Cataclasite dike, Sierra de Hualfin, northwestern Argentina

Sierra de Hualfin is a spectacular basement-cored uplift in the Sierras Pampeanas in northwestern Argentina. Exposures of Eocene ‘basement’ granite form the core of the mountain. The outside flanks still have a veneer of Tertiary sedimentary rocks (basin sediments, hundreds of meters thick) that are uplifted, faulted, folded, and draped over the edge of the uplift. What’s great is that you can go below the sediments and see clear exposures of faults in the granite. Along one of these is a cataclastic dike, which ostensibly resembles and igneous dike, but actually is crushed fine grained cataclastically deformed granite carried upward by solutions in a fault zone, forming a special kind of dike.

Pilar Garcia studied this area as the focus of her PhD dissertation at The University of Arizona.  I visited her in her field area back in 1999, to walk/hike/climb the geology with her.  On one of the traverses we made we encountered these features.   

Age & Formation
A product of faulting in the Tertiary, probably in the Miocene, ~20 Ma.