Home About us Contact
|
Home About us Contact | ||
|
|
||
Fault Rocks (fault + rock)
Selected AbstractsUp-temperature flow of surface-derived fluids in the mid-crust: the role of pre-orogenic burial of hydrated fault rocksJOURNAL OF METAMORPHIC GEOLOGY, Issue 5 2006C. CLARK Abstract The Walter-Outalpa shear zone in the southern Curnamona Province of NE South Australia is an example of a shear zone that has undergone intensely focused fluid flow and alteration at mid-crustal depths. Results from this study have demonstrated that the intense deformation and ductile shear zone reactivation, at amphibolite facies conditions of 534 ± 20 °C and 500 ± 82 MPa, that overprint the Proterozoic Willyama Supergroup occurred during the Delamerian Orogeny (c. 500 Ma) (EPMA monazite ages of 501 ± 16 and 491 ± 19 Ma). This is in contrast to the general belief that the majority of basement deformation and alteration in the southern Curnamona Province occurred during the waning stages of the Olarian Orogeny (c. 1610,1580 Ma). These shear zones contain hydrous mineral assemblages that cut wall rocks that have experienced amphibolite facies metamorphism during the Olarian Orogeny. The shear zone rock volumes have much lower ,18O values (as low as 1,) than their unsheared counterparts (7,9,), and calculated fluid ,18O values (5,8,) consistent with a surface-derived fluid source. Hydrous minerals show a decrease in ,D(H2O) from ,14 to ,22,, for minerals outside the shear zones, to ,28 to ,40,, for minerals within the shear zones consistent with a contribution from a meteoric source. It is unclear how near-surface fluids initially under hydrostatic pressure penetrate into the middle crust where fluid pressures approach lithostatic, and where fluid flow is expected to be dominantly upward because of pressure gradients. We propose a mechanism whereby faulting during basin formation associated with the Adelaidean Rift Complex (c. 700 Ma) created broad hydrous zones containing mineral assemblages in equilibrium with surface waters. These panels of fault rock were subsequently buried to depths where the onset of metamorphism begins to dehydrate the fault rock volumes evolving a low ,18O fluid that is channelled through shear zones related to Delamerian Orogenic activity. [source] Role of fluids in the metamorphism of the Alpine Fault Zone, New ZealandJOURNAL OF METAMORPHIC GEOLOGY, Issue 1 2001J. K. Vry Abstract Models of fluid/rock interaction in and adjacent to the Alpine Fault in the Hokitika area, South Island, New Zealand, were investigated using hydrogen and other stable isotope studies, together with field and petrographic observations. All analysed samples from the study area have similar whole-rock ,D values (,DWR = ,56 to ,30,, average = ,45,, n = 20), irrespective of rock type, degree of chloritization, location along the fault, or across-strike distance from the fault in the garnet zone. The green, chlorite-rich fault rocks, which probably formed from Australian Plate precursors, record nearly isothermal fluid/rock interaction with a schist-derived metamorphic fluid at high temperatures near 450,500°C (,D of water in equilibrium with the green fault rocks (,DH2O, green) ,,,18,; ,D of water in equilibrium with the greyschists and greyschist-derived mylonites (,DH2O, grey) , ,19, at 500°C; ,DH2O, green , ,17,; ,DH2O, grey , ,14, at 450°C). There is no indication of an influx of a meteoric or mantle-derived fluid in the Alpine Fault Zone in the study area. The Alpine Fault Zone at the surface shows little evidence of late-stage retrogression or veining, which might be attributed to down-temperature fluid flow. It is probable that prograde metamorphism in the root zone of the Southern Alps releases metamorphic fluids that at some region rise vertically rather than following the trace of the Alpine Fault up to the surface, owing to the combined effects of the fault, the disturbed isotherms under the Southern Alps, and the brittle,ductile transition. Such fluids could mix with meteoric fluids to deposit quartz-rich, possibly gold-bearing veins in the region c. 5,10 km back from the fault trace. These results and interpretations are consistent with interpretations of magnetotelluric data obtained in the South Island GeopHysical Transects (SIGHT) programme. [source] FAULT-RELATED SOLUTION CLEAVAGE IN EXPOSED CARBONATE RESERVOIR ROCKS IN THE SOUTHERN APENNINES, ITALYJOURNAL OF PETROLEUM GEOLOGY, Issue 2 2001A. Billi The deformation associated with a number of kilometre-scale strike-slip fault zones which cut through outcropping carbonate rocks in the Southern Apennines was investigated at regional and outcrop scales. These faults trend roughly east-west and were studied at the Gargano Promontory on the Adriatic Coast (in the Apulian foreland) and in the Matese Mountains, about 120 km to the west (within the Apenninic fold-and-thrust belt). The fault zones are 200,300 m wide and typically comprise a core surrounded by a damage zone. Within fault cores, fault rocks (gouges and cataclasites) typically occur along master slip planes; in damage zones, secondary slip planes and solution cleavage are the most important planar discontinuities. The protolith carbonates surrounding the fault zone at Gargano show little deformation, but they are fractured in the Matese Mountains as a result of an earlier thrust phase. Cleavage surfaces in the damage zone of the studied faults are interpreted to be fault-propagation structures. Our field data indicate that cleavage-fault intersection lines are parallel to the normals of fault slip-vectors. The angle between a fault plane and the associated cleavage was found to be fairly constant (c. 40") at different scales of observation. Finally, the spacing of the solution cleavage surfaces appeared in general to be regular (with a mean of about 22 mm), although it was found to decrease slightly near a fault plane. These results are intended to provide a basis for predicting the architecture of fault zones in buried carbonate reservoirs using seismic reflection and borehole data. [source] Low-Temperature Plasticity of Naturally Deformed Calcite RocksACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 3 2002LIU Junlai Abstract Optical, cathodoluminescence and transmission electron microscope (TEM) analyses were conducted on four groups of calcite fault rocks, a cataclastic limestone, cataclastic coarse-grained marbles from two fault zones, and a fractured mylonite. These fault rocks show similar microstructural characteristics and give clues to similar processes of rock deformation. They are characterized by the structural contrast between macroscopic cataclastic (brittle) and microscopic mylonitic (ductile) microstructures. Intragranular deformation microstructures (i.e. deformation twins, kink bands and microfractures) are well preserved in the deformed grains in clasts or in primary rocks. The matrix materials are of extremely fine grains with diffusive features. Dislocation microstructures for co-existing brittle deformation and crystalline plasticity were revealed using TEM. Tangled dislocations are often preserved at the cores of highly deformed clasts, while dislocation walls form in the transitions to the fine-grained matrix materials and free dislocations, dislocation loops and dislocation dipoles are observed both in the deformed clasts and in the fine-grained matrix materials. Dynamic recrystallization grains from subgrain rotation recrystallization and subsequent grain boundary migration constitute the major parts of the matrix materials. Statistical measurements of densities of free dislocations, grain sizes of subgrains and dynamically recrystallized grains suggest an unsteady state of the rock deformation. Microstructural and cathodoluminescence analyses prove that fluid activity is one of the major parts of faulting processes. Low-temperature plasticity, and thereby induced co-existence of macroscopic brittle and microscopic ductile microstructures are attributed to hydrolytic weakening due to the involvement of fluid phases in deformation and subsequent variation of rock rheology. During hydrolytic weakening, fluid phases, e.g. water, enhance the rate of dislocation slip and climb, and increase the rate of recovery of strain-hardened rocks, which accommodates fracturing. [source] | ||||||||||