Ductile Substratum Research Articles

The structure of composite volcanoes unravelled by analogue modelling: A review

Volcanic edifices are unstable accumulations of eruptive and intrusive products with complex internal structures. A documentation of these structures facilitates the assessing of the various hazards associated with volcanoes, e.g. flanks instabilities and eruptive events. However, structures are not easily assessed in the field, as volcanoes are regularly covered by eruptive products and are rapidly eroded. In the case of recent and active volcanoes, internal structures are generally extrapolated from the external morphology of the edifice, which can be documented using Digital Elevation Models (DEM) and geodetic data. Interpretation of such data greatly benefits from analogue models, which are idealised representations of reality used to test the impact that various processes may have on the shape of a volcanic edifice. This short review focuses on the contribution of analogue models to our comprehension of the internal structure of composite volcanic cones. The other volcanic morphologies (e.g. caldera) are excluded from this review, as they have distinct structure and evolution that have already been thoroughly reviewed. The reviewed models were used to investigate the impact that regional faults, magma injections and ductile substratum, among other processes, have on the internal structure and surface morphology of many volcanic edifices. These models were also used to locate unstable flanks and eruption sites.

Comparing multi-scale cracking mechanisms in man-made composites and natural materials

Abstract Man-made composites, like reinforced and fiber-reinforced concrete elements, behave similarly to natural composite structures (e.g., brittle rocks lying on a ductile substratum). In both cases, the cracking phenomenon of brittle layers depends on the scale of observation and is ruled by the Golden Ratio. Thus, a unique size-effect relationship, herein called Golden Scaling Law (GSL), is introduced and used to predict the crack pattern of concrete and rock structures. In accordance with several experimental data, GSL permits to calculate the values of crack width and crack spacing when the geometry of crack patter is known at a lower scale. Moreover, GSL can be applied to large composite structures without knowing the material performances, but by testing prototypes of lower dimensions.

Deep-seated gravitational slope deformations in western Sicily: Controlling factors, triggering mechanisms, and morphoevolutionary models

A study of deep-seated gravitational slope deformation (DSGSD) phenomena affecting areas of various geological and geomorphological settings in western Sicily is described. Western Sicily is underlain by a thin-skinned imbricate wedge of Meso–Cenozoic carbonate and siliciclastic rocks that formed by the stacking of several thrust nappes over the Iblean foreland. Locally, the original thrust sheets are folded and cut by high-angle faults. Large areas of western Sicily now display high relief energy due to Plio-Pleistocene block-faulting and uplifting, and the Quaternary morphogenetic phases are characterised by incision, thereby triggering widespread DSGSDs. To identify controlling factors and triggering causes and to develop reliable morphoevolutionary models for the DSGSDs of western Sicily, a geomorphological study based on field surveys and aerial-photography interpretations was performed. Previous geomorphological data relating to well-known examples of DSGSDs were reconsidered, leading to remarkable revisions of the interpretative models in certain cases. New data were subsequently collected, enabling recognition of additional DSGSD phenomena. The whole body of data involves a total of 27 DSGSDs affecting areas in two specific geological settings: (1) areas with flat thrust surfaces, where differential settlements, back-tilting, lateral spreads in competent rocks overlying marls and clays, large topples, and/or block-type slope movements may develop; and (2) areas where deep-rooted carbonate units come into lateral contact with clayey–marly units along high-angle faults, where lateral spreads in brittle homogeneous rocks, sinking, and/or rock flows may occur. These DSGSD phenomena are associated with different evolutionary stages, allowing a morphoevolutionary model to be defined for the two geological conditions. For these two different morphoevolutionary models, the following structural features play an important role in the development of DSGSDs: (1) where carbonate bodies overlie clayey–marly rocks, triggering of the DSGSDs traces back to deformation of a ductile substratum that follows the exhumation of the flat thrust planes and the underlying clayey–marly rocks due to block-faulting and/or stream incision; and (2) where homogeneous carbonate rocks hundreds of metres thick crop out, the DSGSDs are triggered by very high relief energy and tensile stress that follow the combined actions of Quaternary block-faulting and stream deepening associated with differential erosion.

Controls of listric normal fault styles in the northern Gulf of Mexico: Insights from experimental models

Passive margins such as the Gulf of Mexico are characterized by two distinct styles of faulting. Homogenous sand/shale packages in offshore Texas mostly display basinward-dipping listric normal faults with associated rollover structures cut by synthetic and antithetic faults. The fault traces are generally long and show a linear trend. Stratigraphic packages with a ductile substratum (salt) in offshore Louisiana are characterized by basinward and landward-dipping, short arcuate faults detaching within the salt. The structures consist of a series of half-grabens, with the movement of salt from the front to the back of each fault block. Clay experimental models are used to study the controls of fault geometries in the two structural styles and their interaction to form complex transfer zones. The surface of the clay cake is laser-scanned to enable 3D visualization and accurate measurements of structures. The results suggest that within homogeneous sand-shale packages, the dips of the faults and their locations are primarily dependent on the direction of the drop down of the basal detachment along pre-existing discontinuities, with the slope of the basal discontinuity and the direction of extension providing secondary controls. On the other hand, the dips of fault systems in packages underlain by a ductile substratum are primarily controlled by the slope of the basal detachment. Therefore, the more common regional Roho systems typically form above salt sheets with initial basinward slopes, whereas counter-regional fault systems form above salt sheets with initial landward slopes. The direction of extension and the presence of small pre-existing discontinuities impart only secondary controls when ductile basal units are involved. The faults initiate at the head of ductile layer and propagate downslope. Complex transfer zones develop at the boundary of the ductile substratum due to interference between the two fault styles.

The influence of edifice slope and substrata on volcano spreading

Gravitational volcano spreading is caused by flow of weak substrata due to volcanic loading, and is now a process known to affect many edifices. The process produces extension in the upper edifice, evidenced by gräben and normal faults, and compression at the base, seen in strike–slip faults and thrusts. Where spreading is identified, host volcanoes have a range of fault densities, variable rift and gräben shapes, and different degrees of structural asymmetry. Previous studies have suggested a link between edifice shape and structure and the proportion of brittle to ductile material in the substrata or lower edifice. We study this link using refined sand cone analogue models standing on a brittle–ductile/sand–silicone substrata. Two scenarios have been investigated, the first mainly represents oceanic volcanoes with a ductile layer within the edifice (type I), where there is an outer ductile free surface. The second represents most continental volcanoes that have ductile substrata (type II). We apply the model results to natural examples and develop quantitative relationships between slope, brittle–ductile ratio fault density, spreading rate and structural style. Displacement fields calculated from stereophotogrammetry show significant differences between different slope models. We find that more faults are produced when the cone is initially steeper, or when the brittle substratum is thinner. However, the effect of the brittle layer dominates over that of slope. The strike–slip movements are found to be an essential feature in the spreading mechanism and the gräben are in fact transtensional features. Strike–slip and graben faults make a conjugate flower pattern. The structures produced are well-organised for type II edifices, but they are poorly organised for type I models. Type I models represent good analogues for oceanic volcanoes that are commonly affected by large slumps bounded by an extensional zone and lack of well-formed sector gräben. The well-observed connection between oceanic volcano rifts and large landslide-slumps is confirmed to be a consequence of spreading.

Shortening of analogue models with contractive substrata: Insights into the origin of purely landward-vergent thrusting wedge along the Cascadia subduction zone and the deformation evolution of Himalayan–Tibetan orogen

Origin of certain uncommon fold-and-thrust belts, such as the purely landward-vergent (rearward-vergent) thrusting wedge along the Cascadia subduction zone and the wide flat-toped Himalayan–Tibetan orogen, were proposed to be closely related to their ductile substrata. Results of a series of sandbox models with contractive substrata of various contracting velocities and different contracting directions, designed to study the influences of horizontal contraction of ductile substrata on deformation evolution and structural style of fold-and-thrust belts, show that purely rearward-vergent thrusting wedges can be reproduced under the condition of syn-directionally horizontal contraction of the ductile substrata, without other requirements of forward-dipping backstops, landward dipping detachments and strong wedges relative to weak bases. Therefore syn-directionally horizontal contraction of the ductile substrata can be considered an important cause for the formation of purely landward-vergent thrusting wedge along the Cascadia subduction zone. Our modeling results, together with the results of some other previous physical and numerical models, reinforce the opinion that the deformation evolution of the Himalayan–Tibetan orogen were likely governed by its weak lower crust and suggest that the north and south edges of the Himalayan–Tibetan orogen may have undergone almost synchronous Cenozoic deformation evolution owing to the existence of weak lower crust. It is also proposed that: (1) development of thrusts in the Himalayan–Tibetan orogen may not necessarily occur in a frontward sequence; (2) much later uplift deduced from Cenozoic sedimentary records in the basins along the northeastern margin of the Himalayan–Tibetan orogen does not necessarily imply much later initiation of uplift and deformation at the northeastern margin of the orogen.

Digital photogrammetry as a tool in analogue modelling: applications to volcano instability

Three techniques of digital photogrammetry have been applied successfully to laboratory analogue models to study surface displacements caused by various volcano deformation types. Firstly, side-perspective videos are used to differentiate profile displacements between cryptodome intrusion models and models deforming by ductile inner-core viscous flow. Both models show similar morphologic features including a bulged flank and an asymmetric upper graben. However, differences in displacement trajectories of the bulge crest reflect upward intrusion push contrasting with essentially downward displacement vectors of weak core models. The other two techniques use vertical views correlated automatically either as time-sequence monoscopic views or as coeval stereoscopic pairs. This exploits to a maximum the method’s potential by imaging surface displacements over the whole model. Successive monoscopic photograms, because they suffer only moderate numerical processing for topographic effect removal, can detect very small displacements occurring early in deformation processes. As illustrated by analysis of intrusion models, the monoscopic method allows prediction of fault locations and main displacement locations. It can also anticipate the principal strain directions, and separate different deformation stages. On the other hand, the stereo-photogrammetry technique, although more complicated, provides topography and volume changes, as well as pictures of surface displacements in three dimensions. Results are presented for the spreading of volcano models on a ductile substratum and viscous cored cones. We have found digital photogrammetry to be a useful tool for analogue modelling, because it provides quantitative data on surface displacements, including movement invisible to the eye, as well as topographic changes. It is a good method for investigating and comparing different deformation mechanisms. It is especially useful for interpretation of displacement patterns obtained from monitoring of natural active volcanoes. In fact, results of the methods used in the laboratory can be directly compared with field data from geodetic or photogrammetric surveys, as at Mount St. Helens in 1980.

Gravity synclines

The Scurano‐Vetto‐Carpineti syncline in the northern Apennines (Italy) is a small, isolated, bowl‐shaped syncline made of arenaceous rocks resting on a clayey substratum. This syncline was thought to have formed during lower Pliocene. Our work shows instead that it is much younger and that gravitational tectonics is important in its formation. The syncline forms a mesa‐like relief, with the top surface shaped as a “bowl”. The outer slopes of this “mesa” are very steep and are cut by strata dipping inward. All around the mesa is a zone of maximum tectonic deformation in the clayey substratum. We rule out regional tectonics in the generation of the syncline because (1) the structures follow the mesa boundary instead of the regional trends; (2) there are tectonic contacts that prove the existence of substratum clay diapirs around the mesa; and (3) there are marine abrasion planes predating the deformation. Our deductions are supported by modeling: fluid−mechanical analysis shows that when there is a brittle plate overlying a ductile substratum, gravitational deformation is inevitable, leading to a synformal structure. In addition, a silicone‐and‐sand experiment shows that the deformation generates a complex syncline with a horst‐and‐graben structure in the sand plate and with diapirs in the silicon substratum. Thus our syncline forms in a local extensional tectonic regime dominated by gravitational processes, in contrast to “common” synclines, which are the result of regional compressive tectonics. From the order‐of‐magnitude strain‐rate of the syncline we estimate the viscosity of the ductile substratum to be 1018 Pa s.

Reconstruction of sedimentary basins formed on ductile substrata

A simple reconstruction technique for sedimentary basins formed on ductile substrata is introduced. This technique is based on backstripping method and assumes the preservation of bed lengths. First, a seismic section for a sedimentary basin is interpreted and converted into a depth section. The interpreted horizons are divided by tick marks, from the basin axis out toward the basin margins, into a number of equally-spaced line segments. Then, each sediment layer is removed by moving the tick marks on the lower boundary of the layer to the corresponding tick marks on the upper boundary, thereby preserving the bed lengths. Decompaction correction is made to add the amount of expansion after removal of each sediment layer. The seafloor or bathymetry is adjusted after decompaction correction. Backstripping, decompaction correction, and seafloor adjustment are repeated until the deepest sediment layer is left in the basin. This technique is applicable to the analysis of monotonously subsiding basins formed by withdrawal of ductile substrata. It may also be applicable, to a limited extent, to intracratonic basins with simple subsidence histories.

Bench scale investigations of bacterial regrowth in drinking water distribution systems

Laboratory reactors operated under conditions relevant for drinking water distribution systems were used to examine the impact of support surface, chloramine residual, biodegradable organic matter (BOM) concentration, shear, and temperature on the growth of heterotrophic microorganisms. In terms of both heterotrophic bacterial growth-supporting and corrosion characteristics, mild steel and stainless steel/polycarbonate substrata bracket metallic pipe materials, such as ductile iron. Results of gradually increasing disinfectant dosage studies suggest that a 0.5 mg/L free or combined chlorine residual on polycarbonate surfaces, and 0.5 mg/L free chlorine or 2.0 mg/L combined chlorine residual on ductile iron substrata would be needed to reduce biofilm HPC numbers to approximately 103 CFU/cm2. Regression analysis suggests low or very low correlation between biofilm and suspended HPC numbers.

Styles of volcano-induced deformation: numerical models of substratum flexure, spreading and extrusion

The gravitational deformation of volcanoes is largely controlled by ductile layers of substrata. Using numerical finite-element modelling we investigate the role of ductile layer thickness and viscosity on such deformation. To characterise the deformation we introduce two dimensionless ratios; Π a (volcano radius/ductile layer thickness) and Π b (viscosity of ductile substratum/failure strength of volcano). We find that the volcanic edifice spreads laterally when underlain by thin ductile layers ( Π a>1), while thicker ductile layers lead to inward flexure ( Π a<1). The deformation style is related to the switch from predominantly horizontal to vertical flow in the ductile layer with increasing thickness (increasing Π a). Structures produced by lateral spreading include concentric thrust belts around the volcano base and radial normal faulting in the cone itself. In contrast, flexure on thick ductile substrata leads to concentric normal faults around the base and compression in the cone. In addition, we show that lower viscosities in the ductile layer (low Π b) lead to faster rates of movement, and also affect the deformation style. Considering a thin ductile layer, if viscosity is high compared to the failure strength of the volcano (high Π b) then deformation is coupled and spreading is produced. However, if the viscosity is low (low Π b) substratum is effectively decoupled from the volcano and extrudes from underneath it. In this latter case evidence is likely to be found for basement compression, but corresponding spreading features in the volcano will be absent, as the cone is subject to a compressive stress regime similar to that produced by flexure. At volcanoes where basement extrusion is operating, high volcano stresses and outward substratum movement may combine to produce catastrophic sector collapse. An analysis of deformation features at a volcano can provide information about the type of basement below it, a useful tool for remote sensing and planetary geology. Also, knowledge of substratum geology can be used to predict styles of deformation operating at volcanoes, where features have not yet become well developed, or are obscured.

Bench scale investigations of bacterial regrowth in drinking water distribution systems

Laboratory reactors operated under conditions relevant for drinking water distribution systems were used to examine the impact of support surface, chloramine residual, biodegradable organic matter (BOM) concentration, shear, and temperature on the growth of heterotrophic microorganisms. In terms of both heterotrophic bacterial growth-supporting and corrosion characteristics, mild steel and stainless steel/polycarbonate substrata bracket metallic pipe materials, such as ductile iron. Results of gradually increasing disinfectant dosage studies suggest that a 0.5 mg/L free or combined chlorine residual on polycarbonate surfaces, and 0.5 mg/L free chlorine or 2.0 mg/L combined chlorine residual on ductile iron substrata would be needed to reduce biofilm HPC numbers to approximately 103 CFU/cm2. Regression analysis suggests low or very low correlation between biofilm and suspended HPC numbers.

Scaled experiments of volcanic spreading

Experiments were conducted to study the spreading of volcanic constructs. Volcanoes are simulated by a sand cone, and the volcanic substratum is simulated by a sand layer (brittle substratum) overlying a silicone layer (ductile substratum). Similarity conditions between natural volcanoes and experimental prototypes led to the definition of dimensionless π numbers. Experiments determine π values which predict whether or not spreading takes place. Of particular importance are the ratio between the thickness of the brittle substratum and the height of the volcano (π2) and the brittle/ductile ratio of the substratum (π3). π2 indicates that the volcano must be large enough to “break” the substratum before spreading occurs, whereas π3 controls the style of deformation. During spreading, these dimensionless numbers change with time, reaching values that tend toward those observed for stable configurations. Experimental values are compared with those from well‐constrained natural examples. It is found that an essential requirement for volcanic spreading is the presence of a low‐viscosity layer within the substratum. Flow of the weak layer away from the excess load is responsible for the spreading. The overlying edifice displays radial intersecting grabens, due to concentric stretching, dissected summit areas; concentric zones of thrusts and folds form in the substratum around the edifice, and diapirs of the ductile substratum rise within the fault zones.

Multiple detachment levels in thrust tectonics: Sandbox experiments and palinspastic reconstruction

Abstract Sandbox models of thrust tectonics with multiple detachment levels allow alternative interpretations of fault patterns and evolution, even though they are well controlled experiments. Palinspastic reconstruction resolves these ambiguities and leads to refined interpretations. It reveals that if there are thick viscous interlayers, all non-fault related contraction can be accommodated by folding, without tectonic compaction. It also confirms the complete decoupling of faulting above and below a thick weak viscous layer, and the initial straightness and regular spacing and dips of thrusts above a decollement of low basal friction. A weak elasto-plastic interlayer or decollement merely has a lubricating effect, without complete decoupling. Many natural thrust systems are underlain by a ductile substratum and have more than one decollement level. These examples almost always exhibit a complex thrust belt geometry and are apparently formed by different thrusting events that can be difficult to interpret and to time relative to one another. Weak decollement materials and interlayers have a strong effect on the structural style of thrust belts at the moment of detachment. However, other material properties, in particular the viscosity, play a dominant role during the structural development following detachment (the overthrust phase). The objective of this study was to improve structural interpretation of data from areas of thin-skinned thrust tectonics with weak ductile detachment horizons (i.e. excluding large scale basement involvement). The approach involved: • the use of sandbox experiments to provide controlled structural models with different decollement rheologies, and • the refinement and validation of selected cross-section interpretations with palinspastic reconstruction. The result is a set of better constraints on structural interpretation of cross-sections in thrust tectonics. In natural cases, the most common weak decollement lithologies are over-pressured shales and evaporites, in particular rock salt. In our experiments we have simulated rock salt by using silicone putty (‘SP’), a viscous material that can flow slowly as a Newtonian fluid. We modelled overpressured and/or very soft clay with an oil-water emulsion (‘OWE’), which is a ductile, weak elasto-plastic, non-viscous material in which localized shear zones can form, as in any other elasto-plastic solid.

Rheological and tectonic modeling of salt provinces

Abstract Technological improvements and a more-rigorous research approach have led the petroleum industry to renew and expand its interest in physical modeling of faulting associated with halokinesis. Salt tectonics and the formation of structural hydrocarbon traps can be reproduced in models using viscous polymers for the substratum and frictional plastic granular materials for the overburden. The fault patterns simulated in these dynamically scaled models are remarkably similar to those in depth-converted seismic sections. Scale models are, therefore, useful for generating new concepts and for reconstructing the structural evolution of hydrocarbon traps. Although even unsealed models may help generating ideas, the full transfer of quantitative data requires that models be dynamically scaled to their natural prototypes. Previously, published scaling theory for salt tectonics has erroneously assumed fluid behaviour of the overburden. This paper provides a sound scaling approach for geological deformations involving brittle overburdens on ductile substrata. Rheological data from overburden rocks and salt are compiled and compared to model rheologies using dimensions of laboratory analogs and other, intrinsic material properties. Scaling approaches are compared and rules are formulated for achieving dynamic similarity in models of salt tectonics, including those with synkinematic sedimentation.

Restoration of cross-sections to constrain deformation processes of extensional terranes

Restoring balanced cross-sections constrains the possible deformation processes affecting extensional terranes with a ductile substratum, such as salt, and syndeformational sedimentation. Cross-sections are divided into fault blocks and shear domains, each of which can be undeformed with a different set of parameters. The success of restorations is judged by compatibility between the blocks and the reasonableness of the restored structures. Fault blocks in physical models restore with simple shear at about 60° dipping in the direction which removes extension, corresponding to an antithetic orientation in the hangingwalls of normal growth faults and subparallel to the fault in the footwall. Other components of the deformation are rigid rotation, translation and decompaction. Shear angles inferred from natural examples are more varied, ranging from vertical to moderately inclined, nearly always dipping in a direction that removes extension.

Tectonic deformation by rotating parallel faults: the “bookshelf” mechanism

In various tectonic environments, extension of crustal parts and shear distortion has been accommodated by the rotation of an array of parallel faults. Because of its kinematic resemblance to the tilting of a row of books on a shelf, the tectonic process may be referred to as a “bookshelf” mechanism. Its most important manifestions are cross-faulting between normal faults or parallel wrench faults, the extension of deltaic slope deposits and all forms of tilted block tectonics. Kinematically, there are two basic modes of the mechanism: (a) the “dilational” mode, where rotating faults tend to open up and possibly become paths for fluid migration and (b) the “domino” style where reclining faults remain closed. In both cases, formation and rotation of the faults may take place in one tectonic event. Fault displacements may therefore often remain small and difficult to detect. From the mechanical point of view, bookshelf operations that accommodate shearing between parallel and predominantly parallel moving rock boundaries (‘bookshelf’ mechanisms proper) have to be differentiated from those operations responding to an imposed crustal extension or shortening. After a review of geological evidence the process is addressed from a geomechanical point of view in order to determine the geological operating conditions and controlling parameters. The crucial mechanical factor in the bookshelf mechanism is the component of normal stress that acts in the direction of the imposed displacement component parallel to the driving boundary plates. In contrast to the usual assumption, the driving shear stress on the boundary plates not only depends on the normal stress on the plates, but it also varies with the value of the normal stress that acts inside the sheared material in the direction of the plate-parallel displacement component. It is the relationship between the two normal stresses that also determines the type of rotating parallel faults. Large-scale rotation of parallel normal faults in domino style (tilted block tectonics) is primarily associated with the extension of ductile substrata. From mechanical arguments and sand box experiments it has been inferred how the process and in particular, the dip direction of the parallel faults, is controlled by the way substratal extension progresses, by means of the direction of a substratal squeeze flow, the presence of a surface slope, the strength anisotropy associated with fore-set beds and by the configuration of competent boundaries of the extending overburden.

Three‐dimensional seismic inhomogeneities in the lithosphere and asthenosphere: Evidence for decoupling in the lithosphere and flow in the asthenosphere

Newly discovered three‐dimensional seismic velocity anomalies in the earth's lithosphere and asthenosphere are interpreted in terms of decoupling in the lithosphere and flow in the asthenosphere. A rough estimate of flow in the asthenosphere under California was made using Stokes' law on the basis of the size of anomalies and density contrast estimated from the velocity contrast. The result depends on the cause of velocity anomalies. If they are due to any effect which obeys Birch's law, the flow is as rapid as that of Morgan's (1972) plume driving the plate motion. On the other hand, if they are due to partial melts in thin cracks or at grain boundaries, the flow is negligible in accordance with Elsasser's (1971) model of plate tectonics driven quietly by the ridge‐push and slab‐pull in which the asthenosphere under the base of the plate is only passively dragged. The result of Cockerham and Ellsworth (1979, 1980) that the low‐velocity zone in the mantle under central California appeared to have preserved the shape of the downgoing slab for up to 10 m. y. supports Elsasser's rather than Morgan's model. The shapes of low‐velocity bodies found by Iyer and his colleagues under several geothermal areas in the western United States may be interpreted in terms of decoupling between the upper brittle lithosphere and the ductile substratum. The misalignment of the axis of the low‐velocity column from the center of the granite outcrop in the Roosevelt Hot Springs area, Utah, and the inclined low‐velocity column under the Geysers‐Clear Lake region, California, indicate a decoupled lithosphere in these areas, whereas the upright column found under Yellowstone National Park suggests an intact lithosphere there.