Mean Characteristic Remanent Magnetization Research Articles

New paleomagnetic pole and magnetostratigraphy of the Cauvery Basin sediments, southern India

Magnetostratigraphy or magnetic polarity stratigraphy (MPS) is constructed with paleomagnetic parameters for the Campanian–Maastrichtian (Upper Cretaceous) onshore sediment sequences of the Cauvery Basin, southern India. Twenty-nine sedimentary outcrops in the vicinity of Ariyalur district of Tamil Nadu, India were studied using AF (5–150 mT) and thermal demagnetization (100–700°C). The observed remanence carrier is attributed to hematite through rock magnetic experiments. The virtual geomagnetic pole (VGP) latitudes were computed using the acquired characteristic remanent magnetization (ChRM) directions. The mean ChRM produce Dm= 338, Im= –38, (α95=23.91°, k = 15.73, N=60). The mean VGP is estimated at 51.33°N, 292.71°E deriving a paleolatitude of 21.3°S. The constructed magnetic polarity stratigraphy (MPS) is correlated with the standard geomagnetic polarity time scale (GPTS). The composite MPS of these sequences comprises of 12 magnetozones (6 normal and 6 reversed events) that are corroborated with Chron C33n to Chron C30n of the GPTS. The derived paleolatitude position from the present study places Indian subcontinent at little shallow southern latitudes indicating moderately higher drift velocities during Upper Cretaceous.

The importance of multidisciplinary dating in rescue excavations: The case of Santhià, Northern Italy

We present the results of a combined archaeomagnetic and thermoluminescence (TL) investigation on a baked clay kiln discovered near Santhià, Northern Italy. The site, brought to light during a rescue excavation for the installation of methane tubes, provides important evidence for the presence of a brick production workshop in the area. Standard thermal demagnetization was applied to isolate the direction of the Characteristic Remanent Magnetization (ChRM). The mean ChRM direction was used for the archaeomagnetic dating of the kiln, suggesting that it was abandoned around the 16th century CE. TL analysis carried out on the same kiln further confirms this dating. The comparison between the archaeomagnetic and TL results shows excellent agreement, underlining the great potential of these two techniques in dating baked clays. Moreover, thanks to the independent TL dating, the new archaeomagnetic data can be used as a reference point for the reconstruction of the past secular variation path in Italy. Such combined multidisciplinary dating is extremely important in the case of rescue excavations, particularly in urban centers where the rapid expansion of new constructions inevitably causes the destruction of our cultural heritage and the loss of important information about our past.

53–43 Ma Deformation of Eastern Tibet Revealed by Three Stages of Tectonic Rotation in the Gongjue Basin

AbstractThe Gongjue basin from the eastern Qiangtang terrane is located in the transition region where the regional structural lineation curves from east‐west‐oriented in Tibet to north‐south‐oriented in Yunnan. In this study, we sampled the red beds in the basin from the lower Gongjue to upper Ranmugou formations for the first time covering the entire stratigraphic profile. The stratigraphic ages are bracketed within 53–43 Ma by new detrital zircon U‐Pb ages constraining the maximum deposition age to 52.5 ± 1.5 Ma. Rock magnetic and petrographic studies indicate that detrital magnetite and hematite are the magnetic carriers. Positive reversals and fold tests demonstrate that the characteristic remanent magnetization has a primary origin. The Gongjue and Ranmugou formations yield mean characteristic remanent magnetization directions of Ds/Is = 31.0°/21.3° and Ds/Is = 15.9°/22.0°, respectively. The magnetic inclination of these characteristic remanent magnetizations is significantly shallowed compared to the expected inclination for the locality. However, the elongation/inclination correction method does not provide a meaningful correction, likely because of syn‐depositional rotation. Rotations relative to the Eurasian apparent polar wander path occurred in three stages: Stage I, 33.3 ± 3.4° clockwise rotation during the deposition of the Gongjue and lower Ranmugou formations; Stage II, 26.9 ± 3.7° counterclockwise rotation during deposition of the lower and middle Ranmugou formation; and Stage III, 17.7 ± 3.3° clockwise rotation after 43 Ma. The complex rotation history recorded in the basin is possibly linked to sinistral shear along the Qiangtang block during India indentation into Asia and the early stage of the extrusion of the northwestern Indochina blocks away from eastern Tibet.

Paleomagnetic dating of magmatic phases at the Cantung tungsten deposit, Northwest Territories, Canada

The Cantung tungsten–copper (W–Cu) skarn orebodies are hosted by Proterozoic and Lower Cambrian metasedimentary rocks next to the Cretaceous “Mine Stock” monzogranite. Paleomagnetic analyses of 283 specimens from the Open Pit scheelite–chalcopyrite orebody (17 sites) and from adjacent host rocks including the aplite dikes (11 sites) isolated a stable characteristic remanent magnetization (ChRM), mostly by alternating field and then thermal step demagnetization. The step demagnetization results along with rock magnetic analyses of the W concentrate show that the main remanence carriers are single- or pseudosingle-domain pyrrhotite, titanomagnetite, and (or) magnetite. There is no statistically significant difference at 95% confidence between the site mean ChRM directions for the W–Cu ore, the host rock, or the aplite dikes populations. This result indicates that the intrusion of the Mine Stock is coeval with the genesis of the scheelite skarn ore and with dike emplacement to give an overall mean ChRM direction of declination 342.9°, inclination 82.0° (N = 22 sites, radius of cone of 95% confidence α95 = 4.2°, precision parameter k = 54.7) that defines a paleopole at 76.2°N latitude, 212.2°E longitude (radius of cone of 95% confidence A95 = 8.1°). This paleopole is concordant with the coeval 98 Ma North American paleopole at 92% confidence, which provides strong evidence that the eastern Selwyn Basin has been an autochthonous part of North America since the mid-Cretaceous.

Paleomagnetism of the Crocker Formation, northwest Borneo: Implications for late Cenozoic tectonics

Tectonic models for Borneo’s Cenozoic evolution differ in several aspects, particularly in the extent to which they include paleomagnetic data suggestive of strong counterclockwise rotation between 30 and 10 Ma. Key areas are undersampled. We present the results of a paleomagnetic study of Eocene to Early Miocene sandstones from northwest Sabah, principally from the Crocker Formation. We obtained reliable site means from 11 locations along a 250 km northeast-southwest transect using thermal demagnetization to isolate characteristic remanent magnetization (ChRM) directions. The Crocker Formation sandstones are pervasively remagnetized; pyrrhotite dominates the ChRM signal. Locations can be grouped into different domains on the basis of the relative sense of rotation about a vertical axis. Mean ChRM directions for seven locations between Kota Kinabalu and Keningau (declination, dec 12°–19°; inclination, inc –22°–23°) indicate minor clockwise rotation and modest tilting, whereas two locations near Tenom (dec 321°–345°, inc –6°–24°) record counterclockwise rotation and modest tilting. Although we cannot precisely date the age of remagnetization, the results of fold tests from 4 locations, interpreted within the regional structural framework, strongly indicate that remagnetization occurred between 35 and 15 Ma, the waning stages of the Sarawak orogeny to an early phase of the Sabah orogeny. Our results pose serious difficulties for current tectonic models in which Borneo rotates 50° counterclockwise as a rigid block between 30 and 10 Ma. With respect to prior paleomagnetic studies, we suspect that an early episode of strong regional counterclockwise rotation (before 35 Ma) was overprinted not only by differential clockwise rotation of crustal blocks during opening of the South China Sea (32–23 Ma), but also locally by a younger (after 10 Ma) counterclockwise rotation.

Palaeomagnetism of the Howards Pass Zn-Pb deposits, Yukon, Canada

SUMMARY The Howards Pass district is located in the Selwyn Basin, Yukon. The district consists of >15 laminated clastic-dominated (CD) sphalerite–galena (Zn–Pb) deposits in the ‘zinc corridor’ that trends northwest–southeast and extends for ∼35 km. The stratiform mineralized zone, the Active Member, is hosted in carbonaceous cherts and black shales of the Early Silurian Road River Group. Using mostly thermal and then alternating field step demagnetization isolated a stable characteristic remanent magnetization (ChRM) by the core-magnetization-angle method from 18 sites (339 specimens) in 18 variably oriented exploration drill hole cores from 6 Zn–Pb mineralized panels. Rock magnetic analyses show that the main remanence carriers are single- or pseudosingle-domain pyrrhotite and titanomagnetite. The deposits’ mean ChRM direction yields a pole position of either ∼170 Ma on the North American apparent polar wander path or ∼162 Ma on a corrected path for the Intermontane Belt (IMB) terranes. A negative palaeomagnetic fold test indicates that the mineralization's ChRM is post-folding, setting a minimum age for regional metamorphic deformation of ∼170 ± 20 Ma and supporting an Early Jurassic arrival for the IMB's collision and accretion to North America. An autochthonous or para-autochthonous North American tectonic model is favoured for the ChRM of the metamorphosed Selwyn Basin strata rather than an allochthonous IMB model. Further, the palaeomagnetic age indicates that the coarse-grained Zn–Pb mineralization in fine fractures that cut the laminated fine-grained Zn–Pb mineralization at Howards Pass at least was formed by remobilization during Middle Jurassic metamorphism.

New paleomagnetic results from the Lhasa block: Revised estimation of latitudinal shortening across Tibet and implications for dating the India–Asia collision

Knowing how and when the Tibetan Plateau formed is of global concern as high topography modifies climate circulation patterns and alters species migration paths. Previous paleomagnetic work suggested that intracontinental shortening from deformation related to the India–Asia collision played a major role in building the Tibetan Plateau. However, these studies were derived from red bed facies, which are often afflicted by inclination shallowing, leading to overestimates of continental shortening. We report new paleomagnetic data from the Lhasa block, the southern continental margin of Asia before the India–Asia collision. A high temperature, characteristic remanent magnetization (ChRM) was isolated from 43 sites of Late Cretaceous red beds, 32 sites of Late Cretaceous lava flows, and nine sites of Eocene tuffs. All the ChRM directions are significantly different from the present-day geomagnetic field, and the mean directions of red beds, lava flows and tuffs pass the fold test. The distribution patterns of ChRM directions derived from volcanic rocks are consistent with a secular variation model, suggesting that secular variation was likely averaged out. In contrast, the red bed ChRM directions exhibit an east–west elongated distribution pattern, indicative of inclination flattening. The mean ChRM inclination of the red beds, 23.5°±2.5°, is consistent with previous results, but significantly shallower than that of 41.9°±4.4° carried by the coeval lava flows. An elongation―inclination analysis of the red bed ChRM directions yielded an unflattened mean inclination of 39.7°, with 95% confidence limits between 37.7° and 41.9°. The paleolatitudes of the Late Cretaceous volcanic rocks and inclination-corrected sediments suggest about 10° less shortening occurred between Lhasa and Siberia than previous estimates, with total shortening lying between 3.3°±2.6° and 7.3°±5.4° since the Late Cretaceous. The mean ChRM inclination derived from the Eocene tuffs, 51.8°±5.2°, is indistinguishable from the expected reference direction. Our new results imply that India–Asia convergence was accommodated mainly through consumption of the India plate and yield a much younger age (ca. 43Ma) for the initial India–Asia collision than previously estimated by paleomagnetic data.

Paleomagnetism of the world-class Century Zn–Pb–Ag deposits, Australia

Paleomagnetic results are reported from the Century Zn–Pb–Ag SEDEX deposits in northwestern Queensland, Australia. The stratiform mineralization occurs in fine parallel lamellae in ∼ 1595 Ma siltstones and black shales of the Lawn Hill Formation in the Proterozoic McNamara Group. Galena has given a Pb/Pb model age of ∼ 1570 Ma. Paleomagnetic analysis of 333 specimens from ore zones (15 sites), hanging wall sandstones or siltstone (4 sites), and footwall siltstones (5 sites) using mostly thermal and then alternating field step demagnetization, isolates a stable characteristic remanent magnetization (ChRM) for ore sites only. Step demagnetization and rock magnetic analyses of ore, Zn and Pb concentrates and tailings show that the main remanence carriers are single- or pseudosingle-domain inclusions of titanomagnetite in sphalerite and gangue, and pyrrhotite in galena with modern goethite and/or hematite from the weathering of siderite. A paleomagnetic fold test using the ChRM directions of the ore sites is positive, showing that the ore ChRM predates the D 2 deformation of the ∼ 1595 to 1500 Ma Isan orogeny. The orogeny folded the main-stage mineralization, indicating that the ore retains a primary magnetization. The optimum 80% tilt-corrected unit mean ChRM direction for the ore yields a Mesoproterozoic paleopole at 1558 ± 4 Ma on the northern Australian apparent polar wander path. Thus this result both constrains the timing of mineralization and provides an upper age limit for D 2 deformation in the orogeny.

Stratigraphy of the Fort Selkirk Volcanogenic Complex in central Yukon and its paleoclimatic significance: Ar/Ar and paleomagnetic data

Brunhes, Matuyama, Kaena, and Mammoth age basaltic lava flows (Tertiary–Quaternary Selkirk Volcanics) were sampled in west-central Yukon. The mean characteristic remanent magnetization (ChRM) direction of the flows sampled in this and previous studies has a declination of 348.7° and an inclination of 70.8° (n = 42, k = 99.6, α95 = 2.2°) (all on lower hemisphere). The time range represented in this study (ca. 3.25 to ca. 0.004 Ma) is great enough to have confidently averaged secular variation. Sediment associated with the basalt has a mean declination of 7.6° and inclination of 78.8° (n = 5, k = 5.6, α95 = 35.7°). A new 40Ar–39Ar date on the reversely magnetized basal basalts at Ne Ch’e Ddhäwa places the eruption in the Mammoth subchron of the Gauss Normal Chron. The newly dated basal basalt at Ne Ch’e Ddhäwa precedes the initial continental glaciation in Yukon and is older than the Fort Selkirk vent (Lower Mushroom), which was previously thought to be the oldest eruption at Fort Selkirk Volcanic Complex (FSVC). This basal flow at Mushroom is dated at 1.82 ± 0.03 Ma and the uppermost flow is reproducibly dated at 1.36 ± 0.04 Ma. Till on the flanks of a subglacial volcanic mound called Ne Ch’e Ddhäwa (informal) is older than previously thought; its reverse magnetization indicates an Early Pleistocene age rather than the Reid glaciation, which falls during the Brunhes Normal Chron. The paleomagnetism of Tertiary–Quaternary Selkirk Volcanics outcrops outside the FSVC was studied for the first time. The ChRM direction of basalt at the northern edge of the northern Cordillera volcanic province agrees with FSVC directions, suggesting that this flow reflects the same period of volcanism. This suggests that an Eocene K–Ar date, previously thought to be unreliable, may well be correct.

Paleomagnetism of fluorite veins in the Devonian St. Lawrence granite, Newfoundland, Canada

Paleomagnetic results are reported from the Devonian St. Lawrence granite and the fluorite veins within the granite or its related porphyry dikes. Paleomagnetic analysis of 359 specimens from 29 sites shows that there is no statistically significant difference at 95% confidence between the site mean characteristic remanent magnetization (ChRM) directions carried by pyrrhotite and (or) magnetite of the granite and fluorite veins. No post-granite metamorphic event is known in the area. Thus, these concordant directions indicate that the granite and fluorite veins are coeval and retain a primary remanence. However, the mean ChRM direction of 25 vein and granite sites yields an Early Pennsylvanian paleopole of ∼316 Ma on the North American apparent polar wander path at 35.5°N, 129.2°E (A95 = 3.4°) that is younger than the 374 ± 2 Ma Devonian U–Pb zircon age for the granite. Two intersecting vertical vein sets indicate that the St. Lawrence granite has not been tilted since emplacement of the fluorite veins. Therefore, the difference between the measured and expected paleopoles is interpreted to result from a postemplacement counterclockwise rotation of ∼17° of the St. Lawrence granite region about a vertical axis. Analysis of Carboniferous paleopoles in the northern Appalachian orogen indicates the rotation at the St. Lawrence area is Late Devonian during the Acadian orogeny.

Palaeomagnetism and 40Ar/ 39Ar age from a Cretaceous volcanic sequence, Inner Mongolia, China: Implications for the field variation during the Cretaceous normal superchron

An integrated palaeomagnetic and 40Ar/ 39Ar dating study was carried out on an Early Cretaceous volcanic lava sequence from the Suhongtu section, Inner Mongolia, to determine the field behavior within the Cretaceous normal superchron (CNS). 40Ar/ 39Ar ages were obtained from 12 lava flows, indicating that the studied lava was formed around 114.1 ± 0.3 Ma for the lower interval and 110.6 ± 0.1 Ma for the upper interval. Rock-magnetic experiments and electron microprobe analyses indicate that the primary Fe–Ti oxides are the main magnetic carriers. All lava flows carry normal palaeomagnetic directions, which can be grouped into 31 units by an F-test, with a Fisher mean characteristic remanent magnetization (ChRM) of D/ I = 12.8/58.6° ( α 95 = 2.3°). The corresponding palaeomagnetic pole is located at 80.3°N and 200.3°E ( A 95 = 3.2°, K = 64.4), which is indistinguishable at the 95% confidence level from the Eurasia pole derived from the apparent polar wander path for the early Cretaceous. Using a modified Thellier palaeointensity method with stringent acceptance criteria, we obtained two time-series of palaeointensity records from 15 independent palaeomagnetic units (total 136 samples). The virtual dipole moment (VDM) values varied from 2.53 × 10 22 Am 2 to 9.92 × 10 22 Am 2. The mean VDMs for the upper and lower intervals are (5.38 ± 2.06) × 10 22 Am 2 and (4.61 ± 2.67) × 10 22 Am 2, respectively. The observed time-series of palaeointensity, together with the previously available data for the CNS, suggest that magnetic field strength during the CNS fluctuated significantly with time.

Paleomagnetism of Late Paleozoic Strata and Mineralizationin the Tri-State Lead-Zinc Ore District

Paleomagnetic results are reported from five Upper Mississippian and 19 Upper Pennsylvanian limestone and black shale sites (272 samples) and from 42 boulders, oriented with respect to their bedding, of Mississippi Valley-type (MVT) mineralization from three mine waste piles (136 samples) in the Tri-State MVT district and along a northward transect to the Kansas City region. Thermal and alternating field-step demagnetization plus saturation isothermal remanence analysis show that the remanence of the samples is carried mostly by single to pseudosingle domain pyrrhotite and/or magnetite. Both outcrop and boulder samples carry a modern A component from a viscous remanent magnetization in the pyrrhotite and/or magnetite and from hematite produced by weathering. The site mean characteristic remanent magnetization (ChRM) from the five Mississippian outcrops (the D component), when combined with data from seven previously published sites, give a middle to Late Mississippian pole position (330 ± 6 Ma, 1 σ ), which indicates that the host rocks for the Tri-State MVT ores retain a primary chemical remanent magnetization. Similarly the site mean ChRM directions for the 19 Pennsylvanian outcrops (the C component) give a Middle Pennsylvanian pole position (313 ± 8 Ma, 1 σ ). Thus, despite the common presence of trace to minor MVT mineralization in a majority of these sites, they are also deemed to retain a primary chemical remanent magnetization. The MVT mineralization in the boulders yields a ChRM (the M component) with an average inclination of 7.4° ± 1.9°, which constrains ore genesis to an equatorial paleolatitude and which was true only from the Middle Pennsylvanian to the Middle Triassic for the study area. Two interpretations are given, using the paleomagnetic data that support epigenetic MVT ore genesis during either the Late Pennsylvanian (304 ± 6 Ma, 1 σ ) or the Late Permian (257 ± 10 Ma, 1 σ ).

Paleomagnetic constraints on Zn?Pb ore genesis of the Pillara Mine, Lennard Shelf, Western Australia

The Pillara Zn–Pb deposit is the largest of several known Mississippi Valley-type (MVT) deposits in the Lennard Shelf of the Canning Basin. Paleomagnetic and rock magnetic measurements are reported for 294 specimens from 23 sites in mineralization and its carbonate host rocks from the deposit as well as on 15 artificial specimens of zinc and lead concentrate and of tailings. Pyrrhotite carries the characteristic remanent magnetization (ChRM) in nearly all specimens. The ChRM postdates most faulting as shown by breccia tests and most minor regional tilting as shown by the degraded fit on tilt correction. The mean ChRM direction for all sites is D=20.6°, I=−27.5° (N=23, α95=5.3°, k=34.1), yielding an age of 358±5 Ma (2σ) that is similar to the comparable age of 354±8 Ma (2σ) for the Kapok MVT deposit. Host rock diagenesis with attendant secondary remagnetization yields an age of 361±5 Ma (1σ) and the MVT mineralization with a primary chemical remanent magnetization gives an age of 356±3 Ma (1σ), co-eval with a published Rb–Sr sphalerite age of 357±3 Ma. Interpretation of this temporal data suggests that the MVT deposits of the southeastern Lennard Shelf originated during extension, probably in response to rift-related topography-driven fluid flow.

Late Laramide dolomite recrystallization of the Husky Rainbow "A" hydrocarbon Devonian reservoir, northwestern Alberta, Canada: paleomagnetic and geochemical evidence

The Rainbow Field is in reefal carbonates of the Middle Devonian Keg River Formation in the Western Canada Sedimentary Basin. Alternating field and thermal step demagnetization was done on specimens from unoriented core from a vertical, an inclined, and a horizontal well core in the dolomite reservoir. Although they had no viscous remanent magnetization component to use for orientation, most specimens had a well-defined characteristic remanent magnetization (ChRM) that resides in single to pseudosingle domain magnetite. By rotating the mean ChRM direction around its core axis, a small circle can be generated for each core and the small circles intersect in the true ChRM direction of D = 168°, I = -73.5o (α95 = 5.8°, k = 32.2). Its paleopole of 164°E, 83°N (A95 = 10°) defines a Tertiary age with one sigma limits of Middle Eocene to Middle Miocene age. Petrographic examination defines four generations of dolomite. Matrix dolomite has 60-100 µm diameter crystals that were later recrystallized to 200-400 µm. Dolomite cements are represented by vug-filling coarse dolomite (100-200 µm) and saddle dolomite (1000 µm). All four generations of dolomite give similar δ18O values of -10.7 to -16.5‰ (Peedee Belemnite, PDB), δ13C values of +0.7 to +3.2‰ (PDB), and Sr isotopic ratios of 0.70826 to 0.70846 that do not match the expected Middle Devonian carbonate or seawater values. We interpret these data to indicate that mixed pre-Laramide basinal fluids, heated by burial during the Laramide Orogeny, were present during late Laramide time when the dolomites were recrystallized and (or) precipitated prior to petroleum migration and accumulation in the Rainbow "A" reservoir. Thus the combined use of paleomagnetism, geochemistry, and petrography has been proven to be a useful technique to date and identify dolomitization events and pathways for the migration of hydrocarbons.

Paleomagnetic and geobarometric study of the Late Cretaceous Mount Lorne stock, southern Yukon Territory

The ~75 Ma Mount Lorne monzodiorite stock has been studied paleomagnetically to estimate the tectonic motion of the northern Canadian Cordillera since the Late Cretaceous. The stock is one of several paleomagnetic studies currently underway at the University of Windsor Paleomagnetic Laboratory which are funded by the Lithoprobe - Slave northern Cordillera Lithospheric Evolution (SNORCLE) Project. Al-in-hornblende geobarometry and plagioclase-amphibole geothermometry data suggest that the stock has not been tilted since emplacement but does provide evidence that the stock has a normal fault trending north-south through its centre with the east side uplifted relatively by ~1000 m. Paleomagnetic measurements from 12 of 19 granitoid sites yield a well-defined characteristic remanent magnetization direction that is south-southwest and up, and a further six sites yield a direction that is north-northeast and down. Data from a mafic dike yield a negative contact test, suggesting that the dike is coeval with the stock. Combining the 18 granitic site mean characteristic remanent magnetization directions yields a paleopole at 69.1°W, 78.3°N (dp = 4.1°, dm = 4.5°) which suggests that the host Stikine Terrane has been translated poleward by 10.5 ± 3.5° (1170 ± 390 km) and rotated clockwise by 57 ± 11° relative to the North American craton between 75 and 50 Ma. Except for the estimate from the nearby coeval Carmacks Group volcanic rocks, the Mount Lorne estimate is consistent with all other paleomagnetic results within the Stikine Terrane. These latter estimates are also consistent with plate-tectonic models that suggest the Pacific oceanic plates had an increased velocity northwards during the Late Cretaceous.

Diagenesis and remanence acquisition in the Lower Pliocene Trubi marls at Punta di Maiata (southern Sicily): palaeomagnetic and rock magnetic observations

Abstract Three new profiles (LCM, PMD and CMD) through the lower Cochiti polarity zone allow comparison of the effects of weathering and diagenesis on the magnetic properties of the Lower Pliocene Trubi marls. These marls have been well dated, by means of astronomical calibration, and are of particular interest in terms of polarity zonation and regional tectonics. The freshest exposures (LCM profile) carry a remanence associated with pyrrhotite, which is particularly enhanced in some grey layers. This is the first time that pyrrhotite has been documented as the main remanence carrier in these marls. Previously, single-domain (SD) magnetite was recognized as the fundamental ferromagnetic phase. The mean characteristic remanent magnetization (ChRM) direction from the LCM record does not show the expected 30–35° clockwise rotation found in other studies, nor does it record the normal Cochiti interval (i.e. the sampled interval is entirely reverse polarity), suggesting a relatively late origin for the natural remanent magnetization (NRM). Standard rock magnetic techniques are used to investigate the nature of the magnetization in the three laterally equivalent profiles at Punta di Maiata. The entire dataset seems to be better explained by a complex diagenetic history involving a multistage redox history that invokes a ‘late’ anoxia event. The previously postulated iron migration model was put forward for the ‘suboxic situation’ and does not take into account the presence of magnetic sulphides, which are clearly demonstrated in the zone under study. The ‘extended diagenetic model’ inferred here proposes a pathway that accounts for most of the new observations. The salient feature is the postulated existence of a diagenetic phase that has altered the original NRM in some areas (dissolution of magnetite) resulting in a later-stage acquisition of chemical remanence that does not date to the timing of deposition of the sediments. The extension and the detailed mechanisms by which such a process has taken place deserve further research. None the less, the present investigation accounts for previously unrecognized features in the Trubi marls from Sicily and outlines how intricate the nature of the mechanisms contributing to the blocking of remanence in sediments can be.

Large rotation of the Easter microplate as evidenced by oriented paleomagnetic samples from the ocean floor

One of the goals of the Pito93 cruise along the perimeter of the Easter microplate was to obtain oriented rock samples from the ocean floor for paleomagnetic study. Using a new orienting device, the Geocompass, 14 oriented blocks were obtained during 6 dives conducted using the Ifremer DSRV Nautile. Paleomagnetic analysis shows that 13 of these 14 blocks give consistent results in thermal and AF demagnetizations, and reveals a mean Characteristic Remanent Magnetization (ChRM) direction within each block, with both normal and reverse polarities. After elimination of data from 5 blocks situated outside the microplate, and one block which bears an anomalous direction, we have computed a mean ChRM direction at D m = 49.0°, I m = −39.0° (k = 10.2, α 95 = 19.8 °, n = 7). Although the mean inclination is in agreement with the inclination of the recent dipole magnetic field (D = 0°, I = −43°), the average declination is significantly different from this direction, which we interpret as due to a bulk clockwise rotation of 48.5° ± 11° (after the bivariate average) of the Easter microplate over the past 2–3 Ma. This value is in excellent agreement with previous estimates based on remote geophysical measurements and models.