Tracing mantle‐reacted fluids in magma‐poor rifted margins: The example of Alpine Tethyan rifted margins

Abstract

The thinning of the crust and the exhumation of subcontinental mantle in magma‐poor rifted margins is accompanied by a series of extensional detachment faults. We show that exhumation along these detachments is intimately related to migration of fluids leading to changes in mineralogy and chemistry of the mantle, crustal, and sedimentary rocks. Using field observation and analytical methods, we investigate the role of fluids in the fossil distal margins of the Alpine Tethys. Using Cr‐Ni‐V, Fe, and Mn as tracers, we show that fluids used detachment faults as pathways and interacted with the overlying crust and sediments. These observations allow us to discuss when, where, and how this interaction happened during the formation of the rifted margin. The results show that: (i) serpentinization of mantle rocks during their exhumation results in the depletion of elements and migration of mantle‐reacted fluids that are channeled along active detachment system; (ii) in earlier‐stages, these fluids affected the overlying syntectonic sediments by direct migration from the underlying detachments;(iii) in later‐stages, these fluids arrived at the seafloor, were introduced into, or “polluted” the seawater and were absorbed by post tectonic sediments. We conclude that a significant amount of serpentinization occurred underneath the hyperextended continental crust, and that the mantle‐reacted fluids might have modified the chemical composition of the sediments and seawater. We propose that the chemical signature of serpentinization related to mantle exhumation is recorded in the sediments and may serve as a proxy to date serpentinization and mantle exhumation at present‐day magma‐poor rifted margins.

Thermal Evolution of Asymmetric Hyperextended Magma‐Poor Rift Systems: Results From Numerical Modeling and Pyrenean Field Observations

“…However, Jammes et al () and Masini et al () suggested that the Labourd massif might represent the western lateral prolongation of the basement located below the northern Mauléon basin. In this area, the authors described evidence for fluid and reaction assisted brittle fault rocks and associated tectonosedimentary breccias, diagnostic of extensional detachment faults in hyperextended settings (Jammes et al, ; Pinto et al, ; Tugend et al, ). The description of depositional contacts between Albo‐Cenomanian sediments and exhumed midcrustal granulites (Jammes et al, ) and the reworking of these granulites in the overlying sediments argue for laterally discontinuous and allochthonous pre‐rift cover sequences separated by exhumation windows (Figure ).…”

Controls on the Thermomechanical Evolution of Hyperextended Lithosphere at Magma‐Poor Rifted Margins: The Example of Espirito Santo and the Kwanza Basins

“…We use a friction coefficient of 0.01 to simulate a weak serpentinized mantle under high pore fluid pressure (Hilairet et al, 2007). Observations of exhumed serpentinized mantle in the Alps (Pinto et al, 2015) show that mantle serpentinization is of the order of 60%, and we decrease the density accordingly. For the continental crust, feldspar and wet quartz rheology are controlled by dislocation creep laws (Rybacki et al, 2008(Rybacki et al, , 2010 with a minimum viscosity of 10 19 Pa.s.…”

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 multiphase and diachronous rift evolution resulted in a complex final crustal architecture that includes: a proximal 30‐km‐thick crust separated by a necking zone from a thinned continental crust (<10 km), followed by a wide transition zone between the continental and oceanic crusts where mantle exhumation occurred (Figure c). Importantly, no strong metamorphic overprint took place during the Alpine orogenesis allowing the primary sedimentary and geochemical features of the rock volumes to be preserved (Froitzheim & Manatschal, ; Manatschal, Marquer, & Frueh‐Green, ; Masini, Manatschal, Mohn, & Unternehr, ; Pinto et al, ).…”

“…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, ).…”