Sheath folds as discriminators of bulk strain type

Alsop, G. Ian; Holdsworth, R. E.

doi:10.1016/j.jsg.2006.05.005

Cited by 92 publications

(54 citation statements)

References 91 publications

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“…Abbreviations: Bt = biotite, Crd = cordierite, Gm = groundmass, Qz = quartz A sheath fold is a highly non-cylindrical fold in which the hinge curvature exceeds 90°within the axial surface (Cobbold and Quinquis 1980;Skjernaa 1989). Although sheath folds have been shown to form under pure and general shear at varying scales with wavelengths from less than 1 mm to several kilometres (Alsop and Holdsworth 2006), a strong simple shear component is considered the most important bulk strain type to form these structures (Van der Pluijm and Marshak 2004;Fossen 2010). Generally, the development of highstrain bulk simple shearing, stretching lineation and mylonitic fabric seems to represent favourable conditions for sheath fold formation (Cobbold and Quinquis 1980).…”

Section: Discussionmentioning

confidence: 99%

3D modelling and sheath folding at the Falun pyritic Zn-Pb-Cu-(Au-Ag) sulphide deposit and implications for exploration in a 1.9 Ga ore district, Fennoscandian Shield, Sweden

2016

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Altered and mineralized rocks at the Falun pyritic Zn-Pb-Cu-(Au-Ag) sulphide deposit, situated in the Palaeoproterozoic Bergslagen ore district in the southwestern part of the Fennoscandian Shield, have been metamorphosed at low-pressure, amphibolite-facies conditions and affected by ductile deformation. Using combined surface mapping of lithology and structure, drill core logging and microstructural work, the polyphase (D 1 and D 2 ) ductile deformation is demonstrated and a 3D model for the deposit created. Mineral associations include quartz, biotite, cordierite, anthophyllite, and minor almandine, andalusite and chlorite in silicate-rich altered rock, calcite or dolomite in marble and tremolite-actinolite or diopside-hedenbergite in skarn. The silicate minerals show varying growth patterns during the different phases of the tectonothermal evolution, with considerable static grain growth occurring between D 1 and D 2 , and even after D 2 . F 2 sheath folding along axes that plunge steeply to the SSE, parallel to a mineral stretching lineation and the dip direction of the S 2 foliation, is suggested as a key deformation mechanism forming steeply plunging, cone-to rod-shaped mineralized bodies. This contrasts with a previous structural model invoking fold interference. A major shear zone with talc-chlorite-(quartz-biotite) mineral association separates the northern and southern structural domains at the deposit and bounds the polymetallic massive sulphides to the north.

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Section: Discussionmentioning

confidence: 99%

3D modelling and sheath folding at the Falun pyritic Zn-Pb-Cu-(Au-Ag) sulphide deposit and implications for exploration in a 1.9 Ga ore district, Fennoscandian Shield, Sweden

2016

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“…Furthermore, sheath and conical folds with steep axes appear frequently in the NW, especially in Sottunga, which also suggests that the sub-horizontal noncoaxial component was less significant than in the SE parts. Using the variation in the aspect ratios from the outer to inner rings of the sheath folds (the R 0 value; Alsop and Holdsworth 2006) as an indicator of the bulk strain type, the values of R 0 \ 1 for the Sottunga sheath folds (Table 2) fit the type C, 'cat's-eye' sheath folds of Alsop and Holdsworth (2006). Type C sheath folds indicate that the bulk strain type was simple and general shear rather than pure shear and the steep attitude of the folds shows that the simple shear component was sub-vertical.…”

Section: Interpretationsmentioning

confidence: 99%

From ductile to brittle deformation: structural development and strain distribution along a crustal-scale shear zone in SW Finland

Torvela

Ehlers

2009

Int J Earth Sci (Geol Rundsch)

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This study demonstrates the impact of variations in overall crustal rheology on crustal strength in relatively high P-T conditions at mid-to lower mid-crustal levels. In a crustal-scale shear zone, along-strike variations in the rheological competence result in large-scale deformation partitioning and differences in the deformation style and strain distribution. The structural behaviour of the crustal-scale Sottunga-Jurmo shear zone (SJSZ) in SW Finland is described. The shear zone represents a discontinuity between the amphibolite-to-granulite facies, dome-and-basin style crustal block to the north and the amphibolite facies rocks with dominantly steeply dipping structures to the south. The overall deformation style and resulting strains along the shear zone are greatly affected by the local lithology. The results of this study also have implications for the current tectonic models of the Palaeoproterozoic Fennoscandia. The most important implication is that the SJSZ, together with other structurally related shear zones, compartmentalised the far-field stresses, so that the late ductile structures within and south of the SJSZ can be allocated to a convergence from the south as late as *1.79 Ga rather than to the Nordic orogeny from the west-northwest. It is further suggested that at *1.79 Ga the stress regime was still compressive/ transpressive and that the *1.79 Ga magmatism in Å land at least initiated in a compressive setting. No extension or orogenic collapse, therefore, occurred in the Å land area while the rocks still were within the ductile regime.

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“…In terms of the sheath fold clas si fi ca tion pro posed by Alsop and Holdsworth (2006), the sheath folds dom i nantly dis play cats-eye pat terns. Based on a few cases where com plete sheath fold cross-sec tions out crop, it can by con cluded that the R xy el lip tici ty pa ram e ter (see Alsop and Holdsworth, 2006;Alsop et al, 2007) can be very high, even over 20 (com pare folds in Fig.…”

Section: Geometry and Distribution Of Folds In The K£odawa Salt Strucmentioning

confidence: 99%

“…Based on a few cases where com plete sheath fold cross-sec tions out crop, it can by con cluded that the R xy el lip tici ty pa ram e ter (see Alsop and Holdsworth, 2006;Alsop et al, 2007) can be very high, even over 20 (com pare folds in Fig. 4A).…”

Section: Geometry and Distribution Of Folds In The K£odawa Salt Strucmentioning

confidence: 99%

“…The sheath folds de vel oped in the rock salt com plexes display all the types of ge om e tries dis tin guished by Alsop and Holdsworth (2006): cats-eye, bull-eye and anal o gous-eye patterns (Alsop and Holdsworth, 2006;Alsop et al, 2007). The cats-eye type folds of high el lip tici ty (R xy up to 20) dom i nate amongst the older fold gen er a tion as well as in mar ginal parts of both the KSS and rock salt com plexes.…”

Section: Folds In the Rock Salt Complexmentioning

confidence: 99%

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Heterogeneity of folding in Zechstein (Upper Permian) salt deposits in the Kłodawa Salt Structure, central Poland

Burliga

2014

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Anal y sis of folds car ried out in the K³odawa Salt Struc ture (cen tral Po land) showed that the Up per Perm ian (Zechstein) siliciclastic-evaporitic suc ces sion is het er o ge neously folded, re sult ing from dif fer ences in bed com pe tence and stra tig ra phy. Rock salt and pot ash-rich suc ces sions of each Zechstein cy cle are in ter nally folded and con tain mul ti ple sheath folds, orig inated dur ing lat eral flow of salt at an early stage of de for ma tion. These folds have been re folded by up right sheath folds within thick PZ1-PZ2 rock salt com plexes. The lack of superposed folds in PZ3-PZ4 salt com plexes and un con formi ties between the PZ1-PZ2 and PZ3-PZ4 units in di cate that the rock salt beds were in ter nally folded prior to diapirism, in de pendently in each salt bed. The PZ1-PZ2 rock salt beds welded dur ing lat eral flow and were folded to gether into up right sheath folds dur ing the up ward flow of salt. These superposed folds clus tered into subordinary diapirs and pierced through the younger de pos its. The lat ter sunk in salt masses to form large-scale synclinoria, the strata of which con tain early tec tonic and weakly de formed sed i men tary struc tures.

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Sheath folds as discriminators of bulk strain type

Cited by 92 publications

References 91 publications

3D modelling and sheath folding at the Falun pyritic Zn-Pb-Cu-(Au-Ag) sulphide deposit and implications for exploration in a 1.9 Ga ore district, Fennoscandian Shield, Sweden

3D modelling and sheath folding at the Falun pyritic Zn-Pb-Cu-(Au-Ag) sulphide deposit and implications for exploration in a 1.9 Ga ore district, Fennoscandian Shield, Sweden

From ductile to brittle deformation: structural development and strain distribution along a crustal-scale shear zone in SW Finland

Heterogeneity of folding in Zechstein (Upper Permian) salt deposits in the Kłodawa Salt Structure, central Poland

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