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Results for “Journal of Geophysical Research, Solid Earth”

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At least 865 records · Page 48Linked to original sources
USGS1998

Carbon dioxide and helium emissions from a reservoir of magmatic gas beneath Mammoth Mountain, California

Carbon dioxide and helium with isotopic compositions indicative of a magmatic source ( δ 13 C = −4.5 to −5‰, 3 He/ 4 He = 4.5 to 6.7 R A ) are discharging at anomalous rates from Mammoth Mountain, on the southwestern rim of the Long Valley caldera in eastern California. The gas is released mainly as diffuse emissions from normal‐temperature soils, but some gas issues from steam vents or leaves the mountain dissolved in cold groundwater. The rate of gas discharge increased significantly in 1989 following a 6‐month period of persistent earthquake swarms and associated strain and ground deformation that has been attributed to dike emplacement beneath the mountain. An increase in the magmatic component of helium discharging in a steam vent on the north side of Mammoth Mountain, which also began in 1989, has persisted until the present time. Anomalous CO 2 discharge from soils first occurred during the winter of 1990 and was followed by observations of several areas of tree kill and/or heavier than normal needlecast the following summer. Subsequent measurements have confirmed that the tree kills are associated with CO 2 concentrations of 30–90% in soil gas and gas flow rates of up to 31,000 g m −2 d −1 at the soil surface. Each of the tree‐kill areas and one area of CO 2 discharge above tree line occurs in close proximity to one or more normal faults, which may provide conduits for gas flow from depth. We estimate that the total diffuse CO 2 flux from the mountain is approximately 520 t/d, and that 30–50 t/d of CO 2 are dissolved in cold groundwater flowing off the flanks of the mountain. Isotopic and chemical analyses of soil and fumarolic gas demonstrate a remarkable homogeneity in composition, suggesting that the CO 2 and associated helium and excess nitrogen may be derived from a common gas reservoir whose source is associated with some combination of magmatic degassing and thermal metamorphism of metasedimentary rocks. Furthermore, N 2 /Ar ratios and nitrogen isotopic values indicate that the Mammoth Mountain gases are derived from sources separate from those that supply gas to the hydrothermal system within the Long Valley caldera. Various data suggest that the Mammoth Mountain gas reservoir is a large, low‐temperature cap over an isolated hydrothermal system, that it predates the 1989 intrusion, and that it could remain a source of gas discharge for some time.

Journal of Geophysical Research B: Solid Earth↗
USGS1998

Introduction to special section: Stress triggers, stress shadows, and implications for seismic hazard

Many aspects of earthquake mechanics remain an enigma as we enter the closing years of the twentieth century. One potential bright spot is the realization that simple calculations of stress changes may explain some earthquake interactions, just as previous and on going studies of stress changes have begun to explain human-induced seismicity. This paper, which introduces the special section “Stress Triggers, Stress Shadows, and Implications for Seismic Hazard,” reviews many published works and presents a compilation of quantitative earthquake interaction studies from a stress change perspective. This synthesis supplies some clues about certain aspects of earthquake mechanics. It also demonstrates that much work remains before we can understand the complete story of how earthquakes work.

Journal of Geophysical Research B: Solid Earth↗
USGS1998

Nonlinear refraction and reflection travel time tomography

We develop a rapid nonlinear travel time tomography method that simultaneously inverts refraction and reflection travel times on a regular velocity grid. For travel time and ray path calculations, we apply a wave front method employing graph theory. The first-arrival refraction travel times are calculated on the basis of cell velocities, and the later refraction and reflection travel times are computed using both cell velocities and given interfaces. We solve a regularized nonlinear inverse problem. A Laplacian operator is applied to regularize the model parameters (cell slownesses and reflector geometry) so that the inverse problem is valid for a continuum. The travel times are also regularized such that we invert travel time curves rather than travel time points. A conjugate gradient method is applied to minimize the nonlinear objective function. After obtaining a solution, we perform nonlinear Monte Carlo inversions for uncertainty analysis and compute the posterior model covariance. In numerical experiments, we demonstrate that combining the first arrival refraction travel times with later reflection travel times can better reconstruct the velocity field as well as the reflector geometry. This combination is particularly important for modeling crustal structures where large velocity variations occur in the upper crust. We apply this approach to model the crustal structure of the California Borderland using ocean bottom seismometer and land data collected during the Los Angeles Region Seismic Experiment along two marine survey lines. Details of our image include a high-velocity zone under the Catalina Ridge, but a smooth gradient zone between. Catalina Ridge and San Clemente Ridge. The Moho depth is about 22 km with lateral variations. Copyright 1998 by the American Geophysical Union.

Journal of Geophysical Research B: Solid Earth↗
USGS1998

Episodic fluid flow in the Nankai accretionary complex: Timescale, geochemistry, flow rates, and fluid budget

Down-hole geochemical anomalies encountered in active accretionary systems can be used to constrain the timing, rates, and localization of fluid flow. Here we combine a coupled flow and solute transport model with a kinetic model for smectite dehydration to better understand and quantify fluid flow in the Nankai accretionary complex offshore of Japan. Compaction of sediments and clay dehydration provide fluid sources which drive the model flow system. We explicitly include the consolidation rate of underthrust sediments in our calculations to evaluate the impact that variations in this unknown quantity have on pressure and chloride distribution. Sensitivity analysis of steady state pressure solutions constrains bulk and flow conduit permeabilities. Steady state simulations with 30% smectite in the incoming sedimentary sequence result in minimum chloride concentrations at site 808 of 550 m M , but measured chlorinity is as low as 447 m M . We simulate the transient effects of hydrofracture or a strain event by assuming an instantaneous permeability increase of 3–4 orders of magnitude along a flow conduit (in this case the décollement), using steady state results as initial conditions. Transient results with an increase in décollement permeability from 10 −16 m 2 to 10 −13 m 2 and 20% smectite reproduce the observed chloride profile at site 808 after 80–160 kyr. Modeled chloride concentrations are highly sensitive to the consolidation rate of underthrust sediments, such that rapid compaction of underthrust material leads to increased freshening. Pressures within the décollement during transient simulations rise rapidly to a significant fraction of lithostatic and remain high for at least 160 kyr, providing a mechanism for maintaining high permeability. Flow rates at the deformation front for transient simulations are in good agreement with direct measurements, but steady state flow rates are 2–3 orders of magnitude smaller than observed. Fluid budget calculations indicate that nearly 71% of the incoming water in the sediments leaves the accretionary wedge via diffuse flow out the seafloor, 0–5% escapes by focused flow along the décollement, and roughly 1% is subducted.

Journal of Geophysical Research B: Solid Earth↗
USGS1998

Seismotectonics of the Loma Prieta, California, region determined from three-dimensional Vp, Vp/Vs, and seismicity

Three-dimensional V p and V p / V s velocity models for the Loma Prieta region were developed from the inversion of local travel time data (21,925 P arrivals and 1,116 S arrivals) from earthquakes, refraction shots, and blasts recorded on 1700 stations from the Northern California Seismic Network and numerous portable seismograph deployments. The velocity and density models and microearthquake hypocenters reveal a complex structure that includes a San Andreas fault extending to the base of the seismogenic layer. A body with high V p extends the length of the rupture and fills the 5 km wide volume between the Loma Prieta mainshock rupture and the San Andreas and Sargent faults. We suggest that this body controls both the pattern of background seismicity on the San Andreas and Sargent faults and the extent of rupture during the mainshock, thus explaining how the background seismicity outlined the along-strike and depth extent of the mainshock rupture on a different fault plane 5 km away. New aftershock focal mechanisms, based on three-dimensional ray tracing through the velocity model, support a heterogeneous postseismic stress field and can not resolve a uniform fault normal compression. The subvertical (or steeply dipping) San Andreas fault and the fault surfaces that ruptured in the 1989 Loma Prieta earthquake are both parts of the San Andreas fault zone and this section of the fault zone does not have a single type of characteristic event.

Journal of Geophysical Research B: Solid Earth↗
USGS1998

Deformation across the rupture zone of the 1964 Alaska earthquake, 1993–1997

A linear array of 15 geodetic monuments was installed in 1993 across the rupture zone of the 1964 Alaska earthquake ( M w = 9.2). The array extends from Middleton Island (at the edge of the continental shelf and 80 km from the Alaska‐Aleutian trench) to north of Palmer, Alaska (380 km from the trench), in the approximate direction of Pacific‐North American plate convergence (N15.5°W). The array was surveyed in June 1993, May 1995, and June 1997. The changes between surveys are a measure of the deformation of the continental margin across the subduction zone in southern Alaska. Measured relative to the interior of the North American plate, the horizontal velocities on the outer plate margin are parallel to the direction of plate convergence (N15.5°W ) and reach a maximum (58 mm yr −1 ) about 150 km from the trench. Beyond about 300 km from the trench the observed horizontal velocities are small. A narrow (halfwidth 50 km) zone of significant uplift (10 mm yr −1 maximum) is observed about 300 km from the trench, coinciding roughly with the locus of maximum coseismic subsidence associated with the 1964 Alaska earthquake. Although the deformation is roughly described by the conventional model of deformation at a subduction zone (deformation due to virtual back slip on the main thrust zone at the 55 mm yr −1 plate convergence rate), a better fit is given with a 65 mm yr −1 virtual back (normal) slip rate. This higher rate is attributed to continued postseismic relaxation. The model does not explain the relatively high uplift rate and low N15.5°W velocity observed at Middleton Island. That anomalous motion is attributed to continued thrusting on postulated upward trending splays from the subduction zone beneath the island.

Alaska↗
USGS1998

Paleomagnetism of the Miocene intrusive suite of Kidd Creek: Timing of deformation in the Cascade arc, southern Washington

Paleomagnetic study of the intrusive suite of Kidd Creek in the southern Washington Cascades (23 sites in dikes and sills) was undertaken to help determine if these rocks are comagmatic and whether they postdate regional folding of the volcanic arc. Fission track and 40 Ar- 39 Ar age determinations indicate an age of ∼12.7 Ma (middle Miocene) for these rocks. The similarity of normal-polarity characteristic directions for most samples corroborate the available geochemical data indicating that these rocks are most likely comagmatic. Reversed-polarity directions for samples from four sites, however, show that emplacement of Kidd Creek intrusions spanned at least one reversal of the geomagnetic field. The paleomagnetic directions for the dikes and sills fail a fold test at the 99% confidence level indicating that the Kidd Creek rocks postdate regional folding. The mean in situ direction also indicates that the Kidd Creek and older rocks have been rotated 22°±6° clockwise about a vertical or near-vertical axis from the expected Miocene direction. Compression and regional folding of the Cascade arc in southern Washington therefore had ended by ∼12 Ma prior to the onset of deformation resulting in rotation of these rocks.

Journal of Geophysical Research B: Solid Earth↗
USGS1998

Time functions of deep earthquakes from broadband and short-period stacks

To constrain dynamic source properties of deep earthquakes, we have systematically constructed broadband time functions of deep earthquakes by stacking and scaling teleseismic P waves from U.S. National Seismic Network, TERRAscope, and Berkeley Digital Seismic Network broadband stations. We examined 42 earthquakes with depths from 100 to 660 km that occurred between July 1, 1992 and July 31, 1995. To directly compare time functions, or to group them by size, depth, or region, it is essential to scale them to remove the effect of moment, which varies by more than 3 orders of magnitude for these events. For each event we also computed short-period stacks of P waves recorded by west coast regional arrays. The comparison of broadband with short-period stacks yields a considerable advantage, enabling more reliable measurement of event duration. A more accurate estimate of the duration better constrains the scaling procedure to remove the effect of moment, producing scaled time functions with both correct timing and amplitude. We find only subtle differences in the broadband time-function shape with moment, indicating successful scaling and minimal effects of attenuation at the periods considered here. The average shape of the envelopes of the short-period stacks is very similar to the average broadband time function. The main variations seen with depth are (1) a mild decrease in duration with increasing depth, (2) greater asymmetry in the time functions of intermediate events compared to deep ones, and (3) unexpected complexity and late moment release for events between 350 and 550 km, with seven of the eight events in that depth interval displaying markedly more complicated time functions with more moment release late in the rupture than most events above or below. The first two results are broadly consistent with our previous studies, while the third is reported here for the first time. The greater complexity between 350 and 550 km suggests greater heterogeneity in the failure process in that depth range.

Journal of Geophysical Research B: Solid Earth↗
USGS1998

Evolution of the Gorda Escarpment, San Andreas fault and Mendocino triple junction from multichannel seismic data collected across the northern Vizcaino block, offshore northern California

The Gorda Escarpment is a north facing scarp immediately south of the Mendocino transform fault (the Gorda/Juan de Fuca-Pacific plate boundary) between 126°W and the Mendocino triple junction. It elevates the seafloor at the northern edge of the Vizcaino block, part of the Pacific plate, ∼1.5 km above the seafloor of the Gorda/Juan de Fuca plate to the north. Stratigraphy interpreted from multichannel seismic data across and close to the Gorda Escarpment suggests that the escarpment is a relatively recent pop-up feature caused by north-south compression across the plate boundary. Close to 126°W, the Vizcaino block acoustic basement shallows and is overlain by sediments that thin north toward the Gorda Escarpment. These sediments are tilted south and truncated at the seafloor. By contrast, in a localized region at the eastern end of the Gorda Escarpment, close to the Mendocino triple junction, the top of acoustic basement dips north and is overlain by a 2-km-thick wedge of pre-11 Ma sedimentary rocks that thickens north, toward the Gorda Escarpment. This wedge of sediments is restricted to the northeast corner of the Vizcaino block. Unless the wedge of sediments was a preexisting feature on the Vizcaino block before it was transferred from the North American to the Pacific plate, the strong spatial correlation between the sedimentary wedge and the triple junction suggests the entire Vizcaino block, with the San Andreas at its eastern boundary, has been part of the Pacific plate since significantly before 11 Ma.

Journal of Geophysical Research B: Solid Earth↗
USGS1998

Poroelastic rebound along the Landers 1992 earthquake surface rupture

Maps of surface displacement following the 1992 Landers, California, earthquake, generated by interferometric processing of ERS-1 synthetic aperture radar (SAR) images, reveal effects of various postseismic deformation processes along the 1992 surface rupture. The large-scale pattern of the postseismic displacement field includes large lobes, mostly visible on the west side of the fault, comparable in shape with the lobes observed in the coseismic displacement field. This pattern and the steep displacement gradient observed near the Emerson-Camp Rock fault cannot be simply explained by afterslip on deep sections of the 1992 rupture. Models show that horizontal slip occurring on a buried dislocation in a Poisson's material produces a characteristic quadripole pattern in the surface displacement field with several centimeters of vertical motion at distances of 10–20 km from the fault, yet this pattern is not observed in the postseismic interferograms. As previously proposed to explain local strain in the fault step overs [ Peltzer et al ., 1996b], we argue that poroelastic rebound caused by pore fluid flow may also occur over greater distances from the fault, compensating the vertical ground shift produced by fault afterslip. Such a rebound is explained by the gradual change of the crustal rocks' Poisson's ratio value from undrained (coseismic) to drained (postseismic) conditions as pore pressure gradients produced by the earthquake dissipate. Using the Poisson's ratio values of 0.27 and 0.31 for the drained and undrained crustal rocks, respectively, elastic dislocation models show that the combined contributions of afterslip on deep sections of the fault and poroelastic rebound can account for the range change observed in the SAR data and the horizontal displacement measured at Global Positioning System (GPS) sites along a 60-km-long transect across the Emerson fault [ Savage and Svarc , 1997]. Using a detailed surface slip distribution on the Homestead Valley, Kickapoo, and Johnson Valley faults, we modeled the poroelastic rebound in the Homestead Valley pull apart. A Poisson's ratio value of 0.35 for the undrained gouge rocks in the fault zone is required to account for the observed surface uplift in the 3.5 years following the earthquake. This large value implies a seismic velocity ratio V p /V s of 2.1, consistent with the observed low V s values of fault zone guided waves at shallow depth [ Li et al ., 1997]. The SAR data also reveal postseismic creep along shallow patches of the Eureka Peak and Burnt Mountain faults with a characteristic decay time of 0.8 years. Coseismic, dilatant hardening (locking process) followed by post-seismic, pore pressure controlled fault creep provide a plausible mechanism to account for the decay time of the observed slip rate along this section of the fault.

Journal of Geophysical Research B: Solid Earth↗
USGS1998

Synthetic aperture radar interferometry coherence analysis over Katmai volcano group, Alaska

The feasibility of measuring volcanic deformation or monitoring deformation of active volcanoes using space-borne synthetic aperture radar (SAR) interferometry depends on the ability to maintain phase coherence over appropriate time intervals. Using ERS 1 C band (λ=5.66 cm) SAR imagery, we studied the seasonal and temporal changes of the interferometric SAR coherence for fresh lava, weathered lava, tephra with weak water reworking, tephra with strong water reworking, and fluvial deposits representing the range of typical volcanic surface materials in the Katmai volcano group, Alaska. For interferograms based on two passes with 35 days separation taken during the same summer season, we found that coherence increases after early June, reaches a peak between the middle of July and the middle of September, and finally decreases until the middle of November when coherence is completely lost for all five sites. Fresh lava has the highest coherence, followed by either weathered lava or fluvial deposits. These surfaces maintain relatively high levels of coherence for periods up to the length of the summer season. Coherence degrades more rapidly with time for surfaces covered with tephra. For images taken in different summers, only the lavas maintained coherence well enough to provide useful interferometric images, but we found only a small reduction in coherence after the first year for surfaces with lava. Measurement of volcanic deformation is possible using summer images spaced a few years apart, as long as the surface is dominated by lavas. Our studies suggest that in order to make volcanic monitoring feasible along the Aleutian arc or other regions with similar climatic conditions, observation intervals of the satellite with C band SAR should be at least every month from July through September, every week during the late spring/early summer or late fall, and every 2–3 days during the winter.

Journal of Geophysical Research B: Solid Earth↗
USGS1999

Mapping the rupture process of moderate earthquakes by inverting accelerograms

We present a waveform inversion method that uses recordings of small events as Green's functions to map the rupture growth of moderate earthquakes. The method fits P and S waveforms from many stations simultaneously in an iterative procedure to estimate the subevent rupture time and amplitude relative to the Green's function event. We invert the accelerograms written by two moderate Parkfield earthquakes using smaller events as Green's functions. The first earthquake ( M = 4.6) occurred on November 14, 1993, at a depth of 11 km under Middle Mountain, in the assumed preparation zone for the next Parkfield main shock. The second earthquake ( M = 4.7) occurred on December 20, 1994, some 6 km to the southeast, at a depth of 9 km on a section of the San Andreas fault with no previous microseismicity and little inferred coseismic slip in the 1966 Parkfield earthquake. The inversion results are strikingly different for the two events. The average stress release in the 1993 event was 50 bars, distributed over a geometrically complex area of 0.9 km 2 . The average stress release in the 1994 event was only 6 bars, distributed over a roughly elliptical area of 20 km 2 . The ruptures of both events appear to grow spasmodically into relatively complex shapes: the inversion only constrains the ruptures to grow more slowly than the S wave velocity but does not use smoothness constraints.

Journal of Geophysical Research B: Solid Earth↗
USGS1998

Seismic structure of the southern Cascadia subduction zone and accretionary prism north of the Mendocino triple junction

Four multichannel-seismic reflection profiles, collected as part of the Mendocino triple junction seismic experiment, image the toe of the southern Cascadia accretionary prism. Today, 250–600 m of sediment is subducting with the Gorda plate, and 1500–3200 m is accreting to the northern California margin. Faults imaged west and east of the deformation front show mixed structural vergence. A north-south trending, 20 km long portion of the central margin is landward vergent for the outer 6–8 km of the toe of the prism. This region of landward vergence exhibits no frontal thrust, is unusually steep and narrow, and is likely caused by a seaward-dipping backstop close to the deformation front. The lack of margin-wide preferred seaward vergence and wedge-taper analysis suggests the prism has low basal shear stress. The three southern lines image wedge-shaped fragments of oceanic crust 1.1–7.3 km in width and 250–700 m thick near the deformation front. These wedges suggest shortening and thickening of the upper oceanic crust. Discontinuities in the seafloor west of the prism provide evidence for mass wasting in the form of slump blocks and debris fans. The southernmost profile extends 75 km west of the prism imaging numerous faults that offset both the Gorda basin oceanic crust and overlying sediments. These high-angle faults, bounding basement highs, are interpreted as strike-slip faults reactivating structures originally formed at the spreading ridge. Northeast or northwest trending strike-slip faults within the basin are consistent with published focal mechanism solutions and are likely caused by north-south Gorda-Pacific plate convergence.

Journal of Geophysical Research B: Solid Earth↗
USGS1999

Coseismic slip resolution along a plate boundary megathrust: the Nankai Trough, southwest Japan

Geodetic survey measurements are used to estimate the coseismic slip distribution in the 1944 Tonankai (M w =8.1) and 1946 Nankaido (M w =8.3) earthquakes and to assess quantitatively the degree to which this slip is resolved on the plate boundary megathrust. Data used include 798 angle changes from triangulation surveys, 328 leveling section differences, and 5 coseismic tidal gage offsets. Many of the nominally coseismic triangulation data span ∼50 years, nearly half the earthquake cycle, and correction for interseismic deformation using post-1950 observations is applied. Microseismicity is used to define the configuration of the plate boundary interface and approximate it with a continuous, multisegment fault model. Because the onshore geodetic data have very limited resolving power for offshore fault segments, offshore coseismic slip was constrained by Satctke's [1993] estimation based on tsunami data. The majority of the coseismic slip occurs between 15 and 25 km depth. Although resolution declines toward the trench axis, it is sufficiently good to define two distinct high-slip regions, one off southeastern Shikoku Island (11 m maximum) and the other offshore of Kii Peninsula (3 m maximum). The slip magnitude off southeastern Shikoku, coupled with the plate convergence rate, would imply an recurrence interval of about 270 years, much-longer than the average repeat time of ∼120 years for historical great earthquakes on the Nankai Trough. However, the maximum coseismic slip is sensitive to the assumed fault geometry, and slippage on trough-parallel splay faults could significantly decrease the maximum slip to about 6 m.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Strain accumulation on the San Andreas Fault near Palmdale, California

Precise distance measurements of a 10×25 km 15‐station trilateration network that spans the San Andreas fault west of Palmdale, California, have been repeated annually in the period 1971–1975. The network appears to be deforming under simple uniform tensor shear of about 0.21±0.03 μstrain/yr with the direction of maximum right‐lateral shear parallel to the local strike of the San Andreas fault. Comparison of trilateration with triangulation surveys of the same network shows that the rate of strain accumulation has been constant over the past 40 yr. The strain accumulation can be explained by conventional dislocation models (i.e., slip at depth beneath a locked section) of the San Andreas fault with 30‐ to 50‐mm/yr slip. Leveling surveys along a 16‐km line that crosses the fault at Palmdale indicate significant changes in tilt but with frequent reversals, so that no net tilt has accumulated in the overall period 1935–1975.

California↗
USGS2012

Paleomagnetic results from the Lassiter Coast, Antarctica, and a test for oroclinal bending of the Antarctic Peninsula

Paleomagnetic results from 17 magnetically stable units of Upper Cretaceous (‘Andean’) plutons and dikes of the Lassiter Coast, on the southern Antarctic Peninsula, define a mean paleomagnetic pole at 87°S, 131°W (α 95 = 11.5°). This indicates that little latitudinal movement of the southern Antarctic Peninsula has occurred during the past 100 m.y. All magnetically stable intrusives are normally polarized and are believed to have been emplaced during the Late Cretaceous epoch of predominantly normal polarity. There is no evidence of postemplacement remagnetization. The uncertainty in declination at the 95% confidence level is computed for both the Lassiter Coast data and those data available from other Andean sites in the Antarctic Peninsula. Within the limits of uncertainty, data from four localities north of 68°S support the contention of Dalziel et al. (1973) that there has not been any apparent post‐Late‐Cretaceous oroclinal bending in the northern half of the peninsula. For sites to the south in the Lassiter Coast, the uncertainty in declination, due to steep inclinations, is too large to support reliably or deny any large‐scale structural bending.

Antarctic Peninsula, Lassiter Coast↗
USGS2012

Geodimeter measurements of strain during the Southern California Uplift

A review of geodimeter measurements made along the ‘big‐bend’ section of the San Andreas fault in southern California indicates no significant increment in strain during the period of major uplift (late 1959 to mid‐1963). Specifically, no evidence of an increment in compressional strain normal to the San Andreas fault at the time of the uplift was found. Geodolite measurements at four networks along the big bend independently indicate that the strain rate during the 1974–1977 episode of subsidence was essentially a uniaxial north‐south compression at the rate of about ⅓ μstrain/yr. Whether the 1974–1977 rate is significantly different from earlier rates determined by triangulation is not clear owing to a rather large variability in the earlier determinations.

California↗
USGS2012

Revised geomagnetic polarity time scale for the interval 0–5 m.y. B.P.

A change in the constants used in K‐Ar dating and a significant increase in new data have made a recompilation and recomputation of data used to define the Late Cenozoic K‐Ar polarity time scale highly desirable at this time. All available data in the range 0–5 m.y. have been recalculated using the refined constants, with 354 data points in this time interval now meeting the minimum criteria for acceptability. Recalculation of the major polarity epoch boundaries has yielded ages of 0.73 m.y. for the Brunhes‐Matuyama, 2.48 m.y. for the Matuyama‐Gauss, and 3.40 m.y. for the Gauss‐Gilbert boundaries. A revised polarity time scale has been constructed based on available K‐Ar data and information obtained from marine magnetic anomalies and deep‐sea sedimentary cores.

Journal of Geophysical Research B: Solid Earth↗