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At least 775 records · Page 43Linked to original sources
USGS2012

A seismic refraction study of the Oregon Cascades

A 275‐km‐long reversed refraction profile in the Oregon Cascades, two shallow earthquakes of magnitude 5 in southern Washington, a shallow earthquake of magnitude 4.6 in northern California, and a previously published analysis of the Bouguer gravity field are used to develop a crustal P wave velocity model for the Oregon Cascades. Travel time analysis of the refraction profile indicates a crustal structure characterized by surface layers with P wave velocities that vary from 2.9 to 5.2 km/s and thicknesses that vary from 2.5 to 5.0 km, upper crustal velocities of 6.1 to 6.5 km/s between the depths of 3 and 29 km, lower crustal velocities near 7.0 km/s between the depths of 29 and 44 km, and a mantle reflector at a depth of 44 km. Comparison with synthetic seismograms supports this structure and shows that the lower crustal and Mono transitions can be better modeled by continuous velocity gradients than by first‐order discontinuities. Arrival times from two shallow earthquakes in southern Washington in 1981 across 14 telemetry stations in the Oregon Cascades show an apparent velocity of 7.62 km/s. On the basis of a previously published gravity analysis, the mean north‐south component of the dip to the crust‐mantle boundary is estimated at 1° down dip to the south. The apparent velocity from the earthquakes and the dip estimate from gravity indicate that the true Pn velocity is 7.70 km/s. Arrival times from a shallow earthquake in northern California in 1978 across telemetry stations in the Washington Cascades are consistent with this model and indicate that the upper mantle velocity reaches 8.20 km/s at a depth of approximately 100 km.

Oregon↗
USGS2012

Introduction and digest to the Special Issue on Chemical Effects of Water on the Deformation and Strengths of Rocks

The important role of pore pressure in promoting such brittle processes as cataclasis, hydraulic fracturing, large‐scale faulting, and earthquakes within the crust is widely accepted in geology and geophysics [, 1957; , 1959; , 1958; ., 1963; , 1968; ., 1968; ., 1976; , 1973, 1980; , 1981]. Provided that fluid pressure is fully communicated with rock pore space, the effective normal stresses that control crack growth, macroscopic fracture, and friction are reduced by the magnitude of the fluid pressui'e. Beyond this physical effect of pore fluids, there are chemical effects of water on the strength of rocks that are also important in governing differential stresseg and flow in the continental crust. Some of these chemical effects of water on rock deformation have long been recognized.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Experimental deformation of topaz crystals: Possible embrittlement by intracrystalline water

Crystallographically oriented single‐crystal prisms of gem quality topaz (composition AlSiO (OH− F) where x = 0.04 ± 0.01) were deformed at a confining pressure of 1.50 GPa, a temperature of 800°C, and a strain rate of 2×10 s. Under nearly identical conditions, all crystals of anhydrous rock‐forming minerals that have been tested to date, such as olivine, quartz, feldspars, pyroxenes, and refractory oxides, deform plastically; in contrast, our topaz crystals failed by brittle fracture regardless of the orientation of the compression direction. No optical evidence for plastic deformation was detected. Another suite of experiments with compression perpendicular to the (001) cleavage at = 100°–950°C and a strain rate of 2×10 s displayed two regimes of behavior: (1) at >400°C, fracture strength was independent of temperature, and fracture occurred on one or two surfaces parallel to {103}; (2) at <400°C, the fracture strength increased rapidly with decreasing temperature, no macroscopic stress drop was observed, and many closely spaced conjugate fractures formed on (103) and (103). The anomalous brittleness of topaz compared to anhydrous silicate and oxide crystals indicates that intracrystalline “water” plays a role in the embrittlement. We suggest that water within the topaz crystals promotes fracture in ways similar to the mechanisms of slow crack growth aided by environmental moisture.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Effects of compression direction on the plasticity and rheology of hydrolytically weakened synthetic quartz crystals at atmospheric pressure

A hydrothermally grown synthetic quartz crystal with 370±60 ppm hydroxyl impurity was cut into right rectangular prisms in eight crystallographic orientations. We compressed the prisms under constant axial force corresponding to a uniaxial stress of 140.0±0.5 MPa, and temperatures of 510° and 750°C. All but one of the samples sustained permanent axial strains of 2–3%. We established the operating slip systems from specimen shape change, slip bands and dislocation etch pits on polished surfaces, crystallographic orientation changes, stress optical features in thin sections, and transmission electron microscopy. The observed creep behavior and plasticity divided the samples into three groups: (1) Crystals compressed at 45° to [0001] and [ ] and those compressed ⊥( ) and ⊥( ) deformed principally by slip parallel to [0001]. Creep rates were relatively high and were not strongly sensitive to test temperature. Dislocation arrays approximately parallel to ( ) are common. Dislocation loops are elongate parallel to [0001], indicating that the edge segments were more mobile than the screw segments. (2) The second group of samples were loaded normal to [0001] in three orientations: ⊥( ), ⊥( ), and at 45° to ( ). These samples deformed primarily by { } 〈a〉 slip with some evidence for secondary slip on the other systems. They were more creep resistant than the first group and displayed a much higher sensitivity of creep rate to test temperature. Dislocation loops are very elongate parallel to [0001], indicating that the screw dislocation segments were much more mobile than the edge segments. (3) A sample compressed parallel to [0001] at 750°C crept at a barely detectable rate (∼10 s) and no optical scale slip features were observed. These results confirm our earlier work on one orientation each from groups 1 and 2, which indicated a strong creep anisotropy for this same crystal. This creep anisotropy parallels a remarkably similar anisotropy in the diffusivity of impurities in quartz, suggesting a causal relationship between impurity diffusion and creep associated with hydrolytic weakening. Appendix Tables A1‐A5 are available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document B84‐004; $2.50. Payment must accompany order.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Deformation in the White Mountain seismic gap, California-Nevada, 1972-1982

A 100×40 km trilateration network extending from Bishop, California, to near Hawthorne, Nevada, crosses the east end of the Long Valley caldera, site of renewed magma inflation in the 1979–1980 interval, and spans most of the White Mountain seismic gap. The network was surveyed in 1972, 1973, 1976, 1979, 1980, and 1982. The 1980 survey may be contaminated by a scale error. In addition, leveling surveys across the caldera have been run in 1932, 1957, 1975, 1980, 1982, and 1983. Interpretation of the deformation is complicated by the occurrence of the May 1980 Mammoth Lakes earthquake sequence (four earthquakes M L ≥6) at the south edge of the caldera as well as other moderate earthquakes within the White Mountain seismic gap. The vertical deformation is largely accounted for by 0.10‐ to 0.15‐km 3 expansion of a spherical magma chamber 8–10 km beneath the resurgent dome within the Long Valley caldera sometime between July 1979 and September 1980 with an additional expansion of perhaps 0.05 km 3 between September 1980 and July 1982. Some additional sources of deformation within the aftershock zone of the Mammoth Lakes earthquakes seem to be required to explain the horizontal deformation. We show that right‐lateral slip on vertical faults extending WNW from each of the three largest earthquakes in the Mammoth Lakes sequence provides the required additional deformation, but this solution is by no means unique. There are simply too few data to define the rather complex deformation that apparently occurred within the aftershock zone. There is little doubt, however, that inflation of a magma chamber beneath the resurgent dome within the Long Valley caldera was involved in the deformation.

California↗
USGS2012

Earthquake swarm in Long Valley caldera, California, January 1983: Evidence for dike inflation

The 1982–1983 deformation observed by trilateration and leveling surveys across the Long Valley caldera is apparently related to the 8.5‐km‐long by 8‐km‐deep vertical rupture surface defined by the January 1983 earthquake swarm that occurred in the south moat of the caldera. The observed deformation can be explained as follows. In late 1982, 0.03 km 3 of magma was injected into a dike that dips 30° northward from the bottom of the rupture surface. The downdip dimension of this dike is 8 km. The dike inflation accounts for the uplift observed across the caldera as well as some of the horizontal deformation. Inflation of the dike generated a tension of about 3 bars across the vertical plane that was to become the rupture surface of the January swarm. This reduced the frictional stress on the rupture plane and perhaps triggered the slip that caused the January swarm. Right‐lateral slip (0.22 m) on the uppermost 2 km of the rupture plane during and after the January swarm accounts for the additional horizontal deformation observed. The model can be improved marginally if strike slip is admitted over the entire rupture surface and 0.006 km 3 of magma is injected along that surface in the depth interval 3–8 km. The improvement in the model fit, however, is not sufficient to require shallow injection of magma. Thus we conclude that inflation of a dike at depth (8–12 km) dipping northward beneath the resurgent dome plus shallow right‐lateral slip on the rupture surface is a simple, but not unique, explanation of the observed deformation and seismicity.

California↗
USGS2012

Mineralogy and geochemistry of a sediment‐hosted hydrothermal sulfide deposit from the Southern Trough of Guaymas Basin, Gulf of California

Samples dredged from a 15‐m‐high hydrothermal mound atop the flat turbidite pond in the Southern Trough of Guaymas Basin consist of pyrrhotite‐rich massive sulfide, barite, barite + calcite, talc, and opaline silica as well as substrate material composed of fossiliferous, clay‐rich ooze. An 11‐m‐long sediment core taken near the dredge site shows increasing hydrothermal alteration with depth; anhydrite‐filled fractures near the base of the core appear to be channels for hydrothermal discharge. Oxidation of the sulfide‐rich samples to an assemblage of geothite, lepidocrocite, and amorphous Fe oxyhydroxide is ubiquitous. Compared to other massive sulfide deposits on sediment‐starved oceanic ridges, the hydrothermal deposit dredged in Guaymas Basin has a high pyrrhotite/pyrite ratio, a low Zn sulfide and combined ore metal (Cu + Zn + Pb + Ag + Cd) content, and a greater abundance of sulfate, carbonate, and silicate phases. Venting hydrothermal solutions are alkaline with moderately high p H; high Ca, Ba, and SiO 2 content; low ƒS 2 and ƒo 2 ; and very low transition metal content. Disequilibrium assemblages of pyrrhotite and sulfate minerals form during rapid mixing of this evolved vent fluid with ambient bottom waters at the discharge site. Talc is formed at a temperature near 270°C by mixing or entrainment of Mg‐rich bottom water or pore fluid with upwelling hydrothermal fluid that is saturated with silica. Calcite may precipitate from the alkaline, Ca‐rich fluid during degassing of CO 2 . The minimum temperature range for sulfide and nonsulfide deposition is approximately 190°–326°C. The composition of hydrothermal deposits, vent solutions, and altered sediment requires that circulating fluids evolve during deep penetration into the basaltic basement complex, further interaction with the organic‐and carbonate‐rich sediment pile, and near‐surface mixing with ambient seawater. Although the stable assemblage albite‐epidote‐clinochlore present at depth in the sediment pile requires very low dissolved Mg and Fe in the altering fluid, the addition of Mg to deeply buried sediment indicates significant recharge of the system by Guaymas Basin bottom water.

Southern Trough of Guaymas Basin↗
USGS2012

A plate flexure approximation to postseismic and interseismic deformation

The rather large postseismic deformation that is associated with two‐dimensional dip‐slip faulting in the lithosphere is related to the bending of a free plate generated by dip‐slip faulting. In the absence of gravity, asthenosphere relaxation eventually permits the faulted lithosphere to assume the dihedral configuration of a faulted free plate. For thrust faulting, the faulted area is depressed into the asthenosphere, and the flanks of the plate slope uniformly upward. In the presence of gravity, buoyancy forces act upon the plate, and the ultimate ( t →∞) postseismic uplift is approximated ( y /α > 0.2) by w 1 exp (− y /α) (cos y /α ‐ sin y /α), where w 1 is an explicit function of the fault parameters and lithosphere properties, y is the horizontal distance from the downdip end of the fault, and α is the flexural parameter for the lithosphere plate. The relaxed‐asthenosphere response is the sum of the coseismic deformation (unrelaxed‐astheriosphere response) and this ultimate postseismic deformation. The annual, steady deformation associated with strain accumulation at a subduction zone is simply the relaxed‐asthenosphere response to virtual, normal slip on the main thrust zone of an amount equal to the annual plate convergence. From these relations we have estimated the deformation (strain and uplift) expected along the Pacific coast of northern Honshu. The estimated deformation is about twice the observed deformation, but the predicted geographical distribution of the deformation is similar to the observed deformation.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

The Steens Mountain (Oregon) geomagnetic polarity transition: 1. Directional history, duration of episodes, and rock magnetism

The thick sequence of Miocene lava flows exposed on Steens Mountain in southeastern Oregon is well known for containing a detailed record of a reversed‐to‐normal geomagnetic polarity transition. Paleomagnetic samples were obtained from the sequence for a combined study of the directional and intensity variations recorded; the paleointensity study is reported in a companion paper. This effort has resulted in the first detailed history of total geomagnetic field behavior during a reversal of polarity. A comparison of the directional variation history of the reversed and normal polarity intervals on either side of the transition with the Holocene record has allowed an estimate of the duration of these periods to be made. These time estimates were then used to calculate accumulation rates for the volcanic sequence and thereby provide a means for estimating time periods within the transition itself. The polarity transition was found to consist of two phases, each with quite different characteristics. At the onset of the first phase, a one‐third decrease in magnetic field intensity may have preceded the first intermediate field directions by about 600 years. Changes in field direction were confined near the local north‐south vertical plane when the actual reversal in direction occurred and normal polarity directions may have been attained within 550±150 years. The end of the first phase of the transition was marked by a brief (possibly 100–300 years) period with normal polarity and a pretransitional intensity which suggests a quasi‐normal dipole field structure existed during this interval. The second phase of the transition was characterized by a return to very low field intensities with the changes in direction describing a long counterclockwise loop in contrast to the earlier narrowly constrained changes. This second phase lasted 2900±300 years, and both normal directions and intensities were recovered at the same time. Both directional and intensity data document very erratic geomagnetic field behavior during the polarity transition. Changes in magnetic field direction were variable and occurred either (1) in a regular, progressive manner, (2) with sudden, extremely rapid angular changes (58°±21°/year), or (3) with little or no movement for periods of the order of 600±200 years. Changes in magnetic intensity occurred in a like manner and were sometimes correlated with changes in direction, but during other periods both directional and intensity changes occurred independently. Directional changes following the polarity transition occurred in a seemingly normal manner, although intensity fluctuations attest to some instability of the newly reestablished dipole.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

The Steens Mountain (Oregon) geomagnetic polarity transition, 2. Field intensity variations and discussion of reversal models

We carried out an extensive paleointensity study of the 15.5±0.3 m.y. Miocene reversed‐to‐normal polarity transition recorded in lava flows from Steens Mountain (south central Oregon). One hundred eighty‐five samples from the collection whose paleodirectional study is reported by Mankinen et al. (this issue) were chosen for paleointensity investigations because of their low viscosity index, high Curie point and reversibility, or near reversibility, of the strong field magnetization curve versus temperature. Application of the Thellier stepwise double heating method was very successful, yielding 157 usable paleointensity estimates corresponding to 73 distinct lava flows. After grouping successive lava flows that did not differ significantly in direction and intensity, we obtained 51 distinguishable, complete field vectors of which 10 are reversed, 28 are transitional, and 13 are normal. The record is complex, quite unlike that predicted by simple flooding or standing nondipole field models. It begins with an estimated several thousand years of reversed polarity with an average intensity of 31.5±8.5 μT, about one third lower than the expected Miocene intensity. This difference is interpreted as a long‐term reduction of the dipole moment prior to the reversal. When site directions and intensities are considered, truly transitional directions and intensities appear almost at the same time at the beginning of the transition, and they disappear simultaneously at the end of the reversal. Large deviations in declination occur during this approximately 4500±1000 year transition period that are compatible with roughly similar average magnitudes of zonal and nonzonal field components at the site. The transitional intensity is generally low, with an average of 10.9±4.9 μT for directions more than 45° away from the dipole field and a minimum of about 5 μT. The root‐mean‐square of the three field components X , Y , and Z are of the same order of magnitude for the transitional field and the historical nondipole field at the site latitude. However, a field intensity increase to pretransitional values occurs when the field temporarily reaches normal directions, which suggests that dipolar structure could have been briefly regenerated during the transition in an aborted attempt to reestablish a stationary field. Changes in the field vector are progressive but jerky, with at least two, and possibly three, large swings at astonishingly high rates. Each of those transitional geomagnetic impulses occurs when the field intensity is low (less than 10 μT) and is followed by an interval of directional stasis during which the magnitude of the field increases greatly. For the best documented geomagnetic impulse the rapid directional change corresponds to a vectorial intensity change of 6700±2700 nT yr −1 , which is about 15–50 times larger than the maximum rate of change of the nondipole field observed during the last centuries. The occurrence of geomagnetic impulses seems to support reversal models assuming an increase in the level of turbulence within the liquid core during transitions. The record closes with an estimated several thousand years of normal polarity with an average intensity of 46.7±20.1 μT, agreeing with the expected Miocene value. However, the occurrence of rather large and apparently rapid intensity fluctuations accompanied by little change in direction suggests that the newly reestablished dipole was still somewhat unstable.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Implications of silicic vent patterns for the presence of large crustal magma chambers

On the basis of the distribution of silicic vents, many volcanic fields can be grouped with (1) igneous systems that may be small and whose vent locations are controlled by regional tectonics, (2) those that include sizable crustal magma bodies which erupt at sites determined by their anomalous local stress fields, or (3) relatively small volume systems that are transitional between categories 1 and 2. Linear vent patterns that are aligned normal to the regional least principal stress (σ 3 ) commonly are associated with absence of evidence for large, shallow magma bodies. The Coso volcanic field and the Inyo‐Mono domes in California and probably the South Sister area in Oregon are examples of such systems. The 1960 dacitic fissure eruption at Cordón Caulle in southern Chile evidently is linked to tectonic stress relaxation associated with the great earthquake that occurred 48 hours before the eruption began. Large, shallow magma chambers are thought to perturb the local stress field so that areal patterns of silicic vents are diffuse, radial, or arcuate. Such systems may erupt great volumes of pyroclastic material catastrophically and produce large calderas. Well‐preserved examples of late precaldera leaks of silicic magma occur at Long Valley, California, and Mount Mazama (Crater Lake), Oregon. Other possible examples are noted. Some of these, those which formed calderas smaller than Crater Lake, apparently were preceded by silicic eruptions from aligned vents. Bearing in mind that there exists a transitional group between tectonically controlled small systems and very large magma chambers, vent distributions can be useful in evaluating potential volcanic hazards for silicic volcanic fields.

California↗
USGS2012

Magmatic inclusions in silicic and intermediate volcanic rocks

Fine‐grained ellipsoidal inclusions from a few millimeters to over l m in size are present in many intermediate to silicic lava flows and domes. Only recently has it become widely accepted that such inclusions are chilled blobs of magma. Their magmatic origin is manifested by vesicularity and high groundmass porosity, by ellipsoidal shapes, by mingling at contacts with the host, and by textural evidence for groundmass crystallization in an undercooled state. Groundmass textures reflect the degree of undercooling, which is a function of thermal contrast (Δ T ) between inclusions and host magmas before mingling. Compositions of inclusions are invariably more mafic than their hosts and indicate that the magmatic systems were compositionally zoned. Many inclusions were formed from hybrid magmas. Four examples are given in order to document effects of decreasing Δ C (compositional contrast) and Δ T , as hosts for mafic andesitic to andesitic inclusions vary from high‐silica rhyolite (HSR) to silicic andesite. In the HSR example from the Coso volcanic field, California, inclusions track the evolution of zonation in a long‐lived magmatic system. At the other extreme, andesitic inclusions in silicic andesite from Crater Lake, Oregon, demonstrate zonation of a system of low Δ T and Δ C and show effects of in situ differentiation by gas filter‐pressing of residual liquid. Inclusions form when a relatively small proportion of mafic magma comes into contact with a larger proportion of silicic magma. Several mechanisms by which the magmas mix have been proposed: buoyant rise of vesiculated mafic magma or convective stirring at the interface between mafic and overlying silicic magma, forcible injection of mafic magma, diking of fractured silicic magma, and mixing during ascent in a conduit. No single mechanism explains all field observations. Many inclusions in plutons are analogous to magmatic inclusions in volcanic rocks but are likely to have suffered greater chemical and textural change during comparatively slow postentrapment cooling. Magmatic inclusions are evidence of open system behavior of the differentiated parts of magma bodies, and the inclusions themselves may behave as open systems where Δ T is low.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Paleomagnetism of Middle Tertiary volcanic rocks from the Western Cascade Series, northern California

The Western Cascade Series (WCS) is a 3.5‐km‐thick, crudely homoclinal (east dipping) calcalkaline volcanic sequence of mid‐Oligocene to early Miocene age that crops out near the southern tip of the Cascade Range in northern California. The mean direction of remanent magnetization in the WCS is D, 4.9°; I, 57.6° (N, 53; k, 14.4; α 95 , 5.3°). When compared to a reference direction for the North American craton, the WCS direction indicates that the southern Cascade Range has rotated 14.0° ± 9.0° since the WCS accumulated. A difference in mean direction between the lower and upper halves of the WCS suggests that much of this rotation occurred during the late Oligocene. Six other paleomagnetic studies of rock units of an age roughly comparable to the WCS also are available from western Oregon, northwestern California, and southwestern Washington. All show statistically significant clockwise rotation (inclinations of all but one are concordant). Comparison of directions indicates that the Pacific Northwest did not rotate as a rigid body, but neither did it behave as a collection of small, independently rotating domains. The amount of rotation found throughout the area increases to the west or northwest, suggesting a driving force for rotation operating at the continental margin.

California↗
USGS2012

Paleomagnetic evidence for the timing of collapse and resurgence of the Lake City Caldera, San Juan Mountains, Colorado

Rocks of the 23.1‐m.y.‐old Lake City caldera consist of the compositionally zoned Sunshine Peak Tuff, postcollapse intracaldera lava flows, and resurgent quartz syenite intrusions. Declinations of reversely magnetized ( I = −45° to −75°) Sunshine Peak Tuff change from easterly ( D = 93°–130°) throughout most of the tuff to southerly ( D = 195°–207°) within the late eruptive phases. The postcollapse lava flows are also reversely magnetized, and their emplacement appears to have spanned resurgence of the caldera. In contrast, the uppermost levels of the resurgent intrusion record two distinct components of magnetization: a reversed component isolated by thermal demagnetization typically at temperatures greater than 400°C and a normal component defined by thermal demagnetization at lower temperatures. A baked‐contact test implies that emplacement and initial cooling of the intrusion occurred during a time of reversed polarity and that continued cooling at lower blocking temperatures took place during normal polarity. Assuming that the reversed directions of the extrusive and intrusive rocks record a single period of reversed polarity as suggested by the paleomagnetic and radiometric age data, the time for caldera development (from eruption of the ash flow tuffs to emplacement of the resurgent intrusion) was less than about 300,000 years on the basis of best estimates of durations of reversed polarities in the early Miocene. The estimated 300,000‐year time span for the development of the Lake City caldera is very close in duration to the radiometrically determined time spans of much younger (Pleistocene) calderas having similar dimensions and similar volumes of erupted material.

Colorado↗
USGS2012

Sedimentary deposits in the northern lowland plains, Mars

The lowland plains on Mars have surfaces marked by large polygonal fracture patterns. It was recently proposed that the fracture patterns were developed on sedimentary deposits from outflow channels. We support this hypothesis because of the following observations. (1) Polygonal fracture patterns tend to occur in low areas on Mars that apparently received influx of sediments; the area of northern plains, in which the fractures occur, also forms a regional low. (2) The fracture patterns are concentrated near reentrants of the plains projecting into the southern highlands. Not only are these reentrants associated with outflow channels, but channels and polygonally fractured deposits have similar ages. (3) Sinuous ridges at the mouths of Martian outflow channels resemble ridges in Antarctic ice streams and ice shelves; the similarity suggests that the Martian material was transported from the southern highlands toward the northern plains, as the Antarctic material is transported from the continent toward the adjacent ocean. Overall, it appears that a massive transfer of material took place midway in Martian history, either through the outflow channels or elsewhere along the northern highland scarp, and that this material probably formed the deposits now characterized by polygonal fracture patterns.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Paleomagnetism of the Tertiary Clarno Formation of central Oregon and its significance for the tectonic history of the Pacific Northwest

The Clarno Formation, a mostly Eocene and partly early Oligocene sequence of andesitic lavas and volcaniclastic rocks, is the oldest Tertiary formation exposed in north central Oregon. Remanent magnetization directions at 46 sites in the lavas provide a paleomagnetic pole at 84°N, 278°E with a 95% confidence cone of 7°. Comparison of this pole with the North American reference pole for Eocene time indicates that the Clarno Formation has rotated 16° clockwise with an uncertainty of 10° but has undergone no significant latitudinal displacement. Further comparison with paleomagnetic data from the Miocene Columbia River Basalt Group and Steens Basalt shows that large‐scale block rotations in the eastern Columbia Embayment ceased sometime between 38 m.y. and 15 m.y. A paleogeographic reconstruction for 38 m.y. is offered which is consistent with the observed rotation of the Clarno and with the other paleomagnetic data from the Pacific Northwest, which are briefly reviewed. This reconstruction shows that it is possible to account for virtually all of the paleomagnetically indicated rotations in pre‐Miocene Tertiary rocks of the Pacific Northwest by an extensional tectonic model. As a consequence, only part of the rotations in the pre‐Tertiary rocks of the eastern Columbia Embayment need to be the result of accretionary tectonics. The initiation of magmatism in the eastern Columbia Embayment that is represented by the Clarno Formation itself may have been contemporaneous with the beginning of crustal extension in the Pacific Northwest.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Petrology and geochemistry of alkali gabbronorites from Lunar Breccia 67975

Clasts of an unusual type of lunar highlands igneous rock, alkali gabbronorite, have been found in Apollo 16 breccia 67975. The alkali gabbronorites form two distinct subgroups, magnesian and ferroan. Modes and bulk compositions are highly varied. The magnesian alkali gabbronorites are composed of bytownitic plagioclase (OrAn), hypersthene (WoEn), augite (WoEn), a silica mineral, and trace Ba‐rich K‐feldspar. The ferroan alkali gabbronorites are composed of ternary plagioclase (OrAn), pigeonite (WoEn), augite (WoEn), Ba‐rich K‐feldspar, and a silica mineral. Trace minerals in both subgroups are apatite, REE‐rich whitlockite, and zircon. The magnesian and ferroan alkali gabbronorites appear to have formed by progressive differentiation of the same, or closely related, parent magmas; the compositional data indicate that these magmas were REE‐rich. The ternary plagioclase is probably a high‐temperature metastable phase formed during crystallization. In composition and mineralogy, the 67975 alkali gabbronorites show many similarities to Apollo 12 and 14 alkali norites, alkali gabbronorites, and alkali anorthosites, and all these rocks together constitute a distinctive alkali suite. In addition, the alkali gabbronorites show some similarities to KREEP basalts, Mg‐norites, and some felsites. These data suggest genetic links between some or all of these types of pristine rocks. Two types of relationships are possible. The first is that alkali‐suite rocks crystallized in plutons of KREEP basalt magma, and KREEP basalts are their extrusive equivalents. The second is that the alkali‐suite rocks and some felsites all crystallized in plutons of Mg‐norite parent magmas, and KREEP basalt magmas formed by remelting of these plutons. Additional studies are needed to resolve which of these hypotheses is correct.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Remagnetization of the Coast Range Ophiolite and Lower Part of the Great Valley Sequence in Northern California and Southwest Oregon

Overprinted magnetizations have been found at four localities in the Middle Jurassic Coast Range ophiolite and the overlying Upper Jurassic and Lower Cretaceous Great Valley sequence in northern California and at one locality in the partially correlative Lower Cretaceous Days Creek Formation in southwest Oregon. At Del Puerto Canyon, on the east side of the Diablo Range, a pilot study of the upper Jurassic Lotta Creek Formation gives in situ paleomagnetic directions grouped around the present geomagnetic field, suggesting magnetic overprinting long after deposition and folding. We suggest that the loss of original magnetization could be the result of long burial at about 7 km depth, followed by Late Tertiary uplift; the possibility of chemical remagnetization, however, can not be excluded. Paleomagnetic data from sandstones from the Lower Cretaceous Great Valley sequence in the Wilbur Springs area suggest Cenozoic remagnetization that could be related to Pliocene and Pleistocene volcanic and hydrothermal activity in the area. The results from calcareous concretions in the Wilbur Springs area, from 21 sites from basalts and overlying sedimentary rocks of the Great Valley sequence at Stonyford, and from several sites in various rock types in the Coast Range ophiolite and Great Valley sequence near Paskenta suggest unblocking of magnetization during the long period of burial at depth of 7 km or more and remagnetization during tectonic uplift in the Late Cretaceous or Tertiary. Chemical remagnetization, however, can not be excluded. The paleomagnetic data from six sites in Lower Cretaceous sedimentary rocks of the Days Creek Formation in southwest Oregon, which overlies a dismembered ophiolite, fail the fold test. In situ paleomagnetic directions group near the expected Tertiary field directions. These sedimentary rocks may have been remagnetized during a major Eocene tectonic event known to have occurred in this region. The paleomagnetic results confirm that the Coast Range ophiolite and the seemingly little-deformed Great Valley sequence in northern California, as well as correlative rocks in southwest Oregon have had a complex tectonic history. Once the nature and timing of these events are better understood, the timing of remagnetization may be further constrained, which, in turn, could give further insight into the nature of the tectonic events

California, Oregon↗