skip to main content


The NSF Public Access Repository (NSF-PAR) system and access will be unavailable from 11:00PM ET on Friday, December 15 until 2:00 AM ET on Saturday, December 16 due to maintenance. We apologize for the inconvenience.

Title: Constraints on Emplacement Rates of Intrusions in the Shallow Crust Based on Paleomagnetic Secular Variation

In small‐volume igneous intrusions, the duration of magmatism can be difficult to determine because assembly of an intrusion from component magma pulses may occur within geochronologic uncertainties. We demonstrate that the paleomagnetic record of short‐term movement of the geomagnetic pole (secular variation) can place constraints on the duration of intrusion assembly over shorter time periods. An analysis of14C data paired with paleomagnetic data from lava flows illustrates this approach. The flows record paleosecular variation that, when combined with the maximum rate of secular variation from the Holocene, returns a minimum time elapsed between any two flows. Data from an Oligocene laccolith indicate that this system records a minimum of 49° of secular variation and therefore took at least 750 years to be emplaced. High‐precision radiometric geochronology would be unable to resolve this assembly, suggesting that the paleosecular variation record in shallow igneous rocks contains valuable temporal constraints on upper crustal magmatism.

more » « less
Award ID(s):
Author(s) / Creator(s):
 ;  ;  ;  ;  ;  ;  
Publisher / Repository:
DOI PREFIX: 10.1029
Date Published:
Journal Name:
Geophysical Research Letters
Page Range / eLocation ID:
p. 12815-12822
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    Statistical analysis of geomagnetic paleosecular variation (PSV) and time‐averaged field has been largely based on global compilations of paleomagnetic data from lava flows. These show different trends in the averaged inclination anomaly (ΔI) between the two hemispheres, with small positive (<2°) anomalies in midsouthern latitudes and large negative (> −5°) anomalies in midnorthern latitudes. To inspect the large ΔI between 20°N and 40°N we augment the global data with a new paleomagnetic data set from the Golan‐Heights (GH), a Plio‐Pleistocene volcanic plateau in northeast Israel, located at 32–33°N. The GH data set consists of 91 lava flows sites: 40 sites obtained in the 1990s and 51 obtained in this study. The chronology of the flows is constrained by 5740Ar/39Ar ages: 39 from previous studies and 18 from this study, which together cover most of the GH plateau. We show that the 1990s data set might be affected by block rotations and does not fully sample PSV. The Plio‐Pleistocene pole (86.3°N, 120.8°E,N= 44,k= 25,α95= 4.4°), calculated after applying selection criteria with Fisher precision parameter (k) ≥ 100 and number of specimens per site (n) ≥ 5 is consistent with a geocentric axial dipole field and shows smaller inclination anomaly (ΔI= −0.4°) than predicted by global compilations and PSV models. Reexamination of the inclination anomaly in the global compilation using different calculation methods and selection criteria suggests that inclination anomaly values are affected by (1) inclusion of poor quality data, (2) averaging data by latitude bins, and (3) the way the inclination anomaly is calculated.

    more » « less
  2. Abstract

    This work presents an extensive directional paleomagnetic database of the Kiaman reversed superchron. It is composed of 1,459 paleomagnetic directions from igneous rocks corresponding to 91 data sets (or paleomagnetic poles). An almost constant behavior of more concentrated and circular distributions for latitudes higher than 10° was found, which contrasts strongly with predictions of the representative models for the past few million years. We searched for simplified and spatially covariant Giant Gaussian Process (GGP) models that best explain the directional distribution of the Kiaman database. We used the mean strength based on the mean of virtual dipole moment (VDM) results for the period drawn from the available databases. Among the tested models, the one that best explains the directional paleosecular variation of the Kiaman database is the covariant type. According to this model, the correlations between the Gaussian coefficients are valid for the last 10 Myr and the Kiaman superchron. The resulting GGP models haveparameters similar to the 0–10 Ma models, which indicates that the relation between symmetric and antisymmetric families appears unchanged in the geological past. The relative variability of the Kiaman field, as inferred from the ratiofrom GGP models, is lower than for the past 10 Myr. Thus, as well as the paleointensity,seems to be a proxy that can be used for evaluating the geomagnetic development along the geological time.

    more » « less
  3. Abstract

    New geochronologic and paleomagnetic data from the North American Midcontinent Rift (MCR) reveal the synchronous emplacement of the Beaver River diabase, the anorthosite xenoliths within it, and the Greenstone Flow—one of the largest lava flows on Earth. A U‐Pb zircon date of 1091.83  0.21 Ma (2) from one of the anorthosite xenoliths is consistent with the anorthosite cumulate forming as part of the MCR and provides a maximum age constraint for the Beaver River diabase. Paired with the minimum age constraint of a cross‐cutting Silver Bay intrusion (1091.61  0.14 Ma; 2), these data tightly bracket the age of the Beaver River diabase to be 1091.7  0.2 Ma (95% CI), coeval with the eruption of the Greenstone Flow (1091.59  0.27 Ma; 2)—which is further supported by indistinguishable tilt‐corrected paleomagnetic pole positions. Geochronological, paleomagnetic, mineralogical and geochemical data are consistent with a hypothesis that the Beaver River diabase was the feeder system for the Greenstone Flow. The large areal extent of the intrusives and large estimated volume of the volcanics suggest that they represent a rapid and voluminous ca. 1,092 Ma magmatic pulse near the end of the main stage of MCR magmatism.

    more » « less
  4. Abstract

    Study of the late Quaternary geomagnetic field contributes significantly to understanding the origin of millennial‐scale paleomagnetic secular variations, the structure of geomagnetic excursions, and the long‐term shielding by the geomagnetic field. A compilation of paleomagnetic sediment records and archeomagnetic and lava flow data covering the past 100 ka enables reconstruction of the global geomagnetic field on such long‐term scales. We use regularized inversion to build the first global, time‐dependent, geomagnetic field model spanning the past 100 ka, namedGGF100k(GlobalGeomagneticField over the past100 ka). Spatial parametrization of the model is in spherical harmonics and time variations with cubic splines. The model is heavily constrained by more than 100 continuous sediment records covering extended periods of time, which strongly prevail over the limited number of discrete snapshots provided by archeomagnetic and volcanic data. Following an assessment of temporal resolution in each sediment's magnetic record, we have introduced smoothing kernels into the forward modeling when assessing data misfit. This accommodates the smoothing inherent in the remanence acquisition in individual sediment paleomagnetic records, facilitating a closer fit to both high‐ and low‐resolution records in regions where some sediments have variable temporal resolutions. The model has similar spatial resolution but less temporal complexity than current Holocene geomagnetic field models. Using the new reconstruction, we discuss dipole moment variations, the time‐averaged field, and paleomagnetic secular variation activity. The new GGF100k model fills the gap in the geomagnetic power spectrum in the frequency range 100–1,000 Ma−1.

    more » « less
  5. Abstract

    Continental flood basalts are more prone to compositional modification from passage through thicker and (or) more felsic crust in comparison to their oceanic counterparts. The Steens Basalt in southeast Oregon (~17 Ma) is among the oldest and most mafic members of the Columbia River Basalt Group and provides a record of the early stages of flood basalt volcanism. We evaluate the balance of mantle sources in time during the onset of Columbia River Basalt Group magmatism and assess the effect of crustal passage using stratigraphically controlled Sr, Nd, Pb, Hf, Os, and O isotopic compositions, as well as whole rock major and trace element data.

    Mixing models indicate that depleted and enriched mantle sources identified by previous workers contribute in varying proportions during the life of the magmatic system, with the greatest contribution by depleted mantle when eruption rate and presumed intrusion rate increase. During waxing, enrichment of δ18O in some flows signals cryptic deep fractionation of abundant clinopyroxene followed by shallow fractionation of olivine ± clinopyroxene ± plagioclase. Os concentrations are among the highest worldwide at a given MgO (0.29–0.86 ppb at 6.0 to 10.9 wt.%). We argue that high Os results from scavenging of sulfides by recharging magmas passing through earlier crystallized magmas. Elevated87Sr/86Sr in the latest stage supports modest assimilation of partial melts from mafic accreted terranes, facilitated by thermal priming of crust by persistent magmatism. This work provides a more detailed schematic view of the Steens Basalt magmatic system, from mantle origin through crustal staging.

    more » « less