Guy Camus

Neogene ignimbrites of the Nevsehir plateau (Central Turkey): stratigraphy, distribution and source constraints

Abstract In Anatolia (Turkey), extensive calc-alkaline volcanism has developed along discontinuous provinces from Neogene to Quaternary times as a consequence of plate convergence and continental collision. In the Nevsehir plateau, which is located in the Central Anatolian Volcanic Province, volcanism consists of numerous monogenetic centres, several large stratovolcanoes and an extensive, mainly Neogene, rhyolitic ignimbrite field. Vent and caldera locations for the Neogene ignimbrites were not well known based on previous studies. In the Neogene ignimbrite sequence of the Nevsehir plateau, we have identified an old group of ignimbrites (Kavak ignimbrites) followed by five major ignimbrite units (Zelve, Sarimaden Tepe, Cemilkoy, Gordeles, Kizilkaya) and two smaller, less extensive ones (Tahar, Sofular). Other ignimbrite units at the margin of the plateau occur as outliers of larger ignimbrites whose main distributions are beyond the plateau. Excellent exposure and physical continuity of the units over large areas have allowed establishment of the stratigraphic succession of the ignimbrites as, from bottom to top: Kavak, Zelve, Sarimaden Tepe, Cemilkoy, Tahar, Gordeles, Sofular, Kizilkaya. Our stratigraphic scheme refines previous ones by the identification of the Zelve ignimbrite and the correlation of the previously defined ‘Akkoy’ ignimbrite with the Sarimaden Tepe ignimbrite. Correlations of distant ignimbrite remnants have been achieved by using a combination a field criteria: (1) sedimentological characterisitics; (2) phenocryst assemblage; (3) pumice vesiculation texture; (4) presence and characteristics of associated plinian fallout deposits; and (5) lithic types. The correlations significantly enlarge the estimates of the original extent and volume of most ignimbrites: volumes range between 80 km 3 and 300 km 3 for the major ignimbrites, corresponding to 2500–10,000 km 3 in areal extent. The major ignimbrites of the Nevsehir plateau have an inferred source area in the Derinkuyu tectonic basin which extends mainly between Nevsehir and the Melendiz Dag volcanic complex. The Kavak ignimbrites and the Zelve ignimbrite have inferred sources located between Nevsehir and Derinkuyu, coincident with a negative gravity anomaly. The younger ignimbrites (Sarimaden Tepe, Cemilkoy, Gordeles, Kizilkaya) have inferred sources clustered to the south between the Erdas Dag and the Melendiz Dag volcanic complex. We found evidence of collapse structures on the northern and southern flanks of the Erdas Dag volcanic massif, and of a large updoming structure in the Sahinkalesi Tepe massif. The present-day Derinkuyu tectonic basin is mostly covered with Quaternary sediments and volcanics. The fault system which bounds the basin to the east provides evidence that the ignimbrite volcanism and inferred caldera formation took place in a locally extensional environment while the basin was already subsiding. Drilling and geophysical prospecting are necessary to decipher in detail the presently unknown internal structure of the basin and the inferred, probably coalesced or nested, calderas within it.

Eruptive and magmatic cycles at Fuego de Colima volcano (Mexico)

The Fuego de Colima volcano displays a pattern of eruptive cyclicity, characterized by lava flows and/or slow effusions in an open crater alternating with short explosive events. Historical accounts, field investigations and petrological data allow us to refine our understanding of the relationships between recurrent acidic (acidic andesite) and “mafic” (andesitic) products and this eruptive behaviour. The second to last (1818–1913) and present (1913-present day) cycles are instructive in this respect: the short explosive events in 1818 and 1913 correspond to the initial mixing stage of a differentiated magmatic body with a new “mafic” input. Mathematical modeling shows that the pyroclastic products of 1913 result from the mixing between an acidic andesite (61.5% SiO2) and an olivine andesite (∼56% SiO2) in various ratios. The final andesite mixture is made of ∼38% basaltic andesite and ∼62% acidic andesite. The long effusive/extrusive phases correspond to the ensuing differentiation stage of the new magmatic body. For example, the 1961 lava flows were derived from the final mixed and homogenized magma after the eruption of 1913 by fractionation of ∼7% plagioclase, ∼9% pyroxene (clinopyroxene + orthopyroxene) and 0.7% titanomagnetite. The fractionation of 11% plagioclase, 9% pyroxene and 1.4% titanomagnetite is required to produce the composition of the 1986 summit lava dome from the same parent. In conclusion, at Fuego de Colima, an eruptive cycle begins with a short, violent explosive event related to a mixing process and continues with a long, effusive phase characterized by magmatic differentiation. This scheme differs from others that have been proposed, which consider that the eruptive cycles end with an explosive event.

Intercomparison of red TL and ESR signals from heated quartz grains

Abstract Red thermoluminescence (TL) and electron-spin-resonance (ESR) measurements were made on identically prepared quartz grains, which had been zeroed by a lava flow of the Gravenoire volcano (Clermont-Ferrand, France) during the last glacial period (Wurm glaciation). Samples from two different sites were studied. The additive technique was used in order to evaluate the palaeodoses and the corresponding dose response curves (DRC), approximating saturating exponentials, were fitted on the basis of DRC obtained with laboratory reset samples. Using this technique of regression, results derived from Al and Ti ESR signals and red TL signals converged better than when only the additive DRC were taken into account in a simple exponential fit. Annealing experiments indicated that the traps involved in red TL, Al and Ti ESR centres had different stabilities vs time and temperature; thus the agreement of the palaeodoses derived from the three signals, within error limits, strengthened confidence in the results for each sample. The ages obtained for the two sites agreed with each other, although the palaeodoses and natural radiation dose rates were significantly different: 62.2 ± 9.0 and 61.3 ± 9.0 ka.