Hypogeal geological survey in the “Grotta del Re Tiberio” natural cave (Apennines, Italy): a valid tool for reconstructing the structural setting

As karst systems are natural windows to the underground, speleology, combined with geological surveys, can be useful tools for helping understand the geological evolution of karst areas. In order to enhance the reconstruction of the structural setting in a gypsum karst area (Vena del Gesso, Romagna Apennines), a detailed analysis has been carried out on hypogeal data. Structural features (faults, fractures, tectonic foliations, bedding) have been mapped in the ”Grotta del Re Tiberio” cave, in the nearby gypsum quarry tunnels and open pit benches. Five fracture systems and six fault systems have been identified. The fault systems have been further analyzed through stereographic projections and geometric-kinematic evaluations in order to reconstruct the relative chronology of these structures. This analysis led to the detection of two deformation phases. The results permitted linking of the hypogeal data with the surface data both at a local and regional scale. At the local scale, fracture data collected in the underground have been compared with previous authors’ surface data coming from the quarry area. The two data sets show a very good correspondence, as every underground fracture system matches with one of the surface fracture system. Moreover, in the cave, a larger number of fractures belonging to each system could be mapped. At the regional scale, the two deformation phases detected can be integrated in the structural setting of the study area, thereby enhancing the tectonic interpretation of the area (e.g., structures belonging to a new deformation phase, not reported before, have been identified underground). The structural detailed hypogeal survey has, thus, provided very useful data, both by integrating the existing information and revealing new data not detected at the surface. In particular, some small structures (e.g., displacement markers and short fractures) are better preserved in the hypogeal environment than on the surface where the outcropping gypsum is more exposed to dissolution and recrystallization. The hypogeal geological survey, therefore, can be considered a powerful tool for integrating the surface and log data in order to enhance the reconstruction of the deformational history and to get a three-dimensional model of the bedrock in karst areas.     

Reduction seismic response with heavily-damped vibration absorbers

It is shown that two of the damping ratios of certain systems composed of a building and a small attachment in resonance are given by the average of the damping ratios of the two independent components. Based on this fact and the fact that the seismic response of a building can always be reduced by increasing its damping, it is demonstrated that the attachment of a small heavily-damped system in resonance can increase the damping of a building and reduce thus its response to earthquake excitations. Numerical solutions are presented to confirm the demonstration, and recommendations are given to calculate the parameters of such systems.     

Travel-time residuals and three-dimensional velocity structure of Italy

Jeffreys-Bullen P and PKP travel-time residuals observed at more than 50 seismic stations distributed along Italy and surrounding areas in the time interval 1962–1979, indicate the complex velocity pattern of this region. Strong lateral velocity inhomogeneities and low velocity zones are required to explain the observed pattern of residuals. In particular, late arrivals of about 1 sec are observed in the Apenninic mountain range, requiring both greater crustal thickness and low velocity layers, coherent with seismic refraction data and surface wave dispersion measurements. The seismic stations located in the Western and Eastern Alps indicate the presence of high velocities. In the Western Alps the strong azimuthal variation of residuals and the high values of early arrivals have a close relationship to the Ivrea body, an intrusive crustal complex characterized by a velocity as high as 7–7.2 km/sec.A travel-time inversion performed with theAki et al. (1977) block model, confirms the peculiar characteristics and the sharp variations in the lithosphere of the whole Italian region, with values of velocity perturbations between many adjacent blocks, ranging in size from 50 to 100 km, and independent from the earth parametrization chosen, reaching values up to 10% in the lithospheric part and 5% in the asthenosphere. 3-D inversion requires also high velocity along the Tyrrhenian coastal margin, equivalent to an uprise of major crustal and lithospheric discontinuities along this part of the Italian peninsula. Moreover low velocity material must be present in the northern part of the Adriatic foreland, in the lithosphere-asthenosphere system, closely related to the stress and seismicity pattern, and the lateral bending of the lithosphere in the same region.     

Discussion of: Ortega-Flores et al. (2018) Provenance analysis of Oligocene sandstone from the Cerro Pelón area,southern Gulf of Mexico.https://doi.org/10.1080/00206814.2018.1476922

ABSTRACT

We discuss the 2018 publication that reports petrographic, heavy mineral data, mineral chemistry, and zircon geochronology for Oligocene sandstones in the Cerro Pelón area in southern Mexico Sureste basin. As the title of their paper says, the goal of their study is to establish the source (s) of the voluminous Cenozoic section in this region, reaching several kilometres in thickness and important as a petroleum system. These authors conclude that Oligocene sandstones of La Laja Formation were mostly sourced from eclogite- to greenschist-facies metasedimentary, metaigneous, and ultramafic rocks of the Guatemala suture complex. Minor contributions from the Chiapas Massif Complex, exposed directly to the south ~60 km of the Cerro Pelón area, were also suggested by the authors. They thus conclude that the Palaeogene stratigraphic record in southeastern Mexico was mostly controlled by the development of the Caribbean–North America plate boundary rather than by orogenic processes at the Pacific margin of North America. Presently, we do not agree with the conclusions of Ortega Flores and colleagues who studied the Cerro Pelón section, thus some discussion is required. Serpentinite bearing Nanchital Conglomerate is well exposed in the Cerro Pelón area, and high- to low-grade metamorphic rocks experienced an uplift in the vicinity of the Cerro Pelón area at the time of deposition of the La Laja Formation. We believe the data are better explained by multiple local sources in southern and eastern Oaxaca as well as sources to the south and southwest, which include the Cenozoic coastal batholith, the Grenvillean/Guichicovi basement complexes, the Chiapas Massif, the Mazatlán schist and other units in the Cuicateco Belt, as well as the Mesozoic cover of these areas (Todos Santos Formation, Cretaceous carbonate rocks, and Paleogene strata such as the Soyaló and Bosque Formations).     

Relationship between Hypoxia and Macrobenthic Production in Chesapeake Bay

Human development has degraded Chesapeake Bay's health, resulting in an increase in the extent and severity of hypoxia (≤2 mg O₂ l^-1). The Bay's hypoxic zones have an adverse effect on both community structure and secondary production of macrobenthos. From 1996 to 2004, the effect of hypoxia on macrobenthic production was assessed in Chesapeake Bay and its three main tributaries (Potomac, Rappahannock, and York Rivers). Each year, in the summer (late July???early September), 25 random samples of the benthic macrofauna were collected from each system, and macrobenthic production in the polyhaline and mesohaline regions was estimated using Edgar's allometric equation. Fluctuations in macrobenthic production were significantly correlated with dissolved oxygen. Macrobenthic production was 90 % lower during hypoxia relative to normoxia. As a result, there was a biomass loss of ~7,320–13,200 metric tons C over an area of 7,720 km², which is estimated to equate to a 20 % to 35 % displacement of the Bay's macrobenthic productivity during the summer. While higher consumers may benefit from easy access to stressed prey in some areas, the large spatial and temporal extent of seasonal hypoxia limits higher trophic level transfer, via the inhibition of macrobenthic production. Such a massive loss of macrobenthic production would be detrimental to the overall health of the Bay, as it comes at a time when epibenthic and demersal predators have high-energy demands.     

SLA–chlorophyll-a variability and covariability in the Algero-Provençal Basin (1997–2007) through combined use of EOF and wavelet analysis of satellite data

Data reduction and signal decomposition techniques have been applied to a large bio-physical remotely sensed dataset covering the decade 1997–2007. The aim was the estimation of the spatial (basin and sub-basin scales) and temporal (seasonal and interannual) variability of sea level anomalies and chlorophyll-a concentration in the Algero-Provençal Basin, as well as the study of their covariability. Empirical orthogonal functions, wavelet analysis, singular value decomposition and correlation maps have been successfully used to individuate the patterns of (co)variability of the investigated fields. The seasonal variability of the phytoplanktonic biomass is divided in two distinct modes, timewise and spacewise separated. Positive interannual events are related to anomalies in 1999 and 2005, while the main (negative) anomaly is that of summer 2003, associated to the European 2003 heatwave. The analysis of the sea level anomalies shows a minimum in the formation of anticyclonic Algerian eddies during that period. The largest anticorrelation between sea level anomalies and phytoplanktonic biomass is found in the central zone of the basin, suggesting a clear biological response to the shoaling/deepening of the isopycnae and so to the nutrient injection into the euphotic layer. The analysis suggests that the driver of the variability of the nutricline depth in this central area is the displacement (seasonal) of the North Balearic Front and the formation/action of the frontal eddies.