Volcano distribution and tectonics: A planetoidic perspective

ABSTRACT Volcanic activity is ultimately controlled by processes that take place many kilometers beneath the surface of a planet. The deeper processes are unlikely to reach the surface without some degree of modification at shallower levels. Nevertheless, traces of those deeper processes may still be found when examining the final products at the surface. In this work, it is shown that it is possible to gain insights concerning the integrated contribution of deep structures through the study of the spatial distribution of volcanic vents at the surface. The method here described relies on the systematic use of increasing smoothing factors in Gaussian kernel estimations. The sequences of probability density functions thus generated are equivalent to images obtained with an increasing wavelength, which therefore have the power to penetrate deeper below the surface. Although the resolution of this method is much smaller than the resolution provided by seismic or other geophysical surveys, it has the advantages of ease of implementation, extremely low cost, and remote application. Thus, the reported method has great value as a first-order exploration tool to investigate the deep structure of a planet, and it can make important contributions to our understanding of the volcano-tectonic relationship, not only on Earth, but also across the various bodies of the solar system where volcanic activity has been documented.

The 1871 Lāna‘i Earthquake in the Hawaiian Islands

The historic Lāna‘i earthquake of 1871 is reappraised based on new science. This 1871 event occurred between O‘ahu and Maui, where currently 80% of the State of Hawai‘i population resides. I focus on expanding the considered modified Mercalli intensities (MMIs) applied in prior seismic hazard analyses and the application of an earthquake MMI–magnitude, duration–magnitude, and fault-scaling relations. Compared with the MMI–distance trend of the 2019 Mw 7.1 Ridgecrest, California, earthquake, the Lāna‘i event appears larger. The 1871 earthquake and other M≥4.9 earthquakes (measured instrumentally) trend along the Moloka‘i fracture zone (MFZ), suggesting a tectonic relationship. The MFZ near Hawai‘i is the boundary between ∼85 and ∼100 Ma age lithosphere. Consistent with the preponderance of available evidence and new science, I propose a hypothetical, revised Mw∼7.5 earthquake model of the 1871 Lāna‘i earthquake associated with the MFZ. This seismic event may credibly be considered as the third largest earthquake statewide in Hawai‘i’s history and the largest historic earthquake northwest of the Big Island of Hawai‘i.

A CONTRIBUTION TO THE GEOLOGICAL STRUCTURE OF CHIOS ISLAND, EASTERN AEGEAN SEA

Τhe island of Chios, in the eastern Aegean Sea, is of great geological interest due to the outcrops of the oldest Paleozoic rocks of the Hellenides. Three main geological units of Paleozoic and Mesozoic age dominate, that have a tectonic relationship: the Autochthonous unit is overthrusted by the Parautochthonous unit, which in turn is overthrusted by the Allochthonous unit. In this work, new geological and tectonic data concerning the Autochthonous unit and especially its part of the Paleozoic - Mesozoic transition are presented and evaluated. Due to the rare outcrops of this transition and the lack of sufficient palaeontological data, there are various and contradictory opinions concerning its normal or discordant character. Based on our field data and lithostratigraphic correlations, we can draw the following main results: Considering the Autochthonous unit, the transition of the Paleozoic formations to the Mesozoic ones is characterized by an angular unconformity as well as by a basal conglomerate. In some places there is a tectonic contact between them, this of a thrust fault. The Lower Triassic formations of the Parautochthonous unit belong to the Autochthonous unit, since they present similar palaeogeographic conditions. Moreover, the presence of the “Hallstatt” limestones in the Autochthonous unit can be explained by their local deposition in lenticular form.

U–Pb zircon ages and field relationships of Archean basement and Proterozoic intrusions, south-central Wopmay Orogen, NWT: implications for tectonic assignments

New field and U–Pb zircon data from the south-central Wopmay Orogen (south of 65°N) establish crystallization ages of basement and plutonic phases and bring to the forefront questions on the architecture of the orogen. The complex and extensive >3300–2575 Ma basement domain was derived from the adjacent Slave craton. Paleoproterozoic supracrustal rocks, dominated by an extensive pelitic–psammitic sequence with basal arenite, conglomerate, and carbonate, unconformably overlie this basement. Pre- to post-kinematic Paleoproterozoic plutons intrude both basement and overlying strata and crystallized at 1877, 1867–1862, and 1858–1850 Ma. The first pulse of plutonism constrains an early generation of metamorphism and deformation to younger than 1877 Ma and is corroborated by metamorphic zircon in Archean basement. The interval from 1867 to 1862 Ma brackets ductile deformation in granite and granodiorite intrusions; although corresponding structure in the metasedimentary strata is equivocal. Post-kinematic ca. 1858–1850 Ma plutons were coeval with localized high-grade metamorphism and concomitant recrystallization of Archean basement. The absence of older (>1880 Ma) phases of the Hepburn intrusive suite in south-central Wopmay Orogen demonstrates a previously unrecognized north–south plutonic dichotomy and that the historical assignment of Calderian metamorphism to ca. 1885 Ma may be too old. The designation of part of the area to a klippe of Hottah terrane is not compatible with field and zircon age data which shows that basement and the overlying strata can be tied directly to the Slave craton. The study raises questions regarding the tectonic relationship between the Hottah terrane and Slave craton.

The Glória quartz-monzodiorite: isotopic and chemical evidence of arc-related magmatism in the central part of the Paleoproterozoic Mineiro belt, Minas Gerais State, Brazil

The Glória quartz-monzodiorite, one of the mafic plutons of the Paleoproterozoic Mineiro belt, is intrusive into banded gneisses, amphibolites, schists and phyllites of the Rio das Mortes greenstone belt, in the southern portion of the São Francisco Craton, State of Minas Gerais, Brazil. The Glória quartz-monzodiorite yields a SHRIMP U-Pb zircon age of 2188 ± 29 Ma, suggesting a tectonic relationship with the pre-collisional phase of the Mineiro belt. According to the Nd isotopic evidence (epsilonNd(T) = -3.4; T DM = 2.68 Ga) the original magmas was formed by a mixture among Archean crustal material and Paleoproterozoic juvenile magma. The Glória quartz-monzodiorite shows metaluminous and calc-alkaline tendency with intermediate K content, comparable to that of volcanic-arc rocks. The primary mineralogical assemblage was partly modified by metamorphism, dated between 2131-2121 Ma in nearby coeval plutons. Such metamorphism is significantly older than the reported metamorphic episodes of the Mineiro belt in the Quadrilátero Ferrífero region (2059-2041 Ma) in the eastern portion of the study area. This evidence, together with chemical and isotopic data from other mafic and felsic plutons coeval with the Glória quartz-monzodiorite, indicate a tectonic and magmatic migration within the Mineiro belt from west to east.

The 1590-1520 Ma Cachoeirinha magmatic arc and its tectonic implications for the Mesoproterozoic SW Amazonian craton crustal evolution

Isotopic and chemical data of rocks from the Cachoeirinha suite provide new insights on the Proterozoic evolution of the Rio Negro/Juruena Province in SW Amazonian craton. Six U-Pb and Sm-Nd analyses in granitoid rocks of the Cachoeirinha suite yielded ages of 1587-1522 Ma and T DM model ages of 1.88-1.75 Ga (EpsilonNd values of -0.8 to +1.0). In addition, three post-tectonic plutonic rocks yielded U-Pb ages from 1485-1389 Ma (T DM of 1.77-1.74 Ga and EpsilonNd values from -1.3 to +1.7). Variations in major and trace elements of the Cachoeirinha suite rocks indicate fractional crystallization process and magmatic arc geologic setting. These results suggest the following interpretations: (1) The interval of 1590-1520 Ma represents an important magmatic activity in SW Amazonian craton. (2) T DM and arc-related chemical affinity supportthe hypothesis that the rocks are genetically associated with an east-dipping subduction zone under the older (1.79-1.74 Ga) continental margin. (3) The 1590-1520 Ma age of intrusive rocks adjacent to an older crust represents similar geological framework along the southern margin of Baltica, corroborating the hypothesis of tectonic relationship at that time.