LATE EOCENE EARLY OLIGOCENE TECTONISM, VOLCANISM, AND FLORISTIC CHANGE NEAR GRAY BUTTE, CENTRAL OREGON

Mid-Tertiary strata near Gray Butte, central Oregon, record volcanism and sedimentation on the margin of a west-tilted basin that was subseq uently disrupted by a northeast-striking fault system. Compositional c haracteristics of the volcanic rocks support correlation of the sectio n to the upper Eocene-Oligocene part of the John Day Formation. The si milar to 1.2-km-thick section contains five fossil floras documenting climatic change in late Eocene-early Oligocene time and a progression between better known Eocene and Oligocene floras in the region. The pr esence of the transitional floras is a consequence of the subsidence o f the Gray Butte basin to accommodate a section that is nearly four ti mes thicker than better studied correlative strata similar to 50 km to the northeast that consist almost entirely of volcanic rocks. The low est flora is within flood-plain facies, whereas the other four are hos ted in lacustrine strata. Although alteration of volcanic rocks most c losely associated with the floras precludes development of a precise i sotopic-age chronology, regional correlations and several accurate iso topic-age determinations indicate that the principal interval of clima tic cooling may have been in late Eocene time (ca. 38-39 Ma) rather th an at the Eocene-Oligocene boundary. The paleoclimate interpretation i s tempered, however, by the low diversity of the floras (12-24 species ) and possible taphonomic biases in comparing flood-plain and lacustri ne environments. Mapping established the presence of the Cyrus Springs fault zone, a large (1.2 km vertical displacement, possibly >7 km dex tral offset) shear zone that is possibly a surface expression of the K lamath-Blue Mountain gravity-anomaly lineament. The orientations of th is fault zone, subsidiary sinistral structures, dikes, and fold axes s uggest that the presumed Mesozoic structure marked by the lineament wa s reactivated as a dextral-normal fault by east-northeast-west-southwe st-oriented compressive stress. This stress is consistent with early O ligocene North America-Farallon convergence but is inconsistent with n orthwest-southeast to north-south compression suggested by structures farther east. Stress either varied temporally or was partitioned into complex local strain domains. The Cyrus Springs fault zone may have be come active at about 28-30 Ma, resulting in uplift and southward tilti ng of part of the Gray Butte basin fill, and ceased activity before de position of the horizontal upper Miocene-Pliocene Deschutes Formation. The Gray Butte area was also an eruptive center for rhyolitic and alk aline-mafic lava and tuff both before and after initiation of movement on the Cyrus Springs fault. Mid-Tertiary volcanism and sedimentation near the western end of the Blue Mountains, heretofore not clearly rel ated to active structures, may have taken place within a regional tran stensional regime associated with stress orientations different from t hose of Neogene time. Local basins with higher subsidence rates accumu lated relatively thick sequences of lacustrine tuffaceous strata, and their fossil floras show progressive climate change through the Eocene -Oligocene boundary interval.