Ordovician accretion of the Argentine Precordillera terrane to Gondwana: a review

Ordovician accretion of the Argentine Precordillera terrane

to Gondwana: a review

William A. Thomasa,*, Ricardo A. Astinib

aDepartment of Geological Sciences, University of Kentucky, Lexington, KY 40506-0053, USAbCatedra de Estratigrafıa y Geologıa Historica, Universidad Nacional de Cordoba, Av. Velez Sarsfield 299, 5000 Cordoba, Argentina

Abstract

The Precordillera terrane of Argentina was rifted as a lithospheric block from the Ouachita embayment of southeastern Laurentia and

accreted to western Gondwana. In various interpretations, the time of rifting of the Precordillera terrane from Laurentia ranges from Early

Cambrian to Late Ordovician, and the time of accretion to Gondwana ranges from Middle–Late Ordovician to Silurian–Devonian. This

review of available data and previous interpretations, leads to the conclusion that rifting from the Ouachita embayment of Laurentia occurred

in the Early Cambrian, and collision with the Famatina arc on the western margin of Gondwana occurred in the Middle–Late Ordovician. In

that context, Silurian–Devonian deformation reflects the accretion of the Chilenia terrane on the outboard side of the Precordillera terrane.

q 2003 Elsevier Science Ltd. All rights reserved.

Resume

El terrano de la Precordillera Argentina se separo como un bloque litosferico del engolfamiento de Ouachita en el sureste Laurentia y se

acreciono a Gondwana occidental. En diferentes interpretaciones el momento de separacion de la Precordillera desde Laurentia varıa entre el

Cambrico Temprano y el Ordovıcico Tardıo y el momento de acrecion a Gondwana varıa entre el Ordovıcico Medio–Tardıo y el Silurico–

Devonico. Esta revision sobre la informacion disponible e interpretaciones previas concluye que la separacion del engolfamiento de Ouachita

en Laurentia ocurrio en el Cambrico Temprano y la colision con el arco de Famatina en el margen occidental de Gondwana ocurrio en el

Orodvıcico Medio–Tardıo. En ese contexto la deformacion silurica y devonica refleja la acrecion del terreno de Chilenia sobre el margen

libre de la Precordillera.

q 2003 Elsevier Science Ltd. All rights reserved.

1. Introduction

General consensus holds that the Argentine Precordillera

is an exotic block that was rifted from the Ouachita

embayment of Laurentia and accreted to the western margin

of Gondwana (Figs. 1 and 2) (e.g. Thomas and Astini,

1996). The timing of both events remains subject to debate.

Interpretations of the time of continental rifting and

separation of the Precordillera from Laurentia range from

Early Cambrian (e.g. Thomas and Astini, 1996) to Middle–

Late Ordovician (e.g. Keller, 1999). Interpretations of the

time of accretion of the Precordillera to Gondwana range

from Middle–Late Ordovician (e.g. Astini et al., 1995) to

Silurian–Devonian (e.g. Keller et al., 1998; Pankhurst and

Rapela, 1998; Rapela et al., 1998; Keller, 1999).

Considering the strong record of Early Cambrian

rifting, Thomas and Astini (1996) propose that con-

tinental breakup led to isolation of the Precordillera

microcontinent by the end of the Cambrian. Independent

drift of the Precordillera microcontinent was followed by

collision with the Famatina arc terrane on the western

margin of Gondwana during the Middle–Late Ordovician

(Thomas et al., 2002). Silurian–Devonian deformation is

attributed to the collision of the Chilenia terrane on the

outboard side of the already accreted Precordillera

(Ramos et al., 1986).

Alternatively, Keller (1999) interprets the documented

Cambrian extension to represent only the stretching of

Laurentian crust, leaving the Precordillera as a marginal

plateau adjacent to the Laurentian margin. Keller (1999)

assigns the breakup and separation of the Precordillera from

0895-9811/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved.

doi:10.1016/S0895-9811(03)00019-1

Journal of South American Earth Sciences 16 (2003) 67–79

www.elsevier.com/locate/jsames

* Corresponding author. Tel.: þ1-859-257-6222; fax: þ1-859-323-1938.

E-mail addresses: [email protected] (W.A. Thomas), rastini@satlink.

com (R.A. Astini).

http://www.elsevier.com/locate/jsames

Laurentia to the Middle–Late Ordovician, largely on the

basis of the same observations of stratigraphy that Astini

et al. (1995) use as evidence of collision of the Precordillera

with Gondwana. An interpretation of Late Ordovician

rifting requires that accretion of the Precordillera to

Gondwana did not occur before the Silurian–Devonian,

either amalgamated with or sequentially with Chilenia (e.g.

Keller et al., 1998; Pankhurst and Rapela, 1998; Rapela

et al., 1998).

In an interpretation somewhat similar to the hypothesis

of a marginal plateau, Dalziel (1997) suggests that

Cambrian extension resulted in a highly attenuated ‘Texas

plateau’ that contained the Precordillera on a distal edge far

from Laurentia. In that context, the Precordillera as the

leading edge of the Texas plateau collided with Gondwana

during Ordovician time and subsequently separated from

Laurentia (Dalziel, 1997).

A continent–continent collision between Gondwana and

mainland Laurentia, which included the Precordillera

(Occidentalia terrane), has been invoked by Dalla Salda

et al. (1992) to explain the Middle–Late Ordovician events

in the Precordillera. This interpretation does not incorporate

a Cambrian extensional event, but does include postcolli-

sional rifting of Laurentia from Gondwana, leaveing the

Precordillera as a fragment of Laurentia attached to

Gondwana.

In this article, we summarize the observations cited as

evidence for the various tectonic events in the history and

timing of accretion of the Precordillera terrane to

Gondwana. Because the time of rifting constrains the age

of collision and accretion, we begin with a summary of

evidence for Cambrian rifting of the Precordillera from

Laurentia. We conclude that the preponderance of evidence

favors Ordovician accretion of the Precordillera to

Fig. 1. (A) Outline map of the late Precambrian–Cambrian rifted margin of Laurentia, showing the Ouachita embayment (the Early Cambrian palinspastic

location of the Argentine Precordillera) in relation to the locations of late Paleozoic thrust belts around southern Laurentia (abbreviations: A ¼ Appalachian;

O ¼ Ouachita) (modified from Thomas, 1991). (B) Map of present location of the Precordillera in the eastern foothills of the Andes (abbreviations:

PR ¼ Precordillera; F ¼ Famatina; WP ¼ western Sierras Pampeanas; f ¼ Sierra de Valle Fertil; p ¼ Sierra de Pie de Palo; pt ¼ Ponon Trehue; sj ¼ Cerro

San Jorge).

W.A. Thomas, R.A. Astini / Journal of South American Earth Sciences 16 (2003) 67–7968

Gondwana and that Silurian – Devonian deformation

reflects the accretion of Chilenia to the outboard side of

the Precordillera. We also discuss comparisons with

alternative interpretations to evaluate the differences.

Initial recognition of the Laurentian affinities of the

Precordillera has been expanded geographically to include

exposures outside the area of early Paleozoic sedimentary

rocks that characterize the Precordillera (sensu stricto)

physiographic province. We consider the exotic Laurentian

terrane to include the Precordillera, as well as outcrops in

the Sierra de Pie de Palo, Ponon Trehue, and Cerro San

Jorge/Limay Mahuida (Fig. 1). These components var-

iously include basement and early Paleozoic cover strata.

We refer to the entire exotic terrane as the Precordillera

terrane (Astini et al., 1995; Thomas and Astini, 1996; cf.

‘Cuyania terrane,’ Ramos et al., 1996).

2. Summary of evidence for Cambrian rifting

Documentation of Cambrian rifting of the Precordillera

terrane from Laurentia comes from both continental

margins (Figs. 1 and 2). The paleomagnetic pole deter-

mined for Early Cambrian redbeds (Cerro Totora For-

mation, Fig. 3) of the Precordillera coincides with the Early

Cambrian pole for Laurentia in a palinspastic reconstruc-

tion that places the Precordillera terrane in the Ouachita

embayment of the Laurentian margin (Figs. 1 and 2)

Fig. 2. Block diagram showing rifting of the Precordillera from the Ouachita embayment of Laurentia, and a summary of data from references cited in text

(abbreviations: SO ¼ Southern Oklahoma fault system; BH ¼ Birmingham graben; MV ¼ Mississippi Valley graben; RC ¼ Rough Creek graben; R ¼ Rome

trough). Precordillera faunal succession is summarized from Benedetto et al. (1995, 1999).

W.A. Thomas, R.A. Astini / Journal of South American Earth Sciences 16 (2003) 67–79 69

(Rapalini and Astini, 1998). Conversely, the Early Cam-

brian pole for the Precordillera in its present location

contrasts with the Gondwanan pole. The paleomagnetic

pole documents the location of the Precordillera terrane

within Laurentia until the Early Cambrian.

The Lower Cambrian Rome Formation on the Alabama

promontory of southern Laurentia and the Lower Cambrian

Cerro Totora Formation in the Precordillera include

evaporite interbeds within a succession of dolomites and

redbeds (Fig. 2) (Astini and Vaccari, 1996; Thomas et al.,

2001). Indistinguishable strontium isotope ratios of the

Rome and Cerro Totora evaporites indicate identical ages

and similar depositional settings (Thomas et al., 2001). The

strontium isotope ratios correspond to the worldwide

strontium seawater curve for Early Cambrian time,

consistent with trilobite biostratigraphy for both formations.

A distribution range to more radiogenic strontium indicates

that a dominantly marine-water base was diluted by

meteoric runoff and associated siliciclastic detritus,

suggesting deposition in fault-bounded basins partially

open to marine circulation. The strontium data are

consistent with continental rifting during Early Cambrian

time.

An array of synrift igneous rocks extends along the

Southern Oklahoma fault system (Hogan and Gilbert, 1998),

which parallels the Alabama–Oklahoma transform fault

boundary between the northern Precordillera terrane and the

Alabama promontory of Laurentia (Fig. 2). The Southern

Oklahoma fault system is interpreted as a leaky transform.

Ages of the synrift igneous rocks range from 530 to 539 Ma,

indicating the time of continental rifting prior to final

breakup (Hogan and Gilbert, 1998; Thomas et al., 2000). No

comparable synrift igneous rocks have been documented in

the Precordillera; however, one mafic sill observed within a

gypsum bed in the Cerro Totora Formation is a candidate

awaiting confirmation of age.

The thermal subsidence history of postrift passive

margins of the Precordillera and the Texas promontory of

Laurentia (western boundary of the Ouachita embayment)

matches the pattern of complementary asymmetry of

subsidence along the conjugate margins in a low-angle

detachment, simple-shear continental rift (Fig. 2) (Thomas

and Astini, 1999). The transition from the synrift Lower

Cambrian Cerro Totora Formation clastic and evaporite

succession to the passive-margin deposition of massive

shelf carbonates (uppermost Lower Cambrian Los Hornos

Formation and equivalent upper La Laja Formation, Fig. 3)

began in late Early Cambrian time, and the Lower

Cambrian–Lower Ordovician passive-margin carbonate

succession is .2000 m data summarized in thick (data

Fig. 3. Chronostratigraphic chart showing the stratigraphic position of Cambrian and Ordovician stratigraphic units across the Argentine Precordillera

(modified from Astini, 1998a). The double arrow labeled LV shows the stratigraphic range of olistoliths in the Las Vacas and Trapiche Formations. The

stratigraphic ranges of remanent carbonates and related strata are shown by blocks labeled C (Las Chacritas Formation), A (Las Aguiditas Formation), and S

(Sassito Formation). The blocks with shading show the interpreted stratigraphic positions of rocks represented in olistoliths in the Los Sombreros Formation

but not exposed in stratigraphic positions in the Precordillera.


summarized in Thomas and Astini, 1999). In contrast, on

the Texas promontory, Precambrian crystalline basement

rocks are overlain by latest Middle Cambrian sandstones at

an unconformity which has substantial paleotopographic

relief, and the passive-margin succession (upper Middle

Cambrian–Lower Ordovician) is ,1000 m thick. In the

Precordillera, an earlier beginning and a greater magnitude

of subsidence, as well as the preservation of a synrift

sedimentary succession, indicate a lower-plate configur-

ation in a low-angle-detachment, simple-shear continental

rift (Astini and Thomas, 1999; Thomas and Astini, 1999).

In contrast, on the Texas promontory, the later beginning

and lesser magnitude of subsidence, as well as the relief on

continental basement and lack of synrift rocks, indicate a

more narrow zone of transitional crust and a residual

thermal uplift in an upper-plate setting. The complemen-

tary asymmetry of the timing of the rift to passive-margin

transition and the magnitude of subsidence indicate

conjugate margins associated with continental breakup in

the late Early Cambrian and the initial opening of a new

ocean.

The record of late Middle Cambrian establishment of

a passive margin along the Ouachita rift margin of the

Texas promontory contrasts with sedimentological indi-

cators of continued synrift fault movement until early Late

Cambrian along the boundary faults of the Mississippi

Valley graben and Birmingham graben on the Alabama

promontory adjacent to the Alabama–Oklahoma transform

fault (Figs. 1 and 2) (Thomas, 1991; Osborne et al., 2000;

Astini et al., 2002). Relatively thick (.1 km), primarily

fineclastic successions fill the grabens. The graben-fill

successions and graben boundary faults are overlapped by

a regionally extensive carbonate-platform succession

of middle Late Cambrian – Early Ordovician age

(Thomas, 1991). Continued extension across the Alabama

promontory until middle Late Cambrian time (Thomas,

1991; Glumac and Walker, 2000) is consistent with the

opening of the Ouachita Ocean and the migration of the

Ouachita mid-ocean ridge along the Alabama–Oklahoma

transform following continental breakup along the Oua-

chita rift (between the Texas promontory and the

Precordillera terrane) (Fig. 2).

The stratigraphic transition from synrift graben-fill

deposits to passive-margin massive carbonates across the

Mississippi Valley and Birmingham grabens in early–

middle Late Cambrian time clearly documents the evolution

of a passive margin entirely around the Alabama promon-

tory–Ouachita embayment–Texas promontory of Laurentia

(Figs. 1 and 2). A corresponding off-shelf passive-margin

succession is well documented in the Ouachita embayment

(Fig. 2), where the oldest exposed deep-water facies are

Late Cambrian in age (Ethington et al., 1989). The oldest

off-shelf passive-margin strata are in thrust sheets in the

Ouachita Mountains (a late Paleozoic orogenic belt, Fig. 1),

and no older strata are known (Viele and Thomas, 1989).

Although the age of initiation of off-shelf passive-margin

deposition is not documented by available data, the

minimum age of that event is Late Cambrian, thereby

constraining the minimum age of continental breakup.

The faunal succession on the Precordillera records

departure from Laurentia (Astini et al., 1995; Benedetto

et al., 1995, 1999; Benedetto, 1998). The Cambrian faunas of

the Precordillera are Laurentian. In Early Ordovician time,

endemic forms progressively replaced Laurentian forms in

the Precordillera fauna (Fig. 2). The demise of dominant

Laurentian forms after Cambrian time indicates the isolation

of the Precordillera terrane from Laurentia within the Iapetus

Ocean.

3. Summary of evidence for Ordovician accretion

of the Precordillera terrane to Gondwana

3.1. Synorogenic clastic wedge

Shallow-marine carbonates at the top of the passive-

margin succession (San Juan Formation) in the Precor-

dillera are overlain by deep-water black shales (Gualca-

mayo Formation) (Figs. 3 and 4). Black shale deposition

began in the northeasternmost part of the Precordillera in

middle Arenig and progressed diachronously westward

and southward, becoming widespread across the north-

eastern Precordillera by Llanvirn time (Figs. 3 and 4)

(Astini, 1994, 1995a). The black shales grade upward into

a flysch-like succession of mudstones, sandstones, and

conglomerates, reflecting progradation of coarser detritus

over the platform (Las Vacas and Trapiche Formations,

Figs. 3–5) (Astini et al., 1995; Astini, 1998a). Rounded

lithic clasts (mostly ,25 cm) in the conglomerates

include igneous rocks (mainly granodiorites, tonalites,

gabbros, and rare felsic rocks), sedimentary rocks (mostly

feldspathic and subfeldspathic sandstones, rare quartz

sandstones, and carbonates), and less common foliated

metamorphic rocks and vein quartz. Interbedded sand-

stones range from lithic arenites to quartz arenites. The

oldest conglomerates are of Llanvirn age, and widespread

deposition of coarse clastic detritus mostly supplanted

black shales by Caradoc time. The clastic succession is

thickest and contains generally coarser lithic clasts in the

northeasternmost Precordillera, and it thins and

fines southward and westward (Astini et al., 1995).

The distal part of the clastic wedge thins out and is

truncated by Silurian strata at the Talacasto–Tambolar

arch (Fig. 5).

The upward transition from shallow-marine platform-

carbonate facies to deep-water black shales, the diachro-

nous base of the black shales, the upward coarsening of the

clastic facies above the black shales, and the semi-radial

thinning of the clastic succession all indicate deposition in a

synorogenic foreland basin undergoing flexural subsidence

in response to subduction beneath a tectonic load (e.g.

Miall, 1995). To identify the Ordovician foreland basin in


the northeastern Precordillera, we propose the name

Guandacol foreland basin (Figs. 3–5). Composition of

the synorogenic clastic wedge suggests within provenance

was a magmatic arc, as well as a collision orogen that

includes deformed and metamorphosed platform cover and

basement rocks. The source of the detritus is interpreted to

be the Famatina arc and deformed metamorphic and

plutonic rocks along a suture zone, probably including

the rocks presently exposed along the Sierra de Valle Fertil

and Sierra de Pie de Palo (Fig. 1). The distribution of the

conglomerates and sandstones indicates basinwide

dispersal of detritus from the extrabasinal source. The

Talacasto–Tambolar arch is interpreted to be the peripheral

forebulge of the Guandacol foreland basin (Fig. 5)

(Astini et al., 1995).

3.2. Limestone olistoliths in Las Vacas and Trapiche

Formations

Large boulders and olistoliths of intrabasinal carbonates

(derived from part of the preorogenic passive-margin

succession) are incorporated in the synorogenic clastic

wedge (Las Vacas and Trapiche Formations) in the eastern

Precordillera (Figs. 3 – 5) (Astini, 1998b). Most of

the olistoliths are blocks of limestone from the San Juan

Formation (Fig. 3), the uppermost part of the platform

succession. The succession also contains a few olistoliths of

Gualcamayo black shale, some of which show tectonic

folding. The stratigraphic interval represented by the

olistoliths represents ,400 m of the platform carbonate

and overlying black shale succession.

The stratigraphic range and sizes of the olistoliths require

substantial syndepositional vertical relief, both to expose the

stratigraphic succession and to gravitationally drive block

falls and slides. The olistoliths are the synorogenic clastic

wedge in the Guandacol foreland basin, indicating that the

mechanism to generate the olistoliths was coeval with part

of the clastic-wedge deposition. Two alternative structural

styles can be suggested to provide the source of the

olistoliths. The relief might be associated with a system of

large-magnitude horsts (source of olistoliths) and grabens

(depositional sites of olistoliths) across the former passive-

margin shelf and subsequent foreland basin (Fig. 4) (e.g.

Astini, 1998a); however, the mechanism is uncertain and

might be (1) postcollisional extension (Astini, 1998a) (2) an

extensional component of bending during flexural sub-

sidence (e.g. Bradley and Kidd, 1991), (3) a brittle

component of tectonic loading and foreland-basin

Fig. 4. Schematic cross section (not to scale) illustrating the collision of the Precordillera terrane with the Famatina magmatic arc along the western margin of

Gondwana in the Middle–Late Ordovician; Summary of data from references cited in text. Enlargements show the interpreted dispersal of the olistoliths.


subsidence (Ramos et al., 1998), or (4) transtensional and/or

transpressional reactivation or inversion of older basement

faults (e.g. Bayona et al., 2001). Alternatively, the olistoliths

may have been shed into the clastic fill of the foreland basin

from the leading edges of carbonate thrust sheets driven

westward from the collision orogen (Fig. 4) (e.g. Astini,

1992). Mixing of intrabasinal carbonate olistoliths with the

extrabasinal clasts in the conglomerates demonstrates that

the lateral extent of intrabasinal fault blocks was insufficient

to disrupt regional dispersal of extrabasinal detritus across

the Guandacol foreland basin.

3.3. Remanent carbonates

Within the regionally diachronous pattern of initial

clastic deposition, some younger carbonate deposits, herein

called remanent carbonates (Las Chacritas, Las Aguaditas,

and Sassito Formations, Figs. 3–5), are scattered locally

around the distal part of the Guandacol foreland basin.

These remanent carbonate deposits include slump and slide

structures in deeper water limestones and shallowing-

upward catch-up successions (Astini, 1995b; Astini and

Canas, 1995; Carrera and Astini, 1998). The Sassito

Formation, unconformably overlying the San Juan For-

mation, evidently represents both erosion of platform

carbonates and later deposition of shallow-marine carbon-

ates (Astini, 1998a; Keller and Lehnert, 1998). The

magnitude of the unconformity indicates local erosion of

as much as 150 m of the upper part of the San Juan

limestone. The locations of the remanent carbonates (Fig. 5)

correspond to the peripheral forebulge (Talacasto–Tambo-

lar arch) of the Guandacol foreland basin, and the

indications of local bathymetric relief suggest flexural-

extension faults along the forebulge (Astini, 1997). The time

of deposition ranges from early Llanvirn (Las Chacritas

Formation) to early Ashgill (Sassito Formation), overlap-

ping temporally with the latest arc plutons in Famatina and

with progradation of synorogenic clastic sediments (Tra-

piche Formation).

3.4. Local angular unconformities

Local angular unconformities and wedging of thickness

document synsedimentary deformation during deposition of

the clastic succession, both between the Las Vacas

and Trapiche Formations in the middle of the Guandacol

foreland basin and between the La Pola and Don Braulio

Formations in the southeastern part of the basin (Figs. 3

and 5) (Astini, 1998b, 2001a). The angular discordances and

local extent of each of the unconformities are consistent

with the geometry of a rotated half-graben or asymmetric

compressional folds of limited lateral extent.

3.5. Famatina arc complex

The Famatina arc complex (Figs. 1 and 4) east of the

Precordillera includes a marine sedimentary and volcanic

succession of Early and Middle Ordovician (Tremadoc–

Llanvirn) age (Astini, 1999a). Arc plutons range in age from

approximately 499 to 468 Ma (Pankhurst et al., 2000). The

ages of arc magmatism are consistent with subduction of the

leading edge of an oceanic (Iapetus)/continental (Precordil-

lera) plate, beginning in latest Cambrian time when the

leading continental margin of the Precordillera terrane was

still far from the subduction zone. Arc plutonism continued

until approximately earliest Llanvirn (468 Ma), shortly after

the beginning of flexural subsidence and the initiation of

black shale (Gualcamayo Formation) deposition on the

Precordillera platform. Apparently, subduction stopped

when the Precordillera terrane entered the subduction

zone, and the Famatina arc became an important source of

extrabasinal clastic sediment. Folds of the lower–middle

Arenig sedimentary-volcanic succession (Famatina Group)

are truncated by an angular unconformity beneath an upper

Arenig, dominantly volcanic succession (Morado Group),

Fig. 5. Generalized map of the Argentine Precordillera showing locations of

Middle–Upper Ordovician conglomerates, carbonate olistoliths in Ordo-

vician conglomerates, Middle–Upper Ordovician remanent carbonates,

Los Sombreros slope deposits, and Silurian olistostrome (modified from

Astini et al., 1995). The ‘approximate crest of peripheral forebulge’ is

placed along the Talacasto–Tambolar arch (e.g. Sierra de Talacasto).


documenting deformation in Famatina as early as middle–

late Arenig (Davila et al., 2001).

3.6. Bentonite beds

Bentonite (volcanic ash) beds are especially numerous

(.170 beds) in the stratigraphic succession of the

Precordillera, ranging through the Arenig and Llanvirn

strata (Fig. 4) (Huff et al., 1998). The radiometric age of one

bentonite is 464 ^ 2 Ma (Huff et al., 1997). Both the age

and the distribution of the bentonites are consistent with

supply from the Famatina volcanoes as the

Precordillera terrane approached the subduction zone

(Astini, 1998c; Huff et al., 1998). Plutons exposed in

Famatina represent the roots of a continental-margin

volcanic arc (Rapela et al., 1998).

3.7. Metamorphic rocks of the eastern Precordillera terrane

Metamorphic rocks in the Sierra de Pie de Palo (Fig. 1),

on the eastern side of the Precordillera terrane, yield a U–Pb

zircon age of ,460 Ma, which is interpreted to be the time

of collision of the terrane with western Gondwana (Fig. 4)

(Casquet et al., 2001). The age is consistent with an40Ar/39Ar hornblende plateau of 464 Ma (Ramos et al.,

1998) and is probably related to east-over-west ductile shear

zones. The ,460-Ma metamorphism overprints the

Grenville ages of basement rocks (McDonough et al.,

1993; Casquet et al., 2001). Some metasedimentary rocks

contain detrital zircons of Grenville age (,1 Ga),

suggesting metamorphism of the sedimentary cover above

the Grenville-age basement in the easternmost Precordillera

terrane along the collision orogen that included the

Famatinian arc system (Casquet et al., 2001). Deformation

and metamorphism at ,460 Ma are consistent with the time

of subsidence and filling of the Guandacol foreland basin

and with the time of Famatinian arc magmatism and ash

falls in indicating the time of collision of the Precordillera

terrane with Gondwana.

3.8. Hirnantian glaciation

Hirnantian glacial deposits in the Precordillera (Don

Braulio Formation, Figs. 3 and 4) (Buggisch and Astini,

1993; Astini, 2001b) are part of an extensive system that

overlaps into adjacent basins in northwestern Argentina and

Bolivia and is widespread in Gondwana. The extent and

nature of the glacial deposits indicate that the Precordillera

had been accreted to Gondwana by the late Ashgill (Fig. 4)

(Astini, 1995c, 1999b, 2001b).

3.9. Faunal succession in the Precordillera

The Precordillera faunal succession, which first docu-

ments departure from Laurentia, continues in younger strata

and records the approach to Gondwana. Early Ordovician

endemism and the abrupt appearance of new forms

demonstrate the isolation of the Precordillera (Astini et al.,

1995; Benedetto et al., 1995, 1999; Vaccari, 1995;

Benedetto, 1998; Albanesi and Barnes, 2000). The earliest

Gondwanan forms in the Precordillera are primitive fish of

Llanvirn (Llandeilo, Middle Ordovician) age, as constrained

by conodont data (Albanesi et al., 1995). Gondwanan

brachiopods are common in the Caradoc (late Middle

Ordovician) strata of the Precordillera (Benedetto and

Sanchez, 1996; Benedetto, 1998). The faunal successions

indicate the approach and docking of the Precordillera

terrane to Gondwana during Ordovician time; however,

endemic faunas persisted into the Caradoc. The delay of

arrival of more Gondwanan faunas in the Precordillera until

late Ordovician suggests that the Andean-type Famatina arc,

along the tectonically active western margin of Gondwana,

was a barrier to faunal migration.

4. Discussion of the interpretation of Ordovician rifting

of the Precordillera from Laurentia

In contrast to the interpretation of Ordovician accretion

to Gondwana, the Ordovician events have been interpreted

to represent rifting of the Precordillera from Laurentia (e.g.

Dickerson and Keller, 1998; Keller et al., 1998; Rapela

et al., 1998; Keller, 1999). The interpretation that the

Precordillera was rifted from Laurentia during Ordovician

time requires that accretion of the Precordillera to

Gondwana, either amalgamated sequentially with Chilenia,

occurred in with or Silurian–Devonian or some later time.

Emphasizing an extensional horst and graben system in

the context of a model for continental breakup, Keller (1999)

suggests that Ordovician extension and rifting of the

Precordillera from Laurentia account for the deposition of

the Ordovician clastic succession and the contained

carbonate olistoliths. An extensional horst and graben

system does account for the structural and bathymetric

relief necessary to generate and emplace the large limestone

olistoliths of the San Juan Formation in the succession of

Ordovician conglomerates (Keller, 1999), however aspects

of the Ordovician stratigraphy of the Precordillera are more

difficult to explain by this mechanism. The initial

subsidence of the Precordillera carbonate platform to

accommodate the upward transition from shallow-marine

platform carbonates to deep-water black shales is inter-

preted to be a result of extension and thinning of the crust

(Keller, 1999). The tectonic setting of the source of lithic

clasts in the Las Vacas and Trapiche conglomerates poses a

problem and has been described as ‘from the hinterland’

(Keller, 1999, p. 96) and as in a ‘rift-valley stage of

sedimentation’ (p. 107). If the Ordovician igneous-clast

conglomerates (Las Vacas and Trapiche Formations)

represent a ‘rift-valley stage of sedimentation,’ the detritus

must have come from erosional unroofing of basement

rocks on high horsts; however, the range of composition of


the conglomerates is more diverse than any known base-

ment rocks and includes very few, if any, clasts that are

similar to the known basement. The olistoliths do not

represent the lower part of the cover stratigraphy above the

basement. We conclude that Precordillera basement rocks

on intrabasinal horsts are not a viable source for the clasts in

the Las Vacas and Trapiche Formations, which instead

include a range of rock types similar to those in Famatina,

Sierra de Valle Fertil, and Sierra de Pie de Palo, indicating

an orogenic source along the eastern margin of the

Precordillera.

In the context of Ordovician rifting, the volcanic source

of the bentonites in the Precordillera stratigraphy is

interpreted to be related to crustal extension (Dickerson

and Keller 1998); however, on the basis of geochemical

characteristics, Huff et al. (1998) conclude that the

bentonites represent an arc in a convergent setting. A single

bentonitic bed in Llanvirn shales in the Marathon embay-

ment (Fig. 1) of southern Laurentia is interpreted to indicate

close proximity to the Precordillera at that time (Dickerson

and Keller, 1998; Keller, 1999); however, Huff et al. (1998)

conclude that the bed is not bentonite. Regardless, a single

bentonite bed in the Marathon stratigraphy does not

resemble the abundant (.170 beds, Huff et al., 1998)

bentonite beds of the Precordillera, and the bentonites are

geochemically incompatible with extension.

Previously, geochemical and isotopic age data from

granitoids in Famatina were interpreted to indicate no

involvement of the Precordillera in Ordovician arc plutons,

thereby favoring collision and accretion to Gondwana in

Silurian–Devonian time (Pankhurst and Rapela, 1998;

Rapela et al., 1998). The same research group subsequently

published a revision (Casquet et al., 2001) U–Pb zircon

ages from the Sierra de Pie de Palo clearly document

Ordovician metamorphism and indicate Ordovician col-

lision with western Gondwana. The Ordovician metamorph-

ism is now considered to represent part of the collision

orogen that included the Famatina volcanic arc (Casquet

et al., 2001), which extends along strike to Ordovician

plutons in the western Sierras Pampeanas (Quenardelle and

Ramos, 1999; Rapela et al., 1998). Keller (1999) did not

have the use of these most recent geochronologic results, but

the Ordovician metamorphism (Casquet et al., 2001) and

west-directed ductile shearing (Ramos et al., 1998), along

with both plutonic and bentonitic expressions of arc

volcanism, are not compatible with continental rifting.

Hirnantian glaciation has been an important indicator of

accretion of the Precordillera to Gondwana. Keller (1999)

suggests that the glacial diamictites of the Precordillera

might have been deposited from icebergs rather than

directly by continental glaciers.

Using faunal composition analyses from Benedetto et al.

(1995) (Fig. 2), Keller et al. (1998) and Keller (1999)

modify the interpretation to indicate separation of the

Precordillera from Laurentia during Arenig time and

maximum isolation of the Precordillera in Caradoc time.

However, the Arenig time of ‘separation’ is later than the

initial decrease of Laurentian forms, and the Caradoc time

of ‘isolation’ is later than the appearance of the first

Gondwanan forms in the Precordillera (as documented by

Benedetto et al., 1995, 1999). In contrast, Benedetto et al.

(1999) identify an isolation stage (Arenig–early Llanvirn),

pre-accretion (Llanvirn–Caradoc), and Gondwana (Ashgill

and later).

5. Ponon Trehue

At Ponon Trehue (Fig. 1), south of the main

Precordillera exposures, Precambrian basement and lower

Paleozoic cover strata are exposed in two north-striking

thrust blocks. In the eastern thrust block, a Middle

Ordovician clastic succession with laterally discontinuous

limestone (Lindero Formation, Llanvirn, Bordonaro et al.,

1996) is lithologically similar to the distal clastic wedge in

the Guandacol foreland basin (La Cantera Formation or

Las Plantas Formation, Fig. 3). Unlike the clastic wedge,

which overlies the Cambrian–Ordovician platform succes-

sion in the northeastern Precordillera, the Lindero For-

mation rests unconformably on basement rocks (Astini,

2002). At Ponon Trehue, the basal conglomerates contain

basement clasts, petrographically similar to the underlying

basement and unlike the predominantly igneous clasts of

the conglomerates of the northeastern Precordillera.

Complete truncation of the Precordillera carbonate-plat-

form succession is consistent with the erosion of a

relatively high intrabasinal horst block, on which structural

relief was sufficient to expose the entire platform succes-

sion to erosion.

In the western thrust block at Ponon Trehue, disoriented

large blocks of Ordovician (Tremadoc–Arenig) limestones

(Ponon Trehue Formation, Bordonaro et al., 1996) lie

directly above basement rocks, and the succession has been

interpreted to record onlap of the carbonate-platform

deposits onto a high basement block in the center of the

Precordillera microcontinent (Keller, 1999). Contrary to an

onlap unconformity, the disharmonic orientation of the

blocks (Ponon Trehue Formation) above the basement and a

tectonic breccia along the contact indicate tectonically

displaced blocks. The Ponon Trehue blocks are similar in

biostratigraphic age and composition to the carbonate

olistoliths in the synorogenic clastic wedge in the Guanda-

col foreland basin. However, the Ponon Trehue blocks rest

on the basement, whereas olistoliths in the Guandacol

foreland basin are within the clastic wedge, stratigraphically

above a complete carbonate-platform succession. The

composition of the tectonic breccia (a ‘tectonic carpet’

beneath the carbonate blocks) suggest shearing of

underlying basement gneissic granites and the arkosic

granite-clast basal conglomerate of the Lindero Formation.

The position of the carbonate blocks and the tectonic breccia

along the basal contact suggest thrusting of an internally


broken carbonate thrust sheet onto a structurally high

basement, that is confirmed by the nearby sub-Lindero

unconformity. If the sub-Lindero unconformity marks a

basement horst, the palinspastic location of the carbonate

thrust sheet must be in a graben where the platform cover

stratigraphy was not eroded.

6. Los Sombreros Formation, western margin of the

Precordillera terrane, and mafic igneous rocks

It is important to distinguish between the olistolith-

bearing conglomerates (Las Vacas and Trapiche For-

mations) that overlie the platform-carbonate succession of

the eastern Precordillera and the olistolith-bearing slope

deposits of the Los Sombreros Formation of the western

Precordillera. The Los Sombreros Formation (Figs. 3–5)

along the western edge of the Precordillera platform-

contains olistoliths of rocks from the platform, slope, and

basement in a matrix of Middle Ordovician deep-water

shales (Astini et al., 1995; Keller, 1999). The Los

Sombreros Formation contrasts with the succession in the

Guandacol foreland basin, where intrabasinal platform-

carbonate olistoliths are enclosed in conglomerates that are

stratigraphically above the platform-carbonate succession of

the eastern Precordillera.

A diverse array of olistoliths in the Los Sombreros

Formation (Figs. 3 and 4) includes Middle Cambrian–Lower

Ordovician platform-carbonate rocks, Lower–Middle Cam-

brian limestone-shale strata, and Ordovician shales, which

represent parts of the exposed stratigraphy of the Precordil-

lera. In addition, the Los Sombreros olistoliths include

quartz-pebble conglomerate; arkosic sandstones; and lithic-

clast conglomerate, which contains rounded clasts of granitic

gneiss, granite, and quartzite in a coarse sandstone matrix.

The lithology and texture of the conglomerate are consistent

with deposition in alluvial fans along fault scarps in an

extensional graben system on continental basement,

suggesting synrift graben fills deposits that underlie the

passive-margin platform-carbonate succession. One large

olistolith contains a succession of redbeds and carbonates

similar to part of the Lower Cambrian Cerro Totora

Formation synrift deposits (Astini and Vaccari, 1996).

Olistoliths of basement granites confirm that the source of

Los Sombreros detritus penetrated the complete cover

succession (Fig. 4). Of special importance, the Los

Sombreros also includes olistoliths of Middle Cambrian

deep-water limestones (Fig. 3) (maximum olistolith size

.1500 m by ,325 m, Keller, 1999) (Bordonaro and

Banchig, 1996; Palmer et al., 1996; Astini, 1998a), which

represent a deep-water slope environment of deposition.

The Los Sombreros Formation deep-water, mud-domi-

nated succession records deposition on the continental slope

and rise west of the Precordillera carbonate platform (Figs. 4

and 5). The western shelf edge of the Precordillera carbonate

platform, now concealed beneath Andean thrust sheets,

evidently marks the western rifted margin of Precordillera

continental crust. The position of the diverse olistoliths in

the muddy slope deposits reflects slope-collapse faults and/

or submarine canyons that fragmented the outer edge of the

carbonate platform along with the underlying synrift

deposits and basement (Fig. 4). The olistoliths of Middle

Cambrian slope deposits in a matrix of Middle Ordovician

slope deposits document a persistent location of the passive-

margin shelf edge and slope along the western Precordillera

since at least Middle Cambrian time. We interpret the

Middle Cambrian slope deposits to be the record of

the western rifted margin of the Precordillera terrane and

the conjugate margin of the Ouachita rifted margin of the

Texas promontory of Laurentia (Fig. 2). Establishment of the

western slope of the Precordillera terrane no later than

Middle Cambrian is compatible with rifting from Laurentia

during the Early Cambrian. In contrast, Keller (1999)

interprets the source of the Los Sombreros olistoliths to be

a fault scarp of 2100þ m relief that was formed during

Middle Ordovician rifting of the Precordillera from

Laurentia. Initiation of the scarp and adjacent slope during

Middle Ordovician time, however, does not account for the

Middle Cambrian deep-water facies recorded in the

olistoliths.

Pillow basalts of Caradoc age in deep-water clastic facies

(Alcaparrosa Formation, Fig. 3) of the western Precordillera

indicate extension and rifting (Astini et al., 1995). The

pillow basalts have been interpreted to record Ordovician

rifting of the Precordillera from Laurentia (Keller, 1999).

Alternatively, the basalts and enclosing sedimentary

succession are west of the western shelf margin of

Precordillera, as defined by the Los Sombreros (Middle

Ordovician) slope, and the basalts may not be directly

related to extension within the Precordillera platform. Other

interpretations suggest that the basalts formed during initial

convergence at a plate margin, possibly Chilenia, distant

from the Precordillera during plate reorganization following

collision of the Precordillera with Gondwana (e.g. Davis

et al., 2000) and/or along a suture between Chilenia and the

Precordillera (Ramos et al., 2000).

7. Silurian–Devonian accretion of Chilenia

Silurian–Devonian deformation of the margins of the

Precordillera terrane is interpreted to reflect compression in

response to the accretion of Chilenia to the western outboard

side of the Precordillera terrane (Ramos et al., 1986; Astini

et al., 1995). A Silurian melange (Fig. 5), including

carbonate olistoliths, documents deformation along the

eastern part of the Precordillera terrane (Ramos et al., 1986;

von Gosen et al., 1995; Astini, 1996). Relatively thick

Silurian and Devonian clastic successions in the eastern

Precordillera are consistent with tectonic reactivation of the

eastern suture of the Precordillera terrane as a result of

the accretion of Chilenia (Astini, 1996). A regional


unconformity at the base of the Silurian clastic succession

truncates the Ordovician clastic succession across the broad

Talacasto–Tambolar arch, and Silurian strata directly

overlie the San Juan limestone across most of the central

Precordillera (Astini et al., 1995; Astini and Maretto, 1996).

8. Conclusions

Several types of support an interpretation of accretion of

the Precordillera terrane to Gondwana during the Middle–

Late Ordovician. Comprehensive data document rifting of

the Precordillera from Laurentia during Cambrian time.

Independent drift of the Precordillera in the Iapetus Ocean

during the Late Cambrian and Early Ordovician is consistent

with the faunal succession and passive-margin stratigraphy

of the Precordillera. Direct evidence of Ordovician accretion

is supplied from foreland-basin stratigraphy and the tectonic

history of the Famatina arc. An integrated interpretation

includes (1) initial flexural downbending and diachronous

drowning of the Precordillera passive-margin platform as the

microcontinent approached the subduction zone (late

Arenig), (2) flexural extension in the foreland basin and

remanent carbonate deposition along the peripheral bulge,

(3) progradation of extrabasinal synorogenic clastic sedi-

ment into the foreland basin (late Arenig–early Ashgill), (4)

arc magmatism coeval with initial subsidence (499–

468 Ma), (5) dispersal of volcanic ash from the Famatina

arc (Arenig–Llanvirn), (6) termination of arc magmatism

approximately coincident with collision, and (7) defor-

mation and metamorphism (,460 Ma) of both the leading

edge of the Precordillera microcontinent and the Famatina

arc complex. The spread of Hirnantian glacial deposits

confirms that the Precordillera was part of Gondwana before

the end of Ordovician time. While recognizing that differing

that different interpretations have been based on similar

observations, we conclude that all of the data in context

support an interpretation of Ordovician collision of the

Precordillera terrane with the Famatina arc along western

Gondwana.

Acknowledgements

This research was supported by a grant from the US

National Science Foundation (EAR-9706735) to W.A.

Thomas. R.A. Astini acknowledges research support from

Consejo Nacional de Investigaciones Cientıficas y Tecnicas

and from Fundacion Antorchas (Argentina). German

Bayona assisted in the design of the illustrations. This is a

contribution to International Geological Correlation Pro-

gram project 436.

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Ordovician accretion of the Argentine Precordillera terrane to Gondwana: a review

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