Rheological evolution during extension at nonvolcanic rifted margins: Onset of serpentinization and development of detachments leading to continental breakup

Abstract

Abstract. Within the continent-ocean transition several nonvolcanic rifted margins exhibit a zone of partially serpentinized peridotites which continue under the thinned and faulted continental crust and are thought to represent subcontinental lithosphere serpentinized by contact with water. As water in sufficient volumes can only come from the surface, we suggest that a major condition for the onset of serpentinization is the embrittlement of the entire crust during progressive extension and hence the development of active crustal penetrating faults acting as fluid conduits. We investigate this possibility by modeling the rheological evolution of the lithosphere during extension and hence determining at what stretching factors the lower crust enters the brittle regime for a variety of different strain rates and lower crustal rheologies. Using an initial thermal structure appropriate for nonvolcanic margins, we find that the entire crust becomes brittle at stretching factors of between -3 and 5, depending on the strain rate. This compares well with the thickness of the crust observed just landward of the onset of partially serpentinized peridotites west of Iberia. The predicted thickness of the serpentinized peridotites (depth of the thermal limit of serpentinite stability beneath the crust-mantle boundary) also compares reasonably well with their observed thickness. As serpentinites are characterized by low friction coefficients, we suggest that the onset of mantle serpentinization controls the development of decollements at the crust-mantle boundary such as the S and H reflectors west of Iberia (leading to crustal separation). The development of thick serpentinites probably contributes to the weakening of the upper lithosphere and hence the localization of final breakup.

Resolving the fine-scale velocity structure of continental hyperextension at the Deep Galicia Margin using full-waveform inversion

“…As the margin extended and thinned at an ultra-slow rate (< 10 mm/yr half spreading rate), it allowed time for the entire crust to cool conductively, resulting in the normally ductile midand lower-crust becoming progressively embrittled (Srivastava et al 2000;Pérez-Gussinyé and Reston 2001;Pérez-Gussinyé et al 2003). Once the crustal thicknesses reached < 10 km, the entire crust became brittle and coupled, a phenomenon known as continental hyperextension.…”

“…A fully embrittled crust enabled normal faults to form through the entire crust, from the seafloor to the underlying mantle (Pérez-Gussinyé and Reston 2001;Pérez-Gussinyé et al 2003;Pérez-Gussinyé 2013). These faults acted as conduits, delivering seawater to the upper mantle and forming a layer of serpentinised mantle, which is an inherently weak material (Pérez-Gussinyé and Reston 2001;Reston et al 2007;. With continued extension these faults soled out into the structurally weak layer of mantle serpentinite, forming a large and low angle (< 20⁰) detachment fault, known as the S-reflector (Fig.…”

“…Once the crustal thicknesses reached < 10 km, the entire crust became brittle and coupled, a phenomenon known as continental hyperextension. A fully embrittled crust enabled normal faults to form through the entire crust, from the seafloor to the underlying mantle (Pérez-Gussinyé and Reston 2001;Pérez-Gussinyé et al 2003;Pérez-Gussinyé 2013). These faults acted as conduits, delivering seawater to the upper mantle and forming a layer of serpentinised mantle, which is an inherently weak material (Pérez-Gussinyé and Reston 2001;Reston et al 2007;.…”

Controls on the location of compressional deformation on the NW European margin

“…The conditions that favour embrittlement are slow extension and low sedimentation rates (a sedimentary layer can have a 'blanketing' effect, increasing temperatures in the underlying crust). Pérez-Gussinyé & Reston (2001) found that stretching factors of between about 3 and 5 were required for embrittlement to occur, depending on the strain rate. The simulations of Rüpke et al (2013) produced similar results when there was no sedimentation, but demonstrated that higher factors were required when the sediment supply was increased.…”

2-D numerical study of hydrated wedge dynamics from subduction to post-collisional phases

“…We neglect the effects of water migration because of the expected very high water velocities compared with the chosen time steps and total computational time (Quinquis & Buiter 2014); thus, hydration can be considered instantaneous in our model. The latent heat of serpentinization is also neglected because it is balanced by the latent heat produced by de-serpentinization and melting at large scales (Pérez-Gussinyé & Reston 2001;Rupke et al, 2004Rupke et al, , 2013. Furthermore, it has been suggested that a latent heat of serpentinization of 2.9 × 10 5 J kg −1 (Emmanuel & Berkowitz 2006) results in an increase in temperature of 300 • C (Rupke et al 2013); therefore, the serpentine would cease to be stable.…”