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A quartz sandstone bed in a dominantly conglomeratic section in the central part of the Lower Carboniferous Horton Group on Petit-de-Grat Island, offshore southern Cape Breton Island, Nova Scotia, contains [...]

The Cenozoic sedimentary basins on the Atlantic margin of northwest Britain contain a remarkable record of tectonically influenced post-breakup sedimentation. We have mapped the distribution and quantified the solid grain volume of four unconformity-bound successions in the region, the Eocene (~6-8 x [10.sup.4] [km.sup.3]), Oligocene (~2 x [10.sup.4] [km.sup.3]), Miocene--lower Pliocene (~4-5 x [10.sup.4] [km.sup.3]) and lower Pliocene--Holocene (-4-5 x [10.sup.4] [km.sup.3]), complementing previous work on the Paleocene succession. Of the total Cenozoic sediment volume on the Atlantic margin of northwest Britain, ~80% was deposited in Eocene and later time. The relative volumes of the Cenozoic succession do not support previous claims that the Paleocene was the main period of Cenozoic uplift and erosion of sediment source areas. Rather, the Cenozoic sedimentary basins on the Atlantic margin of northwest Britain record the detritus of four major episodes of Cenozoic uplift of the British Isles (Paleocene, Eocene--Oligocene, Miocene, and Pliocene--Pleistocene). doi: 10.1130/G30881.1

Microphytobenthos (MPB) and mangrove detritus were labeled with a carbon isotope ([sup.13]C) in separate experiments to quantify their contributions to the nutrition of major faunal components within a mangrove forest. Within 7 d of MPB labeling, crabs (Parasesarma erythrodactyla and Australoplax tridentata) and foraminifera (Ammonia beccarii and Trochammina inflata) were enriched. A. tridentata became more enriched (e.g., hepatopancreas, 522 [per thousand]) than P. erythrodactyla (110 [per thousand]), and A. beecarii (245 [per thousand]) became more enriched than T. inflata (12 [per thousand]). Addition of labeled mangrove detritus (- 11.5 [per thousand] final enrichment vs. -28.8 [per thousand] for controls) to sediment resulted in enrichment of P. erythrodactyla (hepatopancreas, -21.2 [per thousand] vs. -26.6 [per thousand] for controls), A. tridentata (hepatopancreas, -24.2 [per thousand] vs. -27.1 [per thousand]) and A. becearii (-21.0%o vs. -25.1 [per thousand]) within 7 d. Compartment modeling showed that MPB contributed 93% of the nutrition for A. tridentata and 33% of the nutrition for P. erythrodactyla and that MPB provided more nutrition to A. beecarii (14%) than to T. inflata (minimal). There was a complementary estimated contribution of mangrove detritus to the diets of P. erythrodactyla (80%), A. beecarii (97%), and A. tridentata (minimal), although these estimates should be viewed with caution, due to low initial enrichment and the apparent short temporal persistence of [sup.13]C-labeled detritus added to sediments. T. inflata was barely enriched in either experiment and may rely on a carbon source not considered. The combination of isotope labeling and compartment modeling is relatively new to ecology and shows potential for revealing differences in the patterns of use of algae and macrophyte detritus by consumers.

Our study examined the response of leaf detritus--associated microorganisms (both bacteria and fungi) to a 5-yr continuous nutrient enrichment of a forested headwater stream. Leaf litter dominates detritus inputs to such streams and, on a system-wide scale, serves as the key substrate for microbial colonization. We determined physiological responses as microbial biomass and activity expressed per unit mass of leaves and system-level responses by quantifying leaf litter standing crop monthly and expressing responses per unit area of streambed. Physiological (mass-specific) trends differed from system-level (area-specific) trends. Physiological responses to enrichment were generally positive. With the exception of bacterial biomass, nutrients increased all metrics expressed per unit mass leaf litter in the treatment stream relative to the reference (fungal biomass and production, bacterial production, microbial respiration). This positive physiological response to nutrient enrichment was associated with lower leaf litter standing crop in the treatment stream, resulting in less substrate for microbial colonization. Consequently, during most years on a system-level scale, only fungal production and microbial respiration were positively affected by nutrients, whereas fungal biomass was negatively affected. Thus, from a whole-stream perspective, nutrients led to a lower quantity of leaf detritus with greater variation, resulting in net reductions of associated fungal biomass and greater intra-annual variability in both fungal biomass and respiration. Our results demonstrate profound effects of nutrients on heterotrophic pathways that mediate detritus processing in stream ecosystems. Similar effects on heterotrophic microbes and detrital resources may be a widespread consequence of anthropogenic nutrient enrichment.

Avalonia is the largest accreted crustal block in the Appalachian orogen and comprises a collection of late Neoproterozoic volcano-sedimentary sequences that are overlain by a Palaeozoic platformal sedimentary succession. Detrital zircons from the Conception Group are dominated by 570-620 Ma ages and contain a significant component generated by erosion of coeval igneous arc-volcanic rocks. Overlying samples from the Cuckold and Crown Hill formations are dominated by Neoproterozoic populations with ages between 600 and 650 Ma and are interpreted to be derived from the underlying calc-alkaline arc-plutonic rocks. Early Palaeozoic platform units are dominated by c. 620 Ma zircons with lesser Mesoproterozoic and Palaeoproterozoic zircons. The range of detrital zircon ages is inconsistent with a West African provenance and suggests that Avalonia originated along the Gondwanan margin of the Amazon craton. The influx of Mesoproterozoic and Palaeoproterozoic detritus in the Avalonian platform suggests a major change in tectonic regime. The prominent change in provenance is interpreted to be related to separation of Avalonia from Gondwana during the Early Ordovician opening of the Rheic Ocean. The Redmans Formation is interpreted to represent the rift--drift transition of the Rheic Ocean, which imposes important constraints on the palaeotectonic evolution of Avalonia. Supplementary material: U--Pb isotopic data of LA-ICP-MS analysis of detrital zircons are available at http://www.geolsoc.org.uk/SUP18346.

U--Pb ion microprobe detrital zircon provenance investigations of pre-Mesozoic sediment samples from southern Novaya Zemlya and northern Taimyr, Arctic Russia, provide new constraints on the tectonic evolution of northern Eurasia. Sediments from both areas previously assigned a Neoproterozoic age are shown to be Palaeozoic ([less than or equal to] 500 Ma), changing our understanding of the timing of tectonic events and their regional correlation. Samples from Novaya Zemlya provide an a priori Baltica signature, and dominant cumulative probability age peaks between 610 and 530 Ma are consistent with derivation from Timanide sources. Samples from northern Taimyr also contain the unique Timanian 'fingerprint' and are also inferred to be derived from the continental slope of northeastern Baltica, implying that the North Kara block was a part of Baltica in the early Palaeozoic. These data constrain the timing of deformation, metamorphism and development of an unconformity in southern Novaya Zemlya to a restricted period no earlier than late Cambrian--early Ordovician and at least 50 Ma after the peak Timanian event. The unconformity on Novaya Zemlya can be correlated in style and timing with an unconformity on Severnaya Zemlya and may record post-Timanian extensional collapse and onset of spreading in the Uralian Ocean. Supplementary material: U--Th--Pb ion-microprobe analytical results are available at http://www.geolsoc.org.uk/SUP18340.

Detrital zircon analysis of sandstones interbedded with c. 464 Ma ignimbrites in the lower Mweelrea Formation of the South Mayo Trough, western Ireland, suggests Ordovician source-rock provenance that corresponds to two distinct volcanic-arc phases on the Laurentian margin. East-derived sandstones contain a suite of zircons with a mean age of c. 487 Ma that suggests derivation from the Cambrian to early Ordovician Baie Verte Oceanic Tract arc--ophiolite complex, locally represented by the Lough Nafooey arc rocks and the Clew Bay Complex. Zircons from south-derived sandstones within the Bunnacunneen conglomerate fan have average ages of c. 467-474 Ma, and correspond to the Notre Dame arc and locally the Connemara metagabbro and orthogneiss suite. Granite clasts in the Bunnacunneen conglomerate are similar to the Connemara orthogneiss suite, in terms of both their geochemistry and their age (c. 471 Ma). The southerly derived sedimentary strata also include Archaean and Proterozoic zircon age spectra consistent with a Dalradian source. A southern provenance from the Notre Dame arc and Dalradian rocks suggests that the Connemara terrane lay to the south of the South Mayo Trough during middle Llanvirn times, from at least 464 Ma. Supplementary material: U--Pb laser ablation multicollector inductively coupled plasma mass spectrometry data for detrital zircons and details of analytical methods for U--Pb LA-MC-ICPMS and U--Pb isotope dilution thermal ionization mass spectrometry analyses are available at http://www.geolsoc.org.uk/SUP18331.

We applied apatite U-Pb, fission track, and (U-Th)/He triple dating and white mica [sup.40]Ar/[sup.39]Ar thermochronology to syntectonic sedimentary rocks from the central Andean Puna plateau in order to determine the source-area geochronology and source sedimentary basin thermal histories, and ultimately the timing of multiple tectonothermal events in the Central Andes. Apatite triple dating of samples from the Eocene Geste Formation in the Salar de Pastos Grandes basin shows late Precambrian-Devonian apatite U-Pb crystallization ages, Eocene apatite fission track (AFT), and Eocene-Miocene (U-Th)/He (ca. 8-47 Ma) cooling ages. Double dating of cobbles from equivalent strata in the Arizaro basin documents early Eocene (46.2 [+ or -] 3.9 Ma) and Cretaceous (107.6 [+ or -] 7.6, 109.5 [+ or -] 7.7 Ma) AFT and Eocene-Oligocene (ca. 55-30 Ma) (U-Th)/He ages. Thermal modeling suggests relatively rapid cooling between ca. 80 and 50 Ma and reheating and subsequent diachronous basin exhumation between ca. 30 Ma and 5 Ma. The [sup.40]Ar/[sup.39]Ar white mica ages from the same samples in the Salar de Pastos Grandes area are mainly 400-350 Ma, younger than apatite U-Pb ages, suggesting source-terrane cooling and exhumation during the Devonian-early Carboniferous. Together these data reveal multiple phases of mountain building in the Paleozoic and Cenozoic. Basin burial temperatures within the plateau were limited to

The Famatina belt, Central Andes, is part of an ancient accretionary margin built along Western Gondwana in the early Palaeozoic. U-Pb ion microprobe analysis of detritai zircons and Sm-Nd whole-rock analysis of two early Palaeozoic low-grade metasedimentary units record the early evolution of this region. Detrital zircons in the Negro Peinado and Achavil formations have ages ranging from Palaeoproterozoic to Cambrian, consistent with derivation from Gondwanan sources. [T.sub.DM] ages suggest that the sedimentary rocks were derived from a composite source area, which separated from the mantle during the Palaeoproterozoic (c. 1.8-1.6 Ga). Constraints from the youngest detrital grains indicate accumulation in a Mid- to Late Cambrian foreland basin adjacent to the inboard Pampean orogenic tract. The dominance of Cambrian ages in the Negro Peinado Formation suggests derivation principally from the eastern Pampean belt whereas the dominance of late Neoproterozoic ages in the Achavil Formation suggests that input from the Pampean belt was overwhelmed by older sources. The paucity of Palaeoproterozoic ages argues against direct input from older areas such as the Rio de la Plata craton. The predominance of Meso- and Neoproterozoic ages over older sources suggests that a Brasiliano-age magmatic arc developed on a Mesoproterozoic basement, probably a southern extension of the Arequipa-Antofalla massif.

U-Pb dating of detrital zircons shows that the provenance of Cretaceous-Palaeogene sandstones on Hatton and Edoras banks (SW Rockall Plateau) comprises magmatic rocks dated at c. 1800 Ma and c. 1750 Ma, respectively. Their depositional setting, first-cycle mineralogy and unimodal detrital zircon populations suggest that these sandstones are of local origin. The zircon age data are therefore considered to provide constraints on these poorly understood areas of the Rockall Plateau. The U-Pb dates are directly comparable with U-Pb zircon crystallization ages from granitoid rocks reported from the Ketilidian Belt of southern Greenland and from the Rhinns Complex of western Britain. Hf isotopic data from the Edoras Bank sample are consistent with derivation from a juvenile Palaeoproterozoic block. In conjunction with previously reported Sm-Nd [T.sub.dm] model ages from the Ketilidian Belt, Rockall Bank and the Rhinns Complex, these data extend the known distribution of a large juvenile Palaeoproterozoic terrane spanning the southern NE Atlantic. In contrast, Hf isotopic data from the Hatton Bank sample imply a large contribution from Archaean crust. The zircon population from Edoras Bank also contains sparse Mesoproterozoic grains, providing evidence for the presence of volumetrically minor Grenville-age intrusions in the southern part of the Rockall Plateau.

Early Paleozoic sandstones in the Southern Uplands terrane of Scotland were deposited during closure of the Iapetus Ocean between Laurentia and Avalonia. Their tectonic setting and sources are controversial, and different authors have supported subduction-accretion, extensional continental-margin development, or back-arc basin settings. We report new U-Pb detrital zircon ages from five Late Ordovician sandstones from the Northern Belt of the Southern Uplands and test models of their tectonic setting. The U-Pb zircon age distributions are dominated by peaks characteristic of sources in Laurentia and include grains as old as 3.6 Ga, older than any previously recorded in the British Caledonides SE of the Laurentian foreland. Discordant grains in one sample suggest derivation via erosion of metasedimentary rocks incorporated in the Grampian-Taconian orogen. Rare Neoproterozoic grains, previously interpreted as originating from a peri-Gondwanan terrane, may be derived from igneous rocks associated with Iapetan rifting. Only rare zircons are contemporary with the depositional ages. The results are difficult to reconcile with extensional continental-margin and back-arc models, but they support an active continental-margin subduction-accretion model. Close similarities with distributions from the Newfoundland Appalachians are consistent with sinistral transpression during closing of the Iapetus Ocean. Keywords: tectonics, Caledonides, Southern Uplands, detrital zircon geochronology, provenance.

[1] Tertiary convergence, high-pressure-lowtemperature (HP-LT) metamorphism and subsequent exhumation in the internal Western Alps were concurrent with detrital deposition in foreland basins at the external periphery of the orogen. In the present work we have probed the geochronology and chemistry of white mica grains and metamorphic pebbles from the foreland basins of SE France in order to gain additional perspective on exhumation rates and mountain building processes in the internal domain of the Tertiary orogen. Our data indicate that throughout the Tertiary these basins were mainly fed from a relatively low pressure source, dominated by Si-poor micas yielding pre-Alpine [sup.40]Ar/[sup.39] Ar ages. Small amounts of detritus composed of high-pressure minerals and pebbles are first detected in early Oligocene (~30-32 Ma) strata of the Barreme basin. The [sup.40]Ar/[sup.39]Ar geochronology of phengites from garnet-blueschist pebbles from these strata yielded 34 [+ or -] 3 Ma ages indicating rapid ascent and exposure of HP-LT rocks in the internal part of the orogen shortly after metamorphism. To explain the preponderance of pre-Alpine micas alongside the rare presence of Alpine HP-LT detritus we suggest that the general architecture of the Western Alps, whereby inner HP-LT metamorphic units are cut off from the SE France basins by a low-grade lid of Brianconnais back thrusts, was rapidly built in the early Oligocene and did not change significantly since then. Citation: Morag, M., D. Avigad, Y. Harlavan, M. O. McWilliams, and A. Michard (2008), Rapid exhumation and mountain building in the Western Alps: Petrology and [sup.40]Ar/[sup.39]Ar geochronology of detritus from Tertiary basins of southeastern France, Tectonics, 27, TC2004, doi: 10.1029/2007TC002142.