Characteristic Remanence Directions Research Articles

New paleomagnetic data from the sedimentary cover of the Tandilia System: Further geodynamic or geomagnetic complexities in the Late Ediacaran

Anomalous paleomagnetic data have been found worldwide during the Ediacaran period, giving rise to several non-actualistic hypothesis. In order to get more information about this period, paleomagnetic, magnetic fabric and rock magnetic studies were carried out in the Avellaneda Formation (∼570–560 Ma) from two drill cores of the Alicia quarry in the Olavarría area of the Tandilia System, in the Río de la Plata craton (Argentina). Anisotropy of magnetic susceptibility studies indicate a pre-tectonic origin for the magnetic fabric of this Formation. Rock magnetic studies suggest the presence of magnetite and hematite in different proportions as the main ferromagnetic minerals carrying the remanence. After stepwise thermal demagnetization, two different characteristic remanence directions were obtained for the same unit, one corresponding to the marls (“b1”) of the lower section of the Avellaneda Formation, and the other from the claystones (“b2”) of the upper section of this unit. These results were combined with the remanence directions obtained by Franceschinis et al. (2022) for the same unit at La Cabañita quarry, located 10 km away. This procedure allows the calculation of two paleomagnetic poles for the Avellaneda Formation. The AV1 pole is located at: 2.0° S, 311.1° E, A95: 5.0°, N: 58 while the AV2 pole is at: 3.3° N, 348.9° E, A95: 11.7°, N: 7. These results confirms that the Rio de la Plata craton also presents anomalous paleomagnetic data during the Ediacaran, implying extremely fast movements in very short periods of time. This can be interpreted as evidence of two inertial interchange true polar wander events during this time, as was already proposed by other authors. An alternative possibility suggests a non-actualistic behaviour of the Earth Magnetic Field, switching from axial to equatorial positions, with an intermediate stable position between them. The likelihood and implications of these two hypotheses is discussed.

On the early postclassic collapse of Plazuelas (Northern Border of Mesoamerica): Archaeomagnetic evidence based on full geomagnetic vectors

We report a detailed rock-magnetic, archaeomagnetic and absolute intensity investigation on burned in situ remains of the Plazuelas archaeological site (Northern Border of Mesoamerica). Fifty-six specimens were subjected to step-wise alternating field demagnetizations to retrieve characteristic remanence directions. All samples yielded evidence of stable, single-component magnetization carried by magnetite or Ti-poor titanomagnetite. Sixty-four specimens were selected for paleointensity experiments following the Thellier double heating technique, including remanence anisotropy correction. The archaeomagnetic dating, based on full geomagnetic vectors, was performed using the global geomagnetic models and the regional Paleosecular Variation Curves available for Mesoamerica. The most probable age intervals obtained for the burned floor of Site 1 (Casas Tapadas North) are within the interval 972AD – 744 AD. In the case of the burned floor of Site 2 (Casas Tapadas West), the ages obtained fall within 971 AD - 752 AD. In the case of the burned floor of Site 3 (South Sector), the ages obtained fall within the interval 752 AD – 969 AD. The calculation of the most probable age intervals, carried out using the Bootstrap resampling method, provided the dates of 940 ± 6 AD, 896 ± 14 AD and 883 ± 14 AD, respectively. These ages agree well with various archaeological hypotheses on the sudden collapse of Plazuelas as a result of intentional destruction, mutilation and generalized firing due to the local war and/or social unrest.

Paleomagnetism of the Ediacaran Avellaneda Formation (Argentina), part I: Paleogeography of the Río de la Plata craton at the dawn of Gondwana

A paleomagnetic and rock magnetic study was carried out in the Ediacaran Avellaneda (∼570 Ma) and Cerro Negro (∼555 Ma) Formations belonging to the La Providencia Group, in the upper part of the Neoproterozoic sedimentary cover of the Tandilia region, in the Río de la Plata craton. The Avellaneda Formation was studied at outcrop level and in three drill cores, yielding a mean characteristic remanence direction of Dec: 21.4°, Inc: 67.1°, α95: 4.2°, k: 23.9, N: 51 and a paleomagnetic pole at 1.0° S, 313.4° E, A95: 5.9°. The Cerro Negro Formation yielded a mean characteristic direction of Dec: 22.0°, Inc: 68.5°, α95: 10.3°, k: 20.8, N: 11 obtained from a single drill core, from which a paleomagnetic pole at 3.6° S, 307.8°E, A95: 16.6° was computed. Rock magnetic data indicates that magnetic remanence is mainly associated with magnetite and hematite. The paleomagnetic information presented here results in a change in the previously accepted Late Ediacaran apparent polar wander path of the Río de la Plata Craton. The newly obtained poles indicate that Río de la Plata Craton experienced a rapid drift from a low latitudes location (ca. 19° S) at ca. 600 Ma to moderately higher latitudes (between 50° and 42°S) from around 580 to 550 Ma.

Paleomagnetism and tectonics from the late Pliocene to late Pleistocene in the Xalapa monogenetic volcanic field, Veracruz, Mexico

AbstractThe Xalapa monogenetic volcanic field, Veracruz, Mexico is located at the eastern end of the Trans-Mexican Volcanic Belt. This study uses specimens from 0.8 ka to 5.96 ± 0.156 Ma Xalapa lava deposits and from 4.5 ± 0.028 Ma La Concha ignimbrite. The study identifies the magnetic mineralogy by reflected light microscopy, thermomagnetic curves, and coercivity estimates. It further defines the magnetic domain size distribution by hysteresis plots, Day diagrams, and first-order-reversal-curves. Characteristic remanent directions and virtual geomagnetic poles are also calculated. The characteristic remanent directions are used to establish the magnetostratigraphy and to identify the net rotation experienced, at each site, since the emplacement. The results allow the identification of the rotation domains, and R′-, P-, and Y-shears, active since the past ∼2.5 m.y. in the region and a better characterization of the displacement along the Río Actopan fault, active in the Xalapa monogenetic volcanic field.

Constrained Magnetostratigraphic Dating of a Continental Middle Miocene Section in the Arid Central Asia

The Neogene succession of the Aktau Mountains in the Ili Basin, southeast Kazakhstan, is a terrestrial archive well suited for researching the role of Central Asia in Miocene climate evolution. We present an integrated approach for dating the well-exposed Bastau Formation, based on magnetostratigraphy and constraints from cyclostratigraphy and biostratigraphy. Stepwise demagnetization yielded characteristic remanence directions that are consistent with those expected for the Miocene in Central Asia. The reddish-colored alluvial floodplain deposits and grey lacustrine deposits show partly complex magnetic behavior with magnetite and hematite as the main magnetic carriers, with variable demagnetization behavior and non-dipolar normal and reverse polarity directions. The observed magnetic properties are best explained by depositional variability and magneto-mineralogical alteration effects of both dissolution and neo-formation of magnetite, including significant secondary magnetization. The mean of reverse polarity directions is flatter than the expected Middle Miocene Earth magnetic field, which is an indicator for the existence of inclination shallowing that supports a primary origin. Detailed rock magnetic analyses are used to analyze the nature of the characteristic remanent magnetization and to discriminate primary and secondary remanence directions in order to obtain a reliable magnetostratigraphic result. The proposed age of 15.3 Ma to 13.9 Ma for the Bastau Formation corresponds to the known biostratigraphic setting, matches with typical sedimentation rates of foreland basins in Central Asia, and coincides with spectral analysis of geochemical proxies of that section. The resulting age model serves as a robust framework for paleoclimate reconstruction of Neogene climate dynamics in Central Asia.

Cenozoic to Cretaceous paleomagnetic dataset from Egypt: New data, review and global analysis

Different phases of igneous activity took place in Egypt during the Mesozoic and the Cenozoic and oriented samples were collected from three Cenozoic localities (Baharya oasis in the Western Desert, Abu Had in the Eastern Desert and Quseir along the Red Sea coast), and four Cretaceous localities (Toshki & Abu Simbel south of Aswan, and Shalaten & Abu Shihat along the Red Sea coast). Rock magnetic properties of the samples indicate magnetite and titanomagnetite as the main carrier of the remanent magnetization. Following stepwise demagnetization, characteristic remanent directions were identified only for 62% of the samples, a fairly low rate for that type of samples, and 8 new paleomagnetic poles were calculated. All our Cenozoic poles fall clearly off Master Polar Wander Paths proposed for South Africa. Therefore, all paleomagnetic results, previously published for Egypt, were compiled from Cretaceous to Quaternary. The published poles largely overlap, blurring the Egyptian Apparent Polar Wander Path. A new analysis at the site level was then carried out. Only poles having a kappa larger than 50 were selected, and new pole positions were calculated by area and by epoch, when at least 3 sites were available. Even though the selection drastically reduced the number of considered poles, it allows definition of a reliable Cenozoic apparent polar wander trend for Egypt that differs from the South African Master Polar Wander Path by about 10–15 °. If the Cretaceous igneous poles are in good agreement with the rest of the African data, the sedimentary poles plot close to the Cenozoic portion of the South African Master Polar Wander Path, a discrepancy that could be related either to inclination flattening and/or error on age and/or remagnetization in the Cenozoic.

Magnetic Anisotropy and the Paleosecular Variation Record in Stalagmites from Njirak Cave, East Java: The Effect of Detrital Magnetite Emplaced in Small Cavities by Flooding Events

Magnetic anisotropy and remanence were measured for 33 samples from the axis of two stalagmites from Njirak Cave in East Java. All 33 samples were stepwise demagnetized with alternating fields, but only 16 were stable enough to yield reliable characteristic remanence directions. These directions differ, especially in declination, from the present Earth’s field direction and agree for samples of about the same age from both stalagmites, suggesting that the Earth’s field at Njirak Cave maintained a declination averaging 23° and an inclination averaging −23° for most of the past 2,500 years. Anisotropy of magnetic susceptibility is high, averaging 5.8%. The minimum susceptibility axes are nearly horizontal with scattered declinations; whereas the maximum susceptibility axes tend to be steeply inclined (average inclination is 51°). This implies that the long axes of the mainly multi-domain magnetite grains tend to be steeply inclined (rather than horizontal, as expected if the magnetite were in horizontal detrital layers). We suggest that this is due to the magnetite being in flood-emplaced detritus adhering to the steeply-inclined sides of small (2 to 5 mm) cavities that cut across and are sealed off by the stalagmites’ horizontal calcite growth layers. Steeply inclined maximum susceptibility axes should help detect such stalagmites, whose quality of paleosecular variation record may be lowered by magnetite-bearing cavities.

Using palaeomagnetism to determine late Mesoproterozoic palaeogeographic history and tectonic relations of the Sinclair terrane, Namaqua orogen, Namibia

Abstract The Sinclair terrane is an important part of the Namaqua orogenic province in southern Namibia containing well-preserved Mesoproterozoic volcano-sedimentary successions suitable for palaeomagnetic and geochronological studies. The Guperas Formation in the upper part of the Sinclair stratigraphic assemblage contains both volcanic and sedimentary rocks cut by a bimodal dyke swarm with felsic members dated herein by U–Pb on zircon at c. 1105 Ma. Guperas igneous rocks yield a pre-fold direction and palaeomagnetic pole similar to that previously reported. Guperas sedimentary rocks yield positive conglomerate and fold tests, with a maximum concentration of characteristic remanence directions at 100% untilting. The combined Guperas data generate a palaeomagnetic pole of 69.8° N, 004.1° E ( A 95 =7.4°, N= 9). The 1105 Ma post-Guperas dykes yield stable remanence directions with positive baked-contact tests and a palaeomagnetic pole at 62.3° N, 031.9° E ( A 95 =6.9°, N= 26), which is coincident with that of the Kalahari-wide Umkondo large igneous province, demonstrating tectonic coherence of the Sinclair terrane with the Kalahari craton at the time of dyke emplacement. These results show that palaeomagnetic and geochronological studies of the Sinclair terrane can provide kinematic constraints on the tectonic evolution of the Namaqua–Natal–Maud orogenic belt and its role in the formation of the Rodinia supercontinent.

Paleomagnetism of the Miocene Jantetelco Granodiorites and Tepexco Volcanic Group and inferences for crustal block rotations in central Mexico — Reevaluation

Miocene igneous rocks from a location in central Mexico provide paleomagnetic data that allow re-evaluation of previous data by Urrutia-Fucugauchi (1981) that were interpreted to suggest a counter clockwise rotation of a crustal block by about 50°. We sampled 31 sites from the Miocene trondhjemitic Chalcatzingo domes, the Tepexco volcanic group, and the Xalostoc diorite, covering the same area of approximately 50km2 about 90km SE of Mexico City. Magnetic analysis shows that these rocks contain magnetic phases of variable composition, with Curie temperatures characteristic for magnetite, but often accompanied by lower Curie temperature components. Magnetic hysteresis measurements point to the presence of PSD particles, and in combination these properties suggest that these rocks are also suitable for paleomagnetic study. Scanning electron microscope analysis supports the rock magnetic results, indicating the presence of magnetite and high-intermediate titanium titanomagnetite in many samples. Demagnetization experiments showed in most cases characteristic remanence directions of reasonable to good quality, and for 26 sites mean directions could be determined. Of these 14 (12) are of normal (reverse) polarity. Overall mean directions of normal and reverse sites are antipodal and pass a reversal test at the 95% probability level. The paleodirection from 26 sites is D=348.1°E, I=35.7°, α95=7.4°, and the paleopole is located at Lat=78.3°N, Long=184.2°E, A95=7.0°, which is indistinguishable from the 20Ma reference pole for the stable North America plate. These data do not support any tectonic deformation of the sampling area since the Miocene. Based on the number of sites studied, their rock magnetic characteristics, and the quality of the magnetic remanence that results in a positive reversal test, we consider our data to be reliable and we therefore suggest that this result supersedes that of Urrutia-Fucugauchi (1981). The reason for this previous and much different result remains unknown.

Rock-magnetic and paleomagnetic results from the Tepic-Zacoalco rift region (western Mexico)

A rock-magnetic and paleomagnetic investigation was carried out on eleven Pleistocene and Pliocene 40 Ar/ 39 Ar dated lava flows from the Tepic-Zacoalco rift region in the western sector of the Trans-Mexican Volcanic Belt (TMVB) with the aim of obtaining new paleomagnetic data from the study region and information about the Earth’s magnetic field recorded in these rocks. Rock-magnetic experiments including measurement of thermomagnetic curves, hysteresis parameters and isothermal remanence acquisition curves were carried out to find out the carriers of remanent magnetisation and to determine their domain structure. Although some samples were characterised by the presence of a single ferromagnetic phase (magnetite), in most cases more phases were observed. Analysis of hysteresis parameters showed a mixture of single domain and multidomain particles, the fraction of the latter varying between 40% and 80%. Paleomagnetic results were obtained in all sites, although in 7 sites characteristic remanence directions and remagnetisation circles had to be combined in order to calculate site means. The six Pliocene sites not showing intermediate polarity yielded a paleomagnetic pole (latitude  = 81.1, longitude  = 94.3) which roughly agrees with the expected one. Paleomagnetic directions do not indicate significant vertical-axis block rotations in the western TMVB area. Reversed polarities observed can be correlated to the Gilbert chron, normal polarities to the Gauss chron or the Brunhes chron and intermediate polarities to the Cochiti-Gilbert or the Gilbert-Gauss transition. The reversed or intermediate polarity magnetisation recorded in one of the sites (542 ± 24 ka) corresponds either to the West Eifel 4 or the West Eifel 5 excursion, while the reversed

A paleomagnetic study from the Late Jurassic volcanics (155 Ma), North China: Implications for the width of Mongol–Okhotsk Ocean

A new paleomagnetic study has been carried out on the Tiaojishan Formation effusives in the Chengde area, North China Block (NCB). The age of the Tiaojishan Formation volcanics was recently dated from 153 Ma to 161 Ma by U–Pb and 40Ar– 39Ar. A total of 154 samples from 16 sites within the Tiaojishan Formation were collected. Detailed stepwise thermal demagnetization allows us to isolate two components from 77 oriented drill-core samples. A low-temperature component identified in most samples falls close to the present local Earth field direction. Characteristic remanent directions obtained from high-temperature pass-reversal test yield a pole position at 240.3°E, 59.9°N with α95 = 6.8°. Combined with coeval paleopoles obtained from the Siberia Block (SIB), these results show a significant paleolatitude displacement between the NCB and SIB, which indicates that the Mongol–Okhotsk Ocean was still open with ~ 3000 km width in its eastern part at ca. 155 Ma.

Paleosecular variation during the Kiaman Superchron: a 1150 year record from glacial varves of the Seaham Formation, New South Wales

Glacial varves of the Seaham Formation at Abercrombie Quarry in eastern New South Wales, Australia, preserve a reverse-polarity characteristic remanence acquired at high paleolatitude during the early stages of the Kiaman Superchron, the longest known interval of constant geomagnetic polarity. Normal polarity overprinting, including the pervasive Cretaceous overprint commonly observed on the southeastern Australian seaboard, is present but does not obscure the primary magnetisation. Ferrimagnetic grains are in the pseudo-single domain size range, confirming the reliability of the varves as paleo-remanence recorders. Rip-up clasts maintain the original direction of the chararacteristic remanence despite their random orientation following redeposition, a form of conglomerate test that restricts the timing of remanence acquisition to within at most a few years of original sedimentation. Characteristic remanence directions from 11 sites spanning 1150 years of annual layers move through a simple, systematic cycle, resembling patterns of secular variation observed for the modern geomagnetic field. Mean virtual geomagnetic poles (VGPs) calculated for the sites trace part of a secular variation cycle around the Upper Carboniferous paleomagnetic pole for the region. The mean of site poles at this locality is at longitude 132.8°E, latitude 31.1°S, with A 95 = 21.3°, although this pole does not span sufficient secular variation to represent a true paleomagnetic pole. The angular standard deviation of the site VGPs exceeds both the standard Model G phenomenological description of the modern field and the Time-Averaged Field Initiative results for comparable latitudes, and appears to agree with the original interpretation of non-suppressed paleosecular variation from late Kiaman, low latitude sites in France. Large scatter in the Abercrombie Quarry poles supports the interpretation that the primary (dipole) family dominates the geomagnetic field during a superchron, and that this condition was already established within the first 11 million years of its 52 million year extent.

Rapid Early Cambrian rotation of Gondwana

Research Article| August 01, 2010 Rapid Early Cambrian rotation of Gondwana Ross N. Mitchell; Ross N. Mitchell Department of Geology and Geophysics, Yale University, 210 Whitney Avenue, New Haven, Connecticut 06511, USA Search for other works by this author on: GSW Google Scholar David A.D. Evans; David A.D. Evans Department of Geology and Geophysics, Yale University, 210 Whitney Avenue, New Haven, Connecticut 06511, USA Search for other works by this author on: GSW Google Scholar Taylor M. Kilian Taylor M. Kilian Department of Geology and Geophysics, Yale University, 210 Whitney Avenue, New Haven, Connecticut 06511, USA Search for other works by this author on: GSW Google Scholar Geology (2010) 38 (8): 755–758. https://doi.org/10.1130/G30910.1 Article history received: 28 Nov 2009 rev-recd: 21 Mar 2010 accepted: 25 Mar 2010 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Ross N. Mitchell, David A.D. Evans, Taylor M. Kilian; Rapid Early Cambrian rotation of Gondwana. Geology 2010;; 38 (8): 755–758. doi: https://doi.org/10.1130/G30910.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Based on the history of Mesozoic–Cenozoic plate motions, as well as simple dynamical considerations, a “speed limit” for tectonic plates has been suggested at ∼20 cm/yr. Previous paleomagnetic data from the Early Cambrian of Gondwana are conflicting but generally imply rapid motions approaching that limit. Herein we describe results from a continuous paleomagnetic sampling of Lower to Middle Cambrian strata from the Amadeus Basin, central Australia. We find characteristic remanence directions that show an ∼60° declination shift through the section. Assuming a tectonically assembled Gondwana supercontinent by Early Cambrian time, this large vertical-axis rotation of its Australian sector corresponds to an equally large translation across paleolatitudes for its Brazilian and West African sectors. Analysis of all high-quality paleomagnetic data from Gondwana both confirms and constrains the 60° rotation to have occurred toward the end of Early Cambrian time, at rates exceeding 16 +12/–8 cm/yr. These observations suggest that either nonuniformitarian plate tectonics or an episode of rapid true polar wander occurred during the Cambrian “explosion” of animal life. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Palaeomagnetism and magnetic anisotropy of Carboniferous red beds from the Maritime Provinces of Canada: evidence for shallow palaeomagnetic inclinations and implications for North American apparent polar wander

SUMMARY A palaeomagnetic and magnetic anisotropy study was conducted on the lower-middle Carboniferous Maringouin and Shepody red bed formations of the Maritime Provinces of Canada to detect and correct inclination shallowing. Because of the shallow inclinations commonly observed in red beds and the strong dependence of North America's Palaeo-Mesozoic apparent polar wander (APW) on red beds, inclination shallowing may substantially affect large portions of North America's APW path. Hematite is the primary magnetic mineral carrier in these red beds, accompanied by secondary magnetite, maghemite, goethite and pigmentary hematite. Thermal and chemical demagnetization of the Shepody Fm. successfully isolated characteristic remanence directions of D= 177°, I= 20.4°, α95= 6.5°, N= 19 and D= 177.8°I= 17.7°, α95= 6.9°, N= 16, respectively. Thermal demagnetization of the Maringouin Fm. isolated a characteristic remanence direction of D= 178.7°, I= 24.9°, α95= 14.5°, N= 9. High field anisotropy of isothermal remanence followed by alternating field and thermal cleaning on leached samples was used to isolate the fabric of hematite. Individual particle anisotropy was measured directly from magnetic separates using a new technique. Hematite's magnetic fabric and particle anisotropy were used to apply an inclination correction. Our inclination corrections indicate up to 10° of inclination shallowing, corresponding to corrected palaeopole positions of 27.2°N, 118.3°E, A95= 6.2° and 27.4°N, 117.2°E, A95= 13.1° for the Shepody and Maringouin formations, respectively. This correction corresponds to a ∼ 6° increase in colatitude for Carboniferous North America, which translates into approximately a 650 km change in North America's palaeogeographic position. The proposed position of North America supports a Pangea B-type reconstruction.

A Mesoproterozoic paleomagnetic pole from the Yangzhuang Formation, North China and its tectonics implications

A new paleomagnetic study has been carried out on Mesoproterozoic sediments in the Jixian area, North China. Detailed stepwise thermal demagnetization allows us to isolate three components from 141 oriented drill-core samples (14 sites). A low-temperature component (component A) identified from most of samples falls close to the present local earth field direction. A middle-temperature component (component B) is found only in 21 samples from four sites. The component B directions (after bedding correction) yield a paleopole position at 336.0°E, 10.2°N with A 95 = 17.1°, which is close to the Lower Cambrian pole of the North China Block. Characteristic remanent directions with dual polarities obtained from high-temperature (component C) pass fold test. The directions yield a pole position at 190.4°E, 2.4°N with A 95 = 11.9°. This result indicates that the North China Block was located in low latitude at ca. 1.35 Ga. Recently, several studies suggested that Laurentia, Baltica and Siberia were also situated at low latitudes from well dated paleopoles between ca. 1.5 and 1.25 Ga. This would imply that the North China Block, Laurentia and Baltica might drift together or close in the Mesoproterozoic.

A catalogue of Spanish archaeomagnetic data

A total of 58 new archaeomagnetic directions has been determined from archaeological structures in Spain. Together with five previous results they allow the compilation of the first archaeomagnetic catalogue for Spain, which includes 63 directions with ages ranging between the 2nd century BC and the 20th century AD. Characteristic remanence directions have been obtained from stepwise thermal and alternating field demagnetization. The hierarchical structure has been respected in the calculation of the mean site directions. Rock magnetic experiments reveal that the main magnetic carrier is magnetite or titanomagnetite with different titanium contents. The age estimate of the studied structures is generally well justified by archaeological constraints. For six structures the proposed date is also supported by physical methods. The data are in close agreement with the French secular variation (SV) curve. This catalogue represents the first step in the construction of a SV curve for the Iberian Peninsula, which will be of much use in archaeomagnetic dating and in modelling of the Earth's magnetic field in Western Europe.

Comparison of the Coe–Thellier–Thellier and microwave palaeointensity techniques using high-titanium titanomagnetites: results from a Tertiary basaltic intrusion from the Sydney Basin, New South Wales

Conventional Thellier and microwave (MW) palaeointensity experiments, accompanied by rock magnetic tests, were conducted on samples from an early Tertiary (ca. 49 Ma) basaltic intrusion [the Peats Ridge (PR) basalt] from the Sydney Basin. Thermomagnetic and low-temperature susceptibility measurements suggest that the dominant magnetic mineral is Ti-enriched titanomagnetite (∼TM65–TM50) and hysteresis results indicate that the majority of samples exhibit PSD-like behaviour. Some samples show evidence of alteration, during laboratory heating, but only at temperatures in excess of 400 °C. No alteration is observed in the dominant temperature range (~70–300 °C) used for calculation of palaeointensities. Stepwise thermal demagnetisation studies reveal single component (after removal of a small viscous), reverse polarity magnetisations in all samples, with a mean characteristic remanence (ChRM) direction of D/I=197°/+60° ( α 95=5°, k=62). Coe–Thellier–Thellier (CTT) experiments yielded a high success rate, with 68% of samples conforming to standard acceptance criteria. Microwave (MW) palaeointensity experiments, predominantly using the perpendicular applied field technique, produced a greater success rate of 82%. Mean palaeointensities of 27.1±4.4 μT ( N=21) and 27.4±4.0 μT ( N=28) were obtained from the CTT and MW techniques, respectively. These means could not be distinguished statistically, and the respective quality indicators were similar. These results suggest that the new microwave technique for calculating palaeointensities is, in this case, equivalent to the well-established CTT technique. This is an intuitive outcome, given that rock magnetic analyses suggest that alteration does not occur below the Curie temperature of the samples. Our results therefore provide a positive test for the equivalence of the MW and CTT techniques. The Virtual Dipole Moment (VDM) value for the intrusion is 4.7±0.7×10 22 Am 2, suggesting that the dipole field was approximately 59% of the present day value at ca. 49 Ma. This provides a valuable data point from the Southern Hemisphere, for a sparsely covered time period in the global palaeointensity database.

Record of deformation by secondary magnetic remanences and magnetic anisotropy in the Nar/Phu valley (central Himalaya)

Secondary magnetic remanences residing in pyrrhotite and anisotropy of magnetic susceptibility (AMS) were studied in low-grade metamorphic carbonates of the Tethyan Himalaya in Nar/Phu valley (central Nepal) and used for interpretation of tectonic deformations. The characteristic remanence (ChRM) is likely of thermomagnetic origin related to post-peak metamorphic cooling occurring after the Eohimalayan phase (35–32 Ma). The ChRM postdates small-scale folding (main Himalayan folding F1 and F2) as shown by a negative fold test of site mean directions at 99% confidence level, and has been probably acquired between 32 and 25 Ma. Late-orogenic long-wavelength folding associated with the Chako antiform (CA) is recorded by the spatial dispersion of ChRM directions and the distribution of the main axes of the AMS tensor. The mean tilting of the ChRM direction since remanence acquisition (≈20–30°) approximately coincides with the tilting of the CA (31°) at the study area indicating that the pyrrhotite remanence predates the CA (CA formed at <18 Ma according to preliminary U/Pb dating). However, comparison of tilt angles of remanence directions and AMS tensor axes suggests that remanence acquisition was not completed before the onset of the CA formation. This could imply a younger age (Early Miocene or even younger) of the ChRM. Using the distribution of remanence directions along a small-circle as well as the distribution of AMS tensor axes, a clockwise mean rotation of 16° is obtained for a remanence age of ≈30 Ma. An Early Miocene remanence age would not change this result substantially. Compilation of rotations in the Tethyan Himalaya deduced from secondary pyrrhotite remanences reveals an increasing clockwise rotation from the Hidden valley in the W to the Shiar valley in the E (≈150 km distance), incompatible with an oroclinal bending model.

Paleomagnetic study of Deception Island, South Shetland Islands, Antarctica

SUMMARY A paleomagnetic study was carried out on recent volcanic rocks exposed on Deception Island (63.0°S, 60.6°W), Antarctica. Sampling comprised all stratigraphic units exposed on the island, which include basaltic, andesitic and trachytic lavas, basaltic dykes and pyroclastic flows. Following stepwise thermal and alternating field demagnetization procedures, consistent characteristic remanence directions were determined at 21 sites, using principal-component analysis. The overall mean remanence direction for the Deception Island rocks is dec. 348.8°, inc. −73.7°, α95= 4.4°, N= 21, and is consistent within error with the geocentric axial dipole direction at the study locality. All of the studied rocks show normal polarity, indicating a Brunhes Chron age. The only available radiometric date of 153 ± 46 kyr agrees with this and suggests a minimum chronostratigraphic span of 100 kyr for the sampled rocks. The site mean directions show a Fisherian distribution and dispersion values compatible with current palaeosecular variation models. No evidence of the far-sided or right-handed effect is found in our data.

Differences in paleomagnetic interpretations due to the choice of statistical, demagnetization and correction techniques: Kapuskasing Structural Zone, northern Ontario, Canada

The Kapuskasing Structural Zone (KSZ) reveals a section through the Archean lower crustal granoblastic gneisses. Our new paleomagnetic data largely agree with previous work but we show that interpretations vary according to the choices of statistical, demagnetization and field-correction techniques. First, where the orientation distribution of characteristic remanence directions on the sphere is not symmetrically circular, the commonly used statistical model is invalid [Fisher, R.A., Proc. R. Soc. A217 (1953) 295]. Any tendency to form an elliptical distribution indicates that the sample is drawn from a Bingham-type population [Bingham, C., 1964. Distributions on the sphere and on the projective plane. PhD thesis, Yale University]. Fisher and Bingham statistics produce different confidence estimates from the same data and the traditionally defined mean vector may differ from the maximum eigenvector of an orthorhombic Bingham distribution. It seems prudent to apply both models wherever a non-Fisher population is suspected and that may be appropriate in any tectonized rocks. Non-Fisher populations require larger sample sizes so that focussing on individual sites may not be the most effective policy in tectonized rocks. More dispersed sampling across tectonic structures may be more productive. Second, from the same specimens, mean vectors isolated by thermal and alternating field (AF) demagnetization differ. Which treatment gives more meaningful results is difficult to decipher, especially in metamorphic rocks where the history of the magnetic minerals is not easily related to the ages of tectonic and petrological events. In this study, thermal demagnetization gave lower inclinations for paleomagnetic vectors and thus more distant paleopoles. Third, of more parochial significance, tilt corrections may be unnecessary in the KSZ because magnetic fabrics and thrust ramp are constant in orientation to the depth at which they level off, at approximately 15-km depth. With Archean geothermal gradients, primary remanences were blocked after the foliation was tilted to rise on the thrust ramp. Therefore, the rocks were probably magnetized in their present orientation; tilting largely or entirely predates magnetization.