Andrew M. McCaig

Direct geological evidence for oceanic detachment faulting: The Mid-Atlantic Ridge, 15°45′N

From a detailed survey and sampling study of corrugated massifs north of the Fifteen-Twenty Fracture Zone on the Mid-Atlantic Ridge, we demonstrate that their surfaces are low-angle detachment fault planes, as proposed but not previously verified. Spreading-direction–parallel striations on the massifs occur at wavelengths from kilometers to centimeters. Oriented drill-core samples from the striated surfaces are dominated by fault rocks with low-angle shear planes and highly deformed greenschist facies assemblages that include talc, chlorite, tremolite, and serpentine. Deformation was very localized and occurred in the brittle regime; no evidence is seen for ductile deformation of the footwall. Synkinematic emplacement of diabase dikes into the fault zone from an immediately subjacent gabbro pluton implies that the detachment must have been active as a low-angle fault surface at very shallow levels directly beneath the ridge axis. Strain localization occurred in response to the weakening of a range of hydrous secondary minerals at a very early stage and was highly efficient.

Deep fluid circulation in fault zones

Massive fluid circulation in retrogressive ductile shear zones is a well-established but poorly understood phenomenon. In some cases, surface-derived fluids, which must initially have been at hydrostatic pressures, can be shown to have entered shear zones in which fluids would normally be expected to be at lithostatic pressure. In these circumstances, thermal convection is an unlikely driving force for fluid movement. Underthrusting of a surficial fluid reservoir beneath shear zones is a viable mechanism in some cases, but not for metasomatic shear zones in the Pyrenees where insufficient underthrusting has occurred. Seismic pumping provides an alternative mechanism to explain the paradox of fluid access into ductile shear zones; a kinematic analysis of a hypothetical fault zone shows that stress and dilatancy cycles will be out of phase above and below the frictional/quasiplastic transition. If this effect is sufficiently large, hydraulic gradients may force fluid downward across the transition immediately after earthquake rupture through highly permeable microcrack networks. Between earthquake cycles, plastic creep in mylonites will overprint microcrack networks, increase fluid pressure, and promote slow upward movement of fluid at low permeability. For smaller shear zones, such as those in the Pyrenees, seismic pumping could occur down a shallow decollement with subsequent upward fluid flow through shear zones in the hanging wall of the decollement.

The chemistry of brines from an Alpine thrust system in the Central Pyrenees: An application of fluid inclusion analysis to the study of fluid behaviour in orogenesis

Abstract Quartz filled veins and fractures which formed late in the Alpine thrusting of the Central Pyrenees contain inclusions of hypersaline Na-Ca-Cl brines with total dissolved salts of up to 25 wt%. The total salinity is similar in all samples, irrespective of the vein or the wall rocks, but there are large variations (particularly in the Na Ca ratio) in the chemistry of the fluids between samples. With one exception, each sample contains only a single dominant fluid population. Crush-leach extraction and chemical analysis of the inclusion electrolytes for Na, K, Ca, Mg, Ba, B, Li, Sr, Rb, Fe, Mn, Zn, Pb, F, Cl, Br, and Sr and Pb isotopes reveals that the fluid chemistry is strongly influenced by the local rocks. Of the four different lithologies in the thrust stack sampled, the Triassic mudstones and Cretaceous limestones or Silurian phyllites acted as sources for the vein fluids during the late thrusting. The composition of the fluid in the veins was dependent on the proximity to these lithologies. For example, fluids from the Trias were dolomite saturated, whilst those close to limestone were calcite saturated. Strontium and lead isotopic analysis of inclusions and host rocks confirm that the more Narich fluids were in equilibrium with Triassic redbeds while Ca-rich fluids have been in isotopic equilibrium with either Cretaceous limestones or Silurian phyllites. Some samples have intermediate compositions due to mixing of the two endmember fluids prior to trapping as inclusions. The similarity of the Br Cl ratio (approximately twice seawater) and the consistent high salinity of all the inclusion fluids in the thrust stack indicate that they were all originally derived from a single source but progressively changed their cation and isotope chemistry through interaction with different host rocks. This ultimate source is likely to have been Triassic connate waters. We conclude that a local increase in permeability occurred when the veins formed and that fluid movement was over short distances. No evidence was found for a significant input of either surface or metamorphic fluids during thrusting.

Deep fluid circulation in alpine shear zones, Pyrenees, France: field and oxygen isotope studies

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 δ18O 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 δD 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 δ18O shift with muscovitisation and albitisation reactions in granitic wall rocks. However, even though δ18O 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.

Fluid mixing and recycling during Pyrenean thrusting: evidence from fluid inclusion halogen ratios

Abstract Syntectonic fluids have been sampled through fluid inclusion microthermometry and crush-leach analyses (cations and halogens) from a 50 km N-S transect through the central-southern Pyrenees. The fluid inclusions are contained in syntectonic quartz veins in Triassic redbeds, Cretaceous carbonates and Hercynian basement rocks, with some calcite and dolomite data from limestones and evaporites in more external parts of the belt. The main datasets come from (1) Alpine shear zones cutting the Neouvielle granodiorite in the Hercynian Axial Zone at the north end of the transect; (2) An imbricate zone beneath the Alpine Gavarnie Thrust at the Pic de Port Vieux; (3) Several localities in the footwall and hangingwall of the Gavarnie Thrust on the southern margin of the Axial Zone. The inclusion fluids generally decrease in salinity from 27–35% at the northern end of the transect to 7–22% on the southern margin of the Axial Zone. The majority of the inclusions have Cl/Br ratios lower than seawater and are interpreted as relict fluids after seawater evaporation and halite precipitation in the upper Trias. This interpretation is supported by Cl-Br-Na systematics, which are consistent with a change from halite to halite + sylvite precipitation with progressive evaporation. Fluids in the basement shear zones are interpreted to have essentially the same evaporitic origin as those still contained in sedimentary formations, although it is possible that final concentration of brines in the Neouvielle Massif involved retrograde hydration reactions with removal of water by precipitation of hydrous minerals. The fluids are also very similar in salinity and halogen chemistry to those found in veins associated with Mesozoic Pb-Zn-F deposits which predate the thrusting. The lower salinities seen at the southern margin of the Axial Zone are interpreted to reflect mixing of the brines with a higher level fluid (connate or meteoric water) circulating within the Mesozoic carbonates of the higher thrust sheets. At one locality where Triassic evaporites are still present, high Cl/Br ratios at relatively low salinities are present in inclusions within the underlying Triassic redbeds, but low Cl/Br ratios at higher salinities are seen lower in the sequence. This is consistent with dissolution of halite by a dilute fluid, but with limited penetration downwards. We suggest that the fluid history of the Pyrenees evolved through a series of stages: (1) Upper Triassic evaporite formation with sinking of brines into underlying redbeds and fractured basement rocks; (2) Circulation of brines with formation of Pb-Zn deposits along faults at some time between the Triassic and the Upper Cretaceous; (3) Renewed extension with erosion of Triassic rocks in many areas and further drawing down of Triassic brines into the basement; (4) Deposition of U. Cretaceous and Palaeocene carbonates containing connate waters of marine origin; (5) Formation of the Pyrenean thrust belt with overpressuring and expulsion of the brines along shear zones and faults; (6) Creation of topography with a high-level circulation system in the Mesozoic thrust sheets driven largely by topography. At the southern margin of the Axial Zone there was limited mixing of the deeper, overpressured brines with these more dilute, hydrostatically pressured fluids. An important point is that because of their density, hypersaline brines are difficult to expel from the upper crust, and may be involved in a succession of alteration and mineralisation events in the same general area over hundreds of millions of years.

Drilling constraints on lithospheric accretion and evolution at Atlantis Massif, Mid‐Atlantic Ridge 30°N

Expeditions 304 and 305 of the Integrated Ocean Drilling Program cored and logged a 1.4 km section of the domal core of Atlantis Massif. Postdrilling research results summarized here constrain the structure and lithology of the Central Dome of this oceanic core complex. The dominantly gabbroic sequence recovered contrasts with predrilling predictions; application of the ground truth in subsequent geophysical processing has produced self-consistent models for the Central Dome. The presence of many thin interfingered petrologic units indicates that the intrusions forming the domal core were emplaced over a minimum of 100-220 kyr, and not as a single magma pulse. Isotopic and mineralogical alteration is intense in the upper 100 m but decreases in intensity with depth. Below 800 m, alteration is restricted to narrow zones surrounding faults, veins, igneous contacts, and to an interval of locally intense serpentinization in olivine-rich troctolite. Hydration of the lithosphere occurred over the complete range of temperature conditions from granulite to zeolite facies, but was predominantly in the amphibolite and greenschist range. Deformation of the sequence was remarkably localized, despite paleomagnetic indications that the dome has undergone at least 45 degrees rotation, presumably during unroofing via detachment faulting. Both the deformation pattern and the lithology contrast with what is known from seafloor studies on the adjacent Southern Ridge of the massif. There, the detachment capping the domal core deformed a 100 m thick zone and serpentinized peridotite comprises similar to 70% of recovered samples. We develop a working model of the evolution of Atlantis Massif over the past 2 Myr, outlining several stages that could explain the observed similarities and differences between the Central Dome and the Southern Ridge.

Primitive layered gabbros from fast-spreading lower oceanic crust

Three-quarters of the oceanic crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and crystallization between the mantle and the sea floor. Despite the importance of these rocks, sampling them in situ is extremely challenging owing to the overlying dykes and lavas. This means that models for understanding the formation of the lower crust are based largely on geophysical studies and ancient analogues (ophiolites) that did not form at typical mid-ocean ridges. Here we describe cored intervals of primitive, modally layered gabbroic rocks from the lower plutonic crust formed at a fast-spreading ridge, sampled by the Integrated Ocean Drilling Program at the Hess Deep rift. Centimetre-scale, modally layered rocks, some of which have a strong layering-parallel foliation, confirm a long-held belief that such rocks are a key constituent of the lower oceanic crust formed at fast-spreading ridges. Geochemical analysis of these primitive lower plutonic rocks—in combination with previous geochemical data for shallow-level plutonic rocks, sheeted dykes and lavas—provides the most completely constrained estimate of the bulk composition of fast-spreading oceanic crust so far. Simple crystallization models using this bulk crustal composition as the parental melt accurately predict the bulk composition of both the lavas and the plutonic rocks. However, the recovered plutonic rocks show early crystallization of orthopyroxene, which is not predicted by current models of melt extraction from the mantle and mid-ocean-ridge basalt differentiation. The simplest explanation of this observation is that compositionally diverse melts are extracted from the mantle and partly crystallize before mixing to produce the more homogeneous magmas that erupt.

Detachment fault control on hydrothermal circulation systems: interpreting the subsurface beneath the TAG hydrothermal field using the isotopic and geological evolution of oceanic core complexes in the Atlantic

The geology and alteration history of two well-studied and very similar oceanic core complexes (OCCs) along the Mid-Atlantic Ridge are compared: the Atlantis Massif at 30°N (Integrated Ocean Drilling Program Site U1309) and a dome-like massif at 15°45′N (Ocean Drilling Program Site 1275). Both massifs are charac-terized by (1) a fault surface formed by talc-tremolite-chlorite schists; (2) little deformed gabbroic bodies a few kilometers in size, intruded into serpentinized peridotite and affected by mainly greenschist facies alteration; and (3) syntectonic basaltic intrusions within and below the detachment fault. Sr and O isotope data show that seawater-derived fluids were responsible for alteration in the gabbro, but fluid fluxes were moderate to low. Deformation in these “low-temperature” OCCs contrasts with the Atlantis Bank (Southwest Indian Ridge), where deforma-tion dominantly occurred at temperatures >800°C. The trans-Atlantic geotraverse (TAG) hydrothermal field, located ~4 km east of the Atlantic neovolcanic axis, is underlain by a convex-upward zone of seismicity reaching 7 km below seafloor, interpreted as a detachment fault. This suggests cooling to temperatures <700°C by hydrothermal circulation down to this depth. The geology and thermal evolution of the TAG detachment fault and its footwall are predicted on the basis of observa-tions on OCCs in the Atlantic. We suggest that the hydrothermal system is driven by gabbros emplaced at depth (7 km) and rapidly cooled during exhumation due to hydrothermal discharge along the fault at 350–400°C. The difference between low- and high-temperature OCCs is that gabbros in the former are intruded into the roots of an active hydrothermal system.

FLUID PRESSURE AND SALINITY VARIATIONS IN SHEAR ZONE-RELATED VEINS, CENTRAL PYRENEES, FRANCE : IMPLICATIONS FOR THE FAULT-VALVE MODEL

Fluid inclusion microthermometry data is presented from quartz veins associated with high-angle reverse shear zones cutting the Neouvielle Massif in the central French Pyrenees. A complex series of veins were formed during both normal and reverse movement on brittle faults within the shear zone. Brittle structures developed contemporaneously with quasi-plastic deformation in the shear zones. Fluid inclusion data are presented from a variety of vein types and shows that a wide variation in fluid density and salinity is present in the fault system. These variations occur both within individual veins and between different vein types. Temperatures at the time of vein filling are constrained to be in the range 310–360°C by chlorite microthermometry. The variation in homogenisation temperature is too large to be accounted for by variations in the fluid temperature, and is interpreted to reflect variations in fluid pressure between lithostatic (∼ 500 MPa) and hydrostatic (∼ 200 MPa) values at the time of quartz growth in the veins. The shear zones carry modest displacements and may not themselves have been seismogenic. A model is presented in which fluid pressure variations and stress cycling within the steeply dipping shear zones is controlled by the seismogenic cycle on large underlying thrust structures. The model predicts that the highest maximum trapping pressures should occur in fluid inclusions from low-angle veins formed in a high-Pfluid compressional regime immediately before earthquake rupture. Lower maximum pressures are expected in steep veins related to extension and normal fault movement across the shear zones immediately after rupture. The fluid inclusion data support this model when maximum trapping pressures are considered, but the majority of inclusions in all vein types formed at pressures well below lithostatic values. This suggests that the dominant factor causing quartz precipitation was the sudden pressure drop immediately after earthquake rupture, regardless of vein type.

Microstructural and microchemical consequences of fluid flow in deforming rocks

Abstract The different deformation mechanisms possible in rocks impact differently on fluid flow processes because of the range of induced volume changes associated with different deformation histories. Microstructural analysis of deformed rocks can provide crucial information for the identification of fluid flow pathways, determination of the physico-chemical properties of the fluid, quantification of the amount of fluid involved and an assessment of the variation in the open/closed nature of the fluid flow system. A critical factor in the efficiency of fluid flow in deformed rocks is the competition between the processes which maintain the connectivity of the high permeability pathways and those which close such pathways. The range of deformation processes which are involved in this competition will be different depending on the tectonic settings, the deformation conditions and the rock types involved. A brief review of the processes which interact to control fluid flow during deformation in sedimentary basins, crystalline basement under low to moderate grade metamorphism and during prograde metamorphism at moderate to high grades is given.