Paleomagnetisme de la Sierra Madre Occidentale dans les etats de Durango et Sinaloa (Mexique): variations du champ ou rotations de blocs au paleocene et au neogene?

Abstract

A paleomagnetic study of 350 samples (64 sites) of the western andesitic cordillera has been made. This cordillera forms the main part of the Sierra Madre Occidental in Mexico. Igneous rocks have been sampled in Durango and Sinaloa states (along the Durango - Mazatlán highway) and in the North of Mazatlán (mean point: 24° N; 106° W). Two petrological units, showing two distinct directions of magnetization and associated virtual poles, are observed: 1. The older sequence of andesitic lavas and batholitic granodiorites ranges in age from 4 m.y. to at least 100 m.y. This lower system is characterized by a mean direction of magnetization defined by: I = 34°9; D = 3°3 E; R = 21.7672; α95 =8°8. A corresponding virtual geomagnetic pole located at 84°3 N; 41°E (δp= 5.8; δm = 10.1) differs from previously proposed pole positions. This apparent pole is explained by a slight northward shifting and a clockwise rotation of the area since the andesitic sequence overflow. This model agrees with a closing (during Paleocene and Early Eocene) of the occidental arc system (from W to E: oceanic domain western continent marginal basin) which develops in western Mexico since Cretaceous. Then the paleomagnetic results suggest a larger tightening to the North of the cordillera than to the South. 2. The upper sequence, approximately 1000 meters thick, is made of ignimbritic rhyolites (tuffs and ash-flows). These volcanics range in age from 32 to 22 m.y. and cover at least seven field polarity epochs. The magnetization is characterized by a mean direction: I = 33°7; D = 335°8; R = 18.9343; α95= 10°. The corresponding pole located at 66°8 N; 154°8 E (δp = 6.5; δm = 11.4). It is widely different from both the poles deduced from the andesitic system and the mean pole computed from American Oligocene and Miocene volcanic series. The deviation between the two directions of magnetization (corresponding to the two middle Tertiary poles) is about 20° W at the point 24° N - 106°W. The apparent pole for the upper sequence shows a migration opposite from the one recognized using paleomagnetic studies of the lower group. Two hypotheses are presented and discussed: - One implies large spatial variations of very low frequence in the geomagnetic field during Oligocene and Early Miocene. - The second takes into account an anticlockwise rotation of the area since the ignimbrite deposits. In favour of this hypoteshis we note: (i) the occurrence of compressive events during Miocene in western Mexico. (ii) A change in direction of convergence of the oceanic subduction during late Miocene (first W-E directed, then SW-NE). (iii) The results of other paleomagnetic studies in southwestern Mexico (Jalisco State) sugggesting similar movements. This model also suggest that the rotation affecting the area with respect to "stable" North America during Paleocene, has been larger than it appears according to the remanent magnetization of the lower andesitic group, and reached about 40°. Such a value cannot be applied to the entire border. It suggests a probable fragmentation of the "western continent" during the closing with local arrangements of blocks, in response to larger movements.