Deformation-fluid multiphase interaction within the fractures of the Barcelona Plain and the Vallès Basin: influence on fault rocks and diagenesis / Interacció multifàsica entre deformació i fluids a les fractures del Pla de Barcelona i de la Conca del Vallès: influència en les roques de falla i en la diagènesi

Abstract

The faults limiting the Barcelona Plain and the Vallès Half-graben (Catalan Coastal Ranges, NE Spain) have allowed the study of a complex and multiphase tectonic/fluid history from the Hercynian to recent times. Furthermore, this study has allowed to define the factors that control the fluid regime and the fluid flow pathways through faults.

A multidisciplinary methodology involving field and petrological observations and microstructural analyses combined with geochemical data has been used to characterize host rocks, fault rocks and fracture-related cements.

Four tectonic events that encompass seven deformation phases have been established: Hercynian compression, Mesozoic extension (D1-D2), Paleogene compression (D3) and Neogene extension (D4-D7).

Faults affect crystalline Hercynian basement, Triassic dolostones and Miocene detritic rocks. The fault rocks formed in the area include gouges, cataclasites, breccias and pseudotachylytes. Veins are formed by different mineral assemblages that involve calcite, quartz, laumontite, K-white mica, chlorite and iron oxides, depending on the PT conditions and thus, on the age.

During the exhumation of the late-Hercynian granodiorite, after the Hercynian compression, M1 and M2 muscovite and microcline crystallized as result of deuteric alteration in joints, at temperatures between 330ºC and 370ºC.

During the first Mesozoic rifting (Late Permian-Middle Jurassic), faults controlled the thickness and distribution of the Triassic sediments. Fracture-related dolomite cements precipitated from the Triassic seawater during increasing burial in a relatively closed hydrological regime. The second Mesozoic rifting (Late Jurassic-Late Cretaceous) is characterized by precipitation of M3 and M4 phengite together with chlorite and calcite C1 at temperatures between 190 and 310ºC.

During the Paleogene compression, low-temperature meteoric fluids, favored by tectonic uplift, produced calcitization of the Triassic dolomite cements and dolostones. In the Vallès fault, by means of a shortcut, Mesozoic structures were uplifted and a gouge and subvertical stylolites were generated.

During the Neogene syn-rift, hydrothermal fluids up to 190ºC ascended through the faults. In the Hospital fault these fluids upflowed within the relay area during fault growth by tip propagation due to a seismic pumping effect.

The Neogene post-rift in the Barcelona Plain developed at shallow conditions under low-temperature meteoric regime. At the same period, the Vallès fault was dominated by hydrothermal conditions, which remain active until nowadays.

Faults acted as conduits for hot fluids during both Mesozoic and Neogene extensional events. Topographically-driven meteoric fluids warmed at depth and suffered a strong interaction with the host rocks. During the Mesozoic, ascending warm fluids mixed with marine waters in both main faults whereas, during the Neogene, the ascending fluids mixed with marine waters in the Hospital fault and with meteoric waters in the Vallès fault. Pedogenic products precipitated along Neogene faults control cross-fault fluid flow by reducing fault core permeability.

This work highlights the presence of fault reactivation from Hercynian to Recent times. Hercynian structures have demonstrated to play an important role on the localization of later structures. The main factors that control this process are: fault orientation, fabric softening, fluid pressure and cementation hardening.

Las fallas que limitan el Llano de Barcelona y la Cuenca del Vallès permiten estudiar la relación entre tectónica y fluidos desde el Hercínico hasta la actualidad.

Observaciones de micro- a meso-escala combinadas con análisis geoquímicos han servido para caracterizar las rocas encajantes, las rocas de falla y los cementos.

Las fallas afectan a rocas cristalinas Hercínicas, dolomías Triásicas y rocas detríticas Miocenas y generan “gouges”, cataclasitas, brechas y pseudotaquilitas. Los principales cementos son de calcita, cuarzo, laumontita, mica blanca, clorita y óxidos de hierro, dependiendo de las condiciones P-T.

Se han establecido cuatro eventos tectónicos que recogen siete fases deformativas:

1) Durante la exhumación de la granodiorita tardihercínica cristalizaron moscovitas M1-M2 en diaclasas debido a un proceso de alteración deutérica a temperaturas entre 330ºC y 370ºC.

2) Durante el primer rift mesozoico, las fallas controlaron el grosor y la distribución de los sedimentos triásicos. Cementos dolomíticos precipitaron en fracturas a partir de agua marina triásica durante un incremento del enterramiento en un sistema hidrológico cerrado. El segundo rift se caracterizó por la precipitación de mica M3- M4, clorita y calcita a temperaturas entre 190-310ºC.

3) Durante la compresión Paleógena, fluidos meteóricos de baja temperatura calcitizaron los cementos dolomíticos triásicos y las dolomías encajantes. En la falla del Vallès, un “shortcut” levantó las estructuras Mesozoicas y generó una “gouge” y estilolitos subverticales.

4) Durante el sinrift Neógeno, fluidos de hasta 190ºC ascendieron por las fallas. En la falla del Hospital, estos fluidos ascendieron en la zona de relevo durante la propagación de la falla gracias a un efecto de bombeo sísmico.

El postrift Neógeno se desarrolló en condiciones someras bajo un régimen meteórico de baja temperatura en el Llano de Barcelona. En el mismo periodo, la falla del Vallès estuvo dominada por fluidos hidrotermales.

Las principales estructuras que configuran las Cadenas Costero Catalanas han sido reactivadas desde el Hercínico hasta la actualidad. Las estructuras hercínicas jugaron un papel importante en la localización de estructuras posteriores. Los principales factores que controlaron las sucesivas reactivaciones fueron: la orientación de las fracturas, la fábrica mineral, la presión de fluidos y las cementaciones previas.