Once in a summer: Fall history of the JaH 073 strewn field, Sultanate of Oman

Modeling of a prehistoric fall can be successful if a strewn field is very well documented and coordinates, masses, and shapes of all individual stones are recorded. In combination with meteoroid mass and wind model constraints, a detailed scenario of the atmospheric passage is obtained for the ~20 9 6 km-sized JaH 073 L6 strewn field in Oman. The wide mass ranges from 52.2 kg to <1 g together with the large number of ~3500 stones offer the statistical basis to reconstruct the trajectory and the fragmentation sequence. The size of the meteoroid, constrained by noble gas analyses, corresponds to an initial mass of about 12 t at atmospheric entry using an L-chondrite bulk density of 3400– 3500 kg m-3. Assuming typical ablation behavior, these data are compatible with an entry velocity of 20±3 km s-1. The best model fit is achieved for a serial fragmentation scenario starting at an altitude of ~34 km and showing a main fragmentation at 26 km. A resolved event seems to have occurred at 22 km, followed by a more diffuse fragmentation at 19 km. The vertical trajectory angle is calculated at 43 ± 2° and the azimuth at 329 ± 1°. The position of numerous outlying meteorites in the strewn field can only be reproduced by repeated fragmentation with cumulated transverse velocities from explosive events. The wind model adopted from modern data fits surprisingly well and indicates summer monsoon with strong easterly winds during the fall event, consistent with paleoclimatic data

A new perspective on the significance of the Ranotsara shear zone in Madagascar

The Ranotsara shear zone in Madagascar has been considered in previous studies to be a >350-km-long, intracrustal strike-slip shear zone of Precambrian/Cambrian age. Because of its oblique strike to the east and west coast of Madagascar, the Ranotsara shear zone has been correlated with shear zones in southern India and eastern Africa in Gondwana reconstructions. Our assessment using remote sensing data and field-based investigations, however, reveals that what previously has been interpreted as the Ranotsara shear zone is in fact a composite structure with a ductile deflection zone confined to its central segment and prominent NW-SE trending brittle faulting along most of its length. We therefore prefer the more neutral term "Ranotsara Zone”. Lithologies, tectonic foliations, and axial trace trajectories of major folds can be followed from south to north across most of the Ranotsara Zone and show only a marked deflection along its central segment. The ductile deflection zone is interpreted as a result of E-W indentation of the Antananarivo Block into the less rigid, predominantly metasedimentary rocks of the Southwestern Madagascar Block during a late phase of the Neoproterozoic/Cambrian East African Orogeny (c. 550-520Ma). The Ranotsara Zone shows significant NW-SE striking brittle faulting that reactivates part of the NW-SE striking ductile structures in the flexure zone, but also extends along strike toward the NW and toward the SE. Brittle reactivation of ductile structures along the central segment of the Ranotsara Zone, confirmed by apatite-fission track results, may have led to the formation of a shallow Neogene basin underlying the Ranotsara plain. The present-day drainage pattern suggests on-going normal fault activity along the central segment. The Ranotsara Zone is not a megascale intracrustal strike-slip shear zone that crosscuts the entire basement of southern Madagascar. It can therefore not be used as a piercing point in Gondwana reconstruction

The role of the Ranotsara Zone in southern Madagascar for Gondwana correlations

The Precambrian basement of southern Madagascar was reworked at high-grade metamorphic conditions during the East African Orogen (EAO of Stern, 1994) that formed during assembly of Gondwana in late Neoproterozoic/early Paleozoic times. At the end of the EAO, Madagascar is generally thought to be sandwiched between southern India and eastern Africa. Constraints on its paleoposition are often inferred from similarities in structural features on now dispersed continental fragments, in particular high-strain zones. Major zones with (sub)vertical foliation planes can be traced over hundreds of kilometres in southern Madagascar and have been interpreted as major vertical ductile shear zones (e.g. Windley et al. 1994; Martelat, 1998). The NW–SE trending Ranotsara Zone (dashed rectangle in Fig. 1) is regarded as an intracrustal mega strike-slip shear zone with a sinistral sense of shear that formed at the end of the Proterozoic (e.g. Nicollet, 1990; de Wit et al., 2001). A large number of studies have used the Ranotsara Zone to propose Gondwana reconstructions. The Ranotsara Zone has been correlated with various ductile shear zones in southern India, e.g. with the Bhavani Shear Zone or the Moyar Shear Zone (Katz & Premoli, 1979), the Palghat-Cauvery Shear Zone (de Wit et al., 1995), the Karur-Kamban- Painavum-Trichur Shear Zone (de Wit et al., 2001; Ghosh et al. 2004) or with the Achankovil Shear Zone (Windley et al., 1994; Martelat, 1998). Within Madagascar, the Ranotsara Zone has been correlated along strike with the more N–S trending Bongolava Zone in central-western Madagascar (Hottin 1976), and the Bongolava- Ranotsara Zone has been further traced into the Surma Shear Zone (Windley et al. 1994) and its along-strike continuation, the Aswa Shear Zone in eastern Africa (Müller 2000). Chetty (2003) suggested that the Ranotsara Zone is not only a mega shear zone, but also a terrane boundary separating a region with Archean crust to the north from a region with Neoproterozoic crust to the south. Our remote sensing and field studies of southern Madagascar indicate that the Ranotsara Zone is neither a major terrane boundary nor an intracrustal mega strike-slip shear zone and therefore can not be used as a ‘piercing point’ in Gondwana reconstructions...conferenc

Correction to: Ion microprobe dating of fissure monazite in the Western Alps: insights from the Argentera Massif and the Piemontais and Briançonnais Zones

Following publication of the original article (Ricchi et al. 2020), multiple typesetting errors were identified in the article. The updated sections/sentences are given below and the changes have been highlighted in bold typeface

Late Quaternary history of the Vakinankaratra volcanic field (central Madagascar): insights from luminescence dating of phreatomagmatic eruption deposits

The Quaternary Vakinankaratra volcanic field in the central Madagascar highlands consists of scoria cones, lava flows, tuff rings, and maars. These volcanic landforms are the result of processes triggered by intracontinental rifting and overlie Precambrian basement or Neogene volcanic rocks. Infrared-stimulated luminescence (IRSL) dating was applied to 13 samples taken from phreatomagmatic eruption deposits in the Antsirabe-Betafo region with the aim of constraining the chronology of the volcanic activity. Establishing such a chronology is important for evaluating volcanic hazards in this densely populated area. Stratigraphic correlations of eruption deposits and IRSL ages suggest at least five phreatomagmatic eruption events in Late Pleistocene times. In the Lake Andraikiba region, two such eruption layers can be clearly distinguished. The older one yields ages between 109 ± 15 and 90 ± 11ka and is possibly related to an eruption at the Amboniloha volcanic complex to the north. The younger one gives ages between 58 ± 4 and 47 ± 7ka and is clearly related to the phreatomagmatic eruption that formed Lake Andraikiba. IRSL ages of a similar eruption deposit directly overlying basement laterite in the vicinity of the Fizinana and Ampasamihaiky volcanic complexes yield coherent ages of 68 ± 7 and 65 ± 8ka. These ages provide the upper age limit for the subsequently developed Iavoko, Antsifotra, and Fizinana scoria cones and their associated lava flows. Two phreatomagmatic deposits, identified near Lake Tritrivakely, yield the youngest IRSL ages in the region, with respective ages of 32 ± 3 and 19 ± 2ka. The reported K-feldspar IRSL ages are the first recorded numerical ages of phreatomagmatic eruption deposits in Madagascar, and our results confirm the huge potential of this dating approach for reconstructing the volcanic activity of Late Pleistocene to Holocene volcanic provinces