Extension discrepancy at North Atlantic nonvolcanic rifted margins; depth-dependent stretching or unrecognized faulting?

Rifted margins show an apparent discrepancy between the amount of extension measurable from faults and the amount of crustal or lithospheric thinning determined from wide-angle data or subsidence. This extension discrepancy has been interpreted in terms of depth-dependent stretching in which the upper crust is extended and thinned far less than the rest of the lithosphere. Here I show that at those nonvolcanic margins where the velocity structure is well known, the upper and lower crust appear to thin equally toward the margin, implying that the discrepancy is not due to depth-dependent stretching, but more likely due to unrecognized polyphase and top basement faulting.