Alpine Orogenic Cycle Research Articles

Cephalopod limestone deposition on a shallow pelagic ridge: the Tafilalt Platform (upper Devonian, eastern Anti‐Atlas, Morocco)

ABSTRACTAs a result of early Variscan tectonic movements and of differential subsidence, a platform and basin topography was created along the northern margin of the Sahara Craton during the late Devonian. In the Moroccan Anti‐Atlas Mountains, the Tafilalt Platform is an approximately N‐S running ridge which developed since the late Middle Devonian. It separated a slowly subsiding shallow basin in the east (Tafilalt Basin) from a rapidly subsiding furrow in the west (Mader Basin). Platform deposits are characterized by highly reduced thicknesses, shallow subtidal to supratidal deposits in the late Frasnian and by unconformities at the Lower/Upper Frasnian and the Frasnian/Famennian boundaries. After a local transgression over emergent areas in the north, water depth probably never reached more than several tens to about 100 m in the lower Famennian. Cephalopod limestones of this age, deposited on the platform, represent a very diverse facies pattern comprising quartz‐rich brachiopod coquinas, crinoidal limestones, thick‐bedded cephalopod limestones and nodular limestones. Sedimentation rates ranged from 1 to 5 mm/ 1000 yr. In the late Famennian more uniform marl and nodular limestone facies suggest slightly deeper environments. Platform margins are characterized by higher rates of subsidence, debris flow deposits and slump structures. In the relatively shallow Tafilalt Basin, marls with intercalated nodular limestones were deposited. In the Mader Basin, sandy and calcareous turbidites suggest deeper water conditions in the late Devonian. During the Strunian/Tournaisian the whole area was overwhelmed by a thick deltaic sequence. The general facies distribution is in agreement with depositional models of other Upper Devonian and Lower Carboniferous cephalopod limestones in the European Variscan orogenic belts. In all these cases, condensed cephalopod limestones occupy a distinct palaeogeographic position in predictable facies sequences that reflect pre‐orogenic phases in the Variscan geodynamic cycle. Moreover, close parallels exist with condensed sequences in the Triassic and Jurassic that occur in a very similar position within the Alpine orogenic cycle.

“Alpine” mafic and ultramafic rocks — the ophiolite suite: A contribution to the discussion of the paper “the origin of ultramafic and ultrabasic rocks” by P.J. Wyllie

Abstract Ophiolites, though largely ignored in recent discussions of “Alpine” ultramafic rocks, characterize the Tethyan Belt from the Mediterranean east to the Himalayas and perhaps far beyond. The sequence consists fundamentally of a basal ultramafic layer overlain by diabasic rocks which in turn grade upward into extrusive breccias or pillow lavas. A layer of coarse gabbro may occur between ultramafics and diabase. Radiolarian cherts typically are associated with the lavas, and dense, siliceous pelagic limestones may intergrade with the cherts. Equivalent chert-limestone sequences are widespread among non-volcanic Tethyan sedimentary rocks. The Tethyan ophiolites perhaps resulted from diapiric up-welling and partial melting of mantle material during an early, possibly tensile, stage of the Alpine orogenic cycle. This occurred near the end of the Jurassic, when flysch troughs and associated ridges first appeared; lavas were extruded into the deep troughs and residual ultramafic rocks, perhaps lubricated by intergranular melt, moved plastically beneath the volcanic carapace. Gabbro was intruded later between ultramafics and the diabase-lava layer. The ophiolite complex was thus a composite subsea laccolith. Garnetiferous and pyroxenic ultramafic intrusives are known from the Rif and Betic chains. These may represent undifferentiated mantle material similar to that which gave rise to the highly differentiated ophiolite sequence. Ophiolites are now found primarily as disrupted blocks and slabs in more or less chaotic flysch, apparently displaced from the parent flysch trough during post-emplacement uplift and gravity sliding. Field data and geophysical measurements suggest that a large majority of ultramafic masses of “Alpine” type are rootless and allochthonous.

History of development of late Cenozoic volcanism in the Central Caucasus

Violent volcanism developed in the central part of the Caucasus during the last stage of the Alpine orogenic cycle. Three main epochs of volcanic development are here established: the first -late Miocene-early Pliocene; the second-late Pliocene; the third — Quaternary. These epochs of volcanic activity can be subdivided into a series of phases and subphases. The total volume of volcanic products is in the order of 2000 km3. The acidic volcanic rocks are mostly rhyolitic ignimbrites and have a volume larger than 800–820 km3.