Browsing by Author "Gomila, R."
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- ItemFrictional Melting in Hydrothermal Fluid-Rich Faults: Field and Experimental Evidence From the Bolfin Fault Zone (Chile)(2021) Gomila, R.; Fondriest, M.; Jensen, E.; Spagnuolo, E.; Masoch, S.; Mitchell, T. M.; Magnarini, G.; Bistacchi, A.; Mittempergher, S.; Faulkner, D.; Cembrano, J.; Di Toro, G.Tectonic pseudotachylytes are thought to be unique to certain water-deficient seismogenic environments and their presence is considered to be rare in the geological record. Here, we present field and experimental evidence that frictional melting can occur in hydrothermal fluid-rich faults hosted in the continental crust. Pseudotachylytes were found in the >40 km-long Bolfin Fault Zone of the Atacama Fault System, within two ca. 1 m-thick (ultra)cataclastic strands hosted in a damage-zone made of chlorite-epidote-rich hydrothermally altered tonalite. This alteration state indicates that hydrothermal fluids were active during the fault development. Pseudotachylytes, characterized by presenting amygdales, cut and are cut by chlorite-, epidote- and calcite-bearing veins. In turn, crosscutting relationship with the hydrothermal veins indicates pseudotachylytes were formed during this period of fluid activity. Rotary shear experiments conducted on bare surfaces of hydrothermally altered rocks at seismic slip velocities (3 m s(-1)) resulted in the production of vesiculated pseudotachylytes both at dry and water-pressurized conditions, with melt lubrication as the primary mechanism for fault dynamic weakening. The presented evidence challenges the common hypothesis that pseudotachylytes are limited to fluid-deficient environments, and gives insights into the ancient seismic activity of the system. Both field observations and experimental evidence, indicate that pseudotachylytes may easily be produced in hydrothermal environments, and could be a common co-seismic fault product. Consequently, melt lubrication could be considered one of the most efficient seismic dynamic weakening mechanisms in crystalline basement rocks of the continental crust.
- ItemMagnitude, timing, and rate of slip along the Atacama fault system, northern Chile: implications for Early Cretaceous slip partitioning and plate convergence(GEOLOGICAL SOC PUBL HOUSE, 2021) Seymour, N. M.; Singleton, J. S.; Gomila, R.; Mavor, S. P.; Heuser, G.; Arancibia, G.; Williams, S.; Stockli, D. F.Displacement estimates along the Atacama fault system (AFS), a crustal-scale sinistral structure that accommodated oblique convergence in the Mesozoic Coastal Cordillera arc, vary widely due to a lack of piercing points. We map the distribution of plutons and mylonitic deformation along the northern c. 70 km of the El Salado segment and use U-Pb geochronology to establish the slip history of the AFS. Along the eastern branch, mylonitic fabrics associated with the synkinematic c. 134-132 Ma Cerro del Pingo Complex are separated by 34-38 km, and mylonites associated with a synkinematic c. 120-119 Ma tonalite are separated by 20.5-26 km. We interpret leucocratic intrusions to be separated across the western branch by c. 16-20 km, giving a total slip magnitude of c. 54 +/- 6 km across the El Salado segment. Kinematic indicators consistently record sinistral shear, and zircon (U-Th)/He data suggest dip-slip motion was insignificant. Displacement occurred between c. 133-110 Ma at a slip rate of c. 2.1-2.6 km Myr(-1). This slip rate is low compared to modern intra-arc strike-slip faults, suggesting (1) the majority of lateral slip was accommodated along the slab interface or distributed through the forearc or (2) plate convergence rates/obliquity were significantly lower than previously modeled. Supplementary material: Full U-Pb, (U-Th)/He, petrographic, and structural data with locations is available at Thematic collection:This article is part of the Isotopic dating of deformation collection available at:
- ItemSodic-calcic alteration and transpressional shear along the Atacama fault system during IOCG mineralization, Copiapo, Chile(2024) Seymour, N. M.; Singleton, J. S.; Gomila, R.; Arancibia, G.; Ridley, J.; Gevedon, M. L.; Stockli, D. F.; Seman, S. M.The Punta del Cobre district near Copiapo is a center of iron oxide-copper-gold (IOCG) mineralization spatially and temporally associated with regional sodic-calcic hydrothermal alteration, the Atacama fault system (AFS), and two phases of Early Cretaceous magmatism. Here, we investigate the spatiotemporal and geochemical relationships between magmatism, ductile deformation, and hydrothermal alteration along the similar to 200 to 300-m-thick steeply NW-dipping Sierra Chicharra shear zone, interpreted to be the major strand of the AFS. Mylonitic fabrics and oblique sinistral-reverse kinematic indicators together record coaxial flattening in a transpressional regime. Deformation on the AFS took place before, during, and after intrusion of the synkinematic Sierra Chicharra quartz diorite of the Coastal Cordillera arc at similar to 122 Ma and terminated before intrusion of the unstrained similar to 114 Ma Sierra Atacama diorite of the Copiapo batholith. Geochemical data show that the Copiapo batholith was more mafic and more K-rich than the calc-alkaline Coastal Cordillera arc. This time period thus overlaps IOCG mineralization in the Punta del Cobre district (similar to 120 to 110 Ma). Multiple phases of sodic-calcic alteration in and around the AFS shear zone are recognized. Textures of altered rock in the shear zone show both synkinematic assemblages and post-kinematic hydrothermal oligoclase. A similar to 775-m-long andradite vein that cuts the shear zone formed broadly at the end of magmatism in the district (similar to 95 Ma). Oxygen isotope ratios from the vein indicate that hydrothermal fluids were likely magmatically derived. Together, this work shows the AFS-related shear zone and nearby IOCG mineralization developed in a regional transpressional regime produced by SE-directed oblique convergence across a NE-striking shear zone. IOCG-related magmatic-hydrothermal fluids exploited this transcrustal shear zone to produce multiple episodes of regional sodic-calcic alteration formed from fluids exsolved from magmas or driven by the heat of the Coastal Cordillera arc and Copiapo batholith.