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Tectonophysics 319 (2000) 129149www.elsevier.com/locate/tecto

Contrasting nature of deformation along an intra-arc shear

zone, the LiquineOfqui fault zone, southern Chilean AndesJose Cembrano a,*, Elizabeth Schermer b, Alain Lavenu c, Alejandro Sanhueza a

a Departamento de Geologa, Universidad de Chile, Casilla 13518 Correo 21, Santiago, Chile

b Geology Department, Western Washington University, Bellingham, Washington, DC 98225, USA

c Institut Francais de Recherche Scientifique pour le Developpement en Cooperation (ORSTOM), Casilla 53390 Correo Central,

Santiago 1, Chile y Departamento de Geologa, Universidad de Chile, Santiago, Chile

Received 1 April 1999; accepted for publication 22 November 1999

Abstract

The LiquineOfquifault zone (LOFZ) cuts the Patagonian batholith and the modern volcanic arc of southern

Chile for ca 1000 km. The rock fabric along three transects of the LOFZ combined with new and published

geochronological data suggest marked differences in the nature and timing of deformation along strike. In the Liquin e

transect ( 39S), a 1 km wide, northeast-striking subvertical mylonitic zone shows north-plunging stretching lineations.

This mylonitic zone has been juxtaposed by high-angle reverse faulting against a nearly undeformed Miocene granitoid.

Metamorphic assemblages and microstructures in the mylonites indicate greenschist facies conditions and sinistral

reverse displacement. Deformation pre-dates a 1002 Ma undeformed porphyritic dike (hornblende 40Ar39Ar mean

age). In the Reloncav transect (4142S), deformation in Cretaceous and Miocene plutons is predominantly brittle.

Kinematic analysis of two fault populations yields compressional and dextral strike-slip stress regimes, interpreted as

late Miocene in age. In the Hornopiren transect ( 4243S), a 4 km wide mylonitic zone, developed in plutons and

wallrocks, shows subhorizontal stretching lineations and dextral displacement. A single fault population overprinting

the mylonites supports a dextral strike-slip stress regime. Available UPb, KAr and40

Ar39

Ar dates on deformedCenozoic plutons and wallrock range from 9 to 13 Ma on hornblende and from 6 to 3 Ma on biotite. Microstructures

and mineral assemblages indicate that the youngest ductile fabrics in the plutons formed at greenschist facies, similar

to the biotite Ar closure temperature. Subparallel magmatic and solid-state fabrics combined with geochronology

suggest that dextral displacement was continuous during emplacement and cooling of the plutons. Dextral-oblique

subduction of the Nazca plate beneath western South America has driven long-term intra-arc deformation at the

southern Andes plate boundary zone; ridge collision, in turn, has favored dextral displacement at the leading edge of

the continent since the Pliocene 2000 Elsevier Science B.V. All rights reserved.

Keywords: Andes; Cenozoic; faults; magmatic arc; tectonics

1. Introduction vide a unique opportunity to address both tectonicand magmatic phenomena on a crustal scale (e.g.Beck, 1983; Saint Blanquat et al., 1998). A studyRegional-scale fault zones spatially associatedof the nature and timing of both ductile and brittlewith ancient and present-day magmatic arcs pro-deformation along intra-arc fault zones may pro-vide a valuable insight into deformation partition-* Corresponding author. Fax: +56-2-6963050.

E-mail address: [email protected] (Jose Cembrano) ing along and across convergent margins. Many

0040-1951/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved.

PII: S 0 0 4 0 - 1 9 5 1 ( 9 9 ) 0 0 3 2 1 - 2


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130 J. Cembrano et al. /Tectonophysics 319 (2000) 129149

examples of ancient and active intra-arc faults 2. Tectonic setting of the LiquineOfqui fault zone

have been reported (see Fitch, 1972; Dewey, 1980;

Beck, 1983, 1991; Jarrard, 1986; Busby-Spera and The Cenozoic geodynamic setting of the south-

ern Chilean Andes is well constrained, showingSaleeby, 1990; McCaffrey, 1992). However, well-

documented cases are surprisingly rare (e.g. relatively steady right-oblique subduction of the

Farallon (Nazca) plate beneath South AmericaScheuber and Reutter, 1992; Tikoff and Teyssier,

1992; Brown et al., 1993; Tikoffand Greene, 1997). since 48 Ma with the exception of nearly ortho-

gonal convergence from 26 to 20 Ma (Pardo-CasasOne reason for this scarcity may be that most

available evidence for intra-arc faulting comes and Molnar, 1987). At present, the angle of obliq-

uity of the NazcaSouth America plate con-from earthquake fault plane solutions (e.g.

McCaffrey, 1992), while field studies of long-lived vergence vector with respect to the orthogonal to

the trench is ~26 for southern Chile (Jarrard,intra-arc shear zones have been complicated by

plutonic and volcanic activity which partially oblit- 1986). The slab dip is approximately 16 (Jarrard,

1986), and the age of the subducting Nazca plateerates evidence of faulting. One way to address

the complexity of the deformation is to study decreases from ~25 Ma at 38S to virtually 0 Ma

at 46S, where the Chile Ridge is currently subduct-along-strike variations in the nature and timing of

the deformation using systematic field and micro- ing (Herron et al., 1981). The limited seismic data

available suggest that the Chilean forearc betweenstructural studies and geochronological data.

In this paper, we present structural and geochro- 39 and 46S is currently undergoing trench-ortho-

gonal shortening; the volcanic arc is absorbing anological data from three transects across theLiquineOfqui fault zone of the southern Chilean small trench-parallel component (Chinn and

Isacks, 1983; Cifuentes, 1989; Barrientos andAndes. These data constrain part of the long-term

kinematic history of the fault zone and indicate Acevedo, 1992; Dewey and Lamb, 1992; Murdie,

1994). Thus, oblique subduction has been consid-the variation in timing and style of deformation

in different transects. The LiquineOfqui fault ered to be a driving mechanism for the long-term

right-lateral displacement along the LiquineOfquizone is marked by a set of north-northeast-trending

lineaments, faults and ductile shear zones that fault zone (Herve, 1977; Beck, 1988; Cembrano

et al., 1996). Other authors have emphasized theparallel the magmatic arc from near the Nazca

South AmericaAntarctica triple junction north- indenter effect arising from subduction of the Chile

Ridge at the southern termination of the Liquineward for ca 1000 km (Fig. 1 ) ( Herve, 1977; Herve

and Thiele, 1987; Cembrano et al., 1996). Several Ofqui fault zone (Forsythe and Nelson, 1985;Nelson et al., 1994).workers have claimed that the LiquineOfqui fault

zone has played a major role in deformation and

location of the magmatic activity during the

Cenozoic (Parada et al., 1987; Dewey and Lamb, 3. Regional geology of the LiquineOfqui fault

zone1992; Pankhurst et al., 1992). Recently, speculative

models for the LiquineOfqui fault zone kinemat-

ics have been proposed on the basis of paleomag- The Meso-Cenozoic North Patagonian batho-

lith forms most of the southern Chilean Andesnetic data (Garca et al., 1988; Cembrano et al.,

1992; Beck et al., 1993; Rojas et al., 1994) and and consists of heterogeneously deformed granodi-

oritic to tonalitic plutonic rocks and minor unde-upon regional considerations of the LiquineOfqui

fault zone geometry and geology (Herve, 1994; formed two-mica granite and leucogranite (Parada

et al., 1987; Herve et al., 1993a). The batholith isCembrano et al., 1996). However, until now, few

structural and geochronological data were avail- made up of a Late JurassicEarly Cretaceous belt

to the west, a central relatively narrow (30 kmable to constrain the nature and timing of deforma-

tion along the fault zone. Thus, its significance in wide) Mio-Pliocene belt, and an eastern mid-

Cretaceous belt (Pankhurst and Herve, 1994).partitioning oblique subduction of the Nazca plate

has been poorly understood. Available KAr, 40Ar39Ar, and RbSr radiomet-


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131J. Cembrano et al. /Tectonophysics 319 (2000) 129149

Fig. 1. Regional scale geometry of the LiquineOfqui fault zone and geotectonic setting of the southern Chilean Andes. Relative

NazcaSouth America plate motion vector was highly dextral-oblique to the trench between 49 and 26 Ma, nearly orthogonal between

26 and 20 Ma, and slightly dextral-oblique after 20 Ma (modified from Pardo-Casas and Molnar, 1987; Cifuentes, 1989; Cembrano

et al., 1996). Transect locations are shown by small boxes across the Liquin eOfqui fault zone.

ric ages of plutonic rocks in the study area are up an upper Paleozoic accretionary prism (Herve,

1988; Munizaga et al., 1988; Pankhurst et al.,shown in Table 1 and in Figs. 24 (Cembrano,

1990; Carrasco, 1995; Schermer et al., 1995, 1996; 1992). Recent studies show that at least some of

the low-grade wallrocks, particularly someCembrano et al., 1996). The plutonic rocks intrude

low-to medium-grade metamorphic rocks making deformed metabasites and dike swarms, may be


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Fig. 2. Regional geology and local structure of the LiquineOfqui fault zone for the Liquine region, simplified from Moreno and

Parada (1976 ) and Herve (1977 ). Locations of available radiometric age determinations (Herve, 1977; Munizaga et al., 1988) are

shown with symbols. Triangle: KAr dates on biotite, inverted triangle: KAr whole rock dates.

much younger than previously thought and spa- tion. Superposed on the ductile fabric, an anasto-

mosing network of fractures and faults occurstially associated with transtension during mid-

Tertiary times (Herve et al., 1993b, 1995). along conspicuous north-trending lineaments.

Quaternary volcanoes are aligned along the faultThe regional-scale geometry of the Liquine

Ofqui fault zone is characterized by two major zone, as well as in northeast- and northwest-

trending groups oblique to the fault zonenorth-northeast-trending segments joined by a

series of en echelon northeast -trending lineaments (Cembrano and Moreno, 1994; Lopez-Escobar

et al., 1995).at a right step (Herve and Thiele, 1987; Herve,

1994). The resulting arrangement has been inter-

preted as a strike-slip duplex (Cembrano et al.,

1996) similar to those described in Woodcock and 4. Structural transects

Fisher ( 1986) ( Fig. 1). Along the LOFZ, both the

North Patagonian batholith and metamorphic Three transects were mapped across the LOFZ

at Liquine, Reloncav and Hornopiren (Fig. 1).wallrocks have centimeter- to meter-wide high-

strain zones showing penetrative foliations and a Geologic maps of these areas are shown in Figs. 2

4, respectively. Earlier workers (Herve, 1977, 1979)poorly to moderately well-defined stretching linea-


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Table 1

Previously published geochronological data from the three geological transects

Location Rock type Material Method Age (Ma) Observations Source

Liquine Dacytic dyke Whole rock KAr 291 Cross-cuts mylonites Herve et al. ( 1979 )

Liquine Dacytic dyke Biotite Ar Ar, steps 48 Cross-cuts low strain mylonites Schermer et al. (1996)

Liquine Granodiorite Biotite Ar Ar, total fusion 15 Non-deformed, faulted Munizaga et al. ( 1988 )

Reloncav Tonalite Biotite Ar Ar, total fusion 124 Non-deformed, faulted Carrasco ( 1995 )

Reloncav Diorite Biotite Ar Ar, total fusion 119 Non-deformed, faulted Carrasco ( 1995 )

Reloncav Diorite Biotite Ar Ar, total fusion 113 Non-deformed, faulted Carrasco ( 1995 )

Reloncav Tonalite Biotite Ar Ar, total fusion 1210.3 Non-deformed, faulted Carrasco (1995)




Reloncav Granodiorite Biotite KAr 10.7 Non-deformed, faulted Carrasco ( 1995 )

Reloncav Granodiorite Biotite KAr 11.5 Non-deformed, faulted Carrasco ( 1995 )

Hornopiren Granodiorite Biotite ArAr, total fusion 6.60.3 Low strain Cembrano (1990)

Hornopiren Granodiorite Biotite ArAr, total fusion 3.60.3 Low strain Cembrano (1990)

Hornopiren Granodiorite Hornblende ArAr, total fusion 8.70.7 Low strain Cembrano (1990)

Hornopiren Tonalite Biotite Ar Ar, total fusion 6.41.7 Low strain, SC fabrics Cembrano (1990)

Hornopiren Tonalite Hornblende ArAr, total fusion 13.10.1 Low strain, SC fabrics Cembrano (1990)

Hornopiren Tonalite Whole rock RbSr 4.70.6 Low strain, SC fabrics Pankhurst et al. (1992)

Hornopiren Tonalite Zircon UPb 9.90.2 Low strain, SC fabrics Schermer et al. (1996)Hornopiren Diorite Biotite Ar Ar, total fusion 8.81.7 Non-deformed Cembrano ( 1990 )

Hornopiren Diorite Hornblende ArAr, total fusion 8.60.6 Non-deformed Cembrano ( 1990 )

Hornopiren Granodiorite Biotite ArAr, steps 3.590.01 High strain mylonite Schermer et al. (1995)

Hornopiren Tonalite Biotite Ar Ar, steps 3.780.01 High strain mylonite Schermer et al. (1995)

Hornopiren Mica schist Biotite ArAr, total fusion 7.20.7 High strain Cembrano (1990)

Hornopiren Mica schist Muscovite ArAr, total fusion 10.60.1 High strain Cembrano (1990)

Hornopiren Tonalite Biotite Ar Ar, total fusion 3.40.5 Non-deformed Cembrano ( 1990 )

Hornopiren Diorite Hornblende ArAr, total fusion 114.33 Non-deformed Cembrano (1990)

identified and characterized, at the regional scale, Simpson, 1985) and feldspar microstructure

( White, 1975; Simpson, 1985, Tullis and Yund,the main lineaments comprising the fault zone, butlittle detailed structural or geochronological work 1987; Fitz-Gerald and Stunitz, 1993) were docu-

mented to provide a rough estimate of PT condi-was done to document the kinematics, timing and

style of deformation. Dense forest cover prevents tions of deformation. These diagnostic

microstructures, indicative of different physicaldetailed mapping of large areas, but the good

coastal and roadside exposures analyzed during conditions of deformation, were combined, when

possible, with co-existing metamorphic mineralthis study provide the first detailed kinematic

analysis of the LOFZ. We analyzed ductile struc- assemblages to better constrain deformation tem-

peratures. Because other important factors, suchtures in mylonitic rocks on the mesoscopic and

microscopic scales to determine the kinematics and as strain rate and water content, can also affect

rock behavior (e.g. Passchier and Trouw, 1996),approximate conditions of deformation. Kinematic

indicators include SC fabrics (Berthe et al., 1979; our estimates of temperatures by combining micro-

structure and metamorphic mineral assemblagesWhite et al., 1980; Platt, 1984; Shimamoto, 1989),

asymmetric porphyroclast systems (Passchier and have been obtained by assuming that the studied

mylonitic rocks deformed under roughly similarSimpson, 1986), mica fish (Lister and Snoke,

1984), and domino structures (Simpson and strain rate conditions.

Brittle structures were analyzed using the cri-Schmid, 1983). Quartz ribbon microstructure

(types 1, 2 and 3 of Boullier and Bouchez, 1978, teria of Petit (1987). Stress inversion analysis using


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the methods of Carey and Brunier (1974) and 4.1. Liquine transect

Carey (1979) was conducted for the areas where

numerous faults contained unambiguous kine- Previous reconnaissance mapping (Herve et al.,

1974; Moreno and Parada, 1976; Herve, 1977)matic indicators. In this technique, homogeneous

populations of faults (faults which are kinemati- shows that the Liquine region is characterized by

two main, north-trending geologic units: thecally consistent, with striations compatible with a

single stress tensor) were identified by iteration Liquine granitoid and the Liquine gneisses

(Fig. 2). The Liquine granitoid was originallytests based on the following steps: ( 1) faults having

similar strikes, dips, and pitches of striae were assigned to the Jurassic (Herve, 1977), although

recent KAr dating (biotite) suggests a Miocenegrouped together; then a comparison of these

different groups was done qualitatively with the emplacement age (Munizaga et al., 1988). The

Liquine gneisses have been dated as upperpurpose of evaluating their kinematic compatibility

(e.g. dextral and sinistral faults with similar strikes Paleozoic on the basis of a poorly constrained

RbSr errorchron (Herve, 1977). The Liquineare not compatible); (2) consistent faults sets were

regrouped, and a mean stress tensor was calcu- granitoid and the Liquine gneisses are juxtaposed

by the Liquine-Reloncav fault (Moreno andlated. This stress tensor was then applied to the

entire population of faults collected at a given site; Parada, 1976; Herve, 1977). According to Herve

(1977), a 1 km wide zone of cataclasites developed(3 ) according to the distribution of the angular

deviation (ts) taken less than or equal to 30 in the granitoid is exposed west of the fault,

whereas a 1 km wide mylonitic zone occurs to thebetween the orientation of the theoretical striae (t)derived from the computed stress tensor and the east in Paleozoic gneisses. An undeformed dacitic

porphyry, dated as 29 Ma ( KAr, whole rock),measured striae (s), a homogeneous population of

faults were identified (for a complete description cuts the mylonites. We obtained a new 40Ar39Ar

step-heating date on hornblende grains separatedof the method and underlying rationale, see Bott,

1959; Sebrier et al., 1985; Lavenu et al., 1995). from the cross-cutting dyke. This gives a well-

constrained mean age of 1002 Ma (Fig. 5). WeStress tensors are classified as compressional,

extensional or strike-slip according to the spatial interpret this age as the minimum age of deforma-

tion for the mylonites of the Liquine transect.orientation of the principal stress axes (e.g. Ritz

and Taboada, 1993). In simple terms, a stress Another dyke, which cross-cuts a low-strain variety

of the mylonites, has recently been dated at 48 Matensor is compressional when s1

and s2

are nearly

horizontal and s3 is close to vertical; it is exten- (40

Ar39

Ar, biotite) (Schermer et al., 1996).A mesoscopic-scale conjugate system of north-sional when s2

and s3

are nearly horizontal and

s1is vertical. A strike-slip stress tensor shows subh- trending right-separation and east-northeast-

trending left-separation mesoscopic faults wasorizontal s1

and s3

axes, whereas s2

is close to

vertical. In particular, strike-slip stress tensors can identified within and to the east of the mylonitic

zone. This observation, together with a predomi-be further constrained (dextral vs. sinistral) by

examining the sense of obliquity ofs1

with respect nantly steeply dipping mylonitic foliation, was

used to interpret the shear zone as a pre-Oligoceneto the overall trend of the crustal-scale deformation

zone being investigated. For instance, if s1

trends dextral fault zone (Herve, 1977). No stretching

lineations or fault striae were reported in thesenortheast, a dextral strike-slip deformation regime

is expected for a northsouth-trending regional- early studies. Our own mapping along the Liquine

transect partly agrees with that of Herve (1977)scale shear zone.

Fig. 3. Regional geologic map of the LiquineOfqui fault zone for the Reloncav area, simplified from Thiele et al. (1986), Parada

et al. (1987) and Carrasco (1995). Locations of available radiometric age determinations (Drake et al., 1990, 1991; Carrasco, 1995)

are shown with symbols. Solid square: total fusion ArAr dates on biotite; half-filled square: total fusion ArAr dates on hornblende;

triangle: KAr dates on biotite, inverted triangle: KAr whole rock dates.


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slightly elongated quartz-feldspar and mica aggre-

gates, plunge shallowly to moderately to the north

(Figs. 2 and 6a).

Three types of deformed rocks can be distin-

guished across the Liquine transect: widely distrib-

uted quartz-feldspar-mica mylonites, which include

quartz-rich and quartz-poor varieties, and chlorite-

bearing SC mylonites, which are only locally

observed.

The quartz-feldspar rocks show distinctive

microstructures depending upon quartz abun-

dance. Quartz-rich mylonites display a penetrative

schistosity resulting from the parallel alignment of

type 2 quartz ribbons and muscovite-biotite foliae,Fig. 5. 40Ar39Ar apparent age spectra for the Liquine transectwhich wrap around isolated feldspar porphyro-cross-cutting dyke (hornblende bulk mineral separate).

clasts showing both symmetrical and asymmetrical

tails. Among the latter, s and d porphyroclastand indicates that there is a sharp contact betweensystems are present and exhibit a sinistral shear-a brittle, deformed granitoid and high-strainsense geometry (Fig. 7a). This sense of displace-mylonitic rocks formed from the gneisses (Fig. 2).ment is consistent with discrete C surfaces definedHowever, in contrast with previous mapping, weby fine-grained mica aggregates that are obliquefound that deformation in the granitoid is weakto the schistosity.and only locally developed within a few tens of

Quartz-poor mylonites show a less well-devel-meters of the contact with the mylonites. Sinceoped schistosity as quartz-ribbons are scarce, andthere is no thermal aureole developed in the mylo-globular aggregates of nearly equant quartz grainsnites close to the granitoid, the contact betweenare more common. Porphyroclasts of feldspar arethe granitoid and the mylonitized gneisses verymore widespread than in the quartz-rich myloniteslikely corresponds to a brittle fault with a signifi-and are locally surrounded by conjugate sets ofcant east-side-up component of motion, as recog-microshear bands along which fine-grained biotitenized by Herve (1977).occurs (Fig. 7b). The overall fabric of these rocksA detailed structural study carried out within

is markedly symmetric in character (sensuthe mylonitic zone of the Liquine area, east of theChoukroune et al., 1987). The chlorite-bearingmain lineament of the LiquineOfqui faultmylonites show isolated altered plagioclase porph-(Fig. 2), shows that a meter-wide dark greenyroclasts in a very fine-grained matrix of chloritemylonitic strip grades eastward into distinctivecalcitesericite and residual mafic minerals.meter-wide domains of dark gray porphyroclast-Well-developed SC fabrics indicate sinistralfree mylonite. These dark-gray domains alternateshear sense.with domains of porphyroclastic quartz-feldspar-

The most prominent brittle fabrics in themica-bearing, strongly foliated and poorly lineatedLiquine area occur in the Liquine granitoid andmylonite. The penetrative schistosity strikes

correspond to a meter-spaced set of fractures andapproximately north and dips steeply to the west

( Figs. 2 and 6a); stretching lineations, defined by faults striking north-northeast. Some of the faults

Fig. 4. Regional geology and local structure of the LiquineOfqui fault zone for the Hornopiren region, simplified from Herve et al.

(1979) and Cembrano (1990). Locations of available radiometric age determinations (Herve et al., 1979; Drake et al., 1990, 1991;

Schermer et al., 1996) are indicated with symbols. Solid square: total fusion ArAr dates on biotite; half-filled square: total fusion

ArAr dates on hornblende; diamond: RbSr whole rock isochron; triangle: KAr dates on biotite, inverted triangle: KAr whole

rock dates; filled circle: UPb dates on zircon.


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Fig. 6. Lower hemisphere, equal area projections showing the orientation of ductile fabrics from deformed rocks within the Liquin e

Ofqui fault zone: poles to schistosity for Liquine (a) and Hornopiren (c) (crosses); stretching and mineral lineation plots for Liquine (b)

and Hornopiren (d ) (dots).

are high-angle reverse faults; others show evidence have yielded 40Ar39Ar ages that concentrated

within Early Cretaceous and Miocene timesof left or right-lateral motion, as indicated by

kinematic indicators such as small-scale Riedel (Drake et al., 1991; Carrasco, 1995) (Table 1,

Fig. 3). Carrasco (1995) interpreted the slightlyshears and congruous steps (e.g. Petit, 1987).However, the limited number and erratic nature ductilely deformed Miocene rocks as resulting

from high-temperature deformation during plu-of the mesoscopic faults in the Liquine region

prevent a detailed kinematic analysis. Within a tonic emplacement, with only minor, late emplace-

ment sub-solidus deformation. Our observationsfew tens of meters west of the LiquineOfqui fault

zone main lineament (Fig. 2), the Liquine granit- that most plutonic units show little evidence of

high strain ductile deformation, with the exceptionoid displays spaced centimeter-scale fractures that

coalesce locally to define a roughly defined north- of isolated meter-wide septa of annealed mylonitic

rock within largely undeformed plutonic rocks,striking foliation.

agree with those of previous authors (Thiele et al.,

1986; Carrasco, 1995).4.2. Reloncav transectOn both sides of Estuario Reloncav, centimeter-

to decameter-scale striated fault planes cut all thePrevious studies (Thiele et al., 1986; Parada

et al., 1987; Carrasco, 1995) have shown that plutonic units ranging in age from Cretaceous to

Miocene; Quaternary sedimentary and volcanicnortheast-trending granodioritic and tonalitic plu-

tonic units are only slightly ductilely deformed rocks do not show any evidence of faulting. Most

faults are steeply dipping (>60), with striae rang-along the continuation of the major lineaments

trending south from Liquine. The plutonic rocks ing in pitch from subhorizontal to subvertical.


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Fig. 7. Photomicrographs of shear sense indicators from XZ sections of deformed rocks occurring within the Liquin eOfqui fault

zone. The scale bar is 1 mm. (a) s-type porphyroclast system of feldspar and coexisting shear bands oblique to the schistosity from

mylonites of the Liquine transect. The sense of shear is sinistral. (b) Conjugate shear bands and symmetric porphyroclasts from low

strain mylonites of the Liquine transect. The microstructure suggests shear zone-orthogonal contraction, as indicated by arrows. (c)

Microfaulted plagioclase from deformed plutonic rocks of the Hornopiren transect. The domino structure and shear bands indicatedextral shear. (d ) Mica-fish from wallrock pelitic schists of the Hornopiren section, indicating dextral shear sense.

Using the procedure described in Section 4, two nent, although some strike-slip faults are also

included. Faults striking 010080 (mainly 6080)distinct homogeneous populations of faults were

recognized in the Reloncav region. The first is are dextral-reverse slip. Faults striking 115144

(mostly 120130) show sinistral reverse slip.represented in Fig. 8a, which includes faults with

a predominant strike-slip component, and a few The kinematic analysis of the mesoscopic faults

using E.C.G.-Geoldynsoft software (Carey andwith predominant dip-slip component. Faults

whose strike range between 337 and 040 (average Brunier, 1974; Carey, 1979) is consistent with a

NNE-trending dextral strike-slip deformation zone010) show right lateral slip-sense; faults strikingbetween 060106 (average around 080) are left- for the first population of faults: s

1=219, 05;

s2=110, 75; s

3=310, 14 (Fig. 8a). In contrast,lateral; and faults striking west-northwest are

reverse-slip. A second diagram (Fig. 8b) shows the a compressional ~EW-trending stress regime is

suggested by the second fault population:second homogeneous population of faults iden-

tified; most have a significant down-dip compo- s1=275, 05; s

2=184, 12; s

3=25, 77 (Fig. 8b).


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4.3. Hornopiren transect third interpretation is that the ages are reset ages

resulting from shear heating along the LOFZ

(Herve et al., 1979). More recent laser total fusionIn the Hornopiren Region, the LiquineOfqui

fault zone is represented by a 45 km wide zone 40Ar39Ar dates of biotite and hornblende

(Cembrano, 1990; Drake et al., 1991) have beenof heterogeneously deformed granitoids and by

low- to medium-grade metamorphic wallrock interpreted to reflect Late Miocene to Pliocene (6

3 Ma) solid-state recrystallization of these minerals(Fig. 4). The plutonic rocks, mainly hornblende-

biotite tonalite and granodiorite, have been during ductile dextral shear deformation

(Cembrano, 1992). Two recently obtainedgrouped into informal units on the basis of petro-

graphy and poorly mapped field relations (Fig. 4). 40Ar39Ar step-heating dates on recrystallized bio-

tite from mylonitic samples of the Ro MariquitaThe largely inferred contacts between the plutonic

units and wallrocks trend roughly northwest and Cholgo units are in the range of 3.63.8 Ma.

These were interpreted by Schermer et al. (1995)(Cembrano, 1990). Plutonic rocks of the

Hornopiren region appear to be mainly Cretaceous as the age of a greenschist facies ductile deforma-

tion event.and Miocene in age. Granodiorite dated by UPb

zircon at 135 Ma (Schermer et al., 1996) occurs The metamorphic wallrocks in the Hornopiren

region consist of metapelite and metabasite belong-along the northeast flank of the canal, and diorite

with an 40Ar39Ar total fusion age of 114 Ma ing to a Late Paleozoic accretionary prism of

regional distribution (Herve, 1988; Pankhurstoccurs on Isla Llancahue. The diorite is unde-

formed, in contrast to other plutonic rocks of the et al., 1992). These rocks record a progressivewest-to-east change in metamorphic grade fromregion, possibly due to its low quartz content. A

Miocene (9.9 Ma) UPb age was obtained on the greenschist to amphibolite facies as the contact

with intrusive rocks is approached (Cembrano,Cholgo pluton (Schermer et al., 1996) (Fig. 4),

but the emplacement ages of other plutons are 1992). Pelitic rocks near the contact have yielded

a laser total fusion 40Ar39Ar age of 7.20.7 Mauncertain. A RbSr isochron of 4.70.6 Ma has

been considered the age of emplacement of a partly on biotite (Drake et al., 1991) .

The plutonic bodies display a north-northwestgneissic tonalite located within, and to the east of,

the LOFZ (Pankhurst et al., 1992) (Fig. 4). striking, poorly to well-developed subvertical schis-

tosity (Figs. 4 and 6c) defined by the preferredHowever, the fact that the isochron is largely

constrained by the aplitic end-members suggests alignment of plagioclase crystals, hornblende-bio-

tite aggregates, and by flattened interstitial quartzthat it combines unrelated plutonic units. Furthermapping and dating by the authors support this grains. Whereas, in the Cholgo Unit, a mesoscopic

set of centimeter-spaced north-northeast-strikinghypothesis. Abundant Miocene 40Ar39Ar and

whole rock KAr dates have been obtained from shear bands occurs at an angle of 1525 to the

schistosity (Fig. 7c), in the Rio Mariquita Unit,the Hornopiren region ( Fig. 4). The ages on plu-

tonic rocks have been interpreted as emplacement there are spaced meter-wide mylonite zones.

Lineations are typically absent in the granitoids,ages related to the migration of intrusive foci

toward the LOFZ from Jurassic to Neogene or, although isolated meter-wide ultra-mylonitic zones

show subhorizontal stretching lineations ( Fig. 6d ).alternatively, as the result of differential uplift and

static cooling of granitoids in the vicinity of the More common are subhorizontal striae of biotite

flakes lying in the shear bands, similar to thoseLOFZ during the Neogene ( Herve et al., 1979). A

Fig. 8. Lower hemisphere, equal angle projections, showing orientation and kinematic data for mesoscopic faults at Reloncav (a, b)

and Hornopiren (c) computed to obtain the local stress tensor. Arrows attached to the fault traces correspond to the measured slip

vectors. Thick segments on the fault traces show deviations between the measured (s) and predicted (s) slip vector on each plane.

Corresponding histograms show the number of faults (n) as a function of the difference in pitch between s and s. Convergent large

arrows give azimuths of the computed maximum principal stress direction (s1). From the method of Carey and Brunier (1974).


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described by Lin and Williams (1992) for granit- dextral displacement (Fig. 7d). The geometry of

asymmetrical tails on feldspar porphyroclasts alsooids deformed in the brittleductile transition

zone. indicates dextral displacement.

In the Hornopiren section, both ductile andTypical microstructures exhibited in the plu-

tonic rocks include deformation bands and core brittle deformation are well expressed. Dextral

ductile deformation is recorded within plutonicand mantle structure in quartz, which locally

occurs as type 1 and type 2 ribbons (Boullier and units and metamorphic wallrocks. Core and mantle

microstructure in quartz aggregates, type 1 and 2Bouchez, 1978; Simpson, 1985). Plagioclase crys-

tals, which in places show a strong alignment, are quartz ribbons (Boullier and Bouchez, 1978), and

fracturing and bending of plagioclase all indicategenerally microfaulted, bent, and less commonly

recrystallized to a fine-grained aggregate along low-to mid-greenschist facies conditions for the

latest solid-state deformation of the plutonic rocks.edges and internal fractures. The preferred align-

ment of plagioclase could be of an early magmatic Metamorphic wallrocks exhibit synkinematic

amphibolite facies mineral assemblages (quartz-origin, overprinted later by sub-parallel solid-state

fabrics (e.g. Paterson et al., 1989). Hornblende cordierite-biotite-sillimanite) and microstructures

(type 3 quartz ribbons of Boullier and Bouchez,occurs as porphyroclasts surrounded by biotite,

which in turn is extensively recrystallized to a 1978; Simpson, 1985) but do not show any evi-

dence of retrogression, suggesting that deforma-finer-grained aggregate. A conspicuous pattern of

north-northeast-striking dextral and east-north- tion of the wallrocks ended earlier than in the

plutonic rocks.east-striking sinistral microfaults is observed inoriented thin sections. Localized high strain zones The RbSr whole rock isochron age of

4.70.6 Ma has been interpreted as an emplace-exhibit a mylonitic fabric characterized by s-type

porphyroclasts of feldspar floating in a matrix of ment age for adjacent coarse-grained tonalite and

granodiorite (Pankhurst et al., 1992). However,fine-grained recrystallized quartz-muscovite-epi-

dote. Kinematic indicators, such as asymmetrical the locally intense solid-state deformation of these

rocks, the difference in deformation between thetails and domino structures, indicate dextral dis-

placement ( Fig. 7c ). tonalite (foliated ) and granodiorite hosting aplite

(relatively undeformed) , and single-crystal totalMetamorphic wallrocks exhibit a more penetra-

tive foliation than the plutonic rocks. Foliation in fusion ages on hornblende and biotite from the

tonalite of 136 Ma (Fig. 4) suggest that therethe metapelites strikes north-northwest and dips

moderately to steeply to both west and east (Figs. 4 may be two distinct plutons, an older tonalite andyounger granodiorite.and 6c). Quartz-feldspar-rich layers and micaceous

foliae define the schistosity. Subhorizontal stretch- Detailed studies carried out along the main

lineament flanking Hornopiren Canal reveal theing and mineral lineations are present on the

schistosity. Spaced vertical shear bands that strike existence of sharp faults that cut the ductile fabric

of the plutonic rocks. Most faults are vertical tonorth-northeast and cut the schistosity occur

locally. The metabasites show an anastomosing subvertical, have prominent subhorizontal striae

and vary in length from decimeters to decameters.subvertical foliation, which locally wraps around

mesoscopic-scale pods of virtually undeformed The majority of these faults group into a single

homogeneous population, including two families:rock. Linear fabrics have not been recognized in

the metabasic rocks. the most abundant strikes 000030; the other

shows variable strikes between 060 and 080. BothPelitic schists exhibit a penetrative foliation

defined by discontinuous alternating layers of sets of faults display well-developed kinematic

indicators on the fault surfaces, which indicatetype-3 quartz ribbons (Boullier and Bouchez,

1978) and biotite+muscovite foliae. Well-pre- dextral slip for the north-northeast-striking set and

sinistral slip for the east-northeast-striking setserved mica-fish (Lister and Snoke, 1984) define

C-surfaces along interconnecting trails that are (Fig. 8c). A strike-slip deformation regime is

obtained when applying the E.C.G. Geodynsoft tooblique to the quartz ribbon-schistosity, indicating


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these fault sets, giving stress axes ofs1=244, 13; The coexistence of sinistral-reverse displacement

s2=57, 77; s

3=154, 02 (Fig. 8c). Considering and shortening across-strike in the rocks from

that the LOFZ trends roughly northsouth, and Liquine may be the result of a bulk sinistralthat s

1strikes northeast, an overall dextral strike- transpressional deformation that was partitioned

slip tectonics characterizes the fault zone. into orogen-parallel oblique-slip and across-strike

shortening in the anastomosing fashion described

by Bell (1981) and Bell et al. (1986). Alternatively,5. Discussion two discrete episodes of deformation may have

occurred, but no evidence of cross-cutting struc-5.1. Nature and timing of deformation tures was observed.

Diagnostic microstructures such as type-2Heterogeneously deformed rock units exposed quartz ribbons (Boullier and Bouchez, 1978;

in three traverses mapped along the LOFZ docu- Simpson, 1985) and plagioclase crystals withment a complex history of episodic ductile defor- internal fractures and partly recrystallized rims inmation followed by brittle deformation. In the rocks of the Liquine region suggest that defor-comparing the three transects, we emphasized the mation occurred under mid-to upper-greenschistsimilarities and differences in our detailed observa- metamorphic facies conditions ( i.e. temperaturestions, which are separated by kilometers of poorly in the range of 300350C; Simpson, 1985). Theexposed and poorly studied terrain or areas cov- metamorphic mineral assemblage present in these

ered by Quaternary volcanic units. However, rocks (quartz-biotite-muscovite) is compatible withreconnaissance observations between transects the metamorphic conditions suggested from theindicate the structural style is similar for tens of microstructures.kilometers north and south of each transect. Brittle overprinting on the early ductile fabricRegardless of the detailed reasons for along strike

is poorly developed in Liquine and does not showcontinuity (or lack thereof ) in kinematics, style

the consistent geometry and kinematics necessaryand timing of deformation, our new data indicate

to obtain a local stress tensor.that important new distinctions can be made

Some 200 km south of Liquine, in the Reloncavbetween the different parts of the LOFZ. Detailed

region, ductile deformation is almost absent, whileconsideration of timing of deformation must await

brittle deformation is very conspicuous alongfurther geochronological and thermochrono-

north-trending lineaments. Faults cut isotropiclogical study.

plutonic rock ranging in age from mid-CretaceousThe 2 km wide mylonitic belt occurring in the to Miocene. North-northeast-striking dextral andnorthernmost end of the LiquineOfqui fault zone

east-northeast-striking sinistral faults along withrecords an important episode of ductile deforma-west-northwest-striking reverse faults are consis-tion that affected Paleozoic gneisses. Our recentlytent with a horizontal maximum compressionalobtained 40Ar39Ar date of 1002 Ma in amphi-stress direction trending northeast associated withbole phenocrysts from an undeformed dyke, whichan overall dextral strike-slip tectonics. Anothercross-cuts the mylonites in the center of the Liquinefault population of east-northeast-striking dextral-shear zone, suggests that deformation in this regionreverse and west-northwest striking sinistralis pre-late Cretaceous. Kinematic indicatorsreverse faults appears to have been the result ofsuggest sinistral-reverse displacement in mosteastwest compression. The time relationshipdomains. Coexisting symmetric fabrics, such asbetween these two brittle deformational events isglobular quartz porphyroclasts and conjugatenot clear, as cross-cutting relations of distinctiveshear bands suggest that at least part of thestriae are not observed on single fault surfaces.deformation was coaxial within discrete domainsHowever, the absolute timing of the EW compres-(sensu Choukroune et al., 1987). The interpreta-sional event is likely late Miocene or later astion of local coaxial flow from symmetric fabrics,similar faults are observed in both Cretaceous andhowever, may not always be straightforward (e.g.

Passchier and Trouw, 1996 ). Miocene rocks. However, the strike-slip stress


16/21


tensor identified in Reloncav is very similar to of the rock as the closure temperature for biotite

is around 325C, which is slightly higher than thethat obtained in Hornopiren, which may indicate

that both regions record the same post ~3 Ma temperature at which quartz begins to exhibit

ductile behavior. The absence of ductile deforma-deformational event.

The presence of parallel magmatic and subsoli- tion in the Reloncav segment may reflect any of

the following scenarios: (1 ) a different level ofdus fabrics in the Cholgo unit suggests that dextral

displacement started during emplacement or early exposure due to differential uplift along the fault

zone; (2) the lack of widespread pluton emplace-cooling of the plutonic rocks and continued during

cooling and unroofing of the batholith. Single ment and concomitant thermal weakening during

deformation; and (3 ) higher strain rates and/orcrystal total fusion 40Ar39Ar ages of biotite from

plutonic rock ranging from 8.7 to 3.6 Ma (Drake lower fluid supply than that of the Hornopiren

region.et al., 1991; Cembrano et al., 1996; Schermer et al.,

1996) may reflect recrystallization of these minerals

during solid-state deformation and/or cooling 5.2. Geodynamic significance of the LiquineOfqui

fault zonefollowing deformation. The 7.2 Ma 40Ar39Ar bio-

tite age from synkinematic biotite in wallrock

schist represents cooling following wallrock defor- The LiquineOfqui fault zone has been

regarded as an intra-arc strike-slip fault zonemation and appears to be coeval with intrusion

and/or subsequent cooling and deformation of the resulting either from oblique subduction (Herve,

1977; Beck, 1988) or from the indenter effect ofplutonic rocks. The predominance of hornblendeages from 9 to 13 Ma and biotite ages from 3 to the Chile Ridge at the southern termination of the

fault system (Forsythe and Nelson, 1985; Nelson6 Ma suggests that ductile deformation began in

the late Miocene, shortly after emplacement of the et al., 1994). Although both models seem plausible,

previous workers had limited structural data toCholgo Unit at 9.9 Ma, and progressed during

cooling from 500 to 300C during late the relate plate motions to the actual deformation of

the overriding plate. Kinematic data from theMiocenePliocene. Combined structural and

chronological data suggest that the Cholgo Unit LiquineOfqui fault zone obtained in this study

can be used to assess the influence of these parame-emplaced syntectonically and then cooled in a

regional dextral strike-slip displacement regime. ters in the tectonics of an active continental

margin.However, more field and thermochronological data

are needed to test this hypothesis.The brittle deformation in the Hornopiren 5.2.1. Oblique subductionMost authors studying margin-parallel faultingregion overprints the ductile fabric, forming a

population of north-northeast-striking subvertical along oblique subduction zones (Fitch, 1972; Beck,

1983, 1991; Jarrard, 1986; McCaffrey, 1992; Beckdextral faults and less common east-northeast-

striking sinistral faults. As observed in the et al., 1993) agree that there are several factors

controlling the formation of a forearc sliver. TheseReloncav area, the geometry and kinematics are

consistent with a dextral strike-slip deformation include the nature of the overriding plate (conti-

nental versus oceanic) , angle of obliquity, con-regime. In contrast to the Reloncav transect,

however, the eastwest compressional event is not vergence rate, and interplate coupling. The

importance of these factors can be assessed for therepresented, showing that this event may have

been local in character or possibly older and not LOFZ. The best-constrained period of ductile

deformation reported here occurred during therecorded in the younger plutonic rocks of

Hornopiren. The brittle deformation in the Late Miocene to Pliocene, when dextral displace-

ment appears to have dominated the tectonic set-Hornopiren region certainly took place after the

mid-Pliocene since the faults cut rocks with ting of the magmatic arc. Plate reconstructions

show a slightly right oblique convergence at a high40Ar39Ar ages as young as 3.3 Ma. In fact, this

fixes a maximum age for the brittle deformation rate for that time period (Pardo-Casas and


17/21


Molnar, 1987; Engebretson, written communica- fault zone has been obtained from the dextral

strike-slip fault plane solution of two crustal earth-tion, 1995). Although the sense of obliquity isquakes at both extremes of the fault system (Chinnconsistent with the documented sense of shear, theand Isacks, 1983; Barrientos and Acevedo, 1992).angle between the convergence vector and theAlthough sparse and insufficient, these data suggestorthogonal to the trench seems to have been toothat the fault zone is currently active as a dextralsmall (~25) to account by itself for the strike-strike-slip fault absorbing the margin-parallel com-slip tectonics within the magmatic arc. In fact,ponent of the convergence slip vector (Dewey andMcCaffrey (1992) has shown that, for present-dayLamb, 1992). More seismic data are needed tooblique convergent margins, slip on the intra-arcbetter constrain the present-day nature of motionstrike-slip fault does not occur for obliquities lessof the LiquineOfqui fault zone.than 30. For the southern Chilean Andes, we

propose that strong intraplate coupling resulting

from the subduction of young and buoyant oceanic5.2.2. Indenter tectonicslithosphere north of the NazcaSouth America

Forsythe and Nelson (1985) proposed that someAntarctica triple junction plus a thermally weakmotion on the LiquineOfqui fault zone has beencontinental crust appear to be the key factors forthe result of the impingement of the Chile Ridgeintra-arc faulting.on the southern Andes continental margin duringUPb ages on plutons and 40Ar39Ar ages onthe last 14 Ma. According to their model, the Chilemylonites suggest that ductile to near-brittleridge has acted as an indenter causing an outboarddextral displacement occurred during cooling fromsliver, the Chiloe block, to move northward and

9 to 3 Ma and may have been continuous into thebecome detached from the continent along the

post-3 Ma brittle dextral fault regime. Since plateLiquineOfqui fault zone. More recently, Nelson

motions have not changed substantially after theet al. (1994) have assessed the effect of ridge

Pliocene (Pardo-Casas and Molnar, 1987), parti-subduction and oblique subduction at the Peru

tioning of the NazcaSouth America slip vectorChile trench through numerical modeling. Their

into forearc shortening and intra-arc dextral strike-model predicts the trajectory of maximum hori-

slip displacement may have been continuous forzontal stress (s

Hmax) and the distribution of vertical

several million years.strain along and across the continental margin as

In contrast to MiocenePliocene dextral dis-a result of the collision of the Chile ridge at its

placement, pre-Late Cretaceous sinistral-reverse

present-day position. According to Nelson et al.displacement documented in Liquine is too poorly (1994), the current geometry and sense of motionconstrained in time to allow a meaningful correla- of the southern termination of the LiquineOfquition with plate kinematics. In any case, plate fault zone seem to be compatible with the predictedmotion reconstructions are not reliable for the orientation ofs

Hmax. Modeled strain patterns also

South American plate margin before the Early are consistent with limited available short-termCenozoic (Engebretson, pers. commun. 1995). seismic and geodetic data for the southern AndesHowever, Early Cretaceous sinistral displacement forearc (Plafker and Savage, 1970; Barrientos anddeformation has been found along the intra-arc Ward, 1990). However, field evidence on the natureAtacama fault system in northern Chile (Scheuber and timing of long-term and short-term deforma-and Andriessen, 1990; Brown et al., 1993), raising tion along the entire length of the LiquineOfquithe possibility of activity along the LiquineOfqui fault zone is not discussed in detail by Nelsonfault zone extending back into the Mesozoic. If et al. (1994) to test the compatibility of their ridgethat is the case, the fault zone would be a long- subduction model with the long-term evolution oflived structure reactivated at different times in the LOFZ. Therefore, although fairly consistentresponse to an evolving plate kinematic framework with the short-term deformation, Nelson et al.s(e.g. White et al., 1986). The short-term instantan- model does not address the effect of long-term

ridge subduction on deformation along theeous deformation regime at the LiquineOfqui


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LiquineOfqui fault zone, particularly at its north- tion of the Nazca plate and partitioning of defor-

mation in the thermally weakened crust of theern half, which is some 5001000 km away from

the present-day triple junction, and may have been continental margin. The dextral system appears to

characterize the long- and short-term kinematicsunaffected by ridge subduction. None the less, the

oblique subduction component of their model does of the LiquineOfqui fault zone, as evidenced by

both the structural data presented herein andexplain recent dextral motion along the length of

the LOFZ. Consequently, the long-term and steady limited earthquake data.

nature of oblique subduction makes this driving

mechanism more likely to explain the overall tec-

tonics of the LiquineOfqui fault zone rather than Acknowledgementsthe indenter effect of the Chile Ridge, which we

see as a second-order mechanism enhancing the Fondecyt Grants 1931096 and 1950497 to J.C.,PlioceneRecent dextral strike-slip motion along A.L. and E.S. funded most of this research. Partthe southern portion of the LiquineOfqui fault of this paper was written while J.C. was atzone. Dalhousie University under a Killam Doctoral

fellowship. Dr Peter Reynolds and Mr Keith

Taylor were in charge of the 40Ar39Ar age deter-

mination of the dike sample from Liquine. E.S.6. Conclusions acknowledges support from the Bureau of Faculty

Research of Western Washington University. TheStructural and geochronological data presented participation of A.L. has been possible by an

in this paper document a long and complex history agreement between the Geology Department ofof intra-arc deformation in the southern Chilean the University of Chile and ORSTOM, France.Andes. The northern half of the LiquineOfqui Fondecyt Grant 1920914 and 1931096 supportedfault zone exhibits different styles and times of A.S. Judith Oliva prepared some of the figures,deformation in three transects. The northernmost, and Cristina Maureira helped with the manuscript.Liquine region experienced sinistral ductile defor- Discussions and partial revisions of the manuscriptmation prior to 100 Ma and later faulting of by Professors L. Aguirre, M. Beck, R. Burmester,uncertain kinematics. The central, Reloncav seg- N. Culshaw, F. Herve, R. Jamieson, M.A. Paradament shows no clear evidence of ductile deforma- and D. Prior contributed to the clarification of the

tion from Cretaceous to Miocene time, but brittle ideas contained in this paper. Journal co-editor T.fault fabrics are well developed in plutons of both Engelder, and referees E. Nelson and K. Kepleismid-Cretaceous and late Miocene ages. Inversion are thanked for their thorough reviews and helpfulof fault slip data for the stress tensor suggests suggestions.dextral strike-slip and compressional deformation

regimes. The southern, Hornopiren segment exhib-

its evidence for both ductile and brittle deform-Referencesations. Kinematic data from both brittle and

ductile structures suggest dextral displacement;Barrientos, S.E., Ward, S.N., 1990. The 1960 Chile earthquake:published isotopic ages suggest ductile deformation

inversion for slip distribution from surface deformation.during the late MiocenePliocene (133 Ma), with

Geophys. J. Int. 103, 589598.brittle displacement occurring after 3 Ma. The Barrientos, S.E., Acevedo, P., 1992. Seismological aspects of

the 19881989 Lonquimay (Chile) volcanic eruption. J. Vol-older sinistral deformation seen in the northern-canol. Geotherm. Res. 53, 7387.most segment is of uncertain significance but may

Beck, M., 1983. On the mechanism of tectonic transport inbe related to a period of margin parallel sinistralzones of oblique subduction. Tectonophysics 93, 111.

deformation in the Early Cretaceous coeval withBeck, M.E., 1988. Analysis of Late JurassicRecent paleomag-

that of the Atacama fault system. The dextral netic data from active plate margins of South America. J. S.Am. Earth Sci. 1, 3952.structures appear to be related to oblique subduc-


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