FAULT GOUGE DATING IN THE SPANISH PYRENEES: SHORTENING RATES, THERMAL CONSTRAINTS ON CLAY GOUGE FORMATION, AND CATACLASTIC AGE INTERPRETATION

Dating brittle faults by the growth of authigenic illite in fault gouge allows for direct testing of kinematic models of orogenic evolution, such as in-sequence vs. out-of-sequence thrusting, and the presence of orogenic ‘pulses’ of deformation on multiple thrusts simultaneously. We present ages of authigenic illite in fault gouge from 5 thrust faults along the Ter and Freser rivers in the south-eastern Pyrenees fold-and-thrust belt dated by Illite Age Analysis. The ages suggest broadly in-sequence thrusting for the prowedge of the eastern Pyrenees during the Eocene and early Oligocene, ranging from 49.5 + 4.4 Ma for the inboard Freser Antiformal Stack to 31.9 +/- 3.9 Ma for the frontal L’Escala thrust. Unlike many other foreland fold-and-thrust belts, foreland progression of ages is preserved, although overlapping of ages of the frontal thrusts suggests that the interpretation of a pulse of orogenic activity around 35 Ma is also possible, similar to coeval fault activity observed elsewhere in the Pyrenees. We also provide interpretations of the age of the cataclastically-derived component in gouges, often difficult to interpret. The latest Paleozoic ages of the cataclastically-derived component of many of the foreland basin thrusts indicate that Hercynian-age micas were deposited in the Southern Pyrenean foreland basin as early as the late Cretaceous. Recently published thermochronometer and D₄₇ temperatures from syn-faulting calcite veins at the same outcrops allows the temperature of clay gouge formation to be constrained at 50 - 210°C, with the most likely range of temperatures of 90 - 160°C. Shortening rates for the Ter-Freser section range from 1.6 – 3.0 km/Ma, with bulk strain rates ranging from 7.0 x 10^-16 s^-1 to 1.6 x 10^-15 s^-1.