POST-MIDDLE MIOCENE TRANSTENSION IN THE PLOMOSA MOUNTAINS AND DEVELOPMENT OF THE LA POSA PLAIN, WEST-CENTRAL ARIZONA

The northern Plomosa Mountains metamorphic core complex records large-magnitude NE-directed extension in the early to middle Miocene, consistent with other core complexes in the lower Colorado River extensional corridor. Brittle structures associated with this period of extension include the corrugated Plomosa detachment fault and NW-striking normal faults and joints throughout the detachment footwall. The detachment fault is offset a total of ≥320 meters by a map-scale N-striking dextral-normal fault and NNE-striking sinistral-normal fault, which cut moderately-consolidated colluvial and alluvial deposits. These two post-detachment faults are associated with small-scale, subparallel strike-slip to oblique-slip faults and N- to NE-striking barite and carbonate veins which also cut detachment-related brittle structures. Together, the post-detachment brittle structures in the northern Plomosa Mountains record WNW-ESE extension. In the southern Plomosa Mountains, small-scale conjugate strike-slip and normal faults also cut moderately-consolidated alluvial deposits, and record ENE-WSW extension. Regional-scale post-middle-Miocene NW-striking dextral fault systems associated with the Pacific-North America plate boundary are documented north and south of the Plomosa Mountains, but are conspicuously absent across the northern half of the range. We interpret the post-middle-Miocene brittle structures associated with WNW-ESE to ENE-WSW extension in the Plomosa Mountains to be the result of a transtensional stepover between the bounding dextral fault systems. This transtensional deformation provides an explanation for the missing northern limb of the Plomosa detachment fault corrugation, which was likely down-dropped in the La Posa Plain along a NNE-striking normal fault. Furthermore, a drill hole centered in the northern La Posa Plain reveals that the top of the Bouse Formation is ~300 meters lower than determined for the greater Blythe basin, suggesting transtensional basin development occurred during or after deposition of the Bouse Formation, supporting a Pliocene or younger age for the deformation.