8 April 2014

Deformation associated with the intrusion of the Muleros Andesite

Posted by Callan Bentley

Yesterday, I showed off a few views of the contact between the Cretaceous aged Mesilla Valley Formation shale and the hypabyssal Muleros Andesite which intruded into it during the Eocene at Mt. Cristo Rey (on the US/Mexico border where Texas meets New Mexico).

Today, I’d like to look at some of the structure associated with the contact zone. First off, take a look at this image, which is looking orthogonal to the contact, glimpsing bits of the vast andesite laccolith through scraps of a “screen” of the immediately adjacent shale:


Do you notice a pattern to the feldspar phenocrysts on the left side? They’re aligned!


This lineation may well be a magmatic feature, showing how the magma flowed when it intruded, with cooling and crystallization “locking in” the magmatic flow pattern for us to gaze upon >40 million years hence…


Here’s my model for how this would have worked. As magma intruded, the overlying strata bowed upward, increasing their angle of dip along the flanks of the intrusion.


Strata of the Mesilla Valley Formation slumped off the sides, wrinkling and piling up along the margins of the laccolith. This bolstered the edges of the pluton, making the least-stress-direction straight up. So the laccolith swelled and thickened, leading to magmatic lineations along its margins.


As for the deformation in the shale, we saw that too. Here’s a nice contorted section (sandstone layers weather out as blocky, high-relief layers amid the shale) of the Mesilla Valley Formation:


We asked the students to sketch this extraordinary display of deformation…


… and in each of their field notebooks, something like this unfurled:


This deformation isn’t Laramide. It’s syn-magmatic, I think.

…But wait, there’s more. Tomorrow, we’ll take a look at brittle structures which are probably post-intrusion.