Selective melting processes as inferred from experimentally generated pseudotachylytes

PII:

Journal of Asian Earth Sciences, Vol. 16, Nos. 5±6, pp. 533±545, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 1367-9120/98 $ - see front matter S0743-9547(98)00040-3

Selective melting processes as inferred from experimentally generated pseudotachylytes Aiming Lin* and Toshihiko Shimamoto{ Earthquake Research Institute, The University of Tokyo, No. 1-1 Yayoi 1-chome, Bunkyo-ku, Tokyo 113-0032, Japan (Accepted 11 August 1998) AbstractÐHigh-speed friction experiments have been conducted on gabbros and granites to better constrain the process of frictional melting during seismogenic fault motion. Experiments were done on cylindrical specimens of 25 mm in diameter under a normal stress of 1.0±1.5 MPa and at slip rates up to 2 m/s using a rotary-shear high-speed frictional testing machine. The experimentally-generated pseudotachylytes consist of a fused glassy matrix with abundant vesicles and angular or sub-angular to rounded fragments. These locally occur as injected network veins in the fractured rock. X-ray di€raction analysis has revealed the presence of as much as 40±70 wt% glass. The mineral contents of clasts in the experimental pseudotachylyte in granite as determined by X-ray di€raction analysis, indicate that quartz is the most resistant to melting, biotite the least, and feldspar is intermediate. Thus, the SiO2 depletion in natural pseudotachylyte glass is likely to be due to the selective melting of constituent materials during frictional melting. # 1998 Elsevier Science Ltd. All rights reserved

Introduction

1976), or is controlled by the lowest melting points of individual minerals (e.g. Scott and Drever, 1953; Sibson, 1975). The purpose of this paper is to clarify the melting mechanism and melting chemical processes during fault-generated pseudotachylyte formation, based on the results of high-speed friction experiments.

The presence of vesicles and amygdules, and high-temperature minerals occurring in varied morphologies (spherulites, dendrites, needle-like crystals etc.) indicates that some fault-generated pseudotachylytes are formed by the fusion of wall rocks as a result of frictional melting during seismic fault motion (e.g. Philpotts, 1964; Sibson, 1975; Allen, 1979; Toyoshima, 1990; Lin, 1991, 1994a,b; Magloughlin, 1992). The melt origin of natural fault-related pseudotachylyte was demonstrated unequivocally by the con®rmation of the presence of as much as 90% fused glass in the pseudotachylyte from the Fuyun fault, northwest China (Lin, 1991, 1994a,b). Some experimental results (e.g. Spray, 1987, 1988, 1995; Lin, 1991; Lin et al., 1992; Lin and Shimamoto, 1994) and rock drilling during operations (Killick, 1990; Kennedy and Spray, 1992) also show that frictional melting can occur under the conditions similar to that of seismic faulting at a depth shallower than 10 km. There is general agreement in the nature, extent and process of melting: that fusion occurs preferentially in water-rich minerals such as mica and amphibole (e.g. Allen, 1979; Lin, 1991, 1992, 1994a; Maddock, 1992; Spray, 1992, 1993). Some argue that fusion occurs by minimum melting (e.g. Philpotts, 1964), by total melting (e.g. Magloughlin, 1989), by partial melting (Wallace,

Experimental procedure A rotary-shear high-speed frictional testing machine (Fig. 1a, Shimamoto and Tsutsumi, 1994) was used to duplicate frictional processes in samples of granite and gabbro. High-velocity slip is attained by rotating one of the cylindrical specimens (25 mm in diameter and 48±50 mm in length) with a 75 kW AC servo-motor, while the other specimen is kept stationary. The machine is capable of producing slip rates of up to 2 m/s, corresponding to 1500 rpm, under an axial load of up to 1000 kg. The equivalent slip rate, Veq, is de®ned as Veq multiplied by the area of sliding equals the rate of frictional work. Assuming the same frictional coecient over the entire surface, Veq for a cylindrical specimen of diameter (r) is given by Veq ˆ 4=3 p rR where R is the revolution rate of the motor (Simamoto and Tsutsumi, 1994). Thus, a normal stress up to 100 MPa, which is close to the uniaxial strengths of most rocks, can be applied to the cylindrical specimen. Observation of the sliding interface during the experiment can be made through a transparent window cover as shown in Fig. 1a. The runs reported here were performed as a ®rst series of experiments soon after the testing machine was installed at the

* Present address (for correspondence): Department of Earth and Planetary Sciences, Faculty of Science, Kobe University, Nada-ku, Kobe 657-8501, Japan. E-mail: [email protected]. { Present address: Department of Geology and Mineralogy, Graduate School of Science, Division of Earth and Planetary Sciences, Kyoto University, Kitashirakawa-Oiwake cho, Kyoto 606-8502, Japan. 533

534

A. Lin and T. Shimamoto

Fig. 1. Photographs showing (a) specimen assembly in the high-speed frictional testing machine, (b) the plan view of the frictional surface of gabbro specimen after experiment under stereoscope, (c) ¯ow structures on sliding surface of gabbro (SEM), (d) cavernous textures on frictional surface (SEM).

Earthquake Research Institute, University of Tokyo, in, 1990. Accurate measurements of torque could not be made at this time, so this paper will not report mechanical data. The dynamic behavior of a arti®cial fault at high velocities and under large displacements are reported in Tsutsumi and Shimamoto (1994, 1996). The experiments were conducted on two gabbros and three granites. Table 1 gives the bulk compositions as measured by XRF. The gabbro samples consist of two types of grain size, 1 and 2 mm in diameter which are called ®ne-grained and medium-grained for the

convenience of this study. Both gabbro samples consist of plagioclase, pyroxene, hornblende and biotite. The gabbro samples are used to duplicate the melting process in quartz-poor and hydrous mineral-rich rocks. The granite samples can be divided into three types based on average grain size: 1, 2, and 4 mm in diameter, which are called ®ne-grained, medium-grained, and coarse-grained granite samples, respectively, as with the gabbro samples. The granite samples mainly consist of K-feldspar, plagioclase, quartz, pyroxene, biotite, and magnetite. The granite samples are used

Table 1. Bulk compositions of rocks used in frictional melting experiments analyzed by XRF (01: medium-grained gabbro; 02: ®ne-grained gabbro; 03: coarse-grained granite; 04: medium-grained granite; 05: ®ne-grained granite). FeO*. total Fe calculated as FeO

Wt%

01

02

03

04

05

SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2O5 Total

52.51 0.19 18.44 5.97 0.15 6.94 13.25 2.38 0.18 0.01 100.02

49.85 2.85 16.56 10.11 0.15 4.90 8.20 3.13 2.76 1.58 100.09

72.75 0.24 14.07 2.14 0.06 0.37 1.40 3.87 5.01 0.10 100.01

71.93 0.30 15.21 2.39 0.06 0.48 2.37 3.50 3.16 0.09 99.49

71.60 0.49 15.10 2.94 0.07 0.64 2.16 3.05 4.00 0.15 100.20

Selective melting processes for understanding the melting process in quartz-rich acid rocks. Cylindrical specimens were obtained by coring and their end faces were polished with a silicone carbide grinding wheel to make the specimen ends normal to the cylinder axis. The specimens were washed for about 20 min with an ultrasonic cleaner and dried at room temperature. Experiments were conducted for 20±40 s for gabbros and for 10±15 s for granites at the equivalent slip rates of 0.75±1.90 m/s and under the normal stress of 1±1.5 MPa. Higher normal stress could not be applied since the uniaxial strength of the specimens reduces by more than two orders of magnitude during high velocity slip due to thermal fracturing caused by the frictional heating (Ohtomo and Shimamoto, 1994). The total displacement amounts to 15±80 m for gabbros and to 7± 30 m for granites. The experimental conditions are comparable, at least in terms of velocity and stress drops, to those prevailing during a large earthquake at shallow depth (e.g. Kanamori and Stewart, 1976; Kikuchi, 1995). These displacements are much larger than fault displacements during earthquakes (approximately 10 m at the most). The normal stress applied to this study is low (1 0 1.5 MPa) compared to the stress condition estimated for natural fault-related pseudotachylyte generating processes (stress drop e 320 MPa,

535

Sibson, 1975). Since a high normal stress cannot be applied in our tests, we have increased the total displacement to several 10 m so that the total frictional work corresponds to that of much smaller displacements at much higher normal stress. All run products were thin sectioned to observe the microstructures using a polarizing microscope, and an analytical SEM for determining chemical compositions. Six specimens (two ®ne-grained type gabbros and four ®ne-grained type granites) were analyzed using powder X-ray di€raction.

Microstructures Gabbro samples The simulated fault (sliding interface) in gabbro began to spark and produce dust within a few seconds of the initiation of fault motion and changed to cherry-red in color after 5±7 s to produce frictional melt. A pungent burnt rock smell was created during frictional melting and cherry-red melt began to extrude from the fault surface after 9±11 s. The two cylindrical specimens were separated as soon as the experiment ended, before the fault was

Fig. 2. Photomicrographs showing (a) and (b) textures of the experimentally-generated pseudotachylyte veins along the frictional zone and in network veins injected into the host rock (SEM), (c) microstructures of rounded fragments and vesicles in the experimentally-generated pseudotachylyte (SEM), and (d) an enlargement of the same specimen shown in (c). The thin section was cut normal to the shear zone and parallel to the sliding direction. Note that most fragments are rounded in shape and aligned parallel to the main pseudotachylyte vein. Pl:plagioclase; Py:pyroxene.

536

A. Lin and T. Shimamoto

bonded. The circular frictional surface can be divided into three ring zones by its textures: the central zone (A), middle zone (B), and outer zone (C) (Fig. 1b). The heat production rate is small in the central zone, because of low slip rate, and heat is lost from the outer zone, so that temperature is highest in the middle zone, as shown by Tsutsumi and Shimamoto (1994). The central and outer zones (A, C) are the areas of fragment build up cemented with small amounts of fused glass, locally showing glassing lustre. The middle zone (B) occupies about half the radius of the circular surface and shows a morphology similar to fresh lava, exhibiting vesicles, cavities, and roughness (Fig. 1b±d). In one run, the two facing specimens of gabbro were left bonded or observations of the original texture of the frictional zone. The textures observed on the cylindrical frictional faces here are very similar to that of Lin (1991), Lin and Shimamoto (1994) and Spray (1995). SEM±BSE photomicrographs of gabbro (Fig. 2a and b) clearly display that the main frictional melting zone, a few to 120 mm in width and parallel to the simulated fault, branches into veinlets (submicrons to a few microns in width) similar to the fault veins described by Sibson (1975). The main melting zone has irregular boundaries with the host gabbros, indicating fracturing and local spalling-o€ of the fragments from the host rock. The branching veins are quite irregular in form and are generally much thinner than the melt-

ing zone, but can mostly be traced back to the melting zone. The injected vein is wider at the branching point as shown in Fig. 2b. Experimental pseudotachylyte in gabbro comprises sub-angular to rounded clasts of pyroxene and feldspar derived from the host gabbro and glassy matrix (Fig. 2). Porphyroclasts of varied sizes, but smaller than 40 mm in diameter, are generally aligned parallel to the main melting zone (Fig. 2a and b). The matrix, including some ®ne-grained clasts which are generally submicron in size, appears relatively homogeneous, texturally. Most vesicles are circular, but some are elliptical (Fig. 2c and d). Granite samples High velocity friction experiments are much more dicult to perform for granite than for gabbro, since granite contains quartz, which undergoes a±b transition during frictional heating, which results in thermal fracturing (Ohtomo and Shimamoto, 1994). The host granite often fractures into pieces, consequently injection pseudotachylyte veins, such as those generated in the gabbro experiments, are very dicult to produce. Simulated faults in granite sparked dust in the ®rst 5±6 s of the high velocity tests. The fault surfaced glowed cherry red in color and partially cherry red run products were extruded (Fig. 3a). Hair-like run products are also typical of coarse-grained type

Fig. 3. Photographs showing (a) broken-ring shaped melt product in granite specimen thrown out from the simulated fault, (b) transparent and ®brous run products in granite specimen with ®bers elongated parallel to the shear direction as observed under stereoscope, (c) ma®c mineral fused and smeared out on the fault surface of granite, (d) microstructures of run products in granite specimen (SEM).

Selective melting processes

537

Table 2. Chemical compositions of glass matrix of experimental pseudotachylytes in medium-grained gabbro, and analyzed by EDX

Wt%

Ga1

Ga2

Ga3

Ga4

Ga5

Ga6

Ga7

Ga8

Ga9

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O Cr2O3 NiO P2O5 Total

53.09 0.18 16.88 6.15 0.26 7.52 12.27 1.83 0.23 0.05 0.62 0.18 99.26

53.22 0.13 16.28 6.51 0.00 9.30 12.26 1.90 0.27 0.00 0.29 0.43 100.59

53.24 0.16 18.92 5.42 0.11 5.80 12.04 2.54 0.13 0.19 0.00 0.32 98.87

53.94 0.00 24.24 2.46 0.04 3.12 12.21 3.33 0.24 0.03 0.00 0.13 99.72

52.17 0.36 1.20 8.24 0.16 13.60 20.68 0.00 0.11 0.16 0.07 0.19 96.94

52.83 0.00 25.25 2.56 0.04 3.02 12.20 3.43 0.24 0.04 0.00 0.12 99.73

52.36 0.25 14.97 7.94 0.06 8.32 11.80 1.42 0.26 0.16 0.35 0.27 98.16

52.65 0.50 1.11 8.53 0.45 14.42 21.10 0.16 0.00 0.00 0.36 0.21 99.49

52.38 0.35 1.36 9.11 0.16 14.08 20.60 0.01 0.02 0.23 0.23 0.29 98.82

granite (Fig. 3b). The experiments were often terminated because of excessive granite fracturing. It was, thus, dicult to make oriented sections of granite specimens. Some ma®c minerals were fused and ¯owed as black bands parallel to the slip direction on the frictional surface (Fig. 3c). The transparent or translucent hair-like products are elongated parallel to the shear direction. The experimental pseudotachylyte, derived from granite, mainly comprises angular or subangular to rounded quartz, feldspar fragments, and the glassy matrix (Fig. 3d). The ¯ow structures can also be observed in the SEM±BSE photomicrographs.

Chemical compositions Bulk compositions of all samples used in the frictional experiments were analyzed by XRF (Table 1). SEM±EDS analyses were made on C-coated, polished thin sections in a JSM 804 apparatus with Link-systems (AN10/50) energy dispersive X-ray analyzer (EDX). Defocused beam and raster beam areas of about 5 mm in diameter were typically used. The chemical compositions of the run products were analyzed at the same time as the samples and in the same mode for the samples (Tables 2±6). Clasts were avoided for chemical analysis of the glassy matrix. However, ultra®ne clasts could not be avoided,

because of ultra®ne size and similarity of compositions under the SEM±EDX images. Gabbros The glassy matrix derived from the medium-grained type gabbro sample is heterogeneous in chemical composition whereas that of the ®ner grained gabbro exhibits a more homogeneous composition than the medium-grain gabbro (Fig. 4). The chemical compositions of glassy matrices are somewhat lower in SiO2 content than the host rock for the ®ne-grained type gabbro samples, and locally similar to that of plagioclase (Tables 2 and 3, Fig. 4) and pyroxene contained in the host medium-grained type gabbro samples. This indicates that some melts were formed by fusion plagioclase, and not fully- mixed, and were then quenched into glass in the medium-grained type gabbro samples. Granites The glassy matrix of three types of granite, all have SiO2 contents lower, by a few wt% to 30 wt%, than the host granites (Tables 4±6, Figs 5 and 6). The matrix compositions in the coarse-grained type granite are mainly concentrated in two areas as shown in Fig. 5. One is the composition similar to that of original pyroxene contained in the host rocks (Table 4,

Table 3. Chemical compositions of glass matrix of experimental pseudotachylytes in ®ne-grained gabbro, analyzed by EDX

Wt%

1

2

3

SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O Cr2O3 NiO P2O5 SeO Total

47.53 3.54 16.33 9.88 0.16 6.32 6.98 2.64 3.78 0.14 0.08 1.77 0.07 99.22

49.18 3.02 16.86 8.57 0.20 5.18 5.88 2.63 5.14 0.08 0.00 1.43 0.70 98.87

46.91 3.56 16.23 10.11 0.15 5.41 7.46 2.66 2.92 0.00 0.08 1.60 0.26 97.35

Glass matrix in ®ne-grained gabbro sample 4 5 6 47.78 3.28 16.88 9.60 0.22 5.95 7.25 3.02 2.98 0.00 0.26 1.80 0.40 99.42

46.83 2.99 16.40 10.43 0.21 5.74 7.09 3.07 3.16 0.00 0.00 1.59 0.17 97.68

47.27 3.35 16.97 9.96 0.24 5.60 7.37 2.70 3.10 0.00 0.15 1.55 0.20 98.46

7

8

9

47.17 3.23 15.75 10.31 0.11 6.27 7.48 2.80 3.48 0.05 0.07 1.58 0.39 98.69

46.32 3.45 16.40 10.04 0.00 6.28 6.93 2.76 3.54 0.14 0.48 1.45 0.41 98.20

46.62 3.62 16.35 10.28 0.09 6.22 6.92 2.48 3.59 0.06 0.01 1.60 0.34 98.18

538

A. Lin and T. Shimamoto Table 4. Chemical compositions of glass matrix of experimental pseudotachylytes in coarse-grained granite, analyzed by EDX

Wt%

1

2

3

SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O Cr2O3 NiO P2O5 SeO Total

65.56 0.09 18.75 0.94 0.00 0.00 1.02 5.81 5.00 0.00 0.00 0.35 0.60 98.12

65.62 0.20 19.12 2.41 0.00 0.30 1.02 4.02 6.83 0.00 0.10 0.68 0.20 100.50

66.24 0.31 17.26 1.73 0.00 0.04 0.85 4.09 6.29 0.07 0.08 0.11 0.00 97.07

Glass matrix in ®ne-grained gabbro sample 4 5 6 64.18 0.29 18.49 2.18 0.04 0.30 1.02 3.98 6.24 0.09 0.00 0.36 0.19 97.36

columns 8±9). The other is similar to that of feldspar (Table 4, columns 1±7). No pure silica matrix could be found in the run samples. The data of oxides SiO2, Al2O3, FeO, MgO, CaO, Na2O, and K2O also shows, clearly, that both the original pyroxene and feldspar melted, but remained almost unmixed. There is also a relatively large variational range in the chemical compositions of matrices of run products in the ®ne-grained and medium-grained type granite samples (Tables 5 and 6). The Al2O3 content is almost the same as that in the plagioclase contained in the host granite. Generally, the FeO and MgO contents are about 2.0% higher than those in the bulk composition of the host granite samples. The CaO, Na2O and K2O contents are also a little higher than those of the host rocks.

Powder X-ray di€raction analysis In order to determine the content of fused glass and major constituent minerals in the experimental pseudotachylytes (mixture of fused glass and clasts), six samples: two ®ne-grained type gabbro samples and four ®ne-grained type granite samples, were selected for analysis by the powder X-ray di€raction method. Such analyses are impossible with conventional ana-

70.27 0.09 16.56 0.71 0.05 0.18 0.71 4.29 6.20 0.09 0.04 0.54 0.52 100.15

67.40 0.26 17.86 1.01 0.05 0.25 0.93 4.22 7.15 0.05 0.06 0.37 0.62 100.15

7

8

9

68.36 0.32 17.33 1.29 0.02 0.21 0.90 4.38 5.59 0.06 0.03 0.48 0.53 99.50

47.78 11.08 13.67 7.82 1.49 0.38 1.74 2.79 4.67 0.03 0.04 0.68 0.50 92.67

46.72 11.31 13.26 8.23 1.56 0.22 1.73 2.73 4.47 0.04 0.00 1.66 0.52 91.46

lytical methods since fused glass and ultra®ne clasts cannot be separated. An MXP SCIENCE X-ray diffractometer was used to obtain the di€raction patterns. The measurement conditions were: ®ltered CuKa (1.54050AÊ) radiation, X-ray generator 3 kW, 40 kV, 20 mA, sampling width 0.028, scanning speed 3.08 min., divergence slit 1.08, scattering slit 1.08, receiving slit 0.15 mm, 2y position 5.08. Di€raction patterns of experimentally-generated glass materials Figure 7A and B, respectively, show X-ray di€raction spectra of four specimens of ®ne-grained type granite and two specimens of gabbro. The spectra of the gabbro and granite host rocks are also shown in Fig. 7A-a and Fig. 7B-a. All the X-ray di€raction spectrograms of the run products exhibit a very broad band with 2y ranging from 158 to 428, which is very similar to that of glassy volcanic rocks, such as obsidian and fault-generated glassy pseudotachylyte reported by Lin (1994a). There is no broad band in the spectra of the host rocks of gabbro and granite (Fig. 7A-a and 7B-a), and glass or non-crystalline materials are clearly present in the experimentallygenerated pseudotachylytes.

Table 5. Chemical compositions of glass matrix of experimental pseudotachylytes in medium-grained granite, analyzed by EDX

Wt%

1

2

3

SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O Cr2O3 NiO P2O5 SeO Total

61.12 0.51 20.52 3.72 0.18 0.62 3.11 3.71 4.36 0.00 0.00 0.34 0.00 98.19

64.53 0.17 20.37 1.52 0.02 0.34 3.55 5.00 3.67 0.01 0.00 0.14 0.33 99.65

65.51 0.14 20.68 1.74 0.08 0.50 3.59 4.98 3.06 0.10 0.10 0.28 0.07 100.83

Glass matrix in ®ne-grained gabbro sample 4 5 6 61.67 0.42 19.78 3.80 0.01 0.83 3.07 3.48 4.51 0.00 0.00 0.28 0.10 97.90

63.12 0.48 20.54 3.29 0.00 0.55 2.59 3.92 3.40 0.05 0.03 0.29 0.00 98.26

63.10 0.35 21.39 3.65 0.01 0.67 3.60 4.54 3.26 0.03 0.05 0.26 0.00 100.91

7

8

9

60.81 0.49 20.58 3.76 0.08 0.72 2.95 3.56 5.14 0.00 0.06 0.37 0.11 98.63

67.31 0.29 17.15 2.65 0.10 0.66 2.68 3.59 3.40 0.06 0.06 0.49 0.59 99.02

41.13 1.85 20.21 18.45 0.74 8.70 2.20 1.58 3.16 0.03 0.00 0.15 0.17 98.37

Selective melting processes

539

Table 6. Chemical compositions of glass matrix of experimental pseudotachylytes in ®ne-grained granite, analyzed by EDX

Wt%

1

2

3

SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O Cr2O3 NiO P2O5 SeO Total

65.70 0.41 17.48 2.48 0.04 0.44 2.56 4.18 3.64 0.00 0.01 0.05 0.04 97.03

61.58 0.45 19.98 3.56 0.05 0.79 2.85 3.88 4.58 0.01 0.01 0.06 0.06 97.86

63.53 0.47 17.87 3.85 0.00 0.69 2.57 3.70 4.40 0.01 0.00 0.05 0.07 97.21

Glass matrix in ®ne-grained granite sample 4 5 6 62.43 0.37 19.32 3.20 0.05 0.74 2.56 4.00 4.93 0.01 0.00 0.04 0.04 97.69

The di€raction peaks of pyroxene and feldspar are present both in the host gabbro and its run products, and those of feldspar and quartz are present in the granite and its run products, although the integrated intensities of all peaks are smaller than those of their host rocks (Fig. 7). The mica peaks in the 2y position of 98±108, clearly observed in both the gabbro and granite host rocks, cannot be recognized in the X-ray spectra of run products, indicating that mica crystals were preferentially melted into fused glass even though other minerals were only partially melted. Quantitative analysis Quantitative analyses of fused glassy and mineral clasts have been performed based on the di€raction data for standard specimens in Fig. 8A. The glass for standard specimens was prepared by fusing the ®negrained granite and ®ne-grained gabbro at 15008C for 1 day and quenching in water. It was con®rmed by SEM and powder X-ray di€raction methods that some

Fig. 4. SiO2±Al2O3 ``All other components'' diagram showing the compositional variation of the glassy matrix generated in ®ne-grain gabbro, medium-grain gabbros, and the host gabbros used in the experiments. Pl and PyL plagioclase and pyroxene included in the host gabbro.

61.26 0.48 19.82 3.38 0.18 0.60 2.82 3.86 4.86 0.00 0.00 0.04 0.06 97.36

61.73 0.52 19.97 3.61 0.08 0.57 2.91 3.81 4.40 0.00 0.00 0.07 0.04 97.71

7

8

9

64.02 0.25 19.93 1.95 0.01 0.17 3.21 4.76 3.43 0.00 0.00 0.01 0.02 97.76

64.86 0.52 18.61 3.59 0.03 0.72 2.42 3.26 4.46 0.00 0.02 0.06 0.02 98.57

50.07 1.04 19.61 11.98 0.48 1.11 3.87 3.40 2.76 0.00 0.01 0.05 0.03 94.41

vesicles exist, but no fragments remained in the fused glass. Six standard samples made up of 0, 10, 20, 50, 75, and 100 wt% arti®cially-fused glass were prepared by weighing and mixing the powders of the arti®ciallyfused glass and the host rocks for the gabbro and granite samples, respectively. Their X-ray di€raction spectra are shown in Fig. 8A and B. All the spectra, except those of the host rocks of gabbro and granite samples (no glass, Fig. 7A-a and 7B-a), show a broad band with 2y ranging from 128 to 428. The areas of the broad bands indicating the integrated intensity generated by the glassy material, clearly increase with increasing content of glass, as shown in Fig. 8 from spectra b to e. The peak intensities for the crystalline materials in the same 2y position, in these spectra, decrease with an increase of the glass content. The integrated intensities of glass (Ig) in the standard samples were measured using the triangle approximation method, as shown in Fig. 8B, and were plotted against the glass content (Wg: weight percent) in the Ig±Wg diagram, as shown in Fig. 9. The straight lines

Fig. 5. SiO2±Al2O3 ``All other components'' diagram showing the compositional variation of the glassy matrix generated in coarsegrain granite and the host granite used in the experiments.

540

A. Lin and T. Shimamoto

Fig. 6. SiO2±Al2O3 ``All other components'' diagram showing the compositional variation of the glassy matrix generated in ®ne-grain granite, medium-grain granite, and the host granites used in the experiments.

of calibration curves were obtained by using the method of least squares. The integrated intensities of the experimentally-generated pseudotachylytes in gabbro and granite were

measured and plotted on the calibration curves of Fig. 9A and B, respectively. The glass contents (Wg) read directly from the calibration curves are 44.5±52.3 wt% glass matrix in gabbro samples and 43.4±62.5 wt% glass matrix in granite samples. This means that there are about 50 wt% fragments mixed in the melt material, as observed under the microscope and the electron microscope (Fig. 2). Crystals of the main materials, such as quartz and feldspars, remaining in the melt material of the granite samples were also analyzed quantitatively using the method as described in Lin (1994a). The integrated intensities (Iq) of quartz peaks in the 2y position of 268±278 in the spectrograms b±f shown in Fig. 8A, which are the strongest quartz peaks, were measured and plotted against the quartz content contained in the standard samples. It was found that the intensity of quartz peaks in the same 2y position increase with an increase in amount of crystal material as a linear function. Using the total original quartz crystal content contained in the host granite samples as 100 wt% and the total original quartz crystal content contained in complete glass standard sample as 0 wt% in the calibration curve, the calibration curve was obtained, as shown in Fig. 10A. The integrated intensities of the quartz peaks in the same 2y position in the run products were measured and plotted on the calibration

Fig. 7. X-ray di€raction spectra of experimentally-generated pseudotachylytes in (A) ®ne-grain granite and (B) ®ne-grain gabbro. Di€raction data in (a) are for the host rocks and those in (b)±(f) for experimentally-generated pseudotachylyte. Ma:mica; Pl:plagioclase; Qz:quartz; Py:pyroxene.

Fig. 8. X-ray di€raction spectra of standard samples for (A) ®ne-grain granite and (B) ®ne-grain gabbro. Data for (a) to (f) correspond to the glass contents of 0, 10, 20, 50, 75, and 100 wt%, respectively. Ma:mica; Pl:plagioclase; Qz:quartz; Py:pyroxene.

Selective melting processes 541

542

A. Lin and T. Shimamoto

Fig. 9. Calibration curves for the content (Wg) of glassy matrix relative to the bulk specimen for (A) ®ne-grain gabbro and for (B) ®ne-grain granite. Vertical axes show the integrated intensities in X-ray di€raction spectra.

curve. The quartz contents in the run products read directly are 45.5, 71.9, 75.8, and 82.9 wt% of the original quartz content in the granite samples. This means that about 17±54 wt% original quartz crystals contained in the host rocks were melted in the run products. In the same way, the calibration curve (Fig. 10B) was also obtained for the feldspars (using the plagioclase in the peak of 2y position of 278±288) from the spectra of standard samples shown in Fig. 7. The integrated intensities of the peaks of plagioclase crystal were measured and plotted in the calibration curve, and the plagioclase contents read directly from the curve are 30.5, 35.0, 34.5, and 39.0 wt% (Fig. 10B) of those contained in the host rocks.

Discussion and conclusions Vein geometry Most of the melting-originated pseudotachylytes described in the literature have striking similarities: dense and aphanitic appearance, occurrence as irregular veins intruded into country rocks in both simple

and complex networks, and generally, thicknesses of a few mm to a few cm. It was Shand (1916) who ®rst described and sketched the occurrence of irregular, branching pseudotachylyte veins in the Vredefort region, South Africa. Sibson (1975) classi®ed fault-generated pseudotachylyte veins into two fundamental classes of veins: (1) fault veins, lying along markedly planar shear fractures on which the pseudotachylyte had been generated, and (2) injection veins, intruded into the country rocks and often appearing as dilational veins along which there are no lateral o€set of markers. The di€erent styles of fault vein possessing variable thicknesses and injection veins showing complex networks and pinch and swell are successfully simulated by the high-speed frictional melting test in this study. The question is whether the pseudotachylyte injection veins are formed by rapid intrusion of melt or by the rapid intrusive-like spraying in a gas-solid-liquid system during seismic faulting, as suggested by Lin (1996, 1997a). Injection veins of crushing-originated pseudotachylyte and fault gorge occur in the Iida± Matsukawa fault zone, and the Nojima fault, Japan (Lin, 1996, 1997a; Lin et al., 1994, 1998), which are similar to those found in the melting-originated pseu-

Fig. 10. Calibration curves for the contents of (A) quartz crystal and (B) feldspar (albite) crystal in ®ne-grain granite specimens. Vertical axes show the integrated intensities of albite peaks in X-ray di€raction spectra.

Selective melting processes dotachylyte injection veins (e.g. Shand, 1916; Sibson, 1975; Lin, 1994a,b). Field occurrences, powder X-ray di€raction patterns, chemical compositions, and the size-distribution patterns of clasts show that these injection veins have the same source materials as the fault veins and the granitic country rock (Lin, 1996, 1997a). Therefore, it is suggested that the injection occurred by rapid ¯uidization of ®ne-grained clasts generated in the shear zone during seismic faulting. Fluidization is de®ned as ``the mixing process of gas and loose ®ne-grained materials so that the whole ¯ows like a liquid'' (Glossary of Geology, Bates and Jackson, 1987), and was ®rst introduced to explain certain geologic phenomenon by Reynolds (1954). Although the term ¯uidization is applied speci®cally to gas±solid systems, it is strikingly applicable to a suspension of solid particle in an upward ¯owing stream of liquid which has a lower density than that of the particles. The authors have also observed that clasts may exceed 600 70% in volume in some injection veins of the typical melting-originated pseudotachylytes, such as those described by Sibson (1975), Toyoshima (1990), Lin (1994a,b), and Camacho et al. (1995). It is impossible that the solid clasts intruded into the cracks a few meters beyond the source fault plane by ¯owing only with 30±40% liquid (melt) without pressured gas. It is also shown that there are about 50 wt% fragments mixed with the melt material in the experimental pseudotachylyte. These arguments support the suggestion that the clasts mixed with the melt were injected into open space generated during seismic slip by rapid intrusive-like spraying in a gas± solid±liquid system. Substantial cavity development may accompany seismic slip in strong rocks at depths of several kilometers (Sibson, 1984). These cavities form transitory low-pressure channels and are particularly favorable sites for the rapid passage of ¯uidized particles. Melting textures The evidence obtained from natural pseudotachylytes (e.g. Sibson, 1975; Lin, 1994a,b) and experimentally-generated pseudotachylytes (e.g. Spray, 1987, 1988, 1992, 1995; Killick, 1990; Lin, 1991, this study), undoubtedly, demonstrates that the melting-originated pseudotachylytes can form by frictional heating under a shallow depth of Earth's crust. The structures developed in this study clearly show that the frictional melting can form even under a normal stress (as small as 1.0±1.5 MPa) at high slip rates (1.0±2.0 m/s) corresponding to that of seismic faulting under a shallow depth of the Earth's surface. Furthermore, the vesicles and angular±subangular to rounded clasts which are found in natural pseudotachylyte are also generated in the experimentally-generated pseudotachylytes. The roundness of the clasts included in the pseudotachylyte veins is suggested as a possible indicator of melting (Lin, 1997b). The roundness (Rd) is de®ned in one plane as Rd = S(ri/R)/n, where ri is the radius of curvature of the corner, R is the radius of the maximum inscribed circle in the plane of measurement, and n is the number of corners of the fragment in the given plane. Roundness is destroyed or diminished by fracturing and chipping, and a high degree of roundness

543

is, therefore, often an indication of gentle conditions of wear relative to the size, hardness, and toughness of the fragments. The roundness of quartz and plagioclase fragments included in the well-known meltingoriginated pseudotachylytes described by Sibson (1975), Toyoshima (1990), and Lin (1991, 1994a), and the crushing-originated pseudotachylyte as well cataclastic rocks, were measured by Lin (1997b). The measured results show that the roundness (Rd) is lower than 0.4 in the crushing-originated pseudotachylyte and cataclastic rocks, and varies from 0.1 to 0.9 in the melting-originated pseudotachylytes. This suggests that the high degree of roundness (Rd>0.4) probably formed by frictional melting rather than fracturing or chipping (Lin, 1997b). Some fragments included in the experiment pseudotachylytes, as shown in Fig. 2, have a high degree of roundness (Rd>0.4). This shows that these rounded fragments probably formed by frictional melting during the experiment. There are many more clasts in fault veins than there are in injection veins, as shown in Fig. 1. This is suggested as a criterion for distinguishing fault veins from injection veins in natural pseudotachylytes (Magloughlin and Spray, 1992). Microlitic structures found in natural pseudotachylytes (e.g. Lin, 1994b) and experimental pseudotachylyte (Spray, 1988) could not be simulated in this experiment. The cooling times may have been too short to form the microlites at a low normal stress.

Melting and chemical processes The average chemical compositions of natural pseudotachylytes are commonly similar to those of the rocks in which they occur. It has been suggested that frictional fusion involves total melting rather than selective melting of the country rock (e.g. Philpotts, 1964; Ermanovics et al., 1972; Masch et al., 1985). Because it is impossible to free the ®ne-grained clasts from the pseudotachylyte, the average composition of matrix and fragments has a similar composition to the country rock. However, it is found that the chemical composition of the matrix of pseudotachylytes generally has a lower SiO2 component than that of the host rock in which the pseudotachylytes occur (e.g. Shand, 1916; Sibson, 1975; Toyoshima, 1990; Lin, 1991, 1992, 1994a; Spray, 1992, 1993; Lin and Shimamoto, 1994). The detailed SEM±EDX analytical data show that the matrices of the arti®cially-generated and natural pseudotachylytes are relatively more basic and hydrous than their host rocks (Spray, 1992, 1993). Lin (1994a) found that the average chemical composition of the Fuyun fault-related pseudotachylyte are very similar to that of the granitic country rock, but the chemical composition of glassy matrices are 5±10 wt% lower in SiO2 components than that of the granitic country rock. He also found that 11 wt% quartz fragments of the granitic country rock remained, and micas and feldspars completely disappeared in the glassy pseudotachylyte, on examination, using powder X-ray di€raction analysis. From these data, it was concluded that the Fuyun pseudotachylytes formed mainly by preferential melting of hydrous minerals rather than total melting or

544

A. Lin and T. Shimamoto Table 7. Formation depth estimated for the fault-generated pseudotachylytes reported in literature so far

Location

Formation in depth (km)

Temperature (8C)

References

Outer Hebrides Thrust, Scotland Woodro€e Thrust, Australia

4±5 <5

1100

Alpine Fault, New Zealand

2.2 2±7 1.6 4 1.5

Sibson (1975) Allen (1979) Camacho et al. (1995) Seward and Sibson (1985) Wallace (1976) Maddock et al. (1987) Toyoshima (1990) Lin (1991, 1994a, b)

Ikevaqu, Greenland Hidaka Metamorphic Zone, Japan Fuyun Fault Zone, China

partial melting of the country rocks (Lin, 1991, 1994a; Lin et al., 1992). In the ®ne-grained type gabbro experiments, the chemical compositions of the matrices are heterogeneous, but the chemical compositions of glassy matrices are 5±10 wt% lower in SiO2 components than that of the host rock. This can be explained by the fact that the original quartz crystals included in the host rock were less melted and the SiO2-poor hydrous minerals were preferentially melted. Most of the chemical compositions of glass matrices are di€erent to the bulk compositions of the host rocks but similar to that of the original plagioclase and pyroxene contained in the medium-grained type gabbro sample (Fig. 4). This shows that the plagioclases and pyroxenes melted without chemical reaction. Most of pseudotachylytes found in the world occur in granitic rocks, therefore, the frictional melting experiments in granitic rock provide important information for understanding the melting process of natural fault-related pseudotachylyte. This kind of experiment has also been made previously on Westerly granite by Spray (1992, 1993). It was observed that SiO2 contents are about 5 wt% lower than that of the protolith (Spray, 1993). It is not clear, however, whether quartz crystals were much less melted than feldspar in the granitic rocks during frictional melting, because the 5 wt% SiO2-poor could be formed by preferential melting of SiO2-poor hydrous minerals. In our experiments, the chemical compositions of glassy matrices have low SiO2 components which are a few wt% to 30 wt% lower than that of the host granite (Figs 5 and 6). It was shown that rapid heating due to friction induces the beginning of melting of plagioclase by selective fusion of albite components (Johannes, 1989). It was also observed that biotite and amphibole are the ®rst materials to melt, and the residual materials are quartz and feldspar in rocks melted at atmospheric pressure (Bossiere, 1991). The analyses of the mineral contents of clasts in experimentally-generated pseudotachylyte in granite in this study show that only 20±50% original quartz crystals were melted and 60±70% original feldspar crystals were melted during the experiments. These indicate that quartz is the most resistant to melting, biotite the least resistant, and feldspars are the intermediate. Thus, the SiO2 depletion in the pseudotachylyte glass is likely to be due to such selective melting of constituent minerals during frictional heating. The experiments successfully simulated the chemical compositions of glass matrices reported by Lin (1994a). It is known that the frictional melting is not an equilibrium phenomenon (Lin, 1991; Lin et

1200 750 1100 >1400

al., 1992; Spray, 1993; Shimamoto and Lin, 1994). The experimental results and the characteristics of chemical composition in natural pseudotachylytes show that the frictional melting mainly occurred by preferential melting of low melting point minerals with a chemical nonequilibrium process rather than by total melting or partial melting. This same conclusion was also reached by Lin (1991, 1992, 1994a), Lin et al. (1992) and Spray (1987, 1992, 1993). Melting temperature The melt temperature during pseudotachylyte formation has been estimated using the mineral geothermometer of microlites (e.g. Maddock, 1983; Toyoshima, 1990; Lin, 1994b) or the chemical compositions of matrices (e.g. Wallace, 1976; Lin, 1991, 1994a). The estimated melt temperatures of natural and experimental pseudotachylytes reported so far are between 750± 14008C, as shown in Table 7. These estimations are made assuming an equilibrium melting process. As stated above, the melting occurs under non-equilibrium conditions: these estimated temperatures are likely to be much lower than the real temperatures. Temperature along simulated faults increases above 10008C within several seconds and is perhaps very heterogeneous within a fault zone. The powder X-ray diffraction analyses show that all micas disappeared and some quartz crystals melted in the granite experiments. This means that the melting temperature is higher than the melting point of micas (6508C), and locally higher than the melting point of quartz (17238C) in the granite experiment under a low normal stress (about 1 MPa in this study) during a short time (<30 s). It was also measured directly by setting a temperature sensor in the cylindrical specimen and contacting it on the frictional surface (end face of the cylindrical specimen). This indicates that the frictional melting temperature was at least 11698C in the granite experiment and 11458C in the gabbro experiment (Tsutsumi and Shimamoto, 1994). In this study, the melting temperature estimated in the gabbro frictional experiments is higher than 1100±15508C, which embrace the melting points of feldspar and pyroxene. Acknowledgements ÐProfessors T. Matsuda (now in Seinangakuin University) and T. Fujii of the University of Tokyo are thanked for helpful discussions and comments during this study. Thanks to Dr John Spray and Dr Arch Reid for their constructive comments and suggestions which helped improve the manuscript. Present research was supported by the Science Research Grant (02452068) from the Ministry of Education of Japan.

Selective melting processes

REFERENCES Allen A.R. (1979) Mechanism of frictional fusion in fault zones. J. Struct. Geol. 1, 231±243. Bates R.L. and Jackson J.A. (1987) Glossary of Geology. 3rd edn. American Geological Institute, Alexandria, Virginia. Bossiere G. (1991) Petrology of pseudotachylytes from the Alpine fault of New Zealand. Techtonophysics 196, 173±193. Camacho A., Vernon R.H. and Fitz Gerald J.D. (1995) Large volumes of an hydrous pseudotachylyte in the Woodro€e Thrust, eastern Musgrave Ranges, Australia. J. Struct. Geol. 17, 371±381. Ermanovics I.F., Helmstaed H. and Plant A.G. (1972) An occurrence of Archean pseudotachylite from Southeastern Manitoba. Can. J. Earth Sci. 9, 257±265. Johannes W. (1989) Melting of plagioclase-quartz assemblages at 2 kbar pressure. Contrib. Miner. Petrol. 103, 270±276. Kanamori H. and Stewart D.S. (1976) Modes of strain release along the Gibbs Fracture Zone, Mid-Atlantic Ridge. Phys. Earth Planet. Interiors 11, 312±332. Kennedy L.A. and Spray J.G. (1992) Frictional melting of sedimentary rock during high-speed diamond drilling: an analytical SEM and TEM investigation. Tectonophysics 204, 323±337. Kikuchi M. (1995) Source mechanism of the 1995 Kobe Earthquake inferred from teleseismic data. Chisitsu News 486, 12±15 (In Japanese, with English Abstract and Captions). Killick A.M. (1990) Pseudotachylite generated as a result of a drilling ``Burn±in''. Tectonophysics 171, 221±227. Lin A. (1991) Origin of fault-generated pseudotachylites. Ph.D. Thesis. University of Tokyo, p. 108. Lin A. (1992) Glassy and microlitic pseudotachylytes from the Fuyun Fault Zone, northwest China. 29th IGC, Kyoto Abstract, 169. Lin A. (1994a) Glassy pseudotachylytes from the Fuyun Fault Zone, northwest China. J. Struct. Geol. 16, 71±83. Lin A. (1994b) Microlite morphology and chemistry in pseudotachylite, from the Fuyun Fault Zone, northwest China. J. Geol. 102, 317±329. Lin A. (1996) Injection veins of crushing-originated pseudotachylytes and fault gouge formed during seismic faulting. Engineering Geology 43, 213±224. Lin A. (1997a) Fluidization and rapid injection of crushed ®negrained materials in fault zones during episodes of seismic faulting. Proc. 30th Int. Cong. 14, 27±40 VSP. Lin A. (1997b) Roundness of fragments in pseudotachylytes as an indicator of frictional melting. Struct. Geol.. Journal of Tectonic Research Group of Japan 42, 47±56 (In Japanese, with English Abstract and Captions). Lin A. and Shimamoto T. (1994) Chemical composition of experimentally-generated pseudotachylytes. Struct. Geol. Journal of Tectonic Research Group of Japan 39, 84±101 (In Japanese, with English Abstract and Captions). Lin A., Matsuda T. and Shimamoto T. (1994) Pseudotachylyte from the Iida-Matsukawa fault, Nagano Prefecture: pseudotachylyte of crush origin? Struct. Geol. Journal of Tectonic Research Group of Japan 39, 51±64 (In Japanese, with English Abstract and Caption). Lin A., Shimamoto T. and Iwamori H. (1992) Experimentally-generated pseudotachylytes. 29th IGC, Kyoto Abstract, 167. Lin A., Shigetomi M., Shimamoto T., Miyata T., Takemura K., Tanaka H., Uda S. and Murata A. (1998) Cataclastic rocks formed along the Nojima fault in Awaji Island, Japan and their implications for the tectonic history. The Island Arc (in press). Maddock R.H. (1983) Melt origin of fault-generated pseudotachylytes demonstrated by textures. Geology 11, 105±108. Maddock R.H. (1992) E€ects of lithology, cataclasis and melting on the composition of fault-generated pseudotachylytes in Lewisian gneiss, Scotland. Tectonophysics 204, 261±278. Maddock R.H., Grocott J. and Van Nes M. (1987) Vesicles, amygdules and similar structures in fault-generated pseudotachylytes. Lithos 20, 419±432.

545

Magloughlin J.F. (1989) The nature and signi®cance of pseudotachylite from the Nason terrane, North Cascade Mountains, Washington. J. Struct. Geol. 11, 907±917. Magloughlin J.F. (1992) Microstructural and chemical changes associated with cataclasis and frictional melting at shallow crustal levels: the cataclastic-pseudotachylyte formation. Tectonophysics 204, 243±260. Magloughlin J.F. and Spray J.G. (1992) Frictional melting process and products in geological materials: introduction and discussion. Tectonophysics 204, 197±206. Masch L., Wenk H.R. and Preuss E. (1985) Electron microscopy study of hyalomylonites-evidence for frictional melting in landslides. Tectonophysics 115, 131±160. Ohtomo Y. and Shimamoto T. (1994) Signi®cance of thermal fracturing in the generation of fault gouge during rapid fault motion: An experimental veri®cation. Struct. Geol. Journal of Tectonic Research Group of Japan 39, 135±144 (In Japanese, with English Abstract and Captions). Philpotts A.R. (1964) Origin of pseudotachylites. Am. J. Sci. 262, 1008±1035. Reynolds D.L. (1954) Fluidization as a geological process, and its bearing on the problem of intrusive granites. Am. J. Sci. 252, 577± 614. Scott J.S. and Drever H.I. (1953) Frictional fusion along a Himalayan thrust. Proc. R. Soc. Edinburgh 65, 121±142. Seward D. and Sibson R.H. (1985) Fission-track age for a pseudotachylite from the Alpine fault zone, New Zealand. J. Geol. Geophys. 28, 553±557. Shand S.J. (1916) The pseudotachylyte of Parijs (Orange Free State) and its relation to `trap-shotten gneiss' and `¯inty crush-rock'. Q. J. Geol. Soc. Lond. 72, 198±221. Shimamoto T. and Lin A. (1994) Is frictional melting equilibrium melting or non-equilibrium melting? Struct. Geol. Journal of Tectonic Research Group of Japan 39, 79±84 (In Japanese, with English Abstract and Caption). Shimamoto T. and Tsutsumi A. (1994). A new rotary-shear highspeed frictional testing machine: its basic design and scope of research. Struct. Geol. Journal of Tectonic Research Group of Japan. (In Japanese, with English Abstract and Captions) 39. pp. 65±78. Sibson R.H. (1975) Generation of pseudotachylyte by ancient seismic faulting. Geophys. J. Royal Astro. Soc. 43, 775±794. Spray J.G. (1987) Arti®cial generation of pseudotachylyte using frictional welding apparatus: simulation of melting on a fault plane. J. Struct. Geol. 9, 49±60. Spray J.G. (1988) Generation and crystallization of an amphibolite shear melt: an investigation using radial friction welding apparatus. Contrib. Miner. Petrol. 99, 464±475. Spray J.G. (1992) A physical basis for the frictional melting of some rock-forming minerals. Tectonophysics 205, 19±34. Spray J.G. (1993) Viscosity determinations of some fractionally generated silicate melts: implications for fault zone rheology at high strain rates. J. Geophy. Res. 98 (B5), 8053±8068. Spray J.G. (1995) Pseudotachylyte controversy: Fact of ®ction? Geology 23, 1119±1123. Toyoshima T. (1990) Pseudotachylite from the main zone of the Hidaka metamorphic belt, Hokkaido, northern Japan. J. Meta. Geol. 8, 507±523. Tsutsumi A. and Shimamoto T. (1994) An attempt to measure the temperature of frictional melts of rocks produced during rapid fault motion. Struct. Geol. Journal of Tectonic Research Group of Japan 39, 103±114 (In Japanese, with English Abstract and Captions). Tsutsumi A. and Shimamoto T. (1996) Frictional properties of monzodiorite and gabbro during seismogenic fault motion. The Journal of the Geological Society of Japan 102, 240±248. Wallace R.C. (1976) Partial fusion along the Alpine fault zone, New Zealand. Geol. Soc. Am. Bull. 87, 1225±1228.