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Fast exhumation rate during late orogenic extension: the new timing of the Pilat detachment fault (French Massif Central, Variscan belt)
Gondwana Research ( IF 6.1 ) Pub Date : 2021-10-18 , DOI: 10.1016/j.gr.2021.10.007
Véronique Gardien 1 , Jean-Emmanuel Martelat 1 , Philippe-Herve Leloup 1 , Gweltaz Mahéo 1 , Benoit Bevillard 2 , Pascal Allemand 1 , Patrick Monié 3 , Jean-Louis Paquette 4 , Anne-Sabine Grosjean 5 , Michel Faure 2 , Cyril Chelle-Michou 6 , Clémentine Fellah 1
Affiliation  

A leucogranite sill in the footwall of the extensional Pilat shear zone (PSZ) Eastern French Massif Central was emplaced and sheared in a short time interval, between 301.8±3.1 and 303.0±1.6 Ma (zircon U–Pb and mica 40Ar/39Ar ages, respectively). Extension ended at 298.5±1.5 Ma (40Ar/39Ar on mica) as shown by a non deformed dike cross cutting the sheared zone. On the hanging wall of the fault, the opening of the St. Etienne Basin filled by coarse clastic sedimentation confirms the existence of a relief south of the PSZ. U-Pb dating of a lithic-rich ignimbrite at the bottom of the basin and of a volcanic layer at the top delivered ages of 310.5±3.4 Ma and 299.3±1.3 Ma, respectively. Close U-Pb and 40Ar/39Ar ages from syn- to post-tectonic granites indicate very fast cooling (∼150 °C/Myr) and exhumation rate (6 mm/yr) corresponding to the emplacement of granites in the middle crust immediately followed by their fast tectonic exhumation and cooling into the upper crust. The development of the Pilat fault in the thermally softened crust is responsible for the rapid exhumation of the Velay dome, a Metamorphic Core Complex localized in the footwall of the sheared zone. Here we propose that its rapid exhumation was due to localized sub crustal lithospheric mantle dripping. These new data show that there was only one extensional phase in the PSZ coeval with the opening of the St. Etienne coal Basin. Finally, the new chronology obtained in this study is challenging previous ages suggesting 20 Ma activity for the Pilat shear zone.



中文翻译:

造山运动晚期伸展过程中的快速折返率:皮拉特拆离断层的新时间(法国地块中部,瓦利斯坎带)

法国东部地块中部伸展皮拉特剪切带 (PSZ) 下​​盘的白质花岗岩基石在 301.8±3.1 和 303.0±1.6 Ma(锆石 U-Pb 和云母40 Ar/ 39 Ar年龄,分别)。延伸结束于 298.5±1.5 Ma(云母上40 Ar/ 39 Ar),如横切剪切区的非变形堤防所示。在断层的上盘上,充满粗碎屑沉积物的 St. Etienne 盆地的开口证实了 PSZ 以南的地势的存在。盆地底部富含岩屑的熔凝岩和顶部火山层的 U-Pb 测年分别为 310.5±3.4 Ma 和 299.3±1.3 Ma。接近 U-Pb 和40 Ar/39从同构造到后构造花岗岩的 Ar 年龄表明非常快的冷却 (~150 °C/Myr) 和剥脱速率 (6 mm/yr) 对应于花岗岩在中地壳的侵位,紧接着它们的快速构造剥脱和冷却进入上地壳。热软化地壳中皮拉特断层的发育导致 Velay 圆顶的快速折返,这是一个位于剪切带下盘的变质岩心复合体。在这里,我们提出它的快速折返是由于局部地壳下岩石圈地幔滴落。这些新数据表明,随着 St. Etienne 煤盆地的开放,PSZ 同期只有一个伸展阶段。最后,本研究中获得的新年表挑战了之前的年龄,表明皮拉特剪切带有 20 Ma 活动。

更新日期:2021-10-19
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