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For muscovite in ancient Proterozoic and Archean terranes, quantitative modelling of age spectrum morphology shows that a significant plateau segment does not form in an age spectrum unless there was rapid cooling that either began before and continued through, or which initiated at, the time given by the plateau age. Further, the modelling shows that a well-developed plateau is not preserved unless such rapid cooling continues until ambient temperatures are reached that are <∼220–200 °C. It thus follows that the age of a well-developed plateau segment in a muscovite age spectrum is an accurate constraint on when the rock was first exhumed to (and/or last cooled at) shallow crustal levels. In a sample held at a higher ambient temperature, argon continues to be lost from the lattice at a significant rate, well past the time of so-called 'isotopic closure' predicted by application of Dodson's formula. Parametric modelling shows that monotonic cooling paths do have a well-defined point, at which isotopic closure takes place, but to form an age plateau: (1) the rate of cooling must be rapid; and (2) cooling must continue at the same or similar rates, until temperatures are reached that are low enough to inhibit significant further loss of argon to the environment. Quantitative modelling shows how Dodson's formula begins to fail once ambient temperatures rise above these numerically computed values for isotopic closure. Yet these numerically computed closure temperatures are far below the nominal closure temperature for muscovite, typically quoted as ∼ 350–400 °C. Therefore, these results have significant implications for models that utilise closure temperatures based on estimates using Dodson's formula, when attempting to invert thermochronological datasets to obtain constraints on the temperature–time path. KEY POINTS: Quantitative modelling of age spectrum morphology can constrain possible temperature–time (T–t) paths because the shape of the age spectrum itself provides considerable information. Well-formed age plateaux developed in argon spectra from muscovite imply very rapid cooling towards ambient temperatures of ∼ 220–200 °C, which are well below the nominal closure temperatures for this mineral, typically quoted as 150–200 °C higher. There are multitudinous circumstances in which Dodson's formula fail badly, when used to produce an estimate of closure temperature. Successful applications of this formula requires fast cooling to have continued to relatively low ambient temperatures. T–t paths should not be constructed by joining the dots provided by age estimates linked to closure temperatures inferred using Dodson's formula since such T–t paths may be counter to the mathematics underlying the concept of isotopic closure. [ABSTRACT FROM AUTHOR]

The article discusses the phenomenon of turtle races in the United States and its potential threat to North American turtle populations. Turtle races involve the racing of locally collected box turtles, with the practice originating in 1924. The article highlights the widespread nature of turtle races, the impact on turtle populations, and the need for reform or elimination of the practice to ensure the conservation of turtle species. Additionally, the article presents a case study of gastroesophageal intussusception in the Japanese Giant Salamander, a threatened species in Japan, providing insights into the physiological responses of amphibians to ingesting unsafe substances. [Extracted from the article]

Byline: M. A. Forster (1), G. S. Lister (1) Abstract: Several relatively thin tectono-metamorphic slices have been recognized in the Cycladic eclogite--blueschist belt, through detailed studies on Ios, Sifnos, Syros, and Tinos. A sequence of distinct metamorphic mineral growth events has been documented. These recur in each tectonic slice, although individual slices are dominated by different events. To constrain the timing of these processes, the method of asymptotes and limits has been used to reanalyze published 40.sup.Ar/ [.sup.39]Ar apparent age spectra. This reanalysis supports the concept that there were separate and quite distinct high-pressure metamorphic mineral growth events, and allows potential constraints as to the timing of some of these events to be developed. M.sub.1B eclogite-facies metamorphism is estimated to have occurred at some time in the period 53--49 Ma, the M.sub.1C blueschist-facies metamorphic event at some time in the period 44--38 Ma, and the M.sub.1D transitional blueschist-facies metamorphic event is estimated to have occurred at some time in the period 35--30 Ma. A kinematic model is proposed to explain the geometry of a thinly sliced tectono-metamorphic stratigraphy, as observed, and the reason as to why individual tectonic slices in this 'tectono-metamorphic stratigraphy' should display distinctive patterns of fabrics and micro-structures, as well as characteristic temperature-time curves as inferred by 40.sup.Ar/ [.sup.39]Ar geochronology. Author Affiliation: (1) Research School of Earth Sciences, The Australian National University, 0200, Canberra, ACT, Australia Article History: Registration Date: 23/08/2005 Received Date: 09/03/2005 Accepted Date: 18/08/2005 Online Date: 12/11/2005 Article note: Communicated by A. Hofmann

Despite extensive research in the last 150 years, the regional tectonic reconstruction of the Western Alps has remained controversial. The curved orogenic belt consists of several ribbon-like continental terranes (Sesia/Austroalpine, Internal Crystalline Massifs, Brianconnais), which are separated by two or more ophiolitic sutures (Piemonte, Valais, Antrona?, Lanzo/ Canavese?). High-pressure (HP) metamorphism of each terrane occurred during distinct orogenic episodes: at -65 Ma in the Sesia/Austroalpine, at ~45 Ma in the Piernonte zone and at -35 Ma in the Internal Crystalline Massifs. It is suggested that these events reflect individual accretionary episodes, which together with kinematic indicators and the speed and direction of plate motions, provide constraints for the discussed reconstruction model. The model involves a prolonged orogenic history that took place during relative convergence of Europe and Adria (here considered as a promontory of the African plate). The first accretionary event involved the Sesia/Austroalpine terrane. Final closure of the Piemonte Ocean occurred during the Eocene (~45 Ma) and involved ultra-high-pressure (UHP) metamorphism of the Piemonte oceanic crust. Incorporation of the Brianconnais terrane in the accretionary wedge occurred thereafter, possibly during or after subduction of the Valais Ocean in the late Eocene (45-35 Ma). This subduction was terminated at ca. 35 Ma, when the Internal Crystalline Massifs (i.e. the assumed internal parts of the Briangonnais terrane) were buried into great depths and underwent HP and UHP metamorphism. Keywords: Alps; Alpine Orogeny; Reconstruction; Ophiolite complexes; Subduction

[1] Reconstruction of the evolution of the Tyrrhenian Sea shows that the major stage of rifling associated with the opening of this basin began at ~10 Ma. It involved two episodes of back are extension, which were induced by the rollback of a west dipping subducting slab. The first period of extension (10-6 Ma) was prominent in the northern Tyrrhenian Sea and in the western part of the southern Tyrrhenian Sea. The second period of extension, mainly affected the southern Tyrrhenian Sea, began in the latest Messinian (6-5 Ma) and has been accompanied by subduction rollback at rates of 60-100 km My[r.sup.-1]. Slab reconstruction, combined with paleomagnetic and paleogeographic constraints, indicates that in the central Apennines, the latest Messinian (6-5 Ma) arrival of a carbonate platform at the subduction zone impeded subduction and initiated a slab tear and major strike-slip faults. These processes resulted in the formation of a narrow subducting slab beneath the Ionian Sea that has undergone faster subduction rollback and induced extreme rates of back are extension. INDEX TERMS: 3640 Mineralogy and Petrology: Igneous petrology; 8109 Tectonophysics: Continental tectonics extensional (0905); 8010 Structural Geology: Fractures and faults; 9604 Information Related to Geologic Time: Cenozoic; KEYWORDS: Tyrrhenian Sea, Apennines, Sicilian Maghrebides, subduction rollback, reconstruction. Citation: Rosenbaum, G., and G. S. Lister (2004), Neogene and Quaternary rollback evolution of the Tyrrhenian Sea, the Apennines, and the Sicilian Maghrebides.

Lower plate rocks exposed in the Buckskin-Rawhide metamorphic core complex, west-central Arizona, underwent a pronounced increase in cooling rate 7-12 m.y. after the onset of extensional tectonic denudation at ca. 27 Ma. A detailed 40Ar/39Ar study of lower plate rocks exposed near Planet Peak, in the southwest part of the core complex, indicates that the highest-level mylonitic rocks exposed in the complex cooled below 350 [degrees] C prior to 20 [+ or -] 1 Ma, at an average rate of [less than or equal to]15 [degrees] C/m.y. Thereafter, the cooling rate of these rocks increased progressively, peaking at 28080 [degrees] C/m.y. prior to initial exposure of the core between 15.1 and 13.3 Ma. Published K-Ar, 40Ar/39Ar, and fission-track ages for the most recently unroofed lower plate rocks, exposed at the northeastern end of the core complex, are consistent with a similar cooling history, although the abrupt increase in cooling rate occurred as many as 5 m.y. later in this area. Because the increase in the cooling rate appears to have occurred progressively later toward the northeast, it is unlikely to reflect changes in slip rate. It appears to have coincided with cooling below 350-300 [degrees] C, and thus is interpreted to reflect steepening of the detachment system above the brittle-ductile transition. Thermochronological data from lower plate rocks in the Planet Peak area, together with constraints on their initial exposure at the surface, are consistent with an increase in the dip of the detachment from [less than or equal to]5 [degrees] at midcrustal depths (where it was represented by a shear zone) to [less than]24 [degrees] in the upper crust. At temperatures below 200-150 [degrees] C, lower plate rocks at both ends of the core complex probably cooled at rates [greater than]130 [degrees] C/m.y., which can not be reconciled with the average dip ([less than or equal to]25 [degrees]) and slip rate (6-12 mm/yr) for the detachment fault, given the likely geothermal gradient ([approximately]25 [degrees] C/km) during the early stages of extension. This implies that cooling did not keep pace with denudation at upper crustal levels. The very high cooling rates are interpreted to have primarily reflected the development of a pronounced thermal discontinuity across the detachment fault (possibly [greater than]100 [degrees] C), as hot lower plate rocks were transported to shallow crustal levels. Rapid cooling of the lower plate at these levels may have been enhanced by interaction with fluids derived from the upper plate, consistent with models that link the shallow-level ([less than]2-3 km), moderate-temperature ([less than]330 [degrees] C) hydrothermal mineralization along detachment faults to the rapid uplift of hot lower plate rocks.

The formation of metamorphic core complexes may be triggered by plutonic activity during episodes of continental extension. Pulses of ductile deformation have taken place during short-lived thermal events initiated by the heat input from intruded plutons, sills, or dikes. Such intrusions may be the underlying cause for differential uplift of the footwall during tectonic denudation of metamorphic core complexes. Fast cooling inferred from [.sup.40Ar]/[.sup.39Ar] apparent ages may have taken place after periods of magma arrival, and need not be the result of rapid erosional or tectonic denudation. Heterogeneity of [.sup.40Ar]/[.sup.39Ar] apparent ages can be explained by rapid cooling of deforming mylonites formed at shallow crustal levels in the thermal aureoles of intruded sills (or sill swarms). Under such circumstances, apparently isothermal decompression paths may actually link pressure-temperature points set during periods of transient mineral growth during and after periods of igneous activity.

The metamorphic basement, or granodiorites, and shear zone rocks of D'entrecasteaux Island in Papua New Guinea show a rapid cooling of the orogenic belt. Active extension at the western end of the propagating Woodlark Basin leads to the formation of metamorphic core complexes. A thermochronologic study reveals that the gneisses found in the core of the dome is cooling rapidly and that the retrogression of the metamorphic basement movement on the outer shear zone coincides with emplacement of granodiorites. This has resulted in the formation and the unroofing of the domes in the north-south directed extension of the region.