Analysis of deformation paths in shear zones

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.

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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.

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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.

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浙闽沿海晚中生代伸展构造的岩石学标志:东极岛镁铁质岩墙群

在浙江舟山群岛之东极岛上,出露大规模的镁铁质岩墙群,它们沿晚中生代花岗岩和火山碎屑岩的节理侵位,产状近于直立且与围岩截然接触,指示岩墙岩浆以主动方式沿构造裂隙侵入。东极岛镁铁质岩墙群的岩性主要是角闪辉绿岩,全岩Ar-Ar年龄为93.4Ma。在地球化学上,它们属弱碱性系列,SiO_2含量为46.88%～52.55%,CaO 5.40%～7.82%,Al_2O_316.30%～17.31%,TiO_2 1.53%～1.18%,(Na_2O+K_2O)4.62%～6.88%,Mg~#=34～42;具有轻稀土元素和大离子亲石元素(LILE,如Sr、K、Rb、Ba等)富集、高场强元素(HFSE,如Nb、Ta等)亏损等特点,它们的I_(Sr)为0.7084～0.7098,ε_(Nd)(t)为-3.21～-8.11,与福建沿海的镁铁质岩墙群及浙闽沿海晚中生代基性岩浆岩(辉长岩/辉绿岩和玄武岩)相似。它们由起源于受消减作用影响的岩石圈地幔部分熔融而产生并受地壳混染的玄武岩浆,在伸展构造环境中形成,是浙闽沿海晚中生代伸展构造的重要岩石学标志。     

中国东南部晚中生代构造伸展作用——来自海南岛基性岩墙群的证据

论文在系统地分析了海南岛文市、叉河、三亚地区的三处基性岩墙群的LA-ICP-MS锆石U-Pb年代学和元素地球化学特征的基础上,探讨了岩墙群的来源和其反映的地球动力学背景。文市岩墙群形成于约101Ma,叉河和三亚岩墙群形成于约93Ma;主量元素化学特征显示它们属碱性系列,微量元素化学特征为富集轻稀土元素和大离子亲石元素(Sr、K、Rb、Ba、Th),亏损高场强元素(Nb、Ta、Ti);Sr-Nd同位素组分显示其源区具有EMⅡ特征。这两期岩墙群的存在,显示了海南岛地区在早白垩世晚期和晚白垩世早期存在两次构造伸展事件。海南岛90Ma左右的岩墙群与广东、福建同时代的岩墙群构成中国东南沿海90Ma广泛发育的、呈近北北东向展布的岩墙群带,它们具有相同成因机制,指示中国东南部在90Ma左右时经历了强烈区域性拉张作用;这些岩墙群虽来源于不同的地幔源区,但均与俯冲流体交代作用有关。     

Digital mapping in structural geology — Examples from Namibia and Greece

Many problems in geology, especially structural geology, can only be solved by detailed mapping. Presently, mapping is still mainly carried out on paper using techniques from the 19^th Century. However, tools are now available to carry out most mapping tasks on microcomputers in the field without any need of paper. This speeds up geological mapping and reduces the errors involved in the mapping process. Digital mapping also allows work in featureless areas and areas of great structural complexity that would not be possible using paper maps. We present two practical examples of the new technology of digital mapping using microcomputers, from Namibia and Greece.     

Tectonic significance of deformation patterns in granitoid rocks of the Menderes nappes, Anatolide belt, southwest Turkey

Deformation fabrics in Proterozoic/Cambrian granitic rocks of the Çine nappe, and mid-Triassic granites of the Bozdag nappe constrain aspects of the tectonometamorphic evolution of the Menderes nappes of southwest Turkey. Based on intrusive contacts and structural criteria, the Proterozoic/Cambrian granitic rocks of the Çine nappe are subdivided into older orthogneisses and younger metagranites. The deformation history of the granitic rocks documents two major deformation events. An early, pre-Alpine deformation event (D_PA) during amphibolite-facies metamorphism affected only the orthogneisses and produced predominantly top-to-NE shear-sense indicators associated with a NE-trending stretching lineation. The younger metagranites are deformed both by isolated shear zones, and by a major shear zone along the southern boundary of the Çine submassif. We refer to this Alpine deformation event as D_A3. D_A3 shear zones are associated with a N-trending stretching lineation, which formed during greenschist-facies metamorphism. Kinematic indicators associated with this stretching lineation reveal a top-to-south sense of shear. The greenschist-facies shear zones cut the amphibolite-facies structures in the orthogneisses. ²⁰⁷Pb/²⁰⁶Pb dating of magmatic zircons from a metagranite, which crosscuts orthogneiss containing amphibolite-facies top-to-NE shear-sense indicators, shows that D_PA occurred before 547.2ǃ.0 Ma. Such an age is corroborated by the observation that mid-Triassic granites of the Çine and Bozdag nappes lack D_PA structures. The younger, top-to-south fabrics formed most likely as a result of top-to-south Alpine nappe stacking during the collision of the Sakarya continent with Anatolia in the Eocene.     

The reliability of asymmetric c-axis fabrics of quartz to determine sense of vorticity

Asymmetric c-axis fabrics of quartz are commonly used to determine sense of vorticity in ductile shear zones. This method seems to work if the fabric pattern resembles a model fabric proposed by Lister and Hobbs (1980). Usually, however, c-axis fabrics are rather vague. The reliability of such vague fabrics was tested in a major shear zone with known sense of vorticity. Only 62% of the c-axis fabrics predict the correct sense. Great care should therefore be taken in applying this method to determine sense of vorticity.     

Flow in natural shear zones—the consequences of spinning flow regimes

The ancient flow regime in natural shear zones is often considered to have followed a deformation path comparable to that in theoretical shear zones, i.e. progressive simple shear between rigid wall rocks with a persistent flow plane orientation parallel to the edges of the zone. This is often based on the presence of monoclinic fabric elements in the zones which indicate a dominantly non-coaxial flow regime, though not necessarily persistent simple shear. The deformation fabric of a shear zone from the Pyrenees illustrates that, even at a kinematic vorticity number ofW′ = 1 (simple shear) along the entire deformation path, obliqueness of the flow planes with the edges of the zone is possible for some time if incremental stretching axes were rotating with respect to zone edges. This implies that ductile deformation must have taken place in the wall rock of the zone. Such a flow regime may be difficult to recognize with the fabric criteria used at present but leads to an unusual kinematic significance of the shear zone involved; in extreme cases the zone may have acted as a passive marker in a ductilely deforming medium. It also means that not onlyW′ must be known to reconstruct the ancient flow regime in natural shear zones, but at least also the time dependence of the orientation of the incremental stretching axes.     

The distribution of macrozoobenthos in the southern North Sea in relation to meso-scale bedforms

This study investigates the distribution of macrozoobenthos in relation to meso-scale bedforms in the southern North Sea. Three sites on the Dutch Continental Shelf were sampled that are representative of large areas of the North Sea and show diverse morphological settings. These sites are (i) part of a shoreface-connected ridge, (ii) the lower part of a concave shoreface and (iii) a sandwave area. Within these sites, two or three different morphological units were distinguished. Sampling was undertaken in two seasons for two consecutive years. The species composition was analysed for differences in benthic assemblage within sites, based on the meso-scale morphology, and between sites and seasons. In addition, the benthic assemblage was correlated to water depth, median grain size (D50) and sorting (D60/D10). Results show that significant differences in the benthic assemblage can be found related to meso-scale bedforms, but macro-scale morphological setting and seasonal effects are more important.