Tectono-thermal evolution of a part of a Variscan magmatic arc: The Odenwald in the Mid-German Crystalline Rise

Geothermobarometric studies of plutonic and medium to high grade metamorphic rocks of the Odenwald (SW Germany) provide data for the construction of PT-paths of four different crustal sections within a part of a magmatic arc of the Mid-European Hercynian orogen.The evolution of the thermal structure of the Odenwald is characterized by an early geotherm of 35–40°C/km after crustal stacking involving rocks with medium pressure relics. Peak PT-conditons reached 4–5 kbar and 650°C. Only rocks of the northernmost Odenwald show evidence for PT conditions related to a higher geotherm at this time. Early uplift occurred during normal oblique sinistral strike shear. The central Odenwald was uplifted into a level corresponding to 3 kbar during intrusion of calcalkaline magmas. The northernmost Odenwald subsided at the same time, as proved by an anticlockwise PT-path. Subsequently, in the southern and eastern Odenwald, rapid uplift caused a further increase of the geotherm to 60–80°C/km. This is concomitant with extension and granitoid intrusions.Crustal thickening in the Odenwald is restricted to an early phase during the Devonian. The magmatic arc developed within a »pull-apart«-like structure during the Lower Carboniferous.

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Zusammenfassung Geothermobarometrische Untersuchungen an Plutoniten und mittelbis hochgradigen Metamorphiten des Odenwaldes ermöglichen die Konstruktion von PT-Pfaden für vier Krustenabschnitte in einem Teil eines variszischen magmatischen Gürtels.Zu Beginn der Entwicklung der thermischen Struktur des magmatischen Gürtels stellte sich ein Geotherm von 35–40°C/km nach einer Krustenstapelung ein, in die Gesteine mit Mitteldruckrelikten einbezogen wurden. Die PT-Bedingungen zum Höhepunkt der Metamorphose erreichten 4–5 kbar und 650°C. Nur Gesteine des nördlichen Odenwaldes belegen PT-Bedingungen, die auf einen höheren Geotherm zu dieser Zeit hinweisen. Frühe Hebung fand entlang von Schrägabschiebungen statt. Der zentrale Odenwald wurde bei gleichzeitiger Intrusion kalkalkaliner Magmen in ein Niveau entsprechend 3 kbar gehoben. Der nördliche Odenwald sank gleichzeitig ab, wie durch einen gegen den Uhrzeigersinn gerichteten PT-Pfad belegt wird. Im südlichen und östlichen Odenwald bewirkte danach schnelle Hebung einen weiteren Anstieg des Geotherms auf 60–80°C/km verbunden mit Dehnung und Intrusion granitoider Magmen.Während Krustenverdickung auf eine frühe Phase im Devon beschränkt ist, entwickelt sich der magmatische Gürtel im Bereich des Odenwaldes im Unterkarbon innerhalb einer » pull apart «-ähnlichen Struktur.

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Résumé Dans l'Odenwald (sud-ouest de l'Allemagne), l'étude géothermobarométrique de roches plutoniques et de roches métamorphiques de degré moyen à élevé fournit les données qui permettent la construction de trajets (P, T) relatifs à quatre sections crustales dans une partie d'un arc magmatique de l'orogène varisque d'Europe.Le début de l'évolution de la structure thermique de l'Odenwald a été marqué par un gradient géothermique de 35–40°C/Km, en suite à un empilement crustal impliquant des roches qui contiennent des reliques de pression moyenne. Les conditions (P, T) du métamorphisme maximal furent de 4–5 Kbar et 650°C. Seules les roches de l'Odenwald septentrional témoignent d'un gradient plus élevé à cette époque. Un soulèvement hâtif s'est produit le long de zones de cisaillement sénestres. Concomitamment à l'intrusion de magmas calcoalcalins, l'Odenwald central a été le siège d'un soulèvement jusqu'à un niveau correspondant à 3 Kbar. En même temps, l'Odenwald septentrional s'affaissait, comme en témoigne uu trajet (P, T) anti-horlogique. Par la suite, dans l'Odenwald méridional et oriental, une montée rapide associée à un processus d'extension et à l'intrusion de magmas granitoïdes a amené le gradient géothermique à des valeurs de 60 à 80° /Km.L'épaississement crustal est cantonné à une phaseprécoce d'âge dévonien, tandis que l'arc magmatique s'est développé dans une structure ra pullapart « au cours du Carbonifère inférieur.

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Latest precambrian (Cadomian) zircon ages,Nd isotopic systematics and P-T evolution of granitoid orthogneisses of the Erzgebirge,Saxony and Czech Republic

Single zircons from two orthogneiss complexes, the Grey Gneiss and Red Gneiss, the lowermost tectonic units in the Erzgebirge, were dated. The grey Freiberg Gneiss is of igneous origin and has a ²⁰⁷Pb/²⁰⁶Pb emplacement age of 550±7 Ma. A quartz monzonite from Lauenstein contains idiomorphic zircons with a mean ²⁰⁷Pb/²⁰⁶Pb age of 555±7 Ma as well as xenocrysts ranging in age between 850 and 1910 Ma. Red gneisses from the central Erzgebirge contain complex zircon populations, including numerous xenocrysts up to 2464 Ma in age. The youngest, idiomorphic, zircons in all samples yielded uniform ²⁰⁷Pb/²⁰⁶Pb ages between 550±9 and 554±10 Ma. Nd isotopic data support the interpretation of crustal anatexis for the origin of both units. _Nd(t) values for the grey gneisses are –7.5 and –6.0 respectively, (mean crustal residence ages of 1.7–1.8 Ga). The red gneisses have a wider range in _Nd(t) values from –7.7 to –2.8 (T _DM ages of 1.4–1.8 Ga). The zircon ages document a distinct late Proterozoic phase of granitoid magmatism, similar in age to granitoids in the Lusatian block farther north-east. However, Palaeozoic deformation as well as medium pressure metamorphism ( 8 kbar/600–650° C) are identical in both gneiss units and distinguish these rocks from the Lusatian granitoids. The grey and red gneisses were overthrust by units with abundant high-pressure relicts and a contrasting P-T evolution. Zircon xenocryst and Nd model ages in the range 1000–1700 Ma are similar to those in granitoid rocks of Lusatia and the West-Sudetes, and document a pre-Cadomian basement in parts of east-central Europe that, chronologically, has similarities with the Sveconorwegian domain in the Baltic Shield.     

Impact of partial steps and momentum advection schemes in a global ocean circulation model at eddy-permitting resolution

Series of sensitivity tests were performed with a z-coordinate, global eddy-permitting (1/4°) ocean/sea-ice model (the ORCA-R025 model configuration developed for the DRAKKAR project) to carefully evaluate the impact of recent state-of-the-art numerical schemes on model solutions. The combination of an energy–enstrophy conserving (EEN) scheme for momentum advection with a partial step (PS) representation of the bottom topography yields significant improvements in the mean circulation. Well known biases in the representation of western boundary currents, such as in the Atlantic the detachment of the Gulf Stream, the path of the North Atlantic Current, the location of the Confluence, and the strength of the Zapiola Eddy in the south Atlantic, are partly corrected. Similar improvements are found in the Pacific, Indian, and Southern Oceans, and characteristics of the mean flow are generally much closer to observations. Comparisons with other state-of-the-art models show that the ORCA-R025 configuration generally performs better at similar resolution. In addition, the model solution is often comparable to solutions obtained at 1/6 or 1/10° resolution in some aspects concerning mean flow patterns and distribution of eddy kinetic energy. Although the reasons for these improvements are not analyzed in detail in this paper, evidence is shown that the combination of EEN with PS reduces numerical noise near the bottom, which is likely to affect current–topography interactions in a systematic way. We conclude that significant corrections of the mean biases presently seen in general circulation model solutions at eddy-permitting resolution can still be expected from the development of numerical methods, which represent an alternative to increasing resolution.     

Changes in the abundance of the seagrassesZostera marina L. (eelgrass) andRuppia maritima L. (widgeongrass) in San Diego, California, following and El Niño Event

Changes in environmental conditions can be accompanied by shifts in the distribution and abundances of organisms. When physical factors become unsuitable for growth ofZostera marina (eelgrass), which is a dominant seagrass species in North America, other more ruderal seagrass species, includingRuppia maritima (widgeongrass), often increase in abundance or replace the dominant species. We report the proliferation of widgeongrass into eelgrass beds in Mission Bay and San Diego Bay in San Diego, California, during the 1997 to 1998 El Niño Southern Oscillation (ENSO). Widgeongrass persisted in these eelgrass beds at least one year after a return to non-ENSO conditions and an increase in eelgrass density. We suggest that a warming of the water in two bays in San Diego by 1.5–2.5°C could result, in a permanent shift in the local seagrass vegetation from eelgrass to widgeongrass. This shift, could, have substantial ecosystem-level ramifications.     

Time of formation and peak of Variscan HP-HT metamorphism of quartz-feldspar rocks in the central Erzgebirge,Saxony, Germany

The Variscan Erzgebirge represents an antiform with a core of gneisses and mica schists, surrounded by a phyllitic mantle. The Gneiss-Eclogite Unit (GEU), in the central part, is a composite tectonometamorphic assemblage characterized by a HP-HT imprint and comprises migmatitic para- and orthogneisses, HT mylonites, HP granulites, eclogites and garnet peridotites. It is tectonically sandwiched between two major units with distinctly lower PT histories. The GEU experienced a characteristic “kinked” retrograde PT path after HP-HT equilibration with: (1) strong near-isothermal decompression at high temperatures; (2) extensive re-equilibration at medium pressures, followed (3) by rapid cooling during continued uplift. We dated zircons (Pb-Pb evaporation) from granitoid orthogneisses and metapelites of the GEU. The orthogneisses contain euhedral, long-prismatic zircons of igneous origin that provided protolith ages between 470 and 524 Ma. Metapelites retain well-preserved granulite-facies mineral assemblages and contain spherical, multifaceted metamorphic zircons that grew near the peak of HP/HT metamorphism. Inclusions of prograde HP phengite (∼15 kbar) and rutile are included in one such zircon. Metamorphic zircons of three samples from different localities yielded identical ²⁰⁷Pb/²⁰⁶Pb ages of 340.5 ± 0.7 Ma, 341.2 ± 0.5 Ma and 341.6 ± 0.5 Ma respectively. Consideration of these zircon ages with published ³⁹Ar/⁴⁰Ar white mica ages suggests fast cooling and uplift rates in excess of 50 °C/Ma and 4 km/Ma. This is typical for large-scale extensional tectonic unroofing of the ultra-deep part of a fossil, thickened Variscan continental crust (>60 km) during continuing continental collision and orogenic collapse. Received: 5 June 1997 / Accepted: 7 January 1998