Compressional Tectonic Setting Research Articles

On the architecture of intra-formational Mass-Transport Deposits: Insights from the carbonate slopes of Great Bahama Bank and the Apulian Carbonate Platform

Large sub-tropical carbonate platforms shed huge volumes of sediment onto their adjacent slopes. Resulting carbonate successions are comprised of gravity-flow beds (turbidites, debrites), which are occasionally reworked into mass-transport deposits (MTDs) by the failure of inconsistently lithified portions of slope strata. Despite their significance in the depositional record, the internal architecture of MTDs along relatively tectonically stable and sediment-producing carbonate platforms is poorly understood in comparison with counterparts in active compressional tectonic settings. This study compares carbonate-slope-derived MTDs imaged in seismic reflection data with those observed in outcrop. The internal architecture of MTDs from two case studies is presented: the seismically-imaged MTDs in Miocene-Recent carbonate-slope deposits from the Bahamas, and MTDs off the Cretaceous Apulian carbonate margin exposed in outcrops of the Ionian Basin (Albania). Understanding the large-scale (kilometers to tens of kilometers) internal architecture of MTDs in seismic is crucial for identification of these structures in outcrop. In turn, only outcrop studies allow the detailed analysis of the reworking style within these massive sediment bodies at a smaller scale (meters to kilometers). The internal architecture of the studied carbonate MTDs testifies to the existence of an along-slope continuum of deformation structures, characterized by vertical partitioning into distinct mass flows during single mass-transport events. Mass-wasting processes are inferred from strain-partitioning structures, recording along-slope and slope-parallel architectural heterogeneities. Three deformation domains are recognized: extensional headwall domain, translational body domain, and contractional toe domain. Confined MTDs are limited by steep and deep lateral margins and frontal ramps that mark the boundaries between failed deposits and undisturbed strata. Vertical strain-partitioning is pronounced in the toe domain. Upward intensification of deformation within MTDs is common and corresponds to a top-down reorganization of failed strata during downslope transport. Intra-formational MTDs along carbonate slopes that are lacking exotics, such as the ones studied here, differ from extra-formational MTDs occurring in compressional tectonic settings that form sedimentary mélanges. Ductile-layer deformation structures are predominant in carbonate MTDs as opposed to block-in-matrix fabrics in sedimentary mélanges. The development of shear surfaces, rather than ductile shear zones, in the body and toe domains presents predictive (in modern environments) and diagnostic criteria (in fossil examples) of mass-wasting events along carbonate slopes.

Non-monotonic decrease of coal permeability with depth in highly compressional tectonic settings: A case study in the Bide-Santang Basin, SW China

Abstract Coal permeability dependence on depth has been studied and discussed for a long time. However, the scattered distribution of coal permeability versus depth, and specifically the non-monotonic permeability decrease, in highly compressional tectonic settings has not been fully understood. This paper takes the Bide-Santang Basin, SW China, as a case study to discuss the non-monotonic decrease of coal permeability with depth. The results show that three regime intervals, including the uppermost normal regime interval, the middle mixed-regime interval and the deep normal regime interval, are identified in the Bide-Santang Basin. A tri-segmented linear model provides a better fit of coal permeability than a single linear model does. Coal permeability in the uppermost normal regime interval is characterized by exceptionally high values and quick decrease rate with increasing depth, in comparison with that in other intervals. The middle mixed-regime interval is characterized by a permeability increasing trend with depth, related primarily to the increase of horizontal differential stress in this interval. In the deep normal regime interval, the horizontal differential stress does not show any obvious trend, and coal permeability resumes its decreasing trend with depth. This study reveals the primary role of the horizontal differential stress, besides the effective stress, in controlling coal permeability in highly compressional tectonic settings characterized by stress regime transitions.

Permo–Triassic granitoids, Hainan Island, link to Paleotethyan not Paleopacific tectonics

Abstract Hainan Island may have lain at the boundary between the Tethyan and Pacific domains since the late Paleozoic. Permo–Triassic granitoids outcropped on the island provide insight into Mesozoic tectonic evolution and potential dynamics. Based on analyses of zircon U-Pb geochronological and Hf-O isotopic data, along with whole-rock elemental and Sr-Nd isotopic data, the Permo–Triassic granitoids are divided into three groups: Late Permian (256–252 Ma) gneissic granitoids (Group 1), Early–Middle Triassic (247–244 Ma) massive granitoids (Group 2), and Middle-Late Triassic (242–225 Ma) massive granitoids (Group 3). Groups 1 and 2 exhibit similar elemental and Sr-Nd isotopic compositions with εNd(t) values ranging from –6.71 to –3.25. Group 1 is characterized by gneissic foliation and calc-alkaline I-type geochemical affinities with negative Nb-Ta and Ti anomalies; it has low εHf(t) (–4.2∼+0.2) and high δ18O values (9.35–10.46‰), possibly related to derivation of a metabasite source in a continental arc setting. The Group 2 samples are peraluminous massive granitoids with slightly high A/CNK ratios of 1.00–1.31, δ18O values (8.85–11.75‰), and high Sr/Y ratios but low εHf(t) (–8.7∼+1.6) values. They may have originated from the mixed source of greywacke and metabasite, indicating a compressive tectonic setting. The Group 3 high-K calc–alkaline granitoids have high zircon temperatures (842–867 °C) and show the geochemical signatures of A-type granites. They are featured by slightly low whole-rock εNd(t) and zircon εHf(t) values, suggestive of the derivation from a metabasite–greywacke source in an extensional setting. In combination with other geochronological and geochemical data from Hainan Island, it is proposed that the Permian–Triassic igneous magmatism in Hainan Island reflects a continuous amalgamation process from back-arc consumption (272–252 Ma) to orogenic assembly (251–243 Ma) and orogenic collapse (242–225 Ma) between the South China and Indochina Blocks. Such a temporal–spatial pattern for the Hainan granitoids is consistent with that along the Truong Son, Jinshajiang–Ailaoshan–Song Ma tectonic zone, suggesting dominant control of Paleotethyan domain instead of Paleopacific tectonics.

Provenance and paleogeography of the Jurassic Northwestern Qaidam Basin (NW China): Evidence from sedimentary records and detrital zircon geochronology

The Qaidam Basin is the largest Meso–Cenozoic petroliferous basin on the northeastern Tibetan Plateau and was situated at the northern margin of the Eastern Tethys during the Mesozoic. Jurassic strata are the main source rocks and reservoirs in the northern and western segments of this basin. However, due to exceptionally thick overlying Cenozoic strata, intensive post-depositional modification and limited deep exploration data, several crucial problems regarding this basin remain controversial, including provenance supply, original sedimentary extent, and overall properties in the Jurassic period. This hinders further understanding of regional geological evolution and hydrocarbon exploration. This paper focuses on comparison of residual Jurassic strata in the northwestern Qaidam Basin, the adjacent Saishiteng Mountains and the Suganhu area to the north based on sedimentology and provenance analyses. Detrital zircon geochronology was employed to examine the provenance of Jurassic sediments in different areas and to reconstruct their dispersal patterns. Results show that the Lower Jurassic deposits are characterized by near provenance and rapid facies shift. Fan deltas, braided river deltas, and lacustrine deposits are commonly developed. The Lower Jurassic source area was mainly located in the south Altyn Tagh Mountains. Sedimentary extent and depocenters in the Middle and Late Jurassic transferred northwards to the current piedmont of the Saishiteng and South Qilian mountains. The residual Jurassic in the Saishiteng Mountains is similar and comparable with coeval strata in the Suganhu area as well as the inner Qaidam Basin. Detrital zircon ages indicate multiple provenances in the Middle Jurassic, of which the southern Qilian Mountains are the most important. In the Middle Jurassic, deposition occurred across large areas of the Saishiteng Mountains which belonged to the same basin as the inner Qaidam Basin and the Suganhu areas. A series of small-scale continental rift basins developed in the western part of northwestern Qaidam in the Early Jurassic and then developed into a larger depression from the late Early Jurassic to the Late Jurassic. Post-collision extension was conducive to the development of rift basins in the Early Jurassic. Subduction of the northern Mongolian–Okhotsk and southern Central Tethys Ocean interactively led to an extensional tectonic setting and the development of a depression basin in late Early to Middle Jurassic. The continuous northward compression of the Qiangtang Block mainly resulted in a compressive tectonic setting in the Late Jurassic.

Stratigraphic and structural reconstruction of an offshore sector of the Hyblean Foreland ramp (southern Italy)

Through the seismo-stratigraphic analysis of new high- resolution seismic data acquired along the southwestern offshore of the Hyblean Plateau, this work aims to improve the knowledge about the stratigraphic and structural setting in the marine area connecting the Hyblean Plateau Foreland to the Gela-Catania Foredeep.Two main goals have motivated the acquisition of new seismic profiles along the southern coastline of Sicily, in the Marina di Ragusa offshore:(i) to obtain a better comprehension of the Cenozoic stratigraphic and structural setting of the area, with a particular attention to the characterization of the Pliocene-Holocene sedimentary deposits and their areal distribution;(ii) to identify, in the offshore area, the possible prolongation of the main structural lineaments outcropping in the hinterland (Scicli-Ragusa-Irminio Line), and verify the presence of structures, responsible for the tectonic activity affecting the area.Our model shows that an extensional fault system, characterized by a main NE-SW orientation, affects the highly deformed Oligo-Miocene, or older, substratum, originating structural highs and down-faulted sectors. These faults are inferred to record the early history of the Scicli Line and of the polyphase kinematic evolution of the N50° oriented regional fault systems.The Gessoso-Solfifera deposits (Messinian, Late Miocene) have been recognized in several sectors of the study area, showing a very peculiar seismic facies, and occupying deep erosional channels probably resting within down-faulted sectors. This feature is in good agreement with isolated onshore areas of the Hyblean Plateau (Licodia Eubea and south Vittoria villages) where the Gessoso-Solfifera deposits are associated to normal faults, thus documenting that Messinian evaporites are not restricted to compressive tectonic setting, such as the marginal sub-basins and the thrust top mini-basins of the Appennine-Maghrebian belt.A peculiar seismic facies has been associated to the Trubi Fm., unconformably lying upon the older succession.The undeformed seismic units, onlapping the older substratum, has been associated to the post-Trubi-Holocene deposits, arranged into seven seismic units that reflect the youngest depositional evolution of the nearby Catania-Gela Foredeep. These deposits, generally represented by plane-parallel seismic facies, are in turn separated by an unconformity highlighted by an onlap termination of the yougermost terms upon the older. In some cases they are interrupted by gas rising from the deeper succession that reaches, at times, also the seafloor.

Igneous petroleum systems in the Malargüe fold and thrust belt, Río Grande Valley area, Neuquén Basin, Argentina

Abstract The Rio Grande Valley (Neuquen Basin, Argentina) is a large oil field where voluminous igneous sill complexes are emplaced in different levels of the sedimentary sequence in a fold and thrust belt setting. These sills are predominantly emplaced in organic-rich units such as Vaca Muerta and Agrio Formations and are critical elements of the defined petroleum system, as they affect the thermal maturity of the surrounding source rocks, and also act as naturally fractured reservoirs. In this contribution, we integrated structural sections, borehole data, core descriptions, petrophysical laboratory tests and organic geochemistry measurements to characterize the impact of the magmatism in the petroleum system of the Rio Grande Valley oil field. Especially, we focused in evaluating the impact of the sill-complexes in hydrocarbon generation, migration and accumulation. Our data shows that: (1) intrusions are preferably emplaced in organic rich units; (2) a large number of intrusions is observed in areas intensely deformed by imbricated thrusts; (3) an increase in the number of intrusions drives stronger and more extensive maturation processes in the Vaca Muerta formation, enhancing notably the cracking of kerogen to hydrocarbon within this unit; (4) where the number of intrusions increases, the concentration of total gas and methane also increases, given that all sills are suitable reservoirs; (5) igneous rocks provide a low resistance path for fluids flow and act as the main reservoirs in the study area; (6) the reservoir quality of igneous intrusions is dominated by the combination of matrix and fracture framework properties; (7) zeolites are the most common type of cement in these reservoirs; (8) deep sills located in the Vaca Muerta formation produce larger volumes of hydrocarbons relative to shallower intrusive reservoirs. The Rio Grande Valley oil field offers a world-class example of the implications of igneous complexes in hydrocarbon generation, migration and accumulation in a compressive tectonic setting of fold and thrust belt.

Compressional metamorphic core complexes, low-angle normal faults and extensional fabrics in compressional tectonic settings

AbstractMetamorphic core complexes (MCCs) are interpreted as domal structures exposing ductile deformed high-grade metamorphic rocks in the core underlying a ductile-to-brittle high-strain detachment that experienced tens of kilometres of normal sense displacement in response to lithospheric extension. Extension is supposedly the driving force that has governed exhumation. However, numerous core complexes, notably Himalayan, Karakoram and Pamir domes, occur in wholly compressional environments and are not related to lithospheric extension. We suggest that many MCCs previously thought to form during extension are instead related to compressional tectonics. Pressures of kyanite-and sillimanite-grade rocks in the cores of many of these domes are c. 10–14 kbar, approximating to exhumation from depths of c. 35–45 km, too great to be accounted for solely by isostatic uplift. The evolution of high-grade metamorphic rocks is driven by crustal thickening, shortening, regional Barrovian metamorphism, isoclinal folding and ductile shear in a compressional tectonic setting prior to regional extension. Extensional fabrics commonly associated with all these core complexes result from reverse flow along an orogenic channel (channel flow) following peak metamorphism beneath a passive roof stretching fault. In Naxos, low-angle normal faults associated with regional Aegean extension cut earlier formed compressional folds and metamorphic fabrics related to crustal shortening and thickening. The fact that low-angle normal faults exist in both extensional and compressional tectonic settings, and can actively slip at low angles (< 30°), suggests that a re-evaluation of the Andersonian mechanical theory that requires normal faults to form and slip only at high angles (c. 60°) is needed.

3D litho-constrained inversion model of southern Sierra Grande de San Luis: New insights into the Famatinian tectonic setting

Unravelling the orogenic structure of the Sierra Grande de San Luis (Argentina) is essential to understand the geological evolution of the Famatinian orogen and the western margin of Gondwana. However, its structural organization is still poorly known and widely debated. In this contribution, a new interpretation of the complex geological structures in the southern sector of the Sierra Grande de San Luis is presented. This work is based on a litho-constrained joint inversion of gravity and magnetic datasets combining lithological, structural and rock properties information. Results are consistent with a large-scale doubly-vergent structure caused by a compressional tectonic setting as a consequence of the collision between an allochthonous terrane (Cuyania/Precordillera) and the proto margin of Western Gondwana. A convex morphology of the indenter is proposed to be responsible for producing a significant clockwise rotation of the horizontal deviatoric stresses. This stress field caused a small sinistral component of movement and gave rise to the doubly-vergent transpressive mega-structure.

Modelo estructural del Campo Escuela Colorado a partir de modelos análogos

La cuenca del Valle Medio del Magdalena es una cuenca intermontana (Cooper, et al., 1995; Taboada, et al., 2000; Sarmiento-Rojas, et al., 2006), que se caracteriza por tener estructuras con doble vergencia y está limitada por las cordilleras Central y Oriental (Córdoba, et al., 2000). En el sector sureste del Valle, asociado al frente de deformación occidental de la Cordillera Oriental, se localiza el Campo Escuela Colorado de la Universidad Industrial de Santander. Los estudios previos (Ecopetrol, 2003; Chajín, et al., 2013; Zafra, 2013) en dicho campo difieren en cuanto al modelo estructural y su relación con las estructuras regionales (por ejemplo, la falla de La Salina). En este trabajo se presentan los resultados de 15 modelos de simulación de una cuña crítica para definir las características estructurales en un ambiente compresivo. Los resultados de los modelos muestran un frente de deformación dominado por fallas con vergencia hacia el promontorio y retrocabalgamientos asociados. El apilamiento de material causó elevación topográfica en la zona axial de la cuña. Con base en la tendencia observada en los 15 experimentos realizados, se concluyó que la geometría de las estructuras está dominada por la escamación delgada y que el modelo estructural que define al Campo Escuela Colorado es una zona triangular controlada por unretrocabalgamiento. © 2018. Acad. Colomb. Cienc. Ex. Fis. Nat.

Petrogenesis of Jurassic granitoids at the northeastern margin of the North China Craton: New geochemical and geochronological constraints on subduction of the Paleo-Pacific Plate

At the junction between the North China Craton (NCC) and the Central Asian Orogenic Belt (CAOB), northern Liaoning province, NE China, there are widespread Jurassic igneous rocks. The tectonic setting and petrogenesis of these rocks are unresolved. Zircon U–Pb dating, whole-rock geochemistry, and Hf isotopic compositions of Jurassic granitoids were investigated to constrain their ages and petrogenesis in order to understand the tectonic evolution of the Paleo-Pacific Ocean along the northeastern margin of the NCC. Geochronological data indicate that magmatism occurred between the early and late Jurassic (180–156 Ma). Despite the wide range in ages of the intrusions, Jurassic granitoids were likely derived from a similar or common source, as inferred from their geochemical and Hf isotopic characteristics. Compared to the island arc andesite-dacite-rhyolite series, the Jurassic granitoids are characterized by higher SiO2, Al2O3, and Sr contents, and lower MgO, FeOT, Y, and Yb contents, indicating that the primary magmas show typical characteristics of adakitic magmas derived from partial melting of thickened lower crust. These findings, combined with their εHf(t) values (+1.4 to +5.4) and two-stage model ages (1515–1165 Ma), indicate the primary magmas originated from partial melting of juvenile crustal material accreted during the Mesoproterozoic. They are enriched in large-ion lithophile elements (e.g., Rb, K, Th, Ba, and U) and light rare-earth elements (REE), and depleted in high-field-strength elements (e.g., Nb, Ta, Ti, and P) and heavy REE. Based on these findings and previous studies, we suggest that the Jurassic adakitic granitoids (180–156 Ma) were formed in an active continental margin and compressive tectonic setting, related to subduction of the Paleo-Pacific Plate.

Gas hydrate formation in compressional, extensional and un-faulted structural settings – Examples from New Zealand's Hikurangi margin

We investigate gas hydrate formation processes in compressional, extensional and un-faulted settings on New Zealand's Hikurangi margin using seismic reflection data. The compressional setting is characterized by a prominent subduction wedge thrust fault that terminates beneath the base of gas hydrate stability, as determined from a bottom-simulating reflection (BSR). The thrust is surrounded by steeply dipping strata that cross the BSR at a high angle. Above the BSR, these strata are associated with a high velocity anomaly that is likely indicative of relatively concentrated, and broadly distributed, gas hydrates. The un-faulted setting—sedimentary infill of a slope basin on the landward side of a prominent thrust ridge—is characterized by a strong BSR, a thick underlying free gas zone, and short positive polarity reflection segments that extend upward from the BSR. We interpret the short reflection segments as the manifestation of gas hydrates within relatively coarse-grained sediments. The extensional setting is a localized, shallow response to flexural bending of strata within an anticline. Gas has accumulated beneath the BSR in the apex of folding. A high-velocity zone directly above the BSR is probably mostly lithologically-derived, and only partly related to gas hydrates. Although each setting shows evidence for focused gas migration into the gas hydrate stability zone, we interpret that the compressional tectonic setting is most likely to contain concentrated gas hydrates over a broad region. Indeed, it is the only setting associated with a deep-reaching fault, meaning it is the most likely of the three settings to have thermogenic gas contributing to hydrate formation. Our results highlight the importance of anisotropic permeability in layered sediments and the role this plays in directing sub-surface fluid flow, and ultimately in the distribution of gas hydrate. Each of the three settings we describe would warrant further investigation in any future consideration of gas hydrates as an energy resource on the Hikurangi margin.

Spatial analysis of an intra-plate basaltic volcanic field in a compressional tectonic setting: South-eastern Australia

The Newer Volcanics Province (NVP) is a Pliocene to Recent intra-plate basaltic volcanic field (BVF) that has formed in a compressive tectonic setting (σv<σhmin<σHmax) and is not readily attributed to a single geodynamic process. A comprehensive spatial analysis of both monogenetic eruption centres and coeval vents of the NVP constrain factors that control the distribution and emplacement of volcanoes. A point-set of 434 eruption centres totalling 726 vents are divided into three geographical sub-provinces for analysis. Kernel density estimation and Poisson nearest neighbour analysis are used to scrutinize the distribution of eruption centres. A simple and novel fitted regression line method is used to determine the orientation of coeval vents, and Hough transform and two-point azimuth methods are used to identify alignments and alignment trends between eruption centres. The distribution of eruption centres and their relative spatial density corresponds with the extent of thinner lithosphere. Eruption centres of the NVP have a clustered distribution whilst smaller sub-sets of eruption centres are distributed more uniformly. The main alignment trends between coeval vents related to individual dikes and between eruption centres related to successive temporally discrete dikes are primarily oriented nearly parallel with pre-existing crustal fault trends. The frequency of volcanic alignments shows faults oriented nearly parallel to the orientation of the regional maximum horizontal compressive stress (σ1) are favourably utilised by intrusions over other fault trends. The depth from which pre-existing faults facilitate dike propagation is not constrained. We interpret they are likely important in preventing dikes from stalling and forming sills under a compressive stress field in the case of the NVP. It is also observed that coeval vent alignments are more strongly aligned in areas overlying consolidated basement relative to areas of poorly consolidated basin sediments. This could be explained by poorly aligned groups of vents being fed by shallow sills, preferentially forming in layered basin sediments.

Geochronological Framework and Geodynamic Implications of Mafic Magmatism in the Liaodong Peninsula and Adjacent Regions, North China Craton

AbstractMafic rocks are widespread on the Liaodong Peninsula and adjacent regions of the North China Craton. The majority of this magmatism was originally thought to have occurred during the Pre‐Sinian, although the precise geochronological framework of this magmatism was unclear. Here, we present the results of more than 60 U‐Pb analyses of samples performed over the past decade, with the aim of determining the spatial and temporal distribution of mafic magmatism in this area. These data indicate that Paleoproterozoic–Mesoproterozoic mafic rocks are not as widely distributed as previously thought The combined geochronological data enabled the subdivision of the mafic magmatism into six episodes that occurred during the middle Paleoproterozoic, the late Paleoproterozoic, the Mesoproterozoic, the Late Triassic, the Middle Jurassic, and the Early Cretaceous. The middle Paleoproterozoic (2.1–2.2 Ga) mafic rocks formed in a subduction‐related setting and were subsequently metamorphosed during a ca. 1.9 Ga arc–continent collision event The late Paleoproterozoic (ca. 1.87–1.82 Ga) bimodal igneous rocks mark the end of a Paleoproterozoic tectonothermal event, whereas Mesoproterozoic mafic dike swarms record global‐scale Mesoproterozoic rifting associated with the final breakup of the Columbia supercontinent The Late Triassic mafic magmatism is part of a Late Triassic magmatic belt that was generated by post‐collisional extension. The Middle Jurassic mafic dikes formed in a compressive tectonic setting, and the Early Cretaceous bimodal igneous rocks formed in an extensional setting similar to a back‐arc basin. These latter two periods of magmatism were possibly related to subduction of the Paleo‐Pacific plate.

Structure and evolution of volcanic plumbing systems in fold-and-thrust belts: A case study of the Cerro Negro de Tricao Malal, Neuquén Province, Argentina

Magma ascent and emplacement in compressional tectonic settings remain poorly understood. Geophysical studies show that volcanic plumbing systems in compressional environments are vertically partitioned into a deep level subject to regional compression and a shallow level subject to local extension. Such vertical partitioning has also been documented for the plumbing systems of mud volcanoes, implying common, yet unresolved, underlying processes. In order to better constrain the mechanisms governing this depth partitioning of emplacement mechanisms, we studied the structure and evolution of the Cerro Negro intrusive complex emplaced in the Chos Malal fold-and-thrust belt in the foothills of the Neuquen Andes, Argentina. The Cerro Negro intrusive complex consists of sills and N-S–striking dikes that crosscut the sills. The most prominent structures in the study area are N-S–trending folds, and both E- and W-vergent thrusts. We provide new U-Pb ages of 11.63 ± 0.20 Ma and 11.58 ± 0.18 Ma for sills and 11.55 ± 0.06 Ma for a dike, which show that the Cerro Negro intrusive complex was emplaced in a short period of time. Our ages and field observations demonstrate that the emplacement of the Cerro Negro intrusive complex was coeval with the tectonic development of the Chos Malal fold-and-thrust belt. This implies that the dikes were emplaced perpendicular to the main shortening direction. The systematic locations of the dikes at the anticlinal hinges suggest that their emplacement was controlled by local, shallow stresses related to outer-arc stretching at the anticlinal hinge. We conclude that folding-related outer-arc stretching is one mechanism responsible for the vertical partitioning of igneous plumbing systems in compressional tectonic settings.

Deposits associated with ultramafic–mafic complexes in Mexico: the Loma Baya case

Several ultramafic–mafic complexes of assumed ophiolitic affinity that contain ore deposits occur mostly along the Pacific margin of Mexico and the boundaries of the Guerrero composite terrane, which have been described in association with various tectonomagmatic settings. These are (1) roots of volcanic arcs, (2) ophiolites, (3) oceanic basins, (4) continental fragments underlying a magmatic arc, and (5) subduction complexes or rift-transform magmatic suites. However, the petrogenesis of such ultramafic–mafic complexes and the associated ore deposits remains ill defined to date, as their comprehensive study virtually lacks an appropriate geochemical approach. Ultramafic–mafic complexes with known or estimated ages in Mexico are mostly of Paleozoic and Mesozoic ages, and very few of them formed once the crustal dynamics of the Mexican Pacific margin switched from generally extensional to compressional tectonic settings during the Late Cretaceous. A few deposits, aged Late Cretaceous to Paleocene, were interpreted to have formed in association with tectonomagmatic settings different from most deposits in the Guerrero composite terrane. To move forward in the understanding on how these mafic–ultramafic complexes and their associated ore deposits formed, the genesis of the chromitite deposits within the Loma Baya ultramafic–mafic complex (southernmost Guerrero composite terrane) is hereby endeavored. The combination of the geochemical study of both major and trace elements in chromite (including platinum-group elements; PGE), the mineralogy, PGE geochemistry and radiogenic isotopes (i.e., Re–Os) in platinum-group minerals (PGM), provides new assessment on the genesis of this ultramafic–mafic complex. From the obtained results it stems that the Loma Baya complex and the chromitite deposits associated with it are closely related with the formation of a back-arc basin in an active supra-subduction zone during the Lower Cretaceous, and the parental melts resulted from re-melting of an ultramafic protolith of Paleozoic (or older) age. The melts that originated in such back-arc basin were of tholeiitic affinity and produced Al-rich chromitites with a moderate enrichment in PGE, associated with rates of partial melting of a depleted mantle source of at least 20%. Such geological framework is in frank disagreement with previous work, thus highlighting the necessity of further research in other Mexican ultramafic–mafic complexes to better constrain their geodynamic setting of formation. The obtained Re–Os model ages in PGM of the Loma Baya chromitite deposits (ca. 300 and 130Ma) suggest a possible correlation between these deposits and those in the Californias, in the sense that both areas record similar geological processes (mantle rock melting, re-melting, and chromitite emplacement) within similar tectonomagmatic environments and time frame for their occurrence. Such possibility further highlights the necessity to carry on similar studies in chromitite deposits elsewhere along the Pacific margin in Mexico and southwestern USA, in order to better reconstruct its tectonomagmatic evolution during the Mesozoic.

Generation of tectonic over-pressure inside subducting oceanic lithosphere involving phase-loop of olivine–wadsleyite transition

We conducted a two-dimensional numerical model to analyze the generation of tectonic over-pressure, which is a positive deviation from lithostatic pressure, for deep slabs which are anchored at the 660 km phase boundary. The formation of the ductile shear zone under a compressional tectonic setting induces tectonic over-pressure. We first propose that an apparent shear zone originated from an elastic heterogeneity in the phase loop, which is the two-phase (i.e., olivine and wadsleyite) coexistence interval around the 410 km boundary within subducting oceanic lithospheres, can cause tectonic over-pressure with a range from 0.3 to 1.5 GPa. This over-pressure significantly impacts the formation of the olivine–wadsleyite phase transition. The flattening of the olivine–wadsleyite interface by over-pressure is well-resolved. Therefore, we argue that the over-pressure should be applied when analyzing the phase boundary within the subducting lithosphere. Our results provide a new insight on the interplay among the phase transition, shear zone formation and tectonic over-pressure.

Cold seeps at the salt front in the Lower Congo Basin II: The impact of spatial and temporal evolution of salt-tectonics on hydrocarbon seepage

This study investigates the distribution and evolution of seafloor seepage in the vicinity of the salt front, i.e., the seaward boundary of salt-induced deformation in the Lower Congo Basin (LCB). Seafloor topography, backscatter data and TV-sled observations indicate active fluid seepage from the seafloor directly at the salt front, whereas suspected seepage sites appear to be inactive at a distance of >10 km landward of the deformation front. High resolution multichannel seismic data give detailed information on the structural development of the area and its influence on the activity of individual seeps during the geologic evolution of the salt front region. The unimpeded migration of gas from fan deposits along sedimentary strata towards the base of the gas hydrate stability zone within topographic ridges associated with relatively young salt-tectonic deformation facilitates seafloor seepage at the salt front. Bright and flat spots within sedimentary successions suggest geological trapping of gas on the flanks of mature salt structures in the eastern part of the study area. Onlap structures associated with fan deposits which were formed after the onset of salt-tectonic deformation represent potential traps for gas, which may hinder gas migration towards seafloor seeps. Faults related to the thrusting of salt bodies seawards also disrupt along-strata gas migration pathways. Additionally, the development of an effective gas hydrate seal after the cessation of active salt-induced uplift and the near-surface location of salt bodies may hamper or prohibit seafloor seepage in areas of advanced salt-tectonic deformation. This process of seaward shifting active seafloor seepage may propagate as seaward migrating deformation affects Congo Fan deposits on the abyssal plain. These observations of the influence of the geologic evolution of the salt front area on seafloor seepage allows for a characterization of the large variety of hydrocarbon seepage activity throughout this compressional tectonic setting.

Zircon U-Pb dating and Lu-Hf isotope study of intermediate-mafic sub-volcanic and intrusive rocks in the Lishui Basin in the middle and lower reaches of Yangtze River

Eight Mesozoic sub-volcanic and intrusive rocks in the Lishui Basin have been selected for zircon U-Pb dating and Lu-Hf isotopic analysis. The Laohutou, Datongshan, Daweizhuang, Yeshanao, Yanwaqiao, Xibeishan, Changshantou and Jianshan porphyrites yield concordant early Cretaceous ages of 130.5 ± 1.6 Ma, 136.0 ± 3.4 Ma, 132.7 ± 2.7 Ma, 127.0 ± 1.9 Ma, 129.4 ± 1.7 Ma, 133.2 ± 2.1 Ma, 131.1 ± 2.3 Ma and 127.4 ± 1.8 Ma, respectively. Zircon e Hf(t) values for these rocks range from −3.54 to −9.11, mostly between −3.54 and −5.93, suggesting sources similar to those for coeval igneous rocks in other volcanic basins along the middle and lower reaches of Yangtze River. Considering the geochemical characteristics of coeval magmatic rocks in the middle and lower reaches of Yangtze River and its adjacent areas, this paper proposes a model of slab rollback to explain the Mesozoic magmatism in the east-central part of China continent. In the middle-late Jurassic to early Cretaceous (170–135 Ma), the Pacific plate started to subduct beneath the Eurasian continent, producing a compressive tectonic setting, and magmatism progressed from coast to the inland. The magmas of this period were derived mostly from partial melting of ancient crust. After ~135 Ma, the subduction weakened, and the rollback of the subducted Pacific plate produced an extensional environment. This led to the formation of volcanic basins, the partial melting of enriched mantle sources and less ancient crust, and the intrusion of voluminous intermediate-mafic igneous rocks.

Genesis of Permian granites along the Kangguer Shear Zone, Jueluotage area, Northwest China: Geological and geochemical evidence

Abstract Permian mafic and felsic intrusions occur along the Kangguer Shear Zone (KSZ) in the Jueluotage area, Eastern Tianshan, northwest China. This study focuses on the Permian felsic intrusions (288–276.2 Ma). Systematic geological and geochemical investigations show that granite plutons in the western KSZ (Dikan, Guandao and Hongshi) have lower Al and higher alkali contents than those in the eastern KSZ (Longdong, Huangshan and Baishandong). The western granite plutons experienced higher degrees of crystal fractionation than the eastern granite plutons. Both of the western and eastern Permian granite intrusions along KSZ were derived from juvenile crust, the Nd–Pb isotopic characteristics of which exhibit mixed compositions of DM and EMII type mantles. Both of the western and eastern Permian granites along KSZ were formed within post-collision background. The Dikan, Guandao and Hongshi plutons in the west KSZ sector were formed in an extensional tectonic setting, whereas the Longdong, Huangshan and Baishandong plutons in the east KSZ formed in a compressional tectonic setting.

Mineralogical feature and geological significance of muscovites from the Longyuanba Indosinian and Yanshannian two-mica granites in the eastern Nanling Range

Emplacement P-T condition estimations using granites are important for understanding metamorphic and erosional processes of orogenic belt. Granites are widespread in South China and a majority of them are peraluminous. Particularly, over 91% of the Indosinian granites exposed in the region are peraluminous in composition. It is extremely hard to determine the pressure of intrusion of these peraluminous granites due to the absence of amphibole, a good mineral barometer commonly identified in metaluminous granites. Muscovite is a common mineral in peraluminous granites, certain kind of it could be used as a mineral barometer to constrain the emplacement pressure of peraluminous granites. In this paper, results of petrographic and geochemical studies of muscovites from the Indosinian and early Yanshanian two-mica granites at the Longyuanba in the eastern Nanling Range are reported. Based on petrographic studies, the primary muscovite can be discriminated from the secondary muscovites. Muscovites from the Indosinian two-mica granites are enriched in Ti, Al, Mg, and Na, and depleted in Fe and Mn. Geochemically, these muscovites were considered as primary, whereas those from the Yanshanian two-mica granites fall into the area of secondary muscovite on discrimination diagrams. Barometer estimations show that pressures calculated for primary muscovites are accurate, but those calculated for secondary muscovites are overestimated. The average pressure of emplacement of the Longyuanba Indosinian two-mica granites is 5.9 kbar, corresponding to ∼19 km in depth, suggesting that the Indosinian granitic magmas were probably generated by partial melting of a thickened crust root in a compressional tectonic setting.