Marvin H. Beeson

Intracanyon flows of the Columbia River Basalt Group in the lower Columbia River Gorge and their relationship to the Troutdale Formation

The Priest Rapids Member (Wanapum Basalt) and the Pomona Member (Saddle Mountains Basalt) of the Columbia River Basalt Group crossed the Miocene Cascade Range into western Oregon and Washington as intracanyon flows about 14 m.y. and 12 m.y. ago, respectively. A thick, allogenic, bedded palagonite complex underlying the Priest Rapids intracanyon flow originated when the first Priest Rapids flow (Rosalia chemical type) interacted with a shallow lake on the Columbia Plateau, displacing water that flushed hyaloclastic debris into an incipient ancestral Columbia River channel. The Priest Rapids flow then overfilled the canyon, forcing the river northward, where it established the Bridal Veil channel. The Bridal Veil channel, part of a river system that extended across the Columbia Plateau, was only partly filled by the Pomona flow, allowing the river to remain in this channel through the Cascade Range. The Troutdale Formation is made up of (1) older alluvial gravels deposited below and above the Pomona intracanyon flow while the ancestral Columbia River occupied the Bridal Veil channel and (2) younger, more varied alluvial deposits characterized by clastic and hyaloclastic debris from Boring and High Cascade volcanism, spread over a much wider area, when aggradation overfilled the Bridal Veil channel less than 6 m.y. ago. The shift of the lower Columbia River to its present course took place during this time of maximum alluviation, aided by scattered eruptions of younger volcanoes of the Boring and High Cascade Lavas. The present Columbia River Gorge was formed in post-Troutdale time by entrenchment of the Columbia River during rapid uplift of the Cascade Range of northern Oregon during the past 1 to 2 m.y.

New aeromagnetic data reveal large strike-slip (?) faults in the northern Willamette Valley, Oregon

High-resolution aeromagnetic data from the northern Willamette Valley, Oregon, reveal large, northwest-striking faults buried beneath Quaternary basin sediments. Several faults known from geologic mapping are well defined by the data and appear to extend far beyond their mapped surface traces. The Mount Angel fault, the likely source of the Richter magnitude (M L ) 5.6 earthquake in 1993, is at least 55 km long and may be connected in the subsurface with the Gales Creek fault 25 km farther northwest. Northeast of the Mount Angel fault, a 60-km-long, northwest-striking anomaly may represent a previously unrecognized dextral-slip fault beneath the towns of Canby and Molalla. Vertical offsets along the Mount Angel fault increase with depth, indicating a long history of movement for the fault. Dominantly northwest-trending, relatively straight faults, consistent stepover geometries, offset magnetic anomalies, and earthquake focal mechanisms suggest that these faults collectively accommodate significant dextral slip. The 1993 earthquake may have occurred on a left-stepping restraining bend along the Mount Angel–Gales Creek fault zone.

Tectonic setting of the Portland-Vancouver area, Oregon and Washington: Constraints from low-altitude aeromagnetic data

Seismic activity in the Portland-Vancouver metropolitan area may be associated with various mapped faults that locally offset volcanic basement of Eocene age and younger. This volcanic basement is concealed in most places by young deposits, vegetation, and urban development. The U.S. Geological Survey conducted an aeromagnetic survey in September 1992 to investigate the extent of these mapped faults and possibly to help identify other seismic and volcanic hazards in the area. The survey was flown approximately 240 m above terrain, along flight lines spaced 460 m apart, and over an area about 50 × 50 km. These magnetic data indicate a pronounced northwest-striking magnetic lineation east of the Willamette River in downtown Portland associated with a fault concealed beneath Quaternary sedimentary deposits and previously inferred from shallow well data. The magnetic lineation confirms the existence of the fault and suggests that it has had a prolonged history: (1) Although well data indicate <200 m of vertical offset of underlying volcanic basement, models based on the aeromagnetic data from downtown Portland suggest reverse faulting with up to 1 km of offset deeper in the section. (2) The magnetic lineation associated with this fault extends southeast to the Clackamas River drainage, a distance of 50 km and considerably beyond the mapped extent of the fault. A northwest-striking magnetic anomaly located southwest of the Tualatin Mountains corresponds closely with another mapped fault and with mixed reverse and strike-slip faulting during a seismic swarm (M ≤ 3) in 1991. We believe these and other anomalies in the aeromagnetic data reflect the Portland Hills fault zone, believed to be the southwestern boundary of a structural basin now occupied by Portland and Vancouver. The postulated northeastern boundary of the basin, the Frontal fault zone, is also evident, although less well represented in the aeromagnetic data. Aeromagnetic anomalies, geologic mapping, and earthquake focal-plane solutions demonstrate a complex deformational history in the Portland-Vancouver area since middle Miocene time that includes elements of compression, extension, and dextral slip. These complexities reflect Portland-Vancouvers unique position within a north-south transition in tectonic styles along the Cascadia margin, from compressional in the north to extensional in the south.

Gravity Study through the Tualatin Mountains, Oregon: Understanding Crustal Structure and Earthquake Hazards in the Portland Urban Area

A high-resolution gravity survey through the Tualatin Mountains (Port- land Hills) west of downtown Portland exhibits evidence of faults previously iden- tified from surface geologic and aeromagnetic mapping. The gravity survey was conducted in 1996 along the 4.5-km length of a twin-bore tunnel, then under con- struction and now providing light-rail service between downtown Portland and com- munities west of the Portland Hills. Gravitational attraction gradually increases from west to east inside the tunnel, which reflects the tunnels location between low- density sedimentary deposits of the Tualatin basin to the west and high-density, mostly concealed Eocene basalt to the east. Superimposed on this gradient are several steplike anomalies that we interpret as evidence for faulted contacts between rocks of contrasting density. The largest of these anomalies occurs beneath Sylvan Creek, where a fault had previously been mapped inside the tunnel. Another occurs 1200 m from the west portal, at the approximate intersection of the tunnel with an aeromag- netic anomaly associated with the Sylvan fault (formerly called the Oatfield fault). Lithologic cross sections based on these gravity data show that the steplike anomalies are consistent with steeply dipping reverse faults, although strike-slip displacements also may be important. Three gravity lows correspond with topographic lows directly overhead and may reflect zones of shearing. Several moderate earthquakes (M 3.5) occurred near the present-day location of the tunnel in 1991, suggesting that some of these faults or other faults in the Portland Hills fault zone are seismically active.

Geochemical modelling of basalts from DSDP Leg 65, East Pacific Rise, Gulf of California

Abstract Major and rare earth element (REE) data for basalts from Holes 483, 483B, and 485A of DSDP Leg 65, East Pacific Rise, mouth of the Gulf of California, support a simple fractional crystallization model for the genesis of rocks from this suite. The petrography and mineral chemistry (presented in detail elsewhere) provide no evidence for magma mixing, but rather a simple multistage cooling process. Based on its lowest TiO 2 content (0.88%), FeO ∗ MgO ratio (0.95 with total Fe as FeO), and Mg# (100 Mg Mg + Fe ″ = 70 ), sample 483-17-2-(78–83) has been selected as the most primitive primary magma of the samples analyzed. This is supported by the REE data which show this sample has the lowest total REE content, a La Sm cn (chondrite-normalized) = 0.36, and Eu Sm cn = 1.05. Because other samples analyzed have higher SiO 2 , lower Mg#, and a negative Eu anomaly ( Eu Sm cn as low as 0.89), they are most likely derivative magmas. Wright-Doherty and trace element modelling support fractional crystallization of 14.1% plagioclase (An 88 ), 6.7% olivine (Fo 86 ), and 4.7% clinopyroxene (Wo 41 En 49 Fs 10 ) from 483-17-2-(78–83) to form the least differentiated sample with Mg# = 63. The La Sm cn of this derivative magma is almost identical to the parent magma (0.35 to 0.36), but the other samples have higher La Sm cn (0.45 to 0.51), more total REE, and lower Mg# (60 to 56). Both Wright-Doherty and trace element modelling indicate that the primary magma chosen cannot produce these more evolved samples. For the major elements, the TiO 2 and P 2 O 5 are too low in the calculated versus the observed (1.38 to 1.90; 0.11 to 0.17, respectively, for example). Rayleigh fractionation calculates a lower La Sm cn and requires about 60% crystal removal versus 40% for the Wright-Doherty. These more evolved samples must be derived from a parent magma different from the one selected here and, unfortunately, not sampled in this study. A magma formed by a smaller degree of partial melting with slightly more residual clinopyroxene left in the mantle than for sample 483-17-2-(78–83) is required.