[1] We present a highly sensitive and accurate method for quantitative detection and characterization of noninteracting or weakly interacting uniaxial single domain particles (UNISD) in rocks and sediments. The method is based on high-resolution measurements of first-order reversal curves (FORCs). UNISD particles have a unique FORC signature that can be used to isolate their contribution among other magnetic components. This signature has a narrow ridge along the H c axis of the FORC diagram, called the central ridge, which is proportional to the switching field distribution of the particles. Therefore, the central ridge is directly comparable with other magnetic measurements, such as remanent magnetization curves, with the advantage of being fully selective to SD particles, rather than other magnetic components. This selectivity is unmatched by other magnetic unmixing methods, and offers useful applications ranging from characterization of SD particles for paleointensity studies to detecting magnetofossils and ultrafine authigenically precipitated minerals in sediments.

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Paleomagnetic inclinations in sedimentary formations are frequently suspected of being too shallow. Recognition and correction of shallow bias is therefore critical for paleogeographical reconstructions. This paper tests the reliability of the elongation/inclination (E/I) correction method in several ways. First we consider the E/I trends predicted by various PSV models. We explored the role of sample size on the reliability of the E/I estimates and found that for data sets smaller than ∼100-150, the results were less reliable. The Giant Gaussian Process-type paleosecular variation models were all constrained by paleomagnetic data from lava flows of the last five million years. Therefore, to test whether the method can be used in more ancient times, we compare model predictions of E/I trends with observations from five Large Igneous Provinces since the early Cretaceous (Yemen, Kerguelen, Faroe Islands, Deccan and Paraná basalts). All data are consistent at the 95% level of confidence with the E/I trends predicted by the paleosecular variation models. The Paraná data set also illustrated the effect of unrecognized tilting and combining data over a large latitudinal spread on the E/I estimates underscoring the necessity of adhering to the two principle assumptions of the method. Then we discuss the geological implications of various applications of the E/I method. In general the E/I corrected data are more consistent with data from contemporaneous lavas, with predictions from the well constrained synthetic apparent polar wander paths, and other geological constraints. Finally, we compare the E/I corrections with corrections from an entirely different method of inclination correction: the anisotropy of remanence method of Jackson et al.

[1] Anomalously shallow paleomagnetic inclinations from Tarim basin red beds have suggested more than 1000 km of northward translation of the Tarim block since the Cretaceous. This is in conflict with geologic observations that indicate only a few hundred kilometers of crustal shortening north of the Tarim basin. To determine whether a rock magnetic effect could be the cause of the shallow inclinations, samples were collected from the Cretaceous Kapusaliang Group red beds. Both thermal and chemical demagnetization were employed to isolate the characteristic remanence (ChRM). The ChRMs pass the reversals test, as well as a local fold test. The anisotropy of magnetic susceptibility of the ChRM-bearing particles was isolated by chemical demagnetization and had an oblate fabric with minimum axes perpendicular to bedding and foliations of $1.035. A 10-20% remanent anisotropy was obtained by comparing the saturation isothermal remanent magnetization for subsamples drilled parallel and perpendicular to bedding planes. The correlation of AMS and remanent anisotropy parameters yielded a value for the individual particle magnetic susceptibility anisotropy between 1.05 and 1.62. A particle anisotropy of 1.0638 allowed the best fit between corrected data and theoretical correction curves. An inclination correction corrected the mean Kapusaliang direction from D = 16.3°, I = 29.0°, a 95 = 7.4°to D = 14.1°, I = 61.5°, a 95 = 6.4°. The inclination correction reduced the paleomagnetically predicted latitudinal offset from more than 1000 km to less than the mean direction's 95% confidence limits, suggesting that paleomagnetic inclination shallowing is the cause of low inclinations recorded by the red beds from the Tarim basin.

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A synthetic sediment comprised of kaolinite, distilled water and either equidimensional or acicular magnetite was given a post-depositional remanent magnetization (PDRM) by being stirred in a magnetic field. This sediment was compacted under pressures which varied continuously from 0 to 0.14 MPa in a water-tank consolidometer and to higher pressure steps (~2 . 5 3 MPa) in a standard soil consolidometer. Compaction took place in the same magnetic field in which the sample was given its PDRM. The compaction caused shallowing of the sample's magnetic inclination.This shallowing was found to be a function of the sample's initial magnetic inclination and the degree of sample compaction;where IR is the remanent inclination after compaction, A V is the volume change, I . is the initial magnetic inclination, and a is an empirically derived constant. The data show a maximum inclination shallowing of 12" for an initial inclination of 54", in good agreement with the maximum inclination shallowing predicted by the above equation.We propose an electrostatic mechanism to be the cause of the inclination shallowing. In this model positively charged magnetite grains adhere to the surface of negatively charged clay grains with their long dimension parallel to the clay grain's surface. As the clay grains become reorientated due to compaction the easy axes of magnetization are rotated away from the axis of compression.Alternating field demagnetization data reveal that our samples have shallower characteristic magnetizations than their post-compaction NRMs, implying that the smaller, higher coercivity grains are most affected by the compaction process. These data support our model since a surface charge mechanism for inclination shallowing would predict that the smallest magnetic grains would be preferentially affected.

[1] Red, hematite-bearing sedimentary rocks are an important source of paleomagnetic data, particularly for continental apparent polar wander paths during the Paleozoic. This study presents magnetic anisotropy data from the Mississippian Mauch Chunk Formation of eastern Pennsylvania, indicating that these red beds have suffered from a significant amount of paleomagnetic inclination shallowing. Fourteen oriented block samples were collected from normal and reversed polarity strata identified in a previous study. Thermal demagnetization isolated the characteristic remanent magnetization at seven normal polarity horizons at unblocking temperatures greater than 670°C (mean direction, D = 354.0°, I = À18.4°, a 95 = 10.2°). Anisotropy of remanence measurements (anisotropy of isothermal remanent magnetization and thermal remanent magnetization) indicate a bedding parallel, foliated magnetic fabric with foliations ranging from 1.1 to 1.35. Thermal demagnetization at 670°C of the isothermal remanent magnetization isolates the magnetic fabric of the characteristic remanence-carrying grains and indicates stronger foliations (1.15-1.48). Anisotropy of magnetic susceptibility (AMS) also indicates bedding parallel magnetic foliations of 1.01-1.04, typical of red beds. Chemical leaching isolates the AMS of the characteristic remanence-carrying grains to range from 1.02 to 1.07. This AMS fabric was used to correct the characteristic remanence inclination isolated by thermal demagnetization. The critical parameter needed for an accurate inclination correction, the individual particle anisotropy a c , was determined from the correlation between the normalized principal axes of remanence and susceptibility anisotropy to be 1.06. The corrected Mauch Chunk direction, D = 354.5°, I = À56.4°r esults in a corrected Mauch Chunk Formation paleopole (12°N, 108°E) that is consistent with a European igneous paleopole.

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