Abstract
AbstractEvidence of hydrothermal activity is reported for the Mesozoic pre‐ and syn‐rift successions of the western Adriatic palaeomargin of the Alpine Tethys, preserved in the Western Southalpine Domain (NW Italy). The products of hydrothermal processes are represented by vein and breccia cements, as well as dolomitization and silicification of the host rocks. In the eastern part of the study area, interpreted as part of the necking zone of the continental margin, Middle Triassic dolostones and Lower Jurassic sediments are crossed by veins and hydrofracturing breccias cemented by saddle dolomite. The precipitation of dolomite cements occurred within the stratigraphic succession close to the sediment–water interface. Despite the shallow burial depth, fluid inclusion microthermometry and clumped isotopes show that hydrothermal fluids were relatively hot (80–150°C). In the western part of the study area, interpreted as part of the hyperextended distal zone, a polyphase history of host‐rock fracturing is recorded, with at least two generations of veins cemented by calcite, dolomite and quartz. Vein opening and cementation occurred at shallow burial depth around the time of deposition of the syn‐rift clastic succession. Fluid inclusion microthermometry on both quartz and dolomite cements indicates a fluid temperature of 90–130°C, again pointing to hydrothermal fluids. Both in Fenera‐Sostegno and Montalto Dora areas, O, C and Sr isotope values, coupled with fluid inclusion and clumped isotope data, indicate that hydrothermal fluids derived from seawater interacted with crustal rocks during hydrothermal circulation. Stratigraphic and petrographic evidence, and U–Pb dating of dolomitized clasts within syn‐rift sediments, document that hydrothermal fluids circulated through sediments from the latest Triassic to the Toarcian, corresponding to the entire syn‐rift evolution of the western portion of the Adriatic palaeomargin. The documented hydrothermal processes are temporally correlated with regional‐scale thermal events that took place in the same time interval at deeper crustal levels.
Highlights
Fluid circulation in the continental crust is an active and effective process (e.g. Pirajno, 2009; Yardley & Bodnar, 2014)
This study investigates hydrothermal fluid circulation within the upper crust of the Adriatic continental margin of the Alpine Tethys during Jurassic extensional rifting
Hydrothermal fluids were relatively hot, 80–1 30°C, overpressured and circulated in a plumbing system developed along rift-related fault and fracture zones penetrating for some kilometres into basement rocks
Summary
Fluid circulation in the continental crust is an active and effective process (e.g. Pirajno, 2009; Yardley & Bodnar, 2014). The purpose of this study is to investigate, with a multidisciplinary approach, the pre-to syn-rift sedimentary successions of the Western Southalpine Domain, a portion of the Alpine chain where the metamorphic overprint is very slight or completely absent It preserves the signature of the flow of fluids related to tectonic activity present during the individualization of the Adriatic continental margin and the transition to mantle exhumation and birth of the Liguria– Piemonte ocean. Pre-to syn-rift, mainly carbonate sediments of such successions, are highly sensitive to the flow of hot fluids, and offer the opportunity to gain information on the occurrence, features, timing and spatial evolution of the related hydrothermal systems This information allowed to improve our understanding of time and space variations of tectonically controlled fluid circulation in rifted continental margins, confirming the model recently defined in a different sector of the same palaeomargin (Incerpi et al, 2017; Incerpi, Martire, et al, 2020). Festa et al (2020) documented the direct superposition of the Middle Triassic dolostones on the Lower Permian volcanites, due to the
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