Chemical remanent magnetizations (CRMs) can form by many diagenetic mechanisms, including alteration associated with migration of fluids (e.g. Elmore et al. 1993), maturation of organic matter (e.g. Banerjee et al. 1997; Blumstein et al. 2004) and clay diagenesis (e.g. Katz et al. 2000; Woods et al. 2002; Tohver et al. 2008). Palaeomagnetic analysis can be useful in deciphering the timing of such events by determining the direction of the CRM and plotting the corresponding pole position on the apparent polar wander path (APWP). This can be combined with geochemical and other data in an integrated study to understand the origin of diagenetic events.
Many studies have inferred that fluids were responsible for CRMs (e.g. McCabe & Elmore 1989; McCabe et al. 1989; Lu et al. 1991; Saffer & McCabe 1992), but relatively few studies have focused on testing for a connection between the fluids and CRMs by conducting contact tests on the fluid conduits (e.g. Elmore et al. 1993, 1998; Elmore 2001; Blumstein et al. 2005). These studies have found that remagnetizing fluids have moved through conduits and remagnetized only the conduits and surrounding rocks. Other studies (e.g. Gillett & Karlin 2004) have found that CRMs can be pervasive in many units but questions remain as to how these rocks were remagnetized. Burial remagnetization mechanisms may be possible in some cases (Fruit et al. 1995; Banerjee et al. 1997; Katz et al. 2000; Blumstein et al. 2004; Moreau et al. 2005) but cannot explain all pervasive remagnetizations (Gillett & Karlin 2004). The overall objective of this study is to investigate how fluids move through rocks and cause remagnetizations. This will be accomplished by testing if the Alamo Breccia in Nevada, a potential palaeoaquifer, was in fact a conduit for remagnetizing fluids that caused the widespread remagnetization (Gillett & Karlin 2004) in the Basin and Range area of the southwestern United States.
The Devonian Period Alamo Breccia is a thick (up to c. 100 m; Warme & Sandberg 1996) breccia unit in southern Nevada that is bracketed above and below by shallow marine carbonates. Because of presumed higher permeability, the breccia is likely to have acted as a conduit for fluids which may have caused remagnetization compared to the rocks above and below. It is interpreted as a breccia formed from a bolide impact onto shallow marine carbonate shelf. The brecciated rocks were reworked by tsunamis resulting from the impact (Warme & Kuehner 1998). At some localities, the breccia contains extensive thin calcite veins that are probably related to post-impact deformation events.
To test if the Alamo Breccia was a conduit we conducted an integrated palaeomagnetic and geochemical study to identify, date and determine the origin of remagnetizations present in the unit. Samples from within the breccia and from the carbonates above and below the breccia were collected to conduct a contact test. The test was conducted at two different locations in south central Nevada (Hancock Summit and Mt Irish; Fig. 1). At Hancock Summit, samples were also collected from around c. 1–2 cm thick veins within the breccia to perform vein contact tests. Samples were subjected to a low-temperature treatment and detailed thermal demagnetization to isolate the magnetizations present. Rock magnetic analysis was conducted to investigate the mineralogy of the magnetic minerals. Petrographic and geochemical studies were performed to determine the nature of the diagenetic alteration and help determine the origin(s) of the remagnetizations.
The Alamo Breccia is found in at least 25 mountain ranges throughout south and central Nevada and western Utah (Pinto & Warme 2008). The area contains Palaeoproterozoic Era metamorphic rocks and igneous intrusions that were overlain by a thick wedge of mainly carbonate rocks deposited during the Neoproterozoic Era through the Mid-Devonian Period. The wedge of carbonate and siliciclastic rocks is up to almost 9000 m thick with c. 1800 m deposited during the Devonian Period (Stewart 1980; Gillett & Karlin 2004). From the Late Devonian Period to the Early Mississippian Epoch there was compressional tectonic activity due to the Antler Orogeny, and then the Sonoma Orogeny in the Late Permian Period to the Early Triassic Period. In the Mid-Jurassic Period to the end of the Mesozoic Era, the Sevier Orogeny caused additional compressional tectonic activity. In the middle of the Cenozoic Era, the area began undergoing extensional tectonics that created the present-day basin and range topography (Stewart 1980; Gillett & Karlin 2004). Due to this younger tectonic deformation, the precise centre of the impact crater is not known. Miocene Epoch and post-Miocene Epoch anticlockwise vertical axis rotations of around 10° have been inferred for the study area (Hudson et al. 1998) and particularly the Hancock Summit location (Gillett & Karlin 2004).
The Alamo Breccia is one of the largest known carbonate megabreccias and is calculated to be c. 1300 km3 in volume (Pinto & Warme 2008). The breccia is interpreted as a megasedimentary deposit formed by the ejecta curtain, tsunamis and other processes resulting from a bolide impact event on or near a shallow marine carbonate platform (Warme 2004). Conodonts have been used to date the Alamo Breccia as being deposited during early Frasnian punctata Zone (Sandberg & Warme 1993; Warme & Sandberg 1995, 1996). Taking into consideration radiometric dating of the Late Devonian Period (Kaufmann et al. 2004; Trapp et al. 2004), this converts to an age of c. 382 Ma (Morrow & Sandberg 2005) for the breccia.
Evidence that the Alamo Breccia is an impact deposit includes hematite-studded shocked quartz (Leroux et al. 1995; Morrow et al. 2005), accretionary lapilli (Warme et al. 2002) and a minor iridium anomaly. While the iridium anomaly is fairly weak (133–139 ppt), it is still noteworthy when compared to the background iridium levels of ≤25 ppt (Morrow et al. 2005).
The Alamo Breccia is the middle member of the Guilmette Formation. The Alamo Breccia Member is underlain by the Yellow Slope-forming Member and the Carbonate Platform Facies (member), which contain cyclic limestones and dolostones. The Alamo Breccia is overlain by the Upper Guilmette Member, which consists of the Slope Facies Member topped with quartzose sandstones (Warme et al. 2008).
The Alamo Breccia is divided into four stratigraphic divisions based on changes in lithology and the processes that formed each division (Fig. 2). The lowest section is Unit D, which consists of a thin monomict limestone breccia that separates the megaclasts of Unit C from the lower members of the Guilmette Formation. This unit is interpreted as fluidized rock that was preserved along a detachment surface only in locations where the large megaclasts of Unit C are maintained in nearly their original positions over it. Unit C consists of large (up to c. 500 m long by c. 80 m thick) megaclasts that are parallel–subparallel to bedding. Unit B is a chaotic, polymict, matrix-supported calcite-cemented breccia with clasts up to hundreds of metres long that resembles a debris-flow deposit. Unit B overlies Unit C and, where Units C and D are missing, extends to the underlying Carbonate Platform Facies. The uppermost division is Unit A, which is a coarse–fine-grained, graded, polymict breccia. This unit is less chaotic than Unit B. Unit A usually exhibits at least two intervals of normally graded beds at the top of the unit (Warme & Sandberg 1996; Warme & Kuehner 1998; Pinto & Warme 2008).
At Hancock Summit, Unit A contains numerous veins. The veins range in size from c. 100 µm up to a few centimetres wide. The veins are generally widely spaced but are very closely grouped in some locations. There were two sets of vein orientations: south-southeasterly (161°) and westerly (270°). The westerly veins are generally smaller in size than the southeasterly veins. Some of the larger veins are over 1 m in length, and are oriented in an en echelon pattern.
The Alamo Breccia has not been the subject of palaeomagnetic study. Gillett & Karlin (2004) conducted a regional palaeomagnetic study of Palaeozoic Era carbonate rocks in the Basin and Range area of the southwestern United States including the Guilmette Formation, and found that the rocks contained two ancient components. An intermediate-temperature component with relatively steep positive inclinations and generally north declinations was found between c. 230 °C and 350 °C and was interpreted to reside in magnetite and possibly pyrrhotite at some sites. At higher temperatures (320–500 °C), a low-inclination (LI) component was identified; it was interpreted to reside in magnetite and has south–SE declinations (in stratigraphic coordinates).
The timing and origin of both components were difficult to interpret. The intermediate component was interpreted to have been acquired during the Cretaceous Period normal superchron, which overlaps with timing of the Sevier Orogeny. At several sites however, this Cretaceous Period origin was difficult to justify and the component was interpreted as more likely being Cenozoic Era in age despite the normal polarity. Gillett & Karlin (2004) interpreted the intermediate component to be polygenic in origin and probably reflected local Cenozoic Era or Mesozoic Era events.
The timing and origin of the LI component was also problematic. The negative inclinations and southeasterly declinations suggested the magnetization had been acquired during the Palaeozoic Era Kiaman reversed superchron but some sites, based on the directions, suggest the remagnetization was as young as the Early Triassic Period. Gillett & Karlin (2004) suggested three possible origins for this component: brine mobilization during the Sonoma Orogeny (Late Permian Period–Early Triassic Period); brine mobilization during the Ancestral Rockies uplift (Late Palaeozoic Era); and smectite destruction/magnetite authigenesis. Both of the mechanisms involving brine mobilization are unable to fully account for the geographic extent of remagnetization, while the destruction of smectite does not fully account for the stratigraphic extent unless coupled with thermal resetting of stratigraphically lower units. This led to the conclusion that the LI component was also polygenetic in origin.
Several palaeomagnetic studies have used contact tests to investigate units acting as palaeoaquifers for fluid migration events. For example, Elmore et al. (1998) used palaeomagnetic and petrographic/geochemical methods on Palaeozoic Era rocks in the Arbuckle Mountains to determine timing of fluid-flow events through the Reagan Sandstone, a basal palaeoaquifer. The base of the Reagan Sandstone and the upper section of the underlying Colbert Rhyolite were found to be chemically altered and contained a Late Palaeozoic Era CRM that was interpreted to have been caused by alteration associated with a fluid migration event. The underlying Colbert Rhyolite was also interpreted to carry a primary magnetization, whereas the upper portion of the Reagan Sandstone and the overlying Honey Creek Formation contained a Cambro-Ordovician Period magnetization (Elmore et al. 1998).
A similar study conducted in the Appalachian Mountains showed the presence of fluid-related CRMs located in a Devonian Period palaeoaquifer, the Oriskany Formation (Elmore et al. 2001). The Oriskany and the underlying aquitard, the Helderberg Group, both contain CRMs that reside in magnetite. The Oriskany Formation contained geochemical evidence that orogenic fluids had migrated through it, and the CRM was interpreted as related to the fluids. The Helderberg Group contained no geochemical evidence for externally derived fluids, and the CRM was interpreted as being related to burial diagenetic processes.
Other studies have used contact tests around faults and fractures to date fluid-flow events. For example, Blumstein et al. (2005) studied the rocks in and around the Moine Thrust Zone in northern Scotland; the thrust zone was interpreted to have been a conduit for multiple fluid-flow events. This method can also be applied to the smaller scale of veins. Elmore et al. (1993) found a pervasive Carboniferous Period CRM residing in magnetite in the Viola Limestone in the Arbuckle Mountains, but also found a Permian Period CRM residing in hematite that was localized around veins within the Viola. The Permian Period CRM was found in rocks near the veins that had elevated 87Sr/86Sr values. The 87Sr/86Sr values decrease with distance from the vein to coeval values, and the Permian Period CRM decreased until only the pervasive Carboniferous Period CRM was present. This suggests that the fluids that altered the rocks also caused the CRM in magnetite and that the CRM dates the fluid-flow event.
Cores were collected from the breccia for palaeomagnetic, rock magnetic, geochemical and petrographic analyses using a portable gasoline-powered drill and oriented using an inclinometer and a Brunton compass. Bedding and vein orientation were also measured using a Brunton compass. Cores were also collected from inside and outside of the breccia in order to perform a breccia contact test. A contact test is performed by sampling within the hypothesized fluid conduit, and then sampling the overlying and underlying units. If the breccia was a conduit for remagnetizing fluids, a magnetization would be present within the breccia but would decrease away from the breccia. The intensity of the magnetizations might also be higher in the breccia as opposed to the overlying and underlying units.
During initial sampling cores were collected from throughout the breccia at Hancock Summit (Fig. 1) including individual breccia clasts, matrix and coarse–fine-grained polymict breccia at the top of the unit, as well as sites above and below the breccia. The breccia was also sampled at Mt Irish (Fig. 1), including accretionary lapilli clasts in the upper section of the Alamo Breccia (Warme et al. 2002). The rocks were resampled to confirm the results from the initial sampling and to perform additional demagnetization procedures. To resample, cores were collected from 9 sites (6–28 cores per site) within the Alamo Breccia at the Hancock Summit. Eight sites (2–8 cores per site) were also sampled from up to c. 11 m above the breccia and 11 sites (2–3 cores per site) were sampled from down to c. 50 m below the breccia. At Mt Irish, cores were collected from 7 sites (2–8 cores per site) in rocks overlying the Alamo Breccia up to c. 12 m and 5 sites (1–4 cores per site) from within the breccia. The underlying beds were not exposed.
Contact tests were also performed on veins within the breccia at Hancock Summit. Cores were collected from the veins and with increasing distance from the veins and analysed to determine if there were any connections between the veins and the magnetic characteristics (i.e. magnetic intensities, directions). As for the breccia contact test, if this test was positive there would be a component present at/near the vein and it would decrease with distance from the vein. Eight separate vein contact tests were performed with 97 cores.
In the palaeomagnetic laboratory at the University of Oklahoma, the cores were cut to standard 2.2 cm lengths using a double-blade core saw. The natural remanent magnetizations (NRMs) were measured using a three-axis 2 G Enterprises cryogenic magnetometer with DC SQUIDs (superconducting quantum interference device) in the magnetically shielded lab. Most cores collected during the initial sampling were only subject to stepwise thermal demagnetization. Some specimens were subjected to low-temperature demagnetization (LTD) in order to test its effectiveness and, as a result, all the specimens from the second sampling were subjected to two LTD steps by immersion in liquid nitrogen for a minimum of 30 min. This process is used to remove the magnetization held in multi-domain (MD) magnetite (Dunlop & Argyle 1991). Specimens were then subjected to thermal demagnetization at increasing temperature steps between 100 °C and 600 °C using an ASC Scientific Thermal Specimen Demagnetizer (Model TD-48 SC). The specimens did not exhibit stable decay above 500–540 °C.
The demagnetization data were analysed using the Super IAPD (Interactive Analysis of Paleomagnetic Data) software program (www.geodynamics.no/software.htm) and plotted on Zijderveld diagrams (Zijderveld 1967). Principal component analysis (Kirschvink 1980) was used to determine the magnetic components in specimens which had mean angular deviations (MADs) of less than 15°, although most had MADs of less than 10°. Mean directions of the magnetic components were determined using Fisher (1953) statistics. Because of different bedding attitudes at Hancock Summit (N020/40W) and Mt Irish (N168/22W), a regional tilt test was performed using the directional correction (Enkin 2003) and the Watson & Enkin (1993) tilt tests. Both tilt tests were performed using the Geological Survey of Canada Paleomagnetism Analysis program package (Enkin, version 4.2).
Isothermal remanent magnetization (IRM) acquisition and thermal decay measurements were collected to aid in determining the magnetic mineralogy present in the specimens. An IRM was induced in 14 representative specimens with an impulse magnetizer using a stepwise progression in 25 steps from 0 to 2500 mT. The IRM acquisition curves were analysed using the IRMUNMIX 2.2 (Isothermal Remanent Magnetization unmix) software program (Heslop et al. 2002) which was followed by cumulative log-Gaussian (CLG) analysis (Kruiver et al. 2001) to model the different coercivity contributions (Heslop et al. 2004). The specimens were subjected to alternating field (AF) demagnetization up to 120 mT and were given IRMs (2500 mT, 500 mT and 120 mT) in three perpendicular directions prior to nine of the specimens being thermally demagnetized to obtain the tri-axial decay curves (Lowrie 1990).
Petrographic analysis was conducted to identify magnetic minerals as well as diagenetic features. Fifty polished thin sections from inside and outside the breccia were examined using reflected and transmitted light.
Strontium isotope analysis was conducted at the University of Texas at Austin using the methodology described by Gao et al. (1992) to determine if the rocks had undergone chemical alteration. Analysis was performed on seven specimens from Hancock Summit: two limestone specimens from outside the breccia, four specimens from within the breccia and one calcite vein from within the breccia. Each specimen consisted of c. 200 mg aliquots. The NIST SRM 987 Sr (National Institute of Standards and Technology Standard Reference Materials) standard was measured to a mean value of 0.710248±0.000008. The measured strontium isotope values were normalized to the National Bureau of Standards' NBS 987=0.71014 and then compared with the coeval seawater values for Late Devonian-age limestones (McArthur et al. 2001).
Examination of Zijderveld diagrams of thermal demagnetization results for specimens allowed for the identification of three magnetic components based on unblocking temperatures (Fig. 3a) in most specimens. A low temperature component with northerly declinations and steep down inclinations is found below 250 °C. Based on the direction and the unblocking temperatures, this component is interpreted as a modern viscous remanent magnetization (VRM) acquired in the current geomagnetic field. Since this component is considered to be noise from a palaeomagnetic point of view (Butler 1992), it is ignored for the remainder of this paper. An intermediate-temperature (IT) magnetization has northwesterly declinations and positive inclinations. In most specimens it is removed between 250 °C and 340 °C (Fig. 3a–f) although, in some specimens (c. 30%), it is not removed until up to 500 °C (e.g. site MI2i, Table 1). The IT component is found in sites within, above and below the breccia. Most specimens from sites below the breccias at Hancock Summit only contain this component (Fig. 3d). A high-temperature (HT) component with southeasterly declinations and shallow inclinations is removed between 340 °C and 540 °C in most specimens from within the breccia as well as above and below the breccia (Fig. 3a, c, e, & f). It is found in only three specimens below the breccia at Hancock Summit (site HSB, Table 1). NRM intensities are also similar for the specimens within, above and below the breccia.
Most (c. 73%) specimens from breccia clasts and matrix (HSBrclasts, MI3i; Table 1), the coarse–fine-grained polymict breccia at the top of the unit (AB1, AB3; Table 1) and a lapilli clast (MI1i; Table 1) from the first sample set contain both the IT and HT components. The breccia and surrounding rock were resampled to further test the distribution of the HT component. In addition, some specimens from the second sampling were initially subjected to LTD; they showed a notable inclination shallowing in the HT component compared to the components in specimens from the first sampling, which only underwent thermal demagnetization (Fig. 3b, c). This is consistent with removal of a modern overprint due to the LTD treatment. Consequently, all the specimens from the second sampling were subjected to LTD. Because of the possibility of contamination, only the specimens subjected to LTD were used to calculate the mean directions and for the tilt test.
Analysis after LTD showed that the NRM intensity decreased c. 17% on average. Whereas the sites for non-LTD specimens were not used in the statistical calculations, the mean directions are similar to some of the LTD sites (Table 1) and lend support to the presence of both components throughout the breccia. It is also interesting to note that the LTD treatment did not appear to affect the IT component in most specimens (Fig. 3b, c).
Using specimen directions for the IT and HT components, group mean directions (Table 1) at Hancock Summit were calculated for the breccia (AB, HSV1, HSV2, HSV3 and HSV7), above the breccia (HSA) and below the breccia (HSB). At Mt Irish, group means (Table 1) were calculated for the breccia (MI1, MI2) and above the breccia (MIA).
One site (MIA7) at the top of the section at Mt Irish contains a high-intensity magnetization (>15 mA m−1) with southwesterly declinations and negative inclinations (Table 1). The site is interpreted as having been hit by a lightning strike (Butler 1992) and is not considered further. One site above the breccia at Hancock Summit (HSA9; Table 1) had a HT component with a direction that is distinct from the other sites and was not considered in the calculation of the mean directions.
Using the site means from Hancock Summit and Mt Irish, a regional tilt test was performed to determine when the magnetization was acquired relative to the deformation, which is interpreted to be related to Basin and Range extension. For the IT component the site means group best between geographic and stratigraphic coordinates (Fig. 4a), suggesting this component was acquired during tilting. The Watson & Enkin (1993) tilt test shows the best grouping at 19% (±6%) untilting (Fig. 4b; Table 2) with declination 324° and inclination 58° (k=119.4, N=10, α95=4.4). Because error calculations for this test may produce unrealistically small errors (Enkin 2003), the directional correction (DC) test (Enkin 2003) was also performed. The DC test also yielded a syndeformational result (Table 2) with the best grouping for the site means at 18% (±14%) unfolding. The pole position (61°N, 168°E; semi-axes of the 95% ellipse of confidence around the pole: dp=5°, dm=7°) for the best grouping direction plots off the apparent polar wander path (APWP) for North America (Fig. 5a). Correcting for a possible anticlockwise block rotation (c. 10°) in the study area (Hudson et al. 1998) would move the pole (69°N, 165°E) closer to the Early Cretaceous Period part of the APWP.
For the HT component or characteristic remanent magnetization (ChRM) the site means group best in stratigraphic coordinates (Fig. 4a), suggesting this component was acquired before deformation. The Watson & Enkin (1993) tilt test shows the best grouping at 107% (±6%) unfolding (Fig. 4c; Table 2) with declination 134° and inclination −14° (k=33.2, N=10, α95=8.5). The DC test (Enkin 2003) also yielded a pre-deformational result (Table 2), with the best grouping at 97% (±10) unfolding. When examining the site means for the ChRM in stratigraphic coordinates (Fig. 4a), the sites do not group well even at the best grouping. Because of the possibility of block rotation around a vertical axis at the study area (Hudson et al. 1998) which would affect the declination, an inclination-only test was performed. This test yielded a best grouping at 100% (±5%) unfolding (Table 2).
The age of the HT component was found by plotting the pole position for the 100% untilting direction on the APWP for North America (Fig. 5b). The pole plots at 38°N, 129°E (dp=4°, dm=9°), which lies in the Mid-Pennsylvanian Epoch. Due to the aforementioned possibility of block rotation, a 'corrected' pole was also calculated using the inclination from the inclination-only tilt test results and the declination corrected for the 10° inferred rotation (Hudson et al. 1998). This pole plots at 45°N, 119°E on the Early Permian Period part of the APWP (Fig. 5b). This is the preferred pole for the HT component because of the possibility of block rotation at Hancock Summit.
There is some concern that the poles used to construct the APWP did not take into consideration the effects of inclination shallowing. Bilardello & Kodama (2010a, b) have shown that inclination correction could cause the late Palaeozoic Era path to shift down in latitude up to 10°, but more commonly 4–5°. This would shift the pole towards the Late Permian Period.
One of the major objectives of this research was to determine if the breccia acted as a conduit for remagnetizing fluids. At Hancock Summit, the overlying rocks fail the contact test (Fig. 6). The same IT component and ChRM are found in specimens from within the breccia and in specimens from above the breccia. Most of the specimens (sites HSB and AB8; Table 1) from below the breccia contain the IT component, with only three specimens containing the ChRM (site HSB; Table 1). At Mt Irish, the IT and HT components are both present in specimens from within the breccia as well as above the breccia (Fig. 6). These results suggest that the breccia was not a focused conduit, although it is possible that the sampling did not extend far enough above or below the breccia to be out of the alteration zone.
Eight vein contact tests were performed at Hancock Summit in order to test if the thin veins were conduits for fluids that caused remagnetization. Of the eight vein sites at Hancock Summit, four sites (HSV1, HSV2, HSV3 and HSV7; Table 1) have specimen directions similar to the IT and HT components (Table 1). There is no relationship between distance from the veins and the magnetic intensities or directions. One site, HSV8 (Fig. 7), has streaked directions between the IT component direction (northwesterly) and HT component direction (southeasterly). This is interpreted to be because of vector addition between the two components. The results from remaining vein sites contain scattered specimen directions that are not well grouped and mean directions were not calculated.
The CLG analysis (Kruiver et al. 2001; Heslop et al. 2004) of the IRM acquisition data indicates that the specimens contain three components (Fig. 8; Table 3). A preliminary interpretation based on the coercivities is that they are magnetite, pyrrhotite and goethite (Table 3). The tri-axial decay of the high-coercivity IRM component shows a large decrease at 100 °C which suggests the presence of goethite, although this phase is not interpreted as carrying a stable remanence based on the thermal demagnetization results. The decay of the low-coercivity IRM component shows a slight drop in intensity at 325–350 °C, suggesting the presence of pyrrhotite (Fig. 9). This low-coercivity component is removed by 580 °C, suggesting the presence of magnetite. Tri-axial decay also shows that a higher coercivity phase, namely hematite, is present until c. 680–700 °C. This phase is not interpreted as carrying a stable remanence. The CLG analysis and the thermal decay of the IRMs are consistent with the interpretation that the IT component and ChRM reside in pyrrhotite and magnetite, respectively.
The specimens of the Alamo Breccia are classified as crystalline dolomites, fossiliferous grainstones and packstones, and mudstones. The veins mostly contained calcite, but one vein contained calcite and quartz. Using reflected light some iron oxides (e.g. hematite, goethite) were identified; magnetite and pyrrhotite were not identified however, possibly to due to their small size. Pyrite was also observed using reflected light.
Strontium isotope analysis results were used as an alteration indicator. The 87Sr/86Sr values were plotted on a coeval seawater curve (Fig. 10) for the Late Devonian Period (McArthur et al. 2001). The carbonate breccia samples are elevated in strontium, indicating that they were altered by externally derived fluids. The value from one calcite vein from within the breccia is highly elevated, indicating that externally derived fluids also precipitated the vein.
The Alamo Breccia and the surrounding carbonate rocks at Hancock Summit and Mt Irish contain two ancient magnetic components: a syn-tilting IT component with northwesterly declinations and positive inclinations is present between 250 °C and c. 340 °C; and a pre-tilting ChRM with southeasterly declinations and negative inclinations is found between 340 °C and 540 °C. These components are interpreted to reside in pyrrhotite and magnetite, respectively, based on maximum unblocking temperatures and rock magnetic analysis. Although the maximum unblocking temperatures for the IT component are high for pyrrhotite, Rochette et al. (1990) have shown that pyrrhotite can have an unblocking temperature up to 350 °C. The lack of effect that LTD had on the IT component is consistent with this interpretation. In a few specimens, the IT component is interpreted to reside in magnetite because the unblocking temperatures extend up to 500 °C.
Although the ChRM is pre-tilting, it is a secondary magnetization because the pole position suggests an age younger than the Devonian Period impact that formed the breccia (Fig. 5b). The Alamo Breccia has been completely remagnetized. These results are consistent with those found in Gillett & Karlin's (2004) regional palaeomagnetic study of Palaeozoic Era rocks in Nevada.
Several different origins of these components are possible. The conodonts in the Alamo Breccia have a CAI (Conodont Alteration Index) of 3.0 (Jared Marrow, pers. comm. 2008) which corresponds to a burial temperature of 110–200 °C (Epstein et al. 1977). The IT component, which resides predominantly in pyrrhotite, has a maximum unblocking temperature of 340 °C. Based on this low burial temperature, the component's maximum unblocking temperature and the pyrrhotite relaxation time-unblocking temperature curves for single-domain (SD) grains (Dunlop et al. 2000), the pyrrhotite component cannot be a thermoviscous remanent magnetization (TVRM). The magnetization is instead interpreted as a CRM. Many previous studies have interpreted similar components as TVRMs in magnetite (e.g. Hudson et al. 1998; Enkin et al. 2000). Because of the unblocking temperature, the rock magnetic results, and the lack of effect that LTD had on this component, this magnetic mineral is interpreted as pyrrhotite.
The ChRM is also considered to be a CRM based on the maximum unblocking temperature of 540 °C and the SD magnetite relaxation-time–unblocking-temperature curves (Dunlop et al. 2000). The relaxation-time–unblocking-temperature curves for SD grains are applicable for this study since the MD contribution has been removed by liquid nitrogen treatment.
The ages for the two CRMs correlate fairly well with the ages proposed by Gillett & Karlin (2004) for the two components found in their study. The pole position for the pyrrhotite CRM in this study plots off the APWP (Fig. 5a), although accounting for the block rotation moves the pole close to the Early Cretaceous Period portion of the path. Other factors such as secular variation not being averaged out or tectonic complications due to the remagnetization being acquired during deformation could also account for the pole being off the path. The age of the IT CRM is interpreted to be Cretaceous Period or perhaps Cenozoic Era. The HT CRM is interpreted to be Permian Period in age (Fig. 5b).
There are a number of diagenetic processes that could have caused the CRM in magnetite. Alteration by fluids would not appear to be a likely mechanism because the results of the contact tests suggest that the breccia was not a focused conduit for fluids. But burial remagnetization mechanisms are also not likely mechanisms. For example, maturation of organic matter (e.g. Fruit et al. 1995; Banerjee et al. 1997; Blumstein et al. 2004) is not likely because the carbonates are not source rocks. The carbonates do contain some clay, and clay diagenesis (Katz et al. 2000; Moreau et al. 2005) is a possibility but is not considered likely because many of rocks with the CRM are not clay rich. The strontium isotope analysis indicates that the breccias, as well as the rocks around the breccias, were altered by externally derived fluids (Fig. 10). One hypothesis supported by the data is that the fluids were not constrained by the breccias and moved pervasively through the rocks and altered larger intervals of the section. Alteration by fluids was deemed very likely by Gillett & Karlin (2004) who found the CRM in many rock types. It should be noted, however, that they did not conduct geochemical studies. The interpretation that pervasive flow of fluids caused the CRM differs from other studies (e.g. Elmore et al. 1993, 1998; Elmore 2001; Blumstein et al. 2005) where remagnetizing fluids were only found to have migrated through conduits and locally remagnetized a contact zone around the conduits.
There are several chemical mechanisms that could explain the pyrrhotite CRM. Pyrrhotite authigenesis could be due to pre-existing magnetite reacting with pyrite as a result of burial metamorphism under reducing conditions (Gillett 2003). Salmon et al. (1988) suggested the formation of diagenetic pyrrhotite due to the oxidation of pre-existing pyrite. Thermochemical sulphate reduction (TSR) can also produce pyrrhotite (Pierce et al. 1998) but there is no evidence (e.g. sulphur-rich hydrocarbons, sphalerite, barite) that it occurred in these rocks. Hydrocarbon migration (Machel & Burton 1991) and gas hydrates (Housen & Musgrave 1996; Larrasoaña et al. 2007) are also ruled out as a chemical remagnetization mechanism due to being mostly absent from the rocks. The release of trapped pore fluid (Urbat et al. 2000) is a possibility but, due to lack of evidence, cannot be proven or rejected. As for the magnetite CRM, externally derived fluids can also be a possibility for pyrrhotite CRM.
The results of the vein contact tests at Hancock Summit do not suggest a connection between the veins and the CRMs. It is however unclear why some of the sites around the veins contain scattered directions.
The Alamo Breccia has been completely remagnetized by two CRMs: a syn-tilting IT CRM residing in pyrrhotite acquired in the Cretaceous Period; and a pre-tilting HT Late Palaeozoic Era CRM residing in magnetite. The breccia is not interpreted as a conduit for focused fluids. Both CRMs are found within the breccia and beyond the inferred contact zone in the surrounding rocks. Results from the vein contact tests at Hancock Summit indicate that the veins did not influence the CRMs in the surrounding rocks.
There are a number of possibilities for the origin of the CRMs. The most likely mechanism for imparting the CRMs is interpreted as external fluids based on the elevated 87Sr/86Sr values, which suggest that the rocks have been altered by fluids with a radiogenic signature. Other origins of CRM impartment cannot be ruled out however, such as clay diagenesis causing the CRM in magnetite or the pyrrhotite CRM forming due to the reaction of pre-existing magnetite with pyrite.
The results of this study suggest that the Alamo breccia was not a conduit for focused flow of remagnetizing fluids, but that the fluids moved pervasively through the rocks. This is surprising considering other studies that indicate that such fluids were localized in conduits (e.g. Elmore et al. 1993, 1998; Elmore 2001; Blumstein et al. 2005).
We thank M. Elwood Madden, K. Keranen and two anonymous reviewers for comments on the manuscript. We also thank the AAPG Grants-in-Aid for awarding the Jon R. Withrow grant to SCE. We are also grateful to J. Warme, J. Pannalal and P. Kincaid for their help in the field, and D. Dennie and S. Dulin for their previous work on the Alamo Breccia.
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text:
Суднова́ надбудо́ва () — конструкція, утворена продовженням бортових перекриттів основного корпусу судна, палубою та поперечними перегородками, що знаходиться вище від верхньої палуби.

Палубну конструкцію, що розташовується на відстані до борта більшій ніж 4 % ширини судна, називають рубкою.

Класифікація надбудов
Надбудови поділяються на:
 залежно від протяжності:
 суцільні;
 розділені;
 залежно від висоти:
 одноярусні;
 багатоярусні;
 за місцем розташування:
 носова (бак);
 середня;
 проміжна (зміщена у корму приблизно на ¾ довжини судна);
 кормова (ют);
 а також комбінації вищеперелічених (середня надбудова зливається з носовою або кормовою):
 подовжений бак (не менший від 25 % довжини судна);
 подовжений ют;
 за функційним призначенням:
 житлова (зазвичай багатоярусна; служить для розташування житлових, побутових та службових приміщень);
 нежитлова (наприклад, високий бак, що призначений для підвищення морехідних характеристик судна).

Архітектурно-конструктивні типи суден
Кількість, розташування й форма надбудов визначають архітектурно-конструктивний тип суден, які бувають одно-, дво- і триострівні. Найбільшого поширення мають два останніх типи. Триострівні судна мають бак, ют і середню надбудову. Двоострівне судно, зазвичай, має одноярусну носову надбудову (бак) і багатоярусну кормову (ют); триострівне — одноярусні бак і ют і багатоярусну середню надбудову. Окремі надбудови можуть зливатися, при цьому кажуть, що судно має подовжений бак чи ют. Якщо надбудова поширюється на всю довжину судна, його називають шельтердековим. У таких суден головною є не верхня палуба надбудов, а друга, до якої доходять водонепроникні перебірки.

Бак і ют, зазвичай є одноярусними, їх обриси є продовженням обрисів корпусу. Вони у першу чергу призначені для захисту палуби від заливання хвилями, що підвищує мореплавність судна.

Середня надбудова може служити для захисту від хвиль сходів і світлових люків машинного відділення на суднах із середнім розташуванням машинного відділення.

Судно, яке не має бака і юта, називають гладкопалубним.

Судна з надбудовами, що відступають одна від одної не більше ніж на 30 % довжини судна, називають колодязними.

Надбудови збільшують запас плавучості й покращують морехідні характеристики судна. Вони мають водонепроникні двері, ілюмінатори і люки.

Див. також
 Бак (морський термін)
 Рубка (елемент судна)
 Ют

Примітки

Джерела

 Особливості проектування морських транспортних суден: Навчальний посібник / О. В. Бондаренко, О. І. Кротов, Л. О. Матвєєв, С. О. Прокудін. — Миколаїв: УДМТУ, 2003. — Ч. 1. — 72 с.
 Барабанов Н. В. Конструкция корпуса морских судов. Л.: Судостроение, 1981. — 259 с.

Деталі кораблів

redpajama_set_name: RedPajamaWikipedia


text:
Bitcoin Alternatives Galore, but What's the Difference?
By Zach Schonfeld On 2/21/14 at 2:20 PM EST
Kimcoin, a crypto-currency based around Kim Kardashian, hopes to follow the dog-themed Dogecoin to success ZimoNitrome/YouTube
Tech & Science Bitcoin
There isn't a BuzzFeed quiz to determine which cryptocurrency you are. But at the rate Bitcoin alternatives have been flooding the Internet in recent weeks and months, it can't be long, can it?
The newest cryptocurrency, inspired by the aborted Coinye, a Kanye West themed-currency, is Kim Coindashian, or "Kimcoin," a digital currency pegged around — yes — Kim Kardashian. It's so new it doesn't entirely exist yet, but its developers swear they're not joking. And they are convinced the reality TV star will take to the cryptocurrency treatment better than her fiancé, who shut Coinye down faster than you could say "My Beautiful Dark Twisted Currency." Can this one be different?
"When 'Kanye-coin' came out, I thought it was a huge business opportunity and he actually should have been flattered," said Kimcoin co-developer Azeem, who works at a start-up in Boston and asked that his last name not be identified. "But they were really trying to provoke him. How we're trying to approach it is being very earnest and saying, 'This is a legitimate business opportunity and we'd love to sit down with you to talk about it.'"
Maybe it doesn't seem like a "legitimate business opportunity," considering that of thousands of potential celebrity targets, Azeem went after the one celebrity who's closest to the only celebrity who has already filed a cease and desist notice regarding all of this cryptocurrency mumbo-jumbo. But like most "legitimate business opportunities" — most of the fun ones, at least — it started as a joke. Then, several weeks ago, Azeem and his partner spotted a window of opportunity and began acting on it. Now they're filing trademarks and looking for a software engineer.
"It could serve the interest of a lot of people — the cryptocurrency people on one side and obviously Kim Kardashian and her brand on the other," he told Newsweek. "We're essentially bringing two sides together who would not interact otherwise."
Not that it would be the strangest digital currency floating around in 2014 — or the first to draw success from only semi-serious beginnings. Dogecoin, which fulfills some ancient Mayan prophecy by forging an unholy alliance between an Internet meme and a usable currency, has seized the Internet's ever-flickering attention as of late, surging past alternatives like Namecoin (which has been around since the Dark Ages of 2011), Litecoin (which is the most valuable cryptocurrency after Bitcoin), and Florincoin (which purportedly lets users attach messages to their transactions) in the process.
Dogecoin, of course, has something none of its competitors has. It has dogs.
Dogs that peer out from its logo, faces lovingly splattered with Doge's trademark Comic Sans proclamations. Dogs — real, live Shiba Inus — who reportedly pop up at Manhattan meetups and party alongside Doge-costumed enthusiasts when they take a break from manning their digital wallets or moderating subreddits. Dogs who, thank Basegod, don't file cease and desist requests. Not yet, at least.
Still, it should surprise no one that this mutant form of canine-inspired tender also started as a joke. Or so says Ben Doernberg, an involved citizen of the Dogecoin Foundation who also, yes, moderates the Dogecoin subreddit and claims to be "monitoring a future based on blockchain technology," referring to the public database of Bitcoin ownership and transactions.
The hazy origin story goes like this: there was "a guy named Jackson Palmer in Australia" who reached out to "a guy from Portland who goes by the name Billy Markus," who programmed the coin, and "quickly, this huge community arose around the coin and it started to actually be used."
And somehow, strangely, it has evolved into the second most popular cryptocurrency in terms of dollars being transacted. And it's particularly useful for tipping, Doernberg said. Not at Olive Garden, but online: "If you're a content creator and you make a video or a GIF that people like, they can send you a hundred doge or a thousand doge as a way to say, 'Nice GIF.' [Or] they can send them if someone is particularly helpful on an Internet forum."
But its higher purpose, if it has such a thing, is to lighten the stodgy Bitcoin mood a bit. "Bitcoin is something that for most people involved is extremely serious," Doernberg said. "The founding of Bitcoin is sort of a combination of a technological and economic project meant to have geopolitical implications. I think they just thought it'd be really funny to take something so serious and put a fun, goofy spin on it."
"[But] it became clear very quickly that there are a lot of people sort of interested in that," he added.
Meanwhile, roughly 160 miles southwest of Dogecoin's New York hub, in the middle of Pennsylvania's Amish Country, a registered nurse named Joe White runs the largest continual mining pool for FreiCoin, a digital currency whose developers include a former NASA rocket scientist named Mark Freidenbach.
Founded in late 2012, FreiCoin makes use of the same protocol rules Bitcoin uses, but White says it's meant to complement rather than contend with Bitcoin, "to encourage daily trading and local economic growth." Last spring, White quit his day job to work with FreiCoin full time.
Lately, he claims, interest has been picking up. Sort of.
"In recent days, FreiCoin has seen a rise in valuation and is seeing its increase back to the level it was earlier this year," White, who has christened himself the "Pool Op," or "pool operator," told Newsweek. "It's currently about 0.04 USD, but in my knowledge of cryptos I see the value being more like 1:1 with the USD."
He also explained that a minor subsidy — or demurrage charge, paid by all FreiCoin holders — keeps transaction fees as low as possible. That's another advantage over Bitcoin: "With Bitcoin it'd cost maybe a dollar or two to send a grand. I saw one [FreiCoin] transaction and it was about an eighth of a cent for something like 10 or 20 thousand U.S. dollars."
Still, in a part of Pennsylvania where White's neighbors are more likely to be found churning butter than mining Bitcoin, yet another digital currency with a harder-to-pronounce name is a tough sell. But White expresses no regret about throwing himself into it wholesale.
"I probably spend about 80 to 90 percent of my life keeping up with the security and code," he said. "And I left my good-paying job to come do this. I've been really living frugally to get this all going."
Bitcoin of the Realm

redpajama_set_name: RedPajamaCommonCrawl


text:
Lee: The Gender Politics of Ballroom Dancing
Abby Lee November 8, 2018
For our first Yale ballroom team lesson, we were divided into two groups.
"Guys and girls, separate out," one of the student instructors told us.
"You mean leaders and followers," another one reprimanded gently.
"Yeah, yeah, sorry. Leaders and followers."
This marked the beginning of the delicate process of navigating the gendered dichotomy of ballroom dancing in a time and at a school where gender is no longer considered a binary.
Ballroom dancing, particularly at the upper levels, isn't merely about the motions of the body or the placement of the feet: it's about telling a story through movement, about a narrative in which the partnership between two people is more important than the individual, and the partnership with the music is most important of all.
Unlike contemporary dance, ballroom relies heavily on tradition and history. At the class, we learn that the dances were traditionally employed as devices of hierarchies, some through as acts of courtship, others through displays of status. Part of appreciating the history of ballroom dancing is learning to grapple with its history as well.
Men lead. Women follow. This is the model that has been passed down for centuries through almost all forms of communication and media. Ballroom perpetuates these roles, particularly with smooth and standard dances such as waltz or foxtrot. Younger instructors handle the question of leading and following with more poise, emphasizing the partnership and the balance of the dancing relationship. Older instructors dispense with a lot of this tact and caution. "Be a man," they tell the newbie leaders (who eventually, over the course of several months, have filtered out to be all male students). "Man up! Be strong and be firm."
There is a lot of wincing and laughter when the instructors say something like that. Laughter because she encourages the leaders to beat on their chests like gorillas when repeating the refrain. Winces because it ascribes to several stereotypes that we are used to fighting against. Men as the bolder sex, the natural leaders, the ones who seize control.
Outside these cases, however, the gender binary of ballroom is beginning to break down. There are a few of pairs of women leaders and followers on the Yale team, and competitions frequently feature couples like this. The trend of female-female pairs reflects positive progress in the pop culture trend which encourages women to seize traditionally male roles.
However, at the competition we attended in early October, I was surprised to see that there were zero pairs of male leader and male follower. Women can step up and take masculine roles, but men still cannot take on the typically feminine roles. It's humiliating to cede power once you have it. Because of this, no one even mentions the lack of female leader and male follower couples. Even if the women are willing to lead, the men aren't willing to follow.
This imbalance of couples reflects a deeper failing of American gender politics and is certainly not unique to ballroom dancing. How can we claim equality when traditionally feminine roles or powers are still considered embarrassing or even shameful? We aren't making progress towards making the roles of leader and follower truly neutral or outside the gender binary: we are just encouraging women to step up and be leaders.
Clinton can wear pantsuits and run for president, but men in politics must still go to extreme lengths to prove their masculinity, whether through threatening their daughter's boyfriend in a campaign ad, implying that the manhood of another candidate is lacking, or equating issues like the right to bear arms with strength (ergo manliness). Women technically can be (almost) all things, even if there is a tinge of shame or contempt surrounding typically female roles. But ultimately, men can only be men, as a result of both the failure of American society to value traditionally female roles and the reek of homophobia that still permeates our culture. It's demeaning to follow in a dance, but just as bad: it's gay.
I'm not attributing these problems to ballroom dancing or claiming that traditional dances create sexism. Ballroom, like so many other forms of art, is learning to come to terms with its traditional patriarchal gender roles while preserving the beauty of its history. It's progressing in some ways, and failing in others- just like literature or film or music or visual art.
Artistic expression both reflects and creates our ideas about issues including gender, power, and hierarchies. We use art as a force of change and a force of reflection, sometimes at the same time. But most days, we just use art as a way to express ourselves. Sometimes, we challenge the patriarchy and fight the stereotypes and strive for progress. Sometimes we just want to dance.
Familiar Avalos: AMLO, Trump, and the Caravan


Having first discovered G. Love's The Hustle as a spry 17 year old high school sophomore, it's safe to say I was pleased to see he came out with his latest release Fixin' To Die. The title track has that old school distinctive dirty G. Love sound, which is a good thing. I found my self tapping my foot and really getting into the rhythm of the song.
As I continued to listen to the album I continued tapping my foot as he changes tempo from an upbeat Get Goin to the slower paced Just Fine. Throughout the album he keeps the Blues/Soul sound going with his funky harmonica solos. Overall I enjoyed a fresh sound of an old favorite of mine and would recommend it to long time fans as well as those who have yet to discover G. Love.
In light to the fact that this blog tries to focus on the ability to find free music for one and all, I felt it necessary to include that by signing up for G. Love's mailing list you can get loads of free stuff. Multiple live recordings as well as access to video blogs are available free of charge at http://philadelphonic.com/free. Hope you enjoy it as much as I did.
After reading this post by Jordan, I went to the site linked above and signed up for G.Love's mailing list. After that I had access to 50 FREE live recorded songs. Great deal if you are a G. Love fan.


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