647Revista de la Asociación Geológica Argentina 61 (4): 647-657 (2006)

INTRODUCTION

On 17 December 1949, two Ms 7.8 earth-quakes (Mercali Intensity VIII) struck theIsland of Tierra del Fuego and the sou-thern end of South America within a timespan of nine hours. Damage and casualtieswere slight due to the sparse population andlack of urban development existing in the

region at that time. Based on the surfacewave magnitudes of these events, they areamong the largest instrumentally recordedearthquakes with onshore surface rupturein South America. The ruptures reported inTierra del Fuego, are located along the Ma-gallanes-Fagnano fault. This structure re-presents the on-shore transform boundarybetween the South America and Scotia pla-

tes (Fig. 1), where sinistral-normal slip pre-vails, as indicated by focal mechanisms,seismic reflection data, GPS and field ob-servations (Dalziel et al. 1975, Bruhn et al.1976, Dalziel 1989, Pelayo and Weins 1989,Klepeis 1994, Lodolo et al. 2002, 2003,Smalley et al. 2003). Morphologies related toHolocene faulting on this fault system havealso been described on the Chilean side of

PALEOSEISMIC OBSERVATIONS OF AN ONSHORE TRANSFORM BOUNDARY: THE MAGALLANES-FAGNANOFAULT, TIERRA DEL FUEGO, ARGENTINA

C. H. COSTA1, R. SMALLEY2, Jr., D. P. SCHWARTZ3, H. D. STENNER3, M. ELLIS2, 4, E. A. AHUMADA1, 5 and M.S. VELASCO 2, 6

¹ Departamento de Geología, Universidad Nacional de San Luis, Chacabuco 917, 5700 San Luis, Argentina.E-mail: costa@unsl.edu.ar, eahumada@unsl.edu.ar2 Center for Earthquake Research and Information, The University of Memphis, 3892 Central Ave, Suite 1, Memphis, TN, 38152,USA. E-mail: smalley@ceri.memphis.edu3 United States Geological Survey, Earthquake Hazards Team, 345 Middlefield Rd, Menlo Park, CA 94025, USA.E-mail: dschwartz@usgs.gov, hstenner@usgs.gov4 now at National Science Foundation, Email: mellis@nsf.gov5 CONICET6 now at University of Arizona

RESUMEN: Observaciones paleosismológicas en un margen transformante en el continente: La falla Magallanes-Fagnano, Tierra del Fuego, Argentina.Se presenta información sobre las evidencias geomorfológicas y paleosísmicas observadas al este del lago Fagnano, relacionadas con dos terremo-tos de Ms 7,8, que ocurrieron en Tierra del Fuego el 17 de diciembre de 1949. Se efectuaron relevamientos en las escarpas observadas en los alre-dedores de Tolhuin y Estancia La Correntina-Río San Pablo. En este último sitio, se excavó una trinchera en un trazo secundario de la fallaMagallanes-Fagnano con el propósito de analizar el registro paleosísmico de esta estructura. Con el apoyo de dataciones radiocarbónicas se reco-noció la estratigrafía correspondiente a los últimos 9 ka, interpretándose por lo menos dos eventos sísmicos con rupturas superficiales previos alsismo de 1949 en este trazo de la falla. Se reconocieron escarpas asociadas de hasta 11 m de altura en depósitos atribuibles al Pleistoceno superior-Holoceno (?), pero la componente vertical observada o reportada de los sismos de 1949 nunca fue mayor de 1 m. Ello sugiere la participación ensu génesis de varios eventos previos durante el Cuaternario. A lo largo del sector investigado, la componente horizontal de la ruptura de 1949 noha sido mayor de 4 m y probablemente menor de 0,4 m, lo cual puede ser consistente con la localización de este sector de la estructura en unazona transtensiva, o en el extremo de una zona de ruptura trancurrente.

Palabras clave: Falla Magallanes-Fagnano, Paleosismología, Tierra del Fuego, Terremotos 1949.

ABSTRACT

We present preliminary information on the geomorphologic features and paleoseismic record associated with the ruptures of two Ms 7.8earthquakes that struck Tierra del Fuego and the southernmost continental margin of South America on December 17, 1949. The fault scarpwas surveyed in several places east of Lago Fagnano and a trench across a secondary fault trace of the Magallanes-Fagnano fault `was exca-vated at the Río San Pablo. The observed deformation in a 9 kyr-old peat bog sequence suggests evidence for two, and possibly three pre-1949 paleoearthquakes is preserved in the stratigraphy. The scarp reaches heights up to 11 m in late Pleistocene-Holocene(?) deposits, but thevertical component of the 1949 events was always less than ~1 m. This observation also argues for the occurrence of previous events duringthe Quaternary. Along the part of the fault we investigated east of Lago Fagnano, the horizontal component of the 1949 rupture does notexceed 4 m and is likely lower than 0.4 m, which is consistent with the kinematics of a local releasing bend, or at the end of a strike-sliprupture zone.

Keywords: Magallanes-Fagnano fault, Paleoseismology, Tierra del Fuego, 1949 earthquakes.

0004-4822/02 $00.00 + $00.50 C 2006 Revista de la Asociación Geológica Argentina

648 C. H. COSTA, R . SMALLEY, JR . , D. P. SCHWARTZ, H. D. STENNER, M. ELLIS, E . A . AHUMADA AND M.S. VELASCO

the Island of Tierra del Fuego. (Winslow1982, Winslow and Prieto 1991).Although coseismic surface faulting wasmentioned in many papers (Dalziel 1989,Klepeis 1994, Winslow 1982, Winslow andPrieto 1991, Lodolo et al. 2002) and repor-ted by locals (J. Caibul, A. Goodall, R. Su-therland, pers. comm.) little is known onthe characteristics of the faulting or thepaleoseismic record of this plate-boundaryfault. In this paper we report on initial field

observations and results east of LagoFagnano, where evidence of the 1949 rup-tures is preserved in the morphology andQuaternary deposits.

TECTONIC AND GEOLO-GIC SETTING

The study zone is located in the northernboundary of the Scotia plate (Fig 1). Thisplate is a principal part of the Scotia Arc,

which is principally composed of youngoceanic crust. The Scotia Arc formed overthe past 30 myr as a response to changes inthe relative motion between the South A-merica and Antarctic plates (Barker et al.1991).The Scotia Plate is bounded by transformfaults on the north and south that definethe mostly submarine North Scotia Ridge(Scotia-South America plate boundary) andSouth Scotia Ridge (Scotia-Antarctic plate

Figure 1: Regional tectonic setting of the Tierra del Fuego Island with the location of the main plates. NSR: North Scotia Ridge; SSR: South ScotiaRidge; SFZ: Schackleton Fracture Zone, TJ: Triple Junction. The red stars in the inset below point out the epicenters of the 1949 events according toForsyth (1975), Winslow (1981) and Pelayo and Wiens (1989), whereas the Magallanes-Fagnano fault trace is indicated in red. Location index: 1. SanJulián, 2. Río Gallegos, 3. Magellan Strait, 4. Punta Arenas, 5. Río Grande, 6. Seno Almirantazgo, 7. Lago Deseado, 8. Lago Fagnano, 9. LasifashajValley, 10. Ushuaia, 11. Beagle Channel, 12. Río Irigoyen-Cape Leticia area, 13. Malvinas Islands.

649Paleoseismic observations of an onshore transform boundary..

boundary) respectively (British AntarcticSurvey 1985) (Fig. 1). The North and SouthScotia Ridges are the sites of a small num-ber of earthquakes with east-west orientednodal planes and left-lateral fault-plane so-lutions (Forsyth 1975, Pelayo and Wiens1989, Thomas et al. 2003).The direction of the current relative mo-tion between the South America and An-tarctica plates is approximately parallel tothe northern and southern boundaries ofthe Scotia plate. Relative plate velocity va-ries along the plate margins, and occurs at arate of 20-24 mm.yr-1 along the portion ofthe Antarctic-South America plate boun-dary where the Antarctic plate subductsbeneath the western margin of the SouthAmerica plate between the Chile rise (46°30'S) and the Magellan Strait (52°S) (Chase1978, Minster and Jordan 1978, Dalziel1989). The South America and Antarcticplates also interact along the SchackletonFracture Zone (SFZ), a small strike-slip in-terplate boundary (Fig 1).The island of Tierra del Fuego can be divi-ded in two structural domains (Klepeis1994; Kraemer 2003; Ghiglione and Cris-tallini in press): A northern, thin-skinneddomain and a southern basement-domainlocated in the Scotia plate side. It has beenproposed that the development of theFueguian Andes is the result from thrustingepisodes from the late Cretaceous to theOligocene (Caminos et al. 1981, Ghiglioneet al. 2002, Kraemer 2003, Ghiglione andRamos, 2005).Lithologic boundaries and older structuresproduced by previous episodes of subduc-tion-related compressional tectonics nowstrike parallel to the active transform plateboundary, with many of these older faultsbeing reactivated as strike-slip faults sinceOligocene times (Winslow 1981, Klepeis1994).

THE MAGALLANES FAG-NANO FAULT SYSTEM

A western continuation of the North ScotiaRidge traverses the continental crust of theisland of Tierra del Fuego as the Magalla-nes-Fagnano fault system (Winslow 1982,Dalziel 1989, Klepeis 1994, Olivero andMalumián 1999, Olivero and Martinioni

2001, Lodolo et al. 2002, 2003 among manyothers). In addition, several major sub-para-llel faults, may accommodate part of theplate motion, but amount of partitioning ofmotion on these faults is unknown. As theMagallanes-Fagnano fault continues wes-tward across Tierra del Fuego it returns toa subaqueous setting in Lago Fagnano, Se-no Almirantazgo and the Straits of Mage-llan, ending as one limb of a complex, dif-fuse, unstable trench-trench-transform tri-ple junction between the Antarctic, SouthAmerica and Scotia plates (Forsythe 1975,Cunningham 1993).The Magallanes-Fagnano fault can be tra-ced along a distance ranging from 400 to800 km according to different sources(Winslow 1982, Dalziel 1989, Klepeis 1994,Lodolo et al. 2002) and many authors reportevidence for left-lateral offset along itstrace. Lack of piercing points has hinderedestimation of strike-slip displacements anddisplacement rates. According to Winslow(1982) the Magallanes-Fagnano fault appa-rently offsets the western margin of thePatagonian batholith by 80 km Klepeis(1994), however, claims that cumulative si-nistral separation in the Mount Hope area isonly 2-25 km and has shown that theCenozoic strike-slip tectonics are more dis-tributed in nature, involving other structu-res such as the Deseado Fault Zone.According to Klepeis (1994) the kinematicconditions that led to the establishment ofMagallanes-Fagnano fault as a major inter-plate structure, started with or even beforethe Oligocene birth of the Scotia plate.Wrench deformation has dominated eversince, and is structurally and temporallysuperimposed on the older deformation(Caminos et al. 1981, Ghiglione et al. 2002,Lodolo et al. 2002, 2003, Galeazzi 1996). Indetail however, since the plate boundarydoes not follow a perfect small circle, thereare segments that also exhibit extensionalor compressional tectonics (Lodolo et al.,2003). Transpres-sional convergence hasbeen proposed for the central eastern partof the North Scotia Ridge (Pelayo andWiens 1989, Bry et al. 2004) although side-scan sonar studies south of the MalvinasIslands indicate current motion is pure stri-ke-slip and deformation associated withcurrent motion is superimposed on eviden-

ce for earlier convergence (Cunningham etal. 1998).Lodolo et al. (2003), Yagupsky et al. (2004)and Tassone et al. (2005) have seismicallyimaged a half-graben like structure offsho-re Tierra del Fuego interpreting the Ma-gallanes-Fagnano fault zone to be compo-sed by several segments in an echelon pat-tern along which pull-apart depocentershave formed. On the Atlantic coast the 5km-wide Irigoyen pull-apart basin is empla-ced along the plate boundary (Ghiglioneand Ramos 2005). Furthermore, Lodolo etal. (2002) interpret the 102 x 2-11 km Fa-gnano depression that hosts Lago Fagnanoas a pull-apart basin. West of Lago Fagna-no, the plate boundary turns northwestthrough Seno Almirantazgo and the Straitsof Magellan in a restraining bend geometry,although geological studies have proposedboth transtensional and transpressionalinterpretations for development of thestructures in this region (Klepeis 1994).The location of the Euler pole for theScotia Plate is poorly constrained by sprea-ding rates, transform azimuths and earth-quake slip vectors and its magnitude isroughly determined by closure (Pelayo andWiens 1989, Thomas et al. 2003). The Eulerpole predicts a left-lateral slip rate acrossthe Magallanes Fagnano fault of ~5mm.yr-1 , which is in agreement with poorlyconstrained geologic estimates based onoffset of geologic units (Olivero andMartinioni 2001). More recently, GPS mea-surements across Tierra del Fuego detect6.6 ± 1.3 mm.yr-1 of motion across theMagallanes-Fagnano fault (Smalley et al.2003). The GPS results also suggest thatthe plate boundary may be a wider zonewith plate motion partitioned between theMagalla-nes-Fagnano fault system and aregion of distributed, but much slower,deformation to the north, similar to the SanAndreas-Basin and Range distributed plateboundary in North America (Smalley et al.2003).Before and during the last glacial maximum,glaciers carved out many of the presumablyweak fault zones leaving long deep valleys(Rabassa and Clapperton 1990, Rabassa etal. 2000). Many of these valleys are nowdrowned fiords, lakes or bays. Determiningif the valleys contain active faults, if such

650 C. H. COSTA, R . SMALLEY, JR . , D. P. SCHWARTZ, H. D. STENNER, M. ELLIS, E . A . AHUMADA AND M.S. VELASCO

faults are locked or creeping, the amount ofoffset, and the relative contribution to theirformation of transtensional tectonics ver-sus glacial carving is therefore difficult.Where the traces of the faults outcrop sub-aerially, the surface is generally covered witheither very young glacial deposits or peat,further limiting geologically addressing thequestion of the rate of motion.In spite of these difficulties, geomorpholo-gic evidence of active deformation associa-ted with earthquakes along the fault hasbeen observed (Olivero and Malumián1999, Schwartz et al. 2001, 2002, Oliveroand Martinioni 2001, Lodolo et al. 2002,2003) and no evidence has been reportedfor fault creep. Olivero et al. (1995) estima-ted that a minimum displacement of 30 kmhas occurred during the past 30 myr. Anumber of geomorphic features; such asdisplacement and rotation of tertiary foldaxes, offset streams, truncated meanders,sag ponds, linear truncation of vegetation(Winslow 1982); fresh fault scarps, andlandslides (Dalziel 1989), have been cited asevidence of neotectonic movements mainlyalong the fault trace in Tierra del Fuego.Winslow and Prieto (1991) also noted thatthe 1949 earthquakes caused major landsli-des, uplifting and subsidence, with drownedforests, raised beaches and scarps in thepeninsula Brunswick area (Chile). Theseauthors have indicated that Holocene raisedbeaches suggest uplift rates of 1-5 mm.yr-1,although the rates vary abruptly across faultboundaries.On the Argentinean side of Tierra delFuego the Magallanes-Fagnano fault can betraced through fault-related morphologies

almost continuously from Lago Fagnanoeastward all the way to the Atlantic coastthrough the Irigoyen river valley (Ghiglioneet al. 2002, Lodolo et al. 2003) (Fig 1).Although homogeneous at a regional scale,this structure is composed by several para-llel-subparallel sections, particularly in itseastern half.The geology of the Lago Fagnano area isdominated by Quaternary glacier-relateddeposits underlain by Mesozoic and Ce-nozoic rocks. The lake occupies a glacialvalley, which has been recurrently invadedduring the Pleistocene from the west by gla-ciers descending from the ice-capped Dar-win Cordillera (Bujalesky et al. 1997,Rabassa et al. 2000).

THE SEISMIC EVENTS OF17 DECEMBER 1949

The earliest report of seismic activity in Ti-erra del Fuego was made by Bridges (1879)who reported an earthquake, that was feltstrongly in Ushuaia on 2 February 1879.Lomnitz (1970) estimated the event to havehad a magnitude of Ms 7.0-7.5. A morerecent earthquake of Ms 7.0 was re-cordedoffshore to the east on 15 June 1970 andbody-wave inversion for the focal mecha-nism indicates left-lateral slip on a sub ver-tical east-west oriented plane (Pelayo andWiens 1989). Recorded seismicity since1930 is quite low (most events < M 3.5) andshallow as revealed by recording stationsinstalled in the middle nineties (Vuan et al.1999), with infrequent major (M > 7.0)earthquakes (Forsyth 1975, Winslow 1982,Pelayo and Wiens 1989).

The first Ms 7.8 event of 17 December1949 ocurred at 03:58 local time as reportedby the Servicio Meteorológico Nacionaland the Observatorio de La Plata in Ar-gentina and was most strongly felt in theArgentinean territory in eastern Tierra delFuego where the collapse of a ranch buil-ding killed a policeman. In Ushuaia, thewharf collapsed and damage of varying se-verity was reported for many buildings.After many aftershocks, a second event ofsimilar size ocurred at 12:12. The informa-tion reported in newspapers suggests thatthe second event was farther to the west asit was felt with higher intensity than the firstevent in Punta Arenas (VIII SMM) than inUshuaia (see Fig. 1). In Punta Arenas, manyhouses collapsed, fissures and cracks wereobserved in most construction and threepeople were killed by the second event. Anumber of witnesses reported strong seawaves pushed many boats and small shipstoward the beach. These earthquakes werealso strongly felt in Rio Gallegos, 350 km tothe north, and moderate intensities werereported in San Julian, almost 600 kmnorth (see Fig. 1). A significant number ofaftershocks were felt throught Tierra delFuego for several months after the events.The epicenters for these events are verypoorly constrained with reported locationsin area of the Beagle channel, or north ofLago Fagnano and depths of 10 km(Forsyth 1975, Pelayo and Wiens 1989) (Fig.1), which although not very well controlledare reasonable for a transform boundaryevent.Preliminary modeling of the focal mecha-nisms of these earthquakes indicate a stri-

Figure 2: Landsat TM image ofthe area east of Fagnano Lago,showing the location of scarps atthe Lago fagnano area (A), thetrench site (at Río San Pablo) andfences at San Pablo and Olivasites. The Magallanes-Fagnanomain trace is highlighted by recti-linear features and scarps contro-lling main drainage lines. Thewhite rectangle corresponds tothe area showed in figure 3.

651Paleoseismic observations of an onshore transform boundary...

ke-slip regime for both of them (P. Al-vara-do, pers. comm.)We interviewed a number of eyewitnessesto the earthquake who are still living in thearea. Mr. Jacinto Caibul, who was in theHosterÍa Kaiken area on the eastern shore-line of Lago Fagnano, reported that thefirst event formed an ~1m high, east stri-king scarp that ruptured the road beingbuilt along the berm on the eastern shore ofthe Lago. Mr. Caibul cannot remember anylateral offset across the road. The forma-tion of the scarp was followed by a largedust cloud and retreat of the Lake from theshore, with the arrival of a seiche soonafterwards. According to his testimony, thewave reached the current position of Route3, leaving a shallow flooded area that remai-ned flooded for several months (Figs. 2 and3). The inundation killed ma-ny trees; mostof which are still standing. The earthquakealso affected the area where the Rio Turbioenters Lago Fagnano and left a perma-nently flooded area on the landward side ofthe berm.Mr. Adrian Goodall was in Rio Grande, Ar-gentina, area during the earthquake. He,and a number of other eyewitnesses, reportthat the radio station in Punta Arenas,Chile, announced a second earthquakewould occur at 11 am (there is some confu-sion between reports about the time of theearthquakes as Argentina and Chile are indifferent time zones and some people livingin the area used Argentine time, whileothers used Chilean time). He and his bro-ther Thomas went outside just before thetime predicted and waited for the secondearthquake, which happened at the "predic-ted" time. He saw waves coming down thestreet and then felt the shaking. He visitedLago Fagnano soon afterwards and obser-ved the scarp in the road along the easternside of the Lago, which he recalled it to beabout 1 m high. He also described pheno-mena which might account for liquefactionin the Rio Fuego between Viamonte andRio Grande associated with the first event.Mr. Robert Sutherland and a business part-ner were in Punta Maria, a hotel and gene-ral store just north of Viamonte, during thefirst earthquake and at a farm further inlandduring second. They spent the night atPunta Maria and while Sutherland was tur-

ning off the alarm clock at 4:00 am theearthquake hit. There was a rumble thatgrew in strength for a few seconds and thenstopped. After a short time it started again,this time with a sideways movement. Hispartner tried to open the window duringthe first period of shaking but it jammed.They managed to open the window duringthe quiet interval between the two periodsof shaking, probably the P and S surfacewave arrivals. During the sideways shakingthe armoire "walked" across the floor bet-ween the beds. About 500 cans fell off theshelves in the store. Noticeable aftershockscontinued for about 20 minutes after thefirst large shock. Sutherland also reportedcracks cutting a corduroy road by the RioFuego, and that a "blue mud" (liquefaction)came up through 15 cm wide cracks inroad.The second earthquake occurred aboutmid-day while Sutherland was loading logsonto a truck. The pile of logs on the truckwas about 8' high when the second earth-quake hit and everything fell off the truck.Sutherland managed to throw himself ontothe top of the cab of the truck. He alsoreports that two hours before the secondearthquake, there was an announcement byradio that there would be a second, weaker,earthquake in two hours. The second earth-quake happened exactly at the time an-nounced. An interesting observation is thatas this was the only experience with earth-quakes for many of these people, who werebasically a small group of pioneers, they tellthe story as if is it is perfectly normal tohave announcements of upcoming earth-quakes on the radio. In their only experien-ce it is true.Sutherland also went to visit Lago Fagnanoafterwards and found the flooded area onthe east side of the gravel bar (the lake sho-reline gravel berm became a gravel bar withthe formation of the permanently floodedarea fed by the Río Turbio on the east) hadsubsided about 5 m during the earthquake.He also reports observing the scarp acrossthe gravel bar.In 1950 Sutherland was part of a search andrescue team looking for a missing airplanefrom the Argentine Army. They followedthe fault trace from Lago Fagnano to thecoast. Along the Rio Irigoyen near the coast

he saw post and wire fences, that were builtstraight using a compass, with 6 m offsets.While crossing the Río Irigoyen one of thehorses fell into a "great crack" in the bed ofthe river. One of the members of thesearch party went diving to find the bottomof the crack and it was deeper than hecould dive. There were many such cracks inthe river bottom. He also reports a "tre-mendous fault" with the peat all churnedup. He only remembers horizontal displace-ment, no vertical displacement along thissection of the fault. In the higher groundleaving the river, he saw many broken cor-duroy roads with horizontal offsets.Mr Sutherland subsequently met with a ger-man geologist, a Mr. Klausen, who workedin the area for four years performing atopographic survey for the Argentine Navy.Mr. Klausen mentioned his surveying mea-surements showed a 6 m horizontal displa-cement between the mountains CerroJeujepen (704 m) and Cerro Kashem-Michi(672 m) south and north of the fault res-pectively.Based on the latest work on the controver-sial field of seismic scaling relationships(Zeng et al. 2005), plate boundary strike-slipearthquakes have a slip to length ration of~.5 to ~1.7 10-5. An average horizontal slipof 4-5 m would therefore be expected tooccur on a fault plane extending from 250to 1000 km in length, a very wide rangealthough the available evidence supports arupture length, at least subaerially, near thelower end. It is unknown if the rupturepropagated further east than the coast, orfurther west than the eastern shore ofFagnano Lago, but the vertical only offsetobserved there would be consistent with itbeing near the western end of the rupturezone and/or on a releasing bend Thismeans that the fault plane that ruptured,continued seaward along the North Scotiaridge at least for 300 km. Unfortunatelythere are no reports of offset, either by eye-witnesses or subsequent geological studies,associated with the earthquake in the seacliffs around Río Irigoyen-Cape Leticia,where the fault is thought to go out to sea.The second earthquake was approximatelythe same magnitude and was reported tohave been stronger in Punta Arenas, Chile,to the west. This suggests that the morning

652 C. H. COSTA, R . SMALLEY, JR . , D. P. SCHWARTZ, H. D. STENNER, M. ELLIS, E . A . AHUMADA AND M.S. VELASCO

event was to the east and the noontimeevent was further west. Based on thelengths expected from scaling relationshipsat this type of crustal setting (i.e. Zeng et al.2005), the two events together could havebroken the complete Magallanes-Fagnanofault system from the triple junction to theAtlantic.

THE SURFACE RUPTURES

The ruptures related to the 1949 earthqua-kes were visited at the Lago Fagnano shore-line and east of the Río San Pablo (Fig. 2).These are the areas which provide betterground conditions and visibility to accessthe fault trace.

LAGO FAGNANO AREA

The fault trace here has an ESE trend andseparates an uplifted block to the northfrom a depressed and flooded area to thesouth (Figs. 2 and 3). The clearest exposureof the scarp is located over the shorelineroad, where a rounded step in the topo-graphy coincides with the place where theinterviewed eyewitnesses pointed out theseismic rupture. The scarp cuts unconsoli-dated gravels of modern and paleo-shoreli-nes of the lake. It appears quite degradedand its height ranges from 0.50 m to 1.00 mon the old section of Route N° 3. Thedownthrown area shows many dead threesin upright position, as a result of the seichewhich inundated the area immediately afterthe earthquake. In aerial images the faulttrace is also highlighted by a sudden changein the vegetation cover with Nothofaguswoods in the up thrown side and grassylands in the southern block (Fig. 3). Theshoreline pattern also shows a noticeablechange in its geometry and morphology,being more sinuous and irregular south ofthe fault trace, whereas slight channelentrenchment in the submerged area mightaccount for elastic recovery after the quake.Caibul also mentioned that the old shoreli-ne road (the original Route N° 3) had to berectified and elevated between the faulttrace and Hostería Kaiken and a bridge hadto be constructed for draining the floodedarea. No diagnostic evidence for strike-slipsurface movements were identified here.

RÍO SAN PABLO AREA

This area is located 30 km eastwards fromthe village of Tolhuin, through the road toEstancia La Correntina (Fig. 2). Along thissegment of the fault, observed scarpheights range from 5 to 11 m. Despite den-se vegetation, the Magallanes-Fagnano faultstands out clearly on aerial images as a per-sistent linear trace. The northern upliftedblock exposes Quaternary glaciofluvialcoarse deposits. The uplift on the northside and subsequent fluvial entrenchmentof the down dropped south side of thefault resulted in the development of severalterraces on the north side. In a 5-10 m widearea along the ESE trending fault trace, ten-sion gashes with an echelon arrangement,

coaxial grabens, sag ponds and mole trackshave been identified (Schwartz et al. 2001,2002). Many trees were uprooted and fellinto these along-strike depressions duringthe 1949 earthquakes and their dendrochro-nological record is currently being investi-gated with the aim of recognizing pre 1949surface faulting associated to this structure(F. Roig and J. Rabassa, pers. comm..).The vertical slip associated with the 1949event seems to have been 50 cm or less. Asthe scarp is much higher than this in gene-ral, it highlights that the scarp developedthrough many seismic events which ruptu-red or deformed the surface. Despite thegeneral strike-slip character of this structu-re, diagnostic mesoscopic coseismic eviden-ce of strike-slip offset has been quite elusi-ve at the present reconnaissance level. The

Figure 3: Aerial image of the Magallanes-Fagnano fault scarps and surface ruptures at LagoFagnano shoreline. The vertical arrows point out the south-facing scarp and the area within das-hed lines corresponds to the flooded area due to the earthquakes-related seiche, according to eye-witnesses testimonies. The road number indicates the present position of route 3, which in 1949was located along the shoreline. The horizontal arrow indicates the surface rupture site at the sho-reline. See figure 2 for location.

653Paleoseismic observations of an onshore transform boundary ...

majority of surface disturbance is locatedwithin 30 m of the main fault scarp andlarge tension gashes (typically 2.5 m long,0.30 m deep, 0.50 m wide) as well as discon-tinuous small scarps (<50 cm) exist alongthe main fault trace. We found two fenceswith offsets where they cross the fault trace.We surveyed both fences with the aim ofdetermining the strike-slip component ofslip during the 1949 rupture. There is someuncertainty, however, about the originalgeometry of the fences. At the San Pablofence (Figs. 2 and 4), Mr. Pedro Oliva, thelandowner, could not remember whether ornot this fence was built before the earth-quake and/or if it was restored after theearthquake. He guessed though, that theoriginal geometry was not straight where itcrossed the fault from the higher dry sideon the north, to the submerged turba to thesouth, due to difficulty of working in thetorn up ground of the scarp. The fence alsochanges from post and rail on the north topost and barbed wire to the south. Thequestion about the original geometry addssignificant uncertainty to making a trus-tworthy reconstruction.As more reliable piercing points have notbeen found along the fault trace so far, wedecided to attempt to restore the fence geo-metries as well as possible, considering theknown problems with the aim of constrai-ning the likely maximum and minimum slipamount for the coseismic strike-slip com-ponent. We developed three probable mo-dels, which of course do not preclude otherinterpretations. A left lateral componentranging from 0 to 4 m can be interpreted atthe San Pablo Fence site, according to thedifferent models displayed in figure 4, al-though it is difficult to determine what thecorrect choice might be, due to the reasonsdiscussed above. Note that sudden bends ofthe fence coincide with slope breaks andwith the main fault trace. In fact, if theseoffsest were tectonic, they are the onlyobjective evidence that can be used to esti-mate the horizontal component of slip inthis area.The fence identified as "Oliva Fence" (Figs.2 and 5) was built before the earthquake,and according to Oliva no restoration workwas done after the earthquake. The fencewas constructed by felling trees, stripping

the branches and moving the trunks slightlysuch that the fallen trunks form the fence.The original geometry was therefore quiteirregular and unknown, but Mr. Oliva poin-ted out one section of the fence that hereported was disrupted by the earthquake.This section looked disrupted compared tothe adjoining sections and was thereforesurveyed. The overall fault scarp here is hig-her than at fence site at Río San Pablo, andthere is no evidence of significant coseis-

mic landsliding. The fault zone is quite con-tinuous, showing sag ponds, tension gashes,scarps and other minor related morpholo-gies. Assuming an original linear fenceacross the main fault, its present geometryexcludes a coseismic lateral component lar-ger than 1m. The most conspicuous featurethat might account as an evidence for sinis-tral strike-slip displacement is a 0.40 m stepin the fence, close to a fault scarp (Fig. 5).

Figure 4: Topographic profile (above) and plan view of the surveyed fence at Río San Pablo.Main scarps and secondary scarplets observed at the main fault zone have been sketched. The dashed lines show possible restoration models, assuming this fence undergone deformation atthe 1949 earthquakes, with a maximum left-lateral component of 4m (see scale).

Figure 5: Topographic profile and plan view of the Oliva fence. Several subparallel scarplets arepresent at the main scarp slope. Possible strike-slip component of the 1949 events have beeninterpreted for the area within the circle.

654

TRENCH INVESTIGATIONAT THE RÍO SAN PABLO

Ten meters east of where the Río San Pablocrosses the Fagnano fault trace, a hand-excavated trench allowed observation ofthe deformation produced by the earthqua-ke on a secondary fault trace. Surface ex-pression of the main fault here is expressedby a 1 m scarp probably related to the 1949seismic events. The excavated fault tracehas no noticeable surface expression at thetrench site. The trench stratigraphy is com-posed by four main units labeled A throughD in figure 6. The oldest exposed unit (UnitA) corresponds to a grey glacio-fluvial gra-vel of unknown base and a maximum expo-sed thickness of 60 cm. This is partly over-lain by a gray clay with slate clasts (Unit B1)and by a gray to brownish clay (Unit B2).The third unit (C) corresponds to well laye-red peat bog, with alternate layers of black,dark brown, reddish and orange-yellowish

color. Boundaries among these beds arevery well defined and fine layers such thinas 1 cm can be identified. These layers thic-ken to the south toward the main fault trace(not excavated) containing a variable andminor amount of silt and clay. A grey teph-ra layer is interbedded within the peat level.The ages of the various units have beenconstrained through 14C analyses and aredisplayed in Fig. 7. At the trench scale (Fig6), many minor divisions within unit C canbe traced, although only those relevant interms of fault slip have been sketched. Thewhole sequence is capped by a present darksoil and a brownish-greenish active peatformation.Rupture geometry is exposed on both wallsof the trench and shows two main fault sur-faces merging or truncating upward withdifferent geometric patterns and significantopen spaces (up to 6 cm) along them (Fig.6). The sharp bends along fault F1 in theeast wall makes it difficult to accommodate

vertical slip component through conservati-ve displacement. Many of the features onthe south side of fault F1 suggest a dragalong F1 with a downthrown side to thesouth, which is consistent with topographicrelations exposed at the main fault scarp.Fault slip produced thickness changes inthe stratigraphic units near the deformationzone, being some of them even stripped atone or both sides of the fault trace.The oldest faulting event recorded in thetrench is constrained by Units C6 and C7and according to the radiocarbon ages itmight have taken place between 7420 +40BP and 4560 +40 BP. This event appears tobe related to fault strand F2 since Unit C5and C6 are not exposed on the up-thrownside (as seen at the west wall) and Unit C7overlies it with almost no disturbance.Cumulated slip at the base of Unit C4 is 17cm (west wall), although it is not clear ifthis slip is related to this event only or alsoto a previous event or events.

Figure 6: Logging and interpretation ofboth trench walls on a secondary faulttrace at Río San Pablo. Black boxes indi-cate the sampling sites for 14C dating,whose results are given in calibratedradiocarbon years. The west wall has beenreversed. See text for details.

655

The next seismic event horizon identifiedimplied slip along fault strand F1 after thedeposition of Unit C10 (4420 +40 BP) andprevious to Unit C11 (4160 +40 BP at itsbase). This is inferred based on the notablechange in thickness of unit C10 across faultF2 in the west trench wall, while on the eastwall this unit has been eroded. This sug-gests the reactivation of the south-facingscarp favoring erosion at the uplifted side.A sudden decrease in the accumulated slipafter deposition of Unit C11(4050 +40 BP)is recorded at both trench walls, particularlywithin the Unit C14 (1060 +40 BP), where

9 cm of displacement was measured at itsbase (east wall) and less than 2 cm at its top(both walls), in addition to a considerablethickness change between the foot-wall andhanging-wall. A truncated layer within thisunit in the foot wall (west wall) (black layerin Fig. 6) advocates for a faulting event pre-1879, followed by erosion prior to the pre-sent soil development. Thus, this eventmight have taken place between 780+40 BPand 430 +40 BP. It is also possible to assu-me aseismic creep along fault strand F1 asan alternative interpretation. The upward graben-like culmination of the

fault surface (F1) in the east wall, affects thebase of current peat bog formation (UnitD1; 430 +40 BP ) with 5-8 cm of verticaldisplacement. This slip barely reached thesurface, probably as tension gashes associa-ted to the main rupture during the 1879 or1949 events, expressing a rather small surfacerupture along this fault strand at this point.The events recorded in this trench are cer-tainly not complete with respect to addres-sing earthquake recurrence during theHolocene, because the interpreted paleo-seismic record focuses only on a secondaryfault plane which might not have have rup-tured during all significant earthquakesalong this section of the fault. Conversely,coseismic evidence at the sub-meter scaleand other stratigraphic relations related toseismic event horizons could be better pre-served. Evidence for three, and possiblyfour, seismic events, however for the last 8ka, roughly means an estimated maximumaverage recurrence interval of 2 ka for e-vents rupturing at surface at this secondarytrace. Considering the limitations mentio-ned above, this should be regarded only asthe first estimate for the maximum timespan between surface rupturing earthqua-kes. Paleomagnitudes of the interpretedevents can not be assessed as only the ver-tical component of slip could be determi-ned. The magnitude threshold for produ-cing surface rupture in this type tectonicsetting is probably larger than magnitude M6.5 (Ambraseys 1983, Wells and Cop-pers-mith 1994).

CONCLUSIONS

Field work conducted along two sections ofthe Magallanes-Fagnano fault that sufferedsurface rupture related to one of both ofthe Ms 7.8 earthquakes of 17-12-1949 allowa preliminary determination of a maximumvertical component of 1 m related to themain scarp. Total length of the rupture andassociated coseismic offset at the srufacestill remain unknown. A significant part ofthe rupture could have continued into LagoFagnano to the west and, given the shortdistance between Lago Fagnano and thecoast, it probably also continued offshoreto the east. Except for the unconfirmedreports of 4-6 m of offset further to the

Paleoseismic observations of an onshore transform boundary ...

Figure 7: Calibrated 1 4C obtained through the calibration program OxCal v3.8 (Bronk Ramsey2002) following the atmospheric data provided by Stuiver et al. (1998). Dating results orderedaccording to laboratory number.

656 C. H. COSTA, R . SMALLEY, JR . , D. P. SCHWARTZ, H. D. STENNER, M. ELLIS, E . A . AHUMADA AND M.S. VELASCO

east by the eyewitnesses, the magnitude ofthe strike-slip component in the surveyedarea is poorly constrained and seems to beless than the vertical offset. No typical,diagnostic strike-slip morphologies havebeen so far recognized along these ruptu-res, or in remote sensing images, whosegeneral sense of movement is sinistral-nor-mal. The fault parallel geologic structures,and the unconsolidated nature of the gla-ciar deposits and peat, combined with theheavy precipitation, conspire to obscuredevelopment of morphologic evidence ofstrike slip offset. The young age of theexposed surface material also limits totaloffset since recession of the glaciers to lessthan 100 m.Reconstructing the geometry of preexistingfences yielded results for coseismic offsetswith large uncertainties. Our observations,and reports of eyewitnesses of the 1949events, indicate that along the eastern sho-reline of Lago Fagnano vertical offsets ran-ged from 0 to 1 m with no coseismic hori-zontal strike-slip displacements. Paleoseis-mic evidence from a trench across a secon-dary fault trace at Rio San Pablo, suggestsrupture occurred there in at least two pre-1949 events during the last 8 kyr. Ap-parently, the fault surfaces exposed havemoved individually, and no further move-ment has taken place on fault F2 during thelast 4160 +40 years BP. The observationsindicate that the left-lateral component offault slip during the 1949 events in the areaimmediately east of Lago Fagnano wasmost likely < 1m, and probably absent inmany places, which would be compatiblewith either of the end member interpreta-tions that Lago Fagnano sits in a releasingbend of the South America-Scotia trans-form boundary or was at the western mostend of the rupture zone.

AKNOWLEDGEMENTS

We thank P. Oliva, J. Caibul, A. Goodall, R.Sutherland, C. Navarro, O. Pedersen, A.Coronato, C. Roig, J. Rabassa and theTolhuin Firemen and Police Station fortheir collaboration. The final paper wasimproved by helpful reviews and sugges-tions from M. Ghiglione and V. Ramos. Weare also grateful to G. Seitz (Liver-more

Laboratories, USA) for radiocarbon analy-ses.

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Recibido: 30 de junio, 2006Aceptado: 15 de noviembre, 2006