ABSTRACT

Magmatic activity tends to concentrate at tectonic plate bound-aries. At rapidly convergent margins, such as the Andes, intense mag-matic activity is coeval with strong tectonic shortening, and some vol-canoes and magmatic intrusions have been emplaced near activecompressional structures, usually major thrust faults. In order tounderstand the links between magmatic systems and compressionaldeformation structures in the upper crust, we describe the structureof an active volcano (Tromen, Argentina) and an exhumed intrusion(Boulder Batholith, U.S.A.) emplaced during compressional deforma-tion. In those examples, magmatic systems and thrust faults exhibitgeometrical and chronological relationships. We also present resultsof experimental modelling of magma emplacement during compres-sion. The comparison between geological examples and experimentsshow close similarities. That suggests that the presence of magmainfluences the deformation pattern in the brittle crust. The influenceof deep magma bodies is also to be explored at the scale of the wholecrust during the development of active margins.

KEY WORDS: magma-controlled tectonics, compressionaltectonics, Tromen volcano, experimental modelling.

RIASSUNTO

Tettonica controllata dai magmi in contesti collisionali:approfondimenti da esempi geologici e modelli sperimentali.

L’attività magmatica tende a concentrarsi ai margini delle plac-che litosferiche. Lungo i margini soggetti a rapida convergenza,come le Ande, l’intensa attività magmatica è coeva con elevato rac-corciamento tettonico, ed alcuni vulcani e intrusioni magmatiche sisono messi in posto in corrispondenza di strutture compressionaliattive, solitamente i sovrascorrimenti principali. Per comprendere ilegami tra i sistemi magmatici e le strutture di deformazione com-pressiva nella crosta superiore, descriviamo qui le strutture di unvulcano attivo (Tromen, Argentina) e di un’intrusione esumata(Boulder Batholith, U.S.A.) messi in posto durante deformazionecompressiva. In questi esempi, sistemi magmatici e sovrascorrimentimostrano relazioni geometriche e cronologiche. Presentiamo anche irisultati di modelli sperimentali di messa in posto di magmi in regi-me compressivo. Il confronto tra esempi geologici ed esperimentianalogici dimostra strette similitudini. Ciò suggerisce che la presen-za di magma influenzi la configurazione della deformazione nellacrosta fragile. L’infuenza di corpi magmatici profondi deve quindiessere esplorata alla scala dell’intera crosta durante l’evoluzione deimargini attivi.

TERMINI CHIAVE: tettonica controllata dai magmi, tettonicacompressiva, vulcano Tromen, modellazione sperimentale.

INTRODUCTION

Magmatic activity mostly occurs at plate boundaries,where tectonic deformation also concentrates. Becausemagmatic bodies and their country rock have very con-trasting rheological properties, one might expect deforma-tion to be influenced by the presence of magmatic bodiesat depth (e.g. BUROV et alii, 2003). Although this problemhas been addressed in the lower crust (e.g. HOLLISTER &CRAWFORD, 1986; DAVIDSON et alii, 1992; BROWN & SOLAR,1998; BROWN & SOLAR, 1999; ROSENBERG & HANDY, 2000;BARRAUD et alii, 2001), very little is known about theprocesses of such an interaction between magmatism andcountry rock deformation in the brittle upper crust. Whichone comes first? Which one controls the other? Most ofprevious research has focused on deformation controllingmagmatism (e.g. HUBBERT & WILLIS, 1957; MARRETT &EMERMAN, 1992). Here we also attempt to consider theopposite mechanism, i.e. magma-controlled deformation.

At rapidly convergent margins, such as the Andes, onemight expect that horizontal compression prevents therise of magma through the brittle upper crust (HUBBERT

& WILLIS, 1957; HAMILTON, 1995). Nevertheless, volcanicactivity is also common in compressional environments.Such a contradiction highlights the lack of understandingof the mechanical interplay between magmatism anddeformation. We therefore address the processes ofmagma-controlled deformation in compressional settings.We first describe two geological examples of magmaticcomplexes emplaced in such settings, Tromen volcano,Neuquén basin, Argentina, and the Boulder batholith,Montana, USA. Subsequently, we present results of exper-imental modelling of magma emplacement during short-ening. Thus, we show that magma can transport in ashortening crust, and that magma-controlled deformationprocesses can play an important role in the structuraldevelopment of the upper crust.

GEOLOGICAL OBSERVATIONS

It is well known that magmatic activity is common atconvergent margins. However, only a few studies haveaddressed the association between magmatic complexesand thrust faults (e.g. HOLLISTER & CRAWFORD, 1986;PARRY et alii, 1997; TIBALDI, 2005). Noticeable examplesare Tromen volcano, Neuquén province, Argentina (GAL-LAND et alii, 2007b), and the Boulder batholith, Montana,USA (KALAKAY et alii, 2001). Tromen is a Pleistocene-Holocene back-arc volcano, located in the northern seg-ment of the Southern Andes (fig. 1). It lies in a thick-

Boll.Soc.Geol.It. (Ital.J.Geosci.), Vol. 127, No. 2 (2008), pp. 205-208, 3 figs.

Magma-controlled tectonics in compressional settings:insights from geological examples and experimental modelling

OLIVIER GALLAND (*), (**), PETER R. COBBOLD (*), ERWAN HALLOT (*) & JEAN DE BREMOND D’ARS (*)

(*) Géosciences-Rennes (UMR 6118), CNRS et Université deRennes 1, Campus de Beaulieu - 35042 Rennes Cedex, France.

(**) Physics of Geological Processes (PGP), Universitet i Oslo,Physics building, third floor, Sem Selands vei, 24 - NO-0316 Oslo,Norway (Fax: +47 22 85 51 01; E-mail: olivier.galland@fys.uio.no).

Queste bozze, corrette e accompagnate dall’al-legato preventivo firmato e dal buono d’ordine,debbono essere restituite immediatamente allaSegreteria della Società Geologica Italianac/o Dipartimento di Scienze della TerraPiazzale Aldo Moro, 5 – 00185 ROMA

206 O. GALLAND ET ALII

Fig. 1 - Two geological examples of magmatic complexes emplaced in compressional tectonic setting: a) Simplified geological map of Tromenvolcano, Neuquén basin, Argentina. Tromen is Andean alkaline back-arc Quaternary volcano, located on top of arcuate east-verging thrust. Itbuilt up during thrusting deformation. Modified after GALLAND et alii (2007b); b) Simplified geological map of Boulder batholith, Montana,USA. Boulder batholith was emplaced into Sevier fold-and-thrust belt, during thrusting deformation. Locally around Boulder batholith,thrust front exhibits strongly arcuate trace (Helena salient). Modified after KALAKAY et alii (2001). Structures of both Tromen volcano andBoulder batholith suggest control of magmatic complexes on deformation.– Due esempi geologici di complessi magmatici messi in posto in contesto tettonico collisionale: a) Carta geologica semplificata del vulcanoTromen, bacino di Neuquén, Argentina. Tromen è un vulcano quaternario andino alcalino di retro-arco, collocato sulla sommità di unsovrascorrimento arcuato vergente a Est. Si è sviluppato durante la deformazione che ha prodotto il sovrascorrimento. Modificato da GALLAND etalii (2007b); b) Carta geologica semplificata del batolite di Boulder, Montana, USA. Il batolite di Boulder si è messo in posto nella catena a pieghee sovrascorrimenti di Sevier, durante la deformazione che ha prodotto i sovrascorrimenti. Localmente attorno a questo batolite il fronte disovrascorrimento mostra un contorno molto arcuato (Helena salient). Modificato da KALAKAY et alii (2001). Le strutture del vulcano Tromen edel batolite di Boulder suggeriscono un controllo dei complessi magmatici sulla deformazione.

Fig. 3 - a) Photograph of map view of typical model without injection. Piston (left) deformed model made of compacted silica powder.Straight thrusts accommodated shortening. Straight line locates cross section; b) Photograph and corresponding schematic drawing of crosssection. Offset of horizontal markers locates faults (thrusts). Straight thrusts form at base of piston; c) Photograph of map view of typicalmodel with injection. Straight and arcuate thrusts accommodated shortening. Poorly deformed plateau laid between straight and arcuatethrusts. Injected molten oil erupted along trace of arcuate thrust; d) Photograph and corresponding schematic drawing of cross section.Intruding oil (gray) forms basal sill. Straight thrusts form at base of piston. Arcuate thrust nucleate at leading edge of sill. Plateau lies abovesill.– a) Immagine fotografica dall’alto di un tipico modello senza iniezione. A sinistra un modello deformato a pistone, composto di silice in polvere.I sovrascorrimenti rettilinei hanno accomodato il raccorciamento. La linea retta individua la sezione verticale; b) Fotografia e corrispondentedisegno schematico della sezione verticale. La dislocazione dei riferimenti orizzontali individua le faglie (sovrascorrimenti). Alla base del pistonesi formano sovrascorrimenti rettilinei; c) Immagine fotografica dall’alto di un tipico modello con iniezione. I sovrascorrimenti rettilieni e arcuatihanno accomodato il raccorciamento. Tra i sovrascorrimenti rettiliei e arcuati si trovano plateau poco deformati. Il liquido iniettato è eruttatolungo le tracce del sovrascorrimento arcuato; d) Fotografia e disegno schematico della sezione verticale. L’olio che s’intrude (grigio) forma sillbasali. Alla base del pistone si formano sovrascorrimenti rettilinei. Alla terminazione frontale del sill nucleano sovrascorrimenti arcuati. Il plateausi trova al di sopra del sill.

skinned fold-and-thrust belt in the western margin of theNeuquén basin (COBBOLD & ROSSELLO, 2003). Its vol-canic products are unconformable upon Mesozoic strataof the basin. It built up above the hanging-wall of a majoreastward-verging thrust fault (fig. 1). The Boulderbatholith is a Cretaceous intrusive complex, east of themajor Idaho-Bitterroot batholith (KALAKAY et alii, 2001).It was emplaced in the upper crust, within the Sevierfold-and-thrust belt (fig. 1). It has a flat-lying tabularshape, and an estimated thickness between 5 and 12 km.

Both Tromen volcano and the Boulder batholith haveclose chronological and structural relationships with theirsubstrata (fig. 1):

1) They lie close to major thrust faults.2) Their emplacement was coeval with thrusting.3) The thrust fronts have strongly arcuate shapes

around the volcano or batholith.

Geological observations on Tromen volcano and theBoulder batholith show close relationships betweenthrusting and magmatism (KALAKAY et alii, 2001; LAGE-SON et alii, 2001; GALLAND et alii, 2007b). They show thatmagma can ascend and be emplaced in compression andthat thrust faults are likely to control magma transport.In addition, the arcuate thrusts around the volcano orbatholith may result from the influence of magma upon

the deformation pattern. The following experimentalresults illustrate how magmatic activity may control com-pressional deformation.

MAGMA-CONTROLLED TECTONICS IN COMPRESSIONAL SETTINGS 207

Fig. 2 - Schematic drawing of experimental setup (see text for expla-nations).– Disegno schematico dell’impianto sperimentale (vedi il testo per lespiegazioni).

Fig. 3.

EXPERIMENTAL MODELLING

In order to study the mechanical interactions betweencompressional deformation and magmatic intrusion, weresorted to laboratory experiments, in which an analogueof the brittle crust shortened, while melt was intruding(fig. 2). We used (1) a cohesive fine-grained silica powderto represent the brittle crust, and (2) a molten low-viscos-ity vegetable oil to represent magma (GALLAND et alii,2006). In the experiments, horizontal shortening andinjection were coeval but independent. Shorteningresulted in thrust faults, while overpressured oil formedtabular intrusions.

In those experiments where there was no injection,shortening resulted in a classical thrust wedge, in whichthrusts had straight traces and were 5-6 cm apart (fig. 3;GALLAND et alii, 2003; GALLAND et alii, 2007a); the apicalangle of the wedge was about 15º. In the other experiments,where there was injection, oil formed a basal sill, and thestructure of the wedge was very different. Once in place, thesill lubricated the base of the model, so that arcuate thrustsformed at the leading edge of the sill (fig. 3). The distancebetween thrusts increased, defining a non-deformedplateau. The apical angle of the wedge was smaller than 10º.Uplift of the plateau promoted further intrusion of oil atdepth. In general, the pattern of deformation and intrusiondepended on the kinematic ratio R between rates of short-ening and injection (GALLAND et alii, 2007a). The lengthsof the basal sill and plateau increased with decreasing R.Thus, from our experiments we infer that a small amount ofmagma in a deforming brittle crust strongly modifiesthe deformation pattern. Intrusions control the formationof arcuate thrusts and slightly deformed plateaus by lubri-cating their bases.

DISCUSSION AND CONCLUSIONS

There are close similarities between Tromen or theBoulder batholith and our experimental results. Accord-ing to the geological observations, melts rose and wasemplaced during thrusting. In addition, thrusts have simi-lar arcuate shapes around the magmatic complexes,which are in the hanging walls of the arcuate thrusts.Thus we infer that arcuate structures around Tromen vol-cano and the Boulder batholith have resulted from theinteraction between compressional deformation and non-solidified magma. Similar relationships between thrustsand active volcanoes exist at Guagua Pichincha, Ecuador(LEGRAND et alii, 2002), Socompa, Chile (VAN WYK DE

VRIES et alii, 2001), and Taapaca, Chile (CLAVERO et alii,2004). We therefore suspect that similar processes were atwork in those volcanoes.

Our experimental results show that magmatic sys-tems submitted to compression can control the formationand the shape of thrust faults in a upper brittle crust.Such magma-controlled processes are likely to be of first-order importance in the development of compressionalactive margins, such as the Andes, and possibly beyondthe scale of the upper crust. At large scale, the potentialmechanical impact of deep magmatic intrusions shouldbe explored in models of active margins.

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Received 30 October 2007; revised version accepted 28 February 2008