Geometry Of Nappes Research Articles

The Theodul Glacier Unit, a slab of pre-Alpine rocks in the Alpine meta-ophiolite of Zermatt-Saas, Western Alps

The Theodul-Glacier-Unit (TGU) is a 100 m thick and 2 km long slab of pre-Alpine schist, gneiss and mafic rocks tectonically emplaced in the eclogite-facies Zermatt-Saas meta-ophiolite nappe (ZSU). The meta-sedimentary rocks occur mostly as garnet-phengite schists with locally cm-sized garnet porphyroblasts. The metavolcanic basic rocks are present as variably retrogressed eclogites showing a continental basalt signature and contain abundant zircon, which is unusual for basalts. The zircons dated with the U–Pb system yield an upper intercept age of 295 ± 16 Ma and a lower intercept age of 145 ± 34 Ma. The early Permian age is interpreted to represent the age of high-grade granulite facies metamorphism, evidence of which is also preserved in the cores of garnet porphyroblasts of the Grt-Ph schists. The lower intercept age corresponds to the time of continental breakup and the initiation of the Tethys in the Mid-Jurassic; these events may have created the TGU as an extensional allochton. Eclogite facies metamorphism recorded by the TGU rocks occurred during Alpine subduction at 57 Ma, the Lu–Hf age of TGU eclogite garnets. The TGU reached a depth of about 53 km at P–T conditions of 1.7 GPa and 520 °C derived from both, eclogite and Grt-Ph schist. This is in contrast to the ZSU surrounding the TGU with a reported subduction depth of more than 80 km at 43 Ma. It is proposed here that TGU and ZSU were subducted separately out of sequence. After juxtaposition of the two units during late Alpine thrusting and folding forming the present day geometry of nappes in the Zermatt-Saas region both units were progressively metamorphosed to about 650 MPa and 470 °C. This late prograde metamorphism at 34 Ma produced oligoclase + magnesio-hornblende in the matrix of Grt-Ph schists and eclogites. The derived TGU data document a complete Wilson Cycle.

Lithostratigraphy and U-Pb zircon dating in the overturned limb of the Siviez-Mischabel nappe: a new key for Middle Penninic nappe geometry.

Detailed field work and zircon analysis have improved the knowledge of the lithostratigraphy at the base of the Siviez-Mischabel nappe in the Mattertal (St-Niklaus-Törbel area). They confirm the existence of an overturned limb and clarify the structure of the St-Niklaus syncline. The following formations can be observed: This coherent overturned sequence can be observed from the St-Niklaus area to the Moosalp pass to the north. Detailed mapping revealed that the St-Niklaus syncline is symmetrical and connects the overturned limb of the Siviez-Mischabel nappe to the normal series of the Upper Stalden zone. U-Pb zircon geochronology on magmatic and detrital zircons allowed constraining ages of these formations. Detrital zircons display ages ranging from 2900 ± 50 to 520 ± 4 Ma in the Törbel Formation, and from 514 ± 6 to 292 ± 9 Ma in the Moosalp Formation. In addition, the Permian Randa orthogneiss is intrusive into the polymetamorphic gneisses and into the Permo-Carboniferous metasediments of the overturned limb of the Siviez-Mischabel nappe. This revision clarified also the lithostratigraphy of the nearby and subjacent Lower Stalden zone composed of an overturned limb with Permo-Carboniferous formations. This has critical implications for the tectonic setting of the nappes in the region, speaking for few recumbent folds with well preserved normal and overturned limbs instead of a succession of imbricate thrust sheets in a normal stratigraphic position.

Köli nappes in the north-central Swedish Caledonides – new views on stratigraphy and structural evolution

The Köli Nappes of the Scandinavian Caledonides are composed of oceanic and island-arc terranes of Cambro–Silurian age and represent the upper part of the major Caledonian Upper Allochthon. Field, structural, microscopic, geophysical and geochemical data, together with a review of published information show new aspects on stratigraphy, structural evolution, nappe geometry and late Ordovician greywacke provenance in the Köli Nappes of southwestern Västerbotten and northern Jämtland counties, Sweden. It is proposed here to divide the lower part of the Köli succession of this area, i.e. the correlative of the Seima Formation, into three formations, the Kvemoen Mica Schist, the Ankarede Volcanite, and the Gilliks Formations. Trace element geochemistry of Ordovician Gilliks Formation greywackes indicates a provenance from a continental magmatic arc. New data also show that the overlying quartzite and limestone succession interpreted as the Vojtja and Slätdal Formations here is representing the top of the Stikke Nappe succession, which thus now reaches up into the highest Ordovician. The early repetition and inversion of the stratigraphy is an effect of D1-D2 deformation. The subsequent D3 episode comprises thrusting of, for example, the Middle Köli Nappe over the Lower Köli Nappe. A new aspect is that fault bends in D3 thrusts may give rise to fault-bend folds in overlying units, which are also D3 structures. The D3 episode also includes movement of the Köli units relative to the underlying Seve Nappes with top-to-the-west-directed shearing at the Seve-Köli boundary. It is succeeded by D4 regional-scale folds, related to nappe stacking in the underlying allochthons. The east-dipping limbs of these folds are thus locked for later, top-to-the-west shear deformation. Therefore, at least these parts of the Seve-Köli boundary cannot be regarded as re-activated during late (D5) and post-Caledonian gravitational deformation. Further work will have to show, whether the observed latest stage deformation represents late Caledonian tectonics or is related to subsequent extensional tectonics.

Nappe geometry in the Western Swiss Alps

Detailed geological mapping during the last 20 years in the Western Swiss Alps has shown clearly that most of the lower basement nappes are fold nappes possessing normal and inverted limbs. Moreover their cores are made of strongly deformed gneisses indicating that important ductile strain took place during the formation of the fold nappes. It is therefore probably wrong to imagine deep basement nappes as rigid slices as often actually claimed, especially when interpreting seismic profiles. True ‘brittle type’ thrust nappes involving basement rocks only occur in the internal and upper parts of the belt. Cover nappes, on the contrary, are in most parts of the Alpine belt thrust sheets following more or less the rules of thin-skinned tectonics. Many basement fold nappes lost part of their sedimentary cover during or just before their formation, by décollement along ductile horizons. The result is that many cover thrust nappes in the external part of the Alps are directly related to their original basement fold nappes.

Nappe geometry, basement structure and normal faulting in the central Scandinavian Caledonides; kinematic implications

(1991). Nappe geometry, basement structure and normal faulting in the central Scandinavian Caledonides; kinematic implications. Geologiska Foreningen i Stockholm Forhandlingar: Vol. 113, No. 2-3, pp. 265-269.

Caledonian nappe geometry in north-central Sweden and basin evolution on the Baltoscandian margin

AbstractStructural analysis of the Lower Allochthon in the north-central Scandinavian Caledonides has allowed the construction of restorable cross-sections consistent with the development of a foreland-propagating linked thrust system. The internal geometry of an antiformal stack, the Njakafjäll duplex, within the Lower Allochthon demonstrates tectonic shortening of c. 50% and suggests an overall predeformational width for the Lower Allochthon in this area of at least c. 130 km, and possibly considerably greater if the buried trailing edge of the Lower Allochthon lies in a comparable position to that farther south in Tröndelag. These results, combined with a stratigraphic analysis of the imbricates within the Lower Allochthon and of the adjoining Autochthon and Middle Allochthon, indicate the development, from Proterozoic through Cambrian times, of two sedimentary basins on the c. 200 km wide continental margin of Baltica bordering the Iapetus Ocean. The basins were separated by a region of basement relief, the Børgefjell domain, above which a reduced sequence of Vendian to Cambrian rocks accumulated. This Børgefjell basement high, and the similar Njakafjäll basement high to the east, subsequently became the sites of antiformal stack development. It is argued that the frequent incorporation of basement into the thrust sheets, together with the thin sedimentary fill of these basins, compared with the much greater fill in basins to the south in Jämtland and to the north of Finnmark, implies major palaeogeographic changes along the Baltoscandian margin, possibly related to early rift geometries. The apparent lack of subsequent foreland basin development in north-central Scandinavia compared with areas to the south may indicate a deeper level of thrust detachment beneath the Middle Allochthon to the north, such that any foreland basin sediments have been removed in the hangingwall and subsequently eroded. An alternative possibility is a primary absence of foreland basin development that may relate to a differing response to thrust loading by continental lithosphere which had been variably thinned during the earlier rift regime.

EFFECTS OF VISCOSITY AND SURFACE TENSION ON SLOT WEIRS FLOW

An experimental investigation of the flow over vertical weirs, referred to as “slot weirs” on account of their markedly elongated rectangular shape, was carried out at the laboratory of the Institute of Hydraulics and Hydraulic Constructions of the University of Naples. Experiments on weirs 3 to 20 mm broad showed that, for heads greater than 6-7 cm and for each weir the coefficient of discharge remains practically constant as the head increases, the constant value being greater the narrower the weir. This finding is accounted for by the combined influence, on the discharge process, of two physical properties of water, namely surface tension and viscosity, and by a possible modification of the kinematic process of contraction, due to the presence of the singular point of contact between outflowing nappe and weir edge. The influence of the above properties of water is enhanced with very small heads (lower than 6 cm), to such an extent as to give rise to sharply different behaviours, depending on the experimental conditions. Specifically, a significant drop in the coefficient of discharge is observed for free nappe weirs, and a sharp increase for fully submerged weirs, both features becoming increasingly more marked as the head decreases. The experimental results are discussed in the light of hydrodynamical considerations on the influence of surface tension and viscosity on the nappe geometry.

The internal geometry of nappes: criteria for models of emplacement

Summary The geometric and stratigraphic properties of thrust sheets and fold nappes are controlled by the mechanisms by which they were initiated and emplaced. Thrust sheets that have developed by gravity gliding have their internal stratigraphies truncated by the basal thrusts which cut up-section at the trailing edges. In contrast thrust sheets that have developed by gravitational spreading have their internal stratigraphies truncated by the overlying thrust sheets, and their basal thrusts cut up-section at the leading edges. Fold-nappes have complicated geometries and correlation of stratigraphy between successive nappes is difficult. The basic geometric and stratigraphic properties of thrust sheets and fold-nappes developed by different mechanisms may be used as criteria for testing the applicability of these mechanisms to particular areas of orogenic belts. The Valley and Ridge Province of the Appalachians has geometric and stratigraphic properties consistent with thrust sheet development by gravitational spreading. The western nappes of the Central Scandinavian Caledonides are more complex but have properties suggesting development by buoyant upwelling of the orogen infrastructure. There is, however, a lack of detailed cross-sections of orogenic belts based on geophysical and/or borehole data in addition to the geological data. The interpretation of geological data in isolation is often too dependent on subjective information coloured by the mechanism preferred by the individual concerned.

Geometry and emplacement of nappes in the Central Scottish Highlands

Summary A deformation history related to polyphase nappe formation is presented for the Central Scottish Highlands as a model for the Dalradian tract. The genesis of a four-fold pattern of tectonic belts recognisable throughout this tract is discussed. The fan structure of the Tummel steep belt is argued to be a deep-seated rotation zone. Movements within this zone spanned the climax of Barrovian metamorphism and were responsible for creating the major belt of inverted rocks—the Loch Tay inversion—which forms the lower limb of the Tay (fold) nappe. The early-formed Tay nappe, nucleated at a relatively high tectonic level, was translated as a result of movements within the deeper rotation zone progressively towards the SE principally by simple shear. The core of the Tay nappe is recognised within the Highland Border steep belt at Aberfoyle, but does not root within the deeper levels of the Central Highlands Dalradian. The Highland Border steep belt at Dunkeld was formed by late orogenic downbending of the previously flat-lying inverted limb of the Tay nappe probably resulting from sub-vertical movements within the basement along the ancestral line of the Highland Boundary fault. A kinematic model explains the nappe geometry by density inversion of the Dalradian succession consequent on the pre-orogenic evolution of the Dalradian (marginal) basin.

Nappe displacement in the Scandinavian Caledonides

Large areas of the Scandinavian Caledonides are eroded to the level of the basement/cover contact. Relationships between the Precambrian crystalline basement (largely Svecofennian-Dalslandian, 1800-1000 m.y.) and cover sequences are exposed both in transverse profiles through the mountain belt and along the belt in the various windows. These relationships provide an unique opportunity for studying the basement configuration, character of basement involvement and general nappe geometry. Major allochthonous units of the central part of the Scandinavian Caledonides — the Offerdal, Särv and Seve-Köli Nappe Complex — have been shown to wedge out westwards, having been displaced eastwards from environments along and west of the present Norwegian coast. Recent investigations have shown that these units (the Offerdal, Särv and Seve) reappear in western Norway as major pinch-and-swell structures, the lenses reaching thicknesses in the order of 2 km and with long axes of several tens of kilometres. Within the western parts of the Swedish Caledonides the thicker parts of the lenses approximately coincide with the axes of the late synforms which fold basement and cover together. Further west, in Norway, the tensing appears to be unrelated to the geometry of these major folds. This evidence increases estimates of nappe displacement distance (now thought to be in the order of at least 1000 km). At the same time it emphasizes that translation may account for only about half of this amount, the rest being achieved by stretching of the nappes. Apparently, a nappe sequence built up in the west which subsequently collapsed, leading to continued displacement eastwards on to the Baltoscandian Platform. Whereas basement shortening is of the order of several tens to perhaps hundreds of kilometres in the western part of the central Scandes, it is of lesser importance from central Trøndelag eastwards, a distance of about two hundred kilometres, to the Caledonian Front. Biostratigraphic evidence from the late-orogenic intramontane basins, taken in relation to the youngest units involved in the nappes, requires nappe translation into western Norway to have occurred after the Llandoverian (Köli Supergroup) and prior to the (Ludlovian?) Downtonian (Hitra Formation) deposition in the intramontane basins. The nappes contain sequences derived from a variety of probable oceanic and continental margin environments, and this translation may greatly exceed the minimum estimate of five hundred kilometres. Further displacement eastwards occurred during uplift of seaboard Norway and accompanied sedimentation both in the intra- and extramontane basins. The latter were not finally influenced by the décollement tectonics until after the Early Devonian. This evidence suggests that a compressive regime dominated the early phases of orogenesis during basement shortening, build up of the nappe pile and translation of these denser units on to the western margin of the Baltoscandian Platform. This compression subsequently gave way to a gravity regime, collapse and stretching of the nappes dominating the late phases of displacement on to the Baltoscandian Platform.