Precollisional, multistage exhumation of subducted continental crust: The Sesia Zone, western Alps

作者: J. Babist , M. R. Handy , M. Konrad-Schmolke , K. Hammerschmidt

DOI: 10.1029/2005TC001927

关键词: BlueschistGeologyContinental crustContinental marginGeomorphologyNappeSubductionGeochemistryRiftSesiaShear zone

摘要: [1] The Sesia Zone within the Tertiary arc of western Alps is a relic subducted part Adriatic continental margin along SE border Tethyan ocean. The comprises three basement nappes which individuated during Late Cretaceous (65–80 Ma) subduction to different depths at high-pressure (HP, blueschist, eclogite facies) conditions (peak pressures 1.0–1.2, 1.0–1.5, and 1.5–2.0 GPa). thrusts bounding these developed where crust was previously thinned Jurassic rifting. Crustal-scale shear zones partly overprinted early exhumed coherent slices containing HP rocks. Initial exhumation internal accreted involved thrusting (D1) transpressional shearing (D2) subvertical, E-W trending mylonitic zone under retrograde blueschist- greenschist-facies conditions. This nearly isothermal depth about 25 km, were juxtaposed. Subsequent common 15–20 km occurred in footwall greenschist-facies, top-SE extensional (D3) preserved some highest mountain peaks Zone. New Rb-Sr mineral ages constrain D2 have 60–65 Ma D3 45–55 Ma. Thus broadly coeval with Eocene, directed Liguro-Piemont oceanic lithosphere beneath margin. Slow cooling erosional denudation from 45 30 hanging wall Gressoney (D4), itself contributed Eocene ultra-HP rocks its footwall. By Ma, intruded by shallow granitic plutons eroded redeposited volcanoclastic sediments. Oligo-Miocene Insubric backfolding (D5) only northeastern parts Zone, metamorphism absent or temperature-dominated metamorphism. Most therefore preceded Alpine collision coincided Early lithosphere. transition interpreted mark change high-stress, oblique accretion distal NW retreating, low-stress

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