Mantle-derived serpentinites have been detected at magma-poor rifted margins and above subduction zones, where they are usually produced by fluids released from the slab to the mantle wedge. Here we ...show evidence of a new class of serpentinite diapirs within the external subduction system of the Calabrian Arc, derived directly from the lower plate. Mantle serpentinites rise through lithospheric faults caused by incipient rifting and the collapse of the accretionary wedge. Mantle-derived diapirism is not linked directly to subduction processes. The serpentinites, formed probably during Mesozoic Tethyan rifting, were carried below the subduction system by plate convergence; lithospheric faults driving margin segmentation act as windows through which inherited serpentinites rise to the sub-seafloor. The discovery of deep-seated seismogenic features coupled with inherited lower plate serpentinite diapirs, provides constraints on mechanisms exposing altered products of mantle peridotite at the seafloor long time after their formation.
The Dead Sea Fault (DSF) is a crustal‐scale continental transform fault separating the African and the Arabian plates. Neogene to Quaternary volcanic activity is well‐spread in Northern Israel. Yet, ...the origin of the magmas that fed the eruptions is still unpinned. Our local earthquake tomography depicts velocity distributions typical of rifting settings. At 9 km depth, a prominent high Vp/Vs anomaly marks the presence of cooling melts. We propose that protracted transtension along the DSF caused crustal thinning promoting the emplacement of magmatic bodies. Crustal emplacements of magmas in Northern Israel reconcile multiple observations, including the high geothermal gradient, the prominent magnetic anomalies and the traces of mantle‐derived fluids in the springs across the Sea of Galilee. We provide a compelling evidence for rifting in segments of the DSF and identify the potential source of magmatism that fed part of the volcanic activity of the area.
Plain Language Summary
The Dead Sea Fault (DSF) is a deep‐reaching fault separating the African and the Arabian plates. Geologically recent volcanic activity is well‐spread in Northern Israel but the origin of the magmas that fed the eruptions is yet to be found. We propose that protracted extensional motion along the DSF caused crustal thinning facilitating the emplacement of magmatic bodies in the crust. Our local earthquake tomography depicts velocity distributions typical of spreading margins. At 9 km depth, a prominent anomaly marks the presence of cooling melts. Crustal emplacements of magmas in Northern Israel reconcile multiple observations that are normally not common in sedimentary environments. The occurrence of magmas at depth would release fluids that would be compatible with the seismicity that sporadically affects the region. We provide a compelling evidence for rifting in segments of the DSF and identify the potential source of magmatism that fed part of the volcanic activity of the area. Our findings hold major implications for revisiting the natural hazard assessment of the Levant region.
Key Points
Magma emplaced along the Dead Sea Fault (DSF)
Localized rifting along the segment of the DSF
Occurrence of cooling magmas may reconcile several geophysical, geological and geochemical observations in around the Sea of Galilee
We analyzed the structure and evolution of the external Calabrian Arc (CA) subduction complex through an integrated geophysical approach involving multichannel and single‐channel seismic data at ...different scales. Pre‐stack depth migrated crustal‐scale seismic profiles have been used to reconstruct the overall geometry of the subduction complex, i.e., depth of the basal detachment, geometry and structural style of different tectonic domains, and location and geometry of major faults. High‐resolution multichannel seismic (MCS) and sub‐bottom CHIRP profiles acquired in key areas during a recent cruise, as well as multibeam data, integrate deep data and constrain the fine structure of the accretionary wedge as well as the activity of individual fault strands. We identified four main morpho‐structural domains in the subduction complex: 1) the post‐Messinian accretionary wedge; 2) a slope terrace; 3) the pre‐Messinian accretionary wedge and 4) the inner plateau. Variation of structural style and seafloor morphology in these domains are related to different tectonic processes, such as frontal accretion, out‐of‐sequence thrusting, underplating and complex faulting. The CA subduction complex is segmented longitudinally into two different lobes characterized by different structural style, deformation rates and basal detachment depths. They are delimited by a NW/SE deformation zone that accommodates differential movements of the Calabrian and the Peloritan portions of CA and represent a recent phase of plate re‐organization in the central Mediterranean. Although shallow thrust‐type seismicity along the CA is lacking, we identified active deformation of the shallowest sedimentary units at the wedge front and in the inner portions of the subduction complex. This implies that subduction could be active but aseismic or with a locked fault plane. On the other hand, if underthrusting of the African plate has stopped recently, active shortening may be accommodated through more distributed deformation. Our findings have consequences on seismic hazard, since we identified tectonic structures likely to have caused large earthquakes in the past and to be the source regions for future events.
Key Points
Overall geometry, tectonic processes, and kinematics of the subduction complex
Segmentation of the continental margin in two different lobes
Location and geometry of active faults absorbing plate motion
The Mediterranean Sea hosts two subduction systems along the convergent Africa-Eurasia plate boundary that have produced strong ground shaking and generated tsunamis. Based on historical descriptions ...and sedimentary records, one of these events, in 365 CE, impacted a broad geographical area, including tsunami evidence for distances of 700-800 km from the source event, qualifying it as a 'megatsunami'. Understanding how megatsunamis are produced, and where they are more likely, requires a better understanding of the different secondary processes linked to these events such as massive slope failures, multiple turbidity current generation, and basin seiching. Our sedimentary records from an extensive collection of cores located in distal and disconnected basins, identify turbidites which are analyzed using granulometry, elemental (XRF), micropaleontological, and geochemical data in order to reconstruct their coastal or marine source. The results show that the 365 CE basin floor sediments are a mixture of inner shelf and slope materials. The tsunami wave produced multiple far-field slope failures that resulted in stacked basal turbidites. It also caused transport of continent-derived organic carbon and deposition over basal turbidites and into isolated basins of the deep ocean. The composition of sediment in isolated basins suggests their deposition by large-scale sheet like flows similar to what has been caused by the Tohoku earthquake associated tsunamis. This is significant for rectifying and resolving where risk is greatest and how cross-basin tsunamis are generated. Based on these results, estimates of the underlying deposits from the same locations were interpreted as possible older megatsunamis.
Identification of catastrophic events recorded as resedimented deep marine deposits can be challenging because of multiple possible triggering mechanisms. This study investigates seismo‐turbidites ...(STs) deposited in the Ionian Sea as a consequence of major historical earthquakes related to the Calabrian Arc subduction system. Taking advantage of the available sedimentological reconstructions, we focused our analysis on high‐resolution X‐ray fluorescence core scanner (XRF‐CS), organic carbon and isotopic data to define geochemical signatures characterizing the ST units. The relationships between geochemical and sedimentological proxies were statistically tested using Pearson correlation and principal component analysis (PCA). Up to ∼78% of the total variance in the data set can be reduced to three principal components which identified four elemental ratio groups associated to the degree of terrestrial/coastal influence in each major depositional unit (i.e., pelagic, ST sandy stacked units, homogenites, tsunamite‐seiche laminites, and tsunamite backwash). The sample score results were evaluated together with organic carbon data in order to assess geochemical variability throughout the composite turbidite structure in different basins settings. The basal parts of the ST contain coarse‐grained sediment stacks whose sources can be traced back and sedimentary processes (surficial sediment erosion/massive slope failures) can be defined using geochemical data. The topmost parts of the STs exhibit a mixed compositional character suggesting basin‐wide processes such as seiche oscillations and tsunami wave erosion/backwashing. The application of selected XRF‐CS based elemental ratios as proxies in paleoseismological studies can help reconstruct the seismic history of a continental margin.
Plain Language Summary
Submarine paleoseismology is a widely applied method to reconstruct seismicity back in time in tectonically active regions. The Calabrian Arc in the Ionian Sea is a subduction system related to the convergence between Africa and Eurasia plates which produces uplifting coastal mountains, enhances discharge of sediments on the continental shelf and induces the frequent occurrence of earthquakes. Seismic shaking is often associated with submarine slope failures, tsunamis and deposition of seismo‐turbidites which represent more than 90% of total sedimentation in the deep Mediterranean basins. We analyzed physical and geochemical data of sediment samples through a statistical approach to identify elemental ratio groups bringing information on the difference between the interseismic and co‐seismic sedimentary processes, as well as the degree of terrestrial/coastal influence in marine sediments. The proposed statistical approach can be used for sediment characterization, in absence of other analytical data, providing fast information on sedimentary processes and ultimately on the seismic history of the continental margin.
Key Points
Analysis of elemental, organic carbon, and isotopic data from seismo‐turbidites provides information on source to sink processes
Statistical analysis of physical and geochemical variables enhances correlation of earthquake‐triggered deposits
The application of selected geochemical proxies in paleoseismological studies can be very effective in reconstructing past seismicity
The 1999 Mw 7.4 earthquake triggered a tremendous human tragedy and had a great social impact over the population of the İzmit Bay, one of the most industrialized area of Turkey. Although the ...successive environmental disasters were well documented, information on its sedimentary record is lacking. The present research aims at filling this gap, through the analysis of organic contaminants (PCBs, PAHs, and PBDEs) in a dated sediment core collected in the depocenter of the Karamürsel Basin in 2005. Profiles of total PCBs and total PAHs overlap the timing of industrialization in the area (starting in the 1960s) with values increasing as the population and the number of industrial plants grew larger. Profiles for PBDEs are in accordance with increasing urban inputs but are probably affected by processes of natural formation and post-depositional mixing. The continuous sedimentary record is interrupted at a level dating back to 1980 due to the erosion caused by the 1999 earthquake, having removed a 5–7cm thick sediment layer. Contaminant concentrations in the deepest 10–15cm of a 30cm thick seismo-turbidite unit, triggered by the 1999 event, increase with the progressive fining up and evidence massive transport of sediments from coastal, more polluted sites of the north-eastern Karamürsel shelves and shores. Additional inputs of PAHs are also evident, originating from a fire at the oil refinery that followed the shaking. The effects of the earthquake generated tsunami, its backwash fluxes and the following seiches are not uniquely displayed by each class of contaminants, and they could probably reflect successive inputs deriving from different parts of the basin that are subject to anthropogenic impacts of different nature. Concentrations measured at the top of the core are consistent with an unvaried input of pollutants in the period 1980–2005.
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•On August 17th 1999 an earthquake of Mw=7.4 hit the İzmit Bay in the Marmara Sea.•PCBs, PAHs, and PBDEs were measured in a sediment core from the Marmara Sea in 2005.•Contaminant profiles well overlap the timing of industrialization in the area.•The massive transport of contaminated sediments is evidenced in the seismo-turbidite.•A scenario of unvaried inputs of pollutants is defined for the period 1980–2005.
The Calabrian Arc (CA) subduction complex is located at the toe of the Eurasian Plate in the Ionian Sea, where sediments resting on the lower plate have been scraped off and piled up in the ...accretionary wedge due to the African/Eurasian plate convergence and back arc extension. The CA has been struck repeatedly by destructive historical earthquakes, but knowledge of active faults and source parameters is relatively poor, particularly for seismogenic structures extending offshore. We analysed the fine structure of major tectonic features likely to have been sources of past earthquakes: (i) the NNW-SSE trending Malta STEP (Slab Transfer Edge Propagator) fault system, representing a lateral tear of the subduction system; (ii) the out-of-sequence thrusts (splay faults) at the rear of the salt-bearing Messinian accretionary wedge; and (iii) the Messina Straits fault system, part of the wide deformation zone separating the western and eastern lobes of the accretionary wedge. Our findings have implications for seismic hazard in southern Italy, as we compile an inventory of first order active faults that may have produced past seismic events such as the 1908, 1693 and 1169 earthquakes. These faults are likely to be source regions for future large magnitude events as they are long, deep and bound sectors of the margin characterized by different deformation and coupling rates on the plate interface.
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•Offshore record of high-energy sedimentary events triggered by seismic activity.•Megaturbidites related to far field tsunamigenic earthquakes in the Hellenic Arc.•Megaturbidites ...triggered by Calabrian Arc subduction-type events.•Recurrence time of large plate-interface earthquakes is 2–3 ka.•The last subduction-thrust event dates back to 2.9 ka and this requires an update of the seismic hazard assessment in the region.
The Calabrian Arc subduction-rollback system hosts seismogenic faults capable of generating earthquakes exceeding magnitude 7. Since earthquakes are the result of long-term geodynamic processes, documenting seismic activity during a sufficiently long time interval is of fundamental importance for hazard scenarios. Instrumental and historical data provide critical information on seismogenesis, but they cover time periods shorter than the recurrence times of large earthquakes, especially in areas with low deformation rates such as Calabria. If onshore paleoseismological studies are fundamental to compile earthquake catalogs, they are sometime affected by the relatively poor continuity of sedimentation in the subaerial environment.
In this study we applied the paleoseismological approach to the submarine environment to reconstruct the record of high-energy sedimentary events triggered by seismic activity. We analyzed three gravity cores collected in disconnected sedimentary basins to reconstruct resedimentation processes during the Holocene, integrating inland information for a better assessment of tectonic activity and seismogenesis. Multiproxy analyses of the sedimentary record constrained by radiometric dating allowed reconstructing event stratigraphy and linking resedimented deposits to specific earthquakes.
Onshore and offshore data allow to identify large-magnitude earthquakes in the central Calabrian Arc subduction system during the Holocene, with inferred epicenters located either along normal faults onshore and/or related to the slab dynamics. The turbidite record reveals 20 major events during the last 10 ka, with sources including crustal faults in Calabria (i.e. Lakes, Rossano and Cittanova faults). Analyses of sediment samples and high-resolution seismic reflection images allowed identification of different types of resedimented deposits during the last 30–50 ka. The basin-wide occurrence of three megaturbidites/homogenites suggests they are related to megatsunamis sourced by far field earthquakes along the Hellenic Arc. Megaturbidites with a more limited spatial extent are interpreted as subduction-type events in the Calabrian Arc, while thinner seismo-turbidites record the activity of crustal structures including faults onshore. Results suggest a recurrence time of 2–3 ka for major Calabrian Arc events that needs to be considered for a reliable hazard assessment in the Mediterranean region.
The Calabrian Arc subduction system is part of the Africa–Eurasia plate boundary, is one of the most seismically active regions in the Mediterranean Sea, and has been struck repeatedly by destructive ...historical earthquakes. In this study, we investigate the effects of historical earthquakes on abyssal marine sedimentation through the analysis of the turbidite record. We collected gravity cores in tectonically controlled basins where the eastern Mediterranean pelagic sequence is interbedded with resedimented units. Textural, micropaleontological, geochemical, and mineralogical signatures reveal three turbidite events in the last millennium. We dated the turbidite sequences from two different cores using different radiometric methods, whereas the average time interval between successive turbidite beds was estimated from pelagic sediment thickness and sedimentation rates; chronologies were refined through age modeling that provided age ranges (2σ) of each turbidite bed. The results suggest that turbidite emplacement was triggered by three historical earthquakes recorded in the area (i.e., the 1908, 1693, and 1169 events); their magnitude, epicentral location, and associated tsunamis support causative faults located in the Ionian Sea. The source for all the turbidites, as inferred from their mineralogy, is the metamorphic basement outcropping in southern Calabria and/or northeastern Sicily. Turbidite composition and cable breaks for the 1908 event have been used to infer likely traveling paths and seismogenic faults in the subduction system. Our findings suggest that Ionian Sea turbidites represent more than 80% of sedimentation and may be seabed archives of paleo‐earthquakes capable of reconstructing seismicity back in time, during several earthquake cycles.
Key Points
We examine interplay between historical seismicity, mass failures and turbidites
We reconstruct chronology of earthquake triggered turbidites in the Ionian Sea
Turbidite composition has been used to reconstruct sediment source
Twelve anomalous layers, marked by a high concentration of displaced epiphytic foraminifera (species growing in vegetated substrates like the
Posidonia oceanica) and subtle grain-size changes were ...found in a 6.7
m long, fine sediment core (MS-06), sampled 2
km off the shore of the Augusta Harbor (Eastern Sicily) at a depth of 72
m, recording the past 4500
yrs of deposition. Because concentrations of epiphytic foraminifera are quite common in infralittoral zones, but not expected at −
72
m, we believe that these anomalous layers might be related to the occurrence of tsunamis causing substantial uprooting and seaward displacement of
P. oceanica blades with their benthic biota.
Our approach involved the study of geophysical data (morphobathymetry, seismic reflection, and seafloor reflectivity) and sediment samples, including X-ray imaging, physical properties, isotopic dating, tephrochronology, grain-size and micropaleontology.
Correlations between anomalous layers and tsunami events have been supported by a multivariate analysis on benthic foraminifera assemblage and dates of historical tsunami records. We found that four out of the eleven layers were embedded in age intervals encompassing the dates of major tsunamis that hit eastern Sicily (1908, 1693, and 1169) and the broader Eastern Mediterranean (Santorini at about BP 3600). One more layer, even if less distinct than the others, was also defined and may be the evidence for the AD 365 Crete tsunami.
► We explored offshore multidisciplinary approaches for paleotsunami research. ► High resolution analyses highlighted subtle anomalies in the core sediments. ► Twelve anomalous (grain-size and foraminifera) layers deposited in the past 4500 yrs. ► Anomalous layers can be related to high energy events only. ► Tsunami backwash could be the effective mechanism.
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