29 May 2020

Friday fold: Lopez Island tension gashes

Posted by Callan Bentley

Darrel Cowan returns with a Friday fold for you! He writes with these

awesome fractures. They formed in slightly metamorphosed sandstone on Lopez Island in the San Juan Islands, Washington. The rocks are probably Late Jurassic-Early Cretaceous in age and part of the Lopez Structural Complex [I can give a reference for the setting: GSA SP 221]. The sandstone exhibits a weak foliation imparted by dissolution and reprecipitation of quartz; the quartz in the fractures is evidence of the role of fluids.

You can interpret what we see. I tell my students that these formed in a localized shear zone, even if the boundaries are not expressed in the outcrop, which is glacially polished and striated, by the way. I challenge my students to infer the sense of shear and to explain why the fractures are sigmoidal and thicker in the middle [they kept opening while rotating and extending their wings outward!]. The youngest tips are oriented at 45° to the boundaries of the shear zone, as predicted.

I’ve wanted to go back and visit these outcrops, but access on Lopez is extremely difficult. Someday, after covid-19.

Best wishes to all. On several levels, the Friday fold feels like a frivolous indulgence in such dark times for the nation, but perhaps its a reminder of the sort of content that acts as a beacon of normalcy. It’s the content I would prefer to focus on, were pandemic disease, racial injustice, culture war, environmental degradation, and looming facist oppression not occupying my thinking instead.