Middle Jurassic Pole Research Articles

A new paleogeographic configuration of the Eurasian landmass resolves a paleomagnetic paradox of the Tarim Basin (China)

New paleomagnetic data from Permian red beds and Middle Jurassic limestones from the Tarim Basin pose a paradox. Their declinations are similar to Upper Carboniferous to Neogene rocks collected from the same sections, and their inclinations parallel present values. When assuming that lower than expected inclinations in continental sedimentary rocks arise from inclination shallowing effects, then the paleolatitudes of all Upper Carboniferous to Present rocks from Tarim are indistinguishable. Local vertical axis block rotations occurring in the last 20 million years explain why declinations vary at different localities in the basin. Our Middle Jurassic data positions Tarim 23.6 ± 8.4° farther south than that predicted from the coeval reference pole for Eurasia; however, no geologic argument exists to support the closure of a large ocean basin between Tarim and Siberia since the Middle Jurassic. Thus the paradox: are the rocks from Tarim totally overprinted, or is the middle Mesozoic part of the reference Eurasian apparent polar wander path erroneous? Several lines of evidence suggest the Tarim rocks are not remagnetized. We conclude that Tarim has experienced little or no apparent polar wander since the Carboniferous. Moreover, our Middle Mesozoic reconstruction of Eurasia using the new Middle Jurassic pole from Tarim results in a more geologically compatible solution for the eastern Asian blocks over previous reconstructions.

Paleomagnetism and magnetic fabric of Middle Jurassic dykes from Western Patagonia, Argentina

Jurassic volcanism in Patagonia is widespread. Although associated dyke swarms are conspicuous, they have been almost neglected in previous geologic studies of this region. Radiometric, paleomagnetic and magnetic anisotropy studies are reported from a Middle Jurassic basic to intermediate dyke swarm exposed in the Sierra de Mamil Choique (Western Patagonia) in an area of over 350 km 2. Two whole-rock K/Ar determinations indicate that these dykes were intruded at around 170 Ma. An anisotropy of magnetic susceptibility (AMS) study on 15 dykes (74 block samples) shows that they carry different kinds of magnetic fabric (both normal and inverse), apparently governed by compositional differences. A paleomagnetic study of these samples suggests that Ti-poor titanomagnetite is the probable carrier of the characteristic remanence. Mean site characteristic directions pass a reversal test (grade C). A paleomagnetic pole, computed by averaging VGPs from each individual dyke, is situated at 70.2°S, 190.4°E ( N=13, α 95=9.7°), not substantially apart from other Middle Jurassic poles from South America. Further refinement of the Jurassic South American apparent polar wander path is needed to establish whether or not the Mamil Choique dykes were affected by a small tectonic rotation.

Jurassic paleomagnetic constraints on the collision of the North and South China Blocks

This paper reports a paleomagnetic study of Jurassic rocks from the Ordos Basin in the North China Block (NCB). A recent remagnetization and a high‐temperature and/or high‐coercivity component with dual polarities have been isolated. The Middle Jurassic pole (74°N, 233°E, A95=5°) is roughly consistent with previous results, and the Lower Jurassic pole (82°N, 286°E, A95=7°) is located between the late Triassic and Middle Jurassic poles. These results, combined with paleomagnetic results from adjacent blocks, suggest that 1) the pole for the NCB suffered an angular displacement of 47°+/− 8° between the late Triassic and Middle Jurassic, 2) the SCB underwent final accretion to the NCB in the Middle Jurassic, and 3) accretion of the NCB to Siberia was not complete until the late Jurassic and possibly even until the early Cretaceous.

Palaeomagnetism of Mesozoic igneous rocks from the Maranha˜o Basin, Brazil, and the time of opening of the South Atlantic

From Middle-Upper Jurassic volcanics at the western margin of the Maranha˜o Basin (6.4°S, 47.4°W) 15 sites (121 samples) have a mean magnetization direction D = 3.9°, I = −17.9° with α 95 = 9.3° , k = 17.9 after AF cleaning (all sites have normal polarity). This yields a pole (named SAJ 2) at 85.3°N, 82.5°E ( A 95 = 6.9° ) which is near to the other known Middle Jurassic South American pole. For 21 sites (190 samples) from Lower Cretaceous basalt intrusions from the eastern part of the Maranha˜o Basin (6.5°S, 42°W) the mean direction is D = 174.7°, I = +6.0° with α 95 = 2.8° , k = 122 (all sites have reversed polarity) yielding a pole (SAK 9) at 83.6°N, 261°E ( A 95 = 1.9° ) in agreement with other Lower Cretaceous pole positions for South America. Comparing Mesozoic pole positions for South America and Africa in the pre-drift configuration after Bullard et al. [13] one finds a significant difference (with more than 95% probability) for the Lower Cretaceous and Middle Jurassic poles and also a probable difference for the mean Triassic poles indicating a small but probably stationary separation of the two continents from the predrift position in the Mesozoic until Lower Cretaceous time which may be due to an early rifting event.