Asynchronous extension of the late-Hercynian crust in Calabria

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• 作者：V. Festa^a ; ^{v.festa@geomin.uniba.it} ; A. Fornelli^a ; A. Paglionico^a ; A. Pascazio^a ; G. Piccarreta^a ; R. Spiess^b
• 关键词：Calabrian crust ; Decompression ; Extension ; Microstructures ; Pseudosection ; Geothermobarometry
• 刊名：Tectonophysics
• 出版年：2012
• 期刊代码：82_00401951
• 类别：ear
• 出版时间：20 January, 2012
• 卷：518-521
• 期：Complete
• 页码：29-43
• 文件大小：3840 K

摘要

Quartz-monzodioritic dykes have intruded the Hercynian continental lower crust of Calabria at 323 卤 5 Ma. Pseudosection and average PT calculations have been performed using THERMOCALC. The obtained P (average PT) and T (pseudosection and average PT) values indicate that the quartz-monzodioritic dykes have intruded the 750 掳C hot metapelitic-migmatitic country-rocks at a depth of about 32 km, with a temperature of ca. 850 掳C. Microstructural analysis suggests that deformation of the dykes started at hypersolidus conditions during emplacement and continued after cooling at sub-solidus granulite facies conditions together with the country rocks. During this deformation event, which started not later than 323 卤 5 Ma ago, the main S_n + 1 schistosity evolved. Vorticity analysis of the quartz-metadioritic dykes and of the metapelitic country rocks shows that the S_n + 1 schistosity evolved during a general shear deformation characterized by 60 to 70%of pure shear. This high percentage of pure shear suggests that the main S_n + 1 schistosity evolved in the hot lower crust undergoing extension during vertical shortening. A horizontal attitude of this extensional schistosity is confirmed by retro-deformation of the today obliquely SE-ward dipping S_n + 1 schistosity. Extension continued with time and propagated upwards within the crust, affecting the middle crust in a time interval of 306 卤 1 to ca. 300 Ma when huge masses of granitoid rocks intruded it. The deepest batches of these granitoid bodies show a gneissic schistosity which developed under ductile conditions, while granitoids at higher levels became deformed in a brittle fashion. Our whole data set is consistent with a late Hercynian evolution in Calabria characterized by a progressively crust overlying a mantle that gradually rose during lithospheric extension. This extension was asynchronous, starting at 323 卤 5 Ma in the lower crust and affecting higher crustal levels at progressively later times.