Description: Rapidly developing methods of digital acquisition, visualization and analysis allow highly detailed outcrop models to be constructed, and used as analogues to provide quantitative information about sedimentological and structural architectures from reservoir to subseismic scales of observation. Terrestrial laser-scanning (lidar) and high precision Real-Time Kinematic GPS are key survey technologies for data acquisition. 3D visualization facilities are used when analysing the outcrop data. Analysis of laser-scan data involves picking of the point-cloud to derive interpolated stratigraphic and structural surfaces. The resultant data can be used as input for object-based models, or can be cellularized and upscaled for use in grid-based reservoir modelling. Outcrop data can also be used to calibrate numerical models of geological processes such as the development and growth of folds, and the initiation and propagation of fractures.

Description: Published

Description: 87–98

Description: 3.3. Geodinamica e struttura dell'interno della Terra

Description: N/A or not JCR

Description: reserved

Description: A statistical study of flux ropes and traveling compression regions (TCRs) during the Time History of Events and Macroscale Interactions during Substorms second tail season has been performed. A combined total of 135 flux ropes and TCRs in the range GSM X ∼ −14 to −31 RE were identified, many of these occurring in series of two or more events separated by a few tens of seconds. Those occurring within 10 min of each other were combined into aggregated reconnection events. For the purposes of this survey, these are most likely the products of reconnection occurring simultaneously at multiple, closely spaced x-lines as opposed to statistically independent episodes of reconnection. The 135 flux ropes and TCRs were grouped into 87 reconnection events; of these, 28 were moving tailward and 59 were moving Earthward. The average location of the near-Earth x-line determined from statistical analysis of these reconnection events is (XGSM, Y*GSM) = (−30RE, 5RE), where Y* includes a correction for the solar aberration angle. A strong east-west asymmetry is present in the tailward events, with 〉80% being observed at GSM Y* 〉 0. Our results indicate that the Earthward flows are similarly asymmetric in the midtail region, becoming more symmetric inside −18 RE. Superposed epoch analyses indicate that the occurrence of reconnection closer to the Earth, i.e., X 〉 −20 RE, is associated with elevated solar wind velocity and enhanced negative interplanetary magnetic field BZ. Reconnection events taking place closer to the Earth are also far more effective in producing geomagnetic activity, judged by the AL index, than reconnection initiated beyond X ∼ −25 RE.

Description: We present two transpolar arc events where for the first time we are able to analyze changes in field-aligned currents associated with high-latitude transpolar auroral arcs on time scales of a few minutes. This is accomplished through the use of highly accurate multipoint magnetic field measurements provided by the Space Technology 5 mission, which consists of three microsatellites in low-Earth orbit. In the first event we examine measurements of an arc that is part of a highly dynamic auroral pattern, that of a hook-shaped arc. In the second event, a more stable dusk oval-aligned arc is analyzed. These events illustrate the dynamic nature of arc formation and show the usefulness of high-resolution multipoint measurements. Minimum variance analysis is used to determine the appropriateness of the infinite current sheet approximation and to calculate arc alignment angles which are then compared with those estimated from UV images or precipitating particle data.

Description: A detailed Re-Os molybdenite, pyrite, and chalcopyrite geochronology at five shear-hosted Au occurrences (Kenge, Mbenge, Porcupine, Konokono, and Dubwana) in the Lupa goldfield, southwestern Tanzania, is reported in this paper. Au occurrences within the Lupa goldfield share many geologic similarities with the orogenic Au deposit type and are situated within a Paleoproterozoic magmatic arc that intruded the Archean Tanzanian cratonic margin. Pyrite ± chalcopyrite ± molybdenite-bearing fault-fill veins and mylonitic shear zones crosscut granitic host rocks and are associated with the highest Au grades. Re-Os sulfide ages are deemed a suitable proxy to constrain the timing of Au based on the occurrence of Au-bearing minerals as inclusions within pyrite and chalcopyrite, whereas Au-bearing minerals filling pyrite fractures may represent a younger and undated metallogenic event. Molybdenite at Kenge occurs as ultrafine disseminations within fault-fill veins (1953 ± 6 Ma; n = 3) that possess nominally older weighted average Re-Os ages than molybdenite hosted by stylolite-like veins (1937 ± 8 Ma; n = 7). Both sample sets are ca. 70 m.y. older than a weighted average Re-Os pyrite age from the mylonitic shear zones at Kenge and Mbenge (1876 ± 10 Ma; n = 13), which contain fault-fill veins and record the timing of mylonitization. Molybdenite at Porcupine occurs as ultrafine disseminations within quartz veins and mylonitized granite samples (1886 ± 6 Ma; n = 4) that are broadly equivalent in age to weighted average Re-Os ages of molybdenite occurring as stylolite-like veins (1873 ± 5 Ma; n = 6) and pyrite within oblique-extension veins (1894 ± 45 Ma; n = 2). Weighted average Re-Os pyrite model ages at Konokono (1880 ± 14 Ma; n = 9) and Dubwana (1905 ± 25 Ma; n = 2) are also consistent with the ca. 1.88 Ga event observed at Kenge, Mbenge, and Porcupine. Gold occurrences in the Lupa goldfield therefore record a protracted hydrothermal history (1.95–1.87 Ga) comprising at least three temporally distinct hydrothermal events (ca. 1.95, 1.94, and 1.88 Ga), which are each represented in detail by a complex vein history that occurred at a time scale less than the resolution of the Re-Os method. The sampling of broadly contemporaneous sulfides from five shear zones suggest that mylonitic shear zones represented an interconnected network of midcrustal permeable fluid conduits at ca. 1.88 Ga that permitted the transportation and deposition of gold. Comparison between Re-Os sulfide and high-precision U-Pb zircon ages for the granitic host rocks provides unequivocal evidence for sulfidation concomitant with magmatism. However, the range of Re-Os ages argues against an intrusion-related deposit model whereby metallogenic fluids are solely derived from an individual intrusion. The regional ca. 1.88 Ga metallogenic event identified as part of this study occurred concurrently with eclogite facies metamorphism during the Ubendian orogenic cycle and provides one of Earth’s earliest temporal links between subduction zone processes and orogenic Au deposit formation during the Paleoproterozoic.

Description: Au mineralization in the western Lupa goldfield, southwestern Tanzania was associated with transpression and reverse sinistral slip along a network of steeply S dipping shear zones with non-Andersonian geometries. Slip was accommodated by: (1) frictional failure and sliding during emplacement of quartz ± Au-bearing veins; and (2) crystal plasticity and fluid-assisted diffusive mass transfer. The Kenge mineral system is situated along a NW-SE-trending shear zone and is characterized by ≤10-m thick, Au-bearing fault-fill veins hosted by well-developed phyllosilicate-rich mylonites. The broadly contemporaneous Porcupine mineral system is situated along an ENE-WSW– to E–W-trending shear zone, which is characterized by narrow, discontinuous mylonitic shear zone within a silicified and nonfoliated granitoid protolith. Au mineralization at Porcupine occurs within steeply dipping fault-fill and subhorizontal extension/oblique-extension veins. Three-dimensional frictional reactivation theory provides a self-consistent explanation for the different vein styles at Kenge and Porcupine and extends the classic fault valve model to the general case of oblique slip along multiple, arbitrarily oriented shear zones. Analysis of the differential stress required for frictional reactivation suggests the following: (1) the Kenge shear zone was intrinsically weaker than the Porcupine shear zone, consistent with the lack of well-developed mylonites at Porcupine; and (2) frictional reactivation of the Kenge shear zone occurred under suprahydrostatic but sublithostatic pore fluid pressures, whereas frictional reactivation of the Porcupine shear zone occurred under near-lithostatic fluid pressures. We hypothesize that near-lithostatic pore fluid pressures relieved effective normal stresses at grain-grain contacts, helping to preserve intragranular and fracture porosity at the Porcupine orebody. As such, these pore spaces may be important microstructural sites for Au mineralization. Low effective normal stresses can also explain the poorly developed phyllosilicate-rich mylonites and limited degree of shear zone weakening at Porcupine.

Description: Few studies have focused on the geological characterization of exhumed subsurface faults and fractures within continental flood basalt provinces. We present field and microstructural observations of basalt-hosted fractures and faults from the Faroe Islands, NE Atlantic margin. For a given displacement, the thickness of these highly mineralized faults varies by over three orders of magnitude. Fault-zone thickness and displacement data from the Faroe Islands span nearly four orders of magnitude in displacement, but there is no strong positive correlation between fault-zone thickness and displacement. Fault-rock characterization reveals important breccia distinctions, including collapse/infill, crush/wear/abrasion, and implosion breccias, each with a respective increase in sealing potential. Collapse/infill breccias indicate sustained fluid-migration pathways, as they require open, subterranean cavities that are formed faster than mineral precipitation can seal them. Crush/wear/abrasion and implosion breccias record crack-seal behavior during successive slip events. Despite having distinctly different fault-rock assemblages, fault-zone thickness and displacement data from basalt-hosted faults are indistinguishable from comparable data obtained from sediment-hosted faults. This observation suggests that the first-order controls on fault development are the same in layered basalts and sediments, namely, fault surface bifurcation and linkage, asperity removal, and the accommodation of geometrically necessary strains in the wall rocks.