Delineating Weak Zones in Limestone based on Borehole Drilling and Electrical Resistivity Tomography

This study is focused on imaging of weak zones in subsurface using borehole and geophysical datasets. These weak zones are present within Jhill limestone of Miocene age across the northern, Karachi. A total of forty-nine core samples were collected from eleven boreholes about 30 m deep within the study area. The core analysis reveals presence of cavities in fractured limestone at shallow and deep levels. The lateral extension and thickness of these weak zones are well imaged by the electrical resistivity tomography (ERT) dataset. The 2D tomographs of the six profiles show variability in the ground resistivity response. The ERT profiles are interpreted using on hand samples collects from boreholes. These tomographs reveal relatively high resistivity values interpreted as intercalation of dry clay and marl beds within limestone. The medium resistivity values suggest presence of clay and sand in highly fractured limestone or surficial dry features. The low resistivity values are interpreted to be originated 24 by the weak zones filled with lithologies having high moisture content within limestone. The collected core samples were analysed for geotechnical parameters. The integration of borehole and ERT datasets delineated weak zones in the northern and central regions, which should be well28 cemented to avoid any geohazard.

Plume-Induced Subduction Initiation: Revisiting Models and Observations

Subduction initiation induced by a hot and buoyant mantle plume head is unique among proposed subduction initiation mechanisms because it does not require pre-existing weak zones or other forces for lithospheric collapse. Since recognition of the first evidence of subduction nucleation induced by a mantle plume in the Late Cretaceous Caribbean realm, the number of studies focusing on other natural examples has grown. Here, we review numerical and physical modeling and geological-geochemical studies which have been carried out thus far to investigate onset of a new subduction zone caused by impingement of a mantle plume head. As geological-geochemical data suggests that plume-lithosphere interactions have long been important - spanning from the Archean to the present - modeling studies provide valuable information on the spatial and temporal variations in lithospheric deformation induced by these interactions. Numerical and physical modeling studies, ranging from regional to global scales, illustrate the key role of plume buoyancy, lithospheric strength and magmatic weakening above the plume head on plume-lithosphere interactions. Lithospheric/crustal heterogeneities, pre-existing lithospheric weak zones and external compressional/extensional forces may also change the deformation regime caused by plume-lithosphere interaction.

Probabilistic Modelling of Bending Strength of Timber Beams with the Help of Weak Zones Model

The variability of longitudinal bending strength of timber beams due to the presence of knots and other defects is analyzed. The weak zones model of timber beam bending strength used in the analysis consists of short weak zones (knots or group of them) and strong sections of clear wood. The load bearing capacity of timber beams is defined as an extreme (minimum) value problem or as a first downcrossing problem. Assuming a marked Poisson random field with zero correlation between all random variables, cumulative probability distribution functions of the load carrying capacity of timber beams were determined by analytical methods for typical load cases: pure bending, midspan point load and one-third point load. These results, as well as the marked Poisson random field as a model of longitudinal variability of the bending strength of timber beams, can be applied in the reliability analysis of timber structures. Furthermore, the analytical formulae for the cumulative distribution functions of load carrying capacity of timber beams can provide a good reference for numerical analysis conducted with Monte Carlo simulation to determined statistics for specific timber members.

The Subhercynian Basin: an example of an intraplate foreland basin due to a broken plate

The Late Cretaceous intraplate shortening event in central western Europe is associated with a number of marine basins of relatively high amplitude and short wavelength (2–3 km depth and 20–100 km width). In particular, the Harz Mountains, a basement uplift on a single, relatively steeply dipping basement thrust, have filled the adjacent Subhercynian Cretaceous Basin with their erosive product, proving that the two were related and synchronous. The problem of generating subsidence of this general style and geometry in an intraplate setting is dealt with here by using an elastic flexural model conditioned to take account of basement thrusts as weak zones in the lithosphere. Using a relatively simple configuration of this kind, we reproduce many of the basic features of the Subhercynian Cretaceous Basin and related basement thrusts. As a result, we suggest that overall, it shares many characteristics with larger-scale foreland basins associated with collisional orogens on plate boundaries.

Assessing the Vertical Displacement of the Grand Ethiopian Renaissance Dam during its Filling using DInSAR technology and its Potential Acute Consequences on the Downstream Countries

The Grand Ethiopian Renaissance Dam (GERD), formerly known as the Millennium Dam, is currently under construction and has been filling at a fast rate without sufficient known analysis on possible impacts on the body of the structure. The filling of GERD not only has an impact on the Blue Nile Basin hydrology, water storages and flow but also pose massive risks in case of collapse. Rosaries Dam located in Sudan at only 116 km downstream of GERD, along with the 20 million Sudanese benefiting from that dam, would be seriously threatened in case of the collapse of GERD. In this study, through the analysis of Sentinal-1 satellite imagery we show concerning deformation patterns associated with different sections of the GERD’s Main Dam (structure RCC Dam type) and the Saddle Dam (Embankment Dam type). We processed 109 descending mode scenes from Sentinel-1 SAR imagery, from December 2016 to July 2021, using the Differential Synthetic Aperture Radar Interferometry technique to demonstrate the deformation trends of both - the GERD’s Main and Saddle Dams. The time-series generated from the analysis clearly indicates different displacement trends at various sections of the GERD as well as the Saddle Dam. Results of the multi temporal data analysis on and around the project area show inconsistent subsidence at the extremities of the GERD Main Dam, especially the west side of the dam where we recorded varying displacements in the range of 10 mm to 90 mm at the crest of the dam. We conducted the current analysis after masking the images with a coherence value of 0.9 and hence, the subsequent results are extremely reliable and accurate. Further decomposition of the subsiding rate has revealed higher vertical displacement over the west side of the GERD’s Main Dam as compared to the east side. The local geological structures consisting of weak zones under the GERD’s accompanying Saddle Dam adds further instability to its structure. We identified seven critical nodes on the Saddle Dam that match the tectonic faults lying underneath it, and which display a varying degree of vertical displacements. In fact, the nodes located next to each other displayed varying displacement trends: one or more nodes displayed subsidence since 2017 while the other node in the same section displayed uplift. The geological weak zones underneath and the weight of the Saddle Dam itself may somewhat explain this inconsistency and the non-uniform vertical displacements. For the most affected cells, we observed a total displacement value of ~90 mm during the whole study period (~20 mm/year) for the Main Dam while the value of the total displacement for the Saddle dam is ~380 mm during the same period (~85 mm/year). Analysis through CoastSat tool also suggested a non-uniformity in trends of surface water-edge at the two extremities of the Main Dam.

Two-phase intracontinental deformation mode in the context of India-Eurasia collision: Insights from a structural analysis of the West Kunlun-southern Junggar transect along the NW margin of the Tibetan Plateau

The India-Eurasia convergence since early Cenozoic has established the Tibetan Plateau and the Circum-Tibetan Plateau Basin and Orogen System (CTPBOS). When and how the convergence-driving strain has propagated into the CTPBOS is of significant importance in deciphering the growth process of the Tibetan Plateau. In this study, we conduct a structural analysis of the West Kunlun-southern Junggar transect along the NW margin of the Tibetan Plateau to establish the deformation propagation and through this to determine the plateau growth processes. The results suggest a two-phase deformation mode. The first stage features deformation confined in pre-existing weak zones like the West Kunlun orogen, Buchu Uplift and Tian Shan orogen during Paleogene, in which the intracontinental strain was speculated to be mainly consumed by shortening of these weak zones. The second stage is characterized by deformation propagating into foreland regions since early Miocene, in which shorting along foreland fold-and-thrust belts of a scale of tens of kilometers and decreasing basinwardly plays a key role in absorbing intracontinental strain. We suggest that this two-phase deformation mode possibly reflects a shift of governing mechanism of the expansion of the Tibetan Plateau from a rigid-block manner to a critical wedge taper style.Thematic collection: This article is part of the Fold-and-thrust belts collection available at: https://www.lyellcollection.org/cc/fold-and-thrust-belts

Influence of zones of pre-existing crustal weakness on strain localization and partitioning during rifting: Insights from analogue modeling using high resolution 3D digital image correlation

The factors controlling the selective reactivation of pre-existing crustal structures and strain localization process in natural rifts have been studied for decades but remain poorly understood. We present the results of surface strain analysis of a series of analogue rifting experiments designed to test the influence of the size, orientation, depth, and geometry of pre-existing crustal weak zones on strain localization and partitioning. We apply distributed basal extension to crustal-scale models that consist of a silicone weak zone embedded in a quartz sand layer. We vary the size and orientation (θ-angle) of the weak zone with respect to the extension direction, reduce the thickness of the sand layer to simulate a shallow weak zone, and vary the geometry of the weak zone to reflect a range of anticlinal, either linear or curvilinear natural weak zone geometries. Our results show that at higher θ-angle (≤ 60o) both small- and large-scale weak zones localize strain into graben-bounding (oblique-) normal faults. At lower θ-angle (≤ 45o), small-scale weak zones do not localize strain effectively, unless they are shallow. We observe diffuse, second-order strike-slip internal graben structures, which are conjugate and antithetic under orthogonal and oblique extension, respectively. In general, the changing nature of the rift faults (from discrete fault planes to diffuse fault zones, from normal to oblique and strike-slip) highlights the sensitivity of rift architecture to the orientation, size, depth, and geometry of pre-existing weak zones. Our generic models are comparable to observations from many natural rift systems like the northern North Sea and East Africa, and thus have implications for understanding the role of structural inheritance in rift basins globally.

Delineation of Fractures Using a 2D GPR Processing Strategy for 3D Imaging: Weak Zones within Carbonates at the Archaeological Site of Xochicalco in Mexico

The use of GPR data multipath summation on data acquired over parallel study lines is presented here within the framework of a study on the effects of natural hazards on cultural heritage areas in order to image weak zones within carbonates, such as fractures and caverns. This study was realized at the archeological site of Xochicalco in Mexico, where fractures and caverns are potential sources of the degradation of the archeological remains. Dense parallel GPR study lines spaced every 0.25 m were surveyed using a 400 MHz monostatic antenna with the aim to image possible weak zones in three dimensions. We used a 2D imaging approach, namely, the method of multipath summation, which efficiently focused the scattered energy within the GPR sections. The study revealed, at depths of 1.6m and 1.8m, several linear events attributed to fractures, leading to the preliminary conclusion of this on-going project that cracks on the walls of the Quetzalcoatl Temple after a large earthquake in 2017 are prone to instability of carbonates rocks.