Determining the Paleolatitude of Baltica During the Permo-Triassic to Test Existing Pangea Models
Baltica, the stable part of the European continent, was located in a paleolatitude range of 9-45 degrees N during the Permo-Triassic. When compared with paleolatitudes inferred for Gondwana's northern margin (up to 20 degrees N), the classical Pangea-A configuration (Bullard et al., 1965) is not possible, because Gondwana and Laurussia would overlap some 11 degrees (Muttoni et al., 2003). To avoid such an overlap, Irving (1977) and Muttoni et al. (2003) opted to place Gondwana farther east by more than 3000 kilometers with respect to Laurussia in a Pangea B configuration. The Pangea B configuration, however, requires a large (3500 km) Triassic megashear in order to have the Atlantic open from a Pangea-A fit; yet there is little geological evidence for such a displacement. In order to better constrain Baltica's position within Pangea, we conducted a paleomagnetic study of Permo-Triassic dikes from the Oslo graben. The age of the dikes has been previously determined to be 246-238 Ma (Torsvik et al., 1998) but new 40Ar/39Ar age determinations are in progress. For the study, we sampled 56 sites including 39 dikes, 13 contact limestones and 4 host rocks. The dikes and the contact limestones have the same SW and up direction; however, the contact test is inconclusive, since the host rocks far away from the intrusions also have the same direction. Furthermore, the magnetization in studied samples is carried by magnetite as well as sulfides. The paleomagnetic mean inclination of 46 degrees implies a paleolatitude of Oslo of 27 degrees, which leaves more room for the North-South configuration of Pangea A than before.
Paleogeographic Reconstructions in the Mediterranean - A Paleomagnetic Study of Jurassic Sediments From Sardinia
The paleogeography and tectonic history of the Corsica-Sardinia block and the opening of the Liguro-Provençal ocean since Oligocene times is based on a wealth of geologic, geophysical, and paleomagnetic studies and relatively well understood (Gattacceca et al. 2007, Vigliotti and Langenheim 1995). Conversely, the paleogeography of Sardinia and the surrounding regions during the Mesozoic is much less clear due to the absence of paleomagnetic data, except for a single study on Jurassic sediments from eastern Sardinia (Horner and Lowrie 1981). Consequently, pre-Oligocene deformations of Sardinia remain virtually undated. Recent paleomagnetic studies of dykes of Late Carboniferous and Permian age as well as Permian sediments have revealed significant counterclockwise rotations between Northern, Central and Southern Sardinia (Emmer et al., 2005). The geodynamic context these rotational movements are related to, however, is still far from being clear. In an attempt to contribute to better time constraints for tectonic motions within Sardinia, a total of 208 oriented core samples from 24 sites of predominantly Jurassic age have been collected from the Nurra region (1), the Gulf of Orosei (2) and the Tacchi region (3). Unfortunately, samples taken from the northwest of Sardinia (1) proved to be too weakly magnetized and did not yield any stable directions. Primary directions of magnetization, passing the reversal test, were recovered from regions (2) and (3), yielding overall mean directions of D=284.8°, I=46.6° (N=36, α95=9.9, k=32.1) and D=267.0°, I=49.9° (N=68, α95=12.3, k=13.5) for the Gulf of Orosei and the Tacchi region, respectively. Taking into account error limits, these directions are not significantly different from each other and confirm and expand the limited data set of Horner and Lowrie (1981). Based on these new results, we conclude that no post-Jurassic deformation has affected the region. This suggests that the counterclockwise rotations previously observed in Permian rocks by Emmer et al. must be pre-Jurassic in age and cannot be related to subduction rollback tectonics during the Oligocene to Miocene as suggested by Helbig et al. (2006).
Persistently low Asian paleolatitudes: implications for the Indo-Asia collision
Ongoing controversies on the timing and kinematics of the Indo-Asia collision arise from uncertainties in
paleomagnetically-determined paleolatitudes of Asia and of terranes bounding the Indo-Asia suture zone. We
provide here (1) new Cenozoic paleomagnetic poles from Mongolian volcanics, (2) shallowing-corrected
paleomagnetic data from Chinese Paleogene sediments and (3) a careful selection of paleomagnetic
datasets over Asia and the collision zone. This confirms that Asian latitudes are 5-10° lower than
predicted by Apparent Polar Wander Paths between 50 and 20 Ma. To explain this discrepancy, Asian
latitudinal motions or time-dependant octupolar field contributions may be invoked. Despite differences in
reconstructions according to these end-members, both imply that (1) the collision occurred between 50 and 60
Ma, (2) following collision, large (~2000 km) intra-continental convergence within India and Asia requires
subduction of continental lithosphere and, (3) the total India-Asia convergence (~4000 km since collision)
slows down until ~40 Ma concurrently with slab break-off.
Palaeomagnetic Constrains on the Timing and the Geographical Distribution of Tectonic Rotations in the Betic Chain, Southern Spain. A Review
The Betic Cordillera is the northern branch of the Betic-Rifean orogen, the westernmost segment of the Mediterranean Alpine orogenic system. Several palaeomagnetic studies have enhanced the important role that block rotations about vertical axes have played in the tectonic evolution of the region. In this work we present a review of published palaeomagnetic data. According with the rotational deformation, the Betics are divided into the central-western area and the eastern Betics. A sequence of rotations for the two regions is also proposed. In central and western Subbetics almost constant clockwise rotations of about 60º are documented in Jurassic limestones. The existence of a pervasive remagnetization of Jurassic limestones, which was coeval with the folding of the studied units and dated as post-Palaeogene, constrains the timing of tectonic rotations in western Subbetics. New palaeomagnetic data from Neogene sedimentary sequences in central Betics indicate that palaeomagnetic clockwise rotations continued after late Miocene. A similar pattern of 40º CW rotations occurred after 20-17 Ma was obtained from the study of the Ronda-Malaga peridotites (western Internal Betics). In eastern Subbetics a more heterogeneous pattern, including very high CW rotations has been observed. But recent rotational deformation in the Internal part of eastern Betics is CCW and related to the left-lateral strike-slip fault systems. Proposed kinematics models for the Betics are discussed under the light of the present available palaeomagnetic information.
Anomalous Magnetization of Brecciated Samples from the Taiwan Chelungpu-fault Drilling Project (TCDP)
A negative pulse-like geomagnetic fluctuations had been detected for more than a month prior to the Taiwan Chi-Chi earthquake (M7.6) in 1999 (Yen et al., 2004). Freund (2003) proposed the hypothesis from his experimental results that earthquake-related electric currents along the fault zones caused the geomagnetic fluctuations. If such electric currents discharged, the fault zone rocks might have recorded the strong current- induced magnetic field. Here, we present paleomagnetic evidence for the hypothesis from a thousand-meter deep drilled fault brecciated samples of Taiwan Chelungpu-fault Drilling Project (TCDP). Three major fault zone around 1136m, 1194m, 1243m showed anomalously strong natural remanent magnetizations (NRM) which were 10~100 times larger than those of host rocks, and the ratio (REM) of NRM and saturation isothermal remanences of brecciated rocks show a one-order-of-magnitude higher value of 0.12 than that of normal volcanic lavas in thermal origin. To estimate the REM value if such lithofacies samples acquired the thermal remanent magnetization (TRM) accompanying the fault motions, we carried out an examination to make host rock samples acquire TRM. In the result, the REM value of a sample heated to 600Ž and cooled in 100/muT (twice as strong as earth field) is 0.03, so that 0.12 cannot be considered TRM acquired in earth field. Additionally, the paleomagnetic directions for these samples showed a much shallower or deeper paleomagnetic inclination data which didn't correspond to the geomagnetic inclination at Taiwan. These suggest the presence of strong ambient fields induced from lightning-like currents despite beneath a deep crust if the magnetic carrier is magnetite. To confirm the magnetic mineral carries the remanent magnetization, we examined the magnetic mineralogy by Electron Backscatter Diffraction (EBSD) and scanning magneto-impedance (MI) microscopy. As a result, we identified magnetite as the magnetic carrier. Because such strong magnetic field cannot be produced by geomagnetic dynamo, these data may provide a natural example of earthquake-associated underground strong currents.