DIRECT DATING OF FAULT ROCKS OF THE SEVIER THRUST BELT (UT-ID-WY)

Quantification of fault-related illite neomineralization in gouge allows timing of fault activity to be directly dated. Fault-related neoformed clays are distinguished from preexisting detrital clays on the basis of polytypism (1M_d=neoformed, 2M₁=detrital) or discrete illite (detrital) and I-S (neoformed). Gouge samples were collected along major faults in Utah, Idaho and Wyoming. Samples were separated into at least three size fractions, and the proportion of 1M_d to 2M₁ and discrete illite to I-S was determined using X-ray diffraction powder pattern analysis. Size fractions were dated using the encapsulated ⁴⁰Ar/³⁹Ar method. The percentages of detrital illite are plotted against the ages of the size fractions, and the age of the neoformed material, representing the timing of fault activity, is extrapolated. We analyzed the Willard Thrust (67 Ma +/- 2), the Meade Thrust (66 Ma +/- 2), the Absaroka Thrust (47 Ma +/- 2), the Darby Thrust (46 Ma +/- 2) and the Bear Thrust (46 Ma +/-2). Our findings indicate that ages derived from polytype quantifications are generally more reliable than ages derived from discrete illite and I-S in these samples. For comparison, host rock alteration of sediments in the frontal section of the belt, away from fault contacts, yielded ages of ~88 Ma. The coeval ages indicate that regional pulses of fault activity in the UT-ID-WY segment of the Sevier belt occurred in the latest Cretaceous and earliest Middle Eocene. Thus, rather then individual displacements, our ages indicate that regions of the thrust belt were (re)actived at the same time. These ages record the youngest period of major fault activity, as syn-orogenic sediments were initially shed from the Absaroka sheet at ~85 Ma, and from the Willard-Meade system at ~90 Ma, and syntectonic mineral veins near that fault have ages of ~130-110 Ma. Notably, ages from the nearby Canadian segment of the belt are slightly older (by a few m.y.), but show similar regional patterns and deformation pulses as the US segment of the thrust belt.