Fernando Poblete

The curved Magallanes fold and thrust belt: Tectonic insights from a paleomagnetic and anisotropy of magnetic susceptibility study

The Magallanes fold and thrust belt (FTB) presents a large-scale curvature from N-S oriented structures north of 52°S to nearly E-W in Tierra del Fuego Island. We present a paleomagnetic and anisotropy of magnetic susceptibility (AMS) study from 85 sites sampled in Cretaceous to Miocene marine sediments. Magnetic susceptibility is lower than 0.0005 SI for 76 sites and mainly controlled by paramagnetic minerals. AMS results indicate that the sedimentary fabric is preserved in the undeformed areas of Tierra del Fuego and the more external thrust sheets units, where an incipient lineation due to layer parallel shortening is recorded. Prolate AMS ellipsoids, indicating a significant tectonic imprint in the AMS fabric, are observed in the internal units of the belt. AMS results show a good correlation between the orientation of the magnetic lineation and the fold axes. However, in Peninsula Brunswick, the AMS lineations are at ~20° counterclockwise to the strike of the fold axes. Pretectonic stable characteristic remanent magnetizations (ChRM) were determined in seven sites. A counterclockwise rotation (21.2° ± 9.2°) is documented by ChRM data from four sites near the hinge of the belt in Peninsula Brunswick and near Canal Whiteside while there is no evidence of rotation near the nearly E-W oriented Vicuna thrust within Tierra del Fuego. The curved shape of the Cenozoic Magallanes FTB is not related to vertical axis rotation, and thus, the Magallanes FTB can be considered as a primary arc.

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.

Provenance and age constraints of Paleozoic siliciclastic rocks from the Ellsworth Mountains in West Antarctica, as determined by detrital zircon geochronology

This work was supported by the Anillo Antartico nACT-105 and Instituto Antartico Chileno (INACH) n161 projects. We greatly appreciate the help of our nskilled mountaineer Bernabe Lopez and the Twin nOtters crew of the Grupo 6, Chilean Air Force. Field nwork campaigns to the Ellsworth Mountains were nmade thanks to INACH, with support from Antarctic nLogistics & Expeditions (ALE). Sample EAM1001B, nnotes, and photos were provided by the Chilean Antarctic Expedition 2002–2003, supported by VERTICAL SA, Chile. We also thank Juan Pablo Lacassie, nMauricio Duran, and Cristian Vasquez for their field nassistance; Mauricio Calderon for sample descriptions; Shane Paxon for sample preparation; Thomas nHaber, David Elliot, and John Goodge for discussion; nand Matthew Callaghan for checking the English ngrammar.

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.