Exhumation and deformation history of the lower crustal section of the Valstrona di Omegna in the Ivrea Zone, southern Alps

Abstract

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 40Ar/39Ar 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 (c. 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 40Ar/39Ar 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.

Structural and sedimentary records of the Oligocene revolution in the Western Alpine arc

“…The Ivrea body, which has been extensively investigated using geophysical methods (Roure et al, 1996;Schreiber et al, 2009 and refs. herein) and which outcrops at the western termination of the Southern Alps (Siegesmund et al, 2008), is an Adriatic lithospheric mantle and crustal fragment overthrusting the European Moho. The lower crustal section yielded Eocene ZFT ages (Siegesmund et al, 2008) which mark the N-directed thrusting stage (Handy et al, 1999) over the oceanic (Sesia) and continental European crust (Briançonnais) during the continental subduction stage.…”

“…herein) and which outcrops at the western termination of the Southern Alps (Siegesmund et al, 2008), is an Adriatic lithospheric mantle and crustal fragment overthrusting the European Moho. The lower crustal section yielded Eocene ZFT ages (Siegesmund et al, 2008) which mark the N-directed thrusting stage (Handy et al, 1999) over the oceanic (Sesia) and continental European crust (Briançonnais) during the continental subduction stage. The western boundary of the Ivrea body, corresponding to lithospheric mantle, trends approximately N-S beneath the southern part of the Western Alpine arc (Waldhauser et al, 2002;Kissling et al, 2006;Lardeaux et al, 2006), that is in strong discrepancy with the present arc shape.…”

Interpretation of zircon coronae textures from metapelitic granulites of the Ivrea–Verbano Zone, northern Italy: two-stage decomposition of Fe–Ti oxides

“…Thus, the sampled gneiss is a restite, resulting from migmatization at peak granulite-facies conditions (e.g., Ewing et al, 2013). Partial melting is also responsible for apparent layering of host stronalites (Bea and Montero, 1999;Siegesmund et al, 2008), which are composed of alternating leucocratic and melanocratic layers.…”

“…Ubiquitous faulting of restitic material and both faulting and folding of host stronalite is due to intensive deformation taking place in granulitic metapelites during a long-time span after peak metamorphism (Siegesmund et al, 2008). According to Siegesmund et al (2008) brittle and ductile deformation acted simultaneously during formation of shear zones, and their close interactions resulted in complex deformation microstructures.…”

“…According to Siegesmund et al (2008) brittle and ductile deformation acted simultaneously during formation of shear zones, and their close interactions resulted in complex deformation microstructures.…”