Underplating-related finite-strain patterns in the Gran Paradiso massif, Western Alps, Italy: heterogeneous ductile strain
superimposed on a nappe stack
OsamaK. Kassem
Uwe Ring
10.6084/m9.figshare.3454226.v1
https://geolsoc.figshare.com/articles/dataset/Underplating-related_finite-strain_patterns_in_the_Gran_Paradiso_massif_Western_Alps_Italy_heterogeneous_ductile_strain___superimposed_on_a_nappe_stack/3454226
<p>A finite-strain study in the central and northern Gran Paradiso massif of the Italian Western Alps has been carried out to
elucidate whether ductile strain shows a relationship to nappe contacts and to shed light on the nature of the subhorizontal
foliation typical of the gneiss nappes in the Alps. The <em>R</em><sub>f</sub>/φ and Fry methods were used on feldspar porphyroclasts from 98 samples of the Gran Paradiso unit (upper tectonic unit of
the Gran Paradiso massif) and 12 samples from the underlying Erfaulet unit (lower unit of the Gran Paradiso massif). Microstructures
and thermobarometric data show that feldspar ductility at temperatures higher than <em>c</em>. 450 °C occurred only during high-pressure metamorphism, when the rocks were underplated beneath the overriding Adriatic
plate. Therefore, the finite-strain data can be related to high-pressure metamorphism in the Alpine subduction zone. The augen
gneiss was heterogeneously deformed and axial ratios of the strain ellipse in <em>xz</em> sections range from 2.1 to 69.8. The long axes of the finite-strain ellipsoids trend west–WNW and the short axes are subvertical,
associated with a subhorizontal foliation. The strain magnitudes do not increase towards the nappe contacts. The data indicate
flattening strain type in the Gran Paradiso unit and constrictional strain type in the Erfaulet unit and prove deviations
from simple shear. We conclude that nappe stacking occurred early during subduction probably by brittle imbrication and that
ductile strain was superimposed on and modified the nappe structure during high-pressure underplating in the Alpine subduction
zone. The accumulation of ductile strain during underplating was not by simple shear and involved a component of vertical
shortening, which caused the subhorizontal foliation in the Gran Paradiso massif.
</p>
2016-06-21 11:59:09
thermobarometric data show
Gran Paradiso massif
subhorizontal foliation
Gran Paradiso unit
xz sections range
constrictional strain type
ductile strain
Alps
nappe contacts
Erfaulet unit
Alpine subduction zone
Geology