Lithospheric Scale Research Articles

Cambrian mafic magmatism in the Kékem area, NW edge of the Adamawa-Yadé domain, Central African Fold Belt: Implications for Western Gondwana dynamics

The tectonic evolution of the Central African Fold Belt (CAFB) has been intensely studied, but the timing of the end of the Pan-African orogenic cycle is poorly constrained, leading to contrasting tectonic models. In order to address the nature of magmatic products associated with and the timing of the end Pan-African orogenic cycle, we use mineral chemistry, whole-rock geochemistry and Sr − Nd isotopes, and zircon trace element and U − Pb geochronology of northern and southern gabbro that cut the post-collisional Central Cameroon Shear Zone; results are discussed in the tectonic framework of the CAFB. Clinopyroxene and whole-rock chemistry show that the northern and southern gabbros are mainly subalkaline, with a tholeiitic/transitional affinity. Northern and southern gabbros have REE and trace element patterns, characterized by enrichment in LREE, and depletion in HREE, Th, U, Nb, Ta, and Pb. Their Sr − Nd isotope data show low (87Sr/86Sr)i of 0.703793–0.704608 and slightly negative ɛNd(t) values ranging from −3.20 to −0.66 with Paleoproterozoic Nd model ages of 2149–1612 Ma. Moreover, the northern and southern gabbros yield U − Pb zircon ages of 486 ± 15 Ma and 497 ± 13 Ma, respectively. Our combined data is consistent with the northern and southern gabbroic magma having originated at (or near) a deep, probably asthenospheric mantle source and may have interacted with Paleoproterozoic subduction metasomatized lithospheric mantle during ascent as a result of lithospheric extension associated with lithospheric scale shearing along the shear zone. In concert with previous studies, we suggest that the evolution of the CAFB was characterized by subduction and eventual collision at 660–600 Ma followed by a 600–570 Ma post-collisional stage and final extension from 560 − 486 Ma. The extensional regime was associated with mafic to felsic magmatism with geochemical signatures evolving towards a tholeiitic/transitional or alkaline affinity. Correlation of the CAFB with the Borborema Province of the Brasiliano fold belt suggests that both shared a similar tectonic evolution and orogenic cycle characterized by early crustal thickening followed by transcurrent tectonics and ultimate lithospheric extension.

Large-scale, crustal-block vertical extrusion between the Hines Creek and Denali faults coeval with slip localization on the Denali fault since ca. 45 Ma, Hayes Range, Alaska, USA

AbstractOblique convergence along strike-slip faults can lead to both distributed and localized deformation. How focused transpressive deformation is both localized and maintained along sub-vertical wrench structures to create high topography and deep exhumation warrants further investigation. The high peak region of the Hayes Range, central Alaska, USA, is bound by two lithospheric scale vertical faults: the Denali fault to the south and Hines Creek fault to the north. The high topography area has peaks over 4000 m and locally has experienced more than 14 km of Neogene exhumation, yet the mountain range is located on the convex side of the Denali fault Mount Hayes restraining bend, where slip partitioning alone cannot account for this zone of extreme exhumation. Through the application of U-Pb zircon, 40Ar/39Ar (hornblende, muscovite, biotite, and K-feldspar), apatite fission-track, and (U-Th)/He geo-thermochronology, we test whether these two parallel, reactivated suture zone structures are working in tandem to vertically extrude the Between the Hines Creek and Denali faults block on the convex side of the Mount Hayes restraining bend. We document that since at least 45 Ma, the Denali fault has been bent and localized in a narrow fault zone (<160 m) with a significant dip-slip component, the Mount Hayes restraining bend has been fixed to the north side of the Denali fault, and that the Between the Hines Creek and Denali faults block has been undergoing vertical extrusion as a relatively coherent block along the displacement “free faces” of two lithospheric scale suture zone faults. A bent Denali fault by ca. 45 Ma supports the long-standing Alaska orocline hypothesis that has Alaska bent by ca. 44 Ma. Southern Alaska is currently converging at ~4 mm/yr to the north against the Denali fault and driving vertical extrusion of the Between the Hines Creek and Denali faults block and deformation north of the Hines Creek fault. We apply insights ascertained from the Between the Hines Creek and Denali faults block to another region in southern Alaska, the Fairweather Range, where extreme topography and persistent exhumation is also located between two sub-parallel faults, and propose that this region has likely undergone vertical extrusion along the free faces of those faults.

Seismic evidence of two cryptic sutures in Northwestern Australia: Implications for the style of subduction during the Paleoproterozoic assembly of Columbia

Plate tectonics, including rifting, subduction, and collision processes, was likely to have been different in the past due to the secular cooling of the Earth. The northeastern part of the West Australian Craton (WAC) has a complex Archean and Paleoproterozoic tectonic history; therefore, it provides an opportunity to study how subduction and collision processes evolved during the emergence of plate tectonics, particularly regarding the assembly of Earth's first supercontinent, Columbia. Because the northeastern boundary of the WAC and the southwestern boundary of the North Australian Craton (NAC) are covered by the Phanerozoic Canning Basin, the regional tectonic evolution has remained enigmatic, including how many tectonic elements were assembled and what may have driven rifting and subsequent collision events. Here, we use new passive-source seismic modeling to identify a seismically distinct segment of the lithosphere, the Percival Lakes Province, which lies east of the Pilbara Craton and is separated by two previously unknown southeast-trending lithosphere scale Paleoproterozoic sutures. We interpret that the northeastern suture, separates the Percival Lakes Province from the NAC, records the amalgamation of the WAC with the NAC. The southwestern suture separates the PLP from the reworked northeastern margin of the Pilbara Craton, including the East Pilbara Terrane and the Rudall Province. A significant upper mantle dipping structure was identified in the southwestern suture, and we interpret it to be a relic of subduction that records a previously unknown Paleoproterozoic collision that pre-dated the amalgamation of the WAC and NAC. By comparing our findings with previously documented dipping features, we show that the Paleoproterozoic collisions are seismically distinguishable from their Phanerozoic counterparts.

New Constraints on the Structure and Composition of the Lithospheric Mantle Beneath the Slave Craton, Nw Canada from 3-D Magnetotelluric Data – Origin of the Central Slave Mantle Conductor and Possible Evidence for Lithospheric Scale Fluid Flow

New Constraints on the Structure and Composition of the Lithospheric Mantle Beneath the Slave Craton, Nw Canada from 3-D Magnetotelluric Data – Origin of the Central Slave Mantle Conductor and Possible Evidence for Lithospheric Scale Fluid Flow

The use of «native» wavelet transform for determining lateral density variation of the Volgo-Uralian subcraton

In the last two decades in conjunction with the development of satellite gravimetry, the techniques of regional-scale inverse and forward gravity modeling started to be more actively incorporated in the construction of crustal and lithospheric scale models. Such regional models are usually built as a set of layers and bodies with constant densities. This approach often leads to a certain difference between the initially used measured gravity field and a gravity field that is produced by the model. One of the examples of this kind of models is a recent lithospheric model of the Volgo-Uralian subcraton. In the current study, we are applying the method of «native» wavelet transform to the residual gravity anomaly for defining the possible lateral density variations within the lithospheric layers of Volgo-Uralia. Keywords: wavelet transform; gravity field inversion; forward gravity modeling; Volgo-Uralian subcraton; satellite gravimetry.

Elastic anisotropies of deformed upper crustal rocks in the Alps

Abstract. The crust within collisional orogens is very heterogeneous both in composition and grade of deformation, leading to highly variable physical properties at small scales. This causes difficulties for seismic investigations of tectonic structures at depth since the diverse and partially strong upper crustal anisotropy might overprint the signal of deeper anisotropic structures in the mantle. In this study, we characterize the range of elastic anisotropies of deformed crustal rocks in the Alps. Furthermore, we model average elastic anisotropies of these rocks and their changes with increasing depth due to the closure of microcracks. For that, pre-Alpine upper crustal rocks of the Adula Nappe in the central Alps, which were intensely deformed during the Alpine orogeny, were sampled. The two major rock types found are orthogneisses and paragneisses; however, small lenses of metabasites and marbles also occur. Crystallographic preferred orientations (CPOs) and volume fractions of minerals in the samples were measured using time-of-flight neutron diffraction. Combined with single crystal elastic anisotropies these were used to model seismic properties of the rocks. The sample set shows a wide range of different seismic velocity patterns even within the same lithology, due to the microstructural heterogeneity of the deformed crustal rocks. To approximate an average for these crustal units, we picked common CPO types of rock forming minerals within gneiss samples representing the most common lithology. These data were used to determine an average elastic anisotropy of a typical crustal rock within the Alps. Average mineral volume percentages within the gneiss samples were used for the calculation. In addition, ultrasonic anisotropy measurements of the samples at increasing confining pressures were performed. These measurements as well as the microcrack patterns determined in thin sections were used to model the closure of microcracks in the average sample at increasing depth. Microcracks are closed at approximately 740 MPa yielding average elastic anisotropies of 4 % for the average gneiss. This value is an approximation, which can be used for seismic models at a lithospheric scale. At a crustal or smaller scale, however, local variations in lithology and deformation as displayed by the range of elastic anisotropies within the sample set need to be considered. In addition, larger-scale structural anisotropies such as layering, intrusions and brittle faults have to be included in any crustal-scale seismic model.

Accommodation of compression and lateral extension in a continental crust: Analogical modeling of the Central Atlas (eastern Algeria, Tunisia) and Pelagian sea

At the eastern boundary of the North African Alpine Chain (NAAC), coexisting grabens, thrusts, and strike-slip faults form a complex structural pattern in the general context of Nubia-Eurasia plates convergence. In a context where the root of several major structures is poorly constrained, debates exist on the possible roles of orthogonal to shortening extension, décollement layer at the base of the Mesozoic cover, and lithospheric scale processes such as delamination. We present analog modeling experiments of shortening and/or orthogonal extension applied to an upper-crust consisting of sand lying on weighted silicon. When considered, a second silicon layer mimics a décollement at the basement-cover boundary.Our results highlight: 1) The regional structural pattern results from both crustal shortenings related to convergence between Nubia and Eurasia plates and orthogonal extensions due to a far-field pull of the Western-Hellenic/Dinaride subduction. 2) Several large-scale structures such as the Zaghouan thrust faults, the Sicilia Channel rift system, and the Gafsa Fault zone root deep into the continental crust. 3) The Atlas Triassic décollement layer is responsible for thin-skin tectonics in the Atlassic domain leading to the formation of surface folds called “Djebels”, extension in the “Fossés” area, and also favoring underplating of upper-crust slices in northern Tunisia. 4) Misfit on dominating extension direction at the wedge top between model and nature highlights the role of processes at the scale of the lithosphere such as the formation of a crustal root below the Atlassic wedge and opposite vertical motions between Atlas and Pelagian areas as a consequence of lithospheric delamination.

Exploring the links between volcano flank collapse and the magmatic evolution of an ocean island volcano: Fogo, Cape Verde

Mass-wasting of ocean island volcanoes is a well-documented phenomenon. Massive flank collapses may imply tens to hundreds of km3 and generate mega-tsunamis. However, the causal links between this large-scale, low-frequency instability, and the time–space evolution of magma storage, crystal fractionation/accumulation, lithospheric assimilation, and partial melting remains unclear. This paper aims at tracking time variations and links between lithospheric, crustal and surface processes before and after a major flank collapse (Monte Amarelo collapse ca. 70 ka) of Fogo volcano, Cape Verde Islands, by analysing the chemical composition (major, trace elements, and Sr–Nd–Pb isotopes) and age-controlled stratigraphy (K–Ar and Ar–Ar dating) of lavas along vertical sections (Bordeira caldera walls). The high-resolution sampling allows detecting original variations of composition at different time-scales: (1) a 60 kyrs-long period of increase of magma differentiation before the collapse; (2) a 10 kyrs-long episode of reorganization of magma storage and evacuation of residual magmas (enriched in incompatible elements) after the collapse; and (3) a delayed impact at the lithospheric scale ~ 50 kyrs after the collapse (increasing EM1-like materiel assimilation).

Gravity modeling of the Alpine lithosphere affected by magmatism based on seismic tomography

Abstract. The southern Alpine regions were affected by several magmatic and volcanic events between the Paleozoic and the Tertiary. This activity undoubtedly had an important effect on the density distribution and structural setting at lithosphere scale. Here the gravity field has been used to create a 3D lithosphere density model on the basis of a high-resolution seismic tomography model. The results of the gravity modeling demonstrate a highly complex density distribution in good agreement with the different geological domains of the Alpine area represented by the European Plate, the Adriatic Plate and the Tyrrhenian basin. The Adriatic-derived terrains (Southalpine and Austroalpine) of the Alps are typically denser (2850 kg m−3), whilst the Alpine zone, composed of terrains of European provenance (Helvetic and Tauern Window), presents lower density values (2750 kg m−3). Inside the Southalpine, south of the Dolomites, a well-known positive gravity anomaly is present, and one of the aims of this work was to investigate the source of this anomaly that has not yet been explained. The modeled density suggests that the anomaly is related to two different sources; the first involves the middle crust below the gravity anomaly and is represented by localized mushroom-shaped bodies interpreted as magmatic intrusions, while a second wider density anomaly affects the lower crust of the southern Alpine realm and could correspond to a mafic and ultramafic magmatic underplating (gabbros and related cumulates) developed during Paleozoic extension.

Interpretation of ship-track magnetic data near Narcondam and Barren Island volcanoes revisited

We interpret the legacy ship-track magnetic and bathymetric profile data to investigate the role of the lithospheric scale West Andaman Fault (WAF) in the dormant Narcondam and active Barren Island volcanoes. We use data-based automated interpretation of total magnetic intensity (TMI) profiles using a contact model in order to delineate geometrical and physical parameters of several faults including the WAF and then investigate its implications in recent volcanism at the Barren Island. Our data-based interpretation of TMI data requires transformation of it into the first order analytical signal and tilt angle, which in turn, requires estimation of the first order horizontal and vertical derivatives of TMI data. We ensure robust estimation of the first order horizontal and vertical derivatives using Savitzky–Golay derivative filter and Hilbert–Noda transformation, respectively. We have identified several anomaly segments of the TMI anomaly profiles and used data-based quantitative interpretation in each segments with a contact model. We then make a comprehensive interpretation using bathymetry, geological setup and the results from quantitative data-based interpretation of magnetic data, and ascertain possible role of the WAF in Barren Island volcanism.

Upper mantle anisotropy in the northwest Himalaya and Ladakh-Karakoram zone based on SKS splitting analysis

Seismic anisotropy of the upper mantle beneath the northwest Himalaya and eastern part of the Ladakh-Karakoram zone has been investigated using shear-wave splitting of core-mantle refracted SKS waveforms from 94 teleseismic earthquakes recorded at 28 broadband seismic stations. The SKS splitting parameters (Φ and δt) show a significant strength of anisotropy with a wide range of delay time of split waves (∼0.75−2.94 s). The distinct variations in Fast Polarization Directions (FPDs) along the study section suggest a complex source of anisotropy that cannot be explained by a simple model. In the frontal part of the Himalaya, the FPDs are mostly parallel or sub-parallel to the strike of the Himalayan orogeny suggesting deformation in the shallow lithospheric mantle under compression owing to the India-Asia collision. On the other hand, FPDs observed in the Lesser, Higher, and Tethyan Himalaya largely follow NE-oriented absolute plate motion (APM) of the Indian plate which can be attributed to basal shear as the Indian plate moves over the asthenospheric mantle with a minor contribution from shallow lithospheric deformation. A complex anisotropy pattern is observed in the Indus Suture Zone. The FPDs near the Karakoram Fault Zone (KFZ) are parallel or sub-parallel to the strike of the KF. The strike-slip or transpressional deformation in the lithospheric mantle in the KFZ is considered as the major source of anisotropy beneath the KFZ. The study envisaged that the KF is a lithospheric scale fault that largely accommodates the India-Asia collision and extrusion in the Tibetan Plateau.

Hybrid local and teleseismic P-wave tomography in North Tanzania: role of inherited structures and magmatism on continental rifting

SUMMARY While local earthquake tomography is typically used to image the crust, this technique has restricted depth penetration due to short receiver-source distances. Regional tomography however aims to image the upper mantle from teleseismic events but suffers from poor resolution from 0 down to 40 km depth. We present here a hybrid method that combines the two approaches taking advantage of the short-wavelength resolution within the crust to better constrain the ray path at depth, and thus to improve the lithospheric imaging. Using this new method enhances the continuity or disruption of mantle anomalies towards the surface. Such hybrid tomographic images of crust-to-upper mantle structures are then critical to understand the relation and interplay between the thermal and mechanical lithospheric processes and the role in the localization of the deformation at the surface. We apply our approach to the North Tanzanian Divergence (NTD), where those processes interact with a cold cratonic lithosphere. Our new tomographic images clearly demonstrate the impact of deep-seated processes on surface features. First, strong lateral velocity anomalies and clustered seismicity in the crust are consistent with the surface geology of the NTD (rifted basins, volcanoes and border faults). Then, at a lithospheric scale, the velocity distribution highlights the major role of inherited structures in guiding the rift opening. In particular, our study suggests a strong influence of the Masai cratonic block, south of the NTD, in the rift evolution. The transition from the north–south axial valley into three diverging rift arms (Eyasi, Natron-Manyara and Pangani) is likely due to the change in rheology and to the presence of magma along inherited sutures between the craton and the mobile belts.

Possible Source of the Deep Earthquake of October 15, 2018 in the Sea of Azov

Earthquakes in the Sea of Azov are quite commonplace. Until now, the idea of ​​their nature has focused on tectonic processes in the Earth’s crust. In our work, using morphotectonic analysis, we examined on a lithospheric scale the geodynamic situation in the collision zone of the East Black Sea Plate and the margin of the East European Platform, in order to show that not only in the crust, but also in the lower lithosphere, the necessary tectonic prerequisites exist for the accumulation of seismic energy with subsequent release in the form of earthquakes. Based on a case study of an earthquake that occurred on October 15, 2018, in the Sea of Azov, we showed the possible causes of its occurrence for two options of solving the seismological problem, obtained by the Geophysical Service of the Russian Academy of Sciences and including in the solution according to V.Yu. Burmin’s method. The following focal parameters are published on the Geophysical Survey website: latitude, 46.36; longitude, 37.18; depth, 5 km; mb= 4.3. The focal parameters were obtained for the solution using the Burmin method: latitude, 46.42564; longitude, 37.24274; H = 133.4 km. The mechanisms of two solutions were constructed; for a source with a depth of 5 km, the focal mechanism was obtained: a reverse fault, with the source confined to the intersection of Kalmius-Dzhiginsky and the Main Azov faults; for 133.4 km, a strike-slip fault, with the source confined to the intersection of the Gubkinsky-Central Azov, Kalmius-Dzhiginsky, and the Main Azov faults. The constructed model of collision of the continental lithosphere of the East European Platform and the subducting East Black Sea Plate shows that under conditions of weak compressive forces from the side of the Arabian indenter, a pseudosubduction situation is formed, as a result of which shear stresses occur in the deformable frontal part of the continental lithosphere of the East European Platform, transitioning to a reverse fault along the line of shear. During this process, seismic energy is accumulated, accompanied by weak and moderate earthquakes, of a reverse-fault type for the earth’s crust in the region of the Azov Rise, and a normal-fault type in the lower part of the wedge of the continental lithosphere as a result of its destruction. Regional morphotectonic analysis did not allow us to choose the most probable focal mechanism for the earthquake. However, it is noteworthy that the results of our research have shown that the occurrence of deep earthquakes has all the necessary tectonic prerequisites, despite the well-established opinion of seismologists about the exclusively crustal location of earthquakes in this region.

Petrology, geochemistry and geochronology of granites and granite gneisses in the SE Karakoram, India: Record of subduction-related and pre- to syn-kinematic magmatism in the Karakoram Fault Zone

The granites and granite gneisses from a tectono-metamorphic complex exposed along the Shyok Valley in the Karakoram region, India, forms the southern margin of Asian plate in the India-Asia collision zone. These rocks have been subjected to mineralogical, geochemical, and U-Pb zircon geochronological investigations to constrain the petrogenetic and geodynamic evolution of the Karakoram terrane. Outcrop-scale observations reveal the presence of pre- and syn-kinematic leucogranite bodies intruded within the granites and granite gneisses. The foliation-parallel deformed leucogranite sill shows the dextral shear sense in an extensive metamorphic complex that is in concordance with the Karakoram Fault (KF) in the Ladakh region, NW India. Whole-rock elemental and biotite chemistry equivocally suggest subduction-related metaluminous (I-type) calc-alkaline nature for most of the host granites and gneisses, and generation of syn-collisional peraluminous (S-type) leucogranites through crustal anatexis of pre-existing rocks. The obtained U-Pb zircon ages for the granites, granite gneisses, deformed and undeformed leucogranites vary widely from ca. 160 Ma to 15 Ma and reveal: (a) the initiation of subduction of the Neo-Tethyan oceanic crust beneath the southern Asian Plate at least ~160 Ma ago, and (b) existence of continuous or intermittent deformation along ~1000 km long lithospheric scale dextral KF during ~27−15 Ma.

Past hot fluid flows in limestones detected by Δ47–(U-Pb) and not recorded by other geothermometers

Abstract Geothermometers are commonly used to reconstruct the diagenetic and thermal history of rocks. However, characterizing the timing, origin, and temperature of paleofluid flow remains challenging because it must be assessed indirectly through the analysis of microscopic cements that precipitate and fill intergranular spaces during fluid circulation. Here, we measure both the clumped isotope (Δ47) temperature and in situ U-Pb age of individual diagenetic calcite cements within a sedimentary section of the Paris Basin (France), whose thermal history has been previously inferred to be <60 °C. We show that cementation occurred during two stages associated with major events at the western European lithospheric scale: (1) the Bay of Biscay rifting (Late Jurassic–Early Cretaceous), and (2) north-south Pyrenean compression (Eocene) followed by east-west extension during the European Cenozoic rift system event (Oligocene). Related to both events, we report unexpectedly hot fluids, up to 110 °C, contrasting with the lower temperatures inferred from other geothermometers (e.g., fluid inclusions, clay minerals, apatite fission tracks, maturity of organic matter by Rock-Eval pyrolysis, or vitrinite reflectance). These high temperatures (>70 °C) have been measured for calcite cements containing single-phase aqueous fluid inclusions, challenging the commonly accepted assertion that the absence of nucleation of a vapor phase indicates crystallization at low temperature (∼<70 °C). We suggest that the kinetics of mineralization events prevented the recording of short-lived hot fluid flows by other geothermometers.

Seismic Azimuthal Anisotropy for the Southeastern Tibetan Plateau Extracted by Wave Gradiometry Analysis

AbstractWave Gradiometry (WG) is a dense array data processing method used to extract seismic velocity and other properties of the earth. In this study, we propose using WG for a plane wave field to measure azimuthal anisotropy by fitting the phase velocities from different back azimuths. We applied this new method to the Temporary Western Sichuan Array and obtained the isotropic and azimuthal anisotropy for period bands centered at T = 20, 40, and 60 s in the southeast Tibetan Plateau. Our results show that the fast orientations of anisotropy are well consistent with the strikes of the fault system and orogenic belt. The weak anisotropic magnitude in the northern part of the south Chuandian subblock and a consistence of fast orientation in boundary fault zone (BFZ) at different periods may be attributed to anisotropic relics associated with the Emeishan Large Igneous Province. Comparing our results with those of previous studies, we found that our anisotropic results are consistent with anisotropic structures measured by the Pms phase in the plateau area, while a systematical angular difference exists in the BFZ, Sichuan Basin, and south Chuandian subblock where high Rayleigh wave phase velocity was observed. The Lijiang‐Xiaojin fault and Longmenshan fault appear to form a deep‐rooted strong boundary at the lithospheric scale, as they serve as a boundary for anisotropic structures, isotropic velocity, and the other three WG parameters.

Magmatic and tectonic segmentation of the intermediate-spreading Costa Rica Rift—a fine balance between magma supply rate, faulting and hydrothermal circulation

SUMMARY 3-D tomographic modelling of wide-angle seismic data, recorded at the intermediate-spreading Costa Rica Rift, has revealed a P-wave seismic velocity anomaly low located beneath a small overlapping spreading centre that forms a non-transform discontinuity at the ridge axis. This low velocity zone displays a maximum velocity anomaly relative to the ‘background’ ridge axis crustal structure of ∼0.5 km s−1, has lateral dimensions of ∼10 × 5 km, and extends to depths ≥2.5 km below the seabed, placing it within layer 2 of the oceanic crust. We interpret these observations as representing increased fracturing under enhanced tectonic stress associated with the opening of the overlapping spreading centre, that results in higher upper crustal bulk porosity and permeability. Evidence for ongoing magmatic accretion at the Costa Rica Rift ridge axis takes the form of an axial magma lens beneath the western ridge segment, and observations of hydrothermal plume activity and microearthquakes support the presence of an active fluid circulation system. We propose that fracture pathways associated with the low velocity zone may provide the system through which hydrothermal fluids circulate. These fluids cause rapid cooling of the adjacent ridge axis and any magma accumulations which may be present. The Costa Rica Rift exists at a tipping point between episodic phases of magmatic and tectonically enhanced spreading. The characteristics inherited from each spreading mode have been preserved in the crustal morphology off-axis for the past 7 Myr. Using potential field data, we contextualize our seismic observations of the axial ridge structure at the whole segment scale, and find that the proposed balance between magmatic and tectonically dominated spreading processes observed off-axis may also be apparent along-axis, and that the current larger-scale magma supply system at the Costa Rica Rift may be relatively weak. Based on all available geophysical observations, we suggest a model for the inter-relationships between magmatism, faulting and fluid circulation at the Costa Rica Rift across a range of scales, which may also be influenced by large lithosphere scale structural and/or thermal heterogeneity.

Gold in the lithosphere of the western South China Block, SW China: Insights from quartz porphyries from the giant Zhenyuan gold deposit

Numerous Au-rich deposits in the western South China Block, SW China, provide insights into the gold enrichment process at a lithospheric scale. Zhenyuan is a giant gold deposit in the western South China Block, but its genesis remains controversial with competing hypotheses of magmatic–hydrothermal and metamorphic–hydrothermal development. Here, we present new petrographic, major and trace element, and zircon U–Pb age data for quartz porphyry from the Zhenyuan deposit to evaluate the specific gold enrichment process. Zircon SHRIMP and LA–ICP–MS results demonstrate that the Zhenyuan quartz porphyries crystallized at ca. 255–247 Ma. This age is broadly consistent with the Re–Os age of gold-bearing pyrite measured in previous studies. The porphyries are characterized by high-K calc-alkaline compositions and exhibit significant enrichment in LILEs and LREEs, and strong depletion in HFSEs. They share the same source (Au-rich lower crust) with Cenozoic Au-bearing intrusions. The initial melts had a weak oxidation state (ΔFMQ = –0.15–0.55), which favored Au accumulation and migration. However, the gold was precipitated with the crystallization of Ti–Fe oxides in the middle crust, resulting in the emplacement of Au-poor magma in the upper crust. The gold orebodies in the Zhenyuan deposit formed due to Cenozoic hydrothermal processes, as evidenced by our observations of ore-bearing pyrite and Ar–Ar ages of phlogopite measured in previous studies. Combined with regional mineralization data, we propose lithosphere-scale processes for gold accumulation, migration, and precipitation from the Neoproterozoic to Cenozoic. Specifically, we predict an Au-rich middle crust beneath the western South China Block.

Lithospheric SH Wave Velocity Structure Beneath the Northeastern Tibetan Plateau From Love Wave Tomography

AbstractThe northeastern (NE) Tibetan plateau has been a prime site to understand the dynamic processes responsible for the rise and lateral growth of the Tibetan plateau. Here we construct a high‐resolution lithospheric scale isotropic SH wave velocity model (down to the depth of 130 km) for the NE Tibetan plateau and its adjacent regions based on the measurements of fundamental‐mode Love wave dispersions (20–100 s) with seismic data recorded by ChinArray II and China Digital Seismic Array using two‐plane wave method. Prominent slow SH wave velocity (VSH) is observed in the midlower crust of the NE Tibetan plateau, specifically in the Qilian Orogenic Belt and the Songpan‐Ganzi Terrane regions, coincident with previously indicated significantly slow SV wave velocity (VSV), probably implying the existence of partial melts in the midlower crust. This low SH wave velocity anomaly could be traced down to the uppermost mantle, indicating a weak and warm lithosphere, which we interpret as a consequence of asthenosphere upwelling after lithospheric mantle removal in the NE Tibetan plateau. The asthenosphere upwelling and associated deep‐seated thermal buoyancy could explain the occurrence of partial melting in the mid‐lower crust and account for parts of high elevations in the NE Tibetan plateau. The western Qinling orogenic belt is characterized with a high velocity anomaly in most of lithosphere, which is inconsistent with the existence of channel flow within the lithosphere along the Qinling orogenic belt from the Tibetan plateau.

Interplay between volcanic and plutonic systems: A case study of the early Miocene Solarya Volcano-plutonic Complex in NW Anatolia (Turkey)

Linking volcanic and plutonic systems and examining them, together is critical to evaluate magma chamber processes and mantle-crust, interactions in lithospheric scale. Solarya Volcano-plutonic complex, (SVPC) is one of the major volcano-plutonic complexes located in NW, Turkey and it is remarkable for its coeval and cogenetic plutonic, hypabyssal and volcanic rocks. Solarya pluton (22 Ma) is an epizonal, pluton which consists of three main rock groups; K-feldspar, megacrystalline granodiorite, microgranodiorite and haplogranite. Plutonic rocks pass into hypabyssal association, represented by, granodiorite/quartz-diorite porphyry sheet intrusive rocks which were, injected into the ring and radial faults concurrently with the, emplacement of the pluton. Hypabyssal and volcanic rocks cut one another, and fragments of hypabyssal rocks are found in volcanic rocks. Volcanic, association, is formed from trachyandesite, andesite and dacite lavas, with associated pyroclastic rocks. New 40Ar/39Ar radiometric ages of, volcanic and hypabyssal associations display that, they have 40Ar/39Ar, biotite ages changing from 23.1 ± 0.2 to 21.0 ± 0.2 indicating that they, are coeval with Solarya Plutonic Association. Petrographical features of, SVPC display disequilibrium textures such as oscillatory zoning, sieve, and honeycomb textures in plagioclase, reverse mantled biotite and, hornblende crystals as indicatives of open system magma chamber processes, (mixing/mingling and/or AFC). Plutonic, hypabyssal and volcanic, associations of SVPC show similar major-trace and Sr-Nd-Pb-O isotopic, compositions indicating a common magma source and similar magma chamber, processes. They are medium to high-K calc-alkaline rocks and display, enrichment in LILE and depletion in Nb, Ta, P and Ti. The geochemical and, isotopic characteristics of SVPC are considered to reflect an enriched, mantle (EMII) or sub-continental lithospheric mantle source. Bulk mixing, model between ƐNd and 87Sr/86Sr isotopic data suggests that, the crustal, contribution to the enriched mantle source is about 25–45%. Evaluation of all these data suggests that, Solarya Volcano-plutonic, Complex with early Miocene age was developed under syn-convergent, extensional regime concurrent with the exhumation of other major volcanoplutonic, and core complexes (eg. Kozak, Menderes, Kazdağ, Çataldağ) in NW, Anatolia.