Does the British Isles Paleocene dike swarm reflect the former location of the Iceland hotspot?

ABSTRACT The original location and tectonic setting of the prominent Paleocene dike swarm in the British Isles are reconstructed for a “tight fit” of the North Atlantic region prior to any Cenozoic opening of the ocean basin between Greenland and Europe. The present-day northwest-southeast–oriented swarm originally trended toward southern Greenland and the locations of magmatic rocks of comparable age along the eastern and western margins of Greenland and approximately the position of the Iceland hotspot at 70–60 Ma in a “fixed hotspot” model. This raises the possibility that the northeast-southwest–oriented extensional stress field in which the dikes and associated central igneous complexes were emplaced may have been generated by impingement on the base of the lithosphere by a rising plume beneath present-day West Greenland. It is speculated, on the basis of seismic tomography and three-dimensional modeling, that the Paleocene igneous activity in the British Isles may have resulted from flow of a hot “finger” of upper mantle outward from the plume, perhaps controlled by preexisting lithospheric structures and the distant location of a second Paleocene volcanic province in central Europe.

Provenance of Jurassic Sediments from Yuqia Sandstone-Type Uranium Deposits in the Northern Margin of Qaidam Basin, China and Its Implications for Uranium Mineralization

The Yuqia uranium deposit is a newly discovered sandstone-type uranium deposit in the northern margin of the Qaidam Basin. Concerning the sources of sediment in the basin, most scholars have focused on the study of Cenozoic sediment sources in the northern part of the basin, rather than on the study of Jurassic provenance and its implications for uranium mineralization. In this paper, the Jurassic sandstones in the area were selected for petrography, petrogeochemistry and electron microprobe analysis (EPMA), and the significance of sedimentary provenance and uranium metallogeny were further discussed, based on the previous data of detrital zircon chronology. It is reported here for the first time that coffinite and pitchblende are the main contributors. Independent uranium minerals in the region mainly occur in strawberry pyrite, xenotime, and margins or of quartz. The rocks in the source area, Jurassic sandstones, were mainly formed under the tectonic setting of the active continental margin; however, the sediments are mainly derived from the Indosinian and Paleozoic granitoids exposed in the northern margin of the Qaidam Basin. The uranium-rich granites in the source area led to the preconcentration of uranium in the sandstone of the target layer, and the uranium was brought into the ore-bearing target layer through uranium-bearing oxygenated water. The reduction reaction occurred under the action of reducing matter, and finally, the U6+ was reduced to U4+ in the formation of coffinite and pitchblende.

The Influence of Channel Planform and Slope Topography on Turbidity Current Overbank Processes: The Example of the Acquarone Fan (Southeastern Tyrrhenian Sea)

Overbank deposits provide a potentially valuable record of flows that have passed through a submarine channel. The architecture of overbank deposits has generally assumed to relate to autogenic processes related to channel construction. In previous models, which are largely based on passive margins, the distribution and geometry of these deposits is relatively simple, and hence generally predictable. Here, we show how the interaction of different flow types with the complex morphology on a highly-tectonically modified margin can profoundly affect overbank depositional processes, and hence also the resultant deposit geometry and architecture. Our case study is the Acquarone Fan, located in the intraslope Gioia Basin in the south-eastern Tyrrhenian Sea, whose topography is mainly controlled by the presence of the Acquarone structural ridge, which results in the confinement of the left south-west side of the channel-levee system. The research is carried out through analysis of multibeam bathymetric and high-resolution Chirp sub-bottom profiler data. Seven depositional units (Units I-VII) record the recent depositional history of the fan; their thickness has been mapped and their parent flow-types have been interpreted through their seismic response. According to unit thickness maps, two main patterns of deposition are recognized in the overbank area. Their depocenters coincide with different extensive sediment wave fields developed in specific tracts of the right levee and in the frontal splay area. We show that the location of the depocenters varies in time according to the prevalent flow-type and by its interaction with the surrounding seafloor topography and channel planform. We interpret that the lateral confinement of the channel by the structural high generates episodic rebound of the overspilling flow and the inversion of the channel asymmetry. The vertical stratification of the flow strongly influences the overbank deposition where the channel planform has a non-linear shape such as bends and knick-points. In particular, the vertical stratification influences the hydraulic jump size that conditions the amount of overspill and thus the location of overbank depocenters. This study highlights that variations in the sediment distribution and composition on the overbank can be related to the way different flows interact with tectonic setting.

Geochemical Markers as a Tool for the Characterization of a Multi-Layer Urban Aquifer: The Case Study of Como (Northern Italy)

The analysis of geochemical markers is a known valid tool to explore the water sources and understand the main factors affecting natural water quality, which are known issues of interest in environmental science. This study reports the application of geochemical markers to characterize and understand the recharge areas of the multi-layer urban aquifer of Como city (northern Italy). This area presents a perfect case study to test geochemical markers: The hydrogeological setting is affected by a layered karst and fractured aquifer in bedrock, a phreatic aquifer hosted in Holocene sediments and connected with a large freshwater body (Lake Como); the aquifers recharge areas and the water geochemistry are unknown; the possible effect of the tectonic setting on water flow was overlooked. In total, 37 water samples were collected including water from two stacked aquifers and surface water to characterize hydrochemical features. Moreover, six sediment samples in the recent palustrine deposits of the Como subsurface were collected from cores and analyzed to understand the main geochemistry and mineralogy of the hosting material. The chemical analyses of water allow to observe a remarkable difference between the shallow and deep aquifers of the study area, highlighting different recharge areas, as well as a different permanence time in the aquifers. The sediment geochemistry, moreover, confirms the differences in trace elements derived from sediment-water interaction in the aquifers. Finally, an anomalous concentration of As in the Como deep aquifer was observed, suggesting the need of more detailed analyses to understand the origin of this element in water. This study confirms the potentials of geochemical markers to characterize main factors affecting natural water quality, as well as a tool for the reconstruction of recharge areas.

Deciphering the Late Paleozoic–Cenozoic Tectonic History of the Inner Central Andes Forearc: An Update From the Salar de Punta Negra Basin of Northern Chile

We integrated new and existing geological, geochronological, thermochronological, and two-dimensional (2D) seismic data from the Salar de Punta Negra Basin to define the Late Paleozoic–Cenozoic tectonic evolution of the inner Andean forearc of northern Chile more precisely. Our results indicate that this region experienced early Late Paleozoic–Mesozoic crustal extension, creating several basement half-graben structures bounded by east- and west-dipping master faults. These extensional basins were filled by Upper Permian to Jurassic volcanic and sedimentary (continental and marine) syn-rift deposits. The genesis of these structures is related to the early breakup of the western Gondwana continent and the development of the large Tarapacá Basin in northern Chile and southern Perú. Subsequently, Late Cretaceous to Paleocene contraction occurred, which led to the tectonic inversion of the pre-existing rift system and the uplift of the Paleozoic–Mesozoic syn-rift deposits. Seismic data show that Upper Cretaceous and Paleocene synorogenic deposits accumulated along and over inversion anticlines, recording the initial contraction and marking the change from an extensional to a contractional tectonic setting. During the final episodes of basin inversion, crustal shortening was accommodated by the Eocene to recent basement reverse faulting accompanied by the rapid exhumation of basement pre-rift blocks, which served as the principal sources for the sediments that filled the pre-Andean basins during the Late Cenozoic. Finally, the exhumed basement pre-rift blocks and the reverse faults compartmentalized the contractional intermontane basins, which constitute the main low topographic relief of the inner forearc of northern Chile.

A Detailed Earthquake Catalog for Banda Arc–Australian Plate Collision Zone Using Machine-Learning Phase Picker and an Automated Workflow

The tectonic setting of Timor–Leste and Eastern Indonesia comprises of a complex transition from oceanic lithosphere subduction to arc-continental collision. To better understand the deformation and convergent-zone structure of the region, we derive a new catalog of earthquake hypocenters and magnitudes from a temporary deployment of five years of continuous seismic data using an automated processing procedure. This includes a machine-learning phase picker, EQTransformer, and a sequential earthquake association and location workflow. We detect and locate ∼19,000 events during 2014–2018, which demonstrates that it is possible to characterize earthquake sequences from raw seismic data using a well-trained machine-learning picker for a complex convergent plate setting. This study provides the most complete catalog available for the region for the duration of the temporary deployment, which includes a complex pattern of crustal events across the collision zone and into the back-arc, as well as abundant deep slab seismicity.