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Shear markers and shear sense indicators from the Bhavani shear zone, a member of the shear system separating the northern Archaean and southern Pan African granulite terranes of the South Indian shield, with differently exhumed crustal blocks, suggest multiphase reactivation. This is revealed by textural and geochemical characteristics of mylonites which indicate an event of prograde epidote-amphibolite facies metamorphism of the previously retrograded protolith of shear zone rocks. The amphibole varies progressively from an initial magnesio-hornblende to ferrotschermakitic hornblende with increasing Al and K while the plagioclase maintains oligoclase composition (An21-25). P-T estimates suggest metamorphic conditions of 634°C - 720°C at 3.6±0.5 to 5.8±0.5 kbar. Strain partitioning during the reactivation of the shear zone prior to the Pan African event, is responsible for the gradation in the intensity of mylonitic fabric as well as some of the opposing shear sense indicators. 相似文献
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Michael McCaig 《地球表面变化过程与地形》1983,8(3):239-252
Analysis of hydrographs from a 4·3 hectare stream head catchment indicates that storm runoff is generated from dynamic source areas. The volume and timing of contributions from different parts of the catchment show, when compared with the extent of surface saturation, that pipeflow generated from areas not saturated at the soil surface is a significant component of the quickflow hydrograph. A simple model of pipeflow generation and contribution is discussed in the light of field results. 相似文献
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Andrew McCaig 《Tectonophysics》1986,129(1-4)
A comparison is made between the Gavarnie thrust and the Mérens Fault in the Axial zone of the Pyrenees. The former has a gentle dip and quite a large displacement (at least 12 km) but does not cut through either Hercynian or Alpine isograds. The latter has a smaller displacement (~ 5 km) but dips steeply and cuts through both Hercynian and Alpine isograds at a high angle. On this basis and on the basis of shear zone geometries immediately north of it, it is proposed that the Mérens Fault nucleated as a steeply (65°–80°) dipping structure, while the Gavarnie thrust nucleated with a shallow attitude. The Mérens Fault is not a backward-rotated thrust fault, nor is it the root zone for any major nappe structure. Similar steep ductile structures occur within the Gavarnie nappe and may reflect considerable internal strain in basement lithologies.The relationship between steep and shallow structures is not yet clear; the shear zones may pre-date the thrusting in which case they may be thick-skinned structures affecting the whole lithosphere, or they may be contemporary with thrusting reflecting only local thickening above a décollement.Rheological models can be used to test proposed geometrical and kinematic models for the lithosphere-scale evolution of the Pyrenees. Suggested models are dominated by a cool, rigid, high-level mantle wedge beneath the North Pyrenean zone which probably controlled the location of north-dipping thrust faults. Thick-skinned shortening is possible in thick crust in the Axial zone but is very unlikely in the North Pyrenean zone where steeply rooted structures would have to cut through the strongest part of the lithosphere. 相似文献
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Kongsvegen, a surge‐type glacier in Spitsbergen, Svalbard, shares a tide‐water margin with the glacier Kronebreen. The complex has been in retreat since a surge advance of Kongsvegen around 1948. The surface of Kongsvegen displays suites of deformational structures highlighted by debris‐rich folia. These structures are melting out to form a network of sediment ridges in the grounded terminal area. The structures are also visible in a marginal, 1 km long, 5–20 m high cliff‐face at the terminus. Current models for the evolution of deformational structures at Kongsvegen divide the structures into suites based on their orientation and dip, before assigning a mechanism for genesis based on structure geometry. Interpretation of aerial photographs and field mapping of surface structures suggest that many structures were reorientated or advected during the surge. We suggest that many of the deformational structures highlighted by debris‐rich folia represent reorientated, sediment‐filled crevasses. Some evidence of thrusting is apparent but the process is not as ubiquitous as previously suggested. Many deformational structures also appear to have been offset by more recent structures. Mechanisms of structural development must, therefore, be considered within the context of distinct stages of glacier flow dynamics and multiple surge episodes. Furthermore, evidence for thrusting and folding within the glacier systems of Svalbard has been used as the basis for interpreting Quaternary glacial landforms in the UK. The findings of this paper, therefore, have implications for interpretations of the Quaternary record. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
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A.M. McCaig 《Earth and Planetary Science Letters》1983,63(3):459-473
A high-temperature contact is described between the basal pargasite-bearing spinel-lherzolites of the Bay of Islands ophiolite complex and underlying garnet-granulite facies metagabbros of its dynamothermal aureole. Three distinct high-temperature hydrous assemblages occur in the basal mylonites of the peridotite, and spinel- and garnet-bearing corona textures indicative of increase in pressure under constant or increasing temperature conditions are described for the first time from the uppermost part of the aureole. On the basis of garnet-clinopyroxene geothermometry and garnet-forming reactions in metabasic rocks, P?T conditions of 7–11 kbar, 750–850°C are estimated for rocks on both sides of the contact. Steep inverted gradients in both temperature and pressure of equilibration occur in the aureole, which most likely represents a thinned, overturned and metamorphosed section through an ophiolite sequence. It is proposed that the aureole formed in a low-angle shear zone cutting the oceanic crust and upper mantle.Age data shows that the Bay of Islands Complex was 30–40 Ma old and therefore relatively cold at the time of formation of the aureole. Prolonged (> 1 Ma) shear heating must therefore have occurred at high shear stresses and movement rates (≥ 1 kbar, 10 cm/yr) to produce the high contact temperatures. The displacement surface probably initiated as a discrete fault, evolving into a viscous shear zone with time. Downward movement of the locus of shearing into weaker lithologies and finally thrusting of the ophiolite-aureole complex over cold sediments accounts for the preservation of steep metamorphic gradients in the aureole.The observed pressures at the ophiolite-aureole contact are 3–7 kbar in excess of the expected load pressure from the present thickness of the ophiolite. The cause of the pressure excess was removed before formation of lower-grade parts of the aureole. Possible explanations are tectonic thinning of the ophiolite during displacement or more likely emplacement of nappes on top of the ophiolite before formation of the aureole. A model involving detachment of the ophiolite slice from below a subduction zone can account for the high pressures, rapid uplift and erosion during displacement, and the coincidence of K-Ar ages of amphiboles from the aureole and the sheeted dyke complex of the ophiolite. 相似文献
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Andrew M. McCaig Stephen M. Wickham Hugh P. Taylor Jr 《Contributions to Mineralogy and Petrology》1990,106(1):41-60
A combined field, stable isotope, and whole-rock chemical study was made on late Cretaceous to Tertiary metasomatic shear zones cutting Hercynian gneisses in the Aston Massif, Pyrenees, France. Mylonitisation occurred during the early stages of Alpine compression under retrograde conditions at 400–450°C and about 10 km depth. Whole-rock 18O values of (+11 to +12 in the gneisses) was lowered to +5 to +9 in the shear zones, with the quartz-muscovite 18O/16O fractionations of about 2 to 4 essentially unchanged. These 18O/16O systematics, together with D muscovite=-40 to-50 indicate that large volumes of formation waters or D-rich meteoric waters passed through the shear zones during deformation. The same fluids also redistributed major elements, as shown by the correlation of 18O shift with muscovitisation and albitisation reactions in granitic wall rocks. However, even though 18O was universally lowered within the shear zones, the 18O/16O ratios were not homogenised, nor do they correlate in detail with the presence or absence of muscovitisation, suggesting that fluid flow was probably fracture-controlled and episodic. Field mapping shows that, along the length of a particular shear zone, muscovitisation of granite gneiss dies out 150m above the contact with underlying sillimanite gneiss. Thus, muscovitisation and albitisation of granite gneiss in shear zones and their wall rocks probably occurred during re-equilibration of acidic, chloride-rich, aqueous fluids that had previously moved upward within the shear zones through underlying sillimanite gneiss. Extremely high material-balance fluid-rock ratios (103) are required to explain the extent of muscovitisation along this shear zone, implying integrated fluid mass fluxes of about 108 kg/m2; this is probably close to the maximum value for other shear zones in the network. Similar volumes of a more chemically evolved fluid must have passed through the unmuscovitised mylonites, showing that the absence of alteration cannot necessarily be used to infer low values of fluid flux. For reasonable pressure gradients and time scales of fluid movement, effective permeabilities of 10-15 to 10-17 m2 are required. Such values can be accounted for by short-lived, widely-spaced cracks produced during seismic activity. A model is presented in which formation waters were seismically pumped down an underlying, shallow, southward-dipping decollement and then upward through the steeply-dipping shear zone network.Contribution No 4711: Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA 相似文献
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Fast diffusion along mobile grain boundaries in calcite 总被引：1，自引：0，他引：1
Andrew McCaig Stephen J. Covey-Crump Walid Ben Ismaïl Geoffrey E. Lloyd 《Contributions to Mineralogy and Petrology》2007,153(2):159-175
Experimental measurements of grain boundary diffusion are usually conducted on static boundaries, despite the fact that grain
boundaries deep in the Earth are frequently mobile. In order to explore the possible effect of boundary mobility on grain
boundary diffusion rates we have measured the uptake of 44Ca from a layer of 44Ca-enriched calcite powder during the static recrystallization of a single crystal of calcite at 900°C. A region about 500 μm
wide adjacent to the powder layer is heterogeneously enriched in 44Ca, and complex zoning patterns, including sharp steps in composition and continuous increases and decreases in 44Ca content, are developed. In metamorphic rocks, these would normally be interpreted in terms of changes in pressure or temperature,
Rayleigh fractionation, or episodic fluid infiltration. These explanations cannot apply to our experiments, and instead the
zoning patterns are interpreted as being due to variations in grain boundary migration rate. We have applied an analytical
model which allows the product of grain boundary diffusion coefficient and grain boundary width (D
GB
δ) to be calculated from the grain boundary migration rate and the compositional gradient away from the powder layer. The value
of D
GB
δ in the mobile grain boundaries is at least five orders of magnitude greater than the published value for static boundaries
under the same conditions. In order to allow the scale of chemical equilibrium (and hence textural evolution) to be predicted
under both experimental and geological conditions, we present quantitative diffusion-regime maps for static and mobile boundaries
in calcite, using both published values and our new values for grain boundary diffusion in mobile boundaries. Enhanced diffusion
in mobile boundaries has wide implications for the high temperature rheology of Earth materials, for geochronology, and for
interpretations of the length- and time-scales of chemical mass-transport. Moreover, zones of anomalously high electrical
conductivity in the crust and mantle could be regions undergoing recrystallization such as active shear zones, rather than
regions of anomalous mineralogy, water- or melt-content as is generally suggested. 相似文献
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Seven mylonitic samples and two coarse muscovites from the central Pyrenees have been dated by the 40Ar-39Ar method. Whole rock specimens of mylonite were cut out of thin-section chips allowing complete characterisation of mineralogy and texture. Several specimens showed rising staircase patterns in the range 50–90 Ma, with much higher ages in the highest temperature steps. This is believed to reflect mixing of argon released from micas with excess argon contained in plagioclase and released mainly at high temperatures. One biotite-quartz mylonite gave a plateau age of 93 ± 2 Ma. Other inferred mica ages are about 60–73 Ma for biotite and 50–60 Ma for muscovite; it is probable that biotite contains excess argon and that 50 Ma approximates to the cooling age in the mylonites. One coarse muscovite collected immediately below the major Mérens shear zone gave a Hercynian plateau age, while another collected within the Mérens zone gave a partially reset Hercynian age.Taken together, the data indicate that the shear zones were active in Alpine times < 100 Ma and probably about 50 Ma ago. They are believed to have formed during the early stages of Eocene compression in the Pyrenees. Deformation and resultant uplift probably terminated an important thermal event in this part of the Pyrenean basement, which may have begun at the time of the mid-Cretaceous North Pyrenean metamorphism (90–100 Ma). 相似文献
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