Pierrick Roperch

Counterclockwise rotation of late Eocene–Oligocene fore-arc deposits in southern Peru and its significance for oroclinal bending in the central Andes

The results of a paleomagnetic study along the fore arc of southern Peru (15xad18°S) and northern Chile (18xad19°S) are reported from middle to late Miocene ignimbrites (7 sites), late Oligocene to early Miocene ignimbrites (72 sites), Paleogene sediments (20 sites), and Mesozoic and Paleocene volcanics and intrusions (31 sites). Comparison of locality-mean directions with expected paleomagnetic directions indicates vertical axis rotations ranging from 5.2 ± 11.3° clockwise to 55.6 ± 7.0° counterclockwise. Spatially, the magnitude of counterclockwise rotations increases northward from ~0° within the Chilean fore arc south of 18°30S to >45° north of 16°30S. In southern Peru, paleomagnetic rotations recorded in Paleogene red beds decrease from late Eocene to late Oligocene, whereas Miocene ignimbrites display no evidence of rotation. These new results confirm that the rotations recorded in the fore arc of southern Peru were acquired at least before ~15 Ma, and probably before 25 Ma, and thus prior to the late Neogene shortening of the sub-Andes. The onset of major Andean shortening in the Eastern Cordillera during the latest Eocenexadearliest Oligocene is interpreted to have triggered the bending of the Peruvian fore arc. The region of the Peruvian fore arc with the largest rotations appears to be the fore-arc counterpart of the Abancay deflection, a remarkable NE-SW offset in the axis of the Eastern Cordillera induced by a major regional preorogenic structure. We underline that the Abancay deflection should be seen as the northwestern boundary, and therefore as a key element, of the Bolivian Orocline.

Clockwise block rotations along the eastern border of the Cordillera de Domeyko, Northern Chile (22°45′–23°30′S)

Abstract We report new paleomagnetic results from a study of 38 sites in Cretaceous to Early Tertiary red beds and volcanic rocks belonging to the Purilactis Group, which outcrop along the western border of the Salar de Atacama Basin. After detailed thermal demagnetization, characteristic directions were determined for 32 units. In most cases, red bed sediments from the lower members of the Purilactis Group have a well-defined normal polarity magnetization probably carried by hematite forming an early diagenetic cement. A large decrease in the dispersion of the paleomagnetic directions upon tilt correction demonstrates that this magnetization is a pre-tectonic magnetization. We interpret the dominant occurrence of the normal polarity direction as evidence for acquisition of the magnetization during the Cretaceous normal polarity superchron. Large deviations of the paleomagnetic declinations from the expected ones for stable South America provides new evidence for clockwise tectonic rotations associated with Tertiary deformation in the Cordillera de Domeyko. These data confirm that clockwise tectonic rotations are one of the most significant structural characteristics of the north Chilean Andes. This study, however, indicates spatial variation in the magnitude of the rotation with rotations >60° in the Cerro Totola area. These rotations have occurred in conjunction with transpressional deformation that affected large tracts of the Cordillera de Domeyko during Eocene deformation. The systematic observation of clockwise rotations contemporaneous with sinistral displacements in the Cordillera de Domeyko can be explained by shear-traction at the base of the brittle crust.

Paleogene clockwise tectonic rotations in the forearc of central Andes, Antofagasta region, northern Chile

[1]xa0For the Central Valley of northern Chile (Antofagasta region), a paleomagnetic analysis of data from 108 sites, mainly in Mesozoic and Paleogene volcanic rocks, has yielded stable remanent magnetization directions for 86 sites. From these data, we infer clockwise tectonic rotations of up to 65° within the forearc domain of the central Andes. The apparent relationship between tectonic rotations and structural trends suggests that rotations occurred mainly during the Incaic orogenic event of Eocene–early Oligocene age. A few paleomagnetic results obtained in Neogene rocks do not show evidence of clockwise rotations. Hence the development of the Bolivian orocline during late Neogene time cannot be explained by simple bending of the whole margin. These results demonstrate that tectonic rotations within the forearc and pre-Cordillera are key elements of early Andean deformation, which should be taken into account by kinematic models of mountain building in the central Andes.

Paleomagnetism and tectonics of the southern Atacama Desert (25–28°S), northern Chile

We report paleomagnetic results for 131 sites from the modern forearc of northern Chile (25°S and 28°S). Remanent magnetization in volcanic and intrusive rocks is mostly primary, while a secondary magnetization is observed in sedimentary rocks. Comparison of locality-mean directions with expected paleomagnetic directions indicates vertical axis rotations from -7.3° +/- 21.6° counterclockwise to 52.7° +/- 17° clockwise. Jurassic to Early Cretaceous rocks from the Coastal Cordillera and Cretaceous to Paleocene rocks from the Central Depression show similar magnitude (>30°) clockwise rotations, while more variable rotations occur in Mesozoic to Eocene rocks of the Precordillera. Clockwise rotations in Mesozoic and Paleogene rocks occur in the Chilean Frontal Cordillera south of 27°30S. Paleomagnetic results in three large Miocene ignimbrite sheets overlying rotated and nonrotated older rocks in the Precordillera and Pre-Andean Depression which show no relative rotation between sites indicate that most rotations within the study area occurred prior to 18 Ma (early Miocene) and likely during and after the ``Incaic tectonic event, which affected large tracts of the central Andes. The postulated onset of rotations in the north Chilean forearc was contemporaneous with the beginning of horizontal shortening and uplift of the Eastern Cordillera in Bolivia and northwestern Argentina. Rotation of the Chilean forearc, enhancement of the curvature of the central Andes, and the formation of the Bolivian Orocline seem to be, for the most part, closely linked to the evolution of the Eocene-Oligocene tectonics of the Eastern Cordillera.

Magmatic Gradients in the Cretaceous Caleu Pluton (Central Chile): Injections of Pulses from a Stratified Magma Reservoir

The Caleu pluton (Central Chile) extending over 338 km2 and with more than 1, 400 m of vertical relief intrudes the N-S trending Lower Cretaceous volcano-sedimentary and volcanic successions at a depth equivalent to a pressure of 2 kb. The host, stratified volcanic successions, are tilted about 30°–40° E, whereas the pluton shows paleomagnetic evidence of either tilting of <15° E or clockwise rotation by few degrees. n nA gradient of westward increasing SiO2 content is recognized across the pluton, giving rise to three N-S elongated zones: Gabbro/Diorite Zone (GDZ), Tonalite Zone (TZ) and Granodiorite Zone (GZ). Biotite and hornblende compositions also exhibit a westward decreasing gradient in Mg/(Mg+Fe), indicating that the more mafic the zone is, the more oxidizing is its crystallization condition. Horizontal inward gradients of progressively less evolved rocks are recognized across GDZ and TZ, whereas no horizontal gradients were found in the GZ. Vertical compositional gradients are recognized in the GDZ and TZ, which consist of an upward increase in SiO2 and decrease in MgO, FeO, Fe2O3, and compatible trace elements. A vertical compositional boundary was recognized along a traverse across the TZ separating two magma pulses with similar trends of compositional variations. n nThe three zones of the Caleu pluton were derived from a common isotopically (Sr-Nd) depleted source. Each zone probably evolved independently, as their compositional characteristics would have not been acquired in situ. The resulting compositional characteristics of the zones would have been developed prior to the intrusion, in a subjacent stratified reservoir placed at about seven kilometers below the pluton.

First Paleomagnetic Constraints on the Latitudinal Displacement of the West Burma Block

Cenozoic collision between India and Eurasia produced the Himalayan-Tibetan orogen, which is commonly considered as the archetypical orogen for continent-continent collision systems. However, there is still no consensus on the amount and mechanism of post-collisional convergence, as well as on the roles of the numerous tectonic terranes comprising the orogen (Jagoutz et al. 2015, 2016; Replumaz et al. 2013, Royden et al. 2008, van Hins-bergen et al. 2011). The West Burma block exhibits a unique geodynamic evolution within this system, influenced by oblique subduction of the Indian plate and significant strike-slip motions along the dextral Sagaing Fault. Furthermore, it is at a key location for paleoenvi-ronmental reconstructions (Cai et al. 2016, Licht et al. 2013). Despite this, robust paleo-magnetic data from the West Burma block is largely absent.Here we report new paleomagnetic, petrological and U-Pb age data to constrain the latitudinal displacement of West Burma. To this end, 45 sites were drilled in the intrusives, extrusives and sediments of the Wuntho arc, Myanmar. Paleomagnetic results were obtained at 30 sites. In addition, 135 paleomagnetic results were obtained from a Late-Eocene mono-clinic sedimentary section in the Chindwin basin, Myanmar.Wuntho arc U-Pb ages cluster in the range 110–90 Ma, indicating a Late-Cretaceous age. Paleomagnetic results from this area show declination values of around 50°–100°, im-plying clockwise rotation of the overall arc dispersed by local-block rotations related to faulting, and inclination values close to zero, corresponding to near-equatorial paleolatitude. Tilt corrections are not available for sites in intrusive rocks. However, the sampling is dis-tributed over a large area (1000 km2) and the results are found inconsistent with regional tilt-ing of the arc. The occurrence of remagnetization after tilting of the country rocks in several sites by the intrusive batholith also support the clockwise rotations and the low paleolati-tude. In the Late-Eocene sediments, normal and reverse polarity magnetizations, alongside the occurrence of numerous ~10 cm thick siderite-rich layers with stable magnetizations, in-dicate a primary detrital or a very early diagenetic origin for the acquisition of the magneti-zation. The sediments constrain a low inclination after tilt correction, which is coherent with the inferred near-equatorial position from the older Wuntho arc rocks. Based on these re-sults, we suggest that accretion of the West Burma block occurred at near-equatorial lati-tude, and that it subsequently underwent significant clockwise rotation and northward translation during the Cenozoic.

FIRST PALEOMAGNETIC CONSTRAINTS ON THE LATITUDINAL DISPLACEMENT OF THE WEST BURMA BLOCK

Cenozoic collision between India and Eurasia produced the Himalayan-Tibetan orogen, which is commonly considered as the archetypical orogen for continent-continent collision systems. However, there is still no consensus on the amount and mechanism of post-collisional convergence, as well as on the roles of the numerous tectonic terranes comprising the orogen (Jagoutz et al. 2015, 2016; Replumaz et al. 2013, Royden et al. 2008, van Hins-bergen et al. 2011). The West Burma block exhibits a unique geodynamic evolution within this system, influenced by oblique subduction of the Indian plate and significant strike-slip motions along the dextral Sagaing Fault. Furthermore, it is at a key location for paleoenvi-ronmental reconstructions (Cai et al. 2016, Licht et al. 2013). Despite this, robust paleo-magnetic data from the West Burma block is largely absent.Here we report new paleomagnetic, petrological and U-Pb age data to constrain the latitudinal displacement of West Burma. To this end, 45 sites were drilled in the intrusives, extrusives and sediments of the Wuntho arc, Myanmar. Paleomagnetic results were obtained at 30 sites. In addition, 135 paleomagnetic results were obtained from a Late-Eocene mono-clinic sedimentary section in the Chindwin basin, Myanmar.Wuntho arc U-Pb ages cluster in the range 110–90 Ma, indicating a Late-Cretaceous age. Paleomagnetic results from this area show declination values of around 50°–100°, im-plying clockwise rotation of the overall arc dispersed by local-block rotations related to faulting, and inclination values close to zero, corresponding to near-equatorial paleolatitude. Tilt corrections are not available for sites in intrusive rocks. However, the sampling is dis-tributed over a large area (1000 km2) and the results are found inconsistent with regional tilt-ing of the arc. The occurrence of remagnetization after tilting of the country rocks in several sites by the intrusive batholith also support the clockwise rotations and the low paleolati-tude. In the Late-Eocene sediments, normal and reverse polarity magnetizations, alongside the occurrence of numerous ~10 cm thick siderite-rich layers with stable magnetizations, in-dicate a primary detrital or a very early diagenetic origin for the acquisition of the magneti-zation. The sediments constrain a low inclination after tilt correction, which is coherent with the inferred near-equatorial position from the older Wuntho arc rocks. Based on these re-sults, we suggest that accretion of the West Burma block occurred at near-equatorial lati-tude, and that it subsequently underwent significant clockwise rotation and northward translation during the Cenozoic.