Garry Quinlan

Deep seismic reflection profile across the northern Appalachians

A marine seismic reflection line across the northeast extremity of the Canadian Appalachians in Newfoundland indicates a collisional suture(?) in the lower crust beneath the central ophiolitic Dunnage terrane. The thrust and fold belt (miogeocline) above the Grenville basement, and the Dunnage and Gander tectonostratigraphic terranes all appear to be allochthonous with respect to lower crustal basement. The Gander-Avalon terrane boundary to the east is a near-vertical feature that penetrates the crust. The data also suggest that the ancient passive margin of North America extends eastward under the Dunnage terrane for about 70 km. The Newfoundland deep seismic experiment indicates major tectonic differences compared to results for the southern Appalachians.

Tectonic model for crustal seismic reflectivity patterns in compressional orogens

A pattern of crustal seismic reflectivity tentatively dated as Ordovician- Silurian recurs along strike in the Newfoundland Appalachians. Comparable fault patterns are produced in sandbox models of asymmetric compressional systems where oppositely dipping step-up shear zones are localized above a discontinuity in basal velocity. The reflectivity pattern may be produced by a mechanically analogous process in which a stress discontinuity results from detachment and underthrusting of mantle and possibly lower crustal lithosphere during con- tinent-continent collision. Similar seismic reflectivity patterns exist elsewhere in the Appalachian-Caledonide mobile belt as well as in older and younger compressional orogens, suggesting that detachment and underthrusting leading to asymmetric crustal deformation may be a common feature of compressional orogens.

Lithoprobe east onshore-offshore seismic refraction survey: constraints on interpretation of reflection data in the Newfoundland Appalachians

Abstract Combined onshore-offshore seismic refraction/ wide-angle reflection data have been acquired across Newfoundland, eastern Canada, to investigate the structural architecture of the northern Appalachians, particularly of distinct crustal zones recognized from earlier LITHOPROBE vertical incidence studies. A western crustal unit, correlated with the Grenville province of the Laurentian plate margin thins from 44 to 40 km and a portion of the lower crust becomes highly reflective with velocities of 7.2 km/s. In central Newfoundland, beneath the central mobile belt, the crust thins to 35 km or less and is marked by average continental velocities, not exceeding 7.0 km/s in the lower crust. Further east, in a crustal unit underlying the Avalon zone and associated with the Gondwanan plate margin, the crust is 40 km thick, and has velocities of 6.8 km/s in the lower crust. Explanations for the thin crust beneath the central mobile belt include (1) post-orogenic isostatic readjustment associated with a density in the mantle which is lower beneath this part of the orogen than beneath the margin, (2) mechanical thinning at the base of the crust during orogenic collapse perhaps caused by delamination, and (3) transformation by phase change of a gabbroic lower crust to eclogite which seismologically would be difficult to distinguish from mantle. Except for a single profile in western Newfoundland, velocities in the crust are of typical continental affinity with lower-crustal velocities less than 7.0 km/s. This indicates that there was no significant magmatic underplating under the Newfoundland Appalachians during Mesozoic rifting of the Atlantic Ocean as proposed elsewhere for the New England Appalachians. A mid-crustal velocity discontinuity observed in the Newfoundland region does not coincide with any consistent reflection pattern on vertical incidence profiles. However, we suggest that localized velocity heterogeneities at mid-crustal depths correspond to organized vertical incidence reflections.

Roots of the Labradorian orogen in the Grenville Province in southeast Labrador: Evidence from marine, deep‐seismic reflection data

Marine, deep-seismic reflection data obtained as part of the Eastern Canadian Shield Onshore-Offshore Transect (ECSOOT) Lithoprobe project from offshore southeast Labrador are evaluated using geodynamic and gravity models, and a tectonic interpretation is developed. The geodynamic model explains the seismic reflection data in terms of doubly vergent tectonism resulting from southward directed underthrusting of mantle and lowermost crust. Middle and upper crustal seismic reflectors are considered to be proshears and retroshears and correlated through potential field data with major mylonite zones at terrane boundaries mapped onshore. As there is little evidence of subsequent severe structural modification (apart from strike-parallel Grenvillian dextral transpression, which does not alter the crustal geometry along the line of the seismic transect), the seismic reflectors are considered to represent Labradorian structures, inferred from onshore geochronological data to be mostly related to a ∼ 1665 Ma (Labradorian) accretionary event. The southward directed underthrusting of the mantle and lowermost crust is consistent with existing models for southward Labradorian subduction. Coast-parallel gravity and magnetic anomalies are interpreted as an expression of a Neoproterozoic to early Phanerozoic extensional basin, the existence of which had little effect on the underlying crystalline basement, other than to downfault it relative to that exposed onshore.

Relationship between deeper lithospheric processes and near-surface tectonics of sedimentary basins

Interactions between upper lithosphere deformation, as manifested in the structure and evolution of sedimentary basins, and lower lithospheric deformation presumably reflect large-scale regional or global tectonic events. We review the current state of knowledge regarding these interactions and suggest areas of research which may lead to an improvement in this knowledge. Understanding the development of sedimentary basins requires insight into the processes responsible both for basin subsidence and also for distribution and preservation of sediments within the basins. While some classes of basins are better understood than others, there remain significant gaps both in our ability to account for available data and also in the data available to test current models and develop new ones. Subsidence in some classes of basins can be related to lithospheric scale processes occurring as part of the overall plate tectonic paradigm. For example, it is widely accepted that passive margin basins subside as a result of lithospheric extension (McKenzie, 1978) and that foreland basins are flexural consequences of collision between a continent and outboard terranes (Beaumont, 1981). On the other hand, models for the origin of intracratonic, forearc and trans-tensional basins are essentially still at cartoon level, involving processes that are difficult to quantify and evaluate. In the following we review a number of fundamental questions of vital interest for understanding of the relationship between deeper lithospheric processes and near-surface tectonics.

A collisional crustal fabric pattern recognised from seismic reflection profiles of the Appalachian/ Caledonide orogen

Abstract Deep seismic reflection profiles across the Appalachians, and across the once contiguous Caledonides around Britain and Ireland, show a broadly similar pattern of crustal reflection fabrics. The pattern is of two adjacent fabric “domains”, each characterized by dips towards the other, bounded by an interface which dips in conformity with one of the fabrics. The domain interface strikes parallel to the orogen, suggesting a genetic relationship between the fabrics and the orogen. From the North Sea to the Long Island platform, the domain boundary dips to the northwest. In the southern Appalachians, the pattern appears with less consistency. Between the Charlotte and Carolina slate belts the pattern appears with the same polarity as further north, but below the coastal plain it occurs with opposite polarity—the interface dips to the southeast. An attractive, but not exclusive, hypothesis for explaining the fabric domain contrast is that it results from detachment of all or most of the crust from underlying mantle lithosphere during collisional orogenesis.

Dipping shear zones and the base of the crust in the Appalachians, offshore Canada

Abstract The Seismological character of the Appalachian mobile belt offshore from eastern Canada includes an unusual concentration of northwest-dipping reflectors which appear to dip through sub-horizontal reflectors at the base of the crust. The geometry of these structures has been characterised by ray-trace migration. They are not point diffractors. They appear to dip at between 25 and 45°, but flatten upwards towards a mid-crustal “bright” band and disappear downwards into the mantle. We interpret the reflectors as shear zones which ramp through the lower crust from a mid-crustal detachment to a diffuse and less reflective set of shears in the mantle. It is suggested that the reflectors were caused by the collision of continental blocks at the final closure of the Iapetus Ocean, but may have been subject to later reactivation, especially during late Carboniferous strike-slip.