Alpine subduction zone metamorphism in the Paleozoic successions of the Monti Romani (Northern Apennines, Italy)

The hinterland of the Cenozoic Northern Apennines fold-and-thrust belt exposes the metamorphic roots of the chain, vestiges of the subduction-related tectono-metamorphic evolution that led to the buildup of the Alpine orogeny in the Mediterranean region. Like in other peri-Mediterranean belts, the tectono-metamorphic evolution of the Paleozoic continental basement in the Apennines is still poorly constrained, hampering the full understanding of their Alpine orogenic evolution. We report the first comprehensive tectono-metamorphic study of the low-grade metasedimentary (metapsammite/metapelite) succession of the Monti Romani Complex (MRC) that formed after Paleozoic protoliths and constitutes the southernmost exposure of the metamorphic domain of the Northern Apennines. By integrating fieldwork with microstructural studies, Raman spectroscopy on carbonaceous material and thermodynamic modelling, we show that the MRC preserves a D1/M1 Alpine tectono-metamorphic evolution developed under HP-LT conditions (~ 1.0-1.1 GPa at T ~400 °C) during a non-coaxial, top-to-the-NE, crustal shortening regime. Evidence for HP-LT metamorphism is generally cryptic within the MRC, dominated by graphite-bearing assemblages with the infrequent blastesis of muscovite ± chlorite ± chloritoid ± paragonite parageneses, equilibrated under cold paleo-geothermal conditions (~ 10 °C/km). Results of this study allow extending to the MRC the signature of subduction zone metamorphism already documented in the hinterland of the Apennine orogen, providing further evidence of the syn-orogenic ductile exhumation of the HP units in the Apennine belt. Finally, we discuss the possible role of fluid-mediated changes in the reactive bulk rock composition on mineral blastesis during progress of regional deformation and metamorphism at low-grade conditions.

Oblique convergence causes both thrust and strike-slip ruptures during the 2021 M 7.2 Haiti earthquake

A devastating magnitude 7.2 earthquake struck Southern Haiti on 14 August 2021. The earthquake caused severe damages and over 2000 casualties. Resolving the earthquake rupture process can provide critical insights into hazard mitigation. Here we use integrated seismological analyses to obtain the rupture history of the 2021 earthquake. We find the earthquake first broke a blind thrust fault and then jumped to a disconnected strike-slip fault. Neither of the fault configurations aligns with the left-lateral tectonic boundary between the Caribbean and North American plates. The complex multi-fault rupture may result from the oblique plate convergence in the region that the initial thrust rupture is due to the boundary-normal compression and the following strike-slip faulting originates from the Gonâve microplate block movement, orienting towards the SW-NE direction. The complex rupture development of the earthquake suggests that the regional deformation is accommodated by a network of segmented faults with diverse faulting conditions.

Nonrigid Bookshelf Kinematics of Northeastern Tibet: Constrains from Fault Slip Rates around the Qinghai Lake and Chaka-Gonghe Basins

The Elashan fault (ELSF) and Qinghainanshan fault (QHNF), two major faults developed around the Qinghai Lake and Chaka-Gonghe basins, are of great importance for investigating the deformation model of the internal northeastern Tibetan Plateau. However, their late Pleistocene slip rates remain poorly constrained. In this study, we combine high-resolution topography acquired from unmanned aerial vehicles (UAV) and geomorphological dating to calculate the slip rates of the two faults. We visited the central ELSF and western QHNF and measured displaced terraces and stream channels. We collected 10Be samples on the surface of terraces to constrain the abandonment ages. The dextral slip rate of the central segment of the Elashan fault is estimated to be 2.6±1.2 mm/yr. The uplift rates since the late Pleistocene of the Elashan and Qinghainanshan faults are 0.4±0.04 mm/yr and 0.2±0.03 mm/yr, respectively. Comparing the geological rates with the newly published global positioning system (GPS) rates, we find that the slip rates of the major strike-slip faults around the Qinghai Lake and Chaka-Gonghe basins are approximately consistent from the late Pleistocene to the present day. The overall NE shortening rates by summing up the geological slip rates on major faults between the East Kunlun and Haiyuan faults are ~3.4 mm/yr, smaller than the geodetic shortening rates (~4.9 to 6.4 mm/yr), indicating that distributed deformation plays an important role in accommodating the regional deformation. By analyzing the geometrical and kinematic characteristics of the major faults surrounding the basins, we suggest that the kinematic deformation of the internal northeastern Tibet is a nonrigid bookshelf model that consists of counterclockwise rotation (~0.8° Myr-1) and distributed thrusting.

Soft sediment deformation features in Dauki Fault region: Evidence of paleoearthquakes, Shillong Plateau, NE India

The northeastern region (NER) of India has a number of complex regional geological structures, out of which the Dauki fault (DF) is a prominent one. The E-W trending reverse DF, which is referred to go through the southern margin of Shillong Plateau (SP), have played major role in the regional deformation of the adjoining areas and was believed to be active during the Late Quaternary time. Previous paleoseismological studies conducted on the eastern and western part of the DF, Bangladesh, revealed that the fault ruptured in AD 849–920 and AD 1548 respectively. However there were no studies on the DF from southern side of the SP, India. For the first time, from Indian side, soft sediment deformation structures (SSDS) are reported from five trenches in and around the DF zone, SP. Close to the Dauki village, five trenches in the eastern part of the DF, SP, show presence of micro faulting, sand dykes, disturbed strata, and water escape structures. The detailed investigation of SSDS indicates that the origin for deformation is seismic trigger. The 14C AMS dating of deformation structures generated coseismically by earthquakes suggest three seismic events occurred between 130 and 920 year BP, 5415 to 9140 year BP, and at about 4285 year BP. This study confirms that DF is indeed active, at least, since the mid-Holocene. More trenching and dating of seismically induced deformation features are needed to accurately calculate the recurrence interval of major earthquakes that can strike the fast-expanding urban areas in India and Bangladesh.

Vein topology, structures, and distribution during the prograde formation of an Archean gold stockwork

The Archean Cheechoo stockwork gold deposit is hosted by a felsic intrusion of tonalitic-granodioritic composition and crosscutting pegmatite dikes in the Eeyou Istchee James Bay area of Quebec, Canada (Archean Superior craton). The evolution of the stockwork is characterized herein using field relationships, vein density, and connectivity measurements on drill core and outcrop zones. The statistical distribution of gold is used to highlight mechanisms of stockwork emplacement and gold mineralization and remobilization. Two statistical populations of gold concentration are present. Population A is represented by gold grades below 1 g/t with a lognormal cumulative frequency. It is widespread in the hydrothermally altered (albite and quartz) and mineralized facies of the pluton. It is controlled by the development of quartz-feldspar-diopside veins as shown by the similar lognormal distribution of grades and vein density and by the correspondence of grades with network connectivity. Diopside and actinolite porphyroblasts in deformed veins within sodic and calcsilicate alteration zones are evidence for auriferous vein emplacement prior to the amphibolite facies peak of metamorphism. Population B (>1 g/t) is erratic and exhibits a strong nugget effect. It is present throughout the mineralized portion of the pluton and in pegmatites. This population is interpreted as the result of gold remobilization during prograde metamorphism and pegmatite emplacement following the metamorphic peak. The pegmatites are interpreted to have scavenged gold emplaced prior to peak metamorphism. These results show the isotropic behavior of the investigated stockwork during regional deformation and its development during the early stages of regional prograde metamorphism.

Structural Style and Kinematic History of the Colombian Eastern Cordillera

Fold-and-thrust belts and their associated structures are among the most common geological features of convergent margins. They provide significant information about crustal shortening and mountain-building processes. In subaerial belts, where the erosional rates are high and the growth strata are mostly eroded, methodologies such as that presented here can provide insights into to their formation. Two 2D cross-sections located in the Eastern Cordillera of Colombia are presented in this research. These sections extend from the Bogota Savanna to The Llanos, parallel to the regional deformation direction. Section construction was carried out using commercial surface data, and seismic information provided by Ecopetrol. Published thermochronometric data, gravel-clast petrography analysis, and paleoflora analysis were used to construct a viable tectono-evolutionary history of the study area. This evolutionary model is presented here in two palinpastic restorations from the Early Paleogene to Recent (∼65 Ma to Present-day). Section 1 and Section 10 accumulated 17.3 km and 19.5 km of shortening, respectively. The section reconstruction displays two major tectonic events – post-rift subsidence during the Early-Mid Paleogene, and positive inversion from the Oligocene to Recent (∼33 Ma to Present-day). This investigation focuses on the compressional period, where the structural analysis evidences an acceleration in the shortening rate, as well as a progressive migration of the deformation from northwest to southeast. This research discusses the extent and limitation of this methodology, as well as the principal structural aspects of the reconstruction.

Evolution of melt-bearing shear zones during cooling within an upper crustal aureole: the Calamita Schists (Island of Elba, Italy)

<p>The Calamita Schists in the aurole of the Late Miocene Porto Azzurro pluton underwent partial melting and HT metamorphism at P < 0.2 – 0.3 GPa and T > 650 – 700 °C, coeval with regional deformation. Deformation produced a network of shear zones that evolved from melt-present conditions to the brittle-ductile transition. Shearing at high temperature in the presence of melt allowed deformation to remain relatively distributed in wide high-strain zones. As the thermal pulse associated with the intrusion progressively faded away, deformation localized into anastomosing, mylonitic greenschist-facies shear zones surrounding lozenges of high-grade migmatitic schist. Mylonitic shear zones formed at low-angle with respect to the well-established high grade foliation preserved as a relic, oblique foliation. We show that such an extreme strain localization was determined by strain hardening of the no longer melt-bearing quartz-feldspar schist, localized embrittlement on precursory shear bands, and fluid-enhanced reaction softening that caused the breakdown of Al-silicates and the development of phyllosilicate-rich mylonitic bands. Consequently, tectonic structures with different orientation developed under the same kinematic regime, as a result of the changing physical and mechanical properties of the cooling rock volume.</p>

Sedimentary breccias formed during extensional tectonics: facies organization and processes

<p>Sedimentary breccias formed during extensional tectonics are spatially associated with large-throw normal faults. They result from the creation of a steep topography that becomes unstable, producing major rockfalls. The studied breccias, in Crete and in the Pyrenees, are up to 300 meters thick and are characterized by poorly sorted polygenic deposits of pebbles to boulders composed of highly angular plurimillimetric to plurimetric carbonate clasts. A lateral evolution is observed, with pebble-size clasts found near the normal fault and boulder-size clasts away from the fault. This evolution is related to the rockfall process as the total kinetic energy acquired by the small clasts during the fall is lower than that acquired by the bigger ones; as a result, the latter are able to travel farther. Interestingly, the fact that the smallest clasts are proximal while the bigger ones are more distal is contrary to the distribution found in alluvial fan systems, making it possible to differentiate from one another. The studied breccias commonly show disorganized layers and/or no noticeable layering across large distances. We interpret this feature as related to the movement on the normal fault, which progressively tilts the breccia layers and favours their gliding along the slope. Gliding is an important internal process to take into account in rockfall systems because it may disorganize the layering, create specific geometries like onlap around olistoliths, and produce deformation inside the breccia layers; the latter feature could be mistakenly interpreted as resulting from post-deposition regional deformation.</p><p>According to our observations, active normal faults with large throws provide the conditions for the formation and preservation of great volumes of sedimentary breccias through the following processes: i) footwall uplift, creating a pronounced topography with steep slopes, giving rise to major rockfalls, ii) hangingwall rapid subsidence, which allows the accumulation and preservation of the breccias without clast reworking by drainage systems. The latter is reinforced by the fact that, during the early stages of extension, the main watersheds point in a direction opposite to the fault slope whereas only small, discontinuously distributed watersheds flow in the direction of the fault slope. Upon ongoing extension, the size of these small watersheds increase. At one point, the sedimentary flow coming from these watersheds becomes more important than rockfall processes. Part of the breccia body is then eroded, reworked, and replaced by conglomerates of an alluvial fan deposited unconformably above the breccias.</p><p>Summing up, sedimentary breccias are readily formed as thick syn-tectonic deposits during early stages of extensional basin development. Thus, they may be considered as a typical lithology, and a marker, of continental extension.</p>