Cenozoico Transpresional Nazca

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Late Cenozoic transpressional ductile deformation north of

the NazcaSouth AmericaAntarctica triple junction

Jose Cembrano a,*, Alain Lavenu b,c, Peter Reynolds d, Gloria Arancibia c,Gloria Lopez c, Alejandro Sanhueza c

aDepartamento de Ciencias Geologicas, Universidad Catolica del Norte, Avda Angamos 0610, Antofagasta, Chileb

Institut Francais de Recherche Scientifique pour le Developpement en Cooperation (IRD),Casilla 53390 Correo Central, Santiago 1, Chile

cDepartamento de Geologa, Universidad de Chile, Casilla 13518, Correo 21, Santiago, ChiledDepartment of Earth Sciences, Dalhousie University, Halifax, Nova Scotia, Canada B3H 3J5

Received 23 January 2001; accepted 26 June 2002

Abstract

The southern Andes plate boundary zone records a protracted history of bulk transpressional deformation during the

Cenozoic, which has been causally related to either oblique subduction or ridge collision. However, few structural and

chronological studies of regional deformation are available to support one hypothesis or the other. We address along- and

across-strike variations in the nature and timing of plate boundary deformation to better understand the Cenozoic tectonics ofthe southern Andes.

Two east west structural transects were mapped at Puyuhuapi and Aysen, immediately north of the Nazca South

AmericaAntarctica triple junction. At Puyuhuapi (44jS), northsouth striking, high-angle contractional and strike-slip ductile

shear zones developed from plutons coexist with moderately dipping dextral-oblique shear zones in the wallrocks. In Ayse n

(4546j), top to the southwest, oblique thrusting predominates to the west of the Cenozoic magmatic arc, whereas dextral

strike-slip shear zones develop within it.

New 40Ar 39Ar data from mylonites and undeformed rocks from the two transects suggest that dextral strike-slip, oblique-

slip and contractional deformation occurred at nearly the same time but within different structural domains along and across the

orogen. Similar ages were obtained on both high strain pelitic schists with dextral strike-slip kinematics (4.4F0.3 Ma, laser on

muscovitebiotite aggregates, Aysen transect, 45jS) and on mylonitic plutonic rocks with contractional deformation (3.8F0.2

to 4.2F0.2 Ma, fine-grained, recrystallized biotite, Puyuhuapi transect). Oblique-slip, dextral reverse kinematics of uncertain

age is documented at the Canal Costa shear zone (45j

S) and at the Queulat shear zone at 44j

S. Published dates for theundeformed protholiths suggest both shear zones are likely Late Miocene or Pliocene, coeval with contractional and strike-slip

shear zones farther north. Coeval strike-slip, oblique-slip and contractional deformation on ductile shear zones of the southern

Andes suggest different degrees of along- and across-strike deformation partitioning of bulk transpressional deformation.

The long-term dextral transpressional regime appears to be driven by oblique subduction. The short-term deformation is in

turn controlled by ridge collision from 6 Ma to present day. This is indicated by most deformation ages and by a southward

increase in the contractional component of deformation. Oblique-slip to contractional shear zones at both western and eastern

0040-1951/02/$ - see front matterD 2002 Elsevier Science B.V. All rights reserved.P I I : S 0 0 4 0 - 1 9 5 1 ( 0 2 ) 0 0 3 8 8 - 8

* Corresponding author. Fax: +56-55-355977.

E-mail address: [email protected] (J. Cembrano).

www.elsevier.com/locate/tecto

Tectonophysics 354 (2002) 289 314


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margins of the Miocene belt of the Patagonian batholith define a large-scale pop-up structure by which deeper levels of the crust

have been differentially exhumed since the Pliocene at a rate in excess of 1.7 mm/year.

D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Ductile deformation; Late Cenozoic; Nazca South America Antarctica triple junction; Transpression; Liquine Ofqui fault zone

1. Introduction

Overall transpressional deformation is expected at

continental margins where the convergence vector is

oblique with respect to the plate boundary zone (e.g.

Sanderson and Marchini, 1984; Dewey et al., 1998;

Fossen and Tikoff, 1998). Other first-order factors

controlling the tectonics of convergent margins are the

age of the subducting plate, the nature and thermal

structure of the overriding plate, the existence of

major trench-parallel faults and ridge subduction

(e.g. Fitch, 1972; Jordan et al., 1983; Jarrard, 1986;

Beck, 1991; Nelson et al., 1994).

Kinematic models show that transpressional defor-

mation arising from oblique convergence is accom-

plished by distinctive structural styles along and across

different plate boundaries, mostly depending on the

angle of obliquity, defined as the angle between the

convergence vector and the normal to the trench (e.g.Jarrard, 1986; McCaffrey, 1992). For small angles of

obliquity, transpression is homogeneously distributed

as in the case of the AustralianPacific plate boundary

in New Zealand. For large angles of obliquity, complete

partition of transpression is expected. This is the case of

the Pacific North America plate boundary of the west-

ern US, where the San Andreas Fault accommodates

most of the simple shear component (Teyssier et al.,

1995). The general case, however, will be that of

heterogeneous transpression in which discrete domains

across the plate boundary accommodate wrench-domi-nated or pure-shear dominated transpression (e.g.

Fitch, 1972; Fossen et al., 1994; Tikoff and Greene,

1997). Bulk transtension has also been reported at

convergent plate margins, especially at those that were

actively retreating (e.g. Grocott et al., 1994).

In the case of ductile shear zones at obliquely

convergent plate margins, the strike-slip component

of transpression is not taken up by slip on a single

fault but within zones of distributed shear (e.g. Fossen

et al., 1994; Jones and Tanner, 1995).

However, the nature and degree of deformation

partitioning will not only depend on the angle of

obliquity. For instance, thermally weak intra-arc

shear zones can accommodate a significant part of

the bulk transpressional deformation arising from

oblique convergence affecting the predictions of

kinematic models (e.g. Saint Blanquat et al., 1998).

The southern Chilean Andes provides a natural

laboratory to investigate the nature of long-term

and short-term transpressional deformation across

an obliquely convergent continental margin. A major

problem when trying to establish the nature of the

relationship between the tectonics of the overriding

South American plate and the Cenozoic plate kine-

matic framework has been the almost complete lack

of detailed structural and thermochronological stud-

ies in the northern Patagonian region. With the

exception of regional geology and petrology-oriented

work (e.g. Bobenrieth et al., 1983; Bartholomew andTarney, 1984; Herve et al., 1995), there has been no

systematic study of the tectonic evolution of this

segment of the Andes. The regional geology and

structure of the foreland, however, has been recently

studied (Flint et al., 1994; Suarez and De La Cruz,

2001). One important feature of this segment of the

Andes is the absence of a Meso-Cenozoic regional

fold and thrust belt inland of a 1000-km-long Cen-

ozoic intra-arc fault zone, which has been assumed

to accommodate much of the convergent deforma-

tion.Field studies have been difficult to conduct in the

western slope of the Patagonian Cordillera because

of dense vegetation and generally poor weather

conditions. However, a large network of valleys

and fjords cuts the region, allowing access by boat

and basic coastal outcrop mapping to be carried out.

Furthermore, it is possible to study several eastwest

transects across the orogen by traveling along certain

fjords and channels. In this paper, we present and

discuss field, structural and thermochronological data

J. Cembrano et al. / Tectonophysics 354 (2002) 289314290


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from two transects across the southern Andes, at

latitudes 44j and 46j, the Puyuhuapi and Aysen

transect, respectively (Figs. 1 and 2). Previous work

by Schermer et al. (1995, 1996) and Cembrano et al.

(2000) presented a summary of along-strike contrasts

in the nature and timing of deformation along the

segment immediately to the north, from 39jS to

43j

S.

Fig. 1. Regional-scale tectonic setting of the Southern Andes. Northeast-trending, trench-parallel lineaments correspond to the Liquin eOfqui

fault zone (LOFZ). Current position of the NazcaSouth AmericaAntarctica triple junction is shown. S eries of small arrows represent

convergence vectors between the Nazca and South American plates for the last 48 Ma. (Modified from Pardo-Casas and Molnar, 1987;

Cembrano et al., 1996; Bourgois et al., 1996; Somoza, 1998 ). Boxes show transect locations.

J. Cembrano et al. / Tectonophysics 354 (2002) 289314 291


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Fig. 2. Regional-scale geologic map of the southern Chilean Andes between 43jS and 47jS. (modified from SERNAGEOMIN, 1980;

Bobenrieth et al., 1983; Forsythe and Nelson, 1985; Pankhurst and Herve, 1994; Herve et al., 1995; Pankhurst et al., 1999).



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1.1. Tectonic and geologic setting

An active spreading center, the Chile Ridge, is

currently subducting under western South Americaand a trench-parallel fault system, the LiquineOfqui

fault zone (LOFZ), has developed within the Ceno-

zoic magmatic arc (Fig. 1). The Cenozoic geodynamic

setting of the southern Chilean Andes is well con-

strained, showing relatively steady right-lateral obli-

que subduction of the Farallon (Nazca) plate beneath

South America since 48 Ma with the exception of

nearly orthogonal convergence from 26 to 20 Ma

(Pardo-Casas and Molnar, 1987; Somoza, 1998). At

present, the angle of obliquity of Nazca South Amer-

ica plate convergence vector with respect to the

orthogonal to the trench is f26j for southern Chile

(Jarrard, 1986). The slab dip is approximately 16j

(Jarrard, 1986) and the age of the subducting Nazca

plate decreases from f25 Ma at 38jS to virtually 0

Ma at 46jS, where the Chile Ridge is currently

subducting (Herron et al., 1981; Cande and Leslie,

1986; Bourgois et al., 1996). The limited available

seismic data suggest that the Chilean forearc between

39jS and 46jS is currently undergoing trench-orthog-

onal shortening whereas the volcanic arc is absorbing

a small trench-parallel component (Chinn and Isacks,

1983; Cifuentes, 1989; Barrientos and Acevedo,1992; Dewey and Lamb, 1992; Murdie, 1994).

The North Patagonian Batholith (NPB) occupies a

central belt, 1000-km long and 200-km wide, within

the Patagonian Cordillera. The NPB marks the axis of a

the relatively low-altitude (1 2 km) Main Range

flanked to the west by the southward continuation

of the Central Depression or Longitudinal Valley of

Central Chile (e.g. Herve, 1994; Lavenu and Cem-

brano, 1999). The Central Depression is a north south

trending belt characterized by numerous islands and

fjords composed of basement metamorphic rocks anda younger unit of patchy metasediments and pillow

metabasalts that lie closer to the NPB. The latter unit,

named Traiguen Formation, is generally flat-lying west

of a major northsouth-trending lineament (Canal de

Moraleda) and highly deformed at and to the east of

the channel where they are rare (Figs. 1 and 2).

The basement is mostly composed of metasedi-

mentary rock, greenstone and chert. Godoy et al.

(1984) suggested that the basement was Late Paleozoic

in age based upon field and geochronologic reconnais-

sance. Herve (1988) interpreted the metamorphic base-

ment as an accretionary prism, deposited and deformed

in situ during Late Paleozoic times although much

younger ages (Jurassic) have been obtained recently(Herve, 1998). Basement rocks are locally deformed

along northeast-striking ductile shear zones, which are

meters to hundreds of meters in width.

Based upon regional mapping and basic geochro-

nologic dating, the North Patagonian Batholith has

previously been divided into three roughly defined,

orogen-parallel belts: A western Jurassic to Creta-

ceous granodioritic belt, a central Miocene belt of

granodioritediorite with few granites, and an eastern

mid-Cretaceous belt constituted by monzogranites and

granites (Pankhurst et al., 1992, 1999; Herve et al.,

1993; Pankhurst and Herve, 1994), (Fig. 2).

Bartholomew and Tarney (1984) proposed that the

northsouth trending strip of volcanoclastic and sedi-

mentary rock (Traiguen Formation) located between

the Paleozoic basement and the main outcrop of the

Patagonian Batholith represented a Late Cretaceous

Tertiary intra-arc basin. According to their model,

crustal thinning arising from east west extension

was sufficient for true oceanic floor to develop. Their

model is supported mainly by the geochemical signa-

ture of the volcanic rocks: tholeiitic rather than calc-

alkaline in character. Field evidence documenting thegeometric and kinematic relationship between the

intra-arc basin deposits and the batholith is very

limited. They proposed that, during a Miocene com-

pressional event, the basin was inverted and under-

thrust below the plutonic complex. Easterly dipping

shear zones outcropping along the margins of the basin

were presented as evidence for underthrusting. The

fact that plutonic and metamorphic rocks representing

lower crustal levels crop out to the east of Canal de

Moraleda was used as evidence for west-verging

regional thrust faults (Bartholomew and Tarney, 1984).Herve et al. (1995) proposed an alternative model

for the origin of the pillow basalts and associated

metasediments. Based on geologic mapping of the

Traiguen Formation at Isla Magdalena, they suggested

that a pull-apart basin nucleated along a leaky trans-

form fault, represented now by the LOFZ. However,

as in the former model, little structural and chrono-

logical evidence was provided to relate coeval dextral-

strike-slip motion on the fault to basin formation and

sedimentation. A major difficulty has been to date the



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metabasalts and metasediments, which for the most

part, have yielded poorly constrained Miocene ages

(Herve et al., 1995).

1.2. Structural setting

Cenozoic continental margin deformation along the

southern Andes is mostly restricted to the magmatic arc,

where a major fault-system, the LOFZ runs for more

than a 1000 km in a north south direction (e.g. Herve et

al., 1979; Forsythe and Nelson, 1985; Dewey and

Lamb, 1992; Herve, 1994; Cembrano et al., 1996).

The forearc and foreland regions show very little evi-

dence of regional-scale deformation. The region under

study does not have a foreland fold-and-thrust belt as do

the Andean regions immediately south and north (e.g.

Ramos, 1989). The development of the Patagonian

foreland fold-and-thrust belt south of 47jS is attributed

to the northward migration of the subducting Chile-

Ridge over the last 14 Ma (Ramos and Kay, 1992).

According to Herve (1994), the absence of foreland

deformation north of 47jS hasresulted from the accom-

modation of a significant part of the convergent defor-

mation along the Liquine Ofqui fault zone.

2. Geometry and kinematics of discrete ductileshear zones of the Puyuhuapi and Aysen transects

Detailed structural work was performed in several

high strain zones developed from rocks of different

nature and age along and across the magmatic arc in

two studied transects. Some of these shear zones are

located along the eastern lineament of the LOFZ,

whereas others occur along the western lineament.

Other, less-well exposed shear zones occur between

the two main crustal lineaments (Figs. 1 4). Bartho-

lomew and Tarney (1984) previously identified someof these shear zones at the regional scale, during

reconnaissance work. However, they provided no data

on the geometry of stretching lineations or the kine-

matics of deformation.

For each shear zone, we analyzed ductile structures

in mylonitic rocks on the mesoscopic and microscopic

scales to determine the kinematics and approximate

conditions of deformation. Observed kinematic indi-

cators include C/S fabrics (e.g. Berthe et al., 1979),

asymmetric porphyroclast systems (e.g. Passchier andSimpson, 1986), mica fish (e.g. Lister and Snoke,

1984), and domino structures (e.g. Simpson and

Schmid, 1983). Quartz ribbon microstructure (e.g.

Simpson, 1985) and feldspar microstructure (Simpson,

1985; Tullis and Yund, 1987; Fitz-Gerald and Stunitz,

1993) were documented to provide a rough estimate of

deformation temperatures. Deformation microstruc-

tures were combined, when possible, with co-existing

metamorphic mineral assemblages to better constrain

temperature conditions during deformation.

Figs. 3 and 4 show the spatial distribution and

geometry of solid-state fabrics of the Andean region

between 44j and 46j south along with stereoplots of

foliations and lineations. The regional structural map

and the stereoplots show variable dips of foliations

and shallowly to steeply plunging lineations through-

out. Most foliations dip to the east along the western

margin of the Miocene plutonic belt of the North

Patagonian Batholith whereas they dip to west along

its eastern margin.

2.1. Puyuhuapi quarry shear zone

This shear zone deforms mingled diorite and gran-

odiorite belonging to a 16.7 Ma pluton (RbSr, whole

rock; Pankhurst and Herve, 1994) (Figs. 3 and 6). The

plutonic rocks show a northeast-striking, steeply dip-

ping magmatic foliation and down-dip mineral line-

ation defined by euhedral amphibole crystals. Rocks

are heterogeneously deformed across a 50-m-wide,

well-exposed, quarry wall located along the eastern

lineament of the LOFZ. Centimeter- to meter-wide

shear zones affect the plutonic rocks. The mylonites

are green-colored and extremely fine-grained; only afew porphyroclasts remain visible to the naked eye.

The solid-state fabric is subparallel to the magmatic

fabric and overprints it at the outcrop scale. Steeply

plunging stretching lineations are defined by quartz

ribbons and elongate porphyroclast systems of feldspar

and recrystallized tails. Outcrop-scale kinematic indi-

Fig. 3. Regional geology and structure of the Puyuhuapi transect showing major ductile shear zones along with respective stereoplots of

foliations (crosses) and lineations (dots). Location of samples selected for 40Ar 39Ar analysis is shown with stars. The Queulat shear zone

roughly coincides with the eastern boundary of the Miocene belt of the Patagonian Batholith at these latitudes.



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Fig. 4. Regional geology and structure of the Aysen transect showing major ductile shear zones along with respective stereoplots of foliations

(crosses) and lineations (dots). Location of samples selected for 40Ar 39Ar analysis is shown with stars. The Canal Costa shear zone marks the

western boundary of the Miocene belt of the Patagonian Batholith and the limit between the Central Depression to the west and the Main Range

to the east.



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cators such as C/S fabrics and asymmetric porphyr-

oclasts suggest top-to-the-west dextral-oblique ductile

thrusting. Consistent kinematic indicators, such as

amphibole fish, were found at the microscopic scale

(Fig. 5A). Diagnostic microstructures such as recrystal-

lized aggregates of quartz and highly strained, frac-

tured plagioclase crystals indicate greenschist facies

conditions for the mylonitic deformation. Actinolite

occurs around the edges of hornblende porphyroclasts

(Fig. 5A) and within the foliated matrix where it

coexists with fine-grained biotite. This mineral associ-ation is consistent with greenschist facies conditions of

metamorphism, as suggested by the microstructure.

2.2. Queulat shear zone

This shear zone, hundreds of meters wide, is com-

posed of weakly to moderately deformed metasedi-

ments and metavolcanics that probably belong to the

Traiguen Formation (Bobenrieth et al., 1983) (Figs. 2

and 3). The Queulat shear zone roughly coincides with

the eastern limit of the Miocene plutonic belt of the

Patagonian batholith in the Puyuhuapi transect. Folia-

tions, defined by flattened conglomerate pebbles and

layers of mica and chlorite, strike north-northeast and

dip moderately to steeply to the west. Stretching line-

ations show variable pitches but consistently plunge

to the southwest (Fig. 3). Shear sense is consistently

dextral-reverse (top to the northeast) along north south

trending, moderately dipping, ductile shear zones.

2.3. Canal Jacaf shear zone

This deformation zone, several tens of meters wide,

juxtaposes the metamorphic basement to the west and

the Traiguen Formation to the east (Fig. 3). The shear

zone is characterized by both a penetrative west-

dipping, steep to subvertical mylonitic foliation and

poorly defined down-dip stretching lineation. Kine-

matic indicators at the outcrop-scale suggest top to the

east dextral-oblique thrusting of the metamorphic

basement over the Traiguen Formation.

Fig. 5. Photomicrographs of high strain rocks from the Puyuhuapi Quarry shear zone (A); Rio Cisnes shear zone (B,C) and Canal Costa shear

zone (D). Sections cut perpendicular to the foliation and parallel to the stretching lineation. Scale bar is 1 mm. Mineral abbreviations:

ac=actinolite; hb=hornblende; pl=plagioclase, ms=muscovite, bt=biotite. Fabric abbreviations: C: shear band, S: schistosity. Shear sense is

indicated on each photograph.



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2.4. Rio Cisnes shear zone

This shear zone is oriented fN 60jE and is repre-

sented in aerial photographs and satellite images as af30-km-long lineament following the length of Rio

Cisnes (Figs.2 and 3). In thefield, the shear zone affects

both the 10 Ma Puyuhuapi granite (Rb Sr, whole rock,

Herve et al., 1993) and metamorphic basement wall-

rocks of possible Paleozoic age (Fig. 3). High strain

shear zones in the pluton and wallrock are centimeter to

hundreds of meters in width, strike eastnortheast and

dip moderately to the northwest. The stretching line-

ation, defined by quartz-ribbons and streaks of musco-

vite and biotite, plunges shallowly to moderately to the

southwest. Mesoscopic-scale kinematic indicators,

such as C/S fabrics and asymmetric porphyroclast

systems, document dextral-oblique shear with a dip-

slip, normal component of motion. Shear bands, folia-

tion fish and mica fish (Fig. 5B,C) also indicate dextral

shear with a normal, dip-slip component of motion.

Recrystallized quartz aggregates, and the development

of C/S fabrics suggest that deformation took place

under mid-greenschist facies conditions (e.g. Shima-

moto, 1989; Lister and Snoke, 1984).

2.5. Islas Cinco Hermanas shear zone

This high strain shear zone developed from meta-

morphic rocks of the metamorphic basement at Islas

Cinco Hermanas (Fig. 4). A north northeast striking

steep foliation is defined by flattened and stretched

quartz ribbons and mica foliae. A subhorizontal

stretching lineation is very well developed. C/S fab-

rics, z-shaped folds and sigmoidal porphyroclasts are

seen on horizontal surfaces parallel to the lineation and

perpendicular to the foliation. Shear sense is consis-

tently dextral. Metamorphic conditions of mylonitiza-

tion were probably greenschist facies, as documentedby very fine-grained aggregates of re-crystallized

quartz and mica.

2.6. Canal Costa shear zone

This major shear zone follows the north south

trend of the Canal deforming granitic rocks of the

Patagonian Batholith and volcanic and sedimentary

rocks of the Traiguen Formation (Figs. 2 and 4). At a

regional scale, the Canal Costa represents a morpho-

logic boundary between the so-called Central Depres-

sion to the west and the Main Range to the east,

dominated by outcrops of the Traiguen Formation and

the Patagonian Batholith, respectively. Moreover,west of the channel, volcanic and sedimentary rocks

are almost flat-lying whereas they are highly de-

formed at the Canal Costa shear zone and do not

occur extensively to the east of the channel (Figs. 2

and 4). High strain cataclastic and mylonitic rocks

occur as a discontinuous strip along the eastern flank

of the channel. Most of them show north south

trending foliations dipping steeply to the east. Stretch-

ing lineations in the mylonites plunge moderately to

the north, although other directions of plunge are

observed (Fig. 4). Kinematic indicators document

top to the southwest dextral-reverse ductile shearing

(Fig. 5D); a few kinematic indicators showing dip-slip

normal motions are locally found.

The regional structural maps and the stereoplots

show variable dips of foliations and shallowly to steeply

plunging lineations throughout(Fig. 4). Most foliations

dip to the east along the western margin of the Miocene

plutonic belt of the North Patagonian Batholith whereas

they dip to west along its eastern margin. Kinematic

indicators in east-dipping mylonitic zones of the Canal

Costa shear zone document top-to-the-west thrusting of

the plutonic complex onto the Traiguen Formation. Atthe eastern side (Puyuhuapi and Queulat shear zones),

kinematic indicators of ductile thrusting and dextral

transpressional deformation predominate.

3. Age of deformation

High strain rocks commonly contain a complex mix

of relic grains (porphyroclasts) in a matrix of dynam-

ically recrystallized minerals. Moreover, the relic grains

are usually partly recrystallized around their margins(core and mantle structure) whereas the so-called

recrystallized grains often contain domains in which

dynamic recrystallization has not been fully accom-

plished as revealed by detailed SEM studies (e.g.

Trimby et al., 1998). This means that a bulk separate

of a specific mineral species may contain both relic and

recrystallized isotopic signatures. Conventional step

heating is likely to produce a geologically meaningless

average age, unless age resolution is permitted by the

fact that the two different generations of this mineral



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species have distinctive argon retention properties (e.g.

West and Lux, 1993). Scheuber et al. (1995) dated a

continuous series of rocks from the undeformed proto-

lith to a high-strain mylonite in the middle of a kilo-meter-wide ductile shear zone in northern Chile. In this

study, they found that40Ar 39Ar ages are fully reset by

deformation-induced recrystallization in the high-strain

mylonites while mixed ages in the form of staircase

spectral patterns were obtained in the low strain zones

where both relic and recrystallized minerals coexist.For

those samples in which both pre- and synkinematic

grains of the same mineral species coexist at the sample

scale, the 40Ar 39Ar laserprobe technique is one way to

effectively date deformation (e.g. Reddyet al., 1996). In

this method, the laser targets mineral grains (or parts

thereof), which are interpreted to have been dynami-

cally recrystallized at or below the closure temperature

of the mineral. Thus, proper identification of targeted

mineral species is critical, and the laser must be able to

isolate these from other phases. If the target minerals

have relatively high potassium concentrations, the laser

analyses will be relatively insensitive to the inadvertent

outgassing of contaminants (e.g. low K hornblende).

For the present study, a total of 12 representative

samples, all with good structural and kinematic control,

were selected for dating. For 10 of these, mineral

separates were prepared for conventional step heating;the remaining two were prepared for laserprobe analy-

sis. For irradiation in the McMaster University reactor,

separated mineral concentrates were wrapped in Al foil.

Laserprobe analyses were carried out on polished slabs

(f80 Am thick) that were irradiated at the same time as

the mineral separates. The flux monitor was the horn-

blende standard, MMhb-1 (assumed age=520F2 Ma).

An internal tantalum resistance furnace of the double-

vacuum type was used to carry out the step heating.

Laserprobe analyses were made with a NdYAG

system operated in the pulsed mode (12 kHz). Atargeted area on a rock section was fused by moving

the sample chamber under the laser beam by means of

an xy translation stage. All isotopic analyses were

made using a VG 3600 mass spectrometer. Both unde-

formed and high strain varieties of plutonic rocks were

analyzed. For the latter, apparent ages of biotite and/or

muscovite are likely to represent the age of deformation

because mylonitic deformation was shown to have

taken place at low to medium greenschist facies con-

ditions (e.g. Kligfield et al., 1986; Dunlap et al., 1991).

3.1. Results of the ArAr dating

Apparent age data from step heating are reported

with their 1r uncertainties; mean ages are providedwith 2r errors, the latter including the uncertainty in

the parameter J, but not allowing for error in the

assumed age of the flux monitor. Lithology, location,

material dated, intensity of deformation and ArAr

analytical data for each sample are listed in Table 1.

For the following discussion, a plateau is defined as a

set of contiguous steps that together contain at least

50% of the total 39Ar released, and for which the

apparent ages are indistinguishable.40Ar 39Ar apparent age spectra (and 37Ar/39Ar

spectra for hornblendes) are shown in Fig. 6 for

undeformed (low strain) rocks and in Fig. 7 for high

strain rocks. Age data as discussed below are sum-

marized in Table 1.

Samples 95JC6, 95JC4, 95JC1 and 95JC12 are from

the Queulat plutonic unit at the Puyuhuapi Quarry

(Figs. 3 and 6). Sample 95JC6, an undeformed diorite,

yielded well-defined ages of 14.4F0.6 and 14.4F0.3

for hornblende and biotite, respectively (Fig. 6a,b).

These are interpreted as cooling ages of the pluton

immediately following emplacement perhaps at about

16 Ma (RbSr whole rock age, Pankhurst and Herve,

1994). Biotite from sample 95JC4, a low strain varietyof mylonitic granodioritediorite, yielded a spectrum

that has relatively low ages with high errors in the early

part of the gas release (Fig. 6c). Ages over the latter part

of the release are better defined and have an average

value of 5.3F0.3 Ma. Because this is a low strain rock,

we interpret this age only as an upper limit to the time of

solid-state deformation. Hornblende from 95JC1, a low

strain mingled granodiorite diorite, yielded discordant

age and 37Ar/39Ar spectra, possibly an indication of the

presence of more than one generation of amphibole. We

suggest that the mean age, 20.2F

0.2 Ma, relates to theearly cooling history of this rock. Biotite from 95JC12,

a low strain variety of granodiorite, yielded a spectrum

that is only slightly discordant with a mean age of

13.3F0.2 Ma. We interpret this as a cooling age,

perhaps partially reset by later deformation. Biotite

from 95JC14, a low strain biotite granite from Islas

Cinco Hermanas in the Aysen fjord (Figs. 4 and 6f),

yielded a mildly discordant spectrum with a mean age

of 5.7F0.2 Ma. As for 95JC4 above, we interpret this

as a partially reset age.



12/26

Table 1

Summary of 40Ar 39Ar analytical data for low strain and high strain rocks of the Puyuhuapi and Aysen transects

T (jC) mV39Ar 39Ar% Age (Ma)F1r %ATM 37Ar/39Ar 36Ar/40Ar 39Ar/40Ar %IIC

95-JC-1 Biotite. Low strain mingled granodiorite diorite. Puyuhuapy transect550 41.5 3.0 128.3F3.8 960.0 67.57 0.031041 0.276388 12.75

600 33.8 2.4 56.9F4.7 308.0 32.82 0.010247 0.151868 16.28

650 41.4 2.9 25.9F2.7 167.0 15.91 0.005628 0.108746 18.44

700 79.5 5.7 5.8F1.3 124.0 5.80 0.004197 0.178930 31.51

750 89.9 6.5 2.1F0.1 115.0 3.88 0.003871 0.308042 59.86

800 77.6 5.6 1.3F1 110.0 3.37 0.003713 0.337276 81.72

850 82.9 6.0 0.4F0.7 95.0 2.40 0.003192 0.474628 177.89

900 84.1 6.0 1.2F0.7 84.5 1.91 0.002839 0.494858 47.30

950 136.1 9.8 3.1F0.4 57.1 0.94 0.001926 0.560082 9.43

975 140.8 10.1 3.2F0.3 46.1 0.72 0.001561 0.687272 7.13

1000 160.2 11.5 3.6F0.2 33.2 0.53 0.001131 0.748429 4.57

1025 132.8 9.6 3.5F0.3 31.8 0.55 0.001091 0.791464 4.97

1050 105.2 7.6 3.5F0.3 30.9 0.61 0.001068 0.796960 5.52

1100 156.8 11.3 3.9F0.2 28.8 0.39 0.000978 0.742279 3.131200 17.5 1.2 1.4F3.2 105.0 2.39 0.003493 0.170124 55.31

1450 1.6 0.1 104.4F225.2 103.0 33.92 0.003492 0.001414 8.30

Mean age (950 1100 jC)=3.5F0.2 Ma; J=0.00232F0.0000232 (1%)

95-JC-1 Hornblende. Low strain mingled granodiorite diorite. Puyuhuapi transect

650 10.1 3.3 62.5F10.8 88.5 2.83 0.002999 0.007514 1.58

750 12.3 4.0 15.2F7.6 94.7 2.19 0.003209 0.014355 4.63

850 13.4 4.4 2.9F4.8 97.0 2.88 0.003293 0.041715 30.37

950 42.6 14.1 19.5F2.1 80.2 9.99 0.002721 0.041969 16.61

975 44.0 14.5 21.7F1.5 64.3 12.98 0.002194 0.067942 19.48

1000 49.0 16.2 17.5F1.3 60.8 13.18 0.002087 0.092551 24.39

1025 19.1 6.3 14.0F2.7 66.8 11.38 0.002315 0.097334 26.10

1050 8.2 2.7 14.2F5.3 75.3 10.24 0.002603 0.071654 23.16

1075 6.3 2.0 10.0F9.0 91.2 13.15 0.003115 0.036222 41.87

1100 7.3 2.4 15.0F9.2 90.9 15.88 0.003095 0.024947 34.08

1125 12.4 4.1 18.4F6.2 88.8 15.90 0.003017 0.025098 28.00

1150 12.0 3.9 27.0F5.8 84.1 15.80 0.002856 0.024399 19.24

1200 33.9 11.2 31.5F2.7 73.7 15.71 0.002503 0.034417 16.50

1250 11.8 3.9 21.9F6.4 88.0 15.88 0.002989 0.022554 23.58

1350 13.7 4.5 12.7F8.3 96.7 15.88 0.003275 0.010757 40.15

1450 5.0 1.6 53.4F35.2 103 14.03 0.003498 0.002658 7.46

Mean age (950 1350 jC)=20.2F2 Ma; J=0.00232F0.0000232 (1%)

95-JC-2 Biotite. Low strain mingled granodiorite diorite. Puyuhuapi transect

550 19.6 1.5 132.6F8.2 1634.0 71.00 0.043443 0.387645 12.82

600 10.0 0.7 116.6F11.8 1634.0 60.12 0.036556 0.364203 12.81

650 12.9 1.0

56.2F

8.2 289.0 31.42 0.009248 0.132213 15.80700 32.4 2.5 11.0F3.2 137.0 7.72 0.004587 0.140649 21.76

750 106.8 8.3 0.3F0.8 103.0 1.66 0.003469 0.593526 192.52

800 273.9 21.3 1.7F0.2 39.8 0.43 0.001367 1.394780 7.76

850 268.2 20.9 1.7F0.1 24.1 0.32 0.000865 1.719379 5.82

900 157.0 12.2 1.3F0.2 39.7 0.47 0.001381 1.747207 10.91

950 60.6 4.7 0.5F0.6 124.0 1.27 0.003841 1.877213 88.76

975 35.7 2.7 1.5F0.6 374.0 1.97 0.006809 3.500425 42.98

1000 48.9 3.8 0.1F0.4 100.0 1.22 0.003034 2.374611 12370.68

1025 56.8 4.4 0.7F0.3 62.3 0.89 0.002059 1.846029 39.47

1050 52.6 4.1 0.5F0.3 69.5 0.83 0.002234 1.837870 44.83

1100 109.0 8.5 1.8F0.2 39.8 0.20 0.00131 1.315038 3.51



13/26


95-JC-2 Biotite. Low strain mingled granodiorite diorite. Puyuhuapi transect

1200 37.1 2.8 1.7F1.6 87.8 0.28 0.002891 0.289009 5.22

1450 0.9 0.0 66.2F463.1 101.0 36.56 0.003419 0.000922 15.31

Mean age (800900 jC)=1.6F0.2 Ma; J=0.00232F0.0000232 (1%)

95-JC-4 Biotite. Low strain mingled granodioritediorite. Puyuhuapi transect. Puy. Quarry shear zone

550 31.7 2.2 98F6.8 417.0 61.95 0.013616 0.132815 16.38

600 22.3 1.5 52.2F7.4 240.0 32.65 0.007926 0.109694 17.81

650 28.9 2.0 17.0F5.6 126.0 13.11 0.004261 0.065689 23.51

700 56.6 4.0 1.0F0.3 101.0 3.82 0.003426 0.080565 132.82

750 84.0 5.9 2.4F1.7 88.4 2.10 0.002972 0.196596 27.22

800 69.3 4.9 2.0F1.9 89.5 2.43 0.003004 0.214004 38.11

850 84.5 6.0 3.6F1.2 69.5 1.72 0.002334 0.345849 15.06

900 77.2 5.5 3.9F1.1 59.0 1.43 0.001988 0.427411 11.66

950 102.7 7.3 4.9F0.8 48.1 0.91 0.001621 0.432076 5.90

975 125.7 8.9 5.1F0.5 29.9 0.71 0.001028 0.558256 4.411000 204.8 14.6 5.3F0.3 18.1 0.34 0.000624 0.62939 2.05

1025 225.6 16.1 5.6F0.2 11.0 0.24 0.000386 0.644185 1.36

1050 150.2 0.7 5.4F0.2 15.1 0.31 0.000525 0.637975 1.84

1100 95.8 6.8 5.1F0.5 28.5 0.46 0.000965 0.561897 2.85

1200 35.8 2.5 3.5F2.4 77.8 1.27 0.002574 0.256589 11.42

1450 4.7 0.3 30.0F46.9 104.0 14.31 0.003515 0.006162 14.21

Mean age (950 1100 jC)=5.3F0.3 Ma; J=0.00232F0.0000232 (1%)

95-JC-4 hornblende. Low strain mingled granodioritediorite. Puyuhuapi transect. Quarry shear zone

650 18.1 5.8 49.9F8.0 124.0 37.38 0.004218 0.021157 21.42

750 15.2 4.8 29.4F9.8 109.0 25.49 0.003707 0.014044 25.81

850 14.1 4.5 16.5F7.6 107.0 20.37 0.003633 0.019659 37.64

950 14.8 4.7 14.0F6.0 110.0 18.72 0.003713 0.030779 41.23

975 9.7 3.1

13.5F

7.7 110.0 25.13 0.003716 0.032562 57.31

1000 13.4 4.3 1.7F5.7 101.0 25.99 0.003413 0.033072 504.01

1025 30.5 9.7 13.5F2.7 85.6 24.43 0.002894 0.043870 57.80

1050 59.7 19.0 25.7F1.9 76.3 19.54 0.002582 0.038096 24.98

1100 18.6 5.9 25.2F3.3 64.8 18.46 0.002200 0.057060 23.96

1125 4.6 1.4 18.3F10.7 81.0 24.55 0.002718 0.041820 43.30

1150 6.2 1.9 26.5F8.3 74.2 25.56 0.002503 0.039426 31.63

1175 12.9 4.1 34.7F4.8 68.1 22.51 0.002304 0.037658 21.64

1200 11.9 3.8 38.4F5.2 65.9 22.33 0.002229 0.036261 19.48

1250 38.5 12.3 37.5F2.1 63.5 21.07 0.002150 0.040068 18.81

1350 34.1 10.8 37.2F2.4 63.7 21.83 0.002158 0.040106 19.62

1450 10.1 3.2 58.0F7.3 69.3 23.52 0.002335 0.021646 14.05

Mean age (11751350 jC)=37.2F3 Ma; J=0.00232F0.0000232 (1%)

95-JC-6 Biotite. Undeformed granodiorite. Puyuhuapi transect. Seno Queulat

550 1.3 0.0 18.2F41.6 86.5 36.29 0.002733 0.027781 64.46

600 7.1 0.3 15.9F12.4 78.1 6.47 0.002558 0.055596 13.08

650 36.4 1.9 15.5F3.3 63.6 1.34 0.002120 0.096896 2.79

700 108.0 5.8 14.8F0.9 41.5 0.47 0.001390 0.163844 1.03

750 204.6 11.1 14.6F0.3 16.7 0.23 0.000558 0.236413 0.50

800 310.8 16.9 14.3F0.2 7.5 0.14 0.000252 0.267607 0.31

850 196.9 10.7 14.4F0.2 7.8 0.26 0.000265 0.264022 0.58

900 102.9 5.6 14.2F0.4 15.3 0.71 0.000518 0.244703 1.60

950 107.3 5.8 14.3F0.6 20.7 0.70 0.000695 0.228504 1.59

(continued on next page)

Table 1 (continued)



14/26


95-JC-6 Biotite. Undeformed granodiorite. Puyuhuapi transect. Seno Queulat

1000 104.9 5.7 14.2F

0.5 17.2 0.77 0.000582 0.240189 1.751050 209.7 11.4 14.2F0.4 11.2 0.43 0.000380 0.258885 0.97

1100 287.1 15.6 14.3F0.2 8.4 0.37 0.000286 0.265203 0.84

1200 150.3 8.2 14.3F0.3 14.1 0.78 0.000482 0.247193 1.76

1450 3.0 0.1 250.4F47.7 53.7 454.86 0.001918 0.007100 82.78

Mean age (650 1200 jC)=14.4F0.3 Ma; J=0.00232F0.0000232 (1%)

95-JC-6 Hornblende. Undeformed granodiorite. Puyuhuapi transect. Seno Queulat

650 18.0 2.1 16.0F3.9 79.1 0.68 0.002686 0.054145 1.38

750 61.6 7.3 11.3F1.0 54.8 0.23 0.001862 0.166933 0.65

850 61.5 7.3 10.9F0.9 47.1 0.46 0.001606 0.201649 1.36

900 29.5 3.5 12.7F1.6 50.3 1.60 0.001730 0.162285 4.04

950 33.0 3.9 15.5F2.0 61.7 5.19 0.002115 0.102282 10.81

975 51.1 6.0 17.5F1.4 56.3 8.24 0.001936 0.102916 15.24

1000 225.6 26.8 15.2F0.4 47.0 9.23 0.001609 0.144158 19.551025 152.1 18.0 14.3F0.5 28.2 9.07 0.001030 0.204592 20.37

1050 59.9 7.1 14.3F0.8 23.3 6.49 0.000926 0.215059 14.55

1075 40.1 4.7 12.9F1.0 32.2 3.65 0.001172 0.213963 9.04

1100 42.2 5.0 12.9F1.3 47.5 5.63 0.001668 0.166737 13.99

1125 23.9 2.8 12.9F2.6 66.9 9.81 0.002314 0.104947 24.31

1150 7.9 0.9 7.8F8.0 91.9 12.08 0.003140 0.042667 49.06

1200 6.5 0.7 0.4F11.9 99.0 12.65 0.003390 0.023506 840.20

1250 4.3 0.5 5.7F19.4 102.0 12.56 0.003467 0.015287 69.57

1350 13.4 1.5 1.2F8.7 99.0 11.29 0.003373 0.015062 290.56

1450 10.0 1.1 8.5F15.8 98.6 2.44 0.003342 0.006368 9.12

Mean age (10001125 jC)=14.4F0.6 Ma; J=0.00232F0.0000232 (1%)

95-JC12 Biotite. Undeformed granodiorite. Puyuhuapi transect. Seno Queulat

550 6.8 0.4

115.6F

12.5 544.0 69.12 0.016499 0.145725 14.89

600 15.7 0.9 18.6F5.2 129.0 19.28 0.004327 0.065755 31.57

650 57.6 3.5 9.8F1.4 76.4 3.23 0.002568 0.099398 10.50

700 196.4 12.0 13.4F0.3 36.6 0.54 0.001231 0.196102 1.29

750 207.8 12.7 13.2F0.2 13.2 0.39 0.000443 0.272260 0.95

800 283.2 17.3 13.5F0.1 7.9 0.25 0.000268 0.282850 0.60

850 140.5 8.6 13.1F0.2 10.4 0.40 0.000351 0.281216 0.98

900 90.2 5.5 12.8F0.4 15.4 0.62 0.000521 0.270966 1.56

950 79.4 4.8 12.6F0.4 19.9 0.56 0.000665 0.262184 1.44

975 51.4 3.1 12.6F0.5 15.8 0.81 0.000540 0.271966 2.07

1000 86.0 5.2 13.1F0.3 14.0 0.41 0.000468 0.269544 1.01

1025 79.9 4.8 13.3F0.3 9.3 0.24 0.000308 0.279619 0.58

1050 94.9 5.8 13.7F0.2 7.1 0.03 0.000230 0.279282 0.07

1100 197.8 12.1 13.9F

0.2 8.9 0.00 0.000290 0.272419 0.021200 43.3 2.6 13.2F0.9 34.9 0.14 0.001140 0.202711 0.35

1450 1.2 0.0 96.8F314.6 102.0 10.89 0.003449 0.001018 2.92

Mean age (850 1200 jC)=13.3F0.2 Ma; J=0.00232F0.0000232 (1%)

96GA01 Biotite. Mylonite. Puyuhuapy transect. Puyuhuapi Quarry shear zone

550 3.9 0.3 32.7F20.6 92.0 0.07 0.003107 0.010492 0.08

600 12.2 1.0 17.9F9.1 94.4 0.01 0.003193 0.013503 0.03

650 30.1 2.5 11.6F4.0 93.6 0.02 0.003168 0.023668 0.07

700 49.2 4.2 6.8F2.2 92.7 0.11 0.003134 0.046644 0.57

750 49.3 4.2 6.6F1.8 92.0 0.02 0.003113 0.051777 0.13

800 48.1 4.1 9.7F2.6 92.9 0.00 0.003144 0.031672 0.02

Table 1 (continued)



15/26



850 81.2 6.9 7.3F

1.6 91.6 0.03 0.003099 0.049540 0.15900 88.0 7.5 6.3F0.8 79.5 0.03 0.002686 0.140882 0.16

950 170.6 14.6 5.0F0.4 74.4 0.01 0.002514 0.222908 0.12

1000 263.8 22.6 4.1F0.2 51.6 0.02 0.001742 0.512718 0.21

1050 213.1 18.3 4.2F0.1 29.5 0.07 0.000997 0.730535 0.59

1100 143.1 12.3 4.4F0.2 24.9 0.01 0.000840 0.730962 0.12

1150 8.7 0.7 12.9F1.3 24.5 0.28 0.000818 0.248039 0.73

1200 1.3 0.1 60.6F24.6 64.4 2.32 0.002145 0.024916 1.39

1450 0.7 0.0 206.6F163 87.9 2.27 0.002965 0.002414 0.49

Mean age (10001100 jC)=4.2F0.2 Ma; J=0.002438F0.000018 (0.7%)


600 12.1 1.3 9.6F6.3 95.5 0.21 0.003231 0.020024 0.74

700 56.8 6.5 6.7F2.2 94.6 0.31 0.003203 0.034368 1.55

750 38.2 4.4 5.1F1.9 94.2 0.26 0.003186 0.049435 1.74800 42.7 4.9 6.9F2.0 92.9 0.26 0.003144 0.044138 1.24

850 65.8 7.5 9.2F1.6 90.3 0.16 0.003054 0.045940 0.59

900 60.8 7.0 8.9F0.8 71.4 0.12 0.002415 0.138857 0.45

950 99.3 11.4 4.4F0.5 74.6 0.10 0.002524 0.246991 0.76

1000 187.4 21.5 3.9F0.2 49.4 0.07 0.001672 0.559732 0.64

1050 176.8 20.3 3.8F0.2 40.6 0.36 0.001384 0.67938 3.18

1100 103.9 11.9 3.5F0.3 58.1 0.39 0.001970 0.521886 3.75

1150 18.0 2.0 5.0F1.8 79.5 1.19 0.002690 0.175269 7.78

1200 3.0 0.3 5.5F14.6 96.1 5.33 0.003246 0.030209 32.26

1450 2.9 0.3 13.4F34.5 97.9 6.78 0.003311 0.006694 17.01

Mean age (10001100 jC)=3.8F0.3 Ma. J=0.002434F0.000018 (0.7%)

95GA04 Biotite. Low strain mingled granodioritediorite. Puyuhuapi transect. Puy. Quarry shear zone

550 2.4 0.2 14.4F13.8 89.3 0.49 0.002990 0.031890 1.15

600 13.4 1.1 4.5F2.9 93.0 0.20 0.003137 0.066126 1.45

650 30.1 2.5 6.1F1.5 88.2 0.12 0.002980 0.083825 0.65

700 52.7 4.5 5.6F0.8 82.5 0.11 0.002786 0.134880 0.69

750 59.2 5.0 5.9F0.7 79.7 0.06 0.002694 0.148544 0.36

800 54.6 4.6 6.3F1.3 89.8 0.11 0.003038 0.069375 0.58

850 81.8 7.0 6.2F0.7 83.2 0.06 0.002813 0.117719 0.35

900 80.7 6.9 4.9F0.4 67.7 0.07 0.002286 0.282460 0.52

950 136.4 11.6 4.6F0.2 53.6 0.05 0.001811 0.433324 0.38

1000 275.4 23.5 4.0F0.1 33.2 0.04 0.001124 0.711898 0.35

1050 235.7 20.1 4.1F0.1 28.8 0.18 0.000978 0.743037 1.45

1100 131.0 11.2 4.3F0.1 27.0 0.08 0.000914 0.732523 0.61

1200 10.3 0.8 3.6F2.7 87.1 1.20 0.002920 0.153202 11.08

1450 4.2 0.3 23.0F

14.5 92.3 1.07 0.003117 0.014366 1.59Mean age (950 1050 jC)=4.2F0.1 Ma; J=0.002432F0.000018 (0.7%)

96-GA-26 Muscovite. High strain bt-ms schist. Puyuhuapi transect. Puerto Cisnes shear zone

550 2.4 0.1 7.8F6.2 76.9 0.00 0.002385 0.111454 0.00

600 5.6 0.3 8.6F1.7 44.8 0.11 0.001571 0.272863 0.44

650 14.0 0.7 7.3F1.2 44.7 0.00 0.001536 0.318981 0.02

675 17.2 0.9 6.9F1.1 44.8 0.03 0.001538 0.338126 0.18

700 21.7 1.2 5.1F1.5 66.2 0.03 0.002259 0.278627 0.20

725 31.8 1.7 5.9F1.0 52.4 0.02 0.001786 0.335482 0.11

750 43.0 2.3 5.6F0.7 48.6 0.01 0.001658 0.384821 0.08

Table 1 (continued)

(continued on next page)



16/26


96-GA-26 Muscovite. High strain bt-ms schist. Puyuhuapi transect. Puerto Cisnes shear zone

775 67.2 3.7 5.8F

0.7 61.9 0.00 0.002101 0.273282 0.01800 242.4 13.4 6.2F0.2 51.5 0.00 0.001745 0.324493 0.01

825 323.1 17.8 6.3F0.1 25.6 0.00 0.000869 0.489688 0.00

850 223.4 12.3 6.3F0.1 18.5 0.00 0.000627 0.535572 0.00

900 275.6 15.2 6.3F0.1 25.0 0.00 0.000848 0.496512 0.00

950 158.7 8.7 6.2F0.3 37.3 0.00 0.001266 0.421508 0.00

1000 92.6 5.1 6.1F0.5 46.4 0.00 0.001575 0.362133 0.04

1100 209.0 11.5 6.1F0.3 43.5 0.00 0.001474 0.381153 0.02

1200 74.3 4.1 5.9F0.7 63.0 0.01 0.002139 0.258096 0.06

1450 5.9 0.3 64.8F32.5 104.0 0.23 0.003525 0.002732 0.10

Mean age (7501000 jC)=6.2F0.2 Ma; J=0.00232F0.0000232 (1%)

95-JC-14 Biotite. Undeformed granite. Aysen transect. Isla Cinco Hermanas

550 11.2 0.6 67.8F6.8 356.0 39.55 0.011387 0.150647 16.09

600 43.8 2.5

4.6F

1.8 111.0 6.14 0.003754 0.109710 42.33650 149.8 8.7 4.3F0.5 78.2 1.21 0.002629 0.206765 8.77

700 297.2 17.3 5.8F0.2 42.6 0.44 0.001430 0.408531 2.41

750 366.1 21.3 6.2F0.1 24.6 0.29 0.000825 0.503958 1.50

800 200.6 11.7 5.9F0.2 42.6 0.44 0.001424 0.400065 2.36

850 87.4 5.1 4.7F0.4 40.3 0.95 0.001342 0.509703 6.28

900 67.1 3.9 4.5F0.5 47.2 1.13 0.001561 0.477224 7.97

950 84.7 4.9 5.2F0.5 51.0 0.72 0.001688 0.387755 4.43

975 70.5 4.1 4.9F0.7 66.7 0.68 0.002213 0.278840 4.35

1000 76.0 4.4 5.3F0.4 38.0 0.66 0.001251 0.478727 3.99

1025 70.9 4.1 6.0F0.4 35.4 0.22 0.001146 0.443894 1.20

1050 56.8 3.3 6.4F0.5 38.6 0.02 0.001240 0.394752 0.07

1100 92.0 5.3 6.3F0.5 43.6 0.06 0.001430 0.367440 0.32

1200 35.8 2.0 4.6F1.3 73.2 0.72 0.002397 0.237803 4.92

1450 1.4 0.0

106.8F

202.5 103.0 27.84 0.003495 0.001509 6.62

Mean age (7001200 jC)=5.7F0.2 Ma; J=0.00232F0.0000232 (1%)

95GA17 Muscovitebiotite bands. Laser spots on a polished slab. High strain schist. Aysen transect. Isla Cinco Hermanas shear zone

Spot mV 39Ar Age (Ma)F2r %ATM 37Ar/39Ar 36Ar/40Ar 39Ar/40Ar %IIC

1 24.7 6.8F0.4 34.5 0.04 0.001169 0.034083 0.01

2 44.3 7.7F0.3 39.4 0.05 0.001333 0.027900 0.02

3 34.5 6.2F0.5 61.6 0.10 0.002087 0.021826 0.05

4 45.2 4.6F0.3 59.4 0.10 0.002010 0.030686 0.06

5 40.2 8.0F0.3 38.2 0.09 0.001295 0.027313 0.03

6 38.4 5.6F0.4 63.4 0.16 0.002146 0.023076 0.09

7 48.7 4.6F0.2 47.1 0.16 0.001594 0.039999 0.10

8 67.6 5.9F0.2 40.6 0.07 0.001376 0.035211 0.03

9 26.3 7.5F0.4 45.0 0.15 0.001525 0.025958 0.06

10 30.9 5.3F0.4 52.0 0.14 0.001761 0.031881 0.08

11 26.2 9.2F0.4 25.9 0.19 0.000876 0.028424 0.07

12 25.0 8.5F0.5 42.2 0.20 0.001429 0.024006 0.07

Mean age (spots 1 12 )=6.4F0.6 Ma; J=0.000197F0.00002 (10.1%)

95-GA-19 Muscovite. High strain schist. Aysen transect. Isla Cinco Hermanas shear zone


550 6.6 0.4 6.1F8.1 92.2 0.04 0.003119 0.053169 0.21

600 54.0 3.4 3.1F0.8 68.3 0.05 0.002312 0.415424 0.56

Table 1 (continued)



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Biotites from sample 96GA01 and 96GA03, both

high strain mylonites, gave spectra with scattered,

poorly defined ages over the first half of the gas

release, but with plateaus over the final high temper-ature heating steps. The plateau ages are 4.2F0.2 and

3.8F0.2 Ma, respectively (Fig. 7a,b). We interpret

these as the time of high strain solid-state contractio-

nal ductile deformation that affected the Queulat

plutonic unit. Sample 96GA26 is from a high strain

pelitic schist in the Rio Cisnes shear zone (Table 1,

Fig. 3). A muscovite separate from this rock gave a

spectrum with a very well-defined plateau (11 steps,

over > 95% of the gas release) at an age of 6.2F0.2

Ma (Fig. 7c). This age could be interpreted as the time

when these rocks cooled to muscovite closure temper-

atures following intrusion of the Puerto Cisnes pluton

(at ca. 10 Ma). More likely, it dates solid-state defor-

mation and recrystallization of muscovite because therocks were deformed under greenschist facies condi-

tions, and the muscovite grains define extensional

crenulation cleavage indicating dextral transtensional

deformation.

Samples 95GA17 and 95GA19 are from high

strain quartzmica schists collected from Islas Cinco

Hermanas in the Aysen fjord (Fig. 4). A fine-grained

muscovite separate from 95GA19 yielded a spectrum

that has ages generally increasing from a low off3

4 Ma to a high off8 Ma (Fig. 7d). This may result


650 86.4 5.6 3.4F0.3 37.9 0.04 0.001286 0.749365 0.41675 80.1 5.1 3.8F0.3 29.7 0.04 0.001010 0.755144 0.35

700 72.4 4.6 3.4F0.3 37.1 0.04 0.001260 0.767762 0.40

725 71.8 4.6 3.8F0.2 13.0 0.03 0.000447 0.940648 0.32

750 75.8 4.9 4.0F0.2 15.6 0.03 0.000533 0.877467 0.26

775 110.9 7.1 4.9F0.2 22.5 0.02 0.000765 0.659609 0.18

800 89.3 5.7 5.7F0.2 13.1 0.02 0.000448 0.635116 0.14

825 106.8 6.9 5.0F0.2 17.7 0.02 0.000603 0.682395 0.13

850 114.5 7.4 4.9F0.2 20.4 0.02 0.000695 0.671934 0.13

900 217.5 14.1 5.5F0.2 22.6 0.01 0.000767 0.578451 0.08

950 204.9 13.2 5.5F0.2 27.1 0.01 0.000918 0.552298 0.10

1000 138.6 8.9 7.2F0.3 24.6 0.02 0.000834 0.431066 0.11

1100 90.9 5.8 8.6F0.5 43.9 0.08 0.001487 0.270910 0.29

1200 21.6 1.4 9.8F3.4 87.6 0.37 0.002965 0.052524 1.23

Total gas age=5.2F0.2 Ma; J=0.00232F0.0000116 (0.5%)

95GA19 Muscovite biotite bands. Laser spots on a polished slab. High strain schist. Aysen transect. Isla Cinco Hermanas shear zone

Spot mV 39Ar Age (Ma)F2r %ATM 37Ar/39Ar 36Ar/40Ar 39Ar/40Ar %IIC

1 2.0 7.4F6.4 75.1 0.38 0.002541 0.011825 0.16

2 1.9 4.9F4.7 74.8 0.34 0.002530 0.017946 0.20

3 3.7 4.7F1.8 63.0 0.23 0.002130 0.027820 0.14

4 21.3 4.6F0.3 43.7 0.06 0.001480 0.042544 0.04

5 20.1 4.2F0.6 74.6 0.13 0.002525 0.021266 0.09

6 19.2 4.0F0.4 63.1 0.01 0.002136 0.032294 0.01

7 11.1 4.9F0.9 67.2 0.01 0.002273 0.023639 0.00

8 10.5 4.4F0.9 68.8 0.01 0.002327 0.024947 0.00

9 5.4 5.9F

7.1 91.4 0.17 0.003093 0.005105 0.0810 2.9 4.2F5.2 85.4 0.42 0.002891 0.011999 0.29

11 1.3 13.9F64.2 93.9 2.34 0.003179 0.001532 0.60

12 9.5 5.3F1.7 79.6 0.21 0.002696 0.013489 0.11

Mean age (spots 112)=4.7F1.0 Ma; J=0.000197F0.00002 (10.1%)

Analysis are from bulk mineral separates unless otherwise indicated (laserspots on oriented slabs). Mean ages are reported with a 2r error37Ar 39Ar, 36Ar 40Ar and 40Ar 39Ar ratios are corrected for mass spectrometer discrimination, interfering isotopes and system blanks.

%IIC=interfering isotopes correction.

Table 1 (continued)

95-GA-19 Muscovite. High strain schist. Aysen transect. Isla Cinco Hermanas shear zone



18/26

Fig. 6. 40Ar 39Ar apparent age spectra for undeformed (low strain) samples of the two transects. Samples 95JC6, 95JC4, 95JC1, 95JC12 are

from the Puyuhuapi transect, sample 95JC14 is from the Aysen transect. Mean and plateau (for 95JC6 Bt) ages as discussed in text are indicated.



19/26

Fig. 7. 40Ar 39Ar apparent age spectra for samples of high strain rocks of the two transects. Samples 96GA01, 96GA03 and 96GA26 are from

the Puyuhuapi transect, samples 95GA17 and 95GA19 are from the Ayse n transect. Plateau ages as discussed in text are indicated.



20/26

from the partial resetting of old muscovite grains by a

high strain deformation event atf34 Ma. Other-

wise, the spectrum might represent the mixture of two

populations of grains: (i) a relict population repre-sented by older ages, and (ii) a recrystallized popula-

tion represented by the youngest ages. Results from

the laserprobe dating of these two samples, described

below, favors the second of these two hypotheses.

The laser dating, carried out on selected thick

sections, targeted in each case muscovite biotite

fine-grained aggregates in sigmoidal lenses parallel

to the foliation. Results for each sample are shown in

Fig. 7e,f on plots of apparent age versus 39Ar abun-

dance. For 95GA19, many of the spot/area analyses

yielded relatively small amounts of gas (i.e.


21/26

the Patagonian Batholith and wallrocks. The bulk of

the transpressional deformation took place at around 4

Ma as documented by 40Ar 39Ar dating on recrystal-

lized biotite and muscovite from several greenschistfacies shear zones. A slightly older dextral transten-

sional event may have taken place in the region at

about 6 Ma.

The present-day spatial distribution of geologic

units, the overall topography of the orogen, and the

mostly contractional/dextral strike-slip nature of their

boundaries strongly suggests the development of a

transpressional pop-up structure defining the Main

Range of the Patagonian Cordillera (Fig. 8). The

Cenozoic magmatic arc rocks are thrusted westwards

over the basement/Traiguen Formation and appear to

be thrusted eastward over the Cretaceous belt of the

Patagonian batholith. This regional-scale pop-up like

structure is very similar to that obtained through three-

dimensional numerical dynamic modeling of trans-

pressional deformation at obliquely convergent plate

margins (Braun and Beaumont, 1995). Recent exper-

imental analog modeling of transpressional deforma-

tion by Schreurs and Colletta (1998) also shows

striking similarities with the field data. Furthermore,

the series of en echelon structures joining the two

main boundaries of the Cenozoic plutonic belt in

southern Chile resembles the duplex-like structureobserved in the same analog models. The observation

that the Cenozoic belt of the NPB, which constitutes

the axis of the Main Range, has been differentially

exhumed with respect to the Central Depression to the

west and the foreland region to the east has a

fundamental bearing on orogenic processes operating

at convergent margins. A long-lived transpressional

magmatic arc, such as the one described here, con-

centrates the bulk of deformation by crustal thicken-

ing and strike-slip movements giving rise to a

topographic relief by double-verging thrust zones asdepicted by Braun and Beaumonts numerical model.

Ridge collision and oblique subduction have been

proposed as alternative driving mechanisms of trans-

pressional deformation at the leading edge of the

South American plate. Collision of successive seg-

ments of the Chile Ridge took place between 6 and 3

Ma, close to the present position of the triple junction

(Fig. 9). Dextral right-oblique subduction, in turn, has

prevailed during most of the Cenozoic (Fig. 1).

Theoretically, ridge subduction favors margin-orthog-

onal contraction close to the indenter and oblique-slip

to strike-slip deformation a few hundred kilometers

away from it (Tapponier and Molnar, 1976; Nelson et

al., 1994) (Fig. 10a). On the other hand, obliquesubduction is thought to produce overall transpres-

sional deformation along ancient and present-day

plate boundaries (e.g. Jarrard, 1986; Beck, 1991;

McCaffrey, 1992; Teyssier et al., 1995) (Fig. 10b).

We envisage oblique subduction as the long-term

driving mechanism of dextral transpression in the

southern Andes. Previous studies proposed that dex-

tral transtension occurred locally along the magmatic

arc during early Tertiary times when subduction was

Fig. 9. Migration of the Chile Ridge and fracture zones with respect

to the plate boundary during the last 6 Ma (Bourgois et al., 1996).

Several northnorthwest trending short segments of the ridge were

consecutively subducted from around 6 Ma to present-day close to

the Taitao Peninsula.



22/26

highly dextral-oblique and intraplate basin formationaccompanied by tholeiitic basalts may have occurred

in a leaky transform environment (e.g. Herve et al.,

1995). The basin was later inverted in Miocene

Pliocene (?) times when less oblique convergence

took place. Less oblique convergence could have led

to contraction and overthrusting of the Patagonian

Batholith over the basinal deposits.

During the Pliocene, subduction of the Chile Ridge

must have played a more significant role in the

tectonics of the southern Andes than previously rec-

ognized. The series of ridge segments that havecollided with the continent at about the same latitude

from 6 Ma have probably enhanced the contractional

component of dextral-oblique transpressional defor-

mation, close to the ridge indenter. Dextral-strike slip

deformation documented a few hundred kilometers to

the north at 42jS (Cembrano et al., 1996) and dextral

transtensional deformation recorded at 6 Ma along

east northeast trending shear zones at 44jS, are

kinematically compatible with bulk dextral transpres-

sional deformation induced by the indenter effect of

Fig. 10. Models for the tectonic consequences of ridge collision (a), and oblique subduction (b), in the tectonics of plate boundaries. (Tapponier

and Molnar, 1976; Nelson et al., 1994; Beck, 1991).



23/26

the Chile Ridge. Furthermore, coeval zones of obli-

que-reverse slip and strike-slip ductile deformation

zones of similar orientation between 42jS and 47jS

suggest that overall dextral transpression at the

Nazca South America plate boundary zone has been

partitioned into zones of contraction-dominated and

strike-slip dominated kinematics within the magmatic

arc. In contrast, the forearc and foreland regions haveremained nearly undeformed (Figs. 8 and 11).

5. Conclusion

The transect across part of the plate boundary zone

at Puyuhuapi (44jS) revealed several centimeter-to-

hundreds-of-meters wide north-east trending ductile

shear zones that affect Paleozoic metamorphic rocks,

mid-Tertiary stratified rocks and Miocene plutonic

rocks. Some of these shear zones, e.g. the Puyuhuapi

and Queulat shear zones, roughly define the eastern

boundary of the Miocene belt of the North Patagonian

Batholith. Steeply dipping meter-wide mylonite zones

display dextral oblique reverse sense of shear in

the plutons whereas wider, lower strain shear zones,

show top-to-the-east ductile shear kinematics. Two40

Ar

39

Ar step heating analyses on fine-grained bio-tite separates from mylonitic zones within Miocene

plutons yielded 3.8F0.2 and 4.2F0.2 Ma ages. These

were interpreted as the time of a regional Pliocene

dextral transpressional deformation event.

The Aysen transect (45jS) revealed northeast-trend-

ing ductile shear zones developed within Paleozoic

metamorphic rocks, Miocene plutons and mid-Terti-

ary metasedimentary metavolcanic rocks. Dextral,

high-strain shear zones were observed in the Paleo-

zoic basement at Islas Cinco Hermanas. These rocks

Fig. 11. Cartoon of the Nazca South America plate boundary zone showing how overall heterogeneous transpressional deformation arising

from oblique convergence and ridge subduction has been accommodated during the last 6 Ma. Discrete zones of dextral-reverse shear, dextral

strike-slip and margin-orthogonal contraction are found along and across the plate boundary, concentrated in the magmatic arc. The component

of plate boundary contraction is enhanced close to the Chile Ridge collision zone. Dextral strike-slip deformation is favored as the distance from

the collision zone increases. Convergence vector (shown with arrow) has been slightly oblique with respect to the orthogonal to the trench over

the last 6 Ma.



24/26

were analyzed by two methods: (i) conventional step

heating (on a muscovite separate) and (ii) laser heating

(in situ on bands of fine-grained mica). The data

for these rocks suggest that recrystallization accom-panying deformation occurred principally at about 4

4.5 Ma. In general, this later event is more readily

resolved by the laserprobe method than it is by conven-

tional step heating.

Similar40Ar 39Ar dates (4 4.5 and 6 Ma) obtained

from ductile shear zones of different kinematics (rang-

ing from pure strike-slip to pure contractional) strongly

suggest that bulk transpressional deformation has been

partitioned in a complex way along and across the

southern Andes magmatic arc during Pliocene.

The increased contractional component of overall

transpressional deformation in the southern transects,

close to the Pliocene and present-day positions of the

triple junction, suggests a strong causal link between

consecutive episodes of Pliocene ridge collision and

continental margin tectonics. Oblique subduction, a

process that has taken place since 48 Ma, has been a

driving mechanism for long-term dextral transpression

along the continental margin and cannot be causally

separated from ridge collision. Both ridge collision

and oblique subduction produce bulk transpressional

deformation along the leading edge of the continent.

The contractional/dextral-oblique kinematics ofregional scale shear zones at both the western and

eastern boundaries of the Miocene plutonic belt of the

Patagonian Batholith suggest Pliocene bulk transpres-

sional continental deformation has been accommo-

dated through coeval thrusting, oblique-slip and

strike-slip deformation. These orogen-parallel crustal

shear zones have produced differential exhumation of

Miocene plutons and wallrocks with respect to rela-

tively undeformed forearc and foreland regions.

Because the presently exposed intra-arc shear zones

formed atf

350j

C, at least 7 km of rock must havebeen eroded away during the last 4 Ma (for a thermal

gradient of 50 jC/km). This gives an average exhu-

mation rate in excess of 1.7 mm/year for the Miocene

plutons and deformed wallrocks.

Acknowledgements

The present work is part of the first authors PhD

thesis at Dalhousie University, Nova Scotia. A Killam

fellowship funded the stay of JC in Canada. The

Chilean National Science Foundation (FONDECYT)

funded fieldwork and laboratory analyses through

grants 1950497 to JC and 2960019 to AS. FranciscoHerve (Universidad de Chile), Liz Schermer (Western

Washington University) and Bill McClelland (Uni-

versity of California) participated in the early stages of

the fieldwork. Marcos Zentilli, Becky Jamieson and

Nick Culshaw (Dalhousie University) read and made

important suggestions and comments that signifi-

cantly improved the content and scope of this paper.

Dave Prior (University of Liverpool), external exam-

iner of Cembranos (1998) thesis, is thanked for his

enthusiasm and encouraging discussion. C. Androni-

cos and D. Cunningham made important suggestions

to an early version of this paper. Editor K. Hodges,

Eric Nelson and an anonymous reviewer are thanked

for their thorough revisions and suggestions.

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Cenozoico Transpresional Nazca