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Paleomagnetic and rock magnetic microanalysis of fully oriented microspecimens provides a more direct understanding of the source and origin of geologically stable remanent magnetizations. Scanning transmission electron microscopy allows direct observation and characterization of magnetic phases. We provide a brief history of micromagnetic research, describe a functional microspecimen preparation method, evaluate instrumentation, and present results of three combined micromagnetic/STEM studies.

Layered and massive gabbros of the Humboldt Lopolith and extrusive equivalents in west-central Nevada yield well-grouped paleomagnetic data on a site basis. With corrections for attitudes of compositional layering in gabbros and flow contacts in lavas (kcorr/kin situ = 5.5), the mean of 15 site means is: D = 307°, I = 21°, α95 = 9°, k = 17. While K-Ar data imply a Late Jurassic age, we prefer an older, probably Early Jurassic, age for the lopolith and its magnetization based upon magnetization data and relations of these rocks with host strata. Compared with Early Jurassic expected directions, the data reveal a discordancy in declination of −37° (+/− 14-22°) which implies a significant counterclockwise rotation of the lopolith. Local rotation of Fence-maker allochthon units most likely occurred when southeasterly-directed thrust sheets obliquely contacted a generally north-south trending shelf margin. Mesozoic deformation will be more accurately estimated with better age control and measurement of possible Cenozoic crustal rotation.

On the basis of their remanence properties, the lithologies of the Late Archean (2710–2703 Ma) Ghost Range Complex, an east–west-trending layered mafic–ultramafic extrusive sequence in the central Abitibi Greenstone Belt, can be divided into three groups. Group 1 units contain a high-coercivity, high-blocking temperature (greater than 520 °C) magnetite-dominated remanence characteristic of the complex (D = 280°, I = 2°, k = 5.5, α95 = 11.8°, virtual geomagnetic pole = 13°E, 7°S; isolated by both AF and thermal methods), in good agreement with the few previous results bearing on the Late Archean apparent polar wander path for North America. Group 2 units contain a low-coercivity, low-blocking-temperature (less than 310°C) scattered remanence residing in pyrrhotite. Often, both remanences coexist in a single lithology at a given site. Group 3 units contain distributed coercivity and blocking-temperature remanences, again residing in magnetite, that are more scattered but statistically identical to the mean group 1 direction. The order of magnetic blocking appears to have been group 1 before group 3 before group 2. The geologic setting of the Ghost Range suggests that it has remained essentially stable since emplacement and therefore the group 1 direction appears to reliably represent a Late Archean paleomagnetic pole.

Single-domain and pseudo-single-domain titanium-poor magnetite grains are present in silicic vitrophyre glass of an ash-flow tuff from western Nevada. Conventional transmission electron microscopy and analytical electron microscopy corroborate inferences, made solely from rock magnetic and remanence data, that the thermoremanent magnetization in the vitrophyre is carried by such grains. The magnetite grains vary in size, shape, and distribution throughout the glass. They probably crystal lized during violent eruption and welding of the ash flow, at temperatures well in excess of 600 °C.

High-resolution aeromagnetic data from the Larder Lake – Kirkland Lake region have been analyzed to develop optimum maps for geologic interpretation. The use of the apparent susceptibility technique has led to a map that shows clearly the major lithologies in a major synclinorium in the Abitibi Greenstone Belt. Calc-alkaline volcanics of the Blake River group are weakly magnetic, but show ring-like anomalies associated with intrusives where magnetite has been generated in the country rock. Alternating bands of Mg-rich and Fe-rich tholeiites of the Kinojevis Group show a strong magnetization associated with the Fe-rich phases. Gabbroic intrusions underlain by serpentine and peridotite give strong levels of magnetization.These various units have been sampled and it is shown that the susceptibility is not high enough to account for the anomalies. Rather, the anomalies are associated with the mean natural remanent magnetization in the formations sampled. These studies have led to a revised geological map of this region.

Abrupt westerly deflections of north-south trending facies boundaries, the isopleth of (87sr/86Sr)° = 0.076, and arcuate structural trends of Paleozoic, Mesozoic, and Cenozoic age in the west-central Basin and Range Province, have been explained by (1) post-Early Jurassic oroflexural folding and shear, (2) the existence of a pre-Late Jurassic resistant block along the western continental margin, possibly representing late Precambrian breakup, and (3) regional flexure (“Mina deflection”) prior to intrusion of Late Cretaceous plutons which crosscut flexure-related structural trends. Paleomagnetic data gathered from Late Cretaceous plutons and remagnetized metasedimentary rocks along and north of the northern margin of the zone of east-west structural trends are not in support of significant oroflexural folding (and attending major clockwise rotation) since Late Cretaceous time. The data (Water Canyon pluton: D = 1°, I = 57°, α95 = 4°, k = 9, N= 132; Gunmetal pluton: D = 354, I = 60, α95 = 6, k = 21, N = 29; East Garfield Hills pluton: D = 349, I = 74, α95 = 11, k = 14, N = 12; La Panta pluton: D = 22, I = 66, α95 = 10, k = 16, N = 13; Gillis Canyon pluton: D = 354, I = 50, α95 = 4, k = 20, N = 50; and folded units of the Excelsior Mountains: D = 309, I = 79, α95 = 7, k = 12, N = 37) are dispersed from expected Cretaceous directions for the Mina deflection region, but they do not conform to a model of major systematic regional clockwise rotation. The generally small deflections of unit means from expected directions could be explained by irregular components of Late Cenozoic tilting and/or rotation. The paleomagnetic data do not deny the possibility of pre-Late Cretaceous oroclinal bending. Nevertheless, regional structural analysis complimented by the paleomagnetic results indicate that the Mina deflection is in all likelihood a primary feature whose geometry reflects the late Precambrian fragmentation of western North America. Structures related to the accretion of allochthonous assemblages between Middle Devonian and Cretaceous time in this part of the Cordillera also reflect the primary nature of this feature.

Thrust-belt deformation along the eastern edge of the Disturbed Belt in the Helena salient of Montana has not been well dated owing to a lack of syn- or postorogenic strata. In situ paleomagnetic data from alkaline intrusions exposed in the easternmost folds of the salient in the northern Crazy Mountains Basin, previously described as pre-, syn-, or posttectonic with respect to deformation, are well grouped (Dec. = 343°, Inc. = 61°, α95 = 5.0°, k = 46, n = 16 sites); a negative fold test is significant at minimally 95% confidence. Intrusion and magnetization acquisition thus postdate fold and thrust deformation. K-Ar age determinations of selected intrusions range from 52 to 48 Ma. Paleomagnetic and isotopic age data, combined with stratigraphic information, indicate that folding in the northern Crazy Mountains Basin is middle to late Paleocene in age. This age is in agreement with suggested dates for deformation in the northern Montana Disturbed Belt but is recognizably older than the youngest episodes of frontal deformation in the Utah-Idaho-Wyoming salient. Data from this study and existing structural and stratigraphic information demonstrate that deformation in the overthrust belt and foreland provinces of southwest Montana overlapped temporally and spatially, ranged from Late Cretaceous to earliest Eocene in age, and progressed from west to east through time.

Jackson, M,, Van der Voo, R. and Geissman, J.W., 1988. Paleoma~etism of Ordovician alkalic intrusives and host rocks from the Pedemal Hills, New Mexico: positive contact test in remagnetized rocks? Tectonophysics, 147: 313-323. A set of thin dikes from central New Mexico, dated at 469& 7 Ma (Rb-Sr; Loring and Armstrong, 1980), have yielded a virtual geomagnetic pole which lies on the Late Paleozoic segment of the North American apparent polar wander path. The remanence of the dikes appears to be a product of Late Paleozoic hydrothermal alteration. Paradoxically, however, the magnetization of the host rocks is most simply explained in terms of a positive contact test. Samples collected between 0.2 and 0.5 dike-widths from the contact contain a component of remanence parallel to the magnetization in the dikes, with unblocking temperatures which decrease with distance from the dikes. Host rocks from a distance of more than 1 dike-width show no evidence of the characteristic dike magnetization. There are two possible resolutions of this paradox: (1) the magnetization of the host rocks is secondary, despite the apparent positive contact test, and is a product of hydrothe~al fluid migration through the dikes or along the contact zones; or (2) the magnetization of the dikes is primary, but not representative of the Ordovician paleofield for North America. Possible reasons for inaccurate representation include: (a) incomplete averaging of secular variation; (b) tectonic rotation with respect to the stable craton; or (c) erroneous age determination for the rocks. We argue that explanation (1) is the most likely.

Previously, field relations in the Butte District, Montana, have been interpreted to indicate a regional, 40-60° or greater westward tilting of the Late Cretaceous Butte quartz monzonite (BQM), the host rock to mineralization. The tilting was inferred to be Eocene or younger in age, based upon structural features in the overlying Eocene Lowland Creek Volcanics. Extensive paleomagnetic data reveal a more complicated picture. For surface exposure BQM, the in situ mean direction of characteristic magnetization of 13 site means is D = 1.5°, I = 73.3° (K = 27.6, $$\alpha_{95} = 2.6^{\circ}$$). Characteristic directions from BQM affected by early mineralization and high-temperature potassic alteration (antipodes) and two sets of quartz porphyry dikes, which pre-and post-date this mineralization, coincide with results from the surface and Berkeley Pit exposures of BQM. Studies of the magnetic oxides, along with the results of alternating field and thermal demagnetization, indicate a primary thermoremanent origin...

An excursion of the geomagnetic field is recorded by all basaltic lava flows of the Albuquerque Volcanoes, New Mexico. K-Ar age data (weighted mean:155 ± 47 ka) and evaluation of flow surface soil profiles suggest the late Pleistocene (ca. between 100 and 150 ka) as the time of extrusion. Means from 63 sites (eight flows) yield a grand mean of D=101.1°, I=-36.1°, α95=1.2° (α1 95 =0.7, α2 95=1.2), k=2219 (N=8) and a corresponding VGP of 354. IE, 20.2S, dp=0.8°, dm=l.4° (A1 95=0.5, A2 95=1.4), VGP ASD=1.8°. TRM acquisition experiments suggest that the basalts are high fidelity recorders of an ambient field. Because all flows yield statistically indistinguishable directions, an excursion morphology clearly cannot be defined. The Albuquerque Excursion may correlate with other excursions or short polarity episodes of late Pleistocene age (e.g. Blake). Though limited in morphology, well-dated excursions and polarity episodes recorded in volcanic rocks may provide information on the frequency of significant dynamo instabilities.

Upper Carnian (∼225 Ma) strata of the Bluewater Creek Formation of the Chinle Group of the Colorado Plateau, western New Mexico, contain predominantly reverse polarity characteristic magnetizations of high coercivity and distributed unblocking temperature up to 695°C. Data from a syndepositional structure suggest acquisition of a stable remanence during or soon after deposition. The mean paleomagnetic pole for 13 virtual geomagnetic poles (VGP) accepted (of 17 sites collected) is located at 55.2°N, 87.5°E (A95 = 6.7°, K = 39.7). The angular distance between this pole and the mean of recently published middle and late Norian (∼210 Ma) poles from overlying strata is 12.5°, supporting the hypothesis that significant apparent polar wander occurred during Late Triassic time. Furthermore, the Bluewater Creek pole lies to the west (6.1°, angular distance) of the cratonic reference pole for Carnian time supporting the hypothesis of a small Colorado Plateau rotation since the early Mesozoic. Anisian strata of the Moenkopi Formation contain dual polarity magnetizations of high coercivity and distributed unblocking temperatures up to 690°C, The mean VGP of six sites yields is located at 53.1°N, 96.3°E (A95 = 6.4°, K = 109.2), which is similar to previously determined Middle Triassic poles. Triassic paleomagnetic poles from southwest North America from strata in continuous stratigraphic succession, both on and off the Colorado Plateau, are not consistent with the hypothesis of a Late Triassic stand still of the geomagnetic pole.

The Intermountain and Idaho seismic belts within Idaho, Wyoming, and Montana form an unusual parabolic pattern about the axis of the aseismic eastern Snake River Plain (SRP). This pattern is also reflected in the distribution of latest Quaternary normal faults. Several late Cenozoic normal faults that trend perpendicular to the axis of the eastern SRP extend from the aseismic region to the region of latest Quaternary faulting and seismicity. A study of the late Miocene to Holocene displacement history of one of these, the Grand Valley fault system in southeastern Idaho and western Wyoming, indicates that a locus of high displacement rates has migrated away from the eastern SRP to its present location in southern Star Valley in western Wyoming. In Swan Valley the studied area closest to the eastern SRP, isotopic ages, and paleomagnetic data for over 300 samples from 47 sites on well-exposed late Cenozoic volcanic rocks (the tuff of Spring Creek, the tuff of Heise, the Huckleberry Ridge tuff, the Pine Creek Basalt, and an older tuff thought to be the tuff of Cosgrove Road) are used to demonstrate differences in the displacement rate on the Grand Valley fault over the last ∼10 m.y. Tectonic tilts for these volcanic rocks are estimated by comparing the results of paleomagnetic analyses in Swan Valley to similar analyses of samples from undeformed volcanic rocks outside of Swan Valley. Basin geometry and tilt axes are established using seismic reflection profiles and field mapping. Combining these data with the tilt data makes it possible to calculate displacement rates during discrete temporal intervals. An average displacement rate of ∼1.8 mm/yr is calculated for the Grand Valley fault in Swan Valley between 4.4 and 2.0 Ma. In the subsequent 2.0-m.y. interval the rate dropped 2 orders of magnitude to ∼0.014 mm/yr; during the preceding 5.5-m.y. interval the displacement rate is ∼0.15 mm/yr, or about 1 order of magnitude less than the rate between 4.4 and 2.0 Ma. Mapping of fault scarps and unfaulted deposits along the Grand Valley fault system shows that latest Quaternary fault scarps are restricted to the portion farthest from the eastern SRP, the southern part of the Star Valley fault. Surface displacements estimated from scarp profiles and deposit ages estimated from soil development suggest a latest Quaternary displacement rate of 0.6–1.2 mm/yr for the southern portion of the Star Valley fault. Morphologic evidence suggests that this displacement rate persisted on the Star Valley fault throughout most of the Quaternary. The latest Quaternary displacement rate calculated for the southern portion of the Star Valley fault is similar to the rate calculated for Swan Valley during the interval from 2.0 to 4.4 Ma. This similarity, together with evidence for a low Quaternary displacement rate on the fault system in Swan Valley, suggests that the location of the highest displacement rate has migrated away from the eastern SRP. Other normal faults in southeastern Idaho, northwestern Wyoming, and southwestern Montana, while less well described than the Grand Valley fault system, exhibit a similar outward migrating pattern of increased fault activity followed by quiescence. Furthermore, a temporal and spatial relationship between fault activity and the 3.5 cm/yr northeastward track of the Yellowstone hotspot is observable on the Grand Valley fault system and on other north-northwest trending late Cenozoic faults that border the eastern SRP. The temporal and spatial relationship of Miocene to present high displacement rates for other circumeastern SRP faults and the observable outwardly migrating pattern of fault activity suggest that a similar parabolic distribution of seismicity and high displacement rates was symmetrically positioned about the former position of the hotspot. Moreover, the tandem migration of the hotspot and the parabolic distribution of increased fault activity and seismicity are closely followed by a parabolic-shaped “collapse shadow,” or region of fault inactivity and aseismicity. We suggest that the outwardly migrating pattern of increased fault activity (active region) results from reduced integrated lithospheric strength caused by thermal effects of the hotspot. Conversely, the outwardly propagating quiescent region is the result of a reduction or “collapse” of crustal extension rates caused by increased integrated lithospheric strength. Lithospheric strength in this region is increased by addition of mafic materials at the base of the crust and at midcrustal levels. Although the strength of the mantle portion of the lithosphere is reduced, the increased strength of the crust results in a total integrated increase in lithospheric strength. Paradoxically, the surface heat flow data suggest that the region within the interior parabola has a higher heat flow (after accounting for the cooling effects of the eastern SRP aquifer) than the adjacent regions, yet the interior region exhibits significantly lower extension rates. It appears that in this region the surface heat flow is not a good predictor of rates of lithospheric extension.

The use of portable drilling equipment in field sampling for paleomagnetic research has been commonplace for well over 2 decades. Advances in field drilling “technology” in the past 10 years or so have afforded paleomagnetists a quite straightforward, reasonably painless, and very portable means of obtaining independently, accurately oriented samples for their research. Perhaps the last major breakthrough, in particular, with all due respect for the back of the unsuspecting graduate student/field assistant, would be the availability of dehydrated water!

Paleomagnetic and rock magnetic data obtained from azimuthally unoriented core samples, collected at approximately 1- to 3-m intervals, of Continental Scientific Drilling Program core hole VC-1 have prompted reinterpretations of the Quaternary volcanic stratigraphy intersected by the bore and have aided in evaluating the thermal regime within late Paleozoic strata attending fluid circulation and mineral deposition during and after development of the Toledo and Valles calderas. The results from Quaternary units (Banco Bonito Obsidian: I = +35.4°, a95 = 2.8° (inclination only determinations), n = 33; Battleship Rock Tuff: D = 359.6°, I = +42.4°, a95 = 2.8°, n = 5 site means (surface sites); VC-1 Rhyolite: I = +39.2°, a95 = 12.8°, n = 7; Upper VC-1 Tuff: I = +37.2°, a95 = 10.7°, n = 13; Middle VC-1 Tuff: I = +42.1°, a95 = 2.1°, n = 39; South Mountain Rhyolite: D = 350.9°, I = +49.9°, a95 = 3.4°, n = 10 (one surface site)) are consistent with isotopic age data, indicating that the entire moat volcanic sequence intersected is less than 650 kyr. Monitoring of natural remanent magnetization (NRM) intensity, NRM directions, directions of magnetizations isolated during progressive demagnetization, median destructive forces, and rock magnetization parameters has identified systematic variations within the thick Banco Bonito Obsidian and VC-1 Tuff units. The Permian Abo Formation, Pennsylvanian to earliest Permian Madera Limestone, and Pennsylvanian Sandia Formation typically contain a moderate positive inclination magnetization component (Abo Formation: I = +52.2°, a95 = 7.4°, n = 16; Madera Limestone: I = +58.4°, a95 = 2.8°, n = 105; Sandia Formation: I = +53.9°, a95 = 4.8°, n = 21); when residing in magnetite, it is usually unblocked in the laboratory by 350°C; when carried by hematite it is unblocked by 550°C. A moderate negative inclination (e.g., Madera and Abo strata: D = 173.1°, I = −46.6°, a95 = 5.5°; n = 47 samples; assuming a north seeking positive inclination RM providing a useful means of core orientation), oppositely directed magnetization is occasionally isolated and removed at higher laboratory unblocking temperatures (up to 550°C in magnetite-dominated and 600°C in hematite-dominated lithologies). Where present, magnetizations of shallower inclination (e.g., Madera and Abo strata: D = 160.1°, I = −10.9°, a95 = 4.4°, n = 28) are removed over higher ranges of unblocking temperatures, and these are probably of late Paleozoic age. The moderate negative and positive inclination magnetizations are in all likelihood viscous partial thermoremanent magnetizations (TRMs) (VPTRMs). These were activated at moderate (∼300°C) temperatures between 1.45 and 0.97 Ma, attending and following the main stages of Toledo/Valles caldera development during the Matuyama, and at near-present downhole temperatures (∼100°–180+ °C) during the Brunhes and possibly within the last 10 kyr, respectively. Importantly, the preservation of negative inclination RM components in magnetite-dominated lithologies of the Madera Limestone dictates that temperatures during the Brunhes chron (past 730 kyr) did not exceed approximately 300°–350°C. The inferred temperatures for thermal activation of the VPTRMs, based upon the Middleton and Schmidt thermal activation curves for both magnetite and hematite derived following Walton, agree with independent estimates of thermal activity, using fluid inclusion and K-Ar isotopic age determination data, during the Quaternary. The results further illustrate the potential usefulness of thermal activation theories for inferring temperatures of past geologic events, especially when they can be applied to magnetizations of similar age residing in both magnetite and hematite, and of paleomagnetic and rock magnetic studies in general in continuous coreholes in complex geologic environments.

The remanent magnetization of red beds of the Late Permian Dewey Lake Formation from northwest Texas was investigated using thermal, chemical, and alternating field demagnetization. After removal of a small present-day viscous overprint, the Dewey Lake shows univectorial decay of the remanence revealing a dual-polarity characteristic direction of D = 152.4°, I = −11.1°, k = 39.5, α95 = 5.9°, N = 16 sites. Three polarity transitions were observed defining at least four polarity zones. The observed polarity zones are roughly layer parallel and may correlate with the Illawarra magnetozone at the end of the Kiaman. If this is so, K-Ar age determinations of an interbedded ash layer (Kolker and Fracasso, 1985) suggest that the Kiaman Superchron ended before 251±4 Ma. The paleopole, located at 51.4°N–126.2°E, confirms the reliability of previously reported Late Permian poles from southwest United States, which fall significantly off the Late Permian segment of the paleomagnetic Euler pole (PEP) track of Gordon et al. (1984). This implies that a more segmented set of small circles is required to model the North American apparent polar wander path, diminishing the usefulness of the PEP method.