1,721,005 research outputs found
Preliminary results on the structure of ocean crust from new Holes drilled in fast-spread crust during ODP Leg 206
No full textStructure of the Transition Zone in the oceanic crust from the Cocos Plate (Pacific Ocean)
No full textUnusual microstructural features of a subaqueous lava from basaltic crust off the East Pacific Rise (ODP Leg 206, Cocos Plate)
No full textMicrostructural features of a subaqueous lava from basaltic crust off the East Pacific Rise (ODP Site 1256, Cocos Plate)
No full textThis work examines a massive basaltic lava emplaced in a subaqueous environment and drilled at ODP Site 1256. Site 1256 was drilled on the eastern flank of the East Pacific Rise during ODP Leg 206 (6°44N, 91°56W; Guatemala Basin), located in 15 Ma old oceanic crust created by superfast seafloor spreading (ca. 220 mm/yr). The massive lava lies between thin sheet flows and caps pillow lavas, sheet flows with minor hyaloclastite, breccia and dikes. The massive basalt was encountered in two holes, 1256C and 1256D, which are 30 m apart, and has a thickness of 35 m in Hole 1256C and 75 m in Hole 1256D. It can be interpreted as a ponded lava, originated by rapidly erupted lava accumulated in a off axis >3-5 km depression of a steep paleotopography (Teagle et al., 2004).
The Hole 1256 lava pond has been divided into distinct petrographic units and structural units. Five main structural units with different key flow-related textures and syn-magmatic or late magmatic structures were recognized. Ductile and brittle-ductile structures attributed to the flow gives constraints about the emplacement mechanism of the lava and possibly seafloor topography. Unusual textural features related to the flow kinematics were recognized mainly in the top and in the bottom parts of the lava pond, whereas brittle-ductile and brittle deformations occur throughout the whole ponded body. Microstructures in the base may be interpreted as flow-related deformation of hot ductile coalesced spatter clasts erupted during the first stages of emplacement. Alternatively, they
may have been formed during lava drain-back, in the final emplacement stages
ODP-IODP Site 1256 (East Pacific Rise): an in-situ section of upper oceanic crust formed at a superfast spreading rate
No full textThe igneous rocks cored in Holes 1256C and 1256D during Ocean Drilling Program (ODP) Leg 206 and Integrated Ocean Drilling Project (IODP) Expedition 309 are dominated by thin (10's of cm to 3 m) basaltic sheet flows separated by chilled margins, with several massive flows (> 3 m thick), minor pillow basalt and hyaloclastites, and rare small dikes. In Hole 1256D, the percentage of massive flows decreases downhole. One notable feature of both holes cored is the presence of a very thick massive lava (̃35 m thick in Hole 1256C and ̃75 m thick in Hole 1256D) near the top of each hole. The structural aspect of this thick massive unit are here described. It has been interpreted as a lava pond emplaced in a topographic depression. Although the ocean crust drilled at Hole 1256C and Hole 1256D partly fits the "Penrose" model for showing the superposition of volcanics on sheeted dikes and intrusives, the relative thickness of the lava-dike sequence could reflect a combination of two or more of the following processes: local (spatial) heterogeneity along ridge-axis, (temporal) variability, essentially linked to a more or less intense activity of the magma chamber, and off-axis eruptions
Effect of tectonism and magmatic activity on the structure of upper oceanic crust at a superfast spreading rate
No full textCore-Log Integration Approach for characterizing a Shallow Basement Section of the East Pacific Rise
No full textODP Leg 206 and IODP Expeditions 309/312 drilled a complete intact in situ section of the upper oceanic crust formed in a super-fast spreading ridge at Site 1256 in the Guatemala Basin, Pacific Ocean. It starts from a sedimentary cover (~250m thick) followed by an extrusive lavas section that transitions into a sheeted dikes complex and the uppermost plutonic rocks. The shallow igneous basement is characterized by a single cooling unit of basalt, which has been interpreted as a ponded lava flow, commonly referred as the Lava Pond. The high core recovery within the Lava Pond (~93%) has allowed for good correlations between core and downhole logging data providing an understanding of the relationships between local and regional tectonic. Electrical resistivity, photoelectric factor, density, gamma ray, microresistivity, and ultrasonic downhole measurements were used to identify 4 main logging units and 15 subunits within the Lava Pond. These logging units were matched to variations in core-based measurements of magnetic susceptibility, chemistry, and grain size that provide a perspective into the magmatic and tectonic history of the shallow basement in this area. In particular, borehole microresistivity and ultrasonic images provided detailed analyses of shallow, intermediate, and steeply dipping structural features that allowed reorientation of several core pieces. Approximately 800 structural features were identified within the Lava Pond from microresistivity and ultrasonic borehole images. More than 200 structures consisting of veins, shear veins, microfaults, late magmatic veins, and joints were matched to the downhole logs allowing for reorientation of specific core pieces. Overall, changes in mineralogical composition and variations in fracture density provide the basis for the downhole characterization of the Lava Pond. A kinematic analysis using slicken fibers measured on shear veins and microfaults as indicators of movement provide a better understanding of the near field stress environment at this site
Pattern of tectonic stress in the Cocos-Nazca plate system: data from structures measured on cores and borehole electrical-acoustic images (ODP Hole 1256D, Leg 206)
No full textCockade-textured cataclasite and silica gel from damage zone in carbonated ultramafics: markers of cycles of seismic activity?
No full textShallow crustal processes occurring during seismic slips and generating fracture networks are of great interest due to their complex interplay with a spectrum of other geological processes .
Our study focuses on faults with peculiar core textures, similar to those of "cockade breccia" (Genna et al., 1996) and "clast cortex grains" (Rempe et al., 2014), and on their relation with syntectonic hydrothermal alteration linked with Au bearing-quartz and chalcedony veins.
Our work aims to study the enviromental conditions for the formation of such peculiar texture, their relation with the hydrothermal vein system and their potential as shallow seismic indicators.
We present field, microstructural and petrochemical data of a peculiar damage zone of fault rocks located in carbonated peridotites and serpentinites of the Ligurian Alps (Voltri Massif, Italy). These are mainly reverse faults, which are coeval with syntectonic Au-bearing quartz veins and chalcedony veins (Giorza et al., 2010), in which lherzolites occupy the hangingwall of the faults and serpentinites the footwall. The fault rocks show evidence for carbonation, as olivine and serpentine are clearly transformed into an assemblage made of magnesite, dolomite and minor ankerite. The damage zones of the faults are serpentinite-rich and about 10 m in thickness, while the cataclasite cores are carbonate-rich and ca. 1 m thick. The top of the fault core shows the occurrence of a chalcedony shear veins with chatter marks and slikenlines on the surface. The "cockade breccia" is made of spherical aggregates of Fe-Mg carbonates and are 1 mm to 3 cm in size. These aggregates show cores of microcrystalline Fe-Mg carbonates, and concentric outer layers of relatively coarser Fe-Mg carbonates with radial or laminated texture. In some cases, these aggregates show evidence for rotation along secondary slip zones.
We interpret all these features as the products of chemical interaction between the olivine and serpentine initially present within the fault rocks and the hydrothermal fluid that flowed within these faults. These interactions were probably similar to those occurring within the coeval Au- and chalcedony veins.
Field evidence and theoretical considerations indicate that the reverse faults could have experienced stages of fault-valve behaviour (Sibson et al., 1998), which consisted in cycles of fluid pressure build-ups, fault opening, fluid flushing, and mineral precipitation during the seismic failure of the faults. These cycles varied transiently fault permeabilities. During the fluid pressure build up stage the radial coarse grains developed, while during the fluid flushing stage the clast cortex grains developed their laminated texture. The Au-quartz and chalcedony veins could have formed during the stages of fluid pressure drops following fault slip (Sibson et al., 1998, 2004).
References
Capponi G. & Crispini L., 1997, Progressive shear deformation in the metasediments of the Voltri Group (Ligurian Alps, Italy): occurence of structure recording extension parallel to the regional foliation, Boll. Soc. Geol. It., 116, 267-277.
Genna A., Jébrack M., Marcoux E., Milési J.P., 1996, Genesis of cockade breccias in the tectonic evolution of the Cirotan epithermal gold system, West Java, Can. Journ. of Earth Sci., 33, 93-102.
Giorza A., 2010, Late to post-metamorphic hydrotermalism in the Voltri Unit (Lavagnina Lakes Area, NW Alps). Structural-Petrological-Geochemical approach, PhD Thesis, University of Turin.
Rempe M., Smith S. A. F., Ferri F., Mitchell T. M., 2014, Clast-cortex aggregates in experimental and natural calcite-bearing fault zones, Journ. Of Struct. Geol., 68, 142-157.
Sibson R. H., 2004, Controls of maximum fluid overpressure defining conditions for mesozonal mineralization, Journ. of Struct. Geol., 26, 6-7, 1127-1136.
Sibson R. H., Robert F., Poulsen H., 1998, High-angle reverse faults, fluid-pressure cycling, and mesothermal gold-quartz deposits, Geology, 16, 551-555.
Spagnolo C., 2006, Late orogenic tectonics in the eastern sector of the Ligurian Alps, PhD Thesis, Univesity of Genoa
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