Formation of basins and mountain ranges in Attica (Greece): The role of Miocene to Recent low-angle normal detachment faults

In seismically active regions, active low-angle detachment faults are probably more frequent as is commonly thought and may play an important but still underestimated role in the evolution of landforms and basins. We investigate the tectonically active region of Attica (Greece) in the Aegean back arc as a model region to show how basins and mountain ranges commonly thought to be formed by movements on high-angle normal faults in fact reflect the surface expression of displacements on yet undetected, deep-seated, active low-angle normal detachment faults. Inferences are made based on an integrated study of Attica linking the petrology of clastic sediments with geomorphology and structures, and including few new palynological data. From the Miocene to Recent, three sets of normal detachment fault systems were successively active. Shear zones of the 1st (Early Miocene) stage emplaced rocks of the Attic Cycladic high-P metamorphic belt (AC-HP-belt) from depth corresponding to greeschist facies conditions in the brittle, upper crust. In the 2nd stage the WNW dipping Attica low-angle normal detachment fault system between the AC-HP-belt and the un- or weakly metamorphosed rocks of the sub-Pelagonian Zone (SPZ) was active. Clastic sedimentation started in the Late Miocene, during the 2nd stage. Late Miocene and Early Pliocene clastic sediments reveal that during the 2nd stage many areas that presently expose the AC-HP-belt were still covered by the overlying SPZ. Also, now uplifted areas such as the Parnitha mountain range that currently undergo strong erosion were then the sites of sedimentary sinks. The 3rd stage (Late Pliocene through Recent) is associated with dramatic changes in the morphology and recurring steepening of the relief. Reversal of the Parnitha area from the site of deposition into the site of erosion is associated with deposition of coarse conglomerates to the SE of the Parnitha Mt. and S of the Penteli Mt. Sediments of the 3rd stage reflect activity of the here newly described, SSE-dipping Penteli—Athens low-angle detachment fault (PADF) system formed at a high angle to the Attica detachment fault. The outcome of this study is that the present-day geomorphology is to a high degree related to the operation of the PADF system. Steep fault bounding the Athens and Mesogea basins as well as the mountain ranges (Parnitha, Penteli, Hymittos mounts) belongs to its breakaway zone or root into the PADF. Ongoing tectonic movements related to this fault system were responsible for the 1999 Athens (Mw = 6.0) earthquake. We particularly discuss how the PADF may continue into greater depth, the translation magnitude, and how the PADF fits into the wider kinematic framework of the Aegean region.     

Metapegmatites in the western Bohemian massif: ages of crystallisation and metamorphic overprint, as constrained by U–Pb zircon, monazite, garnet, columbite and Rb–Sr muscovite data

Granitic metapegmatites of two crystalline units of the western Bohemian massif, the Zone of Erbendorf-Vohenstrauss in Germany (ZEV), and the Zone of Teplá-Doma?lice in the Czech Republic (ZTD) have been dated by means of U–Pb and Rb–Sr methods. Ages interpreted to reflect emplacement and crystallisation of the pegmatites were found consistently to be approximately 480?Ma, as constrained by U–Pb analyses of primary magmatic zircon, monazite, garnet and columbite, and by Rb–Sr analyses of large pegmatitic muscovites. Later Devonian amphibolite-facies metamorphism caused ductile shearing of the pegmatites, leading to partial recrystallisation of pegmatitic material. A metamorphic, fine-grained generation of muscovite yielded consistent Rb–Sr ages of 371–376?Ma for both the ZEV and the ZTD, interpreted as dating the end of deformational activity. The Rb–Sr system of the large pegmatitic muscovites turned out to remain closed up to metamorphic temperatures of >600?°C. Deformation at elevated temperatures is identified as the dominant mechanism for opening of the Rb–Sr system of primary muscovites: apparent ages grade towards Devonian ages as the muscovites become more deformed and fragmented. The data derived from the metapegmatites point to a similar or common tectonometamorphic evolution for the ZEV and the ZTD since the Ordovician. Furthermore, the magmatic formation of granitic pegmatites implies an upper crustal position of the intruded rocks in Lower Ordovician times, clearly documenting the existence of two distinct metamorphic cycles. In contrast to this, in the neighbouring part of the Moldanubian, only unmetamorphosed granitic pegmatites with intrusion ages around 320?Ma are observed.     

Domainal variations of U–Pb monazite ages and Rb–Sr whole-rock dates in polymetamorphic paragneisses (KTB Drill Core, Germany): influence of strain and deformation mechanisms on isotope systems

Polyphase metamorphic paragneisses from the drill core of the continental deep drilling project (KTB; NW Bohemian Massif) are characterized by peak pressures of about 8 kbar (medium‐P metamorphism) followed by strain accumulation at T >650 °C, initially by dislocation creep and subsequently by diffusion creep. U–Pb monazite ages and Rb–Sr whole‐rock data vary in the dm‐scale, indicating Ordovician and Mid‐Devonian metamorphic events. Such age variations are closely interconnected with dm‐scale domainal variations of microfabrics that indicate different predominant deformation mechanisms. U–Pb monazite age variations dependent on microfabric domains exceed grain‐size‐dependent age variations. In ‘mylonitic domains’ recording high magnitudes of plastic strain, dislocation creep and minor static annealing, monazite yields concordant and near concordant Lower Ordovician U–Pb ages, and the Rb–Sr whole‐rock system shows isotopic disequilibrium at an mm‐scale. In ‘mineral growth/mobilisate domains’, in which diffusive mass transfer was a major strain‐producing mechanism promoting diffusion creep of quartz and feldspar, and in which static recrystallization (annealing) reduced the internal free energy of the strained mineral aggregates, concordant U–Pb ages are Mid‐Devonian. Locally, in such domains, Rb–Sr dates among mm³‐sized whole‐rock slabs reflect post‐Ordovician resetting. In ‘transitional domains’, the U–Pb‐ages are discordant. We conclude that medium‐P metamorphism occurred at 484±2 Ma, and a second metamorphic event at 380–370 Ma (Mid‐Devonian) caused progressive strain in the rocks. Dislocation creep at high rates, even at high temperatures, does not reset the Rb–Sr whole‐rock system, while diffusion creep at low rates and stresses (i.e. low ε/D_eff ratios), static annealing and the presence of intergranular fluids locally assist resetting. At temperatures above 650 °C, diffusive Pb loss did not reset Ordovician U–Pb monazite ages, and in domains of overall high imposed strain rates and stresses, resetting was not assisted by dynamic recrystallization/crystal plasticity. However, during diffusion creep at low rates, Pb loss by dissolution and precipitation (‘recrystallization’) of monazite produces discordance and Devonian‐concordant U–Pb monazite ages. Hence, resetting of these isotope systems reflects neither changes of temperature nor, directly, the presence or absence of strain.     

Emplacement of synkinematic plutons in the Variscan Odenwald (Germany) controlled by transtensional tectonics

In the Odenwald area (SW-Germany) an intermediate level of Variscan magmatic arc is exposed. Between 360 and 335 Ma the »Neunkirchen Intrusive Complex« intruded into an active NE-SW-trending zone of ductile divergent strike shear. After the thermal peak of metamorphism, but before the emplacement of the magmatic rocks shear zones developed in the wall rocks. The microfabrics of these zones infer plastic deformation of feldspar, hornblende and quartz and an increasing degree of postkinematic annealing towards the intrusive complex. During emplacement and solidification, the magmatic rocks experienced bulk strain: The intrusive complex locally crosscut the shear zones of the wall rocks, but show magmatic and also mylonitic foliations which parallel these zones. A change from plastic to brittle behavior of feldspar and amphibole and a varying degree of postkinematic annealing of quartz, suggest that in the intrusive complex strike shear continued during cooling. In wall rocks and plutonic rocks kinematic indicators imply sinistral displacement; in the wall rocks adjacent to the intrusive complex, strike shear was combined with normal displacement. This suggests that a component of horizontal extension which was associated with strike shear (transtension) facilitated ascend and emplacement of the magma. Transtension occurred at the same time, when the Schwarzwald/Vosges area (in the south of the Odenwald) was subjected to NW-SE compression; transtension was probably initiated by lateral escape of crustal fragments during overall convergent plate movements. Transtension is also supposed to be associated with significant uplift of metamorphic rocks.

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Zusammenfassung Im Gebiet des Odenwaldes (SW-Deutschland) ist das mittlere Krustenniveau eines variszischen magmatischen Gürtels aufgeschlossen. Zwischen 360 und 335 Ma intrudierte eine Abfolge magmatischer Gesteine (»Neunkirchener Flasergranitoid-komplex«) synkinematisch in eine NE-SW-streichende, divergente, duktile Blattverschiebungszone: Nach dem thermischen Höhepunkt der Metamorphose, aber noch vor der Platznahme der magmatischen Gesteine entwickelten sich im Nebengestein Scherzonen. Die Mikrogefüge dieser Zonen zeigen plastisches Deformationsverhalten von Feldspat, Hornblende und Quarz und postkinematische Temperung an. Während der Platznahme und Konsolidierung wurden die Magmatite deformiert. Die magmatische Abfolge schneidet lokal die Scherzonen des Nebengesteins, weist aber magmatische und auch mylonitische Foliationen parallel zu diesen Zonen auf. In den Magmatiten überdauerte die Abkühlung die Deformation: Feldspat und Amphibol zeigen einen Wechsel im Deformationsverhalten von plastisch zu spröde, der Grad der postkinematischen Temperung variiert. Im Nebengestein und in der magmatischen Abfolge wurden aus kinematischen Indikatoren sinistrale Blattverschiebungen abgeleitet. In den Nebengesteinen beinhalten die Blattverschiebungen eine deutliche Abschiebungskomponente. Dies läßt darauf schließen, daß horizontale Extension, die mit den Blattverschiebungen assoziiert war (Transtension) Aufstieg und Platznahme des Magmas erleichterte. Transtension fand gleichzeitig mit NW-SE-Kompression in Schwarzwald/Vogesen (südlich des Odenwaldes) statt. Transtension wurde möglicherweise durch »lateral escape« einzelner Krustenfragmente während der Kompression verursacht; sie ist wahrscheinlich auch mit signifikanter Hebung von metamorphen Gesteinen verbunden.

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Résumé Dans l'Odenwald (Allemagne du sud-ouest) affleure le niveau crustal moyen d'un arc magmatique varisque. Entre 360 et 335 Ma, un ensemble de roches magmatiques (le Complexe intrusif de Neukirchen) s'est mis en place de manière syncinématique dans une zone de cisaillement ductile divergent: après le pic thermique du métamorphisme, mais avant la mise en place des roches magmatiques, des shear-zones se sont développées dans les roches encaissantes. Les microfabriques de ces zones montrent une déformation plastique du feldspath, de la hornblende et du quartz et un recuit post-cinématique dont le degré augmente vers le complexe intrusif. Les roches magmatiques ont été déformées au cours de leur mise en place et de leur consolidation. Le complexe intrusif est localement sécant sur les shear-zones des roches encaissantes, mais présente des foliations magmatiques et mylonitiques parallèles à ces zones. Dans les roches magmatiques, la déformation cisaillante s'est poursuivie pendant le refroidissment: en témoignent le passage du comportement plastique du feldspath et de l'amphibole à un comportement rupturel, ainsi qu'un degré variable de recuit postcinématique du quartz. Tant dans les roches plutoniques que dans leurs encaissants, les marqueurs de déformation impliquent un déplacement sénestre; dans les roches encaissantes, le cisaillement est combiné à un déplacement normal. On en déduit qu'une composante d'extension horizontale, associée au cisaillement (« transtension »), a facilité la montée et la mise en place du magma. Cette «transtension» s'est manifestée au moment où la région des Vosges et de la Forêt Noire (au sud de l'Odenwald) était soumise à une compression et une transpression NW-SE; la transtension a été probablement initiée par le départ latéral («lateral escape») de fragments de croûte lors des mouvements de convergence des plaques. Elle est aussi vraisemblablement en liaison avec une montée significative des roches métamorphiques.

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. 360 335 , , NE-SW («Neunkirchener Flasergranitoidkomplex»). , . , , . . , , , . : , . , . . , , ( ), . NW-SE /, .. . , , («lateral escape») . , , .

     

Oblique crustal detachment in the Variscan Schwarzwald,southwestern Germany

The Schwarzwald is part of the central polymetamorphic crystalline belt of the Variscan Orogen (»Moldanubian Belt«). From north to south it consists of four terranes: the metasedimentary Zone of Baden-Baden, the polymetamorphic Central Schwarzwald Gneiss Complex, the sedimentary — metamorphic Zone of Badenweiler-Lenzkirch, and the Hotzenwald Complex. The largest of these terranes is the Central Schwarzwald Gneiss Complex (CSGC) whose rocks record a history of protracted regional metamorphism and anatectic melt generation. During Variscan convergence between 350 and 325 Ma the CSGC became detached from a high-temperature lower crustal substratum and was emplaced southeastward over Paleozoic clastics, volcanic rocks and crystalline slivers of the Zone of Badenweiler-Lenzkirch and the Hotzenwald Complex. Kinematic indicators suggest that these early convergent movements on retrograde shear zones and the concomitant crustal thickening were superseded by movements on divergent shear zones. The ascent of voluminous granitic plutons from a mid-crustal zone of melt generation into the upper crust was probably triggered by a change in the crustal kinematics from overall convergence to overall divergence at about 325 Ma. In detail this process was probably diachronous. Detachment of upper crust and large scale melt generation in the middle crust of the Schwarzwald was probably facilitated by the tectonic stacking of water-rich pelitic clastics and gneiss slivers, with relatively even proportions of crystalline and pelitic materials.     

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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T- . , , , 35–40°/ . T- 650° 4–5 , T- , . . , 3 , - . , T-, . 60–80°/, . , , «pullapart» .