Evidence of hydrothermal fluid flow in a hyperextended rifted margin: the case study of the Err nappe (SE Switzerland)

Abstract

This paper investigates hydrothermal fluid circulation in pre-and syn-tectonic sediments associated with detachments faults. The study area, located in the Err Nappe (SE-Switzerland), preserves a portion of the Adriatic distal margin. Two sites were studied in combining fieldwork, petrography, geochemistry and fluid inclusion analysis: the Piz Val Lunga and Fuorcla Cotschna areas. Both preserve relationships between a spectacularly exposed rift-related extensional detachment fault and its footwall and hangingwall that consist of extensional allochthons and syn-to posttectonic sediments. These areas register a complex fluid flow history characterized by dolomitization, de-dolomitization, calcite cementation, dolomite and quartz veining and diffuse silicification. Meso-and micro-scale observations allow defining two steps in fluid evolution, which are related to Jurassic rift activity. A first carbonate-rich event occurred before the exhumation of the granitic basement, and this was followed by a second event marked by a change in the fluid towards a silica-dominated chemistry. Homogenization temperatures of fluid inclusions (average T h = 120-130°C), negative d 18 O values and a radiogenic 87 Sr/ 86 Sr signatures of carbonate minerals support the hypothesis that both the pre-tectonic rocks constituting the allochthons and the syn-tectonic sediments overlying the detachment fault were crossed by a flux of over-pressured hydrothermal fluids originating from seawater that penetrated into the basement through fault and fracture systems. Field relationships show that this fluid circulation started latest in middle Early Jurassic time, when fault activity migrated from the proximal to the future distal margin. We propose that it evolved chemically as a result of the involvement of the granitic basement forming the footwall of the extensional detachment system. Hydrothermal activity continued until the Middle/Late Jurassic, when tectonic activity shifted outwards leading to the exhumation of mantle rocks. This paper provides an original contribution to better understand the complex evolution of hyperextended continental rift domains and to constrain their thermal regimes.

Tectono‐thermal Evolution of a Distal Rifted Margin: Constraints From the Calizzano Massif (Prepiedmont‐Briançonnais Domain, Ligurian Alps)

“…Subsequent tectonic quiescence of the fault system during the Middle Jurassic, due to lithospheric onset of exhumation, led to the progressive cooling of the distal margin that is recorded by the ZFT data. A similar timing for active hydrothermal fluid systems associated with Jurassic rifting in the distal Alpine Tethys margin has been recently proposed by Incerpi et al ().…”

“…Thus, both the present study and the literature data suggest that relatively high temperatures (between ~200 and 400°C) might have been acquired during crustal/lithospheric thinning driven by fluid activity in the upper crust at different locations along the Alpine Tethys distal margins (e.g., Beltrando et al, ; Incerpi et al, ). Our study demonstrates that such conditions were able to generate metamorphic paragenesis (greenschist facies) at very shallow crustal levels of <2–3 km.…”

“…The process of rift localization leads to the quiescence of tectonically active areas placed over thick continental crust in the proximal parts of the margins and to the activation of narrow sectors in which the crust becomes severely thinned (i.e., future distal margins). During final rifting, the deformation between lower and upper crust becomes “coupled” (see Sutra et al, ), generating new fluid circulation patterns (Incerpi et al, ). The circulation of hydrothermal fluids is typically associated with anomalous thermal gradients that lead to characteristic synrift heating/cooling cycles affecting the whole crustal section in the distal margin (Beltrando et al, ; Seymour et al, ).…”

Hydrothermal fluid flow associated to the extensional evolution of the Adriatic rifted margin: Insights from the pre‐ to post‐rift sedimentary sequence (SE Switzerland, N ITALY)

“…The discovery of hydrothermal vents at present‐day mid ocean ridges, as well as the occurrence of widespread serpentinization processes, shed light on the relevance and spatial abundance of fluid activity in response to the late stage of continental rifting and early seafloor spreading (Edmonds, ; Harding et al, ; Kelley & Shank, ; Kuhn et al, ; McCaig, Cliff, Escartin, Fallick, & Macleod, ). Only recently, attention has been paid to the study of such activity in the more distal parts of the continental margins (Incerpi, Martire, Manatschal, & Bernasconi, ; Pinto, Manatschal, Karpoff, & Viana, ) i.e., where the pre‐mantle exhumation history is preserved. At present, only few ODP or IODP drill holes are available from such domains (e.g., Tucholke, Sibuet, & Klaus, ).…”

“…The remnants of the Alpine Tethys margins exposed in the Alps, represent the best natural laboratory where such studies can be addressed. These are preserved in both metamorphosed (e.g., Beltrando, Rubatto, & Manatschal, ; Beltrando, Frasca, Compagnoni, & Vitale‐Brovarone, ) and unmetamorphosed successions (e.g., Mohn, Manatschal, Müntener, Beltrando, & Masini, , Masini, Manatschal, Mohn, Ghienne, & Lafont, , Haupert et al, , Incerpi et al, , Decarlis et al, ) and represent segments of the European and Adriatic continental margins developed during the Jurassic rifting (e.g., Lemoine et al, ; Froitzheim, Schmid, & Conti, ; Schmid, Fuegenschuh, Kissling, & Schuster, ). This paper aims to show evidence for syn‐rift hydrothermal fluid circulation and related mineralizations along the Adriatic paleo‐rifted margin.…”