Structural analysis of dyke emplacement directions as an aid to palaeomagnetic studies: an example from northern Chile

A major problem in palaeomagnetic studies of intrusive rocks lies in determining whether or not such rocks have been subjected to post-emplacement tilting. Structural analysis of dyke emplacement directions can be used to show the current attitude of the extension direction for the dyke swarm. If the original extension direction, at the time of emplacement, can be deduced from geological evidence, this then provides a field test for post-emplacement tilting of the dyke swarm and its host rocks. In the example given from northern Chile, we were able to make a palaeomagnetic study of and structurally analyse three successively younger dyke swarms that intrude similarly younging plutons. All three dyke swarms yielded extension directions close to horizontal but with markedly different azimuths. It is argued that the similarity in the plunge of the extension directions cannot be coincidental and that the dykes and their host plutons have not suffered significant post-emplacement tilting. This simple technique should be widely applicable in the assessment of post-emplacement tilting of dykes in palaeomagnetic studies.     

Early Triassic palaeomagnetic results from the Ryeongnam Block, Korean Peninsula: the eastern extension of the North China Block

Greenish sandstones in the Early Triassic Nogam Formation of the Ryeongnam Block, Korean Peninsula were collected at 23 sites for palaeomagnetic study. A high-temperature magnetization component with unblocking temperatures of 670–690 °C was isolated from seven sites and yielded a positive fold test at the 95 per cent confidence level. The high-temperature component is interpreted to be of primary origin because the folding age is Middle Triassic. The Early Triassic palaeomagnetic direction for the Ryeongnam Block after tilt correction is D =347.1°, I =23.8° ( α ₉₅=5.5°). The palaeomagnetic pole (62.5°N, 336.8°E, A ₉₅ = 4.7°) shows good agreement with the coeval pole for the North China Block, suggesting that the Ryeongnam Block has been part of the North China Block at least since Early Triassic times. A tectonic history of the Korean Peninsula includes obduction of the eastern part of the South China Block onto the central part of the Korean Peninsula in the Permian, with the Ryeongnam Block geographically isolated from the main part of the North China Block. Collision of the North and South China blocks commenced initially at the Korean Peninsula, and suturing of the two blocks progressed westwards.     

Late Permian to Late Triassic palaeomagnetic data from Iran: constraints on the migration of the Iranian block through the Tethyan Ocean and initial destruction of Pangaea

A palaeomagnetic study of Late Permian to early Jurassic rocks from the Alborz and Sanandaj–Sirjan zones in Iran and a compilation of selected palaeopoles from the Carboniferous to the present provide an updated history of the motion of the Iranian block within the Tethys Ocean. The Iran assemblage, part of Gondwana during the Palaeozoic, rifted away by the end of the Permian. We ascertain the southern-hemisphere palaeoposition of Iran at that time using magnetostratigraphy and show that it was situated close to Arabia, near to its relative position today. A northward transit of this block during the Triassic is shown, with an estimated expansion rate of the Neotethyan ridge of 100–140  km Myr⁻¹. The northward convergence with respect to Eurasia ended during the Ladinian (Middle Triassic), and is marked by a collision in the northern hemisphere with the Turan platform, which was the southern margin of the Eurasian continent at that time. No north–south component of shortening is evidenced north of Iran afterwards. An analysis of the declinations from the Late Permian to the present shows different, large rotations, emphasizing the important tectonic phases suffered since the Triassic. Finally, we propose palaeomagnetic reconstructions of the Tethys area during the Late Permian and the Late Triassic, showing that the Palaeotethys Ocean was narrower than previously thought, and did not widen its gate to the Panthalassa before the Triassic period.     

A palaeomagnetic study of fracture fills in the Holy Cross Mountains of Central Poland and its application in dating tectonic processes

Calcite and sedimentary fills in fractures cutting the Upper Devonian carbonates in the Holy Cross Mountains (HCM) were dated palaeomagnetically by comparison with the apparent polar wander path (APWP). Haematite-bearing calcite possessed well-defined components of natural remanent magnetization (NRM), which were preserved under thermal demagnetization to temperatures of approximately 500 °C, when specimens disintegrated. Although not completely demagnetized, some specimens revealed a stable NRM component before destruction, thus making a component analysis possible. Five components were determined using density point distribution and cluster analysis. One has a mean that is similar to the present-day local geomagnetic vector. The remaining four components yielded palaeomagnetic poles located at: A (70.3°S, 5.5°E), B (71.3°S, 31.2°E), C (48.7°S, 351.0°E, virtual geomagnetic pole), and D (11.6°S, 312.3°E). Antipodal polarities found in the fracture fills, together with dissimilarities in magnetization found in calcite and hosting carbonates, indicate the lack of simultaneous remagnetization, and different times of remanence acquisition for the rocks under comparison. Taking both palaeomagnetically inferred palaeolatitudes and regional tectonics into consideration, a Mesozoic (Cretaceous?) age is estimated for palaeopoles A and B, a Permian age for pole C, and a Carboniferous age for pole D. These age determinations are in line with the calcite ages estimated from isotopic studies. A comparative palaeomagnetic study performed on a well-dated Upper Devonian neptunian dyke of limestone and a Lower Triassic clastic vein yielded virtual geomagnetic poles (VGPs) close to the APWP for Baltica. Generally, the remanence from fracture fills may be useful for dating related tectonics, karst phenomena and mineralization processes.