Anisotropy Of Magnetic Susceptibility Results Research Articles

Remagnetization of Marcellus Formation in the Plateau Province of the Appalachian Basin

An integrated paleomagnetic, magnetic fabric, and petrographic study of two cores (EC: oriented; DB: unoriented) in the Devonian Marcellus Formation from the Plateau Province (PA) indicates the presence of chemical remanent magnetizations (CRMs) and extensive diagenetic alteration. Anisotropy of magnetic susceptibility results indicate a predominately oblate fabric. Some specimens contain a prolate fabric that is interpreted as diagenetically altered. A well-developed viscous magnetization, possibly contaminated by a drilling induced component, is not present in either core, and cannot be used for orientation. An intermediate temperature component with steep down inclinations (ITN) is removed in both cores below ~ 300°C. Specimens in DB also contain a component with steep up inclinations (ITR) that is removed between 300 and 440°C. The ITN is interpreted to reside in magnetite and/or possibly pyrrhotite, and the ITR resides in magnetite. The ITN could be a reversal of ITR, acquired in the Cretaceous to Cenozoic based on a comparison with the expected inclinations. Alternatively, the ITN could be a contaminated by a Brunhes viscous magnetization. Specimens from both cores contain a characteristic remanent magnetization (ChRM) with shallow inclinations that is removed by 500°C in DB and 460°C in EC, and is interpreted to reside in magnetite. The DB ChRM has an inclination of -9.0°, which corresponds to a range of ages between 310-225 Ma based on the expected inclinations. The ChRM in EC has streaked declinations, which may have been caused by core barrel rotation. Shallow inclinations (- 3.7°) in this core suggest a range of acquisition from 305-230 Ma. Rock magnetic data are consistent with the presence of magnetite but provide conflicting evidence for pyrrhotite. The ChRM in DB is interpreted as a CRM whereas the ChRM in EC could be thermoviscous in origin. Diagenetic studies indicate a complex paragenesis with authigenic minerals such as barite, sphalerite, baroque dolomite, and sylvite suggestive of alteration by external fluids. The presence of the CRM in highly altered zones suggests it is related to alteration by external fluids, although its presence in other facies suggests that burial diagenetic processes could also be a cause of remagnetization.

Magnetostratigraphy of the Rabot Formation, Upper Cretaceous, James Ross Basin, Antarctic Peninsula

Problems of endemism and diachronous extinctions make global correlation of coeval strata in the mid Campanian-Maastrichtian of the James Ross Basin problematic. To provide a more precise chronological framework, we present two magnetostratigraphies of Campanian strata from the Rabot Formation that crops out at Hamilton Norte (200 m thick) and Redonda Point (340 m thick) in James Ross Island. Sampled sections consist of poorly-consolidated, drab-colored fine sandstones and mudstones. Bulk susceptibility logs of both sections show a similar pattern of relatively low values at the lower and upper levels with significantly higher values at mid-levels that confirms the lithostratigraphic correlation between sections. Rock magnetic studies suggest that this change is not attributable to a ferrimagnetic fraction but to a paramagnetic contribution of presumed detrital origin. Stepwise thermal demagnetization showed dominant unblocking temperatures higher than 400 °C. Progressive hybrid low-temperature cycling, low-field AF and thermal demagnetization in a controlled N2 atmosphere, reveals a two-polarity characteristic component of possible primary origin. Rock magnetic experiments suggest that detrital titano-magnetite is the most likely remanence carrier. Anisotropy of magnetic susceptibility results show sedimentary fabrics, indicating that beds were not significantly buried or compacted. Magnetostratigraphies produced at each locality demonstrate a consistent change from reverse to normal polarity remanence in the middle of the sections. Biostratigraphic constraints identify this reversal as the C33r/C33n transition, indicating a 79.90 Ma depositional age for this level of the Rabot Formation. The remanence directions yield a mean whose corresponding paleopole is consistent with two recently obtained Upper Cretaceous reference paleopoles for the Antarctic Peninsula. Our data support the lack of tectonic rotation or oroclinal bending of this region since the Late Cretaceous.

Back-thrusting response of continental collision: Early Cretaceous NW-directed thrusting in the Changle-Nan’ao belt (Southeast China)

The SE coastal area of the South China Block (SCB) is generally interpreted as a Cretaceous active continental margin due to subduction of the Paleo-Pacific plate beneath the Eurasian plate. There, the NE–SW striking Changle-Nan’ao belt was previously considered as a major strike-slip fault zone with a large displacement accommodating the northward subduction of the Paleo-Pacific plate. Our new field and laboratory investigations document a NW-directed ductile thrust zone that placed gneiss upon Early Cretaceous foliated volcanic rocks. Structural analyses and Anisotropy of Magnetic Susceptibility results indicate that the ductile fabrics in both units are represented by a NE–SW striking foliation and a NW–SE stretching lineation with top-to-the-NW shear sense. This deformation occurred at ca. 130–105Ma, before the deposition of undeformed (ca. 104Ma) volcanic rocks, and the intrusion of ca. 90Ma isotropic plutons. This continent-ward structure is tentatively interpreted as a back-thrust resulting of the collision of the West Philippines microcontinent with the SCB rather than an effect of a simple oceanic subduction.

Magnetostratigraphy and Anisotropy of Magnetic Susceptibility of the Lulehe Formation in the Northeastern Qaidam Basin

Abstract:The timing of onset of deposition of the Lulehe Formation is a significant factor in understanding the genesis of the Qaidam basin and the evolution of the Tibetan Plateau. Here, we describe a detailed magnetostratigraphic and magnetic fabric study of the middle and lower parts of the Lulehe Formation. A total of 234 samples were collected from 117 sites throughout a thickness of almost 460 m of fluvial and lacustrine deposits at the Xitieshan section in the northeastern Qaidam basin. Out of these sites, 94 sites yielded well‐defined characteristic remanent magnetization components by stepwise thermal demagnetization and were used to establish the magnetostratigraphy of the studied section. Based on correlation with the geomagnetic polarity timescale, the studied section spans the period from 53.8 Ma to 50.7 Ma. Our results show a three‐fold decrease in sedimentation rates as well as marked change in facies from braided river to delta and shore–shallow lake around 52.6 Ma, which suggests tectonic uplift of the northeastern Qaidam basin margin ridge was rapid at the onset of formation of the Qaidam basin and subsequently weakened after 52.6 Ma. The anisotropy of magnetic susceptibility results indicate that tectonic compression stress had reached the northeastern Tibetan Plateau by the early stages of Indo–Eurasian plate collision and that the direction of stress in the study area was NE–SW. Furthermore, a weakening of tectonic compression stress around 52.6 Ma is consistent with sedimentary records. The age of initial deposition of the Qaidam basin (around 53.8 Ma) was almost synchronous with that of the Qiangtang, Hoh Xil, Xining, and Lanzhou basins, which implies that stress was transferred rapidly through the Tibetan Plateau during or immediately after the onset of Indo–Eurasian collision.

New Late Cretaceous and Paleogene paleomagnetic results from south China and their geodynamic implications

To better understand the tectonic evolution of South China Block (SCB) in response to the India‐Asia collision, we present new paleomagnetic results from Late Cretaceous and Paleogene red bed formations of the Hengyang basin of Hunan province, in the interior part of the SCB. Stepwise thermal demagnetization of the rocks isolated a high‐temperature component. The tilt‐corrected mean direction from the Late Cretaceous rocks is D = 15.6°, I = 29.9° with a95 = 5.7°, N = 26 sites, corresponding to a paleopole at 71.9°N, 236.3°E with A95 = 4.7°, which passes reversal tests. The tilt‐corrected mean direction (D = 358.9°, I = 35.4° with a95 = 5.0°, N = 22 sites) from the Paleogene rock, which passes the reversal test, gives a pole at 82.6°N, 300.6°E with A95 = 4.4°. The low‐field anisotropy of magnetic susceptibility results suggest that the red beds have not experienced significant strain due to compaction or tectonic stress, while the anisotropy of isothermal remanence results suggest that postdepositional compaction in these red beds produced no more than 3°–4° of inclination shallowing. These paleomagnetic and rock magnetic tests imply that the remanence is primary. On the basis of paleomagnetic results obtained from coeval basalts in Mongolia and Siberia, we suggest that no significant latitudinal motion has taken place between the SCB and Siberia since the Late Cretaceous. The significant latitudinal discrepancy between the SCB and European block could be due to nonrigid behavior of the Eurasian plate since the Late Cretaceous. A 16.7 ± 5.0° clockwise rotation of the SCB during the Late Cretaceous to the Paleogene could be related to the collision of India and Asia in the west and the subduction of the circum‐Pacific plate to eastern Asia in the east.

First Late Ordovician Paleomagnetic Pole for the Cuyania (Precordillera) Terrane of Western Argentina: a Microcontinent or a Laurentian Plateau

Time and tectonic processes involved in docking of the Argentine Precordillera (Cuyania terrane) against SW Gondwana has been a matter of much debate. A paleomagnetic study on the Early Caradoc Pavón Formation, exposed in the San Rafael block, province of Mendoza, Argentina, is presented. After detailed thermal and alternating field demagnetizations two geologically significant magnetic components were defined. A widespread post-tectonic component (A) is present in most sites of the Pavón Formation, with dual polarities, and is coincident with the characteristic remanence isolated from a Permo-Triassic rhyolitic dome intruding the sediments. Its pole position (83.7°S, 271.0°E, dp = 6.8°, dm = 9.0° N = 11 sites) falls on the Late Permian-Early Triassic South American reference poles suggesting that this component was acquired during the Choiyoi magmatic phase. A second component (B) also shows dual polarities and a positive fold test suggesting a primary origin. Unblocking temperatures and rock magnetic experiments indicate that B is carried either by hematite or magnetite at different sites. Anisotropy of magnetic susceptibility results suggest a depositional fabric and no remanence distortion due to deformation or compaction. A paleomagnetic pole computed from this remanence (PV) falls on 3.6°N, 346.4°E (dp = 2.9°, dm = 4.6° n = 22 samples). It indicates a paleolatitude around 26°S for deposition of Pavón sediments and constrains the paleogeographic evolution of Cuyania during the Ordovician, which was still at subtropical latitudes by the Early Caradoc. PV is consistent with the Laurentian Late Ordovician reference pole if Cuyania remains attached to SE Laurentia for the Early Caradoc, while it shows a significant cw rotation with no paleolatitude anomaly respect to the Gondwana reference pole when kept in its present position in SW South America. These comparisons are interpreted in three possible alternatives for the paleogeographic and tectonic setting of Cuyania in the Late Ordovician.

Errors in the anisotropy of magnetic susceptibility and magnetic remanence of unconsolidated sediments produced by sampling methods

Samples from glacial varves were obtained by cutting cubes and by pushing plastic cubical containers into the sediment. The anisotropy of magnetic susceptibility results show that the mean angular difference in the kmax direction can be altered by 32° by pushing in the sample containers in a north to south versus an east to west direction. The values of lineation and Q are raised or lowered dependent upon the direction of push and the original fabric direction.Mean remanence directions can deviate by as much as 43° in a single clay layer dependent upon the direction in which the sample containers are inserted into the sediment. Corresponding changes in mean intensity of the remanence also demonstrate that the remanence has been affected by pushing in sample containers. The overall results underline the need to adopt trustworthy procedures for the sampling of soft sediments deposited by settling and traction.