chlorite-smectite clay minerals and fault behavior: new ...processes (evans and chester, 1995;...

LITHOSPHERE | Volume 4 | Number 3 | www.gsapubs.org 209

RESEARCH

INTRODUCTION

The slip behavior of faults has been variably attributed to different mineralogical and fl uid processes (Evans and Chester, 1995; Vrolijk and van der Pluijm, 1999; Sibson, 2005; Wib-berley et al., 2008). While fl uid pressure has the capability to reduce the effective stress state and promote slip, mineralogic properties of fault-related rocks can similarly affect fault behavior by introducing weak mineral phases at inter-faces. Increasingly, the presence of smectitic clay minerals has been advanced as evidence for mineralogic weakening in cores collected from the SAFOD project in Parkfi eld, Califor-nia (Schleicher et al., 2006, 2010; Holdsworth et al., 2011; Bradbury et al., 2011), refl ecting the widespread occurrence of phyllosilicates and their localization in fault-rock samples. Indeed, recent laboratory results support the mechanical role and stability of smectitic clays by show-ing low friction coeffi cients in experiments on SAFOD core (Tembe et al., 2006; Morrow et al., 2007; Carpenter et al., 2011; Lockner et al., 2011) and in other natural settings (e.g., Saffer et al., 2001; Bos and Spiers, 2002; Mizoguchi et al., 2006; Smith and Faulkner, 2010; Ikari et

al., 2011). More extensive work on a variety of materials has been an important part of evalu-ating the role of clays in controlling friction (Lupini et al., 1981; Brown et al., 2003; Saffer and Marone, 2003).

In order to determine the change in shear strength and stability of clay minerals with depth, temperature, and time must be consid-ered in addition to the infl uence of fl uid and mineral chemistry. There are two keys in estab-lishing such reaction parameters: (1) Different clay mineral phases change under low tempera-tures, particularly between ~75 °C and 250 °C, and (2) different phases occur in different chemical environments (Velde, 1992). Numer-ous attempts have been made to characterize the low-temperature mineral stability fi elds of clay minerals, which are notably diffi cult to study because of their small size, variable structural composition, and relatively slow rate of forma-tion and alteration (e.g., Ransom and Helgeson, 1993, 1994; Blanc et al., 1997). Despite these diffi culties, reliable information about stability, formation mechanisms, and structural features of smectitic clays has been gained from the study of single-phase specimens (Kloprogge et al., 1999). Whereas the correlation between

experimentally determined stability fi elds and thermodynamic predictions is generally good, many natural phases formed are metastable and depend on chemical parameters such as pH, ele-ment activity, and the fl uid-rock ratio (Essene and Peacor, 1995).

The stability of pure smectite minerals is typically restricted to the uppermost 3–4 km of the upper crust (Huang et al., 1993). In this paper, we will explore the role of mixed-layer chlorite-smectite minerals, including 50:50 chlorite-smectite varieties like corrensite, which have been reported to appear at ~120 °C and persist to temperatures as high as 260 °C with decreasing smectite content (Hillier, 1993). The occurrence of localized, variably ordered chlo-rite-smectite phases in SAFOD fault-related rock would extend the role of fault weakening from clay neomineralization to a greater range of depth and host-rock compositions, infl uenc-ing fault behavior down to the brittle-plastic transition of continental rocks. Whereas weak fault behavior in the deeper crust has been mostly attributed to elevated fl uid pressure (Cox, 2002; Becken et al., 2008; Kurz et al., 2008) or the occurrence of serpentinitic phases (Moore and Rymer, 2007; Wibberley, 2007; Jung et

Chlorite-smectite clay minerals and fault behavior: New evidence from the San Andreas Fault Observatory at Depth (SAFOD) core

A.M. Schleicher1, B.A. van der Pluijm1, and L.N. Warr21DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCES, UNIVERSITY OF MICHIGAN, 1100 NORTH UNIVERSITY AVENUE, ANN ARBOR, MICHIGAN 48109, USA2INSTITUT FÜR GEOGRAPHIE UND GEOLOGIE, ERNST-MORITZ-ARNDT UNIVERSITÄT, F. LUDWIG-JAHN-STRASSE 17A, D-17487 GREIFSWALD, GERMANY

ABSTRACT

Segments of the modern San Andreas fault experience creep behavior, which is attributed to various factors, including (1) low values of effective normal stress, (2) elevated pore-fl uid pressure, and (3) low frictional strength. The San Andreas Fault Observatory at Depth (SAFOD) drill hole in Parkfi eld, California, provides new insights into frictional properties by recognizing the importance of smectitic clay minerals, as demonstrated by analysis of mudrock and fault gouge samples from zones between 3186 and 3199 m and 3295 and 3313 m measured depths. X-ray diffraction (XRD) results show illite, chlorite, and mixed-layered illite-smectite and chlorite-smectite minerals in the faulted mudrock, whereas serpentine, Mg-rich smectite, and chlorite-smectite minerals are concentrated in the southwest deformation zone and the central deformation zone of the two actively creeping sections in the San Andreas fault. These rocks are abundantly coated by shiny clay mineral layers in some cases, refl ecting mineral formation during creep. Secondary- and transmission-electron microscopy (SEM/TEM) and XRD studies of these slip surface coatings reveal thin fi lms of neoformed chlorite-smectite phases, similar to previously described illite-smectite microscale precipitations. The abundance of chlorite-smectite minerals within fault rock of the SAFOD borehole signifi cantly extends the potential role of mineralogic processes to depths up to 10 km, with cataclasis and fl uid infi ltration creating nucleation sites for neomineralization on displacement surfaces. We propose that localization of illitic to chloritic smectite clay minerals on slip surfaces from near the surface to the brittle-ductile transition promotes creep behavior of faults.

LITHOSPHERE; v. 4; no. 3; p. 209–220. doi: 10.1130/L158.1

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al., 2009; Viti and Collettini, 2009), chlorite-smectite phases offer an alternative explanation for these scenarios, because of their probable low frictional strength and stabilities across a broader range of upper-crustal conditions. Chloritic clay commonly forms in areas where mafi c lithologies are present, forming Mg/Fe-bearing chlorite-smectite instead of K-bearing illite-smectite in felsic lithologies. In both cases the hydrous smectitic interlayers affect the fault-slip behavior, but the apparent higher pressure-temperature stability of chlorite-smectite phases described in this paper extends the stability of mixed-layer smectitic clay mineralization to 8–10 km depth. Chlorite-smectite is important here because it is one of the minerals growing together with Mg-rich smectite (including sapo-nite) in the fault gouge of the San Andreas fault (Solum et al., 2006; Lockner et al., 2011), and together with illite-smectite in adjacent fault rocks of the deformation zone (Schleicher et al., 2006, 2010). In this study, we used X-ray diffraction (XRD) combined with NEWMOD modeling (Reynolds and Reynolds, 1996), as well as scanning electron microscopy (SEM) and high-resolution transmission-electron microscopy (HRTEM), combined with analyti-cal elemental microscopy (AEM), to determine microstructures and mineral compositions of chlorite-smectite within phase III fault rocks from SAFOD drilling, including samples from recently active segments of the fault.

San Andreas Fault Observatory at Depth (SAFOD)

The SAFOD drill site is situated on the Pacifi c plate near Parkfi eld, California, ~1.8 km west of the surface trace of the San Andreas fault. It penetrates a segment of the fault that accommodates displacement between the North American plate in the northeast and the Pacifi c plate in the southwest through a combination of aseismic creep and repeating earthquakes (Nadeau et al., 2004; Ellsworth et al., 2007). The SAFOD project drilled a pilot hole and a main hole with the goal to sample and instrument the San Andreas fault at depth in the creeping sec-tion, the transition zone, and the locked section (Hickman et al., 2004, 2005, 2008; Fig. 1). In 2002, the vertical pilot hole was drilled to 2.2 km through Cenozoic sediments into Salinian gran-ite with the transition at ~760 m (Solum and van der Pluijm, 2004). During phase 1 and 2, in the years 2004–2005, the main hole was drilled, fi rst vertical and then deviated by 55°, through Mesozoic Salinian granitoids into arkosic sedi-mentary rocks and alternating layers of sand-, silt-, and mudstone to a fi nal depth of 3987 m measured depth (Draper-Springer et al., 2009;

Bradbury et al., 2007; Holdsworth et al., 2011). Depths are reported here in meters measured depth (m MD), which were measured along the borehole, and they represent the distance below the drill-rig fl oor (http://www.earthscope/org/data/safod). Based on rock texture, composi-tion, and the presence of fossils at ~3360 m MD, these rocks have been identifi ed as the Cretaceous Great Valley Group (K. Mc Dougall, 2006, written commun., in Draper-Springer et al., 2009). Two different localities drilled during phase 2 show pronounced casing deformation, identifi ed through low Vp, Vs, resistivity, and natural gamma signatures in geophysical logs. Those two regions of fault creep are referred to as the southwest deformation zone, located at 3192 m MD, and the central deformation zone, located at 3302 m MD (Zoback et al., 2010; Bradbury et al., 2011). Whereas the cen-tral deformation zone is actively creeping, as documented by casing surveys (Zoback et al., 2010), repeating micro-earthquakes are thought to occur on the southwest deformation zone below its intersection with the SAFOD hole (Thurber et al., 2010). During phase III in sum-mer 2007, three boreholes were sidetracked off the main hole at a relatively high angle. Drill-ing included both deforming zones (central and

southwest deformation zones) and a third zone that cuts the boundary between sedimentary rocks of Mesozoic Salinian granitoids and the Great Valley Group. In total, a 41 m section of whole-rock core was retrieved between 3141.4 and 3153 m MD (hole E, runs 1–3), between 3186 and 3198 m MD (hole G, runs 1–3), and 3294–3312.7 m MD (hole G, runs 4–6).

Sampling and Analytical Methods

Sampling took place on-site during drilling phase III in summer 2007. Core chips, averag-ing ~3–5 cm in size, were collected directly after arrival at the surface, and carefully cleaned by removing surface drilling mud. About 20 rock samples from different depths were taken from hole G, runs 1–6 (Fig. 2). In the laboratory, the rock chips were hand-crushed to powder for bulk-rock XRD analysis. Additionally, the


Chlorite-smectite in mudrocks from the SAFOD drill hole | RESEARCH

oriented samples were used for analyzing whole-rock mineralogy. Oriented samples were used to enhance the intensities of (001) peaks, aiding the identifi cation of clay mineral phases. Air-dried and glycolated mounts were scanned from 2° to 50°, and 2° to 15°, respectively, at 0.02° 2θ step-size intervals. Chlorite, illite, smectite, and mixed-layer illite-smectite and chlorite-smectite determination was based on comparison between air-dried and glycolated samples.

Microstructures and chemistry were inves-tigated on selected samples by SEM, HRTEM, and analytical electron microscopy (AEM). We followed the analytical procedures outlined in Warr and Nieto (1998). Some rock chips from the wall rock and the fault gouge were vacuum impregnated with LR White resin to prevent collapse of the smectite interlayers (Kim et al., 1995). Afterward, the samples were cut and ground to a thickness of

SCHLEICHER ET AL.


sheared siltstones and mudstones with some polished fracture surfaces, and they belong to the foliated phyllosilicate-rich fi ne-grained unit with heterogeneous clasts and/or interlay-ers. Gouge samples between 3296.6 m and 3299.1 m MD are similar to material occurring in the upper active fault zone area. The rock samples are dark-grayish black and intensely sheared with a wavy foliation and a micro-scaly fabric. The rock samples between 3299.1 m and 3311.6 m MD are dark-gray sheared mud-stones with abundant polished striated fractures, belonging to the same unit as above.

Mineral Assemblages

The powder XRD analysis shows primary mineral components of quartz, feldspar (both plagioclase and K-feldspar), mica (both bio-tite and muscovite), and occasional serpentine. Secondary minerals are calcite, oxides, and sul-fi des, especially pyrite. The



(Figs. 3B and 3D); this peak shifts to ~3.1 nm after ethylene glycolation (Chen, 1977). The superlattice is weakly defi ned or nonexistent in a few samples containing the phase, indicat-ing the presence of a poorly ordered form with higher proportions of randomly interstratifi ed layers. Other expandable chloritic phases show 1.4 nm refl ections that broaden to 1.45–1.5 nm after ethylene glycolation. The ~0.7 nm peak is separated into 0.72 nm and 0.74 nm peaks, cor-responding to 002 chlorite/serpentinite and 004 of corrensite, respectively.

Microstructures

Representative SEM and HRTEM images in Figures 4, 5, and 6 show characteristic clay mineral assemblages and microstructural char-acteristics of the fault gouge and adjacent rocks. Confi rming the XRD patterns (Figs. 3A–3D), the dominant clay minerals in the shaly mudrocks adjacent to the central deformation zone and southwest deformation zone are illite-smectite and chlorite-smectite, as well as illite and chlorite. The backscattered SEM image

in Figure 4 reveals notably angular quartz and feldspar clasts in a clay mineral matrix with a scaly microfabric together comprising >50% of the sample. Representative TEM images show that this matrix contains aligned packets of well-developed, 10–50-nm-thick illite crystallites forming adjacent to illite-smectite crystallites

(Figs. 5A–5D). The illite edges are often altered into illite-smectite by dissolution-precipitation reactions (Fig. 5A), possibly resulting in lower concentrations of deformation-induced lattice distortions. The contacts are characterized by low-angle crystallite boundaries

SCHLEICHER ET AL.


packages that are 10–30 nm in average thickness (Fig. 5B). Many illite-smectite packets display a long-range ordering, mostly R1–R3, charac-terized by the reoccurrence of single smectite layers stacked optically parallel between 2–5 illite layers. Deformation features like bending and areas of lattice distortion are abundant, and the distribution of these features indicates that slip occurred along packet boundaries. Chlorite minerals tend to cluster together with chlorite-smectite, forming packets of 10–50 nm thick-ness (Fig. 5C). Chlorite-smectite also occurs as wavy layers with particle thicknesses of less than 30 nm and variable spacing between 2.7 and 2.9 nm and short-range ordering (Fig. 5D). Lenticular micropores are common in this area and are typical of those observed in smectite-rich materials (Alcover et al., 2000).

TEM study of the fault gouge with scaly fab-rics investigated from and near the central and southwest deformation zones showed chlorite and chlorite-smectite packages that are clus-

tered together (Figs. 6A–6D). The packages are in general >30 nm thick, with characteristically straight margins (Fig. 6A). Corrensite com-monly occurs in aggregates coexisting with dis-crete chlorite crystals (Fig. 6A). The thickness of the corrensite packages varies from 30 to 50 nm, with ~2.4 nm single layers that are interstrati-fi ed with chlorite and other chlorite-smectite. The relatively smooth transition between some particle surfaces refl ects direct alteration, likely by dissolution and re-precipitation mechanisms (Peacor, 1992; Lynch et al., 1997; Srodon, 1999; Abad et al., 2003). In Figure 6B, thin and cur-vilinear particles of varying thickness are inter-leaved with lenticular-shaped pores. The total particle thickness ranges between 10 and 30 nm. The thickness of a chlorite and smectite layer together is typically ~2.9 nm, whereas the chlo-rite lattices are ~1.4 nm thick. Some areas in the fault gouge show characteristic wavy chlorite-smectite particles of 10–30 nm average thick-ness, surrounding and partly replacing small

chrysotile minerals (Fig. 6C). Higher-contrast areas of the crystal structure indicate lattice distortion of the chrysotile due to rock deforma-tion. The clay mineral packets that surround the chrysotile tubes are >30 nm in average thickness and show deformation and strong alteration into chlorite-smectite minerals (Figs. 6C and 6D). In some instances, deformed and kinked chlorite grains were fragmented before thin neocrys-tallized chlorite-smectite particles grew in the adjacent pore space (Fig. 6D). Such relation-ships clearly indicate that multiple generations of clay mineral growth occurred in these fault-related rocks.

Chemical Composition of Chlorite-Smectite

Quantitative microchemical analyses, plot-ted in Figure 7A (as wt% oxides), show the composition of MgO/(MgO + FeO) relative to SiO

2 wt% of chlorite-smectite minerals for

14 nm

2.4 nm1.4 nm

Chlorite+

C-S

50 nm

14 nm

1.4 nm

~2.9 nm

~2.9 nm

Pore

~21Å

1.4 nm

1.4 nm

C-SC-S

ChloriteCorrensite

Chlorite+

C-S

Layer termination

C-SC-S

Chrysotile

Chlorite

Chlorite

A B

C D

Figure 6. High-resolution transmission-electron microscopy (HRTEM) lattice fringe images from rock samples within the south-west deformation zone and central deformation zone of hole G, runs 1–6. All samples show chlorite and chlorite-smectite (including corrensite), forming as straight crystals or wavy layers surrounding chrysotile minerals.





two representative fault-rock samples from the southwest deformation zone and the central deformation zone (see also Table 1). For com-parison, the compositional fi elds of chlorite, chlorite-smectite (corrensite), and Mg-smectite are shown from the well-studied hydrothermally altered mafi c rocks of La Palma, Canary Islands (Schiffman and Staudigel, 1995). The 3191 m MD sample from the southwest deformation zone contains chlorite-smectite minerals that are more Fe-rich than the La Palma rock suite, with a narrow range of MgO/(MgO + FeO) val-ues lying between 0.19 and 0.45. In contrast, the 3295 m MD fault-rock sample, positioned close to the central deformation zone, shows a broader range of MgO/(MgO + FeO) values,

between 0.26 and 0.62, refl ecting high Mg con-tent. Based on these variations, together with the broad range of SiO

2 contents for both samples

(~29–54 wt% SiO2), the chlorite-smectites

show a continuous span from chlorite-rich to smectite-rich end members. Such compositional variations in the samples could indicate a lack of equilibrium due to heterogeneous precipita-tion-dissolution mechanisms with transport of materials within the fault zone, or diversely rich Mg- and Fe-rich fl uids that circulated through the faulted rocks.

The large variety of chlorite-smectites in these samples is also seen by plotting the com-positionally modeled percentage of chlorite against its octahedrally coordinated metal con-

tent (Fig. 7B). The lowest abundances of chlo-rite layers, and hence the most smectite-rich varieties, are present in the 3295 m MD fault-rock sample. Over half of the chlorite-smectite particles measured in this sample contain ~30% or less of di,dioctahedrally or di,trioctahedrally coordinated chlorite. The other mixed-lay-ered particles contain 40% or more of the tri,trioctahedral chlorite variety, and these are the most chlorite-rich minerals (90% chlorite, 10% smectite). A similar mixture of chlorite-smectite types was detected in the 3191 m MD sample. However, the chlorite-poor,di, diocta-hedrally coordinated variety appears to be less abundant. These analyses also indicate a lower content of smectite (average ~48%, n = 22) for the chlorite-smectite minerals for this sample, compared to the 3191 m MD fault rock (average ~61%, n = 12).

DISCUSSION

There is now a general consensus on the importance of weak clay minerals in the intensely foliated fault gouge of the two active creeping sections, southwest deformation zone and central deformation zone, of the San Andreas fault zone at Parkfi eld. However, the precise mechanisms of deformation result-ing in fault creep remain a topic of continuing debate. Based on microstructural observations, Schleicher et al. (2006, 2010) and Holdsworth et al. (2011) suggested that the clay mineral fabric and localized stress-induced dissolution-precip-itation contribute to the mechanical weakness of the intensely foliated fault rock. Schleicher et al. (2010) documented in their high-resolution microscopic study the occurrence of smec-titic nanocoatings on closely spaced and inter-connected slip surfaces in isolated samples. Holdsworth et al. (2011) described similar interconnected clay networks at the micron to centimeter scale of observation across the active fault segments. Lockner et al. (2011) considered the presence of metasomatically formed trioc-tahedral Mg-smectite (saponite) in the serpen-tine gouge to explain the creep behavior, with suitably low frictional strength (µ ≤ 0.2) and a close to zero healing rate (Carpenter et al., 2011; Lockner et al., 2011), which could explain the inferred low absolute shear strength of the San Andreas fault (Lachenbruch and Sass, 1980; Zoback et al., 1987). Similar to prior work, the weakness of the material is attributed directly to the abundance of smectite in the bulk rock, but they estimated >60% content based by petro-graphic analysis (Lockner et al., 2011).

A signifi cant problem with the proposal that pure (Mg-)smectite is responsible for creep behavior of the San Andreas fault is that these

Smectite

ChloriteCorrensite

Foliated phyll-rich rock (3191 m)Foliated phyll-rich rock (3295 m)

Smectite Corrensite Chlorite

MgO

/(M

gO+

FeO

)

SiO2 (wt%)

Oct

ahed

rally

coo

rd. m

etal

con

tent 6.0

5.0

4.0

4.5

5.5

Chlorite (tri, tri)

Chlorite (di, tri)

Chlorite (di, di)

Chlorite% in C-S ml0 20 40 1008060

Schiffman and Staudigel(1995)

20 25 60504030 554535

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

A

B

Foliated phyll-rich rock (3191 m)Foliated phyll-rich rock (3295 m)

Figure 7. (A) Chemical composition of chlorite, chlorite-smectite (including correns-ite), and Mg-smectite (saponite) in the southwest deformation zone, the central deformation zone, and the adjacent area (oxide wt%). Saponite-corrensite-chlorite composition range of La Palma metabasites from Schiffman and Staudigel (1995) is shown for comparison. (B) Compositionally modeled percentage of chlorite is plotted against its octahedrally coordinated metal content (Fe2++Mg2++AlVI) based on analytical electron transmission analysis (AEM). The data show di, dioctahe-drally and di, trioctahedrally coordinated chlorite in the southwest deformation zone, the central deformation zone and the adjacent area. Calculations are on the basis of 14 oxygens for chlorite.



SCHLEICHER ET AL.


mineral phases are generally unstable at tem-peratures >150 °C (Inoue and Utada, 1991) and are therefore expected not to occur at depths >4 km (assuming the present geothermal gradi-ent). Within this context, fault weakness below ~4 km remains a question of debate. Conse-quently, additional mechanisms are invoked to explain fault weakness at these depths, such as the presence of other weak mineral phases like talc (Moore and Rymer, 2007; Wibberley, 2007), pore-fl uid pressures (Rice, 1992; Faulkner and Rutter, 2001; Jefferies et al., 2006a, 2006b; Fulton and Saffer, 2009; Wu, 2011), or stress-induced solution-precipitation creep (Holds-worth et al., 2011). Here, we propose the poten-tial role of other clay mineral phases in the fault zone in terms of their formation conditions and stability at depth, and in particular highlight the implications of the chlorite-smectite mixed-layer minerals for understanding faulting mechanisms down to the brittle-ductile transition zone.

Fault Zone Clay Mineralogy and Conditions at Depth

All mudrock samples collected during phase III drilling in the SAFOD borehole and

investigated in this study contain mixed-layer clay minerals of illite-smectite and/or chlorite-smectite (Fig. 3). The samples investigated in the southwest deformation zone and the cen-tral deformation zone (the area of recent casing deformation) contain, in addition to saponite (Lockner et al., 2011) and serpentine minerals, a characteristic assemblage of Mg-rich chlorite and chlorite-smectite. The overall mineralogy in samples taken during phase III is similar to spot-core samples and cuttings taken during drilling phases 1 and 2, as well as to the rocks in the pilot hole. The cuttings from the pilot hole taken between 488 and 914 m and between 1585 and 2012 m MD show chlorite and chlorite-smectite only in the sedimentary rock, but they are absent in granite (Solum and van der Pluijm, 2004). On the other hand, the washed cuttings from the SAFOD main hole exhibit a variety of mineral assemblages, including highly variable amounts of smectite occurring as illite-smectite and chlo-rite-smectite mixed-layer phases (Solum et al., 2006; Bradbury et al., 2007; Draper-Springer et al., 2009; Schleicher et al., 2009a, 2009b). The spot-cores taken from the main hole during phase 2 (3066 m, 3426 m, ~3300 m, ~3200 m MD) show on average similar amounts of

mixed-layer illite-smectite minerals but with varying degrees of ordering and smectite abun-dance, whereas the deepest samples at 3992 m have the lowest amount of smectite in their mixed-layered illite-smectite phases (2%–5%; Schleicher et al., 2009b).

Temperature Conditions

Many studies have been conducted on the occurrence and stability of illite-smectite (e.g., Wu et al., 1975; Junfeng et al., 1997; Agard et al., 1999; Day-Stirrat et al., 2010), but only a few investigations have been done on the stabil-ity of chlorite-smectite and/or chlorite miner-als (e.g., Schiffman and Staudigel, 1995). The average temperature for smectite-dominated, poorly ordered, illite-smectite minerals in burial environments is ~80–150 °C (Velde et al., 1986; Srodon et al., 1986; Huang et al., 1993; Pollas-tro, 1993; Blanc et al., 1997) and up to ~220 °C for illite-smectite in hydrothermal environments (Day-Stirrat et al., 2010). With a normal geother-mal gradient of 25–30 °C/km, this would refl ect stability at depthd down to ~5.5 km (Fig. 8). R3 type illite-smectite minerals with lower quanti-ties of well-ordered interlayered smectite are,

TABLE 1. QUANTITATIVE MICROCHEMICAL ANALYSES OF CHLORITE AND CHLORITE-SMECTITE USING WEIGHT PERCENT OXIDES, CALCULATED BASED ON 14 OXYGENS

Sample depth (m)

SiO2(wt%)

Al2O3(wt%)

FeO(wt%)

MgO(wt%)

MgO/(MgO + FeO)(wt%)

Sample depth (m)

SiO2(wt%)

Al2O3(wt%)

FeO(wt%)

MgO(wt%)

MgO/(MgO + FeO)(wt%)

3191 43.25 26.12 14.73 6.46 0.30 3295 45.24 22.80 10.77 10.46 0.4924.061.0187.3172.1287.4433.084.806.7167.5277.7384.078.906.0197.3223.5472.067.978.6293.3282.9205.011.1103.1110.3224.4422.009.735.8283.3298.9235.072.1109.916.3200.4403.073.0176.4244.4207.9226.085.1170.710.6249.5492.032.0190.5296.3275.0344.086.3181.7124.1273.7343.060.620.2195.5292.8426.016.7136.0146.4229.6333.080.513.0154.6226.7456.056.7124.906.5201.7343.082.649.1141.6228.6462.098.908.7166.3285.7203.082.767.6158.4202.2445.055.0169.802.9123.3592.057.746.8159.4281.9344.017.3118.7150.8173.8333.071.0162.0238.4275.4316.049.856.583.6248.8482.000.0199.5285.2231.1365.090.1155.855.4273.7433.035.0142.1221.5237.2365.065.1142.925.3261.7453.055.996.7121.7249.3305.086.4196.4124.0291.1492.027.917.3204.4239.1303.021.1125.5281.3290.9213.076.917.1229.4222.3365.044.1141.932.1223.9453.077.0102.0216.5239.2316.052.2137.765.4290.7492.060.0168.4244.3234.1315.009.0133.0169.2264.6423.088.0123.3220.4213.1384.003.0112.1188.3216.5442.037.823.8290.3219.82

31.03 23.47 23.87 11.22 0.3247.93 24.84 11.47 5.95 0.3443.72 25.68 14.41 7.47 0.3447.62 27.54 8.31 6.69 0.4542.08 24.57 15.82 7.45 0.3239.35 25.07 19.22 7.05 0.2737.73 24.22 20.09 7.60 0.2737.12 25.42 18.84 8.11 0.3035.04 23.90 20.61 10.80 0.3434.87 20.04 28.07 6.76 0.19





however, stable at higher temperatures and typi-cally persist to temperatures of ~250 °C under prograde burial conditions (Velde et al., 1986).

The conversion of smectite to chlorite, as accommodated through randomly (chlorite-smectite) or regularly ordered (corrensite) interlayers, has been reported from prograde metabasalts (Bettison and Schiffman, 1988; Bevins et al., 1991) and sedimentary sequences (Helmhold and van der Kamp, 1984; Chang et al., 1986). With increasing temperature, the Mg-dominated trioctahedral smectite phase (sapo-nite) is replaced by an ordered, 1:1 mixed-layer

chlorite-smectite (corrensite), or by vermiculite-chlorite. Corrensite is in turn replaced by trioc-tahedral chlorite in the late diagenetic zone or lower anchizone (Inoue et al., 1984; Schiffman and Staudigel, 1995). In hydrothermal systems, chlorite-smectite can form at temperatures as high as 250 or 300 °C (Suchecki et al., 1977; Kristmannsdottir, 1979). Fluid-dominated sys-tems can produce signifi cant amounts of mixed-layered clays by dissolution of higher-temper-ature crystalline phases, such as the deformed and fragmented chlorite and chrysotile grains shown in Figure 6C. Importantly, in such sys-tems, there are typically higher proportions of smectite interlayers developed at these tem-peratures in metabasite lithologies than found in equivalent illite-smectite minerals within mudrock sequences (Velde et al., 1986).

The 2.2-km-deep pilot hole in the San Andreas fault shows a variable, relatively high geothermal gradient that ranges from 27 °C/km up to 40 °C/km, with major transitions at the sediment-basement interface (770 m) and the intersection of a major shear zone at 1380 m (T1 in Fig. 8; Williams et al., 2004; Draper-Springer et al., 2009). The temperature near the bottom of the pilot hole at 2160 m (T2 in Fig. 8) is 92.5 °C. The SAFOD main hole shows very similar temperatures of 93.3 °C at ~2200 m depth (Williams et al., 2005). The average mod-ern geothermal gradient may thus be as high as 38 °C/km in the uppermost crust (C. Williams, 2006, written commun., in Draper-Springer et al., 2009). Blythe et al. (2003) used apatite-fi ssion track and (U/Th)/He analysis in the pilot hole to evaluate the geothermal gradient and exhumation history of the drill-hole site. They suggested a geothermal gradient of 35 °C/km and little or no evidence of recent exhumation. D’Alessio and Williams (2007) explored the exhumation history at the SAFOD site using thermochronometry and concluded that the area has experienced less than 1 km of exhumation or burial since the onset of the San Andreas fault activity at ca. 30 Ma. Using this geothermal gra-dient and limited exhumation, the temperatures that occur in the SAFOD borehole are suitable for the formation of smectite-containing clay minerals. Indeed, secondary illite-smectite and chlorite-smectite mixed layers occur at least down to a depth of 3311 m (Schleicher et al., 2009b, 2010) where a temperature of ~115 °C is to be expected. With a representative geo-thermal gradient of 35 °C/km, it is probable that chlorite-smectite minerals containing sig-nifi cant quantities of smectite will form within the fault zone down to 8–10 km depths, whereas the lower content of expandable components of illite-smectite minerals is expected to have less infl uence (Fig. 8). These depths are consider-

ably greater than the stability depth for smec-tite suggested by Holdsworth et al. (2011), who only considered the thermal stability of pure smectites, such as saponite, and not that of the mixed-layer chlorite-smectite composition in this study.

Fluid-Rock Interaction Processes

The presence of Mg is essential for the for-mation of the type of chlorite-smectite pres-ent in the SAFOD borehole. The reason why chlorite-smectite forms rather than chlorite is related not only to temperature, but also to the activity of Mg within the reactive fl uid and the mechanism of formation (e.g., Bettison-Varga and Mackinnon, 1997). Chlorite can form directly from solutions or as a replacement product of minerals, whereas chlorite-smectite forms primarily by replacing preexisting clay minerals or other mineral phases (Murakami et al., 1999; Leoni et al., 2010). The proportion of smectite to chlorite in mixed-layered minerals is considered to decrease with increasing tem-perature. Corrensite (ordered 50:50 chlorite-smectite), which occurs within the fault zone at Parkfi eld, is considered to be stable between 100 and 200 °C (Velde et al., 1986) and, thus, would be present down to depths of ~5–6 km. Cheshire and Gueven (2005) investigated the conversion of chrysotile to Mg smectite and concluded that fi brous chrysotile is converted in mild organic acids at 200 °C to smectite with a thin foil morphology, refl ecting our observa-tions described earlier. The Mg-rich chrysotile tubes in Figure 6C show deformation and disso-lution features along their surfaces. We surmise that the surrounding packages of 30–50-nm-thick chlorite-smectite precipitated as a lower-temperature reaction product from chrysotile. Similarly, chlorite alters to chlorite-smectite (including corrensite) at these depths.

Although the average amount of smectite present in chlorite-smectite close to or within the creep zones typically varies between 20% and 50%, as determined by XRD, there is very strong heterogeneity in the smectite content of individual chlorite-smectite particles within any one given sample (Fig. 7A). Variations between 10% and 90% smectite (90%–10% chlorite) can occur, together with differences in the octahedrally occupied metal content of chlorite (Fig. 7B). Such a broad range of chlo-rite-smectite types occurring within single fault-related rock samples indicates a complex altera-tion history that most likely resulted from the repeated infl ux of fault-related fl uids of vary-ing temperature and chemistry. It is considered unlikely that the complex assemblages resulted from the exhumation history. As the maximum

1

2

3

4

5

6

7

8

9

10

100 200 300 400

T1

T3

Temperature (°C)

T2

25 °C/km

30 °C/km

35 °C/km 38

°C/km 40

°C/km

R3 I-SCorrensite

Phase 3 core

C-SChlorite

Illite

C-SChlorite

Illite

C-SChlorite

Illite

R3 I-SCorrensite

SaponiteR0/R1 I-S

and

and

and

True

ver

tical

dep

th (k

m)

Figure 8. A temperature (°C) verses depth (km) profi le extrapolated down to 10 km depth show-ing the clay mineral stability fi elds that are to be expected within the fault zone taking into consideration a range of possible geothermal gradients for the San Andreas Fault Observa-tory at Depth (SAFOD) borehole (25–40 °C/km span). Increasing geothermal gradients indicate different mineral formation at different depths. T1, T2, and T3 are the temperatures measured in the SAFOD borehole. The dotted line shows the depth where the cores were drilled during phase III in 2007. R—reichweite, I—illite, C—chlorite, S—smectite.



SCHLEICHER ET AL.


temperature attained in SAFOD rocks is not considered to have been signifi cantly higher than current borehole temperatures, with mean vitrinite refl ectance values not exceeding 0.93% (Kirschner et al., 2006), we suggest a strong chemical control on the formation of the clay mineral phases. The diverse assemblages may result as nonequilibrium assemblages formed under conditions of localized and restricted fl uid fl ow, whereby the transport of Mg formed the limiting factor as to how much interlayered smectite formed. From this point of view, the near-pure-smectite phases represent the close-to-equilibrium phase at this borehole and the mixed-layered chlorite represents nonequilib-rium phases that grew under conditions defi -cient in limiting cations.

Implications for Fault Behavior

It is increasingly recognized that clay min-erals play an important role in fault weakening behavior, which increases with the number of water layers stored in interlayer sheets of smec-tite (e.g., Wu et al., 1975; Saffer and Marone, 2003; Ikari et al., 2007; Morrow et al., 2007; Collettini et al., 2009). In earlier contributions (e.g., Schleicher et al., 2006, 2010), we demon-strated the role of newly grown illite-smectite on displacement surfaces close to the actively creeping segment of the San Andreas fault that was cored down to ~3 km vertical depth (~3.9 km MD). Instead, samples described in this paper contain chlorite-smectite coatings on displacement surfaces, which imply that a range of clay mineral coatings can be formed that are stable down to greater depths than R0 and R1 illite-smectite coatings. Based on today’s geothermal gradient of 25–35 °C/km and the stability of chlorite-smectite, we pre-dict that the formation of clay coatings can occur as deep as 8–10 km when the activity of Mg is appropriately high (Fig. 8). We propose, therefore, that smectite-bearing clay coating, and in particular the chlorite-smectite miner-als formed by the dissolution of mafi c litholo-gies, governs brittle fault behavior down to the brittle-plastic transition.

The frictional behavior of smectite-chlorite phases has not been extensively studied yet, especially at depths where illite-smectite is not expected to be stable (cf. Fig. 8). Published work on smectite-rich fault rocks shows that they are very weak but leaves open questions about the frictional strength of chlorite-smectite phases, such corrensite, at greater depths. The stability of Mg-rich chloritic clays at higher pressure-temperature conditions offers a possible explanation for fault weakness at depth, pointing the way forward for important

experimental and analytical studies. However, laboratory experiments conducted at appropriate temperatures and pressures on hydrated chlorite-smectite phases will be needed to test this hypothesis, which would require artifi cial samples rather than naturally exhumed (or shallowly drilled) samples, which have typically been altered since their initial formation.

ACKNOWLEDGMENTS

The National Science Foundation (EAR-0345985 and EAR-0738435) and the Deutsche Forschungsgemeinschaft (SCHL 1821/1-1 and 1-2) provided fi nancial support for our SAFOD research. We thank our SAFOD colleagues for many stimulating and challenging discussions, especially on the importance of clay in fault behavior. Jafer Hadizadeh, Bob Holdsworth, Diane Moore, and Demian Saffer are thanked for constructive reviews that signifi cantly improved the paper, although we remain solely responsible for the interpretations.

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