Weak Substratum Research Articles

Volcanic spreading forcing and feedback in geothermal reservoir development, Amiata Volcano, Italia

We made a stratigraphic, structural and morphologic study of the Amiata Volcano in Italy. We find that the edifice is dissected by intersecting grabens that accommodate the collapse of the higher sectors of the volcano. In turn, a number of compressive structures and diapirs exist around the margin of the volcano. These structures create an angular drainage pattern, with stream damming and captures, and a set of lakes within and around the volcano. We interpret these structures as the result of volcanic spreading of Amiata on its weak substratum, formed by the late Triassic evaporites (Burano Anhydrites) and the Middle-Jurassic to Early-Cretaceous clayey chaotic complexes (Ligurian Complex). Regional doming created a slope in the basement facilitating the outward flow and spreading of the ductile layers forced by the volcanic load.We model the dynamics of spreading with a scaled lubrication approximation of the Navier Stokes equations, and numerically study a set of solutions. In the model we include simple functions for volcanic deposition and surface erosion that change the topography over time. Scaling indicates that spreading at Amiata could still be active. The numerical solution shows that, as the central part of the edifice sinks into the weak basement, diapiric structures of the underlying formations form around the base of the volcano. Deposition of volcanic rocks within the volcano and surface erosion away from it both enhance spreading. In addition, a sloping basement may constitute a trigger for spreading and formation of trains of adjacent diapirs. As a feedback, the hot hydrothermal fluids decrease the shear strength of the anhydrites facilitating the spreading process.Finally, we observe that volcanic spreading has created ideal heat traps that constitute todays' exploited geothermal fields at Amiata. Normal faults generated by volcanic spreading, volcanic conduits, and direct contact between volcanic rocks (which host an extensive fresh-water aquifer) and the rocks of the geothermal field, constitute ideal pathways for water recharge during vapour extraction for geothermal energy production. We think that volcanic spreading could maintain faults in a critically stressed state, facilitating the occurrence of induced and triggered seismicity.

Relationships between volcano gravitational spreading and magma intrusion

Volcano spreading, with its characteristic sector grabens, is caused by outward flow of weak substrata due to gravitational loading. This process is now known to affect many present-day edifices. A volcano intrusive complex can form an important component of an edifice and may induce deformation while it develops. Such intrusions are clearly observed in ancient eroded volcanoes, like the Scottish Palaeocene centres, or in geophysical studies such as in La Reunion, or inferred from large calderas, such as in Hawaii, the Canaries or Galapagos volcanoes. Volcano gravitational spreading and intrusive complex emplacement may act simultaneously within an edifice. We explore the coupling and interactions between these two processes. We use scaled analogue models, where an intrusive complex made of Golden syrup is emplaced within a granular model volcano based on a substratum of a ductile silicone layer overlain by a brittle granular layer. We model specifically the large intrusive complex growth and do not model small-scale and short-lived events, such as dyke intrusion, that develop above the intrusive complex. The models show that the intrusive complex develops in continual competition between upward bulging and lateral gravity spreading. The brittle substratum strongly controls the deformation style, the intrusion shape and also controls the balance between intrusive complex spreading and ductile layer-related gravitational spreading. In the models, intrusive complex emplacement and spreading produce similar structures to those formed during volcano gravitational spreading alone (i.e. grabens, folds, en echelon fractures). Therefore, simple analysis of fault geometry and fault kinetic indicators is not sufficient to distinguish gravitational from intrusive complex spreading, except when the intrusive complex is eccentric from the volcano centre. However, the displacement fields obtained for (1) a solely gravitational spreading volcano and for (2) a gravitational spreading volcano with a growing and spreading intrusive complex are very different. Consequently, deformation fields (like those obtained from geodetic monitoring) can give a strong indication of the presence of a spreading intrusive complex. We compare the models with field observations and geophysical evidence on active volcanoes such as La Reunion Island (Indian Ocean), Ometepe Island (Nicaragua) and eroded volcanic remnants such as Ardnamurchan (Scotland) and suggest that a combination between gravitational and intrusive complex spreading has been active.

The interaction between volcanoes and strike-slip, transtensional and transpressional fault zones: Analogue models and natural examples

Regional strike-slip faulting can control magma movements, deform volcanoes and may destabilise their flanks. The aim of this study is to address this problem by comparing two natural examples, Basse Terre Island volcanoes, Lesser Antilles and Maderas volcano, Nicaragua, with analogue experiments. The field and remote sensing analyses of their structures reveal that Guadeloupe volcanoes, which developed in a 145°-striking sinistral transtensional fault zone, are dominantly fractured in a 090°–120° direction, which is parallel to the maximum principal horizontal stress and to the elongation direction of the summit graben of analogue models. This graben is bordered by the Sigmoid-I fault, or Y shear structure, and has facilitated the formation of the Beaugendre and Vieux-Habitants valleys by faulting, erosion or collapse. This structure has also influenced the injection of dykes and the transport of hydrothermal fluids. The comparison of Maderas volcano with the analogue models confirms that the volcano has developed parallel to a 135°-striking dextral transtensional fault zone and is also gravitationally spreading over a weak substratum. This study illustrates how regional strike-slip faulting and gravitational loading combine to produce a clear set of structures within volcanic edifices, which control the location of intrusive zones, hydrothermal activity and collapse directions.

Field evidence for flank instability, basal spreading and volcano-tectonic interactions at Mt Cameroon, West Africa

The Mt Cameroon volcano is the highest and most active volcano of the Cameroon Volcanic Line. Little geological information is available for improving the understanding of the structure of this large volcanic system and its relationship to regional tectonics. After reviewing the tectonic evolution of the region, the analysis of a Digital Elevation Model and results from a field campaign dedicated to mapping geological structures in the summit area and at the SE base of Mt Cameroon are presented. Mt Cameroon is a lava-dominated volcano with long steep (over 30°) flanks. It is elongate parallel to its well defined rift zone. The summit plateau is bordered by 10 m high cliffs formed by summit subsidence along normal faults. Geological profiles were measured along rivers cutting through a topographic step at the SE base of Mt Cameroon. This step is associated with deformed Miocene sediments from the Douala basin that are overlain by volcanic products. Weak sediments of this area are deformed by 050°–060° and 020°–030° trending asymmetrical folds verging toward the SE, and thrusts faults related to the spreading of the volcano over its mechanically weak substratum. Combined remote sensing and field observations suggest that spreading is accommodated by summit subsidence and flanks sliding. Both slow spreading movements and catastrophic collapses of the steep flanks are interpreted to result from complex interactions between the growing edifice, repeated dyke intrusions, the weak sedimentary substratum and tectonic structures.

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.

Gravitational spreading causes en-echelon diking along a rift zone of Madeira Archipelago: an experimental approach and implications for magma transport

Many volcanic rift zones show dikes that are oriented oblique rather than parallel to the morphological ridge axis. We have evidence that gravitational spreading of volcanoes may adjust the orientation of ascending dikes within the crust and segment them into en-echelon arrays. This is exemplified by the Desertas Islands which are the surface expression of a 60 km long submarine ridge in southeastern Madeira Archipelago. The azimuth of the main dike swarm (average = 145°) deviates significantly from that of the morphological ridge (163°) defining an en-echelon type arrangement. We propose that this deviation results from the gravitational stress field of the overlapping volcanic edifices, reinforced by volcano spreading on weak substratum. We tested our thesis experimentally by mounting analogue sand piles onto a sand and viscous PDMS substratum. Gravitational spreading of this setup produced en-echelon fractures that clearly mimic the dike orientations observed, with a deviation of 10°–32° between the model’s ridge axis and that of the main fracture swarm. Using simple numerical models of segmented dike intrusion we found systematic changes of displacement vectors with depth and also with distance to the rift zone resulting in a complex displacement field. We propose that at depth beneath the Desertas Islands, magmas ascended along the ridge to produce the overall present-day morphology. Above the oceanic basement, gravitational stress and volcano spreading adjusted the principal stress axes’ orientations causing counterclockwise dike rotation of up to 40°. This effect limits the possible extent of lateral dike propagation at shallow levels and may have strong control on rift evolution and flank stability. The results highlight the importance of gravitational stress as a major, if not dominant factor in the evolution of volcanic rift zones.

Experiments on rift zone evolution in unstable volcanic edifices

Large ocean island volcanoes frequently develop productive rift zones located close to unstable flanks and sites of older major sector collapses. Flank deformation is often caused by slip along a décollement within or underneath the volcanic edifice. We studied how such a stressed volcanic flank may bias the rift zone development. The influence of basal lubrication and lateral flank creep on rift development and rift migration is still poorly constrained by field evidence; here our analog experiments provide new insights. We injected colored water into gelatin cones and found systematic orientations of hydro-fractures (dikes) propagating through the cones. At the base of the cone, diverse friction conditions were simulated. By variation of the basal creep conditions we modeled radial dike swarms, collinear rift zones and three-armed rift systems. It is illustrated that a single outward-creeping flank is sufficient to modify the entire rift architecture of a volcano. The experiments highlight the general unsteadiness of dike swarms and that the distribution and alteration of weak substratum may become a major player in shaping a volcano’s architecture.

A typical “calanchi” landscape on the Eastern Apennine margin (Atri, Central Italy): geomorphological features and evolution

The surroundings of the town of Atri (Abruzzo Region, Central Italy), situated about 10 km from the Adriatic shoreline, can be considered as representative of most part of the Italian seaward peninsular landscape. The subject of this research, the calanchi landscape, often generally considered as “badlands” by non-Italian authors, is, in fact, made up of many singular landforms, each of which ( calanco) corresponds to a more or less extended hydrographic unit. The dynamics are mainly the result of the action of the particularly aggressive climatic factors (such as the Mediterranean rainfall regime) on weak substrata (clay, silt and sand of the Plio–Pleistocene marine cycles). The morphogenetic activity is not only limited to channel erosion, but it is also due to repeated superficial slides, so that it should in fact be better to consider the calanco as the result of a “combined erosion” process ( sensu [Zachar, D., 1982. Soil Erosion. Elsevier, Amsterdam]). Since the mechanism regulating this process remains unclear, this work wants to offer a contribution to this research field. Multitemporal geomorphological analysis has helped us to distinguish the different areas in which the calanchi evolution has been more or less important in the past. In such a way, it has been possible to characterize the landscape evolution as a function of the parameters that influence soil loss, especially in areas, such as that of Atri, where calanchi-like landforms are highly developed. Moreover, this analysis, coupled with the processing of 40 years rainfall data, also taking into account the concurrent land use changes, has clearly shown that the areas affected by calanchi erosion on the Eastern Apennine margin, still tectonically active, are progressively diminishing.

Finite-element analyses of thrust tectonics: Computer simulation of detachment phase and development of thrust faults

The mechanics of thrust fault formation in rectangular and wedge-shaped, homogeneous thrust blocks are examined using finite-element techniques incorporating elasto-plastic material behaviour. Compression is simulated by applying an incremental end displacement parallel to the top of the horizontal or inclined basement. Shear zones (or faults) can form where the material experiences plastic strain softening. These models simulate that thrusts initiate in sequences that are “pushed from behind”, by some tectonic force, over a decollement horizon. In all models, the formation of thrusts is preceded by the formation of a basal slip plane near the base of the block or in a weak substratum. The front of this slip plane does not control the location of thrusts formed subsequently. In the rectangular block, a thrust forms at the pushed end. In the tapered wedge, drained as well as undrained, a fault system composed of a thrust and a backthrust forms. It is argued that the geometry of the block is the main controlling feature for the formation of thrusts within the tapered wedge. The shape of reverse faults depends on the magnitude of shear stresses acting along the basal slip plane. The higher the shear stress, the more pronounced the curvature. In the drained tapered wedge, the intersection of the backthrust with the basal slip plane appears to control the location of the thrust. Under undrained conditions, the backthrust forms subsequent to the thrust and acts only as an accommodation feature.

Deep-Focus Earthquakes and Their Geological Significance

About 10 per cent of all major earthquakes are now known to originate below the "crust" of the earth at depths ranging up to 700 km. After a brief resume of the evidence for the existence of deep earthquakes, the paper describes the environment in which they occur under the headings of structure and composition, density and pressure, rigidity and bulk modulus, temperature, viscosity, and strength. The characteristics of deep earthquakes are then described under the headings of distribution (geographic and depth), energy released, aftershocks, periodicities, correlations, patterns of initial motion at the surface, and stress and strain near the focal region. Analysis of these data leads first to the conclusion that the type of process involved is probably a normal fracturing response to external stress differences, although other possibilities are considered. The possible known sources of stress appear to be inadequate to function as primary causes of deep earthquakes but may be important as "triggers." The meaning of strength is analyzed, and the conclusion is reached that seismic evidence probably points to an increase in strength from the surface of the earth downward at least to the core. The paper concludes that the concept of a weak substratum, within which flow results from extremely slight stress differences, does not appear to be valid.