We present the results of geological mapping and geochronological studies of the Tally Ho shear zone (THSZ) and adjacent rocks. The shear zone crops out near the west margin of Stikinia, an oceanic arc and the largest of the accreted terranes within the Cordilleran orogen of western North America. The hanging wall of the largely flat-lying shear zone consists of coarsely crystalline leucogabbro and cumulate pyroxenite interpreted as the lower crustal and possibly lithospheric mantle roots of a magmatic arc. Rocks in the footwall consist of volcanic and volcano-sedimentary sequences of the Lewes River Arc, a Late Triassic magmatic arc characteristic of Stikinia. Because the shear zone places lower crustal plutonic rocks over a supracrustal sequence, we interpret it as a crustal-scale thrust fault. Kinematic indicators imply top-to-the-east displacement across the shear zone. The geometry of folds of the shear zone is consistent with deformation in response to displacement over ramps in deeper-seated thrust faults kinematically linked to the THSZ. Crystallization of the hanging-wall leucogabbro at 208±4.3 Ma provides a maximum age constraint for deformation, whereas a post-kinematic granitoid pluton that plugs the shear zone and that crystallized at about 173 Ma provides a lower age limit. The THSZ is, therefore, coeval with: (1) a series of latest Triassic–Early Jurassic shear and fault zones that characterize the length of the west margin of Stikinia; (2) the termination of isotopically juvenile arc magmatism of the Lewes River Arc; (3) crustal loading of Stikinia giving rise to a foreland basin that rapidly filled with westerly derived orogenic molasse that includes clasts of ultrahigh-pressure metamorphic rocks; and (4) juxtaposition of Stikinia against continental crust of the Nisling Assemblage of the Yukon–Tanana terrane to the west. These constraints are consistent with a model of deformation in response to the entry of the continental Nisling Assemblage into the trench of the west-facing Lewes River Arc, terminating subduction and imbricating the arc along a series of east-verging thrust faults, including the THSZ.
In the southeastern Cape Breton Highlands Neoproterozoic plutonic and metamorphic rocks outcrop in upland areas whereas Carboniferous sedimentary rocks are found in the river valleys and coastal lowlands. Detailed analysis of the contacts between these two groups of rocks including mapping, geometric constructions of the contact relations, structural geological investigations, petrographic analysis and geophysical map interpretations show that the basement rocks were emplaced by a thrust fault that extends at least from the Baddeck River valley to North River, and possibly includes klippen south and east of the highlands. The thrust fault transported a slab of rock with minimum thickness of 200 m a distance of at least 8 km over Horton and Windsor group rocks. East-directed translation of the thrust block likely occurred during the Alleghanian orogeny, and appears to mirror movement previously identified in the northern and western Cape Breton Highlands, implying that much of the upland geology is allochthonous, but likely rooted in the central highlands as positive flower structure. RÉSUMÉDans les hautes-terres du cap Breton du Sud-Ouest, on trouve des affleurements de roches métamorphiques et plutoniques du Néoprotérozoïque dans les hautes terres, alors que des roches sédimentaires du Carbonifère se trouvent dans les vallées des rivières et les terres basses côtières. L'analyse détaillée des contacts entre ces deux groupes de roches, y compris la cartographie, des constructions géométriques des relations de contact, des études géologiques structurales, une analyse pétrographique et des interprétations des cartes géophysiques, montrent que les roches du socle ont été mises en place par une faille de chevauchement qui s' étend au moins depuis la vallée de la rivière Baddeck jusqu'à la rivière North, et comprend peut-être la klippe au sud et à l' est des hautes terres. La faille de chevauchement a transporté un bloc de roche d'une épaisseur minimum de 200 m sur une distance d'au moins 8 km au-dessus des roches des groupes de Windsor et Horton. La translation de la nappe de chevauchement en direction est s' est probablement produite lors de l' orogenèse alléghanienne et semble refléter le déplacement constaté précédemment dans les hautesterres du cap Breton du Nord et de l'Ouest, ce qui donne à penser qu'une bonne partie de la géologie des terres hautes est allochtone mais qu' elle a, selon toute probabilité, ses racines dans les hautes terres centrales comme structure en fleur positive. The unmistakable change in elevation from lowland sedimentary rocks to highland basement rocks has resulted in speculation as to how these older plutonic and metamorphic rocks have achieved a higher altitude than the adjacent younger sedimentary rocks. The nature of the contact between these two sets of rocks is crucial to the understanding of this problem, and has implications for the Atlantic Geology · Volume 51 · 2015 299Basement-cover relations in the southeastern Copyright © Atlantic Geology, 2015 Cape Breton Highland...
HALIFAX 2022 FIELD TRIPS OVERVIEW This year the 2022 GAC-MAC Annual Meeting returns to 'Canada's Ocean Playground' in the gleaming new convention centre located in the heart of downtown Halifax. Partnered with the Canadian Society of Petroleum Geologists (CSPG) and International Association of Hydrogeologists (IAH-CNC), the meeting also coincides with the 50 th anniversary of the Atlantic Geoscience Society (AGS). What better way to celebrate Atlantic geoscience by attending one of the many field trips on offer from a variety of organizations! Delegates have a total of 12 trips to choose from with a bonus free self-guided walking tour of the geology of the historic Dartmouth Commons that will be offered to all conference participants. Trips range from several hours to multiple days to suit a wide variety of interests and budgets.
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