Diachronic metamorphic and structural evolution of the Connecticut Valley–Gaspé trough, Northern Appalachians

Abstract

The Connecticut Valley-Gaspé (CVG) trough represents a major, orogen-scale Silurian-Devonian basin of the Northern Appalachians. From Gaspé Peninsula to southern New England, the CVG trough has experienced a contrasting metamorphic and structural evolution during the Acadian orogeny. Along its strike, the CVG trough is characterized by increasing strain and polyphase structures, and by variations in the intensity of regional metamorphism and contrasting abundance of ~ 390 to 370 Ma granitic intrusions. In southern Quebec and northern Vermont, a series of NW-SE transects across the CVG trough have been studied in order to better understand these along-strike variations. Detailed structural analyses, combined with phase equilibria modeling, Raman spectrometry and muscovite 40 Ar/ 39 Ar dating highlight a progressive and incremental deformation involving south-north variation in the timing of metamorphism. Deformation evolves from a D 1 crustal thickening event which originates in Vermont and progresses to southern Québec where it peaked at 0.6 GPa/380°C at c. 375 Ma. This was followed by a D 2 event associated with continuous burial in Vermont from 378 Ma to 355 Ma, which produced peak metamorphic conditions of 0.85 GPa/630 o C and exhumation in Quebec from 368 to 360 Ma. The D 3 compressional exhumation event also evolved from south to north from 345 to 335 Ma. D 1 to D 3 deformation events form part of a continuum with an along-strike propagation rate of  50 km/m.y. During D 1 , the burial depth varied by more than 15 km between southern Quebec and Vermont, and this can be attributed to the occurrence of a major crustal indenter, the Bronson Hill Arc massif, in the New England segment of the Acadian collision zone.

Polarity and Timing of the Deformation Along the Jinsha Suture Zone (Yushu Area, Northeastern Tibet)

“…Plateau ages were calculated if 70% or more of the 39 Ar K was released in at least three or more contiguous steps which define apparent ages agreeing, to within 2σ, with the integrated age of the plateau segment. This 70% threshold, a requirement criterion that we use in all our geochronological studies (e.g., Perrot et al., 2020, is much more restrictive than the 50% threshold set subjectively, in their own words, by Fleck et al. (1977) and still adopted by most of the users of the 40 Ar/ 39 Ar method.…”

“…Plateau ages were calculated if 70% or more of the 39 Ar K was released in at least three or more contiguous steps which define apparent ages agreeing, to within 2σ, with the integrated age of the plateau segment. This 70% threshold, a requirement criterion that we use in all our geochronological studies (e.g., Perrot et al, 2020, is much more restrictive than the 50% threshold set subjectively, in their own words, by Fleck et al (1977) and still adopted by most of the users of the 40 Ar/ 39 Ar method. On the other hand, as the amount of 39 Ar K released is not a disqualifying criterion, we have for a long time associated to our restrictive age plateau concept (≥70% of 39 Ar K ) used for "simple" mineral phases, the concept of pseudo-plateau (Cheilletz et al, 1999) which meets the same statistical criteria except for the cut-off threshold (<70% of 39 Ar K ), and which is much more efficient for the analysis of complex mineral phases.…”