Horizontal Tectonics Research Articles

Late events in the tectonic history of the Saxothuringian zone

Summary The Saxothuringian zone lies between the Moldanubian block (largely consolidated in late Precambrian time) to the south and the Rhenohercynian zone to the north. It is characterized by exotic blocks of relatively high-grade metamorphic rocks set among very low-grade Palaeozoic sequences. These ‘Zwischengebirge’ (Münchberg, Wildenfels, Frankenberg) were formerly interpreted as metamorphic ‘diapirs’. Recent investigations have led to a revival of the nappe concepts previously proposed by Suess, Wurm, Kossmat and others. The Münchberg complex is a pile of later Proterozoic to early Palaeozoic volcanic and sedimentary rocks, some now at advanced states of metamophism, in which both stratigraphic sequences and metamorphic grades appear in inverted order. These rocks rest upon a Carboniferous wildflysch, which, in its turn, rests upon an autochthonous Devonian and, locally, a Lower Carboniferous sequence. The flysch material, like the nappes above, was derived from sources in the SE. Special features of the sedimentary facies, the tectonic deformation, and the state of very low-grade metamorphism, combine with the evidence of a well-developed thrust at the base of the wildflysch sequence to suggest that this sequence should be treated as the lowest tectonic unit in the Münchberg pile of nappes. Tectonic deformation of the Palaeozoic sequence began with the production of tight to isoclinal, recumbent, NW-facing folds, accompanied and outlasted by subhorizontal thrusting. It was at this time that the nappe-like tectonic units (already in their metamorphic state) were emplaced. An F 2 refolding produced open, upright to SE-facing folds. A study of illite crystallinity has indicated the significance of a transverse zone, which was the locus of enhanced heat flow throughout the time of deformation, and has confirmed that a metamorphic inversion was introduced when the (relatively strongly metamorphic) wildflysch was thrust over the autochthonous Devonian and Carboniferous. The Saxothuringian zone shows the closest approach to an alpino-type character found in the northern part of the Variscides. Basin development, deformation and metamorphism are best explained in terms of a model based on horizontal tectonism.

High-grade metamorphism and possible overturning of the Tugela Rand layered intrusion in southeast Africa

The Tugela Rand layered intrusion is a metamorphosed, stratiform body located within the Nappe Zone of the mid-Proterozoic Natal mobile belt, South Africa. The intrusion comprises three macrorhythmic units (zones I, II, III), each consisting of the sequence dunite-websterite-gabbronorite. Possible overturning is evidenced by a downward decrease in the proportion of ultramafic to mafic rocks from zone I to zone III, a downward trend of iron enrichment from En 87 to En 76 , and inverted erosion scours within the cumulates. The intrusion has undergone both granulite- and amphibolite-facies metamorphism. The former is attributed to recrystallization in the presence of residual magmatic heat and the latter to regional metamorphism after or during further cooling of the intrusion. Overturning of the intrusion may have occurred during late-stage horizontal tectonics, when the Nappe Zone was transported northward over the southern boundary of the Kaapvaal craton. Overturned sequences have not previously been identified within the Nappe Zone.

What Has Deep-Sea Drilling Project Found Out about Deep Oceans?: ABSTRACT

The drilling vessel Glomar Challenger is completing 6 years of drilling at more than 300 sites in the Atlantic, Pacific, and Indian Oceans, as well as the Gulf of Mexico, Caribbean Sea, Labrador Sea, Mediterranean Sea, Bering Sea, Red Sea, and Antarctic waters. Significant achievements from drilling in the deep-marine environment have been accomplished in the fields of global tectonics, micropaleontology, paleo-oceanography, chemistry of interstitial water, diagenesis, mineralogy, and sedimentology. The oldest sediment recovered from any ocean basin is only Late Jurassic. This fact coupled with the systematic geographic distribution of the ages, constitutes powerful support for the concept of sea-floor spreading and continental drift. In addition to horizontal tectonics, the drilling has revealed areas of uplift and subsidence. Increased core recovery allowed the study of paleoenvironments on an ocean-wide basis. A major period of stagnation occurred in the Early and Late Cretaceous in the North Atlantic and Caribbean resulting in the deposition of carbonaceous sediments. Circulation was renewed in the Late Cretaceous and aerobic conditions prevailed to the present. Bottom water circulation became increasingly vigorous causing hiatuses in sedimentation of up to 70 million years in the Cretaceous-Paleogene section. Recently completed voyages into Antarctic waters traced glaciation on Antarctica to early Miocene. Another important application of drilling is the identification of seismic reflectors in the deep ocean sediments. The nature of these reflectors is varied but commonly includes chert, limestone layers, ash layers, basalts, unconformities, and rarely, what is believed to be a clathrate or gas hydrates. End_of_Article - Last_Page 1830------------