MAGNETIC FABRICS IN AN EOCENE ANDESITIC DIKE, UPPER PLATE, HEART MOUNTAIN SLIDE, WY

The Eocene Heart Mountain landslide was preceded by emplacement of widespread mafic dikes and the magnetic characteristics of these dikes can be used as a tectonic tracer for upper plate block movements. Previous work on mafic dikes that pervade the upper plate of the Heart Mountain Madison Group limestones at Cathedral Cliffs, WY preserve a vertical K_max orientation (n=32) indicating vertical dike intrusion and that dike intrusion did not trigger horizontal upper plate motion. We have sampled a rootless andesitic dike (N90°E, 90°) that intruded into horizontal Madison Group limestone then translated southeastward to the allochthonous Paleozoic rocks of the Madison Formation overlying the ramp at Rattlesnake Mountain. AMS patterns (n=22) record K_max perpendicular to K_min and both with a horizontal orientation at an acute angle to the dike. AF demagnetization (n=10) was clean with linear paleo-vector plots (mean paleopole: 355°, 10°) that require a 35° rotation about a horizontal east-west axis to align with the Eocene paleopole for this latitude. These results indicate that the dike was emplaced at a shallow angle, rather than vertically, and that some upper plate Heart Mountain blocks translated with vertical motion about a horizontal axis.