John W. F. Waldron

Taconian orogeny and the accretion of the Dashwoods block: A peri-Laurentian microcontinent in the Iapetus Ocean

The stratigraphy of the external Humber zone of the western Newfoundland Appalachians records protracted Neoproterozoic–Early Cambrian rifting, followed by development of a passive margin that persisted until late Early Ordovician (Arenigian) time (ca. 475 Ma). However, adjacent metamorphic rocks, derived from the Laurentian margin and preserved in the Dashwoods subzone, were deformed, overthrust by ophiolites, and intruded by arc plutons by 488 Ma. The adjacent Notre Dame subzone also records isotopic evidence of interaction with the margin by 488 Ma. We propose that a microcontinent (Dashwoods microcontinent) was rifted from Laurentia during the Early Cambrian after an earlier, Neoproterozoic opening of the Iapetus Ocean, and was separated from the margin by a narrow oceanic tract (Humber seaway). Attempted subduction of the Dashwoods microcontinent prior to 488 Ma was followed by closure of the Humber seaway in the Taconian orogeny.

Provenance of the Meguma terrane, Nova Scotia: rifted margin of early Paleozoic Gondwana

Detrital zircon ages from the lower part of the Late Proterozoic(?) to Middle Cambrian Goldenville Group in the Meguma terrane of Nova Scotia suggest derivation from local sources in the Avalonian and Pan-African orogens on the margins of Early Cambrian Gondwana. Samples from near the top of the group show a broader distribution, including ages back to Archean. The eNd data show a corresponding trend, from slightly positive in the lower Goldenville Group to highly negative in the upper Goldenville Group and overlying Upper Cambrian to Lower Ordovician Halifax Group. The trends are consistent with deposition of the lower part of the Meguma succession in a rift, in which uplifted rift-flanks were the main source of the early basin fill, whereas subsequent thermal subsidence of rift margins allowed for more widespread sediment sourcing in younger units. The rift was possibly located between Gondwana and Avalonia, and may have been the locus for separation of Avalonia from Gondwana to form part of the Rheic O...

Role of evaporite withdrawal in the preservation of a unique coal-bearing succession: Pennsylvanian Joggins Formation, Nova Scotia

The Pennsylvanian Joggins Formation, in the Cumberland Group of Nova Scotia, is widely regarded as the worlds best exposure of coal-bearing Carboniferous strata. This 1.5-km-thick coal-bearing unit is famous for upright fossil lycopsid trees, and is preferentially preserved in the Athol syncline in the western Cumberland basin. New seismic profiles show that the Athol syncline is atop a salt weld and that the Joggins Formation thins on the flanks of adjacent evaporite-cored anticlines. These observations indicate that during deposition of the Joggins Formation, at least 1 km of syndepositional subsidence was facilitated by flow of underlying salt into the adjacent anticlines. In contrast, halo-kinesis was mainly active during the Mississippian in the eastern Cumberland basin (Tatamagouche syncline); minibasins were filled by Mabou Group sediments, whereas the Cumberland Group is thin and lacks significant coals. This basinwide comparison shows that much of the subsidence responsible for the preservation of the coal-bearing Joggins Formation was the result of salt withdrawal at depth.

Ancient Laurentian detrital zircon in the closing Iapetus Ocean, Southern Uplands terrane, Scotland

Early Paleozoic sandstones in the Southern Uplands terrane of Scotland were deposited during closure of the Iapetus Ocean between Laurentia and Avalonia. Their tectonic setting and sources are controversial, and different authors have supported subduction-accretion, extensional continental-margin development, or back-arc basin settings. We report new U-Pb detrital zircon ages from five Late Ordovician sandstones from the Northern Belt of the Southern Uplands and test models of their tectonic setting. The U-Pb zircon age distributions are dominated by peaks characteristic of sources in Laurentia and include grains as old as 3.6 Ga, older than any previously recorded in the British Caledonides SE of the Laurentian foreland. Discordant grains in one sample suggest derivation via erosion of metasedimentary rocks incorporated in the Grampian-Taconian orogen. Rare Neoproterozoic grains, previously interpreted as originating from a peri-Gondwanan terrane, may be derived from igneous rocks associated with Iapetan rifting. Only rare zircons are contemporary with the depositional ages. The results are difficult to reconcile with extensional continental-margin and back-arc models, but they support an active continental-margin subduction-accretion model. Close similarities with distributions from the Newfoundland Appalachians are consistent with sinistral transpression during closing of the Iapetus Ocean.

Truncation and translation of Appalachian promontories: Mid-Paleozoic strike-slip tectonics and basin initiation

Accreted terranes of the Appalachian Iapetan and peri-Gondwanan realms display structural trends that are mainly concordant with promontories and embayments in the Laurentian margin, indicating that during accretion, the shape of the continental margin acted as a template around which accreted terranes were molded. In North Carolina and New-foundland, post-accretion transcurrent motion appears to be recorded by displaced outboard portions of promontories, no longer concordant with those in Laurentia. A bend in structural trends confined to the peri-Gondwanan realm at the North Carolina–South Carolina state line is interpreted to represent the dextrally displaced outboard portion of the Virginia promontory. In Newfoundland, the Hermitage flexure is interpreted as a dextrally displaced Laurentian promontory that originated along strike to the northeast. In both places, promontories were truncated and dextrally translated for ~220–250 km by a Devonian–Mississippian orogen-parallel transcurrent system, which may well have extended for the length of the eastern Laurentian margin. South of the nearby St. Lawrence promontory, extreme thinning of Appalachian crust beneath the Maritimes Basin is consistent with extension at a releasing stepover. Estimates of extension are consistent with those obtained from promontory offsets.

Anatomy and evolution of a pull-apart basin, Stellarton, Nova Scotia

The Stellarton Basin is a late Paleozoic pull-apart basin located close to the Meguma-Avalon terrane boundary in the Canadian Appalachians, at the stepover between the Cobequid and Hollow strike-slip faults. The basin contains ∼3 km of rapidly deposited Pennsylvanian clastic sedimentary rocks representing lacustrine and deltaic environments, extensively documented through coal-related mining and drilling. Coal seams and oil shales allow stratigraphic correlation within the basin, permitting reconstruction of basin subsidence and structural evolution. Coal seams represent approximately paleo–horizontal surfaces; thickness variations (corrected for tilt and compaction) in the most coal-rich part of the basin fill show that the south basin margin subsided rapidly during deposition, acting as a trap for coarse sediment and allowing coal-forming mires to develop in the north. Abundant soft- sediment deformation structures reflect synsedimentary tectonic activity. North-striking normal faults dissected the basin fill during and soon after deposition, early in the diagenetic history. Contouring of mine plans allows fault heaves to be identified and also shows that both coal seams and faults were folded by east- to northeast-trending folds, consistent with (1) an environment of deformation involving dextral strike-slip motion and (2) clockwise rotation of fault blocks during progressive strain. Later, north-northeast–striking contractional structures within the basin, and a positive flower structure at the northwest margin, probably represent a transition from transtension to transpression. The basin illustrates the role of rotational progressive strain in the reorientation of structures generated during transtension. It displays evidence for changes in tectonic style through time, from transtension to transpression, that may typify basins developed on major strike-slip faults.

Silurian Tectonics of Western Avalonia: Strain-Corrected Subsidence History of the Arisaig Group, Nova Scotia

The Arisaig Group is a Silurian succession of predominantly shallow marine clastic sediments overlying volcanics, exposed in northern mainland Nova Scotia. Sediment accumulation provides a record of the subsidence of western Avalonia during the interval when terranes were being accreted within the Canadian Appalachians. To calculate the amount of subsidence, one must correct the measured thicknesses for the effects of tectonic strain. Deformed fossils on bedding surfaces indicate strain ratios mainly between 1.2 and 1.6. An empirical porosity-depth relationship is used to correct for compaction. The subsidence curves are then adjusted to allow for variations in water depth and eustatic sea level. The resulting curves show significant variations in subsidence rate regardless of which version of the Silurian time-scale is used. An initial episode of rapid subsidence followed eruption of Llandoverian volcanics. Slower subsidence took place in Wenlockian and Ludlovian time, with deeper-water sedimentation during an early Ludlovian eustatic high. This part of the history is consistent with thermal subsidence following an initial extensional event. A rapid increase in subsidence rate occurred during deposition of the Pridolian Stonehouse Formation. This episode was accompanied by a rapid increase in sediment supply, and a change in paleocurrent flow from southwest to northwest. The rapid Pridolian subsidence probably resulted from oblique collision between Avalonia and the Meguma Terrane to the south.

Stratal disruption and development of mélange, Western Newfoundland: effect of high fluid pressure in an accretionary terrain during ophiolite emplacement

Abstract The Bay of Islands Ophiolite was emplaced onto the continental margin of North America during the mid-Ordovician Taconic orogeny, when tectonic slices of continental margin sediments were accreted to the moving allochthon. Tectonic slices grade into and are surrounded by melange. Early fracture in sandstones formed without grain breakage and allowed penetration of liquid petroleum along fracture planes. Other fractures involved cataclastic flow and were sometimes re-activated during formation of later pressure solution cleavage. Shear-fracture and extension-fracture boudinage affect competent strata; extensional veins cut cement in limestone beds and are filled by shale, quartz, calcite and bitumen. Folds also formed, at a time when siltstone and sandstone were at least partially lithified. Melange matrix shows abundant shear and extension fractures in a variety of orientations. Coaxial extension responsible for disruption of bedding can be explained by a brittle accretionary wedge model in which high fluid pressures resulted from tectonic dewatering of shales. Surface slope decreased as fluid pressure rose beneath the ophiolite, causing horizontal extension of the wedge. After escape of excess water the surface slope steepened again as renewed stacking occurred.

Late Paleozoic strike-slip faults in Maritime Canada and their role in the reconfiguration of the northern Appalachian orogen

Major late Paleozoic faults, many with documented strike-slip motion, have dissected the Ordovician-Devonian Appalachian orogen in the Maritime Provinces of Atlantic Canada. Activity alternated between east-west faults (Minas trend) and NE-SW faults (Appalachian trend). NW-SE faults (Canso trend) were probably conjugate to Minas-trend faults. Major dextral movement, on faults with Appalachian trend, in total between 200 and 300 km, began in the Late Devonian. This movement initiated the Maritimes Basin in a transtensional environment at a releasing bend formed around a promontory in the Laurentian margin and thinned the crust, accounting for the major subsidence of the basin. Appalachian-trend strike slip continued in the Mississippian but was accompanied by major movement on E-W Minas-trend faults culminating around the Mississippian-Pennsylvanian boundary, juxtaposing the Meguma and Avalon terranes of the Appalachians close to their present-day configuration. However, strike slip continued during the Pennsylvanian-Permian interval resulting in transpressional deformation that reactivated and inverted earlier extensional faults. A final major episode of transtension, mainly sinistral, occurred during the Mesozoic opening of the Atlantic Ocean. Restoration of movements on these faults, amounting to several hundred kilometers of slip, explains anomalies in the present-day distribution of terranes amalgamated during early Paleozoic Appalachian tectonism. In the restored geometry, the Nashoba and Ellsworth terranes of Ganderia are adjacent to one another, and the Meguma terrane lies clearly outboard of Avalonia. A restored post-Acadian paleogeography, not the present-day geometry of the orogen, should be used as a basis for reconstructions of its earlier Paleozoic history.

Ganderia–Laurentia collision in the Caledonides of Great Britain and Ireland

During terrane convergence, an influx of clastic sediment from an upper plate onto a lower plate is an early indication of terrane juxtaposition. In the Caledonides of Great Britain and Ireland, units accreted to Laurentia during the early Palaeozoic Era include peri-Gondwanan terrane assemblages that earlier separated from West Gondwana. However, the Southern Uplands Terrane contains detrital zircon populations apparently derived entirely from Laurentia, characterized by a large, asymmetric Mesoproterozoic peak and a scarcity of zircon at 600 Ma and 2.1 Ga. In contrast, Cambrian and Ordovician rocks from the Lake District and the Leinster Massif of Ireland show abundant grains with these ages, together with a range of Mesoproterozoic zircon. These characteristics are shared with the Monian terrane of Anglesey and with Ganderia in the Appalachians, indicating probable derivation from Amazonia in West Gondwana. Silurian sandstones from the Lake District show an influx of Laurentia-derived zircon, and lack the peri-Gondwanan signal. This indicates that in the Caledonides, Ganderia was not accreted to the Laurentian margin until c. 430 Ma, in contrast to the Ordovician accretion of Ganderian fragments recorded in the Appalachians, suggesting that the configuration of the closing Iapetus Ocean varied significantly along the strike of the orogen. Supplementary material: Details of sample coordinates, analytical procedure and U/Pb detrital zircon analytical data are available at www.geolsoc.org.uk/SUP18739.