Diapiric Rise Research Articles

The fate of ultramafic-rich mélanges in cold to hot subduction zones: Implications for diapirism (or not) and chemical geodynamics

Buoyant ultramafic-rich (serpentine- or chlorite-rich) mélange diapirs in sediment-starved subduction zones can transport slab material to arc sources. While the buoyancy of chlorite-rich mélanges was previously investigated, serpentine-rich mélanges were never explored. Thus, the overall contribution of ultramafic-rich mélanges to buoyancy, the conditions for diapir formation, and their fate in subduction zones are not well constrained. Here, we investigate the partial melting behavior and the associated density transformations of a serpentine-rich matrix (5–10 wt.% H2O) with minor sediments (9:1 ratio) at fore-arc (∼65 km) to sub-arc (∼95 km) depths (2–3 GPa and 800–1250 °C) and compare to that of chlorite-rich mélanges from the literature. Our results show that the solidus of serpentine-rich matrices is between 1050 and 1100 °C and requires either diapiric rise of the mélange into the hotter mantle wedge or interactions with a hotter asthenosphere through slab tears to partially melt and produce basaltic melts, whether in hot or cold slab channels. Chlorite-rich mélanges may account for the sources of some arc lavas, but partial melting of serpentine-rich mélanges produce melts depleted in CaO, TiO2, alkalis, and are highly enriched in MgO compared to basaltic arc lavas. Both serpentine-rich and chlorite-rich matrices dehydrate to form denser peridotite and lose buoyancy at ∼800 °C and ≥1000 °C, respectively. Even if diapirism initiates in such mélanges near the slab-mantle interface, they would likely lose buoyancy upon ascent into the hotter mantle wedge resulting in stalled or failed diapirs. Diapir growth (τa) is controlled by the interplay of density, thickness and viscosity of the mélange, as well as the timescale of slab subduction (τs) and thermal structure of the subduction zone. We observe that the onset of diapirs in cold subduction zones requires mélanges that may sometimes be thicker than that observed by field and geophysical studies, while hot subduction zones overall require thinner mélanges. Thus, ultramafic-rich mélange diapirs may occur but only under specific conditions and when the diapiric ascent timescale is faster than the thermal equilibration barrier of ∼800–1000 °C (especially at the core of the mélange). Dehydration or partial melting of ultramafic-rich mélanges can affect the large ion lithophile element (LILE), volatiles, and high-field strength element (HFSE) budgets in the mantle wedge. Partial melting (caused by a diapiric rise or slab tear) does not fractionate LILEs from HFSEs at T ≥ 1100 °C and if the mélange has a lower LILE/HFSE to begin with, that signature is transferred to arc sources. Dehydration releases aqueous fluids rich in fluid-mobile elements (LILE and volatiles) relative to HFSE. Thus, the characteristic high LILE/HFSE signature of aqueous fluids is transferred to arc magma sources. Given high LILE/HFSE ratio is a ubiquitous arc magma signature, but slab tears are not, and diapirism in ultramafic-rich mélanges is highly conditional, this study corroborates that aqueous fluids released from sediment-starved mélanges are the predominant mass transfer agents rather than diapirs.

Active and passive salt diapirs: a numerical study

Summary Salt diapirs dominate the structure in many sedimentary basins and control the preservation and migration of hydrocarbon. The formation of salt diapirs generally falls into two endmember models: active (up-building) and passive (down-building) diapirism. In the active model, salt diapirs rise from salt buoyancy to pierce through the sedimentary overburden, whereas in the passive model, salt diapirs result from differential loading of sediments during deposition. These endmember models are mostly conceptual or kinematic, the mechanics of active and passive diapirism, and their relative roles and interactions in the formation of salt diapirs, remain uncertain. Here, we use two-dimensional high-resolution numerical models to investigate the primary factors and critical conditions for active and passive diapirism. Our results indicate that it is improper to use driving mechanisms to classify salt diapirs, because the buoyancy-driven active salt diapirism involves differential loading, while the passive diapirism requires salt buoyancy. The rise of salt diapirs is more sensitive to the effective viscosity of the overburden than to the salt viscosity. Stiff overburdens could prevent the rise of salt diapirs, but they could be pierced by salt diapirs if plastic yield of the overburden is allowed. During deposition, the coupled salt-sediment deformation, driven by both salt buoyancy and differential loading of sediments, can lead to various diapiric salt structures and minibasins. Regional tectonic stress generally promotes salt diapirism by enhancing strain weakening of salts and overburdens. We suggest that the classification of active and passive salt diapirism is an oversimplification in most cases. We propose a general model of the formation of salt diapirs that usually begins with dome initiation driven by salt buoyancy, followed by syndepositional down-building controlled by sedimentation and differential loading, and ends with canopy formation when sedimentation stops.

3D variation of shortened salt walls from the Moroccan Atlas: Influence of salt inclusions and suprasalt sedimentary wedges

We present a field study of salt ridges and minibasins of the Moroccan High Atlas near Rich that shows the along- and across-strike variability of such structures. Salt walls evolved from halokinesis during Jurassic rifting, to shortening during Cenozoic orogeny. Salt wall segments exhibit variable degrees of welding due primarily to the local presence of intrasalt inclusions (basalt and gabbro) rather than orientation with respect to the shortening or position along the ridge. Flanking Jurassic minibasins may be upright and symmetric or tilted; tilting may have started during halokinesis but was largely acquired during shortening. Minibasins tilt away from welded diapiric segments towards inclusion-rich segments, indicating differential diapir rise. The structure of the central parts of the salt walls differs from the lateral terminations. While the central parts are relatively simple with aperture or welding governed by the inclusions, many salt walls end buried by suprasalt sedimentary wedges. These perched wedges were not arched upward during diapir squeezing but were unexpectedly folded into synclines. The folding was formed by the flanking minibasin tilting in one limb and by inhomogeneous diapir rise and lateral salt escape in the other, thus defining a new modality of roof and shoulder folding above salt diapirs.

Detrital signatures of clastic serpentinite in tectonically diverse settings and interpretation of an example from the Northern Apennines

Abstract Serpentine-bearing sediment, a rare sediment type that is formed and deposited in divergent, convergent, transform, and collisional plate-tectonic settings, carries important evidence of sediment provenance. Specific sources of serpentine-rich sediment display grain assemblages of distinct character that can be used to infer the serpentinization condition and sediment formation. This study reports quantitative and qualitative results on serpentine components in sandstones from Ocean Drilling Program Legs 149 (Iberia), 210 (Newfoundland), and 125 (Mariana and Izu–Bonin regions), and from serpentine-rich debris flows and arenitic breccias in deep-water successions in the Northern Apennine fold–thrust belt. We propose a textural scheme that offers a visual guidance for evaluating serpentinite grains that can be broadly adopted, is easily reproducible, and reduces user bias in determining compositional modes that allow comparison of serpentinite grain populations in arenites from different depositional environments, provenance, and associated tectonic settings. These data allow us to define a scheme for serpentine-dominated deposits that demonstrates the presence of two main groups of grain textures (pseudomorphic and non-pesudomorphic) with specific mineralogy and crystal shape as a function of temperature and pressure in the source rocks. The quantitative analysis of the serpentine-rich arenites and fine-grained sediments derived from forearc and rifted continental-margin settings shows that the studied samples are characterized by high percentages (c. ≥ 80%) of serpentine detritus and subordinate dense minerals and other lithic fragments, including basalt. In rifted continental-margin settings, the prevalent textures in serpentinite sandstones consist of polygonal mesh, mesh-core, and hourglass that all belong to the pseudomorphic category, which preserves the pre-serpentine features and mineralogy. These textures are typically formed in low-temperature conditions (< 390°C); lizardite is the most common mineral, along with minor chrysotile and, in rare cases, antigorite. In contrast, in forearc settings, serpentine-rich grain assemblages exhibit dominantly non-pseudomorphic, interlocking, and interpenetrating textures, dominantly composed of lizardite and recrystallization of lizardite by antigorite. Minor preserved ultramafic minerals related to dynamic recrystallization might be associated with the diapiric rise and protrusion of serpentine bodies. The Northern Apennines case study adopted to test this model indicates that the relationship of detrital serpentine texture to setting can be employed in provenance studies. Firstly, serpentine-bearing sediments derived from ophiolites deformed in fold–thrust belts have more variable serpentinite content, ranging from a few percent to < 10% for samples from deep marine environments, to typically c. 20 to ≤ 50% for stream and beach samples. This compositional variation arises from mixing of sediments derived from deeper to shallower oceanic lithosphere (peridotites and serpentinites) with material from overlying volcanic rocks and sedimentary cover. The deep-water serpentine-rich sands of the Northern Apennines display variable compositions with intermediate characteristics. The source of the serpentine-bearing deposits is interpreted to be a residual oceanic lithosphere characterized by subcontinental mantle-lherzolite originated in the Middle–Late Jurassic by mantle delamination. The serpentinite-dominated debris flows and sand beds contain serpentine grains that exhibit compositional and textural transitions from pseudomorphic to non-pseudomorphic categories, along with changes in mineralogy from lizardite to antigorite. Serpentinite with pseudomorphic texture is observed in the mantle section away from the deformed area. On the contrary, the presence of serpentine-rich arenites with dominant non-pseudomorphic textures suggests derivation from tectonized serpentine along fault scarps and or as products of serpentine diapirism. The detailed serpentinite texture scheme used to classify sand grains in this study includes pseudomorphic (often lizardite, minor crysotile) and non-pseudomorphic textures, with the latter attributed to temperature- and pressure-controlled recrystallization (often to antigorite) or shearing during or after serpentinization. For comparison of different detrital-serpentinite populations, a new ternary plot is proposed where counted parameters are grouped into three end members: undeformed, deformed, and recrystallized. This plot appears to discriminate different sources of detrital serpentine by tectonic setting (e.g., Iberia and Newfoundland margins vs. Mariana forearc) and shows the potential complexity of serpentinite sources in the Apennine basin example. Additional texturally based petrographic data sets are needed to determine the usefulness of this plot in provenance studies.

Late quaternary morpho-stratigraphic record of diapir rise in the Cardona salt extrusion, NE Spain. Halokinetic sequences, raised terraces and uplift rates

Active diapirism has received very limited attention from the geomorphological and Quaternary Science perspective, despite the role played by this ground deformation process in the development of landforms and sedimentary environments, and the important practical implications associated with mobile salt structures (e,g., mining, hydrocarbon production, geostorage). The Cardona salt extrusion (NE Spain) was initiated in late Quaternary times from the post-shortening unroofing of the crest of a salt anticline by the entrenchment of the transverse Cardener River. Detailed mapping, outcrop analysis, geophysical data and trenching indicate that the diapir-flanking deposits show halokinetic complexes comprising two types of morpho-stratigraphic units: (1) an older flap of coarse-grained drapefolded colluvial deposits; and (2) younger lacustrine and fluvial deposits in peripheral depressions confined between the upturned colluvial flaps and the country rock slope. These synkinematic units record an overall relief inversion and correspond to the so-called wedge and hook halokinetic sequences, recording ratios between diapir rise rate and sedimentation rate lower and greater than 1, respectively. The radiocarbon ages obtained from several raised late Holocene strath terraces carved in the salt indicate uplift rates within the range of 36.5–12.1 mm/yr, consistent with geodetic data. The spatial variability of the uplift rates can be related to increasing flow rates towards the axis of the salt wall and potential along-strike variations. The relatively high diapir rise rates observed at Cardona diapir, unaffected by contractional displacement loading (tectonic squeezing), is attributed to the youthful stage of the salt extrusion and the low dynamic viscosity of the Cardona Saline Formation, with a low proportion of impurities and a significant amount of potash salts.

Growth of a salt diapir in an anticline - A record from the Cenozoic halokinetic sequences in the Zagros Fold and Thrust Belt, Iran

Salt diapirs in southern Iran provide excellent exposures of the host rock sedimentary sequences that record salt diapiric growth in compressional settings. This study addresses the reactivation of one particular diapir in the core of an anticline of the Zagros Fold and Thrust Belt (ZFTB) using the classical concept of halokinetic sequences. These sequences reflect the relative rise of the salt diapirs with respect to the depositional rate of the sediments in their surroundings. The resulting reconstruction indicates irregular deposition of the total eight sets of host rock sedimentary packages around the diapir during the Oligocene-Miocene that developed in response to the progressive rise of the host anticline. Folding of host rocks was associated with vertical movement of blocks bounded by cross-faults radiating from the boundary of the diapir, specifically around the triangular indenter on the north, deflecting the anticline to a southward advancing salient. The halokinetic sequences record an early stage of diapiric rise in the rhythmic alternation of gypsum and fossiliferous carbonate with siliciclastic material followed by fossiliferous limestone typical for lagoonal to slope environment in a limestone reef setting. The last stage is associated with thin-skinned tectonics of the uppermost halokinetic sequences thrust towards the diapir. Rapid thickness change of the halokinetic strata along the periphery of the diapir in the southern sector that includes also the crestal region of the anticline indicates compressional reactivation of the structure as early as 11 Ma (Gachsaran Formation), likely associated with salt overflow in the anticline crestal region, where the adjacent halokinetic sequences are completely overturned.

Multiple Episodes of Rock-Melt Reaction at the Slab-Mantle Interface: Formation of High Silica Primary Magmas in Intermediate to Hot Subduction Zones

AbstractSiliceous slab-derived partial melts infiltrate the sub-arc mantle and cause rock-melt reactions, which govern the formation of diverse primary arc magmas and lithological heterogeneities. The effect of bulk water content, composition of reactants, and nature of melt infiltration (porous versus channelized) on the rock-melt reactions at sub-arc conditions have been investigated by previous studies. However, the effect of multiple episodes of rock-melt reactions in such scenarios has not been investigated before. Here, we explore mantle wedge modifications through serial additions of hydrous-silicic slab partial melts and whether such a process may ultimately explain the origin of high-Mg# andesites found in arcs worldwide. A series of piston-cylinder experiments simulate a serial addition of silicic slab melts in up to three stages (I through III) at 3 GPa and 800–1050°C, using rock-melt proportions of 75–25 and 50–50. A synthetic KLB-1 and a natural rhyolite (JR-1) represented the mantle and the slab components, respectively. Right from the first rock-melt interaction, the peridotite mantle transforms into olivine-free mica-rich pyroxenites ± amphibole ± quartz/coesite in equilibrium with rhyolitic-hydrous melts (72–80 wt% SiO2 and 40–90 Mg#). The formation of olivine-free pyroxenite seems to be controlled by complex functions of T, P, rock-melt ratio, wedge composition, and silica activity of the slab-melt. Remarkably, the pyroxenites approach a melt-buffered state with progressive stages of rock-melt reactions, where those rhyolitic melts inherit and preserve the major (alkalis, Fe, Mg, Ca) and trace element slab-signature. Our results demonstrate that lithological heterogeneities such as pyroxenites formed as products of rock-melt reactions in the sub-arc mantle may function as melt ‘enablers,’ implying that they may act as pathways that enable the infiltrating melt to retain their slab signature without undergoing modification. Moreover, the density contrast between the products of rock-melt reaction (melts and residues) and the average mantle wedge (~150 to 400 kg/m3) may help forming instabilities and diapiric rise of the slab components into the mantle wedge. However, the fate of the primitive slab-melts seems to be associated with the length of the pathway of mantle interaction which explains the evident wide magma spectrum as well as their degree of slab garnet-signature dilution. This work and the existence of high-Mg# Mexican-trondhjemites indicates that almost pristine slab-melts can make their way up to crustal levels and contribute to the arc magma diversity.

Deformation of salt structures by ice-sheet loading: insights into the controlling parameters from numerical modelling

Subsurface salt flow is driven by differential loading, which is typically caused by tectonics or sedimentation. During glaciations, the weight of an ice sheet represents another source of differential loading. In salt-bearing basins affected by Pleistocene glaciations, such as the Central European Basin System, ice loading has been postulated as a trigger of young deformation at salt structures. Here, we present finite-element simulations (ABAQUS) with models based on a simplified 50-km long and 10-km-deep two-dimensional geological cross-section of a salt diapir subject to the load of a 300-m-thick ice sheet. The focus of our study is to evaluate the sensitivity of the model to material parameters, including linear and non-linear viscosity of the salt rocks and different elasticities. A spatially and temporarily variable pressure was applied to simulate ice loading. An ice advance towards the diapir causes lateral salt flow into the diapir and diapiric rise. Complete ice coverage leads to downward displacement of the diapir. After unloading, displacements are largely restored. The modelled displacements do not exceed few metres and are always larger in models with linear viscosity than in those with non-linear viscosity. Considering the low stresses caused by ice-sheet loading and the long time-scale, the application of linear viscosity seems appropriate. The elastic parameters also have a strong impact, with lower Young's moduli leading to larger deformation. The impact of both the viscosity and the elasticity highlights the importance of a careful parameter choice in numerical modelling, especially when aiming to replicate any real-world observations.

Petrogenesis of the Córrego das Campinas Gabbro-Anorthosite Suite: Characterization of a Neoproterozoic massif-type anorthosite in the Goiás Magmatic Arc and its significance in the evolution of the Brasília belt, Brazil

Córrego das Campinas Anorthosite is a rare example of a massif-type anorthosite developed in the late Proterozoic (∼650 Ma) during the amalgamation of West Gondwana. This undisturbed, oval, and homogeneous massif body primarily consists of labradorite (An49-63) with a porphyritic and cumulatic inequigranular texture. Other rocks are associated with anorthosite, such as a gabbroic body, consisting of olivine gabbro, leucotroctolite and gabbro with granular, ophitic and cumulatic textures. The olivine gabbro presents a more primitive character, showing minerals enriched in Mg and Ca, such as olivine Fo76-78 intercumulates and anorthitic plagioclase (An90-97), albeit with an Al-rich diopside as well. The anorthosite body is also bordered by tonalite and quartz monzodiorite. These leucocratic and granular rocks contain An26-46 plagioclase in tonalite and An12-18 plagioclase in quartz monzodiorite. Given the signature of these rocks, they can be considered residual liquids from anorthosite crystallization, more enriched in Na and Fe. Metamorphosed ferrodiorites, amphibolites and ilmenite-rich levels occur in the form of dikes and veinlets associated with anorthosite and also seem to represent differentiated liquids from anorthosite crystallization. A quartz syenite body also occurs, consisting of perthitic orthoclase cumulates with an albite mantle, whose genesis is also associated with a process of accumulation in a magma chamber. A albite granite plug occurs intrusively. This albite granite presents enriched in enriched in Na, Si, Rb, Th, Ta, Nb, Ce, Zr, Hf, Y and Yb showing typical signature of an anorogenic granite. Zircon U–Pb dating of olivine gabbro, tonalite, quartz monzodiorite, ferrodiorite and albite granite indicate that these rocks are contemporaneous, with crystallization ages ranging from 648 to 661 Ma, showing older zircon grains indicating Neoproterozoic crustal material contamination (∼870–900 Ma) and even older in the genesis of these rocks. Sm–Nd model ages around 0.75 Ga and positive ϵNd(t) suggest that these rocks are juvenile and derive from depleted mantle, whereas another set of results around 1.0 Ga and less positive ϵNd(t) values indicate that these rocks are juvenile but show crustal contamination. The high percentages of plagioclase found in the rocks, combined with high Al2O3, CaO and Na2O concentrations in total rock, indicate that the rocks of this suite derive from calc-alkaline or Al-rich basaltic magmas. The formation of the suite is in accordance with the classical petrological model of plagioclase accumulation in magma chambers and its subsequent diapiric rise. The age of the suite and its positioning indicate a correlation with an important mafic magmatic event that occurred in the Brasília Belt between 670 and 600 Ma. The interpretation of geological data on the Brasília Belt, associated with the present study, indicates that the rocks of this suite initially developed in a back-arc setting during the pre-tectonic stage of an active continental margin and that the bodies ascended to shallower regions of the crust during the post-collisional stage, through pressure relief, at approximately 650 Ma. The evolution of the Brasília Belt was marked by a constantly elevated geothermal gradient and the Córrego das Campinas massif-type anorthosite supports this inference. Therefore, we suggest that the constantly elevated geothermal gradients in the Brasília Belt were caused by multiple, concurrent geodynamic processes during its evolution, such as i) blocked mantle convection currents triggered by oceanic crust subduction, which formed an intraoceanic island arc (Goiás Magmatic Arc) between 900 and 800 Ma; this physical barrier limited the heat release from these currents, generating a “heat trap” and thus accounting for the elevated geothermal gradients; ii) thickened crust caused by collisional and accretionary events; iii) increased heat of Rodinia break-up plume; and iv) elongated microplate of the Goiás Massif.

Understanding deformation band formation across a range of length scales in the Paradox Basin, Utah

In evaporite basins, salt flow can drive complex deformation of high porosity sedimentary units adjacent to salt and this has been demonstrated by recent numerical modeling studies, however field evidence of this deformation has remained elusive. Detailed analysis of high-porosity wall and cover rocks adjacent to a large salt-cored anticline in Salt Valley, UT yields three observable deformation band gradients. A long-wavelength gradient of steeply dipping normal/shear bands is recognized along structural transects up to 5 km away from the Salt Valley anticline axis. A shorter-wavelength (5–60 m) deformation band gradient shows band density increasing toward similarly-oriented brittle normal faults. A third gradient of intense compactional deformation banding is recognized adjacent to the salt diapir. Importantly, cement phases are undamaged in these bands, indicating that they likely experienced plastic critical-state deformation prior to cementation. The steeply-dipping deformation bands and normal faults are interpreted to be broadly contemporaneous and result from outer arc extension of the Salt Valley anticline; the dense compactional band network adjacent to salt likely accommodated compression during diapiric rise contemporaneous with folding. This integration of field and petrologic studies with forward modeling makes a critical step toward developing predictive models for deformation band formation in hydrocarbon systems.

Overburden deformation induced by active diapirism: Discrete element numerical simulations

AbstractActive diapirs are commonly observed in sedimentary basins worldwide. Factors controlling overburden deformation processes and patterns during the active rise of active diapirs, such as sediment cohesion and overburden thickness, are not well understood. We have utilised the discrete element numerical simulation method to simulate active diapirism in models comprised of sediments with varying cohesions and roof thicknesses. We found that sediment cohesion can significantly affect overburden deformation with low‐cohesion sediments favouring the generation of a broad, smooth forced fold and a symmetric graben, while high‐cohesion sediments favour the formation of master reverse faults and preferentially uplifted and rotated fault blocks in their hangingwalls in the central crest, with an overall asymmetric structural geometry. Moreover, overburden thickness does not significantly affect structural styles in the overburden. Our results also suggest that the higher the sediment cohesion, the higher the amplitude (the lower the wavelength) of the forced fold above the rising diapir becomes. Our models produced a wide spectrum of deformation structures that resemble those in nature and reveal a strong link between structural styles and sediment diagenesis in the context of active diapir‐piercing sediments with varying degrees of lithification.

Evolution of the Late Mesozoic Magmatism of the Omulevka Terrane of the North Part of the Verkhoyansk–Kolyma Orogenic Region

This article presents the results of a study of Late Mesozoic intrusive formations of the Omulevka terrane of the Verkhoyansk–Kolyma orogenic region. The research area covers the Selennyakh block of the Omulevka terrane and the territory adjacent to the south. The compositions of rock-forming, accessory and restitic minerals and geochemical features of intrusive rocks are considered. The methods of optical microscopy, microprobe, silicate and spectral analyses were used. There are the following several stages in the evolution of magmatism: (1) the Late Jurassic supra-subduction (gabbro, dolerites), (2) the beginning of the Early Cretaceous-transitional from supra-subduction to marginal-continental (gabbro-diorites, diorites, granodiorites), (3) the Early Cretaceous of active continental margin (granodiorites, granites), (4) the Late Cretaceous postorogenic or continental-riftogenic (alkali-feldspar granites of A-type), (5) the Late Cretaceous continental riftogenic (subalkaline gabbroids and basaltoids). In the process of evolution from stage one to stage four, there was an increase in the silicic acid content, total alkalinity and ferruginousity of rocks with the movement of magmogeneration levels to higher and higher horizons of the lithosphere (calculated pressure from 1.6–1.4 GPa to 0.6–0.9 GPa). At the same time, the preservation of high temperatures of magmogeneration (1000–1150 °C) and crystallization implies the supply of additional heat from an external (deep) source during the formation of granitoid melts. The magmatic activity is completed by the intrusion of subalkaline derivatives of a deep hearth, formed by metasomatized lherzolites. All the studied igneous rocks are either direct mantle fusions, or bear signs of the participation of mantle matter in the generation of parent melts in crustal substrates: the presence of tschermakite in gabbroids, nonequilibrium structures, the composition of early generations of biotites corresponding to biotites of mantle and crust-mantle derivatives, the presence of pyroxenes and accessory minerals characteristic of mantle magmas in granitoids. In the diagram Al-Na-K-2Ca–Fe + Ti + Mg, the composition points of the studied intrusive rocks tend to the mixing trend. In general, the research results suggest that the evolution of the Late Mesozoic intrusive magmatism of the studied territory and the specific matter of rock compositions were caused by the crust-mantle interaction as a result of the rise of mantle diapirs in the crust from a long-existing deep hearth of the main melt.

Tectonic Influence on Speleogenesis of Sea Caves on Biševo Island (UNESCO Global Geopark Vis Archipelago, Adriatic Sea, Croatia)

A geological and speleological investigation was conducted in the famous Blue Cave (Modra špilja) and the Monk Seal Cave (Medvidina špilja) on Biševo Island (Croatia) to promote the island’s geoheritage through the new Visitor Centre. The island is mainly composed of Cretaceous to Paleogene neritic carbonates, which form the bedrock, whereas parts of the island are covered with thin Quaternary sediments. The caves are of small dimensions and a simple layout, composed of the main channel and few shorter side channels, all positioned in the tidal zone. Thus, the caves are semi-submerged sea caves located along the coastline. The Blue Cave and the Monk Seal Cave developed within the bedrock limestones and dolostones, respectively, within a zone of left-lateral NNE–SSW striking strike-slip faults that belong to the Biševo fault system. Conjugated discontinuities within the carbonate bedrock indicate a specific strike-slip tectonic regime. Additionally, the host rocks were probably also deformed and fractured during the rise of salt diapirs that characterise this part of the Adriatic foreland. Tectonic and bedding discontinuities form the fragments of the host rock, that combined with the impacts of the strong southern waves, significantly influenced the genesis of the caves.

Halokinetic modulation of sedimentary thickness and architecture: A numerical modelling approach

AbstractSubsurface salt flow can deform overlying strata and influence contemporaneous sedimentary systems. Studying salt‐sediment interactions is challenging in the subsurface due to poor imaging adjacent to salt, and in the field due to the dissolution of halite. Discrete Element Modelling provides an efficient and inexpensive tool to model stratigraphy and deformation around salt structures, which is advantageous over other modelling techniques as it realistically recreates brittle processes such as faulting. Six 2D experiments were run representing 4.6 Myr to determine the effect of salt growth on syn‐kinematic stratigraphy. Halokinetic deformation of stratigraphic architecture was assessed by varying sediment input rates through time. Results show the realistic formation and evolution of salt‐related faults which define a zone of halokinetic influence ca. 3 times the width of the initial diapir. Outside of this, early diapiric and syn‐kinematic stratigraphy are undeformed. Within this zone, syn‐kinematic strata are initially isolated into primary salt withdrawal basins, onlapping and thinning towards the salt‐cored high. In most models, syn‐kinematic strata eventually thin across and cover the diapir roof. Thinning rates are up to six times greater within 350 m of the diapir, compared to further afield, and typically decrease upwards (with time) and laterally (with distance) from the diapir. Outputs are compared to a subsurface example from the Pierce field, UK North Sea, which highlights the importance of considering local fluctuations in diapir rise rate. These can create stratigraphic architectures that may erroneously be interpreted to represent increases/decreases in sedimentation rate. Exposed examples, such as the Bakio diapir, northern Spain, can be used to make inferences of the expected depositional facies, below model resolution. Our models aid the prediction of sedimentary unit thickness and thinning rates and can be used to test interpretations arising from incomplete or low‐resolution subsurface and outcrop data when building geological models for subsurface energy.

A continental forearc serpentinite diapir with deep origins: Elemental signatures of a mantle wedge protolith and slab-derived fluids at New Idria, California

The New Idria serpentinite body in central California has been interpreted as a diapir that was hydrated at depth in the forearc mantle wedge by aqueous fluids released from the subducted slab. This interpretation is primarily based on structural relationships and petrographic analyses of high-grade metabasaltic tectonic blocks entrained within the serpentinite. The geochemistry of the New Idria serpentinites is largely unknown other than a few boron isotope analyses that are consistent with slab-derived fluid compositions, but not definitive. In this contribution, we present whole-rock elemental analyses and petrographic examinations of a suite of ten serpentinite samples to constrain the tectonic origin of the New Idria serpentinite body. Major, minor, and trace element relationships reflect the signatures of depleted peridotitic protoliths typically associated with serpentinites from mantle wedge settings. These include high MgO/SiO2 vs. low Al2O3/SiO2, high MgO vs. low TiO2, Cr-rich chromites, and enriched Zr/Nb and Hf/Nb ratios. Unlike in abyssal serpentinites, U is not fluid-mobile and is strongly coupled to Th, with a U-Th relationship that is best explained by melt–rock interaction. Rare earth element fractionation trends are also consistent with melt-rock interaction. Fluid-mobile element (FME) concentrations (Cs, Ba, and Rb) are consistent with a slab-derived signature and are notably enriched, featuring some of the highest Cs and Ba concentrations ever reported in serpentinites. No petrographic or geochemical evidence was found to suggest that these enriched FME signatures resulted from systematic post-serpentinization fluid-rock processes or pre-serpentinization melt-rock interactions. In aggregate, the geochemical signatures of the New Idria serpentinites suggest a highly depleted mantle wedge protolith overprinted by melt-rock interactions and subsequent serpentinization by a slab-derived fluid, consistent with the forearc diapir hypothesis. Although subduction at New Idria ended around 25 Ma, geophysical evidence and simple geodynamic calculations suggest that the timing of the proposed diapiric rise plausibly coincides with the final stages of subduction in central California.

Salt–magma interactions influence intrusion distribution and salt tectonics in the Santos Basin, offshore Brazil

AbstractMany sedimentary basins host thick evaporite (salt) deposits. Some of these basins also host extensive igneous intrusion networks. It thus seems inevitable that, in some locations, magma will interact with salt. Yet how interaction between these materials may influence salt tectonics or magma emplacement, particularly at the basin‐scale, remains poorly understood. We use 3D seismic reflection data from the Santos Basin, offshore Brazil to image 38 igneous intrusions spatially related to thick Aptian salt. Based on identified seismic–stratigraphic relationships, we suggest sill emplacement likely occurred during the late Albian‐to‐Santonian. We show intra‐salt sills are geometrically similar to but laterally offset from supra‐salt sills. We suggest ascending magma was arrested by the salt in some areas, but not others, perhaps due to differences in evaporite lithology. Our mapping also reveals most sills occur within and above the presalt Merluza Graben, an area characterized by Albian‐to‐Neogene, salt‐detached extension. In adjacent areas, where there are few intrusions, salt deformation was driven by post‐Santonian diapir rise. We suggest emplacement of hot magma within evaporites above the Merluza Graben enhanced Albian‐to‐Santonian salt movement, but that crystallization of the intrusion network restricted post‐Santonian diapirism. Our work indicates salt–magma interaction can influence salt tectonics, as well as the distribution of magma plumbing systems, and thus could impact basin evolution.

Crustal material recycling induced by subduction erosion and subduction-channel exhumation: A case study of central Tibet (western China) based onP-T-tpaths of the eclogite-bearing Baqing metamorphic complex

AbstractSubduction and exhumation processes, interacting with each other, play a key role in crustal recycling. Downgoing oceanic lithosphere constitutes the dominant input at subduction margins, but subduction erosion, the removal of crustal material from the overriding plate, may add additional ingredients and complexity to the subduction factory. Different exhumation models have been proposed to explain how subducted materials are exhumed and therefore contribute to crustal recycling, e.g., exhumation up the subduction channel versus diapiric rise through the mantle wedge that overlies the subducted plate. The recently discovered Baqing eclogite-bearing high-pressure metamorphic complex, central Tibet, China, provides an excellent opportunity to decode the exhumation process, the origin of subduction-related magmatism, and the crustal structure of the North Qiangtang block, in addition to elucidating processes of crustal recycling. Pressure-temperature-time (P-T-t) paths and zircon U-Pb ages and trace-element compositions for Baqing high-pressure rocks were used to evaluate exhumation processes and to determine the geochemical and tectonic affinity of the Baqing metamorphic complex.The Baqing metamorphic complex is mainly composed of eclogite, gneiss, and schist. It is located between two geologically distinct terranes—the South Qiangtang block, which has early Paleozoic basement, and the North Qiangtang block, which has Proterozoic basement. In the schist, zircon cores with steep heavy rare earth element (HREE) slopes and oscillatory zoning yielded inherited ages that are similar to detrital zircon ages for the South Qiangtang block schist; in contrast, zircon rims with flat HREE slopes yielded metamorphic ages of 224 Ma that are similar to the metamorphic ages obtained for the Baqing eclogite. In contrast, zircons from the gneiss yielded an upper-intercept age of 1033 ± 32 Ma (interpreted as the crystallization age) and a lower-intercept metamorphic age of 198 ± 4 Ma. Field relations indicate that gneiss and eclogite/amphibolite were exhumed together, so the ∼20 m.y. gap between the gneiss and the metabasite metamorphism may indicate a long exhumation duration. In the region, Proterozoic ages of ca. 1000 Ma are known only from the North Qiangtang block; we thus propose that the Baqing gneiss originated from North Qiangtang block Proterozoic basement, which, along with North Qiangtang block Triassic arc magmatic rocks and the discrepancies between ancient and current arc-trench distances, results in estimates of ∼20–170 km of Triassic subduction erosion. Results of P-T analyses show that most eclogite, amphibolite, and schist shared a similar clockwise P-T path, different from that of the gneiss, which records a higher geothermal gradient. The clockwise P-T trajectory, long exhumation duration, lack of significant heating during exhumation, and the South Qiangtang block affinity of the schist (host rock of the Baqing eclogite) are consistent with subduction-channel exhumation rather than diapiric rise through the mantle wedge. Geochemical similarities between the North Qiangtang block Triassic subduction-related rocks and the Baqing gneiss may signal the involvement of unexhumed Baqing metamorphic complex in the recycling of the Qiangtang crust.

A Paleoproterozoic nappe on Meso-Archean gneisses exhumed by a Cretaceous metamorphic core complex in northeastern North China Craton

The Paleoproterozoic Jiao-Liao-Ji Orogenic Belt provides key evidence of evolution from an early rift-basin (or back-arc basin) to final collision of the Longgang block (Eo-Meso Archean granitic gneiss) and the Nangrim block in the Liaodong peninsula, northeastern North China Craton. A structural investigation of schists and phyllites in the Langzishan Formation of the North Liaohe Group reveals two new major structures: (1) a sub-horizontal nappe on Meso-Archean granitic gneiss belonging to a ~ 1.95–1.9 Ga Paleoproterozoic D1-2 fold-thrust system produced by terminal collision; From the south basin to the north basement, the metamorphic gradient transferred from greenschist facies metamorphism into amphibolite facies produced by crustal thickening in the Paleoproterozoic orogenesis; the intensity of deformation was also increasing from south to north, the pure shearing style changed into simple shearing. (2) A metamorphic core complex (The Gongchangling metamorphic core complex) formed during Cretaceous extension and doming enabled delamination and diapiric rise and exhumation in the core complex. This extensional-delamination event affected a wide area of the orogenic belt, as part of the delamination of the northeastern half of the North China Craton. This is the first example of the Paleoproterozoic thrust-nappe tectonics exhumed and preserved by Cretaceous diapirism in northeast China.

Prospectivity of Mesoproterozoic magmatism in the northern Karbi Hills, NE India for porphyry copper mineralization

The Kuthori porphyry copper prospect in the Mesoproterozoic Kaziranga magmato-sedimentary domain of the northern Karbi Hills of Shillong Plateau, NE India, is a recently discovered granitoid-hosted Cu-Fe-S occurrence. We present here a preliminary geological account to appraise the petrography, geochronology and bulk geochemistry of the occurrence, and propose a geological model for copper mineralization in the Precambrian granitiods of Karbi Hills. These largely anorogenic (A-type) granitoids show evidences of multiple injections of magma of varying compositions ranging from granite-granodiorite-quartz monzonite. The granitoids have undergone hydrothermal alteration, resulting in the development of alteration mineral zones characterized by a central zone of potassic alteration, containing quartz–K-feldspar-biotite-sericite, followed by phyllic and argillic alteration showing typomorphic mineral growth away from the central alteration zone. The granitoids have wide range of silica contents (54.69–73.07%); are dominantly metaluminous and calc-alkaline; contain high total alkalis and Fe-rich biotite; and show high abundances of Y, Nb, Zr, high FeO/(FeO + MgO) ratios and low CaO, MgO and Sr abundances. The incompatible trace element distribution pattern, positive zircon εHf(t) (+3.2 to + 9.7), initial whole rock 87Sr/86Sr ratio (0.703) and sulphide minerals with 34S/32S ratio (+4.5 ± 0.05‰), indicate magma and sulphide derivation from an enriched mantle source at varying temperatures (828 °C-688 °C). The overall evidence suggests that granitoid magma production and emplacement took place in postcollision back arc basin, creating avenues for ore deposition due to a rapid diapiric rise of magma, sudden decrease in pressure and temperature, resurgent boiling marked by crackle brecciation and increase in the oxidation state of magma.

Dynamic Accretion Beneath a Slow‐Spreading Ridge Segment: IODP Hole 1473A and the Atlantis Bank Oceanic Core Complex

Abstract809 deep IODP Hole U1473A at Atlantis Bank, SWIR, is 2.2 km from 1,508‐m Hole 735B and 1.4 from 158‐m Hole 1105A. With mapping, it provides the first 3‐D view of the upper levels of a 660‐km2 lower crustal batholith. It is laterally and vertically zoned, representing a complex interplay of cyclic intrusion, and ongoing deformation, with kilometer‐scale upward and lateral migration of interstial melt. Transform wall dives over the gabbro‐peridotite contact found only evolved gabbro intruded directly into the mantle near the transform. There was no high‐level melt lens, rather the gabbros crystallized at depth, and then emplaced into the zone of diking by diapiric rise of a crystal mush followed by crystal‐plastic deformation and faulting. The residues to mass balance the crust to a parent melt composition lie at depth below the center of the massif—likely near the crust‐mantle boundary. Thus, basalts erupted to the seafloor from >1,550 mbsf. By contrast, the Mid‐Atlantic Ridge lower crust drilled at 23°N and at Atlantis Massif experienced little high‐temperature deformation and limited late‐stage melt transport. They contain primitive cumulates and represent direct intrusion, storage, and crystallization of parental MORB in thinner crust below the dike‐gabbro transition. The strong asymmetric spreading of the SWIR to the south was due to fault capture, with the northern rift valley wall faults cutoff by a detachment fault that extended across most of the zone of intrusion. This caused rapid migration of the plate boundary to the north, while the large majority of the lower crust to spread south unroofing Atlantis Bank and uplifting it into the rift mountains.