1,721,143 research outputs found
Regional and local tectonics at Erta Ale caldera, Afar (Ethiopia)
No full textErta Ale volcano lies along the on-shore Red Sea Rift (northern Afar, Ethiopia), separating the Nubia and Danakil plates. Erta Ale has a NNW-SSE elongated caldera, with a subvertical rim scarp, hosting a lava lake. Structural field work was aimed at defining the deformation pattern around the caldera. The caldera consists of along-rim and across-rim structures, resulting from local and regional (maximum extension ~NE-SW) stress fields, respectively. These structures cross-cut each other at high angles, suggesting that the two stress fields remain distinct, each prevailing during rifting or caldera collapse. The local along-rim extensional fractures are gravity-driven structures , that formed due to the retreat of the caldera wall after collapse, and are confined to the region of caldera subsidence. The across-rim structures are mainly located to the N and S of the caldera, where they form rift zones each accommodating a similar amount of extension (~6.3 m), but displaying different trends and extension directions. Analogue models of interacting fractures are consistent with the Southern Rift being representative of the regional fault kinematics, while the Northern Rift is a local perturbation, resulting from the interaction between two right-stepping rift segments along the Erta Ale Range
Corrigendum to ‘Structural control on magmatism along divergent and convergent plate boundaries: Overview, model, problems’ [Earth Sci. Rev. 136 (2014) 226–288]
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Evaluating fracture patterns within a resurgent caldera: Campi Flegrei
No full textUnderstanding deformation of active calderas allows their dynamics to be defined and their hazard mitigated. The Campi Flegrei resurgent caldera (Italy) is one of the most active and hazardous volcanoes in the world, characterized by post-collapse resurgence, eruptions, ground deformation and seismicity. An original structural analysis provides an overview of the main fracture zones. NW-SE and NE-SW fractures (normal or transtensive faults and extensional fractures) predominate along the rim and within the caldera, suggesting a regional control, both during and after the collapses. While the NE-SW fractures are ubiquitous in the deposits of the last 37 ka, NW-SE fractures predominate in the last 4.5 ka, during resurgence. The most recently (<4.5 ka) strained area lies in the caldera centre (Solfatara area), where the faults, with an overall ENE-WSW extension direction, seem associated with the bending due to resurgence. Solfatara lies immediately to the east of the most uplifted part of the caldera (Pozzuoli area), where domes form and culminate both on the long-term (resurgence, accompanied by volcanic activity) and short-term deformation (1982-1984 bradyseism, accompanied by seismic and hydrothermal activity). Here a consistent volcano-tectonic behaviour characterizes the short- and long-term uplifts, and only the intensity of the tectonic and volcanic activity varies, being related to varying amounts of uplift. Seismicity and hydrothermal manifestations occur during the bradyseisms, with moderate uplift, while surface faulting and eruptions occur during resurgence, with higher uplift. The features observed at Campi Flegrei are found at other major calderas, suggesting a consistent behaviour of large magmatic systems
Activating and reactivating pairs of nested collapses during caldera-forming eruptions: Campi Flegrei (Italy)
No full textA structural model for Campi Flegrei Caldera (CFC; Italy), one of Earth’s most hazardous calderas, is provided. Data from the north (outer) rim show that this formed by downsag soon after the Neapolitan Yellow Tuff emplacement (~15 ka). Campanian Ignimbrite's (CFC’s other major eruption; ~39 ka) displacement at depth suggests the activity of a buried caldera fault. During the Campanian Ignimbrite eruption the north rim was activated as caldera-bounding fault and during the Neapolitan Yellow Tuff eruption as downsag. Previously published data reveal an inner collapsed structure, formed during the Campanian Ignimbrite eruption and reactivated during the Neapolitan Yellow Tuff eruption. So, CFC consists of 2 nested depressions, both active during the Campanian Ignimbrite and Neapolitan Yellow Tuff eruptions, but with different amounts of collapse. CFC is a major example of how 2 nested calderas may: (1) form during the same collapse episode; (2) be reactivated together during caldera-forming eruptions
Transform Faults or Overlapping Spreading Centers? Oceanic ridge interactions revealed by analogue models
No full textTransform faults and Overlapping Spreading Centers (OSC) are the two most common types of interaction between oceanic ridge segments. Various factors have been proposed to control one interaction type or the other. However, measurements of several tens of transforms and OSC reveal that a simple ratio, between the total length of the interacting ridge segments (L) and their overstep (S), distinguishes transforms (L/S11). Scaled experiments have been performed to test the possibility that the initial configuration of neighbor ridges controls the interaction type. With L/S12, OSC always develop at earlier stages, independently from any ridge-parallel extension. Ridge-parallel extension, commonly observed along oceanic ridges, may accommodate the increase in length of an oceanic divergent boundary during its migration and expansion, enhancing the development of transforms under suitable L/S conditions. The matching between experimental and natural data suggests that the interaction type between oceanic ridges depends from their initial configuration and, subsequently, the overall growth of the divergent boundary
Understanding caldera structure and development: an overview of analogue models compared to natural calderas
No full textUnderstanding the structure and development of calderas is crucial for predicting their behaviour during periods of unrest and to plan geothermal and ore exploitation. Geological data, including that from analysis of deeply eroded examples, allow the overall surface setting of calderas to be defined, whereas deep drillings and geophysical investigations provide insights on their subsurface structure. Collation of this information from calderas worldwide has resulted in the recent literature in five main caldera types (downsag, piston, funnel, piecemeal, trapdoor), being viewed as end-members. Despite its importance, such a classification does not adequately examine a) the structure of calderas (particularly the nature of the caldera’s bounding faults) and (b) how this is achieved (including the genetic relationships among the five caldera types). Various sets of analogue models, specifically devoted to study caldera architecture and development, have been recently performed, under different conditions (apparatus, materials, scaling parameters, stress conditions). The first part of this study reviews these experiments, which induce collapse as a result of underpressure or overpressure within the chamber analogue. The experiments simulating overpressure display consistent results, but the experimental depressions require an exceptional amount of doming, seldom observed in nature, to form; therefore, these experiments are not appropriate to understand the structure and formation of most natural calderas. The experiments simulating underpressure reveal a consistent scenario for caldera structure and development, regardless of their different boundary conditions. These show that complete collapse proceeds through four main stages, proportional to the amount of subsidence, progressively characterized by: 1) downsag, 2) reverse ring fault; 3) peripheral downsag, 4) peripheral normal ring fault.
The second part of this study verifies the possibility that these latter calderas constitute a suitable analogue to nature and consists of a comprehensive comparison of the underpressure experiments to natural calderas. This shows that all the experimental structures, as well as their progressive development, are commonly observed at natural calderas, highlighting a consistency between models and nature. As the shallow structure of experimental calderas corresponds to a precise architecture at depth, it provides a unique key to infer the deeper structure of natural calderas: recognizing diagnostic surface features within a caldera will thus allow it to be categorized within a precise structural and evolutionary context. The general relationship between the evolutionary stage of a caldera and its d/s (diameter/subsidence) ratio allows such a quantification, with stage 1 calderas characterized by d/s>40, stage 2 by 18<d/s<40, stage 3 by 14<d/s<18 and stage 4 by d/s<14. The consistency between experiments and nature suggests that, in principle, the d/s ratio may permit to evaluate the overall structure and evolutionary stage of a caldera even when its surface structure is poorly known. The volume of erupted magma associated with caldera collapse is poorly dependent on the d/s ratio or evolutionary stage; however, the location of sin- and post-collapse volcanism may depend not only upon the amount of collapse, but also on the roof aspect ratio. As the regional tectonic control is concerned, the experiments explain the ellipticity of a part of natural calderas elongated parallel to the regional extension; the control of pre-existing structures may explain the elongation of elliptic calderas oblique or parallel to the regional structures.
The four stages adequately explain the architecture and development of the established caldera end-members along a continuum, where one or more end-members (downsag, piston, funnel, piecemeal, trapdoor) may correspond to a specific stage. While such a continuum is controlled by progressive subsidence, specific collapse geometries will result from secondary contributory factors (roof aspect ratio, collapse symmetry, pre-existing faults). These considerations allow proposing an original classification of calderas, incorporating their structural and genetic features
Coupling volcanism and tectonics along divergent boundaries: collapsed rifts from Central Afar, Ethiopia
No full textMagma along divergent plate boundaries is erupted from fissures or vents from central volcanoes, with limited impact on rift architecture. Here I summarize the geological and structural features accompanying the eruption of part of a km-thick volcanic sequence (“Stratoids”) along the Red Sea divergent boundary in Central Afar, in the area of Tendaho and Dobi grabens. More than 4700 km3/Ma (per 100 km of rift length) of magma have been produced by repeated fissure eruptions from within Tendaho Graben. The graben sides show distinctive structural features, as steep topographic gradients, coinciding with inward tilted blocks forming dominoes coeval to the emplacement of the km-thick volcanic sequence. Similar features are observed also in the Dobi Graben. This allows proposing an original mechanism, where the distinctive structure of the grabens results from the collapse at the surface induced by magma withdrawal during the emplacement of the volcanic sequence. This portion of Afar shows how rift architecture is shaped by voluminous fissure eruptions, forming collapsed rifts. These occur in continental domains, during the break-up stage (Central Afar) and in oceanic domains, where rifts narrow (East Pacific Rise). Collapsed rifts represent an end-member type of volcano-tectonic activity, where the width of the erupting reservoir balances that of the active rift zone. Along divergent boundaries, the width of the reservoir influences the style of surface deformation: a progressively higher ratio of the width of the reservoir emptied (Re) to that of the active rift zone (Ri) generates, in sequence, axial grabens, calderas and collapsed rifts
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