Characteristic Magnetization Directions Research Articles

Paleomagnetism of ∼1.09 Ga Lake Shore Traps (Keweenaw Peninsula, Michigan): new results and implications

We report paleomagnetic data from a new section of the ∼1.09 Ga Lake Shore Traps exposed on Silver Island (10 flows) and on the adjacent mainland (two flows) along the northwestern coastline of the Keweenaw Peninsula in Michigan. We also present new data from nine additional lava flows, sampled from the tip of the peninsula previously studied by Diehl and Haig in 1994. Samples from all these lava flows yield well-defined characteristic magnetization directions upon thermal demagnetization. After structural tilt correction, the directions from Silver Island (site-mean declination, D = 276.9°; site-mean inclination, I = 44.4°; 95% radius of confidence for site mean, α95 = 2.6°; number of samples, N = 10) and mainland (D = 298.7°, I = 36.0°, α95 = 10.1°, N = 2) flows are close to the directions from equivalent lava flows from the upper (D = 277.8°, I = 41.0°, α95 = 2.3°, N = 17) and lower (D = 300.0°, I = 34.9°, α95 = 2.3°, N = 10) sections of the middle Lake Shore Traps exposed at the eastern tip of the Peninsula, respectively. Testing the paleomagnetic directions for serial correlation shows that some of the sequential lava flows on Silver Island and from the middle Lake Shore Traps at the tip of the Peninsula record the same vector of the geomagnetic field. Combining these correlated directions yielded new mean directions for Silver Island (D = 277.2°, I = 44.1°, α95 = 3.1°, N = 8), and the upper (D = 277.0°, I = 40.4°, α95 = 3.7°, N = 10) and lower (D = 298.6°, I = 33.3°, α95 = 4.3°, N = 5) middle Lake Shore Traps at the tip of the Peninsula. The statistical similarity of paleomagnetic directions obtained from these two locations with significantly different structural trends supports the conclusions of prior studies that the curvature of the Midcontinent Rift is primary. The new paleomagnetic pole for the Lake Shore Traps is located at 23.1°N, 186.4°E (95% confidence for the paleomagnetic pole, Α95 = 4.0°; N = 31) and merits a nearly perfect six-point classification on the paleomagnetic reliability scale.

Palaeomagnetic investigation of Cenozoic volcanic rocks bordering the North Anatolian Fault Zone, Reşadiye and Koyulhisar Districts, central-east Anatolia, Turkey

A palaeomagnetic study is reported from the lavas of Eocene, Miocene and Pliocene age cropping out immediately to the north of the North Anatolian Fault Zone (NAFZ) in the Reşadiye–Mesudiye region of central-eastern Anatolia. Rock magnetic investigations identify a high percentage of multi-domained magnetite as the dominant ferromagnet in these rocks and this probably accounts for a relatively poor response to alternating field and thermal demagnetisation. Thirty of 37 units yielded acceptable groupings of characteristic magnetisation directions. An earlier study indicated small anticlockwise crustal block rotation in this region since Upper Cretaceous times (D/I = 347/50°), and our study indicates that this was overtaken by clockwise rotation in Eocene times (D/I = 40/47°), although sample size control from the Palaeogene is poor. Results from later Miocene (D/I = 2/62°) and Pliocene (D/I = 0/53°) volcanic rocks indicate that no significant tectonic rotation has occurred in the north of the NAFZ in Neogene times. This contrasts with rotations in the weaker crust comprising the Anatolian collage south of the NAFZ, where differential and sometimes large anticlockwise rotations occurred during the latter part of the Neogene.

Holocene Sediment Magnetic Properties along a Transect From Ísafjardardjúp to Djúpáll, Northwest Iceland

ABSTRACTHolocene changes in terrestrial provenance and processes of sediment transport and deposition are tracked along a fjord-to-shelf transect adjacent to Vestfirdir, Iceland, using the magnetic properties of marine sediments. Magnetic susceptibility (MS) profiles of 10 cores (gravity and piston) were obtained onboard using a Bartington MS loop. Remanent magnetizations were measured at 1-cm intervals from u-channel samples taken from six cores on a cryogenic magnetometer. Between six and nine alternating field demagnetization steps were used to isolate the characteristic magnetization directions. The chronologies of the cores used in this study were determined from AMS 14C dates on mollusks and foraminifera and constrained by the regional occurrence of the 10,200 ± 60 cal yr. BP Saksunarvatn tephra. Correlative fluctuations in magnetic concentration are noted between the fjord and shelf sites, though these fluctuations are partially masked by regional variations in carbonate content. The onset of Neoglaciation is interpreted by changes in magnetic properties including an increase in mass magnetic susceptibility that began approximately 3000 cal yr. BP. The maximum angular deviation and the median destructive field (generally <20 mT) suggest that the natural remanent magnetization is carried by a coarse ferrimagnetite mineralogy, likely magnetite or titano-magnetite. Reproducible paleomagnetic inclination values are observed in several records, including a nearly vertical inclination around 8000 cal yr. BP, suggesting that the magnetic pole may have been proximal to Iceland, followed by an interval of much shallower inclination (6000–7000 cal yr. BP).

Lava identification by paleomagnetism: a case study and some problems surrounding the 1631 eruption of Mount Vesuvius, Italy

Abstract Detailed rock magnetic, paleomagnetic and absolute paleointensity studies of lava flows from the disputed 1631 Mount Vesuvius eruption are reported. The magnetic carrier consists of pseudo-single domain state Ti-poor titanomagnetites. Characteristic magnetization directions determined from detailed stepwise alternating field and thermal demagnetizations provide four new well-defined flow unit mean directions, with α95 ranging from 0.7° to 2.6°. Paleodirections for 11 lava flows from 24-four flows studied previously appear to be related to the 1631 eruption, as indicated by their correlation to the early 17th century segment of the Italian paleosecular variation reference curve. This provides new evidence supporting the conclusion that the 1631 episode was an explosive-effusive eruption. The paleointensity results obtained from this study are the first to be published for Mount Vesuvius, with virtual dipole moments of 9.24±1.8 × 1022 and 13.5±0.4 × 1022 Am2 higher than the present-day geomagnetic field strength.

Resultados paleomagnéticos preliminares do Grupo Santa Fé (Paleozóico da Bacia Sanfranciscana): implicações sobre a idade e paleolatitude da sedimentação glacial

A paleomagnetic study was performed on the Late Paleozoic Floresta Formation (Santa Fe Group) of the Sanfranciscana Basin, central Brazil. Thirty nine individual sites were sampled throughout a ten meters section near Santa Fe de Minas city. After detailed demagnetization procedures a unique characteristic remanence component was identified which was carried by both magnetite and hematite. Considering the indication of a previous investigation that hematite was formed during the initial stages of diagenesis it is concluded that magnetization is primary. This characteristic magnetization direction is of reversed polarity as expected for remanences acquired during the Permo-Carboniferous Reversed Superchron (PCRS, 260-315Ma), and the mean inclination indicates a paleolatitude of 44oS for the sediment deposition. The corresponding paleomagnetic pole is located at 326.0oE 62.0oS (a95 = 5.1o; k = 23), and the age interval (260-280Ma) is constrained by the normal polarity events within the PCRS.

Paleomagnetic Dating of Lava Flows of Uncertain Age, Somma-Vesuvius Volcanic Complex (Southern Italy)

Detailed paleomagnetic and rock magnetic studies of 14 previously undated lava flows from the Somma-Vesuvius volcanic complex were undertaken. The magnetic carriers consist of Ti-poor titanomagnetites typified by pseudo-single domains. Characteristic magnetization directions determined from detailed stepwise alternating field and thermal demagnetizations gave well-defined mean directions for each flow unit. Paleodirections for 10 of the 14 lavas appear to be related to the early 17th century segment of the Italian paleosecular variation reference curve; thus their ages are easily retrieved. The remaining four sites yielded distinct mean directions, which cannot be simply correlated to any given sector of the reference curve, within the time frame expected for the activity. The paleomagnetic directions for these lavas may correspond to time intervals outside the period covered by the reference secular variation curve, or they may reflect local structural disturbance and have no geomagnetic significance.

A new Late Cretaceous paleomagnetic pole for the west of Amuria block (Khurmen Uul, Mongolia)

We present new paleomagnetic results obtained at ten sampling sites from six Upper Cretaceous (92 ± 4 Ma) basaltic lava flows from the westernmost part of Amuria block. The samples were collected in 1999 in the Khurmen Uul region (44.0°N, 103.0°E), Gobi desert (S. Mongolia). Stepwise thermal demagnetization isolated a high temperature component (HTC) carried by magnetite. Five out of ten sites exhibit anomalously high Koenigsberger ratios and low inclinations, which likely resulted from lightning remagnetization. However, three of these sites were only partially remagnetized, allowing us to use remagnetization great-circles to help determine a characteristic magnetization direction for this formation. HTC directions of the eight remaining sites from six distinct flows all possess normal polarity and exhibit better clustering after tilt correction of the flows (ks/kg = 129.3 / 36.6, n = 6). This clustering is ascertained by a positive fold test at the specimen scale within a single flow. We therefore propose that the tilt-corrected HTC represents a primary magnetization. The paleopole computed from the six lava flows HTC mean directions lies at λ = 71.5°N, ϕ = 227.2°E (dp/dm = 5.6° / 8.2°). We discuss the possibility that our pole might not have completely averaged paleosecular variation, because it is based on only 6 independent flows. Finally, this pole is far-sided with respect to the 90 Ma reference pole of Eurasia APWP (Δ λ = 10.2° ± 6.1°), but it is consistent, albeit somewhat far-sided too, with other coeval paleopoles from Amuria, and east China.

Palaeomagnetism and magnetostratigraphy of the Permian–Triassic northwest central Siberian Trap Basalts

A detailed palaeomagnetic and magnetostratigraphic study of the Permian–Triassic Siberian Trap Basalts (STB) in the Noril'sk and Abagalakh regions in northwest Central Siberia is presented. Thermal (TH) and alternating field (AF) demagnetisation techniques have been used and yielded characteristic magnetisation directions. The natural remanent magnetisation of both surface and subsurface samples is characterised by a single component in most cases. Occasionally, a viscous overprint can be identified which is easily removed by TH or AF demagnetisation. The resulting average mean direction after tectonic correction for the 95 flows sampled in outcrops is D=93.7°, I=74.7° with k=19 and α 95=3.3°. The corresponding pole position is 56.2°N, 146.0°E. Unoriented samples from four boreholes cores in the same regions have also been studied. They confirm the reversed–normal succession found in outcrops. The fact that only one reversal of the Earth's magnetic field has been recorded in the traps can be taken as evidence for a rather short time span for the major eruptive episode in this region. However, there is evidence elsewhere that the whole volcanic activity associated with the emplacement of the STB was much longer and lasted several million years.

Rock magnetic results from Ocean Drilling Program Leg 192: implications for Ontong Java Plateau emplacement and tectonics of the Pacific

Abstract A rock magnetic study has been performed on rock samples recovered at Ocean Drilling Program Leg 192 sites on the Ontong Java Plateau in the western Pacific. Igneous rocks from the five Leg 192 sites displayed variable rock magnetic properties. The differences in the rock magnetic properties are a function of mineralogy and alteration. Titanomagnetite and titanomaghemite are present in the Ontong Java rocks. Samples with titanomagnetite exhibit Verwey transitions in the vicinity of 120K. Low-temperature curves for samples with multiple magnetic phases do not clearly show the Verwey transition. The inversion of titanomaghemite to a strongly magnetized magnetite is shown by the irreversible thermomagnetic-cooling curve. Despite the geographically widespread locations of the drill sites, variations in rock magnetic properties closely resemble each other, consistent with the fundamental results of the leg that the basement rocks were derived from homogeneous Kwaimbaita-type magma with a single age of approximately 120 Ma. The rock magnetic investigation provides constraints to evaluate the fidelity of the natural magnetic memory in the basalt rocks and corroborates the palaeomagnetic palaeolatitudes determinations for the Ontong Java Plateau. The generally good quality of rock magnetic data exhibited by Leg 192 rocks supports the inference that the characteristic directions of magnetization isolated from the Cretaceous Ontong Java Plateau sites were acquired near the onset of the Cretaceous Long Normal Superchron about 120 Ma. The portion of the Pacific plate containing the Leg 192 sites was in the southern hemisphere during the mid-Cretaceous volcanism.

Evidence for weak geomagnetic field intensity prior to the Cretaceous normal superchron

A palaeomagnetic and geochronologic investigation has been carried out on a sequence of 19 lava flows at Sihetun in northeastern China. Age determinations by K/Ar dating indicate that the sequence spans 124–133 Ma, that is, the time just prior to Cretaceous normal superchron (CNS). Rock magnetic measurements of thermomagnetic curves and hysteresis loops show the primary carrier of remanence to be pseudo-single domain magnetite. Stepwise thermal demagnetization successfully isolated a well-defined direction of characteristic magnetization (ChRM) in all the lavas. The mean palaeodirection determined is D/ I=5.4°/58.7° ( α 95=2.4°) and D/ I=175.9°/−60.0° ( α 95=6.4°) for 15 normally magnetized flows and 3 reversely magnetized flows, respectively. One lava was found to possess an intermediate direction of magnetization. The inferred palaeolatitude of the sampling site is very similar to the present-day position, supporting the contention that suturing between the Mongolia and north China plates was probably completed by the Late Jurassic, and that subsequently little rotation or translation occurred in the region west of the Tanlu fault zone. Thellier–Thellier palaeointensity experiments with systematic partial thermoremanent magnetization (pTRM) checks were conducted on 66 samples from 13 lava flows. Results from the 29 samples (44%) which met strict reliability criteria indicate a range of virtual dipole moments (VDMs) from 2.83 to 4.17×10 22 A m 2 with an average of (3.53±0.02)×10 22 A m 2. This finding helps confirm that a weak geomagnetic field probably occurred just prior to the CNS.

Effect of nesiritide vs. dobutamine on short-term outcomes in the treatment of patients with acutely decompensated heart failure

A palaeomagnetic and geochronologic investigation has been carried out on a sequence of 19 lava flows at Sihetun in northeastern China. Age determinations by K/Ar dating indicate that the sequence spans 124–133 Ma, that is, the time just prior to Cretaceous normal superchron (CNS). Rock magnetic measurements of thermomagnetic curves and hysteresis loops show the primary carrier of remanence to be pseudo-single domain magnetite. Stepwise thermal demagnetization successfully isolated a well-defined direction of characteristic magnetization (ChRM) in all the lavas. The mean palaeodirection determined is D/I=5.4°/58.7° (α95=2.4°) and D/I=175.9°/−60.0° (α95=6.4°) for 15 normally magnetized flows and 3 reversely magnetized flows, respectively. One lava was found to possess an intermediate direction of magnetization. The inferred palaeolatitude of the sampling site is very similar to the present-day position, supporting the contention that suturing between the Mongolia and north China plates was probably completed by the Late Jurassic, and that subsequently little rotation or translation occurred in the region west of the Tanlu fault zone. Thellier–Thellier palaeointensity experiments with systematic partial thermoremanent magnetization (pTRM) checks were conducted on 66 samples from 13 lava flows. Results from the 29 samples (44%) which met strict reliability criteria indicate a range of virtual dipole moments (VDMs) from 2.83 to 4.17×1022 A m2 with an average of (3.53±0.02)×1022 A m2. This finding helps confirm that a weak geomagnetic field probably occurred just prior to the CNS.

Paleomagnetism of the Acambay graben, central Trans-Mexican Volcanic Belt

Paleomagnetic results for Miocene to Quaternary volcanic units of the Acambay graben are used to investigate the Neogene tectonic activity within the central sector of the Trans-Mexican Volcanic Belt (TMVB). Characteristic magnetization directions were obtained for 22 sites, with an overall mean direction (Dec=176.7°, Inc=−34.3°, k=28, α 95=6°) that is concordant with the expected direction calculated from North American reference data. Examination of site-mean directions shows an apparent geographic pattern related to the intersection of the Queretaro-Taxco fault zone with the E–W border fault system of the graben. This pattern is characterized by easterly and westerly declinations arranged into two broad groups. The mean direction for group A (easterly) is ( n=9): Dec=189.9°, Inc=−28.5°, k=53 and α 95=4.8 . The mean direction for group B (westerly) is ( n=13): Dec=167.6°, Inc=−36.2°, k=38 and α 95=4.3°. Group A sites cover two areas, one inside the graben and the other immediately to the south including the Pastores fault. Group B sites are distributed over broad NW and SE areas across the graben, which include the Amealco caldera and Epitacio Huerta and Acambay–Tixmadeje faults and also the Venta de Bravo fault. Eighteen units show reverse polarities, and four units show normal polarity. Absence of normal polarity Quaternary units suggests a pre-Brunhes age for units sampled. K–Ar dating in the Amealco caldera gives a range between 5.7 and 2.2 Ma. A comparison of group A and B mean directions with reference directions calculated from the North American 10 Ma pole and the geographic pole gives rotation parameters between 9.1±5.3° and 9.9±4.6° and between −12.4±5.4° and −12.4±4.3°, respectively. These may be interpreted in terms of vertical-axis rotations associated with regional left-lateral shear in a normal/strike-slip fault environment, which has characterized the Acambay graben during the Neogene.

Tertiary remagnetization of Paleozoic rocks from the Eastern Cordillera and sub‐Andean Belt of Bolivia

Paleomagnetic samples were collected from 98 sedimentary horizons in eight different Devonian to Permian sedimentary units at eight localities in the Eastern Cordillera and the sub‐Andean Belt of Bolivia. For 77 sites, thermal demagnetization allowed determination of a characteristic magnetization (ChRM) with site‐mean 95% confidence limit, α95, ≤15°. The ChRM is carried predominantly or entirely by hematite. Fold and reversal tests from two of the sampled localities indicate that the characteristic magnetization is synfolding, likely acquired during the earliest stages of deformation. Additionally, a modified conglomerate test at one locality and the nearly uniform direction of ChRM across the Devonian to Permian age units clearly reveals the secondary nature of the characteristic magnetization. Finally, the ChRM directions are discordant from any expected Paleozoic directions. Paleomagnetic poles calculated from the ChRM directions fall near the Cenozoic portion of the apparent polar wander path for South America. We interpret these observations to indicate widespread chemical remagnetization of these Paleozoic strata during, but prior to completion of, Cenozoic Andean folding.

Paleomagnetism, paleogeographic origins, and uplift history of the Coast Range ophiolite at Mount Diablo, California

Divergent paleogeographic origins have been proposed for the Coast Range ophiolite of western California which are testable using paleomagnetic methods. Paleomagnetic data for Middle Jurassic pillow lavas and diabase sills of the Coast Range ophiolite at Mount Diablo, northern California, indicate that they contain two components of remanent magnetization. The characteristic directions have normal and reversed polarities and apparently are carried by Ti‐poor magnetite. This magnetization is inferred to have been acquired during emplacement and seafloor alteration at an ancient spreading ridge. The paleolatitude calculated from its structurally corrected mean direction is 20°N ± 9° and agrees with the expected direction for stable North America; this result is also consistent with the concordant paleolatitude (32°N ± 8°) recently determined for Upper Jurassic Coast Range ophiolite at Stanley Mountain in southern California. In addition, clockwise vertical axis rotation of Mount Diablo (143° ± 11°) is indicated by the characteristic magnetization direction. An overprint component is inferred to have been acquired during uplift of Mount Diablo since the Miocene.

Paleomagnetism of Paleoproterozoic mafic dyke swarm from the Uauá region, northeastern São Francisco Craton, Brazil: tectonic implications

This paper presents paleomagnetic analyses of 146 samples collected from 45 mafic dykes (sites) and 18 samples from basement rocks in contact with dykes. Two groups of dykes are recognized: the first (metamorphic dykes-MD) is composed of amphibolites with distinct schistosity and the second group is formed of two main rock types; a BD (basic dykes) type which retains its original texture and mineralogy and a MBD (metamorphic basic dykes) type where the clinopyroxene is almost completely substituted by amphibole. The MBD dykes occur near shear zones which are associated with the metamorphism of these rocks (ages of 2003 ± 86 Ma and 1975 ± 2 Ma). Most K-Ar and 40Ar 39Ar mineral age determinations on the analysed dykes and basement rocks yielded dates in the interval 1.9-2.14 Ga. Rb Sr mineral isochrons on dykes from the BD type suggest two different generations of emplacement, at 2.38 Ga (minimum age) and 1.98 Ga. After AF and thermal demagnetization, a characteristic direction of magnetization (Dm = 17.1 °; Im = 69.8 °; N = 31; K = 40.6; α 95 = 4.1 °) could be isolated for 29 metamorphosed and unmetamorphosed dykes, and basement rocks from two localities, which yielded a paleomagnetic pole situated at 331.4 °E; 23.8 °N (K = 16.8; α 95 = 6.5 °). The paleomagnetic, petrographic, and geochronological data are interpreted to indicate that this characteristic component of magnetization is secondary. This component may have been acquired during the uplift and cooling of the Transamazonian cycle, at about 1.98-1.9 Ga ago. Tectonically, our data when compared with paleomagnetic poles of similar age from the Kalahari Craton are consistent with a model in which the São Francisco/Congo and Kalahari Cratons were part of a single stable continental mass in the late Paleoproterozoic.

Continental deposits, magnetostratigraphy and vertebrate paleontology, late Neogene of Eastern Spain

Abstract Eleven sections have been sampled for magnetostratigraphy in the fossil mammal-productive late Neogene basins of eastern Spain. The characteristic direction of magnetization was determined through progressive thermal and alternating field demagnetization. It was possible to correlate the magnetozones from the various sections to the GPTS as well as to the fossil MN zones within the sediment. The MN zone boundaries based on this correlation are as follows: MN 11/12 ⋍ 8.1 my; MN 12/13 ⋍ 6 my; MN 13/14 ⋍ 4.9 my; MN 14/15 = 4.18 my; MN 15/16 = 3.58 my. New fossil lists are presented for the Fuente del Viso and Loma del Castillo faunas in the Cabriel Basin.

A paleomagnetic study of the lava flows within the Copper Harbor Conglomerate, Michigan: new results and implications

New paleomagnetic data have been obtained from the interbedded lava flows within the Copper Harbor Conglomerate on Michigan's Keweenaw Peninsula. Previous paleomagnetic studies of these lava flows, known collectively as the Lake Shore Traps, have produced contradictory results. To investigate the cause of these conflicting results, 30 sites encompassing the most complete section of lava flows possible were collected and analyzed.Well-defined characteristic directions of magnetization were isolated using either alternating-field or thermal demagnetization or a combination of both. These directions of magnetization are interpreted as primary magnetizations acquired during the original cooling of the lavas. Hysteresis, thermomagnetic, and petrographic analyses suggest the carrier of magnetization is a pseudo-single-domain, titanium-poor magnetite that has undergone some high-temperature oxidation.Site means determined from the 30 lava flows define three distinct directional clusters. Each cluster of directions corresponds to a different stratigraphic package of lava flows with the Copper Harbor Conglomerate. Between-site dispersion for each stratigraphic package or unit is much less than the expected value for Keweenawan-age rocks. Therefore, we suggest that most of the lava flows in each unit were extruded rapidly and that within an individual stratigraphic unit, paleosecular variation has not been adequately sampled. This explains why previous studies on the Lake Shore Traps have produced such different results; each study did not sample the entire range of directions possible in these lava flows.The paleomagnetic pole calculated from the 30 site-mean virtual geomagnetic poles is located at 22.2°N, 180.8°E (k = 35.0; A95 = 6.5°). The new Copper Harbor pole is now located in the appropriate chronological position with respect to the underlying Portage Lake Volcanics and the overlying Nonesuch Shale on the Keweenawan apparent polar wander path. The similarity of our Copper Harbor pole to that of the Portage Lake Volcanics reinforces the idea that the Copper Harbor Conglomerate is more closely related in time to the Portage Lake Volcanics than to the Nonesuch Shale.

Shallow magnetic inclinations in the Cretaceous Valle Group, Baja California: remagnetization, compaction, or terrane translation?

Paleomagnetic data from Albian to Turonian sedimentary rocks on Cedros Island, Mexico (28.2° N, 115.2° W) support the interpretation that Cretaceous rocks of western Baja California have moved farther northward than the 3° of latitude assignable to Neogene oblique rifting in the Gulf of California. Averaged Cretaceous paleomagnetic results from Cedros Island support 20 ± 10° of northward displacement and 14 ± 7° of clockwise rotation with respect to cratonic North America. Positive field stability tests from the Vizcaino terrane substantiate a mid‐Cretaceous age for the high‐temperature characteristic remanent magnetization in mid‐Cretaceous strata. Therefore coincidence of characteristic magnetization directions and the expected Quaternary axial dipole direction is not due to post mid‐Cretaceous remagnetization. A slump test performed on internally coherent, intrabasinal slump blocks within a paleontologically dated olistostrome demonstrates a mid‐Cretaceous age of magnetization in the Valle Group. The in situ high‐temperature natural remanent magnetization directions markedly diverge from the expected Quaternary axial dipole, indicating that the characteristic, high‐temperature magnetization was acquired prior to intrabasinal slumping. Early acquisition of the characteristic magnetization is also supported by a regional attitude test involving three localities in coherent mid‐Cretaceous Valle Group strata. Paleomagnetic inclinations in mudstone are not different from those in sandstone, indicating that burial compaction did not bias the results toward shallow inclinations in the Vizcaino terrane.

Paleomagnetism and electron microscopy of the Emeishan Basalts, Yunnan, China

Many normal and a few reversed characteristic directions of magnetization have been obtained by predominantly thermal demagnetization from ten sites of the Late Permian Emeishan Basalts collected near Kunming, Yunnan Province. The normal magnetization directions pass a fold test at the 99% confidence level and yield declination/inclination = 26°/−12°, k = 46, α 95 = 6° and a paleopole at 50°N, 241°E. However, the reversed-polarity directions, with declination/inclination at 244° + 3° are not antipodal to the normal ones, which is also noted in other studies of the Emeishan Basalts of the Yangtze Paraplatform of the South China Block. Speculations about the cause of this lack of antipodality center on: 1. (1) local relative rotations. 2. (2) incomplete demagnetization. 3. (3) unusually large non-dipole fields, secular variation or asymmetric reversals in the Late Permian. 4. (4) errors in sample orientation. 5. (5) later remagnetization represented by one or the other polarity group. The first two causes are ruled out by our observations, and the third cannot be tested with data from China alone but is unlikely because it has not been observed elsewhere. Errors in sample orientation may be present in those studies that used a magnetic compass, because sample intensities of magnetization are on the high side. However, for the normal-polarity directions, a solar compass has been used in some of the studies and all normal-polarity directions observed in the area are in agreement. Thus, sample orientation errors can be invoked only for the reversed-polarity directions. In order to test cause (5), we have examined the magnetic carriers in eight samples with scanning electron microscopy. Titanium-poor to titanium-rich magnetite, commonly cruciform in crystal habit, indicates primary igneous crystallization in two samples that have NE and WSW declinations. In contrast, six normal-polarity samples with NNE declinations show pervasive replacement of original titanomagnetite by titanium-free magnetite and sphene, indicating a high degree of alteration. We ascribe this alteration to late hydrothermal circulation and argue that it has caused remagnetization in post-Permian times. This study suggests, therefore, that the NE-SW directions are more likely to be representative of the Late Permian paleomagnetic field than the NNE directions.

A Middle Triassic paleomagnetic pole for North America

Two stratigraphic sequences were sampled through the early Anisian Anton Chico Member of the Moenkopi Formation in north-eastern New Mexico. Two polarities of magnetization are present: a normal-polarity interval succeeded by a reversed-polarity interval and followed by a short normal- and reversed-polarity couplet. Detailed thermal demagnetization (10 to 17 steps) was employed to separate magnetic vectors. The secondary magnetization is largely a direction similar to the present-day and/or axial-field directions. Demagnetization above 520°C reveals a near-horizontal characteristic magnetization. The lithology of the stratigraphic sequence is such that the reversed polarity is contained almost exclusively within coarse sandstone lithologies, and hence, the reversed-polarity characteristic magnetization direction is rarely completely separated from the secondary magnetization. Because of this, the paleopole was calculated from only the lower normal-polarity portion of the section. The pole, calculated from the samples of two localities, is located at 121.4°E, 43.2°N, (α 95 = 5.3). This Middle Triassic paleopole is in good agreement with published Triassic paleomagnetic poles for cratonic North America and statistically overlaps the Early Triassic paleopoles. The large-scale relative motion during the Triassic between the magnetic pole and the North American plate is constrained by this study to have begun after early Middle Triassic time; it suggests that as much as 10° of apparent-polar wander occurred between the late Anisian-Ladinian (late Middle Triassic) and the middle to late Carnian (early Late Triassic), a time interval of between 11 and 14 m.y.