Caldera-forming eruptions of island volcanoes generate tsunamis by the interaction of different eruptive phenomena with the sea. Such tsunamis are a major hazard, but forward models of their impacts ...are limited by poor understanding of source mechanisms. The caldera-forming eruption of Santorini in the Late Bronze Age is known to have been tsunamigenic, and caldera collapse has been proposed as a mechanism. Here, we present bathymetric and seismic evidence showing that the caldera was not open to the sea during the main phase of the eruption, but was flooded once the eruption had finished. Inflow of water and associated landsliding cut a deep, 2.0-2.5 km
, submarine channel, thus filling the caldera in less than a couple of days. If, as at most such volcanoes, caldera collapse occurred syn-eruptively, then it cannot have generated tsunamis. Entry of pyroclastic flows into the sea, combined with slumping of submarine pyroclastic accumulations, were the main mechanisms of tsunami production.
In this study we investigate the Late Cretaceous to recent tectonic evolution of the southwestern Baltic Sea based on a dense grid of seismic reflection profiles. This area covers the Baltic Sea ...sector of the salt influenced North German Basin and its transition to the salt free Baltic Shield across the Tornquist Zone. The Upper Cretaceous to recent structural evolution is discussed by means of individual seismic sections and derived high-resolution time-structure maps of the main horizons, i.e., the Upper Cretaceous, Tertiary and Pleistocene. The Upper Cretaceous and Tertiary layers reveal numerous significant faults throughout the study area. Several of these faults propagate upwards across the unconsolidated Pleistocene sediments and occasionally penetrate the surface. The salt influenced North German Basin reveals three major fault trends: NW-SE, N-S and NNE-SSW. Several of these faults are located directly above basement (sub-salt) faults and salt pillows. The majority of these faults are trending N-S to NNE-SSW and parallel the direction of the Glückstadt Graben faults. In the salt free Tornquist Zone, we identify two major shallow fault trends, which are NW-SE and NE-SW. The majority of these faults are located above basement faults, following the direction of the Tornquist Zone. We conclude that generally basement tectonics controls activation and trends of shallow faults. If salt is present, the ductile salt layer causes a lateral shift between the sub- and supra-salt faults. Major plate reorganisation related to the Africa-Iberia-Europe convergence and the subsequent Alpine Orogeny caused reactivation of pre-existing faults and vertical salt movement in the Late Cretaceous. The change of stress orientation from NE-SW to a NW-SE during Neogene caused another phase of fault and salt tectonic reactivation. We explain that the ice-sheet loading and/or present-day stress field may have acted in combination, causing the recent tectonics and upward extension of the faults.
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•Numerous faults cut the Cretaceous and Tertiary layers throughout the SW Baltic Sea.•Several of these faults continued developing during the Pleistocene until today.•Occasionally faults penetrate the surface.•Recent tectonics resulted from ice-sheet loading and/or present-day stress field.
Located on the Hellenic Arc, the Christiana‐Santorini‐Kolumbo (CSK) rift zone represents one of the most active and hazardous volcano‐tectonic systems in the Mediterranean. Although this rift zone ...has been intensively studied, its tectonic evolution and the interplay of volcanism and tectonism are still poorly understood. In this study, we use high‐resolution reflection seismic imagery to reconstruct the opening of the rift basins. For the first time, we relate the activity of individual faults with the activity of specific volcanic centers in space and time. Our analysis shows a pre‐volcanic NNE‐SSW‐oriented paleo basin underneath the CSK volcanoes, representing a transfer zone between Pliocene ESE‐WNW‐oriented basins, which was overprinted by a NE‐SW‐oriented tectonic regime hosting Late Pliocene volcanism that initiated at the Christiana Volcano. All subsequent volcanoes evolved parallel to this trend. Two major Pleistocene tectonic pulses preceded fundamental changes in the volcanism of the CSK rift including the occurrence of widespread small‐scale volcanic centers followed by focusing of activity at Santorini with increasing explosivity. The observed correlation between changes in the tectonic system and the magmatism of the CSK volcanoes suggests a deep‐seated tectonic control of the volcanic plumbing system. In turn, our analysis reveals the absence of large‐scale faults in basin segments affected by volcanism indicating a secondary feedback mechanism on the tectonic system. A comparison with the evolution of the neighboring Kos‐Nisyros‐Yali volcanic field zone and Rhodos highlights concurrent regional volcano‐tectonic changes, suggesting a potential arc‐wide scale of the observed volcano‐tectonic interplay.
Plain Language Summary
How do regional tectonic movements and large volcanoes interact? Seismological studies indicate complex volcano‐tectonic feedback links, but, so far, information on the long‐term interactions between tectonics and volcanism is rarely available. The Christiana‐Santorini‐Kolumbo (CSK) rift zone lies in the Aegean Sea and is notorious for its devastating volcanic eruptions, earthquakes, and tsunamis. This region offers the opportunity to study volcano‐tectonic interactions over several million years. In this study, we use high‐resolution seismic imagery to reconstruct the evolution of the rift basins and the CSK volcanoes. We find that all volcanoes lie in a Pliocene transfer zone connecting extensional basins. Volcanism initiated as this older tectonic regime was intersected by a NE‐SW‐directed fault system. Subsequently, all volcanoes evolved parallel to this trend. Several distinct tectonic reorganizations occurred in the Pleistocene, which had a pronounced influence on the CSK volcanoes. In turn, our analysis indicates that the emergence of volcanism also impacted the tectonic evolution of the rift system hindering the evolution of large‐scale normal faults in the volcanic basins. The observed tectonic reorganizations seem to reflect major changes in the stress regime of the Hellenic Arc, potentially also affecting adjacent volcanic centers whose volcano‐tectonic evolution is only poorly constrained so far.
Key Points
We reconstruct the volcano‐tectonic evolution of the Christiana‐Santorini‐Kolumbo rift zone using multichannel seismic data
The overprint of a Pleistocene NE‐SW striking fault system on a Pliocene E‐W oriented system initiated the emergence of volcanism
Regional tectonics had a primary control on the volcanic plumbing system, while magmatism had a secondary influence on the tectonic system
The S. Jorge Island in the Azores lies on a peculiar setting, the southern shoulder of the Terceira Rift (TR), which raises a series of questions that we address in this study. We first established ...the main volcanic stratigraphy by recognizing, in the field, the main unconformities/discontinuities and their meaning (major erosion surfaces and faults), then we collected critical samples, and finally dated them by K/Ar to calibrate the stratigraphy and the age of inferred large-scale flank collapses. Based on field, geochronological and marine geophysical data: (1) we found much older rocks in S. Jorge than in previous studies (ca. 1.85 Ma), and established a new volcanic stratigraphy (from bottom to top): Old Volcanic Complex (ca. 1.9–1.2 Ma), cropping out in the eastern third of the island; Intermediate Volcanic Complex (ca. 0.8–0.2 Ma), cropping out in the western two thirds of the island and separated from the underlying complex by a major fault; Young Volcanic Complex (<ca. 0.1 Ma), unconformable on both older units. (2) We discuss the most probable mode of destruction of the successive volcanic complexes, and we conclude that a major landslide occurred between ca. 1.2 and 0.8 Ma, which was responsible for the major lateral discontinuity between the Old and the Intermediate Volcanic complexes. (3) We use previous palaeomagnetic data and new seismic data to evaluate the effects of uplift of the TR's southern shoulder on S. Jorge Island, and conclude that south-westward tilting of the oldest lava flows occurred between ca. 1.2 and 0.8 Ma due to rotation of the TR's southern shoulder during uplift. (4) Finally, we used all data to constrain the age of TR initiation to be ca. 1 Ma in this sector of the TR.
•New K/Ar ages indicate that the S. Jorge Island is older than 1.8 Ma.•Most of the early island was destroyed by a large-scale flank collapse.•The early island was tilted by an angle coincident with the tilt of the underlying oceanic crust.•Elastic rebound of the Terceira Rift, Azores Triple Junction, is responsible for the ca. 13° tilt.•We constrain the age of the Terceira Rift to ca. 1 Ma at this location of the rift.
In this study, we investigate the impact of Late Cretaceous to Neogene plate tectonics and Pleistocene ice load on the post-Jurassic strata above the Waabs salt wall within the Eastern Glückstadt ...Graben by integrating reflection seismic, parametric sub-bottom profiler and well data. Previous studies showed that a collapse graben developed above the salt during several tectonic pulses. However, due to the lack of age constrains, the relationship between local salt tectonics and variations of the regional stress field caused by plate tectonics was just little constrained. Here, we introduce an inter-Cenozoic stratigraphy enabling us to infer and date three major salt tectonic phases. During the Late Cretaceous to Eocene, Africa–Iberia–Europe convergence and subsequent Pyrenean orogeny strata above the salt wall were pushed upwards and faulted. Thickness variations of the Upper Cretaceous and Eocene strata imply at least two pulses for this upward push. The second phase includes the major graben collapse in the Neogene, when the stress regime changed from a compressional to an extensional regime. The third phase is a period of locally differing tectonic reactivation likely caused by ice sheet loading and unloading during the Quaternary. Based on time-isochore and time-structure maps, we elucidate how this last glacial phase of salt tectonic movement likely formed the present day Mittelgrund shoal within the Eckernförde Bay. The spatial correlation between shallow faults and freshwater seepage implies further a causal relationship between fluid migration and faulting.
New bathymetric and seismic reflection data from the Santorini–Amorgos Tectonic Zone in the southern Cyclades have been analysed and a description of the morphology and tectonic structure of the area ...has been presented. The basins of Anhydros, Amorgos and Santorini–Anafi have been distinguished together with the intermediate Anhydros Horst within the NE-SW oriented Santorini–Amorgos Tectonic Zone which has a length of 60–70km and a width of 20–25km. The basins represent tectonic grabens or semi-grabens bordered by the active marginal normal faults of Santorini–Anafi, Amorgos, Ios, Anhydros and Astypalaea. The Santorini–Anafi, Amorgos and Ios marginal faults have their footwall towards the NW where Alpine basement occurs in the submarine scarps and their hangingwall towards the southeast, where the Quaternary sediments have been deposited with maximum thickness of 700m. Six sedimentary Units 1–6 have been distinguished in the stratigraphic successions of the Santorini–Anafi and the western Anhydros Basin whereas in the rest area only the upper four Units 3–6 have been deposited. This shows the expansion of the basin with subsidence during the Quaternary due to ongoing extension in a northwest-southeast direction. Growth structures are characterized by different periods of maximum deformation as this is indicated by the different sedimentary units with maximum thickness next to each fault. Transverse structures of northwest-southeast direction have been identified along the Santorini–Amorgos Tectonic Zone with distinction of the blocks/segments of Santorini, Anhydros/Kolumbo, Anhydros islet and Amorgos. Recent escarpments with 7–9m offset observed along the Amorgos Fault indicate that this was activated during the first earthquake of the 7.5 magnitude 1956 events whereas no recent landslide was found in the area that could be related to the 1956 tsunami.
•Description and mapping of active tectonic faults & resulting grabens in the SATZ•History of extension and subsidence in the Santorini–Amorgos Tectonic Zone•Along strike differences and segmentation due to transverse structures in the SATZ•Identification of the activated fault during the first 1956M7.5 Amorgos earthquake•Creation of fault scarps when slip rate is much higher than sedimentation rate.
The southern Baltic Sea is a peculiar area, where the Sorgenfrei‐Tornquist Zone (STZ), stretching from Bornholm into the North Sea, connects to the Teisseyre‐Tornquist Zone (TTZ) that continues SE up ...to the Black Sea. In this study, we show the structure and evolution of this controversially debated area, both on crustal and basin scale, by using three seismic reflection profiles combined with 2‐D potential field data. The results demonstrate that the southern Baltic Sea is underlain by a thick crust of the East European Craton with a Moho depth in the range of 38–42 km. The overall crustal architecture is shaped by three phases of localized stretching in the early Paleozoic, Devonian‐Carboniferous, and Permian‐Mesozoic. The most spectacular feature of the southern Baltic Sea is a zone of thick‐skinned compressional deformation produced by Late Cretaceous‐early Paleogene inversion, including a system of thrusts and back thrusts penetrating the entire crust in an 80–90 km wide inversion zone. ENE‐vergent thrusts are traced from the top of the Cretaceous down to the Moho and they are accompanied by back thrusts of opposite vergence, also reaching the Moho. Inversion tectonics resulted in the uplift of a block of cratonic crust as a pop‐up structure, bounded by thrusts and back thrusts, and the displacement of the Moho within the STZ and TTZ. The similar mechanism of intra‐cratonic inversion was recognized for the Donbas Foldbelt in eastern Ukraine, and it may be characteristic of rigid cratons, where deformation is localized in a few preexisting zones of weakness.
Key Points
The southern Baltic Sea is underlain by thick crust of the East European Craton with a Moho depth in the range of 38–42 km
Late Cretaceous‐Paleocene inversion developed in a 80–90 km wide zone along the preexisting depocenters forming sites of crustal weakness
Inversion zone includes a system of thrusts and backthrusts penetrating the entire crust and forming a crustal‐scale pop‐up structure
The Dom João de Castro seamount in the Hirondelle Basin (Azores) is a central volcano on the ultraslow diverging Terceira Rift axis. The combination of structural and geochemical data provides ...insights into the evolution of central volcanoes in oceanic rift systems above the Azores melting anomaly. The orientation of fault scarps and volcanic structures at D. João de Castro and the adjacent Castro fissure zone indicate that the regional SW‐NE extending stress field dominates the morphology of the NW Hirondelle Basin. The regional tectonic stress field controls the crustal melt pathways and leads to dike emplacement along fissure zones and the prevalent eruption of mafic lavas. The occurrence of mafic to felsic lavas at D. João de Castro gives evidence for both a deep and a shallow crustal melt reservoir generating a subordinate local stress field at the seamount. New Sr‐Nd‐Pb isotope data along with incompatible trace element ratios indicate that D. João de Castro and the Castro Ridges originated from similarly heterogeneous mantle source but did not form simultaneously. Our new model implies that central volcanoes along the Terceira Rift form by the growth of volcanic ridges and transitioned into circular edifices after magmatic systems generate local changes in the regional lithospheric stress field. The geometry of D. João de Castro and other magmatic systems along the Terceira Rift combined with the alkaline nature of the erupted lavas, and the large lithosphere thickness indicates that young oceanic rifts are more similar to continental rifts rather than mid‐ocean ridges.
Plain Language Summary
Dom João de Castro seamount is a large submarine volcano located in the submarine Hirondelle Basin in the Azores archipelago. The Hirondelle Basin is formed as a result of extensional forces in the oceanic crust along the Azorean Terceira Rift that causes rifting of the Eurasian and Nubian plates. The presence of the D. João de Castro volcano and several elongated volcanic ridges inside the basin shows that the extensive magmatic activity in the Azores contributes to the opening of the basin. By quantifying the orientations of the tectonic and volcanic structures in the basin, it can be shown that the formation is controlled by a dominant SW‐NE directed extensional stress combined with extensive magmatic activity. Based on combined structural and geochemical observations, we conclude that the D. João de Castro seamount formed from the growth of elongated volcanic ridges and transitioned into a circular edifice after a magma system generates a local change in the crustal stress field. The geometry and geochemical composition of volcanic rocks from the D. João de Castro magmatic system, as well as other magmatic systems along the Terceira Rift are more similar to continental rift systems rather than oceanic spreading centers.
Key Points
D. João de Castro seamount in the Terceira Rift, Azores is influenced by a SW‐NE regional transtensional and a local radiating stress field
Structural, seismic, and geochemical data imply formation by the growth of volcanic ridges along with local stress field changes
The geometry, chemistry, and rifting rates of the Terceira Rift are more comparable to continental rifts rather than mid‐ocean ridges
The Maldives, a 900 km north-south trending paired chain of atolls that surround a ca. 100 km wide central basin (the Inner Sea) represent a giant isolated carbonate platform in the Indian Ocean. New ...reflection seismic, multibeam and acoustic Doppler current profiler data show that its development exhibits a distinct change from sea-level to mainly bottom current-controlled sedimentation at the end of the Middle Miocene. The post-Middle Miocene intra-basinal fill consists of 9 mega units that have been formed and shaped by strong bottom currents, entering the Inner Sea between the inter-atoll passages.
At the end of the Middle Miocene the Maldives represent a rimmed platform with the Inner Sea forming an empty bucket. A connection to the Indian Ocean existed only in the northeastern part of the platform, namely the Kardiva Channel. Probably concomitant with the onset of the Indian Monsoon, the current pattern around the Maldives changed significantly resulting in the formation of channels in the rimmed platform margin. The latter process was encouraged by local drowning of large platform parts and additionally, by bottom current forced submarine erosion that widened the channels to passages disintegrating the platform into separated atolls of different sizes. The reworked material was dumped into the Inner Sea where it formed large contourite fans. Along-slope bottom currents entering the Inner Sea from the north further redistributed the material leading to a southward shift of these sedimentary bodies. In this way the Inner Sea was filled from west to east (mega units 1–5). Starting with mega unit 6 the opening of a southern gateway introduced a prominent northward flow of bottom waters in the Inner Sea. It leads to the deposition of giant elongated drifts at the eastern flank of the basin, filling it from east to west. Because the current swept away most of the material around the atolls, the system was not able to prograde and the steady subsidence was compensated by aggradation. Accordingly, a fundamental characteristic of a current-controlled carbonate system is that aggradation occurs irrespective of platform sediment export or differently worded the rate of change in accommodation is always positive and greater than the rate of sediment supply.
ADCP measurements in the winter monsoon season show two water masses in the Inner Sea, one at the surface 0–150 m and a second below it that reaches its sea-floor. By the use of the grain size distribution of ODP leg 716 located in the Inner Sea as proxy for the flow intensity of the bottom water, we recognized an inverse proportional relationship to the monsoon strength from late Upper Miocene to present.
Some of the current-controlled deposits form sigmoidal prograding clinoforms, preferably at the edges of the atolls adjacent to the passages, where the velocity of the current entering the Inner Sea is high. The anatomy of these prograding wedges is similar to sea-level controlled clinoforms described for large isolated carbonate platforms in the sequence stratigraphic concept; however our results document, that they are not influenced by sea-level fluctuations. Because of the high current velocities involved in their deposition, they predominately consist of coarse sand sized carbonate fragments exhibiting large pore space and high permeability. Therefore these drift deposits comprise a possible exploration target.
► In this study we investigate the sedimentation processes of an isolated carbonate platform. ► Since late Middle Miocene sedimentation of the Maldives is mainly bottom current-controlled. ► Sigmoidal prograding clinoforms at the platform margins are drift bodies. ► Bottom currents favour platform aggradation and hinder its progradation.
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