Kieran O'Hara

Fluid flow and volume loss during mylonitization: an origin for phyllonite in an overthrust setting, North Carolina U.S.A.

Abstract In the western Blue Ridge province of the southern Appalachians, North Carolina, granite gneisses are converted to phyllitic mylonites (phyllonites) along the base of the Rector Branch thrust sheet. The mylonites show evidence for fluid infiltration and large volume losses. With decreasing distance from the thrust and local shear zones, feldspars undergo grain-size reduction by cracking followed by chemical breakdown to muscovite, sericite, and zoisite. The following reaction was important during mylonitization: 3 K -feldspar + 2 H + = muscovite + 2 K + + 6 SiO 2 Quartz displays evidence for dynamic recrystallization whereas feldspar deforms brittely which together with the secondary mineral assemblages suggest that deformation occurred at temperatures of between 300° C and 500° C in the presence of a fluid. The mylonites are enriched by a factor of at least 2.5 in elements such as TiO 2 , P 2 O 5 , Zr, Y, V, La, Ce and depleted in SiO 2 in comparison to their protolith gneisses. The immobile element enrichments are attributed to enrichment in residual phases such as ilmenite, zircon, apatite and epidote in the mylonites and are interpreted as due to minimum volume losses of 60%. Mass balance calculations, based on major element chemistry, indicate the mylonites lost large amounts of SiO 2 and smaller amounts of alkalies and were infiltrated by fluid/rock weight ratios of between 10 2 and 10 3 . Phyllonitization as a process involves several deformation mechanisms including cataclasis and solution of feldspar as well as dislocation creep of quartz and micas. Reaction-enhanced softening was apparently important in strain localization.

Volume-loss model for trace-element enrichments in mylonites

Mylonites from three widely separated localities along the Fries ductile deformation zone in the Blue Ridge province show substantial enrichments in trace and minor elements (TiO{sub 2}, P{sub 2}O{sub 5}, Zr, Y, and V) and depletions in Rb and Sr relative to the protolith. Two end-member hypotheses can explain the enrichments: one involves partitioning to and from an infiltrating fluid and assumes that all the elements were mobile. The second hypothesis assumes the high-field-strength cations were largely immobile and that their enrichment is due to large volume losses in the mylonites. Modeling indicates that Ti is immobile for fluid/rock ratios as high as 10{sup 4}, and petrographic and modal data support a volume-loss mechanism. Volume loss was accommodated by loss of SiO{sub 2} and alkalies during feldspar dissolution. Calculated fluid/rock volume ratios required to produce the observed SiO{sub 2} losses (assuming Ti mobility) range from 70 to 600. Variable enrichments in P, Zr, Y, and V are attributed to selective mobility of these elements during fluid infiltration.

State of strain in mylonites from the western Blue Ridge province, southern Appalachians: the role of volume loss

Abstract This study examines the state of finite strain in quartzo-feldspathic mylonites from two localities in the western Blue Ridge province of the southern Appalachians. Mineral shape fabric and whole-rock chemical and modal data suggest that the mylonites deformed by inhomogeneous shortening normal to the foliation and progressive shear parallel to the foliation. The deformation involved substantial bulk-rock volume loss. Strain was partitioned between solution transfer processes in feldspar and dislocation creep processes in quartz and micas at temperatures of between 300 and 450°C. Initially feldspars underwent grain-size reduction by cataclasis but at advanced stages of mylonitization alkali feldspar grains display subrounded oblate shapes (0 k k values (0.2 k 60%) bulk volume loss. Density changes were minor. On the basis of feldspar grain shapes and modal and whole-rock chemical data, volume loss was largely accommodated by fluid infiltration and incongruent dissolution of alkali feldspar (producing muscovite) and involved the loss of alkalis and silica to the fluid phase. The mylonites may have nucleated as solution zones which subsequently underwent strain softening and displacement parallel to the zone boundaries.

Major- and trace-element constraints on the petrogenesis of a fault-related pseudotachylyte, western Blue Ridge province, North Carolina

A newly identified 6 cm thick, zoned pseudotachylyte from the western Blue Ridge province of North Carolina is interpreted to be the result of episodic melting on a reactivated brittle fault, previously active in the ductile regime. Kinematic indicators in cataclasites associated with the pseudotachylyte indicate top-to-the-northeast shear, whereas kinematic indicators in mylonites indicate top-to-the-northwest shear. Higher concentrations in the pseudotachylyte of Al2O3, K2O, FeO and depletion in SiO2 are explained by the total fusion of micas and partial fusion of K-feldspar (≈ 60%), leaving quartz and refractory phases as residual. Enrichment of the pseudotachylytes in Ba2+, Rb+, Sr2+ and Eu2+ provide additional evidence for the preferential melting of K-feldspar. The trace element signature of the pseudotachylytes (similar REE patterns, Ba/K, K/Rb, Ba/Rb, Zr/Hf, Ti/Cr ratios) and entrained lithoclasts identify the source of the melt as cataclastically deformed mylonite of the hangingwall. The similarity of the REE patterns in the pseudotachylytes and the source rock is explained by the entrainment and/or dissolution of REE-bearing phases in the melt in proportion to its volume. These data indicate that coseismic flash melting is distinctly different from equilibrium partial melting and that trace elements can be used to identify the source rock and distinguish those phases that melt from those that remain residual.

A pseudotachylyte geothermometer

Pseudotachylytes invariably contain a conspicuous concentration of clasts and lithic fragments in their matrices, which reflect brittle wear processes during frictional melting. The ratio of clasts to matrix is interpreted as the ratio of wear to melt (W/M) and measurements of this ratio show a range of 0.1–0.7. Based on models of melting and wear processes it is shown that the ratio W/M is independent of fault displacement, stress, fault area and mineralogy. Thermodynamic considerations indicate that W/M corresponds to the thermodynamic efficiency of the conversion of work to heat and is defined as w/q=(Thigh−Tlow)/Thigh where Thigh refers to the melt temperature and Tlow is the ambient country rock temperature. This provides a simple new technique for estimating the ambient crustal temperature Tcrust, in degrees Kelvin Assuming a reasonable value for the temperature of the melt, estimates of country rock temperatures Tcrust for pseudotachylytes from four different localities in the USA indicate a range of 123–387°C. Eclogite-facies pseudotachylytes from western Norway yield a mean country rock temperature of 658°C, in good agreement with independent Fe–Mg exchange geothermometry. This new geothermometer appears to be applicable throughout the entire crust and may provide a better understanding of melting and wear processes during seismic faulting.

A fluid inclusion study of fluid pressure and salinity variations in the footwall of the Rector Branch thrust, North Carolina, U.S.A

Abstract Last melting and homogenization temperatures of fluid inclusions from plastically deformed bedding-parallel quartz veins in the footwall of the Rector Branch thrust, North Carolina, were studied as a function of distance from the thrust. Fluid inclusions and microstructures in mylonitic rocks within the thrust zone were also examined. Fluid inclusions in quartz veins which display evidence for intracrystalline plasticity (e.g. subgrain polygonization) occur along subgrain boundaries and have higher homogenization temperatures (Tn) and a wider range (120–320°C) compared to less deformed samples. Maximum Th values, which approach the temperature of deformation (300 ± 20°C), apparently reflect leakage of inclusions along subgrain boundaries. Minimum Th values (120–160°C), on the other hand, record near lithostatic conditions (2.6 kb) at 300°C. Maximum last melting temperatures (Tm) increase from −20 to −4°C with decreasing distance to the thrust, corresponding to a decrease in salinity of the fluid from 23 to 3 wt% (NaCl equivalent). The decrease in salinity towards the fault is interpreted as due to infiltration of the fault at depth (to approximately 10 km) by surface derived waters during periods of fault zone dilatancy. Inclusions along healed microcracks in quartz from the fault zone display higher salinity (17–26 wt% NaCl equiv.) and are interpreted to reflect enhanced fluid-rock interaction in the fault zone due to hydration reactions. The fluid pressure and salinity variations are consistent with a combined dilatancy-hydraulic fracturing model for the Rector Branch thrust. Previously documented bulk rock volume losses for this fault zone are inferred to have been produced by the fluxing of the fault zone with undersaturated surface derived fluids.

The Effect of Deformation on Oxygen Isotope Exchange in Quartz and Feldspar and the Significance of Isotopic Temperatures in Mylonites

Oxygen isotopic compositions of quartz and feldspar in greenschist‐grade mylonites from the Blue Ridge thrust and the Brevard zone in the southern Appalachians were analyzed by laser microprobe to examine the effect of deformation on isotopic behavior. In mylonites, texturally homogeneous polycrystalline quartz ribbons have a constant isotopic composition (δ18O = 12.9 ± 0.0‰, n = 3), whereas monocrystalline quartz ribbons, which display heterogeneous intercrystalline strain and only minor recrystallization, have variable δ18O values (11.6 ± 0.5‰, n = 5). Alkali feldspars in samples that contain fluid inclusion‐decorated microcracks, reflecting heterogeneous deformation, show a range in isotopic composition (8.8 to 10.2; mean = 9.4 ± 0.7‰, n = 3). In contrast, recrystallized myrmekite rims surrounding alkali feldspar augen in Brevard zone mylonites are isotopically heavier by about 1% (9.2 ± 0.1‰, n = 5) compared to the cores 8.3 ± 0.3‰, n = 4), reflecting isotopic homogenization during neocrystallization. Deformation mechanisms that result in heterogeneous strain on the grain scale (either crystal plastic or brittle) are associated with only partial isotopic homogenization, whereas deformation mechanisms that result in homogeneous strain (e.g., recrystallization, neocrystallization) are associated with isotopic homogenization on the grain scale. Agreement between measured quartz‐feldspar isotopic temperatures and calculated temperatures using a finite difference model indicates diffu‐sional exchange occurred between phases during closed system cooling, and that the measured temperatures in the mylonites are maximum temperatures for the deformation. The approximate agreement between measured temperatures in some mylonites and the calculated Dodson quartz closure temperatures indicates that isotopic exchange below Tc quartz was not substantial. The necessary conditions under which isotopic temperatures in mylonites correspond to the deformation temperature are outlined. On the basis of this study and reconsideration of older data, the onset of total dynamic recrystallization in quartz is estimated to be about 350°C in natural shear zones. Together with reaction weakening of feldspar observed in the mylonites, the temperature interval 350‐400°C is likely to be important for weakening of both quartz and feldspar in the continental crust.

Fluid-rock interaction in crustal shear zones: A directed percolation approach

The interplay among mechnical, chemical, hydrological, and thermal processes in the evolution of crustal shear zones makes an analytical approach to their study difficult. As an alternative, a stochastic description (using percolation theory) is used to gain insight into reaction softening and volume loss in ductfle deformation zones. Directed percolation is preferred to ordinary percolation as a model because, in common with natural shear zones, directed percolation clusters have high length/width ratios and anisotropic permeability. In addition, transport along the cluster length (for p > p c is linear with time (modeling fluid advection). The process of strain softening is modeled by subjecting a critical cluster to variable amounts of simple shear, resulting in a geometry similar to that of natural shear zones. A stochastic model for the evolution of porosity with time on a directed lattice displays fixed-point behavior for a range of pore-collapse rates. The observation that natural shear zones from a variety of tectonic settings display mean volume losses of 60%-70%; suggests that the system naturally evolves toward a critical state. This can be explained by assuming that pore collapse is regulated by the tendency for fluid pressure to remain close to lithostatic. Volume loss in crustal shear zones appears to be an example of hydromechanical-chemical self-organized critical behavior.

Regional δ18O gradients and fluid-rock interaction in the Altay accretionary complex, northwest China

The δ 18 O values in syntectonic gold-bearing quartz veins hosted by schist, mylonite, gneiss, and migmatite from the Altay accretionary prism, Xinjiang province, northwest China, range from 3.4‰–17.8‰ along a 20 km traverse. Plots of δ 18 O values in veins hosted by schist across several thrust sheets display a sawtooth pattern in which isotopic values decrease with increasing metamorphic grade across each thrust sheet and show abrupt increases of 5.3‰ to 6.8‰ across faults in the hanging wall. Models in which the host schist or clays buffer the isotopic composition of the veins provide a poor fit to the data. On the other hand, a fluid-buffered model with δ 18 O fluid = 0 to +2‰ (standard mean ocean water, SMOW) provides a good fit to the data over a 5–10 km depth interval (150–300 °C). Posttectonic normal reactivation of the thrusts with offsets averaging 5 km explains the abrupt increase in δ 18 O across the faults in the hanging wall. A model in which seawater is tectonically ingested into the accretionary prism to a depth of 5 to 10 km, followed by hydraulic fracturing and advection of hot fluid back to the sea floor along high-permeability pathways, is presented.

Constraints on the Emplacement and Uplift History of the Pine Mountain Thrust Sheet, Eastern Kentucky: Evidence from Coal Rank Trends

In this paper coal rank trends on both sides of the Pine Mountain thrust in eastern Kentucky are used to place constraints on thrust evolution. Vitrinite reflectance (\documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage{wasysym} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document}