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At least 1,279 records · Page 71Linked to original sources
USGS2020

Dust deposited on snow cover in the San Juan Mountains, Colorado, 2011-2016: Compositional variability bearing on snow-melt effects

Light-absorbing particles in atmospheric dust deposited on snow cover (dust-on-snow, DOS) diminish albedo and accelerate the timing and rate of snow melt. Identification of these particles and their effects are relevant to snow-radiation modeling and thus water-resource management. Laboratory-measured reflectance of DOS samples from the San Juan Mountains (USA) were compared with DOS mass loading, particle sizes, iron mineralogy, carbonaceous matter type and content, and chemical compositions. Samples were collected each spring for water years 2011-2016, when individual dust layers had merged into one (all layers merged) at the snow surface. Average reflectance values of the six samples were 0.2153 (sd, 0.0331) across the visible wavelength region (0.4-0.7 µm) and 0.3570 (sd, 0.0498) over the full-measurement range (0.4-2.50 µm). Reflectance values correlated inversely to concentrations of ferric oxide, organic carbon (1.4-10 wt. %), magnetite (0.05-0.13 wt. %), and silt (PM63-3.9; median grain sizes averaged 21.4 µm) but lacked correspondence to total iron and PM10 contents. Measurements of reflectance and Mössbauer spectra and magnetic properties indicated that microcrystalline hematite and nano-size goethite were primarily responsible for diminished visible reflectance. Positive correlations between organic carbon and metals attributed to fossil-fuel combustion, with observations from electron microscopy, indicated that some carbonaceous matter occurred as black carbon. Magnetite was a surrogate for related light-absorbing minerals, dark rock particles, and contaminants. Similar analyses of DOS from other areas would help evaluate the influences of varied dust sources, wind-storm patterns, and anthropogenic inputs on snow melt and water resources in and beyond the Colorado River basin.

Colorado↗
USGS2022

Mountain rivers reveal the earthquake hazard of geologic faults in Silicon Valley

The 1989, M w = 6.9 Loma Prieta earthquake resulted in tens of lives lost and cost California almost 3% of its gross domestic product. Despite widespread damage, the earthquake did not clearly rupture the surface, challenging the identification and characterization of these hidden hazards. Here, we show that they can be illuminated by inverting fluvial topography for slip-and moment accrual-rates—fundamental components in earthquake hazard assessments—along relief-generating geologic faults. We applied this technique to thrust faults bounding the mountains along the western side of Silicon Valley in the San Francisco Bay Area, and discovered that these structures may be capable of generating a M w = 6.9 earthquake every 250–300 years based on moment accrual rates. This method may be deployed broadly to evaluate seismic hazard in developing regions with limited geological and geophysical information.

California↗
USGS2023

Debris-flow process controls on steepland morphology in the San Gabriel Mountains, California

Steep landscapes evolve largely by debris flows, in addition to fluvial and hillslope processes. Abundant field observations document that debris flows incise valley bottoms and transport substantial sediment volumes, yet their contributions to steepland morphology remain uncertain. This has, in turn, limited the development of debris-flow incision rate formulations that produce morphology consistent with natural landscapes. In many landscapes, including the San Gabriel Mountains (SGM), California, steady-state fluvial channel longitudinal profiles are concave-up and exhibit a power-law relationship between channel slope and drainage area. At low drainage areas, however, valley slopes become nearly constant. These topographic forms result in a characteristically curved slope-area signature in log-log space. Here, we use a one-dimensional landform evolution model that incorporates debris-flow erosion to reproduce the relationship between this curved slope-area signature and erosion rate in the SGM. Topographic analysis indicates that the drainage area at which steepland valleys transition to fluvial channels correlates with measured erosion rates in the SGM, and our model results reproduce these relationships. Further, the model only produces realistic valley profiles when parameters that dictate the relationship between debris-flow erosion, valley-bottom slope, and debris-flow depth are within a narrow range. This result helps place constraints on the mathematical form of a debris-flow incision law. Finally, modeled fluvial incision outpaces debris-flow erosion at drainage areas less than those at which valleys morphologically transition from near-invariant slopes to concave profiles. This result emphasizes the critical role of debris-flow incision for setting steepland form, even as fluvial incision becomes the dominant incisional process.

California↗
USGS2024

Clumped isotopes record a glacial-interglacial shift in seasonality of soil carbonate accumulation in the San Luis Valley, southern Rocky Mountains, USA

Clumped isotope paleothermometry using pedogenic carbonates is a powerful tool for investigating past climate changes. However, location-specific seasonal patterns of precipitation and soil moisture cause systematic biases in the temperatures they record, hampering comparison of data across large areas or differing climate states. To account for biases, more systematic studies of carbonate forming processes are needed. We measured modern soil temperatures within the San Luis Valley of the Rocky Mountains and compared them to paleotemperatures determined using clumped isotopes. For Holocene-age samples, clumped isotope results indicate carbonate accumulated at a range of temperatures with site averages similar to the annual mean. Paleotemperatures for late Pleistocene-age samples (ranging 19–72 ka in age) yielded site averages only 2°C lower, despite evidence that annual temperatures during glacial periods were 5–9°C colder than modern. We use a 1D numerical model of soil physics to support the idea that differences in hydrologic conditions in interglacial versus glacial periods promote differences in the seasonal distribution of soil carbonate accumulation. Model simulations of modern (Holocene) conditions suggest that soil drying under low soil p CO 2 favors year-round carbonate accumulation in this region but peaking during post-monsoon soil drying. During a “glacial” simulation with lowered temperatures and added snowpack, more carbonate accumulation shifted to the summer season. These experiments show that changing hydrologic regimes could change the seasonality of carbonate accumulation, which in this study blunts the use of clumped isotopes to quantify glacial-interglacial temperature changes. This highlights the importance of understanding seasonal biases of climate proxies for accurate paleoenvironmental reconstruction.

Colorado, new Mexico↗
USGS2024

Using stochastic point pattern analysis to track regional orientations of magmatism during the transition to cenozoic extension and Rio Grande rifting, Southern Rocky Mountains

The southern Rocky Mountains in Colorado and northern New Mexico hosted intracontinental magmatism that developed during a tectonic transition from shortening (Laramide orogeny, ca. 75 to 40 Ma) through extension and rifting. We present a novel approach that uses stochastic weighted bootstrap simulations of a large set of new and historical geochronology data to better understand how regional anisotropies responsible for focusing magma emplacement evolved through time. This technique can detect subtle trends in directional distributions, including multi-modal orientations, and can be filtered from regional to local scales. Our results indicate that magmatism followed first the northeast trend of the Colorado mineral belt between 75 and 40 Ma and deviated afterward. These deviations vary depending on the scale of the analysis. At the smallest scale we evaluated (<75 km), the orientation of magmatism from 45 to 30 Ma rotated counter-clockwise before aligning with the north-south trend of the modern Rio Grande rift. Larger, regional-scale analyses indicate magma centers between 40 to 35 Ma and 25 to 20 Ma were dominantly oriented southwest-northeast, whereas magmatism between 35 and 25 Ma had north-south orientation. The large areal footprint of magmatism and shifting regional patterns suggest that ancient zones of weakness in the North American lithosphere accommodated magma flow at different moments in time, rather than controlled by a retreating interface of the Farallon and North American plates.

Tectonics↗
USGS2025

Light absorbing particles deposited to snow cover across the Upper Colorado River Basin, Colorado Rocky Mountains, 2013-16: Interannual variations from multiple natural and anthropogenic sources

Atmospheric particulate matter (PM) as light-absorbing particles (LAPs) deposited to snow cover can result in early onset and rapid snow melting, challenging management of downstream water resources. We identified LAPs in 38 snow samples (water years 2013–2016) from the mountainous Upper Colorado River basin by comparing among laboratory-measured spectral reflectance, chemical, physical, and magnetic properties. Dust sample reflectance, averaged over the wavelength range of 0.35–2.50 μm, varied by a factor of 1.9 (range, 0.2300–0.4444) and was suppressed mainly by three components: (a) carbonaceous matter measured as total organic carbon (1.6–22.5 wt. %) including inferred black carbon, natural organic matter, and carbon-based synthetic, black road-tire-wear particles, (b) dark rock and mineral particles, indicated by amounts of magnetite (0.11–0.37 wt. %) as their proxy, and (c) ferric oxide minerals identified by reflectance spectroscopy and magnetic properties. Fundamental compositional differences were associated with different iron oxide groups defined by dominant hematite, goethite, or magnetite. These differences in iron oxide mineralogy are attributed to temporally varying source-area contributions implying strong interannual changes in regional source behavior, dust-storm frequency, and (or) transport tracks. Observations of dust-storm activity in the western U.S. and particle-size averages for all samples (median, 25 μm) indicated that regional dust from deserts dominated mineral-dust masses. Fugitive contaminants, nevertheless, contributed important amounts of LAPs from many types of anthropogenic sources.

Colorado↗
USGS2012

Evolution of deep structure along the trans-Alaska crustal transect, Chugach Mountains and Copper River Basin, southern Alaska

One of the most important results of the Trans-Alaska Crustal Transect investigations is the discovery that more than one third of the North American plate in southern Alaska (Chugach Mountains and Copper River basin) consists of tectonically underplated oceanic lithosphere. In southern Alaska, exposed accreted tectonostratigraphic terranes, include, from south to north along the transect, the Prince William, Chugach, Peninsular, and Wrangellia terranes. Chief results from seismic refraction data include the following: (1) A sequence of layers of low and high velocity (5.7–7.8 km/s), more than 10 km thick, dips gently northward, with its top at the surface in the southernmost Chugach terrane and at more than 20-km depth beneath the southern Peninsular terrane. A high-velocity layer at the top of this sequence corresponds to metabasalt in the southernmost Chugach terrane. This sequence is interpreted to be tectonically underplated fragments of the Kula plate and its sedimentary overburden. (2) An intermediate-velocity layer (6.35–6.5 km/s) at 9-km depth beneath both the northern Chugach and southern Peninsular terranes appears to extend without offset across the deep projection of the suture between these two terranes, the Border Ranges fault system. (3) A crustal “root” between depths of 19 and 57 km beneath the northern Peninsular and Wrangellia terranes appears to extend without offset across the deep projection of the suture between these two terranes, the West Fork fault system. This crustal root also appears to abut the tectonically underplated sequence to the south. Unfortunately, in results 2 and 3 the apparent crosscutting and abutting relationships can not be unambiguously resolved. Three scenarios, A, B, and C, have been offered to explain the evolution of this structure. Scenarios A and B differ in interpretation of the depth extent of the northern Chugach and southern Peninsular terranes. In scenario A these terranes extend to only 9-km depth, where they rest on an unknown middle crust, interpreted as a deeper part of the Peninsular terrane. In this scenario, the northern Chugach and southern Peninsular terrenes moved landward as a tectonic wedge, during tectonic underplating in the latest Cretaceous or early Tertiary, and uplifted the upper crust of the Peninsular terrane to the north. In scenario B, the northern Chugach and southern Peninsular terranes extend to 20-km depth. In this scenario, they were compressed but not detached and displaced during accretion. Scenarios A, B, and C differ in interpretation of the deep crustal root that appears to abut the tectonically underplated sequence. Since the Peninsular and Wrangellia terranes were at or below sea level prior to the mid-Cretaceous, the root was probably emplaced in mid-Cretaceous or younger times. In scenario A the root is lower crust of North America that moved southward during tectonic underplating of the Kula plate. In scenario B the root is, in part, tectonically underplated rocks similar to the Kula plate sequence to the south. In scenario C the root is, in part, magmatically underplated rocks.

Journal of Geophysical Research↗
USGS2010

The effect of streambed topography on surface-subsurface water exchange in mountain catchments

A numerical hydrological simulation suggested that water exchange between stream channels and adjacent aquifers is enhanced by convexities and concavities in streambed topography. At St. Kevin Gulch, an effluent stream in the Rocky Mountains of Colorado, subsurface hydraulic gradients and movement of ionic tracers indicated that stream water was locally recharged into well-defined flow paths through the alluvium. Stream water-filled flow paths in the alluvium (referred to as substream flow paths) returned to the stream a short distance downstream (1 to 10 m). Recharge to the substream flow paths occurred where stream water slope increased, at the transition from pools (<1%) to steeper channel units (5–20%). Return of substream flow paths to the stream occurred where stream water slope decreased, at the transition from steeper channel units to pools. A net water flux calculation is typically used to characterize water and solute fluxes between surface and subsurface zones of catchments. Along our study reach at St. Kevin Gulch the net inflow of water from subsurface to stream (1.6 mL s −1 m −1 ) underestimated the gross inflow (2.7 mL s −1 m −1 ) by 40%. The influence of streambed topography is to enhance hydrological fluxes between stream water and subsurface zones and to prolong water-sediment contact times; these effects could have important consequences for solute transport, retention, and transformation in catchments.

Water Resources Research↗
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↗
USGS1996

Equatorial origin for Lower Jurassic radiolarian chert in the Franciscan Complex, San Rafael Mountains, southern California

Lower Jurassic radiolarian chert sampled at two localities in the San Rafael Mountains of southern California (∼20 km north of Santa Barbara) contains four components of remanent magnetization. Components A , B ′, and B are inferred to represent uplift, Miocene volcanism, and subduction/accretion overprint magnetizations, respectively. The fourth component ( C ), isolated between 580° and 680°C, shows a magnetic polarity stratigraphy and is interpreted as a primary magnetization acquired by the chert during, or soon after, deposition. Both sequences are late Pliensbachian to middle Toarcian in age, and an average paleolatitude calculated from all tilt-corrected C components is 1° ± 3° north or south. This result is consistent with deposition of the cherts beneath the equatorial zone of high biologic productivity and is similar to initial paleolatitudes determined for chert blocks in northern California and Mexico. This result supports our model, in which deep-water Franciscan-type cherts were deposited on the Farallon plate as it moved eastward beneath the equatorial productivity high, were accreted to the continental margin at low paleolatitudes, and were subsequently distributed northward by strike-slip faulting associated with movements of the Kula, Farallon, and Pacific plates. Upper Cretaceous turbidites of the Cachuma Formation were sampled at Agua Caliente Canyon to determine a constraining paleolatitude for accretion of the Jurassic chert sequences. These apparently unaltered rocks, however, were found to be completely overprinted by the A component of magnetization. Similar in situ directions and demagnetization behaviors observed in samples of other Upper Cretaceous turbidite sequences in southern and Baja California imply that these rocks might also give unreliable results.

Journal of Geophysical Research B: Solid Earth↗
USGS1996

Far-travelled Permian chert of the North Fork terrane, Klamath Mountains, California

Permian chert in the North Fork terrane and correlative rocks of the Klamath Mountains province has a remanent magnetization that is prefolding and presumably primary. Paleomagnetic results indicate that the chert formed at a paleolatitude of 8.6° ± 2.5° but in which hemisphere remains uncertain. This finding requires that these rocks have undergone at least 8.6° ± 4.4° of northward transport relative to Permian North America since their deposition. Paleontological evidence suggests that the Permian limestone of the Eastern Klamath terrane originated thousands of kilometers distant from North America. The limestone of the North Fork terrane may have formed at a similar or even greater distance as suggested by its faunal affinity to the Eastern Klamath terrane and more westerly position. Available evidence indicates that convergence of the North Fork and composite Central Metamorphic-Eastern Klamath terranes occurred during Triassic or Early Jurassic time and that their joining together was a Middle Jurassic event. Primary and secondary magnetizations indicate that the new composite terrane containing these and other rocks of the Western Paleozoic and Triassic belt behaved as a single rigid block that has been latitudinally concordant with the North American craton since Middle Jurassic time.

California↗
USGS1997

Coseismic deformation of the Wrights tunnel during the 1906 San Francisco earthquake: A key to understanding 1906 fault slip and 1989 surface ruptures in the southern Santa Cruz Mountains, California

The Wrights tunnel is an abandoned railroad tunnel that crosses the San Andreas fault in the southern Santa Cruz Mountains in the vicinity of the 1989 Loma Prieta earthquake. The tunnel was damaged and deformed during the 1906 San Francisco earthquake and a plot showing postearthquake measurements made in the tunnel is given by Lawson [1908]. The amount of offset shown on this plot (1.5 m) has been used in several studies as being representative of the amount of fault offset along this segment of the San Andreas fault in 1906. Our historical research shows that different observers reported different amounts of fault offset in the tunnel and that the 1.5 m given on the plot is not a surveyed measurement. In addition, the plot of the tunnel has been interpreted in several previous studies as evidence of a broad (1.5 km) zone of faulting beneath Summit Ridge. Our analysis shows that this plot need not indicate a broad zone of deformation. Our historical research and modeling of the tunnel measurements indicate that faulting was confined to a zone less than 400 m wide and that 60–85% of the coseismic slip occurred across a single fault plane. There is no evidence for offset across a second shear zone beneath Summit Ridge in 1906. This implies that surface fractures reported on Summit Ridge in 1906 were not associated with significant deformation of the tunnel, implying that they were shallow, surficial features. By analogy, the very similar fractures that occurred on Summit Ridge in 1989 were also probably the result of shallow gravitational, rather than deep-seated tectonic, processes. Our modeling also indicates that total coseismic, near-surface slip across the San Andreas fault zone in the Wrights tunnel in 1906 was at least 1.7–1.8 m.

Journal of Geophysical Research B: Solid Earth↗
USGS1996

Mineral nitrogen transformations in and under seasonal snow in a high-elevation catchment in the Rocky Mountains, United States

In an effort to understand sources of nitrate (NO 3 − ) in surface waters of high-elevation catchments, nitrogen (N) transformations in and under seasonal snow were investigated from 1993 to 1995 on Niwot Ridge, an alpine ecosystem at 3,500 m located in the Colorado Front Range of the Rocky Mountains. Ammonium (NH 4 + ) and NO 3 − labeled with 15 N applied as nonconservative tracers to the snow showed no evidence of nitrification in the snowpack. Furthermore, NH 4 + movement through the amended snowpack was highly correlated with a conservative chloride tracer ( r 2 = 0.99). In an unamended snowpack NH 4 + concentrations in meltwater before contact with the ground were highly correlated with NO 3 − concentrations ( r 2 = 0.98), which is consistent with no nitrification in the snowpack. The isotopically labeled 15 NH + 4 applied to the snowpack was found in underlying soils, showing that NH 4 + released from snow can be rapidly immobilized. Resin bag (mixed-bed ion-exchange resins) measurements (n = 22) showed that 80% of the mobile inorganic N in unamended subnivial soils was NO 3 − . Measurements of KCl-extractable inorganic N from surface soils showed that highest values were prior to the initiation of snowmelt and lowest values were during the growing season. The natural δ 15 N abundance of unamended soils was negative and ranged from −12 to −2, suggesting that atmospheric deposition of δ 15 N-depleted N is an important component of N cycling in these alpine soils. These results suggest that soil mineralization under seasonal snow, rather than snowmelt release of NO 3 − , may control NO 3 − concentrations in surface waters of high-elevation catchments.

Water Resources Research↗
USGS2006

Streams in Catskill Mountains still susceptible to acid rain

Precipitation in North America has become less acidic over the past 2 decades because of reduced power plant emissions and compliance with the Clean Air Act Amendments [ Sirois , 19937rsqb;. The 1990 Clean Air Act Amendments were developed to reduce the acidity of sensitive surface waters, which are primarily in upland forested environments, where acidified waters and associated high aluminum concentrations are toxic to many species of aquatic flora and fauna [ Schindler et al. , 1989]. Our studies show that in spite of less acidic precipitation, the buffering capacity of streams in upland forests of the Catskill Mountains in southeastern New York has not increased in recent years. These data suggest that long-term leaching by acid rain has lowered exchangeable calcium ion concentrations in the soil in upland areas, where the underlying, slow-weathering bedrock provides an inadequate supply of cations to neutralize acidity.

New York↗
USGS1998

Three-year decline of magmatic CO2 emissions from soils of a Mammoth Mountain tree kill: Horseshoe Lake, CA, 1995-1997

We used the closed chamber method to measure soil CO 2 efflux over a three-year period at the Horseshoe Lake tree kill (HLTK)—the largest tree kill on Mammoth Mountain in central eastern California. Efflux contour maps show a significant decline in the areas and rates of CO 2 emission from 1995 to 1997. The emission rate fell from 350 t d −1 (metric tons per day) in 1995 to 130 t d −1 in 1997. The trend suggests a return to background soil CO 2 efflux levels by early to mid 1999 and may reflect exhaustion of CO 2 in a deep reservoir of accumulated gas and/or mechanical closure or sealing of fault conduits transmitting gas to the surface. However, emissions rose to 220 t d −1 on 23 September 1997 at the onset of a degassing event that lasted until 5 December 1997. Recent reservoir recharge and/or extension-enhanced gas flow may have caused the degassing event.

Geophysical Research Letters↗
USGS2012

Petrogenesis of garnet two-mica granites in the Ruby Mountains, Nevada

Garnet-two-mica granites of Jurassic (160 ± 3 m.y.) and Cretaceous (83 ± 1.3 m.y.) ages intrude amphibolite facies metasedimentary rocks of Precambrian age and lower Paleozoic sedimentary rocks in the northern part of the Ruby Mountains, Nevada. High initial 87 Sr/ 86 Sr and high values of δ 18 O for minerals suggest a dominantly sedimentary source for the magmas. Total rare-earth contents are low and less fractionated (Ce N /Yb N = 10) than granitic rocks of similar composition in the Sierra Nevada and show a negative Eu anomaly (Eu/Eu* = 0.57). These data are consistent with melting in the stability field of plagioclase with little contribution to the melt from refractory minerals such as zircon, sphene, and apatite. Minor euhedral garnets are manganese-rich and magnesium-poor (al 60 sp 34 py 3.6 gr 2.8 ) compared to garnet (al 79 sp 6 py 13 gr 2 ) in the intruded metamorphic rocks and show an increase in manganese and a decrease in calcium in the outer few microns. Fractionation of magnesium-iron between garnet and biotite, and of sodium-potassium between K-feldspar and plagioclase in the granites suggest submagmatic equilibration temperatures, in the range of 365° to 505°C, whereas oxygen isotope equilibration temperatures for two granites are higher at 480° and 570°C. The compositions of the granites plot near minimum melting compositions in the water-saturated quartz-albite-orthoclase system. Comparison of muscovite plus quartz stability, the water-saturated granite solidus, and experimental garnet-melt equilibria suggests pressures of crystallization no lower than about 3.5 kbar. This pressure is compatible with that estimated from garnet-plagioclase-sillimanite-quartz equilibria for the intruded metasedimentary rocks.

Nevada↗
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↗