Mediterranean tectonics since the Lower Cretaceous has been characterized by a multiphase subduction and collision history with temporally and spatially variable, small‐scale plate configurations. A ...new shear wave velocity model of the Mediterranean upper mantle (MeRE2020), constrained by a very large set of over 200,000 broadband (8–350 s), interstation, Rayleigh wave, phase velocity curves, illuminates the complex structure and fragmentation of the subducting slabs. Phase velocity maps computed using these measurements were inverted for depth‐dependent, shear wave velocities using a stochastic particle‐swarm‐optimization (PSO) algorithm. The resulting three‐dimensional (3‐D) model makes possible an inventory of slab segments across the Mediterranean. Fourteen slab segments of 200–800 km length along‐strike are identified. We distinguish three categories of subducted slabs: attached slabs reaching down to the bottom of the model; shallow slabs of shorter length in downdip direction, terminating shallower than 300 km depth; and detached slab segments. The location of slab segments are consistent with and validated by the intermediate‐depth seismicity, where it is present. The new high‐resolution tomography demonstrates the intricate relationships between slab fragmentation and the evolution of the relatively small and highly curved subduction zones and collisional orogens characteristic of the Mediterranean realm.
Key Points
High‐resolution Rayleigh wave tomography resolves the configuration of slab segments in the Mediterranean upper mantle down to 300 km depth
Slab fragmentation, gaps, and tears are detected and mapped
Highly fragmented slabs subducted over the last 30 Myr reflect the complexity of the recent Mediterranean tectonics
The application of Rietveld texture analysis (RTA) to time‐of‐flight (TOF) neutron diffraction data allows complex materials with many diffraction peaks to be investigated, for example, rocks ...composed of different minerals. At the recently upgraded SKAT texture diffractometer at the JINR in Dubna (Russia), which provides three alternative multidetector systems, resolution and accessible range of lattice d spacings can be adapted to sample requirements. In order to infer the optimum experimental setup and the reliability of texture estimates from complicated TOF patterns, the influence of counting statistics and various spectral resolutions on texture deconvolution was investigated. Comparing the results obtained at different resolutions and from different sections of the d patterns indicates that the textures of a four‐phase sample can be determined, but using a section at small d spacings with a larger number of peak overlaps leads to smoother textures. A complex seven‐phase sample shows orientation differences in addition to the smoothing effect. Weak textures and textures of the minor rock constituents are inaccurate owing to multiple peak overlaps. Consequently, good resolution is essential for RTA on such samples. Grid thinning tests confirmed that no more than 150 diffraction spectra are needed to characterize the texture of a monomineralic sample, and approximately 350 spectra are sufficient for a four‐phase sample. The irregular grid point arrangement caused by the SKAT geometry has no negative consequences.
The Walvis Ridge is one of the major hotspot trails in the South Atlantic and a classical example for volcanic island chains. Two models compete about the origin of the ridge: It is either the result ...of a deep mantle plume or active fracture zones above mantle inhomogeneities. Among other things crustal information is needed to constrain the models. Here, we provide such constraint with a 480km long P-wave velocity model of the deep crustal structure of the eastern Walvis Ridge at 6° E. According to our data the Walvis Ridge stretches across the Florianopolis Fracture Zone into the Angola Basin. Here, we observe a basement high and thick basaltic layers covering the oceanic crust and the fracture zone. We found two crustal roots along the profile: one is located beneath the ridge crest, the other one beneath the northern basement high in the Angola Basin. The crustal thickness reaches 18km and 12km and the lower crustal velocities are 7.2km/s and 7.4km/s, respectively. The bathymetric expression of the ridge along the profile is less pronounced than closer to shore, which is mainly attributable to the absence of a thick layer of volcanic debris, rather than to reduced crustal thickness below the basement surface. Therefore, this part of the ridge was never or only briefly subaerially exposed. The crustal structure suggests that the ridge and the fracture zone formed independently of each other. The oceanic crust north of the fracture zone, which is buried underneath the basalt layer, is younger than the reconstructed age of hotspot volcanism of the Walvis Ridge. We interpret these structures north of the fracture zone to be at least partly a product of late stage volcanism.
•Late stage volcanism covers old oceanic crust north of the Florianopolis Fracture Zone.•No influence of fracture zone on formation of Walvis Ridge at 6° E•Walvis Ridge at 6° E erupted in deep water environment.
We have analysed strain and flow kinematics in the footwall of the South Cyclades Shear Zone (SCSZ), an important tectonic boundary within the Attic-Cycladic Crystalline Complex exposed on Ios ...Island, Cyclades, Aegean Sea. Coarse-grained augen gneisses in the basement unit flooring the SCSZ and forming a metamorphic core complex are excellently suited to measure finite strain using the Fry method and estimate the vorticity number (
W
k
) of flow with the “blocked-object” method. The results show that Oligo-Miocene exhumation of the basement unit during extension brought approximately 70 % N–S crustal stretching and up to 40 % subvertical shortening in a plane strain environment (
k
= 0.99). Linear down-section strain decrease constrains a zone of contact deformation of the SCSZ of about 1.5 km thick. Kinematic vorticity number estimates suggest little deviation from pure shear (
W
k
= 0.26). Finite strain and
W
k
are not correlated, indicating that the Ios basement and the overlying cover units were stretched compatibly. While the SCSZ is a localized zone of high strain, net displacement, however, may be restricted to about ten kilometres. This has important repercussions on large-scale tectonic models for extension in the Aegean.
The Eclogite Zone, of the Tauern Window is an exhumed subduction channel comprising eclogites with different grades of retrogression in a matrix of high-pressure metasediments. The rocks were exposed ...to 600 °C and 20–25 kbars, and then retrogressed during their exhumation, first under blueschist facies and later under amphibolite facies metamorphism. To gain insights into the deformation within the subduction channel during subduction and exhumation, both fresh and retrogressed eclogites, as well as the surrounding metasediments were investigated with respect to their deformation microstructures and crystallographic preferred orientations (CPOs). Pristine and retrogressed eclogites show grain boundary migration and subgrain rotation recrystallization microstructures in omphacite. A misorientation axes analysis reveals the activity of complementary deformation mechanisms including grain boundary sliding and dislocation creep. The omphacite CPOs of the eclogites correspond to dominant SL-fabrics characteristic of plane strain deformation, though there are local variations towards flattening or constriction within the paleosubduction channel. The glaucophane CPOs in retrogressed eclogites match those of omphacite, suggesting that a constant strain geometry persisted during exhumation at blueschist facies conditions. Plastic deformation of the host high-pressure metasediments outlasted that of the eclogites, as indicated by white mica fabrics and quartz CPO. The latter is consistently asymmetric, pointing to the operation of non-coaxial deformation. The microstructures and CPO data indicate a continuous plastic deformation cycle with eclogite and blueschist facies metamorphism related to subduction and exhumation of the different rock units.
•Eclogite deformation is controlled by omphacite grain boundary migration and subgrain rotation recrystallization.•High-pressure deformation in the Eclogite Zone was recorded by distinct foliations and mineral lineations.•Strain geometry remained constant during subduction and exhumation of the eclogites.•Continuous deformation from prograde blueschist to peak eclogite and retrogade blueschist facies metamorphism.•Metasediments hosting the eclogites document the latest deformation/metamorphism stages during exhumation.
Seafloor seepage of hydrocarbon‐bearing fluids has been identified in a number of marine fore arcs. However, temporal variations in seep activity and the structural and tectonic parameters that ...control the seepage often remain poorly constrained. Subduction zone earthquakes, for example, are often discussed to trigger seafloor seepage but causal links that go beyond theoretical considerations have not yet been fully established. This is mainly due to the inaccessibility of offshore epicentral areas, the infrequent occurrence of large earthquakes, and challenges associated with offshore monitoring of seepage over large areas and sufficient time periods. Here we report visual, geochemical, geophysical, and modeling results and observations from the Concepción Methane Seep Area (offshore Central Chile) located in the rupture area of the 2010 Mw. 8.8 Maule earthquake. High methane concentrations in the oceanic water column and a shallow subbottom depth of sulfate penetration indicate active methane seepage. The stable carbon isotope signature of the methane and hydrocarbon composition of the released gas indicate a mixture of shallow‐sourced biogenic gas and a deeper sourced thermogenic component. Pristine fissures and fractures observed at the seafloor together with seismically imaged large faults in the marine fore arc may represent effective pathways for methane migration. Upper plate fault activity with hydraulic fracturing and dilation is in line with increased normal Coulomb stress during large plate‐boundary earthquakes, as exemplarily modeled for the 2010 earthquake. On a global perspective our results point out the possible role of recurring large subduction zone earthquakes in driving hydrocarbon seepage from marine fore arcs over long timescales.
Key Points
We present manifold evidence for active seafloor methane seepage in the rupture area of the 2010 Maule earthquake, Central Chile
Methane seepage is spatially governed by large upper plate faults that crop out at the upper continental slope
Recurring plate‐boundary earthquakes enhance fault zone permeability, and thus seafloor seepage, over multiple seismic cycles
The petrophysical properties of fine-grained marine sediments to a large extent depend on the microstructure and crystallographic preferred orientations (CPOs). In this contribution we show that ...Rietveld-based synchrotron texture analysis is a new and valuable tool to quantify textures of water-saturated fine-grained phyllosilicate-rich sediments, and assess the effects of compaction and tectonic deformation. We studied the CPO of compositionally almost homogeneous silty clay drillcore samples from the Nankai Accretionary Prism slope and the incoming Philippine Sea plate, offshore SW Japan. Basal planes of phyllosilicates show bedding-parallel alignment increasing with drillhole depth, thus reflecting progressive burial and compaction. In some samples calcite and albite display a CPO due to crystallographically controlled non-isometric grain shapes, or nannofossil tests. Consolidated-undrained experimental deformation of a suite of thirteen samples from the prism slope shows that the CPOs of phyllosilicate and calcite basal planes develop normal to the experimental shortening axis. There is at least a qualitative relation between CPO intensity and strain magnitude. Scanning electron micrographs show concurrent evolution of preferred orientations of micropores and detrital illite flakes normal to axial shortening. This indicates that the microfabrics are sensitive strain gauges, and contribute to anisotropic physical properties along with the CPO.
•Crystallographic preferred orientations (CPOs) measured using synchrotron X-rays•Shape preferred orientations determined using scanning electron microscopy•CPOs develop with respect to the experimental shortening axis.•Platy minerals preferentially aligned normal to the experimental shortening axis.
New multibeam bathymetry allows an unprecedented view of the tectonic regime and its along‐strike heterogeneity of the North Chilean marine forearc and the oceanic Nazca Plate between 19 and 22.75°S. ...Combining bathymetric and backscatter information from the multibeam data with subbottom profiler and published and previously unpublished legacy seismic reflection lines, we derive a tectonic map. The new map reveals a middle and upper slope configuration dominated by pervasive extensional faulting, with some faults outlining a >500‐km‐long ridge that may represent the remnants of a Jurassic or pre‐Jurassic magmatic arc. Lower slope deformation is more variable and includes slope‐failures, normal faulting, re‐entrant embayments, and NW‐SE trending anticlines and synclines. This complex pattern likely results from the combination of subducting lower‐plate topography, gravitational forearc collapse, and the accumulation of permanent deformation over multiple earthquake cycles. We find little evidence for widespread fluid seepage despite a highly faulted upper‐plate. An explanation could be a lack of fluid sources due to the sediment starved nature of the trench and most of the upper‐plate in vicinity of the hyperarid Atacama Desert. Changes in forearc architecture partly correlate to structural variations of the oceanic Nazca Plate, which is dominated by the spreading‐related abyssal hill fabric and is regionally overprinted by the Iquique Ridge. The ridge collides with the forearc around 20–21°S. South of the ridge‐forearc intersection, bending‐related horst‐and‐grabens result in vertical seafloor offsets of hundreds of meters. To the north, plate‐bending is accommodated by reactivation of the paleo‐spreading fabric and new horst‐and‐grabens do not develop.
Key Points
Multibeam bathymetric and seismic reflection data image the structure of the North Chilean marine forearc and the oceanic Nazca plate
The structural character and tectonic configuration of the offshore forearc and the oceanic plate change significantly along the margin
The derived pattern of permanent deformation may hold information for studying seismicity or other types of short term deformation
Many blueschists and eclogites are inferred to have formed from oceanic basalts in subducted slabs. Knowledge of their elastic behavior is essential for reconstructing the internal structure of ...subduction zones. The Cycladic blueschist unit, exposed on Syros Island (Greece), contains rocks belonging to an exhumed Tertiary subduction complex. They were possibly part of a subduction channel, a shear zone above the subducting slab in which exhumation is possible during subduction. Intense plastic deformation, forming crystallographic preferred orientations (CPO), accompanied blueschist and eclogite metamorphism. CPO of the constituent minerals in the collected samples was determined by time‐of‐flight neutron diffraction. Two samples are foliated fine‐grained blueschists with strong CPO, rich in glaucophane, zoisite, and phengite. Two coarser‐grained eclogite samples rich in omphacite and clinozoisite, or glaucophane, have weaker CPO. Vp and Vs anisotropies were computed from the orientation distribution function and single‐crystal elastic constants. All samples show velocity maxima parallel to the mineral lineation, and minima normal to the foliation, providing important constraints on orientations of seismic anisotropy in subduction channels. Vp anisotropies are up to 3 times higher (6.5–12%) in the blueschists than in the eclogites (3–4%), pointing to a potentially important lithological control of elastic anisotropy in subducted oceanic crust.
Plain Language Summary
Blueschists and eclogites are high‐pressure rocks which form when the oceanic crust sinks into higher depths within the Earth at so‐called subduction zones. During this process the rocks are deformed, which causes a preferred orientation of the minerals constituting the rocks. We measured the orientation of these minerals and used these data to calculate physical properties of these rocks. The minerals have direction‐dependent properties; hence, a preferred orientation of these minerals causes direction‐dependent properties of the rocks. In this case we calculate the velocity of seismic waves in different direction of the rocks to determine the properties of the rocks currently located at depth today. This is useful information for the interpretation of geophysical data.
Key Points
Calculation of elastic anisotropy of subducted oceanic crust from single‐crystal elastic constants and textures of eclogite and blueschist
Determination of physical rock properties in subduction zone settings to help the interpretation of seismic data
Understanding of subduction zone processes
We present the first comprehensive study of mass wasting processes in the continental slope of a convergent margin of a subduction zone where tectonic processes are dominated by subduction erosion. ...We have used multibeam bathymetry along ∼1300 km of the Middle America Trench of the Central America Subduction Zone and deep‐towed side‐scan sonar data. We found abundant evidence of large‐scale slope failures that were mostly previously unmapped. The features are classified into a variety of slope failure types, creating an inventory of 147 slope failure structures. Their type distribution and abundance define a segmentation of the continental slope in six sectors. The segmentation in slope stability processes does not appear to be related to slope preconditioning due to changes in physical properties of sediment, presence/absence of gas hydrates, or apparent changes in the hydrogeological system. The segmentation appears to be better explained by changes in slope preconditioning due to variations in tectonic processes. The region is an optimal setting to study how tectonic processes related to variations in intensity of subduction erosion and changes in relief of the underthrusting plate affect mass wasting processes of the continental slope. The largest slope failures occur offshore Costa Rica. There, subducting ridges and seamounts produce failures with up to hundreds of meters high headwalls, with detachment planes that penetrate deep into the continental margin, in some cases reaching the plate boundary. Offshore northern Costa Rica a smooth oceanic seafloor underthrusts the least disturbed continental slope. Offshore Nicaragua, the ocean plate is ornamented with smaller seamounts and horst and graben topography of variable intensity. Here mass wasting structures are numerous and comparatively smaller, but when combined, they affect a large part of the margin segment. Farther north, offshore El Salvador and Guatemala the downgoing plate has no large seamounts but well‐defined horst and graben topography. Off El Salvador slope failure is least developed and mainly occurs in the uppermost continental slope at canyon walls. Off Guatemala mass wasting is abundant and possibly related to normal faulting across the slope. Collapse in the wake of subducting ocean plate topography is a likely failure trigger of slumps. Rapid oversteepening above subducting relief may trigger translational slides in the middle Nicaraguan upper Costa Rican slope. Earthquake shaking may be a trigger, but we interpret that slope failure rate is lower than recurrence time of large earthquakes in the region. Generally, our analysis indicates that the importance of mass wasting processes in the evolution of margins dominated by subduction erosion and its role in sediment dynamics may have been previously underestimated.
Key Points
First comprehensive study at margin dominated by subduction erosion
Distribution of mass wasting indicates segmentation
Slope failure processes are globally different
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