| Abstrakt: |
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 versus out‐of‐sequence thrusting, and the presence of orogenic "pulses" of deformation on multiple thrusts simultaneously. We present illite gouge ages from five thrust faults along the Ter and Freser rivers in the south‐eastern Pyrenees fold‐and‐thrust belt dated by Ar‐Ar methods. The ages show in‐sequence thrusting for the south‐eastern Pyrenees fold‐thrust belt 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. Foreland progression of ages is preserved, although overlapping of ages of the frontal thrusts permits the interpretation of a period of more rapid fault displacement around 35 Ma. Shortening rates for the Ter‐Freser section range from 1.2–2.7 km/my, with strain rates ranging from 7.0 × 10−16 s−1 to 1.6 × 10−15 s−1 The latest Paleozoic ages of the cataclastically derived component inferred from the gouge dating methodology are shown to be pooled 2M1 illite/mica ages of wall rock sediments and have geological meaning. These inferred mica ages of the thrust wallrocks in the southeastern Pyrenees indicate Hercynian‐age micas were deposited in the Southern Pyrenean foreland basin as early as the late Cretaceous. Previous thermochronometer and Δ47 temperatures from syn‐faulting calcite veins constrain temperature of clay gouge formation in the Ter‐Freser section at 50–210°C, with most likely temperatures of 50–160°C, consistent with previous studies. Plain Language Summary: Mountain belts are generally thought to grow from the center out toward their margins over geological time. In this model, the oldest faults are at the interior of the range, while the youngest faults are near the edges of the belt. However, observations from several mountain belts worldwide show something different, that mountain belts have many faults of the same age suggesting "pulses" of mountain belt growth lasting a few millions of years, separated by periods of relative quiet. We test this idea by directly dating clay minerals inside the fault rock of several thrust faults in the southeastern Pyrenees mountain belt in Spain. We find that the SE Pyrenees grew like the model predictions, from the inside out, and not like the series of "pulses" observed in other mountain belts such as the Canadian and America Rockies. We also identify the temperature at which these fault gouge clay minerals, most likely 50–160°C. Key Points: SE Pyrenees fold‐thrust belt propagated in‐sequenceClay gouge minerals grew at temperature of 50–210°C, most likely 50–160°CAge of detrital component from gouge dating is shown to be geologically meaningful, and here is a pooled age of foreland basin micas [ABSTRACT FROM AUTHOR] |
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