Paleomagnetic estimates of total rotation in basement thrust sheets, Axial Zone, Southern Pyrenees.

Abstract

Palaeomagnetism is a powerful tool for identifying rotations about any axis. 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 Lo clockwise rotation about a vertical axis of 25’. ‘ihe middle four sheets have rotated clockwise by 35 Lo 60’ and have been Lilted by 40 to 65’ Lo Lhe north. ‘ihe uppermost sheet has experienced no rotation and tilting, indicating that the Lhrust (or reverse fault) had an original attitude similar to its present steep northward dip. This study illustrates Lhe potential of the palaeomagnetic method for discriminating between models of the evolution of the Pyrenees which invoke steep structures in the central region Lo 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. INTROD1LJCTION The geometry of structures in the internal parts of orogenie belts can be difficult to determine since Lhe palaeohorizontal orientation of igneous or polydeformed basament rocks is not known. ‘ihis problem is of great significance in Lhe Pyrenees where conflicting and fundamentally different modeis of the deep structure have been proposed. The surface ex182 E. A. McClelland andA. 311. McCaig pression of Tertiary thrust faults in the Pyrenees shows a fanning geometry which has been described by many authors. Segurel and Daignieres (1986), Seguret (1972), and Choukroune and Seguret (1973) favour a vertical tectonic interpreiation in which the thrusts steepen downwards into a zone of inhomogeneous shortening in the lower crust. Williams ami Fiseher (1984), and Parish (1984) prefer a thin skinned model in which ali thmsts are assumed to have originated at shallow angles and to have been subsequently steepened by backthrusting or by piling of lower imbricates into antiformal stacks. Deramond et al (1985) propose a Lhickskinned model in which thrusts root down to join a basal detachment al the Moho. McCaig (1986) has discussed ihe relative merits of these modeis. In this papen we will demonstrate that palaeomagnetic estimates can be made of total three-dimensional (3-D) rotations in basement thrust sheets, palaeomagnetic methods Lherefore may enable us in future to resolve the confliet bet’ween vertical, Lhin and thick-skinned modeis. Rete we present a preliminary palaeomagnetic study of a suite of late Carboniferous dykes which cut Silurian Lo Carboniferous rocks on the southern margin of the Pyrenean Axial zone, south of the Maladeta granodiorite. In this area post-Hercynian Triassic redbeds have been imbricated and folded into an antiformal stack during Alpine southwards directed thrusting. Where Triassic rocks are preserved overlying the basement Llie palaeomagnetie estimates of tilt about a horizontal axis agree well with the amount of Lilting of the Triassic bedding. GEOLOGICAL BACKGROUND ‘ihe geology of the southern Axial zone, south of the Maladeta granodiorite in the area of Pont de Suert, has been described by Mey, (1969). Fig. 1 shows a much simplified sketch map of the area with sampling localities. Hercynian deformation has affected sediments from Cambro-Ordovician Lo lower Carboniferous ji age. These are cut by intrusives ofthe late Carboniferous Bono complex, which is probably asssociated with the Stephanian volcanies found at Erill Casteli. Unconformably overlying the Hercynian deformed basement are upper Carboniferous, Permian and Triassic conglomerates aid red sandstones. ‘ihe southward directed Alpine thrusting has imbricated sheets of basement aid ocassional Triassic redbeds into an antiformal stack. Six separate thrust can be identified. ‘ihe lowermost sheet (sheet 1) extends southwards to the northern edge of the Nogueras zone (Figs. 6 and 7). Triassic redbeds are preserved overlying the basement at the northern aid southern ends of the exposure of sheet 1. The unconformity is steeply southward dipping aL the southern edge and is shallowly iorthward dipping at the northern edge indic,ating that the P aeom anetic estim ates o to ta lrtation in bsem eni. 83 Z W w t ‘o ~ N ce, <-1 o— o 4 D A