Detachment Fault Research Articles

Tectonic Controls on Block Rotation and Sheeted Sill Emplacement in the Xigaze Ophiolite (Tibet): The Construction Mode of Slow‐Spreading and Ultraslow‐Spreading Oceanic Crusts

AbstractThe internal structure of oceanic crusts is not well understood due to the limitation of deep drilling. However, that of ophiolites, i.e., on‐land ancient analogs of oceanic lithosphere, could be precisely mapped and measured. The Xigaze ophiolite in Tibet has been regarded as “peculiar”, due to the sheeted sill complex in its upper crust, and non‐sheeted diabase sills/dikes crosscutting its mantle and lower crust, which are geometrically different from the primarily vertical sheeted dike complex. Based on extensive field observations, here we present petrological and geochemical data for the Xigaze ophiolite to decipher the origin of sheeted sill complex and its implications for the construction of oceanic crusts. Diabases in the Xigaze ophiolite could be subdivided into sheeted sills, Group 1 non‐sheeted dikes, and Group 2 non‐sheeted sills, based on their orientations. These diabases cut other lithologies, and hence belong to the latest‐stage products. Based on petrological, geochemical, and structural data, we highlight the important role of detachment fault in the generation of sheeted and non‐sheeted sills. During the formation of oceanic crust, large block exhumation, multi‐stage rotations, and foundering are argued here as key mechanisms for the generation of Xigaze sheeted and non‐sheeted dikes/sills, all of which are in the evolution of detachment fault systems. These processes are also not uncommon for asymmetrical segments at modern slow‐spreading and ultraslow‐spreading ridges, but are rare at symmetrical segments. Due to the evolution of detachment fault, the internal structures of (ultra)slow‐spreading ridges are more complex than those at fast‐spreading ridges.

The Succession of Upper Eocene- Upper Miocene Limestone Growth and Corresponding Tectonic Events in Luconia Shelf, Sarawak, Malaysia

Using high quality regional seismic lines, we evidence major structures resulting from successive phases of tectonic events that affected the Luconia shelf from the Upper Cretaceous to Pliocene. Each tectonic event (Classified as Event 1–Event 3) is associated with different episodes of limestone growth in Luconia Province. The successive limestone growths are used as markers in constraining the timing and style of tectonic deformation. The poly-stage closure of the Proto South China Sea (PSCS) from the Upper Cretaceous to Lower Miocene led to the formation of compressional structures in its southern portion (South PSCS) providing elevated topography for the growth of the oldest limestone found in this area during the Upper Eocene to Lower Oligocene (Event 1). Based on contrasting seismic reflectors, morphology, and depositional patterns, the offshore Upper Eocene-Lower Oligocene limestone growth is correlated to the onshore Engkabang-Karap limestone. The southern part of Luconia was subjected to a continuous compression until the Lower Miocene at a time where the northern side of the Luconia Province was experiencing subsidence due to the rifting of the South China Sea (Event 2). The compression in the south generated elevated anticlines, triggering the growth of the Upper Oligocene to Lower Miocene limestone. By the end of the rifting event in the Lower Miocene, tectonic quiescence had enabled widespread carbonate growth in Luconia from the Middle to Upper Miocene. Regional compression due to the major uplift of Borneo hinterland (Event 3) triggered paramount clastic influx (gravity tectonics) to the offshore perturbating the limestone reef growth in Luconia. The impact of these interrelated shortening and stretching phases led to major crustal thickness variations and a prominent tilt of the Luconia platform that may highlight intricate feedbacks at the transition from compression to extension. While the southern side of the Luconia’s crustal fragment was anchored into Borneo hinterland, crustal extension in the northern region of Luconia led to a hyper-stretched crust characterized by low angle detachment faults and highly rotated blocks rising the mantle to its shallowest.

Detachment faulting in the Xigaze ophiolite southern Tibet: New constraints on its origin and implications

The structure and lithological sequences of ophiolites provide critical information for reconstructing their tectonic setting and thus, the history of the paleo-oceanic basins in which they formed. Our systematic mapping of three of the best-preserved massifs in the Xigaze ophiolite from the middle segment of the Yarlung Zangbo Suture Zone (YZSZ) in South Tibet, revealed a nearly intact structure of the ophiolitic sequence that has largely escaped emplacement and post-emplacement dismemberment. One of the most important results is the identification of a nearly E-W striking, serpentinized shear zone (SPSZ) preserved within the sequence. The SPSZ is rooted in the serpentinized front of the upper mantle and is mainly composed of meta-gabbroic and rodingite lenses floating in a serpentinite matrix. Extensive talc-serpentinite metasomatism, shearing and cataclastic deformation in the SPSZ reveal a complex interplay of fluid metasomatism and progressive ductile-brittle deformation in hundreds of meters of shear zone between the mantle and the crust. The structural and petrological features of the SPSZ are very similar to those of detachment faults within modern in situ oceanic core complexes (OCCs) in the Mid-Atlantic Ridge and the Southwest Indian Ridge. Zircon UPb ages from the footwall of the fault, and the age of radiolarian chert overlying it suggest that the detachment faulting took place between ca. 125 to 123 Ma. Combined with the stratigraphic and geochronological relationships between the ophiolite and the overlying sediments, we suggest that the Xigaze ophiolite developed in the supra-subduction zone on the southern margin of the Lhasa terrane during rollback of the Neo-Tethyan oceanic slab. Recognition of detachment faulting in the Xigaze ophiolites, along with other features, suggest that the Xigaze ophiolite formed at a spreading ridge, whereas the other large ultramafic massifs in YZSZ, may have formed in an off-axis location on the slow-spreading ridge.

Ophicalcites from the Upper Tectonic Unit on Tinos, Cyclades, Greece: mineralogical, geochemical and isotope evidence for their origin and evolution

Ophicalcites exposed on the island of Tinos, Greece, occur as ellipsoidal bodies within greenschist-facies phyllites of the Upper Cycladic Unit. Close to their outcrops, blocks of serpentinites, metabasic rocks and metasediments were identified, implying a tectonically dismembered ophiolitic sequence in the study area. The ophicalcites comprise brecciated serpentinites cemented by calcite. Based on textural, mineralogical and deformation features, five ophicalcite varieties were discriminated, reflecting calcite precipitation, sedimentary features and increasing brecciation. Serpentinitic fragments comprise antigorite, while Cr-spinel, magnetite, talc and chlorite are accessory minerals. Carbonate veins consist of calcite and minor dolomite, talc, chlorite, and rarely epidote. Bulk rock chemical compositions and Cr-spinel mineral composition point towards a supra-subduction environment. Carbon and oxygen isotope ratios of calcite imply precipitation from mixed marine and hydrothermal fluids, followed by isotope exchange due to late, greenschist-facies overprint. The Tinos ophicalcites record intraoceanic exhumation of the ultramafics at the seafloor, where faulting and serpentinization caused an extensive network of fractures, healed by carbonates. Such intraoceanic deformation can be attributed either to obduction tectonics expressed by thrusting of oceanic piles, or to transpressional(?) transform faults, or more probably to slip along detachment fault of an oceanic core complex.

Mosaic zircon petrochronology and implications for the ultra-slow spreading process of Southwest Indian Ridge

Here we report the discovery of mosaic zircons in Hole U1473A on the Atlantis Bank in the Southwest Indian Ridge. Oxygen isotope, UPb dating, and geochemical analyses of the zircons were carried out directly in petrographic thin-sections. The mosaic zircons are from diorite and oxide gabbro at shallow depths, whereas the nonmosaic zircons occur in oxide gabbronorite and tonalite at greater depths. The majority of zircons have δ18O of 5.5 ± 0.1‰, which are mantle-like values, reflecting formation by simple fractional crystallization. No correlation exist between UPb ages, δ18O values, and the mosaic microstructures. The mosaic subdomains in a single zircon are well correlated with some trace elements and fractures. Therefore, the variable trace element contents were produced during crystallization, and associated with secondary brittle deformation due to movement on mid-ocean ridge detachment faults. The pervasive brittle deformation weakened the originally rigid zircon and triggered fracturing. The fractures became highly permeable pathways, allowing rapid grain-boundary diffusive loss of Pb, which resulted in the relatively young age of 11.42 ± 0.41 Ma for the mosaic zircons as compared with 12.16 ± 0.14 Ma for the non-mosaic zircons. The zircon trace element patterns are all indistinguishable from global oceanic zircons. However, the trace element abundances and ratios require a significantly depleted normal-type mid-ocean ridge basalt mantle source.

Thermo-kinematic modeling of detachment-dominated extension, northeastern Death Valley area, USA: Implications for mid-crustal thermal-rheological evolution

The deep structure of continental detachment faults remains debated. Thermo-mechanical models generate detachments that either transect the lithosphere or become distributed shear zones in the mid-lower crust, depending on prescribed thermo-rheological conditions. However, these geometries and prescribed conditions remain little constrained by geology-based reconstructions. We present stepwise, balanced reconstructions of a 160 km-long cross-section through two detachment faults in the southwest USA. Reconstructions form the basis of iteratively improved 2D forward thermo-kinematic numerical simulations of detachment fault slip, footwall exhumation, heat advection, and footwall zircon (U-Th)/He cooling ages. Thermo-kinematic model solutions are calibrated iteratively against surface heat flow, pre- and post-extensional geotherms, inferred Moho temperatures, and thermochronometric data from one detachment footwall. Best-fit models predict the thermal and geometric evolution of the crust and detachments, respectively, during extension. The detachment initially rooted into a mid-crustal shear zone (~7.5–12 km depth) and was probably delocalized in the deep middle crust (>12–15 km). The maximum principal stress was likely non-vertical in the middle crust at detachment initiation, possibly due to mantle upwelling. Our reconstructions suggest that the upper crust and lower crust-mantle lithosphere were decoupled by a weak, mid-crustal layer during early detachment faulting. The weak layer was thinned, cooled, partially embrittled, and therefore strengthened by continued detachment slip. This increased lithospheric mechanical coupling and caused the locus of upper-crustal extension to shift. Thinning of a weak mid-crustal layer, as is thought to precede coupled hyperextension and mantle exhumation during rifting, was mostly complete in our study area by ~7–6 Ma.

Detachment fault fluid composition, temperature, and pressure from fluid inclusion analysis in the central Grouse Creek Mountains, Utah

Fluid inclusions were trapped in sealed microfractures in quartz in quartz-muscovite veins in and near the Red Butte pluton. The pluton lies in the footwall of the Ingham Pass detachment fault in the Grouse Creek Mountain portion of the Raft River-Albion-Grouse Creek metamorphic complex. The veins crosscut all metamorphic fabrics in the country rock and the pluton and postdate the pluton intrusion. The aqueous fluids determined from temperature measurements contain 1.8 to 5.6 wt percent salt as NaCl and 5–21 mol percent CO2. The inclusions homogenized at 185 °C–290 °C though many of the inclusions leaked or decrepitated before homogenization was reached. Pressure estimates made from the CO2 content of the fluids ranged from a high of 210 Mpa near lithostatic pressure to 60 Mpa near hydrostatic pressure. Thermal history determined by Ar-Ar dating of K-feldspar, biotite, and muscovite show that the temperature cooled to 315 °C by 21 Ma near the upper fluid inclusion temperature. Basin and Range normal faulting is dated at 13.4 Ma. The fluid inclusion characteristics compared to lithostatic pressure gradients indicate that the upper plate of the detachment fault had thinned from the maximum of near 15 km at the inception of the fault to about 8 km by the time of entrapment of the fluid inclusions. Pressure as high as lithostatic could have facilitated detachment faulting if the elevated fluid pressure was present during detachment fault displacement.

Rifting of Collapsed Orogens: Successive Incision of Continental Crust in the Proximal Margin Offshore Norway

AbstractStructures related to orogenic “collapse” may provide an important template for reactivation during later rifting. In the proximal rifted margin offshore Norway, the tectono‐sedimentary template related to collapse of the Caledonian mountain belt became widely eroded but inherited metamorphic core complexes and detachment faults evolved further in Late Paleozoic and Earliest Mesozoic time and became associated with very significant crustal thinning. The earliest rifting mode was dominated by structures that mimic the Devonian post‐Caledonian template and comprised a suite of Late Paleozoic to Early Triassic metamorphic core complexes, detachment faults, and supradetachment basins. The deep structure indicates necking of brittle over remnant ductile crust, and we document how the main decoupling level utilized by the earliest rifting structures were cut by a suite of steeper, more deeply incising set of seaward‐ and landward‐dipping Triassic faults. The latter exploited deep shear zones related to the previous intracrustal necking, enabling changes in polarity over short distances. The change from a core complex mode of extension to one involving more deeply incising faults took place prior to deposition of the Late Triassic‐Early Jurassic interrift succession and pre‐dated the main phase of lithospheric‐scale necking associated with Jurassic‐Cretaceous extension. The latter phase of large‐magnitude faulting commenced with reactivation of seaward‐dipping, deeply incised faults inherited from early rifting as well as structures related to the phase of Devonian post‐orogenic transtension. The current study demonstrates that great structural complexity and significant magnitudes of extension may be present in proximal margins, in particular those developed along collapsed orogens.

India‐Elan Bank‐East Antarctica Breakup, Crustal Architecture, and Margin Evolution: Results From Constrained Potential Field and Process‐Oriented Gravity Modeling of Conjugate Margin Segments

AbstractWe present here the results of a seismically well‐constrained potential field modeling along with process‐oriented gravity modeling across the Eastern Continental Margin of India (ECMI) and its conjugate East Antarctica Continental Margin (EACM). The study revealed both along‐strike variations in crustal architecture and effective elastic thickness (Te), showing a locally asymmetric pattern. Crustal stretching factors (β) estimated at these margins range between 1.5 and 4.0 with slightly lower values varying over the narrow zone in the Cauvery basin‐western Enderby basin and relatively higher values in other margin segments. The models reveal typical magma‐poor margin characteristics in the Krishna‐Godavari (K‐G) and Cauvery basin segments of ECMI. The ocean‐continent transition (OCT) in the K‐G basin and its conjugate (central Enderby basin) indicates asymmetric rifting marked by the development of a detachment fault separating the combined India‐Elan Bank margin and the EACM. The higher β values and magmatic underplating at OCT in the Mahanadi basin segment of ECMI may be related to the post‐breakup magmatic emplacement associated with the proximal activity of the Kerguelen plume after the separation of Elan Bank from India by a ridge jump during M2/M0 (124–120 Ma). The pre‐rift reconstruction model through the palinspastic restoration of margins suggests that the central part of ECMI (16–20°) is the ideal pre‐rift location for the Elan bank.

Crustal Structure Across the Extinct Mid‐Ocean Ridge in South China Sea From OBS Receiver Functions: Insights Into the Spreading Rate and Magma Supply Prior to the Ridge Cessation

AbstractThe crust near an extinct mid‐ocean ridge provides unique constraints on how its accretion and deformation responded to the cessation of spreading. Here we present crustal thickness and Vp/Vs measurements beneath 11 Ocean Bottom Seismograph sites that cross the South China Sea's extinct spreading axis. We find that the oceanic crust, which generally had only slight thickness changes once spreading started, abruptly thins at sites close to the extinct ridge axis. Abnormally high Vp/Vs ratios are obtained at several sites south of the ridge, indicating the presence of serpentine. These observations imply that, in its final stage, spreading changed to an ultraslow accretion style. As the axial crust thinned, normal faults and/or detachment faults began to form. Water could penetrate more deeply through these faults, and large fault slip could raise ultramafic peridotites to near or at the seafloor, creating favorable conditions for their enhanced serpentinization.

Magmato-tectonic mechanism of Southwest Indian Ridge (49°-50°E) inferred from quantitative morphotectonic analysis based on high-resolution multibeam bathymetry

Southwest Indian Ridge (SWIR) is a typical ultra-slow spreading ridge which has been extensively studied based on geophysical survey, geochemical sampling and morphological feature extraction. In this paper, based on the high-resolution multibeam bathymetric data, we extract seven types of topography, including fault, axial rift, axial volcanic ridge (AVR), axial trough, AVR's remnant, volcanic cone and oceanic core complex (OCC). We perform quantitative morphotectonic statistic, verifying the relations between the topography and magmato-tectonic mechanism of SWIR 49–50°E. The analyzed results of flank topography show that the average seafloor depth of the southern flank is shallower than that of the northern flank. The Total Cumulative Length of the faults in the northern and southern flank is 624.6 km and 948.2 km. The number, Total density, Average Length and Maximum Length of the faults in the southern flank are higher than those in the northern flank. An OCC and detachment faults locate in the southern flank. The asymmetry in flank topography indicates the asymmetric spreading mode of the study area. The analyzed results of the axial topography show that AVRs and troughs develop in the rift. The rift is narrower and shallower in the AVR regions, and wider and deeper in the trough regions. These observations support the partition of magmatic and less-magmatic stretching in the study area, and indicate the relation between the variation of rift's depth-width and the magmato-tectonic accretion.

Resolving mid- to upper-crustal exhumation through apatite petrochronology and thermochronology

Double-dating using the apatite U-Pb and fission-track systems is becoming an increasingly popular method for resolving mid- to upper- crustal cooling. However, these thermochronometers constrain dates that are often difficult to link through geological time due to the large difference in temperature window between the two systems (typically >250 °C). In this study, we apply apatite U-Pb, fission-track, and apatite and whole rock geochemistry to fourteen samples from four tectonic domains common in Cordilleran orogenic systems: (1) basement-cored uplifts, (2) plutons intruded through a thick crustal column, (3) metamorphic core complexes and associated detachment faults, and (4) rapid, extrusive volcanic cooling, in order to provide a link between in situ geochemical signatures and cooling mechanisms. Comparisons of trace element partitioning between apatite and whole rock provide insights into initial apatite-forming processes and/or subsequent modification. Apatite trace element geochemistry and the Th/U and La/LuN ratios provide tools to determine if an apatite is primary and representative of its parent melt or if it has undergone geochemical perturbation(s) after crystallization. Further, we demonstrate that by using a combined apatite U-Pb, FT, trace element, and whole rock geochemistry approach it is possible to determine if a rock has undergone monotonic cooling since crystallization, protracted residence in the middle crust, and provide unique structural information such as the history of detachment faulting. Insights provided herein offer new applications for apatite thermochronology.

Using paleomagnetism to test rolling hinge behaviour of an active continental low angle normal fault, Papua New Guinea

Metamorphic core complexes (MCCs) have been attributed to slip on long-lived detachment faults in extensional environments. At the surface, such faults are typically shallow dipping (<30°). This is inconsistent with Andersonian faulting theory, which requires that normal faults should nucleate and slip at steeper dips. One possible solution to this mechanical problem is a “rolling hinge” evolution for the fault during its slip history. In this model, faults initiate at steep dips (e.g. 60°) and back-tilt to shallower dips due to flexure and isostatic uplift in response to unloading of the footwall during slip. The Mai'iu fault is an active, rapidly slipping (∼1 cm/yr) low-angle (dip 20-22° at the surface) normal fault. Sampling of the footwall Goropu Metabasalt revealed a consistent normal-polarity component of magnetisation with moderate inclination that we interpret to have been acquired during uplift and cooling of the footwall rocks during the Brunhes chron. Comparison of the direction of the normal component with the expected average (geocentric axial dipole) direction at the site latitude indicates 23.9° ± 2.6° (1σ) of back-tilting of the footwall about the strike of the Mai'iu fault, consistent with a rolling hinge style of rotation. This indicates an original fault dip of 41.3-48.5°, as is consistent with independent estimates of original fault dip from fault-bedding cut-off angles and microseismicity at depth. This study is the first of its kind to demonstrate, using paleomagnetism, large-scale horizontal-axis rotations consistent with a rolling hinge evolution for a continental MCC.

The Lavrion Mines: A Unique Site of Geological and Mineralogical Heritage

The Lavrion area corresponds to the western part of the Attic-Cycladic metamorphic belt, in the back-arc region of the active Hellenic subduction zone. Between the Eocene and the Miocene, metamorphic rocks (mainly marbles and schists) underwent several stages of metamorphism and deformation due to collision and collapse of the Cycladic belt. Exhumation during the Miocene was accommodated by the movement of a large-scale detachment fault system, which also enhanced emplacement of magmatic rocks, leading to the formation of the famous Lavrion silver deposits. The area around the mines shows the stacking of nappes, with ore deposition mainly localized within the marbles, at marble-schist contacts, below, within, or above the detachment. The Lavrion deposit comprises five genetically-related but different styles of mineralization, a feature never observed in another ore deposit elsewhere, containing the highest number of different elements of any known mining district. The local geology, tectonic, and magmatic activity were fundamental factors in determining how and when the mineralization formed. Other key factors, such as the rise and the fall of sea level, which resulted from climate change over the last million years, were also of major importance for the subsequent surface oxidation at Lavrion that created an unmatched diversity of secondary minerals. As a result, the Lavrion deposit contains 638 minerals of which Lavrion is type-locality for 23 of them, which is nearly 12% of all known species. Apart from being famous for its silver exploitation, this mining district contains more minerals than any other district on Earth. The unique geological, mineralogical, and educational (mining, archaeological, and environmental) features suggest that it is highly suitable to be developed as a future UNESCO Global Geopark.

Fluid–rock interactions along detachment faults during continental rifting and mantle exhumation: the case of the Urdach lherzolite body (North Pyrenees)

The North Pyrenean Zone corresponds to the palaeopassive margin of the North Iberia plate, at the foot of which subcontinental mantle was exhumed during Albian times. Rare bodies of exhumed mantle rocks associated with strongly sheared lenses of continental crust are scattered among the North Pyrenean Zone metasediments. Significant fluid flow occurred along a major décollement at the basement–Trias interface in the Urdach massif (Chaînons Béarnais). Fluids with a broad range of salinity (10–38 wt.% NaCl equiv.), indicative of mixing between brines and more dilute waters, produced strong silicification of breccias. The brines circulated at c. 240–280°C under lithostatic pressures at c. 6 ± 1 km depth. The fluids became increasingly saline towards the final stages. The syndeposition of Cenomano-Turonian flysch layers then progressively isolated the lower aquifers close to the décollement where Triassic brines were predominant. The release and migration of significant volumes of brines during stretching and squeezing of the Triassic evaporites played a crucial part in the mineralogical and rheological transformations that occurred during the Pyrenean Cretaceous rifting event.

Linkage of deep lithospheric structures to intraplate earthquakes: A perspective from multi-source and multi-scale geophysical data in the South China Block

The relationship between intraplate earthquakes and intraplate deformation is one of the great conundrums faced by plate tectonics theory. In particular, large earthquakes are more common in Eurasia than in other continents around the world. Formed by the Neoproterozoic collision of the Cathaysia Block and the Yangtze Craton along the Jiangnan Orogenic Belt, the South China Block is well known for its relative stability in the Cenozoic era. However, numerous intraplate earthquakes have occurred in the Yangtze Craton and their mechanism is not well known. Incomplete understanding of the deep geological structure and deep-shallow coupling mechanism has led to two hypotheses about the nature and origin of intraplate earthquakes in the area: the far-field effects of plate subduction and the internal deformation of intraplate lithosphere. In this paper we attempt to provide a new perspective to the intraplate earthquake mechanism in the South China Block. In doing so, we have collected and reviewed multi-source and multi-scale high-precision geophysical data to improve the interpretation of the South China Block at depths from 1000 km to 1 km. The data interpretations help clarify the coupling relationship between the lithospheric structures and intraplate earthquakes in the South China Block. This paper emphasizes the strong structural heterogeneity in the South China Block. In plan view, the block is a mosaic structure formed by the assembly of various continental micro-blocks along several weak zones since the Neoproterozoic. In cross section, the structures of different units are segmented by detachment faults. For our analysis, we have used a three-layer model to describe the coupling of the asthenosphere-lithosphere, lithospheric mantle-crust and within the crust in the South China Block. The three cascade effects are coupled to form an intraplate earthquake triggering mechanism in the Yangtze Craton. The subduction effects of the Pacific Plate and the blocking by the Yangtze Craton are considered to cause a large-scale stress concentration in the Huaying Mountain fold-and-thrust belt; combined with three sets of detachment layers as internal factors, this stress results in numerous small-magnitude intraplate earthquakes occurring in the stable craton. This paper suggests that the deep structures can control the shallow tectonic processes and the plate tectonic responses and that the lithospheric strength can affect the triggering of intraplate earthquakes.

GIS-based Mineral Prospectivity Mapping of Seafloor Massive Sulfide on Ultraslow-spreading Ridges: A Case Study of Southwest Indian Ridge 48.7°–50.5° E

With the depletion of mineral resources on land, seafloor massive sulfide deposits have the potential to become as important for exploration, development and mining as those on land. However, it is difficult to investigate the ocean environment where seafloor massive sulfide deposits are located. Thus, improving prospecting efficiency by reducing the exploration search space through mineral prospectivity mapping (MPM) is desirable. MPM has been used in the exploration for seafloor deposits on regional scales, e.g., the Mid-Atlantic Ridge and Arctic Ridge. However, studies of MPM on ultraslow-spreading ridges on segment scales to aid exploration for seafloor massive sulfide have not been carried out to date. Here, data of water depth, geology and hydrothermal plume anomalies were analyzed and the weights-of-evidence method was used to study the metallogenic regularity and to predict the potential area for seafloor massive sulfide exploration in 48.7°–50.5° E segments on the ultraslow spreading Southwest Indian Ridge. Based on spatial analysis, 11 predictive maps were selected to establish a mineral potential model. Weight values indicate that the location of seafloor massive sulfide deposits is correlated mainly with mode-E faults and oceanic crust thickness in the study area, which correspond with documented ore-controlling factors on other studied ultraslow-spreading ridges. In addition, the detachment fault and ridge axis, which reflect the deep hydrothermal circulation channel and magmatic activities, also play an important role. Based on the posterior probability values, 3 level A, 2 level B and 2 level C areas were identified as targets for further study. The MPM results were helpful for narrowing the search space and have implications for investigating and evaluating seafloor massive sulfide resources in the study area and on other ultraslow-spreading ridges.

Geothermal potential of the eastern end of the Gediz basin, western Anatolia, Turkey revealed by three-dimensional inversion of magnetotelluric data

Three-dimensional models of the crustal electrical resistivity were derived from the inversion of magnetotelluric data in the eastern end of the Gediz basin, western Anatolia. In total, 179 MT sites measured in three different areas have been used in modeling. The study aims at providing information about the electrical resistivity distribution in the subsurface and its relations to structural setting and fluid pathways mainly controlling the development of geothermal reservoirs in the upper crust. The electrical resistivity models reveal a surficial low-medium resistivity layer associated with Neogene-Quaternary supradetachment sedimentary succession and a deeper high resistivity layer related to the Central Menderes Massif. The sedimentary cover over the metamorphic basement includes undulations, presenting an asymmetric extensional basin. The thickness of the sedimentary layer varies along strike from west to east and reaches approximately 3 km (max) in the graben. An anomalously conductive zone associated with a clay alteration zone was observed in this layer. The resistivity models also bring out two deep conductive zones along the southern margin of the basin: a conductive zone that could be interpreted as a geothermal reservoir at the easternmost end of the basin, however instead described as a part of a deep low resistivity zone with regional extent and another conductive zone interpreted as a region possessing an interconnected fracture network developed mainly by the extensional tectonic regime and that probably contains geothermal fluids. Moreover, normal faults exposing along the basin margins characterize the geometry of the basin, and particularly the main graben-bounding fault and Gediz detachment fault provide possible fluid pathways in the upper crust.

Fluid Circulation Along an Oceanic Detachment Fault: Insights From Fluid Inclusions in Silicified Brecciated Fault Rocks (Mid‐Atlantic Ridge at 13°20′N)

AbstractThe MAR 13°20′N corrugated detachment fault is composed of pervasively silicified mafic cataclastic breccias, instead of ultramafics and gabbros commonly found at other detachments. These breccias record overplating of hangingwall diabases, with syntectonic silicification due to important influx of silica‐iron‐rich fluids, able to leach alkalis and calcium. Fluids trapped in quartz inclusions show important salinity variations (2.1–10 wt.% NaCl eq.) indicating supercritical phase separation. Fluid inclusions also contain minor amounts of H2 ± CO2 ± CH4 ± H2S, with high H2/CO2 and H2/H2S ratios, signatures typical of ultramafic‐hosted vent fluids. We propose that seawater infiltrated the hangingwall upper crust at the axis adjacent to the active detachment, reaching a reaction zone at the dyke complex base (∼2 km). At &gt;500°C, fluids become Si‐rich during diabase alteration (amphibolite‐facies alteration in clasts), and undergo phase separation. Brines, preferentially released in the nearby detachment fault during diabase brecciation, mix with serpentinite‐derived fluids bearing H2 and CH4. Cooling during detachment deformation and fluid upward migration triggers silica precipitation at greenschist‐facies conditions (quartz + Fe‐rich‐chlorite ± pyrite). Important variations in fluid inclusion salinity and gas composition at both sample and grain scales record heterogeneous fluid circulation at small spatial and short temporal scales. This heterogeneous fluid circulation operating at &lt;2 km depth, extending both along‐axis and over time, is inconsistent with models of fluids channeled along detachments from heat sources at the base of the crust at the fault root. Present‐day venting at detachment footwall, including Irinovskoe, is instead likely underlain by fluid circulation within the footwall, with outflow crossing the inactive detachment fault near‐surface.

Alteration processes recorded by back‐arc mantle peridotites from oceanic core complexes, Shikoku Basin, Philippine Sea

AbstractWe determined the mineralogical and petrological characteristics of ultramafic rocks dredged from two oceanic core complexes: the Mado Megamullion and 23°30′N non‐transform offset massif, which are located within the Shikoku back‐arc basin in the Philippine Sea. The ultramafic rocks are strongly serpentinized, but can be classified as harzburgite/lherzolite or dunite, based on relict primary minerals and their pseudomorphs. Strongly elongated pyroxene porphyroclasts with undulatory extinction indicate high‐temperature (≥700 °C) strain localization on a detachment fault within the upper mantle at depths below the brittle–viscous transition. During exhumation, the peridotites underwent impregnation by magmatic or hydrothermal fluids, lizardite/chrysotile serpentinization at ≤300 °C, antigorite crystallization, and silica metasomatism that formed talc. These features indicate that the detachment fault zones formed a fluid pathway and facilitated a range of fluid–peridotite interactions.