9 April 2014

Faults disrupting the contact between the Muleros Andesite and Mesilla Valley Formation shale

Posted by Callan Bentley

Hark! What gleams on yonder contact?


Well, there’s no glaciers to polish anything ’round these here parts (southernmost New Mexico + westernmost Texas), so I reckon it must be fault polish. Let’s test that hypothesis by looking for slickensides…


Sure enough! There they are!

IMG_2061 anno

Unlike the deformation we saw yesterday, this faulting of the contact between the Muleros Andesite (Eocene) and the Mesilla Valley Formation shale (Cretaceous) into which it intrudes must post-date the intrusion (since you cannot break a magma, but you can break the igneous rock the magma cools down to become). The slickenlines and fault polish would have to be imparted to a solid rock, not a gooshy melt. So therefore, faulting is post-Eocene.

There are other faults to be observed along the contact, too. For instance, the site with the lineated andesite we observed yesterday, the outcrop surface bears both steeply and shallowly dipping faults with approximately perpendicular orientations:



Sweet – within a few minutes, we were able to tease out a sequence of events that must have influenced the rocks at this site, a little glimpse into their long history.