The structural evolution of the southern Apuan Alps: new constraints on the tectonic evolution of the Northern Apennines (Italy)Évolution structurale des Apuanes méridionales : nouvelles contraintes sur l'évolution tectonique des Apennins septentrionaux (Italie)

Structural analysis performed in the southern sector of the Apuan Alps Metamorphic Complex (AAMC) and on the overlaying Tuscan Nappe (TN) pointed out a structural evolution much more complex than that outlined till now. The comparison between the structural evolutions of the two tectonic units could provide new important constraints on the tectonic evolution of the whole belt. The two tectonic units recorded different tectonic evolution during the first stages of compression, while they shared the same deformation history later after the Tuscan Nappe overlapped the AAMC. The coupled tectonic units have been then deformed by two systems of folds, in a compressive tectonic regime. Extensional tectonics affected the units later, at upper crustal levels. To cite this article: R. Carosi et al., C. R. Geoscience 334 (2002) 339–346.     

Geochemistry of Adakites from the Philippines: Constraints on Their Origins

Abstract. We have identified in the Philippine Archipelago 230 samples of Late Miocene to Quaternary intermediate and evolved magmatic rocks or glasses, the compositions of which plot within the adakitic field defined by Defant and Drum-mond (1990) using Sr/Y ratios versus Y contents. These rocks belong to four different subductions, along the Manila Trench (Batan, Northern Luzon, Central Luzon), the Negros and Sulu Trenches (Negros and Western Mindano), the Cotobato Trench (Southern Mindanao) and the Philippine Trench (Eastern Mindanao). Lavas from Central Mindanao overlie the deep remnants of the Molucca Sea Plate, and were emplaced in a post-collision setting.
All these samples show a significant depletion in Y and HREE with respect to their "normal" calc-alkaline equivalents, suggesting that garnet was either a residual phase during partial melting or a fractionating mineral during differentiation or assimilation coupled with fractional crystallisation (AFC). However, only 19 samples out of our set (i.e., 8 %) display very high Sr/Y ratios (100–250). Our preferred model for the genesis of these "typical adakites" is ca. 20 % partial melting of subducted altered oceanic metabasalts converted to eclogite. This melting process could have been triggered by water from the underlying serpentinites. Most of the samples, termed "intermediate adakites", display major and trace element chemical features intermediate between those of the former group and those of normal calc-alkaline lavas. We show that magma mixing between slab-derived adakitic magmas and mafic mantle-derived melts accounts for most of the trends linking typical and intermediate adakites, although an additional contribution of mantle is required in some cases.     

Statistical relations between source parameters for low and intermediate magnitudes (Friuli, 1976)

A data set of three-component short-period digital seismograms recorded in Friuli after the strong earthquake of 6th May 1976, allowed the local magnitude M_L and the seismic moment M₀ to be estimated in the range 0 < M_L < 2. The data set including the same parameters for the higher-magnitude Friuli events (M_L 5) shows two different slopes for the relation Log M₀ = CM_L + d for the two different ranges of M_L. One finds C ~ 1.0 (for 0 < M_L < 2) and C ~1.5 (for5 M_L 6.2), respectively.This implies that apparent stress release increases at low magnitudes, while it appears to be comparatively independent of the magnitude and to have an average value of about 100 bar for higher-magnitude earthquakes. Conversely, the fault dimensions do not appear to be magnitude-dependent for M_L < 2; for higher-magnitude events the linear fault dimensions range from about 1 km at M_L ~ 5 to about 12 km for the strong earthquake of 6th May 1976 (M_L = 6.2).     

Comparative high-resolution chemostratigraphy of the Bonarelli Level from the reference Bottaccione section (Umbria-Marche Apennines) and from an equivalent section in NW Sicily: Consistent and contrasting responses to the OAE2

The Bonarelli Level (BL) from the upper Cenomanian portion of the reference Bottaccione section (central Italy) is characterized by the presence of black shales containing high TOC concentrations (up to 17%) and amounts of CaCO₃ near to zero. In the absence of carbonate and, consequently, of relative carbon- and oxygen-isotopic data, the elemental geochemistry revealed to be a very useful tool to obtain information about the palaeoclimatic and palaeoceanographic evolution of the Tethys Ocean during the OAE2. Based on several geochemical proxies (Rb, V, Ni, Cr, Si, Ba), the BL is interpreted as a high-productivity event driven by increasingly warm and humid climatic conditions promoting an accelerated hydrological cycle. The enrichment factors of peculiar trace metals (Zn, Cd, Pb, Sb, Mo, U) provide further insight about the H₂S activity at the seafloor during the organic-rich sediment deposition and permitted us to evaluate the use of Ba as palaeoproductivity tracer in conditions of high rate of sulphate reduction.By comparing geochemical records from the reference Bottaccione section (central Italy) with those previously obtained for the coeval Calabianca section (northwestern Sicily), different degrees of oceanic anoxia were delineated and ascribed to different abundance and type (degradable or refractory) of organic matter, which are limiting factors in the bacterial sulphate reduction reactions and in subsequent euxinic conditions at seafloor in the Tethys realm. Based on a ciclostratigraphic approach, consistent fluctuations at 100 ky scale in the chemostratigraphic signals from the two sections are inferred to be expression of a strong orbital-climatic forcing driving changes in the oceanic environment during the BL deposition.     

The oceanic substratum of Northern Luzon: Evidence from xenoliths within Monglo adakite (the Philippines)

Abstract   A 8.65 Ma adakitic intrusive sheet exposed near Monglo village in the Baguio District of Northern Luzon contains a suite of ultramafic and mafic xenoliths including in order of abundance: spinel dunites showing typical mantle-related textures, mineral and bulk rock compositions, and serpentinites derived from them; amphibole-rich gabbros displaying incompatible element patterns similar to those of flat or moderately enriched back-arc basin basalt magmas; and amphibolites derived from metabasalts and/or metagabbros of identical affinity. A single quartz diorite xenolith carrying a similar subduction-related geochemical signature has also been sampled. One amphibolite xenolith provided a whole-rock K–Ar age of 115.6 Ma (Barremian). We attribute the origin of this suite to the sampling by ascending adakitic magmas of a Lower Cretaceous ophiolitic complex located at a depth within the 30–35 km thick Luzon crust. It could represent an equivalent of the Isabela-Aurora and Pugo-Lepanto ophiolitic massifs exposed in Northern Luzon.     

Kinematic and geochronological constraints on shear deformation in the Ferriere-Mollières shear zone (Argentera-Mercantour Massif,Western Alps): implications for the evolution of the Southern European Variscan Belt

International Journal of Earth Sciences - In the Western Alps, a steeply dipping km-scale shear zone (the Ferriere-Mollières shear zone) cross-cuts Variscan migmatites in the...     

Contrasting morphological trends of islands in Central Philippines: Speculation on their origin

Abstract The Palawan microcontinental block collided with the Philippine Mobile Belt in the Central Philippine region resulting in the counterclockwise rotation of Mindoro– Marinduque and clockwise rotation of Panay. The collision also brought about the clockwise rotation of north-east Negros, Cebu, north-west Masbate and Bohol (collectively called the Western Visayan block), resulting into their present-day northeast–southwest trend. This suggests a far more dramatic role of the collision than was previously recognized. Furthermore, the south-east Sulu Sea sub-basin is inferred to have also undergone collision-related clockwise rotation which can account for the observed east-west trending magnetic lineations in the basin. Aside from explaining the contrasting morphological trends of the different islands in Central Philippines, the rotation can also explain, albeit in a different way, how the belts of sedimentary basins, ophiolites and arcs in Panay and Negros can extend to Northern Luzon. Published paleomagnetic data suggest that the collision-related rotation commenced during the early to middle Miocene and had ceased by the late Miocene.     

Chemical availability of arsenic and heavy metals in sediments from abandoned cinnabar mine tailings

The understanding of the solid-phase speciation of arsenic in soils and sediments is important in the evaluations of its potential mobility and availability in the environment. The spoil heaps of abandoned mercury mines contain waste materials with high arsenic and heavy metals concentrations. The weathering of these tailings can cause their mobilization to the surroundings. In this work, the mobility and availability of arsenic and some heavy metals were evaluated in sediments from two heavily polluted mercury mining districts in Asturias (NW Spain). For this purpose, a slightly modified version of the Bureau Community of Reference sequential extraction scheme was applied to sediments. The total contents in the operationally defined fractions were analysed by inductively coupled plasma-atomic emission spectrometry. Extremely high total arsenic concentrations were found in all sediments ranging from 4,000 to 24,800 mg kg^?1. High easily mobilizable arsenic contents were found in the first mining area, related to the solubilization of Ca-bearing phases, supported by extracts analyses, X-ray diffraction results and the positive correlation found among the As and Ca fractionations. Ni and Zn were the most mobile among heavy metals, being Cr the least mobile, suggesting an anthropogenic origin due to the metallurgical processes, transport or dispersion generated from neighbour spoil heaps. In the second mining area, the bulk of As was concentrated in the residual phase, as well as Cr, Cu, Fe, Pb, Ti, indicating a mineralogical origin and the low availability of these elements. The strong correlations established between As and Fe and Ti distributions support the hypothesis that As is mainly associated to structural mineral phases in these sediments.     

Chronic oil pollution harms Magellanic penguins in the Southwest Atlantic

Petroleum pollution is a problem for seabirds along the Southwest Atlantic coast. Twenty-five groups from Salvador, Brazil (12 degrees 58'S) to San Antonio Oeste, Argentina (40 degrees 43'S) survey or rehabilitate sick or oiled seabirds. Four groups, one each in Brazil and Uruguay, and two in Argentina, kept counts of birds found alive and in need of rehabilitation. An average of 63.7% of the seabirds found were Magellanic penguins (Spheniscus magellanicus), with 3869 reported since 1987. Mainly adult penguins were found in Argentina (1605 of 2102 penguins of known age class) and Uruguay (158 of 197). Juveniles were most common in Brazil (234 of 325). Oil fouling was the most frequent cause of injury or sickness. The number of oiled penguins reported in their wintering range has greatly increased since the early 1990s and is strongly correlated with petroleum exports from Argentina. Our results show that chronic petroleum pollution is a problem for wildlife from Southern Brazil through Northern Argentina, and regulations and enforcement are failing to protect living resources.     

Sewage-induced polychaete reefs in a SW Atlantic shore: rapid response to small-scale disturbance

The intertidal zone around Mar del Plata’s sewage discharge (38° S, 57° W) is characterized by the presence of the non-indigenous spionid polychaete Boccardia proboscidea. This species has been classified as tolerant to moderate and high levels of organic contamination. During early stages of colonization this species can reach very high densities without suffering from interspecific competition, building biogenic structures such as reef. The aim of this work was evaluate the recovery time of the reef to a small-scale experimental physical disturbance. Five independent rocks with B. proboscidea reefs on them were selected randomly and a small-scale disturbance was generated by corers (16 cm²). At the beginning of the experiment, six corers were collected in each reef to produce the disturbance. The original density of B. proboscidea in each reef was determined in these corers. The reefs with disturbed corer were sampled on successive days to assess the reef recovery time. The number of larvae, juveniles and adults was quantified. Polychaete reefs had very high densities before the disturbance (mean density: 1,021,250 ind m⁻²). Boccardia proboscidea reefs had a fast recovery rate after small-scale disturbance. Four days after disturbance the density reached about 50% of the original density and after 5 days the disturbed reefs could not be differentiated from the undisturbed reef. The initial recolonization of disturbed patches occurs as a result of migration which appears to be driven by larvae and juveniles. These reefs can not be seen as biodiversity hotspots and the presence of the species demonstrates great environmental deterioration.