Thesis
Reference
Late quaternary environmental changes in lago Fagnano, Tierra del Fuego (54°S): reconstructing sedimentary processes, natural hazards
and paleoclimate
WALDMANN, Nicolas
Abstract
Located at ~54°S in the Island of Tierra del Fuego, Argentina/Chile, Lago Fagnano is the biggest (~110 km long), southernmost non-ice covered lake in the world outside Antarctica. In addition to its tectonic origin, the lake evolved under the reach of past Andean glaciations.
Therefore, its sediments contain a unique record of past tectonic and climatic events. The combination of seismic reflection profiling and detailed analyses of long sedimentary cores allowed to address questions regarding past regional environmental changes. The geophysical data from the lacustrine basin with existing geomorphological data around the lake allowed identifying and assigning a seismic sequence to subaquatic moraines within Lago Fagnano. The laminated sequences are occasionally interrupted by homogenous beds several tens of cm thick interpreted as resulting from simultaneously-triggered basin-wide lateral slope failures and thus, fingerprinting paleo-seismicity along the MFT fault zone. Their temporal distribution revealed an 800-1000 years recurrence of slope failure providing unique data about the frequency and plausible magnitude of [...]
WALDMANN, Nicolas. Late quaternary environmental changes in lago Fagnano, Tierra del Fuego (54°S): reconstructing sedimentary processes, natural hazards and
paleoclimate. Thèse de doctorat : Univ. Genève, 2008, no. Sc. 4044
URN : urn:nbn:ch:unige-22758
DOI : 10.13097/archive-ouverte/unige:2275
Available at:
http://archive-ouverte.unige.ch/unige:2275
Disclaimer: layout of this document may differ from the published version.
1 / 1
Late Quaternary environmental changes in Lago Fagnano, Tierra del Fuego (54°S): Reconstructing Sedimentary
Processes, Natural Hazards and Paleoclimate
THESE
Présentée à la Faculté des sciences de l’Université de Genève pour obtenir le grade de Docteur ès sciences, mention Sciences de la Terre
par
Nicolas WALDMANN de
Mendoza (Argentina)
Thèse No 4044
GENEVE
Atelier de production de la Section de physique 2008
Waldmann, N.:
. Terre & Environnement, vol. 82, 150 pp. (2008)
ISBN 2-940153-81-7
Section des Sciences de la Terre, Université de Genève, 13 rue des Maraîchers, CH-1205 Genève, Suisse Téléphone ++41-22-702.61.11 - Fax ++41-22-320.57.32
http://www.unige.ch/sciences/terre/
Late Quaternary environmental changes in Lago Fagnano, Tierra del Fuego (54°S): reconstructing sedimentary processes, natural hazards and paleoclimate
Dedicado a Rosa Braginsky de Elzufán
Table of content
AbsTrAcT ...5
résumé ...7
chApTer 1 ...9
IntroductIon ...9
1.1. Preface ...9
1.2. Lacustrine Sediments as Paleoenvironmental Archives in a Changing World ...11
1.3. Lago Fagnano: Regional setting ...12
1.4 Methodologies used ...16
1.5. General objectives and outline of the thesis ...17
References ...19
chApTer 2 ...23
SeISmIcStratIgraphyof Lago fagnanoSedImentS (tIerradeL fuego, argentIna) - a potentIaL archIveofpaLeocLImatIcchangeandtectonIcactIvItySIncethe Late gLacIaL ...23
Abstract ...23
2.1. Introduction ...24
2.2. Morphological and geological settings of Lago Fagnano ...24
2.3. Methodology ...26
2.4. Results ...28
2.5. Discussions ...32
2.6. Conclusions ...35
Acknowledgments ...35
References ...36
chApTer 3 ...41
geophySIcaL evIdenceof muLtIpLe gLacIer advanceSIn Lago fagnano (54°S, SouthernmoSt patagonIa) SIncethe Lgm ...41
Abstract ...41
3.1. Introduction ...42
3.2. Approaches ...43
3.3. Regional and environmental settings ...45
3.4. Structure and seismic stratigraphy of Lago Fagnano ...46
3.5. The Fagnano Glacier fluctuations since the LGM ...48
3.6. Chronological constrains of the moraines ...54
3.7. Summary and conclusions ...58
Acknowledgments ...60
References ...60
chApTer 4 ...67
hoLocenemaSS-waStIngeventSIntheSedImentaryInfILLof Lago fagnano, tIerradeL fuego (54°S): ImpLIcatIonSforpaLeoSeISmIcactIvItyofthe magaLLaneS-fagnanotranSformfauLt ....67
Abstract ...67
4.1. Introduction ...68
4.2. Lago Fagnano ...69
4.3. Methods ...70
4.4. Mass wasting events ...72
4.5. Conclusions ...88
Acknowledgments ...89
References ...89
chApTer 5 ...95
hoLocene cLImatIc fLuctuatIonSand poSItIonIngofthe Southern hemISphere weSterLIeSIn tIerra deL fuego (54°S), patagonIa ...95
Abstract ...95
5.1. Introduction ...96
5.2. Study area ...96
5.3. Methods ...98
5.4. Seismic sequence stratigraphy ...99
5.5. Sedimentological, petrophysical and geochemical core results ...102
5.6. Cores chronology ...106
5.7. Discussion ...106
5.8. Paleoclimate implications ... 111
5.9. Conclusions ...115
Acknowledgments ...116
References ...117
chApTer 6 ...123
envIronmentaLreSponSetocLImateoScILLatIonSIn tIerradeL fuegoSIncethe Late gLacIaL ....123
Abstract ...123
6.1. Introduction ...124
6.2. Study area ...125
6.3. Materials and methods ...125
6.4. Results ...126
6.5. Interpretation ...129
6.6. Discussion ...130
6.7. Paleolimnological reconstruction from algae assemblages ...131
6.8. Conclusions ...132
Aknowledgments ...133
References ...134
chApTer 7 ...137
concLuSIonSand outLook ...137
7.1. Conclusions ...137
7.2. Outlook ...140
References ...143
Appendix ...147
List of appendixes to be found in the attached CD...147
AcknowledgmenTs ...149
Abstract
_____________________________________________________________________
Lacustrine sediments provide one of the best continental archives to reconstruct environmental and tectonic processes. Located at ~54°S in the Island of Tierra del Fuego, Lago Fagnano is the biggest (~110 km long), southernmost non-ice covered lake in the world outside Antarctica. The lake occupies two basins forming part of a chain of tectonic depressions along the Magallanes-Fagnano transform (MFT). In addition to its tectonic origin, the lake evolved under the reach of past Andean glaciations. Therefore, its sediments contain a unique record of past tectonic and climatic events.
A multi-proxy approach combining seismic reflection profiling and detailed analyses of long sedimentary cores retrieved at discrete locations of the lake allow me to address questions regarding past regional environmental changes. The concepts of seismic sequence stratigraphy have been applied to the sedimentary infill to reconstruct past environmental conditions. The combination of geophysical data from the lacustrine basin with geomorphological studies conducted by others around the lake has allowed identifying and assigning a seismic sequence to subaquatic moraines within Lago Fagnano.
Analyses of selected cores assisted in the calibration of the seismic signal and permitted core-to-seismic correlations in order to understand the seismic architecture of the basin infill. The age model of the sedimentary infill relies on a well-dated tephra layer and several AMS-14C samples obtained on wood and pollen remains. The sediments consist of laminated sequences occasionally interrupted by homogenous beds several tens of cm thick interpreted as megaturbidites. We interpret these units as resulting from simultaneously-triggered basin-wide lateral slope failures and thus, fingerprinting paleo-seismicity along the MFT fault zone. Their temporal distribution revealed an 800- 1000 years recurrence of slope failure providing unique data about the frequency and plausible magnitude of Holocene earthquakes in Tierra del Fuego.
Detailed petrophysical and geochemical studies of the laminated undisturbed sediments reveal fluctuations that display an apparent cyclicity. These results provide a unique dataset that is further compared with other regional marine and continental archives and thus, allowing to identify known intervals of late Holocene climate fluctuations. This exercise serves to validate the outcome of existing ocean and atmospheric climatic models for the Southern Hemisphere and as a first step for interhemispheric climate correlation.
résumé
_____________________________________________________________________
Les sédiments lacustres fournissent une des meilleures archives continentales pour la reconstruction des processus environnementaux. Localisé approximativement à 54°S de l’île de Terre de Feu, le lac Fagnano est le plus grand lac au monde (~110 km) non recouvert de glace situé dans la partie extrême Sud hors de l’Antarctique. Le lac occupe deux bassins faisant partie d’une chaine de dépressions tectoniques le long du système transformant Magallanes-Fagnano (TMF). En plus de son origine tectonique, le lac a évolué sous l’influence de glaciations Andines antérieures. Par conséquent, ces sédiments contiennent les enregistrements uniques d’événements tectoniques et climatiques passés de cette région.
Une approche multidisciplinaire combinant la réflexion sismique et des analyses détaillés de plusieurs grandes carottes sédimentaires extraites du lac Fagnano, ont permis de s’interroger sur les changements environnementaux régionaux passés. Plus de 800 km de relevé géophysique, combinant simultanément 3.5 kHz (pinger) et 1 in3 (airgun) ont permis de visualiser les sédiments les plus récents ainsi que les anciens dépôts réciproquement. Les concepts de séquence stratigraphique sismique ont été appliqués au remplissage sédimentaire afin de reconstruire les conditions environnementales antécédentes. La combinaison de données géophysiques provenant du bassin lacustre associées à des études géomorphologiques ont permis de désigner une séquence subaquatique de moraines dans la stratigraphie sismique du lac Fagnano. Lors du dernier maximum glaciaire, le glacier Fagnano occupait entièrement le bassin du lac et s’étendait à travers trois différents lobes atteignant une longueur totale d’environ 130 km. Une série de moraines frontales submergées recouvertes par du remplissage lacustre identifiés par les relevés sismiques, suggère des ré-avances occasionnelles du paléo-glacier vers l’Est. Des datations au radiocarbone ont permis d’estimer le taux de sédimentation fournissant un âge indirecte pour l’un de ces intervalles de re-avance du glacier. Cette progression majeure du glacier semble être synchrone avec le Antarctic Cold Reversal (ACR) qui était probablement plus important en magnitude dans cette région par rapport au Younger Dryas Chronozone (YDC) dans l’hémisphère Nord.
Les profiles sismiques ont été complétés avec des carottes sédimentaires collectées dans la partie la plus profonde du bassin facilitant la calibration du signal sismique et permettant une corrélation entre les carottes et la sismique. Cet exercice améliore la compréhension de l’architecture sismique du remplissage du bassin.
Plusieurs carottes ont été prélevées dans la partie la plus profonde du lac générant des sections composées pour le bassin Est et Ouest réciproquement. L’âge du modèle s’appuie sur des couches de tephra datés ainsi que plusieurs échantillons AMS-14C obtenu sur des restes de bois et de pollens. Les archives sédimentaires de l’Holocène
ont été reconstruites à l’aide d’une étude multidisciplinaire de carottes sélectionnées.
Les sédiments consistent en des séquences laminés occasionnellement interrompues par des bancs homogènes de plusieurs dizaines de centimètres d’épaisseur interprétées comme des mégaturbidites. Des études pétrophysiques et géochimiques détaillées de ces intervalles laminés révèlent la présence de fluctuations à la fois dans les éléments majeurs et dans les éléments traces, ainsi que dans les données de matiere organique et dans la palynologie, démontrant une cyclicité apparente. Ces résultats fournissent un ensemble de données uniques pouvant être comparés avec d’autres éléments, tel que des enregistrements marins et continentaux régionaux. Cette comparaison permet d’identifier des intervales connus de fluctuation climatiques à la fin de l’Holocène, avec l’apparition des Southern Westerlies dans la région de la Terre de Feu.
La combinaison d’enregistrements géophysiques, sédimentologiques et géochimiques du lac Fagnano révèlent la présence d’événements de mouvements de masse récurrent durant l’Holocène. La distribution spatiale et temporelle de 22 événements méga-turbiditiques a été répertoriée dans le lac. Le déclanchement simultané du pentes latérale ainsi que la formation de méga-turbidites sont interprétées comme étant les empreintes de la paléo-sismicité le long de la zone de faille TMF.
Le modèle d’âge proposé révèle une récurrence de 800 à 1000 ans dans formation de méga-turbidites. Ces résultats fournissent des données uniques sur la fréquence et la plausible magnitude des tremblements de terre dans la région de Terre de Feu à l’Holocène. Par conséquent, ces premiers enregistrements de glisement du terraine passés dans le lac Fagnano représente les premières prémices à des études approfondies sur les risques geologiques dans la région de Terre de Feu.
9 Chapter 1: Introduction
1
chapter 1: Introduction
Introduction
_____________________________________________________________________
1.1. Preface
The Mar del Sur, or South Sea (later known as the Pacific Ocean), was first described by the Spanish explorer Vasco Nuñez de Balboa in 1513. This discovery encouraged many explorers to search for a passage through America seeking the Spice Islands (currently known as the Moluccas archipelago in Indonesia) in order to control the monopoly of spice trade in Europe (Wafer and Winship, 2004). Among many travelers, the Portuguese Ferdinand Magellan, while in the service of the Spanish Crown, was the first to accomplish this the night of 21stof October 1520 when he arrived to the mouth of a passage that he named Canal de Todos los Santos (All Saints Channel), which later would carry his name. Magellan nominated the land north of the passage as Patagonia and the land south as the Land of Fire, or Tierra del Fuego, which he thought was the tip of a new continent (Fig. 1A). During the following centuries, Dutch, Spanish and Portuguese fleets navigated the islands, mapping the surrounding coasts while searching for other passages to the Pacific Ocean. A major English expedition in 1830 of the HMS Beagle under the command of Robert Fitz Roy, along with the young naturalist Charles Darwin, eventually succeeded in finding a passage south of the main island of Tierra del Fuego. This expedition made important scientific discoveries and was a major corner-stone for modern cartography.
While the shores of Tierra del Fuego were already well mapped by the end of the 19thcentury, the island’s interior remained unknown to early western cartographers (Fig. 1B). Not until 1893 did Argentinean explorers enter the Atlantic estuaries of the main rivers in a mission to evangelize the native Amerindian communities that inhabited the central parts of the island (Oyola-Yemaiel, 1999). Several years following this event, the shore of a big lake, named Kami by the local Yamana inhabitants, was first reported by the Argentinean settlers. Shortly afterwards, the lake was renamed Fagnano, memorializing an Italian evangelical that lead those expeditions.
Since the time of the first explorers and settlers, the remoteness and inaccessibility of Tierra del Fuego, along with its unknown interior, puzzled scholars. The geographical isolation of the island provided a new vast territory for geological and environmental studies. Pioneering works by Nordenskjöld (1898), Bonarelli (1917), Caldenius (1932) and Kranck (1932) paved the way for later geological studies in Tierra del Fuego.
Following these early geologists, however, many scholars have studied the geological past of the island. Yet, the proximity of Antarctica and new research initiatives of the
last decades, coupled with the growing interest of modern societies for tracking the impact of fast environmental changes, demand innovative, multidisciplinary research in this remote setting. The island’s isolation, which was the driving force behind the early studies, is as well part of the motivation behind the current work.
A
B
Figure 1: A) An ancient map of Tierra del Fuego (Hondius, 1633). Well-mapped shorelines of the Magellan Strait can be clearly noted, while the rest of the Island of Tierra del Fuego remained unknown to European cartographers. B) A late 19th century map of the Island of Tierra del Fuego (Soldan and Felipe, 1888), showing the contour of the Tierra del Fuego shore- line. The interior of the island was still unknown and unmapped, including the location of Lago Fagnano.
1.2. Lacustrine Sediments as Paleoenvironmental Archives in a Changing World
Despite the latest developments of the industrialized world in shielding modern societies from natural catastrophes, humans are still fragile and weak against their possible impact. During recent years, several natural catastrophic events have struck populations and influenced the public opinion through the widespread media. Some examples for such natural hazards are the extraordinarily warm summer of 2003 in Central and Western Europe, which broke temperature records for the last 500 years and led directly to 15000 deaths in only one month (Luterbacher et al., 2004). During the following year, the Indian Ocean was hit by a devastating tsunami generated by an Mw=9.2 earthquake with an epicenter offshore Sumatra, killing in its path more than 283000 people in eleven countries (Lay et al., 2005). A year later, in 2005, hurricane Katrina struck New Orleans, taking the lives of more than 1800 people while causing severe destruction along its course and an estimated property damage of $81 billion (Pielke et al., 2008). These devastating examples, which occurred during only three consecutive years, illustrate the wide magnitude and important influence of such events over the delicate equilibrium of modern human populations. In order to localize endangered areas, to estimate the frequencies of such events, and to develop future scenarios for natural hazards, including climate and tectonic changes, a high- quality assessment followed by planning are required at different scales. Studying past environmental changes, as registered in the geological record, can lead to a better understanding of present events in order to assess future climate-related hazards. The more complete the picture of former climatic events is in terms of both spatial and temporal resolution, the better will be our understanding of the Earth system and the ability of predicting future environmental trends.
Lacustrine sediments provide one of the best continental archives to reconstruct past environmental processes. Historically documented floods (e.g., Gilli et al., 2003;
Arnaud et al., 2005), sub-aerial land slides (e.g., Sletten et al., 2003), sub-aquatic mass movements (e.g., Schnellmann et al., 2002; Girardclos et al., 2007), earthquakes (e.g., Marco and Agnon, 2005) and abrupt climate fluctuations (e.g., deMenocal et al., 2000) have all been shown to leave permanent traces in lacustrine sediments. Thus, known historic but also unknown prehistoric events can be eventually described by searching after fingerprints of these natural hazards on the lacustrine sedimentary record.
Lago Fagnano, at the southernmost extreme of South America, evolved under a combination of tectonic and glacial forces. The lake is located in a pull-apart basin along a major plate boundary. The last strong (Mw>5) instrumentally recorded seismicity along this plate boundary that affected the Island of Tierra del Fuego occurred in 1949 (Mw=7.8). Although this event did not cause major civil damage, it resulted in the subsidence of large areas close to the lakeshores. Moreover, this seismic event
triggered several landslides along the banks of Lago Fagnano (Menichetti et al., 2001) and also along the westernmost arm of the Strait of Magellan, where tsunamis were observed (Jaschek et al., 1982). Hence, the sedimentary infilling of Lago Fagnano offers an exceptional possibility for reconstructing the paleoseismic history of this region.
In addition to its tectonic origin, the lake evolved under the reach and influence of past Andean glaciations. Its sedimentary infill records climatic fluctuations that formed and shaped those glaciers. Recent advances in the chronology and the environmental significance of Antarctic ice-core records point towards a larger Quaternary climatic heterogeneity than previously thought. Thus, realistic inter-hemispheric correlations rely on the development of well-constrained records with a dense latitudinal coverage.
Climatic records from southernmost Patagonia are hence critical cornerstones to link Antarctic paleoclimatic archives with their South American and Northern Hemisphere counterparts. Moreover, Lago Fagnano is located in a unique position relative to the global atmospheric circulation (e.g., Southern Hemisphere Westerlies winds). Thus, we conclude that the sedimentary infilling of Lago Fagnano can be used to reconstruct both the paleoseismic and the paleoclimatic history of the southernmost extreme of South America, en route to a more comprehensive global assessment of climate change in the Quaternary.
1.3. Lago Fagnano: Regional setting
The island of Tierra del Fuego is located at the southernmost extreme of Patagonia and, other than Antarctica, is the largest landmass at high latitudes in the Southern Hemisphere (Fig. 2A). The archipelago is bounded by the southern Atlantic Ocean to the east, the southern Pacific Ocean to the west, and the Drake Passage in the south;
the southernmost tip of the archipelago at Cape Horn is separated from the Antarctic Peninsula by only ~1000 km (Fig. 2B). At ~54°30’S, lying within the southern part of the island, Lago Fagnano is located within one of the most substantially and extensively glaciated regions of southernmost South America during the late Pleistocene. The lake has also evolved along a major plate boundary (Fig. 2A). With a total area of ~560 km2, a length of ~105 km, a maximal width of ~10 km and at only 26 m above mean sea level, this lake is the southernmost and largest ice-free fresh water body in the world,
~10 degrees further south than the large lakes of New Zealand.
1.3.1. Geology
The bedrock substratum along the southern flanks of the lake includes low- grade metamorphic black shale and whitish-gray tuff sequences, corresponding to an Upper Jurassic marine volcano-sedimentary complex (Le Maire/Tobifera formations;
Borrello, 1969) (Fig. 3A). The northern and eastern margins of the lake are composed
of folded Lower Cretaceous to Tertiary marine greywacke (Yahgan and Beauvoir formations; Caminos et al., 1981; Olivero and Martinioni, 2001). An Upper Cretaceous monzodioritic body (Cerro Hewhoepen batholith) intrudes the formations at the southeast extreme of the lake (Acevedo et al., 2000).
1.3.2. Physiography
Lago Fagnano comprises two sub-basins: a smaller, deeper basin in the east, reaching a maximum depth of ~210 m, and an elongated, shallower basin in the west with ~110 m maximum water depth (Fig. 3B). The southern shores are bordered by the foothills of Sierras de Alvear (eastern part of Cordillera Darwin), while the lower mountain belt of the Sierras de Beauvoir limits the northern margin of the lake. The lake occupies the deepest continental pull-apart basin in a series of graben-shaped, asymmetric tectonic sinks organized in an en echélon arrangement along the Magellan- Fagnano Transform (MFT) (Fig. 2A; Lodolo et al., 2003, 2007; Tassone et al., 2005;
Menichetti et al., 2008). The onset of left-lateral movement along the MFT is not well dated, but is presumed to have started during the Oligocene (Klepeis, 1994; Lodolo et al., 2003). Recent fault scarps and displacement of glacio-lacustrine sediments observed along the transform lineation in and co-linear with the eastern part of the lake indicate ongoing tectonic activity (Dalziel, 1989). Moreover, fluvial drainages in the same region
Figure 2: A) Satellite image of Patagonia with the general tec- tonic setting (in red) and the maximum extension of ice during the LGM (white area) (after (Singer et al., 2004). B) Shuttle Radar Topographic Mission (SRTM; Farr et al., 2007) map of the Island of Tierra del Fuego.
73°W 72°W 71°W 70°W 69°W 68°W 67°W 66°W 65°W
56°S 55°S 54°S 53°S 52°S
0 40 80 120
Km
±
CordilleraDarwinFig. 3
B
Pacific Ocean
Atlantic Ocean
Drake Passage Magellan Strait 60°W
65°W 70°W 75°W
A
Fig. B
55°S 50°S 45°S 40°S
65°S 60°S 35°S
Patagonia
South American
Plate
Antarctic Plate
ChileTrench
Scotia Plate MFT
Shetland Is.
Plate Nazca
Plate
Cape Horn Patagonia
Tierra del
Fuego Lago Fagnano
are influenced by the presence of latitudinal along-strike structures related to the strike- slip fault system (Menichetti et al., 2001).
1.3.3. Climate and Meteorology
Lago Fagnano is fed by several perennial streams draining rainfall arriving from the Pacific Ocean to its watershed (Fig. 3B). The Claro, Milna, Valdez and Turbio are the principal rivers flowing into this lake, whereas the Azopardo River at the western extreme of the lake is the only outlet towards the Straits of Magellan through the Almirantazgo Fjord (Admiralty Sound).
A meteorological station installed in 2004 on the southern shore of the lake (see Fig. 3B for location) has provided the meteorological data used in this study. The climate of this region is sub-polar, with cold and relatively dry winters and mild but windy, short summers (Fig. 4A). The climate of the region is sub-polar, precipitation increases during the summer season with the influence of the Southern Hemisphere
Undeformed Tertiary deposits Deformed Tertiary deposits Patagonian batolith Upper Cretaceous deposits Yahgan & Beauvoir Fms.
Tortuga complex Lemaire/Tobifera Fms.
Basement complex Strike slip fault Thrust fault
69°W 68°W 67°W 66°W
55°S 54°S 53°S
0 40 80
±
A
MFT
Fig. B
Claro R iver
Milna River Azopardo
River
Big Bay
Western basin Eastern basin
Valdez R iver Turbio R
iver Hewhoepen
Mt.
0 10 20
±
69°00’W 68°30’W 68°00’W 67°30’W 67°00’W
54°30’S
B
Sierras de Beauvoir
Sierras de Alvear
Figure 3: A) Simplified geologic sketch of the eastern half of Tierra del Fuego, adapted from Caminos et al. (1981) and Olivero and Martinioni (2001). MFT stands for Magallanes-Fagnano Transform fault. B) The watershed of Lago Fagnano (white area) superimposed over a regional Digital Elevation Model (DEM) map. The bathymetric map of Lago Fagnano with a 25 m con- tour interval overlies the figure (modified from Lodolo et al., 2007). Yellow dots indicate the location of the different water profiles presented in Figure 5; the red dot stands for the location of the meteorological station.
Westerlies belt (SHW), which brings moisture and humidity to the region during austral summer (Fig. 4B). Winters however, are mostly influenced by the Antarctic Oscillation (AAO) (Montgomery et al., 2001). Measurements taken between the years 2004-2008 show the dominant influence of the SHW in the lake area (Fig. 4C). This SHW belt is the main climatic forcing mechanism in southern Patagonia and is further recorded in other regional continental records such as glaciers (Douglass et al., 2005) and lacustrine sediments (Gilli et al., 2005; Haberzettl et al., 2005).
0% 2%
4% 6%
8%
N
NW NE
E
SE S
SW W
MAM JJA SON DJF 0
5 10 15 20 25 30
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Precipitation (mm)
0 2 4 6 8 10 12 14
Temperature (°C)
B C
A
10 8 6 4 2 0 -2 -4 -6
Figure 4: A) The mean monthly precipitation (blue bars) and the mean monthly temperature (blue dots) for the time period 2004-2008 at the meteorological station placed on the southern shores of Lago Fagnano (see Fig. 3B). Precipitation increases during the austral summer (De- cember-January-February) with the arrival of the Southern Hemisphere Westerlies. B) A map of 1000 mb zonal winds during the austral summer over the Southern Hemisphere, calculated for the time period from 1972 to 2005 (Kalnay et al., 1996). Regional precipitation measurements are directly connected to these Southern Hemisphere Westerlies. Note the increase values of zonal winds over Tierra del Fuego. C) The relative frequency (%) of the annual distribution of the wind direction (2004-2008), as recorded by a meteorological station placed on the south- ern shores of Lago Fagnano. Color bars stand for seasons. MAM: March-April-May; JJA: June- July-August, SON: September-August-October and NJF: November-December-January.
1.3.4. Limnology
Relatively little is known about the modern limnology and water ecosystem of Lago Fagnano. Previous surface samplings show an ultra-oligotrophic to oligotrophic lake characterized by transparent water and reduced algal biomass (Table 1; Mariazzi et al., 1987; Quirós et al., 1988; Quirós and Drago, 1999).
TP
(mg/m3) TN
(mg/m3) Chl
(mg/m3) SDL
(m)
Summer 1986 2 410 0.49 11.5
Spring 1984 12 ? 0.27 9.0
Spring 2002 7 163 ? ?
Three water column profiles were acquired using a Niskin bottle sampler during the November 2006 field season (Fig. 5; see Figure 3B for locations). These profiles, which were obtained at three different locations, are characterized by small vertical changes in both pH and temperature. It is important to highlight, however, that conditions during measurement were uncommon with a 1.5 m high lake level and may have influenced the different parameters measured in the water column.
In all water profiles, no significant thermocline is detected, suggesting a well-mixed lake probably related to meteorological conditions that influenced the lake level rise.
However, the profile obtained at the Big Bay site exhibits minimal variations the oxygen content from 6 mg/l in the surface water to 12 mg/l at 30 m depth. Moreover, we note an increase in water temperatures at a depth of 60 m and a decrease in the oxygen content at the depth of 70 m in the same site. These changes may probably be related to the preservation of a local thermocline due to the partial isolation of the Big Bay.
1.4 Methodologies used
The first seismic and coring campaigns on Lago Fagnano were carried out in 2005 and 2006 with the R/V Neecho. Coring and seismic acquisition was restricted to the Argentinean portion of the lake, which is approximately 87% of the total lake surface area. The seismic survey comprised a ~800 km long grid of both single- channel high-resolution 3.5 kHz (pinger) and 1 in3 (airgun) multi-channel seismic data.
Seismic profiles were digitally recorded in SEG-Y format, using a non-differential global positioning system (GPS) with an average accuracy of ±5 m. For travel-time to depth Table 1: Surface water characteristics of Lago Fagnano measured in summer 1986, spring 1984, and spring 2002 (Mariazzi et al., 1987; Quirós et al., 1988). TP: total phosphorous con- centrations, TN: total nitrogen concentrations, Chl: concentrations of chlorophyll type a, SDL:
water transparency.
conversions, an average water column velocity of 1430 m/s was assumed and 1500 m/s for the sedimentary infill.
Based on the seismic data interpretation, a total of 18 piston cores up to 8 m in length and 9 gravity cores up to 1.6 m length were recovered using a Kullenberg-type coring system. All cores were scanned before opening at the Limnogeology laboratory in the ETH-Zurich with a GEOTEK™ multi-sensor core logger (MSCL) to obtain their petrophysical properties (magnetic susceptibility, wet bulk density and p-wave velocity).
Selected cores were subsequently opened, photographed, described and sampled at the Limnological Laboratory of the University of Geneva for further sedimentological, geochemical and isotopic analyses as well as for tephrochronology and radiocarbon dating.
1.5. General objectives and outline of the thesis
The general scientific goal of this thesis is the reconstruction of past environmental changes in Lago Fagnano during the late Pleistocene-Holocene. One of the main objectives is to understand the seismic stratigraphic architecture using independent, but complementary, geophysical methods. The geophysical survey has been
Western basin 10 November 2006 54° 33’ 26.1” S/ 68° 25’ 17.4”W
Big Bay 9 November 2006 54° 35’ 13” S/ 68° 30’ 40”W
Eastern basin 54° 32’ 16.8” S/ 67° 30’ 20.7”W
10 November 2006
0
10 20
30
40
50
60
70
80
4 6 8 10 12 14
0
20
40
60
80
100
120
140
4 6 8 10 12 14 0
20 40 60 80 100 120 140 160 180 200
4 6 8 10 12 14
Temperature (ºC) pH Oxygen (mg/l)
Figure 5: Vertical water profiles in the western sub-basin of Lago Fagnano, the Big Bay site and the eastern sub-basin (on the left, in the center and on the right, respectively). Note the vertical variation in scale between the profiles. For location of the different profiles, see Fig. 3B.
complemented with subsequent analysis of sedimentary cores collected along with the geophysical profiles, which enables the dating of some of the physical changes interpreted in the seismic record. A secondary goal of this thesis is to try to disentangle the tectonic impact on the sediments from the registered climate record. This thesis is organized under four chapters corresponding to either published, submitted or in- preparation independent manuscripts:
After this introductory first chapter, Chapter Two presents the general seismic stratigraphy architecture of the sedimentary infill of Lago Fagnano, coupled with detailed description of the different sequences. The application of the concept of seismic stratigraphy allows the determination of past environmental changes. This information is further coupled with chemical and geochemical analyses obtained from short cores.
This chapter is a discrete article published in Geologica Acta.
Chapter Three contains the seismic stratigraphic architecture and description of the deeper parts of the basin. The basin infill is divided into glacial and lacustrine deposits following observed facies variations. The glacial infill shows prominent structures that are interpreted as terminal moraines formed during previous (timing unknown) advances of the Fagnano glacier. This information is combined with data retrieved from exposed glacial structures, in order to propose a timeframe for some of these glacier advances. This chapter is a discrete paper to be submitted for publication in Quaternary Research.
Chapter Four includes the seismic stratigraphic analysis of the lacustrine infill at high resolution. This chapter emphasizes the impact of earthquakes in the sedimentary infill, while attempting to disentangle it from the climatic record. Seismic stratigraphy is used to demonstrate the spatial distribution of these deposits. This information is combined with core data in order to asses the structure and age of each event. This chapter is a discrete article to be submitted for publication in Basin Research.
Chapter Five presents new paleoclimatic data for the last ~15 ka, as interpreted from chemical variations in several cores from Lago Fagnano. These data are further correlated with other paleoclimate recorders from Patagonia and the Antarctic. The results elucidate the onset and intensity of the SHW belt in Tierra del Fuego. This chapter is a discrete article accepted for publication in the Journal of Quaternary Science.
Chapter Six shows new results from an ongoing work dealing with major environmental changes since the Late Glacial in Tierra del Fuego. The chapter deals with linking between regional precipitation increase during the mid Holocene and the
prevalence of Nothofagus as the dominant taxa in the region. The palynological study is coupled with a study of frequency variations of algal remains as a proxy for the paleo- trophic conditions of the lake since the Late Glacial. This chapter is a discrete article in preparation for publication in Palaeogeography, Palaeoclimatology, Palaeoecology.
Chapter Seven concludes and summarizes the different chapters, while showing new data from Lago Fagnano that are still under preparation. Furthermore, it includes an outlook for the possibility of future studies in Lago Fagnano.
References
Acevedo, R. G., Roig, C. E., Linares, E., Ostera, H. A., Valín-Alberdi, M., and Queiroga-Mafra, Z.
M., 2000. La intrusión plutónica del cerro Jeu-Jepén. Isla Grande de Tierra del Fuego, Republica Argentina. Cuadernos de Laboratorio Xeológico de Laxe, La Coruña España 25, 357-359.
Arnaud, F., Magand, O., Chapron, E., Bertrand, S., Boes, X., Charlet, F., and Melieres, M.-A., 2006. Radionuclide dating (210Pb, 137Cs, 241Am) of recent lake sediments in a highly active geodynamic setting (Lakes Puyehue and Icalma--Chilean Lake District). Science of the Total Environment 366, 837-850.
Bonarelli, G., 1917. Tierra del Fuego y sus turberas. Sección Geología, Mineralogía y Minería 12(3), 1-119.
Borrello, A., 1969. Los geosinclinales de la Argentina. In “Anales de la Dirección de Geología y Minería.” pp. 188, Buenos Aires.
Caldenius, C., 1932. Las Glaciaciones Cuaternarias en Patagonia y Tierra del Fuego Geografiska Annaler Series a-Physical Geography 14, 1-164.
Caminos, R., Hall, M. A., Lapido, O., Lizuain, A., Page, R., and Ramos, V., 1981. Reconocimiento geologico de los Andes Fueguinos, territorio Nacional de Tierra del Fuego. In “IIX Congreso Geologico Argentino.” pp. 759-786, Buenos Aires.
Dalziel, I. W. D., and Brown, R. L., 1989. Tectonic Denudation of the Darwin Metamorphic Core Complex in the Andes of Tierra Del Fuego, Southernmost Chile - Implications for Cordilleran Orogenesis. Geology 17, 699-703.
deMenocal, P., Ortiz, J., Guilderson, T., Adkins, J., Sarnthein, M., Baker, L., and Yarusinsky, M., 2000. Abrupt onset and termination of the African Humid Period: rapid climate responses to gradual insolation forcing. Quaternary Science Reviews 19, 347-361.
Douglass, D. C., Singer, B. S., Kaplan, M. R., Ackert, R. P. J., Mickelson, D. M., and Caffee, M. W., 2005. Evidence of early Holocene glacial advances in southern South America from cosmogenic surface exposure dating. Geology 33, 237-240.
Farr, T. G., Rosen, P. A., Caro, E., Crippen, R., Duren, R., Hensley, S., Kobrick, M., Paller, M., Rodriguez, E., Roth, L., Seal, D., Shaffer, S., Shimada, J., Umland, J., Werner, M., Oskin, M., Burbank, D., and Alsdorf, D., 2007. The shuttle radar topography mission. Reviews of Geophysics 45, RG2004.
Gilli, A., 2003. Tracking late Quaternary environmental change in southermost South America
using lake sediments of Lago Cardiel (49°S), Patagonia, Argentina. Unpublished PhD thesis, Swiss Federal Institute of Technology Zurich.
Gilli, A., Anselmetti, F. S., Ariztegui, D., Beres, M., McKenzie, J. A., and Markgraf, V., 2005.
Seismic stratigraphy, buried beach ridges and conturite drifts: the Late Quaternary history of the closed Lago Cardiel basin, Argentina (49°S). Sedimentology 51, 1-23.
Girardclos, S., Schmidt, O. T., Sturm, M., Ariztegui, D., Pugin, A., and Anselmetti, F. S., 2007. The 1996 AD delta collapse and large turbidite in Lake Brienz. Marine Geology 241, 137- 154.
Haberzettl, T., Fey, M., Lucke, A., Maidana, N., Mayr, C., Ohlendorf, C., Schabitz, F., Schleser, G.
H., Wille, M., and Zolitschka, B., 2005. Climatically induced lake level changes during the last two millennia as reflected in sediments of Laguna Potrok Aike, southern Patagonia (Santa Cruz, Argentina). Journal of Paleolimnology 33, 283-302.
Hondius, H., 1633. America Noviter Delineata (J. Hondius, Ed.), Amsterdam.
Jaschek, E., Sabbione, N., and Sierra, P., 1982. Reubicación de sismos localizados en territorio argentino (1920-1963) (S. Geofísica, Ed.). Observatorio Astronómico de la Universidad Nacional de La Plata.
Kalnay, E., Kanamitsu, M., Kistler, R., Collins, W., Deaven, D., Gandin, L., Iredell, M., Saha, S., White, G., Woollen, J., Zhu, Y., Chelliah, M., Ebisuzaki, W., Higgins, W., Janowiak, J., Mo, K. C., Ropelewski, C., Wang, J., Leetmaa, A., Reynolds, R., Jenne, R., and Joseph, D., 1996. The NCEP/NCAR 40-year reanalysis project. Bulletin of the American Meteorological Society 77, 437-471.
Klepeis, K. A., 1994. The Magallanes and Deseado fault zones: Major segments of the South American-Scotia transform plate boundary in southernmost South America, Tierra del Fuego. Journal of Geophysical Research 99, 22001-22014.
Kranck, E., 1932. Geological investigations in the Cordillera of Tierra del Fuego. Acta Geographica 4, 1-231.
Lodolo, E., Lippai, H., Tassone, A., Zanolla, C., and Menichetti, M., 2007. Gravity map of the Isla Grande de Tierra del Fuego, and morphology of Lago Fagnano. Geologica Acta 5, 307-314.
Lodolo, E., Menichetti, M., Bartole, R., Ben-Avraham, Z., Tassone, A., and Lippai, H., 2003.
Magallanes-Fagnano continental transform fault (Tierra del Fuego, southernmost South America). Tectonics 22, 1076.
Luterbacher, J., Dietrich, D., Xoplaki, E., Grosjean, M., and Wanner, H., 2004. European
seasonal and annual temperature variability, trends, and extremes since 1500. Science 303, 1499-1503.
Marco, S., and Agnon, A., 2005. High-resolution stratigraphy reveals repeated earthquake faulting in the Masada Fault Zone, Dead Sea Transform. Tectonophysics 408, 101- 112.
Mariazzi, A., Conzzono, V., Ulibarrena, J., Paggi, J., and Donadelli, J., 1987. Limnological investigation in Tierra del Fuego, Argentina. Biología Acuática 10, 1-74.
Menichetti, M., Lodolo, E., and Tassone, A., 2008. Structural geology of the Fuegian Andes and Magallanes fold-and-thrust belt - Tierra del Fuego Island. Geologica Acta 6, 19-
42.
Menichetti, M., Lodolo, E., Tassone, A., and Geletti, R., 2001. Neotectonics at the Magallanes- Fagnano fault system (Tierra del Fuego Island). In “Antarctic Neotectonics Workshop.”
pp. 55, Siena.
Montgomery, D., Balco, G., and Willet, S. D., 2001. Climate, tectonics and the morphology of the Andes. Geology 29, 579-582.
Nordenskjöld, O., 1898. Notes on Tierra del Fuego. An account of the Swedish Expedition of 1895-1897. Scottish Geographical Magazine 12, 393-399.
Olivero, E. B., and Martinioni, D. R., 2001. A review of the geology of the Argentinian Fuegian Andes. Journal of South American Earth Sciences 14, 175-188.
Oyola-Yemaiel, A., 1999. The Early Conservation Movement in Argentina and the National Park Service. Universal-Publishers.
Pielke, R., Wigley, T., and Green, C., 2008. Dangerous assumptions. Nature 452, 531-532.
Quirós, R., Baigun, C. R. M., Cuch, S., Delfino, R., de Nichilo, A., Guerrero, C., Marinone, M. C., Menu Marque, S., and Scapini, M. C., 1988. Evaluación del rendimiento pesquero potencial de la República Argentina I: Datos 1. . In “Instituto Nacional de Investigación y Desarrollo Pesquero, Informe Técnico.” (I. T. d. D. d. A. Continentales, Ed.), pp. 55 Buenos Aires.
Quirós, R., and Drago, E., 1999. The environmental state of Argentinean lakes: An overview.
Lakes & Reservoirs: Research and Management 4, 55-64.
Schnellmann, M., Anselmetti, F. S., Giardini, D., McKenzie, J. A., and Ward, S. N., 2002.
Prehistoric earthquake history revealed by lacustrine slump deposits. Geology 30, 1131-1134.
Singer, B. S., Ackert, R. P. J., and Guillou, H., 2004. 40Ar/39Ar and K-Ar chronology of
Pleistocene glaciations in Patagonia. Geological Society of America Bulletin 116, 434- 450.
Sletten, K., Blikra, L. H., Ballantyne, C. K., Nesje, A., and Dahl, S. O., 2003. Holocene debris flows recognized in a lacustrine sedimentary succession: sedimentology, chronostratigraphy and cause of triggering. Holocene 13, 907-920.
Soldan, P., and Felipe, M., 1888. Atlas geografico Argentino. Gobernacion de la Tierra del Fuego e Islas Malvinas. (F. Lajouane, Ed.), Buenos Aires.
Tassone, A., Lippai, H., Lodolo, E., Menichetti, M., Comba, A., Hormaechea, J. L., and Vilas, J. F., 2005. A geological and geophysical crustal section across the Magallanes-Fagnano fault in Tierra del Fuego. Journal of South American Earth Sciences 19, 99-109.
Wafer, L., and Winship, G. P., 2004. A New Voyage and Description of the Isthmus of America. Kessinger Publishing.
2
chapter 2
: seismic stratigraphy of lago FagnanoSeismic stratigraphy of Lago Fagnano sediments (Tierra del Fuego, Argentina) - A potential archive of paleoclimatic change and tectonic activity since the Late Glacial
1_____________________________________________________________________
Abstract
Located at 54°S in the heart of the Island of Tierra del Fuego, Lago Fagnano occupies the deepest of a chain of en-echelon tectonic depressions along the Magallanes- Fagnano Transform system (MFT). A recent geophysical campaign combining 3.5 kHz (pinger) single-channel with 1 in3 airgun multi-channel systems surveyed more than 100 m of glacio-lacustrine sediments filling two main sub-basins. These data provide a unique opportunity to visualize the most recent lacustrine sequence with high- resolution while simultaneously imaging the oldest infill.
A preliminary seismic stratigraphic analysis of the high-resolution 3.5 kHz pinger data allowed the identification of three major seismostratigraphic units (A, the oldest and C, the youngest). While unit A is interpreted as glacially derived sediments, the overlying unit B is interpreted as fining upward sequences of proglacial turbidites reflecting sediment pulses released by the retreating Fagnano glacier during the last deglaciation. A major environmental change occurred during deposition of unit C when pelagic style of sedimentation is intercalated by sequences of downslope mass flow events probably triggered by relatively strong tectonic pulses along the MFT system.
Gravity cores show a regular alternation of light and dark laminae occasionally interrupted by homogenous sedimentary units interpreted as turbidites. Ultra-high resolution X-ray fluorescence micro-profiles show fluctuations in major trace elements at mm scale that may indicate seasonal variations in the sedimentary influx. These core data provide a unique record of decadal changes in regional climate that can be compared with other marine and continental archives to improve our understanding of the forcing mechanisms behind climate change.
1 Published as: Waldmann , N., Ariztegui, D., Anselmetti, F.S., Austin Jr., J.A., Dunbar, R., Moy, C.M. and Recasens, C., 2008. Seismic stratigraphy of Lago Fagnano sediments (Tierra del Fuego, Argentina) - A potential archive of paleoclimatic change and tectonic activity since the Late Glacial. Geologica Acta, 6, 101-110.
2.1. Introduction
Lacustrine sediments provide one of the best continental archives of environmental change (Gierlowski-Kordesch and Kelts, 2000). In addition, many lakes are located in tectonically sensitive regions; therefore, their sedimentary record may also archives regional tectonic events (Chapron et al., 1999). Thus, lacustrine basins can play a key role in reconstructing the paleoclimatic record while disentangling superimposed paleotectonic imprints.
Acoustic methods, such as high-resolution seismic reflection profiling, are crucial tools for studying lacustrine basins, their structure and their sedimentation regime. High- resolution seismic sequence analysis is becoming a routine approach to lake studies.
Scholz and Rosendahl (1988) first applied principles of sequence stratigraphy to large African rift-valley lake systems, and more recently seismic-stratigraphic studies have been successfully used to reconstruct past environmental changes in a variety of lacustrine settings (Moore et al., 1994; Seltzer et al., 1998; Abbott et al., 2000; Ariztegui et al., 2001;
Gilli et al., 2001; Schnellmann et al., 2005; Ariztegui et al., in press). In this paper, we present seismic reflection data from the Argentinean section of Tierra del Fuego’s Lago Fagnano (Argentina/Chile), showing the potential of this basin, the southernmost large body of unfrozen fresh water in the world, to reconstruct past environmental changes. This article further explores Lago Fagnano sediments as a potential archive of earthquake-triggered mass-wasting events, thereby investigating the paleo-earthquake history of the southernmost tip of South America. Both 3.5 kHz and small airgun seismic- reflection data and the corresponding seismic stratigraphic analysis were calibrated with sedimentary cores covering the uppermost part (~1-8 m) of the seismic profiles.
Our preliminary geophysical and sedimentological results support the potential of this lacustrine sequence to reconstruct both the paleo-seismicity history of southernmost South America as well as its paleoclimatic evolution since the Last Glacial. Continuous records of former climates from such far Southern Hemisphere areas are scarce and are essential both for linking continental Patagonia with Antarctic climate records and for comparing the paleoclimate evolution of the Northern and Southern hemispheres.
2.2. Morphological and geological settings of Lago Fagnano
The island of Tierra del Fuego is the southernmost large landmass in the world, other than Antarctica. This island is bounded by the southern Atlantic Ocean to the east and the southern Pacific Ocean to the west. At ca. 54°S, Lago Fagnano (or Lago Kami in the native Yamana language) lies along the southern half of the island. With a total area of about 560 km2, this oligotrophic, latitudinally elongated lake (Mariazzi et al., 1987) of ~105 km length with a maximal width of ~10 km is the southernmost and largest ice-free water body in the world (Fig. 1). Located along a major plate boundary
and under the Andean glaciations, the lake has a combined tectonic and glacial origin.
Sediment accumulation probably covers the entire Holocene and may date back even to the Last Glacial Maximum (LGM) (Bujalesky et al., 1997).
Currently, the climate of this region is alpine, with a strong winter sub polar- Antarctic influence. Lago Fagnano lies at the southernmost limit of the southwesterly wind influence that brings moisture and humidity to the region during austral summer.
This situation, however, may have changed during the Last Glacial and Late Glacial periods, as shown by other southern Patagonian climatic archives (e.g., Gilli et al., 2001;
Douglass et al., 2005).
The lake comprises two sub-basins: a smaller, deeper basin in the east reaching a maximum depth of 210 m, and an elongated, shallower basin in the west exhibiting
~110 m maximum water depth (Fig. 1). The southern shores are bordered by the foothills of Sierras de Alvear (eastern part of Cordillera Darwin), while the lower mountain belt of the Sierras de Beauvoir borders the northern margin of the lake. The Claro, Milna, Valdez and Turbio rivers all drain into this lake, whereas the Azopardo River at the western extreme of the lake is the only outlet towards the Pacific Ocean, through the Almirantazgo Fjord (Admiralty Sound) and the Straits of Magellan.
Patagonia
South America Plate
Scotia Plate Tierra del
Fuego Atlantic
Ocean
Ocean
N N
500 Km 250 0
67° 30’
68° 00’
68° 30’
54° 35’
Lago Fagnano
0 10
Km
100 Km 50 0
N
Claro River
Milna River Azopardo
River
Turbio River FA06-14 Antarctic
Plate
Southe rn Andes
Cordillera Magallanes Fagnano
Transform
Southe rn Chile
Trench
A B
C
Sierras de Alvear
Sierras de Beauvoir
FA05-3
Fig. 3b
Fig. 2 & 3a
Figure 1: A) Satellite image of Patagonia. B) Satellite image of Tierra del Fuego. Dashed line marks the maximum extension of ice during the LGM, while dark line represents the coastline at LGM times (Rabassa et al., 2000). C) Bathymetric map of Lago Fagnano with a 25 m contour interval (modified from Lodolo et al., 2002), showing our entire seismic grid. Thicker lines and solid dots indicate the location of the seismic profiles and sedimentary cores presented in this article, respectively.
The lake occupies the deepest continental pull-apart basin in a series of graben- shaped, asymmetric tectonic sinks organized in an en-echelon arrangement along the Magellan-Fagnano Transform (MFT) (Lodolo et al., 2003, 2007; Tassone et al., 2005;
Menichetti et al., this issue). The onset of horizontal left-lateral movement along the MFT is not well dated, but is presumed to have started during the Oligocene (Klepeis, 1994; Lodolo et al., 2003). Recent fault scarps and displacement of glacio-lacustrine sediments along the transform lineation in and co-linear with the eastern part of the lake indicate ongoing tectonic activity (Dalziel, 1989; Menichetti et al., 2001). Moreover, fluvial drainages in the same region are clearly influenced by the presence of latitudinal structures related to the strike-slip fault system (Menichetti et al., 2001).
The bedrock substratum along the southern flanks of the lake includes low- grade metamorphic black shale and whitish-gray tuff sequences, corresponding to a Late Jurassic marine volcano-sedimentary complex (Le Maire Formation; Borrello, 1969). The northern and eastern margins of the lake are composed of folded Lower Cretaceous to Tertiary marine greywacke (Yahgan and Beauvoir formations; Caminos et al., 1981; Olivero and Martinioni, 2001). An Upper Cretaceous monzodioritic body (Cerro Hewhoepen batholith) intrudes the formations at the southeast extreme of the lake (Acevedo et al., 2000).
The instrumentally recorded seismicity along the transform boundary is low (Mw<3.5) and shallow (Vuan et al., 1999). Strong, historically documented seismic events, however, have also occurred. The last earthquake that affected the Island of Tierra del Fuego occurred in 1949 (Mw=7.8); its epicenter was located along the MFT in the Atlantic several tens of kilometers off the island’s coast. This earthquake caused the subsidence of large areas close to the lakeshores, forming a series of lagoons still connected to the main lake. Moreover, this seismic event triggered several landslides along the banks of Lago Fagnano (Menichetti et al., 2001) and also in the westernmost arm of the Straits of Magellan, where tsunami waves were observed (Jaschek et al., 1982). Hence, the sedimentary infilling of the lake offers an exceptional possibility for reconstructing both the paleoclimatic history of the region as well as the paleoseismic history of the MFT. Here, we discuss for the first time data bearing on climate and tectonic controls on Lago Fagnano sedimentation, using a geophysical approach combining two different reflection seismic systems, along with preliminary sediment characterization as deduced by grab samples, gravity and piston cores (Fig. 1).
2.3. Methodology 2.3.1. Seismic survey
In March 2005, we acquired more than 800 km of geophysical data during
