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Conjugate Faults in the Zuni Mountains

Conjugate fault with annotation showing the maximum compression direction.
(click for a larger version)
Jacob Thacker

Zuni Mountains, NM
— April 14, 2021

Geology is truly a multidisciplinary science. On any given day we might work at the intersections of anthropology to zoology. Rock deformation (how rocks break and bend) is one example where these intersections help us to better understand our Earth. Engineering tests conducted to determine the strength of materials have shown us that applying a compressional stress to both sides of a rock will cause it to break in characteristic patterns. One pattern is known as conjugate faults, where failure of the rock results in a pair of faults that form a 60° wedge, of which the maximum compression direction (i.e., the squeezing direction) bisects the middle of the wedge. Armed with this experimental information, detailed field measurements of conjugate faults and other deformation features can tell us the different states of stress that Earth’s crust has enjoyed. These conjugate faults in the Zuni Mountains, along with other fault data, have been used to interpret an east-northeast/west-southwest compressional field that buckled the crust and caused faults and folds to uplift the range. This compressional event is known as the Laramide orogeny that squeezed the crust from Montana to here in New Mexico approximately 90 to 40 million years ago. The Laramide orogeny occurred many millions of years before the crust was then put under tension to form the Rio Grande Rift. The white notebook in the photo is ~12 cm tall; the photo is taken looking up at the outcrop.

— Jacob Thacker, NMBGMR Field Geologist