Evolution of the Palaeotethys in the Eastern Mediterranean: a multi-method approach to unravel the age,provenance and tectonic setting of the Upper Palaeozoic Konya Complex and its Mesozoic cover sequence (south-central Turkey)

ABSTRACT

Siliciclastic sediments from the Upper Palaeozoic Konya Complex and its Mesozoic cover were studied by a multi-method approach combining thin-section petrography, bulk-rock geochemistry, mineral chemistry of rutile, and U–Pb geochronology of detrital zircons. Provenance sensitive data of samples from the Upper Palaeozoic Hal?c? Formation indicate sediment supply from mainly low- to medium-grade metamorphosed sedimentary rocks of felsic character, while the contribution from volcanic rocks was rare. The detrital zircon record of sediments from the Hal?c? Formation documents sediment supply from different sources and excludes a similar provenance. Some samples show great similarities with Palaeozoic sandstones from the cover sequence of the Saharan Metacraton and the Arabian–Nubian Shield, while the other samples indicate a provenance that must be sought in units with a southern Eurasian affinity. The upper limit for sediment deposition in the Hal?c? Formation is mostly constrained by Early Palaeozoic zircon populations; however, sediment accumulation in Pennsylvanian–Cisuralian time is more likely, contemporaneously with the Upper Palaeozoic succession on the Karaburun Peninsula (western Turkey). The provenance of sediments from the Upper Triassic Ard?çl? Formation remains enigmatic, but the source should be sought nonetheless in units close to the depositional site. In any case, detrital zircon age spectra and compositional data exclude recycling of underlying rock units (i.e. Hal?c? Formation). Overall, our new provenance data reveal great similarities between the Konya Complex and comparable units (Chios, Karaburun) but also highlight distinct differences in terms of sediment composition and provenance.     

Penetration of solar uv radiation and photodissociation in the Jovian atmosphere

We present computations of the photodissociation coefficients for NH₃, N₂H₄, PH₃, and H₂S in the Jupiter atmosphere. The calculations take into account multiple scattering and absorption using the radiative-transfer method known as δ-Eddington approximation. The atmospheric models include two cloud layers of variable thickness and haze layers above the upper cloud and between the clouds. One of the results of the radiative computations deal with the reflectivity of the Jovian atmosphere as a function of wavelength. A comparison with available data on the albedo of the planet gives some important indications about mixing ratios and distributions of gases and aerosols. The results for the photolysis rates are compared with similar rates obtained by considering either the direct flux or the flux determined by the molecular gas absorption alone. The latter is usually the approximation used in aeronomic models. The results of this comparison show that a considerable difference exists with direct flux photodissociation but significant differences with molecular absorption flux exist only in atmospheric regions where photodissociation is relatively small.     

Is the forward problem of ground water hydrology always well posed?

Complex aquifer systems are often modeled with quasi-three-dimensional models, which consider two-dimensional horizontal flow in the aquifers and one-dimensional vertical flow through aquitards. When the aquifer system consists of a phreatic aquifer and one or more semiconfined aquifers connected by aquitards, the discrete model consists of a nonlinear system of algebraic equations, because the transmissivity of the phreatic aquifer depends on the phreatic head. If the water extraction is very high, the phreatic aquifer can be depleted and the equations of the model must be modified accordingly. There are not simple and general criteria to state if the phreatic aquifer is depleted before solving the system of equations. Therefore, the iterative procedures (e.g., relaxation methods), used to find the solution to the forward problem, must handle these particular conditions and can suffer several problems of convergence. These problems can be caused by the choice of the initial head values or of the relaxation coefficient of the iterative algorithms; however, they can also be caused by the nonexistence or nonuniqueness of the solution to the system of nonlinear equations. The study of existence and uniqueness of the general problem is very difficult and, therefore, we consider a simplified problem, for which the discrete model can be handled analytically. The results of the numerical experiments show that the solution to the forward problem can be nonunique. Only for some cases it is possible to invoke physical arguments to eliminate tentative solutions.     

Toe-of-slope of a Cretaceous carbonate platform in outcrop,seismic model and offshore seismic data (Apulia,Italy)

Synthetic seismic models of outcrops in the Early Cretaceous slope of a carbonate platform on the Gargano Promontory (southern Italy) were compared to an offshore seismic section south of the Promontory. Outcrops of the same age on the promontory have the same sequence stratigraphic characteristics as their offshore equivalent, and are the only areas where the transition from platform to basin of Early Cretaceous is exposed on land. Two adjacent outcrop areas were combined into one seismic-scale lithologic model with the aid of photo mosaics, measured sections, and biostratigraphic data. Velocity, density, and porosity measurements on spot samples were used to construct the impedance model. Seismic models were generated by vertical incidence and finite difference programs. The results indicate that the reflections in the seismic model are controlled by the impedance contrast between low porous intervals rich in debris from the platform and highly porous intervals of pelagic lime mudstone, nearly devoid of debris. Finite difference seismic display showed best resemblance with the real seismic data, especially by mapping a drowning unconformity.     

Zeolitization of Oligo-Miocene volcaniclastic rocks from Logudoro (northern Sardinia, Italy)

The present study reports the results of research on volcaniclastic products from Logudoro (northern Sardinia), a reconstructed stratigraphic succession some hundreds of meters thick, comprising two different ignimbritic units separated by an epiclastic layer (generally ˜10 m thick). Clinoptilolite is the most widely distributed authigenic phase in epiclastite and unwelded units, always associated with opal-CT and sometimes with smectite. An adularia-like feldspar, cristobalite and epigenetic quartz are typical phases of welded units. Within the zeolitized units (unwelded ignimbrite and epiclastic units) clinoptilolite and opal-CT constitute the cement deriving from the transformation of the precursor rhyolitic glass, in agreement with a single zeolitization process that developed after the deposition of the entire succession. Silica increases in zeolitized rocks with respect to the precursor material, leading to hypothesize that the secondary mineralization process was favored by mineralized hydrothermal fluids preferentially circulating through the fault system of the area. Quartz veins in welded subunit and K-feldspar (adularia-like) preferentially located near the faults are in agreement with this hypothesis. The interaction of these fluids with the glassy fraction favored its dissolution and the consequent crystallization of clinoptilolite. Furthermore, the pH increase and the silica supersaturation enhanced the contemporary or subsequent precipitation of opal-CT. Received: 5 June 2000 / Accepted: 14 August 2000     

Silicic magma entering a basaltic magma chamber: eruptive dynamics and magma mixing — an example from Salina (Aeolian islands,Southern Tyrrhenian Sea)

The Pollara tuff-ring resulted from two explosive eruptions whose deposits are separated by a paleosol 13 Ka old. The oldest deposits (LPP, about 0.2 km³) consist of three main fall units (A, B, C) deposited from a subplinian column whose height (7–14 km) increased with time from A to C, as a consequence of the increased magma discharge rate during the eruption (1–8x10⁶ kg/s). A highly variable juvenile population characterizes the eruption. Black, dense, highly porphyritic, mafic ejecta (SiO₂=50–55%) almost exclusively form A deposits, whereas grey, mildly vesiculated, mildly porphyritic pumice (SiO₂=56–67%) and white, highly vesiculated, nearly aphyric pumice (SiO₂=66–71%) predominate in B and C respectively. Mafic cumulates are abundant in A, while crystalline lithic ejecta first appear in B and increase upward. The LPP result from the emptying of an unusual and unstable, compositionally zoned, shallow magma chamber in which high density mafic melts capped low density salic ones. Evidence of the existence of a short crystal fractionation series is found in the mafic rocks; the andesitic pumice results from complete blending between rhyolitic and variously fractionated mafic melts (salic component up to 60 wt%), whereas bulk dacitic compositions mainly result from the presence of mafic xenocrysts within rhyolitic glasses. Viscosity and composition-mixing diagrams show that blended liquids formed when the visosities of the two end members had close values. The following model is suggested: 1. A rhyolitic magma rising through the metamorphic basement enterrd a mafic magma chamber whose souter portions were occupied by a highly viscous, mafic crystal mush. 2. Under the pressure of the rhyolitic body the nearly rigid mush was pushed upwards and mafic melts were squeezed against the walls of the chamber, beginning roof fracturing and mingling with silicic melts. 3. When the equilibrium temperature was reached between mafic and silicic melts, blended liquids rapidly formed. 4. When fractures reached the surface, the eruption began by the ejection of the mafic melts and crystal mush (A), followed by the emission of variously mingled and blended magmas (B) and ended by the ejection of nearly unmixed rhyolitic magma (C).     

Instrumental seismological data since 1900 for northeastern Italy

Statistical analyses on a catalogue of instrumental data for earthquakes in northeastern Italy since 1900 are presented. The different types of magnitude, which are the main parameters under study, have been evaluated so as to be as homogeneous as possible. Comparisons of the different magnitude values show linear dependence, at least in the medium magnitude range represented by the available data set. Correlations between the magnitude most significant for this region and chosen macroseismic data indicate a methodology for assessing the macroseismic magnitude of historical earthquakes which seems to be stable.     

Remote sensed data for automatic detection of land-use changes due to human activity in support to landslide studies

Slope instability studies appear to recognize a number of potential superficial slide-producing agents, which may be directly detected and monitored with Earth Observation (EO) data. The main objective of this work is to use conventional EO data and automatic techniques for providing land-use change maps useful in landslide prevention. The idea is to use the detection of changes in areas already involved in landslide events as a precursory sign of variations in the equilibrium status of the slope, independently from other natural triggering events, such as rain and seismic events. Attention is focused on man-induced surface changes, such as deforestation, urban expansion and construction of artificial structures. A historical set of 20 multi-temporal Landsat TM images, covering the period 1987–2000, was analyzed using a supervised change detection technique on a test site affected by slope instability phenomena located in the Abruzzo region in Southern Italy. A change image is obtained by comparing year-specific thematic map pairs. It contains useful information not only on the place where a transition occurred, but also on the specific classes involved in the transitions between two different years. The full set of change images is used to extract class-conditional transition probabilities, to evaluate variations in specific class distribution and the total number of changed pixels in time. Four classes and their transitions were considered in the analysis: (1) arboreous land, (2) agricultural land, (3) barren land, and (4) artificial structures. The quantitative analysis of the class-joint transition probability values of some specific class-transitions that may worsen slope stability showed that in an area prone to landslides the probability of landslide re-activation or first activation is higher where changes have occurred. Although based on a limited number of known events, such a result encourages extensive experimentation of the proposed technique on better documented landslide test sites.     

Seismic demands and analysis of site effects in the Marmara region during the 1999 Kocaeli earthquake

The characteristics of the strong ground motion accelerograms from the 1999 Kocaeli earthquake are investigated in detail in this study. The emphasis is on the comparison of the response spectra for the fault normal (FN) and fault parallel (FP) components of the ground motions. The results show that the near-fault records with directivity effects characterize themselves with increased base shear demands rather than increased displacement demands for both the FN and FP components and a narrower velocity sensitive region for the FN component. This study also shows that the effectiveness of base isolation may vary from site to site and for a given site, from component to component. The site effects in the Marmara region during the 1999 Kocaeli earthquake are examined. Site amplifications are predicted by the classical spectral ratio (CSR) and the receiver function (RF) methods. The CSR method gives higher estimates for the site amplifications compared to the RF method and is in better conformity with the observed damage during the Kocaeli earthquake. The districts of Istanbul that are especially susceptible to site amplification hazard are determined. It is apparent from the results that the site amplification hazard risk is the highest for Avcilar and Bakirkoy districts. This study also shows that for sites which have the risk of soil amplification for long-period structures, liquefaction may not be beneficial as a natural base isolator, and may result in shifting the eigenperiod of the low- and mid-rise structures to the critical periods with high site amplifications. This may be especially the case for Avcilar and Bakirkoy districts. In Fatih, Bakirkoy, and Cekmece districts, the predominant period of the ground motion is calculated to be very close to the eigenperiods of the typical residential buildings. Therefore, these three districts are expected to experience heavier damages in future earthquakes due to resonance effects.