José Achache

Paleomagnetic constraints on the late Cretaceous and Cenozoic tectonics of southeastern Asia

Abstract Many features of the Cenozoic tectonic history of central and southeastern Asia can be understood as direct consequences of the thrust and penetration of India into Asia. Recent indentation experiments with plasticine (Tapponnier et al. [7]) have extended this idea and have led to the prediction of a pattern of large rotations and displacements of continental blocks that can be tested by paleomagnetism. The available Cretaceous and Cenozoic paleomagnetic data from this part of the world have been reviewed and a new APWP for Eurasia has been constructed for reference. The negligible rotation of South China and large clockwise rotation of Indochina are consistent with the model, i.e., with an history of large-scale left-lateral strike-slip motion along the Altyn Tagh and Red River faults. Data from Malaya and Borneo can be reconciled with the model, although in a less straightforward fashion. The large counter clockwise rotation of South Tibet implies that it rotated in sympathy with India during the collision and suggests that future indentation experiments should include this feature. Finally a middle Cretaceous reconstruction of the south margin of Asia is proposed. One interesting result is the restored continuity of geological features in Tibet and Indochina, with active subduction of oceanic (Indian plate) crust taking place to the south at subtropical latitudes.

The late 1960's secular variation impulse: Further constraints on deep mantle conductivity☆

Abstract Analysis of geomagnetic secular variation data from world observatories demonstrates the occurrence of a secular variation impulse in the late 1960s. When a simple mantle conductivity model is used in conjunction with the impulse data, it can be shown that the average conductivity of the lower mantle does not exceed 150 ω −1 m −1 . This is orders of magnitude smaller than recently published values.

Contribution of induced and remanent magnetization to long-wavelength oceanic magnetic anomalies

The Magsat mission has provided very precise measurements of Earths magnetic field which have been used to derive accurate maps of the long-wavelength magnetic anomaly field. Analyses of this field over localized areas have shown that induced magnetization is mainly responsible for the anomalies and that remanent magnetization is rarely detectable at Magsat altitude. A careful analysis of the data, taking into account the various effects due to non lithospheric sources, allows this field to be better described and, consequently, suggests two observations: (1) long-wavelength oceanic anomalies display many similarities with the distribution of seafloor topography, suggesting a relationship between crustal thickening and magnetization enhancement; (2) age-related pattern (i.e., ridge parallel) of anomalies is observed in two of the larger oceanic basins (Indian and North Atlantic oceans). Several forward models are constructed to investigate these correlations. In the first model, the effect of variations in crustal thickness was modeled assuming uniform susceptibility and that thickness varies proportionally to the seafloor topography. In the second model, the field due to remanent magnetization acquired during the Cretaceous Long Normal period was modeled and shown to contribute significantly to the Magsat field. In order to make the modeled and observed maps fully comparable, several effects must be taken into account: a global magnetization contrast exists between oceans and continents and induces widely distributed anomalies because of the truncated spherical harmonic analysis; the spectral content of the observed map in the north-south direction is reduced because of the along-track filtering due to the polar orbit of Magsat, When these effects are taken into account, the model combining the contribution of induced and remanent magnetization display remarkable similarity to observed data. This comparison constrains both induced and remanent magnetization of oceanic lithospheric rocks.