Pierrick Roperch

Tectonic rotations within the Bolivian Altiplano : Implications for the geodynamic evolution of the central Andes during the late Tertiary

Paleomagnetic results from 61 sites in Tertiary red beds and volcanic rocks from the Bolivian Andes allow quantification of tectonic rotations within the Altiplano domain. A total of 16 sites were also obtained in lower Miocene ignimbrites that cover the forearc of the north Chilean Andes near Arica. In the southern Altiplano (Lipez region) a local clockwise rotation of up to 38° is recorded in lower Miocene volcanic rocks. Farther north, near the Salar de Uyuni, there is no evidence of significant rotations. Counterclockwise rotations are observed in the Northern Altiplano. The largest counterclockwise rotation (28°) is found in Eocene-Oligocene red beds (Tiwanaku Formation) along the eastern border of the Corque syncline. Middle Miocene sediments located within the center of the Corque basin record a counterclockwise rotation of only 10°. About 200 km north of the Corque basin, Eocene-Oligocene sedimentary rocks near the locality of Viacha record only 10° counterclockwise rotation. Paleomagnetic results in lower Miocene ignimbrites from the forearc near the Arica bend do not show evidence for late Cenozoic rotation of the forearc. These new results suggest that the 20° to 25° counterclockwise rotation of the southern Peruvian forearc occurred prior to the Miocene. Anisotropy of magnetic susceptibility (AMS) data show a magnetic fabric mostly controlled by bedding, but slight AMS lineations are found almost parallel to the major regional structural trends such as fold axes. The apparent relationship between tectonic rotations, AMS lineations, and structural trends suggests that rotations occured after or during the final stage of folding. Paleomagnetic data obtained in Paleozoic rocks show evidence of clockwise rotation of the southern sub-Andean ranges. Tectonic rotations during the Neogene are mostly localized on the eastern side of the central Andes. The curvature of the occidental margin near Arica was likely acquired prior to the last stage of Andean deformation. Although the magnitude of the rotations may vary from one locality to the other, there is a consistent pattern showing counterclockwise rotations to the north and clockwise to the south. A large-scale regional mechanism is needed to explain this pattern of tectonic rotations within the central Andes. We propose a model in which along-strike segmentation of the Andean foreland and indentation of the Altiplano by a curved forearc are the major factors controlling tectonic rotations within the Altiplano, Eastern Cordillera, and sub-Andean ranges.

Paleomagnetism of Mesozoic rocks from the central Andes of southern Peru: Importance of rotations in the development of the Bolivian orocline

A paleomagnetic study has been undertaken in Jurassic volcanics and Cretaceous intrusions from coastal southern Peru. A secondary magnetization in the Jurassic volcanics was identified at 21 sites from three main localities along the southern coast of Peru (mean virtual geomagnetic pole: 59.5°N, 190.1°E, k=111, α95=3.0). This secondary magnetization was acquired during a tectonomagmatic event that may be related to the emplacement of the Peruvian coastal batholith and/or to the first Andean phase of compressional deformation in Late Cretaceous. If we assume that the remagnetization is of Late Cretaceous age, a regional counterclockwise rotation of about 20° is recorded by the forearc of southern Peru from 16°S to 18°S. Primary remanent magnetizations recorded by the volcanics and the batholith reveal a more complex tectonic history of block rotations. Counterclockwise rotations of 50° to 80° are documented near the localities of Chala (eight sites), Ocona (two sites) and Arequipa (four sites). Preliminary results based on three sites of Late Permian to Early Triassic sediments from northern Bolivia also show a large counterclockwise rotation. The observed differences between the primary and the secondary magnetization indicate that large block rotations occurred prior to the acquisition of the secondary magnetization. Perhaps, this first episode of deformation partially structured the Andean chain in the Central Andes and induced an incipient orocline. During the Cenozoic, a differential shortening along the chain gave the present-day shape of the Bolivian deflection and resulted in a counterclockwise rotation of the forearc system by about 20° as indicated by the remagnetization.

Distribution, timing, and causes of Andean deformation across South America

Abstract The Andean Orogeny in South America has lasted over 100 Ma. It comprises the Peruvian, Incaic and Quechuan phases. The Nazca and South American plates have been converging at varying rates since the Palaeocene. The active tectonics of South America are relatively clear, from seismological and Global Positioning System (GPS) data. Horizontal shortening is responsible for a thick crust and high topography in the Andes, as well as in SE Brazil and Patagonia. We have integrated available data and have compiled four fault maps at the scale of South America, for the mid-Cretaceous, Late Cretaceous, Palaeogene and Neogene periods. Andean compression has been widespread since the Aptian. The continental margins have registered more deformation than the interior. For the Peruvian phase, not enough information is available to establish a tectonic context. During the Incaic phase, strike-slip faulting was common. During the Quechuan phase, crustal thickening has been the dominant mode of deformation. To investigate the mechanics of deformation, we have carried out 10 properly scaled experiments on physical models of the lithosphere, containing various plates. The dominant response to plate motion was subduction of oceanic lithosphere beneath continental South America. However, the model continent also deformed internally, especially at the margins and initial weaknesses.

Vertical axis rotations across the Puna plateau (northwestern Argentina) from paleomagnetic analysis of Cretaceous and Cenozoic rocks

Between 10°S and 30°S, the central Andes are marked by both a major topographic anomaly, the Altiplano-Puna plateau, and a westward concave geometry whose origin remains controversial. The arcuate shape is accompanied by a remarkable pattern of rotations about vertical axes. Indeed, in the central Andes paleomagnetic studies have demonstrated counterclockwise rotations on the northern limb of the arc (throughout Peru, northernmost Chile, and northern Bolivia) and clockwise rotations on the southern limb (throughout southern Bolivia, northwestern Argentina, and northern Chile). To fill a gap in data from northern Argentina and to contribute to the ongoing debate on the origin of rotations in the central Andes, we have undertaken a paleomagnetic study of 373 cores, taken at 29 sites (grouped into seven localities). The samples are from sediments and lava flows of Cretaceous to Tertiary age located in intermontane basins of the Puna plateau in northwestern Argentina. Vertical axis rotations, calculated from paleomagnetic declinations, are clockwise for all localities and confirm the pattern of clockwise rotations associated with the southern central Andes. However, significant variations in the amount of rotation occur from one locality to another, suggesting that they are, at least in part, influenced by local tectonics. As most faults in the Puna plateau have reverse dip-slip components, we infer that the observed differential rotations between blocks are due to scissoring motions on thrust faults. Whether or not this mechanism has operated across the entire area of thickened crust in the central Andes remains to be demontrated. Even if such faulting has locally influenced rotations, Cenozoic oroclinal bending is a likely cause of the remarkable pattern of vertical axis rotations across the central Andes.

Magnetostratigraphy of the Miocene Corque basin, Bolivia: , Implications for the geodynamic evolution of the Altiplano during the late Tertiary

A magnetostratigraphic study of a thick Miocene continental red bed sequence (Totora Formation) located in the north central Altiplano (the Corque syncline) yielded quantitative determinations of the sedimentation rate and better constraints of the timing of the deformation within the Altiplano. Paleomagnetic results obtained in 653 samples from a composite 4.5 km thick red bed sequence across the Corque syncline indicate that most of the Totora sequence was deposited in the middle Miocene from 14 to 9 Ma with high sedimentation rates up to 970 m/Myr from 12 to 9 Ma. The high rate of infill of the Corque basin demonstrates active subsidence of the northern Altiplano during the middle Miocene. Deformation ceased in the northern Altiplano domain only at the end of the Miocene. Two major tuff beds within the sequence have been previously dated by 39 Ar/ 40 Ar of sanidine minerals [Marshall et al., 1992]. The excellent agreement between the magnetostratigraphic dating of the tuffs and the radiometric dating independently supports the age calibration of the geomagnetic reference timescale proposed by Cande and Kent [1995]. The numerous paleomagnetic results obtained in middle Miocene sediments within the Corque syncline, a structure which can be traced for more than 100 km along its axis, demonstrate that the Corque syncline rotated counterclockwise by 10.8°±2.9° since 9 Ma as a consequence of the internal deformation of the Altiplano. Inclination flattening of 17° is recorded in these red bed sediments. This result provides additional evidence that detrital magnetizations, especially in red beds, should be taken with caution when estimating paleolatitudes or long-term variations of the geomagnetic field.

Paleomagnetic study along the southeastern edge of the Altiplano - Puna Plateau: Neogene tectonic rotations

In northwestern Argentina the transition zone between the Puna and the Sierras Pampeanas was deformed in Late Miocene and Pliocene time. Structural observations suggest that local clockwise block rotations might be observed associated with dextral transpression. This study contributes new paleomagnetic results from four sites in Cretaceous rocks and 19 sites in Neogene sedimentary sequences. When compared with reference paleomagnetic data for stable continental South America, the results show an average inclination shallowing error of 10.5° and a pattern of clockwise rotations up to 29°. Study of the anisotropy of magnetic susceptibility (AMS) on 278 specimens provided, for most sites, AMS tensors with an oblate shape controlled by sedimentation and compaction, explaining the observed inclination shallowing. A slight AMS lineation was measured and we argue that this magnetic lineation reflects Late Miocene-Pliocene compression. A correlation observed between magnetic lineations, deduced from the AMS analysis, and rotation estimates from remanent magnetizations, demonstrates that the inferred shortening directions have also been rotated clockwise.

Tectonic rotations and transcurrent deformation south of the Abancay deflection in the Andes of southern Peru

We report new paleomagnetic results from 55 out of 76 sites sampled at different localities along a transect from Nazca to Cuzco where the general structures of the Peruvian Andes are strongly offset across the Abancay deflection. Nine new 39Ar/40Ar ages better constrain the timing of volcanism along the western edge of the Western Cordillera at the latitude of Nazca. A mean paleomagnetic result from 22 sites in the lower Miocene volcanics does not show significant rotation (R = −2.3° ± 7.7°) of the western margin of the Central Andean Plateau since the early Miocene. Within the Western Cordillera we sampled three structural blocks bounded to the north by the Abancay fault system. In the westernmost block, a large counterclockwise rotation (R = −65.0° ± 11.1°) is found in Mesozoic limestones and Paleocene-Eocene red beds. Magnitude of rotation decreases toward the east from (R = −35.6° ± 12.8°) in the central block to (R = −4.5° ± 8.4°) south of the town of Cuzco. The anisotropy of magnetic susceptibility (AMS) recorded by the red beds sediments is the consequence of compaction and tectonic strain during the early stages of deformation. We show that the magnetic lineations were also rotated counterclockwise as the remanent magnetizations. The present study confirms results from the Peruvian fore arc, showing that rotations are not older than circa 40 Ma and likely not younger than circa 20 Ma. The spatial variation in the amount of counterclockwise rotation suggests a large component of shear along the Abancay deflection concomitant with a broad late Eocene-Oligocene oroclinal deformation in southern Peru.

Using anisotropy of magnetic susceptibility to better constrain the tilt correction in paleomagnetism: A case study from southern Peru

[1] We report a combined study of anisotropy of low field magnetic susceptibility (AMS) and paleo-magnetism from 16 sites in a sedimentary sequence of Eocene–early Oligocene red beds in southern Peru. Incipient tectonic strain is recorded during the early stages of deformation. Nonhorizontal magnetic linea-tion in geographic coordinate suggests either non-cylindrical folding and/or interference of two phases of compressive deformation and tectonic rotation. Applying the classic tilt correction results in significant dispersion in paleomagnetic declinations and apparent clockwise and counterclockwise relative tectonic rota-tions. A dispersion in the orientation of the magnetic lineation also arises from a simple classic tilt correction inducing apparent local rotation in paleostress determi-nation. The magnetic lineation is a good proxy to detect a complex history of folding when the finite strain is not large enough to reset the magnetic fabric acquired during the early stages of deformation and when detailed geo-logical field mapping is not available or not possible. In the present study, a double correction rotating first the lineation to the horizontal reduces significantly the dis-persion of the paleomagnetic data with respect to con-ventional tilt correction (Fisher parameter k increases from 14 to 35). The interest of this double correction must obviously be evaluated for each study according to the complexity of the folding and the intensity of the deformation. Assuming a mean age of 40 Ma for the sedimentary sequence, no significant rotation (−4.5° ± 8.4) is observed in this area of the Peruvian Andes. Citation: Roperch, P., V. Carlotto, and A. Chauvin (2010), Using anisotropy of magnetic susceptibility to better constrain the tilt correction in paleomagnetism: A case study from southern Peru, Tectonics, 29, TC6005,