Particle-size distributions of low-angle normal fault breccias: Implications for slip mechanisms on weak faults

Luther, Amy; Axen, Gary J.; Selverstone, Jane

doi:10.1016/j.jsg.2013.07.009

Cited by 19 publications

(12 citation statements)

References 51 publications

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“…Insets show outcrop-scale features observed in the Whipple Mountains core complex. Mini-detachments are brittle, brittle-ductile, or narrow ductile shear zones, with displacements of tens to hundreds of meters that splay off the main detachment (Axen and Selverstone, 1994;Luther et al, 2013;Selverstone et al, 2012). They may form at any depth down to the LDT.…”

Section: Distributed Deformation Zonementioning

confidence: 99%

“…Towards the top of the zone, ductile structures start to overlap with discrete discontinuities and semi-brittle slip surfaces. Small-scale low-angle normal faults known as "mini-detachments" splay from the main detachment and may form at any depth down to the LDT (e.g., Axen and Selverstone, 1994;Luther et al, 2013;Selverstone et al, 2012). Deformational microstructures in quartz are predominantly of Regime 1 or BLG I type; microscale discontinuities and short-range variations in microstructure and grain size are common.…”

Section: Localized Deformation Zonementioning

confidence: 99%

“…5d) or with distinctive planar brittle-ductile shear zones referred to as mini-detachments ( Fig. 5e; Axen and Selverstone, 1994;Luther et al, 2013;Selverstone et al, 2012). Mini-detachments generally lie sub-parallel to the principal slip surface, cut mylonitic rocks in the detachment footwall, and are associated with narrow zones of high ductile strain with microstructures indicating shear stresses up to ∼ 80 MPa.…”

Section: The Whipple Mountainsmentioning

confidence: 99%

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Rheological transitions in the middle crust: insights from Cordilleran metamorphic core complexes

Cooper

Platt

2017

Solid Earth

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Abstract. High-strain mylonitic rocks in Cordilleran metamorphic core complexes reflect ductile deformation in the middle crust, but in many examples it is unclear how these mylonites relate to the brittle detachments that overlie them. Field observations, microstructural analyses, and thermobarometric data from the footwalls of three metamorphic core complexes in the Basin and Range Province, USA (the Whipple Mountains, California; the northern Snake Range, Nevada; and Ruby Mountains-East Humboldt Range, Nevada), suggest the presence of two distinct rheological transitions in the middle crust: (1) the brittle-ductile transition (BDT), which depends on thermal gradient and tectonic regime, and marks the switch from discrete brittle faulting and cataclasis to continuous, but still localized, ductile shear, and (2) the localized-distributed transition, or LDT, a deeper, dominantly temperature-dependent transition, which marks the switch from localized ductile shear to distributed ductile flow. In this model, brittle normal faults in the upper crust persist as ductile shear zones below the BDT in the middle crust, and sole into the subhorizontal LDT at greater depths.In metamorphic core complexes, the presence of these two distinct rheological transitions results in the development of two zones of ductile deformation: a relatively narrow zone of high-stress mylonite that is spatially and genetically related to the brittle detachment, underlain by a broader zone of high-strain, relatively low-stress rock that formed in the middle crust below the LDT, and in some cases before the detachment was initiated. The two zones show distinct microstructural assemblages, reflecting different conditions of temperature and stress during deformation, and contain superposed sequences of microstructures reflecting progressive exhumation, cooling, and strain localization. The LDT is not always exhumed, or it may be obscured by later deformation, but in the Whipple Mountains, it can be directly observed where high-strain mylonites captured from the middle crust depart from the brittle detachment along a mylonitic front.

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Section: Distributed Deformation Zonementioning

confidence: 99%

Section: Localized Deformation Zonementioning

confidence: 99%

Section: The Whipple Mountainsmentioning

confidence: 99%

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Rheological transitions in the middle crust: insights from Cordilleran metamorphic core complexes

Cooper

Platt

2017

Solid Earth

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“…Peak metamorphic conditions of ≤ 950 MPa and 800 • C were reached in Late Cretaceous time, followed by decompression of > 200 MPa and cooling in the Eocene (Hallett and Spear, 2014;Henry et al, 2011;McGrew et al, 2000), before a final phase of exhumation and cooling in the middle Miocene (Colgan et al, 2010b). Peak metamorphism was accompanied by large-scale recumbent folding that is likely to have contributed to crustal thickening (MacCready et al, 1997;McGrew et al, 2000), but the present nearisoclinal recumbent geometry of these folds may reflect subhorizontal shear and vertical shortening related to the early stages of Eocene decompression (Henry et al, 2011). Stretching lineations associated with these folds are weak, trend ∼ N-S, and have been interpreted in terms of subhorizontal flow of lower crustal material into the culmination that now forms the core complex (MacCready et al, 1997).…”

Section: The Ruby Mountains-east Humboldt Rangementioning

confidence: 99%

Reply to review by B. Grasemann

Cooper¹

2017

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High-strain mylonitic rocks in Cordilleran metamorphic core complexes reflect ductile deformation in the middle crust, but in many examples it is unclear how these mylonites relate to the brittle detachments that overlie them. Field observations, microstructural analyses, and thermobarometric data from the footwalls of three metamorphic core complexes in the Basin and Range Province, USA (the Whipple Mountains, California; the northern Snake Range, Nevada; and Ruby Mountains-East Humboldt Range, Nevada), suggest the presence of two distinct rheological transitions in the middle crust: (1) the brittle-ductile transition (BDT), which depends on thermal gradient and tectonic regime, and marks the switch from discrete brittle faulting and cataclasis to continuous, but still localized, ductile shear, and (2) the localized-distributed transition, or LDT, a deeper, dominantly temperature-dependent transition, which marks the switch from localized ductile shear to distributed ductile flow. In this model, brittle normal faults in the upper crust persist as ductile shear zones below the BDT in the middle crust, and sole into the subhorizontal LDT at greater depths. In metamorphic core complexes, the presence of these two distinct rheological transitions results in the development of two zones of ductile deformation: a relatively narrow zone of high-stress mylonite that is spatially and genetically related to the brittle detachment, underlain by a broader zone of high-strain, relatively low-stress rock that formed in the middle crust below the LDT, and in some cases before the detachment was initiated. The two zones show distinct microstructural assemblages, reflecting different conditions of temperature and stress during deformation, and contain su-perposed sequences of microstructures reflecting progressive exhumation, cooling, and strain localization. The LDT is not always exhumed, or it may be obscured by later deformation, but in the Whipple Mountains, it can be directly observed where high-strain mylonites captured from the middle crust depart from the brittle detachment along a mylonitic front.

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“…The CASRG measurements were compared with those from hand drawn boundaries, which were used as gold standard. Although these automatic methods have significantly improved during the last decade, recent studies on this subject emphasized the ongoing requirement for hand tracing, particularly if the identification of particles require the experience in terms of genetic interpretation by the investigator [8].…”

Section: Introductionmentioning

confidence: 99%

Ambiguous versus non ambiguous characterization of components from pictures of cataclasites

Kohlmayer

Grasemann

2013

Open Geosciences

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Abstract:The quantitative analysis of shape parameters of components in cataclastic rocks is often complicated by the fact that the individual components are difficult to identify. Frequently, the components have the same mineralogical composition as the matrix and the size distribution of components and the resolution of the images make it difficult to discriminate between the individual fragments. This work investigates the differences of component shape analysis from processing pictures of cataclasites digitized by different persons. The approach undertaken herein assumes that the ambiguity of the forms is expressed in a diversity of component identification, respectively in different selection of the components. The extent of this diversity is quantified and analyzed as to how far statistical parameters reflect this ambiguity. Components in the samples may be more or less unambiguous with respect to the identification of particle shape parameters. The present study demonstrates that two parameters, the Aspect Ratio and the angularity (Elliptical Parisfactor) might be used in order to reflect this ambiguity in the component identification.

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Particle-size distributions of low-angle normal fault breccias: Implications for slip mechanisms on weak faults

Cited by 19 publications

References 51 publications

Rheological transitions in the middle crust: insights from Cordilleran metamorphic core complexes

Rheological transitions in the middle crust: insights from Cordilleran metamorphic core complexes

Reply to review by B. Grasemann

Ambiguous versus non ambiguous characterization of components from pictures of cataclasites

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