A Mechanism for Tectonic Inheritance at Transform Faults of the Iapetan Margin of Laurentia

Abstract

Transform faults along the Iapetan rifted continental margin of Laurentia offset the continental rift and/or bound domains of oppositely dipping low-angle detachments. Rift-parallel and transform-parallel intracratonic fault systems extend into continental crust inboard from the rifted margin. Ages of synrift igneous rocks, ranging from 765 to 530 Ma, document non-systematic diachroneity of rifting along the Iapetan margin. Synrift sedimentary accumulations show abrupt variations in thickness across transform faults, and some concentrations of synrift igneous rocks are distributed along transform faults and transform-parallel intracratonic fault systems. The greatest thicknesses of Cambrian–Ordovician passive-margin shelf-carbonate deposits are along transform margins and in continental-margin basins along transform faults, as well as along transform-parallel intracratonic fault systems, indicating anomalously great post-rift thermal subsidence along transform faults. Along the Ordovician–Permian Appalachian-Ouachita orogenic belt, a diachronous array of synorogenic clastic wedges fills foreland basins, recording tectonic-load-driven flexural subsidence of the lithosphere. The greatest thicknesses of synorogenic clastic wedges of all ages are consistently in foreland basins along transform margins and inboard from intersections of transform faults with the rifted margin, indicating systematically weaker lithosphere along transform faults. The distinctive and pervasive properties and behaviour of the lithosphere along transform faults in successive tectonic settings suggest fundamental controls on tectonic inheritance at transform faults. Recent models for continental rifting incorporate ductile extension of the mantle lithosphere beneath brittle extension of the crust; the domain of ductile extension of the mantle lithosphere may reach significantly inboard from the rifted margin of the brittle crust, accounting for rift-parallel extensional faults in the crust inboard from the rifted margin. A transform offset of a rift in brittle crust requires a similar offset in ductile extension of the mantle lithosphere, leading to differential ductile flow on opposite sides of the transform and imparting a transform-parallel distributed-shear fabric. Transform-parallel distributed shear in the mantle lithosphere provides a mechanism for brittle transform-parallel fault systems in the continental crust. Studies of seismic anisotropy show fast directions parallel with transform faults, indicating systematic orientation of crystals through transform-parallel distributed shear in the mantle lithosphere.SOMMAIRELes failles transformantes le long de la marge continentale divergente japétienne de la Laurentie décalent le rift continental et/ou les domaines accrétés en des décollements à pendages opposés faibles. Des systèmes de failles intracratoniques parallèles au rift, et parallèles à la transformation, pénètrent vers l’intérieur de la croûte continentale à partir de la marge de rift. Les âges des roches ignées syn-rift, entre 765 Ma et 530 Ma, témoignent d’une activité de rifting diachronique non-systématique le long de la marge japétienne. Des empilements sédimentaires syn-rifts montrent des variations abruptes d’épaisseur d’une faille transformante à l’autre, et des concentrations de roches ignées syn-rifts se répartissent le long des systèmes de failles transformantes et de failles intracratoniques parallèles. Les accumulations les plus épaisses de carbonates de plateforme de marge continentale passive se trouvent le long des marges de cisaillement et dans les bassins de marge continentale le long de failles transformantes, de même qu’au long des systèmes de failles intracratoniques parallèles, évoquant une subsidence anormalement forte le long des failles transformantes. Le long de la bande orogénique ordovicienne-permienne Appalaches-Ouachita, une gamme diachronique de prismes clastiques synorogéniques remplit les bassins d’avant-pays, attestant d’une subsidence par flexure lithosphérique d’origine tectonique. Les plus grandes épaisseurs de prismes clastiques synorogéniques à tous les âges sont toujours situées dans les bassins d’avant-pays le long des marges transformantes, et vers l’intérieur, à partir des intersections des failles transformantes avec la marge de rift, indiquant une lithosphère systématiquement plus fragile le long des failles transformantes. Les propriétés particulières et le comportement généralisés de la lithosphère le long des failles transformantes dans les contextes tectoniques successifs sont la marque de contrôles fondamentaux sur l'héritage tectonique des failles transformantes. Les modèles récents de rifting continental comportent une extension ductile de la lithosphère mantellique sous l’extension cassante de la croûte; le domaine d'extension ductile de la lithosphère mantellique peut s’étendre significativement vers l’intérieur de la marge de divergence de la croûte cassante, d’où les failles d'extension parallèle au rift, à l’intérieur de la croûte de la marge de divergence. Un décalage de transformation de rift de la croûte comporte un décalage du même genre de l’extension ductile de la lithosphère mantellique, ce qui implique un différentiel de flux ductile sur les bords opposés de la transformation, d’où cette fabrique d’extension parallèle à la transformation. L’extension parallèle à la transformation de la lithosphère mantellique fournit un mécanisme qui explique les systèmes de failles transformantes parallèles dans la croûte continentale. Les études de l’anisotropie sismique montre les grandes vitesses de propagation parallèles aux failles de transformations, ce qui indique une orientation systématique des cristaux induite par une extension répartie selon les cassures transformantes dans la lithosphère mantellique.

Geology and Hydrogeology of Faults on Cape Breton Island, Nova Scotia, Canada: an overview

“…In eastern Canada they indicated the presence of similarly oriented features throughout Newfoundland, the Grand Banks, Nova Scotia, and New Brunswick. Thomas (2014) noted a similar trend in transform faults formed during the initial rifting at the Iapetan margin of Laurentia. Both Thomas (2014) and Lefort and Miller (1999) concluded that these faults formed zones of weakness which were subsequently reactivated during rifting in the Mesozoic and Cenozoic with formation of the Atlantic Ocean.…”

“…Thomas (2014) noted a similar trend in transform faults formed during the initial rifting at the Iapetan margin of Laurentia. Both Thomas (2014) and Lefort and Miller (1999) concluded that these faults formed zones of weakness which were subsequently reactivated during rifting in the Mesozoic and Cenozoic with formation of the Atlantic Ocean. The concept was also supported by Thomas (2006) who concluded that these structures were related to transform faults and along with aligned compressional structures show repeated tectonic inheritance through successive Wilson cycles of supercontinent assembly and breakup.…”

Detrital-zircon geochronology and provenance of the Ocloyic synorogenic clastic wedge, and Ordovician accretion of the Argentine Precordillera terrane

“…Other igneous rocks in the Western Sierras Pampeanas, interpreted as Iapetan synrift, have ages of 845, 774, and 570 Ma Casquet et al, 2008;Colombo et al, 2009). Iapetan synrift igneous rocks along the Laurentian margin range in age from 765 to 530 Ma (summary in Thomas, 2014). Metasedimentary rocks in the Sierra de Pie de Palo and the Sierra de Valle Fértil contain detrital zircons with ages between 665 and 515 Ma (Vujovich et al, 2004;McClelland et al, 2005), which may be a source of recycled sediment.…”

Buenellus chilhoweensisn. sp. from the Murray Shale (lower Cambrian Chilhowee Group) of Tennessee, the oldest known trilobite from the Iapetan margin of Laurentia

“…The Ediacaran through lower Cambrian stratigraphic succession of the southern and central Appalachians records this rift-to-drift transition (Figs. 1, 2; Thomas, 1977Thomas, , 2014Mack, 1980;Bond et al, 1984;Simpson and Sundberg, 1987;Eriksson, 1989, 1990). The extensional rift phase is represented in Tennessee by the Neoproterozoic Ocoee Supergroup, which is a sequence of turbidites and mass flow deposits that accumulated in a large intracratonic rift basin (Tull et al, 2010;Thomas, 2014 and references therein).…”

“…1, 2; Thomas, 1977Thomas, , 2014Mack, 1980;Bond et al, 1984;Simpson and Sundberg, 1987;Eriksson, 1989, 1990). The extensional rift phase is represented in Tennessee by the Neoproterozoic Ocoee Supergroup, which is a sequence of turbidites and mass flow deposits that accumulated in a large intracratonic rift basin (Tull et al, 2010;Thomas, 2014 and references therein). The overlying Chilhowee Group represents the basal siliciclastic portion of the initial transgressive depositional cycle (Sauk Sequence; Sloss, 1963) that blanketed the Iapetan margin during the thermal subsidence phase.…”