Paleomagnetic Results Research Articles

Effects of titanomagnetite reordering processes on thermal demagnetization and paleointensity experiments

AbstractTitanomagnetite (Fe3‐xTixO4, 0 ≤ x ≤ 1) is a common, naturally occurring magnetic mineral critical to many paleomagnetic studies. Underlying most interpretations is the assumption that, lacking chemical alteration, Curie temperature (Tc) remains constant. However, recent work has demonstrated that Tc of many natural titanomagnetites varies strongly as a function of thermal history, independent of chemical alteration. This is inferred to arise from reordering of cations and/or vacancies in the crystal structure, and changes occur at temperatures and times relevant to standard paleomagnetic thermal treatments. Because changes take place at T < Tc, they have the potential to dramatically affect thermal remanence acquisition or demagnetization, impacting interpretation of paleomagnetic results. Here we have modeled the effects of reordering on standard thermal demagnetization and paleointensity experiments. Results suggest that Tc changes during laboratory heating make it impossible to accurately measure the unblocking temperature spectrum without modifying it. Samples with a starting Tc0 less than the closure temperature (Tclose) for the reordering process will develop a high‐temperature “tail” that did not exist prior to heating. Samples with a starting Tc0 > Tclose will have their original Tb spectrum truncated at T ≈ Tclose. Predicted behavior during Thellier‐type paleointensity experiments results in only modest deviations in NRM‐lost or pTRM*‐gained from the nonreordering case. Much larger deviations are predicted for pTRM checks. Compared to paleointensity results from titanomagnetite‐bearing pyroclastic deposits, modeled nonideal behavior occurs in the same temperature intervals, but is much more systematic. Reordering is likely one contributing factor to failure of paleointensity experiments.

Examination of the kinematic structures in İzmir (Western Anatolia) with repeated GPS observations (2009, 2010 and 2011)

The Western Anatolia and the Aegean Sea regions are one of the most significant seismically active and rapidly deforming fields in the world. Generally, seismic activities cause deformations and these deformations are monitored with Global Positioning System (GPS) /Global Navigation System (GNSS). In this context, GPS data were used to determine the deformation of İzmir and its surrounding to estimate the relative plate motions. In this study, the kinematic structures of the faults, which control the seismic hazard in İzmir and its surroundings, processing results of the three-year (2009, 2010 and 2011) episodic GPS observations and the estimation of displacements for 21 GPS stations were presented. The aim of this study is to examine interplate motion of the stations and their relations with the tectonic structures, seismicity and paleomagnetism and additionally, to interprete the motions of the study area relative to different block motions. Consequently, the mean motion of the study area was found approximately 25 mm/yr (towards the SSW) in the Eurasia fixed frame solution. The Aegean block fixed frame and the Anatolian block fixed frame solutions were computed relative to Euler vectors. In Aegean and Anatolian block solutions it was determined that the stations move separately, not as a group. In Euler pole solution, some stations are separated from each other and meanwhile some stations are grouped by considering the differences and similarities of the station motions. According to this solution three lines and two regions were described in the study area. The relations between seismicity and paleomagnetic studies and the kinematic structures determined in Anatolian block fixed frame and Euler pole solution were also investigated. When the Anatolian block fixed frame solution and the earthquakes occurred between the years 1973 and 2011 were evaluated together, it was found that in the high seismically active region especially near to Sığacık bay, the motions of GPS stations were different even though their locations were close to each other. As a result of this, the relationship between the vector directions and active tectonism was determined. Additionally, in the Euler pole solution directions of the motion were found to be coherent with the paleomagnetic results, particularly in Urla and its surroundings. Here, the block fixed frame and Euler pole solutions and additionally, relations of them with seismicity and tectonism were mentioned as difference from previous studies. Besides, in this study, high importance was given to locate each station in main geological formations of the study area.

Evaluating the paleomagnetic potential of single zircon crystals using the Bishop Tuff

Zircon crystals offer a unique combination of suitability for high-precision radiometric dating and high resistance to alteration. Paleomagnetic experiments on ancient zircons may potentially constrain the history of the earliest geodynamo, which would hold broad implications for the early Earth's interior and atmosphere. However, the ability of zircons to record accurately the geomagnetic field has not been demonstrated. Here we conduct thermal and alternating field (AF) paleointensity experiments on 767.1 thousand year old (ka) zircons from the Bishop Tuff, California. The rapid emplacement of these zircons in a well-characterized magnetic field provides a high-fidelity test of the zircons' intrinsic paleomagnetic recording accuracy. Successful dual heating experiments on eleven zircons measured using a superconducting quantum interference device (SQUID) microscope yield a mean paleointensity of 54.1±6.8μT (1σ; 42.6±5.3μT after excluding possible maghemite-bearing zircons), which is consistent with high-precision results from Bishop Tuff whole rock (43.0±3.2μT). High-resolution quantum diamond magnetic (QDM) mapping, electron microscopy, and X-ray tomography indicate that the bulk of the remanent magnetization in Bishop Tuff zircons is carried by Fe oxides associated with apatite inclusions, which may be susceptible to destruction via metamorphism and aqueous alteration in older zircons. As such, while zircons can reliably record the geomagnetic field, robust zircon-derived paleomagnetic results require careful characterization of the ferromagnetic carrier and demonstration of their occurrence in primary inclusions. We further conclude that a combination of quantum diamond magnetometry and high-resolution imaging can provide detailed, direct characterization of the ferromagnetic mineralogy of geological samples.

New insights into the Cenozoic lateral extrusion of crustal blocks on the southeastern edge of Tibetan Plateau: Evidence from paleomagnetic results from Paleogene sedimentary strata of the Baoshan Terrane

AbstractIn order to characterize the lateral extrusion of crustal material during the Cenozoic, we conducted a paleomagnetic study of the Eocene‐Oligocene Zhushan Formation and Paleocene Muguahe Formation in the central part of the Baoshan Terrane (BST), on the southeastern edge of Tibetan Plateau. A primary magnetic component and a remagentized component with the Miocene acquisition age were isolated from the Muguahe Formation and Zhushan Formation, respectively. These data indicate that the BST did not commence clockwise rotation in the period of the Paleocene and the Oligocene and that the BST experienced ~80° clockwise rotation relative to East Asia, since the Miocene. Combining with the documented history of crustal boundary strike‐slip faults on the southeastern edge of the Tibetan Plateau shows that the BST and Tengchong Terrane (TCT) first underwent latitudinal crustal shortening in the Oligocene, and the eastward extension was the main type of motion of the crustal material during this period, which induced the initial strike‐slip movement of their boundary faults. Since the Miocene, the main form of crustal motion of BST, TCT, and Simao Terrane was gradually transformed into clockwise rotation, which possibly indicates that the fold and thrusting fault system‐induced crustal shortening and thickening in the southeastern edge of the Tibetan Plateau have reached extremity, and the southeastern part of the Tibetan Plateau has already been uplifted to the similar elevation with today's elevation in the Miocene.

A late Paleoproterozoic key pole for the Fennoscandian Shield: A paleomagnetic study of the Keuruu diabase dykes, Central Finland

We present an updated paleomagnetic pole and a new isotope age for the late Paleoproterozoic (Svecofennian) diabase dykes from Keuruu, Central Finland. The paleomagnetic results reveal a dual-polarity remanent magnetization with asymmetry, which we explain by an unremoved secondary component contaminating both normal (N) and reversed (R) vectors. The ChRM is carried by magnetite, most of which are primary grains crystallized from basaltic magma. The primary nature of the ChRM is further supported by positive baked contact tests. An R polarity dyke shows a U-Pb concordia age of 1870±9Ma. Zircons from another dyke of N polarity reveal a mean Pb-Pb age of 1867±8Ma. The combined N and R mean ChRM direction is D=340.4°, I=35.2°, α95=5.8°, k=31.3, N=21 dykes (yielding a paleomagnetic pole of Plat=45.7°, Plon=230.9°, A95=5.5°. This positions the Fennoscandian Shield at low latitudes (19.4°), and aligns with Superior Craton in a way that would naturally proceed into the 1.8–1.2Ga NENA (North Europe–North America) configuration. Paleosecular variation analysis of N polarity dykes is consistent with the low latitude position of Baltica.

Oroclinal bending in the Alborz Mountains (Northern Iran): New constraints on the age of South Caspian subduction and extrusion tectonics

In this study, we report an extensive paleomagnetic study (76 sites) carried out in the Alborz Mts. (northern Iran), with the aim of reconstructing the rotation history and the origin of curvature of this orogenic chain. The analyzed deposits are the sedimentary successions of the Upper Red Formation (Miocene), Lower Red Formation (Oligocene) and Eocene clastic units. Paleomagnetic results indicate that the Alborz Mts. can be considered a secondary arc that originated as a linear mountain belt that progressively acquired its present day curvature through opposite vertical axis rotations along its strike. The curvature of the arc was entirely acquired after the middle-late Miocene, which is the age of the youngest investigated sediments (Upper Red Formation). Overall, our paleomagnetic data indicate that the Alborz Mts. can be considered an orocline.Our results define, for the first time, the rotational history of the entire Alborz curved mountain belt, and enable us to reconstruct the paleogeographic and tectonic evolution of northern Iran in the framework of Arabia-Eurasia continental deformation. The kinematics inferred by the pattern of paleomagnetic rotations is at odds with the present day kinematics of northern Iran, characterized by the westward extrusion of the South Caspian block, and by a left lateral shear between Central Iran and the central and western sectors of the Alborz Mts. By integrating paleomagnetic data with stratigraphic, thermochronological, structural and GPS information, we propose that the initiation of South Caspian subduction and the activation of westward extrusion of South Caspian block occurred diachronously and that the initiation of the present-day kinematics of northern Iran was quite recent (Lower Pleistocene, <2Ma).

Tectonic evolution of the Malay Peninsula inferred from Jurassic to Cretaceous paleomagnetic results

A primary remanent magnetization is identified in the Jurassic-Cretaceous red bed sandstones of the Tembeling Group in Peninsular Malaysia. This high-temperature magnetic component is unblocked at 680–690°C, revealing a clockwise deflected direction of Ds=56.8°, Is=31.6° (where ks=8.5, α95=11.3° and N=22) in stratigraphic coordinates. The primary origin of this component is ascertained by a positive fold test and a geomagnetic polarity reversal in the Kuala Wau section. Secondary remanent magnetizations are identified in the rocks of the Tembeling and Bertangga basins, which indicate a counter-clockwise deflection in the geographic coordinates (Dg=349.1°, Ig=15.3° where kg=11.8, α95=5.1°, N=72). The comparison with the expected paleomagnetic directions from the 130Maand 40MaEurasian poles indicates two-stages of tectonic movement in the southern Malay Peninsula: (1) a clockwise rotation of 61.1°±11.9° accompanied by a 13.3°±8.1° southward displacement after the Cretaceous; and (2) a subsequent counter-clockwise rotation of 18.5°±5.0° to the present day position. The first stage of rotation is ascribed to tectonic deformation caused by the indentation of India into Asia after 55Ma, while the second stage is attributed to the collision of the Australian Plate with SE Asia after 30–20Ma. The present paleomagnetic results from the Jurassic-Cretaceous Tembeling Group thus reveal impacts of both of these collisions on SE Asia in general and on Peninsular Malaysia in particular.

Remagnetization and folding in the frontal Montana Rocky Mountains

Local-scale folds within the Mississippian Madison Group of the frontal Montana Rockies preserve pre- and synfolding remagnetization data. Paleomagnetic results display inclinations of ∼70°, in contrast to the expected shallower directions for North American Mississippian rocks. The magnetization is chemical in origin, preserved in superparamagnetic to single-domain magnetite grains from fluid activity. Magnetic intensity results in the study area suggest that mineralization was more prevalent in the interior of the fold-and-thrust belt and diminished toward the east, resulting in lower intensities from less magnetite growth in the very frontal portions of the belt into the foreland. Fold test results of individual folds show syn- and prefolding remagnetizations as a function of location across the belt, with synfolding results in more westerly locations and prefolding results in the most frontal folds of the belt. By comparing our synfolding results with previously determined deformation ages for the Rocky Mountains, an Eocene (53.6 Ma) age for the remagnetization can be assigned. Based on the relative timing of remagnetization, a spatial pattern of folding in the study area is revealed. Major folding commenced (i.e., synfolding magnetization) during an Eocene remagnetization event, while the most frontal portion remained undeformed (i.e., prefolding magnetization) and was subsequently folded after regional remagnetization.

High northern geomagnetic field behavior and new constraints on the Gilsá event: Paleomagnetic and 40Ar/39Ar results of ∼0.5–3.1 Ma basalts from Jökuldalur, Iceland

Recent paleomagnetic results of extrusive rocks from high southern latitudes (>60°S) and high northern latitudes (>60°N) have been suggested to reflect a hemispheric asymmetry of the geomagnetic field on time-scales of 105 to 106 yrs, with higher and more stable fields in the north. This interpretation, however, is based on only a few modern-standard paleodirectional data sets and on high northern stable field paleointensity data of rocks that are mainly younger than 100 kyr. The sparsity of modern-standard data questions the validity (and age range) of this potential geomagnetic asymmetry. In 2013 and 2014, we sampled basaltic lava flows in Jökuldalur, north-eastern Iceland, to obtain high-standard paleodirectional and paleointensity data at relatively high-northern latitudes (65.2°N). On average, we sampled >15 cores per site at 51 sites of predominantly Matuyama age. Complete demagnetization was carried out on all samples using AF or thermal demagnetization. We present 45 distinct paleomagnetic directions based on overall N>10 ChRMs per site and α95<3.5°. We obtain a mean direction of D=355.7°, I=76.3°, and α95=3.2 for N=45 sites that is not significantly different from a GAD field. The resulting 45 VGPs distribute around the North Pole, and the global mean paleomagnetic pole (λ¯=87.8°, ϕ¯=224.3°) is coincident with the North Pole within the α95 confidence limit. We calculate a VGP dispersion SB(Mat)=20.523.317.8 and an average inclination anomaly ΔI=−0.91°2.94−4.88 for our 38 Matuyama age data. The dispersion SB overall supports the interpretation of a dependence of SB on latitude during the Matuyama, while the negligible ΔI suggests little deviation from a GAD field. Based on relatively strict cut-off criteria we also present six new field strength estimates from the time interval ∼1.2–1.83 Ma, thus filling a large data gap of the high-northern stable field behavior. We obtain a median VADM of 57±3ZAm2 (VDM of 60±5Am2), which is higher than the median VADM of 16 intensity estimates from Antarctica (39±7 ZAm2) from the same period. A higher northern field is also found when using less strict cut-off criteria resulting in 14 field estimates from Jökuldalur, i.e. we find support for higher field strength in the northern hemisphere as compared to the southern hemisphere during the Matuyama. Finally, we deliver a revised magneto-chronostratigraphic model of Jökuldalur and conduct an investigation of the type sections of the so-called Gilsá normal polarity event around 1.62 Ma. Our revised model is based on 11 new 40Ar/39Ar ages. No evidence is found of the existence of the Gilsá event in Jökuldalur. Instead we find that the normal polarity intervals in the type sections can both be correlated to Olduvai subchron.

New constraints on the magnetic history of the CV parent body and the solar nebula from the Kaba meteorite

Recent paleomagnetic studies of Allende CV chondrite as well as thermal modeling suggest the existence of partially differentiated asteroids with outer unmelted and variably metamorphosed crusts overlying differentiated interiors. To further constrain the magnetic history of the CV parent body, we report here paleomagnetic results on Kaba CV chondrite. This meteorite contains 11 wt% pseudo-single domain magnetite, making it a rock with an excellent paleomagnetic recording capacity. Kaba appears to carry a stable natural remanent magnetization acquired on its parent body upon cooling in an internally generated magnetic field of about 3 μT from temperatures below 150 °C during thermal metamorphism about 10 to several tens of Myr after solar system formation. This strengthens the case for the existence of a molten advecting core in the CV parent body. Furthermore, we show that no significant magnetic field (i.e. lower than ∼0.3 μT) was present when aqueous alteration took place on the Kaba parent body around 4 to 6 Myr after solar system formation, suggesting a delay in the onset of the dynamo in the CV parent body and confirming that nebular fields had already decayed at that time.

Paleomagnetic results from Late Carboniferous to Early Permian rocks in the northern Qiangtang terrane, Tibet, China, and their tectonic implications

Results of a systematic paleomagnetic study are reported based on Late Carboniferous to Early Permian sedimentary rocks on the north slope of the Tanggula Mountains, in the northern Qiangtang terrane (NQT), Tibet, China. Data revealed that magnetic minerals in limestone samples from the Zarigen Formation (CP^ z )are primarily composed of magnetite, while those in sandstone samples from the Nuoribagaribao Formation (P nr ) are dominated by hematite alone, or hematite and magnetite in combination. Progressive thermal, or alternating field, demagnetization allowed us to isolate a stable high temperature component (HTC) in 127 specimens from 16 sites which successfully passed the conglomerate test, consistent with primary remnance. The tilt-corrected mean direction for Late Carboniferous to Early Permian rocks in the northern Qiangtang terrane is D s=30.2°, I s= - 40.9°, k s=269.0, a 95=2.3°, N =16, which yields a corresponding paleomagnetic pole at 25.7°N, 241.5°E (d p /d m =2.8°/1.7°), and a paleolatitude of 23.4°S. Our results, together with previously reported paleomagnetic data, indicate that: (1) the NQT in Tibet, China, was located at a low latitude in the southern hemisphere, and may have belonged to the northern margin of Gondwana during the Late Carboniferous to Early Permian; (2) the Paleo-Tethys Ocean was large during the Late Carboniferous to Early Permian, and (3) the NQT subsequently moved rapidly northwards, perhaps related to the fact that the Paleo-Tethys Ocean was rapidly contracting from the Late Permian to Late Triassic while the Bangong Lake-Nujiang Ocean, the northern branch of the Neo-Tethys Ocean, expanded rapidly during this time.

Ediacaran–Cambrian paleogeography of Baltica: A paleomagnetic view from a diamond pit on the White Sea east coast

The controversial late Ediacaran to Cambrian paleogeography is largely due to the paucity and low reliability of available paleomagnetic poles. Baltica is a prime example of these issues. Previously published paleomagnetic results from a thick clastic sedimentary pile in the White Sea region (northern Russia) provided valuable Ediacaran paleontological and paleomagnetic data. Until recently, Cambrian-age rocks in northern Russia were known mostly from boreholes or a few small outcrops. A recent mining operation in the Winter Coast region exposed >60 m of red sandstone and siltstone of the Cambrian Brusov Formation from the walls of a diamond pit. Paleomagnetic data from these rocks yield two major components. (1) A single-polarity A component is isolated in ∼90% of samples between 200 and 650 °C. The corresponding pole (Pole Latitutde, Plat = 20°S; Pole Longitude, Plong = 227°E, α95 = 7°) agrees with the Early Ordovician reference pole for Baltica. (2) A dual-polarity B component is identified in ∼33% of samples, mostly via remagnetization circles, isolated from samples above 650 °C. The corresponding pole (Plat = 12°S; Plong = 108°E, α95 = 5°) is close to other late Ediacaran data but far from all younger reference poles for Baltica. We argue for a primary magnetization for the B component and the secondary origin of the other Cambrian poles from Baltica. This in turn requires a major reshuffling of all continents and blocks around the North Atlantic. The early stages of Eurasia amalgamation and models for the evolution of the Central Asian Orogenic Belt require revision.

Dust to dust: Evidence for the formation of “primary” hematite dust in banded iron formations via oxidation of iron silicate nanoparticles

Conventional models for the deposition of banded iron formations (BIFs) envisage the oxidation of upwelled ferrous iron and the precipitation of ferric oxide/hydroxide particles in surface waters that settled to form laterally extensive layers of iron-rich sediment. A fundamental tenet of this model is that fine-grained hematite (so-called dusty hematite) in least-altered BIFs represents the dehydration product of original oxide/hydroxide precipitates. However, this premise has never been proven. We have investigated the origin of the earliest-formed iron oxides in chert in well-preserved BIFs of the 2.63–2.45 billion-year-old Hamersley Group, Australia. We find that laminated chert in BIFs show progressive stages of in situ alteration from gray–green chert, containing iron-silicate nanoparticles, to red chert with abundant hematite dust. Analysis of textures by transmission electron microscopy of samples from the transition zone between gray–green and red chert reveals that dusty hematite formed after partial dissolution of iron-silicate nanoparticles by the precipitation of iron oxides in resulting cavities. These observations suggest that hematite dust is not a relict of an original seawater precipitate but the end-product of post-depositional oxidation. Our observations are consistent with paleomagnetic results from the Hamersley Group, which record two major phases of magnetic remanence carried by hematite that post-date deposition by more than 200 million years. Our results may provide an alternative explanation for the origin of jasper in BIFs deposited before the start of the Great Oxidation Event about 2.4 billion years ago. If correct, it follows that hematite dust is not a reliable proxy for paleoenvironmental conditions or biological processes in early Precambrian seawater. Furthermore, our results suggest that the primary iron precipitate in BIFs was iron-silicate mud that was silicified at or just below the sediment–water interface, a hypothesis that requires neither dissolved oxygen nor photosynthetic life, but was an inorganic, chemical process, reflecting anoxic oceans enriched in iron and silica.

Early Carboniferous paleomagnetic results from the northeastern margin of the Qinghai–Tibetan plateau and their implications

We conducted paleomagnetic investigations on limestone from the Lower Carboniferous Huaitoutala Formation in the Qaidam Basin near Delingha City, Qinghai Province, China. The characteristic remanent magnetization (D=5.8°, I=−25.7°, k=114.3, α95=4.8°) passes a fold test and indicates a paleopole position of −39.2°N, 90.4°E and a paleolatitude of 13.5°N for the Qaidam Block for the early Carboniferous. Based on global tectonic reconstructions and paleontological evidence, we suggest that the Qaidam Block was adjacent to, but independent from, the North China, South China, Alashan–Hexi and Tarim blocks at this time. This result suggests that Pre-Carboniferous sutures reported around the Qaidam Basin represent collisional events within Gondwana, rather than the final sutures that gave rise to the present tectonic configuration.

Late Miocene extension in coastal Sonora, México: Implications for the evolution of dextral shear in the proto-Gulf of California oblique rift

The timing, kinematics, and processes responsible for the rapid transition from subduction to oblique rifting and the localization of the Pacific-North America plate boundary in the Gulf of California are not well understood. Well exposed volcanic rocks deposited between ~15 and 10Ma in the Sierra Bacha (coastal Sonora, México) preserve a record of late Miocene deformation on the eastern rifted margin of the Gulf of California and offer new insights into the timing and kinematic evolution of oblique rifting. Detailed geologic mapping, fault kinematic analysis, U–Pb and 40Ar/39Ar geochronology, and paleomagnetic data reveal that the >2km-thick composite volcanic section is cut by a series of southwest-dipping, domino-style normal faults and uniformly tilted down-to-the-northeast. Palinspastic cross-section restoration suggests that the region experienced ca. 55–60% northeast-southwest-directed extension between ~11.7 and ~10–9Ma. Fault kinematic data reflect relatively minor dextral transtension either following or during the later stages of extension. Paleomagnetic results indicating modest clockwise vertical-axis block rotation suggest that dextral shear was concentrated in the southwest of the study area near the modern coastline.These results support an emerging model in which dextral strain was not ubiquitous across Sonora and did not initiate immediately following the ~12.5Ma transition from subduction to oblique rifting. Instead, strain east of the Baja California microplate at this latitude evolved from extension-dominated transtension prior to ~8Ma to dextral shear-dominated transtension by ~7–6Ma. The onset of dextral shear in coastal Sonora likely resulted from an increase in rift obliquity due to a change in relative plate motion direction at ~8Ma. The increase in rift obliquity and resultant onset of significant strike-slip faulting played a crucial role in facilitating subsequent plate boundary localization and marine incursion in the northern Gulf of California by ~6Ma and continental rupture at ~2–1Ma.

Palaeomagnetism of the Upper Miocene- Lower Pliocene lavas from the East Carpathians: contribution to the paleosecular variation of geomagnetic field.

Investigations of the paleosecular variation of the geomagnetic field on geological timescales depend on globally distributed data sets from lava flows. We report new paleomagnetic results from lava flows of the East Carpathian Mountains (23.6°E, 46.4°N) erupted between 4 and 6 Ma. The average virtual geomagnetic pole position (76 sites) includes the North Geographic Pole and the dispersion of virtual geomagnetic poles is in general agreement with the data of the Time Averaged geomagnetic Field Initiative. Based on this study and previous results from the East Carpathians obtained from 0.04–4 Ma old lava flows, we show that high value of dispersion are characteristic only for 1.5–2.8 Ma old lava flows. High values of dispersion during the Matuyama chron are also reported around 50°N, in the global paleosecular variation data set. More data are needed at a global level to determine if these high dispersions reflect the behaviour of the geomagnetic field or an artefact of inadequate number of sites. This study of the East Carpathians volcanic rocks brings new data from southeastern Europe and which can contribute to the databases for time averaged field and paleosecular variation from lavas in the last 6 Ma.

Paleomagnetism of Devonian dykes in the northern Kola Peninsula and its bearing on the apparent polar wander path of Baltica in the Precambrian

Mafic dykes and large alkaline and carbonatite intrusions of Middle-Late Devonian age are widespread on the Kola Peninsula in NE Fennoscandia. These magmatic rocks are well characterized with petrographic, geochemical and geochronological data but no paleomagnetic results have been reported yet. We studied dolerite dykes from the northern part of the Peninsula and isolated three paleomagnetic components in these rocks. A low-temperature component is aligned along the present-day field, while a major constituent of natural remanent magnetization is an intermediate-temperature component (Decl.=79.6°, Inc.=78.5°, α95=5,9°, N=17 sites) that is present in most Devonian dykes but is found in some baked metamorphic rocks and Proterozoic dykes too. Finally, a primary Devonian component could be reliably isolated from two dykes only. Rock-magnetic studies point to presumably primary low-Ti titanomagnetite and/or pure magnetite as the main remanence carriers but also reveal alteration of the primary minerals and the formation of new magnetic phases.The directions of a major component differ from the Middle Paleozoic reference data for Baltica but closely match those for the 190±10Ma interval recalculated from the apparent polar wander path of the craton. We assume that this Early Jurassic component is a low-temperature overprint of chemical origin. The main impact of the new results is not to mid-Paleozoic or Early Mesozoic times but to much older epochs. Analysis of paleomagnetic data shows that the directionally similar remanences are present in objects with the ages ranging from 500Ma to ~2Ga over entire Fennoscandia. Hence we argue that an Early Jurassic remagnetization is of regional extent but cannot link it to a certain process and a certain tectonic event. If true, this hypothesis necessitates a major revision of the APWP for Baltica over a wide time interval.

Composition of sediment records late Quaternary paleogeographic evolution of Santa Clara Valley, California

Gravel and sand samples from alluvium in five wells in the Santa Clara Valley, California (USA), have particle compositions that help constrain patterns of Quaternary basin evolution and sediment dispersal within the valley. Samples were collected from depths as great as 307 m, and paleomagnetic results obtained by other workers suggest that the sediment sampled ranges in age to ca. 800 ka. The gravel contains common to abundant Mesozoic graywacke and metavolcanic rocks; less common clast types indicate derivation from other Mesozoic rocks. Clasts having lithologies that match Cenozoic source rocks are minor in amount. The abundance of metavolcanic rocks, as well as serpentinite, blueschist, and eclogite, among the less common gravel clasts, generally increases with depth in each well. Sand samples contain a variety of mostly metamorphic heavy minerals, including some distinctive blueschist facies minerals. Blue amphibole and chrome spinel are rare to common in the sand, and their abundance also increases with depth. The most useful clasts for determining the location of the sediment source are the metavolcanic rocks and some of the less common clasts. Metavolcanic rocks, common in modern outcrops only in the Santa Cruz Mountains south and west of the Santa Clara Valley, also are common in the gravel. In contrast, with only a few exceptions at shallow levels in the northeasternmost wells, distinctive gravel clasts with sources east of the valley are nearly absent. These observations suggest that most of the sediment was derived from the south and west. Thus, the topographic asymmetry of the valley, with its modern axial drainage east of the valley center, evidently has persisted for at least the past 800 k.y. The increased abundance of pebbles of serpentinite, blueschist, and eclogite and of sand-sized particles of blue amphibole and chrome spinel with depth suggests a change in source through time. The source for these clasts in the deeper levels, and perhaps also some of the metavolcanic clasts, evidently was a basement high within the valley that was gradually buried as the basin filled with sediment. The gravel in the wells has a composition different from that of any previously described outcrops of Pliocene and Pleistocene nonmarine gravels, including the Santa Clara Formation, exposed around the margins of the Santa Clara Valley. This difference suggests that the Santa Clara Formation and similar units are not present in the parts of the valley penetrated by the wells.

Paleomagnetic study on the Triassic rocks from the Lhasa Terrane, Tibet, and its paleogeographic implications

We present paleomagnetic results from the newly discovered Early-Middle and Late Triassic marine sediments of the Lhasa Terrane. Orientated samples were collected from 32 sites (330 samples) on the north side of the Dibu Co Lake (84.7°E, 30.9°N), Coqên County, in the western region of the Lhasa terrane. Rock magnetic data revealed that most of the samples were dominated by magnetite and/or pyrrhotite. The stepwise demagnetization curves illustrated three-components: a low temperature component (Component A) near the present-day field (PDF), a secondary remanent magnetization (Component B) that may be from the Cretaceous Period, and a high-temperature component (Component C). The Component C were isolated from the Early-Middle Triassic rocks in 8 sites (47 specimens) and from the Late Triassic rocks in 6 sites (37 specimens). The Component C of the Early-Middle Triassic rocks passed a reversal test (B class, 95% confidence level) and a fold test (99% confidence level), that of the Late Triassic rocks passed a fold test (95% confidence level). The corresponding paleopoles for the Early-Middle and Late Triassic periods of the Lhasa Terrane were at 18.9°N, 208.4°E with A95=3.9° and 19.6°N, 211.8°E with A95=10.7°, respectively. We suggest that the Lhasa Terrane maintained a relative stable latitude (16.5±3.9°S and 18.4±10.7°S) in the southern hemisphere during the Triassic Period before moving northwards and amalgamating with the main body of Eurasia. The Qiangtang and Lhasa terranes, which were located at the mid-low latitudes of the southern hemisphere, might have been isolated between Eurasia and Gondwanaland since the Early Triassic Period. The Meso-Tethys, potentially represented by the Bangong–Nujiang suture zone (BNS) between the Lhasa and Qiangtang terranes, opened up in the Early-Middle Triassic Period and expanded during the entire course of the Triassic Period.

Paleomagnetism and dating of a thick lava pile in the Permian Bakaly formation of eastern Kazakhstan: Regularities and singularities of the paleomagnetic record in thick lava series

Paleomagnetic results on thick lava series are among the most important sources of information on the characteristics of ancient geomagnetic fields. Most paleo-secular variation data from lavas (PSVL) are of late Cenozoic age. There are far fewer results from lavas older than 5Ma. The Central Asia Orogenic Belt that occupies several million square kilometers in Asia is probably the world’s largest area of Paleozoic volcanism and is thus an attractive target for PSVL studies. We studied a ca. 1700m thick lava pile in eastern Kazakhstan of Early Permian age. Magmatic zircons, successfully separated from an acid flow in this predominantly basaltic sequence, yielded an Early Permian age of 286.3±3.5Ma. Oriented samples were collected from 125 flows, resulting in 88 acceptable quality flow-means (n⩾4 samples, radius of confidence circle α95⩽15°) of the high-temperature magnetization component. The uniformly reversed component is pre-tilting and arguably of a primary origin. The overall mean direction has a declination=242.0° and an inclination=−56.2° (k=71.5, α95=1.8°; N=88 sites; pole at 44.1°N, 160.6°E, A95=2.2°). Our pole agrees well with the Early Permian reference data for Baltica, in accord with the radiometric age of the lava pile and geological views on evolution of the western part of the Central Asia Orogenic Belt. The new Early Permian result indicates a comparatively low level of secular variation especially when compared to PSVL data from intervals with frequent reversals. Still, the overall scatter of dispersion estimates that are used as proxies for SV magnitudes, elongation values and elongation orientations for PSVL data is high and cannot be fitted into any particular field model with fixed parameters. Both observed values and numerical simulations indicate that the main cause for the scatter of form parameters (elongation values and elongation orientations) is the too small size of collections. Dispersion estimates (concentration parameter and standard angular deviation) are more robust, and their scatter stems from other sources, which may include non-stochastic features of datasets like clusters, loops etc., or non-stationary behavior of secular variation magnitude over time intervals of many million years.