Seismic tomography models of the three-dimensional upper mantle velocity structure of the Mediterranean-Carpathian region provide a better understanding of the lithospheric processes governing its geodynamical evolution. Slab detachment, in particular lateral migration of this process along the plate boundary, is a key element in the lithospheric dynamics of the region during the last 20 to 30 million years. It strongly affects arc and trench migration, and causes along-strike variations in vertical motions, stress fields, and magmatism. In a terminal-stage subduction zone, involving collision and suturing, slab detachment is the natural last stage in the gravitational settling of subducted lithosphere.

http://www.mantleplumes.org/WebDocuments/Wortel2000.pdf

Subduction and Slab Detachment in the Mediterranean-Carpathian Region


 The spectral ratio between horizontal and vertical components (H/V ratio) of microtremors measured at the ground surface has been used to estimate fundamental periods and amplification factors of a site, although this technique lacks theoretical background. The aim of this article is to formulate the H/V technique in terms of the characteristics of Rayleigh and Love waves, and to contribute to improve the technique. The improvement includes use of not only peaks but also troughs in the H/V ratio for reliable estimation of the period and use of a newly proposed smoothing function for better estimation of the amplification factor. The formulation leads to a simple formula for the amplification factor expressed with the H/V ratio. With microtremor data measured at 546 junior high schools in 23 wards of Tokyo, the improved technique is applied to mapping site periods and amplification factors in the area.

Ground Motion Characteristics Estimated from Spectral Ratio between Horizontal and Verticcl Components of Mietremors.

[1] The western Mediterranean subduction zone (WMSZ) extends from the northern Apennine to southern Spain and turns around forming the narrow and tight Calabrian and Gibraltar Arcs. The evolution of the WMSZ is characterized by a first phase of orogenic wedging followed, from 30 Ma on, by trench retreat and back-arc extension. Combining new and previous geological data, new tomographic images of the western Mediterranean mantle, and plate kinematics, we describe the evolution of the WMSZ during the last 35 Myr. Our reconstruction shows that the two arcs form by fragmentation of the 1500 km long WMSZ in small, narrow slabs. Once formed, these two narrow slabs retreat outward, producing back-arc extension and large scale rotation of the flanks, shaping the arcs. The Gibraltar Arc first formed during the middle Miocene, while the Calabrian Arc formed later, during the late Miocene-Pliocene. Despite the different paleogeographic settings, the mechanism of rupture and backward migration of the narrow slabs presents similarities on both sides of the western Mediterranean, suggesting that the slab deformation is also driven by lateral mantle flow that is particularly efficient in a restricted (upper mantle) style of mantle convection.

https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2002TC001488

Lateral slab deformation and the origin of the western Mediterranean arcs

[1] We study the upper mantle P wave velocity structure below the Euro-Mediterranean area, down to 1000 km depth, by seismic travel time tomography. We invert summary residuals constructed with both regional and teleseismic first arrival data reported by the International Seismological Centre (ISC) (1964–1995), introducing some alternative strategies in the travel time tomographic approach and a new scheme to correct teleseismic data for global mantle structure. Our high-resolution model PM0.5 is parameterized with three-dimensional (3-D) linear splines on a grid of nodes with 0.5° spacing in both horizontal directions and 50 km vertical spacing. We obtain about 26% root-mean-square (RMS) reduction of residuals by inversion in addition to roughly 31% reduction after summary rays formation and selection. Sensitivity analyses are performed through several test inversions to explore the resolution characteristics of the model at different spatial scales. The distribution of large-scale fast anomalies suggests that two different stages of a convection process presently coexist in very close regions. The mantle dynamics of western central Europe is dominated by blockage of subducted slabs at the 660 km discontinuity and ponding of seismically fast material in the transition zone. Contrarily, in the eastern Mediterranean, fast velocity material sinks into the lower mantle, suggesting that the flow of the cold downwelling here is not blocked by the 660 km discontinuity. On a smaller scale, the existence of tears in the subducted slab (lithospheric detachment) all along both margins of the Adriatic plate, as proposed by some authors, is not supported by our tomographic images.

http://www.mantleplumes.org/WebDocuments/Piromallo%20and%20Morelli%202003%20(Tomography%20Mediterranean%20J%20Geophys%20Res).pdf

P wave tomography of the mantle under the Alpine-Mediterranean area

Geothermal systems ancient and modern: a geochemical review

An edited version of this paper was published by the American Geophysical Union. Copyright 2000, AGU.
See also:
http://www.agu.org/pubs/crossref/2000/2000JB900024.shtml;
http://atlas.geo.cornell.edu/morocco/publications/calvert2000.htm

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000JB900024

Geodynamic evolution of the lithosphere and upper mantle beneath the Alboran region of the western Mediterranean: Constraints from travel time tomography

A Loop Current anticyclonic ring ∼330 km in diameter and extending to a depth of >1500 m was observed to collide in January of 1984 against the continental shelf slope of the western Gulf of Mexico between 21.5° and 23°N. The collision occurred precisely at the time we conducted our Argos 84−1 hydrographic cruise in the western gulf (26°00′ to 19°20′N) aboard the R/V Justo Sierra. The Caribbean Subtropical Underwater (SUW) was used as a tracer to identify the Loop Current anticyclonic ring within the western gulf. The collision was identified from temperature and salinity distributions and from the dynamic topography distribution relative to 500 m. The ring's collision zone was identified by the presence of a horizontal baroclinic flow divergence, to the east of Tamiahua, that divides the surface circulation into northward and southward baroclinic currents parallel to the western gulf's continental shelf break, with speeds of 85 and 32 cm s−1, respectively. Horizontal divergence and vertical convergence (ring asymmetries) resulted at the focus of the anticyclonic ring's collision and originated the alongshore self advection and northward translation of the colliding anticyclone. Upon colliding the anticyclonic ring shed approximately one third of its volume (∼2 × 104 km3), mass, and transferred angular momentum to the south flanking water mass, thus generating a cyclonic ring to the south of the collision zone. The observed alongshelf southward current results from mass conservation and volume continuity requirements associated with the anticyclonic ring's volume shedding and most probably constitutes the colliding ring's potential vorticity conservation mechanism. The weakening of the anticyclonic ring's relative vorticity due to the collision is most likely made up by gain of vorticity from lateral shear in the northward and southward current jets parallel to the continental shelf break. The core of both the anticyclonic and cyclonic rings had typical SUW salinity (>36.5‰) and temperature (∼22.5°C) values. The rings were separated by a 5 × 104 km2 divergence zone occupied by Gulf Common Water (GCW). The SUW was absent within the collision zone to the east of Tampico (22.3°N, 97.8°W). The GCW within this divergence zone resulted from the convective mixing and dilution of the SUW with less saline (36.1 ≤ S ≤ 36.3‰) water from the uppermost layer of the thermocline. Hence the collision of Loop Current anticyclones against the western continental shelf slope of the gulf constitutes a primary mechanism by which 30 Sv of SUW are converted to GCW in the Gulf of Mexico. On the other hand, the coastal and continental shelf water temperature and salinity distributions that resulted from the ring's collision indicate that the offshore GCW mass intrudes the continental shelf to the east of Tamiahua and is diluted by low-salinity coastal water within the western continental shelf. This GCW mass intrusion most probably constitutes a principal and efficient exchange mechanism between the western gulf's continental shelf and offshore waters.

Collision of a loop current anticyclonic ring against the continental shelf slope of the western Gulf of Mexico

In situ observations of submarine hydrothermal activity have been conducted in Punta Banda, Baja California, Mexico, approximately 400 m from the coast and at a seawater depth of 30 m. The hydrothermal activity occurs within the Agua Bianca Fault, a major transverse structure of Northern Baja California. Hot springwater samples have been collected and analyzed. Marked differences exist between the submarine hot springwater, local land hot springwaters, groundwater, and local seawater. SiO2, HCO3−, Ca, K, Li, B, Ba, Rb, Fe, Mn, As, and Zn are enriched in the submarine hot springwater, while Cl, Na, SO42−, Mg, Cu, Ni, Cd, Cr, and perhaps Pb are depleted in relation to average and local seawater values. Very high temperatures, at the hydrothermal vents, have been recorded (102°C at 4-atm pressure). Visible gaseous emanations rich in CH4 and N2 coexist with the hydrothermal solutions. Metalliferous deposits, pyrite, have been encountered with high concentrations of Fe, S, Si, Al, Mn, Ca, and the volatile elements As, Hg, Sb, and Tl. X ray dispersive spectrometry (1500-ppm detection limit), X ray diffraction, and scanning electron microscopy of the isolated metalliferous precipitates indicate that the principal products of precipitation are pyrite and gypsum accompanied by minor amounts of amorphous material containing Si and Al. Chemical analyses and XRD of the reference control rocks of the locality (volcanics) versus the hydrothermally altered rocks indicate that high-temperature and high-pressure water-rock interactions can in part explain the water chemistry characteristics of the submarine hydrothermal waters. Tritium dating of the hydrothermal solutions places them in the prebomb period of 1953 with an undetermined old age. Their long residence time, the occurrence of an extensive marine sedimentary formation, their association with CH4, and their similarities with connate waters of oil and gas fields suggest that another component of their genesis could be in cation exchange reactions within deeply buried sediments of marine origin. The approximate volume discharge of the hydrothermal system has been measured (330,000 m3/yr), and the overall convective heat flux over the mapped submarine hydrothermal area has been calculated, the value of which exceeds 105 times the reported average conductive flux through the sea floor. Our data and observations indicate that metal-rich marine sediments and marine ore-forming processes are a concurrent reality and that ores are being formed today in submarine areas of high convective heat flow where hydrothermal activity is the precursor for their occurrence. At the same time the results of our investigations have demonstrated that submarine hydrothermal activity and submarine hot springs exist in the ocean in regions characterized by relative quiescence and that technically ‘active’ ridge environments are not an exclusive prerequisite for their existence.

Coastal submarine hydrothermal activity off northern Baja California

In this study we used relatively velocity independent methods (Wadati diagrams) to estimate and check the focal depth of the seismic activity in a sector of the Betic Cordilleras-Alboran Sea in Southern Spain. To understand the behaviour of the seismic activity, a total of 335 earthquakes have been analyzed. The most interesting feature found in the analyzed sector is the strong variation of the focal depth. On average, the focal depth distribution in the Granada basin is restricted to h ≤ 20 km. The lower cut-off in the seismic activity, which is interpreted to mark the brittle-ductile transition. This cut-off is coincident with a midcrustal reflector detected in the zone by previous seismic refraction and deep reflection experiments that was interpreted as an extensional detachment. The seismic activity in the Malaga zone reaches depths up to 100 km. In this case the seismic activity affect to the crust and upper mantle. The deepest activity coincides with low values of heat flow, possibly thus indicating a thicking of the lithosphere towards the W. In the centre of the Alboran Sea the activity is shallower and corresponds with a higher heat flow. We think that our results support models that have proposed active delamination towards the West. Such a model may explain the increase in the lithosphere thickness in this direction, which would be congruent with the depth of the earthquakes.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97GL03306

The depth of the earthquake activity in the Central Betics (Southern Spain)

This paper estimates fundamental translational period and damping ratio parameters and examines the changes in dynamic characteristics of a set of low-to-medium rise buildings in Lorca town (SE of Spain) affected by the May 11th, 2011 earthquake. These building parameters have been calculated analysing structural dynamic response from ambient vibration measurements recorded at top RC buildings pre- and post earthquake, using the Fast Fourier Transform and the Randomdec technique. The empirical expression relating period $$(T)$$
 and number of floor $$(N)$$
 here obtained analysing ambient noise recorded on 59 healthy buildings before the earthquake is $$T= (0.054\pm 0.002)\, N$$
 , very similar to others empirical period–height relationships obtained for RC structures in the European built environment but quite different from code provisions. Measurements performed in 34 damaged buildings show a period elongation after the quake according to $$T^{*} =(0.075\pm 0.002)\,N$$
 expression. Moreover, we found a rise of the fundamental period with the EMS’s grade of damage of buildings. In contrast to natural frequency, damping ratio $$(\xi )$$
 do not shows a significant variation with earthquake damage degree and the product $$\xi \, T$$
 remains near constant.

F. Vidal

Papers

Geodynamic evolution of the lithosphere and upper mantle beneath the Alboran region of the western Mediterranean: Constraints from travel time tomography

Collision of a loop current anticyclonic ring against the continental shelf slope of the western Gulf of Mexico

Coastal submarine hydrothermal activity off northern Baja California

The depth of the earthquake activity in the Central Betics (Southern Spain)

Changes in dynamic characteristics of Lorca RC buildings from pre- and post-earthquake ambient vibration data