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Analysis of deformation paths in shear zones

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Zusammenfassung

Die Geometrie duktiler Scherzonen kann dazu verwendet werden, regionaltektonische Probleme zu lösen, sofern der im Material abgebildete Deformationsweg dieser Zonen genügend verstanden wird. Der Flow in vielen Scherzonen mag sich aus einfacher Scherung ableiten lassen, daher genügen Daten über den letzten Deformationszustand wie z. B. der finite Strain und die Volumenänderung nicht für die Rekonstruktion des Deformationsweges, auch nicht bei konstanten Fließparametern während der fortschreitenden Deformation. Zusätzliche Daten über die Verwirbelungszahl (Rotationszahl) sind nötig. Sie lassen sich ableiten aus verschiedenartigen Gefügeelementen wie Gruppen gefaltetboundinierter Gänge, rotierten Porphyroblasten und »verklemmten«, starren Objekten. Vorgestellt werden anhand von Gefügedaten Konstruktionen am Mohr'schen Spannungskreis zur Bestimmung der Deformationsparameter, um damit den Deformationsweg graphisch darzustellen und daraus die Fließparameter abzuleiten. Änderungen in der Rotationszahl (Vorticity) oder der Geschwindigkeit in der Volumenänderung während progressiver Deformation erlauben den Deformationsweg zumindest teilweise zu rekonstruieren. Verwendung finden hierbei diejenigen Gefügeelemente, die sowohl die durchschnittliche Deformation als auch die letzten Deformationsereignisse registriert haben.

Abstract

The geometry of ductile shear zones can be used to solve problems of regional tectonics if the deformation path of material in the zones is sufficiently understood. Flow in many shear zones may have deviated from simple shear and consequently data on the final deformation state such as finite strain and volume change are insufficient for reconstruction of the deformation path, even if flow parameters were constant during progressive deformation. Additional information on the flow vorticity number is also needed and can be obtained from fabric elements such as sets of folded-boudinaged veins, rotated porphyroblasts and blocked rigid objects. Mohr circle constructions are presented as a tool to calculate deformation parameters from fabric data, to represent the deformation path graphically and to reconstruct flow parameters from the shape of this path. If the vorticity number or the volume change rate changed during progressive deformation, the deformation path can be partly reconstructed using sets of fabric elements which register mean and final values of these parameters.

Résumé

La géométrie des shear zones ductiles peut être utilisée pour résoudre des problèmes de tectonique régionale, pour autant que l'histoire de la déformation des matériaux de ces zones soit suffisamment bien comprise. Il peut arriver, dans beaucoup de shear zones, que le processus ductile se soit écarté du modèle du glissement simple et qu'en conséquence, les caractères finals de la déformation, tels que l'ellipsoïde de la déformation finie, ou le changement de volume, s'avèrent insuffisants pour pouvoir reconstruire l'histoire de la déformation et ce, même si les paramètres de fluage sont restés constants au cours du processus. Il est alors nécessaire de disposer d'informations supplémentaires quant à la vorticité; ces informations peuvent être fournies par certains éléments structuraux tels que des groupes de veines plissées-boudinées, des porphyroblastes qui ont toruné et des objets rigides bloqués. Au moyen de constructions appliquées au cercle de Mohr, il est possible de calculer les paramètres de la déformation à partir des données structurales, de représenter graphiquement l'histoire de la déformation et de retrouver les paramètres de fluage à partir de la forme de cette représentation. Si, au cours de la déformation progressive, la vorticité ou le taux de variation de volume se modifient, l'histoire de la déformation peut être reconstituée partiellement par l'utilisation de groupes d'éléments structuraux qui enregistrent les valeurs moyenne et finale de ces paramètres.

Краткое содержание

Если при исследовани и пород линия деформа ции пластичных зон скола достаточно хорошо из учена, то на основании формы этих зон скола удаетс я решить тектонически е проблемы региональ ного характера. Направление течения во многих зонах скола можно вывести на осно вании простого смеще ния и, следовательно, по со стоянию их последней деформации. Однако, конечная дефо рмация и изменение объема оказываются н едостаточными для ре конструкции пути деформации, если даже параметры потока остаются неиз менными во время посл едовательно усиливающейся дефор мации. Необходимы еще дополнительные д анные о скорости тече ния, которую можно вывест и из элементов тексту ры, как напр.: систем жил, см ятых в складку и преобразованные в будины закрученных порфиробластов и «заклиненных» непо движных объектов. Из данных строения текс туры в поле напряжени я Мора определяют параметр ы деформации, по котор ым строят графики пути е е течения. Колебания ч исла вращения, или скорост и изменения объема во время продолжающейся дефо рмации разрешают, хот я бы частично, реконструи ровать пути ее протек ания. При этом опираются на те структурные элеме нты, которые отмечены, как при деформации всей д анной породы в среднем, так и во время последнего п роцесса деформации.

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  1. C. W. Passchier

    Present address: Instituut voor Aardwetenschappen, Budapestlaan 4, Utrecht, The Netherlands

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    Passchier, C.W. Analysis of deformation paths in shear zones. Geol Rundsch 77, 309–318 (1988). https://doi.org/10.1007/BF01848692

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