Pierre Mitouard

An accreted continental terrane in northwestern Peru

Abstract A paleomagnetic study of over 250 cores from 26 sites sampled in Early to Late Cretaceous and Paleogene volcanic, plutonic and sedimentary formations of the Lancones basin in the Piura province of northern Peru, indicates that most of these lithologies carry a stable primary remanent magnetization whose direction is significantly different from that of coeval formations of stable South America. A clockwise rotation ranging from 90° for the lowermost units to 35° for the uppermost ones has been documented, although the lack of precise chronology has not allowed a detailed temporal description. Four sites from Late Carboniferous (Pennsylvanian) formations in the Amotape-Tahuin Range also show a 110° clockwise rotation and yield evidence for a northward displacement. When considered together with previous geological studies, these data are consistent with the hypothesis of the accretion of an Amotape-Tahuin continental terrane to the Peruvian margin in Neocomian times. The accretion was followed by in situ rotation, suggesting a dextral shear regime. These results indicate that the geodynamical evolution of northern Peru is more closely related to the processes observed in Ecuador than to those classically assumed for the Central Andes of Peru.

Late Cainozoic rotation of the Peruvian Western Cordillera and the uplift of the Central Andes

Abstract A palaeomagnetic study was carried out in the region of Lima (Peru), on 55 sites (564 cores) of middle Cretaceous to Late Miocene age, distributed along three northeast-southwest transverses which cross the Western Cordillera of the Central Andes. Over the entire studied area, the results consistently yield westerly declinations corresponding to post-Cretaceous and post-Eocene counterclockwise rotations with respect to stable South America, of 16° ± 6° and 13.7° ± 8.1°, respectively, when compared to the ill-defined reference poles of this continent. These new results are in good agreement with previous ones from the Central Peruvian Andes and support the model recently proposed for the uplift of the Andean Cordillera.

Post-Oligocene rotations in southern Ecuador and northern Peru and the formation of the Huancabamba deflection in the Andean Cordillera

Abstract A paleomagnetic study of more than 410 samples from 41 sites from Paleocene to early Oligocene volcanic and intrusive formations in southern Ecuador and northern Peru reveals a pattern of rotations in opposite senses on both sides of the Huancabamba deflection. The amplitude of the rotations is ∼ 25° clockwise north of the deflection and ∼ 20° anticlockwise south of it. North of the deflection, the results do not allow us to ascertain whether the rotation arises from oroclinal bending or from block rotation in a distributed shear. South of it, the absence of geological evidence for widespread strike-slip faulting suggests that the results reflect a rotation of the Peruvian margin which could be related to the shortening documented in Southern Peru and to the uplift of the Central Andes.

Paleomagnetic study of an arcuate fold belt developed on a marginal orogen: The Cajamarca deflection, northern Peru

Abstract A paleomagnetic study of over 500 cores from 48 sites sampled in Albian to Turonian sedimentary formations along the Cajamarca deflection in northern Peru indicates that these lithologies carry a stable remanent magnetization. In the two major segments of the deflection, positive fold tests establish that the magnetization predates the folding, except for five sites in the western part which appear to be remagnetized by the intrusion of Paleocene granites. The eastern part of the deflection does not appear to have undergone any significant rotation, whilst a post-late Eocene counterclockwise rotation of about 25° is recorded in the central and western parts. This angular difference only accounts for about half of the difference in the strikes of structures in the two sections of the deflection. When considered with the available tectonic, sedimentological and geodynamic data, these results indicate that the arcuate shape of the Cajamarca fold belt results from complex processes. Early multi-phased compressional events of Paleocene-Eocene age created the folds and built an arcuate structure with non-rotational mechanisms. Then, in post-late-Eocene times the structure reached its present form after rotation of its central and western parts. The bearing of these results on the geodynamic evolution of the Andean Cordillera is discussed.

Paleomagnetic Rotations in the Coastal Areas of Ecuador and Northern Peru

We report a paleomagnetic study of about 850 cores from over 80 sites sampled in Paleozoic to Tertiary volcanic, plutonic and sedimentary formations in western Ecuador and Northern Peru. Most of the sampled lithologies carry a stable primary remanent magnetization whose direction is significantly different from that of coeval formations of stable South America. In western Ecuador the results are consistent with the progressive disappearance of a marginal basin accompanied by a clockwise rotation of about 70°. In Northern Peru four sites from Late Carboniferous (Pennsylvanian) formations from the Amotape-Tahuin range in the Piura province show a 110° clockwise rotation and yield evidence for a northward displacement. The results from the Cretaceous to Paleogene formation from the same region indicate a clockwise rotation ranging from 90° for the lowermost units to 25° for the uppermost ones. When considered together with previous geological studies, these data are consistent with the hypothesis of the accretion of an Amotape-Tahuin continental terrane to the Peruvian margin in Neocomian times. The accretion was followed by in-situ rotation, suggesting a dextral shear regime. Preliminary results obtained here from Cretaceous and Paleogene formations in the Central Andes and published results from other authors indicate that this pattern of clokwise rotation changes to anticlockwise south of the major Huancabamba deflection (4° S). This rotational pattern could be related to a general shear regime, dextral north of the deflection and sinistral south of it, or to a recently proposed mechanism involving along-strike variations in the amount of late Cenozoic shortening.