Palaeomagnetic Estimates of Rotations in Compressional Regimes and Potential Discrimination Between Thin-Skinned and Deep Crustal Deformation

Abstract

Palaeomagnetism is a powerful tool for identifying rotations about any axis. Palaeomagnetic study has shown that a thrust sheet in ORS red marls from SW Dyfed, Wales has rotated during emplacement about an inclined axis plunging 55° to southwest in response to regional transpression across the Hercynian orogenic front and local climb of the thrust sheet over a lateral ramp. A palaeomagnetic study in basement thrust sheets in the southern Axial zone, Pyrenees, has demonstrated that 3-D rotations can be identified in basement terrain where no other palaeohorizontal markers exist. Three different movement histories are described within a stack of six thrust sheets. The lowermost unit has experienced doming and tilting about a horizontal axis at a different time to clockwise rotation about a vertical axis of 25°. The middle four sheets have rotated clockwise by 35 to 60° and have been tilted by 40 to 65° to the north. The uppermost sheet has experienced no rotation and tilting, indicating that the thrust (or reverse fault) had an original attitude similar to its present steep northward dip. This study illustrates the potential of the palaeomagnetic method for discriminating between models of the evolution of the Pyrenees which invoke steep structures in the central region to have had an originally steep attitude, either rooted into a zone of inhomogeneous shortening in the lower crust or decolled on a shallow fault, and those which invoke originally shallow structures which have been subsequently back-steepened.