Southern Segment Research Articles

Preliminary Magnetic Susceptibility Characteristics of the Bharati Promontory (Grovness Peninsula), Larsemann Hills, Prydz Bay Region, East Antarctica

Abstract: The coastal tract of the Prydz Bay region in the East Antarctica exposes Archean to Late Proterozoic magmatic and medium- to high grade (amphibolite – granulite facies) metamorphic rocks. The para- and ortho gneisses from the Bharati promontory (Grovness Peninsula) forming a part of the Larsemann Hills in the southern segment of Prydz Bay were investigated for magnetic characterization. In this small peninsula the upper amphibolite facies gneisses occur as NE-trending bands. The para-gneisses show a range of mineral assemblages (± cordierite ± sillimanite ±garnet) while ortho-gneiss mineralogy includes quartz, feldspar, biotite, garnet. All the lithological units in Bharati promontory contain ubiquitous magnetite, however, with wide variation in the volume proportions. This has resulted in a wide range in magnetic susceptibility (10−4 to 10−2 SI). Magnetic foliations show a correspondence with the general trend of lithounits (050° NE) and define a resulting geometry of mainly D1 and D2 foliations. The magnetic lineations show a preferred orientation with moderate easterly plunge (mean vector 093/36). The findings have implications for the magnetic field survey because such fabrics would impart a strong horizontal component of induced magnetization.

Two-phase subduction and subsequent collision defines the Paleotethyan tectonics of the southeastern Tibetan Plateau: Evidence from zircon U-Pb dating, geochemistry, and structural geology of the Sanjiang orogenic belt, southwest China

A curved belt consisting of terranes or blocks and associated late Paleozoic to early Mesozoic sutures are features of the southeastern part of the Tibetan Plateau. The southeastern Tibetan Plateau Paleotethyan belt is divided into a northwestern segment (central Tibetan Plateau) and a southern segment (the Sanjiang orogenic belt). The Paleotethyan tectonic history of this belt remains a subject of considerable debate. A major controversy concerns whether or not it evolved as a single genetic element. This paper provides new geochronological, geochemical, and structural data to constrain the Paleotethyan tectonics of this belt. A detailed field study across three sections of the Sanjiang orogenic belt has identified two volcaniclastic successions separated by a regional angular unconformity in the northwestern segment, and one continuous volcaniclastic succession in the southern segment. New zircon U-Pb ages reveal that: (1) the lower succession of the northwestern segment was produced by three volcanic phases at ca. 272 Ma, 253–247 Ma, and 240–235 Ma; (2) the upper succession of the northwestern segment formed during a shorter period along the Yushu–Yidun arc from ca. 227 to 211 Ma; and (3) the single volcaniclastic succession of the southern segment was produced continuously from ca. 260 to 210 Ma. Whole-rock geochemical data including trace elements show that the late Paleozoic to early Mesozoic magmatic activity has a continental arc–like affinity. The spatial distribution and field relationships of the magmatic rocks define two continuous volcanic arcs in the southeastern Tibetan Plateau, which correspond to two phases of arc-like magmatism. The Late Triassic Yushu–Yidun arc belt is exposed along the northern half of the northwestern segment to the south of the western Jinsha–Garze–Litang suture. Southward subduction along the western Jinsha–Garze–Litang suture is likely to have generated the Yushu–Yidun arc. The second arc (Jomda–Weixi–Yunxian) was diachronous and is continuous through the southeastern Tibetan Plateau. The northwestern part of the Jomda–Weixi–Yunxian arc belt is overlain by the Yushu–Yidun arc from the north. From northwest to southeast, the cessation of magmatism in the Jomda–Weixi–Yunxian arc varies from the Middle Triassic (ca. 235 Ma) to the Late Triassic (ca. 210 Ma), whereas the onset of magmatism appears to have been in the Early Permian throughout the arc. Zircon Lu-Hf isotopic data suggest that large degrees of melting of a depleted mantle source produced the earliest magmas in each segment of the Jomda–Weixi–Yunxian arc. The similarity in zircon ages of the ophiolitic units and their distribution imply that the Longmu Co–Shuanghu suture is a continuation of the Changning–Menglian suture. The spatial relationship between the Jomda–Weixi–Yunxian arc and the Longmu Co–Shuanghu–Changning–Menglian suture suggests that diachronous eastward subduction of part of the Paleotethys along the suture produced the Jomda–Weixi–Yunxian arc belt. Widespread folds of the continental arc–volcaniclastics with orogen-parallel hinges reflect compression perpendicular to the orogen, subsequent to arc magmatism. As such, the Paleotethyan tectonics of the southeastern Tibetan Plateau are characterized by two-phase subduction and subsequent two-phase continent-continent collision.

Tectonostratigraphic complexes of the southern Kronotskii paleoarc (Eastern Kamchatka): Structure, age, and composition

The eastern peninsulas of Kamchatka are mostly composed of tectonostratigraphic complexes, which were formed within the Late Cretaceous-Eocene Kronotskii-Kamchatka arc. The accretion of this paleoarc to the Kamchatka margin of northeastern Asia in the terminal Cenozoic represented the last collisional event in the formation of the present-day structure of Kamchatka. The article presents new data on the age, composition, and structure of the tectonostratigraphic complexes constituting the southern segment of the Kronotskii-Kamchatka paleoarc. It is shown that the oldest rocks of these complexes are the Campanian in age and represented by volcano-sedimentary rocks that were formed in different geodynamic environments. The investigated igneous rocks are attributed to two types: (1) the tholeiite series of a mid-oceanic ridge (MOR) (Vetlovaya Complex); (2) tholeiite and calc-alkaline series of island arcs (Shipunskii Sequence of the Kronotskii Group).

Geochronology of granitic rocks from the Ruangwa region, southern Tanzania – Links with NE Mozambique and beyond

New U–Pb zircon LA-ICP-MS data are presented for 4 granitoid bodies which intrude high grade gneisses of the previously unmapped Ruangwa region in southern Tanzania. The study area forms part of the late Neoproterozoic East African Orogen (EAO). The oldest unit, a coarse-grained migmatitic granitic orthogneiss gave an early Neoproterozoic (Tonian) crystallization age of 899±9/16Ma, which is similar to, but significantly younger than, Stenian-Tonian basement ages in areas relatively nearby. Crust of this age may extend as far north as the major Phanerozoic Selous Basin, north of which Archaean protolith ages predominate (the “Western Granulites”), except for the juvenile Neoproterozoic “Eastern Granulites”, which are not represented in the study area. To the south, the Tonian crust of the study area provides a tentative link with the Marrupa Complex in NE Mozambique. A granite pluton, dated at 650±5/11Ma is broadly coeval with the main Pan-African tectono-thermal event in the East African Orogen that is recorded across Tanzania north of the Selous Basin. Zircons in this granite contain inherited cores at ca. 770Ma. This age is within the range of dates obtained from south and west of the study area from juvenile granitoid orthogneisses which might be related to a widespread, but poorly understood, early phase of Gondwana assembly along an Andean-type margin.South of the study area, in NE Mozambique, the latest orogenic events occurred at ca. 550Ma, and are sometimes attributed to the Ediacaran-aged “Kuunga Orogeny”. While metamorphic dates of this age have been recorded from the EAO north of the Selous Basin, magmatic rocks of this event have not been recognized in Tanzania. The two youngest granitoids of the present study are thus the first 500–600Ma igneous rocks reported from the region. A weakly deformed very coarse-grained granite pluton was dated at 591±4/10Ma, while a very late, cross-cutting, undeformed granite dyke gave an intrusive age of 549±4/9Ma.The granitoids ages presented in this study contain elements that are characteristic of the northern, Tanzania-Kenya, segment of the East African Orogen and of the southern, Mozambique, segment. The Tonian orthogneiss sample is typical of (but somewhat younger than) the Marrupa Complex of NE Mozambique. No zircon inheritance was recorded in the sample, typical of the juvenile Marrupa Complex. On the other hand, the ca. 650Ma granite pluton has an age that is typical of the northern segment of the orogen; this is the first recorded granite of that age intruded into the Tonian-dominated crust of southern Tanzania or NE Mozambique. The two younger granites have provided dates that are typical of the southern segment of the orogen, and that of the Kuunga Orogen. The study area thus appears to represent an area of transitional crust straddling two complex and contrasting segments of the East African Orogen, with elements of both segments present and evidence for a ca. 770Ma event which appears to be quite widespread and may relate to the early phases of Gondwana amalgamation in southern East Africa.

Relative tectonic activity assessment of the Tuz Gölü Fault Zone; Central Anatolia, Turkey

The Tuz Gölü Fault Zone is one of the major tectonic structures in Central Anatolia that can produce seismic hazards. This study provides geomorphic analyses of fault-generated mountain fronts and stream channel gradients that cross different segments of the fault zone. The mountain front sinuosity (Smf) and the valley-width to valley-height ratio (Vf) analyses suggest moderate activity for the fault segments. The stream channel gradient analysis (SL) reveals that streams crossing the fault segments have higher channel gradients characterized by knickpoints or knickzones. Their longitudinal profiles reflect the transient character of the landscape due to ongoing differential uplift along the fault zone. Along strike variations of the Smf, Vf and SL values are compatible with general relief of the mountain front. The topographic swath profile of the TGFZ implies that the maximum relief and incision are focused at the central part of the fault zone. Based on these geomorphic analyses, the central segments might provide higher seismic risk with respect to the northern and southern segments of the fault zone, and need to be evaluated more carefully for the regional seismic hazard.

A 3400-year-long paleoseismologic record of earthquakes on the southern segment of Anninghe fault on the southeastern margin of the Tibetan Plateau

The Anninghe fault (ANHF) is an active left-lateral strike-slip fault along the southeastern margin of the Tibetan Plateau. Previous studies suggested the ANHF was divided into two segments, which herein we named the northern and southern segments respectively. Multiple trenches were excavated on the northern segment, revealing well-constrained paleoseismic events by radiocarbon ages. However, until now there is no paleoseismic result on the southern segment, which terminates at the Zemuhe fault (ZMHF) around Xichang where there were multiple historical records of large earthquakes. In this paper, we used high-resolution images for mapping fault traces on the southern segment of the ANHF and found a small depression. Through trenching in the depression, five paleoseismic events are identified and named E1 through E5 from youngest to oldest at 1750AD-present, 1430–1870AD, 940–1150AD, 700–1000AD, and 1400–500BC respectively. Comparing with the historical record earthquakes around Xichang, we suggest that the latest event E1 is associated with one of the 1850AD and 1952AD events, and event E2 is interpreted as the 1536AD earthquake; event E4 is possibly associated with the 814AD earthquake. The average recurrence interval of earthquakes on the southern segment is about 600–800yr, the interval between E1 and E2 is 416yr or 314yr, and 386–596yr between E2 and E3. The interval between E3 and E4 is shorter at 126–336yr, but much longer at 1314–2214yr between E4 and E5. These surface-faulting events on the southern segment of the ANHF appear to be unevenly spaced in time. Furthermore, integrating the paleoseismic sequence of the northern segment of the ANHF, the two segments appear to be ruptured individually or interactively triggered within a narrow time range, or co-ruptured during one paleoseismic event, indicating that the ANHF possibly shows a cascading rupturing behavior.

Variations of shear wave splitting in the 2013 Lushan Ms7.0 earthquake region

In this paper, variations of shear wave splitting in the 2013 Lushan Ms7.0 earthquake sequence were studied. By analyzing shear wave particle motion of local events in the shear wave window, the fast polarization directions and the delay time between fast and slow shear waves were derived from seismic recordings at eight stations on the southern segment of the Longmenshan fault zone. In the study region, the fast polarization directions show partition characteristics from south to north. And the systematic changes of the time delays between two split shear waves were also observed. As for spatial distribution, the NE fast polarization directions are consistent with the Longmenshan fault strike in the south of focal region, whereas the NW fast direction is parallel to the direction of regional principal compressive stress in the north of focal region. Stations BAX and TQU are respectively located on the Central and Front-range faults, and because of the direct influence of these faults, the fast directions at both stations show particularity. In time domain, after the main shock, the delay times at stations increased rapidly, and decreased after a period of time. Shear-wave splitting was caused mostly by stress-aligned microcracks in rock below the stations. The results demonstrate changes of local stress field during the main shock and the aftershocks. The stress on the Lushan Ms7.0 earthquake region increased after the main shock, with the stress release caused by the aftershocks and the stress reduced in the late stage.

Large mantle magma reservoir of Afar, Ethiopia

Most of the Earth’s crust is created at midoceanic ridges where two tectonic plates move away from each other, known as constructive plate boundary system and extends for ∼60,000 km around the globe (Wright et al., 2006). It creates characteristic extensional structures, wherein tectonic stress is primarily released by rifting processes, which thus forms major system of normal faults and volcanoes. Such tectonic settings are dominated by vertical sheets of dyke systems, along which magma is injected into the crust. To understand the rifting process and the associated magmatism at these places a direct observation is often difficult, primarily, because on Earth only 2% of the mid-ocean ridge system is above sea level (Smith and Cann, 1999; Wright et al., 2006; Buck, 2013; Desissa et al., 2013). However, Afar (East Africa) and Iceland are the rare places where such observations can be directly examined on land (Buck, 2013; Desissa et al., 2013). Therefore, the past observations at these sites have suggested that they share a very similar driving mechanism (Wright et al., 2006). This usually follows a pattern wherein magma is sporadically stowed in the crust and there it is stored at numerous positions and levels. This is followed by injection through various dyke systems within the brittle upper crust (Wright et al., 2006). Further, it is commonly believed that unavailability of magma in the crust results in the quiescence between different dyking episodes (MacGregor et al., 1998; Canales et al., 2000; Heinson et al., 2000; Singh et al., 2006; Key et al., 2013). For example, the historical record for the northern spreading segments of Iceland suggests 200–300 years of quiescence between dyke episodes; however, the southern segments have largely been quite for 1000 years (Buck, 2013). The recent results of Desissa et al. (2013), and some previous findings at spreading centers (e.g., Carbotte et al., 2006) have questioned this widely accepted view. These studies argue that a large quantity of magma is probably always available in the mantle and shallow crust at many spreading centers, which suggests that some other mechanism is required to understand the control and frequency of dyking events. Desissa et al. (2013) have used magnetotelluric techniques, a geophysical method particularly sensitive to high conductivity (low resistivity) materials (non-organic) fluids, magmas and partial melts, to understand the dynamics of Dabbahu magmatic segment in Afar, Ethiopia. It is therefore a very useful procedure to imagemagma and partial melt in the crust and upper mantle (Tolstoy et al., 2006). Since 2005, the Dabbahu magmatic segment was active, as demonstrated by repeated magma intrusions and is believed to suggest a late stage of continental rifting process (Wright et al., 2006; Buck, 2013; Desissa et al., 2013). Tectonically, the magmatic activity at the 60-km-long Dabbahu spreading segment is part of the Red Sea spreading process, which accommodates the separation between the Arabian and African plates (Buck, 2013). Magnetotelluric data of Desissa et al. (2013) in Afar shows a very high electrical conductivity region, which is 30-km-wide and 35 km deep. It is interpreted as an area, wherein a large of volume of magma (∼500 km3) extends well into the mantle with a 13% melt fraction. The large dimensions of the magma chamber allows these authors to argue that it must have stayed there for quite a while and probably for several tens of thousands of years, however, it remains unclear why it stayed at that depth within both the crust and mantle and for so long without coming out through dyking processes (Buck, 2013). Further, geochemical analyses of samples taken from the Dabbahu segment (Ferguson et al., 2013) suggest that melting of mantle rocks occurs at greater depths (>80 km). This allows to understand that large accumulations of melt should not occur at depth, primarily, because of the low density of the magma should force it to move upwards to percolate into the crust. However, the study of Desissa et al. (2013) clearly suggests that a large quantity of magma is stored within the crust and mantle beneath the Dabbahu plate spreading segment without large dyking events. Such a large volume of melt could supply 100 typical magmatic dyking events and should aid plate rifting for tens of thousands of years to come. This is contrary to the observations (Buck, 2013), and clearly indicates that initiation of dyking is not controlled by the volume of magma but some other process, which needs to be investigated. Further, the resolution of geophysical data (e.g., seismic, gravimetric, magnetotelluric methods) is not always enough to image accurately magma bodies and the limitation of available MT method offers uncertainties regarding the dynamics of mamgmatic dyking in hot-spot influenced crust and such estimates from Iceland are equally questionable (e.g., Bjornsson

Relationship between Tanlu Fault and hydrocarbon accumulation in Liaozhong Sag, Bohai Bay, eastern China

Tanlu fault (Liaozhong segment) goes straight through Liaodong Bay along NNE direction and it is divided into three segments, i.e., the northern segment, the middle segment and the southern segment, according to the differences in structural features. There are obvious changes in deposit thickness, sag structure, tectonic nature and other aspects of the layers. Tanlu fault is a special controlling factor concerning the reservoir formation in Liaozhong sag. First, its activities affect paleogeomorphology and paleogeographic framework and control the distribution of the sedimentary facies and then they proceed to control the distribution of the source rock and reservoir sand bodies. Second, its activities affect the formation and deformation of the structure, control the formation of abundant traps and cause the destruction of some traps. Third, its activities also affect the juxtaposition relation among the fault, the sand bodies and unconformity surfaces and control the function and efficacy of the three as the main hydrocarbon translocation system. In this paper the hydrocarbon reservoir formation process of JZ21-1, JX1-1 and LD27-2 oil and gas fields, the representatives of Tanlu strike slip fault zone are mainly analyzed. The modes of hydrocarbon reservoir formation can be generalized as follows—hydrocarbon source acts as “soil” and oil and gas as “nutrient”; hydrocarbon expulsion relies on “roots”; hydrocarbon migration relies on “trunks”; reservoir forms in “brunches” and the whole process follows the pattern that “hydrocarbon accumulates in strike slips and oil and gas reservoir forms like the growth of trees”.

Controls on segmentation and morphology along the back‐arc Eastern Lau Spreading Center and Valu Fa Ridge

AbstractBack‐arc spreading centers increasingly depart from mid‐ocean ridge (MOR) characteristics with proximity to the arc volcanic front. The close association of these departures with slab‐derived materials in erupted lavas suggests that subduction‐related chemical effects are their primary cause. The Eastern Lau Spreading Center (ELSC) and Valu Fa Ridge (VFR) are type examples of this process. Together they constitute a first‐order spreading center in the Lau back‐arc basin that progressively converges on the Tofua arc volcanic front from north to south. Here we use ship multibeam and deep‐towed side‐scan sonar data to examine variations in axial morphology and volcanism at the second‐ and third‐order segment scale along these ridges and develop a model for the processes that control them. Closest to the arc, VFR, and the southern segment of the ELSC shoal toward second‐order segment ends, in contrast to MORs. Northward and beyond ~70 km from the arc, the axis becomes abruptly deeper and flatter and no longer shoals toward second‐order segment ends. At VFR, along‐axis topographic highs correlate with the location of arc volcanoes along slab flow lines. These correlations are weaker along the southernmost ELSC segment and absent along ELSC segments farther north. The observations show a modulation of back‐arc segmentation with arc proximity that rapidly diminishes with distance. They support a model of the mantle wedge with a strongly hydrous domain within ~70 km of the arc within which the arc and ridge interact and a much less hydrous domain farther from the arc without evident arc‐ridge interactions.

Petrologic, tectonic, and metallogenic evolution of the southern segment of the ancestral Cascades magmatic arc, California and Nevada

Ongoing arc magmatism along western North America was preceded by ancestral arc magmatism that began ca. 45 Ma and evolved into modern arc volcanism. The southern ancestral arc segment, active from ca. 30 to 3 Ma, adjoins the northern segment in northern California across a proposed subducted slab tear. The east edge of the Walker Lane approximates the east edge of the southern arc whose products, mostly erupted from stratovolcanoes and lava dome complexes arrayed along the crest of the ancestral arc, extend down the west flank of the Sierra Nevada. Southern arc segment rocks include potassic, calc-alkaline intermediate- to silicic-composition lava flows, lava dome complexes, and associated volcaniclastic deposits. Northern and southern segment rocks are similar to other convergent-margin magmatic arc rocks but are compositionally distinct from each other. Southern segment rocks have lower TiO 2 , FeO*, CaO, and Na 2 O contents and higher K 2 O contents, and exhibit less compositional-temporal variation. Compositional distinctions between the northern and southern segment rocks reflect the composition and thickness of the crust beneath which the associated magma systems were sourced. Northern segment rock compositions are consistent with generation beneath thin, primitive crust, whereas southern segment rocks represent magmas generated and fractionated beneath thicker, more evolved crust. Although rocks in the two arc segments have similar metal abundances, they are metallogenically distinct. Small porphyry copper deposits are characteristic of the northern segment whereas significant epithermal precious metal deposits are most commonly associated with the southern segment. These metallogenic differences are also fundamentally linked to the tectonic settings and crustal regimes within which these two arc segments evolved.

Paleomagnetic secular variation study of Ar–Ar dated lavas flows from Tacambaro area (Central Mexico): Possible evidence of Intra-Jaramillo geomagnetic excursion in volcanic rocks

More than 350 oriented paleomagnetic cores were obtained for rock-magnetic and paleomagnetic analysis from radiometrically dated (40Ar–39Ar) magmatic rocks occurring in the southern segment (Jorullo and Tacámbaro areas) of the Michoacán-Guanajuato Volcanic Field in the Trans-Mexican Volcanic Belt. Most of the lavas (37) stem from monogenetic volcanoes dated at less than 4Ma. Two additional sites were sampled from the plutonic basement dated at 33–30Ma. Primary remanences carried by low-Ti titanomagnetites allowed to determining 34 reliable site-mean directions of mostly normal (27) but also reversed (7) polarities. The mean directions of these two populations are antipodal, and suggest neither major vertical-axis rotations with respect to the North America craton nor tilting in the region for the last 4Ma (rotation and flattening of the inclination parameters being less than −5.9±3.8 and 0.1±3.9, respectively). The corresponding paleomagnetic pole obtained for Pliocene–Pleistocene times is PLAT=83.4°, PLON=2.4° (N=32, A95=2.7°). Virtual geomagnetic poles also contribute to the time averaged field global database and to the paleosecular variation (PSV) investigations at low latitudes from lavas for the last 5Ma, showing a geomagnetic dispersion value that is in agreement with available PSV models. When comparing the magnetic polarities and corresponding radiometric ages of the studied sites with the Cenozoic geomagnetic polarity time scale (GPTS), a good correlation is observable. This finding underscores the suitability of data obtained on lavas in Central Mexico for contributing to the GPTS. Furthermore, the detection of short-lived geomagnetic features seems possible, since the possible evidence of Intra-Jaramillo geomagnetic excursion could be documented for the first time in these volcanic rocks.

Rhyacian evolution of subvolcanic and metasedimentary rocks of the southern segment of the Mineiro belt, São Francisco Craton, Brazil

The Tiradentes suite in conjunction with the coeval Estação Tiradentes metasedimentary sequence makes up a distinct tectonic element within the Paleoproterozoic framework of the Mineiro accretionary belt in the Southern São Francisco Craton. This suite is the second occurrence of volcanic and subvolcanic rocks in the Mineiro belt and shows petrographic and geochemical similarities with the nearby Serrinha suite (2.23–2.21Ga) - the first reported Rhyacian volcanism in the Mineiro accretionary belt.The Tiradentes suite is represented by mafic andesites, dacites, granophyres and tonalites and shows petrographic features of volcanic rocks, such as spherulites, amygdales, granophyric intergrowths and porphyritic textures with plagioclase phenocrysts. The rocks of this suite are metaluminous to peraluminous, enriched in alkaline elements, compatible with low-Al2O3 trondhjemites and show primary fractionation of plagioclase, apatite, hornblende, biotite and ilmenite. High HREE content coupled with incipient negative Eu anomaly and high Y values are consistent with a garnet–amphibolite protolith with plagioclase and hornblende as present in the residual phases. The samples of the Tiradentes suite show geochemical signatures of magmatic arcs, which are also observed in rocks from the Serrinha suite.Rocks of the Tiradentes suite yield the following U–Pb zircon crystallization ages: (i) mafic andesite: 2217±23Ma; (ii) dacitic dike: 2204±11Ma; (iii) subvolcanic tonalite: 2213±9Ma. The deposition of the Estação Tiradentes metasedimentary sequence probably took place after ca. 2088±12Ga (although it may be as young as 1990±39Ma), according to the U–Pb detrital zircon geochronology of selected samples (diamictite, ferruginous quartzite and carbonaceous phyllite). The Tiradentes rocks show three TDM model ages ranging between 2.4 and 2.3Ga, indicating short crustal residence for the magmatic protoliths. This inference is also supported by the ɛNd(2.2Ga) values (+1.1 to +2.3), whereas the respective 87Sr/86Sr(t) ratios (0.7018–0.7032) are consistent with the involvement of low Rb/Sr material in the magma genesis, such as a Paleoproterozoic oceanic lithosphere.From a tectonic perspective, the above data are consistent with the idea that a Paleoproterozoic seafloor developed outboard of the Archean–Siderian passive-margin setting (Minas Basin) of the São Francisco proto-Craton. The 2.23–2.21Ga volcanic–subvolcanic rocks of the Tiradentes and Serrinha suites were associated with a Rhyacian oceanic arc. The Estação Tiradentes metasedimentary sequence (turbidites and distal phyllites) probably formed in back-arc setting concomitant with the Tiradentes suite, as indicated by detrital zircon geochronology.

Three‐dimensional lithospheric electrical structure of Southern Granulite Terrain, India and its tectonic implications

AbstractThe crustal as well as the upper mantle lithospheric electrical structure of the Southern Granulite Terrain (SGT) is evaluated, using the magnetotelluric (MT) data from two parallel traverses: one is an ~ 500 km long N‐S trending traverse across SGT and another a 200 km long traverse. Data space Occam 3‐D inversion was used to invert the MT data. The electrical characterization of lithospheric structure in SGT shows basically a highly resistive (several thousands of Ohm meters) upper crustal layer overlying a moderately resistive (a few hundred Ohm meters) lower crustal layer which in turn is underlain by the upper mantle lithosphere whose resistivity shows significant changes along the traverse. The highly resistive upper crustal layer is interspersed with four major conductive features with three of them cutting across the crustal column, bringing out a well‐defined crustal block structure in SGT with individual highly resistive blocks showing correspondence to the geologically demarcated Salem, Madurai, and Trivandrum blocks. The 3‐D model also brought out a well‐defined major crustal conductor located in the northern half of the Madurai block. The electrical characteristics of this south dipping conductor and its close spatial correlation with two of the major structural elements, viz., Karur‐Oddanchatram‐Kodaikanal Shear Zone and Karur‐Kamban‐Painavu‐Trichur Shear Zone, suggest that this conductive feature is closely linked to the subduction‐collision tectonic processes in the SGT, and it is inferred that the Archean Dharwar craton/neoproterozoic SGT terrain boundary lies south of the Palghat‐Cauvery shear zone. The results also showed that the Achankovil shear zone is characterized by a well‐defined north dipping conductive feature. The resistive block adjoining this conductor on the southern side, representing the Trivandrum block, is shown to be downthrown along this north dipping crustal conductor relative to the Madurai block, suggesting a northward movement of Trivandrum block colliding against the Madurai block. The lithospheric upper mantle electrical structure of the SGT up to a depth of 100 km may be broadly divided into two distinctly different segments, viz., northern and southern segments. The northern lithospheric segment, over a major part, is characterized by a thick resistive upper mantle, while the southern one is characterized by a dominantly conductive medium suggesting a relatively thinned lithosphere in the southern segment.

Preface to the Focus Section on the 20 April 2013 Magnitude 6.6 Lushan, China, Earthquake

Research Article| January 01, 2014 Preface to the Focus Section on the 20 April 2013 Magnitude 6.6 Lushan, China, Earthquake Huajian Yao; Huajian Yao aLaboratory of Seismology and Physics of Earth’s Interior, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui, 230026, Chinahjyao@ustc.edu.cn Search for other works by this author on: GSW Google Scholar Zhigang Peng Zhigang Peng bSchool of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia 30332 U.S.A.zpeng@gatech.edu Search for other works by this author on: GSW Google Scholar Author and Article Information Huajian Yao aLaboratory of Seismology and Physics of Earth’s Interior, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui, 230026, Chinahjyao@ustc.edu.cn Zhigang Peng bSchool of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia 30332 U.S.A.zpeng@gatech.edu Publisher: Seismological Society of America First Online: 14 Jul 2017 Online ISSN: 1938-2057 Print ISSN: 0895-0695 © 2014 by the Seismological Society of America Seismological Research Letters (2014) 85 (1): 6–7. https://doi.org/10.1785/0220130184 Article history First Online: 14 Jul 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Huajian Yao, Zhigang Peng; Preface to the Focus Section on the 20 April 2013 Magnitude 6.6 Lushan, China, Earthquake. Seismological Research Letters 2014;; 85 (1): 6–7. doi: https://doi.org/10.1785/0220130184 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search The 20 April 2013 Lushan earthquake (moment magnitude Mw 6.6, surface‐wave magnitude Ms 7.0) ruptured part of the southern segment of the Longmenshan fault zone in western Sichuan, China. The Longmenshan fault zone is a tectonic boundary that separates the rigid Sichuan basin to the east and the eastern Tibetan plateau to the west. The Lushan earthquake was a typical thrust event initiated at a relatively shallow depth of about 12 km. Although this earthquake did not produce obvious surface ruptures, it still caused strong shaking around its epicenter and resulted in more than 200 deaths or missing... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The Incidence of an Oil Glut: Who Benefits from Cheap Crude Oil in the Midwest?

Beginning in early 2011, crude oil production in the U.S. Midwest and Canada surpassed the pipeline capacity to transport it to the Gulf Coast where it could access the world oil market. As a result, the U.S. “benchmark” crude oil price in Cushing, Oklahoma, declined substantially relative to internationally traded oil. In this paper, we study how this development affected prices for refined products, focusing on the markets for motor gasoline and diesel. We find that the relative decrease in Midwest crude oil prices did not pass through to wholesale gasoline and diesel prices. This result is consistent with evidence that the marginal gallon of fuel in the Midwest is still imported from coastal locations. Our findings imply that investments in new pipeline infrastructure between the Midwest and the Gulf Coast, such as the southern segment of the controversial Keystone XL pipeline, will not raise gasoline prices in the Midwest.

The 2013 Lushan Earthquake in China Tests Hazard Assessments

Online Material: Earthquake hazard map; figures showing geodynamic and elastic block models; and table of M s>6 earthquakes used in the study. Following the devastating 2008 Wenchuan earthquake that ruptured the central–northern segments of the Longmenshan fault in Sichuan, China, many studies assessed its impact on other major faults in this region (e.g., Parsons et al. , 2008; Toda et al. , 2008). On 20 April 2013, the M w 6.6 Lushan earthquake ruptured the southern segment of the Longmenshan fault, allowing these assessments to be tested. The 2008 Wenchuan earthquake ( M w 7.9) came as a surprise, because the Longmenshan fault zone, which separates the growing Tibetan Plateau from the rigid Sichuan basin (Fig. 1), slips slowly (less than 3 mm/yr; Shen et al. , 2009); with only moderate seismicity (Burchfiel et al. , 2008; Wang et al. , 2010). The earthquake‐hazard map by the Global Seismic Hazard Assessment Program shows the entire Longmenshan fault zone as being relatively safe, as shown in ![Graphic][1] Figure S1 (available in the electronic supplement to this paper). Figure 1. Topography (color contours), faults (lines), and seismicity (dots) of eastern Tibetan Plateau and Sichuan basin. Gray dots, historic events; red dots, instrumentally recorded events; beach ball, fault plane solution and aftershock sequence of the 2008 Wenchuan earthquake are in light blue; those for the 2013 Lushan earthquake are in dark blue. Epicenters are from the China Earthquake Data Center (http://data.earthquake.cn/data, last accessed April 2013). Inset map shows the location of the map region. The same cannot be said for the M w 6.6 Lushan earthquake, which occurred in the morning of … [1]: /embed/inline-graphic-1.gif

The 2013 Lushan Ms 7.0 Earthquake: Varied Seismogenic Structure from the 2008 Wenchuan Earthquake

On 20 April 2013, nearly 5 years after the Wenchuan M s 8.0 event of 12 May 2008, an earthquake of M s 7.0 struck the Longmenshan fault zone in Lushan county of Sichuan province, China. Because the epicenters of the two events were only 90 km apart, and the aftershock areas were separated (Fig. 1), there has been great interest in both how the seismogenic structures of the two events interrelate, and whether events of M s 7.0 or greater could recur in the southern segment of the Longmenshan fault zone. However, there are few data available on the late Quaternary activity of this section, and the degree of activity is disputed (Zhao et al. , 1990; Yang et al. , 1999; Densmore et al. , 2007; Institute of Geology, China Earthquake Administration, 2009; Zhang and Li, 2010; Chen et al. , 2013a). Since the 2008 Wenchuan earthquake, we have investigated the area repeatedly, acquiring evidence of fault activity. In particular, in the disaster area of the Lushan earthquake, we have undertaken 1:50,000 mapping, trenching, and shallow‐seismic exploration. Based on our observations, this paper discusses the seismogenic structure of the Lushan event and its relation to the 2008 Wenchuan earthquake. Figure 1. Map showing (a) the Longmenshan thrust system and (b) major faults in the disaster area of the Lushan event. (1) Yutian, Xinjiang M s 7.3 of 20 March 2008; (2) Wenchuan, Sichuan M s 8.0 of 12 May 2008; (3) Yushu, Qinghai M s 7.1 of 14 April 2010; (4) Lushan, Sihchuan M s 7.0 of 20 April 2013. Red lines indicate the surface rupture of the 2008 Wenchuan earthquake. Yellow circles show relocated aftershocks of the Lushan event, provided by L. H. Fang. WMF, Wenchuan–Maoxian fault; BYF, Beichuan–Yingxiu fault; PGF, Pengxian–Guanxian fault; RFF, Range‐Front buried fault; YWF, Yanjin–Wulong fault (southern segment of the BYF); …

S–Pb isotopic geochemistry, U–Pb and Re–Os geochronology of the Huanggangliang Fe–Sn deposit, Inner Mongolia, NE China

The Huanggangliang Fe–Sn deposit is located in the eastern section of the Central Asian Orogenic Belt (CAOB) and in the southern segment of the Great Hinggan Range polymetallic belt. This deposit is the largest tin-based polymetallic ore deposit in the northern part of the North China plate. The Fe–Sn mineralization principally occurs in skarn zones, which formed in the contact areas between granite intrusions and marbles. Twenty-one δ34SV-PDB values for sulfides from the Huanggangliang Fe–Sn deposit range from −2.3 to +2.7‰, with an average of +0.38‰, and these values are interpreted to reflect a deep magmatic source of the sulfur contained within the ore minerals of the deposit. The distribution of lead isotopic compositions from sulfides in the Huanggangliang Fe–Sn deposit exhibits a generally concentrated field, indicating that those sulfides have homogeneous lead isotopes and may derive from the same metallic source. The existence of such identical Pb isotopic compositions of sulfides and host granites suggests that granitic magmatism was probably largely responsible for the Huanggangliang mineralization.Zircon LA-ICP-MS U–Pb dating from the Huanggangliang granite exhibits a tight cluster on the Concordia plot and yields a weighted mean 206Pb/238U age of 139.96±0.87Ma, which is considered to be the best estimate of the crystallization age of the Huanggangliang granite. Results obtained from the molybdenite Re–Os isotopic analysis indicate Re–Os model ages of 133.6 to 141.5Ma and a weighted mean age of 135.1±2.3Ma. The molybdenite Re–Os dating yields an isochron age of 134.9±5.2Ma and an initial 187Os/188Os ratio of −0.22±0.35. The molybdenite Re–Os isochron age is largely consistent with the zircon U–Pb age, indicating that both granite intrusion and Fe–Sn mineralization were associated with the Mesozoic magmatism in NE China.Based on ore deposit geology, isotope geochemistry and geochronology of the Huanggangliang Fe–Sn deposit and regional geodynamic evolution, the formation of this deposit is believed to be related to lithospheric thinning, which was caused by delamination and subsequent upwelling of the asthenosphere under the intra-continental extension in NE China. The dynamic setting for the geological processes should be linked to the NNW subduction of the Izanagi plate beneath the Eurasian plate during the Mesozoic. This subduction triggered intensive magmatism and mineralization events in the Great Hinggan Range of NE China, making this district an important Cu–Mo–Sn–Fe–Ag–Pb–Zn metallogenic province.

Regional structure of the southern segment of Central Iberian Zone (Spanish Variscan Belt) interpreted from potential field images and 2.5 D modelling of Alcudia gravity transect

This study presents a structural interpretation of potential field data at the southern segment of the Spanish Central Iberian Zone, where little is known about its geometry and physical properties in depth. We used ground gravity and aeromagnetic data from the Spanish Geological Survey (IGME) databases, together with a gravity survey recently acquired along the Alcudia deep seismic reflection profile, and also petrophysical data from rock samples, in part specifically acquired for this work. The aim is to characterize with potential fields the depth structure and physical properties of a complex area, focusing on features of the upper crust where the potential fields can provide a useful insight, as these features appear to be transparent in the images of seismic surveys carried up to date. After building a 2.5 D model of the magnetic susceptibility and density distribution for the whole crust with the constraints of the Alcudia seismic transect, relationships between lower and upper crustal structures can be better established, and surface geophysical maps allow estimating the lateral extensions of the main structures characterized by the potential field modelling. Interpreted gravity and magnetic Alcudia features consist of: 1) long-wavelength anomalies due to changes in crustal thickness and structure, identified along the Alcudia seismic reflection profile; some crustal boundaries and the seismic Moho have been slightly modified in the potential field model to fit the observed data; and, 2) short wavelength anomalies related to upper crustal features as large NW–SE faults and folds, and plutonic alignments. Northern and southern Alcudia segments show relevant lower-middle crust shortening structures underneath extensive Variscan plutons (Mora, Pedroches) and folded basins (Pedroches). Potential field imaging and modelling of these structures have provided a better understanding of the subsurface geology, particularly in non-reflective areas, unravelling the deep structure of the southern Central Iberian Zone.