Richard M. Conrey

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 subsequently disrupted by a northeaststriking fault system. Compositional characteristics of the volcanic rocks support correlation of the section to the upper Eocene‐Oligocene part of the John Day Formation. The ~1.2-kmthick 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 presence of the transitional floras is a consequence of the subsidence of the Gray Butte basin to accommodate a section that is nearly four times thicker than better studied correlative strata ~50 km to the northeast that consist almost entirely of volcanic rocks. The lowest flora is within flood-plain facies, whereas the other four are hosted in lacustrine strata. Although alteration of volcanic rocks most closely associated with the floras precludes development of a precise isotopic-age chronology, regional correlations and several accurate isotopic-age determinations indicate that the principal interval of climatic cooling may have been in late Eocene time (ca. 38‐39 Ma) rather than at the Eocene-Oligocene boundary. The paleoclimate interpretation is tempered, however, by the low diversity of the floras (12‐24 species) and possible taphonomic biases in comparing flood-plain and lacustrine environments. Mapping established the presence of the Cyrus Springs fault zone, a large (1.2 km vertical displacement, possibly >7 km dextral offset) shear zone that is possibly a surface expression of the Klamath‐Blue Mountain gravity-anomaly lineament. The orientations of this fault zone, subsidiary sinistral structures, dikes, and fold axes suggest that the presumed Mesozoic structure marked by the lineament was reactivated as a dextralnormal fault by east-northeast‐west-southwest‐oriented compressive stress. This stress is consistent with early Oligocene North America‐Farallon convergence but is inconsistent with northwest-southeast to northsouth 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 become active at about 28‐30 Ma,resulting in uplift and southward tilting of part of the Gray Butte basin fill, and ceased activity before deposition of the horizontal upper Miocene‐Pliocene Deschutes Formation. The Gray Butte area was also an eruptive center for rhyolitic and alkaline-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 related to active structures, may have taken place within a regional transtensional regime associated with stress orientations different from those of Neogene time. Local basins with higher subsidence rates accumulated relatively thick sequences of lacustrine tuffaceous strata, and their fossil floras show progressive climate change through the Eocene-Oligocene boundary interval.

40Ar/39Ar geochronology, paleomagnetism, and evolution of the Boring volcanic field, Oregon and Washington, USA

The 40 Ar/ 39 Ar investigations of a large suite of fine-grained basaltic rocks of the Boring volcanic field (BVF), Oregon and Washington (USA), yielded two primary results. (1) Using age control from paleomagnetic polarity, stratigraphy, and available plateau ages, 40 Ar/ 39 Ar recoil model ages are defined that provide reliable age results in the absence of an age plateau, even in cases of significant Ar redistribution. (2) Grouping of eruptive ages either by period of activity or by composition defines a broadly northward progression of BVF volcanism during latest Pliocene and Pleistocene time that reflects rates consistent with regional plate movements. Based on the frequency distribution of measured ages, periods of greatest volcanic activity within the BVF occurred 2.7–2.2 Ma, 1.7–0.5 Ma, and 350–50 ka. Grouped by eruptive episode, geographic distributions of samples define a series of northeast-southwest–trending strips whose centers migrate from south-southeast to north-northwest at an average rate of 9.3 ± 1.6 mm/yr. Volcanic activity in the western part of the BVF migrated more rapidly than that to the east, causing trends of eruptive episodes to progress in an irregular, clockwise sense. The K 2 O and CaO values of dated samples exhibit well-defined temporal trends, decreasing and increasing, respectively, with age of eruption. Divided into two groups by K 2 O, the centers of these two distributions define a northward migration rate similar to that determined from eruptive age groups. This age and compositional migration rate of Boring volcanism is similar to the clockwise rotation rate of the Oregon Coast Range with respect to North America, and might reflect localized extension on the trailing edge of that rotating crustal block.

Holocene volcanism of the upper McKenzie River catchment, central Oregon Cascades, USA

To assess the complexity of eruptive activity within mafic volcanic fields, we present a detailed geologic investigation of Holocene volcanism in the upper McKenzie River catchment in the central Oregon Cascades, United States. We focus on the Sand Mountain volcanic field, which covers 76 km 2 and consists of 23 vents, associated tephra deposits, and lava fields. We find that the Sand Mountain volcanic field was active for a few decades around 3 ka and involved at least 13 eruptive units. Despite the small total volume erupted (∼1 km 3 dense rock equivalent [DRE]), Sand Mountain volcanic field lava geochemistry indicates that erupted magmas were derived from at least two, and likely three, different magma sources. Single units erupted from one or more vents, and field data provide evidence of both vent migration and reoccupation. Overall, our study shows that mafic volcanism was clustered in space and time, involved both explosive and effusive behavior, and tapped several magma sources. These observations provide important insights on possible future hazards from mafic volcanism in the central Oregon Cascades.

Comment [on “Evidence suggests slab melting in arc magmas” by Defant and Kepezhinskas]

The recent summary of the adakite hypothesis by M. Defant and P. Kepezhinskas (Eos, 5 February 2001, p. 65) addresses several important issues that have arisen in regard to interpreting arc rocks during the past decade. Defant and Kepezhinskas imply that slab melting is a common process and that many arc volcanoes are composed dominantly of adakite that has interacted with the mantle wedge on its way to the surface. The interactions are further interpreted as the cause of much compositional (metasomatic) variation in wedge peridotite. I regard many of the statements made by Defant and Kepezhinskas as controversial. The main problems of the adakite model follow: 1) The definition is too broad and ambiguous and does not uniquely identify slab melts. For example, high-Mg andesite made by simple basalt-rhyolite mixing can easily be mistaken for adakite. And Defant and Kepezhinskas regard any silicic melt that is derived from a garnet-bearing source as an adakite. They offer no means of distinguishing slab melts from melts of garnet-bearing lower crust. Defant and Kepezhinskas do not reference geophysical data in their definition, but such data provide a way to discriminate between the two possibilities. In areas where geophysical interpretation strongly suggests lower crustal temperatures high enough for melt production, there is no compelling reason to maintain the adakite hypothesis.