Modelling of interactions between dykes, inclined sheets and faults at Santorini volcano

<p>Dykes and inclined sheets are known occasionally to exploit faults as parts of their paths, but the conditions that allow this to happen are still not fully understood. Here we report field observations from a well-exposed dyke swarm of the Santorini volcano, Greece, that show dykes and inclined sheets deflected into faults and the results of analytical and numerical models to explain the conditions for deflection. The deflected dykes and sheets belong to a local swarm of 91 dyke/sheet segments that was emplaced in a highly heterogeneous and anisotropic host rock and partially cut by some regional faults and a series of historic caldera collapses, the caldera walls providing, excellent exposures of the structures. The numerical models focus on a normal-fault dipping 65° with a damage zone composed of parallel layers or zones of progressively more compliant rocks with increasing distance from the fault rupture plane. We model sheet-intrusions dipping from 0˚ to 90˚ and with overpressures of alternatively 1 MPa and 5 MPa, approaching the fault. We further tested the effects of changing (1) the sheet thickness, (2) the fault-zone thickness, (3) the fault-zone dip-dimension (height), and (4) the loading by, alternatively, regional extension and compression. We find that the stiffness of the fault core, where a compliant core characterises recently active fault zones, has pronounced effects on the orientation and magnitudes of the local stresses and, thereby, on the likelihood of dyke/sheet deflection into the fault zone. Similarly, the analytical models, focusing on the fault-zone tensile strength and energy conditions for dyke/sheet deflection, indicate that dykes/sheets are most likely to be deflected into and use steeply dipping recently active (zero tensile-strength) normal faults as parts of their paths.</p>

Origin and evolution of the lowermost lava successions at Santorini volcano (Greece): insights from major and trace element composition of rocks from the submarine caldera wall

<p>Santorini volcano in the central sector of the South Aegean volcanic arc is one of the most active and potentially dangerous magmatic systems in Europe having had twelve Plinian eruptions over the last 350 ka of which at least four eruptions were accompanied by caldera collapses. The well-known Late Bronze Age eruption (~3.6 ka<sup>A</sup>) for example is considered to rank as one of the largest eruptions since the Late Miocene.</p><p>The main focus of research thus far has been on the comparatively young and subaerial deposits, whereas older stages of volcanism have been poorly studied. Our study comprises samples from the submarine caldera flanks and gives new insights into the early evolutionary stages of Santorini volcano, contributing to a better understanding of its eruptive history and potential risks. The submarine lava successions were sampled along the inner caldera wall by a remotely operated vehicle (ROV) during R/V POSEIDON cruise 511 in 2017.</p><p>The investigated lavas can be divided into two magmatic series: a low-K basaltic series overlain by medium- to high-K series, including basaltic andesites, andesites and occasional dacites. First results of <sup>40</sup>Ar/<sup>39</sup>Ar dating reveal ages of ~250 ka for the andesites. For the presumably older basalts, no reliable age data could be obtained.</p><p>Major and trace element compositions and mineral zoning patterns suggest that fractional crystallization was the dominant process controlling magma evolution. In addition, repeated magma mixing played an important role as indicated by characteristic zonation patterns within plagioclase and clinopyroxene ante- and phenocrysts. Comparison of the major and trace element compositions with published data from subaerial deposits show a strong similarity between our lavas and the ~528-308 ka<sup>A</sup> old deposits of Peristeria volcano, a composite stratocone in the north of the volcanic field and whose subaerial deposits are found on northern Thera only<sup>B</sup>. This similarity is also supported by the Sr-Nd-Pb isotopic compositions of our lavas. Our results indicate both an extended age range of Peristeria activity and a much wider geographic distribution of its lava flows than previously recognized.</p><p> </p><p><sup>A</sup> T. H. Druitt et al. (1999), Santorini Volcano, Geological Society of London Memoir</p><p><sup>B</sup> T. H. Druitt et al. (2015): Field guide to Santorini Volcano</p>

Magma accumulation beneath Santorini volcano, Greece, from P-wave tomography

Despite multidisciplinary evidence for crustal magma accumulation below Santorini volcano, Greece, the structure and melt content of the shallow magmatic system remain poorly constrained. We use three-dimensional (3-D) velocity models from tomographic inversions of active-source seismic P-wave travel times to identify a pronounced low-velocity anomaly (–21%) from 2.8 km to 5 km depth localized below the northern caldera basin. This anomaly is consistent with depth estimates of pre-eruptive storage and a recent inflation episode, supporting the interpretation of a shallow magma body that causes seismic attenuation and ray bending. A suite of synthetic tests shows that the geometry is well recovered while a range of melt contents (4%–13% to fully molten) are allowable. A thin mush region (2%–7% to 3%–10% melt) extends from the main magma body toward the northeast, observed as low velocities confined by tectono-magmatic lineaments. This anomaly terminates northwest of Kolumbo; little to no melt underlies the seamount from 3 to 5 km depth. These structural constraints suggest that crustal extension and edifice loads control the geometry of magma accumulation and emphasize that the shallow crust remains conducive to melt storage shortly after a caldera-forming eruption.

The Late Bronze Age Eruption of Santorini Volcano and Its Impact on the Ancient Mediterranean World

The Late Bronze Age eruption of Santorini occurred 110 km north of Minoan Crete (Greece). Having discharged between 48 and 86 km3 of magma and rock debris, the eruption ranks as one of the largest of the last 10,000 years. On Santorini, it buried the affluent trading port of Akrotiri. Modern volcanological research has reconstructed the eruption and its regional impacts in detail, while fifty years of archaeological excavations have unraveled the events experienced by the inhabitants of Akrotiri during the months that led up to the eruption. Findings do not favour a direct relationship between the eruption and the decline of the Minoan civilization, although tsunamis and atmospheric effects may have weakened Cretan society through impacts on shipping, trade and agriculture.

Santorini Volcano and its Plumbing System

Santorini Volcano is an outstanding natural laboratory for studying arc volcanism, having had twelve Plinian eruptions over the last 350,000 years, at least four of which caused caldera collapse. Periods between Plinian eruptions are characterized by intra-caldera edifice construction and lower intensity explosive activity. The Plinian eruptions are fed from magma reservoirs at 4–8 km depth that are assembled over several centuries prior to eruption by the arrival of high-flux magma pulses from deeper in the sub-caldera reservoir. Unrest in 2011–2012 involved intrusion of two magma pulses at about 4 km depth, suggesting that the behaviour of the modern-day volcano is similar to the behaviour of the volcano prior to Plinian eruptions. Emerging understanding of Santorini's plumbing system will enable better risk mitigation at this highly hazardous volcano.