December 16, 2011 in Fieldwork
First, I’d like to thank Evelyn Mervine of Georneys for the geo-pic a day idea. Not only do we all love looking at photos of great geologic features, but it frees us from the work of actually writing real blog posts. Let’s do this again sometime.
For my final geo-pic, I give you faulted radiolarian cherts from a Costa Rican oceanic complex. Ophiolites can be found on the Santa Elena and Nicoya Peninsulas (where this picture is from) and are wonderful because you can go from mantle peridotite to pillow basalts to turbidites to radioalarian cherts in a fairly short distance.
The direction of maximum principle stress is easy to determine, because it tends to bisect the twinned conjugate faults you see above. Also note that these rocks were faulted very close to the brittle-plastic transition. In fact, the thick layer at the bottom looks like it has some distinctly ductile features. See below for a figure from Structural Geology by Fossen (Cambridge) showing conjugate faults and the orientation of the maximum principle stress.
What? No more geo-pictures of the day? I’m sure this will be back sometime, however we now return you to your regularly scheduled programming.