Unblocking Temperature Spectra Research Articles

Thermo-tectonic history of the Tethyan Himalayas deduced from the palaeomagnetic record of metacarbonates from Shiar Khola (Central Nepal)

Palaeomagnetic measurements were carried out on low-grade metamorphic carbonates, of Mesozoic age from the Shiar area (85.1°E, 28.6°N) of the Tethyan Himalaya (TH) in north central Nepal. Two characteristic remanence components carried by pyrrhotite (ChRM1) and magnetite (ChRM2) could be identified by their unblocking temperature spectra of 270–340 and 430–580°C, respectively. Fold tests are not significant, due to the uniform bedding of all sites. However, according to results from other areas of the TH, the pyrrhotite component has been probably acquired as a secondary (p)TRM during exhumation and cooling; thus the age of remanence acquisition can be related to the last cooling event (25–17Ma in the surrounding areas). The inclination of the magnetite component matches the value expected from the Indian APWP. This may the primary origin of the ChRM2.Pyrrhotite site-mean directions show a small-circle distribution, with a best fit parallel to the N–S direction. Backtilting to the expected inclination (Iexp) by intersection of the remanence small-circle with the small-circle of constant Iexp yields a clockwise block rotation of 30–35° with respect to the Indian Plate. Characteristics of the pyrrhotite component (small-circle distribution of site-means, secondary origin, (p)TRM with unblocking temperatures below about 300°C), allow the interpretation of the chronologic order of the thermo-tectonic history: (i) an earlier main folding phase at elevated temperatures; (ii) a later event of cooling through about 300°C coinciding with the acquisition of ChRM1; (iii) clockwise block rotations with respect to the Indian Plate and (iv) long-wave folding as the youngest tectonic event.

Paleomagnetism of a Late Cretaceous island arc complex from South Sakhalin, East Asia: Convergent boundaries far away from the Asian continental margin?

The Hokkaido‐Sakhalin fold system stretches for ∼1500 km along the eastern coast of Asia and consists of several N‐S trending tectonic belts. Studies in South Sakhalin show that the northern part of the Tonino‐Aniva Peninsula (Ozersk unit) is a counterpart of the Tokoro belt on Hokkaido, Japan. In the eastern part of the Ozersk unit, 195 hand samples were sampled at 20 sites from Campanian‐Maastrichtian tuffaceous siltstones and sandstones of the Chayka Formation of island arc affinity. Stepwise thermal demagnetization isolates a postfolding low‐ to intermediate‐temperature component (B) of normal polarity. A high‐temperature component (A) is isolated from about half the samples. Because of strong overlap of unblocking temperature spectra of these two components in other samples, direct observations and remagnetization circles were combined for calculation of site‐mean directions. Component A is mostly of reversed polarity; a few samples of normal polarity are found at three sites. The presence of two polarities with approximately antipodal directions and the positive fold test imply a prefolding, and most probably primary, origin of component A. A formation‐mean inclination of 45.0°±6.4° calculated with the aid of inclination‐only statistics corresponds to a latitude of 26.6°±5.2°N. A similar inclination is derived from a late Cretaceous island arc complex from the Tokoro belt on Hokkaido. Since both mean inclinations are ∼30° lower than the coeval Eurasian reference value, a large‐scale northward transport of the entire Tokoro island arc is inferred. We exclude the possibility of displacement with the Kula plate and coast‐parallel transport; instead, intra‐oceanic motion with the Pacific plate and docking at the Eurasian margin at circa 30 Ma are inferred. Combined paleomagnetic data from the Tokoro belt, the Nemuro belt and Kamchatka region imply that a system of intra‐oceanic island arcs existed in the northwest Pacific in Late Cretaceous time.

Further constraints for the Plio-Pleistocene geomagnetic field strength: New results from the Los Tuxtlas volcanic field (Mexico)

A rock-magnetic, paleomagnetic and paleointensity study was carried out on 13 Plio-Pleistocene volcanic flows from the Los Tuxtlas volcanic field (Trans Mexican Volcanic Belt) in order to obtain some decisive constraints for the geomagnetic field strength during the Plio-Pleistocene time. The age of the volcanic units, which yielded reliable paleointensity estimates, lies between 2.2 and 0.8 Ma according to the available K/Ar radiometric data. Thermomagnetic investigations reveal that remanence is carried in most cases by Ti-poor titanomagnetite, resulting from oxy-exsolution that probably occurred during the initial flow cooling. Unblocking temperature spectra and relatively high coercivity point to ‘small’ pseudo-single domain magnetic grains for these (titano)magnetites. Single-component, linear demagnetization plots were observed in most cases. Six flows yield reverse polarity magnetization, five flows are normally magnetized, and one flow shows intermediate polarity magnetization. Evidence of a strong lightning-produced magnetization overprint was detected for one site. The mean pole position obtained in this study is Plat = 83.7°, Plong = 178.1°, K = 36, A95 = 8.1°, N = 10 and the corresponding mean paleodirection is I = 31.3°, D = 352°, k = 37, α95 = 8.2°, which is not significantly different from the expected direction estimated from the North American apparent polar wander path. Thirty-nine samples were pre-selected for Thellier palaeointensity experiments because of their stable remanent magnetization and relatively weak-within-site dispersion. Only 21 samples, coming from four individual basaltic lava flows, yielded reliable paleointensity estimates with the flow-mean virtual dipole moments (VDM) ranging from 6.4 to 9.1 × 1022 Am2. Combining the coeval Mexican data with the available comparable quality Pliocene paleointensity results yield a mean VDM of 6.4 × 1022 Am2, which is almost 80% of the present geomagnetic axial dipole. Reliable paleointensity results for the last 5 Ma are still scarce and are of dissimilar quality. Additional high-quality absolute intensity determinations are needed to better constraint the geomagnetic field strength during the Plio-Pleistocene time.

Paleomagnetic and paleointensity study of Oligocene volcanic rocks from Chihuahua (northern Mexico)

A detailed rock-magnetic, paleomagnetic and paleointensity study was carried out on Oligocene volcanic formations from Chihuahua (northern Mexico) in order to obtain some decisive constraints for the tectonic evolution of the southern Cordilleran Orogenic Belt of North America and for the geomagnetic field strength during Oligocene time. Age of the volcanic units lies between 33 and 25 Ma according to available radiometric data. Rock-magnetic experiments reveal that remanence is carried in most cases by Ti-poor titanomagnetite, resulting of oxy-exsolution of original titanomagnetite during the initial flow cooling. Unblocking temperature spectra and relatively high coercivities point to ‘small’ pseudo-single domain magnetic grains for these (titano)magnetites. Single-component, linear demagnetization plots were observed in most cases. Nine sites yield reverse polarity magnetization, three are normally magnetized and one unit shows intermediate polarity magnetization. An evidence of strong lightning-produced magnetization overprint was detected for one site. Combining all paleomagnetic data currently available for northern Mexico, we obtained a well-defined Eocene–Oligocene mean paleomagnetic direction with I=48.5°, D=337.1°, k=20, α 95=6.8°, N=24, which deviates counterclockwise from the expected direction estimated from the North American apparent polar wander path. This suggests a vertical-axis tectonic counterclockwise rotation of about 16° relative to stable North America. Transition from Laramide compression to Basin and Range extension occurred during Oligocene, around 32–30 Ma. Tectonic rotation may then reflect east–northeast extension in the mid- to late-Cenozoic. Twenty-three samples were pre-selected for Thellier palaeointensity experiments because of their low viscosity index, stable remanent magnetization and reasonably reversible continuous thermomagnetic curves. Only 12 samples, coming from three individual basaltic lava flows, yielded reliable paleointensity estimates with the flow-mean virtual dipole moments (VDM) ranging from 3.96 to 4.65×10 22 Am 2. Combination of Mexican data with the available comparable quality Oligocene paleointensity results yield a mean VDM of 4×10 22 Am 2, which is comparable or slightly higher than the mean VDMs calculated for the Mesozoic low field period, but significantly lower than present day value. This low intensity may correlate with the relatively high paleosecular variation rate invoked around 30 Ma by several authors.

Paleomagnetism of Siluro‐Devonian sequences, NE Spain

The paleogeography of northeastern Spain is addressed in a detailed paleomagnetic study of Siluro‐Devonian boundary sediments from the Catalan Coastal Ranges (CCR) at Santa Creu d'Olorda, Barcelona, and the eastern Pyrenees at Baèn. The sequences studied are all well dated biostratigraphically and have undergone only very minor deformation, mainly during the late Carboniferous, and extremely low grades of metamorphism (anchizone or less). After stepwise thermal demagnetisation of samples and principal component analysis of the data, up to two paleomagnetic directions can be clearly identified after removal of a present‐day magnetic overprint. The first component (labeled B) is identified only in samples from the CCR with intermediate unblocking temperature spectra (300°–400°C). An overall mean direction of 187°/−03°, semi angle of cone of 95% confidence α95 = 9.0°, and precision parameter k = 43.1 in situ, and 195°/03° α95 = 24.4°, and k = 7.1 (51 samples at seven sites) after bedding correction is obtained for B. The exact age of this secondary component is uncertain, but when compared to published paleomagnetic data, a late Carboniferous to Early Triassic age could be inferred. A stable endpoint direction (labeled C) can be identified between 400° and 600°C in samples from both regions. For the CCR it is southwesterly directed with intermediate inclination values in situ, and yields a mean direction of 208°/48°, α95 = 12.8° and k = 28.4 (six sites) after bedding correction. It passes the fold test, is considered to be primary in origin, and is thus indicative of the Siluro‐Devonian paleofield direction in the CCR. In the Pyrenean samples the C direction is northwesterly directed in situ, with a mean direction of 240°/51°, α95 = 6.5° and k = 86.5 (seven sites) after bedding correction. Although there is a clear improvement in grouping of these site mean directions, this is not significant at the 95% level of confidence. Combining the site mean directions obtained for component C from both regions yields an overall mean direction of 224°/51°, α95 = 8.5° and k = 24.6 (13 sites) after bedding correction. The improvement in grouping is significant at the 99% level of confidence at 90% unfolding, reinforcing the interpretation that this is a primary magnetisation. Adopting the reversed polarity option, the inclination value obtained translates into paleolatitudes of 30°S, placing NE Spain in the same latitudinal belt as the Armorican Massif and the Barrandian terrane of the Bohemian Massif (i.e., the Armorican Terrane Assemblage) in latest Silurian/earliest Devonian times.

Palaeomagnetism and magnetostratigraphy of uppermost Permian strata, southeast New Mexico, USA: correlation of the Permian-Triassic boundary in non-marine environments

Continental red sandstone and siltstone rocks of the Dewey Lake (Quartermaster) Formation at Maroon Cliffs, near Carlsbad, New Mexico, are characterized by two components of magnetization with partially overlapping laboratory unblocking temperature spectra. Both magnetizations display high coercivities (>100 mT), probably residing in haematite. A north-directed magnetization with steep positive inclination unblocks between 100 and 650 °C, isolating a predominantly northwest-directed magnetization, with shallow inclination, of near uniform normal polarity and maximum unblocking temperatures of 680 °C. We collected samples from 24 palaeomagnetic sites (i.e. individual beds) from a ~60 m thick section of flat-lying strata disconformably overlying carbonate and evaporite rocks of the Rustler Formation. The upper member of the Rustler Formation contains a Late Permian (early Changxingian) marine invertebrate and conodont fauna. Of the sampled sites, four yield only steep magnetizations, interpreted to be recent overprints. Eight sites did not yield well-grouped site means and were excluded from the final calculations. The formation mean (dec=337.7°, inc=9.2°; k=31.6, α95=7.8°, N=12 sites) defines a palaeomagnetic pole located at 55.2°N, 117.5°E, in good agreement with other Late Permian North American cratonic poles. Correlation of the short polarity sequence of this section of Dewey Lake strata is unambiguous. Compared with the polarity stratigraphy of marine sections in Asia, and supported by isotopic age determinations on a widespread bentonite bed in Dewey Lake strata in west Texas (approximately 251 Ma) and fossil data for the underlying Rustler Formation, the magnetostratigraphy is consistent with deposition of the Dewey Lake Formation during the latest Changxingian (Late Permian) stage.

Paleomagnetism and paleothermometry of the Sydney Basin 2. Origin of anomalously high unblocking temperatures

The Milton Monzonite of southeastern Australia was thermoviscously remagnetized as a result of Cretaceous burial and uplift. Thermal demagnetization separates the low unblocking temperature (LT) overprint from the high unblocking temperature (HT) primary remanence, with a relatively sharp junction between LT and HT components in vector projections. For single‐domain grains, the junction temperature TL between two such vectors corresponds to the maximum blocking temperature Tr reactivated in nature, apart from a correction for the difference between natural and laboratory timescales. However, measured TL values are distributed over an implausibly wide range (>250°C) for burial remagnetization of an untilted intrusion like the Milton Monzonite. Furthermore, many TL values are anomalously high compared to the predictions of single‐domain theory. Multidomain grains are the cause of these anomalies. Samples pretreated before thermal demagnetization by zero‐field cycling to liquid nitrogen temperature, so as to erase multidomain remanence and isolate single‐domain remanence, do have the theoretically expected TL values. In these samples, realistic remagnetization time and temperature (tr, Tr) conditions in nature are predicted using the t‐T contours of Pullaiah et al. [1975]. The anomalously high TL values before low‐temperature treatment are due to multidomain grains, which carry ≥50% of the LT overprint. The LT thermal demagnetization curve in samples dominated by multidomain grains is quasi‐exponential in shape with a high‐temperature tail extending almost to the Curie point, as predicted by multidomain theory. These high LT unblocking temperatures, which are much greater than plausible remagnetization temperatures reached in nature, overlap and mask the lower part of the HT unblocking temperature spectrum, driving up TL values and leading to inflated estimates of Tr. Although multidomain remanence is a sufficient explanation of anomalously high unblocking temperatures of thermoviscous overprints in the Milton Monzonite, chemical overprinting may be a factor in other lithologies and tectonic settings.

Preliminary Rock Magnetic Study of Archaeomagnetic Samples from Bulgarian Prehistoric Sites.

Rock magnetic properties of archaeomagnetic samples taken from ovens are studied. Thermal demagnetization of saturation remanences and other studies reveal the presence of: (titano)magnetite, iron oxyhydroxides, and maghemite with T b ∼ 650°C which is stable with respect to inversion. During thermal demagnetization the soft IRM component (0.06 T) is always the strongest one, indicating the importance of the (titano)magnetite contribution. A break-up in the values of coercivity H cr , with respect to initial mass-specific magnetic susceptibility (X) occurs at X ∼ 1.5 x 10 -6 m 3 /kg and separates poorer from better quality samples in the palaeointensity experiment. Separation of bad and good Thellier experiments is also observed in the relation between the coercivities and the concentration-independent parameter SIRM/ X . Thus, the poor quality of the palaeointensity experiments for some samples can probably be ascribed to the presence of weathering products and MD magnetite grains while the good quality of these experiments is related to soft hysteresis properties and a broad unblocking temperature spectrum.

An improved methodology for Thellier‐type paleointensity determination in igneous rocks and its usefulness for verifying primary thermoremanence

We propose a new methodology for Thellier‐type paleointensity experiments. In standard paleointensity experiments the blocking temperatures of most stable remanences are often not reached before thermal alteration of magnetic mineralogy begins, and subsequent data have to be discarded. We emphasize that when alteration begins at low temperature and continues throughout the experiment (which then fails in standard analyses), the resulting unblocking temperature (Tub) spectrum of the alteration product does not necessarily overlap with the whole Tub spectrum of natural remanent magnetization (NRM), because Tub depends on physical characteristics of the new grains, not the temperatures at which they were formed. NRM may survive in an uncontaminated higher Tub window. If alteration remanence only has low Tub then partial thermoremanent magnetization (pTRM) checks at temperature step Tj‐1 can be used to correct for alteration that has occurred at Tj, and furthermore, thermal demagnetization of a full TRM acquired in the laboratory at the end of the progressive Thellier‐Thellier experiment should reveal the true, uncontaminated Tub spectrum of the NRM in higher temperature intervals. We propose a new experimental sequence which allows monitoring and correction of alteration and provides two semi‐independent estimates of palaeointensity in the circumstances described above. We illustrate the issues involved through new experimental work on igneous rocks from a Paleozoic charnockitic syenite, where we unravel TRM from multidomain isothermal remanence despite massive alteration during laboratory heating. Finally we point out the value of these experiments for verifying primary TRM in igneous rocks, indicating properties that distinguish TRM from other remanence.

Magnetic discrimination of pyrrhotite‐ and greigite‐bearing sediment samples

By using bulk samples, rock magnetic measurements were performed to discriminate between pyrrhotite‐ and greigite‐bearing shallow marine sediments that are now uplifted above sea level in southwestern Taiwan. Thermal demagnetization of a composite isothermal remanent magnetization (IRM) was found to be effective in differentiating between the two types of sediments. To check the thermal instability and estimate the true unblocking temperature (TB) spectra of sediments containing these minerals, saturation IRMs (SIRMs) were imparted at each temperature step during demagnetization. While pyrrhotite‐bearing samples showed unambiguous TB temperature spectra, greigite‐bearing samples underwent considerable alteration which is responsible for most of the decrease in magnetization during thermal demagnetization. Such thermal instability of greigite is a practical and important clue for its identification. Zero‐field warming of IRM from 5 to 300 K sensitively indicates the presence of pyrrhotite and trace magnetite in bulk samples without any magnetic separation.

High-Unblocking-Temperature Haematite Magnetizations of Late Palaeozoic Red Beds From the Okcheon Zone, Southern Part of the Korean Peninsula

The Late Carboniferous and Permo-Triassic red beds of the Pyeongan Supergroup in the Okcheon zone, southern part of the Korean Peninsula, were found to contain three components of remanence. These components, which are resolved by thermal demagnetization, are characterized by high (Hhc for Late Carboniferous and Hhpc for Permo-Triassic), intermediate (Hm) and low (HI) unblocking temperatures. As the maximum unblocking temperature spectra of the Hhc (Hhpt) component often overlapped the Hm spectra above 680° C, 3° to 5° C steps were needed above 670° C to resolve the Hhc-(Hhpt-) component magnetization. The tilt-corrected mean direction of the Hhc component red beds is D=255.5°, 1=-2.4° (α95=20.8°), and that of the Hhpt component is D=271.9°, 1=-33.4° (α95=16.5°). The precisions of mean directions of Hhc and Hhpt components improve after tilt correction and allow the directions to be distinguished from each another. This indicates that both Hhc and Hhpt component magnetizations were acquired during or shortly after the deposition of the red beds. According to micropetrographic observations, the haematite in the red beds can be divided into three types: detrital specularite (A), non-detrital specularite (B), and pigmental haematite (C). Type A grains are tens of micrometres in size, and many of them are martite. The remanent magnetization of type A is possibly depositional remanent magnetization (DRM), post-DRM or chemical remanent magnetization (CRM) acquired close to the deposition time, whereas that of types B and C is of CRM origin acquired during and after folding, respectively. The time of folding is well established as a Triassic to Jurassic event. We believe that the remanent magnetization carried by type A grains is primary, and is correlated to Hhc-(or Hhpt-) component magnetization.

Rock magnetism and magnetic surveys in the iron ore deposit of El Encino, Mexico

In this study, we report results of petrophysical properties such as bulk (low field) magnetic susceptibility, remanent magnetization, Köenigsberger ratios, density, unblocking temperature spectra, isothermal remanent magnetization acquisition, and thermomagnetic analysis from surface and borehole samples of rock units and iron ore bodies in the iron mining district of El Encino, Jalisco, southwestern Mexico. Iron mineralization consists mainly of magnetite (from 65% to 90% volume) and hematite. It is associated with granodioritic intrusions and volcanic units in a mid-late Cretaceous volcano-sedimentary sequence. The magnetite in the iron ore has generally large grain size and a multidomain behavior. Stable remanent magnetization components associated with pseudo- or single domain behaviors are also observed. Iron ore bodies and associated intrusives show large dipolar magnetic anomalies. For the iron ore bodies examined, either the induced or the remanent component is responsible for the magnetic anomaly response. Data on bulk susceptibility, remanent intensity, and direction provide important constraints for modeling the magnetometric anomalies i.e., by quantifying the relative contributions of induced and remanent magnetization components; and thus permitting a better control of the geometry of source bodies.

A primary magnetization fingerprint from the Cretaceous Laytonville Limestone: Further evidence for rapid oceanic plate velocities

Paleomagnetic data from the Cretaceous (101–88 Ma) Laytonville Limestone of northern California are of geodynamic interest because they suggest a rate of absolute plate motion (> 15 cm/yr) that exceeds those of all present‐day plates. Such findings assume that a primary magnetization is preserved in these accreted pelagic limestone outcrops. This assumption is supported by a trend in the paleomagnetic inclination data that matches, in magnitude and sign, the expected sense of motion predicted by comparing the mean paleolatitude value with the North American apparent polar wander path. The pervasiveness of remagnetizations seen in shallow water carbonates of North America and elsewhere, however, raises questions as to whether the Laytonville Limestone is remagnetized, regardless of tests indicating a primary magnetization. The remagnetized carbonates have a distinctive “wasp‐waisted” magnetic hysteresis signature, thought to reflect a bimodal distribution of magnetite grain sizes. This signature can be quantified by the relationship between saturation remanence (Mr), saturation magnetization (Ms), coercivity (Hc), and the coercivity of remanence Hcr: Mr/Ms = 0.89(Hcr/Hc)−06. This power law is compared with reference data from Pacific deep‐sea carbonates (Deep Sea Drilling Project Sites 167, 317, and 463; Ocean Drilling Program Site 806) and Tethyan pelagic carbonates (Valle del Mis section, Southern Alps) which together define a different curve: Mr/Ms = 0.53(Hcr/Hc)−1.0. The Laytonville Limestone falls in two categories. On the basis of unblocking temperature spectra, resistance to alternating field demagnetization and behavior during the acquisition of isothermal remanent magnetization, red Laytonville Limestone is thought to contain pigmentary hematite in addition to magnetite. Hysteresis curves from red Laytonville Limestone are wasp waisted, but because such curves also typify magnetite‐hematite mixtures, these data do not provide an unambiguous test for remagnetization. Some white Laytonville Limestone, however, appears to contain negligible amounts of hematite and can be directly compared with the remagnetized carbonates. Hysteresis parameters derived from white samples are indistinguishable from those of the reference pelagic limestones. Such samples have a stratigraphic significance, occurring where paleolatitude data indicate an equatorial crossing. These samples may have depleted pigmentary hematite due to increased magnetic dissolution as the limestone entered the equatorial belt of high productivity. The lack of a remagnetization fingerprint in these white samples, together with paleomagnetic, lithologic, and paleontologic data, form a consistent data set supporting the primary nature of the Laytonville Limestone magnetization and rapid oceanic plate velocities. A small plate size coupled with the unusually vigorous mantle plume volcanism of the Cretaceous Pacific basin could have combined to reduce the effectiveness of asthenospheric drag, accounting for the rapid motion.

Experiments to investigate the origin of anomalously elevated unblocking temperatures

A fundamental problem in rock magnetism and paleomagnetism is the physical origin of anomalously elevated unblocking temperatures. To investigate this problem, we have studied viscous remanent magnetization (VRM) acquired at 225°C by 1–2 μm magnetite particles synthesized with the glass‐ceramic method. Experiments were performed with two primary goals: (1) to determine which particular conditions produce anomalously elevated unblocking temperatures, and (2) to compare various observed aspects of viscous behavior to predictions based on two different physical mechanisms that might raise the unblocking temperature: the reduction of magnetic relaxation time by the applied field (the Neel medium‐field mechanism), and directional ordering of defects within domains and domain walls. When compared to model predictions, the present results indicate that neither the Neel medium‐field mechanism nor the directional ordering mechanism satisfactorily accounts for anomalously high unblocking temperatures. Surprisingly, however, the unblocking temperature spectrum of VRM is critically sensitive to a sample's initial state. When samples first are thermally demagnetized from 580°C to 225°C, VRMs acquired subsequently at 225°C exhibit pronounced high‐temperature “tails” extending to 450°C. Yet when samples first are demagnetized in an alternating field at room temperature before acquisition at 225°C, their VRMs unblock by approximately 300°C. Despite these two very different thermal responses, VRM acquisition curves at 225°C are virtually identical for the two initial states. Furthermore, zero‐field storage prior to acquisition suppresses VRM acquisition coefficients by similar amounts for both initial states. Significantly, standard thermal demagnetization to room temperature greatly attenuates the high‐temperature tail of a VRM acquired subsequently at 225°C. We conclude that thermal response of domain structure, as determined by thermomagnetic history, plays a critical role in the thermal unblocking process.

Paleomagnetism and Neogene clockwise rotation of the Northern Cady Mountains, Mojave Desert of southern California

The early to middle Miocene Hector Formation of the northern Cady Mountains consists of about 400 m of syntectonic alluvial fan and volcaniclastic sediments deposited in the central Mojave desert. Paleomagnetic samples were collected from 60 sites throughout this unit. Samples from each site were treated using both alternating field and thermal demagnetization. These studies and isothermal remanent magnetization experiments indicate that the natural remanent magnetization (NRM) is complex and that characteristic directions can be carried in either high coercivity or continuous unblocking temperature spectra depending upon the sample. A positive reversals test and concordant directions of the alternating field (AF) and low and high temperature components suggest a stable magnetization. Although a Unique magnetostratigraphic correlation to the time scale is difficult, the section studied may represent chrons C6B to C5C. After screening of individual components and calculation of site means using Fisher statistics, 48 of the original 60 sites were used to calculate an overall formation mean direction, i.e., declination 18.6°, inclination 45.4° (α95 = 6.0°). Using a 20‐m.y. paleomagnetic pole for North America [Irving and Irving, 1982] the Hector Formation has undergone 20.6°±7.6° (R±ΔR) of clockwise rotation around a vertical axis. This amount of rotation is considerably less than reported for some early Miocene rocks elsewhere in the Mojave Block. Least squares regression of the 48 site mean declinations versus stratigraphic position indicates no trend throughout the section, suggesting that there was no syndepositional rotation of this unit. The late Cenozoic structural history of the Mojave Block is considerably more complex than was originally believed, and currently available paleomagnetic data indicate that many blocks within this region have experienced different degrees of rotation. There were at least two phases of tectonic rotation in this region: (1) early Miocene (about 25 Ma), probably related to the initiation of extension within the Mojave Rift; and (2) post middle Miocene‐Pliocene (between about 16 Ma to the present), possibly related to movement along principal structural features such as the San Andreas fault zone. The exact timing of the younger phase of tectonic activity cannot be better constrained until paleomagnetic data become available for late Cenozoic rocks within the Mojave Block.

Paleomagnetism of Devonian sedimentary rocks and mafic sills from the Meguma Terrane, Nova Scotia.

Paleomagnetic data were obtained from the Lower-Middle Devonian Torbrook Formation (7 sites) which consists of fossiliferous shale, siltstone and quartzite and mafic intrusions (10 sites) composed mainly of gabbro sills. Some but not all the sills intrude the Torbrook Formation. Thus, some of these intrusives are of Silurian age while others are Late Devonian. The natural remanent magnetization site-mean directions are scattered but discordant from the present Earth's field for both the sedimentary rocks and the sills. Stepwise alternating field demagnetization up to 100mT and thermal treatment up to 625°C for the Torbrook Formation revealed the presence of a high unblocking temperature (400°-550°C) and an intermediate coercive force (20-50mT) direction (tilt corrected) at 342°, -10° with a corresponding paleopole at 38°N, 138°E. An additional direction (in situ) is observed at 26°, -72° with lower or overlapping unblocking temperature and coercivity spectra. This appears to be a post-folding magnetization and the corresponding ill-defined pole is 14°S, 101°E. The mafic sills yielded the following directions of magnetization with corresponding poles: a) 6 of the 10 sites gave 323°, -65°[139°E, 7°S], b) 4 sites with a component at 96°, -25°[150°W, 13°N], c) 9 sites with a component at 322°, -37°[152°E, 16°N] and with closely overlapping unblocking temperature and coercivity spectra. The age of intrusion of six sills is probably Early Silurian; their site-mean tilt corrected direction and paleopole are 320°, +17°[175°E, 38°N]. The age of intrusion of the other sills ranges from Silurian to Late Devonian. The pole positions are compared with those of the same age from Laurentia, Gondwana and the Avalon Terrane. During the Silurian period, the Meguma and Avalon zones were apparently not part of the same landmass. By Early-Middle Devonian time, both the Meguma Terrane and Avalon Zone had docked unto the North American craton. The dissimilarity in the apparent polar wander paths and paleolatitudes of Africa and the Meguma Terrane does not support a North African provenance for the Meguma Zone.

Multi-component remanent magnetization in the Aulis volcanics, the Lesser Himalaya, Nepal.

Detailed paleomagnetic investigation was carried out on 33 trachyte and trachyandesite rock samples collected at 5 sites from the Aulis volcanics in the Tansen area, the Lesser Himalaya, Nepal. Analysis of magnetic behavior by stepwise alternating field (up to 53mT) and then thermal demagnetization shows complex multi-component remanence composed of at least four components as follows: 1) Component A (D=001°, I=47°)—of recent field origin, 2) Component B (D=339°, I=32°)—interpreted as acquired during early Miocene, 3) Component C (D=330°, I=-49°)—probably acquired during Late Cretaceous, and 4) Component D, poorly defined, with southerly declination and downward inclination of about 55°—probably acquired prior the acquisition of component C. All these components show in situ grouping suggesting their secondary nature.Thermomagnetic analysis showed double-Curie point curves indicating two major magnetic phases with Curie temperatures around 565°C and 360°C. These phases are interpreted as of Ti-poor magnetite and titanomaghemite, respectively, which were formed due to low-temperature oxidation of original titanomagnetite. Analysis of unblocking temperature spectra suggests that the 360°C phase contributes to the component B magnetization.

Synfolding and prefolding magnetizations in the Upper Devonian Catskill Formation of eastern Pennsylvania: Implications for the tectonic history of Acadia

The Upper Devonian Catskill Formation was sampled for paleomagnetic study in east‐central Pennsylvania (41°N, 76°W). In one area the dominant component of magnetization (SE) is revealed over a broad spectrum of demagnetization temperatures ranging to at least 660°C. A conventional fold test is positive at the 99% confidence level. However, statistical analysis of dispersion with incremental bedding tilt correction shows a significant peak in the precision parameter after about 3/4 unfolding. The magnetization is therefore secondary, with a mean direction of D = 166.6°, I = −1.8°. The corresponding pole position (48.1°N, 124.1°E, a95 = 4.0°) is indistinguishable from paleopoles from earlier studies of the Catskill, which therefore can also be regarded as representing Permo‐Carboniferous remagnetizations. A second component of magnetization (SW, pole position 32.8°N, 90.0°E, a95 = 7.2°) with discrete unblocking temperature spectra and southwesterly declination was isolated in a few samples from the first area and in most samples from a second area. A prefolding origin of this magnetization is supported by a positive fold test on five samples from the first area. If this magnetization does represent a Devonian magnetization then the true paleolatitude for east‐central Pennsylvania is 16° +/− 7.2°S, which is consistent with the paleolatitude observed in many of the Upper Devonian rock units in the Acadia region, although inconsistent with others. The question of the position of Acadia relative to North America in the Upper Devonian is therefore still open.

Paleomagnetic study of Devonian rocks from Ste. Cécile‐St. Sébastien Region, Quebec Appalachians

Three Devonian rock units were collected for paleomagnetic study from the Ste. Cécile‐St. Sébastien region located in the Gaspé‐Connecticut Valley synclinorium of Québec Appalachians: 15 sites (48 samples) from the metasediments (subgreenschist facies‐siltstones, sandstones) of the Compton Formation (early Middle to late Early Devonian age based on fossil plants) which were tightly folded during Middle Devonian Acadian orogeny; 14 sites (61 samples) from the Ste. Cécile‐St. Sébastien granitic body (362 m.y., K‐Ar age) which intruded the metasediments; 14 sites (62 samples) from the hornfels created subsequent to the emplacement of the intrusive. Detailed thermal cleaning of the metasediments yielded a mean remanence direction characterized by linear vector trajectories, after tilt correction, at D = 209°, I = +20°, k = 30, α95 = 7.2° with unblocking temperature spectrum, 300°–550°C, carried probably by magnetite. The predeformational origin of this remanence is tested at 99% confidence level as well as antipodal normal and reverse polarity directions. The corresponding paleomagnetic pole, MS, falls at 28°N, 77°E (dp, dm = 6°, 11°) in agreement with the recently reported poles which are mostly Early Devonian in age and derived from the southeastern margin of the presumed proto‐Atlantic ocean. AF demagnetization of granites resulted in a univectorial mean remanence at: (1)D = 332°, I = − 39°k = 31 for N = 5 sites in fine‐grained granite (FG) unit; (2) D = 313°, I = −25°; K = 19 for N = 9 sites in medium‐grained granite (MG) unit; and (3) D = 319°, I = −30°, K = 17 for all the total (N = 14) granite (G) sites. AF demagnetization of hornfels revealed a post intrusive mean remanence direction, HF, at D = 346°, I = −22°, K = 8 for N = 14 sites. The individual site‐mean remanence directions (with coercivity spectra: 10–60 mT) from both granites and hornfels suffer from a large declination spread of about 60° but the confidence ovals for the FG and HF remanences overlap. This indicates that the FG and HF remanences are indistinguishable from each other and therefore are proximal in time and probably originated during Middle to Late Devonian period. Interpretation of this kind is speculative in the absence of a convincing evidence for a two (FG younger than MG) or three phase intrusive history of the pluton. Since the pole positions of G (18°N, 150°E), MG (18°N, 158°E), FG (18°N, 137°E) and HF (32°N, 125°E) spread over 40° in longitude, these data cannot confirm or negate the possibility of a Mid‐Paleozoic transcurrent motion proposed for northern Appalachians. We regard only the pole, MS, with probable late Early or early Middle Devonian age as reliable for the apparent polar path construction.

Further paleomagnetic results from the Seal Group igneous rocks, Quebec

This paleomagnetic study involves further demagnetization (alternating field, thermal) of specimens from part of the previously published Seal Group (volcanics and sills). A previous study uncovered two stable magnetizations nominated M2 (herein B) and M3 (herein A). In the present work, the A magnetization has been better defined, found to contain reversals of direction, and has been found also in the sills (6 sites, specimen unit weight; 289°, +11°; α95 = 11°) as well as the volcanics (5 sites; 293°, +13°; α95 = 13°) for a combined direction with respect to bedding of 291°, +12 °(11 sites, R = 10.76, α95 = 7°). Of the total number of specimen directions (63; 1 or 2 per sample), 17 were obtained by vector subtraction (VS); all but four of the latter were the means of at least two determinations. It is suggested that the A magnetization residing in hematite was acquired first, and B, residing in magnetite, later, as a result of remagnetization during the Grenville 'Orogeny'. At several sites, where there is no evidence of hematite, a reverse B direction may be present. The A pole is at 17°N, 195°E (δp = 4°, δm = 7°), near the Seal Group red bed pole (05°N, 205°E), and the B pole (direction with respect to horizontal) at 17°S, 158°E (7°, 9°).It is apparent in this and other studies that specimens that contain both normal and reverse magnetizations display a very characteristic behaviour under demagnetization. This behaviour and its implications for deriving good approximations to the true direction by the use of VS—even when remanent coercive force or unblocking temperature spectra overlap substantially—are revealed by means of a model using artificial data.