Southern Segment Research Articles

Evaluation of fine aggregates from the Budhi Gandaki-Narayani River, central Nepal for mortar and concrete

Huge amounts of construction materials are required to fulfill construction works, which are being undertaken by local and Government levels. In this regard, searching for good quality construction materials is a tough job. The Budhi Gandaki-Narayani River is rich in carrying natural fine aggregates from the Higher and the Lesser Himalayas, which are extensively exposed along its bank. Currently ongoing and future hydro power projects, will require a tremendous amount of fine aggregates. In light of this, the present study was made to evaluate fine aggregates of the Budhi Gandaki-Narayani River. The gradation curves of fine aggregates from this river varied from well graded to uniform graded. Compositional analysis shows that quartz is dominant over other minerals. Rock fragments are composed of gneiss, schist, quartzite, metasandstone, phyllite, shale, granite, etc. Deleterious materials excluding organic matter range from 0.3 % to 1.5 % and organic matter present in range from 0.57 % to 1.11 %. The trend of presence of inorganic deleterious material and organic matter is increasing towards southern segments of the river, but they remain below 2%. Mean roundness of particles ranges from 1.88 to 2.54. Bulk dry density of fine aggregate is below 2 gm/cm³. Similarly, the specific gravity ranges between 1.49 and 1.79. Fineness modulus ranges between 1.36 and 3.50 showing that, coarse, medium, fine and very fine sediments are distributed along the stretch of river. Water absorption value of fine aggregate ranges from 0.48 % to 2.87 %. When using fine aggregates from southern segments of the Budhi Gandaki-Narayani River, reduction of mica and beneficiation process will enhance and provide better result for smooth plastering, rendering and screeding.

Heterogeneous Behavior of the Campotosto Normal Fault (Central Italy) Imaged by InSAR GPS and Strong-Motion Data: Insights from the 18 January 2017 Events

On 18 January 2017, the 2016–2017 central Italy seismic sequence reached the Campotosto area with four events with magnitude larger than 5 in three hours (major event MW 5.5). To study the slip behavior on the causative fault/faults we followed two different methodologies: (1) we use Interferometric Synthetic Aperture Radar (InSAR) interferograms (Sentinel-1 satellites) and Global Positioning System (GPS) coseismic displacements to constrain the fault geometry and the cumulative slip distribution; (2) we invert near-source strong-motion, high-sampling-rate GPS waveforms, and high-rate GPS-derived static offsets to retrieve the rupture history of the two largest events. The geodetic inversion shows that the earthquake sequence occurred along the southern segment of the SW-dipping Mts. Laga normal fault system with an average slip of about 40 cm and an estimated cumulative geodetic moment of 9.29 × 1017 Nm (equivalent to a MW~6). This latter estimate is larger than the cumulative seismic moment of all the events, with MW > 4 which occurred in the corresponding time interval, suggesting that a fraction (~35%) of the overall deformation imaged by InSAR and GPS may have been released aseismically. Geodetic and seismological data agree with the geological information pointing out the Campotosto fault segment as the causative structure of the main shocks. The position of the hypocenters supports the evidence of an up-dip and northwestward rupture directivity during the major shocks of the sequence for both static and kinematic inferred slip models. The activated two main slip patches are characterized by rise time and peak slip velocity in the ranges 0.7–1.1 s and 2.3–3.2 km/s, respectively, and by ~35–50 cm of slip mainly concentrated in the shallower northern part of causative fault. Our results show that shallow slip (depth < 5 km) is required by the geodetic and seismological observations and that the inferred slip distribution is complementary with respect to the previous April 2009 seismic sequence affecting the southern half of the Campotosto fault. The recent moderate strain-release episodes (multiple M~5–5.5 earthquakes) and the paleoseismological evidence of surface-rupturing events (M~6.5) suggests therefore a heterogeneous behavior of the Campotosto fault.

Morphometric evidences of recent tectonic deformation along the southeastern margin of the Hyblean Plateau (SE-Sicily, Italy)

The present work focuses on the tectonic characterization of the southeastern portion of the Hyblean Plateau (SE-Sicily, Italy). Along the southeastern margin of this raised crustal block, a clear recent antiformal deformation, coupled with diffuse landscape rejuvenation, affects the summit surface. Many studies, using merely qualitative geomorphological approach, refer the recent morphotectonic evolution of this region to motion along an active border fault, located at the base of the well-developed, arc-shaped Avola mountain front. This paper provides new insights resulting from the morphometric analysis across the Avola region, in order to detail on the relationships between the landscape geomorphic unsteadiness and the long-term tectonic deformation. We perform both drainage system and relief investigations, consisting of the exploration of different geomorphic indices (e.g. hypsometric integral, topographic relief and dissection, SL index, ksn), long-profile analysis, marine terraces age model definition, field-based geomorphological and structural surveys. The results and the spatial distribution of morphometric indexes evidence the occurrence of two incipient NNE trending en-échelon fault segments, here named Cassibile-Noto Fault, responsible for the relief building along the southeastern border of the Hyblean Plateau. The northern fault segment directly controls the northern portion of the Avola mountain front, whereas the southern segment diverges from the main scarp, cutting through the low-standing areas of the coastal plain. In addition, the centrifugal arrangement of fluvial captures and the half-elliptical envelope of the downstream projections of coeval paleo-long profiles, as well as the distribution of Pleistocene marine terraces, well evidence the long-term WNW-ESE-trending low-amplitude bow-shaped deformation affecting the footwall of the Cassibile-Noto normal fault. We relate this convex-upward trend to the Late Pleistocene (<125 ka) tectonic deformation cumulated, at rate of ~0.80 ± 0.05 mm/a, along the active fault. This incipient tectonic structure represents an extensional tectonic boundary that accommodates, together with the Ispica Fault, the active NW-ward motion of the entire eastern sector of the Hyblean Plateau. Overall, the proposed updating of the tectonic setting of the study area, framed in the regional geodynamic picture of the eastern Sicily, would contribute to define a more suitable geological risk assessment of the region.

Characteristics and Classification of the Geothermal Gradient in the Beijing–Tianjin–Hebei Plain, China

Study of the characteristics and classification of geothermal gradients can effectively guide the exploration and development of geothermal resources. In this paper, we systematically compare the relationship between the distribution of the geothermal gradient and its basin structure, and numerically model the heat conduction course of typical geothermal–geological profiles. The results show that the distribution of the geothermal gradient in the Beijing–Tianjin–Hebei Plain exhibits the characteristics of east–west zonation, north–south segmentation, and vertical stratification. From west to east, the basin structure can be divided into five belts, viz. the western sag, the central bulge, the eastern sag, the Cangxian uplift, and the Cangdong sag belts. The heat flow and the geothermal gradient of the cap rocks in the basal bulge/uplift belts are higher than in the sag belts. From north to south, the basin structure of the Jizhong depression is subdivided into the northern, middle, and southern segments by the Xushui-Anxin and Hengshui Faults. It also shows an obvious distribution of low and high geothermal gradient interphases, which are similar to their upper and lower basement. Vertically, the geothermal gradient in the bulge/uplift belts exhibits an asymmetrical “mirror reflection” structure. Abnormal geothermal gradient distributions are observed, being confined to the basement uplift belts caused by the tectonic movement which led to the heterogeneity of the horizontal and vertical heat conduction capacity. Therefore, according to the corresponding relationship between the amplitude of the basement uplifting and the size of the geothermal gradient of the cap rock, the geothermal gradient of the Beijing–Tianjin–Hebei Plain can be divided into five categories, of high convex, convex, low convex, deep buried, and exposed types. Among these, the high convex and convex types are the primary targets for geothermal exploration and development.

Heterogeneous strain regime at the west of the Ogasawara Plateau in the Western Pacific Ocean from inversion of earthquake focal mechanisms

The Ogasawara Plateau has a major control on the geology of the region between the Izu-Bonin and Mariana subduction zones. This region is important for understanding active collisional zones. We attempt to illustrate the stress field in the Wadati-Benioff zone of this region through seismic estimations. 49 events (M ≥ 3.5) are chosen for waveform inversion by the ISOLA software to distinguish the stress field. According to available focal mechanism solutions, the heterogeneous stress field parameters are determined for several depth ranges. Based on the epicentral distribution and seismic stress field, the subduction system at the west of the Ogasawara Plateau can be divided into three segments separated by 26.5°N and 24°N. At depths of 0–50 km in the outer-trench slope, the slab is characterized by a varying stress filed. The best-fit stress model with horizontal σ3 and vertical σ1 is related to the plate bending and preexisting faults. For the central segment, the solutions present a compressional stress regime in the outer-trench slope without preexisting faults, induced by the resistance of the buoyant plateau. The solutions for the depth range of 0–50 km in the forearc region, σ1 coincides with the plate convergence and σ3 dips steeply to the west in the northern and central segments, controlled by the friction effect and convergence of the plates. However, this depth range in the southern segment shows an extensional stress range, associated with subduction of older slab segment, relatively low rate of plate convergence and the morphological adjustment of the slab between different segments. At depths of 50–200 km, the inhomogeneous stress field exhibits a complicated extensional stress system with strike-slip focal mechanisms. At depths of 200–400 km, the seismicity presents a pronounced seismic gap and the stress field cannot be determined. The seismic gap may be attributed to the missing or tear of the subducting slab, which is caused by the subduction of the buoyant Ogasawara Plateau. The mechanism solutions are characterized by an inhomogeneous strain regime below 400 km, driven by gravity and strong contortion of the slab and resistance to penetration of the 660-km discontinuity. In contrast, there are little earthquakes below 400 km in the southern region of the Ogasawara Plateau.

Over 100 years of faults interaction, stress accumulation, and creeping implications, on Chaman Fault System, Pakistan

The curve-shaped Chaman transform fault between Pakistan and Afghanistan demarks the western boundary of the Indian plate, which is the locus of many catastrophic earthquakes, and this is poorly studied region to understand the earthquake/fault interaction and hazard assessment. Based on earthquake triggering theory, we investigate a sequence of earthquakes comprising five earthquakes that occurred in the Chaman Fault System since 1892. Our results elucidate that three out of four earthquakes are triggered by the preceding earthquakes. The 1935 earthquake of magnitude Mw 7.7 is somewhat an independent earthquake in this sequence, slightly influenced by the 1892 event. The 1935 earthquake significantly loaded either terminus of its rupture with positive stress, which later triggered the 1975 earthquake with 35% of its rupture length where ΔCFS is above triggering threshold value, and 1990 earthquake with 44% of its rupture length in positive stress lobe, on Ghazaband Fault, in the south lobe. Since the 1935 earthquake significantly increased the ΔCFS on either terminus of its rupture, the 2013 earthquake with magnitude Mw 7.7 occurred on Hoshab Fault in the positive stress zone with 23% of its rupture length where ΔCFS is positive. Moreover, the northern locked segment of CF, the southern segment of GBF, southeastern segment of HF and PAF demonstrate a high value of ΔCFS. It could trigger seismicity in these regions, thus requiring arrangements to mitigate any seismic hazards in future.

The seismogenic structure of the southern segment of the Longmen Shan thrust belt, eastern Tibetan Plateau, SW China: A comprehensive analysis of surface geology and deep structure

Since the Ms 7.0 earthquake occurred in Lushan on April 20, 2013, five years after the Wenchuan Ms 8.0 earthquake, the seismogenic potential of southern Longmen Shan has been a considerable concern. Comparative studies of active faults and deep structures of the Lushan and Wenchuan earthquakes are important to evaluate Longmen Shan potential. Terrace measurements, geophysical surveys and borehole drilling reveal that late Quaternary fault activity in the southern segment of the Longmen Shan thrust belt is weaker than that along the central segment. Magnetotelluric data across the southern segment of the Longmen Shan thrust belt show that the central fault and the front-range fault are the electrical boundaries of the crust and that the Longmen Shan thrust belt is a high-resistivity body. The Lushan earthquake occurred at the boundary zone between the high- and low-resistivity bodies, and the crustal structure of the southern Longmen Shan thrust belt matches the multiunit combination seismogenic model, indicating a potential large earthquake in Longmen Shan. Compared to conditions along the central segment, the low-resistivity layer of the upper and lower crust on the western side of the Songpan-Ganzi block develops along the Xianshuihe fault, which may reduce the strain accumulated from the Songpan-Ganzi block to the southern Longmen Shan thrust belt. This interpretation suggests that the main crustal shortening is accommodated by foreland thin fold structures, whereas the structural deformation accommodated by the thrust belt accounts for only a small proportion; thus, the seismogenic potential of southern Longmen Shan is weaker than that of the central segment.

Basin evolution, configuration styles, and hydrocarbon accumulation of the South Atlantic conjugate margins

The basins of the South Atlantic passive margins are filled with early rifting stage lacustrine sediments (Barremian, 129–125 Ma), transitional lacustrine and marine sediments (Aptian, 125–113 Ma), and drift stage marine sediments since early Cretaceous (Albian, 113 Ma). The South Atlantic margins can be divided into three segments by the Rio Grande Fracture Zone and the Ascension Fracture Zone according to variations in the basin evolution history and configuration style. The lacustrine shale and marine shale source rocks are developed in the rift stage and drift (post-rift) stage in the South Atlantic passive margins, respectively. The southern segment of the margins is dominated by the lacustrine sedimentary filling in the rifted stage overlain by a thin marine sag system as a regional seal, where the hydrocarbons are mainly accumulated in the structural-stratigraphic lacustrine reservoirs formed in the rift stage. The middle segment developed salty rift-sag-type basins with rift and sag systems and with salt deposited in the transitional intercontinental rift stage, where the lacustrine shale in the lower part of the rifted lacustrine sequence and the marine shale in the lower part of the sag sequence formed in the marine post-rift stage are high-quality source rocks. This segment in the middle is mainly dominated by pre-salt lacustrine carbonate and post-salt marine turbidite plays. The northern segment is characterized by sag-type basins with a narrowly and locally distributed rifted lacustrine system and its overlying widely distributed thick marine sag systems. Gravity-flow (mostly turbidite) marine sandstones as good reservoirs were extensively developed in the sag stage due to the narrow shelf and steep slope. The post-rift marine shales in the lower part of the sag sequence are the main source rocks in the northern segment and the hydrocarbons generated from these source rocks directly migrated to and accumulated in the deep marine turbidite sandstones in the same sag sequence formed in the drift stage. From southern segment to northern segment, source rocks and hydrocarbon accumulations tend to occur in the stratigraphically higher formations. The hydrocarbon accumulations in the southern segment are mainly distributed in the rifted lacustrine sequence while that in the northern segment primarily occur in the post-rift marine sequence.

Transformation of coseismic faults in the northern Longmenshan tectonic belt, eastern Tibetan Plateau: Implications for potential earthquakes and seismic risks

Increasingly frequent earthquakes in the Longmenshan (LMS) tectonic belt are linked to the ongoing eastward growth of the Tibetan Plateau during the Late Cenozoic. Following the devastating 2008 Ms 8.0 Wenchuan earthquake in the central LMS and the 2013 Ms 7.0 Lushan earthquake in the southern segment, an unexpected Ms 5.4 Qingchuan earthquake occurred on September 30, 2017 in the northern LMS. We synthesize focal mechanism solutions, relocated aftershocks, regional geology, and seismic reflection profiles in order to determine the pre-existing faults and active fault system in the northern LMS, thus to better assess potential seismic hazards. At a seismically resolvable scale, we identify a large structural wedge at the basement that is cross-cut by numerous multiphase and multi-behavior active faults. Our seismotectonic model indicates that the seismogenic fault responsible for the 2017 Ms 5.4 Qingchuan earthquake is a pre-existing thrust fault with a dip of ∼34° in the frontal thrust ramp. Reactivation of this fault then triggered the activity of the co-seismic strike-slip fault that was formed in the 2008 Wenchuan event. The seismic interpretation, focal mechanism solutions, and aftershock distribution indicate that two sub-segment faults with different geometries are distinguished along the northern LMS. Accordingly, a transformation of the coseismic fault with different geometry and kinematics is formed. With the constraints that the mainshock and aftershocks occurred on two different faults, we propose that both the frontal ramp thrust fault and the recently-formed strike-slip fault are tectonically active. This study highlights the importance of the influence of the pre-existing fault system on the evolution of the northern LMS during the Late Cenozoic. The rapid uplift of the eastern Tibetan Plateau and the complex fault system indicate the potential for seismic risks in the northern LMS.

Crustal architecture and Moho topography beneath the eastern Indian and Bangladesh margins – new insights on rift evolution and the continent–ocean boundary

The thick sedimentary cover (≤18 km) along the Eastern Continental Margin of India (ECMI) and over the Bay of Bengal has hindered understanding of the mechanisms of rifting that operated on the eastern Indian and Bangladesh margins. Analysis of multichannel seismic reflection data, together with 3D gravity inversion and 2D gravity forward modelling, illuminates basement configuration, crustal thickness and Moho topography, thereby revealing the modes of rift evolution and the location of the continent–ocean boundary (COB) along these margins. The basement, particularly in the shelf and slope regions of eastern peninsular India, is traversed by nearly coast-perpendicular graben, and their seaward continuity is delineated up to 125 km from the coastline. Three different types of continental margin are present along the ECMI: (1) sheared rift on the southern segment of the ECMI, up to 14° N latitude (offshore the Southern Granulite Terrain); (2) hyper-extended rift in the central segment between 14° N and 17° N (offshore the Dharwar Craton); and (3) hypo-extended rift on the northern segment between 17° N and 20° N (offshore the Eastern Ghats Mobile Belt). The graben terminations, crustal thickness and Moho topography of both the ECMI and offshore Bengal Basin clearly suggest that the COB runs nearly parallel to the coastline of peninsular India, but takes an orthogonal turn offshore in the Bengal Basin and connects the palaeo-continental shelf and the Rajmahal–Sylhet Line onshore in the Bengal Basin. Crustal thickness (8–10 km) and depth to the Moho discontinuity (≤28 km) beneath the Bangladesh margin are unusual because the crustal thickness increased through interaction of the Kerguelen plume with the existing oceanic lithosphere. The Moho deepened largely due to the load of the Bengal Fan sediments. The absence of rifted crustal blocks on the Bangladesh margin and the continuity of the COB into the onshore Bengal Basin together imply that the present Bangladesh region was under marine conditions at least until the beginning of Bengal Fan sedimentation (i.e. no less than 23 Ma).

The influence of base-salt relief, rift topography and regional events on salt tectonics offshore Morocco

Abstract This study integrates borehole-calibrated 2D and 3D seismic interpretation with numerical models to provide a regional analysis of the complex salt tectonics offshore central Morocco. We investigate the mechanisms controlling along-margin structural variations, the effects of thick-skinned shortening and the sequential evolution of allochthonous sheets. Additionally, we analyse how base-salt relief generated complex flow kinematics and alternation of extensional and contractional domains by causing flux variations at both autochthonous and allochthonous salt levels. The area is divided into three structural provinces, with the central, Essaouira segment, having greater downdip translation, structural complexity and volume of allochthonous salt, which suggests the salt was originally thicker and better connected across multiple syn-rift structures. The southern segment, Agadir, is dominated by up-right squeezed diapirs formed by early load-driven rise and late shortening; limited salt and overburden translation and no allochthonous sheets. The northern segment, Safi, is narrower and has a smaller number of salt structures that were affected by abrupt translation due to steep detachment gradient. Late, oblique thick-skinned shortening generated contractional structures in the entire salt basin, which are most prominently developed in Essaouira due to a favourably-oriented NW-SE syn-rift structure, the Talfeney Accommodation Zone. Allochthonous salt sheets formed during four phases from the Albian, Late Cretaceous, Paleocene to Oligo-Miocene by two main mechanisms: contraction and basinward salt expulsion. This work improves understanding of the structural configuration along the Moroccan margin guiding the identification of potential sub-salt plays and contributing to a better comprehension of salt-related deformation along rifted passive margins worldwide.

How Far Upstream: A Review of Estuary-Fresh Water Fish Movements in a Large Neotropical Basin

The present study comprises the first systematic record of upstream migratory limits for estuarine migratory fish species of the Patos Lagoon basin (Southern Brazil). The Sinos River sampling program included 85 sites sampled from 2004 to 2015, covering lowland stretches and headwaters. The Guaiba sampling program was performed from February 2011 to March 2013, comprising 59 independent samples. Six estuarine migratory fish species are present in upper Patos basin, covering distances in the range of 250-450 km from the Patos Lagoon inlet with the Atlantic Ocean. Parapimelodus nigribarbis comprised animals in the range of 11.3 to 18.9 cm (Median=13.3 cm), corresponding to adult individuals (Sinos: n=459; Guaiba, n=149). The species was captured almost 500 km from the Patos inlet, reaching 180 km upstream in the Sinos main channel. In the Guaiba lake, the species was captured mainly in autumn, although more abundant in Sinos River in winter and spring. Most of Micropogonias furnieri individuals (n=63) were juveniles (10.7-33.4 cm, median=14.2 cm). The species was not captured in Sinos basin. In the Guaiba Lake the species occurred predominantly in the southernmost part (downstream) and was more abundant (>10x) during the winter. Lycengraulis grossidens was captured in Sinos basin restricted to low stretches in summer/spring months (n=7). In the Guaiba Lake (n=134) the species presented widespread distribution all over the lake surface and year around, although ten times more abundant in spring. The majority of the specimens were adults (8.2-26.9cm, median=20.0 cm). Genidens genidens (n=27) was captured only in summer in the southern (downstream) part of the Guaiba Lake. Most individuals were adults (24.5-34.0 cm, median=27.8 cm). Genidens barbus (n=7) was restricted to spring-summer months and was captured only on southern (downstream) segments of the Guaiba Lake, comprising mainly large juveniles and adults (31.5 – 68.0 cm, median=37.9). Mugil liza appeared in spring in relative low abundances (n=13) but was widespread in the entire Guaiba Lake during the summer. The species was not captured in Sinos sampling program. Size profile comprise specimens in the borderline of maturity (34.3-45.5 cm, median=38.9 cm). General descriptive models for fish displacement in the basin are presented.

Viscoelastic behavior of the crust around the Longmenshan Thrust Belt inferred from post-seismic deformation of the 2013 Lushan earthquake, China

Seven continuous GPS stations are located on the hanging wall of the Longmenshan Thrust Belt, and post-seismic deformation data of the 2013 Lushan earthquake have been recorded for nearly four years following the earthquake. We transform the data from a global reference frame to the South China Block reference frame, and then, the secular motion and post-seismic deformation of the Wenchuan earthquake are excluded using an exponential fit model. Based on our results, the viscosities of the low velocity zone at a depth of ∼25 km and of the middle and lower crust around the Longmenshan Thrust Belt are obtained as 2.75×1017Pa·s and 2.56×1018Pa·s, respectively. The post-seismic rebound, which was observed near the Lushan epicenter, may represent the occurrence of overslip during the earthquake. The more complex structural style may have produced weaker fault activity along the southern segment of the Longmenshan Thrust Belt than along its middle and northern segments.

Episodic bimodal magmatism at an active continental margin due to Paleo-Pacific Plate subduction: A case study from the southern segment of the Tan-Lu Fault Zone, eastern China

Volcanic rocks in the southern part of the Tan-Lu Fault Zone (TLFZ), eastern China, provide key evidence for the connection between spatially and temporally related pulses of Cretaceous magmatism, activity on the TLFZ, and the dynamics of the Paleo-Pacific Plate. We identify two stages of bimodal volcanism in the Chaohu–Lujiang segment, southern TLFZ, based on lithological assemblages and precise isotopic ages. The Early Cretaceous (125–121 Ma) volcanic rocks are fractionated aluminous A-type rhyolites and basalts, whereas the Late Cretaceous volcanic rocks (97–83 Ma) range in composition from porphyritic andesites and trachydacites to rhyolites. The Early Cretaceous rhyolites display strongly negative Nd-Hf isotopic compositions (εNd(t) = −16.3 to −16.1, εHf(t) = −24.1 to −17.9). The most likely source rocks of the fractionated aluminous A-type rhyolites are metaigneous rocks of the Feidong Complex. The Early Cretaceous potassic basalts (K2O/Na2O = 1.5–3.4, εNd(t) = −9.0) exhibit more enriched isotopic compositions than the Late Cretaceous sodic porphyritic andesites (K2O/Na2O = 0.6–0.8, εNd(t) = −5.6, εHf(t) = −15.1 to −6.8), indicating derivation from heterogeneously enriched lithospheric mantle. This heterogeneity is ascribed to derivation from more fertile and isotopically enriched mantle sources with lower solidus temperatures. Modeling of selected elements reveals that the Late Cretaceous trachydacites (εNd(t) = −12.2 to −8.2, εHf(t) = −16.5 to −6.4) originated by mixing between coeval mafic and felsic magmas. The most negative (εNd(t) = −18.8 to −17.1, εHf(t) = −20.5 to −4.7) isotopic compositions, along with Neoproterozoic inherited zircons within the Late Cretaceous rhyolites, indicate derivation from lower crust of the South China Block. We suggest that the Early Cretaceous bimodal volcanic rocks formed in an extensional backarc setting induced by retreat of the subducting Izanagi slab. However, the TLFZ became a focus of Late Cretaceous bimodal volcanism due to weak far-field extension caused by the slow northward motion of the Paleo-Pacific Plate. Moreover, the high velocity and low angle of subduction of the Izanagi slab were the main driving mechanisms behind an Early Cretaceous transpressional event that separated the two stages of bimodal volcanism.

Petrogenesis and geodynamic implications of Late Carboniferous sanukitic dikes from the Bieluagaxi area of West Junggar, NW China

Sanukitic dikes, mainly consisting of pyroxene diorite porphyrite, diorite porphyry, quartz diorite porphyrite and quartz diorite, were widely distributed in the Bieluagaxi area, southern West Junggar. In this contribution, we present new LA-ICP-MS zircon U-Pb ages, major and trace elements, and Sr-Nd-Hf isotopes for these dioritic dikes. These rocks formed in the early Late Carboniferous (ca. 319–315 Ma), coeval with abundant magmatic rocks in the region, and exhibit high MgO, Cr and Ni concentrations and K/Na ratios, and also show strong enrichment in large ion lithophile elements relative to high field strength elements, with pronounced negative Nb, Ta, and Ti anomalies, analogues to those of Cenozoic sanukitoids of the Setouchi Volcanic Belt. Compositionally, the sanukitic dikes are divided into two groups. Group 1 have MgO contents greater than 6 wt% (6.18–9.34 wt%), with Mg# values ranging from 58.2 to 73.7. They also have relatively low SiO2 (51.7–55.7 wt%) and Sr (356 ppm in average) contents, and low Sr/Y (15.1–25.4) ratios, but high concentrations of Cr (6.2–490 ppm) and Ni (12.8–163 ppm). Compared to the Group 1, the Group 2 samples have slightly lower MgO (2.12–5.58 wt%; Mg# = 45.8–68.5), Cr (10.1–329 ppm) and Ni (2.84–136 ppm), but relatively higher SiO2 (58.6–65.7 wt%) and Sr (304–521 ppm) concentrations and Sr/Y (19.2–41.0) ratios, making them akin to adakitic rocks elsewhere. These dikes also exhibit depleted Sr-Nd-Hf isotopic signatures, with (87Sr/86Sr)i = 0.70353–0.70480, εNd(t) = +5.03 to +7.61, εHf(t) = +10.9 to +15.3. Geochemical and Sr-Nd-Hf isotopic data suggest that the Group 1 rocks are similar to the typical sanukitoids of the Setouchi Volcanic Belt, and were likely derived from partial melting of a depleted mantle source with subducted sediment melts. While the Group 2 rocks show a remarkable geochemical affinity with the Northwestern Karamay sanukitoids, and were derived from partial melting of a depleted mantle source metasomatized by slab-derived adakitic melts and fluids. Combined with the identified and reported subduction-related magmatism, i.e., adakites, sanukitoids, Nb-enriched basalts, lagged arc volcanic rocks, and I-type granitoids in the Western Junggar region and taking into consideration the previous achievements, therefore, we consider a multi-stage subduction-accretionary orogeny model may account for the difference of subduction duration between the northern segment and southern segment of Western Junggar.

Geometry and tectonic deformation of the seismogenic structure for the 8 August 2017 &lt;i&gt;M&lt;/i&gt;&lt;sub&gt;S&lt;/sub&gt; 7.0 Jiuzhaigou earthquake sequence, northern Sichuan, China

To reveal the geometry of the seismogenic structure of the Aug. 8, 2017 MS 7.0 Jiuzhaigou earthquake in northern Sichuan, data from the regional seismic network from the time of the main event to Oct. 31, 2017 were used to relocate the earthquake sequence by the tomoDD program, and the focal mechanism solutions and centroid depths of the ML ≥ 3.5 events in the sequence were determined using the CAP waveform inversion method. Further, the segmental tectonic deformation characteristics of the seismogenic faults were analyzed preliminarily by using strain rosettes and areal strains (As). The results indicate: (1) The relocated MS 7.0 Jiuzhaigou earthquake sequence displays a narrow ~ 38 km long NNW-SSE-trending zone between the NW-striking Tazang Fault and the nearly NS-striking Minjiang Fault, two branches of the East Kunlun Fault Zone. The spatial distribution of the sequence is narrow and deep for the southern segment, and relatively wide and shallow for the northern segment. The initial rupture depth of the mainshock is 12.5 km, the dominant depth range of the aftershock sequence is between 0 and 10 km with an average depth of 6.7 km. The mainshock epicenter is located in the middle of the aftershock region, showing a bilateral rupture behavior. The centroid depths of 32 ML ≥ 3.5 events range from 3 to 12 km with a mean of about 7.3 km, consistent with the predominant focal depth of the whole sequence. (2) The geometric structure of the seismogenic fault on the southern section of the aftershock area (south of the mainshock) is relatively simple, with overall strike of ~150° and dip angle ~75°, but the dip angle and dip-orientation exhibit some variation along the segment. The seismogenic structure on the northern segment is more complicated; several faults, including the Minjiang Fault, may be responsible for the aftershock activities. The overall strike of this section is ~159° and dip angle is ~59°, illustrating a certain clockwise rotation and a smaller dip angle than the southern segment. The differences between the two segments demonstrate variation of the geometric structure along the seismogenic faults. (3) The focal mechanism solutions of 32 ML ≥ 3.5 events in the earthquake sequence have obvious segmental characteristics. Strike-slip earthquakes are dominant on the southern segment, while 50% of events on the northern segment are thrusting and oblique thrusting earthquakes, revealing significant differences in the kinematic features of the seismogenic faults between the two segments. (4) The strain rosettes for the mainshock and the entire sequence of 31 ML ≥ 3.5 aftershocks correspond to strike-slip type with NWW-SEE compressional white lobes and NNE-SSW extensional black lobes of nearly similar size. The strain rosette and As value of the entire sequence of 22 ML ≥ 3.5 events on the southern segment are the same as those of the MS 7.0 mainshock, indicating that the tectonic deformation here is strike-slip. However, the strain rosette of the entire sequence of 10 ML ≥ 3.5 events on the northern segment show prominent white compressional lobes and small black extensional lobes, and the related As value is up to 0.52, indicating that the tectonic deformation of this segment is oblique thrusting with a certain strike-slip component. Differences between the two segments all reveal distinctly obvious segmental characteristics of the tectonic deformation of the seismogenic faults for the Jiuzhaigou earthquake sequence.

P Wave Azimuthal Anisotropic Tomography in Northern Chile: Insight Into Deformation in the Subduction Zone

AbstractBased on a large data set of local body wave travel times, we determined the first 3‐D model of azimuthal P wave anisotropic tomography of northern Chile in order to study the deformation in the subduction zone. Our results indicate different deformation patterns in the overriding lithosphere, mantle wedge, and the subducting Nazca slab as well as significant along‐arc variations. Radiating fast velocity directions (FVDs) around the rupture zone of the 2014 Iquique earthquake (Mw 8.2) are notable in the crust, which may reflect the specific crustal extension induced by a point‐like stress source in the form of a fully locked asperity at the plate interface. In most of the study region, FVDs in the mantle wedge are trench normal, which we interpret to have resulted from mantle wedge flow driven by the oceanic plate subduction. However, trench‐parallel FVDs are found beneath the northern segment, which may imply the occurrence of B‐type olivine fabrics in the cold forearc mantle. Strong along‐arc variations are also observed in the subducting slab, which may reflect intraslab heterogeneity and change in the slab geometry. In the northern segment, slab bending induces generally trench‐normal extension and leads to trench‐normal FVDs in the upper part of the slab. In contrast, in the southern segment, flat slab subduction and thus slab unbending dominate, producing trench‐normal compression and trench‐parallel FVDs instead. Along the subducting slab interface, FVDs change from trench‐parallel to trench‐normal in the downdip direction, which may indicate the first‐order transition from compression in the locking area to dominant shear in the decoupling area along the slab interface.

Strain Field Associated With a Component of Divergent Motion Along the Southern Dead Sea Fault: Insights From Magnetic Fabrics

AbstractIn order to reconstruct the strain field along the southern segment of the Dead Sea Fault (DSF) plate boundary, we analyzed the magnetic fabrics of carbonate rocks, outcropping along it. The magnetic fabrics provide a microstructural indicator that help to approximate the principal strain directions in the rocks. Our analysis includes ~900 cores from 58 sampling localities, along ~400 km of the southern DSF. We measured the magnetic fabrics of (1) pure calcite‐bearing limestones that consist diamagnetic fabrics and (2) chalks with composite fabrics, which we further separated into diamagnetic and paramagnetic subfabrics, using measurements of Anisotropy of Magnetic Susceptibility at low temperatures. The results show that 87% of the diamagnetic fabrics and subfabrics are of tectonic origin. The orientations of the maximum Anisotropy of Magnetic Susceptibility axes (K3 axes) approximately align with the maximum horizontal shortening directions along the southern segment of the DSF, differ from the remote stress direction, and are largely parallel to the main trace of the DSF. This parallelism is not related to local variations in the geometry of the faults. We suggest that the deflection of the maximum horizontal shortening parallel to the transform plate boundary is a kinematic consequence of the Sinai‐Arabia relative plate motion, which expresses a component of divergence along the southern segment of the DSF. We conclude that magnetic fabrics of carbonate rocks are sensitive and reliable microstructural indicators for determination of the strain field along major fault systems.

VERIFICATION OF MATERIALS OF REMOTE SENSING OF THE EARTH IN CONNECTION WITH THE SEARCH OF HYDROCARBON DEPOSITS (on example of the Southern segment of the URAL-Kazakh foredeep)

VERIFICATION OF MATERIALS OF REMOTE SENSING OF THE EARTH IN CONNECTION WITH THE SEARCH OF HYDROCARBON DEPOSITS (on example of the Southern segment of the URAL-Kazakh foredeep)

Analysis of Southern Segment of Sumatran Fault Monitoring Bench Mark as Preliminary Approach in Updating Earthquake Hazard Map

Sumatran Fault is one of hazard located in Sumatra Island. Southern segment of Sumatran fault is one of sources of earthquakes in Lampung Province. Hazard map is used as consideration in developing region. The source of hazard comes from stress accumulation of crust which can be derived from movement of points in surface. The study of points or monitoring bench mark to accommodate more precise fault movement or slip and stress accumulation is important for sustainable development in Lampung Province. Tool used for analysis is geographic information system especially buffer analysis. Available monitoring bench mark is analysed so that each bench mark can be classified based on its contribution of fault movement based on distance from segment of current fault. High class bench marks are prioritized to be used as survey sites to monitor fault movement. The other analysis is analysis to obtain region that lack bench mark to monitor segment of current fault or even discover new fault which is branch of segment of current fault. The result of this research is there are four high class bench marks. 58.54% of total segments of 164.020 km long Sumatran fault in Lampung Province is segments with no monitoring bench marks. Three most possible district to build bench mark are Airhitam, Lemong, and Ngambur District.