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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

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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

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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

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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

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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.

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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

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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.

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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

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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

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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