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At least 1,171 records · Page 65Linked to original sources
USGS2012

Magnetic susceptibility and relation to initial 87Sr/86Sr for granitoids of the central Sierra Nevada, California

Measurement of the magnetic susceptibility of more than 6000 samples of granitic rock from the Mariposa 1° by 2° quadrangle, which crosses the central part of the Sierra Nevada batholith between 37° and 38°N latitude, shows that magnetic susceptibility values are above 10 −2 SI units in the east and central parts of the batholith and drop abruptly to less than 10 −3 SI units in the western foothills. In a narrow transitional zone, intermediate values (10 −3 to 10 −2 ) prevail. Magnetic susceptibility appears to decrease slightly westward within the zones of both high and low values. Magnetic susceptibility in plutonic rocks is chiefly a function of the abundance of magnetite, which depends, in turn, on the total iron content of the rocks and their oxidation ratio. Lower magnetic susceptibilities of felsic members of Sierran intrusive suites and of some felsic rock units relative to adjacent mafic rock units commonly reflect differences in total iron content, but the differences of magnetic susceptibility that define the regional pattern generally are much larger and are determined chiefly by the oxidation ratios of the rocks. The relatively unaltered condition of the samples and restriction of δ 18 O to the range of +7 to +10.3‰ indicate that neither hydrothermal fluids nor subsolidus alteration were important in modifying oxidation ratios. Correlations of magnetic susceptibility with initial 87 Sr/ 86 Sr suggest that oxidation ratios have been inherited from the source regions for the magmas from which the rocks crystallized. Reduction of Fe 3+ to Fe 2+ by organic carbon or other reducing substances may also have affected magnetic susceptibility.

Journal of Geophysical Research↗
USGS2012

Strain accumulation across the Wasatch Fault near Ogden, Utah

Deformation of a 70 by 40‐km trilateration network spanning the north trending Wasatch fault near Ogden, Utah, has been monitored from 1972 through 1990. All but nine of the 200 measurements are consistent with deformation that is linear in time. We presume that those nine observations are contaminated by some blunder in making the measurements and that deformation is linear in time. The strain rate over the 1972–1990 interval across a 40‐km‐wide zone west of the Wasatch fault (hanging wall block) is adequately approximated by a 0.04 ± 0.01 μstrain/yr N85°E±5° uniaxial extension. Adequate coverage of the footwall block is available only over the 1981–1990 interval, and the strain rate across a 30‐km‐wide zone there is approximated by a 0.07 ± 0.03 μstrain/yr N20°E±12° uniaxial extension. The observed extension of the hanging wall block in the direction normal to fault strike suggests that the Wasatch fault may be listric; a planar fault would produce a zone of contraction normal to the fault at the surface of the hanging wall block. The component of extension parallel to fault strike observed on the footwall block is unexplained.

Utah↗
USGS2012

Seismicity and geometry of a 110‐km‐long blind thrust fault 1. The 1985 Kettleman Hills, California, earthquake

The August 4, 1985, Kettleman Hills earthquake was the third in a sequence of moderate shocks to occur beneath the northern half of a 110‐km‐long fold chain bounding the eastern California Coast Ranges. The 1982 M W =5.4 New Idria, 1983 M W =6.5 Coalinga, and 1985 M W =6.1 Kettleman Hills events define a southward progression of seismic activity beneath the fold. We use teleseismic waveforms, geodetic modeling, hypocenters relocated in a three‐dimensional velocity model, and subsurface structural data to investigate the Kettleman Hills earthquake. The main shock results from motion on a shallowly dipping thrust fault buried at ∼10 km depth. Aftershocks and coseismic fault slip extend 20 km along the fold axis, nearly the full extent of the Kettleman Hills North Dome anticline. Aftershocks occur primarily several kilometers in front of the fault tip and in the core of the anticline. The main shock and several foreshocks occurred at a 2‐km right step in the Quaternary fold axis, which also corresponds to the southern end of the 1983 Coalinga and northern end of the 1985 Kettleman Hills aftershock zones. From this we infer that the step in the fold is caused by an offset or tear in the underlying fault. The scalar seismic moment is 1.6 × 10 18 N m, consistent with the geodetic deformation, and the duration of rupture is 16 s, 3–4 times greater than for the average earthquake with this scalar moment. The slow rate of moment release provides an explanation for the low level of ground shaking and low local magnitude reported for the event. The peak of the geodetic uplift is located 5 km perpendicular to the Quaternary fold axis. We argue that the fault is propagating northeast into the undeformed San Joaquin Valley sediments and that the overlying fold is growing at about 0.5 mm/yr.

California↗
USGS2012

Application of continuum models to deformation of the Aleutian Island Arc

Continuum models were constructed to describe large‐scale deformation of the Aleutian Island Arc over the past 5 m.y. These models consider the island arc as a continuum in the horizontal plane with the velocity boundary condition at the Pacific edge stated as a fraction of Pacific plate convergence transferred to the arc. First, a simple model of uniformly distributed strain is formulated to illustrate the mechanics of continuous deformation. Lineaments along the arc massif rotated about a vertical axis are matched by small‐element rotation calculated from the model. However, this model does not predict across‐arc variations in deformation and produces an unrealistic amount of crustal thickening after 5 m.y. A physically more meaningful model of deformation is the thin viscous sheet model based on averages of stress and rheology throughout the lithosphere. The amount of motion transferred from the Pacific plate to the arc is constrained by the rotated lineaments, while the effective stress‐strain exponent ( n ) and the ability the lithosphere has to sustain crustal thickness contrasts (the Argand number) are independent variables. Primarily, bathymetry, earthquake focal mechanisms, and styles of faulting are used to evaluate the models. The preferred model is one where the amount of motion transferred from the Pacific plate is greater in an arc‐parallel direction than in an arc‐normal direction, producing stresses consistent with strike‐slip faulting at the far western end of the arc and tensional stresses consistent with transverse normal faulting elsewhere in the arc massif. This model agrees with observations of slip vectors by Ekström and Engdahl (1989), who conclude that a portion of the arc‐parallel component of relative plate motion is taken up in the overriding plate. This model implies that compressive stress transferred to the arc is small in comparison to along‐arc shear stress and that stresses conducive to strikeslip faulting are prevalent throughout the arc.

Alaska↗
USGS2012

Use of weather types to disaggregate general circulation model predictions

General circulation models (GCMs) simulate climatic conditions with a grid cell resolution on the order of 100,000 km 2 . This resolution is inadequate to assess the effects of climatic change on water resources at a regional scale. A method has been developed that uses weather-type analysis as a tool to spatially disaggregate GCM predictions to make them useful for water resource studies. The method has been applied to the Delaware River basin to predict the effects of doubling atmospheric carbon dioxide on precipitation patterns in the region. An application of the technique to the Delaware River basin indicates that future climatic conditions will show minimal changes in weather-type frequency, implying that air circulation patterns will remain unchanged. Results of this study indicate that changes in regional precipitation patterns under a doubling of atmospheric carbon dioxide will be a result of within-type changes in weather characteristics.

Journal of Geophysical Research↗
USGS2012

Reconciliation of stress and structural histories of the Tharsis region of Mars

We present a new compilation of the structural and stratigraphic evolution of the Tharsis region of Mars that incorporates recent advances in understanding its stratigraphy, and we introduce a lithospheric deformation model that can account for the observations. The first period in the formation of Tharsis occurred in Late Noachian/Early Hesperian time with the deposition of volcanic plains materials throughout the surrounding highlands (e.g., Lunae Planum) and on the Tharsis rise (which includes the giant volcanoes and surrounding, elevated lava fields). Extensive radial normal faulting occurred on the rise, locally extending outward at Valles Marineris and Tempe Terra, and concentric wrinkle ridges formed along the edge of the rise. This regional deformation appears to have been modulated by a global compressional stress field due to rapid planetary cooling and contraction. The second period occurred during the Late Hesperian/Amazonian with Tharsis volcanism centered on the rise and radial extensional deformation that extended from the center of the rise for thousands of kilometers. We propose a model in which the lithosphere beneath Tharsis consists of a thin elastic crustal cap on the rise that is mechanically detached from the strong upper mantle by a volcanically thickened, hot, weak lower crust. These layers merge into a single cooler, strong lithospheric layer around the edges of the rise. This model is capable of generating large extensional hoop stresses throughout much of the western hemisphere, in agreement with observations. The tectonic interpretation of the stresses predicted by this model requires the reconciliation of extensional strain within narrow grabens and compressional strain within wrinkle ridges with (1) processes in the deeper lithosphere, (2) the sparsity of strike-slip faults, and (3) other global or locally important stress fields. Stresses predicted by global models affect the entire thickness of the lithosphere, and they can be reconciled with narrow, closely spaced grabens that accommodate large amounts of extensional strain in the upper few kilometers of the lithosphere if the grabens are underlain and kinematically linked with dikes or other tension cracks, such as hydrofractures. Deeper levels of the lithosphere can accommodate this strain by elastic expansion if grabens are spaced far apart (many tens to hundreds of kilometers). Mechanical considerations suggest that deformation on faults beneath wrinkle ridges could extend through a significant thickness of the brittle crust. A number of factors, including stresses generated by the addition of overburden, intrusion of dikes, weakness of geologic materials under extension, and the laterally constrained nature of a single lithospheric plate, may have inhibited the formation of strike-slip faults on Mars. Stresses generated from the removal of overburden could have augmented planetwide wrinkle ridge formation during the Late Noachian/Early Hesperian and in Kasei Valles and western Chryse Planitia during the Early Amazonian. The nonuniform distribution of tectonic features around Tharsis can be understood in terms of the concentration of regional stresses and strain near weaker volcanotectonic centers.

Journal of Geophysical Research↗
USGS2012

Limitations of quantitative analysis of deep crustal seismic reflection data: Examples from GLIMPCE

Amplitude preservation in seismic reflection data can be obtained by a relative true amplitude (RTA) processing technique in which the relative strength of reflection amplitudes is preserved vertically as well as horizontally, after compensating for amplitude distortion by near-surface effects and propagation effects. Quantitative analysis of relative true amplitudes of the Great Lakes International Multidisciplinary Program on Crustal Evolution seismic data is hampered by large uncertainties in estimates of the water bottom reflection coefficient and the vertical amplitude correction and by inadequate noise suppression. Processing techniques such as deconvolution, F-K filtering, and migration significantly change the overall shape of amplitude curves and hence calculation of reflection coefficients and average reflectance. Thus lithological interpretation of deep crustal seismic data based on the absolute value of estimated reflection strength alone is meaningless. The relative strength of individual events, however, is preserved on curves generated at different stages in the processing. We suggest that qualitative comparisons of relative strength, if used carefully, provide a meaningful measure of variations in reflectivity. Simple theoretical models indicate that peg-leg multiples rather than water bottom multiples are the most severe source of noise contamination. These multiples are extremely difficult to remove when the water bottom reflection coefficient is large (>0.6), a condition that exists beneath parts of Lake Superior and most of Lake Huron.

Great Lakes↗
USGS2012

Interseismic deformation at the Nankai Trough, Japan, subduction zone

Uplift along the coast of southwestern Japan following the 1944 Tonankai ( M s = 8.0) and 1946 Nankaido ( M s = 8.2) earthquakes has been inferred from the 1950–1985 tide gage records. Although uplift rates vary with geographic position, the temporal dependence at each site can be described as the superposition of an exponentially decaying (time constant ∼5 years) transient, significant only in the first decade postseismic, and a steady interseismic rate. The steady interseismic uplift rate is defined only over the 1956–1985 interval, but the linearity in time ofthat uplift record is such that uplift over the remainder of the interseismic cycle (∼80 years) might reasonably be expected to occur at the same rate. Moreover, the spatial pattern and magnitude of the steady uplift rate are reasonably close to those predicted by a simple dislocation model of subduction. However, the currently observed steady uplift rate is not consistent with the uplift rate inferred from geodetic leveling for the final half of the previous earthquake cycle. Thus, either the hypothesis of a steady interseismic uplift rate after the first decade postseismic is wrong or the strain accumulation pattern for one earthquake need not be the same as for the preceding earthquake on the same segment.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Seismic constraints on the nature of lower crustal reflectors beneath the extending Southern Transition Zone of the Colorado Plateau, Arizona

We determine the reflection polarity and exploit variations in P and S wave reflectivity and P wave amplitude versus offset (AVO) to constrain the origin of lower crustal reflectivity observed on new three-component seismic data recorded across the structural transition of the Colorado Plateau. The near vertical incidence reflection data were collected by Stanford University in 1989 as part of the U.S. Geological Survey Pacific to Arizona Crustal Experiment that traversed the Arizona Transition Zone of the Colorado Plateau. The results of independent waveform modeling methods are consistent with much of the lower crustal reflectivity resulting from thin, high-impedance layers. The reflection polarity of the cleanest lower crustal events is positive, which implies that these reflections result from high-velocity contrasts, and the waveform character indicates that the reflectors are probably layers less than or approximately equal to 200 m thick. The lower crustal events are generally less reflective to incident S waves than to P waves, which agrees with the predicted behavior of high-velocity mafic layering. Analysis of the P wave AVO character of lower crustal reflections demonstrates that the events maintain a constant amplitude with offset, which is most consistent with a mafic-layering model. One exception is a high-amplitude (10 dB above background) event near the base of lower crustal reflectivity which abruptly decreases in amplitude at increasing offsets. The event has a pronounced S wave response, which along with its negative AVO trend is a possible indication of the presence of fluids in the lower crust. The Arizona Transition Zone is an active but weakly extended province, which causes us to discard models of lower crustal layering resulting from shearing because of the high degree of strain required to create such layers. Instead, we favor horizontal basaltic intrusions as the primary origin of high-impedance reflectors based on (1) The fact that most xenoliths in eruptive basalts of the Transition Zone are of mafic igneous composition, (2) indications that a pulse of magmatic activity crossed the Transition Zone in the late Tertiary period, and (3) the high regional heat flow observed in the Transition Zone. The apparent presence of fluids near the base of the reflective zone may indicate a partially molten intrusion. We present a mechanism by which magma can be trapped and be induced to intrude horizontally at rheologic contrasts in extending crust.

Arizona↗
USGS2012

Paleomagnetism and geochronology of 23 Ma gabbroic intrusions in the Keku Strait, Alaska, and implications for the Alexander Terrane

Samples of Tertiary gabbro from 24 sites in the Keku Strait, Alaska, help constrain the displacement history of the Alexander terrane. Step heating experiments on a plagioclase separate from these previously undated intrusions indicate a discordant 40 Ar/ 39 Ar age of 23.1 ± 1.7 Ma. The characteristic magnetization resides in magnetite, is easily isolated by thermal and alternating field demagnetization, and has both normal and reversed polarities. The mean paleomagnetic pole, with no structural correction, is latitude 87.1°N, longitude 141.6°E, A 95 = 10.1°, with N = 20 sites. This pole indicates insignificant tectonic displacement (0.5° ± 8.2° southward) and rotation (0.6° ± 15.2° counterclockwise). We therefore conclude that any northward displacement or vertical axis rotation of the Alexander terrane, and/or tilting in the vicinity of the Keku Strait must have occurred before 23 Ma.

Alaska↗
USGS2012

Paleomagnetism of the Late Triassic Hound Island Volcanics: Revisited

The collision and accretion of the Alexander terrane profoundly influenced the geologic history of Alaska and western Canada; however, the terrane's displacement history is only poorly constrained by sparse paleomagnetic studies. We studied the paleomagnetism of the Hound Island Volcanics in order to evaluate the location of the Alexander terrane in Late Triassic time. We collected 618 samples at 102 sites in and near the Keku Strait, Alaska, from the Late Triassic Hound Island Volcanics, the Permian Pybus Formation, and 23-Ma gabbroic intrusions. We found three components of magnetization in the Hound Island Volcanics. The high-temperature component (component A) resides in hematite and magnetite and was found only in highly oxidized lava flows in a geographically restricted area. We think it is primary, or acquired soon after eruption of the lavas, principally because the directions pass a fold test. The paleolatitude indicated by this component (19.2° ± 10.3°) is similar to those determined for various portions of Wrangellia, consistent with the geologic interpretation that the Alexander terrane was with the Wrangellia terrane in Late Triassic time. We found two overprint directions in the Hound Island Volcanics. Component B was acquired 23 m.y. ago due to intrusion of gabbroic dikes and sills. This interpretation is indicated by the similarity of upper-hemisphere directions in the Hound Island Volcanics to those in the gabbro. Component C, found in both the Hound Island Volcanics and the Permian Pybus Formation, is oriented northeast and down, fails a regional fold test, and was acquired after regional deformation around 90 to 100 Ma. This overprint direction yields a paleolatitude similar to, but slightly higher than, slightly older rocks from the Coast Plutonic Complex, suggesting that the Alexander terrane was displaced 17° in early Late Cretaceous time. The occurrence of these two separate overprinting events provides a satisfying explanation of the earlier puzzling results from the Hound Island Volcanics (Hillhouse and Grommé, 1980). Finally, great-circle analysis of the paleomagnetic data from the Pybus Formation suggests the Alexander terrane may have been in the northern hemisphere in Permian time.

Alaska↗
USGS2012

Inferred depth of creep on the Hayward Fault, central California

A relation between creep rate at the surface trace of a fault, the depth to the bottom of the creeping zone, and the rate of stress accumulation on the fault is derived from Weertman's 1964 friction model of slip on a fault. A 5 ± 1 km depth for the creeping zone on the Hayward fault is estimated from the measured creep rate (5 mm/yr) at the fault trace and the rate of stress increase on the upper segment of the fault trace inferred from geodetic measurements across the San Francisco Bay area. Although fault creep partially accommodates the secular slip rate on the Hayward fault, a slip deficit is accumulating equivalent to a magnitude 6.6 earthquake on each 40 km segment of the fault each century. Thus, the current behavior of the fault is consistent with its seismic history, which includes two moderate earthquakes in the mid-1800's.

Journal of Geophysical Research↗
USGS2012

Paleoseismology along the 1980 surface rupture of the Irpinia Fault: Implications for earthquake recurrence in the southern Apennines, Italy

The Irpinia fault was the source of the M s 6.9 1980 Irpinia earthquake and produced the first unequivocal historical surface faulting in Italy. Trenching of the 1980 fault scarp at Piano di Pecore, a flat intermontane basin about 5 km south of the 1980 instrumental epicenter, provides the first data on earthquake recurrence intervals, slip per event, and slip rate on a major normal fault in the Southern Apennines fault zone. The trenches exposed evidence of four pre-1980 paleoearthquakes that occurred during the past 8600 years. A best estimate average recurrence interval is 2150 years, although the time interval between individual events varies by as much as a factor of 2. Each paleo earthquake is similar to the 1980 surface rupture in amount of slip and style of deformation, which suggests that the 1980 event is characteristic for the Irpinia fault. Slip per event values average 61 cm. The net vertical displacement of 2.12–2.36 m since 8600 cal year B.P. observed in the trenches gives a vertical slip rate of 0.25–0.35 mm/yr, a dip slip rate of 0.29–0.40 mm/yr, and an extension rate of 0.14–0.20 mm/yr. Although fault behavior data are only available for the Irpinia fault they provide a starting point for evaluating earthquake recurrence and rates of deformation in southern Apennines. They suggest that (1) fault specific earthquake recurrence intervals based on the historical seismic record overestimates the occurrence of large magnitude ( M 7) earthquakes and (2) the Holocene rate of extension across the Apennines is ≤1 mm/yr. The 1980 earthquake and the paleoseismologic observations show that repeated and localized surface faulting occurs in southern Apennines and leaves subtle but distinct geomorphic evidence that can be detected with detailed and careful investigation.

Apennine Mountains↗
USGS2012

Deformation from 1973 to 1987 in the epicentral area of the 1959 Hebgen Lake, Montana, earthquake (Ms = 7.5)

A 40‐km aperture trilateration network centered on the 1959 Hebgen Lake earthquake epicenter has been surveyed in 1973, 1974, 1976, 1978, 1981, 1984, and 1987. The deformation inferred from those surveys is described roughly by a uniaxial, 0.266 ± 0.014 μstrain/yr, N15°E ± 1°extension that is uniform in both time and space. That extension is orthogonal to the strike (N78°W ± 5°) of the 1959 rupture plane inferred from focal mechanism solutions. The absence of strain accumulation in the N75°W direction suggests that the source of deformation must be greatly elongated in that direction. Looked at in greater detail, the deformation is found to be concentrated in the northern part of the network. The zone of concentrated deformation coincides with a part of a well‐defined trend in seismicity that extends at least 100 km N75°W from the Sour Creek resurgent dome in the Yellowstone caldera. Extension perpendicular to that trend is indicated by focal mechanism solutions. Thus, the zone of rifting identified within the Hebgen Lake network apparently extends east southeast to the Yellowstone caldera. About 8 mm/yr extension is observed across a 20‐km width within the zone.

Montana, Wyoming↗
USGS2012

The geology and distribution of impact craters on Venus: What are they telling us?

Magellan has revealed an ensemble of impact craters on Venus that is unique in many important ways. We have compiled a data base describing the 842 craters on 89% of Venus' surface mapped through orbit 2578. (The craters range in diameter from 1.5 to 280 km.) We have studied the distribution, size‐density, morphology, geology, and associated surface properties of these craters both in the aggregate and, for some craters, in greater detail. We find that (1) the spatial distribution of craters is highly uniform; (2) the size‐density distribution of large craters (diameters ≥35 km) is similar to the young crater populations on other terrestrial planets but at a much lower density that indicates an average age of only about 0.5 Ga (based on the estimated population of Venus‐crossing asteroids); (3) unlike the case on other planets, the density of small craters (diameters ≤35 km) declines rapidly with decreasing diameters because of atmospheric filtering; (4) the spectrum of crater modification differs greatly from that on other planets: 62% of all craters are pristine, only 4% are embayed by lavas, and the remainder are affected by tectonism, but none are severely and progressively depleted (as extrapolated from the size‐density distribution of larger craters); (5) large craters have a progression of morphologies generally similar to those on other planets, but small craters are typically irregular or multiple rather than bowl shaped; (6) diffuse radar‐bright or ‐dark features surround some craters, and 367 similar diffuse “splotches” with no central crater are observed; and (7) other crater features unique to Venus include radar‐bright or ‐dark parabolic arcs opening westward and extensive outflows originating in crater ejecta. The first three of these observations are entirely unexpected. We interpret them as indicating that the planet's cratering record was erased by a global resurfacing event or events, the latest ending about 0.5 Ga, after which volcanic activity declined (but did not cease entirely). Since the last resurfacing event, a maximum of 10% of the planet has been resurfaced and only about 4% of the craters have been obliterated. Convective thermal evolution models support this interpretation (Arkani‐Hamed and Toksoz, 1984). Observations 3–7 confirm quantitatively the expectation that the dense atmosphere of Venus has strongly affected the production of craters. Large impactors have been relatively unaffected, intermediate‐sized ones have been fragmented and have produced overlapping or multiple craters, a narrow size range has produced shock‐induced “splotches” but no craters, and the smallest bodies have had no observable effect on the surface. The number of craters eliminated by the “atmospheric filter” is enormous, about 98% of the craters between 2 and 35 km in diameter that Magellan might have observed on a hypothetical airless Venus. Unique crater‐related features such as parabolas and outflow deposits demonstrate the roles of Venus' high atmospheric density and temperature in modifying the crater formation process. Finally, heavily fractured craters and lava‐embayed craters are found to have higher than average densities along the major fracture belts and rifted uplands connecting Aphrodite Terra and Atla, Beta, Themis, and Phoebe regiones. These craters thus provide physical evidence for recent volcanic and tectonic activity at a low level.

Journal of Geophysical Research E: Planets↗
USGS2012

Effects of hydrothermal alteration on the magnetization of the Oligocene Carpenter Ridge Tuff, Bachelor Caldera, San Juan Mountains, Colorado

Intracaldera Oligocene Carpenter Ridge Tuff fills the Bachelor caldera in the central San Juan caldera complex and hosts mineral deposits of the Creede mineral district. The Carpenter Ridge Tuff and unaltered portions of its intracaldera Bachelor Mountain Member, have strong, high‐coercivity, reverse magnetizations with average magnetic susceptibility (MS) and natural remanent magnetization (NRM) of 6 × 10 −3 volume SI and 8 A/m, respectively. Oxide phenocrysts in these rocks are titanomagnetite and ilmenite; however, magnetization appears to be controlled by microcrystic titanomaghemite based on thermal demagnetization unblocking temperatures and Curie temperatures between 580°C and 620°C. Much of the intracaldera tuff was affected by potassic metasomatism, a type of hydrothermal alteration characterized by addition of K and loss of Ca and Na, between 27.3 Ma and 25.1 Ma. Potassic metasomatism resulted in the replacement of original feldspars by potassium feldspar and quartz, oxidation of the original oxide phenocrysts to hematite and rutile, and consequent suppression of MS and NRM by a factor of 5. Also present, however, are metasomatized rocks that have high magnetizations even though their original oxide phenocrysts were destroyed; values of MS and NRM are similar to those of outflow tuff. Such rocks are suspected of containing secondary magnetite on the basis of (1) Curie and thermal unblocking temperatures at and below 580°C, indicating that magnetization is carried by magnetite but that primary microcrysts of maghemite were destroyed, and (2) observation in one sample of magnetite rimming cores of hematite+rutile. The presence of secondary magnetite in metasomatized rocks requires a local shift to more reducing fluid chemistry, although we are unable to determine whether this shift occurred during potassic metasomatism or during later alteration or ore deposition. These altered rocks may record a change from a pervasive alteration by alkaline, oxidizing fluids to a vein‐controlled alteration dominated by more reduced fluids in which magnetite was stable.

Colorado↗
USGS2012

Broad trends in geomagnetic paleointensity on Hawaii during Holocene time

Paleointensity determinations have been obtained from 22 basaltic lava flows on the island of Hawaii using the Thelliers' method. Radiocarbon dating indicates that these flows erupted at intervals ranging from about 200 to 1000 years, and results of the experiments provide an estimate of broad trends in geomagnetic paleointensity during Holocene time in the vicinity of Hawaii. Most of the samples were obtained from quickly cooled flow margins and, as a consequence, typically contain two titanomagnetite populations. Only two of the 79 samples analyzed failed to yield an estimate of the paleofield, demonstrating that paleointensities can be obtained from such samples if carefully selected. Virtual dipole moments calculated for the flows are compared with a published curve of dipole field intensity that was constructed using worldwide archeomagnetic data. The large nondipole fields that were previously postulated for the vicinity of Hawaii are confirmed, and the present data indicate that they were present from about 5000 years B.P. to perhaps as recently as the past 200 years. The data indicate, however, that these nondipole sources must have been virtually absent between about 12,000 and 5000 years B.P. as they are at the present time.

Hawaii↗
USGS2012

Magnetic grain-size variations through an ash flow sheet: influence on magnetic properties and implications for cooling history

Rock magnetic studies of tuffs are essential to the interpretation of paleomagnetic data derived from such rocks, provide a basis for interpretation of aeromagnetic data over volcanic terranes, and yield insights into the depositional and cooling histories of ash flow sheets. A rhyolitic ash flow sheet, the Miocene-aged Tiva Canyon Member of the Paintbrush Tuff, contains both titanomagnetite phenocrysts, present in the magma prior to eruption, and cubic Fe-oxide microcrystals that grew after emplacement. Systematic variations in the quantity and magnetic grain size of the microcrystals produce large variations in magnetic properties through a section of the ash flow sheet penetrated in a borehole on the Nevada Test Site. Natural remanent magnetization varies from less than 1 × 10 −4 to more than 8 × 10 −4 A m 3 kg −1 , and in-phase magnetic susceptibility varies from less than 1 × 10 −6 to more than 10 × 10 −6 m 3 kg −1 . The microcrystals, which include both magnetite and maghemite, have Curie points and maximum unblocking temperatures between 580°C and 640°C. Rock magnetic data, including in-phase and quadrature magnetic susceptibilities as well as hysteresis parameters, demonstrate that these microcrystals are of superparamagnetic and single-domain sizes. Titanomagnetite phenocrysts are the dominant remanence carriers in the central 50 m of the section, whereas microcrystals are important contributors to remanent magnetization and magnetic susceptibility in two 15-m-thick zones at the top and bottom. Within these zones the size of microcrystals decreases both toward the quenched margins and toward the interior of the sheet. The decrease in microcrystal size toward the interior of the sheet is interpreted to indicate the presence of a cooling break; possibly represented by a concentration of pumice. Laboratory heating produces changes in magnetic properties which vary complexly from sample to sample. Such changes include (1) decrease in saturation magnetization, (2) both increase and decrease in in-phase and quadrature magnetic susceptibilities, and (3) shifts in coercivity spectra. The complex sample-to-sample variation in response to heating is attributed to differences in initial grain-size distributions.

Journal of Geophysical Research↗