In the southernmost part of the Southern Brasília Orogen (SE Brazil), regional ultra-high temperature metamorphism has been reported mainly in garnet-bearing granulites and (garnet)-orthopyroxene-bearing leucosomes at the base of the Socorro-Guaxupé Nappe, but detailed petrological studies focusing on garnet-absent mafic granulites at the upper crustal levels of this terrain are still scarce and the tectonic setting in which these extreme thermal conditions were attained are still under debate. In this study, we focus on reconstructing the metamorphic P–T paths of (garnet-absent) mafic granulites at intermediate crustal levels of the Socorro-Guaxupé Nappe using petrography, compositional maps, phase diagram modelling and Ti-in-quartz geothermometry. Our results indicate that the mafic granulites record peak P–T conditions of ~ 970 °C and 9.0–9.5 kbar (thermobaric ratio of ~ 1078 °C GPa−1) and their retrograde paths are characterized by both decompression and cooling, down to ~ 840–850 °C and 5.5–6.0 kbar. Our peak temperature results are consistent with other estimates for granulites and orthopyroxene-bearing leucosomes throughout the nappe, but the (peak) pressure results indicate points to its decrease towards the structural top. The similarity of retrograde paths throughout the Guaxupé Nappe suggests similar exhumation dynamics at different crustal depths. Furthermore, a prominent decompression vector along with the thermal peak, indicated by intergrowth microtextures and chemical zoning, coupled with compositional isopleths, suggests regional metamorphism during the continental collision between the Paranapanema and the São Francisco paleoplates.
The correct estimation of seismic hazards is a touchstone of seismic risk assessments. However, there is no quantitative or standard methodology to include the impacts of geological (i.e., seismo-tectonic) features of active faults or fault zones, and current classification schemes are not useful in hazard evaluations. Therefore, an attempt has been made to develop a methodology that integrates seismo-tectonic parameters of active faults to better inform urban and regional planning decisions. Fault rating system (FRS) provides a comparative review of faults/fault zones using a rating-based approach. In this approach, seven seismo-tectonic parameters are used to classify the fault/fault zone. Each of the seven parameters is assigned a value corresponding to the seismo-tectonic characteristics. The sum of the seven seismo-tectonic parameters is the fault index (FI) value, which lies in the range 0–100. A total of 64 important faults/fault zones were statistically analyzed to determine the best correlations with FI and moment magnitude (Mw) and peak ground acceleration (PGA). It was found that the FI values provide strong correlations with maximum Mw and PGA. It is proposed urban and regional planners use FRS to ensure a consistent approach in characterizing key aspects of active faults in earthquake-prone regions and in estimating ground motion parameters.
The September 26, 2019 Silivri earthquake (MW = 5.8) occurred along the North Anatolian Fault beneath the Marmara Sea and its epicenter was in an identified seismic gap. Coseismic stress calculations demonstrate that the 1999 İzmit earthquake (MW = 7.4) caused stress increase from 0.057 to 0.114 bars at its hypocenter, depending on the various reported rupture parameters. In addition, over 20 years following the 1999 earthquake, and constituting the main difference from previous studies, viscoelastic postseismic stress computations indicate stress increase from 0.081 to 0.135 bars at the hypocenter. In spite of the positive stress transfer, the 2019 earthquake occurred long after the end of the computed aftershock time span (~ 16 years) of the 1999 earthquake. Plots of the seismicity around selected points within the gap also show that the background seismicity level following the 1999 earthquake was reached in 2003. Therefore, it is suggested that the 2019 earthquake was not an aftershock but rather an independent event, and its occurrence was hastened about 4 years due to stress loading. Further analysis of the seismicity between 1978 and 2020 indicates that the b value increased from a range of 1.0–1.1 to 1.6–1.8 till 2002, then progressively decreased to 0.9–1.0, which is consistent with positive stress transfer. The stress increase ranging from 0.19 to 2.52 bars on the segments within the gap brought forward their seismic cycles about 33 and 2 years from east to west, respectively. These additional clock advances in the seismic cycles due to stress load urgently require risk mitigating actions.
Frequent magmatism was a major event causing the mass extinction across the Permian/Triassic Boundary. In the current study, we determined magmatism characteristics from the Pingdingshan section at the Permian/Triassic Boundary using conodont-based geochemical proxies at a high-resolution scale (~ 50 kyr). Integrated trace elements (Mn, Sr, Rb, and Th) and stable/radioactive isotopes (δ18O, δ13C, and 87Sr/86Sr) revealed that conodonts provided an ideal proxy for chemostratigraphic signatures of ancient seawater. The conodont-based, high-resolution 87Sr/86Sr isotopes from the studied interval (250.50–252.00 Ma) displayed three decreasing cycles upwardly against a long-term increasing background, reflecting three episodes of magmatism. As a contrast, the conodont-based, high-resolution δ18O isotopes from this interval exhibited no episodic pattern, indicating that the δ18O isotopes of conodonts were limitedly influenced by marine magmatism. The high-resolution δ13C values of micrites displayed a pattern consistent with the trend of long-term background, revealing that the δ13C signatures of episodic magmatism were not inherited by micrites. The magmatism associated with the EPME event exerted great influences upon the Chihsian source rocks by introducing massive heat through hydrothermal fluids and causing pervasive oceanic anoxia.
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