Variscan tectonic

Inhalt
Teil 1 siehe: https://www.academia.edu/40826934/Feldbuch_gro%C3%9Fe_Exkursion_IVREA_Zone_9._-_19.6.2014_Teil_1

Überblick Tabelle Gesteine – Täler                                                            2
Überblick Tabelle Täler – Gesteine                                                            3
Wichtigste Abbildung                                                                                    4
Überblick Liste Exkursionstage:                                                                  5
09.06.2014 Val Cannobina, Strona-Ceneri-Zone                                      5
10.06.2014 Val Sesia (Varallo), Val Strona d’Omegna, Val Mastallone 6
11.06.2014 Val Strona d’Omegna,                                                              9
- Standort Rosarolo                                                                                        9
- Standort Bachbett                                                                                        10
- Standort Forno                                                                                              11
- Standort Strona                                                                                            12

Hier Teil                                                                                                              2
12.06.2014 Val Cannobina                                                                              2
- Standort: oberhalb Finero, Straßenaufschluß                                            2
- Straßenaufschluß über dem Paß vorm Penninikum                                3
- Standorte Finero und insubrische Linie                                                      5
13.06.2014 Val Strona d’Omegna, Forno, Marmi, Campello Monti          8
14.06.2014 Val Cannobina vom Pass bis zum Tunnel und zwischen den
Tunneln: Cannobio, Oresso, Finero, Probennahme                                    10
15.06.2014 Val Strona d’Omegna, Campello Monti                                    11
16.06.2014 Val Sesia - Steinbruch über der Brücke bei Solivo                  11
17.06.2014 Val Sessera - Strassenaufschluß zwischen Coggiola und Bialla sowie Anfahrt zum Kraftwerk                                                                                    11
18.06.2014 Val Sessera – Kraftwerk                                                            11
18.06.2014 Val Strona di Postua                                                                  11
19.06.2014 Val d’Ossola                                                                                12
20.06.2014 Steine sortieren, Val Cannobina Straßenaufschluß              12
oberhalb Finero, Grossglockner Rückfahrt
Anhang: einige Abbildungen, Kartierungen, Literatur                                12

10-tägige Geologisch-Paläontologische Exkursion nach Südfrankreich
26. August bis 4. September 2013, Leitung: Prof. Dr. T. Martin und Dr. R. Schellhorn, Universität Bonn, Steinmann-Institut, Bereich Paläontologie, Vorbereitungsseminar, Vortrag am 10.6.2013 - handout Vorbereitungsseminar, 5 Seiten

Referent: Manfred Bundschuh, Sander Höhe 5, 51465 Bergisch Gladbach
Tel.mobil: 0174 613 4911, Email: [email protected]

Inhalt
I. Geographische Lage
II. Geologische Gliederung und Deckeneinheiten
III. Die Varisziden

Geologische Entwicklung :

IV. 1. Plattentektonik
V. 2. Alter und Strukturen
VI. 3. Tektonik mit Faltung und Überkippung
VII. 4. Profile

VIII. Zusammenfassung
IX. Literaturverzeichnis

LA-MC-ICP-MS U–Pb zircon ages and wholerock geochemical data obtained from volcanic rocks erupted in the northern margin of Gondwana provide new insights on the polyphase magmatic evolution of the NW Iberian domain during the establishment of passive margin conditions in Lower Paleozoic times. The U–Pb data show crystallization ages of ca. 455 Ma for two calc-alkaline rhyolites sampled in the Upper Parautochthon of the eastern Galicia—Trás-os-Montes Zone (GTMZ) and for an intraplate basalt intruded into Middle Ordovician slates of the autochthonous series of the Central Iberian Zone (CIZ). Together with previous data, the ages obtained reveal a periodic magmatic activity across the northern Gondwana margin during the Lower Paleozoic, which is comparable to that observed in NE Iberia and in other massifs of the Mediterranean realm. Both geochronological and geochemical data reinforce paleontological and stratigraphic evidences for paleogeographic proximity between these domains and contribute to the recognition of extensional-related magmatism along the northern margin of Central Gondwana associated with the opening of the Rheic Ocean.

The late Paleozoic collision between Gondwana and Laurussia resulted in the polyphase deformation and magmatism that characterizes the Iberian Massif of the Variscan orogen. In the Central Iberian Zone, initial continental thickening (D 1 ; folding and thrusting) was followed by extensional orogenic collapse (D 2) responsible for the exhumation of high-grade rocks coeval to the emplacement of granitoids. This study presents a tectonometamorphic analysis of the Trancoso-Pinhel region (Central Iberian Zone) to explain the processes in place during the transition from an extension-dominated state (D 2) to a compression-dominated one (D 3). We reveal the existence of low-dipping D 2 extensional structures later affected by several pulses of subhorizontal shortening, each of them typified by upright folds and strike-slip shearing (D 3 , D 4 and D 5 , as identified by superimposition of structures). The D 2 Pinhel extensional shear zone separates a low-grade domain from an underlying high-grade domain, and it contributed to the thermal reequilibration of the orogen by facilitating heat advection from lower parts of the crust, crustal thinning, decompression melting, and magma intrusion. Progressive lessening of the gravitational disequilibrium carried out by this D 2 shear zone led to a switch from subhorizontal extension to compression and the eventual cessation and capture of the Pinhel shear zone by strike-slip tectonics during renewed crustal shortening. High-grade domains of the Pinhel shear zone were folded together with low-grade domains to define the current upright folded structure of the Trancoso-Pinhel region , the D 3 Tamames-Marofa-Sátão synform. New dating of syn-orogenic granitoids (SHRIMP U\ \Pb zircon dating) intruding the Pinhel shear zone, together with the already published ages of early extensional fabrics constrain the functioning of this shear zone to ca. 331–311 Ma, with maximum tectonomagmatic activity at ca. 321–317 Ma. The capture and apparent cessation of movement of the Pinhel shear zone occurred at ca. 317– 311 Ma.

This contribution provides a case example on the generation of large-scale recumbent folds in syn-orogenic gran-itoids. We analyze the progressive reworking of extension-related structures into later ones after a period of crustal thickening. The Padrón migmatitic dome formed after the climax of the Gondwana-Laurussia collision in the late Paleozoic. Petrostructural analysis carried out in the eastern flank of this dome reveals that extensional flow resulted in progressive exhumation of mainland Gondwana, which rested under peri-gondwanan alloch-thonous terranes and a suture zone during maximum crustal thickening. Exhumation proceeded up to upper crust levels (andalusite stability field) along with partial melting of the middle-lower crust and with the generation of granitoid laccoliths during an early extensional stage. Newly-formed lithological and mechanical anisot-ropies, such as the presence of variably-sized sheet-shaped bodies of syn-orogenic granitoids, provided a favorable rheological setting for fold nucleation during the intermediate stages of extension. In extending oro-genic crust, whether recumbent folds occur after significant melt production depends on the lateral/vertical flow ratio, and on the orientation of deforming bodies with regard to kinematic/strain axes. We suggest that subhorizontal extensional flow dominated over vertical flow during the early and intermediate stages of the evolution of the Padrón dome. A component of vertical (diapiric) flow caused progressive tilting of the sheet-like bodies and obliquity respect to strain axes. This resulted in the development of regional-scale folds at the expense of syn-orogenic granitoids, such as in the case of the Portomouro recumbent synform. Extensional ductile flow was oblique to the trend of the orogen during the whole process, and directed to the NNW during the formation of recumbent folds. Non-coaxial shearing favored an (NNW-SSE) elongate shape for the syn-kinematic granitic massifs as well as the subsequent nucleation of recumbent folds. Deformation concentrated along discrete detachments during the late stages of extension.

Correlation of a group of allochthonous terranes (referred to as basal, ophiolitic and upper units) exposed in the NW and SW of the Iberian Massif, is used to propose a new geotectonic zone in the southern branch of the Variscan Orogen: the Galicia–Ossa-Morena Zone. Recent advances in SW Iberia identify most of the former Ossa-Morena Zone as another allochthonous complex of the Iberian Massif, the Ossa-Morena Complex, equivalent to the Cabo Ortegal, Órdenes, Malpica-Tui, Bragança and Morais complexes described in NW Iberia. The new geotectonic zone and its counterparts along the rest of the Variscan Orogen constitute an Internal Variscan Zone with ophiolites and units affected by high-P metamorphism. The Galicia–Ossa-Morena Zone includes a Variscan suture and pieces of continental crust bearing the imprint of Ediacaran–Cambrian events related to the activity of peri-Gondwanan magmatic arcs (Cadomian orogenesis). In the Iberian Massif, the general structure of this geotectonic zone represents a duplication of the Gondwanan platform, the outboard sections being juxtaposed on top of domains located closer to the mainland before amalgamation. This interpretation offers an explanation that overcomes some issues regarding the differences between the stratigraphic and paleontolog-ical record of the central and southern sections of the Iberian Massif. Also, equivalent structural relationships between other major geotectonic domains of the rest of the Variscan Orogen are consistent with our interpretation and allow suspecting similar configurations along strike of the orogen. A number of issues may be put forward in this respect that potentially open new lines of thinking about the architecture of the Variscan Orogen.

Correlation of three high-P and low to intermediate-T belts from NW and SW Iberia allows recognition of a single terrane affected by Late Devonian continental subduction during the Variscan orogeny. This terrane is located in the lower part of the allochthonous complexes of NW Iberia, and in a lower position in most of the Ossa Morena Zone that, therefore, emerges as another allochthonous complex of Iberia. The Ossa Morena Complex together with the allochthonous complexes of NW Iberia form a huge rootless nappe that was transported towards the Variscan foreland during final assembly of Pangea.

The Ossa-Morena zone in SW Iberia represents a section of the northern margin of West Gondwana that formed part of a Cordilleran-type orogenic system during the Neoproterozoic (Cadomian orogeny). The crustal section in this zone preserves the record of rifting that led to the opening of the Rheic Ocean in the early Paleozoic and the collision of Gondwana and Laurussia in the late Paleozoic (Variscan orogeny). We present U-Pb zircon data from three alkaline to peralkaline syenites that intruded Neoproterozoic and Cambrian strata and give crystallization ages ranging between ca. 490 Ma and 470 Ma. Lu/Hf isotopic data from these zircons give positive initial εHf values (0 ≤ εHf(t) ≤ +11.5) that approach the model values for the depleted mantle at the time of crystallization. This suggests that a significant proportion of the magma was derived from the mantle, with limited mixing/assimilation with crustal-derived melts. Alkaline/peralkaline magmatic suites of similar age and chemical composition intruded other sections of the northern margin of West Gondwana and along the boundaries of the continental blocks that today make up Iberia. These blocks are further characterized by the presence of high-pressure metamorphic belts that formed during accretion and subsequent collision of peri-Gondwanan domains against Laurussia during the Devonian and Carboniferous (Variscan orogeny). Our U-Pb and Lu-Hf data set indicates that during the Cambrian−Ordovician transition, lithosphere extension reached a stage of narrow intracontinental rifting, where deeply sourced magmas, probably coming from the lower crust and/or the upper mantle, intruded continental upper crust across various sections of previously stretched crust. We propose that necking of the Gondwana lithosphere into several continental microblocks with fertile mantle beneath them compartmentalized extension (multiblock model), which favored the onset of early Paleozoic peralkaline and alkaline magmas. The boundaries of microblocks represent zones of inherited crustal weakness that were later reactivated during the late Paleozoic as major accretionary faults related to the amalgamation of Pangea during the Variscan orogeny. Our dynamic model provides an explanation for the unusual spatial relationship between peralkaline and alkaline igneous provinces (usually shallow in the crust) and the occurrence of high-pressure rocks. Our observations suggest that Cordilleran-type orogens subjected to extension after long-lived subduction can develop wide continental platforms that feature multiple continental blocks. In addition, the formation of sequenced high-pressure belts in collisional orogens can be explained as the ultimate consequence of multiple necking events within continental lithosphere during previous collapse of a Cordilleran-type orogen.

The Ediacaran–Cambrian transition in Central Iberia occurred in an evolving setting.TDM ages of Ediacaran greywackes indicate an input of arc-derived juvenile material.Older isotopic sources in Cambrian shales point to a more stable continental sources.A complete Ediacaran–Early Cambrian stratigraphic transition can be observed in the southern part of the Central Iberian Zone (Iberian Massif). Two different stratigraphic units, underlying Ordovician series, display geochemical and Sm–Nd isotopic features in agreement with an evolving geodynamic setting. Pusa Shales (Early Cambrian) rest unconformably on greywackes of the Lower Alcudian Formation (Late Ediacaran). Both sequences present minor compositional variations for major and trace element contents and similar REE patterns, close to those of PAAS (Post Archean Australian Shale). Trace element contents and element ratios suggest mixed sources, with intermediate to felsic igneous contributions for both units. Tectonic setting discrimination diagrams for the Ediacaran greywackes indicate that these turbiditic series were deposited in a sedimentary basin associated with a mature active margin (volcanic arc). However, the compositions of the Cambrian shales fit better with a more stable context, a back-arc or retro-arc setting. εNd(T) and TDM ages are compatible with dominance of a similar cratonic source for both sequences, probably the West Africa Craton. εNd565 values for the Ediacaran greywackes (− 3.0 to − 1.4) along with TDM ages (1256–1334 Ma) imply a significant contribution of juvenile material, probably derived from the erosion of the volcanic arc. However, εNd530 values in the Cambrian shales (− 5.2 to − 4.0) together with older TDM ages (1444–1657 Ma), suggest a higher contribution of cratonic isotopic sources, probably derived from erosion of the adjacent mainland. Coeval with the progressive cessation of arc volcanism along the peri-Gondwanan realm during the Cambrian, there was a period of more tectonic stability and increasing arrival of sediments from cratonic areas. The geochemistry of the Ediacaran–Cambrian transition in Central Iberia documents a tectonic switch in the periphery of Gondwana, from an active margin to a more stable context related to the onset of a passive margin.

Flysch-type, syn-orogenic deposits of Carboniferous age occur in relation to the emplacement of a large allochthonous nappe stack in the Variscan belt of NW Iberia. New U–Pb age populations of detrital zircons obtained using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) are considered together with others from previously dated samples to establish the relationships between sedimentation and thrusting. The age populations of four syn-orogenic formations are compared with those of the pre-orogenic sequence in the Autochthon and Parautochthon, representing the Gondwanan passive margin, and in the Allochthon, formed by peri-Gondwanan and oceanic terranes. In addition, a new structural study has been carried out to understand the relationships between the syn-orogenic deposits and the development of Variscan structures. The aims are to identify the sources of sediments and to establish the relationship between Variscan structural evolution and syn-orogenic sedimentation. Development of a forebulge outwards from the allochthonous front, deduced from the structural study, suggests the existence of depocentres that hosted the syn-orogenic sediments. Together with the trend shown by the more recent zircons in each formation, that are younger towards the external zones, the data suggest that sedimentation occurred in progressively migrating depocentres formed in front of the allochthonous wedge during its emplacement. The zircon age populations point to the Allochthon as the main source of detritus for the syn-orogenic basins, with perhaps a limited participation of the Parautochthon and Autochthon in the younger formations.

The post-Variscan uplift of the western Anti-Atlas Precambrian core is studied by zircon fission track (ZFT) analysis of ten samples of granites and schists from the Kerdous and Ifni inliers. All samples yield Carboniferous ZFT ages ranging from
358 Ma to 319 Ma, with nine dates younger than 338 Ma. The weighted mean age calculated for these nine samples is 328 Ma. These results compare with the available K–Ar datings of white mica and biotite from the same rocks or from the overlying Ediacaran-Cambrian low-grade metasediments. The fact that different systems with distinct closure temperatures
yield similar ages suggests the occurrence of a short Carboniferous thermal event followed by rapid cooling. Consistent with the regional geological framework, the thermal event is assigned to the Variscan folding, being followed by rapid exhumation and cooling related to the post-folding erosion.

Strike-slip shear zones of the Variscan orogen are used to derive the evolution of paleostrain and discuss the kinematics of the waning stages of the Gondwana-Laurussia collision during the amalgamation of Pangea. In the Iberian Massif, the recognition of three late Carboniferous deformation events related to strike-slip tectonics (D3, D4, D5) in the Trancoso-Pinhel region (Portugal) reveals that late orogenic transcurrent deformation was episodic and occurred in a short period of time (<15 m.y.). Early stages of strike-slip deformation included dextral and sinistral shear zones and orogen-parallel upright folds (D3; ca. 311 Ma). These structures followed the development of extensional shear zones (D2) during the tectonothermal reequilibration of the orogen. D3 structures were deflected and folded by the sinistral D4 Juzbado-Penalva do Castelo shear zone, dated as ca. 309–305 Ma by SHRIMP (sensitive high-resolution ion microprobe) U-Pb zircon dating of synkinematic granitoids. D3 and D4 structures were folded under east-west compression (D5) influenced by the strike-slip movement of the dextral Porto-Tomar shear zone. Variscan movement along the Porto-Tomar shear zone started ca. 304 Ma (onset of the Buçaco basin and syn-D5 granites), but ceased before ca. 295 Ma (age of the final closure of the Ibero-Armorican arc and crosscutting granites). The contrasting geometry, kinematics, and timing of these strike-slip shear zones are explained by deformation partitioning upon a rheologically inhomogeneous crust with structural and tectonothermal anisotropies generated during previous deformation. The convergence vector between Gondwana and Laurussia during D3–D5 remained the same, and was equivalent to the vector that explains the previous tectonic record (D2) in central and northwestern Iberia.

Inhalt
Hier Teil 1

Überblick Tabelle Gesteine – Täler                         2
Überblick Tabelle Täler – Gesteine                         3
Wichtigste Abbildung                                 4

Überblick Liste Exkursionstage:                 5

09.06.2014 Val Cannobina, Strona-Ceneri-Zone         5

10.06.2014 Val Sesia (Varallo), Val Strona d’Omegna, Val Mastallone 6

11.06.2014 Val Strona d’Omegna,                 8
- Standort Rosarolo                         9
- Standort Bachbett                       9
- Standort Forno                               10
- Standort Strona                               10

Teil 2 siehe https://www.academia.edu/41500666/Feldbuch_gro%C3%9Fe_Exkursion_IVREA_Zone_9.-19.6_2014_Teil_2

12.06.2014 Val Cannobina                2
- Standort: oberhalb Finero, Straßenaufschluß       2
- Straßenaufschluß über dem Paß vorm Penninikum       3
- Standorte Finero und insubrische Linie       5

13.06.2014 Val Strona d’Omegna, Forno, Marmi, Campello Monti       8

14.06.2014 Val Cannobina  vom Pass bis zum Tunnel und zwischen den
Tunneln: Cannobio, Oresso, Finero, Probennahme                 10

15.06.2014 Val Strona d’Omegna, Campello Monti       11

16.06.2014 Val Sesia - Steinbruch über der Brücke bei Solivo       11

17.06.2014 Val Sessera - Strassenaufschluß zwischen Coggiola und Bialla
sowie Anfahrt zum Kraftwerk                     11

18.06.2014 Val Sessera – Kraftwerk             11
18.06.2014 Val Strona di Postua             11

19.06.2014 Val d’Ossola                     12

20.06.2014 Steine sortieren, Val Cannobina Straßenaufschluß     12
oberhalb Finero, Grossglockner Rückfahrt

Anhang:
einige Abbildungen, Kartierungen, Literatur 12

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A complete section of the southern realm of the Variscan orogenic belt can be restored in the Corsica-Sardinia segment. Northern Corsica exposes a nonmetamorphosed Palaeozoic succession lying on Panafrican mica schist related to a microcontinent (most likely Armorica or from a microcontinent from the Hun superterrane) that had drifted away directly from Gondwana. These formations are thrust over the Variscan Internal Zone composed mainly of anatectic high-grade Palaeozoic formations that crop out from central Corsica to northern Sardinia; the metamorphic peak of the eclogite remnants has been dated at c. 420 Ma. The Variscan Internal Zone interpreted here as a collision zone, and also the Eovariscan suture, was intruded in Corsica by Mg-K granite from 345 to 335 Ma. The thrust of this Internal Zone onto the stack of parautochthonous nappes in central Sardinia is cross-cut by the Posada Asinara dextral shear zone. To the south, parautochthonous nappes overthrust the North-Gondwana margin which displays a possible Panafrican basement topped by an Iglesiente-Sulcis nonmetamorphic/anchimetamorphic Palaeozoic succession. To cite this article: P. Rossi et al., C. R. Geoscience 341 (2009). # 2008 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.

Every document reflects the historical context in which it was done and the geological map of 1968 is a clear example. The map is the result of a geological mapping project done in the central Pyrenees from the University of Leiden (Netherlands) just after the Second World War and continued until the 70's. More than a hundred Dutch geologists participate on that project and the final editing result was 10 maps at 1:50.000. The quality of the work done and the accuracy of the cartography are still a reference for the earth scientist in the Pyrenees, referring as "the Dutch school of geology." In this mapping project the Principality of Andorra is split into two maps, the northern sector in sheet 6 and the southern sector in sheet 10, made by HJ Zwart and J.J.A Hartvelt respectively. The geological patterns of both maps are different, because the lithological characteristics of the areas mapped are also different, however the two authors agreed to synthesize the two maps in a single one, with the idea of publishing it in the Principality of Andorra. The idea was frustrated by the lack of institutional interest and the map has been unpublished for almost 40 years in Holland. The efforts of the Marcel Chevalier Foundation and the National Archives of Andorra have made it possible to recover the map for its study. This map is a key map to understand the lithology of the Principality of Andorra as a whole, is an original manuscript with a remarkable scientific value for the Pyrenees. Therefore this presentation should be considered as a tribute to the authors of the map, as well as to the whole Dutch school for its scientific guidance, which in the case of Andorra is especially useful because "the
geology seems to fall you over."

10-tägige Geologisch-Paläontologische Exkursion nach Südfrankreich
26. August bis 4. September 2013, Leitung: Prof. Dr. T. Martin und Dr. R. Schellhorn
Universität Bonn, Steinmann-Institut, Bereich Paläontologie, Vorbereitungsseminar, Vortrag am 10.6.2013

Inhalt
23 Folien

I. Geographische Lage
II. Geologische Gliederung und Deckeneinheiten
III. Die Varisziden

Geologische Entwicklung :

IV. 1. Plattentektonik
V. 2. Alter und Strukturen
VI. 3. Tektonik mit Faltung und Überkippung
VII. 4. Profile

VIII. Zusammenfassung
IX. Literaturverzeichnis

The allochthonous Cabo Ortegal Complex (NW Iberian Massif) contains a ~500 m thick serpentinite-matrix m elange located in the lowest structural position, the Somozas M elange. The m elange occurs at the leading edge of a thick nappe pile constituted by a variety of terranes transported to the East (present-day coordinates; NW Iberian allochthonous complexes), with continental and oceanic affinities , and represents a Variscan suture. Among other types of metaigneous (calcalkaline suite dated at 527–499 Ma) and metasedimentary blocks, it contains close-packed pillow-lavas and broken pillow-breccias with a metahyaloclastitic matrix formed by muscovite–paragonite–margarite–garnet–chlorite– kyanite–hematite–epidote–quartz–rutile. Pseudosection modelling in the MnCNTKFMASHO system indicates metamorphic peak conditions of ~17.5–18 kbar and ~550 °C followed by near-isothermal decompression. This P–T evolution indicates subduction/accretion of an arc-derived section of peri-Gondwanan transitional crust. Subduction below the Variscan orogenic wedge evolved to continental collision with important dextral component. Closure of the remaining oceanic peri-Gondwanan domain and associated release of fluid led to hydration of the overlying mantle wedge and the formation of a low-viscosity subduction channel, where return flow formed the m elange. The submarine metavolcanic rocks were deformed and detached from the subducting transitional crust and eventually incorporated into the subduction channel, where they experienced fast exhumation. Due to the cryptic nature of the high-P metamorphism preserved in its tectonic blocks, the significance of the Somozas M elange had remained elusive, but it is made clear here for the first time as an important tectonic boundary within the Variscan Orogen formed during the late stages of the continental convergence leading to the assembly of Pangea.

A regional study starting from detailed geological mapping has been carried out in the Malpica-Tui Complex of Galicia in NW Spain. The complex is formed by two units representing pieces of the external edge of Gondwana, subducted and exhumed during the Variscan collision. The study shows that synsubduction and early synexhumation structures in continental subduction channels tends to be obscured and even erased once exhumation is complete. Detailed structural analysis, matched with the knowledge of the history, and available data for other Galician basal units have elucidated the major structures developed during the subduction-exhumation process. The results include evidence of the plate convergence causing early Variscan continental subduction of the Gondwana margin. Subduction was followed by exhumation driven by ductile thrusting within the subduction channel, which, in turn, provoked crustal duplication in the subducted slab and modified the initial tectonometamorphic architecture of the subduction wedge. The next step was accretion to the adjacent continental domains, placing the subduction wedge on top of unsubducted parts of the Gondwana margin via ductile thrusting. Thrusting was preceded by progressive propagation of a train of recumbent folds toward the foreland that affected the previous structural stack. Subsequent transference of oceanic (Rheic) and peri-Gondwanan terranes to the Gondwana margin took place by out-of-sequence thrusting followed by crustal extensional collapse and strike-slip tectonics.

CITATIONS 25 READS 122 3 authors: Some of the authors of this publication are also working on these related projects: Master degree thesis: Tectonometamorphic evolution of the Massa Unit in Punta Bianca area (Northern Apennines, Italy) View project Evolution of the Lanterman-Mariner tectonic melange: evidences for the geodynamic of the paleo-Pacific margin of Gondwana and numerical models View project

A model for the P–T–t evolution of NW and Central Iberia is presented to better understand its metamorphic
evolution from initial Barrovian stage through HT–LP conditions. The model is constrained by the well-known
succession of structural events and geochronology of the Variscan deformation, standard physical parameters,
and conservative values for the surface heat flowand radiogenic heat production. The goals are to test an orogenic
model derived from observations against a physical reality constrained by the thermal properties of the
lithosphere, and to explore the implications for Variscan orogenic evolution. A600 km section, parallel to the
axial surface of the Central Iberian arc ismodeled to the scale of the continental lithosphere. The model outcomes
are presented as sections showing the evolution of isotherms over 65 Ma, as the variation of the lithospheric
geotherm and as P–T–t paths for significant parts of the model section and for points originating at different
depths. Profiles of lithosphere strength have been constructed combining the maximum shear stress for brittle
fracture with published flow–stress–depth profiles of characteristic rocks and temperatures obtained for the
different depths in the model.
Quantitative constraints from2-D thermal models suggest that the metamorphic and magmatic evolution can be
explained by the succession of three orogenic stages, namely crustal thickening, a period of thermal relaxation,
and subsequent extensional collapse, providing that the first stage doubles the thickness of the continental
crust. Thrusting of a thick nappe stack, well preserved in NW Iberia, seems to be the likely cause of the high
temperatures reached later during the HT–LP event that occurred during late orogenic extension. The late
addition of heat from asthenospheric upwelling remains a possibility, but the model results suggest that it is
not necessary to explain the Variscan P–T–t paths.

Quartz c-axis fabrics, vorticity of flow and deformation temperatures were analyzed in samples collected in the central sector of the Variscan inner zone in Sardinia (Italy). In order to constrain the kinematics of exhumation of medium-and high-grade metamorphic rocks during D2 post-collisional deformation, we investigated two shear belts with opposing shear senses. These shear belts are developed parallel to the trend of the inner zone which marks the boundary between two metamorphic complexes. Our data suggest that the sinistral shear belt initiated during the early stage of D2 deformation in a simple-sheardominated regime, whereas the dextral shear belt initiated later in a pure-shear-dominated regime. We document a general non-coaxial flow regime with contemporaneous pure and simple shear components. Geometrical relationships reveal the operation of oblique extrusion during a compressive regime where the coaxial component acted sub-horizontal and orthogonal to the trend of the axial zone. Changes in the kinematics, vorticity of flow and strain geometry that occurred during D2 deformation may be related to post-collisional orogen-parallel displacement caught up in the Variscan orogeny.

Große Exkursion Südfrankreich 26.8. – 3.9.2013

1. 26.8.13 Strassbourg, Parcey
2. 27.8.13 Gannat
3. 28.8.23 Menat, Gignac
4. 29.8.13 Gignac, Jura des Causses
5. 30.8.13 Gignac
6. 31.8.13 Montpellier, Esperaza
7. 1.9.13 Esperaza
8. 2.9.13 Ordoviz und Devon/Karbon Grenze, Roquebrun
9. 3.9.13 Montelimar, Macon

The existence of a large allochthonous nappe pile emplaced onto the hinterland of the Iberian Massif and covering most of the basement of NW and SW Iberia proposed by Díez Fernández and Arenas has been called into question on the basis of structural, geophysical, and regional data from SW Iberia. The discussion provided here is based on a revision of the fundamentals of the Variscan geology of NW Iberia and on a more detailed presentation of the geology of SW Iberia. Our counterarguments reveal the reasoning of Simancas et al. against the verisimilitude of the large allochthonous pile as inconclusive, incomplete, or erroneous in some cases. Therefore we maintain that our structural correlation and its implications remain fully valid for the case of the Iberian Massif.

In peneplaned terranes, it is often impossible to get a full 3D view of geological objects. In the case of granitic plutons, for which intrusive relationships between constituent units can provide first order information regarding their petrogenesis, this lack of 3D field evidence is a major issue. Indirect observations can be provided by geophysical surveys. Here, we interpret field gravity data and airborne gamma ray radiometric maps with whole rock geochemistry data in order to obtain information on granite petrogenesis. First, we test our proposed combined geophysical and geochemical approach on the Huelgoat Variscan intrusion (Armorican Massif, France) and we show that ternary radiometric maps are a good proxy for the distribution of K, U and Th radioelements. Then, we apply our method to the Lizio and Questembert Variscan granitic intrusions (Armorican Massif) and show that some features characteristic of the intrusions, such as the feeding zones, can be localised by geophysical imaging. Indeed, radiometric maps constitute a frozen image of the latest stage of the magmatic building of plutons.

The structure of the Ciudad Rodrigo area (Iberian Massif, Central Iberian Zone) has been revisited in order to integrate new geological data with recent models of the evolution of the Iberian Massif. Detailed mapping of fold structures along with a compilation of field data have been used to constrain the geometry and relative timing of ductile deformation events in this section of the hinterland of the Variscan belt. The structural evolution shows, in the first place, the development of a regional train of overturned folds with associated axial planar foliation (D1). Towards the lower structural levels, the deflection of the fold limbs and a subhorizontal crenulation cleavage depict the upper structural boundary of a superimposed low angle shear zone (D2), which extends at least to the deepest parts of the basement exposed in the study area. The amplification and rotation of D1 folds about a horizontal axis also occurred within this shear zone. The flat-lying character of the D2 structures accounts for the attenuation of the previously thickened crust, which developed following gravity gradients during thermal re-equilibration. Subsequent deformation led to the formation of two orthogonal sets of upright folds (D3), representing a new shift between crustal thinning and crustal thickening in the region.

Olivine hornblendites (cortlandtites) form part of the Montnegre mafic complex related to late-Variscan I-type granitoids in the Catalan Coastal Ranges. Two generations of spinel are present in these hornblendites: Spl1 forms euhedral crystals included in both olivine and Spl2. Spl2 forms euhedral to anhedral crystals associated with phlogopite and fibrous colourless amphibole forming pseudomorphs after olivine. Compositions of Spl1 are picotite-Al chromite (Fe#: 77.78-66.60; Cr#: 30.12-52.22; Fe3+/R3+: 6.99-21.89; 0.10< TiO2%<0.62). Compositions of Spl2 are pleonaste (Fe#: 37.86-52.12; Cr#: 1.00-15.45; Fe3+/R3+: 0.31-5.21; TiO2%<0.10). The two types of spinel follow a CrAl trend, mainly due to the substitution (Fe2+)-1Cr-1= MgAl, which is interpreted as the result of mixing between two different mantle-derived melts. The compositions of early Spl1 crystals included in olivine are characteristic of Al-rich basalts. More aluminous Spl2 would result from reaction of olivine with a less evolved, Al and K-rich mantle-derived melt after new refilling of the magma chamber or channel. As a whole, spinels from similar examples of Variscan olivine hronblendites also follow a CrAl trend with high Fe# and starting at higher Cr# than other trends of this type. Cr# heterogeneity in the early spinels from these Variscan hornblendites would be inherited from the variable Al content of the mafic melts involved in their genesis.

thermal modeling of HT-LP metamorphism in NW and Central Iberia: Implications for Variscan magmatism, rheology of the lithosphere and orogenic evolution, Tectonophysics (2015), ABSTRACT A model for the P-T-t evolution of NW and Central Iberia is presented to better understand its metamorphic evolution from initial Barrovian stage through HT-LP conditions. The model is constrained by the well-known succession of structural events and geochronology of the Variscan deformation, standard physical parameters, and conservative values for the surface heat flow and radiogenic heat production. The goal is to test an orogenic model derived from observations against a physical reality constrained by the thermal properties of the lithosphere, and to explore the implications for Variscan orogenic evolution. A 600 km section, parallel to the axial surface of the Central Iberian arc is modeled to the scale of the continental lithosphere. The model outcomes are presented as sections showing the evolution of isotherms over 65 Ma, as the variation of the lithospheric geotherm and as P-T-t paths for significant parts of the model section and for points originating at different depths. Profiles of lithosphere strength have been constructed combining the maximum shear stress for brittle fracture with published flow-stressdepth profiles of characteristic rocks and temperatures obtained for the different depths in the model.

The Variscan continental suture zone exposed
in NW Iberia is examined to uncover the long-lived rheo-
logical control exerted by the strata deposited over the
external parts of Gondwana on its geodynamic evolution.
The suture occurs within a set of allochthonous terranes
whose limits were taken as domain boundaries to interpret
the Variscan stacking of Paleozoic continental domains and
retrodeform the resulting nappe pile. The suture zone
formed due to closure of ocean basins located between
Gondwana and Laurussia during the Late Paleozoic and
consists of relics of oceanic and transitional crust. The
suture zone exhibits a tabular to lens shape due to repeated
tectonic events dominated by non-coaxial deformation
(thrusts and low-angle normal faults). Thrusting and nor-
mal faulting also involved the margins of the continents
bounding the suture. The structure of the continental
blocks, however, is dominated by folds, particularly large
nappe folds with pronounced superimposed flattening. The
upper part of the basal allochthonous units comprises a
rheologically incompetent domain below the suture zone. This domain is typified by the carbonaceous-rich strata,
which are probably Ordovician–Silurian sediments based
on U–Pb detrital zircon populations. The rheology of this
layer determined the location of the first accretionary thrust
that initiated the Late Devonian subduction of the
Gondwana margin below the suture zone. By favoring fault
development, the upper sequence of the basal allochtho-
nous units as a whole influenced the exhumation of deep-
seated continental crust, the transference of the suture zone
over Gondwana, and the re-equilibration of the resulting
overthickened crust.

Excellent exposures of high-pressure rocks developed in a Variscan continental subduction system outcrop in NW Iberia. The kinematic criteria provided by the high-pressure metamorphic fabrics can be used to infer tectonic flow within the deep sections of this system. The dominant trend of the ductile flow is oblique to that of the orogenic belt, indicating oblique continental subduction. Its azimuth, a few tens of degrees clockwise relative to the orogenic trend, suggests dextral transpression between Gondwana and Laurussia during continental subduction that took place at the Upper Devonian, and provides a consistent kinematic reference for the earliest assembly of Pangaea in NW Iberia.

Recent field work and mapping in the lower units of the Cabo Ortegal Complex provided new data about the tectonic mélange that appears in the lowest structural position: the Somozas Mélange. This mélange unit with average thickness of 1800 m is restricted to the eastern part of the complex, and is located at the advancing front of the allochthonous complexes of NW Iberia. Three rock units are involved in the mélange: 1) an ophiolitic mélange consisting of igneous rocks mixed with serpentinites; 2) a metasedimentary unit with phyllites and phyllonites, with scarce conglomerates, marbles and quartzites; 3) high-T metamorphic rocks with varied types of amphibolites and orthogneisses.

The basal units of the allochthonous complexes of NW Iberia represent a fragment of the external edge of Gondwana subducted and subsequently exhumed during the Variscan collision. The structural analysis carried out in orthogneissic massifs of the southern part of one of these, the Malpica–Tui Unit, reveals the generation of recumbent folds and associated axial planar foliation during their exhumation. These folds nucleated in irregular igneous bodies that were initially deformed during the subduction event and show east to southeast vergence. Down-plunge projection of surface data and a series of regularly spaced cross-sections have been used to build 3D models of the two main bodies of orthogneiss, of calc-alkaline and peralkaline compositions respectively. The first is presently a lens-shaped body folded in a recumbent syncline, whereas the peralkaline gneiss, also affected by a train of asymmetric recumbent folds in the south, exhibits in the north a fold-like structure which is interpreted as inherited from its primary geometry, that of a ring dike pluton.

An integrated interpretation of the late Paleozoic structural and geochronological record of the Iberian Massif is presented and discussed under the perspective of a Gondwana-Laurussia collision giving way to the Variscan orogen. Compressional and extensional structures developed during the building of the Variscan orogenic crust of Iberia are linked together into major tectonic events operating at lithosphere scale. A review of the tectonometamorphic and magmatic evolution of the Iberian Massif reveals backs and forths in the overall convergence between Gondwana and Laurussia during the amalgamation of Pangea in late Paleozoic times. Stages dominated by lithosphere compression are characterized by subduction, both oceanic and continental, development of magmatic arcs, (over-and under-) thrusting of continental lithosphere, and folding. Variscan convergence resulted in the eventual transference of a large allochthonous set of peri-Gondwanan terranes, the Iberian Allochthon, onto the Gondwana mainland. The Iberian Allochthon bears the imprint of previous interaction between Gondwana and Laurussia, including their juxtaposition after the closure of the Rheic Ocean in Lower Devonian times. Stages governed by lithosphere extension are featured by the opening of two short-lived oceanic basins that dissected previous Variscan orogenic crust, first in the Lower-Middle Devonian, following the closure of the Rheic Ocean, and then in the early Carboniferous, following the emplacement of the peri-Gondwanan allochthon. An additional, major intra-orogenic extensional event in the early-middle Carboniferous dismembered the Iberian Allochthon into individual thrust stacks separated by extensional faults and domes. Lateral tectonics played an important role through the Variscan orogenesis, especially during the creation of new tectonic blocks separated by intracontinental strike-slip shear zones in the late stages of continental convergence. Units, (6) Upper Allochthonous Units, (7) Beja-Acebuches Ophiolite, and (8) South Portuguese Zone. Gathering of 4, 5, and 6 will be also referred to as Iberian Allochthon (or simply Allochthon). A simple restoration of the cross-section shown in Figure 3 (Fig. 4a) reveals two major suture zones featured with ophiolitic units with different protolith ages. The NNE-dipping suture represented by the Beja-Acebuches Ophiolite (Bard and Moine, 1979; Munhá et al., 1986; Quesada et al., 1994) cuts the suture marked by the Allochthonous Ophiolitic Units (Arenas et al., 1986) and divides the Iberian Massif in two major blocks. The northern block comprises the Cantabrian Zone, and the Iberian Autochthon, Parautochthon and Allochthon, all of which have Gondwanan affinity

Les nouvelles cartes geologiques de Wallonie permettent d’obtenir une vision plus globale de l’organisation des terrains paleozoiques de la zone frontale varisque dans le Sud de la Belgique. Cette bande de terrain d’orientation WSW-ENE, passant au sud d’un axe Charleroi-Namur a ete reconnue a l’origine grâce aux travaux miniers, mais a ete considerablement documentee par des sondages profonds et des profils sismiques. Un schema structural coherent du front varisque est propose ou les unites structurales suivantes sont distinguees, du nord au sud : le Massif du Brabant et la base de sa couverture non affectes par l’orogenese varisque, le Parautochtone brabancon, les Ecailles et les massifs renverses de Haine-Sambre-Meuse (EMR-HSM) et l’Allochtone ardennais. Les terrains devoniens non deformes durant l’episode varisque constituent la limite nord du front varisque. Le Parautochtone brabancon comprend des terrains devono-carboniferes plisses et/ou failles. Les EMR-HSM sont formes par la...