An assessment of the cause of the ‘extension discrepancy’ with reference to the west Galicia margin

Abstract

AbstractA common observation at rifted margins is that the amount of extension measurable from faulting is too little to explain the observed crustal thinning and subsidence. This is the ‘extension discrepancy’. Several causes have been proposed, including depth‐dependent stretching (DDS) or thinning (DDT), sequential faulting, subseismic faulting and polyphase faulting. In this contribution, we explore the different possibilities, with specific reference to the west Galicia margin. If we take the observations at face value, then it seems unavoidable that the upper crust must be stretched and thinned less than the middle and lower crust, i.e. that DDS or DDT has occurred. However it is unclear where the displaced lower crust has gone as there is no inverse discrepancy. Furthermore, independent evidence against large‐scale DDT is provided by seismic velocities and by the occurrence of upper crustal, lower crustal and mantle rocks in close proximity at the deep margin. We thus reject DDT as a sole cause of the extension discrepancy, although recognize that small‐scale local DDT associated with asymmetric faulting is expected. Such small‐scale DDT is an integral part of models of sequential faulting, but these do not predict an overall extension discrepancy, so alone cannot explain one. Subseismic faulting also alone seems inadequate as it cannot explain the magnitude of the extension discrepancy observed at the deep margin. However, as subseismic faulting is a requirement of the fractal distribution of fault sizes and the limited resolution of the seismic method, it must contribute to the extension discrepancy, something that is commonly ignored. Polyphase faulting, in which the thinnest crust has been affected by more than one phase of faulting, resulting in complex and poorly imaged structural architecture, is both predicted at deep margins, and in combination with subseismic faulting, capable of explaining the extension discrepancy. We demonstrate that the west Galicia margin has undergone more than one phase of faulting, with later faults offsetting earlier ones to create complex geometries and, in combination with the expected amount of subseismic faulting, a substantial underestimate of the amount of extension.