David C. Engebretson

A Rifted Margin Origin for the Crescent Basalts and Related Rocks in the Northern Coast Range Volcanic Province, Washington and British-Columbia

An apparatus for harvesting okra or other fruit or vegetable comprises an elongated frame having a handle at one end thereof and a cutter member at the other end. In order to prevent any contact between the user and irritating pods of the okra, a trigger is provided at the handle for remotely operating the cutter. Also, a shroud partially surrounds the handle to protect the hand of the user during cutting. In one embodiment, the cutter comprises a razor blade located beneath a cylindrical guide member that retains the okra pod in position for cutting of the stem while a pair of stabilizing ears steadies the guide member on a branch of the okra plant. In a second embodiment, a rectangular guide member contains a stem holding bracket that maintains the stem of the okra pod in proper position for cutting by a blade. A set of prongs carried by the blade pierces the pod during a cutting stroke. The pod is automatically released during retraction for disposal into a collection receptacle.

Topographic form of the Coast Ranges of the Cascadia Margin in relation to coastal uplift rates and plate subduction

The Coast Ranges of the Cascadia margin are overriding the subducted Juan de Fuca/Gorda plate. We investigate the extent to which the latitudinal trend in average topography of the Coast Ranges is a function of the latitudinal change in attributes related to the subduction process. These attributes include the variable age of the subducted slab that underlies the Coast Ranges and average vertical crustal velocities of the western margin of the Coast Ranges for two markedly different time periods, the last 45 years and the last 100 kyr. These vertical crustal velocities are computed from the resurveying of highway bench marks and from the present elevation of shore platforms that have been uplifted in the late Quaternary, respectively. Topography of the Coast Ranges is in part a function of the age and buoyancy of the underlying subducted plate. This is evident in the fact that the two highest topographic elements of the Coast Ranges, the Klamath Mountains and the Olympic Mountains, are underlain by youngest subducted oceanic crust. The subducted Blanco Fracture Zone in southernmost Oregon is responsible for an age discontinuity of subducted crust under the Klamath Mountains. The northern terminus of the topographically higher Klamaths is offset to the north relative to the position of the underlying Blanco Fracture Zone, the offset being in the direction of migration of the fracture zone, as dictated by relative plate motions. Vertical crustal velocities at the coast, derived from bench mark surveys, are as much as an order of magnitude greater than vertical crustal velocities derived from uplifted shore platforms. This uplift rate discrepancy indicates that strain is accumulating on the plate margin, to be released during the next interplate earthquake. In a latitudinal sense, average Coast Range topography is relatively high where bench mark-derived, short-term vertical crustal velocites are highest. Because the shore platform vertical crustal velocites reflect longer-term, permanent uplift, we infer that a small percentage of the interseismic strain that accumulates as rapid short-term uplift is not recovered by subduction earthquakes but rather contributes to rock uplift of the Coast Ranges. The conjecture that permanent rock uplift is related to interseismic uplift is consistent with the observation that those segments of the subduction zone subject to greater interseismic uplift rates are at approximately the same latitudes as those segments of the Coast Ranges that have higher magnitudes of rock uplift over the long term.

Yellowstone in Yukon: The Late Cretaceous Carmacks Group

The Late Cretaceous Carmacks Group, a thick subaerial volcanic succession that once covered much of southwest Yukon, was deposited on an uplifted terrane and is divisible into a lower fragmental unit and an upper flood basalt unit. Coeval hydrothermal activity resulted in widespread alteration and gold mineralization. The lavas are shoshonites, enriched in large ion lithophile and light rare earth elements, but depleted in high field strength elements. Ankaramitic absarokite flows in the upper Carmacks Group range up to 15 wt% MgO, requiring a high liquidus temperature (1400 °C at 1 bar, dry). High K 2 O contents (>3%) of these magnesian lavas indicate that the potassic character of the volcanic suite was established in the mantle. Although previously interpreted as subduction related, the Carmacks Group was erupted during a Cordilleran-wide magmatic lull and lacks coeval calc-alkalic batholiths. The lavas are petrologically similar to plume-related Eocene to Pliocene potassic lavas of the western United States. New paleomagnetic collections, combined with previous work, place the Carmacks Group 17.2° ± 6.5° (1900 ± 700 km) south of its present position relative to the craton during deposition, near the paleolocation of the Yellowstone hotspot. The spatial coincidence, similarity of tectonic setting, and lithologic similarity of the Carmacks Group and Yellowstone volcanic successions suggest that the Carmacks Group is the 70 Ma effusion of the Yellowstone hotspot. Subsequent northward displacement of the Carmacks Group is attributed to coupling with the Kula plate. Correlation of the Carmacks Group and the Yellowstone hotspot fixes the paleolatitude and the paleolongitude of the terranes of the northern Intermontane belt at 70 Ma.

Relative motions between Eurasia and North America in the Bering Sea region

Knowledge of the relative motion of the North American and Eurasian plates during the late Mesozoic and Cenozoic provides insight into the observed timing and style of deformational events in the Bering Sea region. Periods of strong convergence from ≈ 70 to ≈ 50 Ma (Maastrichtian to Paleocene) are correlated with compressional deformation between the Chukotsk Peninsula and northern Alaska and movement along the Denali fault. The convergence may also be the cause of a previously proposed counterclockwise rotation of western Alaska. Transform motion between these plates from 50 to 38 Ma (Middle to Late Eocene) correlates with subsidence of the Bering Shelf, creation of a series of pull-apart basins along the Bering margin and cessation of calc-alkaline volcanism in western Alaska. Slight compressive convergence from 37 Ma to the present may be responsible for the anticlinal deformation of basin filling sediments in the Anadyr and Khatyrka basins.

Remagnetization during Cretaceous Normal Superchron in Eastern San Juan Islands, WA: implications for tectonic history

Abstract The Eastern San Juan Islands expose an island arc ophiolite sequence and one or more ocean floor terranes. Cover rocks of the former, including red radiolarian tuff and arc-derived sandstone, have no penetrative fabric and lack high-pressure minerals. In contrast, pillow basalts and ribbon cherts of the latter are strongly deformed and the overlying metagraywacke is folded with a penetrative axial plane cleavage. All ocean floor rocks have aragonite in veins, indicative of higher-pressure conditions than experienced by the island arc rocks. A downward magnetization is common to the ocean floor rocks, consistent with widespread remagnetization during the Cretaceous normal superchron. This magnetization is carried by magnetite and, in red rocks, by hematite; fluid flow during or after deformation and high P metamorphism likely facilitated remagnetization at low temperature. The magnetic directions are somewhat scattered but most are moderately to steeply down to the southeast. They are far from directions expected with or without northward transport of Baja BC. Rotation of observed directions into agreement with those expected for either present or more southerly locations also rotate structural elements in the host rocks. Three alternative rotation histories explored here all produce structural reconstructions that differ considerably from the present structure. A tectonic history of the eastern San Juan Islands based on any of these reconstructions would be superior to one based on present attitudes of structures. This is because at least a constraint that the restored paleomagnetic directions match a paleomagnetic field direction is satisfied. Choices among alternative reconstructions would become clearer with better understanding of relative ages of specific structures and their magnetizations. Improved geologic understanding of the regional setting where the rocks could have been deformed and remagnetized would also help.

Paleomagnetism and tectonics of the Crescent Formation, northern Olympic Mountains, Washington

A stable prefolding magnetization has been discovered in pillow basalts of the Eocene lower Crescent Formation of the northern Olympic Mountains. The curved outcrop pattern of the Crescent Formation has been the target of several unsuccessful studies to test for oroclinal bending. The success of this study is due, in part, to the development of a small-diameter electric core drill for sampling the fractured rims of basalt pillows. Thermal demagnetization produced stable endpoints by 580°C in 12 of the 34 sites sampled (large within-site scatter was common in the remaining sites). Among the accepted sites, within-site scatter was small and correction for bedding tilt significantly reduced the scatter between sites. The mean paleomagnetic pole for newly sampled pillow basalts (86.4° north latitude, 170.0° east longitude, A95=16.5°) is indistinguishable from the early to middle Eocene pole expected for North America. Including previous results from sites in subaerial basalt of the upper Crescent Formation in and near the easterm Olympic Mountains results in a more regional paleomagnetic pole (80.7° north latitude, 192.0° east longitude, A95 =8.0°, N=46) that shows no significant rotation (0.8° ± 14.4°) or poleward displacement (−3.6 ± 8.5). Analysis of the magnetic mineralogy suggests that the remanence is early, probably primary. The pole, therefore, should be valid for tectonic interpretation of the region. A circular distribution of virtual geomagnetic poles after correction for bedding tilt supports the hypothesis that the northern Crescent Formation experienced deformation due to the rise, in a domelike fashion, of the sediments of the Olympic Core terrane. Erosion of a partial dome open to the west could have produced the curvature seen in the outcrop pattern of the Crescent Formation. The lack of significant rotation of the northernmost Coast Ranges contrasts with the net clockwise rotation seen to the south. The difference could be that irrotational northward translation (paleomagnetically insignificant yet geologically important), driven by oblique convergence, was accommodated by the north-south trending strike-slip faults to the east of the Olympic Mountains.

Relative Motions between Eurasia and North America in Bering Sea Region: ABSTRACT

End_Page 665------------------------------Knowledge of the relative motion of the North American and Eurasian plates during the late Mesozoic and Cenozoic provide insight into the observed timing and style of deformational events in the Bering Sea region. Periods of strong convergence between the North American and Eurasian plates (approximately 70-50 Ma, Maestrichtian to Paleocene) are correlated with compressional deformation between the Chukotsk Peninsula and northern Alaska and the initiation of development and movement along the Denali fault. The convergence also may be the cause of a previously proposed counterclockwise rotation of the Alaska Peninsula. Transform motion between these plates (approximately 50-37 Ma, middle to upper Eocene) correlates with subsidence of the Bering shelf and creation of a series of pull-ap rt basins (Anadyr, Amak, Bristol Bay, Navarin, Pribilof, and St. George) along the Bering margin. Slight compressive convergence from 37 Ma to present may be responsible for the anticlinal deformation of basin-filling sediments in the Anadyr and Khatyrka basins reported by McLean. A correlation between the velocities with which the two plates moved away from each other in the North Atlantic and the geometry of interaction in the Bering Sea region can be seen for most of the Tertiary. End_of_Article - Last_Page 666------------