Brad S. Singer

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.

Synoptic reconstruction of a major ancient lake system: Eocene Green River Formation, western United States

Numerous 40Ar/39Ar experiments on sanidine and biotite from 22 ash beds and 3 volcaniclastic sand beds from the Greater Green River, Piceance Creek, and Uinta Basins of Wyoming, Colorado, and Utah constrain ∼8 m.y. of the Eocene Epoch. Multiple analyses were conducted per sample using laser fusion and incremental heating techniques to differentiate inheritance, 40Ar loss, and 39Ar recoil. When considered in conjunction with existing radioisotopic ages and lithostratigraphy, biostratigraphy, and magnetostratigraphy, these new age determinations facilitate temporal correlation of linked Eocene lake basins in the Laramide Rocky Mountain region at a significantly increased level of precision. To compare our results to the geomagnetic polarity time scale and the regional volcanic record, the ages of Eocene magnetic anomalies C24 through C20 were recalibrated using seven 40Ar/39Ar ages. Overall, the ages obtained for this study are consistent with the isochroneity of North American land-mammal ages throughout the study area, and provide precise radioisotopic constraints on several important biostratigraphic boundaries. Applying these new ages, average sediment accumulation rates in the Greater Green River Basin, Wyoming, were approximately three times faster at the center of the basin versus its ramp-like northern margin during deposition of the underfilled Wilkins Peak Member. In contrast, sediment accumulation occurred faster at the edge of the basin during deposition of the balanced filled to overfilled Tipton and Laney Members. Sediment accumulation patterns thus reflect basin-center–focused accumulation rates when the basin was underfilled, and supply-limited accumulation when the basin was balanced filled to overfilled. Sediment accumulation in the Uinta Basin, at Indian Canyon, Utah, was relatively constant at ∼150 mm/k.y. during deposition of over 5 m.y. of both evaporative and fluctuating profundal facies, which likely reflects the basin-margin position of the measured section. The most rapid sediment accumulation for the entire system (>1 m/k.y.) occurred between 49.0 and 47.5 Ma, when volcaniclastic materials from the Absaroka and/or Challis volcanic fields entered the Green River Formation lakes from the north. Our new ages combined with existing paleomagnetic and biostratigraphic control permit the first detailed synoptic comparison of lacustrine depositional environments in all the Green River Formation basins. Coupled with previously published paleocurrent observations, our detailed correlations show that relatively freshwater lakes commonly drained into more saline downstream lakes. The overall character of Eocene lake deposits was therefore governed in part by the geomorphic evolution of drainage patterns in the surrounding Laramide landscape. Freshwater (overfilled) lakes were initially dominant (53.5–52.0 Ma), possibly related to high erosion rates of remnant Cretaceous strata on adjacent uplifts. Expansion of balanced-fill lakes first occurred in all Green River Formation basins at 52.0–51.3 Ma and again between 49.6 and 48.5 Ma. Evaporative (underfilled) lakes occurred in various basins between 51.3 and 45.1 Ma, coincident with the end of the early Eocene climatic optima and subsequent onset of global cooling defined from marine record. However, evaporite intervals in the different depocenters were deposited at different times rather than being confined to a single episode of arid climate. Evaporative terminal sinks were initially located in the Greater Green River and Piceance Creek Basins (51.3–48.9 Ma), then gradually migrated southward to the Uinta Basin (47.1–45.2 Ma). This history is likely related to progressive southward construction of the Absaroka Volcanic Province, which constituted a major topographic and thermal anomaly that contributed to a regional north to south hydrologic gradient. The Greater Green River and Piceance Creek Basins were eventually filled from north to south with Absarokaderived detritus at sedimentation rates 1–2 orders of magnitude greater than the underlying lake deposits.

40Ar/39Ar and K-Ar chronology of Pleistocene glaciations in Patagonia

During the Pleistocene, east of Lago Buenos Aires, Argentina, at 46.5 °S, at least 19 terminal moraines were deposited as piedmont glaciers from the Patagonian ice cap advanced onto the semi-arid high plains adjacent to the southern Andes. Exceptional preservation of these deposits offers a rare opportunity to document ice-cap fluctuations during the last 1.2 m.y. 4 0 Ar/ 3 9 Ar incremental-heating and unspiked K-Ar experiments on four basaltic lava flows interbedded with the moraines provide a chronologic framework for the entire glacial sequence. The 4 0 Ar/ 3 9 Ar isochron ages of three lavas that overlie till 90 km east of the Cordillera at Lago Buenos Aires, and another 120 km from the Andes along Rio Gallegos at 51.8 °S that underlies till, strongly suggest that the ice cap reached its greatest eastward extent ca. 1100 ka. At least six moraines were deposited within the 256 k.y. period bracketed by basaltic eruptions at 1016 ′ 10 ka and 760 ′ 14 ka. Similarly, six younger, more proximal moraines were deposited during an ∼651 k.y. period bracketed by an underlying 760 ′ 14 ka basalt and the 109 ′ 3 ka Cerro Volcan basalt flow that buried all six moraines. Coupled with in situ cosmogenic surface exposure ages of moraine boulders, the 109 ka age of Cerro Volcan implies that moraines deposited during the penultimate local glaciation correspond to marine oxygen isotope stage 6. Further westward toward Lago Buenos Aires, six additional moraines younger than the Cerro Volcan basalt flow occur. Surface exposure dating of boulders on these moraines, combined with the 1 4 C age of overlying varved lacustrine sediment, indicates deposition during the Last Glacial Maximum (LGM, 23-16 ka). Although Antarctic dust records signal an important Patagonian glaciation at 60-40 ka, moraines corresponding to marine oxygen isotope stage 4 are not preserved at Lago Buenos Aires; apparently, these were overrun and obliterated by the younger ice advance at 23 ka. Notwithstanding, the overall pattern of glaciation in Patagonia is one of diminishing eastward extent of ice during successive glacial advances over the past 1 m.y. We hypothesize that tectonically driven uplift of the Patagonian Andes, which began in the Pliocene, yet continued into the Quaternary, in part due to subduction of the Chile rise spreading center during the past 2 m.y., maximized the ice accumulation area and ice extent by 1.1 Ma. Subsequent deep glacial erosion has reduced the accumulation area, resulting in less extensive glaciers over time.

Reconciling astrochronological and 40Ar/39Ar ages for the Matuyama-Brunhes boundary and late Matuyama Chron

When five Matuyama-Brunhes (M-B) boundary records from the North Atlantic are placed on isotope age models, produced by correlation of the δ18O record directly or indirectly to an ice volume model, the M-B boundary lies consistently at the young end of marine isotope stage 19 with a mean age for the midpoint of the reversal of 773.1 ka (standard deviation = 0.4 kyr), ∼7 kyr younger than the presently accepted astrochronological age for this polarity reversal (780–781 ka). Two recently proposed revisions of the age of the 40Ar/39Ar Fish Canyon sanidine (FCs) standard to 28.201 ± 0.046 Ma and 28.305 ± 0.036 Ma would adjust 40Ar/39Ar ages applicable to the M-B boundary (and other reversals and excursions back to 1.2 Ma) to ages older than the new astrochronological ages by 8–24 kyr. The variables used to construct the ice volume models cannot account for the discrepancy. The FCs standard age that best fits the astrochronological ages is 27.93 Ma, which is within the uncertainty associated with the commonly used value of 28.02 (±0.16) Ma but younger than the recently proposed FCs ages. The EDC2 and EDC3 age models in the Dome C (Antarctic) ice core yield ages of 771.7 ka and 766.4 ka, respectively, for the 10Be flux peak that denotes the paleointensity minimum at the reversal boundary, implying that the EDC2 (rather than EDC3) age model is consistent with the observations from marine sediments, at least close to the M-B boundary.

Ar/Ar ages from transitionally magnetized lavas on La Palma, Canary Islands, and the geomagnetic instability timescale

[1] A detailed study of 43 lava flows comprising two stratigraphic sequences exposed along the north and south walls of Barranco de los Tilos on the island of La Palma, Canary Islands, reveals a complex, temporally segmented record of geodynamo behavior that contains no less than three distinct geomagnetic events. The Matuyama-Brunhes (M-B) reversal is recorded in five transitionally magnetized lava flows from the north (TN) section. The isochrons obtained from three of the lower four M-B lavas are defined by 14 incremental heating experiments that, together with a previous age determination, yielded a weighted mean of 798.4 ± 6.2 ka (all uncertainties ±2s). In addition, a 780.3 ± 10.3 ka isochron was determined for the overlying transitionally magnetized flow, indicating that it was erupted during a distinctly younger portion of the transition. Near the base of the south (TS) section one finds a sequence of weakly magnetized flows associated with virtual geomagnetic pole (VGP) positions in the southwest Indian Ocean between latitudes 56� S and 65� S, suggesting instability of the geomagnetic field beyond that of typical secular variation. 40 Ar/ 39 Ar isochrons from three of these flows, defined by 11 separate incremental heating experiments, gave a weighted mean of 822.2 ± 8.7 ka. This anomalous field behavior recorded 24 ± 11 kyr prior to the M-B reversal may coincide with an event featured in several marine sediment records. Directly above two normal polarity flows ( 40 Ar/ 39 Ar isochrons of 751.9 ± 18.1 ka and 675.0 ± 15.7 ka) are nine transitionally magnetized lavas having magnetization directions associated with low to midlatitude VGPs spanning 23� –60� N. These flows are then capped by a single flow possessing normal polarity. Based on 12 incremental heating experiments, 40 Ar/ 39 Ar isochrons of five of these nine lavas, along with the uppermost flow, gave a weighted mean age of 580.2 ± 7.8 ka for this period of transitional to normal field behavior. From these same transitional lavas, Quidelleur et al. [1999] reported three unspiked K-Ar ages with a weighted mean of 602 ± 24 ka and proposed a new event called the ‘‘La Palma’’ excursion. However, the 40 Ar/ 39 Ar age presented here is three times more precise than the K-Ar age and is indistinguishable at the 95% confidence level from the 40 Ar/ 39 Ar age of a lava from the Snake River Plain, Idaho, that originally defined the Big Lost event. Transitional field behavior of similar age observed in astronomically dated marine cores further establishes that the Big Lost event recorded at La Palma was indeed global in extent. Rigorous temporal and geomagnetic constraints for several additional periods of geomagnetic field instability during the last several million years will comprise a geomagnetic instability timescale that can be factored confidently into models of the dynamo process. INDEX TERMS: 1560 Geomagnetism and Paleomagnetism: Time variations—secular and long term; 1520 Geomagnetism and Paleomagnetism: Magnetostratigraphy; 1035 Geochemistry: Geochronology; 1513 Geomagnetism and Paleomagnetism: Geomagnetic excursions; KEYWORDS: argonargon, dating, lavas, excursion, paleomagnetism, reversal

Intercalibration of radioisotopic and astrochronologic time scales for the Cenomanian-Turonian boundary interval, Western Interior Basin, USA

We develop an intercalibrated astrochronologic and radioisotopic time scale for the Cenomanian-Turonian boundary (CTB) interval near the Global Stratotype Section and Point in Colorado, USA, where orbitally influenced rhythmic strata host bentonites that contain sanidine and zircon suitable for 40Ar/39Ar and U-Pb dating. Paired 40Ar/39Ar and U-Pb ages are determined from four bentonites that span the Vascoceras diartianum to Pseudaspidoceras flexuosum ammonite biozones, utilizing both newly collected material and legacy sanidine samples of J. Obradovich. Comparison of the 40Ar/39Ar and U-Pb results underscores the strengths and limitations of each system, and supports an astronomically calibrated Fish Canyon sanidine standard age of 28.201 Ma. The radioisotopic data and published astrochronology are employed to develop a new CTB time scale, using two statistical approaches: (1) a simple integration that yields a CTB age of 93.89 ± 0.14 Ma (2σ; total radioisotopic uncertainty), and (2) a Bayesian intercalibration that explicitly accounts for orbital time scale uncertainty, and yields a CTB age of 93.90 ± 0.15 Ma (95% credible interval; total radioisotopic and orbital time scale uncertainty). Both approaches firmly anchor the floating orbital time scale, and the Bayesian technique yields astronomically recalibrated radioisotopic ages for individual bentonites, with analytical uncertainties at the permil level of resolution, and total uncertainties below 2‰. Using our new results, the duration between the Cenomanian-Turonian and the Cretaceous-Paleogene boundaries is 27.94 ± 0.16 Ma, with an uncertainty of less than one-half of a long eccentricity cycle.

Eruptive history, geochronology, and magmatic evolution of the Puyehue-Cordón Caulle volcanic complex, Chile

Forty-three 40 Ar/ 39 Ar age determinations of lava flows, domes, ignimbrites, and dikes, plus 14 C dates from seven distal tephra layers, combined with stratigraphy, geochemistry, and Sr and Th isotope data, establish an eruptive chronology for the Puyehue-Cordon Caulle volcanic complex at 40.5° S in the Andean southern volcanic zone (SVZ). The complex preserves ~131 km 3 of lava and tephra that erupted from numerous vents widely separated in time and space. Approximately 80% of the total volume consists of basaltic to andesitic lava that formed two broad shield volcanoes between 314 and 70 ka. The modern Puyehue stratovolcano was built on the southerly shield during the past 69 k.y. following a hiatus of 25 k.y. Puyehue has erupted ~15 km 3 of basaltic to rhyolitic magma that spans the entire compositional range found in the southern SVZ and evolved via at least six phases including: (1) basaltic andesitic to dacitic lavas between 69 and 32 ka, (2) a shift to bimodal magma compositions that is first expressed by a rhyodacite mingled with inclusions of MgO-rich basaltic andesite at 34 ka, (3) dacitic to rhyolitic flows and domes from 19 to 12 ka, (4) basaltic to basaltic andesitic flows between 15 and 12 ka, (5) subsequent rhyolitic dome growth in several effusive and explosive stages between 7 and 5 ka, followed by (6) a powerful series of phreatomagmatic and sub-Plinian eruptions at ca. 1.1 ka that obliterated the preceding rhyolite domes and formed the present 2.5-km-diameter, 280-m-deep summit crater. Along the Cordon Caulle fissure zone, ~5 km 3 of rhyodacitic to rhyolitic lavas, domes, and cones have formed during the past ~16.5 k.y., including explosive and effusive eruptions in 1921–1922 and 1960. Eruptive rates were nonuniform over time, with background growth at 0.04 km 3 /k.y. or less, punctuated by spurts at up to 0.90 km 3 /k.y. The time-averaged rate, 0.42 km 3 /k.y., is nearly double that at the Tatara-San Pedro complex 500 km to the north during the past 300 k.y. These findings indicate that within a single arc the magmatic and eruptive fluxes at individual frontal volcanoes can be highly variable. The last three stratocone-building events on Puyehue began during periods of deglaciation, suggesting a relationship between unloading of ice and ease of magma ascent. Puyehue basalt exhibits subtle changes in 238 U- 230 Th, 87 Sr/ 86 Sr, and trace element composition over time that signal shifts in the composition and degree of melting of the mantle wedge, or the extent to which basalt was modified by assimilation of heterogeneous crustal melts. The complex has become exceptionally bimodal and more explosive over time with recent rhyolites evolving by extreme crystal fractionation of mafic magma and lesser volumes of andesite and dacite created via mixing of rhyolite and basalt. Despite the high flux of basalt during the past 300 k.y., no large silicic magma reservoir formed in the upper crust. Instead, 238 U- 230 Th data favor rapid ascent of several small bodies of basaltic and silicic magma from the lower crust, promoted perhaps by conduits that reflect strike-slip faulting beneath the complex.

Revised age of Aleutian Island Arc formation implies high rate of magma production

Radioisotopic dating of subaerial and submarine volcanic and plutonic rocks from the Aleutian Island Arc provides insight into the timing of arc formation in the middle Eocene. Twenty-eight 40 Ar/ 39 Ar ages constrain the duration of arc magmatism to the last 46 m.y. Basaltic lavas from the Finger Bay volcanics, the oldest exposed rocks in the arc, gave an isochron age of 37.4 ± 0.6 Ma, which is 12-17 m.y. younger than a widely cited age of 55-50 Ma. Three main pulses of arc-wide magmatism occurred at 38-29, 16-11, and 6-0 Ma, which coincide with periods of intense magmatism in other western Pacific island arcs. Using the geochronology and volumetric estimates of crust generated and eroded over the last 46 m.y., we calculate a time-averaged magma production rate for the entire arc that exceeds previous estimates by almost an order of magnitude.

Elevated shear zone loading rate during an earthquake cluster in eastern California

We compare geodetic velocity to geologic fault slip rates to show that tectonic loading was doubled across the eastern California shear zone (ECSZ) during a cluster of major earthquake activity. New slip rates are presented for six dextral faults that compose the ECSZ in the central Mojave Desert. These rates were determined from displaced alluvial fans dated with cosmogenic 10 Be and from a displaced lava flow dated with 40 Ar/ 39 Ar. We find that the sum geologic Mojave ECSZ slip rate, ≤6.2 ± 1.9 mm/yr, is only half the present-day geodetically measured velocity of 12 ± 2 mm/yr. These rates account for cumulative fault slip and geodetic observations that span the 60-km-wide shear zone; therefore this difference cannot be attributed to postseismic relaxation. Redistribution of tectonic loading over the earthquake cycle at a regional scale suggests that earthquake clustering may be enhanced via feedback with weakening of ductile shear zones.

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.