Determining the timing of formation of the Rawil Depression in the Helvetic Alps by palaeomagnetic and structural methods
G. L. Cardello, B. S. G. Almqvist, A. M. Hirt, N. S. Mancktelow, 2016. "Determining the timing of formation of the Rawil Depression in the Helvetic Alps by palaeomagnetic and structural methods", Palaeomagnetism in Fold and Thrust Belts: New Perspectives, E. L. Pueyo, F. Cifelli, A. J. Sussman, B. Oliva-Urcia
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Anisotropy of magnetic susceptibility, palaeomagnetism and structural methods are used in order to test the relative timing of antiform updoming and formation of the Rawil Depression in the Helvetic Alps. Samples were collected from all nappes currently exposed in the study region. The magnetic fabric is consistent with extension oblique and parallel to the regional fold trend and with palaeostress reconstructions from fault planes and veins. Palaeomagnetic analyses show a stable characteristic remanence (ChRM), with samples recording both normal and reverse polarity. A successful fold test performed across the antiformal dome structure suggests that the palaeomagnetic signal was acquired prior to doming. By comparison with thermochronometric data, the ChRM was acquired between 25 and 10 Ma and is pre- to synfolding. A secondary post-doming palaeomagnetic component (A), whose magnetization is likely to have occurred between 10 and 3.5 Ma, appears to be too steep with regards to the inclination of the Earth’s field, suggesting recent large-scale tilting has occurred in the region. These combined analyses indicate that widespread orogen-parallel extension occurred prior to the formation of the Rawil Depression, which is finally interpreted as the result of a stepover structure at the curvature between Central and Western Alps.
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Palaeomagnetism is a technique used to understand complex deformation patterns in fold-and-thrust belts; it can be used to characterize the distribution, magnitude and timing of vertical axis rotations, an elusive variable using other methods. A combination of palaeomagnetic and structural geology analyses has helped to unravel the geometry and kinematics of fold-and-thrust belts around the world and of different geological ages for more than 50 years. This volume comprises three sections: the first shows thorough overviews of western Mediterranean arcs and the western Carpathians; the second depicts several examples from the Andes, the Alps, Anatolia, Pyrenees, Iberian Ranges and the Atlas; and the third shows the latest research on the use of palaeomagnetism to understand fold-and-thrust belts in 3D and 4D in a more quantitative way and it also includes some methodological proposals to avoid common errors. In the papers of the first two sections, the combination of palaeomagnetic analyses with structural data, AMS or magnetostratigraphic analyses demonstrate the usefulness of palaeomagnetism in deciphering complex deformation patterns in fold-and-thrust belts.