NNW Trending Research Articles

Three-dimensional anisotropic inversion and electrostratigraphic imaging of marine magnetotelluric data to understand the control of crustal deformations by pre-existing lithospheric structures in the Mexican Ridges Fold belt, Southwestern Gulf of Mexico

SUMMARY3-D imaging of the lithosphere in the Mexican Ridges fold belt is important for understanding how the crustal deformations in this basin relate to deep tectonic processes and structures inherited from extinct Jurassic seafloor spreading. Here, we use broad-band (0.0001–0.4642 Hz) marine magnetotelluric data from the basin to reconstruct the 3-D anisotropic resistivities of the lithosphere and their spatial gradients. The resistivity gradients maxima enabled independent definition of important geological boundaries (seen on collocated seismic reflection data) and estimation of crustal thickness. We found anomalous layered zones of low resistivity and high electrical anisotropy at 5–8 km depth (coinciding with the regional detachment zone in Eocene shales in 3-D seismic data) and in the upper mantle which we interpret as indicating intense deformation and/or recent magmatic influence. We also found a banded crystalline basement structure across the fossil spreading centre comprising WSW–ENE trending, 6–10 km wide, electrically resistive subvertical sheets with conductive and anisotropic borders, which merge into a basal resistive stock-like body at 15–20 km depth. These are cut or bounded by later NNW trending major faults. These WSW and NNW structural trends correlate with the previously interpreted transform and normal faults that formed during the Late Jurassic opening of the Gulf of Mexico only if rotated clockwise by 25–30°. Surprisingly, the rugged thrust-related seabed is offset at the projected positions of the steep resistive-conductive basement sheets (which also have spatially coincident high magnetic intensity and seismicity) enabling us to infer they represent magmatic intrusions facilitated by pre-existing faults. Their conductive borders spatially coincide with possibly fluid-filled vertical fracture-sets in the overlying sediments seen in seismic data which we interpret as hydrothermal fluid pathways. We infer that a magmatic body recently intruded our study area, its ascent controlled by pre-existing basement structures, and influenced the deformation of the Neogene sequences and the seafloor topography.

An integrated geophysical and geological 3D model to characterize the feeder system of La Joya Honda and La Joyuela maars complex in central Mexico

La Joya Honda maar is recognized as one of the largest maar craters of the world (∼1200 m x 930 m), and La Joyuela maar, with axes of ∼1160 m x ∼790 m, is located in San Luis Potosí, México. Geophysical characterization using terrestrial magnetometry, aeromagnetic data, satellital gravimetry, and petrographic analysis of rock samples were interpreted jointly to improve the knowledge of both maars, their structural root geometry, and dimension maar-diatreme volcanoes. Magnetic and gravimetric analysis of both maars exhibit high magnetic anomaly values (≥400 nT) related to low gravimetric signatures (-5.2 to 1.5 mGal) and show characteristic geophysical maar behavior, complemented by positive anomalies (0.137 nT/m and 0.019 mGal/m, respectively) in the Analytic Signal result. Additionally, associated with the diverse tectonic and faulting events that the area has been subjected to, lineaments analysis and rose diagrams were used to determine structural tendencies, indicating the regional stress field (NNW trending) related to local fault orientations (E-W, NE-SW), which might be expected for the formation of maars with a comparable fracturing disposition at a depth between La Joya Honda and La Joyuela maars. This investigation shows the first magnetic inversion model in the zone, identified two vertical conduits susceptibility isosurface (2.53x10-4 SI) with an exceeded depth of -12 km below both volcanic structures, where regional analysis of lineaments and Werner solutions determine weak cortical structures. Furthermore, the petrological evidence (xenoliths) supports the geophysical model, which helps to understand regional factors that make the ideal environmental conditions for maar formation. Our new results contribute to the knowledge of the feeder system of the maars complex found in La Joya Honda and La Joyuela maars, demonstrate that the combination of different regional structural and local trends controls the magmatic evolution producing preferential paths for magma ascent.

Insights into the late Cenozoic structural deformation and tectonic stress field of the Qiabuqia region, Gonghe Basin, northeastern Qinghai–Tibetan Plateau

In addition to overall uplift of the Qiabuqia region during the Late Cenozoic, three deformation stages can be identified in the northeastern Qinghai–Tibet Plateau. Specifically, these deformation stages were recognized in areas east of the Gonghe Basin that surrounds Waliguan Mountain and include: 1) Late Miocene to Late Pliocene deformation—dominated by thrust napping with dextral strike-slipping; 2) Early Pleistocene to late middle period of Pleistocene deformation—fault structures were dominated by the dextral strike-slipping with thrust napping; and 3) end of Late Pleistocene to Middle Holocene deformation—thrust napping slowed down and finally braked, while the shallow surface began to loosen and extend at Qiabuqia region’s rear-edge. DR4 borehole drilling data indicate that the Xiangshuihe Formation’s buried depth is ∼1,000 m. The Xianshuihe Formation’s top section is characterized by normal fault–type (NF) tectonic stress, while the maximum principal tectonic stress (σ1) is perpendicular and the minimum principal tectonic stress (σ3) is horizontal. In contrast, the deep basement is characterized by thrust fault (TF) and strike-slip (SS) tectonic stress, while the σ1 is in the NE-SW direction, ranging from 20.1° to 75.3° with an average of 40.0°. Based on the aforementioned observations, it can be inferred that there are three sets of hidden faults in the basement of the Qiabuqia region: 1) the NW–NNW trending compressional–torsional faults; 2) the NE trending tensional–torsional faults; and 3) the NWW trending compressional–torsional faults. The findings in this study can potentially offer a robust geological basis for exploring and utilizing hot, dry rock resources within the Gonghe Basin.

Mafic dikes of the Mariinsky Taiga Alkaline Province, Kuznetsk Alatau terrane, southwestern Siberia: Intraplate alkaline magmatism in the Central Asian Orogenic Belt

The Mariinsky Taiga Alkaline Province (MTAP) is located in the Kuznetsk Alatau terrane of southwestern Siberia, which is part of Altai-Sayan orogenic system, of the Central Asian Orogenic Belt. The MTAP consists of a wide distribution of Paleozoic alkaline-basite magmatism (volcanic, subvolcanic and intrusive), which has been recently characterized with the recognition of at least two main timings, ca. 500 and ca. 400 Ma (herein labeled MTAP-1 and MTAP-2, respectively). We newly characterize the petrology, geochemistry, and age constraints of NNW and NNE trending alkaline dikes which cross-cut the alkaline intrusions and host volcanogenic and carbonate units and which are inferred to belong to MTAP-2. The dikes consist of ultrabasic foidolites (urtites, urtite-porphyry, ijolite-porphyry, microijolites), basic foidolites (plagioclase ijolites), alkaline basites (gabbro-diabase, labradorite porphyrites, camptonites and tamaraites), and nepheline, analcime and alkaline syenites. Their chemical composition is characterized by a wide range in silica (SiO 2 = 38.7–60.7 wt%), and alkalinity (Na 2 O + K 2 O = 4–18.7 wt%; Na 2 O/K 2 O = 0.5–8.3 wt%), low titanium (TiO 2 = 0.07–2.15 wt%) and high alumina content (Al 2 O 3 = 11.1–28.5 wt%), which corresponds to the K-Na differentiates of the basic alkaline magmatic series. The sum of rare-earth elements (ΣREE) increases from alkaline basite (ΣREE = 103.58–218.94 ppm), to the main foidolites (ΣREE = 137.74–217.61 ppm), and then to the ultrabasic foidolites (ΣREE = 55.56–268.88 ppm). Maximum REE content occurs in nepheline syenites and microsyenites (ΣREE = 73.24–1566.96 ppm) and they are the final product of the differentiation of foidite and basite melts. All dike varieties are characterized by low concentrations of the most highly charged elements (Th, Nb, Ta, Hf, and Y), which, according to their concentrations, plot between oceanic island basalt (OIB) and island arc basalt (IAB). The presence of a negative Nb-Ta anomaly and the relative enrichment of Rb, Ba, Sr and U indicate probable interaction of a mantle plume melts with lithospheric mantle metasomatized in an earlier subduction event. The primary isotope ratio 87 Sr/ 86 Sr(t) = 0.705388–0.706025 and the range of ɛNd(t) from +3.5 to +6.53 in the rocks indicate interaction of mantle derived melts with crustal components. Sm-Nd and U-Pb dating of the alkaline-basite dikes matches the boundary of the Early-Middle Devonian (406–389 Ma) and indicates membership in MTAP-2, which can be linked to the regional ca. 400 Ma Altai-Sayan Large Igneous Province/Rift System. Both the NNW and NNE trending dikes are associated with, and probably fed from, nearby alkaline intrusions such as i.e. Kiya-Shaltyr pluton which are also ca. 400 Ma (part of MTAP-2), but are independent of older ca. 500 Ma intrusions (which belong to MTAP-1) such as the University pluton and which are suggested to belong to a older plume-generated regional event. • Characterization of NNW and NNE trending dike belts in the MTAP, western Siberia. • They formed in the Middle Paleozoic (406–389 Ma) in a plume-rifting event. • Isotopic parameters ɛSr(t) indicate a strong contamination by crustal components. • The dike represent intraplate magmas emplaced in a shear setting.

South Tethyan passive margin paleoslope orientation inferred from soft-sediment deformation and fault kinematic analysis: a case study from the Cretaceous of Borj Cedria area, Tunisia

The Cretaceous outcrops of Borj Cedria-Bou Kournine area belongs to the NE-trending Atlas system of northern Tunisia. This area exposes sub-meridian folds associated with numerous N- to NW-trending major fault systems. This study together with previous surveys reveals that this N-trending folding is believed to be related to the inversion of the Jurassic and Early Cretaceous pre-existing fault zones and generated in response to the late compressive deformations. In addition, the study area provides evidence of soft-sediment deformations by good exposures of Cretaceous-aged slump sheets. Slump folds are usually associated with several meso-scale syn-sedimentary normal faulting together with frequent reworked blocs and occasional conglomeratic horizons. All these features indicate sedimentation on irregular seafloor topography. The aim of the present study is to investigate slump folds by applying techniques to reconstruct the contemporaneous slope gradient which has triggered soft-sediment deformations. Moreover, the brittle deformation is quantified using fault kinematic analysis together with the analysis of lithostratigraphic correlation and syn-sedimentary structures. Considerable thickness variations of Cretaceous deposits are interpreted as controlled by normal faulting activities. Likewise, fault kinematic analysis typifies a regional pure extension that trends NNW during Barremain, NNE during Albian, and probably NW during Cenomanian time. Based on slump folds analysis, the inferred sub-marine paleoslope is believed to have a northward dipping during Barremian and a NNE-dipping during Albian time. On the light of the over-mentioned interpretations, it is believed that Cretaceous sedimentation of the study area is highly controlled by major syn-depositional normal faults associated with intra basin growth faulting. These fault systems seem to be related to the Southern Tethyan expansion of the rifted continental passive margin.

Inherited structures controlling the development of the Arista-Ahualulco graben in the Mesa Central, Mexico

The reactivation of inherited structures is of utmost importance in extensional regions such as rifts and basins. We have investigated the possibility that normal faults found in the NNW Arista-Ahualulco graben may have formed due to an inversion of previous NNW trending contractile structures. Available geological information consistently suggests that the Arista-Ahualulco graben has experienced repeated phases of post-Laramide extension, starting with extensional reactivation of the basal Mexican Orogen structures. Our data in the studied area indicate inheritance of Mexican Orogen structures in the following aspects: (a) The Cenozoic normal faults are nearly parallel to the Mesozoic fold and thrust structures; (b) Preexisting faults controlled Cenozoic magmatism. In the preexisting contractile structures, three types of normal faults are observed: newly-formed, reactivated from the reverse faults and reactivated by bed-parallel slip. Based on the above observations, we proposed a two-stage model to explain the Cenozoic gravitation collapse: a late-orogen stage (late Paleogene-Eocene) with a collapse coeval to shortening and post-orogen stage (late Eocene-Miocene) under extensional regime).

Carbon dioxide diffuse emission at the Tolhuaca hydrothermal system (Chile) controlled by tectonics and topography

The Tolhuaca hydrothermal system is one of the few attested geothermal resources in Chile. While recent investigations provided some insights into the depth and temperature of the geothermal reservoirs and the chemical and mineralogical evolution of the hydrothermal system, little is still known about the CO2 degassing of the system and the local and shallow control of fluid pathways. Here, we document the soil CO2 degassing and soil temperature distributions in the southern part of the Tolhuaca hydrothermal system and at one of its northern fumaroles, and provide a first estimate of its total CO2 release. The surveyed area is responsible for a total CO2 emission of up to 30 t d−1. Hydrothermal CO2 emissions (~ 4–27 t d−1) are mostly restricted to the thermal manifestations or generally distributed along NNW trending lineaments, sharing the same orientation as the volcanic vents and thermal springs and fumaroles. Hydrothermal CO2 fluxes, fumaroles and thermal springs are generally encountered in topographic lows, in close vicinity of streams and often in clay-rich pyroclastic units, highlighting a relation between landscape evolution and the activity of the hydrothermal system. We suggest that glacial unloading and incision of the stream inside the clay-rich units have likely enhanced locally the permeability, creating a preferential pathway for the migration of deeper fluid to the surface. As several hydrothermal systems in the Andes are found on the flank of volcanoes hosting glaciers, we propose that they could have had a similar development to that of the Tolhuaca hydrothermal system.

Magnetic characteristics of a part of Closepet Granite, Eastern Dharwar Craton

The exposed granite gneissic complex to the west of Cuddapah basin in Eastern Dharwar Craton (EDC) is studied to understand the magnetic character of various plutons in this region with more emphasis on a part of Closepet Granite (CG). The NW–SE trending plutons in this region show gradual decrease in magnetic intensities from southwest to northeast; CG in the southwest corner of the study area have high mafic content, and there onwards show a depletion in mantle enrichment towards NE. Parts of the CG namely, the main mass and northern intrusions separated by a gap zone are differentiated based on magnetic character deduced by processing the aeromagnetic data. Reduced to the pole (RTP) and its upward continued maps revealed NNW trending eastward convex anomaly highs inferring the magnetic nature of CG due to Porphyritic Monzogranite (PMG). Gap zone is identified to be structurally significant, bounded by deep ENE faults and inferred to be associated with PMG at relatively deeper levels. 2.5D modelling of magnetic anomaly profile substantiated our qualitative analysis of magnetic anomalies over CG, inferring magnetic and non-magnetic units of CG. Known shear zones running north–south are identified with a few inferred shears revealed from magnetic data interpretation within CG.

Remote sensing and structural studies of the Central Asir Shear Zone, Western Arabian Shield: Implications for the late Neoproterozoic E-W Gondwana assembly

The Central Asir Shear Zone (CASZ) is a N- to NNE-oriented inhomogeneous strain zone parallel to the Nabitah shear belt in southern Saudi Arabia. It is ~200 km long and 20 km wide, and traverses the Asir terrane. The deformed terrains in the vicinity of the CASZ have experienced a prolonged history involving three successive phases of deformation: arc-collision, dextral transpression and sinistral strike-slip faulting. Remote-sensing data analysis and field investigation provide insights into the geological and structural features along the CASZ. Image transformation and enhancement reveal faults and folds, and discriminate between granitoids, metavolcanics, gabbros and younger (Ablah Group) sediments. The obtained evidence points to polyphase deformation of the area, characterized by narrow zones of high strain, folding and shearing enclosing slices of less deformed rocks. Lineament analysis shows predominantly NNE, NNW, N and NE trends of folds and shear zones, and NW, NE and E fracture trends. The CASZ has a >100 m.y. long deformation history, including a change from dextral to sinistral shearing. Both shearing senses are found on the older N-S and younger NW-SE fault systems. N-S oriented shearing was accompanied by intrusion of granitoids and localized hydrothermal alteration along N to NNE trending shear zones. Strain analysis shows low strain in the Ablah basin sediments, consistent with Ablah deposition after the tectonic climax in the region.

Multiple isotope (He-Ar-Zn-Sr-Nd-Pb) constraints on the genesis of the Jiawula Pb-Zn-Ag deposit, NE China

• Mantle fluid and atmospheric water are two most important sources for the ore-forming fluid of the Jiawula deposit. • Zn isotopic variation was most likely caused by kinetic Rayleigh fractionation. • Coeval magma played a key role in the formation of the deposit. The large Jiawula Pb-Zn-Ag deposit is located in the Manzhouli area, northeast China. The Pb-Zn-polymetallic mineralization in the Jiawula deposit is present as ore veins distributed in Early Mesozoic sedimentary and magmatic rocks and was controlled by NWW to NNW trending faults. He and Ar isotope analysis results of fluid inclusions hosted in pyrite show that 40 Ar/ 36 Ar ratios are 280.5–300.1, which is close to the atmospheric value (295.5). 3 He/ 4 He ratios vary from 40.15 × 10 −7 to 71.68 × 10 −7 . The calculated mantle He are in a range of 35.8–64.0%, with an average of 51.0%. He-Ar isotopic data reveals that the ore-forming fluids of the Jiawula deposit were derived from mixture of two fluid end-members: (1) mantle-derived fluid, which has provided a considerable proportion of He; and (2) crustal fluid, which is characterized by almost atmospheric Ar and crustal He. The sphalerite from the Jiawula deposit have a narrow range of δ 66 Zn JMC values, varying from 0.15 to 0.307‰, with a mean value of 0.236‰. We suggest that the variation of Zn isotope values of sphalerite from the Jiawula deposit was most likely caused by kinetic Rayleigh fractionation. The Zn isotope analysis result, combined with previous S isotopic data, indicate that the ore-forming material of the Jiawula deposit was derived from a single source. The intrusive rocks in the deposit have low initial 87 Sr/ 86 Sr ratios, relatively high ε Nd (t) values, and young T DM2 Nd model ages, indicating a high proportion of juvenile material in the petrogenesis. Combined with Sr-Nd-Pb isotopic data of intrusive rocks that are coeval with mineralization in the deposit, He-Ar-Zn isotope analysis results, and previous studies, we propose that the mantle component in the ore-forming fluid and material of the Jiawula deposit was most probably derived from the simultaneous magma, which was derived from partial melting of crust with involvement of mantle magma in an extensional setting during the late Jurassic to the early Early Cretaceous.

Neotectonic evidence for Late Quaternary reverse faulting in the northern Chile outer forearc (22.5°S-23°S): Implications for seismic hazard

In the northern Chile outer forearc (19°S to 23°S), extension is the dominant deformation style conducted by crustal faults during the Neogene-Quaternary. Most of this extension has been produced by normal faulting along submeridian branches of the Atacama Fault System (AFS). During the Late Quaternary, several of these faults have built conspicuous metric-scale normal faulting scarps in alluvial deposits; the construction of these scarps has been related, in some cases, to M~7 normal faulting paleoearthquakes. On the contrary, shortening during this time span has only been locally reported. Until now, no metric-scale reverse faulting scarps have been documented along submeridian strands of the AFS. This study presents novel evidence corresponding to reverse fault scarps produced during the Late Quaternary by three NNE and NNW trending faults in the outer forearc between 22.5°S and 23°S: the El Toro (ETF), Caliche (CF), and Sierra Valenzuela (SVF) faults. These scarps, with up to 2.5 m height, developed in basement rocks, pre-Quaternary, and/or Quaternary alluvial deposits. Based on their characteristics and the evidence suggesting that similar fault scarps in the Andean forearc have a seismogenic origin, we interpret that these neotectonic landforms were produced by single reverse faulting paleoearthquakes. Considering the maximum scarp height as a proxy for the coseismic slip and fault length as a proxy for surface rupture length, we have estimated magnitudes between M6.2 and M6.9 for the causative rupture events. To discuss how reverse crustal faulting may be related to the interseismic and/or coseismic stages of the subduction cycle, we perform elastic dislocation models to calculate the horizontal displacement fields (at 0 and 20 km depth) induced by the interplate locking distribution in the area (over a seismic cycle) and three megathrust earthquakes (Antofagasta, 1995, Tocopilla, 2007, and a synthetic rupture). Modeling results suggest that the studied faults may experience reverse slip during both stages of the subduction cycle. We propose that the ETF, CF and SVF are faults capable of producing moderate-to-large earthquakes in the future. Finally, we reinforce the need to address the earthquake potential of the studied faults, and others similar to them, to acquire a complete assessment of the seismic hazard in the northern Chile outer forearc.

Autochthonous origin of the Encruzilhada Block, Dom Feliciano Belt, southern Brazil, based on aerogeophysics, image analysis and PT-paths

The Encruzilhada Block lies between the eastern hinterland and western foreland of the Dom Feliciano Belt (southern Brazil) and its tectonic meaning in the orogenic evolution of the belt is still poorly understood. Its low magnetic signal is similar to that of the foreland supracrustal sequences, whereas the high gammaspectrometric signal appears due to the hinterland post-collisional granitoids. Satellite image lineament analysis points to tectonic similarities between the Encruzilhada Block and the foreland, as the structures in both domains show predominantly NNW and NNE trends, while the hinterland mostly presents NE-trending structures. The pre-orogenic connection between the Encruzilhada Block and the foreland is indicated by the correlation between the Várzea do Capivarita and Cerro da Árvore complexes. Both contain 800 – 780 Ma metaigneous rocks with arc-like geochemistry, and parametamorphic rocks with similar sedimentary provenance. Common syn-orogenic deformation history of both complexes is suggested by progressive to P-T o-th E-W transpressive deformation and similar peak metamorphic age (660 – 640 Ma). PT-paths suggest exhumation of the hinterland Várzea do Capivarita Complex from high-T/low-P conditions of 790–820 °C/4.4–4.8 kbar to 660–720 °C/2.5–3.4 kbar at ca. 630 Ma. On the other hand, the foreland Cerro da Árvore Complex was subject to progressive metamorphism from 555 to 565 °C/5.4–5.7 kbar to 560–580 °C/5.8–6.3 kbar, which is interpreted as a result of orogenic thickening. Evidence suggests that both complexes have originated in a similar, if not a single, basin on an attenuated lithosphere with high geothermal gradient, possibly a (back-arc?) rift. Oblique collision has caused basin inversion and thrusting of the lower crust over the rift margin, which may have been accompanied by lithosphere delamination. Progressive transpressional deformation and voluminous post-collisional magmatism (640 – 578 Ma) controlled by major lineaments have blurred the original tectonic contact and finally caused thermal metamorphism in both complexes at ca. 3 kbar, when the autochthonous Encruzilhada Block was stabilized in its present geological configuration. The intracontinental character of major shear zones limiting hinterland and foreland requires a review of the existing evolutionary model for the Dom Feliciano Belt in the context of Western Gondwana amalgamation.

Deformation conditions and kinematics of fault rocks in the Hatagawa Fault Zone and the Shajigami Shear Zone, Northeast Japan: Insights from calcite and quartz microstructures

Calcite grains are plastically deformed at lower temperatures than are quartz grains, which means that investigating the microstructures of both minerals should provide useful information regarding the deformation history of faults. We investigated the conditions of deformation and kinematics of fault rocks within the Hatagawa Fault Zone and the Shajigami Shear Zone, Northeast Japan, using deformation microstructures and crystallographic preferred orientations of quartz and calcite. The Hatagawa Fault Zone trends NNW–SSE, and the associated foliated granite cataclasites show sinistral shear. Limestone mylonite samples showing sinistral shearing display girdles of calcite c-axes close to the YZ plane and Z-maximum, suggesting relatively high and low temperatures of deformation, respectively. The Shajigami Shear Zone trends NE–SW, and quartz microstructures in granodiorite mylonites of the shear zone indicate dislocation creep at temperatures of 400–500 °C. The limestone mylonites have Z-maximum patterns of calcite c-axes. These fault rocks were deformed during the Cretaceous (before the Cenozoic 40° counterclockwise rotation of Northeast Japan) in a kinematic framework characterized by a shortening direction that rotated from N–S to NW–SE. The kinematics and deformation ages of the fault rocks correspond to a change in the direction of motion of the Izanagi Plate at c. 100 Ma.

Integrating aeromagnetic data and Landsat-8 imagery for detection of post-accretionary shear zones controlling hydrothermal alterations: The Allaqi-Heiani Suture zone, South Eastern Desert, Egypt

The Arabian-Nubian Shield (ANS) underwent a regional carbonation event with CO2 fluxes derived from the mantle and circulated along post-accretionary shear zones, thus affecting the redistribution of gold mineralization. Formerly, the analyses of remotely-sensed data have suggested regions of gold mineralization linked to the post-accretionary structured framework, but the results were insufficient for detailed prospecting in the ANS. In this research, aeromagnetic data and Landsat-8 imagery were integrated for delineating new high potential zones of gold mineralization in the Allaqi-Heiani Suture (AHS) zone, the ANS, South Eastern Desert (SED) of Egypt. Aeromagnetic data were enhanced using the Center for exploration targeting (CET) grid enhancement technique to detect the main structures that control hydrothermal alterations in the study area. Principal Component Analysis (PCA) technique and ratios of spectral bands were applied to Landsat-8 data for mapping zones of hydrothermal alteration associated with gold mineralization. The traced structural elements from RTP, CET and Landsat-8 maps were statistically plotted and interpreted using rose diagrams. These diagrams indicated that the NW and NE trends are the most dominant in the RTP map and that the intersection zones obtained from the CET analysis are aligned along the NW-oriented trend. While the NNW, WNW and NE trends were the most dominant ones in Landsat maps. The integration of the results indicates several new high potential zones of gold and Cu/Ni mineralization types, which are mostly concentrated at the western part of the AHS zone. The hydrothermal alteration zones and associated gold mineralization are strongly linked to NW, NE, E-W and WNW trending lineaments. The approach used in this study can be applied to other parts of the ANS and other regions with similar geological conditions around the world.

3D Coseismic Deformation Field and Source Parameters of the 2017 Iran-Iraq Mw7.3 Earthquake Inferred from DInSAR and MAI Measurements

The coseismic slip on the main fault related to the 2017 Iran-Iraq Mw7.3 earthquake has been investigated by previous studies using DInSAR (differential interferometric synthetic aperture radar) ground deformation measurements. However, DInSAR observation is not sensitive to the ground deformation in the along-track (AT) direction. Therefore, only the one-dimensional (1D) DInSAR coseismic deformation field measurements, derived in the LOS (line-of-sight) direction of radar, was applied in source parameters estimation. To further improve the accuracy of the fault slip inversion, the 3D (three-dimensional) coseismic deformation fields were reconstructed in the first place, by a combined use of the DInSAR and MAI (multiple aperture InSAR) measurements. Subsequently, the LOS and 3D deformation data sets were used as the constraint respectively, to perform a two-step inversion scheme to find an optimal geometry and slip distribution on the main fault. The comparative analysis indicated that the 3D coseismic deformation data sets improved the inversion-accuracy by 30%. Besides, the fault invention results revealed a deep dislocation on a NNW trending fault (the strike is 352.63°) extending about 60 km, along the fault dips 14.76° to the ENE. The estimated seismic moment is 8.44 × 1019 Nm (Mw7.3), which is close to the solution provided by USGS (United States Geological Survey). The slip distributed at the depth between 12 and 18 km, and the peak slip of 6.53 m appears at the depth of 14.5 km left near the epicenter. Considering the geological structure in the earthquake region and fault source-parameters, it comes to a preliminary conclusion that the ZMFF (the Zagros Mountain Front fault) should be responsible for the earthquake. In general, this paper demonstrated the superiority of using the 3D coseismic deformation fields on source parameters estimation.

Strain Accommodation in the Daliangshan Mountain Area, Southeastern Margin of the Tibetan Plateau

AbstractThe Xianshuihe‐Xiaojiang fault system (XXFS) plays a crucial role in accommodating relative motions of the south China block and the expanding Tibetan Plateau. Compared to the narrow southern and northern segments of the XXFS, the central section (also referred to as the Daliangshan mountain area) is broader and dissected by a network of faults. Geologic studies suggest that three major NNW trending faults may dominate slip partitioning of more recent faults in this region. However, due to accessibility issues, sparse geodetic studies have constrained geodetic fault slip rates with contradictory results. In this study, we use 17 new Global Navigation Satellite System sites combined with existing solutions to investigate slip partitioning of the region. We derive a tectonic velocity solution, develop a block kinematic model constrained by Euler pole clustering technique, and calculate geodetic fault slip rates to compare with geologic fault slip rates. We confirm that the Anninghe‐Daliangshan sliver (ADS) is a distinct, rigid block, and we resolve a new, rigid Daliangshan‐Yingjing‐Mabian block (DYB). Kinematic modeling predicts long‐term slip rates that suggest both the ADS and the DYB partition (~10.0–12.0 mm/year) relative motion between the Tibetan Plateau and south China block in the central XXFS, reconciling geologic observations. We suggest that both the continuum and microplate deformation patterns are present in our study area, of which the local tectonics is better explained by the microplate model, but scale of the kinematics makes its motion more consistent with the continuum model.

Structural analysis of T. Sundupalle greenstone belt and surrounding granitoids, Andhra Pradesh, India

This contribution highlights the less widely known T. Sundupalle greenstone belt and surrounding granite–gneiss complexes. The greenstone belt trends NNW and comprises supracrustal assemblages with intense deformation. Four episodes of deformation events are revealed from this present structural study. Younger deformation episodes like igneous intrusions, recrystallization, and partial melting complicates the older fabric elements. Several deformation episodes were recognized by a systematic structural analysis based on superposed fabrics and principle of cross-cutting relationships. However, due to superposed deformation, often the features become obscure. The identification of the deformation episodes through the fabric elements in the shear zone allowed a tectonic interpretation. The initial greenstone belt formation above the thin hot Archaean crust followed by sinking and partial melting to form granitoids and their lit-per-lit emplacement along foliations formed gneissosity and further deformation, folding, and partial remelting generated migmatitic gneiss. The youngest event is the dyke emplacement along different trends during extensional tectonic regime.

Petrogenesis of the 1.85 Ga Sonakhan mafic dyke swarm, Bastar Craton, India

The NNW trending tholeiitic Sonakhan mafic dyke swarm of the Northern Bastar Craton is comprised of basalt to basaltic andesite (SiO2 = 46.3 wt% to 55.3 wt%; Mg# = 37 to 70) dykes. A single basaltic dyke yielded a weighted-mean 207Pb/206Pb baddeleyite age of 1851.1 ± 2.6 Ma. The Sr and Nd isotopes (87Sr/86Sri = 0.70396 to 0.70855; εNd(t) = −5.7 to +2.0) are variable which is a consequence of crustal contamination. Trace element modeling suggests the dykes were likely derived by partial melting of a spinel-bearing mantle source. The Sonakhan dykes are 30 million years younger than the 1.88 Ga Bastar-Cuddapah dykes (Bastanar-Hampi swarm) of the southern and central Bastar Craton indicating they represent a distinct period of magmatism. However, much like the 1.88 Ga dykes, the Sonakhan dykes appear to be correlative with dykes from the Yilgarn Craton (Yalgoo dyke = 1854 ± 5 Ma) of Western Australia. The temporal and compositional similarity of the Sonakhan dykes with the Yalgoo dyke is evidence that they are petrologically related and may represent different branches of the same dyke swarm. The existence of two distinct Paleoproterozoic dyke swarms in the Bastar Craton that each have a correlative unit in the Yilgarn Craton is supportive of a link between India and Australia before 1.9 Ga. Moreover, it suggests that the break-up of India and Western Australia was protracted and lasted for at least 30 million years.

An inhomogeneous across-slab conduit controlled by intraslab stress heterogeneity in the Nankai subduction zone

Nonvolcanic, deep low-frequency tremors and slow-slip events occur simultaneously in the transition zone from locked to continuously creeping fault in the down-dip portion of the Nankai Trough subduction zone, southwestern Japan. The occurrence of these slow earthquakes is discontinuous along the trench and attributed to the effect of high pore pressures at the plate boundary. Here, we show that spatial variations in intraslab stress may control fluid migration from the subducted Philippine Sea slab to the plate boundary. The triaxial normal faulting stress, detected by stress tensor inversion using focal mechanisms in the slab, controls anisotropically permeability that trends NNW–SSE subhorizontally from the subducted Philippine Sea slab to the plate boundary. The inhomogeneous permeability controlled by spatial stress heterogeneities in the subducted Philippine Sea slab controls the intraslab fluid pathway. This hypothesis is consistent with the spatial heterogeneity of slow earthquakes and 3He/4He ratio distributions.

The Miocene Atastra Creek sinter (Bodie Hills volcanic field, California and Nevada): 4D evolution of a geomorphically intact siliceous hot spring deposit

The Atastra Creek siliceous hot spring deposit, or sinter, occurs in the Paramount-Bald Peak alteration zone, due north of the Bodie precious metals mining district in the Miocene Bodie Hills volcanic field, California and Nevada, U.S.A. Distinctive features include its geomorphically intact geyser vent mounds, the presence of growth-fault-stepped sinter terraces that developed westward along a NNW trending fault, and atypical Hg, Sb and As concentrations. High-temperature (>75 °C), subaerial, proximal apron sinter occurs in two settings – (1) radiating botryoidal geyserite in vent mounds and (2) bedded geyserite (columnar, nodular, size-sorted geyser eggs) intercalated with mid- to low-temperature (<60 °C), middle to distal apron terrace sinter. The active geyser vent mounds migrated from south to north across the apron terrace through time. A newly recognized, visually striking type of sinter – termed silica infiltrate herein – formed directly beneath the subaerial geyserite deposits. It is characterized by mainly parallel, thin, irregular beds of white silica with narrow borders of dark blue-gray silica, as well as swirly to globular white silica, all chaotically linked together by silica ‘necks’ and ‘bridges’. The silica infiltrate permeates the pre-existing sinter, and is interpreted to represent super-heated fluid injections into the immediately overlying geyser vent mounds and pools. The adjacent sinter sheets preserve fluid-flow directions of warm-water channels that traversed the discharge apron, including microbial streamer fabric and wavy laminated sinter with flattened bubbles. More distal sinter textures are recorded by rimmed terracettes with mammillated and palisade microbial fabrics and rare, warm pool-related stromatolites. However, plants are absent, probably due to the high metalloids, in particular As. Post-depositional events include an inferred hydrothermal eruption breccia and an interpreted drop in the water table accompanied by incursion of steam acid condensate, as evidenced by pervasive hydrothermal alteration of the Atastra Creek deposits. Hence, the well-exposed sedimentary facies distributions, well-preserved geomorphology, and stratigraphic and structural relationships together allow an unusually complete, paleoenvironmental reconstruction of the Atastra Creek paleohydrothermal deposit.