South Limb Research Articles

Kinematic and tectonic implications of crenulation cleavage, kink bands, and mesoscopic folds in the Baraboo Syncline, Wisconsin (∼1.45 Ga Picuris Orogen)

The Baraboo Syncline of Wisconsin is a south-verging, regional-scale fold that formed during the ∼1.45 Ga Picuris Orogeny. It involves Mesoproterozoic quartzite and phyllite of the Baraboo Quartzite. Phyllite layers preserve two deformational assemblages. D1 includes the regional-scale flexural-slip to flexural-flow syncline and associated mesoscopic parasitic folds. It also includes spaced cleavage in quartzite layers and north-dipping phyllitic cleavage in phyllite layers. On the gently north-dipping south limb of the regional syncline, parasitic folds verge south (out-of-the-hinge). On this limb, D2 includes north-verging monoclinal kink bands as well as south-dipping asymmetric crenulation cleavage; kink bands have not been found on the north limb. Crenulation growth micro-folded phyllitic cleavage domains into asymmetric sigmoids that are shaped like an ‘S' on the Baraboo Syncline's south limb and like a ‘Z' on the fold's north limb (as viewed looking east). The origin of D2 structures has long been controversial. Some authors attribute them to a post-syncline phase of extensional tectonism on north-dipping shear zones, whereas others consider them to be a consequence of progressive crustal shortening that continued after D1 fabrics had developed. Our structural analysis supports the second hypothesis and therefore implies that the manifestation of the Picuris Orogeny in southern Wisconsin did not include a post-syncline extensional phase. This interpretation is compatible with a tectonic model in which the Baraboo Syncline formed during inversion of a rift or pull-apart basin during north-south crustal shortening. Specifically, we found no north-dipping normal-sense shear zones, and we demonstrate that the kink bands and asymmetric crenulation of the south limb accommodated a component of north-south shortening in the plane of phyllitic cleavage. Published analog models show that the north-verging kink bands of the south limb could have developed in the same deformation regime as did south-verging parasitic folds. The geometry of crenulation in the north limb indicates that it could be an extensional crenulation accommodating stretching of the limb once it was steep. Late-stage flattening at a high angle to bedding on the south limb locally rotated the crenulation cleavage domains to a sub-horizontal dip. Outcrops in the Baraboo Syncline emphasize that asymmetric crenulation is a fungible fabric, in that examples with similar appearance can accommodate different strains and displacements depending on local context.

Tethys’s Heat Fluxes Varied with Time in the Ithaca Chasma and Telemus Basin Region

We investigated how lithospheric heat fluxes varied temporally and spatially on the Saturnian moon Tethys, focusing on the region of Ithaca Chasma that overprints Telemus Impact Basin. Our results, derived from flexure associated with Ithaca, indicate elastic thicknesses of 4.1 ± 0.3 km to 6.4 ± 0.4 km and heat fluxes ranging from 12 to 39 mW m−2 assuming a nonporous pure H2O ice lithosphere. Our results for Ithaca’s south limb are similar to previous estimates within the north limb, indicating consistent heat fluxes across a large spatial extent in this area. However, our estimates are lower than those for the older Telemus Basin (>60 mW m−2), revealing evidence that Tethys experienced a substantial temporal variation in heat fluxes in this region. Heat fluxes reflected by Ithaca are similar to previous estimates for Tethys’s two youngest impact basins, Melanthius and Odysseus, suggesting that Ithaca may also be relatively young. If Tethys’s lithosphere is porous, then our heat flux estimates for Ithaca Chasma drop to 12–38 mW m−2, 11–35 mW m−2, and 10–33 mW m−2 for 5%, 15%, and 25% porosities, respectively. If Tethys’s lithosphere includes ∼10% NH3-hydrates, then the estimates are 5–16 mW m−2, 5–15 mW m−2, 4–14 mW m−2, and 4–13 mW m−2 for 0%, 5%, 15%, and 25% porosities, respectively. Although we find that some ground-based reflectance spectra hint at 2.2 μm bands that may result from NH3-bearing species, the detected features are weak and may not result from surface constituents. Consequently, our heat flux estimates that assume a pure H2O ice lithosphere are likely more accurate.

Fracture distribution in a folded fluvial succession: The Puig-reig anticline (south-eastern Pyrenees)

Sedimentary rocks of foreland fold-and-thrust belts typically undergo intensive fracturing as folds grow. The resulting fracture networks can present significant variations depending on the distribution of sedimentary facies and the complex structural characteristics of fold-and-thrust belts. The Puig-reig anticline, located in the south-eastern Pyrenees, mainly exposes proximal fluvial deposits in the north limb and medial fluvial deposits in the rest of the anticline. Thus, this anticline constitutes an excellent case study to investigate the main controls on the distribution of fracture networks in folded fluvial deposits, in terms of structural position and lithofacies variations. Outcrops were selected to be representative of different structural positions, from the fold hinge to its limbs, and of a variety of the main lithofacies, from proximal to medial fluvial deposits. Fracture data were acquired using the linear scanline method. The results indicate that the anticline rocks have been affected by four sets of fractures. The north limb is dominated by thick conglomerate bodies with interlayered sandstones deposited from unconfined flash floods and wide-shallow channel streams in the proximal fluvial fan, and presents large fracture spacing and low fracture intensity but relatively large fracture length and aperture. The crest and the crest-limb transition zones are mainly characterised by interlayered conglomerates, sandstones, siltstones and clays, deposited from braided channel streams and overbanks in the medial fluvial fan, and present fractures with relatively high fracture intensity and variable fracture length and aperture. The south limb, composed of channel filling sandstone layers and stable overbank fine deposits in the medial fluvial fan, is characterised by low fracture intensity and small fracture length and aperture. Based on multiple linear regression analysis, fracture intensity is mainly controlled by the structural position, bedding thickness and lithological associations, with relatively more intense fracturing in thin sandstone layers with multiple interlayers of fine deposits in the anticline crest. The fracture length mainly depends on bedding thickness and is affected lithological associations. The fracture apertures are mainly controlled by lithofacies, with relatively higher apertures affecting conglomerate bodies. The results of this study are relevant for characterising similar systems in the subsurface, where data is scarce.

EFFECT OF TECTONIC SETTING ON DISTRIBUTION OF PELAGIC SEDIMENT UNIT (SHIRANISH FORMATION) IN ZAGROS FORELAND BASIN, IRAQI SEGMENT

Shiranish Formation represents the uppermost Cretaceous pelagic sediment unit of the Iraqi segment within the western Zagros fold and the thrust belt. The occurrence of the formation governed by the northern edge of the Neabo-Arabian plate`s tectonic activity and the distribution has now been interpreted according to the modern tectonic concept of the foreland basin system. Field data and Isopach map have shown that two main basins existed on both sides of the Forebulge (Qamchuqa Formation). Currently, the first one has extended north west-southeast within the foredeep depozone, while the second one, which represents the Backbulge Depozone. The influence of the tectonic rise of the Forebulge Depozone leads to a non-existing pelagic sediments unit of the Shiranish Formation in the area restricted from northwest Amadiya town (the south limb of Mateen Anticline) towards southeast areas (Gara Anticline), thereafter to Aqra areas. Thus, the absence of the formation in the Shikhan area (e.g.) refers to inhomogeneity on the backbulge basin causing exposed or submerged areas during the age of upper Cretaceous.

The large trans-Neptunian object 2002 TC302from combined stellar occultation, photometry, and astrometry data

Context.Deriving physical properties of trans-Neptunian objects is important for the understanding of our Solar System. This requires observational efforts and the development of techniques suitable for these studies.Aims.Our aim is to characterize the large trans-Neptunian object (TNO) 2002 TC302.Methods.Stellar occultations offer unique opportunities to determine key physical properties of TNOs. On 28 January 2018, 2002 TC302occulted amv~ 15.3 star with designation 593-005847 in the UCAC4 stellar catalog, corresponding toGaiasource 130957813463146112. Twelve positive occultation chords were obtained from Italy, France, Slovenia, and Switzerland. Also, four negative detections were obtained near the north and south limbs. This represents the best observed stellar occultation by a TNO other than Pluto in terms of the number of chords published thus far. From the 12 chords, an accurate elliptical fit to the instantaneous projection of the body can be obtained that is compatible with the near misses.Results.The resulting ellipse has major and minor axes of 543 ± 18 km and 460 ± 11 km, respectively, with a position angle of 3 ± 1 degrees for the minor axis. This information, combined with rotational light curves obtained with the 1.5 m telescope at Sierra Nevada Observatory and the 1.23 m telescope at Calar Alto observatory, allows us to derive possible three-dimensional shapes and density estimations for the body based on hydrostatic equilibrium assumptions. The effective diameter in equivalent area is around 84 km smaller than the radiometrically derived diameter using thermal data fromHerschelandSpitzerSpace Telescopes. This might indicate the existence of an unresolved satellite of up to ~300 km in diameter, which is required to account for all the thermal flux, although the occultation and thermal diameters are compatible within their error bars given the considerable uncertainty of the thermal results. The existence of a potential satellite also appears to be consistent with other ground-based data presented here. From the effective occultation diameter combined with absolute magnitude measurements we derive a geometric albedo of 0.147 ± 0.005, which would be somewhat smaller if 2002 TC302has a satellite. The best occultation light curves do not show any signs of ring features or any signatures of a global atmosphere.

Partitioning and distribution of silver in sediment-hosted Cu-Ag deposits: Evidence from the Ghanzi-Chobe Belt portion of the Kalahari Copper Belt

The Khoemacau Mining (KCM) Cu-Ag deposits in northwestern Botswana are comprised of several mineralised zones, including from south to north: the Banana Zone (made up of the Chalcocite Zone, New Discovery, North Limb, South Limb Definition, and the North East Fold), Mango, Zone 5, Zone 5 North, Boseto, Zeta NE, and Zone 6. The KCM Cu-Ag deposits are characterised by two styles of mineralisation that include disseminated and structurally controlled styles of mineralisation, both typified by the scarcity of Ag-bearing minerals. Laser ablation inductively coupled mass spectrometry (LA-ICP-MS) and electron probe microanalysis (EPMA) techniques were employed to (i) track the sources of Ag by assessing the Ag budget of the sulphide phases, (ii) assess the partitioning and distribution of Ag in coexisting sulphide phases, and (iii) outline the mode of occurrence of Ag in the host sulphides. Bornite, chalcocite, wittichenite, tetrahedrite-like mineral, covellite, and galena were observed to be Ag-endowed. Bornite and tetrahedrite-like mineral are the principal hosts of Ag and contain, on average, up to 2090 and 4107 ppm Ag, respectively. But when bornite coexists with chalcocite, Ag preferentially partitions into chalcocite (mean Ag concentrations range from 423 to 1050 ppm) than in bornite (mean Ag concentrations range from 201 to 309 ppm). In the chalcocite-wittichenite assemblage, Ag partitions equally in these phases with mean concentrations of 1890 and 1854 ppm, respectively. Galena from the North East Fold contains a significant amount of Ag (mean 477 ppm) than that one from Zone 5, which is Ag-poor (mean up to 87 ppm). Unlike the spatially associated sulphides, chalcopyrite, however, is a poor host for Ag (mean concentration 1.3–104 ppm). The LA-ICP-MS data demonstrated that Ag is essentially bound in the crystal lattice of bornite, chalcocite, wittichenite, tetrahedrite-like mineral, covellite, and locally galena, as shown by the homogeneous distribution of Ag in the elemental maps, small to medium standard deviations (averaging 2–44% of the mean) and relatively flat time-resolved depth profiles. However, large variations in the Ag concentration between laser spots (averaging 51–70% of the mean) from the same sample, as observed in some bornite II grains from the New Discovery and the North East Fold mineralised zones, may indicate the presence of Ag-rich micro to nanoparticles in bornite II. The initial mineralising fluid is thought to have been saturated in Ag and precipitated Ag-rich phases in the mineralised zones southwest of the KCM Property, including the Chalcocite Zone, New Discovery, North Limb, South Limb Definition, and the North East Fold. Subsequently, it became Ag-under-saturated with time as it precipitated Ag-poor phases in the mineralised zones northeast of the KCM Project (such as Zone 5). The positive correlations (R > 0.60) displayed between Ag and As, Se, Sb, Hg, Te, and Bi in the Cu-(Fe)-sulphides demonstrate that these elements could be used as pathfinder elements in sediment-hosted Cu-Ag deposit exploration.

Comparison and Selection of Construction Methods for Shallow and Submerged Large Section Tunnels in Urban Metro

Shallow-buried and Deep-Excavated large-section tunnels in Urban Metro have many characteristics, such as large span, complex construction procedures, multiple disturbances to surrounding rocks, and difficult to control ground subsidence. In the process of tunnel construction, the surrounding rock geological conditions and construction procedures have a very important impact on the stability and stress distribution of the tunnel. Based on the South limb project of Xiejiacun entrance and exit section of Tiantongyuan East Station of Beijing Metro Line 17, the applicability of dynamic partition method (DPM) and double-sided pilot excavation method is studied by numerical simulation. The ground subsidence, convergence point deformation, surrounding rock stress, initial support stress and extrusion deformation of palm face are taken as the judgment basis for analysis. The conclusions of this paper are as follows: the surface settlement, vault displacement and surrounding rock stress of DPM method are smaller than those of double-sided guide tunnel method, so it is concluded that DPM method is more suitable for this project; however, the difference of displacement and internal force between DPM method and DPM method is even slight, but when DPM (10) and DPM (15) are used in the construction of right guide tunnel, the internal force and displacement will change abruptly and can be strengthened appropriately. Finally, the deformations of the face during the construction of three construction methods are discussed to verify the feasibility and safety of DPM method, which provides a basis and guidance for the smooth implementation of the project and can be used as a reference for similar projects.

Ouansimi copper mineralization (Western Anti-Atlas, Morocco): Paragenetic sequence and circulation of gangue hosted paleofluids.

The Ouansimi copper deposit, located South of the Kerdous inlier in Morocco, is an example of vein-type mineralization hosted in the Adoudou Formation limestone of the Lower Cambrian Taroudant Group. The copper-bearing veins are mainly controlled by Variscan NNW-trending faults and are subdivided here into Eastern, Western, and Central structures. On the basis of mineralogical, metallographic, and fluid inclusion studies, two mineralizing processes are proposed. A primary process is interpreted for the chalcopyrite, digenite, galena, sphalerite, carrollite, and pyrite with quartz and dolomite as gangue minerals. A secondary process is proposed for chalcocite, bornite, covellite, native copper, copper carbonate and oxide. Microthermometric analysis of fluid inclusions was carried out on quartz and dolomite gangues. Primary fluid inclusions trapped in quartz show the existence of fluids with temperatures of 175°C–275 °C and salinities of 10–15 wt % NaCl eq. However, fluid inclusions in dolomite indicate fluids with temperatures of 200°C–250 °C and salinities of 20–25 wt % NaCl equiv. Combining ore mineralogy and fluid inclusion microthermometry, two main hypogene sulphide mineralization stages can be distinguished in the Ouansimi copper deposit. The first Cu–Fe stage, with quartz as a gangue mineral, was related to hydrothermal fluids with moderate temperature and low salinity. The second Cu–Zn–Pb–Co–Ni–Fe stage was associated with dolomite as a gangue mineral and infilled cavities and fractures cutting quartz. This second phase was related to hydrothermal fluids with moderate temperature and high salinity. The tectonic evolution of the Ouansimi deposit was characterized by thrusting of the south limb over the north limb and by dextral and sinistral offsets of the mineralized structures along late-stage ENE-WSW-trending faults.

Research on Gas Occurrence Regularity in Fengcheng Mining Area of Jiangxi Province

The hazard of coal and gas outburst and the utilization of coalbed methane, continuously promotes research on the characteristics of methane distribution. As an important coal production base in China, Fengcheng Mining Area not only is serious in coal and gas outburst but also is one of the most promising coalbed methane development zones, so it is of significance to research on gas occurrence regularity. Based on the analysis of gas data collected from geological exploration and coal mining, the research reveals that: the gas occurrence regularity vary significantly between north limb and south limb of Fengcheng syncline. In north limb, with the increase of the depth of coal seam, gas content of coal seam tend to increase, the relative gas emission rate and the absolute gas emission rate tend to increase, the intensity of coal and gas outbursts tend to increase. But in south limb, the gas contents of coal seam are very low, far below the gas contents in north limb under the same depth. The controlling factors to gas occurrence regularity are discussed in this paper. The features of coal quality, the overlying lithology of coal seam and coal body structure in Fengcheng Mining Area are all favorable to gas generation and gas preservation. Under these conditions, in north limb, with the increase of coal seam's depth and overlying pressure of coal seam, the risk of gas hazard increase, which cause the gas occurrence regularity in north limb of Fengcheng syncline. But in south limb, the Eocene strata of Paleocene is directly overlying above Longtan coal-bearing strata of the Permian, which means that the coal seams of south limb have ever been strongly uplifted and exposed to ground so as to be weathered and eroded for a long time, and a large amount of gas is released, thus the gas contents of B4 coal seam in south limb are very low.

Observation of the period ratio P1/P2 of transversal oscillations in solar macro-spicules

We analyze the time series of oxygen line profiles (Ovi 1031.93 Å and Ovi 1037.61 Å) obtained from SUMER/SOHO on the solar south limb. We calculated Doppler shifts and consequently Doppler velocities in three heights 4″, 14″, and 24″ from the limb on a coronal hole region. Then, we performed wavelet analysis with Morlet wavelet transform to determine the periods of fundamental mode and its first harmonic mode. The calculated period ratios have departures from its canonical value of 2. The density stratification and magnetic twist are two main factors which may cause these departures.

The Hydrogeological Feature and Geological Hazard Prevention of M Coal Mine in Xinjiang Fukang

M coal mine is located in the hilly terrain of mountain front in the southern margin of Junggar Basin in Fukang. The geological structure belongs to a medium type in the mine area where there are surface faults (two larger faults) and structural developments. The stratigraphic dips of south limb of Fukang syncline and southern Fukang anticline are large while that near F5 fault of anticline axis are larger and even upright. Brittle rocks develop fractures. In consideration of meteorology, earthquakes and other factors, mining may lead to such geological hazards as eboulement and surface subsidence, which should be highlighted.

Environmental implications of the snow chemistry from Mt. Yulong, southeastern Tibetan Plateau

Snow chemical records were recovered from fresh and surface snow samples in the Mt. Yulong region. Striking seasonal differences were evident among the major ions. The non-monsoon snow samples show higher ionic concentrations than those of the monsoon season. The observations can be categorized into three groups based on their seasonal behavior and characteristic relationships that were explored using correlation and factor analyses. Calculations of backward trajectory modes suggest that the coarse mode ions (e.g. Ca2+, Mg2+) were mainly transported from Central Asia, through the south limb of the westerlies, in non-monsoon seasons. The Indian monsoon and southeast monsoon are the prevailing air masses in the Yulong region during the monsoon season, and can account for the extensive precipitation and lower snow ion concentrations in this season. Ion balance calculations indicate that there is an excess of cations in snow chemistry due to higher concentrations of Ca2+. In addition, the low snow Na+/Cl− ratio shows an obvious excess of Cl− concentrations possibly due to the precipitation scavenging of gas-phase HCl in the air and/or different sources between Na+ and Cl−. The snow chemical data presented here indicate that Mt. Yulong provides a probably unique record of atmospheric and environmental conditions in southwestern China.

Seasonal evolution of S q current system at sub-auroral latitude

The quiet-time (ΣKp ≤ 3) daily variations of the geomagnetic field at the Indian Antarctic station, Maitri (Geographic Coord.: 70.75°S, 11.73°E; Geomagnetic Coord.: 66.84°S, 56.29°E) during two consecutive years of a solar minimum are considered in order to investigate the characteristics of the solar quiet (Sq) current system. The present work reports the signatures of the south limb of the Sq current loop of the southern hemisphere over a sub-auroral station. It is observed that the seasonal variation of the Sq current strength over Maitri is strongest during the summer months and weakest during the winter months. In spite of the total darkness during the winter months, an Sq pattern is identified at Maitri. The range of the horizontal field variation in the daily Sq pattern during summer is one order higher than that during winter. An interesting feature regarding the phase of the local time variation in the seasonal pattern is found here. A sharp shift in the time of the peak Sq current to later local times (> 1 hour per month) is observed during January–February and July–August, which may correspond to the transition from the complete presence, or absence, of sunlight to partial sunlight. The differences in the incoming solar UV radiation during such transitions can cause a sudden change in the local ionospheric conductivity pattern, and can also trigger some unusual thermo-tidal activity, that might be responsible for modifying the global Sq pattern.

Geometric consequences of ductile fabric development from brittle shear faults in mafic melt sheets: Evidence from the Sudbury Igneous Complex, Canada

Compared to felsic igneous rocks the genetic relationship between brittle and ductile fabric development and its influence on the geometry of deformed mafic melt sheets has received little attention in structural analyses. We explore these relationships using the Sudbury Igneous Complex (SIC) as an example. The SIC is the relic of a layered impact melt sheet that was transformed into a fold basin, the Sudbury Basin, during Paleoproterozoic deformation at the southern margin of the Archean Superior Province. We studied brittle and ductile strain fabrics on the outcrop and map scales in the southern Sudbury Basin, notably in the Norite and Quartz Gabbro layers of the SIC. Here, deformation is heterogeneous and occurred under variable rheological conditions, evident by the development of brittle shear fractures, brittle-ductile shear zones and pervasive ductile strain. The mineral fabrics formed under low- to middle greenschist-facies metamorphism, whereby brittle deformation caused hydrolytic weakening and ductile fabric development. Principal strain axes inferred from all structural elements are collinear and point to a single deformation regime that led to thinning of SIC layers during progressive deformation. Ductile fabric development profoundly influenced the orientation of SIC material planes, such as lithological contacts and magmatic mineral fabrics. More specifically, these planar structural elements are steep where the SIC underwent large magnitudes of thinning, i.e., in the south limb of the Sudbury Basin. Here, the actual tilt component of material planes is likely smaller than its maximum total rotation (60°) inferred from inclined igneous layering in the Norite. Our field-based study shows that ductile fabric development from brittle faults can have a profound influence on the rotational components of primary material planes in deformed igneous melt sheets.

Discussions on the character and interpretation model of Kelasu deep structures in the Kuqa area

Abstract The character and interpretation model of deep structures in the Kelasu belt are discussed from growth strata, thrusting sequence, salt accumulation, and imbricate patterns between the shallow and deep structures. Seismic data shows that the fold hinges of growth strata in the south limb migrate toward north from deep to shallow, the growth axial surface connected along these fold hinges dips southward, and strata thickness in its south are thicker than that in the north. These characteristics of growth strata are completely different from that of basement-involved structures and salt structures. By the kinematic analysis of faults on the southern and northern sides of the Kela-2 structure, the northern fault is considered to thrust northward and form Kela-2 structure firstly, and then be cut through by the southern fault. Strong thrusting of the southern fault caused the Kela-2 structure to uplift several kilometers, and made the peripheral plastic rocks passively flow and accumulate in the footwall, this course is very similar to the pump suction. Based on this model, the sub-salt and sub-coal thrusting systems are interpreted in the deep Kelasu belt. In the sub-salt structural layer, the remaining lower part of Kela-2 structure in the footwall of its southern fault is the northern Keshen anticlinal belt. The discovery of the northern Keshen anticlinal belt and the Jurassic sub-coal thrusting structures greatly widens the exploration scope of the Kelasu belt, showing an enormous exploration potential.

Earthquakes generated from bedding plane-parallel reverse faults above an active wedge thrust, Seattle fault zone

A key question in earthquake hazard analysis is whether individual faults within fault zones represent independent seismic sources. For the Seattle fault zone, an upper plate structure within the Cascadia convergent margin, evaluating seismic hazard requires understanding how north-side-up, bedding-plane reverse faults, which generate late Holocene fault scarps, interact with the north-vergent master-ramp thrust and overlying backthrust of the fault zone. A regional uplift at A.D. 900–930 involved an earthquake that nucleated at depth and included slip on both the master-ramp thrust and the backthrust. This earthquake also included slip on some of the <6-km-deep north-side-up, bedding-plane reverse faults. At locales where the north-side-up reverse faults intersect the Puget Sound coast, an earthquake a few centuries earlier than the A.D. 900–930 regional uplift only uplifted areas within hundreds of meters north of the reverse faults. We infer that the bedding-plane reverse faults are seismogenic because shore platforms near the reverse faults have been abruptly uplifted during earthquakes when other shorelines in the Seattle fault zone were unaffected. Faults of the Seattle fault zone therefore can both produce regional uplift earthquakes, with or without surface displacement on the reverse faults, and produce earthquakes that rupture the bedding-plane reverse faults causing fault scarps and uplift localized to hundreds of meters north of these faults. This latter type of earthquake has occurred at least twice and perhaps three times in the late Holocene, and all these earthquakes preceded the regional coseismic uplift of A.D. 900–930. To account for the paleoseismic observations, we propose that the Seattle fault zone is a wedge thrust, with the leading edge being a fault-bend, wedge thrust fold. The active axial surface of the wedge thrust fold is pinned at the tip of the wedge, and a steeply north-dipping sequence of Tertiary sediment forms the south limb of the wedge thrust fold. Some of these steeply north-dipping, bedding-plane surfaces are seismogenic reverse faults that produce scarps. Earthquakes on the wedge thrust produce the regional coseismic uplift events, and earthquakes within the faultbend fold cause the local uplift earthquakes. Thus, bedding-plane faults can rupture during earthquakes when the wedge thrust does not rupture but instead continues to accumulate seismic energy.

Growth of the Huoerguosi anticline (north Tianshan Mountains) by limb rotation since the late Miocene

The Huoerguosi anticline, located in the north Tianshan Mountains piedmont fold-and-thrust belt, is a trending east-west fault-related fold. In the cross section along the Jingou River, its south limb is composed of the pre-growth strata of the Anjihaihe (E2–3a), the Shawan ((E3-N1)s), the Taxihe (N1t) and the lower part of the Dushanzi (N2d) Formations, trending east-west and dipping to south 55°, and the growth strata of the upper part of the Dushanzi (N2d) and Xiyu ((N2-Q1)x) Formations, dips of which decrease from 55° at the base of the growth strata to 47° at the bottom of the Xiyu ((N2-Q1)x) Formation to ∼0° at the top of the Xiyu ((N2-Q1)x) Formation. The strata at the north limb of the anticline are vertical or over-turned, and are cut by the breakthrough thrusts to result in the drag fold. In the depth, the anticline is symmetric, and its core comprises the Cretaceous and the Jurassic coal-bearing beds. In the seismic profile, the seismic reflectors of pre-growth strata at the south limb of the anticline dip to south constantly, and ones of the growth strata fan southward, whose dips decrease upward. The geometry of the south limb growth strata outcropped along the Jingou River valley and the deep structure of the anticline shown in the seismic profile indicate that the Huoerguosi anticline is a detachment fold anticline growing by limb rotation. Based on the growth model and magnetostratigraphic age, during the growing process of the Huoerguosi anticline, the average shortening rate absorbed by the south limb is ∼0.46 mm/a, and the average uplifting rate of the anticline is ∼0.86 mm/a which exceeds the average deposition rate, which is in accordance with the fact that the top of the anticline is intensely eroded. Considering symmetric geometry of the Huoerguosi anticline and ignoring the breakthrough thrusts, the shortening of the whole anticline should be more than ∼0.92 mm/a, doubling the shortening rate determined from the growth at the south limb.

Stratigraphy, biostratigraphy and C-isotopes of the Permian–Triassic non-marine sequence at Dalongkou and Lucaogou, Xinjiang Province, China

Measured lithostratigraphic sections of the classic Permian–Triassic non-marine transitional sequences covering the upper Quanzijie, Wutonggou, Guodikeng and lower Jiucaiyuan Formations at Dalongkou and Lucaogou, Xinjiang Province, China are presented. These measured sections form the framework and reference sections for a range of multi-disciplinary studies of the P–T transition in this large ancient lake basin, including palynostratigraphy, vertebrate biostratigraphy, chemostratigraphy and magnetostratigraphy. The 121 m thick Wutonggou Formation at Dalongkou includes 12 sandstone units ranging in thickness from 0.5 to 10.5 m that represent cyclical coarse terrigenous input to the lake basin during the Late Permian. The rhythmically-bedded, mudstone-dominated Guodikeng Formation is 197 m and 209 m thick on the north and south limbs of the Dalongkou anticline, respectively, and 129 m thick at Lucaogou. Based on limited palynological data, the Permian–Triassic boundary was previously placed approximately 50 m below the top of this formation at Dalongkou. This boundary does not coincide with any mappable lithologic unit, such as the basal sandstones of the overlying Jiucaiyuan Formation, assigned to the Early Triassic. The presence of multiple organic δ 13C-isotope excursions, mutant pollen, and multiple algal and conchostracan blooms in this formation, together with Late Permian palynomorphs, suggests that the Guodikeng Formation records multiple climatic perturbation signals representing environmental stress during the late Permian mass extinction interval. The overlap between the vertebrates Dicynodon and Lystrosaurus in the upper part of this formation, and the occurrence of late Permian spores and the latest Permian to earliest Triassic megaspore Otynisporites eotriassicus is consistent with a latest Permian age for at least part of the Guodikeng Formation. Palynostratigrahic placement of the Permian–Triassic boundary in the Junggar Basin remains problematic because key miospore taxa, such as Aratrisporites spp. are not present. Palynomorphs from the Guodikeng are assigned to two assemblages; the youngest, from the upper 100 m of the formation (and the overlying Jiucaiyuan Formation), contains both typical Permian elements and distinctive taxa that elsewhere are known from the Early Triassic of Canada, Greenland, Norway, and Russia. The latter include spores assigned to Pechorosporites disertus, Lundbladispora foveota, Naumovaspora striata, Decussatisporites mulstrigatus and Leptolepidites jonkerii. While the presence of Devonian and Carboniferous spores and Early Permian pollen demonstrate reworking is occurring in the Guodikeng assemblages, the sometimes common occurrence of Scutasporites sp. cf. Scutasporites unicus, and other pollen, suggests that the Late Permian elements are in place, and that the upper assemblage derives from a genuine transitional flora of Early Triassic aspect. In the Junggar Basin, biostratigraphic data and magnetostratigraphic data indicate that the Permian–Triassic boundary (GSSP Level) is in the middle to upper Guodikeng Formation and perhaps as high as the formational contact with the overlying Jiucaiyuan Formation.

Fluid Temperature and Salinity Characteristics of the Matagami Volcanogenic Massive Sulfide District, Quebec

Fluid inclusions hosted within various lithologic units of the >40 million metric ton (Mt) Matagami district, Abitibi greenstone belt, preserve samples of Archean volcanogenic massive sulfide (VMS) and postvolcanogenic massive sulfide hydrothermal fluid. Microthermometric measurements on ore-hosted primary two-phase liquid-vapor inclusions from Matagami’s south limb deposits indicate that the VMS hydrothermal fluid was highly saline (16.2 ± 4.7 wt % NaCl-CaCl2 equiv, 1σ, n = 230) and of moderate temperature (trapping temperature, Tt = 208° ± 32°C, 1σ, n = 230). A fluid with these characteristics is capable of transporting ~5 × 10−4 m (30 ppm) Zn as ZnCl3 − and ZnCl4 2− chloride complexes. However, the low temperature of this fluid precluded efficient Cu transport (≤ 3 ppm), which may in part explain the relatively Cu poor nature of the Matagami deposits. Calculated densities of this ore fluid as high as 1.10 g/cm3 are consistent with a bottom-hugging brine model. However, a subset of the data indicate a fluid less dense than ambient seawater, suggesting that buoyant hydrothermal plumes were also present. A microthermometrically determined high CaCl2 content (XNaCl <0.55, molar Na/Ca = 2.3/1) for the VMS ore-hosted primary fluid is consistent with an Archean hydrothermal fluid more Ca-rich than modern-day seawater. Quartz-epidote veins located in the hydrothermal cracking zone of the Bell River Complex host primary liquid-vapor-halite inclusions. These inclusions are interpreted to be samples of the deep-seated equivalent to the VMS ore-hosted hydrothermal fluid described above. Microthermometry indicates that these inclusions trapped a high-temperature brine (Tt = 373° ± 44°C, 1σ , n = 92; 38.2 ± 1.9 wt % NaCl equiv, 1σ , n = 92). We interpret this brine to be a phase-separated product of (modified) model seawater (3.2 wt % NaCl), an exsolved magmatic fluid, or a combination thereof, deep within the hydrothermal system at 650° to 670°C and a near-lithostatic pressure of 90 MPa. Phase separation and subsequent convection lowered the temperature of the brine prior to its entrapment within the hydrothermal cracking zone. The occurrence of high-temperature brine overlain by lower temperature/salinity fluid suggests a two-cell convection model, consistent with metal mass-balance calculations for the south limb. The high salinity of the ore-hosted fluid inclusions indicates two possibilities: (1) a significant amount of brine was incorporated into the upper cell and mixed with heated seawater during convection; (2) Archean seawater itself was very saline and of variable salinity. With the cooling of the Bell River Complex, lower temperature fluids, dominantly of seawater origin, circulated deep within the hydrothermal system. Modified by water-rock interaction, yet not phase separated, these fluids sealed off the remnant permeability of the fracture network of the hydrothermal cracking zone and were locally trapped as secondary liquid-vapor fluid inclusions (homogenization temperature, Th = 242° ± 17°C; 9.1 ± 1.6 wt % NaCl equiv, 1σ , n = 14) hosted within the Bell River Complex quartz-epidote vein material. Post- and/or waning-stage VMS hydrothermal activity is evident from the presence of quartz-epidote veins crosscutting Wabassee Group hanging-wall rocks. Microthermometry on quartz-hosted primary liquid-vapor fluid inclusions suggests that this activity occurred at relatively low temperatures (Th = 76°–177°C, n = 212), over a wide range of salinity (6.0–32.4 wt % NaCl-CaCl2 equiv, n = 212), and with a high apparent CaCl2 content (XNaCl <0.06). These fluid inclusion data illustrate the importance of subsea-floor chemical and physical processes directly related to metal transport and deposition in VMS systems. In particular, phase separation deep within the hydrothermal system is interpreted as a key process for generating saline brines capable of forming significant ore deposits.

Pleistocene Brawley and Ocotillo Formations: Evidence for Initial Strike‐Slip Deformation along the San Felipe and San Jacinto Fault Zones, Southern California

We examine the Pleistocene tectonic reorganization of the Pacific–North American plate boundary in the Salton Trough of southern California with an integrated approach that includes basin analysis, magnetostratigraphy, and geologic mapping of upper Pliocene to Pleistocene sedimentary rocks in the San Felipe Hills. These deposits preserve the earliest sedimentary record of movement on the San Felipe and San Jacinto fault zones that replaced and deactivated the late Cenozoic West Salton detachment fault. Sandstone and mudstone of the Brawley Formation accumulated between ∼1.1 and ∼0.6–0.5 Ma in a delta on the margin of an arid Pleistocene lake, which received sediment from alluvial fans of the Ocotillo Formation to the west‐southwest. Our analysis indicates that the Ocotillo and Brawley formations prograded abruptly to the east‐northeast across a former mud‐dominated perennial lake (Borrego Formation) at ∼1.1 Ma in response to initiation of the dextral‐oblique San Felipe fault zone. The ∼25‐km‐long San Felipe anticline initiated at about the same time and produced an intrabasinal basement‐cored high within the San Felipe–Borrego basin that is recorded by progressive unconformities on its north and south limbs. A disconformity at the base of the Brawley Formation in the eastern San Felipe Hills probably records initiation and early blind slip at the southeast tip of the Clark strand of the San Jacinto fault zone. Our data are consistent with abrupt and nearly synchronous inception of the San Jacinto and San Felipe fault zones southwest of the southern San Andreas fault in the early Pleistocene during a pronounced southwestward broadening of the San Andreas fault zone. The current contractional geometry of the San Jacinto fault zone developed after ∼0.5–0.6 Ma during a second, less significant change in structural style.