Late Variscan Research Articles

Crustal kinematics and deep structure of the northern Rhine Graben, Germany

Surface and subsurface geological data, unpublished information gleaned from industrial seismic surveys, and results from a deep reflection seismic profile across the Tertiary northern Rhine Graben indicate that crustal extension in excess of 6 km was achieved by asymmetric detachment of the upper crust at a depth of about 15 km. Comparison of a simple geometric‐kinematic cross‐sectional model and its depth extrapolation with the DEKORP‐9‐N deep crustal reflection data suggests that extension below the graben was achieved by slip along a gently westward dipping shear zone which truncated and offset lower crustal planar fabrics and which continues into the uppermost mantle. The planar lower crustal reflectors probably originated mainly during late Variscan (Permocarboniferous) crustal extension and/or magmatic underplating.

Late variscan tectonic evolution by thinning of earlier thickened crust. An 40Ar 39Ar study of the Montagne Noire, southern Massif Central, France

40Ar 39Ar age determinations on samples selected on deformational criteria form the basis of a proposal of a new detailed tectonometamorphic history of the Montagne Noire (the southern part of the Massif Central, France). This range is classically divided into a cover and an axial gneissic and migmatitic core which was metamorphosed under high T medium P conditions. The basement is structurally overlain by epimetamorphic Paleozoic sediments in the north. In the south, equivalent rocks form a well-known pile of recumbent nappes. The Variscan history of this area comprises early shortening and thickening of the continental crust. The climax of this event took place around 320 Ma ago, as is shown by previous RbSr data and by 40Ar 39Ar measurements. New developments in structural analyses have led to a better understanding of the late evolution of the area in terms of diapiric uplift of the core. South-southwestwards verging recumbent nappes have been emplaced and were weakly metamorphosed in their lower parts. This movement was accompanied and followed by dextral extensional wrenching of the Paleozoic cover along the northern and southern margins of the axial zone. Attendantly, ductile shear zones were formed. In order to decipher the complex history, three structural levels have been dated with the 40Ar 39Ar method. These levels are: (1) The migmatitic and gneissic series in the axial core, where micas yield ages of c. 315 Ma; (2) Mylonites from the northern and southern shear zones where biotites and muscovites yield ages of c. 310 Ma; (3) Muscovites and biotites from the epimetamorphic nappe structures yielding ages of c. 297 Ma. The later ages may represent a younger thermal pulse. These data are compared and discussed in the light of previous radiometric studies of the area and precise biostratigraphic constraints. A detailed geodynamic evolution of the studied segment is proposed.

The Ghomarides Nappes, Rif Coastal Range, Morocco: A Variscan chip in the Alpine Belt

The Ghomarides nappes are the uppermost basement nappes in the Rifan Alpine belt. Paleozoic rocks constitute the prominent part of these nappes, together with Triassic and younger cover sediments. Four nappes are distinguished, each one with a distinct stratigraphical sequence, especially for the Devonian and Carboniferous periods. Sedimentological and structural arguments plead for an initial location on the northern margin of the African continent. The main folding and metamorphic events are shown to be Eo‐Variscan (late Acadian). Tectonic vergence was to the north (present coordinates). Late Variscan (Hercynian) shortening gave upright, NW‐SE trending folds. Alpine ductile overprints are scarce except at the base of the lowest nappe where K/Ar phengite ages are about 25 Ma. The Ghomarides nappes correlate with the Malaguides nappes (Betic Cordilleras, southern Spain), the Kabylian (northern Algeria) and Calabrian (southern Italy) Paleozoic nappes, the Tuscan‐South‐Alpine basement and the Upper Austro‐Alpine nappes. The proposed correlations allow us to restaure the southern margin of the Hercynian belt along northwestern Africa.

An outline of the geology of the Afghan Pamirs

In the Great Afghan Pamir (Pamir-e Kaland) the following formations can be distinguished, from bottom to top: Wakhan Fm. (3000–4000 m thick anchimetamorphic slates and sandstones with frequent intercalations of quartzites and rare beds of crystalline limestones; conodonts of Lower Triassic); Qal'a-e Panja Tonalite (epimetamorphic, cataclastic); Qal'a-e Ust Gneiss (meta- and orthogneisses); Issik Granodiorite (batholite of the Afghan Pamirs, equivalent to Baba Tangi-Lunkho Granodiorite; xenoliths, flow structures and diaphtoritic portions; Upper Jurassic to Eocene). The tectonics are determined by the Peripheric Southern Fault of the Pamirs and the Wakhan Fault, showing vertical dislocation up to 1000 m and sinistral thrusting in connection with the Western Himalayan Syntaxis. Late Variscan and Alpine deformations and an intensive middle Alpine metamorphism can be observed. Interpretations of satellite pictures lead to an insertion of the regional tectonic features into the model of plate tectonics of the Himalayan arc.

Some problems of geotectonics of Sinkiang district

During Paleozoic time the central Tien Shan crystalline zone, a geanticlinal region with two deep rifts, separated the southern and northern Tien Shan geosynclines. The uplifting movement was primary within this period. Within Sinkiang the southern belt of the western section of Pei Shan is a Late Paleozoic subsiding zone, developed on the basement of the Precambrian folds. Pei Shan fault block is not the eastern extension of the Tien Shan geosyncline nor a branch of it. Rejuvenation of the fold in the northern Kunlun syneclise fold zone was not Caledonian but late Variscan, in the western Kunlun geosynclinal fold zone. –IGR Staff.