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Fluids and thrusting
Chemical Geology, 49 (1985) 353--362 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
FLUIDS AND THRUSTING W.S. F Y F E a n d R. K E R R I C H
Department of Geology, University of Western Ontario, London, Ont. N6A 5B7 (Canada) (Accepted for publication July 16, 1984)
Abstract Fyfe, W.S. and Kerrich, R., 1985. Fluids and thrusting. In: Y. Kitano (Guest-Editor), Water--Rock Interaction. Chem. Geol., 49: 353--362. Recent studies of the structure of the continental crust by COCORP and BIRPS have shown that major thrust structures in the lower crust are ubiquitous. Plate-tectonic processes of continental crust destruction or construction at sites where subduction or collision occurs (the Himalayas or the Andes) involve thrusting o f continental materials or ocean crust beneath continents, and local overthrusting may accompany transcurrent motion. In all such cases massive fluid flow must occur. As first stressed by M.K. Hubbert and W.W. Rubey, thrusting mechanisms require that fluid pressures are close to, or greater than, lithostatic pressures. When thrusting occurs, compaction of the underplate must occur rapidly, expelling pore water. Crustal thickening leads to gradual heating of the underplate with prograde metamorphism near the top and even melting at the base. If the overthrust plate is thick and hot at the base, retrograde metamorphism will occur at the base of the overthrust plate and rising fluids will encounter inverted thermal gradients. In such a region veins need not occur but leaching phenomena may dominate. If an underthrust plate is hydrated, and/or contains sedimentary aquifers, large fluid volumes are expelled through shear zones into the overthrust plate. The scale of fluid-release processes can be large. Thus, in thin-skinned tectonics where the overthrust plate is 10--15 km thick, induced fluid release can easily reach 4 • 109 g m -2 of thrust surface. The flow of such fluids will be controlled by the thrust surface and lithology, both of which will influence hydraulic fracture mechanisms and spacing of fractures. The chemistry of thrust-derived fluids may be highly variable depending on lithology and the time constants of thrusting. Vertical thermal and redox environments will be similarly dependent. In the subduction process, impressive quantities of fluids must pass back up the thrust surface, and such fluids have recently been directly observed during drilling. Dewatering of spilites, serpentinites and sediment layers of the underthrust oceanic lithosphere must produce massive fluid flow back to the surface but some of these fluids may form hydrated minerals in the overlying mantle and, at great depth, flux mantle melting. Extreme metasomatism in blueschist belts must result from such fluids. A case of particular interest involves the cessation of subduction when a high-level slab reaches thermal equilibrium. In general, flow regimes in the thrust and fault zones associated with collisions follow a sequence from conditions of high T--P with locally derived fluids at low water/rock ratios during initiation of the structures, to high fluxes of reduced metamorphic fluids along conduits as the structures propagate and intersect hydrothermal reservoirs. Later in the tectonic evolution, and at shallower crustal levels, there may be incursion of oxidizing near-surface fluid reservoirs into the faults. These fluids may have extremely low 5180values, where mountain ranges form on rebound faults, and high-altitude depleted fluids penetrate down the rebound structure. Extensive mineralization may be associated with such thrust-derived fluids. Examples which will be discussed include the Mother Lode Au deposits of California, U.S.A., and the large U deposits of Lagoa Real, Brazil.
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354 1. Introduction
During the past few years intensified highresolution seismic studies of the structure of continental crust by groups like C O C O R P (see Cook et al.,1980) and BIRPS (Matthews, 1982) have revealed that thrusts are one of the dominant components of major structures. M a n y high-angle near-surface structures refract to become low-angle discontinuities at depth. That such structures are abundant should be no surprise as classic models of the European A1ps and Himalayas in India (see Heim, 1921; Trumpy, 1969; Drury and Shackleton, 1981) demand large thrust faults, as do metamorphic patterns where inverted thermal gradients are comm o n (see Dewey and Windley, 1981). In modern plate tectonics underthmsting at destructive plate margins is the dominant process removing old oceanic lithosphere or causing continent--continent collision and thickening of continental crust. Further Allis (1981) has shown that at conservative (transcurrent) plate boundaries, as along the Alpine Fault of N e w Zealand, local domains of overthrusting are likely and are required by gravity and thermal observations. In this paper we wish to draw attention to the scale of fluid release associated with major thrust processes, and the geochemical processes which m a y be associated with the fluid migration. Hubbert and Rubey (1959) demonstrated that mechanisms for low-angle thrusting required fluids on thrust surfaces essentially at lithostatic pressures to overcome prohibitive sliding friction plus constraints on the magnitude of applied surface forces imposed by the limited strength of rock. Yet despite their elegant analysis, little attention has been given to the origin of such fluids or the evolution of fluid systems associated with thrusts. 2. Subduction When oceanic lithosphere is subducted at ocean trenches by underthrusting continental
or oceanic crust a huge fluid release process is initiated. The angle of underthrusting m a y be shallow as under the Andes or very steep as at the Japan Trench (Uyeda, 1983). In the past decade there have been spectacular advances in the description of trench structures and processes near trenches (see Hilde and Uyeda, 1983). In earlier studies Oxburgh and Turcotte (1970) indicated h o w friction might make a significant contribution to heating the upper levels of the descending lithosphere. But in the light of direct evidence for fluid release up the slab (Anderson, 1981) the significance of frictional heating must be questioned. In fact endothermic dehydration processes m a y contribute to keeping the slab cool as well as maintaining high fluid pressures, thus reducing normal stresses and limiting frictional heating (Fyfe and McBirney, 1975). Recent data on the upper layers o f descending lithosphere indicate that during bending, horst and graben structures may be developed and these have been observed at numerous sites (see Hilde, 1983; Hilde and Uyeda, 1983; Uyeda, 1983). The horst and graben structures m a y lock sediments into the descending material along with the spilitic and serpentinized ocean-floor crust, and such components will carry water to depth in hydrated minerals. There is no density problem associated with transporting some initially light sedimentary material along with the lithosphere, and eventually the eclogite transition will produce a large negative b u o y a n c y (see Molnar and Gray, 1979). In what follows we will assume a structure in the descending crust as shown in Fig. 1. As ocean-floor crust is subducted at an annual rate in the order of 10 km 3 (to balance new creation at ridges) and as the sediment thickness m a y be of the order of 500 m, assuming the spilitic crust contains 5% chemically b o u n d water and the sediments 10% b o u n d water, the water subduction rate is 1.5 • 10 is g yr. -1 or 1.5 km 3 yr. -1. Other volatile phases (CO~ S) are also locked into the slab (Fyfe and Lonsdale, 1981). Initially sediments will
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have a large porosity with an equally large pore-fluid component. In the first stages of underthrusting heating and compression of the underthrust material will commence. Porosity reduction with mild heating will create high pore-fluid pressures and fluid expulsion will either occur back up the slab, as has been directly observed (Anderson, 1981), or will be released b y hydrofracturing into the overplate. These cold fluids (T -~ 20°C) will carry salt water b u t mineral solutes will be slight. It should be noted that if such dewatering processes dominate during the low-temperature stages (say up to 50°C, depth ~- 5 km) then because of the structure of descending lithosphere, at sites such as A (Fig. 1), large fluid volumes may pass over the horst tops where water/rock ratios may be very large.
~obo
Fig. 1. Subduction of lithosphere with typical horst and graben structures (after Uyeda, 1983). Fluids from dewatering of sediments and porous spilites will flow up the slab and into the overplate.
Given the horst and graben structure with grabens full of poorly compacted sediments, a peculiar situation must arise as the load develops on the thrust surface which will have a highly heterogeneous mechanical strength. One would anticipate that a chaotic breccia zone would develop above grabens by collapse of the overriding plate with potential hydrofracturing into the roof. A melange zone m a y develop in the upper part of the grabens (Fig. 2). Depending on the subduction angle this will bring deep crustal rocks
or the mantle into direct contact with grabenfill materials. Once pore water is expelled, prograding o f the top of the slab will involve development of blueschist facies rocks and eventually eclogite. Water expulsion patterns are n o w possibly not predictable because in both the chaotic zone (Fig. 2) and in the spilitic portion of the slab large amounts of anhydrous minerals are still present and partially altered rocks will tend to go to states of complete hydration, ultramafics to serpentine--talc assemblages and the like. As the descent rate is 1--10 cm yr. -1, heating is occurring at rates of I°C per 104 yr. to I°C per 103 yr. At such low rates, mineralogical equilibrium is likely to attain even at very low temperatures. When flow is slow and evolved fluids are being consumed b y anhydrous components in the slab, residual fluids will tend to become highly saline. Such fluids may explain some of the metasomatism seen in blueschist facies rocks. Fluid diffusion m a y dominate over open flow and one would hardly expect open fractures until major facies boundaries are crossed. In compacted materials the diffusion coefficients will be very low and water will tend to diffuse to local anhydrous regions where water pressures are buffered by the hydration reactions. In c o m p a c t e d clays diffusion coef/
~tacje fault
Fig. 2. Detail of the crust of the subducted slab. Sediments trapped in grabens and ridge faults are compacted and dewatered and create a chaotic collapse zone along the moving interface.
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ficients are typically in the range 10-1°--10 -11 m 2 s -1 (Freeze and Cherry, 1979). At much greater depths where basaltic blueschists or amphibolites transform to eclogites, or kaolin--pyrophyllite sediments break d o w n to kyanite rocks, volume relations m a y be such that: Yreactant > Yproduc t + Ywate r
(1)
or Yreactant < Yproduct + Ywate r
(2)
Case (1) may be true for an amphibolite -~ eclogite or basaltic blueschist -~ eclogite transition, case (2) for pyrophyllite -~ kyanite + quartz. For case (1) water m a y not be lost until thermal expansion of the fluid drives hydraulic fracture development b u t for case (2) there is steady expulsion of highpressure fluids. At great depth (> 100 km), amphibole and phlogopite breakdown will trigger zones o f massive melting in the overlying hotter mantle (WyUie and Sekine, 1982). The ultrahigh-pressure fluids coming o f f the subducted material at temperatures above 600°C will transport very large quantities of solutes, up to 50%, and will transport large quantities o f elements like Na, K, U, and species like SiO2 (Ryabchikov and Boettcher, 1980). We would also expect that such fluids would move many interesting trace metals. Neither the fate of fluids in subduction zones nor the extent of their participation in geochemical transport are known in detail. However, there is good reason to suppose that at higher temperatures a significant fraction o f volatiles are returned towards the surface, incorporated in magmas. Taylor (1974, 1979) has shown that the narrow range of - 4 0 to -80%0 in the 8D-values of h y d r o x y minerals in pristine igneous rocks is probably inherited from a fluid--rock prehistory at mid-ocean ridges. During cooling and hydration o f the oceanic crust initially at mid-ocean ridges, and subsequently during translation towards the site of subduction, secondary hydroxyl-
bearing minerals acquire a 8D of - 4 0 to -80%0, which reflects the 8D of marine water (0%o), the mineral--water fractionation coupled with the ambient temperature of hydration (Taylor, 1974, 1979). Such hydration reactions are k n o w n to extend to depths o f > 5 km ( F y f e and Lonsdale, 1981; Gregory and Taylor, 1981). Whereas hydration of the ocean crust generally proceeds under conditions of high water/rock ratios, dehydration in subduction zones occurs under low water/ rock, and although the volume of hydrated crust is relatively small its fluid content is large compared to that of the surrounding mantle and lower crust, which have relatively low hydrogen contents. Thus the 8 D signature of h y d r o x y minerals in subduction-related magmas appears to have been acquired from fluids of dehydration in the downgoing slab, irrespective of the magma source or whether the slab participates in melting. This is part of a fluid cycle in which water is first transferred from the hydrosphere to the ocean crust, then subducted, incorporated into magmas and returned towards the terrestrial surface. A particularly intriguing question involves what happens when the forces driving subduction turn off and subduction ceases. A region where this m a y have occurred involves the Mother Lode and Melones Fault zone west of the Sierra Nevada in California, U.S.A. Schweickert et al. (1980) have reported blueschist facies rocks along this region and recent COCORP seismic studies have revealed a major structure dipping deep to the east. It is tempting to speculate that residual subducted material is involved. If this is the case, slow thermal equilibration of the slab (from 10 ° km -1 to 30 ° km -1) would produce a massive prograde metamorphic event. Large volumes o f fluids would be evolved and could move up the thrust discontinuity. If this occurred, the fluid mass could reach 4 • 1013 g per m of strike length. Does this fluid account for the massive quartz vein generation, carbon dioxide alteration and Au transport in the region? The quan-
357 tities appear to be appropriate for the scale of the phenomenon.
Dostal et al., 1980; Jamieson and Strong, 1980; McCaig, 1980).
3. Thrusts in oceanic crust 4. Large continental overthrusts Thin high-temperature dynamothermal aureoles with associated hydrothermal and geochemical transport are located at the plane of structural discontinuity in the basal peridotite of many ophiolites. McCaig (1980) has shown that the aureoles of the Bay of Islands, Eastern Townships (Quebec), and White Hills (Newfoundland) ophiolites in Canada represent thinned and inverted sections through the oceanic crust. Amphibolitebearing lherzolite mylonites at the Bay of Islands ophiolite complex have pronounced additions of Ti, K and Fe, along with depletions of Mg, Si, Cr and Ni. These chemical changes in mylonites of the thrust are attributed to interaction with high-temperature metasomatic fluids during ophiolite emplacement onto the continental margin (McCaig, 1980). Dostal et al. (1980) describe fluid flow with associated geochemical alteration in mylonites, considered to have accompanied emplacement of the St. Anthony Complex ophiolite in Newfoundland, Canada. Here, the mylonites (biotite amphibolites) "resemble" transitional alkali basalts, but are thought to have experienced additions of K, Rb, Ba, P, Ta, Th, U and LREE, with depletions of Fe, Mg, Na, Ca plus the HREE. These late synmetamorphic mylonites acted as a channel for fluids released during the final stages of metamorphism of the St. Anthony Complex. Estimates of the deformation temperature during formation of the mylonites are relatively high (880--400°C), and it is likely that fluids migrating along the mylonites at such elevated temperatures would contain high solute concentrations of many elements. There is as yet no concensus as to whether the mylonites described above developed during thrusting in an oceanic environment near the ridge crest, or alternatively during obduction-related thrusting (see Malpas, 1979;
Major thrust structures are well-known features in most mountain ranges, and particularly in those involving continental collisions. Hubbert and Rubey (1959) in their classic paper showed how high fluid pressure (Pfluid ~-, or >, lithostatic) is necessary to float off the overthrust block. Recent seismic studies of deep continental structures show that low-angle thrust faults appear to be ubiquitous in deep continental structures. It follows that collision tectonics must be one of the major phenomena in continental crust formation. Dewey (1974) shows the typical models of continent--continent thrusting. Barazangi and Ni (1982) have analyzed recent seismic data from the Himalayas and suggest a simple model for the thrusting of India beneath Tibet producing a crustal thickness of 70 km or so. Himalayan models show thrust slices of varying complexities (Powell, 1975). While much remains to be done t o understand mechanisms of such thrust processes, there is little doubt that a double thickness continental crust can be produced at typical plate-tectonic velocities. In major thrusts as described in the Appalachians or Himalayas, where thrusting distances of hundreds of kilometers are involved, if the thrust plates are thick (15--30 km) thermal equilibrium will not attain in the time of thrusting (at 10 cm yr. -1, 104 yr. km -1 or 106 yr. per 100 km). To understand what may happen one may make a first approximation that as far as the thrust plates are concerned, thrusting is isothermal for the duration of translation. Given this assumption, if one continental block 30 km thick is thrust over another, the temperature distribution after thrusting may appear as in Fig. 3, with a corresponding metamorphic section. As time proceeds the metamorphic
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facies distribution m a y be as in Fig. 3. Equilibrium temperature distribution will be determined b y erosion rates at the t o p and melting at the base. An excellent example o f crustal melting is provided b y the remarkable y o u n g granites of the Himalayas Hamet and All6gre, 1976; Ferrara et al., 1983) with their very high initial Sr ratios. Depth(.m) ~°C 0
t 1 --* t 2 - - *
t3--.
t 4--*
0
z
GS
z
GS
,2..%
A (Y) GS GS
30
-....~.,,~.
G$ ...... thrust plane
z
GS 60
500
A
BS I
A
E (Z)G-mig G
Moho
Fig. 3. Possible changes in metamorphic patterns following a continent--continent collision. At tI immediately after thrusting, temperatures are as s h o w n on the left. Time t2 represents changes before significant erosion or warming of the basal section. B y time t3 melting occurs at the base, producing high-STSr granites (Z = zeolite facies; G S = greenschist; A = a m p h i b o l i t e ; B S = blueschist; E -- eclogite; G -- granulite; m i g = partially m e l t e d rock). F o r further discussion see text.
Consider processes in the various parts of the overthrust structures given that each has an initial thermal gradient of = 20 ° km -1. The overthrust continental block will n o t undergo dehydration reactions b u t the underthrust block will be prograded to amphibolite-granulite. During thrusting, pore water will be expelled from region X and prograde metamorphism will occur over the entire thickness of the underplate. On average, 4% H~O m a y be lost from the underplate and such fluids will penetrate the overthrust plate (or flow along it during thrusting) by hydraulic fracture mechanisms producing vein swarms and shear zones. Per km 2 o f thrust surface, 4.5 km 3 of fluid will pass through the thrust surface. Fluids rising on a path X -~ Y will cause retrograde metamorphism in zone Y as the thermal gradient is reversed. As the once near-surface rocks
of zone X will be relatively more oxidized, such fluids m a y also tend to oxidize the overplate as described b y Beach and Fyfe {1972) and will not precipitate SiO~ in veins, b u t rather dissolve SiO2. So while "anti-quartz veins" are forming near region Y, massive veining will occur along path Z ~ X with the possible formation o f Au--quartz deposits (Fyfe and Kerrich, 1983). Different parts o f the crustal section will follow different metamorphic paths with time. Thus in region Y, the time path may be amphibolite facies-~ greenschist facies; in region X, zeolite facies -~ greenschist amphibolite. In deep regions (> 40 km), after thrusting and before thermal equilibration, blueschists and eclogites may form transiently to be replaced later by amphibolites or granulites with partial melting and formation o f granite liquids. The exact path will depend on the rate of erosion o f f the mountain structure. Temperature structure will pass through a series of intermediate states with smaller regions of inverted gradient as time progresses so that major mineralization processes will migrate with time. But it will be in the regions near the thrust surface that the most dramatic prograde and retrograde processes occur. There will also be p r o f o u n d changes in the mechanical properties of the rocks; those undergoing retrograde processes will be strong while those prograding will be weak. Once melting commences in. the base o f the section, both the latent heat of fusion of granitic materials and the scavenging o f heat-producing elements b y the melts, will prevent the base from attaining extreme temperatures but will transfer heat to prograde higher levels even more. Clearly there is good reason for intensive study o f such processes in regions such as the Himalayas. The possibilities for mineral deposits are o f great interest (Sillitoe, 1979). We have recently described U mineralization in a structure believed to represent an overthrust (Lobato et al., 1983) in Bahia, Brazil. In this region o f Lagoa Real there is clear structural evidence for Proterozoic rocks
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dipping under amphibolite-granulite facies Archean basement. Across the exposed fault region metamorphic facies are inverted with Proterozoic amphibolites overlying greenschists. In the overlying basement, highly retrograded shear zones are developed where epidote-amphibolite facies albitites carry U. The shear zones are characterized by Na metasomatism, removal of quartz, oxidation developing hematite and U mineralization. Oxygen isotope data show that quartz (81so + 11.3 to +12.1%o), plagioclase (+8.5 to +8.8%0) and magnetite (+1.0 to +1.5%o) in the parent gneisses are not isotopically concordant, but approach isotopic equilibrium in shear zones, signifying deformation temperatures of 500--540°C in the presence of fluids with ti 1sO + 5.8 to +6.4%0 and under conditions of low water/rock ratios: the fluids are interpreted to be of metamorphic origin, indigenous to the host gneisses. In domains of U mineralization, shear zones are intensely oxidized, desilicified, and characterized by the reaction of K-feldspar plus plagioclase to albite. All minerals undergo a shift of -10%o, indicating discharge of fluids 81sO -4%o at 540°C. The reservoir involved is interpreted to be release of meteoric water formation brines in the underlying Proterozoic sediments up through the Archean gneisses, during overthrusting: 1000 km 3 of solutions is estimated to have passed through these structures. The U reserves in these rocks exceed l 0 s t*. This could be in accord with transport by 1000 km 3 driven from the underplate (see Lobato et al., 1983). It is interesting that in the same region late veins carry quartz, which may reflect return to a normal geothermal gradient. Metamorphic mineralogy in both altered shear zones and the underplate indicate that the thrust plate had a thickness of the order of 10 km, similar to that proposed for the Appalachian overthrusts (Cook et al., 1980). Two further examples of fluid flow and *1 t = 1 m e t r i c t o n n e = 103 kg.
associated geochemical transport involved with thrust faults have been described by Kerrich et al. (1983), with reference to the Grenville front (Ontario, Canada) and Yellowknife thrusts (North West Territories, Canada), respectively. Translation at the Grenville front, at = 109 yr. was accommodated along two mylonite zones and an intervening boundary fault. The high- (MZ II) and lowtemperature (MZ I) mylonite zones formed at 580--640°C and 430--490°C, respectively, in the presence of fluids with ~lSO +9.5%0 (MZ II) and +7.5%o (MZ I). These fluids are interpreted to be of metamorphic origin, indigenous to the immediate rocks. A population of post-tectonic quartz veins occupying brittle fractures, characterized by 51SOquartz - 1 % o , were precipitated from fluids o f - 9 % o to -14%o at 200--300°C. The water may have been derived from downwards penetration into fault zones of low-lSO precipitation on a mountain range induced by continental collision, with uplift accommodated at deep levels by the mylonite zones coupled with rebound on the boundary faults. At Yellowknife a series of large-scale brittle ductile shear zones transect metabasalts and juxtaposed granodiorite of the Yellowknife greenstone belt (~- 2 . 8 . 109 yr.). Initial development of shear zones involved volume dilatation during brittle fracturing with migration of 5 wt.% volatiles into the shear zone from surrounding metabasalts. Early deformation involved no departures in redox state or whole-rock 5180 from background states, of Fe2÷/ZFe = 0.72 and +7 to +7.5%0, respectively, attesting to conditions of low water/rock. Shear zones subsequently acted as high-permeability conduits for pulsed discharge of > 9 km 3 of hydrothermal fluids at 360--450°C, precipitating quartz--Au lodes. Mineralized conduits in shear zones are highly reduced (Fe2÷/Z Fe > 0.9). Fluid ~lSO-values of +8%o are interpreted to signify a metamorphic origin of the hydrothermal reservoir discharging through the shear zones. This reservoir also streamed pervasively through the entire outer 2 km of the granodiorite
360
batholith bordering greenstones, shifting the 5~SOquartz from +9%o in the pluton interior to +13%o at the margins. The West Bay Fault a late major transcurrent structure contains massive bull quartz (+11.4 to +12.6%o) that grew at 200--300°C from fluids of +2 to +6%0, possibly precipitated by discharge of formation brines. Thus, as speculated for the case of the Mother Lode, some examples of fracture-related Au deposits in greenstone belts may be associated with fluid release from metamorphic prograding of an underplate. In general, flow regimes in the continental fault and shear zones follow a sequence from conditions of high T--P with locally derived fluids at low water/rock, during initiation of the structures, to high fluxes of reduced metamorphic fluids along conduits as the structures propagate and intersect hydrothermal reservoirs, or induce dehydration in the underplate via thermal inversion. Later in the tectonic evolution and at shallower crustal levels, there was incursion of oxidizing near-surface fluid reservoirs into the faults. Hyndman (1980, 1983) and other workers on "metamorphic core complexes" have described involvement of granitic rocks or gneiss domes along with their cover rocks in mylonite zones and chlorite breccia, along structural discontinuities. The fluid regimes associated with these features constitute an interesting area for future research.
5. Transform faults It is well known that fluids are associated with movement on major transform faults such as the Alpine Fault of New Zealand and the San Andreas Fault of California, U.S.A. Sibson et al. (1975) have discussed what is termed seismic pumping. In this process groundwaters gradually migrate into fractures and pore spaces which progressively dilate during the long-term stress build up prior to a major seismic event, and are
suddenly forced out by pore volume collapse accompanying stress release after the catastrophic seismic failure. A spectacular example of fluid release during seismic activity came from study of the Matsushiro earthquake swarm in Japan (see Tsuneishi and Nakamura, 1970; Fyfe et al., 1978). It is of note that both in California and New Zealand, the major transforms involve thick low-grade graywacke piles which may be reaching thermal equilibrium following a trenchforming subduction episode (MacKinnon, 1983). A recent paper by Allis (1981; see also Walcott and Cresswell, 1979) presents a most interesting model of the structure along the Alpine Fault of New Zealand. From gravity, seismic and thermal data they show that the crust has attained thicknesses exceeding 40 km. They propose that the Indian plate is underthrusting the Pacific plate, creating an overthickened crust over a zone ~ 100 km wide. The vigorous hotspring activity along the fault can be no surprise as the root zone will be undergoing profound prograde metamorphism. These rather small amounts of crustal thrusting associated with transform motion may be of considerable significance for fault plane mineralization and metamorphism. Before thermal effects much influence the underplate, loading could induce blueschist metamorphism. This will rapidly prograde unless uplift rapidly follows. Such mechanisms could be of interest in the distribution of eclogites and blueschists in the Franciscan of California (see Bailey et al., 1964) and may offer explanation of the formation and preservation of aragonitic rocks which require rapid uplift for their preservation (Brown et al., 1962). Varying degrees of overthrusting along a moving transform could lead to rapid pressure--depth fluctuations. References Allis, R.G., 1981. Continental underthrusting beneath the Southern Alps of N e w Zealand. Geology, 9: 303--307.
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Anderson, R.N., 1981. Surprises from the Glomar Challenger. Nature (London), 293: 261--262. Bailey, E.H., Irwin, W.P. and Jones, D.L., 1964. Franciscan and related rocks and their significance in the geology of western California. Calif. Div. Mines, Bull., 183: 89--112. Barazangi, M. and Ni, J., 1982. Velocities and propagation of Pn and Sn beneath the Himalayan arc and Tibetan plateau: Possible evidence for underthrusting of Indian continental lithosphere beneath Tibet. Geology, 10: 179--185. Beach, A. and Fyfe, W.S., 1972. Fluid transport and shear zones at Scourie, Sutherland: evidence of overthrusting? Contrib. Mineral. Petrol., 36: 175--179. Brown, W.H., Fyfe, W.S. and Turner, F.J., 1962. Aragonite in California glaucophane schists, and the kinetics of the aragonite--calcite transition. J. Petrol., 3: 566--582. Cook, F., Brown, L. and Oliver, J., 1980. The Southern Appalachians and the growth of continents. Sci. Am., 243: 156--168. Dewey, J.F., 1974. Plate tectonics. In: F. Press and R. Siever (Editors), Planet Earth. W.H. Freeman, San Francisco, Calif., pp. 123--135. Dewey, J.F. and Windley, B.F., 1981. Growth and differentiation of the continental crust. Philos. Trans. R. Soc. London, Ser. A, 301: 189--206. Dostal, J., Strong, D.F. and Jamieson, R.A., 1980. Trace element mobility in the mylonite zone within the ophiolite aureole, St. A n t h o n y Complex, Newfoundland. Earth Planet. Sci. Lett., 49: 188--192. Drury, S.A. and Shackleton, R.M., 1981. The history of the earth's crust. In: D.G. Smith (Editor), The Cambridge Encyclopedia of Earth Sciences. Cambridge University Press, London, pp. 250-275. Ferrara, G., Pisa, B., Torino, L. and Tonarini, S., 1983. Rb/Sr geochronology of granites and gneisses from the Mount Everest region, Nepal Himalaya. Geol. Rundsch., 72: 119--136. Freeze, R.A. and Cherry, J.A., 1979. Groundwater. Prentice-Hall, Englewood Cliffs, N.J., 604 pp. Fyfe, W.S. and Kerrich, R., 1983. Gold concentration processes. Proc. Gold 82 Conf., Harare, Balkema, Rotterdam, pp. 99--127. Fyfe, W.S. and Lonsdale, P., 1981. Ocean floor hydrothermal activity. In: C. Emiliani (Editor), The Sea, Vol. 7. Wiley, New York, N.Y., pp. 589--638. Fyfe, W.S. and McBirney, A., 1975. Subduction and the structure of andesitic volcanic belts. Am. J. Sci., 275-A: 285--297. Fyfe, W.S., Price, N.J. and Thompson, W.S., 1978. Fluids in the Earth's Crust, Elsevier, Amsterdam, 383 pp. Gregory, R.T. and Taylor, H.P., 1981. An oxygen
isotope profile in a section of Cretaceous oceanic crust, Samail ophiolite, Oman: evidence for ~ 8 0 buffering of the oceans by deep (> 5 km) seawater hydrothermal circulation at mid-ocean ridges. J. Geophys. Res., 86: 2737--2755. Hamet, J. and All~gre, C.J., 1976. Rb--Sr systematics in granite from central Nepal (Manaslu): significance of the Oligocene age and high 87Sr/8'Sr in Himalayan orogeny. Geology, 4: 470--472. Heim, A., 1921. Geologie der Schweiz. Tauehnitz, Leipzig, 476 pp. Hilde, T.W.C., 1983. Sediment subduction versus accretion around the Pacific. In: T.W.C. Hilde and S. Uyeda (Editors), Convergence and Subduction. Tectonophysics, Spec. Iss., 99(2-4): 381-397. Hilde, T.W.C. and Uyeda, S. (Editors), 1983. Convergence and Subduction. Tectonophysics, Spec. Iss., Vol. 99, No. 2-4,400 pp. Hubbert, M.K. and Rubey, W.W., 1959. Role of fluid pressure in mechanics of over-thrust faulting. Geol. Soc. Am. Bull., 70: 115--166. Hyndman, D.W., 1980. Bitterroot d o m e - S a p p h i r e tectonic block, an example of a plutonic-eore gneiss-dome complex with its detached suprastructure. Geol. Soc. Am. Mere., 153: 427--443. Hyndman, D.W., 1983. The Idaho batholith and associated plutons, Idaho and western Montana. Geol. Soe. Am. Mem., 159: 213--239. Kerrieh, R., La Tour, T.E. and Willm0re, L., 1983. Fluid participation in deep fault zones: evidence from stable isotope and fluid inclusion evidence. Proc. Conf. on The Role of Water in Rock Deformation. Monterey, Calif., July 1982. Lobato, L.M., Forman, J.M.A., Fyfe, W.S., Kerrieh, R. and Barnett, R.L., 1983. Uranium enrichment in Archaean crustal basement associated with overthrusting. Nature (London), 303: 235--237. MacKinnon, T.C., 1983. Origin of the Torlesse terrane and coeval rocks, South Island, New Zealand. Geol. Soe. Am. Bull., 94: 967--985. Malpas, J., 1979. The dynamothermal aureole of the Bay of Islands ophiolite suite. Can. J. Earth Sei., 16: 2086--2101. Matthews, D.H., 1982. BIRPS: deep seismic reflection profiling around the British Isles. Nature (London): 298: 709--710. McCaig, A.M., 1980. Dynamothermal aureoles of ophiolites and ultramafie bodies in the Canadian Appalachians. M.Se. Thesis, University of Western Ontario, London, Ont. (unpublished). Molnar, P. and Gray, D., 1979. Subduction of continental lithosphere: some constraints and uncertainties. Geology, 7: 58--63. Oxburgh, E.R. and Turcotte, D.L., 1970. Thermal structures of island arcs. Geol. Soc. Am. Bull., 81: 1665--1688. Powell, C., 1975. Tectonic models of the Tibetan plateau. Geology, 3: 727--731.
362 Ryabchikov, I.D. and Boettcher, A.L., 1980. Experimental evidence at high pressure for potassic metasomatism in the mantle of the earth. Am. Mineral., 65: 915--919. Schweickert, R.A., Armstrong, R.L. and Harakal, J.E., 1980. Lawsonite blueschist in the northern Sierra Nevada, California. Geology, 8: 27--31. Sibson, R.H., McMoore, J. and Rankin, R.H., 1975. Seismic pumping -- a hydrothermal fluid transport mechanism. J. Geol. Soc. London, 131: 653--659. Sillitoe, R.H., 1979. Speculations on Himalayan metallogeny based on evidence from Pakistan. In: A. Farah and K.A. De Jong (Editors), Geodynamics of Pakistan, Geol. Surv. Pakistan, Quetta, pp. 167-179. Taylor, H.P., 1974. The application of oxygen and hydrogen isotope studies to problems of hydrothermal alteration and ore deposition. Econ. Geol., 69: 843--883.
Taylor, H.P., 1979. Oxygen and hydrogen isotope relationships in hydrothermal mineral deposits. In: H.L. Barnes (Editor), Geochemistry of Hydrothermal Ore Deposits. Holt, Rinehart and Winston, N e w York, N.Y., 2nd ed., pp. 236--277. Trumpy, R., 1969. Die helvetischen Decken der Ostschweiz. Eclogae Geol. Helv., 62: 105--142. Tsuneishi, J.S. and Nakamura, K., 1970. Faulting associated with the Matsushiro swarm earthquakes. Bull. Earthquake Res. Inst. Tokyo Univ., 48: 29--38. Uyeda, S., 1983. Comparative subductology. Episodes, pp. 19--24. Walcott, R.I. and Cresswell, M.M., 1979. The origin of the Southern Alps. R. Soc. N.Z. Bull., No. 18, 147 pp. Wyllie, P.J. and Sekine, T., 1982. The formation of mantle phlogopite in subduction zone hybridization. Contrib. Mineral. Petrol., 79: 375--780.
FLUIDS AND THRUSTING W.S. F Y F E a n d R. K E R R I C H
Department of Geology, University of Western Ontario, London, Ont. N6A 5B7 (Canada) (Accepted for publication July 16, 1984)
Abstract Fyfe, W.S. and Kerrich, R., 1985. Fluids and thrusting. In: Y. Kitano (Guest-Editor), Water--Rock Interaction. Chem. Geol., 49: 353--362. Recent studies of the structure of the continental crust by COCORP and BIRPS have shown that major thrust structures in the lower crust are ubiquitous. Plate-tectonic processes of continental crust destruction or construction at sites where subduction or collision occurs (the Himalayas or the Andes) involve thrusting o f continental materials or ocean crust beneath continents, and local overthrusting may accompany transcurrent motion. In all such cases massive fluid flow must occur. As first stressed by M.K. Hubbert and W.W. Rubey, thrusting mechanisms require that fluid pressures are close to, or greater than, lithostatic pressures. When thrusting occurs, compaction of the underplate must occur rapidly, expelling pore water. Crustal thickening leads to gradual heating of the underplate with prograde metamorphism near the top and even melting at the base. If the overthrust plate is thick and hot at the base, retrograde metamorphism will occur at the base of the overthrust plate and rising fluids will encounter inverted thermal gradients. In such a region veins need not occur but leaching phenomena may dominate. If an underthrust plate is hydrated, and/or contains sedimentary aquifers, large fluid volumes are expelled through shear zones into the overthrust plate. The scale of fluid-release processes can be large. Thus, in thin-skinned tectonics where the overthrust plate is 10--15 km thick, induced fluid release can easily reach 4 • 109 g m -2 of thrust surface. The flow of such fluids will be controlled by the thrust surface and lithology, both of which will influence hydraulic fracture mechanisms and spacing of fractures. The chemistry of thrust-derived fluids may be highly variable depending on lithology and the time constants of thrusting. Vertical thermal and redox environments will be similarly dependent. In the subduction process, impressive quantities of fluids must pass back up the thrust surface, and such fluids have recently been directly observed during drilling. Dewatering of spilites, serpentinites and sediment layers of the underthrust oceanic lithosphere must produce massive fluid flow back to the surface but some of these fluids may form hydrated minerals in the overlying mantle and, at great depth, flux mantle melting. Extreme metasomatism in blueschist belts must result from such fluids. A case of particular interest involves the cessation of subduction when a high-level slab reaches thermal equilibrium. In general, flow regimes in the thrust and fault zones associated with collisions follow a sequence from conditions of high T--P with locally derived fluids at low water/rock ratios during initiation of the structures, to high fluxes of reduced metamorphic fluids along conduits as the structures propagate and intersect hydrothermal reservoirs. Later in the tectonic evolution, and at shallower crustal levels, there may be incursion of oxidizing near-surface fluid reservoirs into the faults. These fluids may have extremely low 5180values, where mountain ranges form on rebound faults, and high-altitude depleted fluids penetrate down the rebound structure. Extensive mineralization may be associated with such thrust-derived fluids. Examples which will be discussed include the Mother Lode Au deposits of California, U.S.A., and the large U deposits of Lagoa Real, Brazil.
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354 1. Introduction
During the past few years intensified highresolution seismic studies of the structure of continental crust by groups like C O C O R P (see Cook et al.,1980) and BIRPS (Matthews, 1982) have revealed that thrusts are one of the dominant components of major structures. M a n y high-angle near-surface structures refract to become low-angle discontinuities at depth. That such structures are abundant should be no surprise as classic models of the European A1ps and Himalayas in India (see Heim, 1921; Trumpy, 1969; Drury and Shackleton, 1981) demand large thrust faults, as do metamorphic patterns where inverted thermal gradients are comm o n (see Dewey and Windley, 1981). In modern plate tectonics underthmsting at destructive plate margins is the dominant process removing old oceanic lithosphere or causing continent--continent collision and thickening of continental crust. Further Allis (1981) has shown that at conservative (transcurrent) plate boundaries, as along the Alpine Fault of N e w Zealand, local domains of overthrusting are likely and are required by gravity and thermal observations. In this paper we wish to draw attention to the scale of fluid release associated with major thrust processes, and the geochemical processes which m a y be associated with the fluid migration. Hubbert and Rubey (1959) demonstrated that mechanisms for low-angle thrusting required fluids on thrust surfaces essentially at lithostatic pressures to overcome prohibitive sliding friction plus constraints on the magnitude of applied surface forces imposed by the limited strength of rock. Yet despite their elegant analysis, little attention has been given to the origin of such fluids or the evolution of fluid systems associated with thrusts. 2. Subduction When oceanic lithosphere is subducted at ocean trenches by underthrusting continental
or oceanic crust a huge fluid release process is initiated. The angle of underthrusting m a y be shallow as under the Andes or very steep as at the Japan Trench (Uyeda, 1983). In the past decade there have been spectacular advances in the description of trench structures and processes near trenches (see Hilde and Uyeda, 1983). In earlier studies Oxburgh and Turcotte (1970) indicated h o w friction might make a significant contribution to heating the upper levels of the descending lithosphere. But in the light of direct evidence for fluid release up the slab (Anderson, 1981) the significance of frictional heating must be questioned. In fact endothermic dehydration processes m a y contribute to keeping the slab cool as well as maintaining high fluid pressures, thus reducing normal stresses and limiting frictional heating (Fyfe and McBirney, 1975). Recent data on the upper layers o f descending lithosphere indicate that during bending, horst and graben structures may be developed and these have been observed at numerous sites (see Hilde, 1983; Hilde and Uyeda, 1983; Uyeda, 1983). The horst and graben structures m a y lock sediments into the descending material along with the spilitic and serpentinized ocean-floor crust, and such components will carry water to depth in hydrated minerals. There is no density problem associated with transporting some initially light sedimentary material along with the lithosphere, and eventually the eclogite transition will produce a large negative b u o y a n c y (see Molnar and Gray, 1979). In what follows we will assume a structure in the descending crust as shown in Fig. 1. As ocean-floor crust is subducted at an annual rate in the order of 10 km 3 (to balance new creation at ridges) and as the sediment thickness m a y be of the order of 500 m, assuming the spilitic crust contains 5% chemically b o u n d water and the sediments 10% b o u n d water, the water subduction rate is 1.5 • 10 is g yr. -1 or 1.5 km 3 yr. -1. Other volatile phases (CO~ S) are also locked into the slab (Fyfe and Lonsdale, 1981). Initially sediments will
355
have a large porosity with an equally large pore-fluid component. In the first stages of underthrusting heating and compression of the underthrust material will commence. Porosity reduction with mild heating will create high pore-fluid pressures and fluid expulsion will either occur back up the slab, as has been directly observed (Anderson, 1981), or will be released b y hydrofracturing into the overplate. These cold fluids (T -~ 20°C) will carry salt water b u t mineral solutes will be slight. It should be noted that if such dewatering processes dominate during the low-temperature stages (say up to 50°C, depth ~- 5 km) then because of the structure of descending lithosphere, at sites such as A (Fig. 1), large fluid volumes may pass over the horst tops where water/rock ratios may be very large.
~obo
Fig. 1. Subduction of lithosphere with typical horst and graben structures (after Uyeda, 1983). Fluids from dewatering of sediments and porous spilites will flow up the slab and into the overplate.
Given the horst and graben structure with grabens full of poorly compacted sediments, a peculiar situation must arise as the load develops on the thrust surface which will have a highly heterogeneous mechanical strength. One would anticipate that a chaotic breccia zone would develop above grabens by collapse of the overriding plate with potential hydrofracturing into the roof. A melange zone m a y develop in the upper part of the grabens (Fig. 2). Depending on the subduction angle this will bring deep crustal rocks
or the mantle into direct contact with grabenfill materials. Once pore water is expelled, prograding o f the top of the slab will involve development of blueschist facies rocks and eventually eclogite. Water expulsion patterns are n o w possibly not predictable because in both the chaotic zone (Fig. 2) and in the spilitic portion of the slab large amounts of anhydrous minerals are still present and partially altered rocks will tend to go to states of complete hydration, ultramafics to serpentine--talc assemblages and the like. As the descent rate is 1--10 cm yr. -1, heating is occurring at rates of I°C per 104 yr. to I°C per 103 yr. At such low rates, mineralogical equilibrium is likely to attain even at very low temperatures. When flow is slow and evolved fluids are being consumed b y anhydrous components in the slab, residual fluids will tend to become highly saline. Such fluids may explain some of the metasomatism seen in blueschist facies rocks. Fluid diffusion m a y dominate over open flow and one would hardly expect open fractures until major facies boundaries are crossed. In compacted materials the diffusion coefficients will be very low and water will tend to diffuse to local anhydrous regions where water pressures are buffered by the hydration reactions. In c o m p a c t e d clays diffusion coef/
~tacje fault
Fig. 2. Detail of the crust of the subducted slab. Sediments trapped in grabens and ridge faults are compacted and dewatered and create a chaotic collapse zone along the moving interface.
356
ficients are typically in the range 10-1°--10 -11 m 2 s -1 (Freeze and Cherry, 1979). At much greater depths where basaltic blueschists or amphibolites transform to eclogites, or kaolin--pyrophyllite sediments break d o w n to kyanite rocks, volume relations m a y be such that: Yreactant > Yproduc t + Ywate r
(1)
or Yreactant < Yproduct + Ywate r
(2)
Case (1) may be true for an amphibolite -~ eclogite or basaltic blueschist -~ eclogite transition, case (2) for pyrophyllite -~ kyanite + quartz. For case (1) water m a y not be lost until thermal expansion of the fluid drives hydraulic fracture development b u t for case (2) there is steady expulsion of highpressure fluids. At great depth (> 100 km), amphibole and phlogopite breakdown will trigger zones o f massive melting in the overlying hotter mantle (WyUie and Sekine, 1982). The ultrahigh-pressure fluids coming o f f the subducted material at temperatures above 600°C will transport very large quantities of solutes, up to 50%, and will transport large quantities o f elements like Na, K, U, and species like SiO2 (Ryabchikov and Boettcher, 1980). We would also expect that such fluids would move many interesting trace metals. Neither the fate of fluids in subduction zones nor the extent of their participation in geochemical transport are known in detail. However, there is good reason to suppose that at higher temperatures a significant fraction o f volatiles are returned towards the surface, incorporated in magmas. Taylor (1974, 1979) has shown that the narrow range of - 4 0 to -80%0 in the 8D-values of h y d r o x y minerals in pristine igneous rocks is probably inherited from a fluid--rock prehistory at mid-ocean ridges. During cooling and hydration o f the oceanic crust initially at mid-ocean ridges, and subsequently during translation towards the site of subduction, secondary hydroxyl-
bearing minerals acquire a 8D of - 4 0 to -80%0, which reflects the 8D of marine water (0%o), the mineral--water fractionation coupled with the ambient temperature of hydration (Taylor, 1974, 1979). Such hydration reactions are k n o w n to extend to depths o f > 5 km ( F y f e and Lonsdale, 1981; Gregory and Taylor, 1981). Whereas hydration of the ocean crust generally proceeds under conditions of high water/rock ratios, dehydration in subduction zones occurs under low water/ rock, and although the volume of hydrated crust is relatively small its fluid content is large compared to that of the surrounding mantle and lower crust, which have relatively low hydrogen contents. Thus the 8 D signature of h y d r o x y minerals in subduction-related magmas appears to have been acquired from fluids of dehydration in the downgoing slab, irrespective of the magma source or whether the slab participates in melting. This is part of a fluid cycle in which water is first transferred from the hydrosphere to the ocean crust, then subducted, incorporated into magmas and returned towards the terrestrial surface. A particularly intriguing question involves what happens when the forces driving subduction turn off and subduction ceases. A region where this m a y have occurred involves the Mother Lode and Melones Fault zone west of the Sierra Nevada in California, U.S.A. Schweickert et al. (1980) have reported blueschist facies rocks along this region and recent COCORP seismic studies have revealed a major structure dipping deep to the east. It is tempting to speculate that residual subducted material is involved. If this is the case, slow thermal equilibration of the slab (from 10 ° km -1 to 30 ° km -1) would produce a massive prograde metamorphic event. Large volumes o f fluids would be evolved and could move up the thrust discontinuity. If this occurred, the fluid mass could reach 4 • 1013 g per m of strike length. Does this fluid account for the massive quartz vein generation, carbon dioxide alteration and Au transport in the region? The quan-
357 tities appear to be appropriate for the scale of the phenomenon.
Dostal et al., 1980; Jamieson and Strong, 1980; McCaig, 1980).
3. Thrusts in oceanic crust 4. Large continental overthrusts Thin high-temperature dynamothermal aureoles with associated hydrothermal and geochemical transport are located at the plane of structural discontinuity in the basal peridotite of many ophiolites. McCaig (1980) has shown that the aureoles of the Bay of Islands, Eastern Townships (Quebec), and White Hills (Newfoundland) ophiolites in Canada represent thinned and inverted sections through the oceanic crust. Amphibolitebearing lherzolite mylonites at the Bay of Islands ophiolite complex have pronounced additions of Ti, K and Fe, along with depletions of Mg, Si, Cr and Ni. These chemical changes in mylonites of the thrust are attributed to interaction with high-temperature metasomatic fluids during ophiolite emplacement onto the continental margin (McCaig, 1980). Dostal et al. (1980) describe fluid flow with associated geochemical alteration in mylonites, considered to have accompanied emplacement of the St. Anthony Complex ophiolite in Newfoundland, Canada. Here, the mylonites (biotite amphibolites) "resemble" transitional alkali basalts, but are thought to have experienced additions of K, Rb, Ba, P, Ta, Th, U and LREE, with depletions of Fe, Mg, Na, Ca plus the HREE. These late synmetamorphic mylonites acted as a channel for fluids released during the final stages of metamorphism of the St. Anthony Complex. Estimates of the deformation temperature during formation of the mylonites are relatively high (880--400°C), and it is likely that fluids migrating along the mylonites at such elevated temperatures would contain high solute concentrations of many elements. There is as yet no concensus as to whether the mylonites described above developed during thrusting in an oceanic environment near the ridge crest, or alternatively during obduction-related thrusting (see Malpas, 1979;
Major thrust structures are well-known features in most mountain ranges, and particularly in those involving continental collisions. Hubbert and Rubey (1959) in their classic paper showed how high fluid pressure (Pfluid ~-, or >, lithostatic) is necessary to float off the overthrust block. Recent seismic studies of deep continental structures show that low-angle thrust faults appear to be ubiquitous in deep continental structures. It follows that collision tectonics must be one of the major phenomena in continental crust formation. Dewey (1974) shows the typical models of continent--continent thrusting. Barazangi and Ni (1982) have analyzed recent seismic data from the Himalayas and suggest a simple model for the thrusting of India beneath Tibet producing a crustal thickness of 70 km or so. Himalayan models show thrust slices of varying complexities (Powell, 1975). While much remains to be done t o understand mechanisms of such thrust processes, there is little doubt that a double thickness continental crust can be produced at typical plate-tectonic velocities. In major thrusts as described in the Appalachians or Himalayas, where thrusting distances of hundreds of kilometers are involved, if the thrust plates are thick (15--30 km) thermal equilibrium will not attain in the time of thrusting (at 10 cm yr. -1, 104 yr. km -1 or 106 yr. per 100 km). To understand what may happen one may make a first approximation that as far as the thrust plates are concerned, thrusting is isothermal for the duration of translation. Given this assumption, if one continental block 30 km thick is thrust over another, the temperature distribution after thrusting may appear as in Fig. 3, with a corresponding metamorphic section. As time proceeds the metamorphic
358
facies distribution m a y be as in Fig. 3. Equilibrium temperature distribution will be determined b y erosion rates at the t o p and melting at the base. An excellent example o f crustal melting is provided b y the remarkable y o u n g granites of the Himalayas Hamet and All6gre, 1976; Ferrara et al., 1983) with their very high initial Sr ratios. Depth(.m) ~°C 0
t 1 --* t 2 - - *
t3--.
t 4--*
0
z
GS
z
GS
,2..%
A (Y) GS GS
30
-....~.,,~.
G$ ...... thrust plane
z
GS 60
500
A
BS I
A
E (Z)G-mig G
Moho
Fig. 3. Possible changes in metamorphic patterns following a continent--continent collision. At tI immediately after thrusting, temperatures are as s h o w n on the left. Time t2 represents changes before significant erosion or warming of the basal section. B y time t3 melting occurs at the base, producing high-STSr granites (Z = zeolite facies; G S = greenschist; A = a m p h i b o l i t e ; B S = blueschist; E -- eclogite; G -- granulite; m i g = partially m e l t e d rock). F o r further discussion see text.
Consider processes in the various parts of the overthrust structures given that each has an initial thermal gradient of = 20 ° km -1. The overthrust continental block will n o t undergo dehydration reactions b u t the underthrust block will be prograded to amphibolite-granulite. During thrusting, pore water will be expelled from region X and prograde metamorphism will occur over the entire thickness of the underplate. On average, 4% H~O m a y be lost from the underplate and such fluids will penetrate the overthrust plate (or flow along it during thrusting) by hydraulic fracture mechanisms producing vein swarms and shear zones. Per km 2 o f thrust surface, 4.5 km 3 of fluid will pass through the thrust surface. Fluids rising on a path X -~ Y will cause retrograde metamorphism in zone Y as the thermal gradient is reversed. As the once near-surface rocks
of zone X will be relatively more oxidized, such fluids m a y also tend to oxidize the overplate as described b y Beach and Fyfe {1972) and will not precipitate SiO~ in veins, b u t rather dissolve SiO2. So while "anti-quartz veins" are forming near region Y, massive veining will occur along path Z ~ X with the possible formation o f Au--quartz deposits (Fyfe and Kerrich, 1983). Different parts o f the crustal section will follow different metamorphic paths with time. Thus in region Y, the time path may be amphibolite facies-~ greenschist facies; in region X, zeolite facies -~ greenschist amphibolite. In deep regions (> 40 km), after thrusting and before thermal equilibration, blueschists and eclogites may form transiently to be replaced later by amphibolites or granulites with partial melting and formation o f granite liquids. The exact path will depend on the rate of erosion o f f the mountain structure. Temperature structure will pass through a series of intermediate states with smaller regions of inverted gradient as time progresses so that major mineralization processes will migrate with time. But it will be in the regions near the thrust surface that the most dramatic prograde and retrograde processes occur. There will also be p r o f o u n d changes in the mechanical properties of the rocks; those undergoing retrograde processes will be strong while those prograding will be weak. Once melting commences in. the base o f the section, both the latent heat of fusion of granitic materials and the scavenging o f heat-producing elements b y the melts, will prevent the base from attaining extreme temperatures but will transfer heat to prograde higher levels even more. Clearly there is good reason for intensive study o f such processes in regions such as the Himalayas. The possibilities for mineral deposits are o f great interest (Sillitoe, 1979). We have recently described U mineralization in a structure believed to represent an overthrust (Lobato et al., 1983) in Bahia, Brazil. In this region o f Lagoa Real there is clear structural evidence for Proterozoic rocks
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dipping under amphibolite-granulite facies Archean basement. Across the exposed fault region metamorphic facies are inverted with Proterozoic amphibolites overlying greenschists. In the overlying basement, highly retrograded shear zones are developed where epidote-amphibolite facies albitites carry U. The shear zones are characterized by Na metasomatism, removal of quartz, oxidation developing hematite and U mineralization. Oxygen isotope data show that quartz (81so + 11.3 to +12.1%o), plagioclase (+8.5 to +8.8%0) and magnetite (+1.0 to +1.5%o) in the parent gneisses are not isotopically concordant, but approach isotopic equilibrium in shear zones, signifying deformation temperatures of 500--540°C in the presence of fluids with ti 1sO + 5.8 to +6.4%0 and under conditions of low water/rock ratios: the fluids are interpreted to be of metamorphic origin, indigenous to the host gneisses. In domains of U mineralization, shear zones are intensely oxidized, desilicified, and characterized by the reaction of K-feldspar plus plagioclase to albite. All minerals undergo a shift of -10%o, indicating discharge of fluids 81sO -4%o at 540°C. The reservoir involved is interpreted to be release of meteoric water formation brines in the underlying Proterozoic sediments up through the Archean gneisses, during overthrusting: 1000 km 3 of solutions is estimated to have passed through these structures. The U reserves in these rocks exceed l 0 s t*. This could be in accord with transport by 1000 km 3 driven from the underplate (see Lobato et al., 1983). It is interesting that in the same region late veins carry quartz, which may reflect return to a normal geothermal gradient. Metamorphic mineralogy in both altered shear zones and the underplate indicate that the thrust plate had a thickness of the order of 10 km, similar to that proposed for the Appalachian overthrusts (Cook et al., 1980). Two further examples of fluid flow and *1 t = 1 m e t r i c t o n n e = 103 kg.
associated geochemical transport involved with thrust faults have been described by Kerrich et al. (1983), with reference to the Grenville front (Ontario, Canada) and Yellowknife thrusts (North West Territories, Canada), respectively. Translation at the Grenville front, at = 109 yr. was accommodated along two mylonite zones and an intervening boundary fault. The high- (MZ II) and lowtemperature (MZ I) mylonite zones formed at 580--640°C and 430--490°C, respectively, in the presence of fluids with ~lSO +9.5%0 (MZ II) and +7.5%o (MZ I). These fluids are interpreted to be of metamorphic origin, indigenous to the immediate rocks. A population of post-tectonic quartz veins occupying brittle fractures, characterized by 51SOquartz - 1 % o , were precipitated from fluids o f - 9 % o to -14%o at 200--300°C. The water may have been derived from downwards penetration into fault zones of low-lSO precipitation on a mountain range induced by continental collision, with uplift accommodated at deep levels by the mylonite zones coupled with rebound on the boundary faults. At Yellowknife a series of large-scale brittle ductile shear zones transect metabasalts and juxtaposed granodiorite of the Yellowknife greenstone belt (~- 2 . 8 . 109 yr.). Initial development of shear zones involved volume dilatation during brittle fracturing with migration of 5 wt.% volatiles into the shear zone from surrounding metabasalts. Early deformation involved no departures in redox state or whole-rock 5180 from background states, of Fe2÷/ZFe = 0.72 and +7 to +7.5%0, respectively, attesting to conditions of low water/rock. Shear zones subsequently acted as high-permeability conduits for pulsed discharge of > 9 km 3 of hydrothermal fluids at 360--450°C, precipitating quartz--Au lodes. Mineralized conduits in shear zones are highly reduced (Fe2÷/Z Fe > 0.9). Fluid ~lSO-values of +8%o are interpreted to signify a metamorphic origin of the hydrothermal reservoir discharging through the shear zones. This reservoir also streamed pervasively through the entire outer 2 km of the granodiorite
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batholith bordering greenstones, shifting the 5~SOquartz from +9%o in the pluton interior to +13%o at the margins. The West Bay Fault a late major transcurrent structure contains massive bull quartz (+11.4 to +12.6%o) that grew at 200--300°C from fluids of +2 to +6%0, possibly precipitated by discharge of formation brines. Thus, as speculated for the case of the Mother Lode, some examples of fracture-related Au deposits in greenstone belts may be associated with fluid release from metamorphic prograding of an underplate. In general, flow regimes in the continental fault and shear zones follow a sequence from conditions of high T--P with locally derived fluids at low water/rock, during initiation of the structures, to high fluxes of reduced metamorphic fluids along conduits as the structures propagate and intersect hydrothermal reservoirs, or induce dehydration in the underplate via thermal inversion. Later in the tectonic evolution and at shallower crustal levels, there was incursion of oxidizing near-surface fluid reservoirs into the faults. Hyndman (1980, 1983) and other workers on "metamorphic core complexes" have described involvement of granitic rocks or gneiss domes along with their cover rocks in mylonite zones and chlorite breccia, along structural discontinuities. The fluid regimes associated with these features constitute an interesting area for future research.
5. Transform faults It is well known that fluids are associated with movement on major transform faults such as the Alpine Fault of New Zealand and the San Andreas Fault of California, U.S.A. Sibson et al. (1975) have discussed what is termed seismic pumping. In this process groundwaters gradually migrate into fractures and pore spaces which progressively dilate during the long-term stress build up prior to a major seismic event, and are
suddenly forced out by pore volume collapse accompanying stress release after the catastrophic seismic failure. A spectacular example of fluid release during seismic activity came from study of the Matsushiro earthquake swarm in Japan (see Tsuneishi and Nakamura, 1970; Fyfe et al., 1978). It is of note that both in California and New Zealand, the major transforms involve thick low-grade graywacke piles which may be reaching thermal equilibrium following a trenchforming subduction episode (MacKinnon, 1983). A recent paper by Allis (1981; see also Walcott and Cresswell, 1979) presents a most interesting model of the structure along the Alpine Fault of New Zealand. From gravity, seismic and thermal data they show that the crust has attained thicknesses exceeding 40 km. They propose that the Indian plate is underthrusting the Pacific plate, creating an overthickened crust over a zone ~ 100 km wide. The vigorous hotspring activity along the fault can be no surprise as the root zone will be undergoing profound prograde metamorphism. These rather small amounts of crustal thrusting associated with transform motion may be of considerable significance for fault plane mineralization and metamorphism. Before thermal effects much influence the underplate, loading could induce blueschist metamorphism. This will rapidly prograde unless uplift rapidly follows. Such mechanisms could be of interest in the distribution of eclogites and blueschists in the Franciscan of California (see Bailey et al., 1964) and may offer explanation of the formation and preservation of aragonitic rocks which require rapid uplift for their preservation (Brown et al., 1962). Varying degrees of overthrusting along a moving transform could lead to rapid pressure--depth fluctuations. References Allis, R.G., 1981. Continental underthrusting beneath the Southern Alps of N e w Zealand. Geology, 9: 303--307.
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