Jonathan M. G. Glen

AGE OF ETENDEKA FLOOD VOLCANISM AND ASSOCIATED INTRUSIONS IN SOUTHWESTERN AFRICA

Detailed 40Ar/39Ar laser step-heating analyses of mineral separates from five volcanic units in Namibia and Angola and five intrusions in Namibia yield important geochronological data for the Etendeka igneous province. Ten plateau dates on plagioclase, hornblende, and biotite between 131.7 ± 0.7 and 132.3 ± 0.7 Ma were obtained, and a late syenite from the Messum intrusive complex yielded a slightly younger hornblende plateau date of 129.3 ± 0.7 Ma. Magnetostratigraphy of the volcanic rocks in three sections up to 700 m thick, laterally spanning more than 100 km, suggests that the flows record only two geomagnetic polarity reversals. Precise temporal coincidence with the Parana flood volcanic province in South America indicates that Etendeka volcanism does not represent a significantly younger phase of magmatism that migrated from northwest to southeast over 10 m.y., as has recently been proposed. The duration of intrusive activity was at least 2–3 m.y. longer than recorded by volcanism, and its total duration awaits further constraints.

Large-scale fractures related to inception of the Yellowstone hotspot

During middle Miocene time, western North America was subject to flood-basalt volcanism, dike-swarm injection, and broad-scale fracturing and folding of the crust. We propose a simple model to account for these events and for a regional pattern of geologic and geophysical features. Aeromagnetic maps reveal some of the most important elements of this pattern, which are several narrow, arcuate anomalies, here referred to as the Northern Nevada rifts. These rifts extend hundreds of kilometers across Nevada and are likely caused by highly magnetic, middle Miocene mafic dikes. With the aid of filtering techniques, the anomalies can be traced into Oregon. Together with other geologic features, such as fold axes, dike swarms, and faults, they produce a spoke-like pattern fanning over 2208 of arc that converges toward a point near the Oregon-Idaho border (lat ;448N). A possible cause for this pattern is a point source of stress at the base of the crust related to the formation of the Yellowstone hotspot. The spoke-like pattern, however, does not persist at large distances from the emerging hotspot; several hundred kilometers to the south, the Northern Nevada rifts deviate significantly (.308) from a radial trend. We show that a simple model—imposing a point source of stress at the base of the crust and a regional stress field aligned with the presumed middle Miocene stress direction—fits the observed fracture pattern. It thus accounts for both the radial pattern present near the nascent hotspot and the far-field pattern due to regional stresses.

Magma flow inferred from anisotropy of magnetic susceptibility in the coastal Paraná-Etendeka igneous province: Evidence for rifting before flood volcanism

The Parana-Etendeka igneous province is one of the largest flood volcanic provinces in the world; peak magmatic activity at 132 Ma is believed to have occurred about 5 m.y. before the birth of south Atlantic sea floor and development of rift basins along the Brazilian coastal margin. Anisotropy of magnetic susceptibility (AMS) measurements on 283 samples (28 flows and 3 sills) from the Etendeka igneous province of Namibia and 180 samples (21 flows) from the Parana province in Brazil reveal remarkably consistent fabric orientations with maximum susceptibility (K1) axes subhorizontal and parallel to the rifted margin. The AMS results are most likely due to shape anisotropy reflecting magma flow directions, suggesting that lava flows and intrusive conduits near the eventual rifted margin were controlled by structures having topographic expression in existence at the time of peak flood volcanism. These results imply that rifting preceded flood volcanism, at least in the portion of the magmatic province within 100 km of the nascent Mid-Atlantic Ridge.

Differentiating induced and natural seismicity using space‐time‐magnitude statistics applied to the Coso Geothermal field

A remarkable characteristic of earthquakes is their clustering in time and space, displaying their self-similarity. It remains to be tested if natural and induced earthquakes share the same behavior. We study natural and induced earthquakes comparatively in the same tectonic setting at the Coso Geothermal Field. Covering the preproduction and coproduction periods from 1981 to 2013, we analyze interevent times, spatial dimension, and frequency-size distributions for natural and induced earthquakes. Individually, these distributions are statistically indistinguishable. Determining the distribution of nearest neighbor distances in a combined space-time-magnitude metric, lets us identify clear differences between both kinds of seismicity. Compared to natural earthquakes, induced earthquakes feature a larger population of background seismicity and nearest neighbors at large magnitude rescaled times and small magnitude rescaled distances. Local stress perturbations induced by field operations appear to be strong enough to drive local faults through several seismic cycles and reactivate them after time periods on the order of a year.

Paleointensity of the Earth's magnetic field in Early Cretaceous time: The Paraná Basalt, Brazil

Paleointensity experiments were carried out with the Coe and Thellier methods on Early Cretaceous basalt (133 My old) from the Parana basin in southern Brazil. Paleointensity estimates could be obtained from only six lava flows: three of normal and three of reversed polarity, of the 71 sampled. Moreover, the quality of the determinations is fairly low, with q ratio usually less than 10, mainly as a result of chemical and/or physical changes of the magnetic oxides during heating. This study strongly underscores the difficulty of finding natural rocks suitable for paleointensity experiments even so the directional results previously obtained were good. The most reliable result give an intensity of 28 ± 1 μT, while the others range from 20.8 μT to 37.7 μT. The corresponding virtual dipole moments (VDMs) range from 4.7 x 10 22 to 7.9 x 10 22 Am 2 . This dispersion is attributed mainly to inaccurate estimates of the ancient field rather than to secular variation. Although the number of cooling units for which paleointensity estimates could be obtained is limited, the results nevertheless indicate that the early Cretaceous VDM was significantly lower than that for the recent field but greater than has been reported for this portion of the Mesozoic Dipole Low period.

A detailed record of paleomagnetic field change from Searles Lake, California: 2. The Gauss/Matuyama polarity reversal

This new study of the Gauss/Matuyama transition from Searles Lake, California conjoined with other records from the western United States, provides interesting insights into the structure of the reversing magnetic field. The present study employs improved measurement and data reduction techniques, multiple parallel strings of samples, and a finer sampling interval than was used in the original study by Liddicoat[1982]. Particularly crucial to this investigation was the use of overprint directions to reconstruct declinations, required because the core was rotary drilled. The results of this technique were corroborated by employing an independent method that uses anisotropy of magnetic susceptibility to resolve a sediment fabric: the fabric facilitated the alignment of core segments. The new record reveals that the main swing of the transition occurs over a significantly shorter time span than was found in the original study. In addition, it brings out several small scale variations that were absent in the old record, some of which take the form of relatively rapid jumps in direction that punctuate more steadily varying changes. This alternating steady and rapid field change is similar to behavior observed in volcanic records, which argues that such behavior is not merely an artifact of episodic volcanism. The Searles Lake record is strongly nonzonal and is defined in the Northern Hemisphere by a swath of virtual geomagnetic poles (VGPs) stretching from northern Eurasia to west Africa and to the northwest Pacific. Glen et al. [1994] showed that a collection (spanning >15 Myr) of western North American transition and excursion records displays this same pattern, indicating that the VGP swath is a persistent feature of the transitional field. In addition, the compilation reveals that the swath extends into the Southern Hemisphere, outlining a region marked by an absence of poles that is centered on the Indian Ocean. The fact that this pattern is offset from a similar one seen in global compilations suggests that the persistent fields have a significant nondipolar component. Seven additional records are now available, making the western North America data set perhaps the finest regional set of high-resolution records consisting of both igneous and sedimentary records. The new records, which provide an important test of the existence of the VGP pattern, strongly support the findings that reveal the presence of persistent, long-term (>15 Myr) nondipolar transitional fields.

Eruptions at Lone Star geyser, Yellowstone National Park, USA: 2. Constraints on subsurface dynamics

We use seismic, tilt, lidar, thermal, and gravity data from 32 consecutive eruption cycles of Lone Star geyser in Yellowstone National Park to identify key subsurface processes throughout the geysers eruption cycle. Previously, we described measurements and analyses associated with the geysers erupting jet dynamics. Here we show that seismicity is dominated by hydrothermal tremor (~5–40 Hz) attributed to the nucleation and/or collapse of vapor bubbles. Water discharge during eruption preplay triggers high-amplitude tremor pulses from a back azimuth aligned with the geyser cone, but during the rest of the eruption cycle it is shifted to the east-northeast. Moreover, ~4 min period ground surface displacements recur every 26 ± 8 min and are uncorrelated with the eruption cycle. Based on these observations, we conclude that (1) the dynamical behavior of the geyser is controlled by the thermo-mechanical coupling between the geyser conduit and a laterally offset reservoir periodically filled with a highly compressible two-phase mixture, (2) liquid and steam slugs periodically ascend into the shallow crust near the geyser system inducing detectable deformation, (3) eruptions occur when the pressure decrease associated with overflow from geyser conduit during preplay triggers an unstable feedback between vapor generation (cavitation) and mass discharge, and (4) flow choking at a constriction in the conduit arrests the runaway process and increases the saturated vapor pressure in the reservoir by a factor of ~10 during eruptions.

The Complexity of Reversals

Geomagnetic reversals could be far more complex than even the best, most detailed paleomagnetic record in hand. Sequences of lava flows can give accurate spot readings of the field but yield records that are necessarily shot full of holes, whereas sedimentary recordings smooth temporal variations of the field and may also be subject to hiatuses and delayed, non-uniform lock-in of remanence. Thus, paleomagnetic records are always incomplete and give only lower bounds on how rapidly changing and complex the behavior of the reversing field may have been. Nonetheless, the combined evidence from several high-deposition-rate sedimentary records, multiple lava-flow records of one reversal from the same region, and a reversal record from a dynamically self-consistent numerical geodynamo simulation suggest that at least some reversals are much more complex than typically portrayed, with episodes of oscillatory and very rapid field change.

Geophysical Data Reveal the Crustal Structure of the Alaska Range Orogen within the Aftershock Zone of the Mw 7.9 Denali Fault Earthquake

Geophysical information, including deep-crustal seismic reflection, magnetotelluric (MT), gravity, and magnetic data, cross the aftershock zone of the 3 November 2002 Mw 7.9 Denali fault earthquake. These data and aftershock seis- micity, jointly interpreted, reveal the crustal structure of the right-lateral-slip Denali fault and the eastern Alaska Range orogen, as well as the relationship between this structure and seismicity. North of the Denali fault, strong seismic reflections from within the Alaska Range orogen show features that dip as steeply as 25� north and extend downward to depths between 20 and 25 km. These reflections reveal crustal structures, probably ductile shear zones, that most likely formed during the Late Cretaceous, but these structures appear to be inactive, having produced little seis- micity during the past 20 years. Furthermore, seismic reflections mainly dip north, whereas alignments in aftershock hypocenters dip south. The Denali fault is nonre- flective, but modeling of MT, gravity, and magnetic data suggests that the Denali fault dips steeply to vertically. However, in an alternative structural model, the Denali fault is defined by one of the reflection bands that dips to the north and flattens into the middle crust of the Alaska Range orogen. Modeling of MT data indicates a rock body, having low electrical resistivity (� 10 Xm), that lies mainly at depths greater than 10 km, directly beneath aftershocks of the Denali fault earthquake. The maxi- mum depth of aftershocks along the Denali fault is 10 km. This shallow depth may arise from a higher-than-normal geothermal gradient. Alternatively, the low electrical resistivity of deep rocks along the Denali fault may be associated with fluids that have weakened the lower crust and helped determine the depth extent of the after- shock zone.

Distribution of buried hydrothermal alteration deduced from high‐resolution magnetic surveys in Yellowstone National Park

Yellowstone National Park (YNP) displays numerous and extensive hydrothermal features. Although hydrothermal alteration in YNP has been extensively studied, the volume, geometry, and type of rock alteration at depth remain poorly constrained. In this study, we use high-resolution airborne and ground magnetic surveys and measurements of remanent and induced magnetization of field and drill core samples to provide constraints on the geometry of hydrothermal alteration within the subsurface of three thermal areas in YNP (Firehole River, Smoke Jumper Hot Springs, and Norris Geyser Basin). We observe that hydrothermal zones from both liquid- and vapor-dominated systems coincide with magnetic lows observed in aeromagnetic surveys and with a decrease of the amplitude of short-wavelength anomalies seen in ground magnetic surveys. This suggests a strong demagnetization of both the shallow and deep substratum within these areas associated with the removal of magnetic minerals by hydrothermal alteration processes. Such demagnetization is confirmed by measurements of rock samples from hydrothermal areas which display significantly decreased total magnetization. A pronounced negative anomaly is observed over the Lone Star Geyser and suggests a significant demagnetization of the substratum associated with areas displaying large-scale fluid flow. The ground and airborne magnetic surveys are used to evaluate the distribution of magnetization in the subsurface. This study shows that significant demagnetization occurs over a thickness of at least a few hundred meters in hydrothermal areas at YNP and that the maximum degree or maximum thickness of demagnetization correlates closely with the location of hydrothermal activity and mapped alteration.