Reinterpretation of nappe structures in the Central Alps Evidence from 3D foliation field modelling
Abstract
The Lepontine Alps represent the classic region for both, fold nappe development at mid-crustal levels and fold interference patterns from outcrop to regional scale. They have also been proposed as models for rapid burial and exhumation. However, before such models can be applied or critically assessed, the geometry of the units involved must be accurately known in three dimensions and this is a challenging task in this complex region. The three-dimensional geometry of the Lepontine nappes is not established unequivocally, as the integration and visualisation of structural data reflecting at least five different deformation phases has not yet been achieved satisfactorily. The present study focusses on a combination of new structural mapping in critical and/or contradictory areas, existing observations and their geostatistical assessment with modern computer-based tools (Editeur Géologique, Gocad) in order to develop a testable three-dimensional model of the geometry of the Lepontine Nappes in the Central Alps. Here we present a regional-scale three dimensional model of the southern central part of the Lepontine Region, bordered by the Swiss-Italian frontier in the east and Valle Leventina to the west and by Biasca and Locarno to the north and south. The model visualises a geostatistical calculation of the dominant foliation field in three dimensions, based on field measurements. The present day tectonostratigraphy and nappe geometry are mainly influenced by the three earliest recognisable Alpine deformation phases D_1 to D_3. D_1 structures are associated with first-time crustal nappe emplacement. Lithologies that are interpreted as nappe separators - mainly Mesozoic sediments such as "Bündnerschiefer" - have mostly been interleaved between the nappes during D_1, which entails strong isoclinal folding and pronounced boudinage of the infolded lithologies, with the development of a penetrative foliation S_1. S_1 and the infolded Mesozoic have been intensely refolded by the subsequent deformation event D_2, which also produced large-scale isoclinal folds and the present-day dominant foliation S_2 axial plane to F_2 structures on all scales. The majority of the regionally important fold structures in the northern part of the Penninic realm are assigned to D_2. D_3 produced comparably broad large-scale folds of long wavelength in the north that tighten dramatically in the south as the trend of their fold axial planes swings into parallelism with the Insubric Fault. D_3 only locally produced a new foliation S_3 parallel to its fold axial planes. On the outcrop scale, D_3-folds are poly- to disharmonic and of variable wavelength and amplitude. Regional D_3 structures are mainly defined on the basis of large-scale changes in the orientation of S_2. However, due to the previously mentioned strong tightening in the south, D_3 and later deformations progressively obliterate most D_2 structures, as the Insubric Fault is approached from the north. Our three dimensional model is entirely based on measurements of S_2, i.e. the dominant foliation throughout the study area and therefore only shows the effects of D_3 and later stages of deformation. In addition to well-known regional structures, auch as the Campo Tencia Synform, which dominates the area between northern Valle Maggia and northern Valle Leventina, this model reveals further folds of regional impact that have not previously been given sufficient attention. The most important finding in this context is the existence of a major, mainly east-west striking D_3 synform adjacent to the Insubric Line, which we will refer to as the Locarno-Synform. Lithologies on opposite limbs of the same fold clearly should not be assigned to different tectonic units. The new model is therefore not in accord with the present interpretation of this area, where one major tectonic unit, namely the Cima Lunga Nappe, is sandwiched between the tectonically juxtaposed but supposedly distinct Maggia and Simano Nappes. Instead we conclude that the Maggia and Simano Nappes in fact represent different parts of a single major tectonic unit, repeated on the limbs of a major synform - the Locarno Synform. The recognition of this synform and several other regionally important folds requires a significant reinterpretation of the distribution of the previously mapped tectonic units. Several nappe structures, previously considered independent units, are now interpreted as repetitions on alternate limbs of regional F_2 or F_3 folds, although their detailed geometry is often complicated by the history of multiple refolding.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....3749M