Journal of Structural Geology最新文献

Fault-horizon cut-off data extracted from seismic reflection datasets are used to study normal fault geometry, displacement distribution, and growth history. We assess the influence of three seismic interpretation factors (repeatability, measurement obliquity, and fault cut-off type) on fault parameter uncertainty. Two repeat interpretations resulted in mean differences of 5–15% for throw, 11–42% for heave, 9–31% for displacement, and 7–27% for dip across faults. Measurement obliquity, where faults are interpreted using non-perpendicular transects to fault strike, show increasing uncertainty with increasing obliquity. Uncertainty in throw is 14–24% at obliquities >20° and 6–13% where obliquities <20°. Continuous cut-offs, including non-discrete deformation, generally exhibit greater uncertainties compared to discontinuous (discrete) cut-offs. We consider the effect of interpretation factors on fault parameters used in seismic hazard assessment (SHA) and fault seal, using the established Shale Gouge Ratio (SGR). Even modest measurement obliquities and repeatability errors can affect inputs for SHA, causing large differences in throw- or slip-rate and inferred fault length. Measurement obliquity and repeatability have a variable impact on SGR calculations, highlighting the additional importance of sedimentary layer thickness and distribution. Our findings raise questions about the optimum workflow used to interpret faults and how uncertainties in fault interpretation are constrained and reported.

Analogue modelling of wrench tectonics typically utilizes a rigid basement with a velocity discontinuity under a brittle or brittle-viscous cover, such as in Riedel experiments, which confines fault localization in the overlaying model. However, such a set-up is hardly compatible with modeling brittle-ductile systems such as the upper and lower crust or a brittle sedimentary cover overlying a viscous evaporitic layer. To achieve a more realistic experimental approach, Bruno Vendeville designed an alternative experimental set-up decoupling the basement from the brittle overburden with a viscous layer in which the basement is not involved. In this configuration, strike-slip movement is driven laterally rather than from the base up, facilitated by “weak zones” that preferentially localize the deformation during shortening and enable sliding between compartments. This original approach provides greater flexibility for modeling complex strike-slip settings, allowing for more freedom for strike-slip structures to form and evolve through time.

Although the experiments described in this work were conducted in the late 1990s, the co-authors have chosen to revisit and adapt this earlier work for this Special Issue to underscore Bruno's influence on another aspect of salt tectonics and his pioneering foresight in the field of analogue modelling.

Based on a multi-scale and hydrostructural approach, this study presents the most relevant methodology to be applied to a karst hydrosystem in order to get a full understanding of underground water flow. It implies a complete structural analysis, from the hydrosystem scale to the outcrop scale, including the intermediate scale of the major geological structures. We illustrate the method in the Arcier hydrosystem, in the northwestern border of the Jura fold-and-thrust belt (Eastern France).

Field mapping and structural analysis allow to update the geological vision of the hydrosystem with two kink-type fault propagation folds, including a trishear kinematic model, on either side of a plateau presenting a hollow-and-dome configuration. Fracturing analysis reveals a fault-fracture network that we infer governs the entire hydrosystem. A Riedel pattern is highlighted, characterized by a N–S-striking (N355° ± 5), sinistral strike-slip, regional shear zone. Then, two 3D geological models, at different scales, constructed with MOVE and Visual Karsys softwares are combined with water levels and artificial tracer tests. It reveals a multilayer aquifer and a redefinition of groundwater circulations for the Arcier hydrosystem.

The results demonstrate a strong geological control of karstic hydrosystems on groundwater circulations, proving that classical hydrogeological methods, such as natural and/or artificial tracers, must be combined with rigorous geological analysis. Moreover, the multi-scale approach provides an explanation of groundwater circulation based on the intersection between 3D geometry of impervious layers delimiting the aquifers and their base water level, instead of the 2D view (section or map) requiring systematic recourse to inferred vertical faults to cross permeability barriers vertically or laterally. This study also brings a new vision to the local protection of the water resource.

A graphical method developed for the construction of structure contours for inclined planar surfaces has been extended to curved or folded surfaces with cylindrical geometries and horizontal axes. These structure contours can be used to constrain the outcrop trace of a folded contact on a geological map. The extended method requires, as a minimum, a field of orientation data and the map position of a single elevation on the contact under investigation. The method utilises orientation data collected on and around a partially mapped trace of a folded contact to constrain the outcrop trace throughout a geological map. The method uses a vertical cross-section that is parallel to the profile plane of any folds present. Crest, trough and hinge points identified in the cross-section can be projected on to the geological map in their correct positions and elevations. Similarly, structure contours on fold axial surfaces can be projected on to the geological map and the contours used to locate the axial traces of cylindrical folds with horizontal axes.

Quantification of drainage basins and their networks on fold limbs can lead to a better understanding of spatial relationship between active tectonics and drainage variations. This study aims to evaluate the effect of vertical fold growth on the morphometry of drainage basins and their related networks formed in the Asmari Anticline in the Zagros Fold-Thrust belt. This belt is one of the youngest continental collision zones hosting one of the largest petroleum provinces in the world so that giant oil fields are found in its anticlines. We selected the Asmari Anticline due to the variability of drainage basins and stream networks across the fold's forelimb and backlimb. The basin area (Ba), topographic slope (TS), hypsometric integral (HI), basin shape (Bs), drainage basin orientation (DBO), crescentness index (CI), sinuosity of the main drainage (Smd), spacing ratio (R), and fault density (FD) for 68 drainage basins were calculated. Also, the morphometric characteristics of drainage networks including drainage density (Dd), drainage density of 1st-order streams (Dd1), and drainage frequency (Fs) were analyzed for each limb. Results show that southwestern limb (forelimb) is characterized by high topographic and dip slope, large and relatively circular basins, with high values of CI, Smd and DBO, implying pronounced lateral and headward erosion. Conversely, the smaller and elongated basins, with higher values of hypsometric integral and spacing ratio in the northeastern limb, show lower erosion of backlimb. Higher Dd, Dd1, and Fs values in the southwestern limb (9.54, 6.39, and 39.92 respectively) than the northeastern limb (8.96, 5.69, and 28.33 respectively), suggest higher rates of forelimb erosion, especially where dendritic drainage pattern is developed. This study implies that the fold's divide migration towards NE during fold growth has played a role in the variations of the morphometric parameters in the southwestern and northeastern limbs of the Asmari Anticline. Also, faults and fractures have important effects on the mentioned morphometric parameters. Higher density of faults dominantly with NW-SE trend in the SW limb, especially where trellis drainage pattern is developed, has also facilitated the lateral erosion of the steeply dipping forelimb, resulting in the enlargement of basins with higher values of Ba, CI, DBO, S (spacing of adjacent basins outlets), and Smd.

Thin-skinned fold-thrust belts are defined by an extensive detachment surface, “the sole thrust’. The sole thrust is typically assumed to be sub-planar beneath the fold-thrust belt. The model of a planar sole thrust and, by inference, planar basement is often applied rather uncritically, whereas experimental and field studies show that basement topography is not only varied but crucial for the geometrical and kinematic evolution of fold-thrust belts. Basement topography controls on the thrust dynamics remains the least well understood parameter in fold-thrust belts, and more case studies are needed to underpin further understanding. We present field evidence from the well-exposed Caledonian Thrust Front in Lapland, showing the influence of inherited, orogen-perpendicular basement structures on the subsequent structural evolution of the Caledonian sole thrust and its underlying sedimentary rocks. Inherited orogen-perpendicular basement structures created open corrugations in the foreland that directed thrust allochthons and controlled the geometry and strain state of the sole thrust and associated rocks. We propose that even relatively small-scale structures can have a significant control on the geometry and strain state of an evolving thrust system, and that variations in thrust geometries are not simply explained by inversion or coincidental heterogeneous internal thickening (imbrication) of thrust-related units.

We conducted scaled analogue modelling to show the influence of varying single layer initial orientation on the geometry of folds and boudins in a bulk constrictional strain field. The initial angle between the plane of shortening and the competent layer (θ_Z(i)) was incrementally increased from 0° to 90° by multiples of 11.25°. While the amount of layer thickening decreased with increasing θ_Z(i), the deformation structures produced range from pure dome-and-basin folds to coeval folds and boudins. Based on the attitude of fold axes, there are extension-parallel (F_EPR) and extension-perpendicular (F_EPP) folds, with axes subparallel and subperpendicular to the principal stretching axis (X), respectively. Coeval growth of F_EPR folds and boudins occurred when θ_Z(i) > ca. 25°. The F_EPP folds can be subdivided into a first type which affect the entire layer (if θ_Z(i) ranges between 11.25 and 78.75°) and a second type, referred to as FB_EPP folds, which are affecting pre-existing boudins if θ_Z(i) > 45°. The interlimb angle of all types of folds increases with increasing θ_Z(i). Folds and boudins similar to the ones produced in this study can be found in salt domes and in tectonites of subduction zones.

The physical, mechanical and fracture properties at Stromboli volcano have been integrated at multiple scales to understand whether the interplay between a presumed NE/SW rift zone and the Sciara del Fuoco (SDF) depression has resulted in a zone of weakness able to promote fracturing prone to flank instability. Multiscale fracture quantification by imaging via FracPaQ toolbox both fractures and sample scale fractures has been integrated with rock physics and rock mechanics experiments on cm-scale samples belonging to the Paleostromboli, Vancori, Neostromboli, Pizzo and Present Deposit volcanic cycles that have been taken from within and outside the rift zone. The structural changes to the edifice have been quantitively assessed by mapping at different scale fracture properties such density and orientation within and outside the rift zone allowing to identify the potential damaged zones that could reduce the edifice strength.

Results indicate that basalt textures, microfracture density, porosity, chemical zoning and preferential alignments, despite lithologically dependent, can be related to the NE/SW zone of weakness at the regional scale and to collapsed volumes that have been subject to continuous intrusive activity. Numerical inversion models have been performed to cross correlate fracture density in the basalts at multiple scales.

A link between microfracture density and seismic velocities has been also established via numerical modelling, allowing to interpret in terms of degree of fracturing the results of seismic tomographies at the field scale, providing a novel method to image crack damage evolution within the inner structure of the volcano edifice.

Coulomb critical wedge theory predicts that thrust wedges would grow sequentially from the hinterland to the foreland, meaning that distal deformation occurs last. However, in the northern Tibetan and Iranian Plateaus, far away from the southern collision zones, widespread deformation occurs soon after collisions of Arabia and India with Eurasia. Additionally, despite the prevalence of weak lower crust and distal pre-existing faults or weak zones, their relationship to early distal deformation remains poorly understood. For this reason, we run systematic experiments of discrete element models involving basal décollement layer as well as distal pre-existing fault. Our model results reveal that (1) the presence of pre-existing faults is necessary for the occurrence of early distal deformation; (2) the early deformation of distal pre-existing fault is dependent on basal décollement strength and independent of model width; (3) strong basal décollement fails to activate the distal pre-existing faults, instead weak basal décollement can deform them at the early stage; (4) in the presence of weak basal décollement, a slower shortening rate not only facilitates greater shortening absorption by the distal pre-existing fault at the early stage but also results in a more pronounced deviation from sequentially-forward deformation propagation. These findings demonstrate that the preferential reactivation deformations of distal pre-existing faults are mechanically controlled by a weak basal décollement layer. Together with geological and geophysical observations, we suggest that the early deformations of northern Tibetan and Iranian Plateaus may be the result of the reactivation of pre-existing faults due to the existence of weak lower crust soon after collisions.

Fracture data acquired from drill core and borehole image logs require corrections for the bias due to fracture orientation, that are usually achieved by the Terzaghi correction technique. Previous studies often approximate the wellbore as a one-dimensional scanline, assuming that the length of the core axis within the sampling range is equal to the scanline length. This study refers to the commonly used workflow as the original Terzaghi correction method which is known to perform poorly when the angle (θ) between the core axis and the fracture is small. To address this issue, we propose an extension of the Terzaghi correction method that also considers the core diameter and is a function of both the fractures and the host layer dipping angle. The new method resolves the fracture density problem by selecting a new direction for the scanline perpendicular to the fracture and calculating the projection length of the sampling space in this direction. The fracture spatial arrangements and observed number of sampled fractures mainly affect the performance of this new approach. However, possible errors in the new correction method pertaining to the equidistant fracture density should decrease with increasing number of sampled fractures. It is found that an acceptable correction range exists for equidistant fracture density, though, when the corrected density is not within the acceptable correction range, the observed sampled fractures will not be equal to the true observed value. This means the corrected fracture density would be in an unacceptable range of errors. Moreover, the new method can provide results within the acceptable range when the angle between the fracture and the core axis is less than 20°. At the same time, this new method is free from other disadvantages in the original Terzaghi correction method, particularly, when the ratio of layers’ thickness to the core diameter is low. Therefore, the improved approach presented here is especially applicable to thin layers (no more than two times the core diameter) and conditions where the angle between the fracture and the core axis is less than 20°, which can contribute to fracture density characterization in the subsurface.