Sebastián Oriolo

Timing of deformation in the Sarandí del Yí Shear Zone, Uruguay: implications for the amalgamation of Western Gondwana during the Neoproterozoic Brasiliano–Pan‐African Orogeny

U-Pb and Hf zircon (sensitive high-resolution ion microprobe -SHRIMP- and laser ablation-inductively coupled plasma-mass spectrometry -LA-ICP-MS-), Ar/Ar hornblende and muscovite, and Rb-Sr whole rock-muscovite isochron data from the mylonites of the Sarandi del Yi Shear Zone, Uruguay, were obtained in order to assess the tectonothermal evolution of this crustal-scale structure. Integration of these results with available kinematic, structural, and microstructural data of the shear zone as well as with geochronological data from the adjacent blocks allowed to constrain the onset of deformation along the shear zone at 630–625 Ma during the collision of the Nico Perez Terrane and the Rio de la Plata Craton. The shear zone underwent dextral shearing up to 596 Ma under upper to middle amphibolite facies conditions, which was succeeded by sinistral shearing under lower amphibolite to upper greenschist facies conditions until at least 584 Ma. After emplacement of the Cerro Caperuza granite at 570 Ma, the shear zone underwent only cataclastic deformation between the late Ediacaran and the Cambrian. The Sarandi del Yi Shear Zone is thus related to the syncollisional to postcollisional evolution of the amalgamation of the Rio de la Plata Craton and the Nico Perez Terrane. Furthermore, the obtained data reveal that strain partitioning and localization with time, magmatism emplacement, and fluid circulation are key processes affecting the isotopic systems in mylonitic belts, revealing the complexity in assessing the age of deformation of long-lived shear zones.

Palaeomagnetic data from the Precordillera fold and thrust belt constraining Neogene foreland evolution of the Pampean flat-slab segment (Central Andes, Argentina)

Abstract Available and new palaeomagnetic data reveal transpressional deformation in the Argentine Precordillera fold and thrust belt contemporaneous with Juan Fernández ridge subduction. Localized changes in the orientation of palaeomagnetic directions indicate a vertical axis rotation pattern linked to local, oblique brittle-ductile shear zones that overprint the regional structure. The nearly homogeneous clockwise block rotation pattern from Western-Central Precordillera shows localized rotation nulls along NNW-trending left-lateral oblique belts, revealing that overprinting anticlockwise tectonic rotations could have balanced previous clockwise rotations. Conversely, clockwise rotations in Eastern Precordillera are only localized in the vicinity of these NNW-trending structures, and are controlled by rigid block rotations linked to basement-involved deformation.These results combined with available regional palaeomagnetic data in the forearc would indicate a regional bending of the upper plate margin between 27° S and 33° S that seems to be related to the subduction of the Juan Fernández aseismic ridge.

Basement-involved deformation overprinting thin-skinned deformation in the Pampean flat-slab segment of the southern Central Andes, Argentina

In the southern Central Andes, the Andean foreland was deformed due to Neogene shallowing of the Nazca slab beneath the South America plate. In this 27–33oS Pampean flat-slab segment, the N-trending Argentine Precordillera transpressional fold-and-thrust belt and the Sierras Pampeanas broken foreland developed as a consequence of inward migration of the orogenic front. At 28oS, a NNE-trending westward-dipping, thick Neogene synorogenic sequence is exposed in the Sierra de los Colorados, which shares deformation features of the Precordillera and the Sierras Pampeanas. Integration of new structural and kinematic data and available structural, kinematic, geophysical and palaeomagnetic information allows consideration of the Sierra de los Colorados area as part of the northern sector of the Precordillera during the middle Neogene. At c . 9 Ma, basement block exhumation started with the uplift of the Sierra de Umango-Espinal that was triggered by deformation along the NE-trending Tucuman oblique belt. This stage marked the beginning of compartmentalization of the incipiently deformed Vinchina foreland. Since c . 6.8–6.1 Ma, basement block uplift linked to the Miranda–Chepes and Valle Fertil NNW-trending sinistral transpressional belts, as well as kinking of the Neogene sequence by localized WNW-striking cross-strike structures, resulted in multiple segmentation that produced a complex mosaic of basement-block pieces. The overprint of these regional, basement-involved, oblique, brittle–ductile transpressional and cross-strike megazones could be related to high interplate coupling. Localized mechanical and rheological changes introduced by magmatism favoured this thick-skinned deformation overprint.

Cross-strike structures controlling magmatism emplacement in a flat-slab setting (Precordillera, Central Andes of Argentina)

Abstract Detailed structural, kinematic and geophysical data on the foreland of the Pampean flat-slab segment (Hualilán area, Andean Precordillera of Argentina) have shown that cross-strike structures have had an important role in the evolution of this Andean segment since Miocene times. These structures represent pre-existing crustal fabrics reactivated during the Andean orogeny and could have controlled the emplacement of the Miocene arc-magmatism migrating into foreland due to the flattening of the slab. Likewise, kinematic results obtained for these structures support a similar stress frame to that obtained elsewhere in the Precordillera but showing different motions as a consequence of their high obliquity to the orogen trend. Moreover, they record a reorientation of kinematic axes during Late Miocene–Pliocene times.

Phanerozoic low-temperature evolution of the Uruguayan Shield along the South American passive margin

The crystalline basement of Uruguay was assembled during the Brasiliano Orogeny in the Neoproterozoic Era and was later affected by discrete tectonic activity. A new multi-method low-temperature dataset including (U–Th)/He ages from both zircon and apatite, T–t modelling and K–Ar dating of fine sericite fractions and fault gouge reveal a detailed post-orogenic geological history spanning the Phanerozoic Eon. The juxtaposition of the terranes that compose the area was achieved in the Ediacaran Period, and post-collision was marked by intense exhumation, in which the crystalline basement reached near-surface conditions by the early to mid-Palaeozoic. Regional subsidence promoted sedimentation in the Paraná Basin until the Permian, covering and reheating much of the basement that is at present exposed. Afterwards, deposition and volcanism were mostly confined to its current limits. Regional exhumation of the shield during the Permo-Triassic exposed much of the northern portion of the basement, and the south was further affected by the opening of the South Atlantic Ocean during the Mesozoic. Little exhumation affected the Uruguayan Shield during the Cenozoic, as reflected in its modest topography. The reactivation of inherited Neoproterozoic structures influenced the development of Mesozoic basins and the present-day landscape. Supplementary material: Supplementary data (sample locations, (U-Th)/He data and K-Ar data) are available at https://doi.org/10.6084/m9.figshare.c.3702043

The Nico Pérez Terrane of Uruguay and Southeastern Brazil

The Nico Perez Terrane of Uruguay and southeastern Brazil is characterized by an important component of Archean crustal growth and extensive post-Archean crustal reworking recorded in Paleoproterozoic zircon magmatic crystallization ages in widely distributed granitic orthogneisses. Supracrustal blocks of an older Neoarchean to Siderian sedimentary cover including BIFs, quartzites and marbles are preserved only as minor relics. Additionally, an intraplate Mesoproterozoic record includes anorthosite complexes, metagabbros, amphibolites, felsic volcanic rocks and sediments assumed to correspond to a stable platform cover. Rocks with similar isotopic features occur also as basement inliers and roof pendants in the batholiths of the Dom Feliciano Belt. Two different subterranes are recognized in the Nico Perez Terrane, separated by the north-northeast-trending Cacapava–Sierra de Sosa Shear Zone. The granulite-facies Valentines Rivera and Santa Maria Chico granulitic complexes crop out in the western side of the shear zone and were less reworked during the Neoproterozoic, while the Pavas Block of Uruguay and several basement inliers in the Tijucas Terrane and Pelotas Batholith of Brazil were strongly reworked. Cooling ages, extensive shear zones and granite intrusions document this reworking that was probably facilitated by a thin lithosphere. The Nico Perez Terrane represents a fragment of the Congo Craton separated during the Neoproterozoic.

Provenance of Austroalpine basement metasediments: tightening up Early Palaeozoic connections between peri-Gondwanan domains of central Europe and Northern Africa

New U–Pb and Lu–Hf detrital zircon data together with whole-rock geochemical and Sm–Nd data were obtained for paragneisses of the Austroalpine basement south of the Tauern Window. Geochemically immature metasediments of the Northern–Defereggen–Petzeck (Ötztal–Bundschuh nappe system) and Defereggen (Drauzug–Gurktal nappe system) groups contain zircon age populations which indicate derivation mainly from Pan-African orogens. Younger, generally mature metasediments of the Gailtal Metamorphic Basement (Drauzug–Gurktal nappe system), Thurntaler Phyllite Group (Drauzug–Gurktal nappe system) and Val Visdende Formation (South Alpine Basement) were possibly derived from more distant sources. Their significantly larger abundances of pre-Pan-African zircons record a more advanced stage of downwearing of the Pan-African belts and erosion of older basement when the Austroalpine terrane was part of the Early Palaeozoic Northern Gondwana passive margin. Most zircon age spectra are dominated by Ediacaran sources, with lesser Cryogenian, Tonian and Stenian contributions and subordinate Paleoproterozoic and Neoarchean ages. These age patterns are similar to those recorded by Cambro–Ordovician sedimentary sequences in northeastern Africa between Libya and Jordan, and in some pre-Variscan basement inliers of Europe (e.g. Dinarides–Hellenides, Alboran microplate). Therefore, the most likely sources seem to be in the northeastern Saharan Metacraton and the Northern Arabian–Nubian Shield (Sinai), further supported by whole-rock Sm–Nd and zircon Lu–Hf data.

The Kalahari Craton, Southern Africa: From Archean Crustal Evolution to Gondwana Amalgamation

The Kalahari Craton comprises all Archean to Mesoproterozoic rocks of southern Africa, which are surrounded by Pan-African orogenic belts that resulted from the amalgamation of Gondwana. Progressive crustal growth and accretion of minor crustal blocks is recorded during the Archean and also involved reworking of Hadean crustal remnants, suggesting modern plate tectonics was already operating in the late Neoarchean. The first widespread Paleoproterozoic tectonomagmatic event is recorded by intracontinental magmatism of the Bushveld Complex and the Okwa Terrane, and coeval magmatism in the Limpopo Belt, which separates the Archean Kaapvaal and Zimbabwe cratons. Afterwards, Paleoproterozoic transpression took place along the Limpopo Belt and was contemporaneous with the Magondi Orogeny. Subsequent addition of juvenile Paleoproterozoic crust took place along the western margin of the proto-Kalahari Craton, as recorded by the Rehoboth Basement Inlier. During the Mesoproterozoic, subduction zones were present all around the Archean-Paleoproterozoic proto-Kalahari Craton. The accretion of several microcontinents and island arcs along the southern margin gave rise to the Namaqua-Natal Orogeny. Tonian to Cryogenian intraplate magmatism was finally succeeded by the incorporation of the Kalahari Craton into Gondwana during the protracted Late Neoproterozoic-Early Paleozoic Pan-African Orogeny.

Shear Zones in Brasiliano-Pan-African Belts and Their Role in the Amalgamation and Break-Up of Southwest Gondwana

Crustal-scale shear zones are ubiquitous in most Brasiliano–Pan-African belts of southwestern Gondwana and they resulted from the assembly of the Rio de la Plata, Congo and Kalahari cratons. In the Dom Feliciano Belt, the Sierra Ballena-Dorsal do Cangucu-Major Gercino shear zone system and Sarandi del Yi Shear Zone are the most prominent structures, and they share a common history with shear zones of the Kaoko Belt, such as the Purros and Three Palms Mylonite Zones. The Purros Mylonite Zone, in turn, can be traced further south in the Damara Belt, where it is correlated with the Ogden Mylonite Zone. All these orogen-parallel shear zones underwent ductile deformation mostly at c. 630–580 Ma. However, further shearing is recorded in both the Kaoko and Dom Feliciano Belts at c. 550 Ma, thus being coeval with shearing along the Colenso Fault of the Saldania Belt. Though the Brasiliano–Pan-African Orogeny led to a relative stabilization of the South American and African continental crust by the early Paleozoic, shear zones were subsequently reactivated under brittle conditions during the Phanerozoic. These fault zones were particularly active during the opening of the South Atlantic Ocean in the Cretaceous, controlling magmatism emplacement, basin development and crustal exhumation. Shear zones thus played a major role not only during the Neoproterozoic assembly but also during the subsequent break-up of Gondwana.