Sébastien Castonguay

Tectonometamorphic evolution of the southern Quebec Appalachians: 40Ar/39Ar evidence for Middle Ordovician crustal thickening and Silurian–Early Devonian exhumation of the internal Humber zone

Published age data from the Humber zone of the southern Quebec Appalachians indicate that deformation and metamorphism occurred during Middle to Late Ordovician time. This paper presents the results of a regional-scale geochronological study of 70 amphibole and mica single grains by the 40 Ar/ 39 Ar laser step-heating method, which brings forward a more complete Middle Ordovician–Early Devonian tectonometamorphic evolution of the Quebec Appalachian Humber zone. Biotites have plateau ages varying between 420 and 411 Ma. Amphiboles are often compositionally rimmed and/or contain biotite inclusions. Their age spectra are generally disturbed, but may suggest the occurrence of two distinct events: Silurian (429–424 Ma) and Middle Ordovician (462–460 Ma). Two main groups of muscovite ages have been obtained: Middle Ordovician (469–461 Ma) and Silurian–Early Devonian (431–411 Ma), but Middle to Late Devonian (387–376 Ma) ages are also locally present. Middle Ordovician ages are interpreted to record crustal thickening by nappe emplacement during regional Taconian metamorphism. Silurian–Early Devonian ages are attributed to consequences of backthrusting and normal faulting. A statistical analysis of Silurian–Early Devonian muscovite age spectra reveals an irregular decrease in weighted apparent age maxima across the internal Humber zone. This age-decreasing trend is interpreted to result from the temporal (ca. 431–411 Ma) and possibly spatial (northwest to southeast) progression of deformation, recrystallization, and fluid circulation during tectonic exhumation of the internal Humber zone in southern Quebec. Tectonic models for the Northern Appalachian internal Humber zone should take into account the strong body of evidence for Silurian–Early Devonian hinterland-directed and extensional deformation as documented in southern Quebec.

Structural evolution of the Laurentian margin revisited (southern Quebec Appalachians): Implications for the Salinian orogeny and successor basins

The Laurentian margin of the Appalachians is divided into external and internal zones on the basis of metamorphic and structural contrasts. In the southern Quebec internal zone, Silurian to Early Devonian southeast-verging structures are superimposed on northwest-verging structures, whereas most of the external zone lacks such overprints. Regional backthrust faults define a major upper plate‐lower plate boundary; the external-zone rocks are in the hanging wall, and internal-zone rocks are in the footwall. Metamorphic rocks with Silurian‐Early Devonian 40 Ar/ 39 Ar ages (430‐410 Ma) characterize the lower plate. To the east, the Saint-Joseph fault and the Baie Verte‐Brompton line are southeastdipping normal faults that crosscut the upper plate‐lower plate boundary. Metamorphic rocks with Middle Ordovician 40 Ar/ 39 Ar ages (469‐461 Ma) and rocks of the external zone both occur in the downthrown side of the Saint-Joseph fault and the Baie Verte‐Brompton line. U-Pb and 40 Ar/ 39 Ar ages suggest that the northwest-verging structures are related to ophiolite obduction and crustal thickening during the Taconian orogeny (ca. 480‐445 Ma), whereas the southeast-verging structures formed during Silurian‐Early Devonian backthrusting and normal faulting. The revised structural interpretation has implications for the Salinian orogeny and involves (1) southeast-directed transport of the Taconian crustal wedge of the upper plate, followed by normal faulting and juxtaposition with the lower plate along the Saint-Joseph fault and the Baie Verte‐Brompton line, and (2) the forma

Dating polyphase deformation across low-grade metamorphic belts: An example based on 40Ar/39Ar muscovite age constraints from the southern Quebec Appalachians, Canada

Geochronologic 40Ar/39Ar data of fabric-forming metamorphic minerals, in conjunction with structural and metamorphic studies, are being increasingly used to constrain the deformation and tectonometamorphic evolution of polyphase low-grade orogens. Careful data interpretation is needed to extract meaningful age constraints from neo- and recrystallized minerals affected by isotopic disturbances. In the southern Quebec Appalachians, the Sutton Mountains anticlinorium exposes the metamorphic core of the early Paleozoic continental margin of Laurentia. Orogenesis in this part of the Appalachians was the result of tectonic events that have been classically attributed to the combined effects of the Middle to Late Ordovician Taconian and the Middle Devonian Acadian orogenies; however, evidence of separate and distinct Silurian–Early Devonian tectonism has also been recently documented. Laser step-heating 40Ar/39Ar data on single-grains of muscovite from polydeformed greenschist facies samples of the Sutton Mountains anticlinorium indicate that these tectonometamorphic events are heterogeneously preserved as a prograde Taconian event at ca. 456 Ma and an Acadian overprint at ca. 390 Ma. The integration of 40Ar/39Ar age spectra analyses with structural relationships provides precise age constraints on the duration, propagation, and evolution of Silurian–Early Devonian hinterland-directed deformation as it migrated across the anticlinorium, including back thrusting from ca. 433 Ma to ca. 420 Ma and extensional faulting from ca. 417 Ma to ca. 405 Ma. Along the Laurentian margin of the northern Appalachians, such Silurian–Early Devonian tectonism is currently attributed either to the collapse of the Taconian orogen triggered by the delamination of the subducted slab or to the outboard accretion of peri-Gondwanan terranes during the Salinic orogeny.

Timing of tectonometamorphism across the Green Mountain anticlinorium, northern Vermont Appalachians: 40Ar/39Ar data and correlations with southern Quebec

In the pre-Silurian lithotectonic units of the northern Vermont Appalachians, the timing of orogenesis and tectonometamorphism has traditionally been ascribed to the combined effects of the Middle Ordovician Taconian orogeny and Middle to Late Devonian Acadian orogeny. However, numerous geochronological studies throughout the Northern Appalachians, including neighboring southern Quebec, have obtained Silurian and Early Devonian age data that document more or less continuous tectonometamorphic activity throughout the Ordovician-Devonian. The structural and metamorphic evolution of northern Vermont can be separated into three regional phases, which are characterized by distinct structures, fabrics, and metamorphic parageneses. The first phase (D 1 ), associated with westward emplacement of various thrust slices leading to crustal thickening and regional metamorphism, and the second phase (D 2 ), characterized by bivergent structures and metamorphic overprint, have both been considered to be Taconian. The third phase, the structure and fabric of which are also observed in the Silurian–Devonian rocks to the east, is considered to be Acadian. We present new step-heating and spot fusion 40 Ar/ 39 Ar geochronological data on amphibole and fabric-forming muscovite from samples taken across the Green Mountain anticlinorium, which, coupled with published data, provide improved age constraints on tectonometamorphism of D 1 (latest Cambrian to Middle Ordovician), D 2 (Silurian–Early Devonian), and D 3 (Middle Devonian) events. By comparing structural and metamorphic characteristics, and now timing, these phases are interpreted to be correlative to the tripartite tectonometamorphic evolution documented in southern Quebec, and they further exemplify the along-strike diachronism of tectonism induced by the inherited irregular geometry of the Laurentian margin.