Geology

doi: 10.1130/0091-7613(1986)14<56:NODSZO>2.0.CO;2 1986;14;56-59Geology

 Paul Segall and Carol Simpson Nucleation of ductile shear zones on dilatant fractures  

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Nucleation of ductile shear zones on dilatant fractures

Paul Segall U.S. Geological Survey, 345 Middlefield Road, MS 977, Menlo Park, California 94025

Carol Simpson Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061

ABSTRACT Small shear zones in granitic rocks of the Sierra Nevada, California, and near Roses

(Rosas), northeast Spain, display features which indicate that dilatant fracturing preceded the localization of ductile shear deformation. Many zones have sharp, nearly planar boundaries between their highly deformed interiors and the undeformed wall rock. The Roses shear zones narrow continuously to hairline fractures at their ends. Mineralized microcracks oriented parallel to the shear zones cluster near the zone boundaries and are interpreted as relicts of the earlier fracturing episode. Dynamic recrystallization of minerals filling these microcracks demonstrates that fracturing predated the ductile deformation. Gradients in ductile strain suggest that deformation spread laterally into the wall rocks after nucleaition of the shear zones. We suggest that cracks enhance the wall rock ductility by increasing local fluid fluxes, thereby promoting chemical alteration and/or hydrolytic weakening.

INTRODUCTION Several recent studies have focused on strain

distribution and microstructures in ductile shear zones (e.g., Ramsay and Graham, 1970; Mitra, 1979; Ramsay, 1980). Relatively little is known about how these zones nucleate or evolve once they have formed. In initially homogeneous media, shear deformation local-izes only when the material supports lower stress with increasing deformation. This stress decrease may result from strain, strain rate, or

thermal softening (Bowden, 1970; Poirier, 1980). Changes in deformation mechanism in-duced by progressive changes in fabric, micro-structure, or composition may permit locdiza-tion of shear strain (Poirier, 1980; White et al., 1980). Although most geologists recognize that rocks are rarely homogeneous, little attention has been given to the role of preexisting heter-ogeneities, such as fractures, in the formation of shear zones.

Segall and Pollard (1983a) presented evi-

Figure 1. Small-scale shear zone in granodiorlte, Mount Abbot quadrangle, Sierra Nevada, California. Aplite dike (not shown) is lett-laterally offset 1-3 m by this zone. Foliation is defined by elongate quartz aggregates and chlorite-spidote-rlch bands that trend 10° to 20° from strike of zone. Knife is 9 cm long.

dence that small strike-slip faults (shear zones) in the Sierra Nevada, California, nucleated on preexisting, mineral-filled, dilatant fractures. In this paper we show that the materials within these shear zones have been deformed by pre-dominantly ductile deformation mechanisms. We also show that similar mineral-filled cracks were precursors; to small-scale ductile shear zones within the Roses (Rosas) granodiorite, northeast Spain. Although there are significant differences between the shear zones in the two areas, striking similarities suggest that fracturing played an important role in the formation of both sets of shear zones.

SHEAR ZONES IN THE SIERRA NEVADA

Small-scale, strike-slip shear zones are exposed in the Upper Cretaceous biotite-hornblende granodiorite of Lake Edison, central Sierra Nevada, California. They are typ-ically 1 mm to several centimetres wide and up to several tens of metres long. Secondary dilatant fractures that are often quartz-filled branch obliquely into the two dilatant quad-rants near the ends of many of the zones (Segall and Pollard, 1983a). The boundaries of the zones are usually very sharp, the granodiorite outside the zone appearing undeformed. The materials within (he zones display a foliation defined by alternating quartz and epidote and chlorite-rich bancls (Fig. 1). At the outcrop scale, aplite dikes are sharply offset by these zones. Despite this the shear zones display mi-crostructural features typical of mylonites (Fig. 2), including dynamically recrystallized quartz grains which often form "ribbons," recrystal-

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Figure 2. Photomicrograph of mylonitic tex-ture within shear zone of Figure 1. Note sharp boundary between mylonite and nearly unde-formed granodiorite (arrows). Rounded feld-spar porphyroclasts (F) occur in fine-grained, quartz-chiorite-epidote matrix. Elongate quartz aggregates (q) contain subgrains and dynamically recrystallized grains. Scale bar = 1.0 mm.

lized mica, and rounded to subangular epidote and feldspar porphyroclasts.

The shear zones occur in the same outcrops with and parallel to mineralized fractures across which there has been no shear deformation. Al-teration zones in which the biotite, hornblende, and feldspar of the wall rock are altered to epi-dote, chlorite, and white mica (± calcite) form 3- to 4-cm-wide borders to both sheared and unsheared fractures (Segall and Pollard, 1983a, 1983b). The unsheared fractures contain com-pletely undeformed epidote and chlorite (± zeo-lite) fillings (Segall and Pollard, 1983a), whereas the zones with offset markers always contain deformed mineral fillings, including abundant epidote, chlorite, and quartz. The simplest interpretation consistent with these ob-servations is that the mineralized fractures formed first, and at a later time shear strain lo-calized on some of them (Segall and Pollard, 1983a).

In some cases the granodiorite adjacent to the sheared fracture has been ductilely de-formed. Figure 3 illustrates a shear zone that offsets a series of aplite dikes by ~2 m. A prominent foliation is defined by elongate clots of epidote, chlorite, and biotite, and flattened quartz-grain aggregates. There are marked vari-ations in the degree of foliation development both along and across the strike of the zone. This is particularly vivid near aplite d where

there is a strong foliation on the right-hand (south) side of the zone and little or no folia-tion on the opposite side. In addition, there is a clear gradient in ductile strain along the length of the zone (on its south side) from the highly deformed aplite a to the almost undeformed aplite c. A quartz vein within the shear zone near aplite d is presumed to continue westward and must first undergo either an en echelon step or a double bend in the central part of the map, followed by a deflection to the south. The quartz in the vein exhibits elongate ribbons, undulatory extinction, and dynamic recrystalli-zation, demonstrating that a fracture filled with quartz precipitate preceded at least a large part of the shear deformation. This suggests that the shear zone nucleated on the quartz-filled frac-ture and then spread laterally into the sur-rounding wall rock. The nonuniform strain distribution may reflect deformation concen-trated near bends and steps in the preexisting fracture.

SHEAR ZONES IN THE ROSES GRANODIORITE

Narrow (centimetre-wide) shear zones (Fig. 4) similar to those in the Sierra Nevada occur in subparallel sets in the late Hercynian Roses granodiorite, northeast Spain (Simpson et al., 1982; Simpson, 1983). They exhibit well-developed foliation, defined by aligned biotite clusters, quartz ribbons, and epidote trails, that often spreads out into the wall rock in a manner identical to the shear zones described by Ramsay and Graham (1970). The host granodiorite is slightly deformed even far away from the shear zones (Simpson et al., 1982); nevertheless, the boundaries between wall rock and shear zone are often extremely abrupt as in the Sierran examples. The tips of zones com-monly bifurcate and each strand decreases in width (cf. Simpson, 1983, Fig. 8). Near their tips the zones become cracklike with no visible foliation or offset (Fig. 4).

In thin section, the tips of several Roses shear zones were found to be mineralized dila-tant cracks with no apparent shear offset. Al-though minor ductile strain (undulatory extinction, some dynamic recrystallization) occurs in quartz grains adjacent to the cracks, quartz and feldspar grain boundaries can be matched across the cracks, indicating negligible shear offset, and the epidote crack filling is ap-parently undeformed. The cracklike termina-tions grade smoothly into the more foliated part of the shear zones; the mineral filling shows a progressive increase in ductile strain from the ends toward the main part of the zones.

In both field areas, filled microcracks are commonly observed subparallel to the main mineralized crack or shear zone (Fig. 5). The microcracks crosscut grain boundaries and are not restricted to particular minerals, although

Figure 3. East-west-trending shear zone off-setting series of aplite veins, Mount Abbot quadrangle, Sierra Nevada, California. Note that a, b, and c indicate matching aplites on either side of zone. Fracture containing highly deformed quartz (Q) Is exposed at eastern end of zone and is inferred to continue be-neath covered area (dashed line). Prominent foliation in granodiorite is observed only in vicinity of shear zone. Northeast-striking frac-tures, including some with left-lateral offset, crosscut and therefore postdate shear-zone foliation.

they are more commonly observed in feldspars. There is no evidence for crystallographic con-trol on the microcrack orientations; instead, they are consistently subparallel to and within a few millimetres of the adjacent shear zone.

Fol ia ted Granodior i te

Unfol iated Granodior i te

Covered

Quar tz Vein

F r a c t u r e

GEOLOGY, January 1986 57

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Several of the microcracks show deformation of the mineral filling. Figure 6 shows a micro-crack in feldspar that is parallel to the shear zone of Figure 5. The microcrack is filled with quartz that exhibits elongate subgrains and re-crystallized grains, demonstrating ductile de-formation of the crack filling. The angles that the long axes of the new grains make with the microcrack boundaries are consistent with the known left-lateral displacement across the main part of the shear zone (Simpson and Schmid, 1983, Fig. 10; Lister and Snoke, 1984, Figs. 3d and 9c). These observations again indicate that the brittle cracking and subsequent min-eralization must have predated the ductile deformation.

DISCUSSION The principal evidence for brittle cracking as

a precursor to ductile deformation in the Sier-ran zones is the existence of a set of parallel mineralized fractures, some of which show lat-eral offset and others of which do not (Segall and Pollard, 1983a). Those fractures with no offset contain completely undeformed filling, whereas those with offset contain ductilely de-formed mineral filling. The oblique dilatant fractures at the ends of some zones formed con-temporaneously with shear offset along the zones themselves (Segall and Pollard, 1983a). Thus, macroscopic brittle fracturing both pre-dated and accompanied localized ductile shear-ing within the zones.

Although none of the Roses shear zones are completely undeformed, there is good evidence that fracturing preceded at least some if not all of the ductile deformation within them: (1) Some of the Roses shear zones grade continu-ously along strike into fractures (Fig. 4) that have little or no offset and contain undeformed mineral fillings. That these fractures grade smoothly into the main shear zone without crosscutting the foliation indicates that the frac-turing occurred prior to or synchronously with the ductile deformation. (2) In both field areas, shear zones occur with sharp planar boundaries between the highly deformed zone interior and the undeformed, or less deformed, wall rock (Figs. 1 and 2). The common occurrence of such sharp boundaries is difficult to explain un-less the deformation localized along a preexist-ing planar structure, such as a fracture or mineralized vein. (3) Mineralized microcracks that parallel the main shear zone occur in both areas (Figs. 5 and 6). Similar microcracks are commonly observed adjacent to as well as near the ends of unsheared dilatant fractures in gran-itic rocks (Segall and Pollard, 1983b, Fig. 4). These small cracks form in response to the stress concentrations at the tip of the main frac-ture. Related microcrack arrays, or brittle process zones, are observed near the tips of opening cracks in laboratory rock-fracture experiments (Hoagland et al., 1973; Kobayashi and Foumey, 1978). As the main crack tip propagates through its process zone, micro-cracks are left behind on either side. The mi-crocracks adjacent to the Roses and Sierran shear zones are interpreted as relicts of these process zones and indicate that the earliest stage in the deformation history was dilatant crack-ing. That minerals filling some of these micro-cracks are ductilely deformed (Fig. 6) demonstrates that the ductile deformation post-dated the microcracking.

Figure 4. Map of steeply dipping left-lateral shear zones in Roses granodiorite, northeast Spain. Outcrop surface is close to true strain profile. Feather ornament = foliation in ductile parts of shear zones; stipple = xenoliths; heavy lines = aplite dikes; light lines = cracklike features with no discernible foliation. Numbered circles = drill-hole sites.

Figure S. Subparallel quartz- and epidote-filled microcracks in Roses granodiorite crosscut quartz (Q)/feldspar | F) grain boundaries with no apparent offset. Mineral infill is undeformed. Scale bar = 0.3 mm.

Figure 6. Ductile deformation of quartz f illing microcrack in feldspar. Microcrack is parallel to and about 1.0 mm away from those shown in Figure 5. Scale bar = 0.03 mm.

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b * f i l l e d , u n d e f o r m e d J c r a c k s

o^ 'Z^^X I = = = - • \

/ n u i 1 -O I

i

undeformed or s l ight ly d e f o r m e d c r a c k s

Figure 7. Model tor nucleation of narrow duc-tile shear zones on earlier dilatant cracks. Q = quartz. Details in text.

Observations in the two field areas suggest a general model for the formation of narrow duc-tile shear zones, illustrated schematically in Figure 7. The first stage (Fig. 7a) is the forma-tion of a dilatant fluid-filled fracture and its as-sociated process zone. The fracture propagates through its process zone linking through some of the microcracks but stranding others on its flanks (Fig. 7b). The aqueous fluids within the fractures react chemically with wall rock, alter-ing the granodiorite and depositing the fracture-filling minerals. The chemical exchange may also result in an increased concentration of in-tracrystalline water adjacent to the fracture.

At a later stage the applied stresses are reor-iented, imposing shear stresses on the earlier formed fractures and causing shear strain to lo-calize on the fracture-filling minerals (Fig. 7c). Marginal microcracks may or may not be de-formed at this time. As yet, we have no simple explanation why some fractures are highly de-formed while others remain unsheared. Con-trolling factors might include the mineralogy or width of the initial vein. Unsheared fractures do

tend to be narrower than deformed ones, but this may be in part due to deformation-induced widening of the zone.

With increase in strain, the ductile deforma-tion may spread laterally into the wall rock (Fig. 3), implying enhanced ductility of the ad-jacent granodiorite. This may be due to the ear-lier chemical alteration ("reaction softening," White et al., 1980), and/or to hydrolytic weakening (Griggs and Blacic, 1965; Blacic and Christie, 1984) promoted by increased concen-trations of intracrystalline water in the densely cracked and altered zone adjacent to the main fracture. The latter hypothesis is consistent with the experiments of Kirby and Kronenberg (1984) which indicate that precursory micro-cracking promotes water uptake and hydrolytic weakening in quartz single crystals. This hy-pothesis is also supported by preliminary mea-surements using infrared spectroscopy which showed increased concentrations of water within quartz in the central part of the shear zone shown in Figure 3 relative to the quartz in the surrounding granodiorite (Kronenberg et al., 1984).

CONCLUSIONS Field and microstructural observations of

narrow ductile shear zones in gTanodiorites of Spain and California suggest that brittle fractur-ing preceded ductile deformation in both areas. These observations should not be too surpris-ing, considering that fractures are ubiquitous in the earth's crust and shear deformation is well known to localize along preexisting weak zones. We suggest that mineralized fractures are natural examples of these weak zones.

The spread of deformation into the wall rock following shear-zone nucleation is a problem that is far from resolved. We suggest that en-hanced ductility of the wall rock may result from chemical alteration and/or hydrolytic weakening caused by aqueous fluids in the ear-lier dilatant fractures.

REFERENCES CITED Blacic, J.D., and Christie, J.M., 1984, Plasticity and

hydrolytic weakening of quartz single crystals: Journal of Geophysical Research, v. 89, no. B6, p. 4223-4239.

Bowden, P.B., 1970, A criterion for inhomogeneous plastic deformation: Philosophical Magazine, v. 22, p. 455-462.

Griggs, D.T., and Blacic, J.D., 1965, Quartz: Anom-alous weakness of synthetic crystals: Science, v. 147, p. 292-295.

Hoagland, R.G., Hahn, G.T., and Rosenfield, A.R., 1973, Influence of microstructure on fracture propagation in rock: Rock Mechanics, v. 5, p. 77-106.

Kirby, S.H., and Kronenberg, A.K., 1984, Hydrolytic weakening of quartz: Uptake of molecular water and the role of microfracturing [abs.]: EOS (American Geophysical Union Transactions), v. 65, p. 277.

Kobayashi, T., and Fourney, W.L., 1978, Experimen-tal characterization of the development of micro-crack process zone at a crack tip in rock under load: National Symposium on Rock Me-chanics, 19th, University of Nevada, Reno, Pro-ceedings, p. 243-246.

Kronenberg, A.K., Wolf, G.H., and Segall, P., 1984, Variations in intragranular water within a strain gradient: FTIR traverse across a ductile shear zone [abs.]: EOS (American Geophysical Union Transactions), v. 65, p. 1098.

Lister, G.S., and Snoke, A.W., 1984, S-C mylonites: Journal of Structural Geology, v. 6, p. 617-638.

Mitra, G., 1979, Ductile deformation zones in Blue Ridge basement rocks and estimation of finite strains: Geological Society of America Bulletin, v. 90, p. 935-951.

Poirier, J.P., 1980, Shear localization and shear in-stability in materials in the ductile field: Journal of Structural Geology, v. 2, p. 135-142.

Ramsay, J.G., 1980, Shear zone geometry: A review: Journal of Structural Geology, v. 2, p. 83-99.

Ramsay, J.G., and Graham, R.H., 1970, Strain varia-tions in shear belts: Canadian Journal of Earth Sciences, v. 7, p. 786-813.

Segall, P., and Pollard, D.D., 1983a, Nucleation and growth of strike-slip faults in granite: Journal of Geophysical Research, v. 88, no. Bl, p. 555-568. 1983b, Joint formation in granitic rock of the Sierra Nevada: Geological Society of America Bulletin, v. 94, p. 563-575.

Simpson, C., 1983, Displacement and strain patterns from naturally occurring shear zone termina-tions: Journal of Structural Geology, v. 5, p. 497-506.

Simpson, G, and Schmid, S.M., 1983, An evaluation of criteria to deduce the sense of movement in sheared rocks: Geological Society of America Bulletin, v. 94, p. 1281-1288.

Simpson, C., Carreras, J., and Losantos, M., 1982, Inhomogeneous deformation in Roses grano-diorite, N.E. Spain: Acta Geologica Hispanica, v. 17, p. 219-226.

White, S.H., Burrows, S.E., Carreras, J., Shaw, N.D., and Humphreys, F.J., 1980, On mylonites in ductile shear zones: Journal of Structural Geol-ogy, v. 2, p. 175-187.

ACKNOWLEDGMENTS Supported by National Science Foundation Grants

EAR-8121438 and EAR-8305846 to Simpson. Jordi Carreras provided invaluable logistical support in Spain. He Yongnian aided in the mapping for Figure 3. We thank P. Geiser, S. Kirby, A. Kronenberg, S. Martel, D. Pollard, R. Sibson, J. Tullis, and W. Means for helpful discussions.

Manuscript received May 8,1985 Revised manuscript received September 3, 1985 Manuscript accepted October 1, 1985

GEOLOGY, January 1986 Printed in U.S. A. 59

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