Two vertical and orthogonal systematic joint sets are generally arrayed in a grid pattern on the bedding surface,which are the significant features of flat-lying sandstone terrains.Although extensive researches are re...Two vertical and orthogonal systematic joint sets are generally arrayed in a grid pattern on the bedding surface,which are the significant features of flat-lying sandstone terrains.Although extensive researches are reported on this topic,many fundamental problems have still not been solved.Such mutually perpendicular opening-mode fractures are an obvious manifestation of effective tensile stresses in two orthogonal directions in the horizontal bedding plane.A good understanding of these orthogonal joint systems is a key to structural analysis,landscape interpretation,and guidance of resolving a number of very practical problems in engineering,mining and hydrologic projects.Based on an anatomic investigation on the orthogonal joints in the Potsdam sandstone of Cambrian age at Ausable Chasm(New York State,USA)and Beauharnois(Quebec,Canada),we proposed that the orthogonal joints may result from the auxetic effects of quartz-rich sandstone rather than local or regional rotation of the maximum tensile stress(σ_(3))direction by about 90°.The sandstone beds with negative Poisson's ratios are so fascinating that,when placed under vertical burial compression and layer-parallel extension in one direction(σ_(3)),it becomes stretched in the transverse direction(σ_(2)),producing two orthogonal sets of mutual abutting and intersecting joints(J1 and J2 normal toσ_(3) andσ_(2),respectively),and both are normal to the bedding surface.Joint set J1 is more closely-spaced than J2 by a factor of∼3.3,which is correlated with an average Poisson's ratio of−0.3 for the Potsdam sandstone at the time of joint formation.展开更多
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...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.展开更多
基金Shaocheng Ji thanks the Natural Sciences and Engineering Research Council of Canada for a discovery grant,and Dr.Terry Engelder for helpful discussion.
文摘Two vertical and orthogonal systematic joint sets are generally arrayed in a grid pattern on the bedding surface,which are the significant features of flat-lying sandstone terrains.Although extensive researches are reported on this topic,many fundamental problems have still not been solved.Such mutually perpendicular opening-mode fractures are an obvious manifestation of effective tensile stresses in two orthogonal directions in the horizontal bedding plane.A good understanding of these orthogonal joint systems is a key to structural analysis,landscape interpretation,and guidance of resolving a number of very practical problems in engineering,mining and hydrologic projects.Based on an anatomic investigation on the orthogonal joints in the Potsdam sandstone of Cambrian age at Ausable Chasm(New York State,USA)and Beauharnois(Quebec,Canada),we proposed that the orthogonal joints may result from the auxetic effects of quartz-rich sandstone rather than local or regional rotation of the maximum tensile stress(σ_(3))direction by about 90°.The sandstone beds with negative Poisson's ratios are so fascinating that,when placed under vertical burial compression and layer-parallel extension in one direction(σ_(3)),it becomes stretched in the transverse direction(σ_(2)),producing two orthogonal sets of mutual abutting and intersecting joints(J1 and J2 normal toσ_(3) andσ_(2),respectively),and both are normal to the bedding surface.Joint set J1 is more closely-spaced than J2 by a factor of∼3.3,which is correlated with an average Poisson's ratio of−0.3 for the Potsdam sandstone at the time of joint formation.
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).
文摘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.
