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

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 (<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 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.

伊夫雷亚带（Ivrea Zone，南阿尔卑斯山）是被认为出露下大陆地壳剖面的关键区域之一，此前已有多篇综述类文章对其展开研究。伊夫雷亚-韦尔巴诺带（Ivrea–Verbano Zone）沿因苏布里奇糜棱岩带（Insubric mylonite belt）旋转至直立产状。在东南部，该单元与斯特龙纳-切内里带（Strona Ceneri Zone）接触，后者被认为是穿越二叠纪科萨托-梅尔戈佐-布里萨焦线（CMB线）的上大陆地壳单元。CMB糜棱岩局部被侏罗纪时期活动的波加洛断裂带（Pogallo Line）的糜棱岩和碎裂岩（cataclasites）叠加改造。此外，左旋陡倾的罗萨罗剪切带（Rosarolo shear zone）在从早二叠世（高温超糜棱岩阶段）到新阿尔卑斯期基性岩脉与假玄武玻璃（pseudotachylites）的漫长时间跨度内持续活动，其活动域从韧性域延伸至脆性域。高温糜棱岩对应地壳伸展作用，可能与岩浆底侵（magmatic underplating）形成的正断层相关。折返与倾斜过程中不同地壳层次的再活化，可通过压力温度（P/T）条件降低过程中的应变增量得到记录。其现今的近直立产状形成于新阿尔卑斯期变形阶段。针对该带折返历史的约束条件，源自沿北东-南西向（NE–SW）瓦尔斯特罗纳剖面（Valstrona section）获取的全新⁴⁰Ar/³⁹Ar角闪石年龄、K–Ar黑云母年龄与锆石裂变径迹数据。本次研究还基于独居石（monazite）更高的封闭温度，对已有的铀-铅（U–Pb）独居石年龄开展了重新解译。波加洛断裂带附近测得的独居石年龄最老（约292 Ma）。镁铁质侵入体带来的热输入足以重置铀-铅独居石体系，因苏布里奇线附近测得的最年轻独居石年龄即可佐证这一点。重新解译结果支持以下假说：最老的独居石年龄是二叠纪热事件完全重置体系的结果。⁴⁰Ar/³⁹Ar角闪石年龄与K–Ar黑云母年龄记录了二叠纪热事件后的冷却过程。年龄序列大致平行，从波加洛断裂带（角闪石年龄：271 Ma，黑云母年龄：227 Ma）向因苏布里奇线逐渐降低（角闪石年龄：201 Ma，黑云母年龄：156 Ma）。剖面上部的锆石裂变径迹年龄与黑云母年龄序列基本平行，而向因苏布里奇线方向出现的显著偏离，则被归因于渐新世（Oligocene）基底的倾斜。因苏布里奇线附近测得的锆石裂变径迹年龄约为38 Ma。未在CMB线或波加洛断裂带处观测到年龄偏移，这证实了以下假说：波加洛断裂带为斜向正断层，而CMB线仅发生了极少量垂直位移。不同冷却年龄的匹配结果证明，伊夫雷亚-韦尔巴诺带的倾斜发生于新阿尔卑斯期变形阶段，与二叠纪时期发生倾斜的假说相悖。