Tectonophysics 402 (2005) 37 – 54
www.elsevier.com/locate/tecto
A multi-kilometer pseudotachylyte system as an exhumed
record of earthquake rupture geometry at hypocentral
depths (Colorado, USA)
Joseph L. AllenT
Department of Geology and Physical Sciences, Concord University Athens, WV 24712-1000, USA
Received 10 April 2004; received in revised form 24 September 2004; accepted 25 February 2005
Available online 10 May 2005
Abstract
A system of pseudotachylyte-bearing fault zones preserved along the Proterozoic Homestake shear zone in the southern
Rocky Mountains provides an avenue for investigating earthquake processes at the hypocenter. The results of detailed field
mapping suggest that pseudotachylyte may serve as a dynamic indicator of rupture directivity and yield general estimates of
some earthquake source parameters when examined at the multi-kilometer, fault-system scale. Pseudotachylyte fault veins are
primarily exposed within eight NE-striking, sub-vertical fault zones that have a cumulative length of more than 21 km. The fault
zones are mapped for 7.3 km along strike and fan to the northeast from a 170-m-wide outcrop belt to a maximum cross-strike
width of 2.3 km. Pre-existing structural control on rupture geometry is indicated by concordance between foliation and fault
veins, as well as spatial coincidence between the limbs of map-scale, rootless isoclinal folds and the location of most fault
zones. The central portion of the longest fault zone exhibits evidence for dextral oblique slip that involved more than 2.1 m of
strike-slip offset between four parallel fault veins that are interpreted to have formed in response to a single rupture event. In
addition, an along-strike continuity and systematic distribution of fault zones, a progressive northeastward decrease in
pseudotachylyte volume and maximum vein thickness, and a relative scarcity of cross-cutting relationships further suggests that
the majority of the frictional melt in the system may have developed in response to one (or several) multi-kilometer ruptures, as
opposed to hundreds of shorter ruptures. The similarity of kilometer-scale relationships observed along the Homestake
pseudotachylyte system with the subsurface slip distribution and surface geometry of present-day, strike-slip earthquakes is
interpreted to indicate that frictional melting occurred within a concentrated zone of moment release or an earthquake
hypocenter during one or more M w z 6.3 earthquakes (M 0 z 6.9 1025 dyn cm) that involved northeastward rupture
propagation.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Pseudotachylite; Frictional melting; Homestake shear zone; Paleoseismology; Fault reactivation; Coseismic rupture
T Fax: +1 304 384 6225.
E-mail address: [email protected].
0040-1951/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.tecto.2004.10.017
38
J.L. Allen / Tectonophysics 402 (2005) 37–54
1. Introduction
A persistent question in structural geology is
whether faults may preserve a record of incremental
coseismic displacement distinguishable from aseismic
creep (Sibson, 1989; Cowan, 1999). The generation of
pseudotachylyte in response to frictional fusion on a
time scale of seconds suggests that this fault rock may
be a reliable indicator of specific episodes of seismic
faulting at depth. The geometry of pseudotachylytebearing zones therefore provides a direct record of the
result of earthquake rupture at hypocentral depths,
since most fault-generated pseudotachylytes appear to
have formed in the seismogenic zone where shallow
earthquakes nucleate in continental crust (Sibson,
1975, 1977, 1989; Scholz, 1990). So far, the kilometer-scale geometry of specific earthquake ruptures
is known only from the surface traces of active faults
and from indirect seismic records (e.g., Pavlides et al.,
1999; Lin et al., 2002); however, these data are unable
to resolve fine geometric details of lithologic or
structural heterogeneity at depth.
Pseudotachylyte is commonly reported in the
literature as isolated outcrops of thin veins (millimeter- to centimeter-scale thicknesses) in and near
fault zones; in many studies, the length or lateral
connectivity of veins is either not reported or not able
to be determined due to limited exposure (e.g., Park,
1961; Wallace, 1976; O’Hara, 1992; Sherlock and
Hetzel, 2001). Other studies report pseudotachylyte
fault veins to be continuous for meters to tens of
meters, and some have provided photographs or maps
of varying detail of the centimeter- to meter-scale
distribution of veins (e.g., Sibson, 1975; Magloughlin,
1989; Techmer et al., 1992; Lin, 1994; McNulty,
1995; Curewitz and Karson, 1999; Fabbri et al., 2000;
Lin et al., 2003; Lund and Austrheim, 2003; Di Toro
et al., 2005). On a broader scale, Wenk et al. (2000)
report that a pseudotachylyte system in California
might be continuous across a 20- to 200-m-wide by
15-km-long zone, and suggest that fault veins may
potentially be mappable. However, only a few studies
have actually attempted to record the distribution and
connectivity of fault veins for more than ten meters
along strike. These include the work of Grocott
(1981), who provided maps of paired pseudotachylyte
generation zones as long as 67 m that were part of an
unmapped 1-km-long system, and Swanson (1988,
1989), who provided detailed maps of a 60-m-wide by
220-m-long vein system.
Few studies have documented details of 1- to 10km-long fault zones in crystalline rocks (Pachell and
Evans, 2002), and no studies have previously documented the structure of a pseudotachylyte-bearing
fault system at this scale. This study describes a 7.3km-long system of pseudotachylyte-bearing fault
zones that cover an area nearly three orders of
magnitude larger than the system described by
Swanson (1988). Field mapping defines a series of
eight broadly en echelon fault zones that are laterally
continuous along strike within gneisses of the Homestake shear zone in the southern Rocky Mountains.
The system is comparable to some recent earthquake
ruptures in scale, geometry, and displacement. The
results suggest that pseudotachylyte may serve as a
unique paleoseismic tool for investigation of earthquake processes at hypocentral depths.
2. Geologic setting
The Homestake shear zone is a N 10-km-wide belt
of shear zones that are exposed within the northern
Sawatch Range of central Colorado, USA (Fig. 1).
The shear zone is rooted within the Proterozoic
continental lithosphere of southwestern North America, which assembled at ~ 1.7 Ga and was widely
reactivated at ~ 1.4 Ga during intracontinental transpression and syntectonic magmatism (Nyman et al.,
1994; Karlstrom et al., 2002; Shaw et al., 2002). Early
geologic mapping defined the Homestake shear zone
as an anastomosing series of northeast-striking, subvertical, shear zones hosted within Lower Proterozoic
gneisses and Lower to Middle Proterozoic plutonic
rocks of granitic to intermediate composition (Tweto
and Sims, 1963; Tweto, 1974; Tweto and Lovering,
1977). The individual shear zones are commonly
meters to tens of meters wide, and contain a wide
variety of fault rocks, including mylonite, ultramylonite, pseudotachylyte, and subordinate cataclasite
and breccia. All of these fault rocks are not necessarily
found in association with one another in all of the
individual shear zones originally mapped by Tweto
(1974), although the diversity of the fault-rock suite
clearly indicates shear-zone development across a
range of crustal levels through time.
J.L. Allen / Tectonophysics 402 (2005) 37–54
COLORADO
Glenwood
Springs
N
Denver
105˚50'
107˚
39˚45'
Gore
Range
Eagle
Vail
Study Area (Fig. 3)
Z
39˚10'
Leadville
M
o
Ra squ
ng ito
e
Aspen
Sawat
ch Ran
ge
HS
EXPLANATION
Phanerozoic Rocks
Middle Proterozoic granitic rocks of ~1400 Ma age group
Lower Proterozoic granitic rocks of ~1700 Ma age group
Lower Proterozoic metasedimentary gneiss and migmatite
Proterozoic crystalline basement rocks, undifferentiated
10 km
Fig. 1. Location of study area and geologic sketch map of the
northern Sawatch Range; a Laramide, basement-cored, anticlinal
uplift in the Rocky Mountain foreland (modified from Tweto et al.,
1978). The northeastern flank of the range dips gently (108) to the
northeast. HSZ=Homestake shear zone.
The oldest basement-rock unit in the northern
Sawatch Range is a quartz-rich, semi-pelitic gneiss
that hosts most of the shear zone (Fig. 1). The rock is
39
locally coarse grained and schistose where the biotite
content is high, and to the north, near the ~ 1700 Ma
Cross Creek batholith, the gneiss is variably migmatitic. Recent mapping, microstructural analysis, and
monazite dating by Shaw et al. (2001, 2002) document four contrasting ductile deformation events
during the Proterozoic development of the Homestake
shear zone. Deformation initiated during an extended
episode (~ 1710–1630 Ma) of high-temperature
metamorphism, partial melting, and intrusion of the
Cross Creek batholith (Shaw et al., 2001). This early
deformation isoclinally folded the primary gneissic
foliation (defined by migmatitic leucosomes and
biotite) and generated centimeter- to meter-scale
granitic dikes parallel to overprinting asymmetric fold
axes (D1 event of Shaw et al., 2001). As hightemperature deformation progressed, crustal shortening transposed the early foliation into the present
sub-vertical orientation along northeast-striking, highstrain zones (D2 of Shaw et al., 2001). These early
fabrics are overprinted by northeast-striking, northwest-side up, dextral mylonite zones (~ 1372 Ma), and
slightly younger, northwest-side down, dextral ultramylonite zones that are locally associated with both
cross-cutting and ductily sheared pseudotachylyte (D3
and D4 events of Shaw et al., 2001). This pseudotachylyte crops out more than 6 km N–NE of the
system described in this paper and the specific
temporal and spatial relationships between pseudotachylyte in the two locations are unknown. Following a
long history of Proterozoic deformation and exhumation, isolated components of the shear zone were
reactivated and cut by brittle faults during the Late
Cambrian, Early Ordovician, and Late Cretaceous
(Allen, 2004).
3. General characteristics of host rocks and
pseudotachylyte in the Homestake shear zone
3.1. Host rocks
The primary host rock is a sub-vertically foliated,
biotite- and quartz-rich, fine- to medium-grained,
semi-pelitic gneiss that is commonly migmatitic and
locally mylonitic. The gneiss includes meter-scale
bands of coarse-grained, biotite-rich, pelitic schist
that contains garnet and abundant 1- to 5-cm-wide
40
J.L. Allen / Tectonophysics 402 (2005) 37–54
ribbons of alkali feldspar and quartz (Fig. 2A). The
stable assemblage for the gneiss is typical of low
pressure, high-temperature upper amphibolite facies,
and includes quartz + microcline + plagioclase + bio-
tite + magnetite, with variable amounts of sillimanite F muscovite F cordierite (Allen et al., 2002;
Moecher and Sharp, 2004). A steeply plunging
mineral stretching lineation defined by quartz and
J.L. Allen / Tectonophysics 402 (2005) 37–54
prisms or elongate clots of sillimanite is locally
visible on the sub-vertical gneissic foliation. Subordinate host rocks include biotite-rich, sub-vertically
foliated granitic dikes, and hornblende-rich, calcsilicate gneiss. These latter rocks host less than 2.2%
of mapped pseudotachylyte-bearing fault zones
reported in this study.
3.2. Pseudotachylyte
Pseudotachylyte is found in linear to broadly
sinuous veins that are most commonly exposed on
horizontal outcrops. In outcrop, it appears as a
concoidally fractured and jointed, dark-gray to black
or dark brown, aphanitic rock that contains visible
lithic clasts. In many places, the rock is streaked or
zoned and locally includes internal isoclinal folds
with vein-parallel axial surfaces, suggesting Newtonian flow. Veins crop out in a wide variety of
forms along the shear zone (Fig. 2B–D), and can be
generalized as either: (1) fault veins, defined as
tabular bodies of pseudotachylyte of variable thickness (b1 mm to 39 cm) that fill faults with shear
displacement parallel to vein walls; or (2) injection
veins that branch off fault veins at moderate to high
angles and exhibit displacement consistent with
dilatation perpendicular to vein walls. In most places,
several fault veins are parallel and separated from
one another by 1 to 3 m; local splay fractures
diverge at acute angles and connect with an adjacent
fault vein in a manner that is very similar to an
interlinked system of paired dextral boundary shears
and minor bsecondaryQ fractures described by Grocott (1981) from exposures in Greenland. Fault veins
are most commonly solitary and frame local centimeter- to meter-scale isolated lenses (terminology of
41
Kim et al., 2004) along strike. In some areas,
however, fault veins are found in association with
a complex network of interconnected fractures that
form pseudotachylyte zones with decimeter-scale
widths, similar to bType CQ veins described by Lund
and Austrheim (2003). Both fault and injection veins
locally swell to form elongate or bulbous reservoirs
that represent b 1- to 12-cm-wide (or more) pockets
of melt. Veins are abruptly bounded by wall rocks,
and are locally lined by a dense concentration of
lithic clasts (Fig. 2E–F). In all outcrops, pseudotachylyte appears to have been formed due to rupture
of intact rock, as veins are not parallel to a fracture
or joint set, and do not overprint cataclasite or gouge
zones. This observation is supported by oxygen and
hydrogen isotope data, which suggest that the
Homestake pseudotachylytes formed in a closed
system devoid of meteoric water (Moecher and
Sharp, 2004).
In thin section, pseudotachylyte matrix appears
opaque, isotropic, cryptocrystalline, and black to
dark brown. Lithic clasts are matrix bound and
include fragments of quartz and plagioclase. Lithic
clasts are mostly monomineralic and are dominated
by quartz, although some larger gneissic host-rock
fragments were observed in outcrop and some thin
sections. Clasts commonly range in size from b 10
Am to 3 cm, are rounded, and commonly display
corroded and embayed margins and optical halos.
Iron oxides (magnetite) are also present and appear
in three distinct forms. The most common form is
typified by 1 to 5 Am octahedral grains densely
dispersed throughout the matrix. A less common
form is amorphous, rounded in shape, and much
larger (N 10 to 100 Am) in size, and the third form is
present as dendritic microlites that are nucleated
Fig. 2. Representative photographs of pseudotachylyte and host rock; locations indicated on Fig. 3. (A) Sub-horizontal exposure of a SWvergent, rootless isoclinal fold outlined by leucosomes in biotite gneiss, the primary host rock. (B) Solitary fault vein. Arrow shows location of a
fault-wall damage zone forming an isolated lens that resembles the geometry of a dextral sidewall ripout as described by Swanson (1989).
Horizontal exposure of vertical vein. (C) Paired, sub-millimeter-thick fault veins highlighted in white (pen points 0608; horizontal exposure of
vertical veins). Deflection of foliation and leucosome mismatches consistent with dextral oblique slip. Exact magnitude of displacement
unknown, although field relationships suggest a maximum of ~ 10 cm strike-slip offset across the upper vein. (D) Injection veins branching off
of a very thin (sub-millimeter), foliation-parallel fault vein along the right side of the photo. Injections are swollen to form bulbous, off-fault
reservoir. Pen points 0558; horizontal exposure of vertical vein. (E) Polished slab showing ~ 14-cm-thick fault vein; wall rock is along far right
of photo. In three dimensions, all pseudotachylyte in this hand sample is seen to be a continuous and interconnected, single-generation melt as
evidenced by swirled flow lines. Note large rounded clasts of host gneiss. (F) Polished slab showing thick fault vein that exhibits a concentration
of granitic, vein-margin lithic clasts; wall rock at bottom of photo. Slab cut perpendicular to sub-vertical dip. (G) Photomicrograph of dendritic
microlites of magnetite nucleated on larger magnetite grains. Note rounded and embayed quartz clasts and opaque matrix. Field of view = 70 Am
wide.
42
J.L. Allen / Tectonophysics 402 (2005) 37–54
along the margins of larger, amorphous magnetite
grains (Fig. 2G). Small octahedral magnetite crystals
are not present in the host rock and, therefore, are
interpreted as a crystallization product of frictional
melt. These octahedral magnetite crystals, dendritic
microlites, and embayed quartz grains strongly
suggest a melt origin for pseudotachylyte, and the
presence of acicular crystals of mullite reported by
Moecher and Brearley (2004) suggest that the melt
temperature in one thick vein (10 cm) may have
exceeded 1150 8C.
Some studies have suggested that the presence of
vesicular textures in pseudotachylyte is indicative of
frictional melting at very shallow (b 1.6 km) depths,
and the presence of amygdules may be associated
with depths less than approximately 7 km (Masch et
al., 1985; Maddock et al., 1987; Swanson, 1992).
Homestake pseudotachylytes lack either of these
features, suggesting an origin below 7 km. This is
consistent with observations indicating that pseudotachylyte near this system is locally cut by ductile
microshears (Shaw et al., 2001), and the absence of
meteoric fluids during faulting (Moecher and Sharp,
2004). On the basis of circumstantial evidence for an
origin at depths N 7 km and field relationships in
faulted cover strata, Allen et al. (2002) inferred that
the pseudotachylyte must be of Proterozoic rather than
Phanerozoic origin.
4. Kilometer-scale distribution of pseudotachylyte
4.1. Introduction
The results presented in this study are based upon
field mapping of the distribution of pseudotachylyte at
various scales. The kilometer-scale distribution was
mapped on air photos at scales of 1:4930 and 1:2470.
The meter-scale vein geometry was characterized
locally from large-scale mapping completed on
surveyed outcrop grids and from photo mosaics taken
1.5 m above outcrops and subsequently mapped in the
field.
At the kilometer scale, individual pseudotachylyte
fault veins are exposed within eight NE-striking fault
zones that are mapped for 7.3 km along strike (Fig. 3).
Each fault zone is typically less than 5 to 20 m wide
and consists of two or more parallel to sub-parallel,
through-going fault veins and associated splays and
injections. Mapped fault zones locally consist of only
a single fault vein along the northeastern-most segments of most zones. Cross-cutting relationships
between individual veins are uncommon, and could
only be observed between thin veins (b 0.6 cm) in
several areas along the southwestern and central parts
of fault zone IV, where dense linear networks of
pseudotachylyte are common. At least two older,
overprinted generations of pseudotachylyte are locally
present within this fault zone, including some that has
been subjected to crystal-plastic deformation. However, the volume and abundance of older veins
appears to be subordinate.
As a system, the fault zones diverge to the
northeast from a narrow 170-m-wide outcrop belt to
a maximum known cross-strike width of 2.3 km. The
cumulative mapped length of the fault zones is more
than 21 km, and 5 km of this length represents very
well to partly exposed fault veins. In some areas,
linear to sinuous veins within fault zones can be traced
along continuous exposures as much as 130 m long.
Most outcrops are shorter and are separated by a few
meters to a few hundred meters of cover that locally
includes pseudotachylyte-bearing float.
4.2. Structural framework of host rock
The attitude of foliation and lineation within the
host rock defines three structural domains herein
referred to as the northern, central, and southern
domains (Fig. 4; Table 1). The central domain
contains the majority of the pseudotachylyte-bearing
fault zones. The northern domain does not contain any
pseudotachylyte, and the southern domain is host only
to fault zone I, and the northeastern-most segments of
some other fault zones. In all domains, foliation
mostly strikes northeast, except where it defines the
hinges of SW-vergent, map-scale isoclinal folds (e.g.,
Fig. 3, regions 1, 2, 4, and 5). The domains are
primarily differentiated on the basis of foliation dip.
Approximately one quarter of the foliation in the
central and southern domains dips V 758 but in
opposite directions, whereas the southern domain has
more vertical foliation (Table 1).
Both kilometer- and meter-scale relationships show
that the orientation of fault zones is parallel to preexisting, NE-striking, sub-vertical foliation in the
J.L. Allen / Tectonophysics 402 (2005) 37–54
43
B. GEOLOGIC MAP
VIII
VII
region 5
VI
IV
A. INDEX MAP
Area of Fig.5
Country rock lozenge
along fault zone IV
Fig.2b
region 4
V
Fig.2c,
2f, 2g
Fig.2d
III II
Fig.2e
region 2
region 3
I
region 1
NW strand
SE strand
= fold trace
N
EXPLANATION - Map B
39˚ 24’
30”
’
106˚ 26
Quaternary cover
Pseudotachylyte (one or more
fault veins, dashed where inferred)
Gneiss showing foliation traces
Unimproved road
1 km
Fig. 3. (A) Index map and (B) geologic map showing distribution of pseudotachylyte-bearing fault zones (labeled with Roman numerals) within
sub-vertically foliated gneiss. Index map highlights locations referenced in text and shows generalized envelope surface of map-scale, SWvergent isoclinal folds. Region 4 includes elongate bodies of calc-silicate gneiss.
44
J.L. Allen / Tectonophysics 402 (2005) 37–54
Northern Domain
Central Domain
Southern Domain
Fig. 4. Map showing distribution of structural domains in the same area as Fig. 3. Equal-area stereonets show 1% area contours of poles to foliation
in each domain; lineation plotted as points. Structural data are further summarized in Table 1. In the southern domain, foliation is mostly subvertical and distinctly steeper than the central and northern domains, which both have a higher proportion of foliation dipping V 758. The northern
domain does not contain pseudotachylyte-bearing fault zones and is characterized by north-dipping, rather than south-dipping foliation.
Homestake shear zone. The northeast-fanning geometry of the system as a whole reflects both the wedgeshaped geometry of the central domain and the
distribution of SW-vergent folds developed within
the host rock prior to pseudotachylyte generation. An
outcrop-scale analog of the host-rock structure is
provided by Fig. 2A, which shows a meter-scale fold.
From the hinge area above the hammer, the fold limb
includes parasitic minor folds and broadly fans out to
the left of the photograph. The limbs of parasitic folds
generally mimic the distribution of the pseudotachylyte-bearing fault zones mapped in Fig. 3.
4.3. Fault zones I–III
Fault zone I (Fig. 3A) represents the southeasternmost boundary of the pseudotachylyte system and is
mapped for 3.6 km along strike. The zone is interpreted
as a series of four right-stepping fault segments and is
lost beneath cover along strike at the southwestern and
northeastern ends. Region 1 is an example of a rightstep, and the fault zone consists of at least three
concordant fault veins (b 0.8 to 1.5 cm thick) separated
by a distance of 1.5 to 3.5 m. The veins are
anomalously hosted by sub-vertically foliated granitic
J.L. Allen / Tectonophysics 402 (2005) 37–54
45
Table 1
Structural domains
Foliation
Northern domain
Central domain
Southern domain
Lineation
n
General dip
direction
Vector mean
(strike and dip)
% with
dip V 708
% with
dip V 758
n
General
trend
Vector mean
(trend and plunge)
85
331
512
N
S
S
0608, 708N
0678, 808S
0738, 818S
17.6%
15.4%
4.7%
25.9%
23.6%
7.6%
4
66
75
N
S–SE
SW
3498, 688
1748, 748
2478, 708
rock for 360 m. South of the main mapped strand of
fault zone I (regions 1 and 2, Fig. 3) are three short
exposures of fault veins that are solitary or paired and
commonly b 0.5 cm thick. The southeastern-most of
these exposures includes several 3- to 6-m splays that
follow an abrupt foliation change from 0698 (main vein
trend) to 0928 and resemble a horsetail splay at the end
of a dextral fault (e.g., Kim et al., 2004).
Fault zones II and III are mapped for 1.5 km and
1.2 km along strike (Fig. 3A). Both zones diverge
from a 17-m-wide zone on the southwest and follow
foliation northeastward. Individual veins are poorly
exposed in the region of divergence and the fault
zones are primarily mapped on the basis of extensive
float within a platy orange-brown zone of weathered
gneiss and pseudotachylyte that is intensely fractured.
Both zones are lost in cover to the southeast and are
not found in outcrops farther along strike, suggesting
the maximum length of these fault zones is less than
1.6 km. Northeastward from the region of divergence,
fault zone II is locally exposed as a parallel pair of 1 to
2 mm veins separated by 0.2 to 1.5 m. In contrast,
fault zone III locally swells to a 20-m-wide zone
consisting of at least six individual, foliation-parallel
fault veins that are as much as 2 cm thick.
4.4. Fault zone IV (central fault zone)
Fault zone IV is mapped for 5.8 km along strike,
and is the longest and best exposed of the eight fault
zones (Fig. 3). Veins are lost along strike at both ends
of the fault zone within regions of modest exposure,
suggesting that this may closely approximate the
interconnected, along-strike end of the system. The
thickness and density of pseudotachylyte veins within
the fault zone decreases from southwest to northeast
in a manner that is systematic rather than random, and
is the subject of further analysis in a subsequent
section.
Region 3 illustrates two fault strands; a 440-m-long
northwestern strand, and an en echelon southeastern
strand. The northwestern strand consists of a cluster of
4 to 6 very thick (typically 2–21 cm) fault veins that
strike 0608 and are hosted by a band of concordant
foliation. The concordant fault veins are bounded on
both ends by foliation that strikes ~ E–W (oblique to
fault veins), suggesting that discordant foliation may
have served as a local structural barrier to rupture
propagation. The southeastern strand in region 3 is a
series of poorly exposed and discontinuous outcrops
of pseudotachylyte that appear to have a rightstepping, en echelon distribution. Small exposures
indicate that this segment of the fault zone consists of
at least four concordant 1- to 6-cm-thick fault veins
across a 10-m-wide zone.
The middle segment of fault zone IV (region 4) is
highlighted by a well-exposed country-rock lozenge
framed by fault veins that define a structural horse (Fig.
3). Fault veins along the northwestern margin of the
lozenge cross cut foliation at high angles (locally
perpendicular) for 250 m (illustrated by Fig. 2C).
Along the southeastern margin of the lozenge, 4 to 6
fault veins follow the southern limb of a SW-vergent
isoclinal fold, which locally defines the boundary
between the southern and central structural domains
(Fig. 4).
In most areas, the magnitude of displacement and
sense of shear along fault veins is difficult to
establish, mainly because of a scarcity of mutual
cross-cutting relationships. However, a consistent
sense of shear is documented in several locations
along fault zone IV. One example is presented in Fig.
5, which is an outcrop-scale map showing four of six
parallel fault veins that comprise the width of fault
zone IV at this location. One fault vein dextrally
offsets a sub-vertical felsic dike by 1.7 m, and
another dextrally offsets a vertical quartz vein by 0.4
m (fault veins b and d; Fig. 5A). Two fault veins are
46
J.L. Allen / Tectonophysics 402 (2005) 37–54
A
0.4 m
d
1.8 cm
Location
of C
Location
of B
1c
m
c
2.7
0.7 cm
2.2 cm
1 cm
0.8 cm
1.2 cm
cm
2.2 cm
0.8 cm
1.5
1.5 cm
1.7 m
b
cm
1 cm
0.6 m
N
0.11 m
0.35 m
0.3 cm
1m
a
SE
NW
Up
C
covered
NW
1 cm
B
SE
Up
Explanation
Pseudotachylyte
Fault (no pst)
Foliation trajectories
(subvertical)
Subvertical marker
(quartz and feldspar)
covered
5 cm
A
T
15 cm
A
T
Fig. 5. (A) Outcrop map of four foliation-parallel, principle-displacement fault veins that encompass most (75%) of the width of the central fault
zone (fault zone IV) NE of the country rock lozenge. Note dextral offsets (recorded in meters within boxes) and splay faults. Fault veins are
vertical and typically b 1–4 mm thick; local thick lenses of melt are shown with maximum thickness recorded in centimeters. Two partially
exposed veins are located 5.7 and 7 m to the northwest (b 0.7 cm thick; not illustrated) and comprise the remainder of this segment of fault zone
IV. Map A was completed in the field from photo mosaics on a gridded horizontal exposure. Map location indicated on Fig. 3. (B and C) Vertical
cuts illustrating up-to-NW component of displacement, suggesting an overall dextral oblique slip. Circled letters bAQ and bTQ indicate strike-slip
component of displacement away and toward the reader, respectively. Maps B and C were drawn in the field on photo mosaics taken on a
southwest-facing vertical cut.
exposed in rare vertical cuts (Fig. 5B–C), and they
show deflection of foliation consistent with an
upward displacement of the northwestern side
relative to the southeastern side. These observations
indicate a dextral oblique slip at the mid-point of
fault zone IV. Total dextral strike-slip offset must be
N 2.1 m, and the magnitude of up-to-northwest
vertical offset is unknown.
Similar observations on horizontal exposures elsewhere along fault zone IV demonstrate dextral
displacement. These include: (1) a pair of thin fault
veins (Fig. 2C) that show deflection of foliation
J.L. Allen / Tectonophysics 402 (2005) 37–54
<1
<1
<1
<1
consistent with dextral shear; and (2) a band of subvertical quartz-feldspar ribbons that are dextrally offset
by 0.4 m along the northeastern-most segment of fault
zone IV. Prominent millimeter- to centimeter-scale
leucosomes are obliquely cut in many places by fault
veins, although offsets are difficult to determine, even
where other relationships suggest only small (centimeter-scale) displacement. This suggests that a component of dip-slip displacement may be responsible for
mismatches, and is consistent with observations
illustrated in Fig. 5.
4.5. Fault zones V–VIII
<1
<1
1.
8
<1
<1
1.
4
<1
47
5.
5
Maximum fault vein
thickness >6 cm
1
1.
3
Maximum fault vein
thickness 3-6 cm
<1
1.
5
Maximum fault vein
thickness 1-3 cm
<1
<1
<1
39
11 21
1
N
<11
14
<1
3.
2
2.
3
<1
2.
1
<1
3. 4.9
1
2.
3.
4
1
<1
2
1.
9
7
6 7 4 .6
4 3 6
2
.3 8 .2
<1
<1
Country rock lozenge
along fault zone IV
<1
<1
1.
1
Maximum fault vein
thickness <1 cm
Fault zone V is mapped for 3.3 km along strike and
bounds the northwestern limb of a southwest-vergent
isoclinal fold along most of its length. The southeastern limb is bounded by fault zone IV (region 4,
Fig. 3). The fault zone consists of two to nine fault
veins that are sub-parallel, linear to sinuous, and
locally bifurcate along strike to form elongate lenses
that have centimeter- to meter-scale widths and
apparent lengths of several meters to inferred lengths
of as much as 110 m. At least two sub-millimeter
veins near the southwestern-most exposure of the fault
are hosted by a tabular, foliation-parallel band of
mylonite. Some veins are dextrally offset by steeply
dipping, northwest-side-up, microshears that are
parallel or oblique to the foliation.
Fault zones VI, VII, and VIII are mapped for 2.3,
1.5, and 1.8 km along strike, respectively. All three
zones are characterized by a predominance of only a
few very thin (b 0.1–0.5 cm), sub-parallel fault veins
distributed across narrow fault zones (commonly 1–5
m wide). Fault zone VI is a left-stepping, en echelon
fault zone that exhibits thicker fault veins on the
southwest (as much as 1.3 cm), and thinner veins
(0.05–0.4 cm) northeastward. Fault zones VII and
VIII are locally separated by a SW-vergent fold
(region 5) and are parallel to the limbs, similar to
relationships observed between fault zones IV and V.
6
1 km
6
8
3
5. Thickness trends
Fig. 6. Map showing spatial distribution of maximum observed
thickness of pseudotachylyte. Numbers indicate thickness in
centimeters. Mapped area is the same as that mapped in Fig. 3.
The maximum observed thickness of individual
pseudotachylyte fault veins systematically decreases
from southwest to northeast. The thickest veins (3–21
48
J.L. Allen / Tectonophysics 402 (2005) 37–54
log vein thickness (cm)
1. 1.6
11
INDEX MAP
1
10
>1 cm
0
9
N
VIII
VII
<1 cm
-1
10
0
20 m
1 km
position within fault zone (m)
2. 1.6
VI
Pseudotachylyte
1
1.6
0
3
2
-1
3.
IV
V
10
20
30
40 m
1
45
6
7
III
II
0
8
I
1.6
1
0
-1
0
10
20
40 m
8.
1
1
0
0
-1
-1
0
0
10 m
9. 1.6
5. 1.6
1
1
0
0
-1
6. 1.6
30
1.6
4. 1.6
0
-1
10 m
0
10.1.6
1
1
0
0
-1
0
10 m
-1
7. 1.6
0
11.1.6
1
1
0
0
-1
0
10
20 m
-1
0
Fig. 7. Cross-strike transects showing distribution and thickness of pseudotachylyte. Cross sections plot log of vein thickness (cm) vs. position
within fault zone (m); gray horizontal bar shows 1 cm thickness for reference. Position axis (x-axis) on all transects shows distance from the SE
margin of the fault zone on the left (0 m), to the NW margin on the right. All are drawn with the same horizontal and vertical scales. Index map
shows transect locations.
J.L. Allen / Tectonophysics 402 (2005) 37–54
cm) are common at the southwestern-most end of fault
zone IV (Fig. 6), where one 39-cm-thick pseudotachylyte breccia is also present. Maximum vein thickness along fault zone IV decreases towards the
northeast, and thicknesses of 2–6 cm are common
along the middle part of the fault zone. Northeast of
the pseudotachylyte-bounded, country-rock lozenge,
maximum observed thicknesses abruptly decrease.
Other fault zones typically exhibit pseudotachylyte
fault veins that are less than 1 cm in thickness (b 1 to
3 mm veins are common).
In order to characterize the cross-strike distribution
of fault and injection veins within individual fault
zones, eleven cross sections were measured in wellexposed sites where fault-zone boundaries could be
identified. The cross sections are plotted as histograms
showing the log of measured vein thicknesses versus
the position of veins within individual fault zones
along a taped, cross-strike transect (Fig. 7). The
thickness of all veins encountered on transects was
plotted, and a distinction between fault and injection
veins is not made. The cross sections show a southwest
to northeast decrease in fault-zone width, maximum
vein thickness, and cumulative vein thickness for
individual fault zones (Fig. 8). Cross sections 1
through 7 were measured along fault zone IV and
clearly illustrate a northeastward, systematic decrease
in maximum vein thickness from 39 to 1.4 cm, and a
corresponding decrease in fault-zone width from a
mean of 37 m in sections 1 through 3, to a mean of
width 16 m in sections 4 through 7. The cumulative
70
Fault zone width (m)
Thickness (m or cm)
60
Maximum vein
thickness (cm)
Cumulative thk (cm)
50
40
thickness of pseudotachylyte also decreases northeastward along fault zone IV, except at Section 5 southwest of the pseudotachylyte-bounded country-rock
lozenge, which includes 14 veins thicker than 1 cm.
6. Discussion
A crucial question is whether the systematic
distribution of pseudotachylyte-bearing fault zones
in the Homestake shear zone represents the kilometerscale fossil record of one (or several) earthquake
ruptures, or whether it represents dozens (or hundreds)
of ruptures that formed meter- to decameter-scale fault
veins that coalesced along strike to form a multikilometer system. The goal of this discussion is to
address this question and compare the system with
some well-known earthquakes. Kanamori and Heaton
(2000) postulated that large magnitude earthquakes
(M w N 5–6) normally should be influenced by thermal
processes such as frictional melting, and other studies
have interpreted frictional melting to have occurred
during recent earthquakes based upon seismic data
and energy-budget estimates (e.g., Kanamori et al.,
1998; Nadeau and Johnson, 1998). Theoretical
analyses of the generation of frictional melt on a
planar fault (McKenzie and Brune, 1972; Cardwell et
al., 1978) and laboratory experiments (Spray, 1995)
indicate that the rise in temperature (DT) on a fault
plane is directly proportional to displacement (d),
frictional shear stress (r f), and slip velocity. Similarly,
Kanamori and Heaton (2000) considered the gross
thermal budget for a zone of faulting and suggested
that DT is proportional to d 1/2. The seismic moment
(M 0) is a scalar that describes the size of an
earthquake as:
M0 ¼ DlA
30
20
10
0
0
2
4
6
8
10
12
Location: SW (left) to NE (right)
Fig. 8. Plot of thickness versus location along strike for fault zone
width, maximum vein thickness, and cumulative vein thickness for
data and locations presented in Fig. 7. Position axis (x-axis) shows
location of transects 1–11.
49
ð1Þ
where l is the rigidity (shear modulus) of the host
rock, D is the mean displacement, and A is the area of
rupture. Following Hanks and Kanamori (1979), the
seismic moment is related to moment magnitude (M w)
by:
Mw ¼ 2=3log M0 10:7
ð2Þ
Since higher mean displacement generally correlates
with larger seismic moment and corresponding M w,
frictional melting might be more common for large
50
J.L. Allen / Tectonophysics 402 (2005) 37–54
M w earthquakes as suggested by Kanamori and
Heaton (2000), and importantly to this discussion,
more extensive along specific segments of frictionally melted fault zones that underwent greater
displacement.
Seismic inversion has been used to interpret
subsurface slip distribution along several well-known
earthquakes. Some ruptures, such as the 1979 Coyote
Lake earthquake (M w = 5.9) on the Calaveras fault,
California, show maximum slip at the hypocenter and
a progressive decrease in slip upwards and laterally
(Oppenheimer et al., 1990). In contrast, many other
earthquakes exhibit a more variable pattern of slip at
depth that is characterized by maximum slip distal to
the hypocenter (e.g., Hartzell and Heaton, 1983;
Cohee and Berzoa, 1994; Wald and Heaton, 1994).
For example, the 1992 Landers, California earthquake
(M w = 7.3) was characterized by ~ 2 m of slip at the
hypocenter, and by a maximum slip of ~ 6 m at a
distance of 30 km from the hypocenter (Wald and
Heaton, 1994). The rupture propagated northwestward
along three en echelon fault segments that encompassed numerous faults, and each of the three
segments shows an elliptical region of maximum slip
on the southeast (Fig. 9A). These data suggest that
moment release was concentrated in three regions
(Cohee and Berzoa, 1994; Wald and Heaton, 1994). A
similar pattern was observed for the propagation of
the rupture associated with the 1979 Imperial Valley
earthquake (Fig. 9B). These examples illustrate the
known heterogeneity of slip along earthquake ruptures
and provide an analogy for an interpretation of
features observed in the Homestake shear zone.
Several observations suggest that a record of multikilometer-scale rupture may be preserved along the
Homestake shear zone. These include: (1) the alongstrike persistence of fault veins at the outcrop scale,
and pseudotachylyte-bearing fault zones at kilometer
scale; (2) the systematic northeastward decrease in
maximum vein thickness, fault-zone width, and
cumulative vein thickness; and (3) the overall
geometry of the entire system, which fans northeastward similar to a horsetail fan near the terminus of a
dextral fault zone. A similar scale and geometry of
rupture has been observed in recent strike-slip earthquakes (e.g., Tchalenko and Ambraseys, 1970; Hartzell and Heaton, 1983; Cohee and Berzoa, 1994; Lin
et al., 2002). An additional feature of the Homestake
pseudotachylytes is the lack of common and widespread cross-cutting relationships, which is emphasized by the interconnectivity of dextral principal
displacement fault veins and associated dextral splays
at the mid-point of the central fault zone (IV) in Fig. 5.
A remarkably similar geometry was observed by
Grocott (1981) in decameter-scale, paired fault-vein
systems in Greenland that were interpreted to be the
result of a single rupture event, since it is improbable
that a linked system may have sequentially formed
without offsetting or visibly bounding pseudotachylyte within earlier veins. By analogy, the fault veins in
Fig. 5 are also interpreted to have formed during a
unique rupture. This implies that the majority (at least
four of six) of fault veins across the width of the midpoint of fault zone IV formed during a single rupture,
as opposed to an array of coalescing or overlapping
smaller ruptures that sequentially generated pseudotachylyte and grew laterally through time. The
Homestake and Greenland observations contrast with
pseudotachylytes described in many other locations
that clearly show widespread cross-cutting relationships indicative of multiple episodes of melt generation (e.g., Sibson, 1980; Hobbs et al., 1986;
Swanson, 1988; Magloughlin, 1989; Toyoshima,
1990; Koch and Masch, 1992; Maddock, 1992;
Techmer et al., 1992; Camacho et al., 1995; Curewitz
and Karson, 1999; Fabbri et al., 2000; Theunissen et
al., 2002; Lin et al., 2003; Austrheim and Andersen,
2004). Similarly, seismogenic faults preserved in
surficial sediments along active fault zones are
typically offset by faults produced by younger earthquakes (Weldon et al., 2004).
The observations along the Homestake shear zone
suggest that the central fault zone (IV) is largely the
result of a single multi-kilometer coseismic rupture.
Since the fault zones are mutually separated, it is not
possible to determine whether more than one may
have been produced in response to the same seismic
event, and it is clear that there are at least two
subordinate generations of melt along the southwestern half of fault zone IV. However, the along-strike
connectivity of fault veins in each fault zone suggests
that most of the melt volume along any given zone
formed contemporaneously. The systematical decrease
of the abundance and thickness of veins towards the
northeast may reflect a northeastward decrease in
displacement. If so, then the region of thick pseudo-
J.L. Allen / Tectonophysics 402 (2005) 37–54
A
n
erso
NW
p
Cam
lts
Fau
k/Em
Homestead Valley Fault
Roc
0
51
>5m
3-5 m
Joh
SE
nso
n Va
lley
Fau
lt
1-3 m
H
>5m
1-3 m
1-3 m
15 km
5 km
Br
B
aw
ley
10 km
Fa
ult
NW
Imperial Fault
SE
0
0.2-1.0 m
5 km
H
>1 m
5 km
C
eudotachylyte
Homestake ps
system
SW
NE
0
6–12
>12
3–6
H?
1–3
<1
1 km
Fig. 9. Slip distribution and rupture geometry for two California earthquakes (A and B), compared to a hypothetical model of slip in the
Homestake pseudotachylyte system (C). (A) The 1992 Landers earthquake and (B) 1979 Imperial Valley earthquake (simplified from Hartzell
and Heaton, 1983; Archuleta, 1984; Sibson, 1989; Wald and Heaton, 1994; Yeats et al., 1997). Both events had dextral strike-slip focal
mechanisms and exhibit several concentric high-slip zones away from the hypocenter (H). Upper sketch in each image shows rupture pattern
along faults in map view; lower sketch in each image shows an interpretation of slip distribution (in meters) through the seismogenic zone. (C)
Comparative conceptual model outlining inferred slip distribution through the seismogenic zone for a hypothetical multi-kilometer rupture in the
Homestake pseudotachylyte system. Contours show maximum melt thickness in cm rather than slip, since melt thickness is considered as a
proxy for displacement (thickness data compiled from Fig. 6). Vertical (depth) scale is not precisely known; however, microtextural features
discussed in Section 3.2 suggest an origin in the lower half of the seismogenic zone, and a local overprint of mylonitized pseudotachylyte
suggests an origin near the base of the seismogenic zone.
tachylyte on the southwestern end of the system (Fig.
6) may have been formed within a zone of concentrated moment release as shown in Fig. 9C, or it may
represent an actual hypocenter. In this model, a
rupture may have propagated to the northeast, away
from a zone of moment release or the hypocenter.
52
J.L. Allen / Tectonophysics 402 (2005) 37–54
A minimum estimate of seismic moment and
moment magnitude can be obtained from Eqs. (1)
and (2) if it is assumed that the length of a rupture that
produced one or more fault zones was at least equal to
the trace length of the entire system (7.3 km), and the
rupture area, A, is taken as a square. If a typical crustal
value of l is used (3 1011 dyn/cm2; Scholz, 1990),
and mean displacement is approximated by 2.1 m,
which represents the minimum known displacement at
the mid-point of fault zone IV, then M 0 = 6.9 1025
dyn cm, and M w = 6.3. It is quite possible that fault
zone IV may have been produced in response to a
much larger rupture that extended through the entire
seismogenic zone (~ 15 km) and/or for a longer extent
along strike. The minimum magnitude estimate,
however, is within the range (M w = 5 to 7) for which
the theoretical analysis of Kanamori and Heaton
(2000) predicts temperature rise to be sufficient for
frictional melting of non-granulated rock under dry
conditions. Their model further predicts contemporaneous development of a zone of melt-filled faults,
rather than a single melt layer. This phenomenon
appears to be preserved in each of the fault zones of
the Homestake shear zone.
veins and splay faults at the mid-point of the central
fault zone provides local evidence for preservation of
a single rupture event that formed at least 75% of the
fault veins at one location. A paucity of widespread
cross-cutting relationships further suggests that the
central fault zone and perhaps others record at least
one multi-kilometer rupture of M wz6.3. The systematic distribution and geometry of pseudotachylytebearing fault zones, combined with maximum and
cumulative thickness trends suggests a northeastward
rupture directivity.
Determination of earthquake source parameters
from outcrop-scale exposures of pseudotachylytes
has proved to be difficult, primarily because of
complications associated with measurement of displacement (Di Toro et al., 2005). The Homestake
pseudotachylyte system, however, is extensive and is
comparable in scale and geometry to surface ruptures.
This allows for application of traditional methods of
paleoseismology to provide estimates of some basic
source parameters. Mapping at the fault-system scale
in other pseudotachylyte-bearing fault zones where
exposure is sufficient may prove useful as a unique
paleoseismic tool for interpreting earthquake rupture at
hypocentral depths at a scale that has been overlooked.
7. Conclusion
Acknowledgements
Detailed field mapping demonstrates that the
distribution of pseudotachylyte fault veins in the
Homestake shear zone is systematic and defines a
series of eight northeast-striking fault zones. The
thickness of individual fault veins, the width of fault
zones, and the cumulative thickness of pseudotachylyte across strike of individual fault zones decreases
along the trace of the fault system from southwest to
northeast. The entire system exhibits a northeastfanning geometry that is concordant with foliation and
localized along the limbs of map-scale, rootless
isoclinal folds and parasitic minor folds. The orientation of preexisting fabric and structures is therefore
interpreted to have strongly influenced the rupture
geometry that resulted in frictional melting.
Frictional melting at the mid-point of the central
fault zone (fault zone IV) involved an episode of
dextral oblique-slip along sub-vertical faults; a minimum strike-slip offset of 2.1 m is observed. Connectivity of parallel, meter- to decameter-scale fault
Research was supported by grants from the
Andrew W. Melon Foundation administered by the
Appalachian College Association. R.D. Hatcher, Jr.
provided valuable advice in the field and research
space at the University of Tennessee-Knoxville in the
early stages of this project. A.M. Hawthorne Allen, S.
Abshire, and undergraduate participants in several
iterations of the Concord College geology field camp
(’99, ’01, ’03) provided meaningful field assistance.
Olivier Fabbri and John Grocott provided constructive
reviews that helped to improve the final version of the
manuscript.
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