Aulacogens of the Neoproterozoic to Ordovician Laurentian Iapetan Margin

• Published in: Earth-science reviews Vol. 255; p. 104829
• Main Author: Lowe, D.G.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.08.2024
• Subjects: Appalachians; Aulacogen; Basin; Breakup; Continental breakup; Edge driven convection; Humber Zone; Inheritance; Laurentia; Ottawa Graben; Ottawa-Bonnechere Graben; Passive margin; Plume; Potsdam Group; Reactivation; Rifted margin; Rifting; Saguenay Graben; St. Lawrence rift; Taconic Seaway; Ottawa-Bonnechere Graben; Reactivation; Rifting; Saguenay Graben; Edge driven convection; Inheritance; Plume; Aulacogen; Rifted margin; St. Lawrence rift; Taconic Seaway; Passive margin; Potsdam Group; Laurentia; Basin; Continental breakup; Humber Zone; Appalachians; Breakup; Ottawa Graben
• ISSN: 0012-8252
• DOI: 10.1016/j.earscirev.2024.104829

The Ottawa and Saguenay grabens are aulacogens (failed rifts) formed on the Ediacaran Laurentian Iapetan continental margin. They contain a record of syn-rift evolution and passive margin reactivation through an entire Wilson Cycle, yet their origins, the role they played in continental margin evolution, and insights they can provide on breakup processes and passive margin structural inheritance are not known. This review contextualizes the magmatic, stratal, and structural histories of the Ottawa and Saguenay grabens by integrating details of their evolution with a review of the coinciding stratal and plate tectonic evolution of the Laurentian margin from New York to Newfoundland. The results demonstrate that these aulacogens provide a sensitive record of the coeval plate tectonic evolution of the Laurentian continental margin, from rifting processes to peripheral subduction zone evolution. The Ottawa and Saguenay grabens formed as rift branches of the larger St. Lawrence rift system, analogous the East African Rift System, during 620–570 Ma rifting and breakup between Laurentia and Baltica. The much greater volume of magmatism in the Ottawa Graben relative to the Saguenay Graben suggests that it was a more axial part of the rift system, albeit distal to the margin. During subsequent hyperextension leading to the opening of the Taconic Seaway (ca. 570–515 Ma), the Ottawa and Saguenay grabens developed into margin-normal rifts along the final manifestation of the Laurentian passive margin. Alkaline flood basalts erupted at the mouth of the Ottawa Graben and felsic stocks intruded inboard parts of the graben, likely as the result of asthenosphere upwelling due to edge-driven mantle convection below the thinned crust of the Ottawa Graben. Corresponding deltaic progradation at the mouth of the Ottawa Graben records renewed rifting that facilitated focused sediment transport through to the margin. Following Middle Cambrian margin breakup, reactivation of the Ottawa and Saguenay grabens played a significant role in regional strain partitioning and termination, and illustrate a connection to inherited margin-transverse faults and fracture zones. For example, from ca. 510–505, while rifting intensified on the southeastern Laurentian margin, transtension and significant normal faulting in the Ottawa Graben and collinear Franklin Basin imply reactivation along the entire collinear margin-transverse Missisquoi fault zone, which likley occured in response to the termination of regional extension along this first-order rift segment boundary. Subsequent Furongian (ca. 500–495 Ma) reactivation of the Ottawa and Saguenay grabens remains enigmatic, perhaps responding to distant fore-arc spreading in the Iapetus Ocean or to broad cratonic uplift. Tremadocian (ca. 485–482 Ma) aulacogen reactivation is linked to the coeval onset of segmented fore-arc spreading in the Taconic Seaway, which may have made use of fracture zones to accommodate localized extension via strike-slip deformation, which, in part, was terminated in the collinear Ottawa and Saguenay grabens. Finally, Late Tremadocian to Floian reactivation of the Ottawa Graben (ca. 480–475 Ma) is linked to flexural strain from the onset of tectonic loading on the most distal parts of the Laurentian margin.