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The formation of orthogonal joint systems and cuboidal blocks:New insights gained from flat-lying limestone beds in the region of Havre-Saint-Pierre(Quebec,Canada)

查看全文 作　　者：Shaocheng [1]Ji;Yvéric [1]Rousseau;Denis [1]Marcotte;Noah John [2]Phillips 高影响力作者 机构地区：[1]Département des Génies Civil,Géologique et des Mines,École Polytechnique de Montréal,Montréal,Québec,H3C 3A7,Canada;[2]Department of Geology,Lakehead University,Thunder Bay,Ontario,P7B 5E1,Canada高影响力机构 出　　处：《Journal of Rock Mechanics and Geotechnical Engineering》索引2023年第15卷第12期,共15页高影响力期刊 基　　金：The authors express their gratitude to the Natural Sciences and Engineering Research Council of Canada for financial support through a Discovery Grant(Grant No.06408). 摘　　要：Vertical orthogonal joints are a common feature in shallow crustal rocks.There are several competing theories for their formation despite the ubiquity.We examined the exceptional exposures of orthogonal joints in flat-lying Ordovician limestone beds from the Havre-Saint-Pierre Region in Quebec,Canada(north shore of Saint-Lawrence River)to test conceptual models of joint formation in a natural setting.In the region,the spacing of cross-joints is consistently larger than the spacing of systematic joints by a factor of 1.5 approximately.The joint-spacing-to-bed-thickness ratios(s/t)are much larger in these beds(s/t=4.3 for systematic joints,and 6.4 for cross-joints)than those in higher strained strata along the south shore of the Saint-Lawrence River(s/t=1),highlighting the effect of tectonic strain in decreasing fracture spacing and block size.The high values of s/t indicate that cross-joint formation was unlikely caused by a switch from compression to tension once a critical s/t ratio for systematic joints was reached(as hypothesized in previous studies).We proposed a new model for the formation of orthogonal joint systems where the principal stress axes locally switch during the formation of systematic fractures.The presence of ladder-shaped orthogonal joints suggests a state of effective stress withσ_(1)^(∗)≫0>σ_(2)^(∗)>σ_(3)^(∗)and whereσ_(2)^(∗)-σ_(3)^(∗)is within the range of fracture strength variability at the time of fracture.This research provides a new mechanical model for the formation of orthogonal joint systems and cuboidal blocks. 关 键 词：Plato's cuboids Orthogonal joints Fracture spacing LIMESTONE Layered rock mechanics Appalachian geology
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