Active transsection faults in rift transfer zones: evidence for complex stress fields and implications for crustal fragmentation processes in the western branch of the East African Rift

New structural and seismologic evidence from the Rwenzori Mountains, Uganda, indicate that continental rifts can capture and rotate fragments of the lithosphere while rift segments interact, in a manner analogous to the interaction of small-scale fractures. The Rwenzori Mountains are a basement block within the western branch of the East African Rift System that is located at the intersection of two rift segments and is apparently rotating clockwise. Structural data and new seismological data from earthquake epicentres indicate a large-scale, 20-km-long transsection fault is currently detaching the Rwenzori micro-plate on its northern margin from the larger Victoria plate (Tanzania craton), whereas it is already fully detached in the south. We propose that this fault develops due to the rotation of the Rwenzori block. In a numerical model we show how rift segment interaction, block rotation and the development of transsection faults (faults that cut through the Rwenzori Mountains) evolve through time. The model suggests that uplift of the Rwenzori block can only take place after the rift has opened significantly, and rotation leads to the development of transsection faults that connect two rift segments, so that the block is captured within the rifts. Our numerical model suggests that the majority of the uplift has taken place within the last 8 Ma.     

Normal fault displacement dislocating a Roman aqueduct of Ephesos,western Turkey

A 38‐km‐long ancient aqueduct channel that served Roman Ephesos, Turkey was dislocated vertically over 3 m by a single seismic event on a normal fault. A new channel was constructed downstream from the fault in Roman times, next to and partly on top of the original channel. Archaeological investigations and study of carbonate deposits suggest a causative seismic event in the second half of the second century CE, probably in 178 CE, after the original channel had functioned for <35 years. The ?çme Tepe fault was identified as responsible for the displacement and may still constitute a seismic and tsunami hazard for the Turkish west coast, specifically for the city of Ku?adas?. Ancient aqueducts, of which more than 1400 are presently known, are a promising and almost untapped archive for archaeoseismic studies, especially in the Mediterranean area.     

Reconstruction of deformation and flow parameters from deformed vein sets

In any homogeneously deformed material, sets of material lines with a similar stretch history occupy sectors with a scale and orientation that depend on finite strain, area change rate and vorticity of the parent flow. If the size and shape of these “material line sectors” are known, they can be used to determine the sense of shear and to calculate deformation and flow parameters, even if actual stretches along material lines are unknown. In rocks which have undergone near-homogeneous deformation on a large scale, the orientation of sets of boudinaged folded and folded-boudinaged or boudinaged-folded veins can be used for such calculations. A general theoretical background to the problem is presented and a Mohr circle construction to carry out the deformation and flow analysis is introduced. Among other statements, the theory predicts that shortened or folded boudins must develop in non-coaxial monophase deformation, with considerable area change in the plane of observation, or in polyphase deformation.     

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 model for asymmetric reworked segment structures is applied to such features from the Lower Ugab Metaturbidites in NW Namibia. The model suggests that these structures form where the neck veins are stronger than the boudinaged layer, with a significant simple shear component of the bulk flow. The quartz filled necks in the Lower Ugab are therefore stronger than the quartz-rich wall rock in greenschist facies where the progressive deformation occurred. Bone-boudins are usually interpreted to form in transpressional flow, but simulations of the rotation of tension gashes show that they can also form in simple shear or slightly transtensional shear flow.     

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 interface. The simulations suggest that the solid–solid islands become unstable by necking and anti-cracking so that the island-channel interface will develop into a grain-boundary interface. The rough interface of the grain-boundary will coarsen with time so that in the extreme case a smooth thin-film interface will emerge. However the thin-film interface is also not necessarily stable but may develop into a rough grain-boundary where the roughness is initiated by heterogeneous dissolution of the solid. Our numerical experiments suggest that interface structures during dissolution–precipitation creep are transient and that the dominance of one interface structure depends on material properties, the displacement rate, purity of the reacting solid and the scale of observation.     

The generation of ductile and brittle shear bands in a low-angle mylonite zone

In the Saint-Barthélemy Massif, French Pyrenees, a ductile thrust zone developed in gneisses during retrogression from lower amphibolite facies conditions to the upper greenschist facies. The last major structures formed in the zone are isolated shear bands, divided into three types.Anastomosing, inhomogenous ultramylonitic shear bands (Type I) are subparallel to the mylonitic foliation in the gneiss (S_g). Most of these bands developed by ductile deformation processes only.Planar, homogeneous ultramylonite bands (Type II) are usually oblique to S_g. They generated as pseudotachylyte bands by brittle fracturing and underwent strong subsequent ductile deformation.Type III shear bands are planar and oblique to S_g. They consist of pseudotachylyte, weakly affected by ductile deformation.Type I, II and III bands seem to represent progressively younger structures on a local scale, linked to falling P-T conditions. The systematic variation in orientation of the different shear bands with respect to S_g is interpreted as being due to a different response of brittle and ductile structures to the orientation of the kinematic frame and the rock anisotropy.     

云蒙山变质核杂岩抬升过程中伸展拆离和岩浆底辟联合作用的证据

北京地区云蒙山变质核杂岩在白垩纪阶段抬升的早期,伴随着沿四合堂剪切带由北向南的拆离滑脱和大型花岗闪长岩的垂向侵位,晚期变形发生在花岗闪长岩岩基周边及其邻近围岩中,形成云蒙山剪切带,并伴随大量同构造的花岗岩和伟晶岩岩脉灌入。剪切带中所有的岩脉都随时间发生了不同程度的变形,较老的岩脉形成紧闭的圆柱状褶皱,枢纽与剪切带的线理和面理接近平行。岩脉与剪切带中L-S组构的平行化作用主要是由于这种转动的结果。岩脉的成分和长英指数随它们的变形程度发生变化,说明持续的岩浆分异作用与韧性剪切变形是同时发生的。云蒙山剪切带由岩浆底辟引起的上盘岩石重力所驱动,并不断得到同构造侵位岩脉的补充,起到存储和不断改造侵位岩脉的作用。早期伸展体制下形成的四合堂剪切带局部遭受云蒙山剪切带的改造或复合。该地区的岩石、构造和同构造岩脉的变形几何学和运动学证据表明,太古宙结晶基底的抬升是下部岩浆底辟与上部地壳伸展拆离共同作用的结果。     

Writ in water—Unwritten histories obtained from carbonate deposits in ancient water systems

Calcium carbonate deposits from ancient water systems such as aqueducts are a hidden archive for archaeology and environmental sciences. These deposits formed wherever carbonate-rich water was in contact with a water-containing structure and recorded water composition, temperature, biological content, the operation or nonoperation of a water system segment, flow discharge and velocity, the shape of disappeared segments of water structures, the number of years a water supply system was active, disruptions of the water supply and water management such as repairs, adaptations and cleaning. Indirectly, urban development, resilience, population- and socioeconomic dynamics can be studied through the stratigraphy of carbonate in water systems. Carbonate archives can also give insight into long-term changes in paleoclimate and on environmental pollution, deforestation, extreme floods, droughts, earthquakes and volcanic eruptions. Archaeological and environmental investigations of carbonate deposits can provide data with up to daily resolution over decades to centuries. Although absolute dating of carbonate from water systems is still problematic, each study on the aqueduct of an ancient city, together with its carbonate deposits, provides its own microstory in Roman life.