Monte Rosa Nappe Research Articles

The Exhumation of Eclogite-Facies Metamorphic Rocks-a Review of Models Confronted with Examples from the Alps

We summarize recently proposed models for the exhumation of (ultra-) high pressure ([U]HP) metamorphic units within collisional orogens. In terms of mechanics, exhumation is either caused by buoyancy or by externally applied stresses. In terms of kinematics, exhumation is either by erosion, extension, or extrusion. The tectonic scenarios that may lead to the exhumation of (U)HP units are: (1) flow within a low-angle or high-angle corner; (2) entrance of the continental margin into the subduction zone; (3) cessation of subduction; (4) slab breakoff; (5) extension in the backarc; and (6) serpentinization of the mantle wedge. The applicability of these models are discussed for some (U)HP terrains in the Alps, in particular the Koralm Complex of the Austroalpine Unit, and the Internal Penninic Nappes of the Eastern, Central, and Western Alps, (in particular, the Eclogite Zone, the Adula Nappe, and the Monte Rosa Nappe). Within the Austroalpine Koralm Complex of the Eastern Alps, the exhumation of HP rocks see...

The Exhumation of Eclogite-Facies Metamorphic Rocks—a Review of Models Confronted with Examples from the Alps

We summarize recently proposed models for the exhumation of (ultra-) high pressure ([U]HP) metamorphic units within collisional orogens. In terms of mechanics, exhumation is either caused by buoyancy or by externally applied stresses. In terms of kinematics, exhumation is either by erosion, extension, or extrusion. The tectonic scenarios that may lead to the exhumation of (U)HP units are: (1) flow within a low-angle or high-angle corner; (2) entrance of the continental margin into the subduction zone; (3) cessation of subduction; (4) slab breakoff; (5) extension in the backarc; and (6) serpentinization of the mantle wedge. The applicability of these models are discussed for some (U)HP terrains in the Alps, in particular the Koralm Complex of the Austroalpine Unit, and the Internal Penninic Nappes of the Eastern, Central, and Western Alps, (in particular, the Eclogite Zone, the Adula Nappe, and the Monte Rosa Nappe). Within the Austroalpine Koralm Complex of the Eastern Alps, the exhumation of HP rocks seems to be related to the development of a major detachment within the lower crust (Plattengneis shear zone), subsequent to the formation of an orogenic wedge. The central parts of the Koralm Complex were exhumed to a larger extent than the northern, and in particular the southern parts, as indicated by a continuous decrease of pressures toward north and south. Thus, we suggest that the Plattengneis shear zone was not related to the compressional phase during nappe stacking, but to the extensional exhumation of the Middle Austroalpine basement complexes that were previously affected by eclogite facies metamorphism. The tectonometamorphic evolution of (U)HP Internal Penninic Nappes, which are assumed to have originated from the European continental margin, may be characterized by(buoyancy-driven) extrusion of eclogite facies crustal wedges from the subduction channel with a low-angle corner geometry. During the early phase of exhumation, the pressure-temperature path is characterized by cooling in the Dora Maira Massif, the Gran Paradiso, the Monte Rosa Nappe, and the Eclogite Zone; this argues for exhumation within a still-active subduction zone. In the Adula Nappe, decompression was isothermal. At amphibolite- to greenschist facies conditions, the exhumation mechanism of these units changed to crustal extension, accompanied by minor heating during decompression (except for the Adula Nappe). This argues for cessation of subduction and removal of the cooling effect.

EPMA Major and Trace Element Analysis in Garnet and its Petrological Application

A comparison between major and trace element concentrations in garnet performed by electron microprobe (EPMA) technique is reported. Quantitative spot analyses and X-ray maps of major elements (Fe, Mg, Mn, Ca) and the trace element yttrium in garnets from metamorphic rocks are presented. The selected garnet samples come from meta-pelitic and meta-basic specimens belonging to the tectonic unit of the Monte Rosa Nappe (Western Alps). In the metapelites, the quantitative Y distribution maps display a prominent increase at the core, the Y abundance varying by over two orders of magnitude, from about 80 ppm (rim) to over 2100 ppm (core). The Y profiles show well defined patterns with sharp features that do not correlate with major element distributions. A roughly comparable pattern can be supposed only with Mn. The Y distribution suggests that the diffusion of Y through the garnet is very slow compared to the major elements, thus the Y results are suitable for geothermometric estimates. In the metabasites, the Y spatial distribution is characterised by an increasing content from the core to the rim, displaying a zoning pattern opposite to the metapelite garnet. Quantitative EPMA analyses range from 1100 ppm at the rim to values lower than the detection limit at the core. Therefore, the Y content in the garnet can be related to several chemical and physical variables such as the bulk rock composition and the phase assemblage. In particular, in the xenotime-bearing metapelitic system the Y distribution seems to be correlated with metamorphic peak temperature.

Origin of the Monte Rosa nappe in the Pennine Alps—A new working hypothesis

On the basis of lithological and structural evidence, a new solution is proposed for the classical problem of the restoration of folded continental and oceanic nappes in the Pennine Alps of Switzerland and Italy. According to this working hypothesis, the Monte Rosa nappe, a high-pressure metamorphic gneiss unit, represents basement of the northern European margin of Alpine Tethys. Its paleogeographic origin was formerly sought either in the Brianconnais microcontinent or in the southern Adriatic continental margin. The Monte Rosa nappe is enveloped by the lithologically heterogeneous and extremely deformed Furgg zone, interpreted here as a melange zone formed during subduction and collisional closure of the Valais ocean basin, the northern subbasin of Alpine Tethys. In a late stage of the Alpine orogeny (late Eocene), postdating closure of the Valais ocean basin, the European continental margin including the Monte Rosa basement was subducted southward under the Pennine nappe stack and reached eclogite facies depth. From there, the Monte Rosa nappe quickly ascended back toward the surface in an internal (southeastern) position, separated from the other Europe-derived units located farther northwest by the rootless, Brianconnais- derived Bernhard nappe system.

Eo/Oligocene (35 Ma) high-pressure metamorphism in the Monte Rosa nappe (Western Alps): implications for paleogeography

Eo/Oligocene (35 Ma) high-pressure metamorphism in the Monte Rosa nappe (Western Alps): implications for paleogeography

Oligocene gold quartz veins at Brusson, NW Alps; Sr isotopes trace the source of ore-bearing fluid to ore a 10-km depth

The genesis of late metamorphic gold lodes in the northwestern Alps has remained enigmatic. In order to trace the source of the ore-bearing fluid at the Brusson locality and to test current genetic models, 130 Rb-Sr isotope measurements have been performed on hydrothermal vein minerals, their fluid inclusions, and their unaltered country rocks. The data, corrected for in situ grown 87 Sr since vein emplacement at 31 Ma (Sr initial ), reveal Sr isotope disequilibrium between the wall rocks of the veins (Monte Rosa metagranites: Sr initial approximately 0.7580, and paragneisses: Sr initial 0.7107-0.7129; Piemonte metabasites: Sr initial approximately 0.7038) and the vein-forming fluids (Sr initial = 0.7291 - 0.7079). At most vein localities, Sr initial of the vein minerals and fluid inclusions evolves monotonically away from the host-rock signatures with progressive vein growth, due to decreasing local fluid-rock interaction upon wall-rock armoring. These analyses demonstrate that fluid flow in the hydrothermal system was upward and the Sr initial value of the deep source fluid was less than, but probably close to, 0.7100. Since the Monte Rosa nappe has Sr initial values higher than 0.7107 and a vertical thickness of more than 10 km, the ore-bearing fluid must have derived from at least 10 km below the present outcrop level. We conclude that hydrothermal systems which produce Tertiary lode gold deposits in orogenic belts may have large vertical extents, perhaps up to the range recently reported for equivalent Archean deposits. When combined with published fluid inclusion compositions and Pb and He/Ar isotope data, our results rule out three of the previously proposed sources for the Brusson fluids: (1) metamorphic devolatilization of paragneisses and metagranites of the Monte Rosa nappe (vein Sr and Pb are too primitive and vein fluid chemistry is inconsistent), (2) fluid escape from coeval lamprophyres (vein Sr and Pb are too primitive), and (3) incursion of meteoric water (vein 36 Ar/ 3 He values are orders of magnitude too low). The only potential source rocks consistent with the new constraints and known to be abundant beneath the Monte Rosa nappe are calc-schists (0.7093 < Sr initial < 0.7104; model 238 U/ 204 Pb ratio, mu 2 , approximately 9.72) and intercalated metabasites of the Mesozoic Piemonte ophiolite nappe (Sr initial approximately 0.7038; 9.20 < mu 2 < 9.51). The Periadriatic syenogranitic intrusions that crop out 18 km southeast of Brusson have similar Sr initial values and are coeval with the gold deposits, but there is no evidence for the presence of such intrusions below Brusson. Our genetic scenario, in accord with the timing of orogenic events, is that regional Meso-Alpine metamorphism liberated CO 2 -H 2 O-NaCl-KCl-H 2 S fluids from the deep-seated Piemonte calc-schists via devolatilization reactions (yielding fluid Sr initial approximately 0.710, mu 2 approximately 9.72), and that gold was extracted as the fluids reacted with the intercalated Piemonte metabasites prior to rising through the Monte Rosa nappe to the sites of gold deposition.

Alpine and pre-Alpine magmatism in the root-zone of the western Central Alps

The highest grade of metamorphism and associated structural elements in orogenic belts may be inherited from earlier orogenic events. We illustrate this point using magmatic and metamorphic rocks from the southern steep belt of the Lepontine Gneiss Dome (Central Alps). The U-Pb zircon ages from an anatectic granite at Verampio and migmatites at Corcapolo and Lavertezzo yield 280–290 Ma, i.e., Hercynian ages. These ages indicate that the highest grade of metamorphism in several crystalline nappes of the Lepontine Gneiss Dome is pre-Alpine. Alpine metamorphism reached sufficiently high grade to reset the Rb-Sr and K-Ar systematics of mica and amphibole, but generally did not result in crustal melting, except in the steep belt to the north of the Insubric Line, where numerous 29 to 26 Ma old pegmatites and aplites had intruded syn- and post-kinematically into gneisses of the ductile Simplon Shear Zone. The emplacement age of these pegmatites gives a minimum estimate for the age of the Alpine metamorphic peak in the Monte Rosa nappe. The U-Pb titanite ages of 33 to 31 Ma from felsic porphyritic veins represent a minimum-age estimate for Alpine metamorphism in the Sesia Zone. A porphyric vein emplaced at 448±5 Ma (U-Pb monazite) demonstrates that there existed a consolidated Caledonian basement in the Sesia Zone.

Isotope systematics in vein gold from Brusson, Val d'Ayas (NW Italy), 1. Pb/Pb evidence for a Piemonte metaophiolite Au source

The Monte Rosa Gold District (NW Italian Alps) comprises hundreds of late Alpine, epigenetic, gold-bearing quartz veins. We report a lead isotope study on an important one of these veins, Fenilia (Brusson, Val d'Ayas), which reveals a complex isotope geochemistry of petrographically cogenetic vein minerals. Petrographic observations allow to distinguish a quartz-dominated main-stage and a quartz-poor final-stage mineral assemblage. Most of the free gold crystallized in the beginning of the final stage, whereas sulphides were deposited during its waning. Three distinct lead isotope fields result for the paragenetic sequence of vein minerals. The variable lead isotope signature (15.517 < 207Pb/ 204Pb < 15.623) trapped in final-stage free gold, and minor quartz can be interpreted as a variable mixture between a primitive MORB-type and a radiogenic Pb component. It differs distinctly from the radiogenic lead signature of the main-stage mineral assemblage ( 207Pb/ 204Pb > 15.706), as well as from the uniform radiogenic lead (15.649 < 207Pb/ 204Pb < 15.691) characteristic of the waning final stage. We favour a model where the deeply circulating final-stage fluid, the ultimate source of which remains obscure, acquired lead and thus possibly its gold in Piemonte metaophiolitic rocks beneath the Monte Rosa nappe. During ascent, the primitive gold-bearing fluid was variably contaminated by an evolved crustal lead (15.706 < 207Pb/ 204Pb < 15.743) compatible with derivation from Monte Rosa nappe rocks. During deposition of final-stage minerals the Pb isotope composition of the fluid changed to a constant signature becoming again dominated by a radiogenic lead source. Thus within an assemblage defined as cogenetic on petrographic grounds the Pb isotopic signature is not necessarily uniform and source modelling based on lead isotopes of associated minerals may be inappropriate. It remains open at present, whether the inferred Piemonte metaophiolite gold source is confined to the Fenilia vein in Val d'Ayas only, or whether it might be more widespread in the Monte Rosa Gold District.

Horizontal contraction or horizontal extension? Heterogeneous Late Eocene and early Oligocene general shearing during blueschist and greenschist facies metamorphism at the Pennine-Austroalpine boundary zone in the Western Alps

Mylonitic structures related to two orogenic events are described from the upper and lower contacts of the Combin zone and the immediately overlying upper Austroalpine Dent Blanche nappe/Mont Mary klippe and the directly underlying lower Austroalpine Etirol-Levaz slice. The first event, Late Eocene in age, commenced during blueschist facies P-T conditions, but pre-dated the peak of subsequent greenschist facies overprint. The second event, Early Oligocene in age, took place during retrograde greenschist facies conditions. Most sense of shear indicators associated with the retrograde mylonites indicate top SE shearing, but subordinate top NW displacing shear sense indicators have also been mapped. Mylonitic top SE shearing appears to be restricted to the Combin zone and its upper and lower contacts. Within the Dent Blanche nappe and Mont Mary klippe and at the base of the Etirol-Levaz slice, structures were observed which developed during blueschist/greenschist facies conditions and are, in conjunction with the P-T-t history of these rocks, inferred to be older. Associated kinematic data indicate a top NW shear sense. Comparable blueschist/greenschist facies shear sense indicators have not been observed in the Combin zone. Nonetheless, the foliation in the Combin zone shows a progressive evolution from blueschist facies to greenschist facies to retrograde greenschist facies conditions. This indicates that the Combin zone and the immediately over- and underlying Austroalpine units shared a common tectono-metamorphic evolution since the Late Eocene. Finite strain data reveal oblate strain fabrics, which are thought to result from a true flattening strain geometry. Flow path modelling reveals a general non-coaxial deformation regime and corroborates significant departures from a simple shear deformation. In the study area, mylonitic top SE shearing in the Combin zone is attributed to Early Oligocene backfolding and backthrusting of the Mischabel phase. Temperature-time curves suggest slight reheating in the Monte Rosa nappe underneath and cooling in the Dent Blanche nappe above the Combin zone, hence confirming a thrust interpretation for this event. The top NW displacing structures are thought to result from Late Eocene emplacement of the Dent Blanche nappe and the Combin zone onto the Middle Pennine Barrhorn series along the Combin fault. As related structures initiated during mildly blueschist facies conditions in the Dent Blanche nappe and the underlying Combin zone and both were emplaced together onto the greenschist facial Barrhorn series, it is concluded that the structures developed as the nappes moved upward relative to the earth's surface. Thus the Combin fault is regarded as a thrust. The geometry of this structure indicates that the Combin fault is an out of sequence thrust that locally cut down section. Hence, top NW out of sequence thrusting caused local thinning of the metamorphic/structural section in association with horizontal shortening. Out of sequence thrusts cutting down section, and back-thrusts, offer the possibility of explaining the pronounced break in the grade of metamorphism across the Combin fault, i.e. the contact between the eclogite facial Zermatt-Saas zone and the overlying lower grade Combin zone, by contractional deformation.

Forethrusting, backfolding, and lateral gravitational escape in the northern part of the Western Alps (Monte Rosa region)

Research Article| July 01, 1992 Forethrusting, backfolding, and lateral gravitational escape in the northern part of the Western Alps (Monte Rosa region) UWE RING; UWE RING 1Institut für Geologie, Universität Tübingen, Sigwartstrasse 10, D-7400 Tübingen, Germany Search for other works by this author on: GSW Google Scholar OLIVIER MERLE OLIVIER MERLE 2Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar Author and Article Information UWE RING 1Institut für Geologie, Universität Tübingen, Sigwartstrasse 10, D-7400 Tübingen, Germany OLIVIER MERLE 2Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1992) 104 (7): 901–914. https://doi.org/10.1130/0016-7606(1992)104<0901:FBALGE>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation UWE RING, OLIVIER MERLE; Forethrusting, backfolding, and lateral gravitational escape in the northern part of the Western Alps (Monte Rosa region). GSA Bulletin 1992;; 104 (7): 901–914. doi: https://doi.org/10.1130/0016-7606(1992)104<0901:FBALGE>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract A penetrative composite foliation and stretching lineation in the ophiolite of the Zermatt-Saas-Vanzone region developed under eclogite-facies conditions in mid-Cretaceous time. Eclogite-facies mineral parageneses in this foliation were progressively replaced by blueschist-facies and greenschist- to amphibolite-facies mineral assemblages during Late Cretaceous and mid-Tertiary time and were subsequently completely retrogressed. Kinematic indicators associated with the various stretching lineations of the different metamorphic facies reveal that shearing was initially approximately top-to-west (high-P phase) and changed during the greenschist/amphibolite-facies stage into a top-to-west/northwest orientation. Forethrusting was followed by low-angle normal shearing during the final retrograde phase. This sense of shear was upper block-to-southwest/west-southwest. Unambiguous structural relics of the high-pressure event were not recognized in the gneiss of the Monte Rosa nappe, but the close similarity of the metamorphic evolution of the Zermatt-Saas and Monte Rosa units (including the high-P event) makes a common tectonic history probable.In late Oligocene and Miocene time, the Zermatt-Saas-Vanzone region was affected by an initial phase of backfolding (Mischabel phase, predating top-southwest/west-southwest-directed, low-angle normal shearing) toward the south/southeast followed by a second backfolding event (Vanzone phase). The main geometrical and structural features suggest that the Mischabel phase resulted from vertical extrusion and horizontal intrusion of the ductile basement (Monte Rosa) into shallower units (Antrona and Zermatt-Seas ophiolites) and is in accord with a recently proposed kinematic model for the Central Alps. No clear-cut relations between the second phase of backfolding and the retrograde phase were observed; this suggests that both events took place largely simultaneously. Backfolding northeast of the Simplon line postdated retrograde normal shearing and is of late Miocene age. It is concluded that backfolding in the Zermatt-Saas-Vanzone region cannot be causally linked with backfolding northeast of the Simplon line. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Kinematics of ductile shearing from outcrop to crustal scale in the Monte Rosa Nappe, Western Alps

Significant ductile deformation has produced widespread mylonitic structures in the basement orthogneisses and the Mesozoic cover quartzites and marbles of the Monte Rosa nappe, in the inner part of the Western Alps. We summarize here the results of a detailed microstructural study at several scales of observation in the Northern part of the nappe and discuss the kinematic significance and compatibility of such results. Most of the observed deformations can be accounted for by progressive WNW‐vergent shearing during and after the late Eocene, within a ductile shear zone of crustal scale. The displacement of the hanging wall may have been in excess of several tens of kilometers. However, some ESE‐vergent shear criteria cannot be reconciled with this general picture and suggest that local backthrusting occurred later near the toe of the Monte Rosa nappe.

Kilometre-scale sheath fold at Mattmark and implications for transport direction in the Alps

Folds were the first tectonic features of rocks to be studied and as such are taken as a reference for the interpretation of other structures. Intuitively, fold axes were assumed to be subperpendicular to the direction of transport. Nevertheless, many studies of metamorphic belts showing intense ductile deformation describe folds with axes close to the transport direction. In particular, sheath folds1–9, are highly curvilinear folds elongated parallel to the direction of transport. We discuss here a spectacular large sheath fold situated in a crustal shear zone in the Monte Rosa nappe, Swiss Alps. Our analysis implies that both the geometry and significance of folds in the internal Alps must be re-examined. We further demonstrate the existence of crustal ductile shear zones in an area of the Alps where previously emphasis was generally placed on fold nappes; the movements on these shear zones is often tens of kilometres.

Areal geology and petrology of eclogites and associated metabasites of the Piemonte ophiolite nappe, breuil—st. Jacques area, Italian Western Alps

The Breuil—St. Jacques area is located in the upper Valtournanche and Ayas valleys on the north side of the middle Aosta Valley. The principal unit exposed here consists of calc-schists + greenstones of the Piemonte ophiolite nappe. This complex is interposed between the underlying Pennine Monte Rosa nappe and the overlying Austroalpine Dent Blanche + Sesia—Lanzo nappe. The juxtaposed representatives of two structural units occur within the Piemonte section; they differ in terms of lithologic associations, metamorphic assemblages and paleogeographic significance. 1. (1) The structurally lower Zermatt—Saas unit consists of an important basal sequence of largely serpentinized peridotite tectonites, an overlying group of discontinuous metagabbros locally containing magmatic clinopyroxene relics, and a capping series of various types of tholeiitic to slightly alkaline metabasalt. Syn- and post-volcanic sedimentary strata, now predominantly garnetiferous, ankeritic mica schists with associated marbles and minor calcareous, manganiferous metaradiolarites (spessartine-, piemontite- and braunite-bearing metacherts) form a superjacent cover series. The Zermatt—Saas unit exhibits a composite metamorphic character. Eclogites and early glaucophane schists of the eoalpine stage of recrystallization have been incipiently to pervasively replaced by a greenschistic assemblage produced during the Lepontine metamorphic event. This latter recrystallization involved the renewed growth or recrystallization of sodic amphibole. Serpentinite of the Zermatt—Saas unit contains numerous gabbroic dikes, some of which have been partly transformed to fine-grained rodingitic material; other rodingites represent metasomatic reaction rims between ultramafic material and lithologically diverse surrounding rocks. 2. (2) The structurally overlying Combin unit possesses a distinctive stratigraphy which contrasts with that of typical ophiolites and correlative oceanic crust. It consists of locally preserved Upper Permian (?), Triassic and Liassic strata of continental affinities, overlain by a section made up chiefly of regular intercalations of calc-schists, marbles and metavolcanic layers derived from submarine basaltic flows, hyaloclastites, tuffs and/or tuffites. The mafic rocks have been pervasively recrystallized to porphyroblastic albitebearing greenschists (prasinites). This volcanoclastic sequence contains interbeds of manganiferous metaradiolarite and quartz + albite-bearing chlorite schists, in part with associated stratiform Cu-Fe sulfides; it also includes some lenses — tectonic slivers and/or olistostromes — of metagabbro and serpentinite. The Combin unit does not exhibit the characteristic relict eclogitic association of the Zermatt—Saas unit; instead it displays only the effects of greenschist facies recrystallization (including very rare relics of sodic amphibole) attributable to the Lepontine metamorphic event, and corresponding to the greenschist overprinting of both the Zermatt—Saas unit and the lower tectonic member of the Austroalpine nappe. Bulk XRF analyses of seventeen mafic rock samples demonstrate that, although postigneous metasomatism has produced sodium enrichment, original lithologies possessed affinities with oceanic tholeiite. The Zermatt—Saas and Combin metabasites do not exhibit distinctive compositional differences. The relatively high-pressure metamorphic prograde path displayed by Zermatt—Saas mineral assemblages of eoalpine age is characteristic of subduction zone metamorphism, whereas the retrograde P—T trajectory represents nearly adiabatic decompression —hypothesized to have accompanied buoyant return of the imbricated subducted complex towards the surface after its detachment from the downgoing lithospheric slab. The greenschist facies overprinting, which according to isotopic age measurements is connected at least in part with the Lepontine metamorphic event, seems to be related to a post-collisional thermal reequilibration of the pile of nappes, including both Zermatt—Saas Combin units.

Glaucophane Fabric in the Cover of the Monte Rosa Nappe, Zermatt–Saas Fee, Southwest Switzerland

To satisfy experimental data of phase equilibria and rock strength, current plate-tectonic models suggest that blueschist parageneses in Mesozoicophiolites in mountain belts may have formed in subduction-zone environments. However, structural geometry and estimates of overburden thickness in the region of the Mischabel backfold in the western Alps suggest that mesoscopic linear fabrics of glaucophane and other minerals formed at a relatively shallow depth during backfolding. They either grew syntectonically or were rotated as pre-existing minerals into a preferred linear orientation. It is proposed that the glaucophane lineation formed during the Barrovian Lepontine phase of Alpine metamorphism late in, or after, the northward transport of the Monte Rosa nappe, which in turn moved after obduction of its Mesozoic envelope of oceanic ophiolites (with its blueschist parageneses formed in a Tethyan subduction zone) onto European sialic crust that now forms the core of this nappe.