Late quaternary tephra layers along the Cilento coastline (southern Italy)

The results of stratigraphic and tephrostratigraphic analyses of several pyroclastic levels, collected along the coastal sector of the Cilento region (southern Italy), are presented. Some of these levels are here described for the fist time, others, already known in literature, were reconsidered in order to better understand their stratigraphic position and to point out the possible volcanic sources.     

Syn-kinematic emplacement of the Pangong metamorphic and magmatic complex along the Karakorum Fault (N Ladakh)

This paper investigates the age, P–T conditions and kinematics of Karakorum Fault (KF) zone rocks in the NW part of the Himalaya–Karakorum belt. Granulite to greenschist facies assemblages were developed within the KF zone during strike-slip shearing. The granulites were formed at high temperature (800 °C, 5.5 kbar), were subsequently retromorphosed into the amphibolite facies (700–750 °C, 4–5 kbar) and the greenschist facies (350–400 °C, 3–4 kbar). The Tangtse granite emplaced syn-kinematically at the contact between a LT and the HT granulite facies. Intrusion occurred during the juxtaposition of the two units under amphibolite conditions. Microstructures observed within the Tangtse granite exhibit a syn-magmatic dextral S–C fabric. Compiled U–Pb and Ar–Ar data show that in the central KF segment, granulite facies metamorphism occurred at a minimum age of 32 Ma, subsequent amphibolite facies metamorphism at 20–18 Ma. Further shearing under amphibolite facies (650–500 °C) was recorded at 13.6 ± 0.9 Ma, and greenschist-facies mica growth at 11 Ma. These data give further constrains to the age of initiation and depth of the Karakorum Fault. The granulite-facies conditions suggest that the KF, accommodating the lateral extrusion of Tibet, could be at least a crustal or even a Lithosphere-scale shear zone comparable to other peri-Himalayan faults.     

Rb-Sr and K-Ar ages on minerals at temperatures of 300°–400° C from deep wells in the Larderello geothermal field (Italy)

Rb-Sr and K-Ar ages have been obtained on six biotites, two muscovites and one hornblende from samples of micaschist, gneiss and amphibolite of Lower Paleozoic to Precambrian age at a depth exceeding 2,000 m in basement rocks of the Larderello-Travale geothermal region. Most of the data cluster in the range 2.5–3.7 Ma, revealing the existence of a Pliocene thermal event to which the origin of the field may be attributed. The resulting duration of the Larderello geothermal field is unexpectedly long. In the basement levels of the two wells examined, unstabilized minimum temperatures of 290° and 380° C were measured. All the biotites show almost complete ⁴⁰Ar and ⁸⁷Sr retention at the measured well temperatures. Petrologic evidence (stilpnomelane stability) and experimental data (activation energies and diffusion coefficients) also favour a closure temperature above 400° C for Rb-Sr and K-Ar in biotites, in agreement with recent direct experimental determinations.For the last 3 Ma mean geothermal gradients of 120°–150° C/km have been evaluated in the first 2–3 km, and 60°–65° C/km in the underlying 2 km. A rough estimate of total cooling in the last 3 Ma gives a value of 120° C at 2,500 m depth and 50° C at 4,000 m depth in Sasso 22 well. A mean uplift rate of about 0.2 mm/year is calculated independently.Research conducted under a collaboration agreement between the Italian National Research Council (CNR) and the Italian National Electricity Board (ENEL)     

Ar/Ar dating of amphiboles from Zabargad Island (Red Sea) is precluded by interaction with fluids

Stepwise heating data on four amphiboles separated from various Zabargad rocks (a dolerite dike, a peridotite, a gneiss and a pyroxenite) invariably show saddle-shaped age spectra. The age information in the spectrum is generally masked by 1–2 nl/g excess ⁴⁰Ar. The spectrum least affected by excess Ar constrains the emplacement age of the dolerite dike (Z-2137) to be 20 Ma. The greatest concentration of excess Ar is found in the peridotite, implying that the ultramafic rocks were metasomatized by a very ⁴⁰Ar-rich fluid. A complex history of subsolidus reactions with hydrothermal fluids is suggested by the irregularities in the ³⁷Ar/³⁹Ar spectra.     

A new strategy for analyzing the chronometry of constructed rock features in deserts

The western Great Basin contains thousands of constructed rock features, including rock rings, cairns, and alignments. Unlike subtractive geoglyphs, such as the Nasca Lines of Peru, that remove desert pavement, these surface features alter the location and positioning of cobble‐ to bouldersized rocks. The chronology of surface rock features has remained unconstrained by numerical ages because no prior chronometric approach has been able to yield age control. We propose a new strategy for studying these features by analyzing anthropogenic modifications to rock coatings, an approach that permits the use of several dating methods, two of which are assessed here: radiocarbon dating of pedogenic carbonate and rock‐varnish microlaminations. Initial results from Searles Valley, eastern California, suggest that constructed rock features may be as old as early Holocene and terminal Pleistocene. Archaeological surveys of desert areas would be greatly enhanced if they noted altered positions of rock coatings. © 2006 Wiley Periodicals, Inc.     

Decay constants in geochronology

Geologic time is fundamental to the Earth Sciences, and progress in many disciplines depends critically on our ability to measure time with increasing accuracy and precision. Isotopic geochronology makes use of the decay of radioactive nuclides as a help to quantify the histories of rock, minerals, and other materials. Both accuracy and precision of radioisotopic ages are, at present, limited by those of radioactive decay constants. Modem mass spectrometers can measure isotope ratios with a precision of 10-4 or better. On the other hand, the uncertainties associated with direct half-life determinations are, in most cases, still at the percent level. The present short note briefly summarizes progress and problems that have been encountered during the Working Group‘s activity.     

Assessing the age of relief growth in the Andes of northern Chile: Magneto-polarity chronologies from Neogene continental sections

The Andes of northern Chile currently experience a phase of relief rejuvenation as indicated by valleys that are >1000 m dissected into pediplains. However, it has been unclear when this phase of relief enhancement started. This paper discusses the use of palaeomagnetic chronologies from four sections in the Taracapá-Region (northern Chile) to assess this age. The sections are located in distal and proximal positions. Sediment accumulation occurred between c. 22.5 and 8/7.5 Ma with a hiatus that possibly spans a time interval between c. 19.5 and 11 Ma. The magnetic polarity chronologies suggest a preferred age between 8.0 and 7.5 Ma for the time when relief growth started. In proximal positions, however, alternative correlations suggest an age of 8.5 Ma. In addition, the palaeomagnetic data reveal no rotation of the analysed strata, suggesting a minimum age of c. 22.5 Ma for the tectonic block rotation south of the Arica deflection.     

Stratigraphy and K-Ar ages of the Diego Hernández wall and their significance on the Las Cañadas Caldera formation (Tenerife, Canary Islands)

The Las Canadas Caldera corresponds to a volcanic caldera formed after a long period of explosive activity. The structural and lithological variations which can be observed in the caldera wall make clear the complexity of the original Las Canadas Edifice in which different eruptive centres existed. The stratigraphic and structural reconstruction of the Las Canadas Caldera indicates that the Diego Hernández wall, located at the eastern side of the caldera wall, comprises the youngest pre-caldera deposits. Determination of their K-Ar ages has provided the maximum age for the formation of the Las Canadas caldera. The results are internally consistent with stratigraphic relationships and allow two pre-caldera volcanic cycles in the Diego Hernández wall to be differentiated in accordance with geological evidence. The first cycle shows imprecise age limits. The second cycle ranges from 0.35 to 0.17 Ma. This upper pre-caldera age suggests that the Las Canadas caldera and the Teide-Pico Viejo were formed more recently than was previously assumed.     

Mélanges and other types of block-in-matrix formations in the Cantabrian Zone (Variscan Orogen,northwest Spain): origin and significance

Block-in-matrix formations in the Variscan foreland of Spain (Cantabrian Zone) occur in two different geological settings. The major block-in-matrix formations are mélanges, which appear as carpets beneath or ahead of submarine thrust systems. These mélanges may reach up to kilometric thickness and are mostly composed of broken formations (boudinaged sequences) of late Carboniferous age and scattered ‘exotic’ blocks derived from older Palaeozoic formations. Moreover, the mélanges in the Cantabrian Zone also include subordinate debris flow deposits with a chaotic block-in-matrix fabric (olistostromes). The source of the mélange blocks was the front of advancing nappes, chiefly the upper part of the nappe stacks. Therefore, the Cantabrian mélanges are interpreted as originated through submarine sliding and slumping associated with steep slopes at the orogenic front. The different types of rock bodies of these mélanges may be related to the degree of lithification of the sediments or rocks during slumping. So, broken formations are boudinaged sequences where the boudins or blocks resulted from extensional faults developed in lithified or semilithified limestones and sandstones, whereas the unlithified muddy matrix underwent continuous deformation. The scattered ‘exotic’ blocks ranging in age from early Cambrian to early Carboniferous were incorporated into the mélanges as individual blocks from competent well-lithified formations, originally located in the lower part of the nappe stacks. Although the Cantabrian Zone mélanges include olistostromic intervals, most of the olistostromes of this zone occur in a different geological setting. They are usually intercalated in the normal marine deposits of the Variscan foreland basin and, in contrast to the mélanges, they are mostly related to the margins of carbonate platforms, ahead of moving nappes. Finally, other instances of olistostromes are related to slopes generated by limb rotation of growth folds, which developed on submarine wedge-top successions.