Neogene rotations of the Chenoua massif, northern Algeria, from remagnetizations

Abstract

International audienceThe tectonics of the Chenoua massif suggests block rotations of Neogene nappes associated with the African–European plate convergence. To estimate the extent of these rotations, a Paleomagnetic study on rhyolites and andesites of Langhian–Serravallian age and sandstones of Burdigalian age was carried out on 23 sites (200 specimens). The sites are distributed in the northwestern, southeastern and southern Chenoua massif. One or two components of magnetization, mainly carried by magnetite, pyrrhotite and/or hematite, were isolated in sandstones and volcanics. The sandstone sites reveal magnetizations in sandstones from the Cap Blanc syncline that are post-folding. However, both polarities are found, which is consistent with data from Africa during the Upper Miocene. Clockwise and counterclockwise rotations were recorded, dating back to the Neogene times in volcanics and sediments. From the faulted Cap Blanc syncline counterclockwise rotations of 1 ± 4° to 18 ± 28° around a vertical axis occurred in sediments since the Miocene with respect to Africa. In fact, remagnetizations occurred at several periods of time and in different sites, providing information on the evolution of post-tectonic rotations. Some volcanics record counterclockwise rotations of about 30° since the Miocene, whereas others do not show any significant rotation. This can be explained by the direction of the principal compressive stress axis σ 1 and by lateral extrusions related to an indentation model, in which we expect both clockwise and counterclockwise rotations

Strong Neotectonic Block Rotations, Related to the Africa‐Eurasia Convergence in Northern Algeria: Paleomagnetic Evidence From the Mitidja Basin

“…On the northern side of the Mitidja basin, around the Chenoua massif (Tipasa area), paleomagnetic directions (Aïfa, ; this study), mostly obtained within fault zones, show clockwise rotations relatively to stable Africa (Figure ).…”

“…The mean direction with its confidence zone is in black. (b) Sites affected by local rotations, all converted to negative inclinations: this study (red) and from Aïfa, (blue). Continuous (dotted) line for confidence circles correspond to lower (upper) hemisphere.…”

“…Map with arrows indicating the paleomagnetic declinations (all presented as normal polarity), with, in white, their associated confidence cones (Table ), obtained in the different sites: blue arrows for sites with magnetite as magnetization carrier in Aïfa () and red arrows for this study. The black arrow on the bottom right indicates the expected paleomagnetic direction after Besse and Courtillot ().…”

“…In a few sites, where unblocking temperatures are higher than 580 °C, thermomagnetic curve decreases appear also for temperatures higher than 580 °C, confirming the presence of hematite (Figure b, MI327 sample). In the Tipasa area (Figure ), Aïfa () found sulfides and hematite in the lava flows in addition to magnetite. Goethite also likely appears locally on some of his thermomagnetic curves.…”

Indentation Model Using Gravity Anomaly and Anisotropy of Magnetic Susceptibility: Consequences on Nappe Rotations in the Chélif Basin, Algeria

“…It occurred at a reactivated sinistral fault trending NE-SW and 40 km long, with vertical movements reaching locally ~6 m and horizontal shearing movements higher than ~1 m and of shortened by ~3 m [4,13]. 80 km north of the Chélif Basin, the Chenoua Massif was also subjected to clockwise rotation of blocks which could reach values of 20° since the Middle Pliocene [14]. These rotations are interpreted in the frame of a N-S indentation model [3].…”