Presence Of Weak Zones Research Articles

Transient Injection of Flow: How Torn and Bent Slabs Induce Unusual Mantle Circulation Patterns Near a Flat Slab

AbstractTorn and bent slabs are usually associated with flat‐slab subduction where the descending plate develops a horizontal geometry beneath the overlying continent. How such slab dynamics modify the surrounding mantle flow and the overriding plate remains enigmatic. Here, we conduct three‐dimensional subduction numerical experiments to investigate the flat slab to steep‐angle slab transition region and examine the impact of slab geometry changes on mantle flows. The results show that the along‐strike change due to flattening a segment of slab induces oblique flow toward the mantle corner at the transition region where flat slab bends to steep‐subducting slab. Slab tears can occur due to the buoyancy contrast between an oceanic ridge and the surrounding dense oceanic crust and/or presence of weak zones. A vertical tear at the side edge of a flat slab causes toroidal flow around the steep‐angle slab where sub‐slab mantle floods rapidly through the tear. Flow through the tear spreads to all directions including upwelling toward the continental base that may trigger slab and partial melting and thus affect arc magmatism. A horizontal tear that occurs ahead of the flat portion where slab resumes its steep‐angle results in enhanced plate‐motion parallel flow. Notably, the tear‐induced flow behaves as a rapid injection through the slab breach with the peak velocity up to an order of magnitude higher than plate motion, lasting for 1–2 million years after initially tearing. This rapid pulse flow might be recorded in surface tectonics as distinct transient events of topography or metamorphism.

Stability assessment for proposed Hemja-Patichaur road tunnel using empirical and analytical methods, western Nepal

Stability assessment is an important aspect for the safety and proper design of underground structures. This paper aims to analyze the stability issues for the proposed 13 km long road tunnel alignment along the Pokhara-Baglung Highway which stretches from the southeast Hemja (Kaski District) to the northwest Patichaur (Parbat District), Gandaki Province, Nepal. For this purpose, initially, an engineering geological mapping in the field was conducted followed by comprehensive analysis of stability. The study has shown that rock mass class within the proposed tunnel stretch ranges from good to extremely poor as per Q and RMR systems of rock mass classification. In this study, stability assessment was carried out by empirical methods and analytical methods. The sections along tunnel alignment were selected for stability study according to the overburden, rock class and presence of weakness zones. The study concluded that empirical methods has shown squeezing conditions at four sections and non-squeezing conditions at four sections as per Q-values. Similarly, analytical methods have shown the rock bursting condition as stable at the roof at most of the sections except minor spalling at some sections. However, the tunnel wall may suffer severe rock bursting with increased in-situ stress conditions of the rock mass. Further, the instantaneous and final deformation of the tunnel was obtained by Panthi and Shrestha’s approach as less than one percent which is negligible.

Early Modeling of Seismic Velocity Structure P, S, and Vp/Vs Ratio in Bengkulu Region and its Surroundings

Bengkulu is located in the southern region of Sumatra, where the Indo-Australian and Eurasian plates are subducted. As a consequence of the subduction, seismicity in the area increases, as well as volcanic activity and the appearance of structures such as the Sumatran and the Mentawai faults. Seismic velocity structures were reconstructed using earthquake tomography in Bengkulu and the surrounding areas using data from the International Seismological Center (ISC) and the Meteorology, Climatology, and Geophysics Agency (BMKG). The earthquake data collection period was from January 1, 2010 to December 31, 2018. 19 stations recorded 1721 earthquakes. LOTOS-12 is used to do a tomography inversion, starting with a 1D velocity model and wadati’s Vp/Vs ratio and iteratively inverting for Vp, Vs, and Vp/Vs ratios. The bending algorithm is used to calculate the arrival time, and the LSQR method is used to invert it. The Checkerboard Resolution Test (CRT) yielded Tomography results. The results revealed the existence of zones with low-velocity anomalies and low-values of Vp/Vs ratios, indicating the presence of weak zones caused by Sumatra faults, Mentawai faults, and other local faults. Low-velocity anomalies with high-values of Vp/Vs ratios are related to volcanic activity, hereas high-velocity anomalies are associated with the existence of subduction plates and compact rocks. The low-velocity anomalies in the fore-arc at 30-50 km depths coincide with fluid circulation caused by slab dehydration.

Quantitative characterisation of fracture connectivity from high-resolution borehole image logs

Fractures are fundamental structural elements in brittle rocks. They have a primary role in transporting geofluids, particularly in basement and carbonate rocks, where they have been the focus of reservoir characterisation and modelling for decades. Fracture networks are often described by statistical analysis of their geometrical attributes such as orientation, length and aperture. However, the spatial arrangement (topology) of fractures/fracture sets and its temporal evolution is equally important to better understand physical attributes of rocks. How much is a fracture network connected? Can this be quantified? Does connectivity change in different locations, if yes what is the reason for it? What is the brittle strain distribution in a fracture network? How much are fracture density and connectivity influenced by mechanical stratigraphy?This paper attempts to address these relevant questions and test quantitative fracture connectivity analysis on high resolution electrical and ultra-sonic borehole image logs in an appraisal well (Total depth, TD: 2195.36 mMD), drilled into the Utsira High basement, in the extensional Norwegian North Sea rift basin. Fracture connectivity and fracture size attributes were investigated in Permian Zechstein carbonates and Ordovician-Silurian granitic basement and separately for defined fracture zones at the interval of 2050–2195 mMD. The average degree of fracture/fault connection is good (D = 2.6) for the sediments and excellent (D = 3.5) for the basement. Accordingly, the block intensity (R22) is 0.4 for sediments and 0.2 for basement, which imply a moderately and strongly disintegrated host rock, respectively. There is a large fracture connectivity variation in different fracture zones, i.e. completely isolated fractures (Zone 1) vs highly interconnected and strongly disintegrated basement (Zones 9–10). The connectivity variation is governed by lithology, mechanical properties (stiffness), strain distribution and presence of weakness zones (faults). The study also highlights some possible applications of fracture connectivity results to reservoir modelling and future follow-up trends in quantitative spatial characterisation of fracture networks.

Preliminary result of electrical resistivity and electromagnetic methods to determine the bedrock

Infrastructure development such as buildings, especially in multi-story buildings, really requires strong foundation support. In the construction of a multi-story building, usually using a pile that is embedded into a zone that has a hard rock arrangement or bedrock to support the building above it. This is a challenge for multistory buildings in areas of volcanic deposits or pyroclastic deposits. The rock layer below the surface in this area is dominated by less dense rocks such as tuff. This rock layer has more challenging when using for building foundations because it tends to be permeable, so a special method is needed to obtain information on hard rock or bedrock for building foundation needs. In other hand, the presence of weak zones, below the surface can cause sudden land subsidence which of course will damage the buildings on it. This study aims to determine the depth of the bedrock zone, in the pyroclastic deposit area using the integration of geophysical methods, resistivity and electromagnetic (EM) method. The resistivity method is more sensitive for areas that have large types of resistance or areas that are resistive, such as rocks that have low porosity and permeability such as igneous rocks. Meanwhile, the EM method is more sensitive in layers that have high porosity and permeability values or in conductive areas such as sediment layers. Nevertheless, the conductor layer is also influenced by the mineral content in the rock, so a good understanding of geology is needed in matching the data and conditions in the field. This is expected to be a reference in designing building structures, especially in pyroclastic deposit.

Underwater ERT surveying in water with resistivity layering with example of application to site investigation for a rock tunnel in central Stockholm

ABSTRACTWe present an approach to underwater electrical resistivity tomography surveying under conditions with several water layers with different resistivity in the water above the electrode layout. The approach is verified against a synthetic model example and tested in full scale on data from a field survey. The field survey was carried out in central Stockholm as part of pre‐investigations for a new metro train (T‐bana) tunnel planned to pass under seawater. The water passage is associated with major tectonic zones that can potentially be very difficult from a tunnel construction point of view. The aim was to identify variations in depth of the bottom sediments and variations in rock quality including the possible presence of weak zones in the rock. Survey conditions are complicated by boat traffic and electrical disturbances from the power grid and train traffic. The water depth was mapped using sonar combined with recording pressure transducers, and water resistivity as a function of water depth was recorded using geophysical borehole logging equipment. Water resistivity as a function of depth was integrated in the inversion model. The results show that the rather difficult survey conditions could be handled in a satisfactory way thanks to adequate equipment, careful planning, and attention to details. The measured data contain information that is relevant for creating coherent models of the variation in depth to rock, which corresponds well with data from drilling. The results also indicate that information in variation in rock quality that can be of critical importance for planning of underground construction can be derived from the data.

INFLUENCE OF HETEROGENEOUS LITHOSPHERIC STRUCTURE ON CONTINENTAL RIFTING: POSSIBLE IMPLICATIONS FOR THE LIGURE-PIEMONTESE OCEAN

Continental rifting represents the thermo-mechanical process by which continents break. This process is characterised by an high variability in terms of duration, volcanicity, width and symmetry. Models formulated to explain such a structural variability consider differences in (e.g., Davison, 1997; Ziegler and Cloetingh, 2004): initial rheological or thermal structure of the lithosphere, rift kinematics (e.g., oblique vs. orthogonal) and mechanics (e.g., pure vs. simple shear), strain rate, melt volumes, presence of pre-existing weakness zones. Among these processes, the lithospheric structure inherited from previous deformation phases and the presence of weakness zones have been shown to play a major role in controlling continental extension (e.g., Dunbar and Sawyer, 1989; Ziegler and Cloetingh, 2004). As a consequence, rifting processes often affect continental lithospheres characterized by important lateral variations in rheology and these variations are able to influence parameters as the width and symmetry of the rift, the patterns of uplift and subsidence, the location and amount of volcanic products. Extension in the Ligure-Piemontese ocean has been shown to be superimposed onto a previous ercinic chain, thus configuring a lithospheric weakness zone. In this frame, lateral rheological heterogeneities is expected to have played a major role in controlling the structure of the passive margins of the Adria/Europe system. In this work, this influence is investigated through scaled analogue modelling. Small-scale experiments were performed in an artificial gravity field of 31g, by using the Large Capacity Centrifuge of the Tectonic Modelling Lab of the CNR-IGG, settled at the Earth-Science Department of Florence University. The models, with dimensions of 10cm x 16cm x 4cm, were built with suitable analogue materials (sand, silicone, oleic acid) able to reproduce the brittle/ductile stratification of the continental lithosphere and were properly scaled to be directly comparable to natural prototypes. The experimental series investigated the occurrence of three different continental lithospheres: 1) a weak orogenic lithosphere (WL; Thickness: 80-100km; Thermo-mechanic Age: 50 Ma; Effective Elastic Thickness: 20km); 2) a normal 4-layer lithosphere (NL; Th: 120km; TA: 250 Ma; EET: 40km); 3) a strong cratonic lithosphere (CL; Th: 140km; TA: 400 Ma; EET: 70km). Results show that in the simple case of WL, continental extension is taken up by regularly-spaced faulting in the upper brittle layer coupled to flow of the ductile lower crust and uppermost lithospheric mantle. No strain localisation configures with this initial rheological stratification; instead, the deformation style can be ascribed to a wide-rifting mode (e.g., Buck, 1991; Brun, 1999). The base of the lithosphere in these models was rather flat, suggesting that no major asthenospheric upraising took place during deformation. In case of lateral variation between WL and CL, deformation was mostly localised at the craton border and within the weak domain; no deformation developed within the strong lithosphere. The largest normal fault developed at the contact between WL and CL, whereas regularly-spaced faults and lower crustal flow (with development of core complex structures) characterised the weakest portion of the models. Extension was accommodated by downward flexure of the WL and elastic rebound with strong shoulder uplift of CL; limited asthenospheric upraising was mainly localised within the WL. In models with NL and CL separated by a central WL, extensional deformation was strongly localised by the weakness zone. This rheological configuration leaded indeed to a rapid necking of the lithosphere and asthenospheric upraising leading to continental break-up in the central part of model. The modelling results support an important role played by the initial lithospheric structure on rift structure and duration and suggest that lateral rheological variations may have an important influence in controlling the local pattern of uplift/subsidence and faulting in continental rift settings. These results may have also relevance for the opening of the Ligure-Pemontese ocean. Preliminarily, the structural conditions of continental extension between the European and Adriatic margin may be correlated to the experiment reproducing the strength contrast between weak and cratonic lithospheres. In particular, the downward flexure associated with low subsidence rates of the WL may well account for the development of the Calcare Massiccio fm. on the Adriatic margin, whereas the areas characterised by high subsidence at the WL-CL contact may be correlated with the coeval development of the Brianconnaise domain in the European margin.

Why did Arabia separate from Africa? Insights from 3-D laboratory experiments

We have performed 3-D scaled lithospheric experiments to investigate the role of the gravitational force exerted by a subducting slab on the deformation of the subducting plate itself. Experiments have been constructed using a dense silicone putty plate, to simulate a thin viscous lithosphere, floating in the middle of a large box filled with glucose syrup, simulating the upper mantle. We examine three different plate configurations: (i) subduction of a uniform oceanic plate, (ii) subduction of an oceanic–continental plate system and, (iii) subduction of a more complex oceanic–continental system simulating the asymmetric Africa–Eurasia system. Each model has been performed with and without the presence of a circular weak zone inside the subducting plate to test the near-surface weakening effect of a plume activity. Our results show that a subducting plate can deform in its interior only if the force distribution varies laterally along the subduction zone, i.e. by the asymmetrical entrance of continental material along the trench. In particular, extensional deformation of the plate occurs when a portion of the subduction zone is locked by the collisional process. The results of this study can be used to analyze the formation of the Arabian plate. We found that intraplate stresses, similar to those that generated the Africa–Arabia break-up, can be related to the Neogene evolution of the northern convergent margin of the African plate, where a lateral change from collision (Mediterranean and Bitlis) to active subduction (Makran) has been described. Second, intraplate stress and strain localization are favored by the presence of a weakness zone, such as the one generated by the Afar plume, producing a pattern of extensional deformation belts resembling the Red Sea–Gulf of Aden rift system.

Seismogenesis in Eastern Canada

Abstract The pattern of seismicity in eastern Canada depends on the presence of weak zones from previous major tectonic orogenies and how these weak zones are reactivated by local and regional stress fields and geophysical processes. Within the Canadian Shield, away from seismotectonic trends, there is a low level of seismicity and earthquakes tend to be small, less than M5. However, along seismically active trends, earthquakes as large as M7 have occurred. The seismotectonic features fall into four main categories: positive (uplift) continental basement linears; grabens formed by old plate separation; passive rifted margins offshore; and extinct spreading ridges. Two of the positive seismotectonic trends are the Boothia Uplift-Bell Arch that transects the northeastern part of the craton and northeastern Baffin Island, where the effects of postglacial rebound on the upper crustal stress field are the most pronounced. The St. Lawrence Valley (and interconnecting grabens) is a seismically active graben system that contains the most seismically active region (the Charlevoix zone) in eastern Canada. The extinct spreading ridge along the Labrador Sea and the Mesozoic rifted margin along Baffin Bay and Labrador Sea contain clusters of moderate seismicity. There are diffuse zones of moderate seismicity over some geological provinces (e.g. Central Metasedimentary Belt in western Quebec) apart from major tectonic features, a confined seismic zone (within an intrusion) in the Miramichi region and seismicity at the intersection of faults in northern Ontario. In the Nahanni region, which is situated near the boundary between the northeast Cordillera and the Interior Platform, the commencement of a noteworthy earthquake sequence with magnitude up to Ms 6.9 indicates considerable stress-strain build-up over a large area. There is an anticline in the epicentral area that is bounded by thrust faults and mountain ranges. In order to enhance our understanding of causative factors of current seismicity, it is necessary to determine in greater detail the tectonic forces and geophysical processes that are reactivating pre-weakened faults along the seismotectonic trends and over broad, diffuse seismogenic regions. Some of these factors are the rate of stress build-up, stress concentration at the intersection of faults and between mountain ranges, residual stress, the role of pore fluids, individual block movement, whether this movement is due to postglacial rebound or to other underlying viscoelastic phenomena and the rate of sediment deposition along the continental slope. Paleoseismicity is useful not only for the reconstruction of old large earthquakes but also for providing insight as to why surface fault offsets have not been observed in regions where large earthquakes (and associated high rate of microseisrnicity) have occurred within the past several hundred years.

Geophysical study of the easternmost Walvis Ridge, South Atlantic: Deep structure

Research Article| December 01, 1975 Geophysical study of the easternmost Walvis Ridge, South Atlantic: Deep structure J. GOSLIN; J. GOSLIN 1Institut de Physique du Globe de Paris, Laboratoire de Géophysique Marine, 4 Avenue de Neptune, 94100, St. Maurdes-Fossés, France Search for other works by this author on: GSW Google Scholar J. C. SIBUET J. C. SIBUET 2Centre Océanologique de Bretagne, B.P. 337, 29273, Brest, France Search for other works by this author on: GSW Google Scholar Author and Article Information J. GOSLIN 1Institut de Physique du Globe de Paris, Laboratoire de Géophysique Marine, 4 Avenue de Neptune, 94100, St. Maurdes-Fossés, France J. C. SIBUET 2Centre Océanologique de Bretagne, B.P. 337, 29273, Brest, France Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1975) 86 (12): 1713–1724. https://doi.org/10.1130/0016-7606(1975)86<1713:GSOTEW>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation J. GOSLIN, J. C. SIBUET; Geophysical study of the easternmost Walvis Ridge, South Atlantic: Deep structure. GSA Bulletin 1975;; 86 (12): 1713–1724. doi: https://doi.org/10.1130/0016-7606(1975)86<1713:GSOTEW>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract A seismic-refraction study of the sedimentary structure of the South West African continental shelf was carried out between lat 17°S and 24°S using expendable sonobuoys. Striking differences exist both in the topography and sedimentary structure between the shelf north and south of the Walvis Ridge. South of the ridge, as far as lat 23°S, the shelf consists of a prograded series, whereas north of the ridge, at least as far as lat 17°S, east-trending canyons cut the shelf sedimentary cover. The steep northern scarp of Walvis Ridge can be traced eastward under the sediment of the continental margin. The southern flank of the ridge is buried under a thicker sedimentary cover and could only be traced eastward to long. 10°E on seismic-reflection records. This flank probably parallels the northern scarp under the continental margin. Two-dimensional structural models, built with the help of seismic-reflection and seismic-refraction results and based on the hypothesis of local isostatic equilibrium, account for the observed gravity profiles. A compensating root consists of light material (density 2.95 g/cm3) and reaches a depth of about 25 km. Gravity results also suggest that the Walvis Ridge does not constitute a superimposed load on the lithosphere; rather, the ridge and its underlying compensating mass were created at approximately the same time as the adjacent ocean basins.The creation of the two aseismic ridges of the South Atlantic — the Rio Grande Rise and Walvis Ridge — by a mantle hot spot and plume is accepted; this theory seems to explain most of the peculiar features of the Walvis Ridge. However, it is probable that the surface expression of the mantle hot spot was controlled by the presence of weak zones in the lithosphere such as transform faults. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.