PALEOMAGNETIC ANALYSIS OF BASALT DIKES AT CATHEDRAL CLIFFS, WY: IMPLICATIONS FOR HEART MOUNTAIN FAULTING

Mafic dikes pervade the upper plate of the Heart Mountain Detachment, yet the dike concentration in the lower plate is sparse. The dikes in the upper plate have been interpreted to be either syntectonic and an intimate part of the gradual extension of the upper plate (continuous allochthon model) or older than and unrelated to faulting (tectonic denudation model). The magnetic fabrics of 32 mafic dikes at Cathedral Cliffs were analyzed using low-field anisotropy of magnetic susceptibility (AMS) as a proxy for flow. These dikes intrude Ordovician-Mississippian carbonates and overlying Eocene volcanic rocks and are truncated below along the nearly horizontal Heart Mountain detachment. The dikes trend between N10W and N20E, are all steeply dipping, and range in width between 0.5 to 3 m. Hysterisis loop data show that multi-domain magnetite grains control the AMS fabric in the dikes with a few samples falling into the pseudo-single domain range. Mean susceptibilities for the dikes range from 1.972 x 10-2 to 4.727 x 10-2 with an overall mean of 3.201 x 10-2. Flow directions for the dikes were determined by the bearing and plunge of the Kmax axes. About 66% of the dikes sampled show typical dike AMS patterns with Kmax and Kint in the plane of the dike and Kmin normal to the dike plane. About 72% of the dikes sampled have Kmax inclinations >40% and thus were emplaced upward; 22% of the dikes have Kmax inclinations of <10% and thus were emplaced laterally. The remaining dikes have intermediate Kmax inclinations. With numerous dikes showing vertical to subvertical emplacement directions and with no source below the detachment nearby indicates the dikes predate emplacement of the upper plate. Therefore, dike intrusion cannot contribute extension as suggested by the continuous allochthon model, nor can they post date emplacement of the upper plate, as suggested by the tectonic denudation model.