Reliability of palaeomagnetic poles from sedimentary rocks

<p>Palaeomagnetic poles form the building blocks of apparent polar wander paths and are used as primary input for quantitative palaeogeographic reconstructions. The calculation of such poles requires that the short-term, palaeosecular variation (PSV) of the geomagnetic field is adequately sampled and averaged by a palaeomagnetic dataset. Assessing to what extent PSV is recorded is relatively straightforward for rocks that are known to provide spot readings of the geomagnetic field, such as lavas. But it is unknown whether and when palaeomagnetic directions derived from sedimentary rocks represent spot readings of the geomagnetic field and sediments are moreover suffering from inclination shallowing, making it challenging to assess the reliability of poles derived from these rocks. Here, we explore whether a widely used technique to correct for inclination shallowing, known as the elongation-inclination method (E/I), allows us to formulate a set of quality criteria for (inclination shallowing-corrected) palaeomagnetic poles from sedimentary rocks. The E/I method explicitly assumes that a sediment-derived dataset provides, besides flattening, an accurate representation of PSV. We evaluate the effect of perceived pitfalls for this assumption using a recently published dataset of 1275 individual palaeomagnetic directions of a >3 km-thick succession of ~69-41.5 Ma red beds from the Gonjo Basin (eastern Tibet), as well as synthetic data generated with the TK03.GAD field model. The inclinations derived from the uncorrected dataset are significantly lower than previous estimates for the basin, obtained using coeval lavas, by correcting inclination shallowing using anisotropy-based techniques, and by predictions from tectonic reconstructions. We find that the E/I correction successfully restores the inclination to values predicted by these independent datasets if the following conditions are met: the number of directions N is at least 100, the A95 cone of confidence falls within a previously defined A95<sub>min-max</sub> reliability envelope, no negative reversal test is obtained and vertical-axis rotation differences within the dataset do not exceed 15°. We propose a classification of three levels (A, B, and C) that should be applied after commonly applied quality criteria for paleomagnetic poles are met. For poles with classification ‘A’, we find no reasons to assume insufficient quality for tectonic interpretation. Poles with classification ‘B’ could be useful, but have to be carefully assessed, and poles with classification ‘C’ provide unreliable paleolatitudes. We show that application of these criteria for datasets of other sedimentary rock types classifies datasets whose reliability is independently confirmed as ‘A’ or ‘B’, and that demonstrably unreliable datasets are classified as ‘C’, confirming that our criteria are useful, and conservative. The implication of our analysis is that sediment-based datasets of quality ‘A’ may be considered statistically equivalent to datasets of site-mean directions from rapidly cooled igneous rocks like lavas and provide high-quality palaeomagnetic poles.</p>

Paleomagnetism of the Upper Cretaceous oceanic red beds in southern Tibet, China: Implications for the extent of Greater India at ~75 Ma

<p>The extent of Greater India with precise and accurate chronological control is a key issue that concerns the spatio-temporal pattern and tectonic models of the India-Asia collision.<strong> </strong>Here we carried out a detailed magnetostratigraphic and paleomagnetic study on the Upper Cretaceous oceanic red beds (CORBs) (Chuangde Formation) exposed in the Tethyan Himalaya terrane. The high temperature (650‒690°C) magnetic components are isolated from two separated sections at Cailangba and display both normal and reverse polarities, which were used to construct magnetic polarity sequences of the sections that can be subsequently correlated to the geomagnetic polarity time scale (GPTS) to better estimate the age of the rocks. With the aid of previously published biostratigraphy by Chen et al. (2011, Sedimentary Geology), the polarity magnetozones of the Cailangba B section are correlated to chron C32r.2r (74.3–74.0 Ma) and the upper part of chron C33n (79.9–74.3 Ma), and the single normal polarity magnetozone of the Cailangba A section is correlated to the upper part of chron C33n (79.9–74.3 Ma). As a result, the CORBs in the Cailangba A and B sections represent the time interval of 76.2–74.0 Ma by magnetobiostratigraphy. Two independent methods of inclination shallowing correction were tested, which all indicate a bias inclination of ~70%. After inclination shallowing correction, the mean inclination increased to ‒35.0°, giving what we propose to be a high-quality Late Cretaceous paleopole of 40.8°N/256.3°E, A<sub>95</sub> =1.8°. Our findings indicate that the Indian passive continental margin was situated at a paleolatitude of 19.4° ± 1.8°S at ~75 Ma. These data suggest that Greater India extended about 715 ± 374 km farther north from the present northern margin of India in the Late Cretaceous, implying a latitudinal width of 3641 ± 308 km for the Neo-Tethys Ocean that still separated the Lhasa terrane of southern part of the Asian plate and the Greater India.</p>

Reliability of palaeomagnetic poles from sedimentary rocks

Summary Palaeomagnetic poles form the building blocks of apparent polar wander paths and are used as primary input for quantitative palaeogeographic reconstructions. The calculation of such poles requires that the short-term, palaeosecular variation (PSV) of the geomagnetic field is adequately sampled and averaged by a palaeomagnetic dataset. Assessing to what extent PSV is recorded is relatively straightforward for rocks that are known to provide spot readings of the geomagnetic field, such as lavas. But it is unknown whether and when palaeomagnetic directions derived from sedimentary rocks represent spot readings of the geomagnetic field and sediments are moreover suffering from inclination shallowing, making it challenging to assess the reliability of poles derived from these rocks. Here, we explore whether a widely used technique to correct for inclination shallowing, known as the elongation-inclination method (E/I), allows us to formulate a set of quality criteria for (inclination shallowing-corrected) palaeomagnetic poles from sedimentary rocks. The E/I method explicitly assumes that a sediment-derived dataset provides, besides flattening, an accurate representation of PSV. We evaluate the effect of perceived pitfalls for this assumption using a recently published dataset of 1275 individual palaeomagnetic directions of a >3 km-thick succession of ∼69–41.5 Ma red beds from the Gonjo Basin (eastern Tibet), as well as synthetic data generated with the TK03.GAD field model. The inclinations derived from the uncorrected dataset are significantly lower than previous estimates for the basin, obtained using coeval lavas, by correcting inclination shallowing using anisotropy-based techniques, and by predictions from tectonic reconstructions. We find that the E/I correction successfully restores the inclination to values predicted by these independent datasets if the following conditions are met: the number of directions N is at least 100, the A95 cone of confidence falls within a previously defined A95min-max reliability envelope, no negative reversal test is obtained and vertical-axis rotation differences within the dataset do not exceed 15°. We propose a classification of three levels (A, B, and C) that should be applied after commonly applied quality criteria for paleomagnetic poles are met. For poles with classification ‘A’, we find no reasons to assume insufficient quality for tectonic interpretation. Poles with classification ‘B’ could be useful, but have to be carefully assessed, and poles with classification ‘C’ provide unreliable paleolatitudes. We show that application of these criteria for datasets of other sedimentary rock types classifies datasets whose reliability is independently confirmed as ‘A’ or ‘B’, and that demonstrably unreliable datasets are classified as ‘C’, confirming that our criteria are useful, and conservative. The implication of our analysis is that sediment-based datasets of quality ‘A’ may be considered statistically equivalent to datasets of site-mean directions from rapidly cooled igneous rocks like lavas and provide high-quality palaeomagnetic poles.

PALEOMAGNETISM OF THE MIDDLE JURASSIC SEDIMENTARY ROCKS OF THE ELGON FORMATION OF THE ULBAN TERRANE

The paper presents new findings of paleomagnetic studies on Middle Jurassic sedimentary rocks of the Elgon Formation of the Ulban Terrane from coastal outcrops along the Ulban Bay (53.5°N, 137.7°E). Demagnetization yielded a pre-folding characteristic component of magnetization. No inclination shallowing effect for the characteristic magnetization vector was revealed in sandstone specimens of the Elgon Formation. The coordinates of the paleomagnetic pole and the paleolatitude at which the studied rocks deposited were calculated: Plat = -34.3 (34.3)°; Plong = 161.2 (341.2)°; dm = 2.7 dp = 5.3, paleolatitude = 0.1° (+2.7°/-2.6°) S. The acquired paleomagnetic data show evidence for the deposition of rocks of the Ulban Terrane in the Middle Jurassic at the boundary between the Asian paleocontinent and the Paleo-Pacific which later formed part of the Sikhote-Alin Orogenic Belt.

Detrital remanent magnetization of single-crystal silicates with magnetic inclusions: constraints from deposition experiments

SUMMARY Quasi-linear field-dependence of remanence provides the foundation for sedimentary relative palaeointensity studies that have been widely used to understand past geomagnetic field behaviour and to date sedimentary sequences. Flocculation models are often called upon to explain this field dependence and the lower palaeomagnetic recording efficiency of sediments. Several recent studies have demonstrated that magnetic-mineral inclusions embedded within larger non-magnetic host silicates are abundant in sedimentary records, and that they can potentially provide another simple explanation for the quasi-linear field dependence. In order to understand how magnetic inclusion-rich detrital particles acquire sedimentary remanence, we carried out depositional remanent magnetization (DRM) experiments on controlled magnetic inclusion-bearing silicate particles (10–50 μm in size) prepared from gabbro and mid-ocean ridge basalt samples. Deposition experiments confirm that the studied large silicate host particles with magnetic mineral inclusions can acquire a DRM with accurate recording of declination. We observe a silicate size-dependent inclination shallowing, whereby larger silicate grains exhibit less inclination shallowing. The studied sized silicate samples do not have distinct populations of spherical or platy particles, so the observed size-dependence inclination shallowing could be explained by a ‘rolling ball’ model whereby larger silicate particles rotate less after depositional settling. We also observe non-linear field-dependent DRM acquisition in Earth-like magnetic fields with DRM behaviour depending strongly on silicate particle size, which could be explained by variable magnetic moments and silicate sizes. Our results provide direct evidence for a potentially widespread mechanism that could contribute to the observed variable recording efficiency and inclination shallowing of sedimentary remanences.

Palaeomagnetism of Late Triassic volcanic rocks from the western margin of Khorat Basin, Thailand and its implication for ambiguous inclination shallowing in Mesozoic sediments of Indochina

SUMMARY Palaeomagnetic constraints are essential factors in the reconstruction of the Mesozoic convergence of Eastern Asia blocks. As one of the key blocks, Indochina was constrained only by sedimentary-rocks-derived palaeomagnetic data. To evaluate whether the palaeomagnetic data used to restore the Late Triassic position of Indochina suffered inclination shallowing effects, we conducted a palaeomagnetic and geochronologic study on a coeval volcanic clastic rocks sequence in the western margin of the Khorat Basin, Thailand. The U-Pb SIMS dating on zircons indicates the age of the sampling section is between 205.1 ± 1.5 and 204.7 ± 1.4 Ma. Site mean directions are Dg/Ig = 217.2°/−39.4° (κg = 45.1, α95g = 10.1°) before and Ds/Is = 209.2°/−44.5° (κs = 43.8, α95s = 10.2°) after tilt correction. The new data set indicates a positive reversal test result at ‘Category C’ level. The characteristic remanent magnetization recorded by the coexistent magnetite and hematite is interpreted to be primary remanence acquired during the initial cooling of the volcanic clastic rocks. The consistence of the corresponding palaeolatitudes derived from the volcanic clastic rocks and the former reported sedimentary rocks suggests that there is probably no significant inclination shallowing bias in the sedimentary-rocks-derived palaeomagnetic data. Therefore, the estimates of the Late Triassic position of Indochina are confirmed to be reliable. The Indochina Block had collided to the southern margin of Eurasia by the Late Triassic and played an important role in the Mesozoic convergence of the Eastern Asia blocks.