FAULT GOUGE AGES FROM THE SOUTH-CENTRAL PYRENEES FOLD-AND-THRUST BELT – DEMONSTRATION OF IN-SEQUENCE THRUSTING AND THE GEOLOGICAL SIGNIFICANCE OF WALL-ROCK PROVENANCE AGES EXTRAPOLATED FROM GOUGE DATING

The mechanics of the propagation of fold-and-thrust belts has been the subject of considerable interest to the structural geological community for the last thirty years. The dating of brittle faults by dating the growth of authigenic illite in fault gouge allows for the direct testing of contrasting kinematic models of orogenic evolution. Recent results from direct dating of fault gouges in the American Cordillera have suggested that that orogenic wedges propagate in a series of ‘pulses’, rather than propagating forward gradually through time, as predicted by widely-accepted theoretical and analogue models of critical wedge behaviour. In contrast to these previous results, we present fault gouge ages from the Pyrenean fold-and-thrust belt from 5 thrusts along a 50 km-long section in the southeastern Pyrenees along the the Ter and Freser rivers that document in-sequence thrusting over a period spanning ~20 Ma. The Ter-Freser section records Middle-to-Late Eocene thrust propagation through a Paleogene to Mid-Eocene sedimentary basin on the southeastern margin of the range. Our gouge ages directly document south-directed in-sequence thrusting from 49.5 +/- 4.4 Ma for the formation of the hinterland Freser antiformal stack to 31.9 +/- 3.9 Ma for the most distal L’Escala thrust at the margin of the foreland basin, as predicted by theoretical models of critical wedge behavior. Preservation of the in-sequence behaviour of the Pyrenean wedge is aided by low convergence rates (0.1 mm/yr) over a long time span (~20 Ma). The formation of new outward thrusts is seen as the consequence of periodic advance of the wedge toe, perhaps in response to orogenic wedge height increasing during slow Eocene convergence of Iberia with Eurasia. The uppermost Paleozoic ages of the detrital component extrapolated from our technique (the pooled age of the wall rocks) are also collectively geologically meaningful and indicate that the late Paleozoic age of the detrital component of many of the foreland basin thrust faults indicate that Hercynian-age micas were deposited in the southern Pyrenees as early as the Eocene.