Faulted calc-silicate and chert layers, Happy Valley, near Tucson, AZ

Original limestone and chert were metamorphosed to a calc-silicate rock, probably during the mid-Tertiary. Deformation of these rocks included both folding and faulting. What is striking about this rock is the preservation of discrete faults that offset a conspicuous ‘microstratigraphy.’ This is the kind of structural system that can be retrodeformed in ways to recreate the original unfaulted relationships, and thus to provide a clear sense of nature and amount of shortening or extension.

I collected this in the mid-1970s in the Rincon Mountain area, while working on metamorphic core complexes.  I like the fact that the gory details of fault attributes are disclosed, revealing so clearly that faults are not simple ‘straight lines’ that neatly cut and displace ‘neat layers.’

Age & Formation
Horquilla Formation of Pennsylvanian age, ~320 Ma.