Microfabric memory of vein quartz for strain localization in detachment faults: A case study on the Simplon fault zone

Haertel, Mike; Herwegh, Marco

doi:10.1016/j.jsg.2014.08.001

Cited by 22 publications

(19 citation statements)

References 85 publications

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“…These conditions align well with reported regional Alpine metamorphic gradients (e.g., Niggli and Niggli, 1965). Only at temperatures as high as ∼ 300 • C or above, onset of ductile deformation in quartz occurs, as can be inferred from the occurrence of dynamic recrystallization in form of bulging recrystallization (e.g., Stipp et al, 2002;Bambauer et al, 2009;Haertel and Herwegh, 2014). However, our quartz-rich basement rocks were mainly deformed in a semi-brittle manner (see.…”

Section: Early Stage Deformation Under Highest Alpine Temperature Consupporting

confidence: 89%

Linking Alpine deformation in the Aar Massif basement and its cover units – the case of the Jungfrau–Eiger mountains (Central Alps, Switzerland)

et al. 2018

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Abstract. The northwest (NW) rim of the external Aar Massif was exhumed from ∼ 10 km depth to its present position at 4 km elevation above sea level during several Alpine deformation stages. Different models have been proposed for the timing and nature of these stages. Recently proposed exhumation models for the central, internal Aar Massif differ from the ones established in the covering Helvetic sedimentary units. By updating pre-existing maps and collecting structural data, a structural map and tectonic section were reconstructed. Those were interpreted together with microstructural data and peak metamorphic temperature estimates from collected samples to establish a framework suitable for both basement and cover. Deformation temperatures range between 250 and 330 °C, allowing for semi-brittle deformation in the basement rocks, while the calcite-dominated sedimentary rocks deform in a ductile manner at these conditions. Although field data allow to distinguish multiple deformation stages before and during Aar Massif's exhumation, all related structures formed under similar P, T conditions at the investigated NW rim. In particular, we find that the exhumation occurred during two stages of shearing in Aar Massif's basement, which induced in the sedimentary rocks first a phase of folding and then a period of thrusting, accompanied by the formation of a new foliation.

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Section: Early Stage Deformation Under Highest Alpine Temperature Consupporting

confidence: 89%

Linking Alpine deformation in the Aar Massif basement and its cover units – the case of the Jungfrau–Eiger mountains (Central Alps, Switzerland)

et al. 2018

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“…The weak CPO in sample CB13-55a more plausibly reflects relatively low strain, which is within reason for its deep structural position (430 m) relative to the detachment fault. This relationship is consistent with weak CPOs at positions structurally removed from the main detachment and/or shear zone in other natural settings (Haertel et al, 2013;Haertel and Herwegh, 2014) or in low finite strains in numerical models (Keller and Stipp, 2011;Lister and Hobbs, 1980). The weak CPO at 384 m (sample CB13-57a) and the dispersed a-axis patterns at 152 and 369 m depth (samples CB13-78 and CB13-55c, respectively) may reflect GBS.…”

Section: Quartzsupporting

confidence: 81%

Deformation conditions during syn-convergent extension along the Cordillera Blanca shear zone, Peru

et al. 2019

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Strain localization across the brittle-ductile transition is a fundamental process in accommodating tectonic movement in the mid-crust. The tectonically active Cordillera Blanca shear zone (CBSZ), a ∼200-km-long normal-sense shear zone situated within the footwall of a discrete syn-convergent extensional fault in the Peruvian Andes, is an excellent field laboratory to explore this transition. Field and microscopic observations indicate consistent top-down-to-the-southwest sense of shear and a sequence of tectonites ranging from undeformed granodiorite through mylonite and ultimately fault breccia along the detachment.Using microstructural analysis, two-feldspar and Ti-in-quartz (TitaniQ) thermometry, recrystallized quartz paleopiezometry, and analysis of quartz crystallographic preferred orientations, we evaluate the deformation conditions and mechanisms in quartz and feldspar across the CBSZ. Deformation temperatures derived from asymmetric strain-induced myrmekite in a subset of tectonite samples are 410 ± 30 to 470 ± 36 °C, consistent with TitaniQ temperatures of 450 ± 60 to 490 ± 33 °C and temperatures >400 °C estimated from microstructural criteria. Brittle fabrics overprint ductile fabrics within ∼150 m of the detachment that indicate that deformation continued to lower-temperature (∼280–400 °C) and/or higher-strain-rate conditions prior to the onset of pervasive brittle deformation. Initial deformation occurred via high-temperature fracturing and dissolution-precipitation in feldspar. Continued subsolidus deformation resulted in either layering of mylonites into monophase quartz and fine-grained polyphase domains oriented subparallel to macroscopic foliation or the interconnection of recrystallized quartz networks oriented obliquely to macroscopic foliation. The transition to quartz-controlled rheology occurred at temperatures near ∼500 °C and at a differential stress of ∼16.5 MPa. Deformation within the CBSZ occurred predominantly above ∼400 °C and at stresses up to ∼71.4 MPa prior to the onset of brittle deformation.

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“…Meter-wide strain gradients formed from a weakly to moderately deformed rim towards a mylonitic central zone and an ultramylonitic shear zone core. Hence, typical shear zone narrowing occurs (e.g., Means, 1995;Ebert et al, 2007;Herwegh et al, 2008;Haertel andHerwegh, 2014, Herwegh et al, 2016). Monomineralic quartz domains in the shear zone center are deformed by dislocation-creep processes, as they have strong crystallographic preferred orientation (CPO) correlated with textures related to subgrain rotation recrystallization in the fine-grained domains.…”

Section: Evolution Of Strain Gradientsmentioning

confidence: 99%

How is strain localized in a meta-granitoid, mid-crustal basement section? Spatial distribution of deformation in the central Aar massif (Switzerland)

Wehrens

Baumberger

Berger

et al. 2017

Journal of Structural Geology

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This study investigates strain distribution in granitoid rocks formerly in the middle crust in the Central Aar massif, Switzerland and places the deformation behavior in the tectonic framework of the Alpine orogeny. Strain is heterogeneously distributed in terms of strain partitioning forming several hundreds of closely spaced shear zones (SZ) (>80 SZ/km with SZ thicknesses <10 cm; about 10 SZ/km with SZ thicknesses of 0.5-10 m) separating 3D bodies of low to moderate background strain. Both the degree of background-strain intensity as well as the number of shear zones increases from granitic to granodioritic host rocks and is controlled by primary variations in the mica content between 10-15 vol% (granodiorite) and <8 vol% (granite). Shear zones evolved from ductile shearing in granodiorites, whereas they often nucleated from fractures in the stronger granites. The majority of the steep shear zones preferentially accommodated upward motion by the southern Block leading to an increase in peak metamorphic conditions from 250° in the North to 450°C in the South of the Aar massif. The shear zones initiated at about 18-20 km depths during a stage of crustal thickening (Handegg phase). Subsequent deformation reactivated some shear zones with a gradual transition from reverse dip-slip over oblique-slip to strike-slip shear zones under local transpressional conditions (Oberaar phase).

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Microfabric memory of vein quartz for strain localization in detachment faults: A case study on the Simplon fault zone

Cited by 22 publications

References 85 publications

Linking Alpine deformation in the Aar Massif basement and its cover units – the case of the Jungfrau–Eiger mountains (Central Alps, Switzerland)

Linking Alpine deformation in the Aar Massif basement and its cover units – the case of the Jungfrau–Eiger mountains (Central Alps, Switzerland)

Deformation conditions during syn-convergent extension along the Cordillera Blanca shear zone, Peru

How is strain localized in a meta-granitoid, mid-crustal basement section? Spatial distribution of deformation in the central Aar massif (Switzerland)

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