The role of serpentinization and magmatism in the formation of decoupling interfaces at magma-poor rifted margins

Abstract

In spite of recent progress in the understanding of magma-poor rifted margins, the processes leading to the formation and evolution of the exhumed mantle domain and its transition toward steady state oceanic crust remain debated. In particular, the parameters controlling the progressive localization of extensional deformation and magmatic processes leading to the formation of an oceanic spreading center are poorly understood. In this paper, we highlight the occurrence of two major decoupling horizons controlling the structural and magmatic evolution of distal magma-poor rifted margins. They are marked by strong seismic reflectors located at about 1 sec TWTT (Upper Reflectors-UR) and 2 sec TWTT (Lower Reflectors-LR) below top basement in domains of exhumed mantle at several magma-poor rifted margins. Both reflection seismic observations and studies on the physical properties of serpentinized mantle suggest that the UR likely results from deformation localized along a rheological interface at about 3 km below top basement, associated with an abrupt change in the mechanical behavior of peridotites once they are serpentinized to ~15%. We suggest that this interface played a major role in successive fault reorganizations during formation of the exhumed mantle domain. The comparison of reflection seismic observations with Alpine field analogues suggests that the LR likely results from an interaction between magmatism, deformation and hydration reactions during final rifting. Based on our results, we suggest that during final rifting the strain distribution is controlled first by hydration and then by magmatic processes in the domain of exhumed mantle.

From rifting to hyperextension: Upper Jurassic–Lower Cretaceous tectono‐stratigraphy of the Porcupine Basin, Irish Atlantic Margin

“…However, in segment 3, only 500 m of K0 sediments were deposited above the location of the Porcupine Arch (Figure 12a) where continental crust was thinned to 2–3 km (O’Reilly et al., 2006), which may reflect axial uplift related to localized serpentinization of the upper mantle (O’Reilly et al., 1996, 2006; Reston, 2009; Reston et al., 2004). Alternatively, uplift of the hyperextended crust could have been driven by a combination of fault block tilting (Gillard et al., 2019) and the flexural isostatic rebound of the Moho and upper mantle as predicted from thermomechanical numerical modelling (Chenin et al., 2019).…”

Hydration of a Mantle Shear Zone Beyond Serpentine Stability: A Possible Link to Microseismicity Along Ultraslow Spreading Ridges?

“…Our results indicate that Mg-hornblende formation in peridotite mylonite shear zones might represent "semibrittle" behavior, with downflow fluid, reaction weakening, and fluid-assisted strain localization in anastomosing shear zones Figure 12. Conceptual illustration of the Mg-hornblende crystallization conditions at a magma-starved or magma-fed ultraslow spreading ridge setting (adapted from Cannat et al, 2019;Gillard et al, 2019). Seawater-derived fluids access the mantle lithosphere along brittle faults to temperatures above the serpentine stability, reaching a mylonitic "semibrittle" domain where synchronous growth and deformation of Mg-hornblende occurs (red-shaded area).…”

The nature of the interface between basalts and serpentinized mantle in oceanic domains: Insights from a geological section in the Alps

“…Deeper roughly flat-lying rheological interfaces have been imaged through geophysical methods at the mantle-crust boundary in distal margins (Perez-Guissinye and Reston, 2001;Reston, 2009) or directly in ultramafic rocks in ultra-distal rifted margins or in oceanic settings. There, they would correspond to the peridotite-serpentinized mantle interface (Escartin et al, 2003) or the 15% serpentinization front (Gillard et al, 2019). Hence this shallow decoupling level should be viewed as an important reflector on seismic profiles and likely would help to recognize the interface between serpentinized mantle rocks and the overlying basalts.…”

“…It is difficult to determine where these normal faults root at depth. They could either branch onto a decoupling level corresponding to the 15% serpentinization rheological interface (Gillard et al, 2019) or onto a level of mafic underplating at the brittleductile transition (Manatschal et al, 2011). This proposed model is fully dependent on the coupling degree between basalts and serpentinites, i.e.…”