Gérard Poupeau

Late exhumation stages of the Alpujarride Complex (western Betic Cordilleras, Spain): new thermochronological and structural data on Los Reales and Ojen nappes

New thermochronological and structural studies were conducted to quantify the cooling and late exhumation histories of the Internal Zone of the western Betic Cordilleras. The study was carried out in the Ojen-Marbella region where the Ojen and Los Reales nappes of the Alpujarride Complex outcrop. 40Ar39Ar single-grain analyses of muscovite and biotite display plateau ages of 19 Ma, on the same rocks. Apatite fission-track datings give ages of 18-16 Ma for the last step of cooling below 110°C, and confined track measurements indicate a very rapid cooling between 110° and <60°C. The combination of both data suggests a fast cooling phase at 19-16 Ma with a gradient higher than 100°C/Ma between 500 ± 50°C and <60°C. This fast cooling was associated with extensional tectonics accommodating thinning of the metamorphic alpine-type pile of nappes. The exhumation rate ranges from 1 to 3 km/Ma during the 19-18 Ma interval and is close to 1 km/Ma in the 18-16 Ma interval. Moreover an uplift of the Ojen and Los Reales units began after the Early Pliocene (5 Ma). The sediments sealing the strike-slip contact forming the boundary between the Alpujarride Complex and the Neomumidian Formation of Early Miocene age contain Early Pliocene planktonic formainiferal assemblages (Zone N18). Benthic formainiferal assemblages of these sediments, actually outcropping at 150 m above sea level, indicate a deposition in the upper bathyal zone ranging from 200 to 600 m water depth. Our conclusions confirm and precise the exhumation processes during the 19 to 16 Ma interval, and uplift of the Internal Zones since 5 Ma.

Large Neogene subsidence event along the Middle America Trench off Mexico (18°N–19°N): Evidence from submersible observations

Exposures of pre-Eocene plutonic rocks located along the inner slope of the Middle America Trench were sampled during deep-sea dives off Manzanillo (lat 18°N–19°N) at depths ranging from 3950 m to 2823 m. Fission-track data show a rapid cooling of these plutonic rocks during the Paleocene. A subaerial unconformity indicates that the plutonic rocks were exposed after Paleocene time. The presence of upper middle bathyal upper Miocene–lower Pliocene marine sediments above the unconformity documents a Neogene subsidence event at a mean rate of 0.35 mm/yr. This subsidence indicates subduction erosion along this segment of the margin.

Late thermal evolution of the Oman Mountains subophiolitic windows: Apatite fission-track thermochronology

In the southern Oman Mountains, high-pressure metamorphic rocks of continental origin crop out beneath the Cretaceous Samail ophiolitic nappe. We collected 14 samples from 15 to 1200 m above sea level and at various structural levels that show apatite fission-track ages between 40 ± 7 and 55 ± 5 Ma. The mean confined track lengths vary from 13.08 to 13.70 µm. Fission-track data optimization shows that the subophiolitic units cooled slowly below 60 °C at ca. 53 Ma, then remained at low temperature until ca. 19 Ma, i.e., during the weak postorogenic subsidence of the obduction belt. The subophiolitic basement was reheated to ∼70–80 °C at ca. 7–4 Ma, before final cooling. The reheating event is correlated with the late Miocene–Pliocene (Zagros) compressional phase. Post-Oligocene heating up to 80 °C is also documented by stable isotope study of the post-nappe Eocene-Oligocene onlap. Both the Paleocene and the Pliocene-Quaternary denudation rates are close to 0.3 mm/yr, and compatible with isostatically assisted erosion processes.

Temporal variations in the cooling and denudation history of the Hunza plutonic complex, Karakoram Batholith, revealed by 40Ar/39Ar thermochronology

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Importance du volcanisme calco-alcalin miocène sur la marge sud-ouest de la Corse (campagne MARCO)

Abstract During the MARCO cruise, systematic exploration along the western and northern Corsican margins was carried out by dredging. The compositions of the dredged rocks range from basalt to amphibole-biotite bearing andesite with a broad calc-alkaline character. A Miocene age has been obtained for the amphibole-biotite andesite sample DR02 both by 40Ar-39Ar dating on hornblende (16.0 ± 0.4 My) and using fission-track method on apatite (17.2 ± 0.8 My). The south-western Corsican volcanic zone represents the direct extension of the Miocene Sardinian graben volcanism north of 42 °00 N. It could either be synchronous with or post-date the oceanic opening event. Such an arc volcanism probably results from the subduction to the north of the Liguro-Piemont ocean beneath Europe, the Provencal-Ligurian basin being in back-arc position with respect to the studied volcanic centres.

Nouvelles données sur la provenance de l'obsidienne des sites néolithiques du Sud-Est de la France

Abstract We analyzed the chemical composition of 22 artifacts considered as obsidians by PIXE in non-destructive mode. The samples were collected from 11 Neolithic stations in southeastern France, between the Rhone valley and the northern Alps. One piece appeared not to be obsidian. Among the others, 20 came from a single source in Sardinia and one from an unknown source. These data confirm the importance of the Sardinian origin of the obsidian found in France, in particular for the Middle Neolithic.

Late Mesozoic and Cenozoic thermotectonic history of the Mexican Pacific margin (18 to 25°N): new insight from apatite and zircon fission-track analysis of coastal and offshore plutonic rocks

Abstract Apatite and zircon fission-track dating was used to constrain cooling histories on granitic samples taken from the offshore Acapulco trench batholith and onshore Manzanillo and Puerto Vallarta batholiths located along the southwestern active margin of Mexico, and from the La Paz batholith of southern Baja California. The apatite data indicate that many of the samples in the Manzanillo, Acapulco trench batholiths cooled rapidly below 60°C shortly after emplacement between 70 and 55 Ma. The La Paz batholith and the sample NM-20-08 of the Acapulco trench batholith experienced an older and slower cooling across the apatite partial annealing zone. The zircon fission-track ages obtained from two samples of the Puerto Vallarta batholith indicate a rapid cooling from 250 to 110°C between 54 and 50 Ma. This non-coeval cooling of batholiths at two different depths is probably due to uplift and erosion associated with the Laramide event in southwestern Mexico. The three samples from the Puerto Vallarta batholith suggest a younger evolution across low temperatures with two flat stages located near the base of the apatite partial annealing zone and above it. The first flat stage suggests that the denudation or rock uplift rate was reduced at a depth corresponding to the base of the apatite partial annealing zone. The younger single apatite ages of the Puerto Vallarta batholith, and the last common cooling of the other batholiths indicate the mild thermal influences of both the Plio-Pleistocene Trans-Mexican Volcanic Belt and the opening of the Gulf of California.

PIXE analysis and fission track dating of obsidian from South American prehispanic cultures (Colombia, Ecuador)

Abstract In the present work we have coupled PIXE with fission track dating to characterise obsidian artefacts from about 40 archaeological sites of Colombia and Ecuador. PIXE analysis, carried out with the external beam line of the AGLAE tandem accelerator, yields the content in about 15 elements with Z xa0>xa08, whereas fission track dating is applied to ages in excess of about 10xa0000 years. About 120 artefacts were investigated by PIXE, of which 50 were dated by fission tracks. Ages and compositions were compared to those of obsidians from all known geological sources of the region. We show that this double characterisation allows us to determine the number of obsidian sources exploited in an ancient past and to give some insight into obsidian circulation.

Histoire thermique du massif de Kétama (Maroc): sa place en Afrique du Nord et dans les Cordillères bétiques

Abstract Fission track dating of apatite and zircon associated with K/Ar isotopic data allow us to identify two thermal events in the Ketama massif (Subrif zone, Morocco) that occurred during the Cretaceous period: they are Berriasian to Hauterivian (126.6 + 2.1 to 131.9 ± 3.2 Ma) and Santonian to Campanian (78.0 ±1.2 to 81.6 ± 4.0 Ma) in age, respectively. These data show that no Cenozoic metamorphism exists in the Ketama massif. The zircon fission track analyses evidence temperatures of about 200 °C for the first thermal event and temperatures spanning 300–350 °C for the second event. The apatite fission track analyses show that residence in the partial annealing zone (50–110 °C) occurred between 13.9 ± 1.8 and 20.0 ±2.4 Ma. Therefore, the main cooling and unroofing (i.e. uplift)phase of the Ketama massif occurred during the Lower to Middle Miocene times.

Trace and rare-earth element characteristics of acidic tuffs from southern Peru and northern Bolivia and a fission-track age for the sillar of Arequipa

Trace-element and REE data of glass and pumices of acidic tuffs and related fall deposits erupted in southern Peru and northern Bolivia between 20 and 0.36 Ma display typical characteristics of subduction related continental arc magmatism of the CVZ with strong LILE/HFSE enrichment and non enrichment of HREE and Y. Geochemical variations of these tuffs are linked to subduction processes and controlled by changes in tectonic regimes which occured with each Quechua tectonic pulse and affected the astenospheric wedge and both the dowgoing and the overriding lithospheres. During Neogene — Pleistocene times, tuffs erupted in northern Bolivia are typically enriched in Zr, Hf, Th, Ba, LREEs and other incompatible elements and incompatible /Yb ratios are much higher relative to those erupted from southern Peru, at a given SiO2 content (65–67 wt. for dacites, 72–73 wt.% for rhyolites). ZrHf ratios increase eastward from 27 to 30 and CeYbN ratios from 11 to 19 reflecting the variation of degree of wedge contribution. Fractionation of the LREE over the HREE and fractionation of incompatible elements may be due to their heterogeneous distribution in the magma source. More highly fractionated REE patterns of Bolivian tuffs than Peruvian tuffs are attributed to variable amounts of contamination of magmas by lower crust. After the Quechua compressional event at ~7 Ma, SrY ratios of tuffs of the same age, erupted at 150–250 km or 250–400 km from the Peru-Chile trench, increase from southern Peru to northern Bolivia. These differences may be attributed to the subduction of a swarm oceanic lithosphere under the Bolivian Alti-plano, leading to partial melting of the sudbucted lithosphere. New FT dating of obsidian fragments of the sillar of Arequipa at 2.42 ± 0.11 Ma. This tuff dates the last Quechua compressional upper Pliocene phase (~2.5 Ma) and confirms that the sillar is not contemporaneous with the Toba 76 tuff or the Perez ignimbrite of northern Bolivia. Geochemical characteristics of tuffs erupted before and after this last compressional phase remained the same and provide evidence that the upper Miocene (~7 Ma) compressional deformations played the most important role on the variability of the geochemical characteristics of the southern Peruvian and northern Bolivian tuffs.