Asymmetric boudins as shear sense indicators—an assessment from field data

Asymmetric boudins are potential but problematic shear sense indicators. They can be divided into two groups, with slip on the inter-boudin surface that is either synthetic (S-slip) or antithetic (A-slip) with respect to bulk shear sense. Since both groups have mirror-image symmetry, independent geometric criteria are needed to distinguish them if they are to be used as shear sense indicators. Investigation of asymmetric boudins in trains parallel to the main foliation from the Kaoko Belt in Namibia and elsewhere indicate that the geometry of both groups is in most cases different. Shearband boudins (formed by S-slip) have a long, curved lenticular shape and large relative displacement and …

Student Author of the Year Award 2009

The ‘‘Student author of the Year Award’’ is made for an outstanding paper in which an undergraduate or graduate student played a key role in the research and publication. The recipient of this award for 2009 is Ruben Diez Fernandez for his contribution, co-authored with Jose R. Martinez Catalan 3D Analysis of an Ordovician igneous ensemble: A complex magmatic structure hidden in a polydeformed allochthonous Variscan unit Journal of Structural Geology, Volume 31, Issue 3, March 2009, Pages 222–236. The Editors have selected this paper out of many student submissions in 2009. The basis of this paper is detailed field mapping, metamorphic petrology and structural analysis. These clearly establ…

The second century CE Roman watermills of Barbegal Unraveling the enigma of one of the oldest industrial complexes

Carbonate deposits provide insights into the operation of the earliest industrial utilization of hydropower by an ancient society.

Analysis of Cipollino Verde marble wall decoration in Ephesos, Turkey, using geological reconstruction

Abstract Slabs of Cipollino Verde, composed of layered and folded marble from Euboea in Greece, decorated the hall of a Roman town house in Ephesos, Turkey in the second century AD. After excavation, the fragmented slabs were restored. Preservation of the dowels in the walls from which the slabs were originally suspended allowed reconstruction of the order in which they had been placed on the walls. The pattern of folded layering in the slabs in turn allowed 3D structural geological reconstruction of the folds, and reconstruction of the arrangement of slabs in the marble block, from where they were serially sectioned, using a slab saw. Investigation of geological fold alignment in the block…

Riedel-shear control on the development of pennant veins: Field example and analogue modelling

Abstract The wall rocks of a crustal scale sinistral ductile shear zone in Namibia, the Purros Mylonite Zone, contain two types of asymmetric quartz veins. Bedding surfaces contain sigmoidal quartz veins with limited thickness along their symmetry axes that can be classified as tension gashes. A second type of veins consists of a striated central fault vein separating pennant-type quartz filled terminations. The tips of these “pennant veins” have a different orientation to those of the tension gashes. Analogue experiments were carried out using a sheet of silicone powder suspended on a slab of poly-dimethyl-siloxane (PDMS), both deformed in simple shear. These experiments produced open frac…

Bivergent extension in orogenic belts: The Menderes massif (southwestern Turkey)

The central Menderes massif is characterized by an overall dome-shaped foliation pattern and a north-northeast-trending stretching lineation. The asymmetry of shear bands and quartz c-axis fabrics on either side of the structural dome demonstrate a top to the north-northeast shear sense in the northern part and a top to the south-southwest shear sense in the southern part of the submassif, i.e., a bivergent downdip movement. This suggests a symmetric collapse of the Alpine Menderes orogenic belt along two extensional shear zones. Conjugate shear bands and symmetric quartz c-axis fabrics in the east-trending transition zone demonstrate a coaxial deformation between the two extension domains.…

Shear zone junctions: Of zippers and freeways

Abstract Ductile shear zones are commonly treated as straight high-strain domains with uniform shear sense and characteristic curved foliation trails, bounded by non-deforming wall rock. Many shear zones, however, are branched, and if movement on such branches is contemporaneous, the resulting shape can be complicated and lead to unusual shear sense arrangement and foliation geometries in the wall rock. For Y-shaped shear zone triple junctions with three joining branches and transport direction at a high angle to the branchline, only eight basic types of junction are thought to be stable and to produce significant displacement. The simplest type, called freeway junctions, have similar shear…

Facies and facies association of the siliciclastic Brak River and carbonate Gemsbok formations in the Lower Ugab River valley, Namibia, W. Africa

Abstract The Neoproterozoic Zerrissene Turbidite Complex of central-western Namibia comprises five turbiditic units. From the base to the top they are the Zebraputs Formation (greywacke and pelite), Brandberg West Formation (marble and pelite), Brak River Formation (greywacke and pelite with dropstones), Gemsbok River Formation (marble and pelite) and Amis River Formation (greywacke and pelites with rare carbonates and quartz-wacke). In the Lower Ugab River valley, five siliciclastic facies were recognised in the Brak River Formation. These are massive and laminated sandstones, classical turbidites (thick- and thin-bedded), mudrock, rare conglomerate and breccia. For the carbonate Gemsbok R…

Cambrian orogeny in the Ribeira Belt (SE Brazil) and correlations within West Gondwana: ties that bind underwater

A Damara orogen perspective on the assembly of southwestern Gondwana

The Pan-African Damara orogenic system records Gondwana amalgamation involving serial suturing of the Congo-Sao Francisco and Ro ´o de la Plata cratons (North Gondwana) from 580 to 550 Ma, before amalgamation with the Kalahari - Antarctic cratons (South Gondwana) as part of the 530 Ma Kuunga-Damara orogeny. Closure of the Adamastor Ocean was diachronous from the Aracuao ´ Belt southwards, with peak sinistral transpressional deformation followed by craton overthrusting and foreland basin development at 580- 550 Ma in the Kaoko Belt and at 545-530 Ma in the Gariep Belt. Peak deformation/metamorphism in the Damara Belt was at 530-500 Ma, with thrusting onto the Kalahari Craton from 495 Ma thro…

Structures along the Orobic thrust, Central Orobic Alps, Italy

A series of regional deformation phases is described for the metamorphic basement and the Permian cover in an area in the central Orobic Alps, northern Italy. In the basement deformation under low-grade amphibolite metamorphic conditions is followed by a second phase during retrograde greenschist conditions. These two phases predate the deposition of the Permian cover and are of probable Variscan age. An extensional basin formed on the eroded basement during the Late Carboniferous, filled with fan conglomerates and sandstones, and rhyolitic volcanic rocks. Well-preserved brittle extensional faults bound these basins. Further extension deformed basement and cover before the onset of Alpine c…

Development of antitaxial fringes during non-coaxial deformation: an experimental study

Strain fringes were modelled experimentally around wooden objects in a simple-shear box containing Polydimethylsiloxane (PDMS). The fringes were made of paraffin wax that was periodically poured into dilated sites next to the core-object and left to crystallise. In the experiments fringes and core-object rotated at similar decreasing rotation rates relative to extensional incremental stretching axes (ISA) of flow and rotated relative to each other by small amounts. Fringes did not necessarily open parallel to ISA due to interactions between fringes and core-object. Therefore, neither displacement-controlled fibres nor object-centre paths can be expected to record the exact orientation of th…

Reverse and normal drag along a fault

An analysis of the theoretical displacement field around a single dip-slip fault at depth reveals that normal and reverse fault drag develop by perturbation flow induced by fault slip. We analytically model the heterogeneous part of the instantaneous displacement field of an isolated two-dimensional mode II fault in an infinite, homogeneous elastic body in response to fault slip. Material on both sides of the fault is displaced and ‘opposing circulation cells’ arise on opposite sides of the fault, with displacement magnitudes increasing towards the center of the fault. Both normal and reverse drag can develop at the fault center depending on the angle between the markers and the fault; norm…

Metamorphic History of Gneiss Terrains

The local metamorphic history is an essential topic in the study of high-grade gneiss terrains. This chapter only deals with those aspects of the metamorphic history that can be studied in the field. We will briefly outline terminology, the methods which can be applied, and the difficulties involved. For more information we refer to textbooks such as Miyashiro (1975), Winkler (1976), Mason (1981), Best (1982), Vernon (1983), Spry (1986), Yardley (1989) and to the references given in the text.

Flame foliation: Evidence for a schistosity formed normal to the extension direction

Abstract Foliations are normally thought to develop approximately parallel to the XY-plane of the finite strain ellipsoid, i.e., perpendicular to the main shortening direction. We present a new type of schistosity named “flame foliation” that forms orthogonal to the main extension direction, approximately parallel to the YZ-plane of finite strain. Flame foliation consists of anastomosing biotite-rich selvedges overprinting S1 in pelitic layers of metaturbitites in NW Namibia. The biotite crystals in the selvedges are peculiar because they are oriented oblique or orthogonal to the flame foliation itself and parallel to the previous S1 cleavage, a feature no other foliation shows. In some cas…

Conflicting shear sense indicators in shear zones; the problem of non-ideal sections

Abstract Deflection of pre-existing planar structures such as foliations or veins by ductile shear zones is geometrically very similar to the curvature of newly developed shear zone-restricted foliations in zones that cut a rock with a random fabric. Sense of curvature of shear zone-restricted foliations can be used to determine shear sense, but the deflection of pre-existing planar structures is less reliable. Two examples are presented of shear zones in Australia where both types of structures seem to represent conflicting shear sense. This conflict can be attributed to a geometrical effect which causes deflection of older structures in an opposite direction to shear sense on outcrop surf…

The influence of matrix rheology and vorticity on fabric development of populations of rigid objects during plane strain deformation

Abstract The influence of vorticity and rheology of matrix material on the development of shape-preferred orientation (SPO) of populations of rigid objects was experimentally studied. Experiments in plane strain monoclinic flow were performed to model the fabric development of two populations of rectangular rigid objects with object aspect ratios (Rob) 2 and 3. The density of the rigid object populations was 14% of the total area. Objects were dispersed in a Newtonian and a non-Newtonian, power law matrix material with a power law exponent n of 1.2. The kinematic vorticity number (Wn) of the plane strain monoclinic flow was 1, 0.8 and 0.6 with finite simple shear strain of 4.6, 3.0 and 0.9,…

The joint sets on the Lilstock Benches, UK. Observations based on mapping a full resolution UAV-based image

Outcrop studies of fracture networks are important to understand fractured reservoirs in the subsurface, but complete maps of all fractures in large outcrops are rare due to limitations of outcrop and image resolution. We manually mapped the first full-resolution UAV-based, Gigapixel dataset and DEM of the wave-cut Lilstock Benches in the southern Bristol Channel basin, a classic outcrop of layer-bound fracture networks in limestones. We present a map of the patterns and age relationships of successive sets of joints in dm-thick limestone layers separated by claystone beds. Using interpretation criteria based on crosscutting relationships, abutting and joint length, up to eight successive s…

Controls on lineation development in low to medium grade shear zones: a study from the Cap de Creus peninsula, NE Spain

Lineations composed of similarly oriented elongate mineral aggregates or grains are a common feature in deformed rocks, but it is unclear which factors control the development of such lineations. Field observations and microstructural analysis of samples, which were taken from discrete greenschist to lower amphibolite facies shear zones of the easternmost Variscan Pyrenees, show that strain is only one of several factors that control the strength and type of a lineation. Dynamic recrystallization, metamorphic reactions and rigid body rotation are also important controlling factors for the development of lineations. The most important of these is dynamic recrystallization. The way in which d…

Dynamics of hydrofracturing and permeability evolution in layered reservoirs

International audience; A coupled hydro-mechanical model is presented to model fluid driven fracturing in layered porous rocks. In the model the solid elastic continuum is described by a discrete element approach coupled with a fluid continuum grid that is used to solve Darcy based pressure diffusion. The model assumes poro-elasto-plastic effects and yields real time dynamic aspects of the fracturing and effective stress evolution under the influence of excess fluid pressure gradients. We show that the formation and propagation of hydrofractures are sensitive to mechanical and tectonic conditions of the system. In cases where elevated fluid pressure is the sole driving agent in a stable tec…

Geochemistry, Isotope Geochemistry and Geochronology: Application to Field Studies

Structural studies make it possible to reconstruct the metamorphic and deformation history of a rock body but do not provide information on the absolute timing of these processes. Likewise, field observations and structural work alone may not unambiguously identify the protoliths of many high-grade gneisses or the compositional changes associated with migmatisation, anatexis and the general action of fluid and/or vapour phases in a rock. Geochemical research can answer many of these problems but, as most of this research is laboratory-oriented, we limit ourselves to some general outlines of the possible lines of research, so that these can be borne in mind during fieldwork.

Disequilibrium melt distribution during static recrystallisation

Melt migration and segregation, and the rheology of partially molten rocks in the upper mantle and lower crust, strongly depend on the grain-scale distribution of the melt. Current theory for monomineralic aggregates predicts a perfectly regular melt framework, but high-temperature experiments with rock-forming minerals + melt show considerable deviations from this predicted geometry. Disequilibrium features, such as fully wetted grain boundaries and large melt patches, have been described; these were mainly attributed to surface-energy anisotropy of the minerals. We present static analogue experiments with norcamphor + ethanol that allow continuous in situ observation of the evolving liqui…

Kinematic analysis — the sense of shear-sense. Discussion of: ‘Essay Review: Kinematic analysis — pure nonsense or simple nonsense’ by D. Flinn

Protomylonite, Mylonite and Ultramylonite

The objective of this chapter is to show how variation of strain intensity can be judged in thin section. Usually this kind of variation can best be observed in low-grade mylonites where the percentage of porphyroclasts decreases progressively with strain intensity. However, the percentage of matrix is highly dependent on mineralogical composition (e.g. quartz and biotite tend to convert to matrix readily). Compositional banding in gneiss can therefore result in mylonitic banding of apparent strain variation, which in fact only reflects variation in composition of the parent rock. Several examples of ultramylonite are derived from quartzitic rocks that tend to form few or no porphyroclasts …

Evolution of mica fish in mylonitic rocks

Abstract Mineral fish are lozenge-shaped porphyroclasts, single crystals in a finer grained matrix, which occur in ductile shear zones and which are commonly used as shear sense indicators. Mineral fish of biotite, tourmaline, K-feldspar, garnet, hypersthene and quartz occur in mylonites but most common are white mica fish. These mica fish can be subdivided into six morphological groups that develop by different mechanisms determined by different initial shapes and orientations. The principal mechanisms of formation are intracrystalline deformation combined with rigid body rotation. Concomitant selective grain size reduction occurs by recrystallisation, cataclastic separation, pressure solu…

Contrasting metamorphic evolution of metasedimentary rocks from the Çine and Selimiye nappes in the Anatolide belt, western Turkey

P-T conditions, mineral isograds, the relation of the latter to foliation planes and kinematic indicators are used to elucidate the tectonic nature and evolution of a shear zone in an orogen exhumed from mid- crustal depths in western Turkey. Furthermore, we discuss whether simple monometamorphic fabrics of rock units from different nappes result from one single orogeny or are related to different orogenies. Metasedimentary rocks from the Cine and Selimiye nappes at the southern rim of the Anatolide belt of western Turkey record different metamorphic evolutions. The Eocene Selimiye shear zone separates both nappes. Metasedimentary rocks from the Cine nappe underneath the Selimiye shear zone…

Carbonates from the ancient world's longest aqueduct:A testament of Byzantine water management

The fourth‐ and fifth‐century aqueduct system of Constantinople is, at 426 km, the longest water supply line of the ancient world. Carbonate deposits in the aqueduct system provide an archive of both archaeological developments and palaeo‐environmental conditions during the depositional period. The 246‐km‐long aqueduct line from the fourth century used springs from a small aquifer, whereas a 180‐km‐long fifth‐century extension to the west tapped a larger aquifer. Although historical records testify at least 700 years of aqueduct activity, carbonate deposits in the aqueduct system display less than 27 years of operation. This implies that the entire system must have been cleaned of carbonate…

Depositional ages and provenance of the Neoproterozoic Damara Supergroup (northwest Namibia): implications for the Angola-Congo and Kalahari cratons connection

Abstract The Damara Orogen is composed of the Damara, Kaoko and Gariep belts developed during the Neoproterozoic Pan-African Orogeny. The Damara Belt contains Neoproterozoic siliciclastic and carbonate successions of the Damara Supergroup that record rift to proto-ocean depositional phases during the Rodinia supercontinent break up. There are two conflicting interpretations of the geotectonic framework of the Damara Supergroup basin: i) as one major basin, composed of the Outjo and Khomas basins, related to rifting in the Angola-Congo-Kalahari paleocontinent or, ii) as two independent passive margin basins, one related to the Angola-Congo and the other to the Kalahari proto-cratons. Detrita…

Stratigraphy of the Neoproterozoic Damara Sequence in northwest Namibia: Slope to basin sub-marine mass-transport deposits and olistolith fields

Abstract The Neoproterozoic Damara Sequence (>1000 m thick) is composed of siliciclastic and carbonate rocks that crop out in the Damara Belt, Namibia. In Damaraland (including the Vrede, Bethanis, Austerlitz and Toekoms farms), these rocks were deformed and metamorphosed under greenschist facies (biotite zone) conditions during the Damara Orogeny. The stratigraphy and paleoenvironments of the Damara Sequence rocks are debated by the scientific community. We use field data, including detailed 1:25,000 geological mapping, elaboration of stratigraphic profiles and observation of preserved primary structures, textures and composition, to identify lithofacies and lithofacies associations, and t…

Shear Sense Indicators

Many geologists study mylonites with the exclusive aim to determine the sense of shear. Obviously this is an important aspect, but it is important to study shear zones first, before shear sense determination is attempted. In order to deduce the correct sense of shear we recommend the following procedure (Fig. 9.1)

Porphyroblasts and Reaction Rims

A volume of rock involved in deformation and metamorphism will continuously undergo changes in structure and mineral content. This chapter treats mineral growth and replacement structures and the way in which their geometry can be used to reconstruct tectonic history. Two types of informative structures are treated: porphyroblasts and reaction rims.

Geometric description of flanking structures

Present nomenclature of faults and flanking structures is ambiguous. This paper presents a system for description of flanking structures, based on geometric parameters and independent of kinematic frame. The description can be made using two levels of accuracy. A qualitative method is described using four geometric features: tilt, slip, lift and roll. This method is suggested for practical use in the field, since it does not involve measurements or complicated procedures. In parallel, a quantitative approach is also presented, based on analytical modelling of Bezier curves. This method requires measurement of geometric features and involves mathematical treatment, but allows comparison betw…

Analysis of a complex high-strain zone at Cap de Creus, Spain

Abstract The structural analysis of a high-strain zone developed in medium- to high-grade metamorphic micaschists from the Cap de Creus area, Spain provides an example of the complex relationships between geometry, strain and kinematics to be found in deep crustal shear zones. This high-strain zone is composed of E-W trending structural domains characterized by different strain intensities and associated with syntectonic emplacement of pegmatite dykes. The main phase of deformation discussed here, D2, affects steeply dipping bedding, boudinaged quartz veins and S1 developed parallel to bedding. D2 deformation of these features and of syn-D2 pegmatite dykes gives rise to fold/boudin structur…

Zipper junctions: A new approach to the intersections of conjugate strike-slip faults

Intersecting pairs of simultaneously active faults with opposing slip sense present geometrical and kinematic problems. Such faults rarely offset each other but usually merge into a single fault, even when they have displacements of many kilometers. The space problems involved are solved by lengthening the merged fault (zippering up the conjugate faults) or splitting it (unzippering). This process can operate in thrust, normal, and strike-slip fault settings. Examples of conjugate pairs of large-scale strike-slip faults that may have zippered up include the Garlock and San Andreas faults in California (USA), the North and East Anatolian faults (Turkey), the Karakoram and Altyn Tagh faults (…

A Framework of Microtectonic Studies

From their first use in the last century, thin sections of rocks have been an important source of information for geologists. Many of the older textbooks on structural geology, however, did not treat microscopic aspects of structures, while petrologists would describe microscopic structures as, for example, lepidoblastic or nematoblastic without paying much attention to kinematic and dynamic implications. During the last decades, however, structural geologists learned to profit from the wealth of data that can be obtained from the geometry of structures studied in thin section, and metamorphic petrologists have appreciated the relation of structural evolution on the thin section scale and m…

Are polymers suitable rock analogs?

To evaluate if a polymer is suitable for analog modeling, it is essential to know the rheological properties of the material. Polymers used in analog modeling exhibit a complex rheological behavior; only part of which has been taken into account in most modeling studies. The mechanical behavior is strongly dependent on strain rate and temperature, and is characterized by specific dependencies of the storage and loss moduli, related to the elasticity and viscosity, on the deformation rate (frequency). We have measured the storage and loss moduli at a broad range of strain rates and strains, using an oscillatory parallel-disk rheometer. Investigated materials are polydimethylsiloxane (PDMS), …

Geoethical education: A critical issue for geoconservation

Some geological outcrops have a special scientific or educational value, represent a geological type locality and/or have a considerable aesthetical/photographic value. Such important outcrops require appropriate management to safeguard them from potentially damaging and destructive activities. Damage done to such rock exposures can include drill sampling by geologist for scientific purposes. In this work, we show how outcrops important to structural geology and petrology can be damaged unnecessarily by drill coring. Unfortunately, regulation and protection mechanisms and codes of conduct can be ineffective. The many resources of geological information available to the geoscientist communit…

Barbegal carbonate imprints give a voice to the first industrial complex of Europe

International audience; The watermill complex of Barbegal is one of the first industrial complexes in the world, and one of the largest such installations known from antiquity. It has been studied through excavations and what is known about the complex, its history and purpose, is based on the remaining stonework of mills and water installations, since no traces of the woodwork or machinery of the mills have been preserved. The archaeological museum in Arles, however, stores 142 pieces of carbonate that formed on the woodwork of the mills. We studied this material by analysis of the shape of the fragments and of stable isotopes and crystallographic fabric of selected carbonate samples. This…

Numerical simulation of fibre growth in antitaxial strain fringes

A two-dimensional computer model (‘Fringe Growth’) is used to simulate the incremental growth of crystal fibres in undeformed antitaxial strain fringes. The user can define the shape of a core-object (e.g. a pyrite crystal), the growth velocity and anisotropy of growing crystals, the rotation of fringes and core-object with respect to a horizontal datum and with respect to each other, and the opening velocity of fringes. Growth is simulated by movement of nodes connecting line segments that define the grain boundaries. Modelling results predict that face-controlled strain fringes will grow around smooth core-objects and strain fringes with displacement-controlled and face-controlled fibres …

Mountain-building under extension

A mechanism is presented which explains how intra-continental rifting can cause large topographic uplift. The effect is sufficient to account for the uplift of rift flanks and the very high and strongly localized uplift of the Rwenzori horst in the Western Branch of the East African Rift System. We propose that the uplift is generated by crustal bending, which is caused by a misfit of the lateral tensile stress between the upper and middle crust. The misfit is a function of different yield mechanisms when the upper crust breaks whereas the middle crust flows. Two independent numerical schemes confirm the suggested uplift mechanism. Both models—a 2 and 2.5 D elastoplastic lattice-particle mo…

Low-Grade Mylonites

The temperature range for these mylonites is thought to be roughly between 250 and 500 °C. There is a gradual transition between cataclasites and low-grade mylonites. Whereas many feldspar porphyroclasts in low-grade mylonites still show fracturing by cataclasis, the quartz is usually deformed by crystal-plastic processes as shown by its change in shape and by undulose extinction. At increasing temperature bulging recrystallisation starts to manifest itself along the lobate contacts and eventually recrystallisation by subgrain rotation takes over (Chapter 10).

Neoproterozoic Rosetta Gabbro from northernmost Arabian–Nubian Shield, south Jordan : Geochemistry and petrogenesis

An Ediacaran mafic intrusion of south Jordan is a distinctive appinitic igneous rock with a possibly unique texture, characterized by spherical clots up to 40 mm in diameter composed of amphibole cores from which plagioclase euhedra radiate; we call it the Rosetta Gabbro. It is exposed as a small (ca. 750 m(2)) outcrop in the Neoproterozoic basement of south Jordan. A second outcrop of otherwise similar gabbro is located about 400 m to the north of the Rosetta Gabbro, but it lacks the distinctive texture. The Rosetta Gabbro could represent a magma pipe. It intrudes the Aqaba Complex (similar to 600 Ma) granitoids and metasediments of the Janub Metamorphic Complex (633-617 Ma). The gabbro is…

WITHDRAWN: Boudin centennial

Development of S-C' type cleavage in Paraffin wax using a circular shear rig

Interpretation of Structures and Fabrics Recognition and Interpretation of Fabric Patterns in Outcrop

High-grade gneisses with a long and complex history have a finite ‘memory’ of past events. This memory is formed by fabric elements such as foliations, lineations, folds, mineral assemblages, boudins and sequences of intrusion. One of the aims of a geologist is to tap this memory as effectively as possible. The memory of rocks, however, is rather inadequate in that it is partly destroyed by the same events which produce the fabric elements that are recorded (Williams, 1983). Strong deformation erases older fabric elements; intrusions, recrystallisation and partial melting do the same (Figs. 1.1; 4.1). One of the purposes of this manual is to help geologists working in high-grade gneiss terr…

δ objects as a gauge for stress sensitivity of strain rate in mylonites

Abstract Our understanding of the flow properties of deforming rocks in the Earth's lithosphere is mainly based on theoretical work and on the extrapolation of high-strain-rate experimental data to the low strain rates of rock deformation in nature. The geometry of structures in naturally deformed rocks can be an additional source of information on the rheology of the lithosphere. Flow experiments show that the geometry of a mantle of recrystallised material around a rigid object can be used to distinguish between a linear or power-law relation of differential stress and strain rate in strongly deformed rocks such as mylonites. This means that it is possible to use geometrical patterns in d…

How to make a transverse triple junction—New evidence for the assemblage of Gondwana along the Kaoko-Damara belts, Namibia

T-shaped orogenic triple junctions between mobile belts usually form in two unrelated stages by subsequent and oblique continental collisions separated by a significant time span. Besides these “oblique triple junctions”, another type, named “transverse triple junctions”, may exist. Such junctions are created by a more complex mechanism of partly contemporaneous convergence of three cratons in a restricted time frame, involving strike slip. The Neoproterozoic–Cambrian Kaoko-Damara junction between the Rio de la Plata, Congo, and Kalahari cratons in Namibia is an example of such a transverse orogenic triple junction, formed by at least four subsequent but partly related deformation events. I…

Laminated carbonate deposits in Roman aqueducts: Origin, processes and implications

Carbonate deposits in Roman aqueducts of Patara and Aspendos (southern Turkey) were studied to analyse the nature of their regular layering. Optical microscopy and electron-backscattered diffraction results show an alternation of dense, coarsely crystalline, translucent laminae composed of bundles and fans of elongate calcite crystals with their c-axes parallel to the long axis, and porous, fine-grained laminae with crystals at near-random orientation. The ?18O and ?13C data show a strong cyclicity and anti-correlation, whereby high and low ?18O values correspond to dense columnar and porous fine-grained laminae, respectively. Geochemical analyses show similar cyclic changes in carbonate co…

Environmental and depositional controls on laminated freshwater carbonates: An example from the Roman aqueduct of Patara, Turkey

Carbonate deposits in aqueducts are a new high-resolution data source for environmental changes during the time of the Roman Empire, notably in the fields of palaeoclimate and spring hydrology. In order to distinguish environmental effects from those related to depositional setting, laminated carbonate deposits were compared along the entire length of an ancient aqueduct channel at Patara, Turkey. The carbonate deposits, up to 80mm in thickness, are composed of lamina couplets up to 1mm thick of alternating porous microspar and dense, columnar sparite. The former formed in the dry, warm season and the latter in the wet, cool season. The presence of biofilms seems to play a role in the devel…

Mapping in Gneiss Terrains

Whatever the purpose of working in gneiss terrains, a prerequisite for success is a thorough understanding of the geometry and relative age relations of rock units and the sequence of deformation and metamorphism. Because of the three-dimensional complexity of deformation and intrusion relations in gneiss terrains, it is in most cases impossible to understand relations by visiting a single outcrop, or even by means of a small number of transects through the area under consideration. If no detailed geological maps are present, or if the subject of study is insufficiently dealt with on available maps, it will be necessary to map an area in some detail.

Experimental modeling of viscous inclusions in a circular high-strain shear rig: Implications for the interpretation of shape fabrics and deformed enclaves

[1] Deformation experiments with initially spherical and prolate viscous inclusions suspended in a viscous Newtonian matrix in a circular high strain annular shear rig provide insights on the shape development of inclusions in high strain shear zones during progressive deformation. Inclusions with a specific viscosity ratio with respect to the matrix material show distinct types of three-dimensional shape development. For instance, at a high viscosity ratio between matrix and inclusion a pulsating ellipsoid develops, which both continuously rotates and changes its shape from a sphere to an ellipsoid and back to a sphere. The experiments show that the shape of an inclusion that has a viscosi…

A strike-slip core complex from the Najd fault system, Arabian shield

Metamorphic core complexes are usually thought to be associated with regional crustal extension and crustal thinning, where deep crustal material is exhumed along gently dipping normal shear zones oblique to the regional extension direction. We present a new mechanism whereby metamorphic core complexes can be exhumed along crustal-scale strike-slip fault systems that accommodated crustal shortening. The Qazaz metamorphic dome in Saudi Arabia was exhumed along a gently dipping jog in a crustal-scale vertical strike-slip fault zone that caused more than 25 km of exhumation of lower crustal rocks by 30 km of lateral motion. Subsequently, the complex was transected by a branch of the strike-sli…

Deformation and fluid flow in the Huab Basin and Etendeka Plateau, NW Namibia

Abstract The Lower Cretaceous Twyfelfontein sandstone formation in the Huab Basin in NW Namibia shows the effects of volcanic activity on a potential reservoir rock. The formation was covered by the Parana-Etendeka Large Igneous Province shortly before or during the onset of South-Atlantic rifting. Deformation bands found in the sandstone trend mostly parallel to the continental passive margin and must have formed during the extrusion of the overlying volcanic rocks, indicating that their formation is related to South-Atlantic rifting. 2D-image porosity analysis of deformation bands reveals significant porosity reduction from host rock to band of up to 70%. Cementation of the sandstone, lin…

Geology of the Bozdag area, central Menderes massif, SW Turkey: Pan-African basement and Alpine deformation

The Menderes massif consists of a Precambrian Core Series that preserves evidence for a polymetamorphic history and a Paleozoic/Mesozoic Cover Series that experienced only the Alpine tectonometamorphic evolution. Structural, petrographic, and geochronologic investigations in the central Menderes massif demonstrate that (a) part of the metamorphic and structural evolution of the Precambrian basement is older than the undeformed 551+/-1.4-Ma-old Birgi metagranite, and (b) inferred Alpine fabrics overprinting the Cover Series largely have the same attitudes as the old structures in the much older Core Series. The inferred Alpine fabrics include both contractional and extensional structures. Co…

Lattice-particle simulation of stress patterns in a Rwenzori-type rift transfer zone

Abstract A new 3D spring lattice computer model has been developed and used to calculate the stress-field in the vicinity of a rift transfer zone. The numerical setup is based on the Rwenzori block, a transfer zone in the Western Branch of the East African Rift Valley. The study has two closely related, yet independent aims: primarily to gain insight into the pattern and the causes of the stress field in the Rwenzori area. The second aim is the evaluation of the model itself, based on a comparison of the model results with local geological structures. The simulations calculate the stress in the brittle part of the crust, at the topographic surface and at a depth of 10 km. The model does the…

Multistage Tectonism and Metamorphism During Gondwana Collision: Baladiyah Complex, Saudi Arabia

Field evidence from the Baladiyah complex in the northern part of the Arabian^Nubian Shield of Saudi Arabia indicates several erosional unconformities separating different high- and medium-grade metasedimentary sequences. This suggests that the collision between East and West Gondwana involved several cycles of exhumation and burial, providing a unique opportunity to study the multiple stages of this orogeny. A mineral equilibria approach and thermodynamic modeling are used to place constraints on the formation conditions of each of these cycles. It is shown that the complex is characterized by three regional metamorphic events followed by a fourth metamorphic event related to shear heating…

Mimicking shear zones: An example from Wadi Filk, Jordan

Abstract Ductile shear zones can develop in at least two ways: (1) a nucleus can grow laterally by free propagation into undeformed host rock, like most faults or joints; (2) the zone may nucleate and grow on or in a planar discontinuity and mimick its orientation. Most small-scale ductile shear zones are mimicking zones, but large-scale ductile shear zones could be free-propagating. The Wadi Filk mylonite zone in Jordan is a two km long, ten meter wide mylonite zone flanked by ultramylonite zones, developed in undeformed Neoproterozoic porphyritic monzogranite. Since mineral and major element composition of mylonite and monzogranite are identical, the structure seems to have formed by free…

Development of branching brittle and ductile shear zones: A numerical study

Continental collision zones are usually associated with large-scale strike-slip shear zones. In most cases these shear zones are complex and consist of multiple strands, varying in width, length, and total displacement. Here we present 2-D numerical models to simulate the formation of such shear zones at different depth levels within the crust, under either brittle (frictional/plastic) or ductile conditions. Localization of shear zones is initiated by a material contrast (heterogeneity) of the material parameters. We systematically test the rate of strain-weakening in brittle and in ductile regimes to understand its influence on the development of shear zone networks. Our simulations sugges…

Pressure solution compaction of sodium chlorate and implications for pressure solution in NaCl

Sodium chloride (NaCl) has been extensively used as a material to develop, test and improve pressure solution (PS) rock deformation models. However, unlike silicate and carbonate rocks, NaCl can deform plastically at very low stresses (0.5 MPa). This could mean that NaCl is less suitable for use as an analogue for rocks that do not deform plastically at conditions where PS is important. In order to test the reliability of NaCl as a rock analogue, we carried out a series of uniaxial compaction experiments on sodium chlorate (NaClO3) at room pressure and temperature (P‐T) conditions and applied effective stresses of 2.4 and 5.0 Mpa. NaClO3 is a very soluble, elastic‐brittle salt, that cannot …

How to resist subduction: evidence for large-scale out-of-sequence thrusting during Eocene collision in western Turkey

Significant along-strike variations have locked large parts of the Alpine subduction complex in the Eastern Mediterranean in the Eocene, and defined the end of high-pressure accretion in western Turkey. Structural analysis reveals that the Anatolide belt in western Turkey formed under greenschist facies metamorphic conditions in the Eocene when a high-pressure metamorphic fragment of the Adriatic plate (the Cycladic blueschist unit) was thrust onto the imbricated mid-crustal units of the Anatolian microcontinent (the Menderes nappes). The contact between the Cycladic blueschist unit and the Menderes nappes, the Cyclades–Menderes thrust, represents an out-of-sequence ramp which cuts up-sect…

The influence of strain localisation on the rotation behaviour of rigid objects in experimental shear zones

Abstract Mica fish and tourmaline fish from natural mylonites were analysed in thin section to determine their orientation distribution. They are oriented with their long axes tilted with respect to the mylonitic foliation, and fish with a small aspect ratio exhibit a slightly larger angle than fish with a large aspect ratio. This orientation seems to be a stable orientation for the mica and tourmaline fish. Analogue experiments with two rheologically different matrix materials were performed to explain the data. One material was PDMS, a linear viscous polymer. The other was tapioca pearls, a granular material with low cohesion and Mohr–Coulomb type behaviour. In contrast to a fairly homoge…

Origin and Evolution of High-Grade Gneiss Terrains

In the previous chapters we have given an impression of the way in which the analysis of high-grade rocks can be approached in the field. Here we conclude with some current larger-scale ideas on the origin and evolution of such rock assemblages.

The fabric attractor

Abstract The nature of fabric accumulation in high strain zones such as ductile shear zones depends on the nature and orientation of flow eigenvectors or apophyses. Some flow apophyses can act as ‘attractors’ of material lines or principal finite strain axes. This paper explains the nature of such attractors and discusses their significance and orientation in different monoclinic flow types. In ductile shear zones, strain values are high enough to show the effect of attractors in deformed rocks clearly. The concept of attractors can be used in deformation modelling, and can help in understanding the accumulation of deformation fabrics in homogeneous and inhomogeneous flow, e.g. around boudi…

Numerical modelling of asymmetric boudinage

Abstract Asymmetric boudinage structures are commonly used as shear sense indicators but their development is incompletely understood. This paper describes the influence of initial shape and kinematic parameters on the evolution of boudin trains using a numerical approach based on the finite difference code FLAC. Boudin trains are simulated as a series of competent objects embedded in a soft matrix subjected to general monoclinic ductile flow. Deformation of boudin trains includes heterogeneous stretching, rotation of boudins and offset along the neck regions. The sense of relative boudin offset is mainly influenced by the initial orientation of the interboudin plane in the boudinaging laye…

Deformation of melt-bearing systems—insight from in situ grain-scale analogue experiments

Abstract The deformation behaviour of partially molten rocks was investigated using in situ analogue experiments with norcamphor+ethanol, as well as partially molten KNO 3 +LiNO 3 . Three general deformation regimes could be distinguished during bulk pure shear deformation. In regime I, above ca. 8–10 vol.% liquid (melt) fraction ( ϕ bulk ), deformation is by compaction, distributed granular flow, and grain boundary sliding (GBS). At ϕ bulk ϕ bulk (regime III), grains form a coherent framework that deforms by grain boundary migration accommodated dislocation creep, associated with efficient segregation of remaining liquid. The transition liquid fraction between regimes I and II ( ϕ LT ) dep…

Contrasting stress fields on correlating margins of the South Atlantic

Made available in DSpace on 2018-12-11T17:25:39Z (GMT). No. of bitstreams: 0 Previous issue date: 2015-01-01 The passiveness of passive continental margins across the globe is currently under debate since several studies have shown that these margins may experience a variety of stress states and undergo significant vertical movement post-breakup. Of special interest is the South Atlantic, because the bounding continents have very different recent geological histories, with Africa experiencing continental rifting whereas South America is influenced by subduction on the Pacific side. It is not clear to what extent the Atlantic continental margins are subject to the same stresses and vertical …

Fabric Development in Gneiss Terrains

This chapter describes some aspects of the development of fabrics in gneiss terrains. It provides essential background information that should be read before attempting to map a gneiss terrain. Inevitably, the interpretations are ‘state of the art’ and not necessarily the absolute answer. Additional information can be found in the cited literature and various specialised journals such as the Journal of Structural Geology, the Journal of Metamorphic Geology, Tectonophysics, Tectonics and Precambrian Research.

The structure of reactive grain-boundaries under stress containing confined fluids

We present numerical experiments on structure development in grain-boundaries during dissolution–precipitation creep. Two solids that are represented by an elastic spring configuration are pressed together with a compressible fluid in the grain-boundary. The solid can dissolve or precipitate depending on elastic and surface energy as well as fluid pressure and concentration of dissolved material in the fluid. We perform a number of numerical experiments with different starting configurations that represent a large-scale island-channel interface with solid–solid contacts across the islands, a rough grain-boundary interface with a fluid along the whole interface and a smooth thin-film interfa…

530Ma syntectonic syenites and granites in NW Namibia — Their relation with collision along the junction of the Damara and Kaoko belts

Abstract The Lower Ugab and Goantagab structural domains are located at the junction between the N–S trending Kaoko and the E–W trending Damara belts (NW Namibia), where Neoproterozoic metavolcano-sedimentary sequences were intruded by several syenitic/granitic plutons. We present here new U–Pb ages on zircon grains from the Voetspoor and Doros plutons. Together with petrological, geochemical and structural data we evaluate the timing of the deformation and relation to the geodynamics during the final stage of Gondwana amalgamation. The plutons are composed of three main rock types: hornblende quartz-syenite, syenodiorite and biotite granite. The two former are predominant and show genetic …

Mohr-cyclides, a 3D representation of geological tensors: The examples of stress and flow

Mohr-circles are commonly used to represent second-rank tensors in two dimensions. In geology, this mainly applies to stress, flow, strain and deformation. Three-dimensional second rank tensors have been represented by sets of three Mohr-circles, mainly in the application of stress. This paper demonstrates that three-dimensional second rank tensors can in fact be represented in a three-dimensional reference frame by Mohr surfaces, which are members of the cyclide family. Such Mohr-cyclides can be used to represent any second rank tensor and are exemplified with the stress and flow tensors.

Flow perturbations: a tool to study and characterize heterogeneous deformation

The classification of the myriad of small-scale structures that are used in tectonic analysis is presently based on their geometry, which makes it difficult to discover transitions and groups amongst them. An alternative would be to classify structures according to the flow type by which they form, but this is difficult. Although most structures form by heterogeneous flow, modelling studies are often focussed on bulk homogeneous flow, since the mathematical treatment of heterogeneous flow is cumbersome. Also, heterogeneous deformation patterns seem to occur in as many types as there are small-scale structures. This paper introduces the use of the geometry of flow perturbations to improve ou…

Fabric attractors in general triclinic flow systems and their application to high strain shear zones: A dynamical system approach

High strain zones may deform by flow with a triclinic symmetry. This paper describes triclinic flow in a reference frame where Instantaneous Stretching Axes (ISA) are fixed. The operation of triclinic flow is described in two ways: first in terms of flow and the nature of flow eigenvectors and in the second part of the paper in terms of finite strain. In monoclinic flow, at least one of the eigenvectors of the flow coincides with one of the ISA and one or two of the eigenvectors act as attractors of foliation or lineation elements. In triclinic flow some flow eigenvectors are undefined since the two largest eigenvalues (controlling the flow) are imaginary. Imaginary eigenvalues are particul…

Pseudo-boudins in Pegmatite, Arunta, Australia

Dominance of microstructural processes and their effect on microstructural development: insights from numerical modelling of dynamic recrystallization

The influence of the dominance of different processes on the microstructural development of a quarzite has been numerically modelled using the modelling system Elle. In the model dynamic recrystallization of a polycrystalline aggregate has been simulated by a combination of viscous deformation, lattice rotation, subgrain formation, rotation recrystallization, nucleation of new grains and recovery. Different combinations of the dominance of processes are considered by variations in values of the grain boundary mobility and the energy threshold value for recrystallization by nucleation of new grains. In addition, two different starting microstructures (fine and coarse grained) are used. Resul…

Crystal-Plastic Deformation, Recovery and Recrystallisation of Quartz

As stated in the introduction, this chapter is included because of the special importance of quartz to estimate metamorphic conditions during and after mylonitisation. The theory behind crystal-plastic deformation is treated elsewhere (e.g. Passchier & Trouw 2005). The main optical expression of crystal-plastic deformation is smooth, non-patchy undulose extinction. Elongated grains with such undulose extinction, sometimes accompanied by deformation lamellae, are indicative for low-temperature deformation. At slightly higher temperatures recovery produces subgrains and recrystallisation tends to substitute the old deformed grains by small new ones. Three types of recrystallisation can be dis…

Cataclastic solution creep of very soluble brittle salt as a rock analogue

Until about the late 1960s, macroscopically ductile deformation of quartz was seen as a microscopically cataclastic process by most geologists (cf. the origin of the name ‘mylonite’). Undulatory extinction, subgrains, recrystallised grains and even crystallographic preferred orientations were interpreted as due to water-assisted brittle deformation processes. Nowadays, by contrast, the occurrence of these optical microstructures is considered as conclusive and unequivocal evidence for dislocation creep. The abundance of these microstructures in naturally deformed rocks lead to the conclusion that dislocation creep is the most important ductile deformation mechanism within the Earth’s crust.…

Stress induced grain boundary migration in very soluble brittle salt

Abstract Grain boundary migration (GBM) was studied in-situ at room temperature, atmospheric pressure and an applied diffmfwerential stress of ∼9.5 MPa under the optical microscope, in a wet aggregate of an elastic-brittle salt (sodium chlorate). The aggregate was previously deformed predominantly by a combination of grain boundary sliding, pressure solution and cataclastic solution creep. After deformation, but when the sample was still under differential stress, undeformed, fracture-free grains were observed to grow at the cost of deformed, intensely fractured grains. GMB rates typically fell in the range 2--10 μm/day. GBM took place only as long as the sample was under stress. Boundaries…

3D visualization of sheath folds in Ancient Roman marble wall coverings from Ephesos, Turkey

Abstract Archaeological excavations and restoration of a palatial Roman housing complex in Ephesos, Turkey yielded 40 wall-decorating plates of folded mylonitic marble (Cipollino verde), derived from the internal Hellenides near Karystos, Greece. Cipollino verde was commonly used for decoration purposes in Roman buildings. The plates were serial-sectioned from a single quarried block of 1,25 m3 and provided a research opportunity for detailed reconstruction of the 3D geometry of meterscale folds in mylonitized marble. A GOCAD model is used to visualize the internal fold structures of the marble, comprising curtain folds and multilayered sheath folds. The sheath folds are unusual in that the…

Fracture and vein patterns as indicators of deformation history: a numerical study

U-Pb and Pb-Pb zircon ages for metamorphic rocks in the Kaoko Belt of Northwestern Namibia: A Palaeo- to Mesoproterozoic basement reworked during the Pan-African orogeny

The Kaoko Belt belongs to the Neoproterozoic mobile belt system of western Gondwana, whose geodynamic evolution is assumed to have resulted from collision between the Congo Craton (present Africa) and the Rio de la Plata Craton (present South America). Several magmatic intrusion periods can be distinguished in the coastal area of this belt, based on conventional U-Pb, SHRIMP and Pb-Pb evaporation analyses on zircons. The prevailing igneous rock types are of granitic to tonalitic composition. A Palaeoproterozoic terrain with U-Pb magmatic emplacement ages between ~2.03 and 1.96 Ga may be correlated with the Eburnian event (~1.8 to 2.0 Ga), which is widespread in Africa. Additionally, two dis…

High-Grade Mylonites

High-grade mylonites are formed at temperatures above 650 °C. They are relatively uncommon, probably because their conservation is problematic. Most mylonites formed under these conditions would tend to fully recrystallise which destroys and masks the mylonitic structure. Mylonitic features are only preserved if grain growth is somehow inhibited in the rock, e.g. by its polymineralic nature.

Mylonites Derived From Parent Rocks Other Than Granites and Gneisses

Most mylonites shown in this atlas are derived from granites and gneisses. This is not a coincidence; the mineralogy of these rocks favours the formation of mylonites because of the contrasting behaviour of quartz and biotite on the one hand (forming matrix) and feldspar and muscovite on the other hand (forming porphyroclasts). Another group of rocks that readily forms mylonites are impure quartzites in which resistant minerals tend to form fish-like structures, again, by strong contrast in rheological behaviour.

Growth of stylolite teeth patterns depending on normal stress and finite compaction

Abstract Stylolites are spectacular rough dissolution surfaces that are found in many rock types. They are formed during a slow irreversible deformation in sedimentary rocks and therefore participate to the dissipation of tectonic stresses in the Earth's upper crust. Despite many studies, their genesis is still debated, particularly the time scales of their formation and the relationship between this time and their morphology. We developed a new discrete simulation technique to explore the dynamic growth of the stylolite roughness, starting from an initially flat dissolution surface. We demonstrate that the typical steep stylolite teeth geometry can accurately be modelled and reproduce natu…

Tectonic transport directions of the Lycian nappes in southwest Turkey constrained by kinematic indicators

The orientation, asymmetry and cross-cutting relationships of the structures along the contact zone between the Lycian nappes and the Menderes Massif suggest the presence of three deformation phases in the Milas region of southwest Turkey. The first deformation phase (D1) is characterized by a ductile deformation with top-to-the-NE sense of shear. Structural data of the first deformation measured along the uppermost part of the Menderes Massif and the base of the Lycian nappes suggest that the lowermost unit of the Lycian nappes was emplaced initially from southwest to northeast onto the Menderes Massif during the Early Eocene. The second deformation phase (D2) is also ductile in nature and…

The transition from single layer to foliation boudinage: A dynamic modelling approach

Abstract Foliation boudinage is a deflection of foliation in the vicinity of a central discontinuity in foliated rocks, mostly filled with vein material. It shows evidence for brittle deformation and void-opening during ductile flow. We used a two-dimensional visco-elastic spring model based on a discrete element approach to study the dynamic development of foliation boudinage and the behaviour of anisotropic visco-elastic material deformed under pure shear conditions. The anisotropies are set by defining rheological heterogeneities in the models with (1) a single layer in a weaker matrix; (2) multi-layers with different elastic properties and (3) random-distributed “micas”, rows of horizon…

Exhumation of high-pressure granulites and the role of lower crustal advection in the North China Craton near Datong

Granulites in the Datong-Huai'an area of North China are characterized by high P-T assemblages (14 16 kbar, -9OOY) that underwent decompression cooling to -7 kbar and -800°C during a 250&2400 Ma tectonic event. Nearly all structures in the grantilites developed during the retrograde exhumation history, and can be subdivided into: (1) the stratigraphically lower, 'lower structural domain' that is characterized by complex folding with 55-10 km wide domes surrounded by concentric troughs, preserving concentric lineation patterns; and (2) the stratigraphically higher 'upper structural domain' that is characterized by a planar gneissic foliation, upright folds and a constant, shallowly SW-plungi…

Intrusion mechanisms in a turbidite sequence; the Voetspoor and Doros plutons in NW Namibia

Abstract Two syntectonic plutons of Cambrian age intruded Neoproterozoic metaturbidites in Namibia at the junction of the NS trending Kaoko and EW trending Damara belts. Sinistral transpression in the Kaoko Belt produced km-scale upright D1 folds overprinted by minor D2 folds. D3 is associated with N–S shortening in the Damara Belt. The plutons show two main pulses of intrusion: hornblende syenite intruded late during D1 or during D2 and biotite granite during D3. Each tectonic event produced a strain shadow defined by the shape of folds and the foliation trend around the plutons. The internal igneous fabric and the arrangement of wall rock xenoliths that locally make up 50% of the intrusio…

Metamorphism of Precambrian–Palaeozoic schists of the Menderes core series and contact relationships with Proterozoic orthogneisses of the western Çine Massif, Anatolide belt, western Turkey

The tectonic setting of the southern Menderes Massif, part of the western Anatolide belt in western Turkey, is characterized by the exhumation of deeper crustal levels onto the upper crust during the Eocene. The lowermost tectonic units of the Menderes Massif are exposed in the Çine Massif, where Proterozoic basement orthogneisses of the Çine nappe are in tectonic contact with Palaeozoic metasedimentary rocks of the Selimiye nappe. In the southern Çine Massif, orthogneiss and metasedimentary rocks are separated by the southerly dipping Selimiye shear zone, preserving top-to-the-S shearing under greenschist facies conditions. In contrast, in the western Çine Massif, the orthogneiss is deform…

Brittle reactivation of ductile shear zones in NW Namibia in relation to South Atlantic rifting

Rifting has occurred worldwide along preexisting mobile belts, which are therefore thought to control rift orientation on a large scale. On a smaller scale, shear zones within mobile belts are reactivated as rift faults. In NW Namibia, shear zones of the Neoproterozoic Kaoko Belt run subparallel to the present-day continental passive margin and are inferred to have been reactivated during the opening of the South Atlantic Ocean. However, the extent of this reactivation and the influence of the reactivated shear zones on South Atlantic rifting are largely unknown. A combined remote sensing and field study was conducted to quantify offsets that are a direct function of shear zone reactivation…

Microstructural, chemical and textural records during growth of snowball garnet

The growth history of two populations of snowball garnet from the Lukmanier Pass area (central Swiss Alps) was examined through a detailed analysis of three-dimensional geometry, chemical zoning and crystallographic orientation. The first population, collected in the hinge of a chevron-type fold, shows an apparent rotation of 360 degrees. The first 270 degrees are characterized by spiral-shaped inclusion trails, gradual and concentric Mn zoning and a single crystallographic orientation, whereas in the last 90 degrees, crenulated inclusion trails and secondary Mn maxima centred on distinct crystallographic garnet domains are observed. Microstructural, geochemical and textural data indicate a…

Complex vein systems as a data source in tectonics: An example from the Ugab Valley, NW Namibia

Abstract Neoproterozoic metaturbidites in the Lower Ugab Domain, Namibia, contain a complex network of four sets of quartz-calcite veins, overprinted by km-scale folds associated with four regional foliations. The veins formed by fluid overpressure predating the main deformation. Deformation structures developed at the junction of two mobile belts during the assembly of Gondwana, the NS Kaoko Belt, and the EW trending Damara Belt. Km-scale NS trending folds were initiated during EW constriction in the Kaoko Belt, while their further development and all subsequent events are related to constriction in the EW-Damara Belt, with coeval sinistral strike slip in the Kaoko Belt. Deformation of the…

Historical and Technical Notes on Aqueducts from Prehistoric to Medieval Times

The aim of this paper is to present the evolution of aqueduct technologies through the millennia, from prehistoric to medieval times. These hydraulic works were used by several civilizations to collect water from springs and to transport it to settlements, sanctuaries and other targets. Several civilizations, in China and the Americas, developed water transport systems independently, and brought these to high levels of sophistication. For the Mediterranean civilizations, one of the salient characteristics of cultural development, since the Minoan Era (ca. 3200-1100 BC), is the architectural and hydraulic function of aqueducts used for the water supply in palaces and other settlements. The M…

Oblique collision and evolution of large-scale transcurrent shear zones in the Kaoko belt, NW Namibia

Abstract Early structures in the central part of the Kaoko orogenic belt of NW Namibia suggest that the initial stage of collision was governed by underthrusting of the medium-grade Central Kaoko zone below the high-grade Western Kaoko zone, resulting in the development of an inverted metamorphic gradient. In the Western zone, early structures were overprinted by a second phase of deformation, which is associated with localization of the transcurrent Puros shear zone along the contact between the Western and Central zones. During this second phase, extensive partial melting and intrusion of ∼550 Ma granitic bodies occurred in the high-grade Western zone. In the Central zone, the second phas…

Strain analysis and vorticity of flow in the Northern Sardinian Variscan Belt: Recognition of a partitioned oblique deformation event

Abstract A field example of strain partitioning has been analysed along the Nurra–Asinara transect of the NW Sardinian Variscan chain (Italy). The section in the Nurra–Asinara area is in a continuous sequence of tectono-metamorphic complexes made of low- to high-grade metamorphic rocks affected by a polyphase tectonic history. The principal fabric of the area is controlled by a D2 progressive deformation phase in which the strain is partitioned into folds and shear zone domains. The D2 stretching lineation and shear sense show a clear change from south to north. The principal meso- and micro-structures, vorticity gauges and a quantitative kinematic analysis of local strain suggest that the …

The Mulgandinnah Shear Zone; an Archean crustal scale strike-slip zone, eastern Pilbara, Western Australia

Abstract A large part of the deformation in the Archean Pilbara granitoid-greenstone terrain is localized in relatively narrow shear zones. The Mulgandinnah shear zone (MSZ) is a major one of these, with a width up to 8 km, that can be followed for over 70 km along strike in the Shaw Batholith in the eastern Pilbara. It forms part of the Mulgandinnah Lineament, that can be traced to the Lalla Rookh Basin and the Carlindi Batholith in the north, giving it a total length of over 150 km. The MSZ contains both mylonites and ultramylonites, both of which have foliations that are subvertical to steeply dipping, with the ultramylonitic foliation overprinting the mylonitic foliation to form more lo…

Medium-Grade Mylonites

The temperature range for the formation of this group of mylonites is approximately 500 to 650 °C. In medium-grade mylonites quartz is usually fully recrystallised, mainly by subgrain rotation, and has grown to a polygonal crystalloblastic fabric of strain free grains with an average grain size exceeding about 50 micrometers.

Monoclinic model shear zones

Abstract Although many ductile shear zones are supposed to have developed by approximately simple shear flow, some must have formed under different conditions. A few types of such ‘non-simple shear zones’ have been proposed in the literature such as transpression-, transtension- and stretching-shear zones. This paper presents a full three-dimensional kinematic model of shear zones with monoclinic flow geometry. Monoclinic shear zone types can be classified according to flow parameters, and according to the geometry and orientation of accumulating finite strain. Modelling of finite strain accumulation shows that a number of unusual and potentially problematic structural features may develop …

The aqueduct of Gerasa – Intra-annual palaeoenvironmental data from Roman Jordan using carbonate deposits

Abstract Calcium carbonate (CaCO3) deposits from Roman aqueducts are an innovative archive to obtain local high-resolution palaeoenvironmental and archaeological data in interdisciplinary studies. Deposits from one of the aqueducts of the Roman city of Gerasa provide a record of 59 years during the 1st to 3rd centuries CE, divided into three sequences separated by plaster layers. Annual carbonate layers show an alternation of sparite, formed in winter, and micrite, formed in summer. Brown bands at the base of many sparite layers probably correspond to large rainstorms in early winter. A fine lamination present in the brown bands may be diurnal in origin. Stable isotope and trace element dat…

Journal of Structural Geology: Student Author of the Year Award for 2013

Bivergent extension in orogenic belts: The Menderes massif (southwestern Turkey): Comment and Reply

Apparent boudinage in dykes

Intrusive rocks may be arranged in the form of strings of lenses or beads, as found on the Cap de Creus Peninsula, NE Spain, and in the South Finland Migmatite-Granite Belt. These structures first appear to be the result of stretching and boudinage of intrusive sheets or dykes. However, closer examination reveals that they are not boudins, but are instead primary intrusive structures. A detailed study was performed on a swarm of pegmatite intrusions at Cap de Creus. Layering is often continuous between beads, and, in some cases, individual beads exhibit a very irregular shape. These observations are shown to be incompatible with an origin by boudinage. Analogue experiments were used to test…

Geometric aspects of synkinematic granite intrusion into a ductile shear zone — an example from the Yunmengshan core complex, northern China

The Cretaceous Yungmengshan core complex in northern China contains a large syntectonic granodiorite batholith that intrudes a slightly older diorite intrusion. A major gently dipping ductile decollement shear zone is developed along the contact of the diorite and granodiorite. The shear zone is invaded by a large volume of granitic and pegmatite veins associated with the main granodiorite batholith during activity of the shear zone under high-grade metamorphic conditions. Progressively older veins are more strongly deformed into tight cylindrical fold structures rotated into parallelism with the lineation and foliation in the shear zone. Parallelism of veins to the foliation is partly due …

Palaeoproterozoic crustal accretion and collision in the southern Capricorn Orogen: the Glenburgh Orogeny

Abstract The Capricorn Orogen in central Western Australia records the Palaeoproterozoic collision of the Archaean Pilbara and Yilgarn Cratons. Until recently only one orogenic event was thought to be the cause of this collision, the 1830–1780 Ma Capricorn Orogeny. However, recent work has uncovered an older event, the Glenburgh Orogeny that occurred between 2000 and 1960 Ma. The Glenburgh Orogeny reflects the collision of a late Archaean to Palaeoproterozoic microcontinent (the Glenburgh Terrane) with the Archaean Yilgarn Craton and is therefore tectonically distinct as well as significantly older than the widespread 1900–1800 Ma tectonothermal events recorded in northern Australia. The Gl…

Shear sense indicators in striped bedding-veins

Striped bedding-veins are veins that lie subparallel to bedding and have an internal layering or lineation at a small angle to the veins’ long axis. They form during bedding-parallel slip and can be used as shear sense indicators. Solid inclusion trails produce the visible internal layering or lineation and track the opening direction of the veins. Elongate quartz crystals however can be oriented at an angle of up to 80° to the opening direction, are non-tracking, and contain almost no information on the shear sense. The striped bedding-veins can be separated into three types according to the geometry of their internal segmentation. Veins of type B opened parallel to jogs oriented at a low …

Geochemistry and P–T–t evolution of the Abu-Barqa Metamorphic Suite, SW Jordan, and implications for the tectonics of the northern Arabian–Nubian Shield

Abstract The Abu Barqa Metamorphic Suite (ABMS) represents the oldest part of the Arabian–Nubian Shield in southern Jordan. It comprises tonalitic gneiss, metasediments including schist and paragneiss, and granitic gneiss, intruded by later granitic bodies. Geochemically, the majority of the schist samples have shale and Fe-shale protoliths, while the paragneisses represent metagreywacke. Tectonic discrimination diagrams indicate that the protolith of the ABMS was deposited at an active continental margin/island arc setting. U–Pb zircon (SIMS) ages from metamorphic and igneous rocks of the ABMS indicate that it evolved between ∼800 and ∼610 Ma. A tonalitic gneiss has a crystallization age o…

The origin of fibrous veins: constraints from geochemistry

Metastable staurolite-cordierite asemblage of the Bossòst dome: Late variscan decompression and polyphase metamorphism in the Axial Zone of the central Pyrenees

Abstract A kilometre-scale shear zone is recognized in the Cambro–Ordovician schist of the Bossost dome, a Variscan metamorphic and structural dome in the Axial Zone of the central Pyrenees. Non-coaxial deformation is recorded by rotated garnet and staurolite porphyroblasts following regional metamorphism M 1 , while coaxial conditions prevailed during later contact metamorphic M 2 growth of andalusite and cordierite. Mineral compositions and bulk rock analyses show that garnet–staurolite–andalusite–cordierite assemblages are significantly enriched in Mg and Mn over the garnet–staurolite assemblage, which lacks sufficient Mg for cordierite to form. The garnet–staurolite assemblage preserves…

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 Cine 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 Cine 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 (DPA) during amphibolite-facies metamorphism affected only the orthogneisses and produced predominantly top-to-NE shear-sense indicators associated with a NE-trending stretchi…

Foliations, Lineations and Lattice Preferred Orientation

Many microstructures in rocks are defined by a preferred orientation of minerals or fabric elements. We distinguish foliations, lineations and lattice-preferred orientation.

Boudinage classification: end-member boudin types and modified boudin structures

In monoclinic shear zones, there are only three ways a layer can be boudinaged, leading to three kinematic classes of boudinage. These are (1) symmetrically without slip on the inter-boudin surface (no-slip boudinage), and two classes with asymmetrical slip on the inter-boudin surface: slip being either (2) synthetic (S-slip boudinage) or (3) antithetic (A-slip boudinage) with respect to bulk shear sense. In S-slip boudinage, the boudins rotate antithetically, and in antithetic slip boudinage they rotate synthetically with respect to shear sense. We have investigated the geometry of 2100 natural boudins from a wide variety of geological contexts worldwide. Five end-member boudin block geome…

Key-ring structure gradients and sheath folds in the Goantagab Domain of NW Namibia

Abstract The concept of deformation phases is one of the corner stones of structural geology but, despite its simplicity, there are situations where the concept breaks down. In the Goantagab Domain of NW Namibia, structures in an area of complex deformation can be subdivided into at least four sets, attributed to four deformation phases on the basis of overprinting relations. Three of these sets of structures, however, formed during the same tectonic event under similar metamorphic circumstances but slightly different flow conditions. These sets of structures show gradational transitions in space that can be understood by a concept of “key-ring structure gradients”, where older D A structur…

Asymmetric continental deformation during South Atlantic rifting along southern Brazil and Namibia

Abstract Plate restoration of South America and Africa to their pre-breakup position faces the problem of gaps and overlaps between the continents, an issue commonly solved with implementing intra-plate deformation zones within South America. One of these zones is often positioned at the latitude of SE/S Brazil. However, geological evidence for the existence of a distinct zone in this region is lacking, which is why it remains controversial and is not included in all modeling studies. In order to solve this problem we present a study of multiple geological aspects of both parts of the margin, SE/S Brazil and its conjugate part NW Namibia at the time of continental breakup. Our study highlig…

Carbonate deposits from the ancient aqueduct of Béziers, France — A high-resolution palaeoenvironmental archive for the Roman Empire

Abstract Carbonate deposits from a Roman aqueduct in Beziers, southern France, record environmental conditions during the late first century C.E. These deposits formed in a steep section of the aqueduct with a high flow velocity, which caused rapid deposition of up to 11 mm of calcite per year over a period of 22–24 years. The microstructure, trace element and stable isotope composition show that regular deposition was interrupted by high-discharge events, probably in response to heavy rainfall during autumn and winter, transporting colloidally- and particle-bound elements and depositing calcite with elevated δ 18 O values. Individual autumn high-discharge events coincide with abrupt decrea…

Polymetamorphism and ductile deformation of staurolite-cordierite schist of the Bossost Dome: indication for Variscan extension in the Axial Zone of the central Pyrenees

The Bossòst dome is an E–W-trending elongated structural and metamorphic dome developed in Cambro-Ordovician metasedimentary rocks in the Variscan Axial Zone of the central Pyrenees. A steep fault separates a northern half-dome, cored by massif granite, from an E–W-trending doubly plunging antiform with granitic sills and dykes in the core to the south. The main foliation is a flat-lying S1/2 schistosity that grades into a steeper-dipping slaty cleavage at the dome margins. Three major deformational and two metamorphic phases can be differentiated. S1/2 schistosity is an axial planar cleavage to W-vergent recumbent folding that probably occurred in mid-Westphalian time. Peak regional metamo…

An active bivergent rolling-hinge detachment system: Central Menderes metamorphic core complex in western Turkey.

Two symmetrically arranged detachment systems delimit the central Menderes metamorphic core complex and define a bivergent continental breakaway zone in the Anatolide belt of western Turkey. Structural analysis and apatite fission-track thermochronology show that a large east-trending syncline within the Alpine nappe stack in the central part of the orogen is related to late Miocene-early Pliocene to recent core-complex formation. The syncline formed as a result of two opposite-facing rolling hinges in the footwalls of each of the two detachments. Back-rotation of the syncline limbs suggests that the detachments rotated from an initial dip of 50 degrees -60 degrees to a currently shallow or…

Modelling of stylolite geometries and stress scaling

International audience; In this contribution we present numerical simulations of stylolite growth to decipher the effects of initial rock heterogeneity and stress on their morphology. We show that stylolite growth in a rock with a uniform grain size produces different patterns than stylolite growth in a rock with a bimodal grain size distribution. Strong pinning of large heterogeneities produce stylolite structures that are dominated by pronounced teeth, whereas a uniform grain size leads to spikes and a roughness that shows variable wavelengths. We compare the simulated stylolites with natural examples and show that the model can reproduce the real structures. In addition we show that stro…

Tectonic evolution of the southern Kaoko belt, Namibia

Abstract The tectonic evolution at the junction of the Panafrican Kaoko and Damara belts is well recorded in the siliciclastic and carbonate successions of the Neoproterozoic Zerrissene turbidite system, metamorphosed to the biotite zone of the greenschist facies. The structures in the turbidites are attributed to two main deformational events. The older one generated two continuous folding phases, D1 and D2, and the younger one resulted in D3 deformation. D1, of dominant E–W shortening, caused upright kilometre-scale folds with well-developed axial planar cleavage, N–S trending axial planes and subhorizontal axes. This phase graded into D2 that refolded the first folds coaxialy and develop…

An outline of shear-sense analysis in high-grade rocks

Ductile shear zones are important in tectonic reconstructions as a source of information on the relative motion of large crustal blocks or plates in the geological past. Methods to interpret fabric in ductile shear zones were mostly developed for low grade rocks where overprinting relations are usually well preserved. However, high grade shear zones are common and dominate in many Precambrian terrains. High grade shear zones should be analysed in a different way from low grade zones. The plane on which shear sense markers should be observed, the vorticity profile plane, is more difficult to find than in low grade shear zones. The most reliable shear sense markers in high grade shear zones a…

Modelling of segment structures: Boudins, bone-boudins, mullions and related single- and multiphase deformation features

Finite element modelling has been used to simulate the development of segment structures, deformed layer segments separated by veins, such as boudins, mullions, and bone-boudins. A parameter sensitivity analysis is used to compare the influence of the nature of the flow, the relative viscosities of veins in necks and the host rock, and the initial geometry of the layer segments. Parameter fields have been determined for the relative viscosity of veins and layers, and the kinematic vorticity number of flow. Reworked segment structures can have several shapes such as bone-, bulging, shortened bone-boudins and their asymmetric equivalents such as domino- and shearband-boudin geometry. The mode…

From Sample to Section

A sound analysis of microstructures relies on correct sampling and on the right choice of the direction in which thin sections are cut from samples. This chapter discusses the steps of sample collection; choice of sectioning plane, and problems involved in the interpretation of three-dimensional structures from two-dimensional sections.