STRUCTURAL EVOLUTION OF THE LATE CRETACEOUS – EARLY PALEOGENE QUITOBAQUITO SHEAR ZONE, ORGAN PIPE CACTUS NATIONAL MONUMENT, SOUTHERN ARIZONA

The Quitobaquito Shear Zone (QSZ) includes a series of Late Cretaceous upper greenschist-lower amphibolite facies thrust faults exposed in the Quitobaquito Hills and Puerto Blanco Mountains. Structurally higher thrusts of the QSZ are visible in the Quitobaquito Hills while structurally lower thrusts are visible in the Puerto Blanco Mountains. The structurally highest thrust, the Quitobaquito thrust, juxtaposes Mesoproterozoic augen gneiss basement against Middle to Late Jurassic supracrustal rocks. Lithologies include metasediments, metavolcanic/volcaniclastics, and metagranites. Mapping across these ranges reveals differences in structural patterns across approximately 5 km of restored depth. Mylonites across the QSZ record an overall top-to-the-north sense of shear and kinematic indicators suggest subsimple shear. Variably developed mylonitic foliation and associated stretching lineation in the Quitobaquito Hills are consistently oriented, with foliation dipping SSE and lineation plunging SSW to SSE. Orientations in the Puerto Blanco Mountains are more variable. The mylonitic foliation in the Puerto Blanco Mountains is folded by E-W trending upright, horizontal folds. Folding causes the stretching lineation to vary in orientation, though it largely plunges NNW. At least two range-scale, tight to isoclinal folds were observed. At outcrop scale, folds range from centimeter-scale crenulations to larger recumbent folds. Most formed in metavolcanic/volcaniclastic and metasedimentary rocks near ~1 – 10 m thick ultramylonite zones. Boundaries of these zones vary from sharp to gradational.

Timing of the QSZ can be constrained by relationships between structures recording deformation and timing of intrusion of the 80 Ma Aguajita Spring Granite and the 67 Ma Senita Basin Granite. The contact between the Aguajita Spring Granite and the augen gneiss unit is an ultramylonite zone and demonstrates emplacement of the Aguajita Spring Granite pre-dates movement of the QSZ. Mylonitic foliation appears to be warped around the Senita Basin Granite, which lacks a solid-state fabric, suggesting emplacement of this pluton post-dated shearing. This suggests the QSZ was active between 80 – 67 Ma, differing from thermochronologic evidence suggesting the Quitobaquito thrust was active ca. 70 – 55 Ma.