Crustal Discontinuities Research Articles

The use of combined C[sbnd]Mg isotope compositions of carbonates from orogenic Sb[sbnd]Au deposits as a tracer of fluid interaction with sea-floor altered crust

The Mesoarchaean Murchison Greenstone Belt is composed of a strongly deformed volcano-sedimentary succession metamorphosed up to amphibolite facies and surrounded by metaluminous and peraluminous granitoids in the north of the Kaapvaal Craton of southern Africa. A circa 40 km-long shear-zone of strongly carbonatised rocks (the Antimony Line, or Sb-Line) oriented along the main trend of the greenstone belt (WSW–ENE) hosts important Sb deposits with accessory Au. These commodities mostly occur as stibnite and native gold hosted by quartz‑carbonate rocks at the centre of the Sb-Line, underlining the importance of CO2-rich fluid flow for mineralisation during deformation and metamorphism.In this study, we compare the elemental and stable isotope (C-O-Mg) composition of carbonates from the Sb-Line with regionally distributed carbonates hosted by the volcano-sedimentary succession distal to mineralisation. The carbonates of the Sb-Line and regional rocks have overlapping major element compositions. Both magnesite and dolomite in most Sb-Line samples have marked light-REE-depleted patterns with variable positive Eu anomalies. Carbon isotope ratios define two clusters, with marked δ13C peaks at ca. -5 ‰ for Sb-Line rocks and ca. -2 ‰ for regional rocks, implying separate C-sources. The peak at ca. -2 ‰ likely represents early carbonatisation through sea-floor alteration, whereas the first peak at ca. -5 ‰ is indicative of deep CO2 (mantle, or magma-derived) introduced during tectonic activity of the Sb-Line. Magnesium isotope ratios of regional rocks reveal limited fractionation (bulk δ26Mg = −0.27 ± 0.10 ‰) that overlap with mantle values, but some carbonate-bearing veins present 26Mg-depleted compositions (bulk δ26Mg = −1.91 to −0.40 ‰). Sb-Line carbonated rocks have more fractionated Mg isotope compositions (bulk δ26Mg = −0.8 to 0.0 ‰) and carbonates display marked negative 26Mg values (δ26Mg from −1.46 to −0.31 ‰). We interpret these results in terms of preferential remobilisation of isotopically light Mg during fluid-rock interaction and dissolution of carbonate in the host-rocks. The lack of correlation between δ13C and δ26Mg indicates decoupling of these isotopic systems, implying contribution from isotopically distinct sources of C and Mg to the mineralised zone.Similar to other structurally controlled AuSb mineralisation in Archaean greenstone belts, metal transport and ore deposition in the Murchison Greenstone Belt was closely related to the deep cycle of carbon, linking C-draw-down during sea-floor alteration, carbonate re-mobilisation during metamorphism and CO2 degassing from deep sources along major crustal discontinuities. In contrast to models for orogenic AuSb deposits invoking a purely intra-crustal, metamorphic origin of mineralising fluids, our results underline the importance of deep-sourced fluids originating from an external source from the volcano-sedimentary succession.

Reactivation and propagation of the transbrasiliano strike-slip system in Tocantins province, central Brazil: From Ediacaran to Cretaceous

Strike-slip zones are formed by the accommodation and displacement of crustal blocks, reflecting dynamic behavior within the lithosphere. Repeated deformation in these zones can obscure earlier geological processes by creating new structural frameworks and reactivating ancient weakness zones. In central Brazil, the Transbrasiliano Lineament - a major continental-scale strike-slip system - has significantly reworked the Tocantins Province, altering key geological elements that constrain the final stages of Western Gondwana's assembly. Although several seismic events have been linked to this large structure, establishing a direct correlation remains challenging due to its crustal framework being largely obscured by Phanerozoic sedimentary basins. This study presents a multiscale analysis of geological, geomorphological and geophysical data to characterize the Transbrasiliano Lineament dynamic in central Brazil. Our findings reveal a deep crustal discontinuity associated with the Goiás-Tocantins seismic zone, characterized by low-velocity anomalies and a thinner crust within the Brazilian Platform. The complex configuration of this strike-slip system, due to successive reactivations, has displaced and reoriented previous magnetic elements, affecting the Araguaia, Brasília, and Paraguay Belts. Consequently, deformation of the Transbrasiliano Lineament has propagated from the southern Bananal Basin to the northern Paraná Basin, exhibiting a horsetail splay geometry. Structural geophysical analysis indicates that this ongoing reactivation has recorded deformation episodes from the Ediacaran-Cambrian to the Upper Cretaceous.

Crustal deformation from GNSS measurement and earthquake mechanism along Pieniny Klippen Belt, Southern Poland

The dynamic geological features of the Pieniny Klippen Belt (PKB) in southern Poland are nowadays a focal point of researchers as it is recognised as an active zone of crustal discontinuity. In the present study, we employed long-term analysis of ground-based global navigation satellite system (GNSS) measurements (from 2004 to 2020) to probe the crustal deformation, strain rates, and rotational rates in the PKB unit and the surrounding region, i.e., Magura Nappe (MN) and Podhale Flysch (PF) units. Measured velocities from GNSS observables are modelled by the auto-regressive integrated moving average (ARIMA) method to comprehend the long-term tectonic deformation. Our results showed that the ARIMA-modelled velocity varied from ~ 0.15 to ~ 8.86 mm/yr, indicating about 8.71 mm/yr difference along all units. Such differences suggest that crustal slip along the active thrusts and folds is the major factor causing regional deformation. The strain rates in PKB are also varying from the western to the eastern part. The rotational rates in PKB show a counterclockwise (CCW) pattern similar to the strain rates. These patterns suggesting that the PKB was rotated in the CCW direction with a large angle during the Miocene period. Finally, we analysed the seismicity for a period from 2004 to 2020 by using Bayesian moment tensor inversion and multivariate Bayesian inversion. The Bayesian inversion was applied based on bootstrapping chain analysis to figure out the earthquake mechanism using moment tensor inversion for the mainshock that occurred in Poland on 20 July 2018. The inversion results for the 2018 earthquake resolved a thrusting mechanism with nodal plane-1 having a strike of 346°, dip of 32°, and rake of 92° and a nodal plane-2 with a strike of 163°, dip of 58°, and rake of 89°. Since the seismicity in the Poland region has experienced less significant earthquakes in the last century, it is reasonable to attribute this lower seismic activity to the correspondingly low slip rates discerned through geodetic monitoring efforts.

Lefkos Shear Zone of Karpathos, Greece (SE Aegean): Pleistocene to recent transtensional forearc basin and paleoseismicity

Pleistocene–recent transtensional forearc basins and associated island uplift could be driven by sinistral–oblique convergence along the eastern Hellenic subduction zone, Greece, or by simultaneous trench retreat attributable to subduction of Permo-Triassic Tethyan lithosphere. Structural mapping and basin analysis of the Lefkos Basin, Karpathos indicate deformation best related to oblique trench retreat in which Karpathos drifts southeastward more slowly than Rhodes. Lefkos Basin, perched on a corner of the offshore Karpathos Basin, comprised two Early Pleistocene marine sub-basins developed under northwest extension. Unusually long-wavelength hummocky cross-stratification (>10 m) and clinoforms (15–20 m high) constitute distinctive marker facies. Facies architecture reflects inter-related fault-controlled subsidence, syn-depositional uplift/tilting, glacioeustacy, and fan-delta-lobe avulsion. A 230U/234Th coral age of ∼600 ka and a deer mandible date a Middle Pleistocene transition to southwestward fan-delta progradation and basin uplift. Following ∼0.5 Ma fan-delta-plain deposition, the Lefkos Slide dropped a ∼3.7 km2 coastal plain. Subsequent steep north–south dextral–normal faults dominate Lefkos Basin, with subordinate steep east–west sinistral–normal and northeast-trending normal–oblique faults. Faults signify wrench domination based on their steepness, rare slip indicators, slip reversals and co-genetic upright isoclinal folds. These structures are integral to a forearc-scale dextral–normal shear zone, the Lefkos Shear Zone, an active crustal discontinuity spanning the southeastern forearc from eastern Crete through Kasos–Karpathos to Rhodes. Vertical coherence exists between upper-crustal dextral–normal shear in the Lefkos Shear Zone, crustal seismic anisotropy and deeper mantle processes. Hydraulic fractures, fluid-expulsion pipes, and ruptured cobbles record intermittent significant seismicity (Mw ≥5–6) that permeated the entire Lefkos basin fill spanning ∼2 million years, ultimately involving Late Roman–Early Byzantine archeological remains through ∼1000 years ago. This study encourages re-evaluation of seismic, landslide and tsunami hazards in the sector of the Hellenic forearc surrounding the Lefkos and Karpathos Basins.

The influence of lithospheric inheritance and Cenozoic magmatism on Phanerozoic rock cooling in the São Francisco Craton and adjacent orogens

The South American Platform encompasses the São Francisco Craton (SFC) and the Araçuaí-Ribeira orogenic system (AROS) located in the eastern Brazil. These geologic domains hold records of differential exhumation, sedimentation, volcanic activity, and post-rift reactivation of faults and shear zones. Therefore, these terranes provide outstanding opportunities for studying the dynamics of lithospheric processes, particularly in their interaction zones, characterized by the transition from cratonic to orogenic lithosphere. To investigate the processes involved in the contrasting behavior of the upper crust, we provide 23 new apatite fission track (AFT) ages and thermal histories from the northern segment of the AROS and from the eastern area of the SFC. The AFT ages range from 154.4 ± 20.1 and 37.1 ± 3.0 Ma and the mean track lengths range from 10.9 and 13.2 μm. We compiled thermal information from previous thermochronological analysis from 357 sites and constructed inverse distance weighted maps that depict the spatial distribution of AFT ages and the temperature evolution from 360 to 30 Ma. The results indicate that the SFC experienced its final exhumation within the apatite partial annealing zone (APAZ) at temperatures ranging from 110 °C to 60 °C during the late Paleozoic. In contrast, the AROS went through its last cooling within the APAZ during the Mesozoic or even in the Cenozoic. This suggests that reactivation and reheating, possibly associated with the South Atlantic rift, influenced the orogenic basement of the passive margin. We highlight that the reheating associated to the Abrolhos magmatic province in the northern segment of the AROS caused the youngest AFT ages of the study area. Our work demonstrates the importance of considering contrasting rheology when analyzing regional exhumation patterns, the structural fabric of orogens through reactivation of crustal discontinuities generating heating and exhumation, and how volcanism can disrupt the thermal record, inducing crustal reheating.

Crustal Structure Beneath Mexico City From Joint Inversion of Receiver Functions and Dispersion Curves

AbstractOver the years, it has been clear to seismologists that the crustal structure beneath Mexico City is highly complex. This fact may influence the seismic waves' behavior when traveling through it. Consequently, many studies have been done to understand this crustal structure. This article presents a high‐resolution S‐wave velocity model obtained from joint inversion of radial receiver functions and group‐velocity dispersion curves beneath Mexico City (VMRFDC). This model includes azimuthal variations observed with receiver functions, which may be interpreted as crustal discontinuities. Receiver functions were retrieved from velocity and acceleration records, and dispersion curves were collected from a previous tomographic study. We performed a geostatistical analysis that revealed a horizontal spatial correlation of the crustal structure for up to 7.5 km. This distance was obtained from the S‐wave velocity omnidirectional semivariogram range per depth. Using ordinary kriging, we utilized this spatial correlation to estimate S‐wave velocities per depth and introduced two novel techniques: a resolution test for kriging and a geostatistical‐dependent smoothing technique. Finally, we compare our results with previous works, making our model consistent with geological and geophysical features reported in the literature.

The Main Himalayan Thrust Beneath Nepal and Southern Tibet Illuminated by Seismic Ambient Noise and Teleseismic P Wave Coda Autocorrelation

AbstractNepal is an actively deforming region due to its tectonic setting that hosts destructive earthquakes including the recent 2015 Mw 7.8 Gorkha earthquake. To better understand the physics of earthquakes and better assess the seismic hazard in the region, a highly resolved 3‐D structure of the crust is essential. This study presents a new 3‐D S‐wave velocity structure of the crust using ambient noise tomography (ANT). In relation to the 2015 earthquake, we show that the lateral variation of S‐wave velocity likely controls the rupture propagation and arrest. This study further constrains crustal discontinuities beneath Nepal including the Main Himalayan Thrust (MHT) using teleseismic P‐wave coda autocorrelation. The MHT geometry correlates well with sizeable low S‐wave velocity zones obtained from the ANT and separated by two ramps. The southernmost first ramp is where the 2015 Gorkha earthquake nucleated that we map up to the source model of the 1934 historical earthquake in Eastern Nepal. The northernmost second ramp is adjacent to the mid‐crustal low velocity zone beneath south Tibet. In between the two ramps, we interpret the horizontal low velocity zone as the interseismic creeping patch of the MHT with down‐dip extent of 50–70 km length and 90–100 km in correspondence with the Mw 7.8 2015 and 8.4 1934 source models, respectively. A systematic mapping of the low‐velocity zones in relation to the MHT integrated with GPS geodesy along with the respective slip from individual past earthquake ruptures can help decipher the paradox of maximum Himalayan earthquake magnitudes.

New insights from an emplacement model for the Santa Eulália Plutonic Complex (SW Iberian Peninsula)

The Santa Eulália Plutonic Complex is formed by two main granites: G0 and G1. The G0 granite hosts metasedimentary carbonate and pelitic rocks (roof pendants) and elongated masses of mafic–intermediate rocks (the M-group). The host rocks form a diverse sequence of igneous, metasedimentary and metamorphic rocks. The study area is constrained by Variscan structures formed in a transpressional/transtensional sinistral tectonic regime (the Tomar–Badajoz–Córdoba sinistral shear zone) and is cut by the Alter do Chão and Assumar faults. The geological complexity of the area makes it difficult to determine the emplacement mechanism of the pluton. We propose a model for the ascent and emplacement of the pluton that can also be applied to similar post-collision Variscan granites. The gravity anomalies suggest that the pluton is slightly asymmetrical and extends to the SSE beneath the host rocks; its main root is >8 km thick in its deepest areas. The available radiometric data for an extended area embracing several regional plutons suggest a west–east magmatic alignment. The Variscan structures probably formed efficient crustal discontinuities enabling the generation and ascent of magma. Our model involves a west–east magmatic axis for magma spreading along extensional fractures (T-fractures) related to the Tomar–Badajoz–Córdoba shear zone. The opening movement along these fractures created divergent forces that allowed the ascent and emplacement of the plutonic rocks. The importance of these fractures is represented well by outcrops of porphyritic biotite granites (the Ervedal, Fronteira and G1 granites).

The Magmatic Rifting of Santos Basin: Aeromagnetic Mapping of Dykes, Terranes and Marginal Structures and the Interplay Between Tectonism and Volcanism

AbstractWe present a new magnetic map integrating continental and oceanic features at the Santos Basin, in order to investigate the connection between onshore and offshore tectonics and magmatism at the early stages of rifting. The magnetic and seismic data evidence the offshore continuity of the Florianópolis and Serra do Mar dyke swarms, while the Ponta Grossa Dyke Swarm is interrupted by the marginal structures. This structural relationship brings to evidence a different timing of formation of the Ponta Grossa Dyke Swarm relative to the early opening of the margin. A very high amplitude magnetic lineament ‐ the Santos Marginal Anomaly (SAMA) reaches up to 1,500 nT and trends NE‐SW, with local E‐W inflections forming the Cananéia and Guaratiba Arcuate anomalies. Our modeling suggests that the SAMA can be associated with dykes and syn‐rift extrusive rocks linked at depth to larger magmatic bodies at the proximal and necking domains. Based on the increasing width and amplitude of the magnetic anomalies oceanwards, we propose a mechanism whereby the igneous accretion (i.e., the amount of magmatism) augmented as rifting evolved from continent to ocean, creating a volcanic rift axis. This rift was segmented by crustal discontinuities that probably nucleated on preexisting rheological contrasts, forming tectonomagmatic segments during the early extensional stages. The onshore‐offshore relationships highlight that where there was an agreement between basement fabric and dyke orientation, inheritance may have influenced their emplacement, although it was not a limiting factor for their formation. The regional geodynamics of the Gondwana opening was the first order control on the emplacements of the dyke swarms, with deviations of the magmatically influenced rift by tectonic inheritance.

Magmatic cycles in Santos Basin (S.E. Brazil): Tectonic control in the temporal-spatial distribution and geophysical signature

The Santos Basin, in the offshore southeastern Brazilian margin, evolved from a continental rift system to a passive divergent margin during the Cretaceous as a onsequence of the Gondwana breakup and the formation of the proto-Atlantic Ocean. The basin exhibits several cycles of magmatism from Valanginian to Eocene times, with a regionalized distribution of igneous rocks. Using seismic, gravity, magnetic, and wellbore data, the magmatic cycles were investigated and connected with the basin's tectonic activity. Our study includes a regional analysis of the various magmatic cycles, with a focus on identifying the temporal variations and spatial differences observed in the basin's many tectonic compartments. The first magmatic cycle, named rift-onset, is observed in both coastal and offshore regions and includes magmatic rocks from ca.135 to ca. 132/130 Ma. Onshore, the rift-onset magmatic rocks are observed in the coastal dike swarm province and in the coastal alkaline province. Offshore, the riftonset is observed as a prolongation of the coastal dike swarms into the proximal basin and as flood basalts in the distal basin. The second cycle includes the synrift (ca.132-130 to 123 Ma) and postrift (ca.123 to 113-112 Ma) magmatic rocks. The separation of intrusive synrift and postrift rocks is challenging in the onshore fieldwork and in the seismic analyses offshore, which is aggravated by the basin's scarcity of geochronological dating. Therefore, these two events have been treated together when the separation is not clear. The third cycle comprises the drift (or post breakup) magmatic rocks, spanning from the Lower Cretaceous to the Tertiary (from ca. 110 to ca. 40 Ma), and is well documented in coastal and offshore domains. Drift magmatism seems to be controlled by tectonics, since magmatic bodies tend to concentrate along lines of crustal discontinuities. Magmatic cycles (rift-onset, syn/postrift, and drift) also display spatial differences, exhibiting variation in the magmatic volume and in the morphology of igneous rocks from the southern to the central/northern Santos Basin. The study of magmatism in the, Santos Basin offers an excellent opportunity to investigate the processes that occurredduring the evolution of a volcanic rifted margin.

Semi-Automatic Image Processing System of Aeromagnetic Data for Structural and Mining Investigations (Case of Bou Azzer Inlier, Central Anti-Atlas, Morocco)

Numerical analysis of geophysical data to uncover Precambrian belts and probably to enclose mineral deposits is becoming once more communal in mining activity. The method is founded on typifying zones branded to comprehend deposits and looking for analogous areas. The proposed work outlines a semi-automatic image processing system for the structural and mining investigation of the Bou Azzer inlier, which varies from preceding approaches as it is centered only on aeromagnetic data. The aeromagnetic signature of what seem to be geologically expressive features are pursued within the aeromagnetic items. Cobalt and associated mineralizations in the Bou Azzer inlier are recognized to arise nearby main crustal discontinuities revealing as significant shear zones, which turn act as drains for mineralizing fluids. Mineralization occurs in sectors of structural complexity beside the shear zones. Developing towards the semi-automatic uncovering of such regions, the furthermost prospective extents are those everywhere inferred structural complexity occurs next to the regions of magnetic discontinuity. The proposed method is mainly based on the approach developed by the center for exploration targeting. The study was led by means of aeromagnetic data from the Bou Azzer inlier, which is considered one of the most productive and prospective regions for minerals and base metal mineralization in Morocco. The combined results obtained from geological and geophysical data prove that prospective areas have a dominant trend of NNE-SSW, NW-SE, NNW-SSE, E-W, and NE-SW directions. The CET Grid and Porphyry Analyses show that the probable porphyry mineral deposit locations mainly concentrated in the center of inlier, the Foum Zguid dyke, and northern and eastern part, which correspond to the Bou Azzer ophiolitic complex and platform deposits of the Lower Neoproterozoic Tachdamt-Bleïda.

Tectonic domains in the NW Amazonian Craton from geophysical and geological data

The Amazonian Craton is one of the largest cratonic areas in the world. In Colombia, a major portion of the Craton is covered by Paleozoic to Cenozoic sedimentary rocks and recent deposits. This, in addition to the difficulty of access and dense tropical rainforest, have made the geology of this area to remain relatively unexplored to this date. Most accepted models for the Proterozoic evolution of the Amazonian Craton indicate that it evolved via successive accretion of orogenic belts and crustal terranes around an ancient nucleus, and that tectonic provinces identified in the southern half of the craton, the Guaporé Shield, extend underneath the Amazon River Basin onto its northern exposure, the Guiana Shield. Nevertheless, recent geologic studies in the W Guiana Shield indicate that its evolution may have been different from the W Guaporé Shield, where these accretionary models were formulated. In this work, we used airborne gravity/magnetic geophysical datasets covering the NW portion of the Amazonian Craton, to better elucidate its structure and tectonic evolution. We apply a multiscale edge detection and 3D modeling to identify and delineate major crustal discontinuities and other geological features. Using this approach, we identified six primary geophysical lineaments that are interpreted as possible crustal boundaries. By combining our geophysical interpretation with all the geological, geochronologic and isotopic information available for the region, we propose the presence of the following tectonic domains: Ventuari-Tapajós, Rio Negro-Juruena (which we further subdivide into Atabapo and Vaupés Belts), Apaporis Graben, and Putumayo. Furthermore, a new U-Pb zircon crystallization age of 1227 ± 8/13 Ma obtained from volcanic rocks of the Piraparaná Formation indicates that extensional tectonics along the Apaporis Graben began at least in the late-Mesoproterozoic. This is significantly older than previously thought, and thus entirely transforms the tectonic significance of the Apaporis Graben structures. Our interpretation of structural limits is in excellent agreement with and provides a more accurate location for previously suggested boundaries, which were until now only loosely constrained by the sparse geological and geochronologic information available. This work provides the first regional reconstruction of crustal-scale features of NW South America, improving the understanding of the regional tectonic architecture of NW Amazonian Craton using geophysical methods.

Crustal structure in and around the East Anatolian volcanic belt by using receiver functions stacking

This study aims to explore the crustal seismic properties across the East Anatolian volcanic belt. Teleseismic receiver functions obtained through deconvolution in the time domain are used to estimate crustal discontinuities and Moho interface. Further, slant stacked method, employed for the first time in the study region, is applied to identify the arrival times of P to S conversion and reverberations (Ps and PpPms) that give Vp/Vs ratio beneath each station. For slant stacking method, radial receiver functions are calculated using deconvolution based on frequency domain. Inversion results indicate that Moho depth varies from at most ∼40 km near the Bitlis-Zagros thrust zone to 45 km in the northeastward of Quaternary volcanic centers. The southwest of study region is characterized overall by a low average crustal Vp/Vs ratio (less than 1.71), implying a felsic crustal composition. A high average crustal Vp/Vs ratio of 1.82–1.87 is found near volcanic centers, characterized by predominantly a mafic crustal composition. The prominent crustal low-velocity zones especially around the Nemrut and Ararat volcanoes are identified below ∼20 km depth and likely imply the presence of partial melt zones or active magma chambers in the region. As a result, the findings of this study provide useful new constraints to the crustal thickness and composition in the study region.

Distinct scheelite REE geochemistry and 87Sr/86Sr isotopes in proximally- and distally-sourced metamorphogenic hydrothermal systems, Otago Schist, New Zealand

The Otago Schist in New Zealand hosts orogenic tungsten (W) mineralization in two types. Proximally-sourced (“proximal”) mineralization in sub- to lower-greenschist facies Mesozoic metasedimentary rocks within the northern flank of the Otago Schist Belt comprise quartz-calcite veins with minor scheelite and few sulfides. Distally-sourced (“distal”) deposits are hosted in major crustal discontinuities within Mesozoic sub-greenschist to upper-greenschist facies metasedimentary rocks and are associated with abundant sulfides ± Au. In-situ trace element (particularly REE, Sr, Mo, Na, As) and in-situ 87Sr/86Sr compositions of scheelite show the scheelite occurring in these different mineralization types to be geochemically distinct. Proximal occurrences are characterised by scheelite with both heterogenous trace element and Sr isotope compositions displaying variations that can be linked to different veins, host rock types and scheelite generations. In contrast, scheelite within distal scheelite deposits tends to have fairly homogenous trace element and Sr isotopic ratios at the deposit to grain scale. The heterogenous compositions of proximal occurrences represent local derivation of diverse components from the sub-millimetre to meter scales and small extents of fluid flow, which resulted in a high sensitivity to sources and local rock compositions and thus limited large-scale equilibration of elemental and isotopic systematics within the scheelite they contain. The larger distal deposits formed by regional leaching by metamorphic fluids, probably at the greenschist-amphibolite facies boundary, followed by homogenisation as they ascended through the crust and leading to deposition of scheelite with rather uniform chemical characteristics. Our data re-enforce the model that tungsten mobility in the Otago Schist involved local-scale mobilisation of W in the shallow crust by breakdown of detrital rutile and scheelite at low metamorphic grade. Burial of these veined scheelite-bearing rocks to temperatures of ∼500 °C enabled remobilisation of W by metamorphic fluids, concurrent with scavenging of Au, As and S, followed by return of the metal-bearing fluids towards mid to upper crustal levels, where they precipitated scheelite.

Pleistocene-Holocene Monogenetic Volcanism at the Malko-Petropavlovsk Zone of Transverse Dislocations on Kamchatka: Geochemical Features and Genesis

The Malko-Petropavlovsk zone of transverse dislocations (MPZ) was formed on the extension of the deep Avachinsky transform fault in perpendicular relation to the subduction trench. It is a natural boundary between variously aged slabs in Kamchatka (103–105 Ma under Southern Kamchatka and 87–92 Ma under the Eastern volcanic belt). Monogenetic cinder cones in the MPZ are randomly distributed along these long-lived rupture zones. Here we present new geochemical and isotopic results of monogenetic volcanism in the MPZ. Based on whole rock and trace element geochemistry, Pb-Sr-Nd isotopic ratios of monogenetic cinder cone magmas were shown to tap the enriched mantle source (low 143Nd/144Nd isotopic ratios (0.512959–0.512999), and changed as 87Sr/86Sr (0.703356–0.703451) and 206Pb/204Pb (18.30–18.45), 208Pb/207Pb (38.00–38.12) isotopic ratios). High Nb/Yb and La/Yb ratios, without significant inputs of the slab’s components (the lowest Ba, Th content), indicate decompression melting predominately. Calculations of the pressure (9–11 kbar) and temperature (1160–1240 °C) conditions using a glass thermobarometer suggest that magma of monogenetic cinder cones resided near the Moho boundary prior to eruption. This correlates with the crustal discontinuity under the MPZ according to geophysical observations (converted-wave seismic exploration and magnetotelluric sounding). The majority of well-preserved monogenetic cinder cones were formed in Holocene, after the last glaciation, but eruptions were not observed historically. This, however, suggests that similar eruptions in the MPZ may occur in the future. Given that the MPZ hosts major population centres of Kamchatka (Petropavlovsk, Elizovo, Vilyuchinks, and Paratunka: ~ 250,000 people or ~ 80% of the whole Kamchatkan population live in the major cities on the coastline of the MPZ), we highlight the urgent need to install a continuous monitoring system around the MPZ cones, geophysical investigation, and more serious attention from the local government and scientists. In particular, a detailed study of the MPZ regarding age, volume, and volcanic hazard assessment (pyroclastic vs extrusive) will help reduce potential risks of eruptions from monogenetic volcanoes for humans and infrastructures.

Geochronology and Petrogenesis of Early Pleistocene Dikes in the Changbai Mountain Volcanic Field (NE China) Based on Geochemistry and Sr-Nd-Pb-Hf Isotopic Compositions

Changbai Mountains intraplate volcanism (NE China) developed above the 500 km deep stagnant portion of the Pacific slab in the last 20 Ma. The more recent activity includes a shield-forming stage (2.8–0.3 Ma), the Tianchi cone construction stage (1.5–0.01 Ma), and a caldera-forming stage (0.2 Ma-present). Detailed studies on the petrogenesis of the volcanic products between the first two stages and the possible role of geodynamics and local tectonics in controlling the volcanism, however, are lacking. Here, we present structural and whole-rock geochemical and zircon Hf isotopic data on Pleistocene dikes of the Changbai Mountains at the transition from the shield-forming to the Tianchi stage with the aim to constrain their age and the source(s) of their parental magma. The dikes represent the shallower feeding system of monogenetic cones and have a NW-SE strike, which is also the preferred strike of the major fault affecting the area and along which the Changbai Mountains monogenetic scoria cones align. The dikes have a potassic affinity and a trachybasaltic composition. Their zircon U–Pb age is 1.19–1.20 Ma (Calabrian). The trachybasalts are enriched in Rb, Ba, Th, U, Nb, Ta, K, Pb, and LREE and slightly depleted in Sr, Zr, Hf, Ti, and HREE with a weak negative Eu/Eu* (δEu = 0.96–0.97). Trace elements and isotopic compositions are compatible with an OIB-type source with an EMI signature. The calculated (87Sr/86Sr)i(=0.705165–0.705324), (143Nd/144Nd)i(= 0.512552–0.512607, εNd(t) = −0.58 to −1.65), and Hf model ages (TDM2) of 1768–1562 Ma suggest that the trachybasaltic dikes were contaminated by a Mesoproterozoic, relatively basic lower crust. The source of the Calabrian trachybasalts consists of asthenospheric melts modified by a subcontinental lithospheric mantle. These melts upwell from depth and stop at the crust-mantle interface where underplating processes favor the assimilation of ancient lower crust material. During the ascent to the surface along deep-seated crustal discontinuities, these magmas weakly differentiate.

Kinematic partitioning in the Southern Andes (39\xb0 S\u201346\xb0 S) inferred from lineament analysis and reassessment of exhumation rates

The Southern Andes are often viewed as a classic example for kinematic partitioning of oblique plate convergence into components of continental margin-parallel strike-slip and transverse shortening. In this regard, the Liquiñe-Ofqui Fault Zone, one of Earth’s most prominent intra-arc deformation zones, is believed to be the most important crustal discontinuity in the Southern Andes taking up margin-parallel dextral strike-slip. Recent structural studies, however, are at odds with this simple concept of kinematic partitioning, due to the presence of margin-oblique and a number of other margin-parallel intra-arc deformation zones. However, knowledge on the extent of such zones in the Southern Andes is still limited. Here, we document traces of prominent structural discontinuities (lineaments) from the Southern Andes between 39° S and 46° S. In combination with compiled low-temperature thermochronology data and interpolation of respective exhumation rates, we revisit the issue of kinematic partitioning in the Southern Andes. Exhumation rates are maximal in the central parts of the orogen and discontinuity traces, trending predominantly N–S, WNW–ESE and NE–SW, are distributed across the entire width of the orogen. Notably, discontinuities coincide spatially with large gradients in Neogene exhumation rates and separate crustal domains characterized by uniform exhumation. Collectively, these relationships point to significant components of vertical displacement on these discontinuities, in addition to horizontal displacements known from published structural studies. Our results agree with previously documented Neogene shortening in the Southern Andes and indicate orogen-scale transpression with maximal vertical extrusion of rocks in the center of the transpression zone. The lineament and thermochronology data call into question the traditional view of kinematic partitioning in the Southern Andes, in which deformation is focused on the Liquiñe-Ofqui Fault Zone.

Diversification of Prochilodus in the eastern Brazilian Shield: Evidence from complete mitochondrial genomes (Teleostei, Prochilodontidae)

The Neotropical fish genus Prochilodus includes five species occurring in the main drainages of the eastern Brazilian Shield: P. argenteus, P. costatus, P. lineatus, P. harttii, and P. vimboides. Multi and single locus phylogenies have questioned the monophyly of P. costatus and P. lineatus, and the biogeographic history of these species in the Brazilian Shield has never been explored. We characterized new mitogenomes for these species to reconstruct their evolutionary history, estimated the timing of Prochilodus cladogenesis, and discussed the implications of past geologic events on species diversification. The phylogeny supports the monophyly of both P. costatus and P. lineatus, and indicates a Miocene divergence of P. vimboides, much earlier than subsequent species diversification. The Early Pleistocene split of P. argenteus and P. harttii (~2.1 million years ago—MYA) is hypothesized to be related to recent Quaternary reactivations of the Rio Araçuaí Fault that promoted river captures between the eastern tributaries of the São Francisco basin and the upper Rio Jequitinhonha in the Serra do Espinhaço mountain range. The time-calibrated phylogeny also indicated a subsequent split of P. costatus and P. lineatus (~1.1 MYA) likely due to Quaternary activities of the Upper Rio São Francisco crustal discontinuity and Estrela Fault that catalyzed river captures between the upper Rio São Francisco and the Rio Grande of the Paraná basin in the Serra da Canastra mountain range. These results provide a temporal context for the diversification of Prochilodus and bring new insights to further study the historical biogeography of other riverine fish groups along the upland basins of the eastern Brazilian Shield.

The Seismotectonic Significance of Geofluids in Italy

There is growing interest in how geofluid emissions are released in the atmosphere by the planet’s geodynamic activity, and how much they contribute to the global budget of greenhouse gases. Many workers are addressing this issue with studies conducted at global scale, so as to get the required global-scale answers. The data available at the global scale on geofluids, faults, earthquakes and volcanoes, however, are generally too coarse to provide these answers. We investigate the relationships between geofluid emissions and tectonics at a more detailed scale. Building on over a century of data on geofluid emissions and on an extensive knowledge of the region’s tectonics and seismicity, we focused on Italy, one of the areas of the globe that experience the largest release of natural CO2and CH4. We systematically overlaid and compared data collected by a number of workers into 13 published countrywide databases concerning geofluid emissions, carbon-bearing deposits, seismogenic faults, historical and instrumentally documented earthquakes, and heat flow observations. Our results indicate that 1) thermal springs and CO2emissions dominate in areas of mantle upwelling and crustal stretching, but also that 2) some of them occur in the extending inner Apennines, generally along major lithospheric chain-perpendicular lineaments that bound the largest normal faults. Conversely, 3) CH4emissions and mud volcanoes dominate in areas undergoing active contraction, where no CO2emissions are observed; in particular, we find 4) that mud volcanoes concentrate where the crests of active anticlines intersect major lithospheric chain-perpendicular lineaments. An overarching conclusion is that, in Italy, the release of geofluids is primarily controlled by deep crustal discontinuities that developed over the course of 5–10 My, and is only mildly affected by ongoing crustal strains. Geofluid emissions bring information on processes that occur primarily in the lower crust, marking the surface projection of generally hidden discontinuities that control the geometry and modes of seismic release. As such they may also provide valuable insight for improving the assessment of seismic hazard in hard-to-investigate seismically active regions, such as Italy.

Geophysical determination of the Jalisco and Michoacán blocks boundaries along the Colima Graben

The Colima Graben is located in the southwestern part of Mexico, extending from the triple junction of the grabens of Citlala (located Southwest of the Chapala Graben), Colima and Tepic-Zacoalco south of Guadalajara, to the Mesoamerican Trench (MAT) south of Manzanillo on the Pacific coast. The Colima Graben is spatially related to the boundary between the Jalisco and Michoacán Blocks, which has been interpreted as an area of crustal rupture that facilitates the rise of upper mantle material, making it difficult to identify surface evidence to determine the limits of the Jalisco and Michoacán Blocks. The use of gravity and magnetic data inversions, make posible the determination of the boundary geometry of the two blocks, evidencing the crustal discontinuity through the contrast in density and magnetic susceptibility over the fracture zones.The Bouguer anomaly and its Analytic Signal allow the identification of a high gravimetric gradient close to the coast, between the Jalisco and Michocán blocks, that is supported by a 3D gravity inversion. The inversion also ilustrated the geometry of the tectonic units continuing from the coast to the triple junction, thus identifiying a clear separation between both blocks. Additionally, it is possible to provide evidence of the different structural trends present along the length of the Colima Graben. This is the first time where a 3D gravimetry and magnetic susceptibility inversion are applied over the full extent of the Colima Graben, resulting in the first clear definiton of the boundaries of both the Jalisco and Michoacán blocks, and showing that the Jalisco Block and Colima Graben do not adjoin each other. Instead a new distribution of the different boundaries of the Colima Graben and the Jalisco and Michoacán blocks is proposed.