NEW GEOLOGIC MAPPING RECORDS LATE MIOCENE TECTONIC DEFORMATION IN NORTHERN HARNEY BASIN, OREGON

New geologic mapping in the Harney 7.5′ Quadrangle, in the northern part of the Harney Basin of southeastern Oregon, provides insight into a complex depositional and tectonic history. Mapped geologic units include an older succession of early to middle Miocene basalt-andesite flows separated by coeval tuffs (units Tmdc 1-4) of the 16-14.88 Ma Dinner Creek Tuff. Overlying this lower mafic sequence are three, large-volume ash-flow tuff sheets, including the 9.6 Ma Devine Canyon Ash-flow Tuff (Tmtd), 8.4 Ma Prater Creek Ash-flow Tuff (Tmtp), and 7.05 Ma Rattlesnake Ash-flow Tuff (Tmtr), each separated by volcaniclastic sedimentary rocks. Volcanic strata in the quadrangle are tilted south (3° to 10°) and cut by a number of steeply dipping, NW-, N-, and NE-trending normal and normal oblique faults, interpreted as conjugate Riedel shears.

We report field and analytical results and quantify tilting rates from stereonet projections of structural attitudes. The orientation of primary sedimentary bedding planes and overlying congruent eutaxitic compaction foliation in the tuffs recorded in individual fault blocks show that older rocks are progressively tilted, creating a series of subtle unconformities: 9° S (Tmdc4), 7° S (Tmtd), 5° S (Tmtp), 2° S (Tmtr). The degree of angular discordance for the period between these emplacement events are: Tmdc4-Tmtd = 2°; Tmtd-Tmtp = 2°; Tmtp-Tmtr = 3°; and Tmtr-present day = 2°. The rate of tilting is calculated by the degree of angular discordance divided by the length of time between the deposition of the volcanic strata: a) Tmdc4-Tmtd: 1°/2.64 m.y.; b) Tmtd-Tmtp: 1°/0.60 m.y.; c) Tmtp-Tmtr: 1°/0.45 m.y.; and d) Tmtr-recent:1°/3.5 m.y. This data reveals that following the emplacement of Tmtd at 9.6 Ma, tilting rates apparently increased up to 7 Ma, then slowed to a rate similar to that recorded prior to Tmtd.

Ongoing geologic mapping by DOGAMI in the basin will provide insights into how stress from the Brothers Fault Zone, Basin and Range Province, John Day Fault, and the Yakima Fold Belt are accommodated in the dynamic volcanic-tectonic environment of the northern Harney Basin. The results of this work will serve as a fundamental basis for ongoing and future geologic, geohydrologic, and geohazard studies in the area.