Origin of hydrous fluids at seismogenic depth: Constraints from natural and experimental fault rocks

摘要: Abstract Fluids control the mechanical behavior of fault zones during seismic cycle. We used geochemical, mineralogical, microstructural, hydrogen isotope compositions and Fourier Transform Infrared (FTIR) investigations to characterize origin hydrous fluids involved in ductile brittle shear at bottom seismogenic crust. Natural samples were collected from exhumed mylonitic cataclasite–pseudotachylyte bearing faults northern Adamello (Italian Southern Alps), which active 9–11 km depth. Pseudotachylytes, solidified coseismic friction-induced melts, testify ancient faults. pseudotachylytes compared with artificial produced high velocity friction experiments simulating slip. Mylonites have elemental ( − 80 ‰ δ D 78 ) similar host tonalite 77 73 ), within analytical error ± 5 . Cataclasites instead mineralogical (chlorite, epidote, K-feldspar, no biotite), major trace elements (enrichment K2O, Ba, Rb; depletion CaO, Na2O, SiO2) 69 60 suggesting interactions a crustal metamorphic fluid. Pseudotachylytes are composed temperature minerals (plagioclase, biotite, dmisteinbergite, cordierite, scapolite) resulting mixing cataclasite. complex microstructures, including: (i) microlitic domains, well crystallized micrometric composition 81 59 cataclasites tonalite; (ii) cryptocrystalline poorly very water content, release upon heating T > 50 ° C low δD value 93 ). The bulk is dominated by domains 103 88 where most hosted. Their compatible adsorption present day rainfall = 95 Artificial same starting 76 74 or cataclasite 68 62 slight decrease values some 85 first ingression fluid occurred cataclastic pseudotachylytes, when not contaminated water, cataclasite, as reproduced dry experiments. dissolved melts likely derived breakdown undergone melting, we could identify infiltration an external earthquakes.