Post-orogenic Collapse Research Articles

Sr-Nd isotope and trace-element geochemistry of late Variscan volcanism in the Pyrenees: Magmatism in post-orogenic extension?

We obtained Sr-Nd isotope and trace-element data on late Variscan volcanic rocks in the Pyrenees: felsic lavas and andesites from the Ossau and the Serra de Cadi massifs and calc-alkaline andesites and alkali basalts from the La Rhune and the Anayet volcanic massifs. The calc-alkaline volcanic rocks have crustal trace-element and Sr-Nd isotopic characteristics. The andesites probably originated from melting of Pan-African paragneisses, with a limited contribution of mantle-derived sources having lithospheric characteristics. The andesitic lavas were more or less contaminated in magma chambers by Palaeozoic sedimentary rocks, which can be interpreted as a recycled Proterozoic crust. The felsic lavas are principally derived from anatexis of the Palaeozoic products, the rhyolites providing even peraluminous characteristics. The Anayet andesites show mantle-derived characteristics, and are interpreted as a mixture between a strongly undersaturated continental alkali basalt and a peraluminous end-member. However, Sr and Nd contents rale out any simple process. The late Anayet alkali basalts show typical asthenospheric Sr and Nd isotopic characteristics. The La Rhune basalts exhibit Sr and Nd sub-continental lithospheric isotopic characteristics, very similar to those of the Triassic Pyrenean continental basalts. It is thought that the granulite facies metamorphism dated at 310 Ma was caused by the ascent of the asthenosphere inducing a thermal anomaly. The mafic magmas ascending from the mantle are responsible for the crustal anatexis. This results in the emplacement of both the Pyrenean composite plutons, such as the Querigut massif, and the Ossau volcanic rocks. The Pyrenean late Variscan volcanic rocks were emplaced in either pull-apart basins opened by the reactivation of the major Variscan Pyrenean fault in a distensive setting or, better, in basins resulting from a post-orogenic collapse. Both these models involve a thinning of the subcontinental lithosphere, until its disappearance. Consequently, the inferred mantle-derived end-member of the Anayet lavas, which occurs in a post-Variscan distensive setting, clearly derives from the asthenosphere.

Intracontinental wedging and post-orogenic collapse in the mesohellenic trough

The Mesohellenic Trough is a 130 km long and about 30 km wide subsiding area which contains a thick sequence of well exposed Late Cenozoic post-orogenic sediments. This intermontane basin, located at the contact between the Apulian and Pelagonian collided margins, provides a good example of the characteristics needed to study the chronology of late orogenic intracontinental structures.

Late Cainozoic geodynamic evolution of Thessaly and surroundings (central-northern Greece)

In the framework of the late Alpide deformation of Greece and of the recent and active extensional tectonism of the Aegean region, the geotectonic evolution of the Thessaly region (central-northern Greece) has been examined, using a quantitative and qualitative structural analysis; stratigraphie, sedimentological, morphotectonic and seismological data. The geometry of the faults, their architecture and the knowledge of the stress pattern are used to explain some aspects of the tectonics and crustal dynamics of Thessaly and the surrounding area. The oldest compressional phases taken into account show a mean ENE-WSW trending direction of shortening and have been defined as late Alpide (early Aquitanian and Langhian). A later (Late Miocene-Pliocene) NE-SW oriented extensional phase has been related to the Hellenic post-orogenic collapse which developed behind the collisional front between the Aegean (Eurasia) and African plates. This phenomenon diachronically migrated from the east (Central Macedonia, Thermaikos Basin) towards the west (Epirus, Albania) where it is still active. As a consequence of this second phase, the area forms a basin and range like structure. The third, and last, phase (Middle Pleistocene-present) is characterized by a N–S direction of extension and affects the entire Aegean region. It generated new E–W trending basins, superimposed on the inherited ones. This gave as a final result, the complex block pattern we can see today. The recent and active right-lateral strike-slip movements along the North Aegean Trough seem to stop in the Sporades Basin and do not affect the uppermost crust of mainland Greece. A further WNW-ESE directed extension observed occasionally in central and northern Greece could be explained by local events or as block-related deformation.

Characteristic reflection patterns in the Southeast Canadian Cordillera, Northern Appalachians and Swiss Alps

There are some striking similarities and some differences between the seismic reflection sections recorded across the fold and thrust belts of the southeast Canadian Cordillera, Quebec-Maine Appalachians and Swiss Alps. In the fold and thrust belts of all three mountain ranges, seismic reflection surveys have yielded high-quality images of: (1) nappes (thin thrust sheets) stacked on top of ancient continental margins; (2) ramp anticlines in the hanging walls of faults that have ramp-flat or listric geometries; (3) back thrusts and back folds that developed during the terminal phases of orogeny; and (4) tectonic wedges and regional décollements. A principal result of the Cordilleran and Appalachian deep crustal studies has been the recognition of master décollements along which continental margin strata have been transported long distances, whereas a principal result of the Swiss Alpine deep crustal program has been the identification of the Adriatic indenter, a crustal-scale wedge that caused delamination of the European lithosphere. Significant crustal roots are observed beneath the fold and thrust belts of the Alps, southeast Canadian Cordillera and parts of the southern Appalachians, but such structures beneath the northern Appalachians have probably been removed by post-orogenic collapse and/or crustal attenuation associated with the Mesozoic opening of the Atlantic Ocean.

The terrane concept and its manifestation by deep reflection studies in the Variscides

The terrane concept is understood as an important extension of plate tectonics and is based on the recognition of allochthonous, mobile geological units. The concept is successfully applied to the Variscides with their wide range of collisional belts. It is mainly supported by the dense deep-seismic network of DEKORP, which reveals certain reflectivity patterns and succeeds in mapping old and new deep fault zones between the terranes. Variscan terranes are rooted in the ductile lower crust and seem to consist of continental crust only, partly exclusively of rigid upper crust. Oceanic terranes, on the other hand, are always rooted in the asthenosphere. The development of continental terranes and their boundaries depends strongly on their thermal and rheological history. In the case of post-orogenic collapse with heating and extension of the lower crust, seismic lamellae develop and often truncate former thrust faults.

Fluid inclusion evidence for basement decompression during Permo-Triassic extension, SE New England, USA

CO2-bearing fluid inclusions in strongly lineated but weakly foliated late Precambrian gneisses within the Hope Valley Shear zone of Connecticut and Rhode Island are of mixed composition (Xco2± 0.1; 7 wt% NaCl equivalent) and variable density (0.59–0.86 g/ml) and occur mainly as isolated inclusions. Also present are dilute (3 wt% NaCl equivalent) aqueous inclusions which occur on healed fractures related to greenschist facies retrograde metamorphism. Isochores for dense isolated CO2-bearing inclusions indicate pressures of 7.5–9 kbar at 500–600° C, the estimated temperature conditions of peak metamorphism. Published 40Ar/39Ar hornblende plateau age spectra indicate cooling through about 500° C at 265 ± 5 Ma. Isochores for low-density CO2-bearing inclusions and aqueous inclusions intersect at the conditions of retrograde metamorphism (325–400° C) and indicate pressures of 3–4 kbar. Published 40Ar/39Ar biotite plateau ages indicate cooling through about 300° C at 250 ± 5 Ma. These data define a P–T uplift curve for the region which is convex towards the temperature axis and indicate uplift rates between 0.4 and 3.3 mm/year in Permian time. Exhumation of basement gneisses was coeval with normal (west-down) motion along the regional basement–cover contact (Honey Hill–Lake Char–Willimantic fault system), and is interpreted as due to post-orogenic extensional collapse of the Alleghanian orogeny.