Stepwise Thermal Demagnetization Research Articles

Experimental Evaluation of Remanence Carriers Using the Microcoercivity‐Unblocking Temperature Diagram

AbstractKnowledge of remanence carriers in recording media is crucial for obtaining reliable paleomagnetic data. In this paper, we report a newly developed technique to determine remanence carriers in natural rock samples that uses anhysteretic remanent magnetization (ARM) spectra in the microcoercivity (hc) and unblocking temperature (TUB) planes. The distribution of ARM carriers was determined via systematic stepwise demagnetization experiments of ARM combining thermal demagnetization and alternating field demagnetization treatments and calculating the change in ARM intensity on the hc‐TUB plane (hc‐TUB diagram). Samples from three basaltic lavas and two welded tuffs were analyzed. Combining the hc‐TUB diagram with the change in ARM coercivity spectra, both decreases and increases in magnetic minerals due to the thermal alteration during laboratory heating could be identified. From experimental samples, ARM carrier components were identified as being original rather than produced as a result of thermal alteration. In the case of the Izu‐Oshima 1986 A lava flow (1986La) sample, it was confirmed from the hc‐TUB diagram that high‐hc components can be separated using stepwise alternating field demagnetization treatment but not stepwise thermal demagnetization treatment. Paleointensity experiments were also conducted on Izu‐Oshima 1986 A lava flow samples. Two of the three paleointensity estimates were greater than the expected field intensity, suggesting a certain rock‐magnetic property that affects the paleointensity estimation. The hc‐TUB diagrams of the Izu‐Oshima 1986 A lava flow samples suggest that the differences in the paleointensity estimates were likely caused by different degrees of the high‐temperature oxidation state.

Paleomagnetic evidence for cold emplacement of eruption-fed density current deposits beneath an ancient summit glacier, Tongariro volcano, New Zealand

The thermal structures of eruption-fed, subglacial volcaniclastic deposits are poorly understood because their emplacement is hazardous to observe or obscured by the glacier. Determining deposit emplacement temperature, however, supports improved understanding of the flow dynamics and emplacement processes of volcaniclastic material beneath a glacier. Understanding the emplacement temperatures of ancient volcanic deposits is also important because they can be used, in combination with field studies, to infer the eruptive environment. Here, we use paleomagnetic techniques to quantify the emplacement temperatures of two ancient, proximal, eruption-fed density current deposits at Tongariro volcano, New Zealand. Stepwise thermal demagnetisation of lithic and recycled juvenile block-sized clasts reveal randomly orientated directions of magnetisation, suggesting that the clasts were rotated within the flow but not heated. Additional data from thermomagnetic, hysteresis, and isothermal remanent magnetisation tests indicate that the principal carrier of magnetic remanence is magnetite, and that the magnetisation directions are a primary remanence rather than post-depositional chemical remanent magnetisations. Following systematic removal of any viscous remanent magnetisation, the post-emplacement equilibrium temperatures for the deposits can be estimated at <150°C. The paleomagnetic data support field evidence for rapid cooling of clasts and waterlain deposition. The deposit-forming eruptions took place beneath a summit glacier where the freshly erupted tephra was efficiently cooled by mixing with meltwater. Lithic blocks and recycled juvenile bombs were entrained and remobilised within the cool currents that drained along meltwater channels beneath the ice. This is the first study in which paleomagnetic data have been used to determine the equilibrium temperatures of subglacial density current deposits. The data provide new insight into volcaniclastic flow dynamics beneath a glacier and current-meltwater interactions.

Paleomagnetism and rock magnetic cyclostratigraphy of the Ediacaran Doushantuo Formation, South China: Constraints on the remagnetization mechanism and the encoding process of Milankovitch cycles

South China provides exceptional and unique sedimentary archives of Earth's fundamental changes in Ediacaran time. However, the lack of robust geochronological constraints renders a poorly-defined chronostratigraphic framework for Ediacaran-aged strata in South China, thus making it difficult to understand Earth's critical changes during this time interval. Magnetostratigraphy and rock magnetic cyclostratigraphy turn out to be a few reliable methods in chronostratigraphic studies, especially in the Precambrian. Here, we present a detailed paleomagnetic and rock magnetic study of the Ediacaran Doushantuo Formation in the Huanglianba section, Guizhou Province, South China. The section consists of ~75 m thick carbonate succession and has a record of one of the largest carbon isotope excursions in the Earth history, known as the Shuram excursion. Stepwise thermal demagnetization of Doushantuo carbonates reveals single-component magnetizations that yield a paleomagnetic pole consistent with the Late Triassic segment of the apparent polar wander path of South China. The secondary origin of Doushantuo carbonates' magnetization is supported by their magnetic mineralogy based on various rock magnetic experiments and scanning electron microscopic analyses. We interpret that the secondary magnetization resides mainly in authigenic pyrrhotite that likely precipitated from the reducing fluids during the late stage of Indosinian orogeny in the Triassic. Interestingly, the companion high-resolution cyclostratigraphic study of a 4-m thick succession at the upper layers of the Doushantuo Formation in the Huanglianba section indicates that the Milankovitch cycles were encoded in the variation of magnetic susceptibility, immune from the influence of remagnetization. Variation of magnetic susceptibility is attributed to changes in the abundance of paramagnetic minerals which, by implication, means that the amount of terrestrial materials transported from the continent was regulated by Milankovitch cycles during the deposition of Doushantuo carbonates. Therefore, our results suggest that even though carbonates are prone to remagnetization, astronomical modulation of deposition could be encoded in rock magnetics of the remagnetized carbonates. As a result, rock magnetic cyclostratigraphy has potential in remagnetized carbonates for providing a floating astronomical timescale that can be applied to constrain sediment accumulation rates, estimate the duration of geological events, and construct high-resolution chronostratigraphies.

Late Cretaceous paleogeography of the Antarctic Peninsula: New paleomagnetic pole from the James Ross Basin

Two paleomagnetic poles of 80 and 75 Ma have been computed from 191 to 123 paleomagnetic samples, respectively, of the marine sedimentary units of the Upper Cretaceous Marambio Group exposed in the James Ross Island, Antarctic Peninsula. Paleomagnetic behaviors during stepwise thermal demagnetization and rock magnetic analyses indicate that magnetization is likely primary and carried by SD-PSD detrital titanomagnetite. Application of an inclination shallowing correction by the elongation-inclination method yielded a significant inclination shallowing affecting the older (ca. 80 Ma) succession exposed in the northwest area of the island. However, the paleomagnetic directions computed from the younger (ca. 75 Ma) succession outcropping in the southeast corner of the island yielded an indeterminate result using the same analysis. The inclination shallowing-corrected 80 Ma paleopole position plus previous ones of ca.110, 90 and 55 Ma were used to construct the Apparent Polar Wander Path (APWP) for the Antarctic Peninsula during the Late Cretaceous-Paleocene. This path confirms that oroclinal bending of the Antarctic Peninsula as well as relative displacement with respect to East Antarctica are negligible since 110 Ma. Comparison with the apparent polar wander path for South America for the 130-45 Ma period suggests that this continent and the Antarctic Peninsula kept a very similar relative paleogeographic position since 110 Ma until 55 Ma, which likely meant a physical link between both continental masses. During that period, both continents underwent a relatively fast southward displacement of around 7° and a clockwise rotation relative to the Earth spin axis that can be bracketed between around 100 and 90 Ma. Oroclinal bending of the Fuegian Andes was likely due to tectonic interactions between the Patagonian-Fuegian Andes and the Antarctic Peninsula promoted, at least partially, by such displacements. By 55 Ma the Antarctic Peninsula probably was starting or about to start its final separation from South America.

Paleomagnetic results of Cretaceous cherts from Zhongba, southern Tibet: New constraints on the India-Asia collision

An oceanic Greater Indian Basin was hypothesized to have formed between ∼120 Ma and ∼65 Ma by regional extension that separated the cratonic India from its northern margin of Tethyan Himalaya (TH). However, the lack of paleomagnetic data from this critical time interval in the TH marks an important gap in our knowledge about the tectonic evolution of the Greater India. We have conducted a paleomagnetic study of a Cretaceous marine succession in Zhongba, southern Tibet that consists of predominantly purple reddish cherts and siliceous limestone. Paleomagnetic samples from 12 sites were subjected to stepwise thermal demagnetization that revealed either two- or three-component magnetizations with the high-temperature component (HTC) typically unblocked at ∼650 °C. The HTCs exhibit dual polarities that yield a positive reversed test at 95% confidence level, suggesting a primary origin of the remanence. With chronologic constraints from the combined analysis of the magnetostratigraphic and radiolarian data, the HTCs yield a mean of Ds = 336.9°, Is = −50.7°, ks = 18.0, α95 = 5.3° for the ∼120.8–93.9 Ma interval of the TH, which corresponds to a paleopole at 25.0°N, 285.7°E (dp/dm = 4.8/7.1°). The apparent polar wander path (APWP) of the TH overall resembles that of cratonic India, suggesting no polarward separation that is sufficiently large to merit an oceanic basin between the TH and cratonic India. Therefore, our new reliable paleomagnetic data from the critical time interval do not support the Greater Indian Basin hypothesis and its implied dual collisional processes between India and Asia. On the other hand, the APWPs exhibit relative shifts for the post-120 Ma segment and the pre-130 Ma segment, indicating relative motions between the TH and cratonic India that most likely resulted from shearing along boundary-parallel faults during the India-Asia collision.

New Paleomagnetic Results From Middle Jurassic Limestones of the Qiangtang Terrane, Tibet: Constraints on the Evolution of the Bangong‐Nujiang Ocean

AbstractTo constrain the relationship between the Qiangtang and Lhasa terranes and the evolution of the Bangong‐Nujiang Ocean (BNO), we carried out a paleomagnetic investigation of the Middle Jurassic limestone from the Shuanghu region in the southern Qiangtang terrane. Stepwise thermal demagnetization succeeded in isolating high‐temperature characteristic directions by either principal component or great circle fitting analyses. The site‐mean direction of the 13 sampling sites was Dg = 334.7°, Ig = 51.2°, kg = 27.9, α95 = 8.0° (in geographic coordinates) and Ds = 245.2°, Is = 54.1°, ks = 113.9, α95 = 3.9° (in stratigraphic coordinates). The paleomagnetic data passed both fold and reversal tests. Both rock magnetic and petrographic investigations suggest a primary magnetization. The paleomagnetic results imply that the paleolatitude of the Qiangtang terrane was 34.6 ± 4.6°N (reference site: 32.7°N, 89.4°E) in the Middle Jurassic. Combined with previous reliable paleomagnetic results, it is suggested that the Qiangtang terrane was situated at a stable paleolatitudinal position and did not undergo obvious N‐S displacement between the Late Triassic and the Middle Jurassic. Comparison with the paleomagnetic results available for the Lhasa terrane implies that the width of the BNO was 2,600 ± 710 km (23.4° ± 6.4°) during the Middle Jurassic. The BNO expanded from the Early to Middle Triassic, reached its maximum width in the Late Triassic, and then shrank until it closed by the Cretaceous.

First report of coupled Early Permian paleomagnetic and geochronologic data from the Dunhuang block (NW China), and implications for the tectonic evolution of the Paleo-Asian ocean

In order to better understand the tectonic relationship between the Dunhuang block (DHB) and its adjacent blocks, and to constrain the timing of the closure of the Paleo-Asian ocean, a combined geochronologic and paleomagnetic investigation has been undertaken on Early Permian tuff, basalt flows and sandstones from the Shuangbaotang Formation in the northwestern part of the DHB. U-Pb zircon dating indicates that the age of the strata is between 280.6 ± 2.9 Ma and 291.4 ± 2.6 Ma. Thermal demagnetization of a three-component isothermal remanent magnetization (IRM), and Curie point experiment suggest that magnetite dominates in the rock samples analyzed. In addition, there is a minor amount of hematite in some sandstones. Stepwise thermal demagnetization successfully isolated stable characteristic remanent magnetization (ChRM) from 11 tuff layers, two lava flows and nine sandstone beds. Two components were isolated from all samples: a lower temperature component (LTC) and a higher temperature component (HTC). The LTC is near the direction of the present-day geomagnetic field and produced a negative fold test, indicating it is a viscous remanent magnetization in the present-day geomagnetic field. Most of the HTC are reverse polarity (Normal = 4, reverse = 158), which is in accordance with the Kiaman Reversed Superchron that spans the Late Carboniferous-Permian interval. In addition, the HTC of all studied sites passed the fold and reverse tests, suggesting that they likely represent primary remanent magnetization. The tilt-corrected mean direction from all sites (tuff, basalts and sandstones) is Ds = 1.7°, Is = 43.1°, ks = 403, α95 = 1.5°, N = 22. The mean paleopole of the site-mean direction-corresponded VGPs lies at 74.5°N, 268.5°E with A95 = 1.6°. Considering the consistent inclination values recorded among the studied tuff, basalts and sandstones, and the low degrees of anisotropy within all samples, we suggest that there is no significant inclination shallowing caused by depositional compaction in the sedimentary layers of the studied section. Taking into account the results from this study as well as previous Late Paleozoic paleomagentic studies from adjacent tectonic blocks, we conclude that the DHB formed part of the amalgamated Dunhuang-North China-Alxa-Qaidam mega-block during the Early Permian, but was separated from the Tarim block by a small ocean (here named the Qiemo-Xingxingxia paleo-ocean) at this time. A comparison of the Early Permian paleolatitudes of these and other adjacent blocks suggest that the Paleo-Asian ocean (sensu lato) was still open at this time. Combined with other geological evidence, a paleogeographic reconstruction of the Paleo-Asian ocean has been reconstructed for the Early Permian.

New Late Jurassic to Early Cretaceous Paleomagnetic Results From North China and Southern Mongolia and Their Implications for the Evolution of the Mongol‐Okhotsk Suture

AbstractTo better constrain the evolution of the Mongol‐Okhotsk suture, we carried out new paleomagnetic studies on Sharilyn Formation (~155 Ma) and Tsagantsav Formation (~130 Ma) in southern Mongolia, Amuria Block (AMU), and Tuchengzi Formation (~140 Ma) and Dadianzi/Yixian Formation (~130 Ma) in the Yanshan belt, North China Block (NCB). A total of 719 collected samples (from 100 sites) were subjected to stepwise thermal demagnetization. After a low‐temperature component of viscous magnetic remanence acquired in the recent field was removed, the stable high‐temperature components were isolated from most samples. The high‐temperature components from each rock unit passed a fold test and a reversal test, indicating their primary origins. The corresponding paleomagnetic poles were thus calculated. For AMU, the ~155 Ma pole is at 74.7°N/232.5°E (A95 = 3.7°), the ~130 Ma pole at 74.6°N/194.7°E (A95 = 2.9°); for the NCB, the ~140 Ma pole is at 82.7°N/208.6°E (A95 = 4.3°), the ~130 Ma pole at 80.5°N/197.4°E (A95 = 2.3°). By combining our new results with the published data, we refined the 155–100 Ma segment of the apparent polar wander paths for AMU and NCB, which can demonstrate that these two blocks have been tectonically coherent (AMU‐NCB) during 155–100 Ma. Comparison of the apparent polar wander paths, however, revealed a latitudinal plate convergence of 14.3° ± 6.9° and ~19.0° relative rotation between Siberia and the AMU‐NCB after ~155 Ma. Large‐scale latitudinal convergence likely ceased by ~130 Ma, although some relative rotation between them continued along the Mongol‐Okhotsk suture until ~100 Ma.

New early Permian paleopoles from Sardinia confirm intra-Pangea mobility

We present new paleomagnetic results for the early and middle Permian (18 sites and 167 samples) from sedimentary and volcanic rocks from northern and central-southern Sardinia (Italy). Characteristic directions magnetization have been retrieved using stepwise thermal demagnetization techniques. The bedding corrected site mean directions for the northern and central-southern Sardinian basins show similar inclinations but differ significantly in declination indicating rotations of up to 55° between the two regions. No indication for inclination shallowing was observed. When corrected for the opening of the Ligurian Sea in the Cenozoic and the Bay of Biscay in the Cretaceous, the resulting paleopoles for northern Sardinia (Latitude 46.6°S, Longitude 46.7°W) and southern Sardinia (Latitude 42.8°S, Longitude 35.0°E), transferred into European coordinates are displaced from the coeval reference poles for stable Europe by ∼30° clockwise and ∼45° counterclockwise, respectively, with a rotation pole located close to the sampling region. Statistical parameters for the inclination-only mean for the 18 sites improve after applying the appropriate tilt corrections, suggesting that the magnetization was acquired before tectonic tilt and therefore allows to date the observed rotations to predate a mid Permian(?) folding event. These new results are in agreement with paleomagnetic data from the Sardinian dyke provinces and support earlier interpretations that the differences in general strike observed there are a secondary feature. Combining the data presented here with published data for the Corso-Sardinian block and the greater Mediterranean realm, we argue that the differential block rotations identified in Permian sediments and volcanic rocks reflect post-Variscan intra-Pangea mobility localized along a wide zone of deformation.

Preliminary paleomagnetic and rock magnetic results from 17 to 22\xa0ka sediment of Jeju Island, Korea: Geomagnetic excursional behavior or rock magnetic anomalies?

Paleomagnetic and rock magnetic investigations were performed on a 64-cm-thick section of nonmarine unconsolidated muddy sediment from the Gosan Formation on Jeju Island, Korea. This sediment was recently dated to have been deposited between 22 and 17 kyr BP calibrated, with a sedimentation rate of 13–25 cm/kyr, based on many radiocarbon ages. Interestingly, stepwise alternating field (AF) demagnetization revealed characteristic natural remanent magnetizations with anomalous directions, manifested by marked deviations from the direction of today’s axial dipole field, for some separate depth levels. On the other hand, stepwise thermal (TH) demagnetization showed more complex behavior, resulting in the identification of multiple remanence components. For all TH-treated specimens, consistently two different components are predominant: a low-temperature component unblocked below 240–320 °C entirely having normal-polarity apparently within the secular variation range of the Brunhes Chron, and a high-temperature component with unblocking temperatures (Tubs) between 240–320 and 520–580 °C that have anomalous directions, concentrated in the ~ 13–34-cm-depth interval (~ 17–19 ka in inferred age) and possibly below ~ 53 cm depth (before ~ 20 ka). Rock magnetic results also infer the dominance of low-coercivity magnetic particles having ~ 300 and ~ 580 °C Curie temperature as remanence carriers, suggestive of (titano)maghemite and/or Ti-rich titanomagnetite and magnetite (or Ti-poor titanomagnetite), respectively. A noteworthy finding is that AF demagnetizations in this study often lead to incomplete separation of the two remanence components possibly due to their strongly overlapping AF spectra. The unusual directions do not appear to result from self-reversal remanences. Then, one interpretation is that the low-temperature components are attributable to post-depositional chemical remanences, associated possibly with the later formation of the mineral phase having Tub ~ 300 °C, whereas the high-temperature components are of primary detrital origin that survived later chemical influence. Accordingly, the unusual directions might record geomagnetic instability within the ~ 17–22 ka period manifested by multiple excursional swings, partly associated with the Tianchi/Hilina Pali excursion. However, further work is needed to verify this interpretation and distinguish it from alternative explanations that invoke rock magnetic complexities as the cause of the unusual directions.

Magnetic Properties and Paleomagnetism of Zebra Rock, Western Australia: Chemical Remanence Acquisition in Hematite Pigment and Ediacaran Geomagnetic Field Behavior

AbstractZebra Rock, a decorative stone remarkable for its unusual pattern of regularly spaced reddish bands and rods with white background, is found within the Neoproterozoic succession in East Kimberley, Western Australia. The unusual pigment distribution suggests that precipitation of hematite, or its precursor phase, occurred in a single episode. Magnetic properties of hematite pigment in Zebra Rock are distinctly different from those of the host shale, with a smaller median particle size and higher degree of structural perfection. The low thermal stability of the Zebra Rock pigment, with onset of thermal alteration at 300°C, suggests that the rocks have not undergone significant metamorphic heating. Stepwise thermal demagnetization reveals multiple magnetization components. Short‐range variability in the relative contributions of the components to the total remanence is indicative of the stochastic nature of the hematite pigment growth process. In addition to seven magnetization components with shallow to intermediate inclinations that can be matched to the Paleozoic Australian apparent polar wander path, Zebra Rock samples contain a distinct steeply dipping magnetization that is not observed in the host shales. The steep magnetization appears to be primary, based on its unique association with the Zebra pattern, dissimilarity with younger directions, and evidence for low degree of thermal alteration of the rocks. The steep characteristic remanence contrasts with previous paleomagnetic indications of low Australian Neoproterozoic paleolatitudes. The characteristic Zebra Rock magnetization is the first Australian example of incompatible magnetization directions that have been reported previously from Ediacaran rocks in Laurentia, Baltica, and Africa.

Paleomagnetism of a well-dated marine succession in South China: A possible Late Cambrian true polar wander (TPW)

The Cambrian true polar wander (TPW) hypothesis remains controversial largely because of the uncertainties in the quality and/or fidelity of the paleomagnetic data as well as their chronological control. Testing the TPW hypothesis requires high-quality paleomagnetic data of sufficient spatial and temporal resolutions. Here, we present paleomagnetic results of a continuous Cambrian shallow marine succession from South China where available detailed biostratigraphy provides exceptional chronological constraints. Forty-three sites of paleomagnetic samples were collected from this limestone-dominated succession. Stepwise thermal demagnetization generally reveals three-component magnetizations. Low- and intermediate-temperature components can be cleaned by ∼330 °C, and the high-temperature component (HTC) was isolated typically from ∼350 to ∼450 °C. A positive fold test and the presence of reversed polarity in the strata, together with rock magnetic data as well as the scanning electron microscopic (SEM) and transmission electron microscopic (TEM) results, collectively suggest that the HTCs are likely primary. A directional shift of the HTCs occurs between the lower-middle Cambrian and the upper Cambrian strata in the succession and is tentatively interpreted to indicate a ∼57° polar wander from ∼500.5 to 494 Ma. Because the rate of polar wander is too fast to be a tectonic origin, this polar wander is interpreted to represent a Late Cambrian TPW. This TPW appears coeval with the Steptoean positive carbon isotope excursion (SPICE) and the major trilobite mass extinctions, suggesting a potential link between the TPW and the Late Cambrian biotic and climatic changes. Because the proposed TPW event is exceptionally well-dated, it should be testable through examination of other worldwide sections.

Paleomagnetism of the Cretaceous Galula Formation and implications for vertebrate evolution

This study uses magnetostratigraphy to help constrain the age of the paleontologically important Galula Formation (Rukwa Rift Basin, southwestern Tanzania). The formation preserves a Cretaceous vertebrate fauna, including saurischian dinosaurs, a putative gondwanatherian mammal, and notosuchian crocodyliforms. With better dating, the Galula Formation and its fossils help fill a temporal gap in our understanding of vertebrate evolution in continental Africa, enabling better evaluation of competing paleobiogeographic hypotheses concerning faunal exchange throughout Gondwana during the Cretaceous. Paleomagnetic samples for this study were collected from the Namba (higher in section) and Mtuka (lower in section) members of the Galula Formation and underwent stepwise thermal demagnetization. All samples displayed a strong normal magnetic polarity overprint, and maximum unblocking temperatures at approximately 690 °C. Three short reversed intervals were identified in the Namba Member, whereas the Mtuka Member lacked any clear reversals. Given the relatively limited existing age constraints, one interpretation correlates the Namba Member to Chron C32. An alternative correlation assigns reversals in the Namba Member to recently proposed short reversals near the end of the Cretaceous Normal Superchron (Chron C34), a time that is traditionally interpreted as having stable normal polarity. The lack of reversals in the Mtuka Member supports deposition within Chron C34. These data suggest that the Namba Member is no older than Late Cretaceous (Cenomanian-Campanian), with the Mtuka Member less well constrained to the middle Cretaceous (Aptian-Cenomanian). The paleomagnetic results are supported by the application of fold and reversal tests for paleomagnetic stability, and paleomagnetic poles for the Namba (246.4°/77.9°, α95 5.9°) and Mtuka (217.1°/72.2°, α95 11.1°) members closely matching the apparent polar wander path for Africa during the Late Cretaceous. These results confidently indicate a Late Creteceous age assignment for the Namba Member of the Galula Formation, a unit that has yielded key crocodyliform (e.g., Pakasuchus; Rukwasuchus) and dinosaur (e.g., Rukwatitan; Shingopana) fossils from eastern Africa.

Africa was still far south in the Late Ypresian: Paleomagnetic study on the early Eocene ‘Minia’ formation in central Egypt

The paleomagnetic study was carried out on three sections of the Late Ypresian “Minia” formation limestone, in order to shed light on the paleolatitude of northeast Africa upon the end of the Early Eocene. The initial study on the anisotropy of magnetic susceptibility [AMS] helped in confining the paleomagnetic sampling to the virtually isotropic limestone beds. The subsequent stepwise thermal demagnetization of the three-axis isothermal remanence acquired in one sample of each sampled site, revealed the limited contribution of goethite and hematite with the main remanence carrier magnetite in most samples.The assessment of the natural remanence during the progressive stepwise thermal demagnetization [15–17 steps] of all samples, elucidated the early decay [<150 °C] of a present- day field [PDF] overprint component scattered around the magnetic field direction of the study area. Despite that, the anchored component in most samples was carried in magnetite, yet hematite was recorded in few sites. The visual inspection of the decay spectra in the orthogonal projections, followed by the determination of the best-fit line of every component using the principal component analysis [PCA] technique, differentiated between the magnetite- and hematite- remanence components:1.The magnetite-anchored component, which was overwhelming in most samples, was antipodal with shallow to medium inclination, yielding a paleomagnetic North poles at 73.8°N/197.5°E. This component, which successfully passed the reversal test at 95% confidence level, was considered as the characteristic primary remanent direction of the “Minia” formation. This paleomagnetic pole was, consequently, considered as representing the African Plate in the Late Ypresian.2.On the other hand, the hematite component had normal north-direction clustered around the present-day field [PDF] in the study area. The hematite paleomagnetic pole at 88.7°N/78.8°E, was considered as a PDF overprint acquired in few samples upon exposure to desert weathering and partial oxidation of their original magnetite to hematite.The reasonable consistence of the obtained pole of the “Minia” formation with synchronous poles of the main tectonic units rotated to the African coordinates, point to its reliability. This pole revealed that upon the end of the Lower Eocene, Northeast Africa was still at a far south paleolatitude with respect to its present-day position. The location of Cairo which is now at latitude 30°N, was at 14.2°N paleolatitude during the late Ypresion. This means that Africa was about 16° of latitude south to its current latitude.

Cenozoic clockwise rotation of the Chuan Dian Fragment, southeastern edge of the Tibetan Plateau: Evidence from a new paleomagnetic study

Late Eocene-early Oligocene red beds of the Ninglang Formation were sampled from 23 sites in the Yongsheng area, which comprises the northwestern part of the Chuan Dian Fragment (CDF) of the South China Block. The higher temperature components (HTC) were isolated by stepwise thermal demagnetization between ∼300–680°C and they passed the fold test. However, all the HTCs are of normal polarity, which appears to conflict with the frequent occurrence of reversed polarities during the late Eocene-early Oligocene, and therefore the possibility of remagnetization needs to be considered. Widespread secondary hematite was detected in the red beds that further indicates the remagnetization of samples. From the magmatic–metallogenic events in the sampling area, we propose an early Oligocene (∼35Ma) remagnetization event that was most likely related to porphyritic intrusions induced by fluid activity. Comparison of the pole calculated from the remagnetized remanent directions with the ∼35Ma paleomagnetic pole for Eurasia indicated that the degree of clockwise rotation in the Yongsheng area is 17.0±4.1° relative to stable Eurasia. The rotation value is consistent at the 95% confidence level with results obtained from Paleogene and Cretaceous strata in other areas of the CDF. Paleomagnetic data indicate that a consistent clockwise rotation of 20.6±6.3° occurred in different areas in the CDF: at Yongsheng, Zhupeng, Bailu, Dayao, Chuxiong west and Jianchuan. Careful analysis of the paleomagnetic data and the geometrical characteristics of the Ailao Shan–Red River shear zone indicates that the rotation process was separated into two discrete intervals: Approximately 11° of quasi-rigid clockwise rotation occurred between ∼35 and 12.7Ma compared to stable Eurasia, which may have been accompanied by strike-slip movement of the Ziyun-Luodian fault, caused by west-to-east compression induced by the northeastern indention of the Eastern Himalayan Syntaxis. Subsequently, a clockwise rotation of about 10° of the CDF with respect to stable Eurasia is inferred from the paleomagnetic results since 12.7Ma, which is consistent with the change in the geometry of the Ailao Shan–Red River shear zone around the western Chuxiong Basin. The later CW rotation was related to shearing movement of the Xianshuihe–Xiaojiang fault.

Origin of false components of NRM during conventional stepwise thermal demagnetization

Many Permian-Triassic dolerite samples from the Siberian Trap Large Igneous Province exposed to conventional stepwise thermal demagnetization at 250-450 °C display mid-temperature remanence (MTC) directed opposite to the high-temperature NRM component. Alternating field (A.C.) demagnetization fails to isolate the antipodal component, but it appears during continuous thermal demagnetization, though in a different temperature range. Laboratory experiments and simulations prove that MTC remanence is an artifact resulting from magnetic memory of self-reversing titanomagnetite grains oxidized at low temperature. This effect can deform the results stepwise thermal cleaning and be responsible for misleading patterns of paleomagnetic directions. Given that oxidized titanomagnetite grains are widespread in volcanic rocks, we suggest to identify true paleodirections by combined continuous and stepwise thermal demagnetization. The extension of our model to the case of NRM2 overprint directed at some angle to partially reversed primary NRM1 component accounts for the difference between the results of stepwise and continuous thermal demagnetization observed in samples of the Steens Mountain basalt (USA).

New Early and Late Carboniferous paleomagnetic results from the Qaidam Block, NW China: Implications for the paleogeography of Central Asia

There is an ongoing debate regarding the paleogeographic evolution of the Qaidam Block (37.3°N/96.4°E) in northwest China during the late Paleozoic. To provide a reliable constraint on the paleogeographic reconstruction and to determine the relationship with the adjacent area, we performed a paleomagnetic investigation of Early and Late Carboniferous sediments from the Qaidam Block. Stepwise thermal demagnetization successfully isolated high unblocking-temperature characteristic directions from the samples. The tilt-corrected mean direction of the Early Carboniferous sediments is Ds/Is=298.6°/−46.4° with α95=4.7° and N=15 sites, corresponding to a paleopole at −3.2°N, 147.5°E with A95=4.8°. The Late Carboniferous paleomagnetic direction is Ds/Is=320.1°/−46.1° with α95=5.3° and N=8 sites, corresponding to a paleopole at −15.1°N, 132.2°E with A95=5.4°. A positive fold test for the Early Carboniferous paleomagnetic directions, and consistency with the Kiaman reversed superchron for the Late Carboniferous paleomagnetic directions, indicates the primary origin of the characteristic remanence of the studied section. The new paleomagnetic results suggest that the Qaidam Block was located at about 27°N and did not undergo significant N-S movement during the Carboniferous. However, the Qaidam Block experienced a counter-clockwise rotation of about 21° during this period. Comparisons of the paleomagnetic results from the major blocks of Central Asia suggest that the Qaidam Block may have experienced a relatively rapid northward movement and collided with the Tarim Block after the Late Permian. Based on the paleomagnetic poles of the Qaidam Block and adjacent blocks, we present a tentative paleogeographic reconstruction for central Asia during the Early and Late Carboniferous.

Thermoremanence acquisition and demagnetization for titanomagnetite under lithospheric pressures

AbstractThe geological sources of large‐scale lithospheric magnetic field anomalies are poorly constrained. Understanding the magnetic behavior of rocks and minerals under the pressures and temperatures encountered at large crustal depths is particularly important in that task. The impact of lithospheric pressure is not well known and most of the time neglected in numerical models of the geological sources of magnetic anomalies. We present thermal remanent magnetization (TRM) acquisition and stepwise thermal demagnetization on synthetic titanomagnetite dispersed powder, within an amagnetic cell under hydrostatic pressure up to 1 GPa. TRM is measured after thermal cycling within a cryogenic magnetometer. Pressure‐dependent increase in the Curie temperature (initially in the 50–70°C range) is observed, mostly between 0.3 and 0.6 GPa, on the order of 20 K/GPa. TRM intensity also increases with pressure up to 200% at 675 MPa, although the pressure variation with temperature inside the cell complicates the interpretation.

Paleomagnetic field reconstruction from mixtures of titanomagnetites

Stepwise thermal demagnetization and alternating field (AF) demagnetization are commonly used in paleomagnetic studies to isolate remanent magnetic components of different origins. The magnetically hardest, i.e. highest unblocking temperature/peak field component is often interpreted as the primary magnetization and magnetically softer components as subsequent remagnetizations due to geological events posterior to the formation of the rock, such as reheating or formation of new magnetic minerals. The correct interpretation of the sequence of the geological events such as tectonic rotations from paleomagnetic data often relies on correctly attributing the observed magnetic directions to the remanence carriers and acquisition mechanisms. Using a numerical model to simulate remanence acquisition and stepwise thermal and AF demagnetization experiments, we show that the presence of mixtures of different magnetic minerals, such as magnetite and titanomagnetites of varying titanium-content can have very significant effects on Zijderveld plots. In thermal demagnetization experiments a spurious third component at intermediate temperatures or a continuous curvature may arise from an overlap of the primary remanence with a subsequent thermal or viscous remagnetization carried by small-grained iron-rich magnetite and large-grained titanium-rich titanomagnetite. AF demagnetization plots of magnetic mixtures are even more complex: primary and secondary remanences carried by different minerals may appear as either three or four components in Zijderveld plots. During alternating field demagnetization the highest coercivity component is not necessarily equivalent to the primary remanence and does not necessarily correspond to the highest temperature component in an analogous thermal demagnetization experiment, i.e., the primary remanence direction cannot be recovered. The effects are shown to be due to the different responsiveness of magnetite and titanomagnetites towards viscous or thermoviscous remanence acquisition: remanent magnetizations with long acquisition times are more effectively recorded by titanium-poor minerals, while short acquisition times are equally well recorded by titanium-rich minerals. In demagnetization experiments on laboratory timescales, the relative contribution of two minerals to Zijderveld plots differs to the relative contribution of remanence acquisition over geological timescales, leading to overlapping components in Zijderveld plots. The model was also used to simulate paleointensity (ancient magnetic field intensity) experiments and it was found that the grain distribution affects the slope of Arai plots, but is negligible compared to the effect of the cooling rate of NRM acquisition. The simulations suggest that for slowly cooled rocks a cooling rate correction of up to 1.5 to 1.6 may be required depending on the mineralogy.

Complex remanent magnetization in the Kızılkaya ignimbrite (central Anatolia): Implication for paleomagnetic directions

Pyroclastic flow deposits, known as ash-flow tuffs or ignimbrites, are invaluable materials for paleomagnetic studies, with many applications for geological and tectonic purposes. However, little attention has been paid to evaluating the consistency and reliability of the paleomagnetic data when results are obtained on a single volcanic unit with uneven magnetic mineralogy. In this work we investigate this issue by concentrating on the Kızılkaya ignimbrite, the youngest large-volume unit of the Neogene ignimbrite sequence of the Central Anatolian Volcanic Province in Turkey, bringing evidence of significant magnetic heterogeneities in ignimbrite deposits (magnetic mineralogy, susceptibility, magnetic remanence, coercivity, etc.) and emphasizing the importance of a stratigraphic sampling strategy for this type of volcanic rocks in order to obtain reliable paleomagnetic data. Six sections were sampled at different stratigraphic heights within the devitrified portion of the ignimbrite. Isothermal remanence measurements point to low-Ti titanomagnetite as the main magnetic carrier at all sites; at some sites, the occurrence of oxidized Ti-magnetite and hematite is disclosed. The bulk susceptibility (km) decreases vertically at two out of six sections: its value for the topmost samples is commonly one order of magnitude lower than that of the samples at the base. In most cases, low km values relate to high coercivity of remanence (BCR) values, which range from 25 to >400mT, and to low S-ratios (measured at 0.3T) between 0.28 and 0.99. These data point to the occurrence of oxidized magnetic phases. We therefore consider the km parameter as a reliable proxy to check the ignimbrite oxidation stage and to detect the presence of oxidized Ti-magnetite and hematite within the deposit. The characteristic remanent magnetization is determined after stepwise thermal and AF demagnetization and clearly isolated by principal component analysis at most sites. For these sites, the site-mean paleomagnetic direction is consistent with data from the literature. At a few other sites, the remanence is more complex: the direction moves along a great circle during demagnetization and no stable end-point is reached. The occurrence of oxidized Ti-magnetite or hematite as well as two remanence components with overlapping coercivity and blocking temperature spectra suggest that the Kızılkaya ignimbrite acquired first a thermal remanent magnetization and then, during the final cooling or a short time later, a secondary remanent magnetization component which is interpreted as a CRM acquired during post-emplacement devitrification processes. Notwithstanding the Kızılkaya ignimbrite is a single cooling unit, its magnetic properties suffered substantial variations laterally and vertically within the deposit. The Kızılkaya case shows that thick pyroclastic deposits should be sampled using a stratigraphic approach, at different sites and different stratigraphic heights at each individual sampling location, otherwise, under-sampling may significantly affect the paleomagnetic results. When sampling is performed on a short duration or on very poorly preserved deposits we recommend drilling the lower-central portion in the most strongly welded and devitrified facies. Such sampling strategy avoids complications arising from the potential presence of a pervasive secondary CRM masking the original ChRM.