Laurie L. Brown

Recent investigations of the 0–5 Ma geomagnetic field recorded by lava flows

We present a synthesis of 0–5 Ma paleomagnetic directional data collected from 17 different locations under the collaborative Time Averaged geomagnetic Field Initiative (TAFI). When combined with regional compilations from the northwest United States, the southwest United States, Japan, New Zealand, Hawaii, Mexico, South Pacific, and the Indian Ocean, a data set of over 2000 sites with high quality, stable polarity, and declination and inclination measurements is obtained. This is a more than sevenfold increase over similar quality data in the existing Paleosecular Variation of Recent Lavas (PSVRL) data set, and has greatly improved spatial sampling. The new data set spans 78°S to 53°N, and has sufficient temporal and spatial sampling to allow characterization of latitudinal variations in the time-averaged field (TAF) and paleosecular variation (PSV) for the Brunhes and Matuyama chrons, and for the 0–5 Ma interval combined. The Brunhes and Matuyama chrons exhibit different TAF geometries, notably smaller departures from a geocentric axial dipole field during the Brunhes, consistent with higher dipole strength observed from paleointensity data. Geographical variations in PSV are also different for the Brunhes and Matuyama. Given the high quality of our data set, polarity asymmetries in PSV and the TAF cannot be attributed to viscous overprints, but suggest different underlying field behavior, perhaps related to the influence of long-lived core-mantle boundary conditions on core flow. PSV, as measured by dispersion of virtual geomagnetic poles, shows less latitudinal variation than predicted by current statistical PSV models, or by previous data sets. In particular, the Brunhes data reported here are compatible with a wide range of models, from those that predict constant dispersion as a function of latitude to those that predict an increase in dispersion with latitude. Discriminating among such models could be helped by increased numbers of low-latitude data and new high northern latitude sites. Tests with other data sets, and with simulations, indicate that some of the latitudinal signature previously observed in VGP dispersion can be attributed to the inclusion of low-quality, insufficiently cleaned data with too few samples per site. Our Matuyama data show a stronger dependence of dispersion on latitude than the Brunhes data. The TAF is examined using the variation of inclination anomaly with latitude. Best fit two-parameter models have axial quadrupole contributions of 2–4% of the axial dipole term, and axial octupole contributions of 1–5%. Approximately 2% of the octupole signature is likely the result of bias incurred by averaging unit vectors.

Structural and temporal requirements for geomagnetic field reversal deduced from lava flows

Reversals of the Earths magnetic field reflect changes in the geodynamo—flow within the outer core—that generates the field. Constraining core processes or mantle properties that induce or modulate reversals requires knowing the timing and morphology of field changes that precede and accompany these reversals. But the short duration of transitional field states and fragmentary nature of even the best palaeomagnetic records make it difficult to provide a timeline for the reversal process. 40Ar/39Ar dating of lavas on Tahiti, long thought to record the primary part of the most recent ‘Matuyama–Brunhes’ reversal, gives an age of 795 ± 7 kyr, indistinguishable from that of lavas in Chile and La Palma that record a transition in the Earths magnetic field, but older than the accepted age for the reversal. Only the ‘transitional’ lavas on Maui and one from La Palma (dated at 776 ± 2 kyr), agree with the astronomical age for the reversal. Here we propose that the older lavas record the onset of a geodynamo process, which only on occasion would result in polarity change. This initial instability, associated with the first of two decreases in field intensity, began ∼18 kyr before the actual polarity switch. These data support the claim that complete reversals require a significant period for magnetic flux to escape from the solid inner core and sufficiently weaken its stabilizing effect.

Volcanism and erosion during the past 930 k.y. at the Tatara-San Pedro complex, Chilean Andes

Geologic mapping, together with 73 new K-Ar and 40Ar/39Ar age determinations of 45 samples from 17 different volcanic units, plus paleomagnetic orientations, geochemical compositions, and terrestrial photogrammetry are used to define the chronostratigraphy of the Tatara–San Pedro complex, an eruptive center at 36°S on the volcanic front of the Andean southern volcanic zone. The Tatara–San Pedro complex preserves ≈55 km3 of lavas that erupted from at least three central vent regions. Remnant, unconformity-bound sequences of lavas are separated by lacunae that include significant periods of erosion. Quaternary volcanism commenced ca. 930 ka with eruption of voluminous dacitic magma, followed 100 k.y. later by the only major rhyolitic eruption. From 780 ka onward, more than 80% of the preserved volume is basaltic andesite (52%–57% SiO2), but petrographically and geochemically diverse dacitic magmas (63%–69% SiO2) erupted sporadically throughout this younger, dominantly mafic phase of activity. A few basaltic lavas (49%–52% SiO2) are present, mainly in portions of the complex older than 230 ka. The number of vents, the petrologic and geochemical diversity, and the temporal distribution of mafic and silicic lavas are consistent with emplacement of many separate batches of mafic magma into the shallow crust beneath the Tatara–San Pedro complex over the past million years. Nearly two-thirds of the preserved volume of the Tatara–San Pedro complex comprises the two youngest volcanoes, which were active between ca. 188–83 ka and 90–19 ka. Repeated advances of mountain glaciers punctuated growth of the complex with major erosional episodes that removed much of the pre-200 ka volcanic record, particularly on the south flank of the complex. Dating the inception of a glaciation on the basis of preserved material is difficult, but the age of the oldest lava above a lacuna may be used to estimate the timing of deglaciation. On this basis, the argon ages of basal lavas of multiple sequences indicate minimum upper limits of lacunae at ca. 830, 790, 610, 400, 330, 230, 110, and 17 ka. These are broadly consistent with global ice-volume peaks predicted by the oxygen isotope-based astronomical time scale and with age brackets on North American glacial advances. Estimated growth rates for the two young volcanoes are 0.2 to 0.3 km3/k.y.; these are three to five times greater than a growth rate estimated from all preserved lavas in the complex (0.06 km3/k.y.). Removal of up to 50%–95% of the material erupted between 930 and 200 ka by repeated glacial advances largely explains this discrepancy, and it raises the possibility that episodic erosion of mid-latitude frontal arc complexes may be extensive and common.

The Santa Rosa Event: 40Ar/39Ar and paleomagnetic results from the Valles rhyolite near Jaramillo Creek, Jemez Mountains, New Mexico

The Jaramillo Event was originally defined by Doell and Dalrymple in 1966 on the basis of K^Ar ages from sanidine in the normally, transitionally and reversely magnetized rhyolite domes named Cerro del Abrigo, Cerro Santa Rosa I and Cerro Santa Rosa II, respectively, that erupted following collapse of the archetypal Valles Caldera, New Mexico. We have collected new paleomagnetic data from the three domes and new 40 Ar/ 39 Ar laser fusion and furnace incremental heating experiments on sanidine crystals from the Cerro Santa Rosa I rhyolite. Step-wise alternating field and thermal demagnetization techniques applied to 52 samples from seven sites indicate that the original paleomagnetic results of Doell and Dalrymple [Science 152 (1966) 1060^1061] are valid. Cerro del Abrigo is normally magnetized, whereas the Santa Rosa I dome is transitional with an inclination of 363‡ toward the east and the Santa Rosa II dome is of reversed polarity. Twenty-five laser fusion experiments on sanidine crystals from the the Cerro Santa Rosa I dome, together with the saddle-shaped spectrum obtained by incrementally heating the sanidine in a furnace reveal that this rhyolite contained a small but significant component of excess argon prior to eruption. Our preferred age of 936 ˛ 8 ka (˛ 2c) for the Santa Rosa I rhyolite is based on the concordant laser fusion isochron and incremental heating plateau ages. This age is significantly older than was inferred on the basis of earlier laser fusion 40 Ar/ 39 Ar results that suggested a trapped component characterized by a 40 Ar/ 36 Ar ratio lower than the atmospheric value of 295.5. Our new age distinguishes the Santa Rosa I dome by 65 kyr from the termination of the Jaramillo normal subchron 1001 ˛ 10 ka and by 37 kyr from basaltic lavas at Haleakala and Tahiti, which record the Kamikatsura Event 899 ˛ 6 ka. Moreover, this determination is a factor of three more precise than the best previous estimate for a proposed geomagnetic event in this time period. The Cerro Santa Rosa I rhyolite dome, once intimately linked with the end of the Jaramillo Event and the acceptance of plate tectonic theory, now defines a highly resolved feature, the Santa Rosa Event, in a terrestrial geomagnetic reversal time scale that is consistent with the global record of magnetic field intensity from marine sediments. fl 2002 Elsevier Science B.V. All rights reserved.

A closer look at remanence-dominated aeromagnetic anomalies: Rock magnetic properties and magnetic mineralogy of the Russell Belt microcline-sillimanite gneiss, northwest Adirondack Mountains, New York

A large, distinct negative aeromagnetic anomaly of over 2000 nT associated with microcline-sillimanite-quartz gneisses in the Russell area, northwest Adirondack Mountains, was previously shown to be remanence-dominated, although the carriers of remanence were not well documented. Russell Belt gneisses have a strong natural remanent magnetization with steep remanence directions, D = 263°, I = −58°, an average intensity of 3.6 A/m, and typical susceptibilities of 10−4 SI. The remanence is thermochemical in origin, acquired during cooling from peak metamorphic conditions of 650°–750°C during the Ottawan Orogen (1050–1080 Ma). The reversed polarity of remanence reflects a reversed paleofield, rather than self-reversed, contrary to earlier suggestions. The gneisses contain up to 3% oxide, predominantly metamorphic titanohematite, which accounts for the low susceptibility values and highly stable remanence. Optical observations show titanohematite grains with multiple generations of ilmenite, pyrophanite, rutile, and spinel exsolution lamellae. Microprobe analyses confirm titanohematite compositions ranging from 72 to 97% Fe2O3 with hematite83 being most typical. In rare samples, inclusions of magnetite were identified. The ubiquitous presence of titanohematite, and the rare occurrence of magnetite, is supported by thermal and alternating field demagnetization studies, saturation magnetization measurements, hysteresis properties, temperature-hysteresis studies, and low-temperature remanence measurements. Numerous crustal granulites have titanohematite as part of the oxide assemblage, and this may contribute a strong remanent component to what have previously been considered to be solely induced anomalies.

Tectonic and geologic evolution of the Espanola Basin, Rio Grande Rift: Structure, rate of extension, and relation to the state of stress in the western United States

Abstract The Espanola basin of the Rio Grande rift began as a broad crustal downwarp in latest Oligocene time. Most of the basin is 2–3 km deep, but localized faulting allowed accumulation of up to 5 km of sedimentary fill in a central sub-basin. The localized early faulting ended before filling of the central Espanola basin was completed about 10 m.y. ago. Movement on faults that define the present western margin of the Espanola basin began ~ 10 m.y. ago. Jemez Mountain volcanism, in the western Espanola basin, also began at about this same time. West tilting of up to 30° occurred due to movement along pervasive N-trending intrabasin faults about 7.5 m.y. ago in conjunction with continued movement along the western border faults. Volcanism continued after this tilting, forming many of the large volcanic constructs of the Jemez Mountains. Regional uplift of the entire northern Rio Grande rift began ~ 7 m.y. ago. Movement on the Pajarito fault zone began about 5 m.y. ago and continues to the present. This fault zone defines the western margin of the velarde graben, a narrow central sub-basin where recent movement has been concentrated. Some volcanism also has occurred within the southern Velarde graben. Total extension across the Espanola basin since ~ 26 m.y. ago is estimated to have been ~ 5.5 km (roughly 10%) or between 3.5 and 8 km assuming high-angle planar faulting. The ~ 0.2 mm/yr averaged long term rate of extension has been separated into three periods of activity: 1. (1) ~ 0.14 mm/yr from 26 to 10 m.y. ago. 2. (2) ~ 0.5 mm/yr from 10 to 5 m.y. ago. 3. (3) ~ 0.14 mm/yr from 5 m.y. ago to present. A change in least principal stress direction from WSW-ENE to WNW-ESE that occurred throughout the western United States about 10 m.y. ago coincides with a roughly 3.5 times increase in the rate of extension, preferential development and movement of N- to NE-trending normal faults, and a few degrees of clockwise rotation of rocks in the western Espanola basin. Similar to the Espanola basin, initial basins of the southern Rio Grande rift were broad downwarps and rifting was greatly accelerated after ~ 10 m.y. ago. Accelerated uplift of the northern Rio Grande rift also occurred at about this time indicating that activity in the entire Rio Grande rift was modulated by this change in extension direction ~ 10 m.y. ago that appears related to Pacific-North American plate interactions. This modulation coupled with major faulting (~ 10 m.y. ago) preceding uplift (~ 7 m.y. ago) in the Espanola basin suggest a passive rifting process for the Rio Grande rift whereby stresses due to plate interactions elsewhere cause faulting in the lithosphere which leads to the development of a “passive” asthenospheric uplift. Furthermore, the roughly 20 m.y. pre-uplift history of sediment accumulation in basins of the central and southern rift, and the inherited character, trend, and geometry of the Rio Grande rift as a whole are also more consistent with a passive rifting process.

40Ar/39Ar chronology of late pliocene and early pleistocene geomagnetic and glacial events in southern Argentina

K-Ar dating and paleomagnetic directions from the lava sequence atop Cerro del Fraile, Argentina, contributed to the nascent Geomagnetic Polarity Time Scale (GPTS), recording the Reunion event, and the Olduvai and Jaramillo subchrons [Fleck et al., 1972]. New stratigraphy, paleomagnetic analyses, 40 Ar/ 39 Ar incremental heating ages, and unspiked K-Ar dating of 10 lava flows on Cerro del Fraile place these eruptions between 2.181±0.097 and 1.073±0.036 Ma and enhance this unique record, which includes seven tills interbedded with the lavas. The Reunion event is recorded by three lavas with transitional, normal, and reversed polarity that yielded identical 40 Ar/ 39 Ar isochron ages and a weighted mean age of 2.136±0.019 Ma. When combined with 40 Ar/ 39 Ar ages from lavas on Reunion Island and a normal tuff in the Massif Central, the age of the Reunion event is 2.137±0.016 Ma and is older by ∼50 kyr than the 2.086±0.016 Ma Huckleberry Ridge event. The onset and termination of the Olduvai are similarly constrained to 1.922±0.066 Ma and 1.775±0.015 Ma, whereas the onset of the Jaramillo occurred 1.069±0.011 Ma. A discordant age spectrum from another transitional lava gave a total fusion age of 1.61 Ma and an unspiked K-Ar age of 1.43 Ma. It is uncertain whether this corresponds to the Gilsa, Gardar, Stage 54, or Sangiran events, or represents an unrecognized period of geomagnetic instability. Deposition of till on the piedmont surface prior to 2.186 Ma and six subsequent tills between 2.186 Ma and ∼1.073 Ma mark frequent glaciations of southern South America during marine oxygen isotope stages 82 to 48.

Matuyama-Brunhes transition recorded in lava flows of the Chilean Andes: Evidence for dipolar fields during reversals

The transitional behavior exhibited by Earth9s magnetic field during polarity reversals holds the key to understanding the complexities of the field. Interpretation of the severely limited reversal data set, however, remains controversial. Discussion centers around the nature of pole paths during transitions and their possible relation to other geophysical phenomena. Paleomagnetic data from Volcan Tatara-San Pedro in the central Chilean Andes (lat 36°S, long 71°W) provide the first record of the Matuyama-Brunhes reversal from South America and only the third transitional record from the Southern Hemisphere. A stratigraphic section of ten lava flows yielded intermediate pole positions that center in Australia; the mean pole is 16.8°S, 133.0°E. K-Ar analyses of two of these flows provided dates of 768 ±8 ka and 763 ±14 ka, ages coincident with the established Matuyama-Brunhes boundary. The clustering of these transitional poles strengthens the hypothesis that Earth9s magnetic field retains a strong dipole component during reversals.

Recurrent eruption and subsidence at the Platoro caldera complex, southeastern San Juan volcanic field, Colorado: New tales from old tuffs

Reinterpretation of a voluminous regional ash-flow sheet (Masonic Park Tuff) as two separate tuff sheets of similar phenocryst-rich dacite erupted from separate source calderas has important implications for evolution of the multicyclic Platoro caldera complex and for caldera-forming processes generally. Masonic Park Tuff in central parts of the San Juan field, including the type area, was erupted from a concealed source at 28.6 Ma, but widespread tuff previously mapped as Masonic Park Tuff in the southeastern San Juan Mountains is the product of the youngest large-volume eruption of the Platoro caldera complex at 28.4 Ma. This large unit, newly named the “Chiquito Peak Tuff,” is the last-erupted tuff of the Treasure Mountain Group, which consists of at least 20 separate ash-flow sheets of dacite to low-silica rhyolite erupted from the Platoro complex during a 1 m.y. interval (29.5−28.4 Ma). Two Treasure Mountain tuff sheets have volumes in excess of 1000 km 3 each, and five more have volumes of 50–150 km 3 . The total volume of ash-flow tuff exceeds 2500 km 3 , and caldera-related lavas of dominantly andesitic composition make up 250-500 km 3 more. A much greater volume of intermediate-composition magma must have solidified in subcaldera magma chambers. Most preserved features of the Platoro complex-including postcollapse asymmetrical trap-door resurgent uplift of the ponded intracaldera tuff and concurrent infilling by andesitic lava flows-postdate eruption of the Chiquito Peak Tuff. The numerous large-volume pre-Chiquito Peak ash-flow tuffs document multiple eruptions accompanied by recurrent subsidence; early-formed caldera walls nearly coincide with margins of the later Chiquito Peak collapse. Repeated syneruptive collapse at the Platoro complex requires cumulative subsidence of at least 10 km. The rapid regeneration of silicic magmas requires the sustained presence of an andesitic subcaldera magma reservoir, or its rapid replenishment, during the 1 m.y. life span of the Platoro complex. Either case implies large-scale stoping and assimilative recycling of the Tertiary section, including intracaldera tuffs.

Paleomagnetic directions and 40Ar/39Ar ages from the Tatara‐San Pedro volcanic complex, Chilean Andes: Lava record of a Matuyama‐Brunhes precursor?

the same volcanic sequence overlain by flows with normal polarity. The 40 Ar/ 39 Ar incremental heating experiments on lavas within the two sections provide nine independent age determinations and yield a weighted mean of 791.7 ± 3.0 ka (±2s) for the paleomagnetic transition. The sections are linked by geological mapping, the precise radioisotopic dating, and geochemical correlations. Alternating field and thermal demagnetization studies, rock magnetic analyses, and petrographic observations indicate that the magnetization is primary and carried by titanomagnetite. The polarity change is characterized by a jump from reverse poles to a quasi-stationary cluster of virtual geomagnetic poles over Australia, followed by a jump to normal polarity latitudes. Magnetization of these lavas is thus consistent with either a brief period when the field was dominated by a subequatorial dipole, or a more complex nondipolar field that may reflect the influence of a long-lived regional lower mantle control over a weakened dynamo. The Quebrada Turbia lavas are circa 16 kyr older than those, dated by exactly the same methods, which record a later more complex portion of the reversal at Haleakala volcano, Maui. Moreover, the 792 ka radioisotopic age of these Chilean lavas is older than most astronomical estimates for the Matuyama-Brunhes reversal suggesting that this section may, in fact, record a precursor to the actual field reversal, that is expressed by low paleointensities in more than a dozen well-studied marine sediment cores. INDEX TERMS: 1535 Geomagnetism and Paleomagnetism: Reversals (process, timescale, magnetostratigraphy); 1030 Geochemistry: Geochemical cycles (0330); 9360 Information Related to Geographic Region: South America; 9604 Information Related to Geologic Time: Cenozoic; KEYWORDS: paleomagnetism, reversal,