Alain Tremblay

Evidence for forearc seafloor-spreading from the Betts Cove ophiolite, Newfoundland: oceanic crust of boninitic affinity

Abstract The Ordovician Betts Cove ophiolite of Newfoundland has a well-developed cumulate sequence, in which is rooted a sheeted dyke complex that grades up into pillow lavas. Dominant chromite + olivine + orthopyroxene cumulate peridotites and orthopyroxenites have phase assemblages and mineral chemistries consistent with crystallization from boninitic magmas. Dykes and lavas have phenocrysts of olivine + high- Cr Al chromite ± orthopyroxene ± low-TiO2 clinopyroxene. They have high SiO2 and MgO contents, and depleted U-shaped trace-element profiles indistinguishable from those of Bonin Islands boninites. Field data imply that cumulates, dykes and lavas all are comagmatic, while geochemical and mineralogical data indicate that all are of boninitic affinity. Since boninites are only found in forearcs, this implies that the Betts Cove ophiolitic crust formed in a forearc. Since the entire oceanic crustal section at Betts Cove is of boninitic affinity, then this implies that true seafloor-spreading can initiate in forearc.

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.

Along‐strike Acadian structural variations in the Québec Appalachians: Consequence of a collision along an irregular margin

The Quebec Appalachians have been shaped by two major orogenies: the Middle to Late Ordovician Taconian and the Middle Devonian Acadian. Thrust faults and nappe structures characterized the Taconian deformation throughout the Quebec Appalachians, whereas structural styles pertaining to the Acadian orogeny differ from southern Quebec to the Gaspe Peninsula. Structural analysis of Late Ordovician to Middle Devonian supracrustal rocks shows two different Acadian deformational regimes: strike-slip tectonics in the Gaspe Peninsula and thrust or dip-slip tectonics in southern Quebec. In southern Quebec, Acadian regional deformation produced folds which vary from upright to overturned and tight to isoclinal from the NW to the SE. Major faults (e.g., La Guadeloupe) are northwestward directed thrust faults marked by highly ductile shear zones outlined by mylonite. In the Gaspe Peninsula, structural trend is NE and major E–W dextral strike-slip faults transect this trend. Folds are open and upright, inclined and tighter near the major faults where they have a clockwise rotation like the regional cleavage. Major faults (e.g., Grand Pabos) follow corridors delimiting high-strain zones where the fabrics developed are indicative of a ductilebrittle regime of deformation. In the Temiscouata region, major structural features of both southern Quebec and the Gaspe Peninsula are recognized. Structures of this region consist of ENE trending dextral strike-slip faults and high-angle WNW verging reverse faults, parallel to major NNE folds. These structures reflect the transition from purely horizontal movement along major strike-slip faults of the Gaspe Peninsula to vertical movement along thrust faults in southern Quebec. Acadian structural variations within the Quebec Appalachians are interpreted in terms of a continental collision of Gondwana along the irregular margin of Laurentian and its Taconian accreted terranes.

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

Supracrustal faults of the St. Lawrence rift system, Québec: kinematics and geometry as revealed by field mapping and marine seismic reflection data

Abstract The St. Lawrence rift system from the Laurentian craton core to the offshore St. Lawrence River system is a seismically active zone in which fault reactivation is believed to occur along late Proterozoic to early Paleozoic normal faults related to the opening of the Iapetus ocean. The rift-related faults fringe the contact between the Grenvillian basement to the NW and Cambrian–Ordovician rocks of the St. Lawrence Lowlands to the SE and occur also within the Grenvillian basement. The St. Lawrence rift system trends NE–SW and represents a SE-dipping half-graben that links the NW–SE-trending Ottawa–Bonnechere and Saguenay River grabens, both interpreted as Iapetan failed arms. Coastal sections of the St. Lawrence River that expose fault rocks related to the St. Lawrence rift system have been studied between Quebec city and the Saguenay River. Brittle faults marking the St. Lawrence rift system consist of NE- and NW-trending structures that show mutual crosscutting relationships. Fault rocks consist of fault breccias, cataclasites and pseudotachylytes. Field relationships suggest that the various types of fault rocks are associated with the same tectonic event. High-resolution marine seismic reflection data acquired in the St. Lawrence River estuary, between Rimouski, the Saguenay River and Forestville, identify submarine topographic relief attributed to the St. Lawrence rift system. Northeast-trending seismic reflection profiles show a basement geometry that agrees with onshore structural features. Northwest-trending seismic profiles suggest that normal faults fringing the St. Lawrence River are associated with a major topographic depression in the estuary, the Laurentian Channel trough, with up to 700 m of basement relief. A two-way travel-time to bedrock map, based on seismic data from the St. Lawrence estuary, and comparison with the onshore rift segment suggest that the Laurentian Channel trough varies from a half-graben to a graben structure from SW to NE. It is speculated that natural gas occurrences within both the onshore and offshore sequences of unconsolidated Quaternary deposits are possibly related to degassing processes of basement rocks, and that hydrocarbons were drained upward by the rift faults.

Acadian metamorphism in the Dunnage zone of southern Québec, northern Appalachians: 40Ar/39Ar evidence for collision diachronism

In New England, the Acadian orogen is divided into Western and Eastern metamorphic belts on the basis of contrasting pressure-temperature ( P-T ) paths, peak metamorphism conditions, and ages. Along-strike correlations of structures indicate that rock units affected by Devonian metamorphism and deformation in southern Quebec belong to the Western Acadian belt, and share the same tectonic evolution as in New England. Muscovite 40 Ar/ 39 Ar ages obtained from greenschist-grade, Ordovician volcanic and plutonic rocks of the Ascot Complex in southern Quebec indicate that Acadian metamorphism and deformation peaked ca. 380–375 Ma. Ordovician muscovite ages of ca. 462 Ma are locally well preserved in the granitic intrusion of the Ascot Complex, and provide constraints on the timing of island-arc plutonism in the Dunnage zone of southern Quebec. Ordovician 40 Ar/ 39 Ar age spectra do not show important losses of radiogenic argon, indicating that Acadian metamorphism has been low grade, with temperatures below the Ar isotopic closure temperature proposed for muscovite. Acadian peak metamorphism is significantly younger, and P-T conditions of regional deformation have been lower in southern Quebec than in amphibolite-grade rocks of the Western Acadian belt of New England. In the northern Appalachians, Acadian metamorphism and deformation are attributed to Devonian age crustal overthickening related to plate convergence. The comparison of Devonian tectonic fabrics in New England and southern Quebec suggests that progression in the timing and P-T conditions of Acadian peak metamorphism in both areas are the result of the irregular geometry of the collision zone and of the northward migration of metamorphic and structural fronts along the orogen.

Origin and incremental evolution of brittle/ductile shear zones in granitic rocks: natural examples from the southern Abitibi Belt, Canada

Metre-scale shear zones developed in the Mooshla granitic pluton exhibit a plethora of internal mesoand microstructures revealing their mode of nucleation, growth and termination. The subvertical E-W brittle/ ductile shear zones are developed in the most isotropic part of the intrusion and consist of phyllonites characterized by mylonitic fabrics and well-developed down-dip mineral lineations. They are exposed on a single, large and flat outcrop perpendicular to the mineral lineation. n nThese brittle/ductile shear zones are in close spatial association with a fracture system along which they have nucleated and propagated. Joints, fractures and brittle faults contain abundant evidence of ductilely deformed and recrystallized minerals suggesting that they underwent ductile shearing after their formation. The deformation was accompanied by the influx of a metasomatic fluid which altered original FeMg-bearing minerals and plagioclase into quartz, epidote, chlorite, sericite and carbonate that precipitated in dilatant fractures and narrow breccia zones. We recognize two different types of lateral terminations: (i) shear zones that terminate into joints and fractures that are parallel or oblique to the shear direction, and (ii) curved segments of shear zones that connected with neoformed splay fractures at high angle to the shear direction. In the latter case, sharp bending and severe perturbation of the shear zone orientation occur at the shear zone termination where ductile shearing is transfered into an adjacent shear zone via dilatational fractures. This demonstrates that shear zones tended to grow towards each other and coalesce to form a well-developed anastomosing network. n nWe propose that narrow dilatational fractures and joints acted as paleo-weakness planes along which fluid-rock interaction and resulting reaction-softening occurred. The deformation scenario involves episodic fluid drawn into dilatant sites responsible for cyclic fluid pressure fluctuation in the system. Dilatancy-related fracturing and reaction-softening lead to deformation under brittle-to-plastic conditions and to shear strain localization.

State of intraplate stress and tectonism of northeastern America since Cretaceous times, with particular emphasis on the New England-Quebec igneous province

Abstract A paleostress analysis based on inversion of fault-slip data has been conducted in the Quebec Appalachians and the St. Lawrence Lowlands in order to characterize the direction and state of stress during and after the emplacement of Cretaceous Monteregian plutons. Two major events with contrasting directions of extension are recognized: (1) an early, widely distributed NE-SW-directed extension (∼ 140 Ma), and (2) a N-S-oriented extension associated with the main phase of magmatism (∼ 125 Ma). Both these extensional paleostress regimes are consistent in direction with the result of statistical analysis of Cretaceous pluton and dyke trends in southern Quebec and New England. The NE-SW-directed far-field tensional stress of the first event favours the reactivation of the earlier E-W-trending Proterozoic Ottawa-Bonnechere Graben and the reorientation of the local stress field. As a result, a restricted zone of N-S-directed extension was developed synchroneously with the emplacement of the Monteregians plutons. Cretaceous intrusions of the studied area are thus interpreted as the result of magmatism along reactivated Proterozoic basement faults, rather than in terms of the North American plate moving over a hot spot. In Late Cretaceous-early Tertiary times, the extensional stress regime changed to an ENE-WSW-directed compressional stress field. This compression is characterized by strike-slip faults and represents the youngest tectonic event in the area. Directions of compressional paleostress axes compare well to the present-day maximum compressive stress in northeastern America. The stress regimes inferred in the Quebec-New England igneous province can be attributed to the Early Cretaceous rifting between Labrador and Greenland. Variations of spreading rate and plate boundary conditions of North America in the Late Cretaceous-early Tertiary led to stress inversion in eastern North America and to the establishment of a durable compressional stress field that is still present today.

Extension versus shortening models for hinterland-directed motions in the southern Quebec Appalachians

Abstract In the Taconian internal zone of southern Quebec, structures related to hinterland-directed motion are found on both limbs of the Sutton-Notre-Dame mountains anticlinorium (D3West and D3East structures) and along the St-Joseph fault. These ductile to brittle-ductile structures were formed during upper- to mid-greenschist grade metamorphic events, thus suggesting that the area occupied a mid-crustal position during hinterland motion. Layer-extensional fabrics along the eastern limb of the Sutton-Notre-Dame mountains anticlinorium can be interpreted either as structures contemporaneous to backthrusting deformation rooted on the western limb of the anticlinorium or as structures formed during normal faulting along the St-Joseph fault. The study of layer-extensional structures found in the Taconian internal zone of the southern Quebec Appalachians shows that it is not obvious to infer wether normal-sense fabrics that occur in orogenic hinterlands are related to regional crustal extension or to local syn-collisional extensional faulting.

Alleghanian paleostress reconstruction in the northern Appalachians: Intraplate deformation between Laurentia and Gondwana

A numerical paleostress tensor analysis using striated fault planes has been conducted in the Quebec reentrant of the northern Appalachians to characterize the stress field of late Paleozoic deformations. Three directions of maximum compressional stress axes (s1) have been found and correlated to (1) an early north-northwest-south-southeast compression, (2) a north-northeast-south-southwest compression, and (3) a late west-northwest-east-southeast compression. Fault populations associated with these stress regimes are present in all tectonic zones of the Quebec and northern New Brunswick Appalachians. Directions of σ 1 axes determined in the northern Appalachians resemble in orientation and in relative chronology layer-parallel shortening fabrics and joint patterns found in the Appalachian foreland of the central Appalachians. The paleostress regimes are interpreted as the record of intraplate deformation related to the indentation of Gondwana into Laurentia during the late Paleozoic Alleghanian orogeny.