Pseudotachylites in the Eastern PeninsularRanges of California
Hans-Rudolf Wenk a,*, Lane R. Johnson a, Lothar Ratschbacher ba Department of Geology and Geophysics, University of California, Berkeley, CA 94720, USA
Received 4 May 1999; accepted for publication 14 February 2000
Abstract
A continuous zone containing pseudotachylites in the Eastern Peninsular Ranges of California extends over morethan 15 km from Deep Canyon in the north to at least Toro Canyon in the south. Pseudotachylites are found inrocks of tonalitic to dioritic composition. While the overall compositions of host rock and pseudotachylite veins aresimilar, veins are characteristically enriched in Fe and Ti and depleted in Si. Many veins are cataclastic, and allcontain fragments, but a large number have a groundmass with skeletal and spherulitic microlites of calcic plagioclase(An 40–50), biotite and ilmenite, indicative of partial melting.
1. Introduction identified as the result of a meteorite impact. Sincethen, pseudotachylites have been documented inmany highly deformed metamorphic rocks and arePseudotachylites have fascinated geologists everoften associated with mylonites.since these enigmatic rocks have been found by
There have been long discussions as to whetherLapworth in Scotland (Lapworth, 1885) and bythey originated by local cataclasis or melting (e.g.Clough (1888) in Wales. They were later describedPhilpotts, 1964; Francis, 1972). Today, a consensusin more detail and named by Shand (1916) inemerges that both processes have been active torocks of the Vredefort structure, which has beenvarying degree, depending on the circumstances(Magloughlin, 1992; Spray, 1995). Melting occursbecause of heat produced during cataclasis. A* Corresponding author. Fax: +1-510-643-9980.
E-mail address: [email protected] (H.-R. Wenk) significant, and often dominant, component of
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pseudotachylites consists of larger and smaller rocks between Palm Springs and MartinezMountain commonly referred to as the Santa Rosa(including submicroscopic) fragments of host rock.
The rest is a fine matrix, often of dark color and mylonite zone (Fig. 1; e.g. Rogers, 1965; Simpson,1984). In the lower part of the sequence, deforma-isotropic, when observed with the petrographic
microscope. In spite of many reports, neither melt tion was largely ductile: granodiorites of the SanJacinto and Santa Rosa Mountains transformednor glass has been documented with any clarity,
except in experiments with frictional sliding sur- to mylonites and ultramylonites with very largestrains, as established, for example, by an analysisfaces (e.g. Killick, 1990; Spray, 1995), in hyalomy-
lonites that formed by frictional heating in of deformed enclaves ( Wenk, 1998). It appearsthat these rocks were deformed during thelandslide surfaces (e.g. Masch et al., 1985), and
pseudotachylites along young faults (Lin, 1994a). emplacement of the various intrusions constitutingthe Peninsular Ranges batholith in the Cretaceous,This is not surprising since glass is highly unstable
and will crystallize over geologic times. While glass accompanied by top-to-the-southwest thrustmovements. Fig. 2 summarizes the availablemay not be preserved, microlites with morpholo-
gies typical of rapid growth in a viscous medium geochronological/geobarometric data from theSanta Rosa mylonite zone and its footwall andare good indicators of a melt origin. They have
been described by numerous authors in tectonic hanging wall. Batholitic rocks of the San Jacintointrusion in the footwall (e.g. Hill and Silver,pseudotachylites (e.g. Francis and Sibson, 1973;
Maddock, 1983; Koch and Masch, 1992; 1980), and related mylonites have both U–Pbzircon ages of about 97 Ma and crystallized atMagloughlin, 1992; Techmer et al., 1992; Lin,
1994b). about 16 km depth. The hanging wall AsbestosMountain tonalite is younger, about 87 Ma, andAn abundance of dendritic, skeletal and spheru-
litic microlites with compositions consistent with crystallized at a deeper crustal level (22 km) (Agueand Brimhall, 1988). Goodwin and Renne (1991)a granitic melt have been found in pseudotachylites
from the Santa Rosa mylonite zone in Southern used K–Ar cooling ages, and Cecil (1990) used40Ar/39Ar ages, to determine the onset of myloniticCalifornia. These pseudotachylites occur in a large,
20–200 m wide, continuous zone, more than 15 km deformation at about 87 Ma.The metamorphic rocks that surround the plu-in length. The production of melt during cataclasis
renders these rocks amenable to date the brittle tons are locally known as the Desert Divide Groupand the Palm Canyon Complex (e.g. Sharp, 1967);deformation in this region and compare it with
the ductile deformation of mylonites (e.g. Goodwin the latter form the hanging wall of the majorductile mylonite. The Palm Canyon Complex isand Renne, 1991).
In this contribution, we will describe the pseudo- composed of a variety of rocks, most importantlyquartzo-feldspathic gneisses, migmatites, pelitictachylites, discuss their possible origin, constrain
their ages by the 40Ar/39Ar dating method, and schists, amphibolites, pyroxenites and marbles,with excellent exposures in the deeply eroded desertcalculate the energy required for melting. We
refrain from a full analysis of the pseudotachylite canyons such as Palm Canyon, Deep Canyon andBear Creek. These rocks are locally stronglycontroversy, whose many facets are well presented
in the special issue of Tectonophysics (Vol. 204, deformed, in both a ductile and brittle fashion. Ahigh amphibolite facies metamorphic grade leading1992). Also, we will not repeat the description of
common features that have been presented many to migmatization is established by index mineralssuch as sillimanite and cordierite (in pelitic schists),times in the literature.forsterite and diopside (in marbles), and orthopy-roxene and spinel (in amphibolites). U/Pb zirconage determintions reveal a complex history but2. Geological settingindicate that migmatization was under way at least10 Ma before the intrusion of the easternThe north-eastern Peninsular Ranges in south-
ern California contain a zone of highly deformed Peninsular range plutons (Fig. 2). It appears that
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Fig. 1. Generalized geological map of the Palm Canyon, Deep Canyon and Sheep Mtn. Area. Highway 74 is indicated for orientation.Pseudotachylite occurrences and the large pseudotachylite zone are shown.
the large ductile deformation occurred after the K/Ar, and 40Ar/39Ar thermochronology (e.g.Armstrong and Suppe 1973; Dokka 1984; Cecil,emplacement of the San Jacinto–Santa Rosa
batholith and during and after the emplacement 1990; Goodwin and Renne 1991; unpublisheddata) shows that at about 70 Ma, both the hangingof the Asbestos Mountain tonalite. Fission track,
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the large right lateral displacements on the pres-ently active San Andreas and San Jacinto faultsystems, that divide the area into blocks, and arewell expressed in the mesoscale fault pattern, areyounger events than those discussed in this paper.
In these deformed rocks, there are numerousoccurrences of pseudotachylites shown on the map(Fig. 1). The term pseudotachylite is used here forextremely fine-grained concordant or discordantveins that are injected into host rock and does notnecessarily imply melting, as will be discussedbelow. Many are isolated clusters scatteredthroughout the area and over a range of tectonicunits. Most have been found in the metamorphicPalm Canyon Complex, but there are a few occur-rences of pseudotachylites in the external roofpendants of the Peninsular Ranges batholith. Sofar, none has been found in the ductile granodio-ritic mylonites overlying the batholith. Of mostinterest is a large continuous zone of pseudotachyl-ites in gneisses of the Palm Canyon Complex thatis about 20–200 m wide and extends from upperDeep Canyon (near Sugarloaf Mountain on
Fig. 2. Cooling curve for the footwall and hanging wall of the Highway 74 to the north) all the way to at leastSanta Rosa mylonite zone, northeastern Peninsular Ranges, Toro Canyon in Coachella Valley, 15 km to thederived from fission track, 40Ar/39Ar, K/Ar, and U/Pb mineral south. In this zone, which may be repeated byages. Data are from Armstrong and Suppe (1973), Hill and
faulting into several branches in the south, pseudo-Silver (1980), Dokka (1984), Cecil (1990), Goodwin and Rennetachylite veins are present at least every 10–50 m(1991), and unpublished work by B.K. Nelson and L.
Ratschbacher. Ages are plotted against the estimated average and can be mapped. In the central part of theor calculated closure temperatures for the different isotopic sys- zone, there are typically five to 10 veins per squaretems. The lower bound of ductile flow reflects the lower bound meter and in a 20×20 m outcrop 3 km SW ofof crystal-plastic flow in quartz.
Lake Cahuilla, pseudotachylites covered about 1%of the outcrop surface. Like many other pseudo-tachylites, veins are generally in the vicinity of awall and footwall blocks had cooled to <300°C,
implying that major ductile deformation had termi- fault (in this case, the contact between the PalmCanyon Complex and the overlying tonalitic rocksnated at that time. Cooling was rapid, accompa-
nied by brittle–ductile and brittle faulting, and, at of the Asbestos Mountain unit), but they rarelyoccur directly on a major fault or branching offless than 250°C, by pseudotachylite formation
(Fig. 2). from a fault. There are a few examples along theCoyote Creek fault south-east of Palm Desert.The region has become a classical site to study
mylonitic deformation at large strain (Simpson Several veins are often associated to form smallnetworks or clusters, but they do not occur inand Schmid, 1983; Simpson, 1984; Erskine and
Wenk, 1985; O’Brien et al., 1987; Todd et al., 1987; massive, breccia-like structures as described, forexample in the Hebrides (e.g. Maddock, 1983).Goodwin and Wenk, 1995; Wenk, 1998). Whereas
deformation of the batholitic rocks is largely duc- Pseudotachylites in this zone are restricted toplagioclase biotite gneisses and are never found intile, metamorphic rocks of the overlying Palm
Canyon Complex show, in addition, pervasive leucocratic gneisses, calcsilicate rocks or maficrocks, contrary to the Ivrea zone (Techmer et al.,brittle deformation. It should be emphasized that
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1992) and the Giles complex in Central Australia(Wenk and Weiss, 1982), for example, where theyhave been described in gabbros. The majority ofveins (~75%) are more or less parallel to thefoliation and compositional banding (Fig. 3a);25% cross-cut the foliation, often at angles rangingfrom 45 to 80°. Occasionally, they branch off froma local fracture surface on which small displace-ments from several millimeters to several centime-ters have taken place, or from a pseudotachylitevein that is parallel to the foliation (Fig. 3B). Insome cases, striations can be observed on pseudo-tachylite surfaces and are generally parallel to theregional lineation. They may indicate the directionof slip during local rupture. Frequently, no signifi-cant offsets can be observed between the two sides,whether veins are parallel or perpendicular to thefoliation, but offsets in the cm range are probablycommon, yet difficult to constrain quantitatively.A complex situation with parallel and cross-cuttingveins in a banded gneiss is shown in Fig. 3C for avein in migmatites of central Deep Canyon withexcellent fresh outcrops. In this case, a pseudo-tachylite vein originates in a mafic band and cross-cuts a quartz vein. There is good evidence that thepseudotachylite is younger than migmatization,pegmatite dikes and ductile deformation, but itprecedes pervasive fracturing, accompanied by dis-placements on the cm scale (including pseudo-tachylites) and chloritic alteration and fracturefilling with prehnite and laumontite. An explor-atory survey of orientations of fault veins (ratherthan injection branches) at two localities in thesouthern part exhibits a very regular pattern, con-sistent with top-to-the-southwest displacementsthat are characteristic of the ductile phase (Fig. 4).
In outcrops and most hand specimens, thepseudotachylites have generally a reddish browncolor due to oxidation. In one outcrop belowHighway 74, they are green. Only in polishedstream-bed outcrops do they appear black. Thatis where most of the samples were collected forthis study.
On a two-dimensional outcrop, individual pseu-dotachylite veins range from 2 mm to 10 cm in Fig. 3. Pseudotachylite structures on a mesoscopic scale. (A)
Concordant ( left) and discordant pseudotachylite veins intrue thickness and extend from 2 cm to 2 m.gneiss from Bear Creek. (B) Network of pseudotachylites in aResults from a statistical analysis on 290 veins inhighly cataclastic gneiss from upper Deep Canyon. (C)the northern and southern part of the zone arePseudotachylite veins originating in a mafic layer (top) andcross-cutting quartz vein in central Deep Canyon.
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Fig. 4. Orientation of pseudotachylite veins in two localities and interpretation of the stress field that produced them. Numbers 1–3designate principal directions of stress (1=compression>2>3=extension), calculated from veins and associated slickenlines, dotsare poles to foliation, also shown as large bold circles, and triangles indicate ductile stretching lineation. Lower hemisphere equalarea projection. (a) dry creek 37°18∞; 116°17∞05◊; (b) canyon 33°35∞12◊; 116°17∞05◊, both in the southern part of the study area, 2.5 kmSW of Lake Cahuilla).
Fig. 5. Size distributions of pseudotachylites. (a) Thickness versus length (different symbols used for southern and northern area).(b) Histogram of observed widths, ranging from 0.1 to 10 cm. (c) Histogram of aspect ratios. (d) Histogram of volumes, assumingthat pseudotachylites have a square cross-section.
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displayed in Fig. 5. Fig. 5a shows a distribution largest veins, up to 10 cm in thickness, wereobserved in the south.( log scale) illustrating that the average is about
1 cm wide and 40 cm long. Short and long dimen-sions vary; short dimensions peak at 1 cm(Fig. 5b), and aspect ratios, determined on planar 3. Mineralogical and chemical compositionoutcrops, are on average 80 (Fig. 5c). In the fewcases where the three-dimensional extent can be All pseudotachylites in the Peninsular Ranges
occur in quartzo-feldspathic rocks. Host rocks areassessed, it is clear that veins approximate sheetswith an equant or pancake geometry, wedging out alkali feldspar biotite gneisses, with considerable
titanite and occasionally hornblende as accessories.at the periphery. Assuming that the two longdimensions are the same, an approximate volume Pseudotachylites have not been observed in leu-
cocratic gneisses, in amphibolites or carbonatewas calculated by multiplying the short with thesquare of the large dimension. An average volume rocks. Samples for which analytical results are
presented in this paper are summarized in Table 1.is 5000 cm3 with considerable variation over morethan six orders of magnitude (Fig. 5d). The size We have analyzed by XRF major elements and
traces in fifteen pairs of veins (with inclusions) anddistributions are consistent with a log-normalpattern. There does not seem to be a large differ- corresponding host rock, and results for major
elements are shown in Table 2 (Pearson, 1996).ence between pseudotachylites from the northernand southern localities in the zone, though the These bulk analyses reveal the usual pattern that
Table 1Location of samples for which analytical data are reported (with host rock, latitude and longitude)
XRF analysesPC162 Lower Horse Thief Creek, chlorite schist, 33°34∞45◊; 116°25∞3.5◊PC181 Deep Canyon, leucocratic gneiss, below Hwy 74, 33°34∞59◊; 116°26∞22◊PC226 Sugar Loaf Mountain, below Hwy 74, migmatite, 33°34∞55◊; 116°26∞7.5◊PC420 Indian Portrero, Palm Canyon, mylonite, 33°41∞30◊; 116°31∞24◊PC428 Central Deep Canyon, gneiss, 33°36∞0◊; 116°24∞40◊PC430 Sugar Loaf Mountain, below Hwy 74, gneiss, 33°34∞58◊; 116°25∞51◊PC444 Hidden Palm Canyon, granodiorite, 33°37∞22◊; 116°24∞9◊PC486 Deep Canyon, Hidden Palms, pelitic gneiss, 33°37∞10◊; 116°24∞20◊PC487 Carrizo Creek, water tower, migmatite, 33°38∞24◊; 116°24∞28◊PC491 Base of Martinez Mtn. Landslide, granodiorite, 33°34∞30◊; 116°16∞30◊PC553 Ridge S of Guadaloupe Creek, gneiss, 33°35∞15◊; 116°17∞30◊PC555 Canyon S of Guadaloupe Creek, mylonitic gneiss, 33°35∞12◊; 116°17∞05◊PChtc Horse Thief Creek, leucocratic gneiss, 33°34∞58◊; 116°25∞3◊PCccs Cactus Springs, gneiss, 33°33∞48◊; 116°23∞7◊PCcst Cactus Springs trail, gneiss; 33°33∞28◊; 116°22∞17◊
Microprobe analysesMM3 Base of Martinez Mtn. Landslide, granodiorite, 33°34∞30◊; 116°16∞30◊PC226 Sugar Loaf Mountain, below Hwy 74, migmatite, 33°34∞55◊; 116°26∞7.5◊PC514 Bottom of Devil Canyon, mylonitic gneiss, 33°35∞55◊; 116°17∞30◊
Ar–Ar age determinationsHW74-17 Sugar Loaf Mountain, below Hwy 74, migmatite, 33°34∞55◊; 116°26∞7.5◊HW 98 Sugar Loaf Mountain, below Hwy 74, migmatite, 33°34∞50◊; 116°26∞5◊MM3 Base of Martinez Mtn. Landslide, granodiorite, 33°34∞30◊; 116°16∞30◊MM9 Canyon N of Martinez Mtn. Landslide, gneiss, 33°34∞20◊; 116°16∞50◊PC162 Lower Horse Thief Creek, chlorite schist, 33°34∞45◊; 116°25∞3.5◊PC553 Ridge S of Guadaloupe Creek, gneiss, 33°35∞15◊; 116°17∞30◊PC556 Canyon S of Guadaloupe Creek, mylonitic gneiss, 33°35∞12◊; 116°17∞05◊
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Table 2XRF analyses of pseudotachylite veins and associated host rocka
a Only total iron was analyzed. For norm calculations, it was assumed that the FeO/Fe2O3 ratio was 1.4 (h, host; p, pseudotachylite;pi, internal part; pe, external part).
the two are compositionally related (e.g. and a mechanical fractionation of melt and quartzclasts. We will discuss this chemical anomalyAinsworth, 1981; Maddock, 1992; Reimold, 1991;
Techmer et al., 1992; Killick, 1994), precluding a below.On the basis of petrographic analysis, the pseu-large transport. QPA, Or–Ab–An and ACF nor-
mative diagrams show that the rocks are aluminous dotachylites from the California Peninsular Rangescan be divided into two groups. The first group,alkali feldspar gneisses (Fig. 6). Like other
researchers we found no systematic pattern for including all of the isolated occurrences, containsveins with larger to smaller fragments and nomost elements. However, consistently, host rocks
relative to veins are enriched in SiO2 and depleted microscopic evidence of melting. They appear tobe the result of dominant cataclasis and are markedin FeO (Fe total ) and TiO2, as illustrated in Fig. 7.
A similar deficiency of SiO2 in veins has been with star symbols on the map in Fig. 1. The secondgroup is restricted to samples from the contiguousobserved by other investigators (e.g. Ainsworth,
1981; Shand, 1916; Sibson, 1975; Techmer, 1989; zone and individual samples are marked with dotson the map. The microstructure of pseudotachyl-Toyoshima, 1990; Spray, 1993; Lin, 1994a) and
has been interpreted as an effect of partial melting ites in this group is characterized by an abundance
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Fig. 6. Chemical composition of host and veins, established by XRF analyses (Table 2). QAP (A) Or–An–Ab (B) and ACF (C)diagrams were calculated using the hornblende norm. Squares are host rocks and crosses veins, connected by tie lines.
of microlites that are small but easily recognized tively large skeletal crystals (20–200 mm in lengthin thin sections, in addition to cataclastic fragments and 1–20 mm in width), identified as calcic plagio-(Fig. 8). Often, skeletal crystals nucleate on clase. The space between plagioclase microlites iscataclastic fragments, with crystallites exhibiting largely filled with much smaller (<10 mm in length)radial growth patterns or spherulitic morphology crystals of a higher birefringence, identified as(Fig. 8a and b). There are at least three types of biotite. In addition, there are small (<5 mm) crys-microlites: one has a low birefringence and rela- tals of ilmenite, particularly around inclusions of
titanite (e.g. in PC 514). The largest and best-preserved microlites of plagioclase and biotite havebeen collected in the canyon just north of theMartinez Mountain landslide (e.g. MM 3). Inaddition to microlites, the veins of this secondgroup contain 10–50% crystal fragments of quartz,plagioclase and, more rarely, orthoclase and titan-ite. The crystal fragments are typically deformed,with fractures and small displacements, visiblealong plagioclase twins (Fig. 9a), and fracturedquartz with fractures filled with fine cataclasticdebris (Fig. 9b). Deformation of fragments is gen-erally more intense along the outer margin.Fragments also include completely comminutedmaterial (Fig. 9c), clearly indicating that cataclasispreceded melting. Both crystal and comminutedfragments act as nucleation sites for radial micro-lite growth. Other microlites probably nucleate onsubmicroscopic features and form spheruliticaggregates. The surfaces of fragments are oftencorroded, with rounded and concave borders(Fig. 8c) indicative of partial melting. There areno fragments of biotite and hornblende in the
Fig. 7. SiO2–FeO (a) and SiO2–TiO2 variation diagrams (b).veins, suggesting that these minerals melted com-Weight percent oxides. Squares are host rocks and crosses veins,
connected by tie lines. pletely. Yet, biotite crystals in the host rock, in
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Fig. 8. Microstructures of microlites, plane polarized light. (a) Spherulitic skeletal crystals of plagioclase with a fine groundmass,mainly consisting of biotite. (b) Crystals nucleating at a fragment of quartz. (c) Fragments with a rounded and concave morphologyconsidered as evidence of partial melting. Skeletal crystals nucleate at the surface of the fragments.
Fig. 9. Fragments in pseudotachylite veins, crossed polars. (a) Twinned plagioclase illustrating kinking and bending. (b) Fracturesquartz inclusion with cataclastic debris along fractures. (c) Fine-grained breccia fragments in a matrix with microlites.
direct contact with the vein, do not show any signs features of microlites and inclusions are evidencefor the presence of melt during the formation ofof partial melting. None of the fragile microlites
shows any sign of deformation, suggesting that, at those pseudotachylites.All pseudotachylites investigated in this studythe time of crystallization, there was minimal shear
stress. There is no alignment of the needle-shaped have undergone alteration, though alteration isgenerally more pronounced in the host rock.crystallites indicative of flow. The morphologic
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Secondary minerals forming along fractures and not be very quantitative. Nevertheless, they con-form to normal igneous biotite with some enrich-at the contact between veins and host are calcite,
chlorite and iron hydroxides. A chemical analysis ment in titanium (Lin, 1994a,b). There are clastsof titanite, surrounded by skeletal needlesof the internal (black) and external (reddish) part
of a large vein (PC553) shows a slight enrichment (Fig. 10c). Titanite has the same composition asthat in Asbestos Mountain tonalite, the surround-in iron in the reddish part (Table 2).
The composition of microlites was determined ing needles are ilmenite, highly enriched in Mn.Due to their small size, no quantitative analyseswith microprobe analyses (Table 3). Plagioclase
compositions are typical for volcanic or plutonic could be obtained for the minute iron oxides orhydroxides in the groundmass.rocks of granodioritic composition (An 40–50).
Interestingly, the plagioclase has a fairly high ironcontent, which was previously noted in pseudo-tachylites (Maddock, 1992). Backscattered images 4. Age determinationsdocument the skeletal shape of the plagioclasecrystals (Fig. 10A). The lighter crystallites are To estimate the age of pseudotachylite forma-
tion, we analyzed seven veins with the 40Ar/39Arbiotite and very small iron oxides. Plagioclase isconsistently slightly more calcic than fragments. method (Table 4, Fig. 11). We selected veins from
Highway 74 and the upper reaches of DeepIn addition, the margin of plagioclase fragmentsis more calcic than the core, indicative of Na loss Canyon, all from the upper part of the Palm
Canyon Complex (Table 1; HW and PC sampleduring melting, and also richer in Fe. Fig. 10Billustrates this reversed zoning, which has also numbers). These samples were complemented with
samples from the southern extent of the zone,been observed in pseudotachylites from theSudbury impact structure (Thompson and Spray, adjacent to the Martinez Mt. landslide; those
samples have the largest plagioclase and biotite1994). Biotite microlites have similar dimensionsto that of the electron beam, and analyses may microlites (MM sample numbers and PC553 and
Fig. 10. Backscattered electron images obtained with a Cameca microprobe. (A) Spherulitic and dendritic plagioclase microlites inMM3. Small bright microlites are biotite. (B) Plagioclase (gray) and quartz (black) inclusions. Note the reverse zoning in plagioclasewith increase in calcium towards the margin (MM3). (C) Inclusion of titanite surrounded by skeletal crystallites of ilmenite. Smalliron oxide (or hydroxide?) particles in the groundmass (bright) (PC 514). Scale bars are 200 mm for (A) and (B) and 20 mm for (C).
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Table 3Microprobe analyses, elemental percentage and formula values (for feldspars)
# SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O Tot Si Al Ca Na K
556). Pieces were broken from the center of the In this study, we employed the conventionalstep-heating technique, instead of laser spot fusionveins while viewing them with a microscope. From
three samples, we analyzed two splits from the or laser step-heating dating. Laser dating, forexample, employed by Kelley and Spray (1997)same vein, one from the center and one nearer the
vein edge, but still well away from the host for dating a pseudotachylite associated with theRochechouart impact structure, also fails torock–vein contact. From one sample, we selected
two laterally separated splits from the center of resolve single matrix mineral phases due to theextremely fine-grained nature of these rocks, andthe same vein.
Samples were analyzed at Stanford University, has not provided the multiple-step, high-precisionspectra, with gas released over a wide range ofand the analytical techniques and line performance
were identical to those described by Hacker et al. temperatures, that are amenable to conventionalstep-heating. In two samples, we performed a(1996). The reported precision of the ages is 1s.
Internally and externally concordant refer to statis- temperature-cycling heating schedule, commonlyused to extract multi-diffusion domain informationtically indistinguishable intra-sample and inter-
sample ages, respectively. (e.g. Lovera et al., 1991).
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Fig. 11. Pseudotachylites from the northeastern Peninsular Ranges. Shown are K/Ca ratios (top), apparent age spectra with 1s
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were calculated using steps with temperatures indicated only (see also Table 4). TFA, total fusion age. MSWD, is the mean squareweighted deviation. Atm. is the 36Ar/40Ar ratio of the atmosphere (1/295.5).
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Table 4Summary of 40Ar/39Ar dataa
Sample Weight J (×107) Isochrone MSWD 40/36Ar Percentage Weighted Total fusion Preferred age(mg) age 39Ar mean age age interpretation
a J is the irradiation parameter, and MSWD is the mean square weighted deviation (Wendt and Carl, 1991), which expresses thegoodness of fit of the isochron (Roddick, 1978). The second number is the expected value for the number of steps. Isochron andweighted mean ages are based on fraction of 39Ar listed under Percentage 39Ar. All analyses are for whole rock.
We dated hornblende and biotite from the discordant spectrum with a total fusion age of59.9±0.6 Ma. Split 2, sampled closer to the veinsouthwestern part of the Asbestos Mt. pluton in
the hanging wall of the pseudotachylite zone margin, degassed at unusually high temperatures,suggesting that it is not a glass. Isotopic ratios(unpublished results). Because 40Ar/39Ar ages for
these minerals should record closure at temper- indicate that the gas is not a simple mixture of anatmospheric and a radiogenic component, and weatures in excess of those during pseudotachylite
formation, 500±50 and 300±50°C, respectively, consider this age to be unreliable. Vein PC162from Horse Thief Canyon was also run in splitsthese ages serve as an upper bound to pseudo-
tachylite formation. The hornblende yielded a from the margin and center. The margin shows arelease spectrum with serially increasing ages, sug-strongly discordant spectrum with a total fusion
age of ~67.5 Ma that has a probable uncertainty gesting that the age of this sample is ~57 Ma. Thecenter shows a saddle-shaped spectrum with aof±5 Ma. The biotite spectrum is less discordant,
and 72±2 Ma is thought to be the cooling age. central plateau with an age of 62 Ma, interpretedas a maximum age for the sample. Two splits ofThis age is identical to the Asbestos Mt. tonalite
K/Ar biotite age of 70±1 (Armstrong and Suppe, MM9 yielded complex spectra; however, both havetotal fusion ages of 56 Ma. The spectrum of the1973), recalculated after Dalrymple (1979).
The pseudotachylites from the Eastern first split of MM3 shows ages that increase from~60±5 Ma, and a central near-plateau with anPeninsular Ranges show complex release spectra
that are internally discordant and, in most cases, age of about 74 Ma; the second split shows withinerror the same age. Macroscopically, this veindo not define good isochrons. Interpreting the
spectra is thus not straightforward, but the four contains the fewest host rock inclusions and micro-scopically the largest microlite biotite and plagio-samples for which we analyzed different parts of
the same vein provide additional constraints for clase crystals. Its age is close to the biotite ages ofthe Santa Rosa mylonite, Palm Canyon Complexthe interpretation.
Pseudotachylite 98 from the uppermost Palm tectonites, and the Asbestos Mt. tonalite. We pro-visionally interpret the low-temperature end of theCanyon Complex close to the contact with the
Asbestos Mt. tonalite was run in two splits. Split spectrum of MM3-1 to indicate Ar loss at60±5 Ma, corresponding to the age given by the1, from the vein center, yielded an internally
269H.-R. Wenk et al. / Tectonophysics 321 (2000) 253–277
release of most of the gas in the majority of the with particulate material that is highly mobile(Weiss and Wenk, 1983).other veins (Table 4). Sample HW74-17, a pseudo-
tachylite from Highway 74 near Sugarloaf The morphology and composition of microlitesprovide some information about the conditionsMountain, and PC556, a sample from lower
Guadaloupe Creek, yielded both internally discor- during melting and cooling. The shape that acrystal assumes during growth is directly controlleddant spectra with weighted mean and isochrone
ages of 59±1 Ma. PC553, a large vein S of by the relative importance of diffusion rate andgrowth rate. This is related to the degree of super-Guadaloupe Creek, shows a saddle-shaped
spectrum with a central plateau with an age of cooling (Dowty, 1980). At high cooling rates, theratio of diffusion to growth rate is small, and40 Ma, which we interpret as a maximum age for
the sample; the vein margins of MM9 and PC162 spherulitic and dendritic crystals form (Lofgren,1980). During rapid quenching, homogeneousshow an approximately coeval low-temperature Ar
loss. Temperature cycling in PC556 and MM3-2 nucleation that needs an incubation period is inhib-ited, and crystallites grow from inclusions in adid not yield any changes in the age spectra.radial geometry. A melt from which calcic plagio-clase (An 40–50), biotite and ilmenite precipitatedrequires temperatures in excess of 1000°C (Spray,1992), but remaining relicts of small and larger5. Discussion and seismic implicationsfragments of quartz, plagioclase, some orthoclaseand titanite (no biotite!) indicate that these temper-Tectonic pseudotachylites from the Palm
Canyon Formation occur both as veins of largely atures only prevailed for very short times, becauseotherwise complete melting would have occurred.cataclastic debris and as veins of a fine groundmass
with skeletal and spherulitic microlites indicative We have mentioned that there is no evidence fordeformation of the very fragile skeletal plagioclaseof crystallization from a melt. Veins with microlites
also contain numerous fragments of cataclasites crystals and no geometric alignment that wouldhave occurred during flow. The melt could notand fractured crystals, implying that cataclasis was
present and preceded melting. This agrees with the have been mobile at the growth stage of themicrolites, suggesting a highly viscous melt, con-experiments and conclusions of Spray (1995) that
comminution is an essential precursor to melting ceivably crystallization from a glass, still at anelevated temperature. Particularly in the thinnerby friction and that both processes are complemen-
tary. As the stressed brittle solid fractures, spall- veins, there is occasional evidence of morphologiczoning with microlites being smaller at the margin,ation occurs, producing an aggregate with a large
surface area. Such a mechanism does not require indicative of higher cooling rates, than in thecenter. Therefore, it is unlikely that microlitelarge and rapid displacements on sliding surfaces
(e.g. Ramsay and Huber, 1987, p. 584; Swanson, growth is the result of late devitrification.Veins are depleted in Si and enriched in Fe and1992). Large displacements may be present in
hyalomylonites such as along Langtang and Kofels Ti. This could be due to preferential partial meltingof ferromagnesian minerals such as biotite andlandslide surfaces (Masch et al., 1985) but are
lacking in most tectonic pseudotachylites. Spray hornblende and extrusion of the melt leavingquartz fragments behind (Spray, 1992). Spray(1995) documents convincingly that the percentage
of melt is related to the strain rate. According to (1993) has observed experimentally that the meltis depleted in silica, and clasts are largely quartz.his experiments, melting is most pervasive at very
high strain rates and large local stresses. It is However, our analyses comprise matrix as well asquartz clasts and still show a depletion in silica.therefore best developed in pseudotachylites that
formed during impact events such as Vredefort Also, no zones enriched in quartz clasts have beenobserved from which vein material could have(Spray et al., 1995), Sudbury (Thompson and
Spray, 1994) or Rochechouart (Kelley and Spray, been extruded. Another explanation could be twoimmiscible liquids (Philpotts, 1982) with a mobile1997). At lower strain rates, cataclasis prevails
270 H.-R. Wenk et al. / Tectonophysics 321 (2000) 253–277
low-viscosity melt of basaltic composition injecting pseudotachylite vein is given byinto fractures. Also, with such a mechanism, there
Ev=EmrV (2)ought to be regions enriched in silica, which wecould not identify. Occasionally, one suspects that where r is the density and V is its volume. For
smaller pseudotachylite veins with a volume ofpseudotachylite veins may have initiated on mafic,biotite-rich layers in these often banded gneisses 50 cm3=5×10−5 m3 (and a density, r, of
2500 kg m−3), the energy required for melting is(Fig. 3C), but we rejected this hypothesis becausebanding is not always present, yet the chemical 1.25×105 J or 1.25×1012 ergs. For larger veins
with a volume of 0.5 m3, the energy calculates topattern is pervasive. The systematic change incomposition may be secondary. It is not uncom- 1.25×109 J or 1.25×1016 ergs.
This is the energy required for melting.mon to find quartz veins that cross-cut pseudo-tachylites, and solutions may have leached out Additional energy is required to create fractures,
and induce spallation and cataclasis. (Note thatsilica of these highly metastable rocks. Similarly,iron and titanium oxides and hydroxides could the energy radiated as seismic waves is considered
separately below.) It is generally assumed that thishave precipitated during alteration and added thedark and rusty color so typical of these veins. mechanical energy is relatively minor, but this is
based on the assumption that melting occurs byAdmittedly, such an explanation also has short-comings. It should be noted that the characteristic friction on a shear plane (Lachenbruch and Sass,
1980; Sibson, 1980; Scholz, 1990). This may notSi–Fe pattern is present in pseudotachylites with amicrolite groundmass as well as in those composed be true for extreme cataclasis with large reduction
in grain size to the sub-micron scale by breakinglargely of cataclastic fragments.Pseudotachylites form during brittle deforma- bonds and corresponding creation of surface area.
The total energy may therefore be considerablytion of rocks at large local stresses. Therefore, theyhave long been associated with seismic activity larger.
There is considerable uncertainty about the(e.g. McKenzie and Brune, 1972; Sibson, 1975). Ifmost mechanical work W during fracture is con- seismic efficiency, g, i.e. the percentage of energy
during a failure event that is released as seismicverted to heat (Scholz, 1990) and making someassumptions about temperatures, then the volume waves. Most estimates are less than 1% (McGarr
et al., 1979; Scholz, 1990) though it could beof a pseudotachylite vein can be related to themechanical work released during an earthquake, conceivably larger (McGarr, 1999). The estimate
that appears to be most independent of an assumedand thus the magnitude. This first statistical invest-igation of pseudotachylite geometry may provide model of fracture is that of McGarr (1976) and
Spottiswoode and McGarr (1975), who compareddata that are pertinent to seismology.Consider the amount of energy, Em, required to the energy released by the closure of a stope in a
deep mine with the energy of associated seismicmelt a unit mass of rockevents and obtained an efficiency of 0.24%. With
Em=CPDT+H (1)this, we can calculate the seismic energy, Es,released during the formation of a single veinwhere Cp is the specific heat at constant pressure,
DT is the difference between the ambient andEs≈gEv=gEmrV=g(CpDT+H )rV. (3)
melting temperature, and H is the heat of fusion.With reasonable values and assuming that melting The seismic energy (in ergs) can then be converted
to magnitude, M, using the Gutenberg–Richteroccurred at 1300°C, i.e. DT=1000°C, e.g. forfeldspar or quartz, Cp=800 J kg−1 °C−1, and relationship M=( log Es−11.8)/1.5 and the scalar
moment, M0, with M0=16.1+1.5 M (Hanks andH=7×105 J kg−1, this yields Em=1.5×106 J kg−1. Assuming that 30% of the veins are Kanamori, 1979) to obtaininclusions, the energy to melt a kilogram of pseu-
log M0=4.3+log Es . (4)
dotachylite reduces to 106 J.An estimate of the energy required to create a Assuming an efficiency of 0.24%, we obtain
271H.-R. Wenk et al. / Tectonophysics 321 (2000) 253–277
Fig. 12. Plots showing surface area as a function of seismic moment for microearthquakes observed along the San Andreas fault nearParkfield (Nadau and Johnson, 1998) (a) and corresponding data from pseudotachylite veins in the Eastern Peninsular Ranges (b,c); (b) assumes that the seismic efficiency is 1% and (c) 0.24%, introducing a slight shift.
for a small pseudotachylite vein M=−1.6, several orders of magnitudes smaller than anythingrecorded, may exist. Admittedly, the measuredlog M0=13.8 dyne-cm and for larger ones, M=
1.1, log M0=17.8 dyne-cm. volume of single veins underestimates the totalvolume of melt produced in a seismic event becauseThis calculation was repeated for each indivi-
dual vein, with the volume of the vein used to of contributions of additional injection veinlets.Still, the magnitudes derived from pseudo-calculate the energy required to melt 70% of the
vein [Ev, Eq. (2)], a fraction of this energy equal tachylite melting are relatively small, but it is likelythat during a seismic event, one or several clustersto the seismic efficiency taken to be the energy
radiated in seismic waves [Es, Eq. (3)], and this of veins form in accordance with the observationsof networks in the field. Assuming that each eventseismic energy then converted to a scalar moment
[M0, Eq. (4)]. In Fig. 12, we plot estimated comprises 100 veins, the magnitudes are increasedby 2. Particularly for large pseudotachylite veins,moments against area, for a seismic efficiency of
1% (b) and 0.24% (c). These results can be com- the measured area may be underestimated. Whilethe thickness can be measured accurately, thepared with completely independent estimates
obtained by Nadau and Johnson (1998), an analy- determined length and area are limited by the scaleof the outcrop, which often does not extend beyondsis of seismic waves from small repeating earth-
quakes along the San Andreas fault near Parkfield 2 m. We also note that larger veins have generallyfewer fragments and perhaps a more complete(Fig. 12a). Using measured moment release rates
and comparing them with rates of surface creep, conversion to heat, so there could be a dependenceof seismic efficiency with size. Calculating the totalthey calculated areas in the range of 103–106 cm2,
corresponding to earthquakes with magnitudes energy, we only considered melting. As mentionedabove, a comminution model requires considerableranging from −0.7 to 1.4. There is a surprising
overlap between the two completely independent additional energy to create surfaces. It is note-worthy that the smallest observed crystalline frag-data sets, particularly for a seismic efficiency of
0.24%, not only in the values of moment but in ments are around 1 mm in size, which may be thelimit at which the energy for melting becomesthe general trend of area versus moment. These
data from the pseudotachylite veins extend the smaller than that for comminution.The comparison between the pseudotachyliteseismic data downward from a moment of 1015 to
1012 dyne-cm, a range that was too small to be veins and the seismic data introduces an earth-quake parameter, which cannot be readily esti-observed in the data available to Nadau and
Johnson (1998). The presence of pseudotachylite mated from the seismic data alone, i.e. thethickness of the zone generating the elastic waves.veins suggests that micro-earthquakes, which are
272 H.-R. Wenk et al. / Tectonophysics 321 (2000) 253–277
Taking advantage of this fact, it is possible to because of extrusion of materials into sidefractures.derive a relationship between the thickness of the
zone and the amount of slip across the zone. In Sibson (1975, 1977) obtained a rather differentexpression, d=436h2, with d and h in cm, from athe calculations given above, the scalar moment
was related to the seismic energy, which was in study of pseudotachylite veins that all had thick-nesses less than 1 cm. However, for the typicalturn related to the volume of the vein through the
seismic efficiency and the melting energy. Letting vein of that study, with a thickness of about0.1 cm, the amount of displacement is similar tothe volume V=Ah where A is the area and h the
thickness of the vein, Eqs. (3) and (4) can be the estimate from Eq. (9).As a mechanism, applicable to both thecombined to obtain:
Peninsular Ranges pseudotachylites and micro-log(M
0)=4.3+log(g)+log(Em)+log(A)+log(h).
seismic events along the San Andreas fault, weanticipate an increase in shear stress that locally(5)reaches the magnitude necessary for failure
However, the moment is defined as M0=mAd (Aki(Grocott, 1981). Contrary to the estimates of
and Richards, 1980), where d is the displacement(McKenzie and Brune, 1972), no large earth-
on the crack that generated the elastic waves, andquakes are necessary to produce pseudotachylites.
m is the shear modulus. Thus, we also have:Rupture occurs with planar fractures and smalldisplacements. Mostly, fractures are not isolated,log(M
0)=log(m)+log(A)+log(d ). (6)
but a primary rupture initiates secondary fractures,These two equations can be combined to yield:
perhaps 10–20 for a single event. It is also likelythat each event triggers other ruptures that are notlog(d )=4.3+log(g)+log(Em)−log(m)+log(h)connected so that a single seismic event may
(7)produce several hundred pseudotachylite veins.Rupture produces spallation, cataclasis and finallyormelting. A large pseudotachylite zone such as that
d=104.3 [(gEm)/m]h. (8)in southern California is the result of a largenumber of seismic events over an extended timeThis shows that there is a direct linear relation-
ship between the displacement on a vein and its period. It is noteworthy that we have neverobserved different generations of veins with cross-thickness.
Note that d/h is essentially the strain that is cutting relationships (as in Central Australia, forexample) or any signs of reactivation of an olderreleased when the vein is produced, so this result
can be interpreted to mean that the rock fails when vein. This may indicate that a vein or group ofveins is locked by the mechanical constraints ofthe strain reaches a critical level. With the numeri-
cal values used earlier for g and Em and taking the surrounding rocks and that it is easier to forma new fracture (i.e. strain hardening). Also, asm=3×1011 dyne/cm2, we have:noted above, there is no evidence for ductile defor-
d=4h. (9)mation of pseudotachylite veins, suggesting thatthey formed at a low temperature. The volume ofThis equation suggests that the overall displace-
ment for a vein of original thickness of 1 cm is melt in a pseudotachylite network is related to themagnitude of the local stress.4 cm. This is very reasonable, given the evidence
for extreme deformation that is found within the While the Peninsular Ranges pseudotachylitesseem to conform to the new broader view of theseveins. Displacements of 1–5 cm are often observed.
A quantitative investigation of the displacement– enigmatic rocks as combined cataclasis and melting(Spray, 1995) and provide insight into earthquakethickness relationship would be important, but
displacements are often difficult to determine mechanisms, they also help to constrain the tec-tonic history of this area. For the latter, we havebecause of a lack of adequate markers, and there
is uncertainty in thickness–volume estimates to analyze what the 40Ar/39Ar system actually
273H.-R. Wenk et al. / Tectonophysics 321 (2000) 253–277
dates. Accurate ages of pseudotachylites, related This corresponds to closure temperatures calcu-lated for 10 mm biotite, resulting from deforma-to crustal faulting, have rarely been reported (see
Kelley et al., 1994, for example), and the difficulties tion-induced reduction in grain size, inultramylonites of the Santa Rosa mylonitesin dating are attributed to complexities of the Ar
system, due, for example, to incomplete outgassing (Goodwin and Renne, 1991).Most of the veins show increasing ages at theof Ar during rapid melting and cooling, incorpora-
tion of clasts from the country rock, and alteration high-temperature end of the spectra, mostly corre-lating with low K/Ca ratios. As some ages exceedand devitrification.
At best, the ages date the time of frictional the U/Pb zircon age of high-temperature metamor-phism and migmatization of the Palm Canyonmelting of Palm Canyon gneisses during cataclasis.
Our pseudotachylites may, in this regard, behave Complex (about 110 Ma, Fig. 2), these ages areprobably due to excess 40Ar. The low K/Ca ratiossimilarly to tectites, in that they are highly siliceous
and contain 2–4 wt% K2O (Table 2) and little exclude host rock minerals such as alkalifeldsparand biotite, and amphibole has not been observedwater. However, the ages may reflect cooling of
minerals in the fragments, growth of neoblastic in fragments.The ages at the low-temperature end of theskeletal minerals, or, worse, a mixture of host rock
and neoblastic mineral gas. Because our samples spectra of some samples may be significant. Theseages are either at 60 Ma, coinciding with the agewere only a few milligrams and were examined in
detail with a microscope, we contend that contami- of most of the pseudotachylite veins, or 40 Ma,consistent with the maximum age of vein PC553.nation by host-rock fragments and alteration is
minor, but cannot be excluded, particularly if the Our preferred interpretation is that the datesfor pseudotachylites from the eastern Peninsularobserved Si depletion and Fe–Ti enrichment is a
secondary chemical exchange. Ranges signify the age of frictional melting at 56–62 Ma of protoliths that had cooled to ≤300°C atK-bearing crystal fragments indentified in our
veins are plagioclase and more rarely orthoclase; about 63–75 Ma. The biotite and hornblende agesfrom the host rock are compatible with such annewly formed minerals are plagioclase and biotite.
The character of both inclusion phases and newly interpretation. The different ages of vein MM3 (ca74 Ma) and PC553 (ca 40 Ma), both very largeformed minerals should be qualitatively traceable
by the K/Ca ratios calculated from the released veins and in proximity in the southern region, mayindicate that veins occasionally have formed earlier39Ar and 37Ar. The mean K/Ca ratios in our
pseudotachylites are between 0.2 and 161, but nine (during ductile deformation) and later, or thesedates may be geochemical anomalies.out of 11 samples yielded ratios of around 1
(Fig. 11). These are greater than the K/Ca ratios Regardless of the meaning of Ar/Ar dates forthese pseudotachylites, i.e. earthquake-triggeredfor the skeletal calcic plagioclase (mean 0.15,
Table 3) and are within the range of skeletal biotite melting and subsequent quenching, or cooling offragments, and/or growth of microlite minerals(1–4, Table 3) and alkalifeldspar (no orthoclase in
the fragments has been analyzed, but the from melt or glass at elevated temperature, weconclude from the considerations above and their39Ar/37Ar ratios in Asbestos Mt. alkalifeldspar
indicate K/Ca ratios of about 12; unpublished age position in the cooling paths of Fig. 2 thatpseudotachylites postdate the stage of large-scaleresults). The small grain size of both skeletal biotite
(<10 mm long) and orthoclase fragments (20– ductile deformation. This is consistent with fieldobservations that veins often cross-cut the myloni-200 mm long, 1–20 mm wide) imply low effective
closure temperatures. Equating the characteristic tic foliation. Ages between 56 and 62 Ma corre-spond to the low-temperature part of the coolingdiffusion distance with the observed grain size,
using the Ar diffusion parameters reviewed in history (Fig. 2) and are in good agreement withthe youngest K/Ar ages obtained from post-tec-McDougall and Harrison (1999) and allowing a
broad band of cooling rates, the calculated closing tonic, fine-grained biotite in ultramylonites(62.4±1.4 Ma, Goodwin and Renne, 1991) andtemperatures for the skeletal biotite are <250°C.
274 H.-R. Wenk et al. / Tectonophysics 321 (2000) 253–277
the fission track ages (60–62 Ma, Dokka, 1984). possibility that both the chemical variation andsome of the peculiar ages reported here may beWe interpret the pseudotachylites as accompanying
late-stage cooling to near surface temperature due to secondary alteration. Also, the strikingresemblance of the seismic pattern established onduring final, post-mylonitic deformation. Contrary
to other occurrences where pseudotachylites are the basis of pseudotachylite volumes and seismicityalong the San Andreas fault may be coincidental,associated with mylonites and where brittle and
ductile deformations are coeval and oscillate [e.g. as there are still many aspects of this comparisonthat are unclear. For instance, the seismic datain the Hebrides, (Maddock, 1986), the Silvretta
nappe, ( Koch and Masch, 1992), in East pertain to repeating events, and the most likelyexplanation for the pseudotachylite veins is thatGreenland ( Karson et al., 1998) and the Musgrave
block in Central Australia, (Moore and Goode, they represent single events. Certainly, the contactbetween the Palm Canyon Complex and the1978; Wenk, 1978; Shimamoto and Nagahama,
1992)], pseudotachylites in the Palm Canyon Asbestos Mountain tonalite is not a large strike-slip fault. However, the similarity in the energyComplex all postdate mylonitic deformation. Note,
further, that there is no break in the cooling curve, distribution pattern seems to indicate that forma-tion of melt is due to local tectonic stresses in asuggesting that mylonitic flow and brittle faulting
form a succession, marking the exhumation of brittle regime and the intrinsic mechanical proper-ties of rocks. The picture that emerges suggestsrocks across and above the brittle–ductile trans-
ition. This is confirmed by our structural studies that neither pseudotachylites nor earthquakesrequire large displacements on a single glidinghighlighting a kinematic continuum between duc-
tile and brittle deformation in this region (unpub- surface, as has been conventionally accepted.Similar conclusions were reached by Killick andlished data).
The transition from quartz-dominated ductile Roering (1998). We still have no good explanationwhy pseudotachylites are rarely observed in directflow to frictional gliding would cause large local
stress differences, particularly at the transition proximity to a fault but are generally distinctlyremoved. Is it conceivable that directly on thefrom homogeneous granitic rocks of Asbestos
Mountain and related units along the margin of fault, deformation is aseismic, whereas in the rigidrocks in the vicinity of a fault, high local stressesCoachella Valley to extremely heterogeneous litho-
logic units such as the rocks of the Palm Canyon can accumulate. A few comments on nomenclatureare in order. ‘Pseudotachylite’ was a name given,complex. This contact is in some places transitional
(Deep Canyon) but, especially in the eastern part, long before these rocks could be adequatelydescribed and interpreted, for impact-related rocksis marked by sharp faults such as in Coyote Creek
where perfectly planar faults are filled with in South Africa (Shand, 1916). Higgins (1971) stillstates that ‘‘hyalomylonite (a term coined by Scottcataclastic gouge. It is in the vicinity of this
contact, but not directly along the fault, where the and Drever, 1953, for pumice-like rocks in theHimalaya) is essentially equivalent to pseudo-continuous zone of pseudotachylites occurs.tachylite’’. Yet, we know now that glassy andvesicular hyalomylonites differ in structure, as wellas origin at the base of landslides (Erismann et al.,6. Conclusions1977), from tectonic pseudotachylites. Perhaps aname such as ‘seismolite’ would more adequatelyThe large but fairly narrow and structurally
well confined zone of pseudotachylites in the describe rocks formed by local cataclasis and melt-ing in association with tectonic-seismic activityPeninsular Ranges of California is providing new
insights. However, like every investigation of these and also distinguish them from impact-generatedpseudotachylites.enigmatic rocks, it leaves open questions and
uncertainties. For example, we are still puzzled by In any case, pseudotachylites from the EasternPeninsular Ranges provide excellent material withthe systematic chemical differences in Si and Fe
between host and vein. We cannot exclude the which to further investigate some of the remaining
275H.-R. Wenk et al. / Tectonophysics 321 (2000) 253–277
Related Rocks Riverside County California. Senior thesis,issues, and we make our samples and detailedUniversity of California, Berkeley, CA, 72 pp.maps available to any researchers who wish to
Clough, C.T., 1888. The geology of the Cheviot Hills: Englandengage in such studies. and Wales. In: Geol. Surv. Mem., Explanation of Sheet 108NE, 22.
Dalrymple, G.B., 1979. Critical tables for conversion of K/ARages from old to new constants. Geology 7, 558–560.
Acknowledgements Dokka, R.K., 1984. Fission-track geochronological evidencefor late Cretaceous mylonitization and early Paleocene upliftof the northeastern Peninsular Ranges, California. Geophys.H.R.W. was supported by NSF EAR-99-02866.Res. Lett. 11, 46–49.The fieldwork was greatly facilitated by logistic
Dowty, E., 1980. Crystal growth and nucleation theory and thesupport from Michael Dunn (Palm Springs), thenumerical simulation of igneous crystallization. In: Har-
US National Forest Service (Greg Lumpkin) and graves, R.B. (Ed.), Physics of Magmatic Processes. Univer-the UC Deep Canyon Research Station (Al Muth). sity Press, Princeton, NJ, pp. 420–485.
Erismann, T., Heuberger, H., Preuss, E., 1977. Der BimsteinMany students contributed ideas that made field-von Kofels (Tirol ), ein Bergsturz-‘Friktionit’. Tschermakswork a challenging pleasure. John Donovan helpedMin. Petr. Mitt. 24, 67–119.with microprobe analyses, and Tim Teague and
Erskine, B.C., Wenk, H.-R., 1985. Evidence for late CretaceousAlene Pearson assisted with XRF analyses. We are crustal thinning in the Santa Rosa mylonite zone, southernappreciative to Jan Cecil for help with the prepara- California. Geology 13, 274–277.tion of the geological map. Brad Hacker (UCSB) Francis, P.W., 1972. The pseudotachylyte problem. Comments.
Earth Sci. Geophys. 3, 35–53.is thanked for introducing L.R. to Ar/Ar geochro-Francis, P.W., Sibson, R.H., 1973. The outer Hebrides thrust.nology when both were at Stanford, and for an
In: Park, R.G., Tarney, J. (Eds.), The Early Precambrianeducating review of the Ar/Ar part of this paper. of Scotland and Related Rocks of Greenland. University ofConstructive reviews by T. Engelder, A.M. Killick Keele, Keele, UK, pp. 95–104.and J.G. Spray helped us to improve the manu- Goodwin, L.B., Renne, P.R., 1991. Effects of progressive
mylonitization on Ar retention in biotites from the Santascript, and illuminating discussions with I.Rosa Mylonite Zone, California, and thermochronologicCarmichael were helpful indeed. L.R. thanks Mikeimplications. Contrib. Mineral. Petrol. 108, 283–297.McWilliams to allow him to use his lab for dating
Goodwin, L.B., Wenk, H.-R., 1995. Development of phyllonitethese extraordinary rocks. L.R. was funded by from granodiorite: Mechanisms of grain-size reduction inDFG-Heisenberg projects 442/4-1, 6-1. Folkmar the Santa Rosa mylonite zone. J. Struct. Geol. 17, 689–707.
Grocott, J., 1981. Fracture geometry of pseudotachylite genera-S. Hauff collected samples HW74-17 and 98 duringtion zones: a study of shear fractures formed during seismichis MS thesis project on the structural history ofevents. J. Struct. Geol. 3, 169–178.the SRMZ. Laura E. Webb helped with trans-
Hacker, B.R., Mosenfelder, J.L., Gnos, E., 1996. Rapid ophio-Atlantic Ar-data communication. lite emplacement constrained by geochronology and thermal
considerations. Tectonics 15, 1230–1247.Hanks, T.C., Kanamori, H., 1979. A moment magnitude scale.
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