.S.A.

Abstract In order to better understand the relationship of clastic influx and organic-matter accumulation and preservation in the Middle Devonian Acadian foreland basin, we conducted a high-resolution provenance analysis of a composite section of the Hamilton Group. The Hamilton Group includes the organic-rich Marcellus Shale, one of the most lucrative unconventional shale gas plays in the world, and the overlying Mahantango Formation. Geochemical analysis of 121 samples from two nearby wells in Monongalia County, West Virginia, reveals subtle changes in clay provenance throughout deposition of the Marcellus Shale and the Mahantango Formation. Major-element and trace-element geochemistry suggest derivation from a felsic igneous, granodioritic, upper-continental crustal source with additional influx from a recycled quartzose sedimentary source. All weathering indices indicate increasing chemical weathering intensities throughout deposition of the Marcellus Shale, followed by consistently moderate chemical weathering associated with accumulation of the Mahantango Formation. Sm-Nd isotopic analysis of ten samples throughout the Hamilton Group yield eNd values ranging from –7.06 to –11.75, and Nd depleted-mantle model ages (τDM) ranging from 1.63 to 1.85 Ga, with ages becoming younger upsection. Our results suggest that the extrabasinal detritus of the Hamilton Group originated from a mixed sediment source, with clay influx from both rocks associated with the Superior Craton to the north and northwest (τDM > 2.7 Ga) and Grenville-sourced sediments derived from the adjacent Acadian fold–thrust belt to the east (τDM ∼ 1.4–1.6 Ga). Older Sm-Nd model ages, felsic composition, and evidence of sediment recycling suggest little to no contribution from the Acadian volcanic arc, indicating that volcanic tuff of the Tioga ash beds represent isolated episodes of volcanic input into the basin. Model ages, eNd values, and trace-element geochemistry indicate increased sediment influx from the Acadian fold–thrust belt throughout deposition of the Hamilton Group, with the highest sediment influx having occurred during Mahantango Formation deposition. Increased clay influx from sediment shed off from the Acadian fold–thrust belt associated with marginal marine regression increased delivery of clastic detritus, diluting the flux of organic matter and likely disrupting basin water stratification that aided in preservation of organic matter in the lower Marcellus Shale. The Middle Devonian Acadian Basin serves as an example of the influence of detrital influx on organic-matter accumulation and preservation in mud-dominated depositional systems.

Provenance and paleogeography of post-Middle Ordovician, pre-Devonian sedimentary basins on the Gander composite terrane, eastern and east-central Maine: implications for Silurian tectonics in the northern Appalachians

Recent mapping in eastern and east-central Maine addresses long-standing regional correlation issues and permits reconstruction of post-Middle Ordovician, pre-Devonian paleogeography of sedimentary basins on the Ganderian composite terrane. Two major Late Ordovician-Silurian depocenters are recognized in eastern Maine and western New Brunswick separated by an emergent Miramichi terrane: the Fredericton trough to the southeast and a single basin comprising the Central Maine and Aroostook-Matapedia sequences to the northwest. This Central Maine/Aroostook-Matapedia (CMAM) basin received sediment from both the Miramichi highland to the east and highlands and islands to the west, including the pre-Late Ordovician Boundary Mountains, Munsungun-Pennington, and Weeksboro-Lunksoos terranes. Lithofacies in the Fredericton trough are truncated and telescoped by faulting along its flanks but suggest a similar basin that received sediment from highlands to the west (Miramichi) and east (St. Croix).Deposition ended in the Fredericton trough following burial and deformation in the Late Silurian, but continued in the CMAM basin until Early Devonian Acadian folding. A westward-migrating Acadian orogenic wedge provided a single eastern source of sediment for the composite CMAM basin after the Salinic/Early Acadian event, replacing the earlier, more local sources. The CMAM, Fredericton, and Connecticut Valley-Gaspé depocenters were active immediately following the Taconian orogeny and probably formed during extension related to post-Taconian plate adjustments. These basins thus predate Acadian foreland sedimentation.Structural analysis and seismic reflection profiles indicate a greater degree of post-depositional crustal shortening than previously interpreted. Late Acadian and post-Acadian strike-slip faulting on the Norumbega and Central Maine Boundary fault systems distorted basin geometries but did not disturb paleogeographic components drastically.

Prelithification and synlithification tectonic foliation development in a clastic sedimentary sequence: COMMENT

Using observations from a single locality (Wine Strand) in the Glashabeg Formation (Dingle Group) of the Dingle Peninsula, southwest Ireland, Meere et al. (2016) conclude that more than 50% shortening of these strata was accomplished during the mid Devonian Acadian event, before lithification was complete. That the late Silurian to Emsian Dingle Group was deformed during the Acadian is not disputed (Todd, 2015, and references therein). Rather, it is the conclusion that the single tectonic foliation is Acadian and was generated before lithification was complete is refuted by several factors, not covered by Meere et al., but which are evident from assessment of the entire area (Todd, 2015). (1) The entire Palaeozoic of the area was affected by the end– Carboniferous Variscan orogenic shortening. Structural observations indicate that Old Red Sandstone (ORS) rocks demonstrably younger than the Acadian unconformity have experienced ~14% shortening by folding and faulting and a Rs yz strain of ~2.5 (or 40% shortening). The strain is associated with a cleavage that is axial planar to the folds in plan and occurs in symmetrical fans on the axial plane in cross section (Todd, 2015). In the west of the peninsula, this Variscan deformation is evidenced by steeply dipping and cleaved Late Devonian ORS on the north coast and inferred from the substantial uplift of the Acadian unconformity above regional. Any interpretation of shortening in the Dingle Group must take account of this overprint. (2) Dingle Group folding is Acadian in age but the single tectonic fabric is a younger overprint. The folding and faulting of the Dingle Group clearly accomplishes more shortening (~40%) than is evidenced by Variscan-only folds (~14%) above the Acadian unconformity (Todd, 2015). However, when strain data from the entire area are considered, there is a smaller difference in the Rs yz strains in mudrocks above (~2.5) and below (~2.8) the Acadian unconformity (Todd, 2015). In other words, as much 88% of the total bulk strain in the Dingle Group is likely due to Variscan shortening, not Acadian. The cleavage transects the axial planes of the Dingle Group folds in both plan and section, commensurate with Variscan overprinting of preexisting and gently tightened Acadian folds and hence is not a product of Acadian transpression (Todd, 2015). (3) Strain patterns are highly dependent on bedding orientation and lithology and fabric of the sediments. At Wine Strand, and other locations where strata are sub-horizontal, mudrocks, when pedogenic calcrete is absent or weakly developed, are typically deformed by constant volume flattening strain with a component of vertical stretching (Meere and Mulchrone, 2006). In coarser lithologies, the calculated Rs yz strains are markedly lower and instead some bulk strain occurred by volume loss along the spaced cleavage folia (Fig. 1A). The cleavage folia in the coarser units become more prominent on depositional fabrics like intraclasts, gutter casts, and channel margins that have formed preferential planes of pressure solution. Where mudrocks are strongly effected by pedogenic calcretes, the pressure solution cleavage is more prominent and carbonate nodules and columns are flattened in it. Where beds are more steeply dipping to sub-vertical in the Dingle Group, more flattening and accentuation of bed-parallel depositional fabrics in coarser rocks is observed, particularly through the rotation and packing of more labile/ductile lithic clasts and pressure solution creating curviplanar instead of point grain/clast contacts (Figs. 1B–1D). Pressure solution folia are usually partitioned to the matrix, often welding clast contacts; occasionally clasts are cut. Mica beards on quartz grains (Meere and Mulchrone, 2006) and competent clast extension by joints/veins have been observed (Fig. 1C). The strain incompatibility between mudrocks and sandstones/conglomerates is more pronounced in subvertical setting, indicated by bed-perpendicular quartz ladder veins (Meere and Mulchrone, 2006). Hence, throughout the area there are strain features associated with the tectonic foliation only consistent with postlithification strain. These features are less common or absent at Wine Strand, but this is a function of the partitioned bulk strain of bed-parallel depositional fabrics in a poorly sorted, loosely packed conglomerate fabric and pressure solution in the domainal cleavage.

Structure of the Dingle Peninsula, SW Ireland: evidence for the nature and timing of Caledonian, Acadian and Variscan tectonics

AbstractThe Palaeozoic rocks of the Dingle Peninsula provide a record of the evolution of the Caledonides, Acadides and Variscides. The succession ranges from Early Ordovician deep-water sediments, through Silurian shallow marine to non-marine sediments and volcanic rocks to an Old Red Sandstone (ORS) succession topped by Carboniferous marine shales. Comparison of structural styles in the unconformity-bounded groups, together with a detailed analysis of fault zones, allows the tectonic history to be deduced. The older rocks record Caledonian processes on the margin of Avalonia during Early Ordovician time and convergence then soft collision with Laurentia during Silurian time. The Dingle Basin was developed during the late Silurian – Early Devonian transtension in the Iapetus suture zone and was inverted in the latest Emsian Acadian orogenic episode. Post-Dingle Group ORS groups in the north of the peninsula were deposited in a syn-rift footwall block to the main Munster Basin. The Acadian transpressional and Munster Basin extensional structures were reactivated or overprinted in the Variscan deformation such that Acadian folds are transected by Variscan cleavage in both plan and vertical views. After Iapetus closure, changes in the tectonic regime are believed to be a result of adjustments in the geometry of subduction of the Rheic Ocean.

The Silurian Salinic Disturbance — newly discovered orogenic event in the northern Appalachians

In northern New Brunswick the Silurian and Ordovician rocks have been affected by two regional deformation events. The first deformation is attributed to the Salinic orogeny. It took place during the late Wenlockian — early Ludlow (426—421 Ma) and produced tectonic folds with northwest striking axial surfaces. Evidence for cleavage and faulting coeval to this event is rare. The second regional deformation took place during the lower to middle Devonian (407—390 Ma) and is know as Acadian orogeny. The Acadian deformation overprinted the Salinic structures in the lower Silurian rocks but produced single fold generation in the upper Silurian rocks. Intensive cleavage and widespread faulting are related to the Acadian revolution. The refolds in the lower Silurian rocks are steeply plunging. The refolds are overprinted by the Acadian cleavage, while the Acadian folds are dextrally transected by the same cleavage. The kinematics of the Salinic deformation is not understood but it is known that the Acadian deformation was achieved during intensive dextral shear, which resulted in transecting cleavage and an echelon arrangement of Acadian folds with respect to the high-strain shear zones. Most of the faults in the northern Appalachians appear to be synchronous or to postdate the Acadian folding. The dextral shear was penetrative on a regional scale and resulted in numerous bedding parallel faults with hidden separation.

40Ar– 39Ar white mica ages reveal Neoproterozoic/Paleozoic provenance and an Alleghanian overprint in coeval Upper Ordovician–Lower Devonian rocks of Meguma and Avalonia

40Ar– 39Ar analyses (IR single grain, total fusion) of white micas from Upper Ordovician–Lower Devonian sedimentary strata in the Meguma (Annapolis section) and Avalon (Arisaig section) terranes of the Canadian Appalachians complement recent detrital zircon studies by providing constraints on local source areas and identifying the age of low temperature (< 350 °C) events along this portion of the northern margin of the Rheic Ocean. In the Annapolis section, pre-depositional muscovite ages range in age from Cambrian to Early Ordovician but occur only in the arenaceous ca. 440 Ma White Rock Formation, reflecting the relatively inert behavior of the quartz sandstone compared to overlying argillaceous rocks. Pre-depositional ages of muscovites from coeval strata in the Arisaig Group range from Cambrian to Middle Ordovician. In younger Arisaig strata, detrital muscovite ages typically range from Late Neoproterozoic–Early Ordovician, but some are similar to the depositional ages of the formations. The detrital muscovites in both Annapolis and Arisaig sections are interpreted to have been derived from neighboring terranes (e.g. Ganderia) during the accretion of Avalonia to Baltica and Laurentia. A remarkable outcome of the study is that many muscovite ages are significantly younger than the depositional age of the strata. They do not record the (Middle–Late Devonian) age of Acadian folding or adjacent intrusions, but instead preserve a precise age of (322.6 ± 2.1 Ma, 16 analyses). Although the sections sampled display no discernable evidence for Carboniferous tectonism, these ages are coeval with documented Alleghanian deformation in both Avalonian and Meguma terrane rocks and we interpret them to reflect distributed fluid flow coeval with dextral shear along the Avalon–Meguma terrane boundary.

The Acadian fold belt in the Meguma Terrane, Nova Scotia: Cross sections, fold mechanisms, and tectonic implications

Structural data and cross sections from the eastern part of the Devonian‐Carboniferous fold belt of the Meguma Terrane give insight into its structural development and crustal tectonics. The sandy Goldenville Formation, the active competent member during buckling, formed a multilayer several kilometers thick that lay beneath the denser, softer muds and silts of the Halifax Formation. Cross sections reveal 2 orders of folds: 11–18 km wavelength folds are interpreted as resulting from buckling under the influence of gravity; 4–6 km wavelength folds reflect the thickness of the multilayer with buckle shortening in the range 32–44%. Depth to detachment derived from the sections (11–13.5 km below present erosion surface) is such that given the depth of the detachment below the paleosurface during the main phase (pre‐Devonian granite) of folding, it would likely have been a relatively soft and thick shear zone that would have influenced profile geometry of the main phase folds at this stage of fold belt development. Granite production that may have been triggered by mantle delamination during continuing convergence accompanying terminal Pangean assembly and ocean basin closure was followed by rapid exhumation. Later localized deformation (Late Devonian–Carboniferous) in the fold belt took place as the Meguma Terrane was displaced along the terrane‐bounding Cobequid‐Chedabucto fault system. The characteristic asymmetric cross sections of structures of this phase are attributed to cooling and strengthening of the detachment that was a consequence of the postgranite uplift and exhumation.

Transpressive duplex and flower structure: Dent Fault System, NW England

Revised mapping along the Dent Fault (northwest England) has improved the resolution of folds and faults formed during Variscan (late Carboniferous) sinistral transpression. A NNE-trending east-down monocline, comprising the Fell End Syncline and Taythes Anticline, was forced in Carboniferous cover above a reactivated precursor to the Dent Fault within the Lower Palaeozoic basement. The Taythes Anticline is periclinal due to interference with earlier Acadian folds. The steep limb of the monocline was eventually cut by the west-dipping Dent Fault. The hangingwall of the Dent Fault was dissected by sub-vertical or east dipping faults, together forming a positive flower structure in cross-section and a contractional duplex in plan view. The footwall to the Dent Fault preserves evidence of mostly dip-slip displacements, whereas strike-slip was preferentially partitioned into the hangingwall faults. This pattern of displacement partitioning may be typical of transpressive structures in general. The faults of the Taythes duplex formed in a restraining overlap zone between the Dent Fault and the Rawthey Fault to the west. The orientations of the duplex faults were a response to kinematic boundary conditions rather than to the regional stress field directly. Kinematic constraints provided by the Dent and neighbouring Variscan faults yield a NNW–SSE regional shortening direction in this part of the Variscan foreland.

Forearc extension and sea-floor spreading in the Thetford Mines Ophiolite Complex

Abstract The Ordovician Thetford Mines Ophiolite Complex (TMOC) is an oceanic terrane accreted to the Laurentian margin during the Taconic Orogeny and is affected by syn-obduction (syn-emplacement) deformation and two post-obduction events (Silurian backthrusting and normal faulting, and Acadian folding and reverse faulting). The southern part of the TMOC was tilted to the vertical during post-obduction deformation and preserves a nearly complete cross-section through the crust. From base to top we distinguish cumulate Dunitic, Pyroxenitic and Gabbroic Zones, a hypabyssal unit (either sheeted dykes or a subvolcanic breccia facies), and an ophiolitic extrusive-sedimentary sequence, upon which were deposited sedimentary rocks constituting the base of a piggy-back basin. Our mapping has revealed the presence of subvertically dipping, north-south- to 20°-striking faults, spaced c. 1 km apart on average. The faults are manifested as sheared or mylonitic dunites and synmagmatic breccias, and correspond to breaks in lithology. The fault breccias are cut by undeformed websteritic to peridotitic intrusions, demonstrating the pre- to synmagmatic nature of the faulting. Assuming that rhythmic cumulate bedding was originally palaeo-horizontal, kinematic analysis indicates that these are normal faults separating a series of tilted blocks. In the upper part of the crust, the north-south-striking fault blocks contain north-south-striking dykes that locally constitute a sheeted complex. The faults correspond to marked lateral changes in the thickness and facies assemblages seen in supracrustal rocks, are locally marked by prominent subvolcanic breccias, and have upward decreasing throws suggesting that they are growth faults. The base of the volcano-sedimentary sequence is a major erosional surface in places, which can penetrate down to the Dunitic Zone. The evidence for coeval extension and magmatism, and the discovery of a locally well-developed sheeted dyke complex, suggest that the TMOC formed by sea-floor spreading. The dominance of a boninitic signature in cumulate and volcanic rocks suggests that spreading occurred in a subduction zone environment, possibly in a forearc setting.

Supra–subduction zone extensional magmatism in Vermont and adjacent Quebec: Implications for early Paleozoic Appalachian tectonics

Metadiabasic intrusions of the Mount Norris Intrusive Suite occur in faultbounded lithotectonic packages containing Stowe, Moretown, and Cram Hill Formation lithologies in the northern Vermont Rowe-Hawley belt, a proposed Ordovician arc-trench gap above an east-dipping subduction zone. Rocks of the Mount Norris Intrusive Suite are characteristically massive and weakly foliated, have chilled margins, contain xenoliths, and have sharp contacts that both crosscut and are parallel to early structural fabrics in the host metasedimentary rocks. Although the mineral assemblage of the Mount Norris Intrusive Suite is albite 1 actinolite 1 epidote 1 chlorite 1 calcite 1 quartz, intergrowths of albite 1 actinolite are probably pseudomorphs after plagioclase 1 clinopyroxene. The metadiabases are subalkaline, tholeiitic, hypabyssal basalts with preserved ophitic texture. A backarcbasin tectonic setting for the intrusive suite is suggested by its LREE (light rare earth element) enrichment, negative NbTa anomalies, and Ta/Yb vs. Th/Yb trends. Although no direct isotopic age data are available, the intrusions are broadly Ordovician because their contacts are clearly folded by the earliest Acadian (Silurian‐Devonian) folds. Field evidence and geochemical data suggest compelling along-strike correlations with the Coburn Hill Volcanics of northern Vermont and the Bolton Igneous Group of southern Quebec. Isotopic and stratigraphic age constraints for the Bolton Igneous Group bracket these backarc magmas to the 477‐ 458 Ma interval. A tectonic model that begins with east-dipping subduction and progresses to outboard west-dipping subduction after a syncollisional polarity reversal best explains the intrusion of deformed metamorphosed metasedimentary rocks by backarc magmas.

Emsian Synorogenic Paleogeography of the Maine Appalachians

The Acadian deformation front in the northern Appalachians of Maine and New Hampshire can now be closely located during the early Emsian (Early Devonian; 408–406 Ma). Tight correlations between paleontologically and isotopically dated rocks are possible only because of a new 408‐Ma time scale tie point for the early Emsian. The deformation front lay between a belt of Lower Devonian flysch and molasse that were deposited in an Acadian foreland basin and had not yet been folded and a belt of early Emsian plutons that intruded folded Lower Devonian rocks. This plutonic belt includes the newly dated Ore Mountain gabbro (U/Pb; 406 Ma), which hosts magmatic‐sulfide mineralization. Along the deformation front, a 407‐Ma pluton that locally truncates Acadian folds (Katahdin) was the feeder to volcanic rocks (Traveler Rhyolite; 406–407 Ma) that are part of the foreland‐basin succession involved in these same folds. The Emsian igneous rocks thus define a syncollisional magmatic province that straddled the deformation front. These findings bear on three alternative subduction geometries for the Acadian collision.

The interplay of regional structure and emplacement mechanisms at the contact of the South Mountain Batholith, Nova Scotia: floor-down or wall-up?

In southern Nova Scotia, the Devonian South Mountain Batholith was emplaced into metasedimentary rocks of the Cambro-Ordovician Meguma Group at ca. 370 Ma. The contact of the eastern end of the South Mountain Batholith transects at a high angle the trace of subhorizontal, upright Acadian (mid-late Devonian) folds formed in the Meguma Group. At two locations, where the contact is well exposed, there are contrasting structures in the country rocks adjacent to Acadian anticlinoria and synclinoria, respectively. Regional folds are affected by ductile deformation where anticlinoria abut the batholith but are undisturbed at the synclinoria. At the anticlinorial contacts, the metasedimentary bedding youngs towards the granite, and granite side-down shear resulted in a belt in which bedding is transposed to a new contact-parallel fabric. Deflection of linear structures that were initially horizontal in the Acadian folds (e.g., intersection lineations) illustrates the granite side-down shear. The reorientation of initially horizontal linear structures gradually diminishes as the contact is followed from the anticlinoria to the synclinoria, where the regional fold geometry is preserved right up to the contact, showing that there is no granite side-down shear in the synclinoria at the present level of erosion. Two models that potentially explain this variation in contact structure are discussed. In the first, it is explained as an artifact of emplacement of the batholith late in the growth of the Acadian folds, in which the horizontal, upright anticlinoria amplified and moved upward relative to the pluton. A shear zone was formed parallel to the contact along the thermally softened tip of the anticlinoria. The synclinoria remained fixed vertically and there was no differential movement between granite and country rock. Thus, regional structures and evidence for stoping are most widely preserved in the synclinoria, where they were not overprinted by the marginal shearing. The second model invokes floor-down emplacement of magma into folds of layered sediments with contrasting mechanical properties. The erosion surface within the synclinoria intersects slates of the Halifax Formation with mechanical properties that favour emplacement predominantly by dyking and stoping. Below the level of erosion, the stratigraphically underlying Goldenville Formation, having different mechanical properties than the Halifax, presumably is displaced downwards predominantly by ductile deformation (pure and simple shear). Within the anticlinoria, where the Goldenville Formation is exposed, the requirement of a level pluton floor necessitates that downward deflection is accompanied by relatively high ductile strains in the wall rock. A third possible model that combines features of the syntectonic and floor-down models is an obvious option.

Alleghanian reactivation of the Acadian fold belt, Meguma Zone, southwest Nova Scotia

Shear zones and northwest-verging folds define a 30 km wide belt of deformation that overprints the Acadian fold belt and telescopes isograds in the Meguma Zone in southwest Nova Scotia. The shear zones appear to form a linked system that accommodated convergence-dominated transpression of the Meguma Zone against an irregular Avalon boundary. Available geochronological data indicate a Mid-Carboniferous (Alleghanian–Variscan) age for the overprinting deformation. The Mid-Carboniferous basement reactivation in southwest Nova Scotia is likely coeval with deformation of Carboniferous strata and reactivation of basement structures (Meguma Group) in the northern Meguma Zone. Together, these Mid-Carboniferous structures may define a wide belt of Alleghanian–Variscan deformation across the northwest (cratonward) margin of the Meguma Zone.

Distribution and characteristics of Taconian and Acadian deformation, southern Québec Appalachians

The regional structure of the southern Quebec Appalachians is redefined in terms of superposed Taconian and Acadian deformation events. Taconian structures and metamorphism dominate in Cambro-Ordovician rocks west of the Baie Verte-Brompton Line (BBL). The Taconian orogen is subdivided into a northwestern external zone, composed of fault-imbricated continental rocks displaying single-phase foreland structures, and a south-eastern internal zone of polydeformed and metamorphosed continental and oceanic rocks. The structural relationships of Cambro-Ordovician rocks astride the BBL demonstrate that the continent-ocean contact was deformed by both southeast-verging late Taconian and upright Acadian folds. East of the BBL, syn- and post-Taconian rocks were mainly affected by Acadian deformation. The Acadian external zone consists of Cambro-Ordovician and post-Ordovician rock units characterized by a single phase of folding. The zone is bounded to the southeast by the Riviere Victoria fault. The Acadian internal zone displays polyphase deformation in the easternmost part of the Quebec Appalachians. The Acadian deformation shows a significant overlap with Taconian structures and metamorphism, and structural windows of the Taconian orogen are found within both the external and internal zones of the Acadian orogen.

Palaeomagnetic study of Llandovery (Lower Silurian) red beds in north-west England

High unblocking temperature magnetization components carried by haematite have been isolated from 12 sites of Llandovery (Lower Silurian) red mudstones from NW England. The principal deformation phase in this area is Acadian (mid-Devonian), however, some faults (with related folds) were also active during the later Variscan orogeny. The site-mean magnetization directions pass a regional fold test at the 95 per cent confidence level, implying that the magnetization components pre-date Acadian folding. The overall mean direction (Declination: 43.4d, Inclination: -24.1d, α95: 12.4d) gives a pole position at 13.6dS, 313.9dE and a palaeolatitude of 12dS. The low palaeolatitude implies closure of the Iapetus Ocean between Eastern Avalonia and Laurentia by Late Llandovery time and is consistent with a recently published palaeolatitude (13dS) for the Wenlock (mid-Silurian) of southern Britain. Comparison of these values with the published palaeolatitude from NW England of 43dS for Caradoc (Late Ordovician) time implies very rapid (at least 15 cm yr−1) northward drift of Eastern Avalonia during Late Ordovician/Early Silurian time. The new pole position from the Llandovery red beds lies close to Late Silurian and Early Devonian poles from Scotland and Wales, but is significantly different from a group of mid-Silurian poles comprising data from Scottish intrusions and from Swedish sediments. This discrepancy may be due to Late Silurian/Early Devonian (pre-Acadian) remagnetization of our sampling sites, or local clockwise rotation of the sampling sites during Acadian and/or Variscan deformation. However, the positive fold test, presence of reversals and local deformation history tend to mitigate against these possibilities. The more likely interpretation is that although Baltica/Scotland were one continental block from the mid-Silurian (Scandian orogeny), Eastern Avalonia did not become part of this block until latest Silurian.

Early Devonian (pre-Acadian) magnetization directions in Lower Old Red Sandstone of south Wales (UK)

SUMMARY Two components of magnetization have been resolved from the Lower Old Red Sandstone of south Wales. All 39 sampled sites lie north of the Variscan front, and all but three are largely unaffected by Variscan folding. Thirteen sites yield a high unblocking temperature magnetization component which pre-dates Acadian (mid-Devonian) folding. The site mean direction for this component (Dec.: 232.2°, Inc.: 31.9°, α95: 8.5°) yields a pole position (Lat.: 7.3°S, Long.: 306.7°E) which can be assigned an Early Devonian age. A lower unblocking temperature component is present at 38 of the sampled sites. This component post-dates both the Acadian and Variscan folding, has reversed polarity and a mean direction (Dec.: 193.8°, Inc.: −7.2°, α95: 2.6°) which corresponds to a pole position (Lat.: 40.4°S, Long.: 338.0°E) consistent with a latest Carboniferous or early Permian age. The Early Devonian pole position confirms the existence of a Siluro-Devonian inflexion in the apparent polar wander path (APWP) for southern Britain which coincides with an inflexion in the APWP for Scotland (Britain north of the Iapetus suture), indicating the closure of the Iapetus Ocean by Siluro-Devonian time. The Early Devonian palaeolatitude (17°S ± 5°) is consistent with an Early Devonian configuration in which Gondwana, Laurentia, Baltica and Avalonia were part of a single supercontinent.

The effects of grain‐scale deformation on the Bloomsburg Formation Pole

Previous paleomagnetic results from the Silurian Bloomsburg Formation in the central Appalachian Valley and Ridge Province suggest that the Bloomsburg's characteristic magnetization is a synfolding magnetization acquired during the Acadian Orogeny. However, there is little geological evidence to support extensive Acadian folding in the central Appalachians. A more favorable explanation is that the characteristic magnetization originated as a prefolding Silurian magnetization and has been altered by grain‐scale deformation to mimic a synfolding Devonian remagnetization. In order to investigate whether penetrative deformation has altered the Bloomsburg Formation's characteristic signal, the relationship between strain and remanence was examined around three central Appalachian Valley and Ridge folds. Structural analysis of mesoscopic and microscopic features along with center‐to‐center finite strain analysis at these three folds indicates a deformation sequence that includes prefolding layer‐parallel shortening overprinted by top‐to‐the‐foreland, bedding‐parallel shear and flexural slip/flow folding. Inclination of the characteristic magnetization varies systematically with the magnitude of finite strains. This relationship between strain and remanence suggests that the characteristic magnetization has been progressively rotated toward shallower inclinations by penetrative deformation analogous to rigid particle rotation in a viscously deforming matrix. Using directions from sites with relatively low finite strain values, a new Bloomsburg Formation pole is calculated at 18.7°N, 107.4°E (K = 58.4, A95 = 8.2°). This new pole falls on the Silurian track of the North American apparent polar wander path between the Silurian Wabash Reef pole and the Silurian‐Devonian Andreas pole. In addition, these results suggest that previously reported 20°–30° difference in declination of site mean directions north arid south of the Pennsylvania salient may also be the result of grain‐scale reorientation of the remanence‐carrying grains and not oroclinal bending.

The Weekend dykes, a newly recognized mafic dyke swarm on the eastern shore of Nova Scotia, Canada

A group of related mafic dykes is present along the eastern shore of Nova Scotia from Halifax to Country Harbour. The dykes are generally vertical, strike about 150°, and have an abundance of carbonate, apatite, and hydrous mafic minerals, indicating that the dykes may have formed from an alkaline (lamprophyric) magma. They postdate the Acadian folding of the Cambro-Ordovician Meguma Group metasedimentary rocks but predate Carboniferous faulting. Abundant exotic gneissic and (meta)plutonic xenoliths, representing probable pre-Meguma Group rocks, are found in half of the presently known dykes.The dykes may have formed in a regional tensional field during a shearing event synchronous with docking between the Meguma and Avalonian terranes or in a more local radial stress field around an igneous body, such as the Bog Island Lake or Ten Mile Lake diatreme in the Liscomb Complex. Similar ages suggest that the dykes may be related to the intrusion of the Devono-Carboniferous granites of the Musquodoboit batholith.

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.