Exhumation and deformation history of the lower crustal section of the Valstrona di Omegna in the Ivrea Zone, southern Alps
S. Siegesmund
P. Layer
I. Dunkl
A. Vollbrecht
A. Steenken
K. Wemmer
H. Ahrendt
10.6084/m9.figshare.3454535.v1
https://geolsoc.figshare.com/articles/dataset/Exhumation_and_deformation_history_of_the_lower_crustal_section_of_the_Valstrona_di_Omegna_in_the_Ivrea_Zone_southern_Alps/3454535
<p>The Ivrea Zone (southern Alps) is one of the key regions interpreted as exposing a section of the lower continental crust
and was the subject of several review-type articles. The Ivrea–Verbano Zone was rotated into an upright position along the
Insubric mylonite belt. In the southeast, this unit is in contact with the Strona Ceneri Zone, which is interpreted as upper
continental crust crossing the Permian Cossato–Mergozzo–Brissagio Line (CMB Line). The CMB mylonites are locally overprinted
by the mylonites and cataclasites of the Pogallo Line, which was active during the Jurassic. In addition, the sinistral, steeply
inclined Rosarolo shear zone was active over a long time span from the ductile into the brittle field, i.e. from the Early
Permian (high-temperature ultra-mylonites) to the Neo-Alpine basic dykes and pseudotachylites. The high-temperature mylonites
accommodated crustal extension and may be related to normal faults generated by magmatic underplating. The reactivation at
different crustal levels during exhumation and tilting is documented by strain increments at decreasing P/T conditions. Its
present subvertical orientation was attained during the Neo-Alpine deformation. Constraints on its exhumation history are
based on new <sup>40</sup>Ar/<sup>39</sup>Ar hornblende ages, K–Ar biotite ages and zircon fission-track data along the NE–SW trending Valstrona section. A re-interpretation
of existing U–Pb monazite ages is included, based on a higher closure temperature for monazite. The oldest monazite ages are
observed in proximity to the Pogallo Line (<em>c</em>. 292 Ma). Heat input by mafic intrusions was sufficient to reset the U–Pb monazite system, as is evidenced by the youngest
ages in the vicinity of the Insubric Line. The re-interpretation favours the hypothesis that the oldest monazite ages are
the result of complete resetting by a Permian thermal event. The <sup>40</sup>Ar/<sup>39</sup>Ar hornblende ages and K–Ar biotite ages document the cooling after Permian heating. Roughly parallel age progressions decrease
from the Pogallo Line (hornblende: 271 Ma vs. biotite: 227 Ma) towards the Insubric Line (hornblende: 201 Ma vs. biotite:
156 Ma). Zircon fission-track ages run parallel to the biotite ages in the upper part of the profile, whereas towards the
Insubric Line a significant deviation from the biotite age progression is attributed to tilting of the basement during the
Oligocene. Zircon fission-track ages around 38 Ma are found close to the Insubric Line. No age offset, neither at the CMB
nor at the Pogallo Line, is observed. This confirms the hypothesis that the Pogallo Line is an oblique normal fault, and that
the CMB Line has accommodated only minor vertical displacement. The capture of the different cooling ages confirms the tilting
of the Ivrea–Verbano Zone during the Neo-Alpine deformation and contradicts the tilting of the Ivrea–Verbano Zone during the
Permian.
</p>
2016-06-21 12:11:27
biotite age progression
Valstrona di Omegna
Strona Ceneri Zone
201 Ma vs
NE
age progressions decrease
Insubric Line
Pogallo Line
271 Ma vs
Insubric mylonite belt
Rosarolo shear zone
Ivrea Zone
CMB Line
monazite ages
Geology