The Role of Serpentinization In Guiding Mantle Upwelling Beneath The Iberia Abyssal Plain
Abstract
Recent geophysical/geological studies of the non-volcanic passive margin beneath the Iberia Abyssal Plain support the existence of a 40-170 km wide zone of exhumed con- tinental mantle (ZECM) at the transition between continental and oceanic crust, where mantle rocks are highly serpentinized and intruded by only small volumes of syn-rift melt (Whitmarsh et al., 2001). This is based on the seismic velocity structure of the ZECM, the occurrence of weak, somewhat linear, but discontinuous magnetic anoma- lies, and geochemical/petrological studies of samples recovered by scientific drilling. The widespread occurrence of tectonic breccia capping exhumed serpentinized peri- dotites throughout the ZECM is interpreted as representing the footwall of a series of sub-horizontal detachment faults, which have successively opened the ZECM, shortly after mantle exhumation was initiated by concave-downward faults (Manatschal et al., 2001). The most recent model, describing the evolution of the Iberian margin, delin- eates a transition from symmetric rifting by pure shear, where extensional deformation was distributed over the entire future margin, to asymmetric rifting by simple shear, where extensional deformation was localised to a series of sub-horizontal detachment faults and mantle exhumation was accomplished by concave-downward faults (Whit- marsh et al., 2001). This transition occurred when the crustal thickness was ~ 7 km. Upper mantle necking, guided by "weakening" in the upper mantle, created a broad upwelling zone which eventually narrowed, increasing the melt volume and triggering the onset of seafloor spreading. This study focuses on the nature of the "weakening" that guided the location of upper mantle necking, testing the hypothesis that it was induced by serpentinization. The scale and relative timing of serpentinization and the mechanical behaviour of serpentinized peridotite is reviewed. We conclude that ser- pentinization preceded mantle exhuamtion and, using centrifuge models, we argue that serpentinization played a key role in localizing mantle necking and therefore exten- sional deformation, because of its low viscosity and density relative to the surrounding mantle and the overlying continental crust. We also note that because deforming ser- pentine has both low and anisotropic permeability, upwards melt migration may have inhibited, which is consistent with the observed lack of melt within the ZECM.
- Publication:
-
EGS General Assembly Conference Abstracts
- Pub Date:
- 2002
- Bibcode:
- 2002EGSGA..27.5503S