Researchers Target a Continental Transform Fault in Tierra del Fuego

Transcription

1 Eos,Vol. 83, No. 1,1 January 2002 VOLUME 83 NUMBER 1 1 JANUARY 2002 EOS, TRANSACTIONS, AMERICAN GEOPHYSICAL UNION PAGES 1-12 Researchers Target a Continental Transform Fault in Tierra del Fuego PAGES 1,5-6 A comprehensive suite of field surveys was carried out by a team of Italian and Argentinean scientists in South America's Tierra del Fuego region to investigate the 600-km-long Magallanes- Fagnano fault system (MFS), a transform-type margin that developed on continental crust (Figure 1). Identifying and analyzing the morphological and structural elements related to the MFS and understanding mechanisms of slip along the fault are the principal objectives of an ongoing project called Tectonic Evolution of the South America-Scotia Plate Boundary During the Cenozoic (TESAC). The MFS, which represents one of the major segments of the South America-Scotia plate boundary, runs from the western arm of the Magallanes Strait to the Atlantic offshore, and substantially splits the Isla Grande, the main island of Tierra del Fuego, into two continental blocks. An integral part of the project is focused on the pull-apart basins, the elongated depocenters that generally form where the principal displacement zone of the fault system is laterally offset. The three-dimensional architecture of the pull-apart basins is strongly controlled by the magnitude and type of motion along the principal displacement zone, and by the geometry of the underlying basement-fault systems. Identifying the deep crustal features within a fault zone and analyzing the sedimentary settings associated with strike-slip motion are thus key elements for reconstructing the geometry and the kinematic development of the transcurrent system. Data gathered during the TESAC project off the Atlantic coast of the Tierra del Fuego, along Lago Fagnano a major depression that hides a significant part of the MFS and the centraleastern part of Isla Grande have imaged the surface and sub-surface structure of the transform fault and its associated basins.the MFS is composed of distinct tectonic lineaments that are segments of the transform system and are represented by mostly near-vertical faults, with polarities that change along the strike of the fault.the sedimentary architecture of the Pacific \Ocean 74 W 70"W 66 W 62"W Fig. 1. This simplified tectonic sketch of the southern tip of South America shows the principal structural features mentioned in the text. Inset box shows the Digital Elevation Model presented in Figure 3. The stippled segment refers to the seismic profile in Figure 4. MFS = Magallanes- Fagnano fault system; IF = Lago Fagnano; MS = Magallanes Strait; CSP = Cabo San Pablo; CC = Cabo Colorado; CL = Cabo Leticia. Original color image appears at the back of this volume.

2 Eos, Vol. 83, No. 1,1 January 2002 Lago Fagnano -68*00* Simplified Battiymetric Map Fig. 2. This bathymetric map shows the central-eastern Lago Fagnano, which reaches a maximum water depth of202 m. The basin is highly asymmetric in shape, with the steepest slope along the northern shore. Original color image appears at the back of this volume. Fig. 3. This relief image of the area comprising the eastern Lago Fagnano was obtained by super imposing the SPOT image onto the Digital Elevation Model map. The clearly recognizable master fault of the Magallanes-Fagnano fault system is associated with a linear valley occupied by the Rio Turbio. Original color image appears at the back of this volume. asymmetric basins formed within the princi pal displacement zone, in which the thick end of the depositional wedge abuts the transform fault, suggests simultaneous strike-slip motion and transform-normal extensions common feature found in other continental trans-tensional environments. The Tierra del Fuego is one of the few places on Earth that offers the opportunity to observe the surface and sub-surface geological features related to a continental transform plate bound ary But it is one of the least known because of its remote location at the southern tip of South America and the difficulties of access.the pres ence of large peat zones, widespread lagoons, forestry covers, and the absence of paths have always impeded routine collection of geophysi cal and geological data along the strike of the transform segment. Project Logistics Four separate campaigns were carried out both onshore and offshore in the Tierra del Fuego region from February 1998 through December The group of geologists and geophysicists was logistically supported during the overall field operations by technicians of the Estacion Astronomica Rio Grande, which provided the Differential-Global Positioning System (D-GPS) data network and supported the researchers in acquiring the 380 gravity and magnetic data points. The main effort was dedicated to reaching some of the outcrops of the central and eastern part of the Isla Grande to conduct the field structural and geological reconnaissance.the bathymetric survey of the Lago Fagnano was completed in collaboration with the Prefectura Naval - Dpto. Lago Fagnano, which provided a Zodiac boat during the measurement phase. The offshore seismic survey was carried out off the Atlantic coast of Isla Grande aboard the A.R.A. Puerto Deseado, an oceanographic vessel owned by the Argentinean navy During the 2 weeks preceding the cruise, which took place in October 1999, a portable, multichannel seismic reflection instrument was installed onboard the ship. It consisted of a 96-channel, 1.2-km-long, solid-state analogue streamer and two air-guns (Generator-Injector system) for the acoustic energization. A total of about 900 km of high-resolution seismic profiles were col lected across the eastward projection of the MFS and on the southern continental margin of the Isla Grande. This survey used previous seismic studies conducted by the research vessel OGS-Explora in the Magallanes Strait to image the along-strike seismic structure of the MFS. In addition to the collected data, multispectral Systeme Probatoire d'observation de la Terre (SPOT) images, Synthetic Aperture Radar (SAR) frames, and aerial photographs helped to identify the main morphostructural lineaments of the study area. W r e n c h Tectonics in the Tierra del Fuego Region The field geological mapping of the Isla Grande was preceded by careful analysis of the remote-sensing images, and in particular of the aerial photos, to select those accessible areas where the majority of outcrops occur along the principal deformation zone of the MFS.The study was conducted in three key areas: along the Ruta Nacional N. 3, the only north-south arterial road of the Isla Grande that connects Rio Grande and Ushuaia; along the northern shore of the Lago Fagnano and its surroundings; and along some sectors of the island's Atlantic coast characterized by noticeable, sub-vertical cliffs. Along the Ruta Nacional N. 3, it is possible to analyze, from north to the south, the progressive deformation affecting the Palaeocene-Eocene rocks pertaining to the Magallanes basin, from sub-horizontal packages of sandstone-dominated beds, to a

3 Eos,Vol. 83, No. 1,1 January 2002 SE NW coast of Tierra del Fuego and along the banks of Lago Fagnano...The epicenter was located presumably on the fault structure which includes Lago Fagnano and the western arm of the Magallanes Straits...The magnitude was 7.5." Important strike-slip activity was also recog nized in many of the steep outcrops that characterize part of the Atlantic coast of Isla Grande, from Cabo San Pablo to Cabo Leticia. In this area, the logistic effort was particularly remarkable due to the absence of paths and the difficulty of access.the Direccion Provin cial de Aeronautica - Tierra del Fuego provided a helicopter during part of the field survey along the coast to reach the most remote areas and transport the camp equipment.the field geological reconnaissance was mainly conducted during the low-standing tide inter vals, which expose the structures of the deformed sedimentary strata. Lago Fagnano: A Large Pull-apart Basin Fig. 4. This section of a multichannel seismic reflection profile was acquired off the Atlantic coast of Isla Grande (location in Figure I). Data image the offshore projection of the master fault of the Magallanes-Fagnano fault system and associated asymmetric basin. series of east-southeast/west-northwest-oriented folds and associated north-verging thrusts (that is, the Magallanes fold and thrust belt). This system developed adjacent to the north ern flank of the southernmost Andes in response to a mid-cretaceous to Tertiary compressional phase, and also involved the Juras sic-cretaceous assemblages of the Rocas Verdes marginal basin [Dalziel, 1989].Several wrenching shear zones, which cut across the deformed sedimentary layers, have been iden tified along the northern shore of the Lago Fag nano. The field evidence includes cataclastic shear bands injected with numerous veins, and brittle sub-vertical faults that form com plex interlocking arrays.the displacements generated by both normal and minor reverse movements in these fault zones produced a series of offset, stepped blocks.these structures testify to the presence of an important compo nent of shear overprinted on the previously deformed sequences. To the east of Lago Fagnano, there is morphological evidence of fault-related Quaternary activity, with linear truncation of river meanders, deflection of stream direc tions, talus slopes, and the formation of a sag pond of the Rio Turbio, the eastern tributary of the Lago Fagnano. Some of these features are very recent, as seen in a cross-stratified glacio-fluvial sand outcrop in a small quarry located just to the east of the eastern shore of Lago Fagnano, where several sets of subvertical, south-dipping normal faults, are pres ent. Some of these features were probably created during the 17 December 1949 earth quake, described by Lomnitz [1970] in these terms: "Landslides occurred along the west The bathymetric map of the 105-km-long, east-west-trending Lago Fagnano was created using 17 echo-sounder profiles acquired, often under prohibitive meteorological conditions, aboard a Zodiac boat.the map (Figure 2) delineates the main submerged morphological expressions of the South America-Scotia plate boundary in this sector of the Isla Grande, and most probably reflects its sub-bottom structure.the basin profile presents a highly asymmetric shape, and the sense of asymmetry changes along its length. Lago Fagnano is probably the surface expression of a large pull-apart basin, formed by strands of the MFS. Its length is comparable to some of the largest strike-slip basins along continental transforms, which are mostly asymmetric in shape [BenAvraham and Zoback, 1992]. Onshore, the strands of the MFS are well recognizable from the SPOT and Digital Elevation Model (DEM) maps, which show the presence of an east-southeast/west-north west major lineament that traverses the central-eastern Isla Grande from the eastern shore of Lago Fagnano to the Atlantic coast. It is constituted by at least two sub-parallel segments in an en-echelon arrangement. The western segment is characterized by a narrow depression, occupied by the Rio Turbio valley (Figure 3); the other, which is morphologically less evident, reaches the coast to the east in correspondence of Cabo Colorado, where a complex deformational system includes thrusts, multilayer folds associated with an axial plane cleavage, and strike-slip faulting. The complete Bouguer anomaly map shows distinct relative minima along the trace of the MFS in the central-eastern part of the Isla Grande.These gravity features may be interpreted as localized depocenters (the pull-apart basins) generated at releasing bends or sidesteps in the main strike-slip fault system. Offshore, these gravity minima may be linked to the east with the major negative gravity anomaly that regionally extends east-west within the Falk land Trough, as seen from satellite-derived data. The cross-section geometry of the MFS and associated basins has been imaged by the

4 seismic lines acquired by the TESAC project off the Atlantic coast of the Isla Grande and those gathered during earlier R/V OGS-Explora Antarctic campaigns in the central and western Magallanes Strait. The profile presented in Figure 4 shows an 8-km-wide asymmetric basin comparable to the depression occupied by Lago Fagnano that is bounded by a nearvertical discontinuity on one side reaching the sea floor and a set of subsidiary normal faults on the other side. The complex sedimentary architecture of the basin may reflect different tectonic mechanisms in which periods of oblique extension can alternate with transformnormal extension [Ben Avraham and Zoback, 1992].Comparable features in other transform environments are known, such as the Dead Sea rift, San Andreas fault, Polochic fault (Guatemala), El Pilar fault (off Venezuela), and the Lupa fault (Rukwa Lake,Tanzania). Trans-tension along the Magallanes- Fagnano Fault System The data gathered both onshore and offshore support the interpretation that the MFS is remarkably trans-tensive in nature and is structurally and temporally superimposed on the older tectonic style of the Tierra del Fuego region. It constitutes diverse segments in an en-echelon arrangement, along which pull-apart depocenters have formed. However, the stress style that characterizes the MFS contrasts with those of the North Scotia Ridge at the central-eastern part of the South America- Scotia plate boundary, where seismic profiles have documented north-south-directed convergence [Ludwig and Rabinowitz, 1982]. The evolution of the MFS is intimately related to the complex tectonic events responsible for the late-oligocene development of the oceanic floor of the western Scotia Sea, which definitively led to the separation of Antarctica from the South American continent.the role of the MFS in accommodating the South America- Scotia relative motion system may have been predominant after cessation of seafloor spreading in the western Scotia Sea (~9 Ma), but some displacements may have occurred a long time before [Cunningham et al., 1995]. Analyses of fault populations conducted on the Chilean side of Isla Grande [Klepeis, 1994] indicate that these fault zones have accommodated left-lateral strike-slip motion Eos,Vol. 83, No. 1,1 January 2002 that may have been present since Cretaceous time [Grunow et al., 1992]. On a larger scale, the regional deformation mechanism has generated impressive topographic lineaments associated with strike-slip displacements in the central region of the Magallanes Strait, with abundant evidence of present activity [Winslow, 1982].These fault trends,which partly exploit early Tertiary and Cretaceous structural trends, represent the diverse segments constituting the MFS. Present-day seismicity along the transform boundary is very low (<3.5 in magnitude) and shallow, as monitored from 1997 to 1999 by an array of portable broadband seismic stations located in the Chilean territory and by a permanent broadband station installed near Ushuaia in 1996 [Vuan etal, 1999]. Historical seismicity is, however, significant, as demonstrated by the 1949 event, and by a preceding, magnitude-7 earthquake that occurred in the western Magallanes Strait in 1879 [Lomnitz, 1970].The low seismicity can be explained by the slow relative motion between the South America and Scotia plates along the boundary, which is less than 0.5 cm/yr, as documented by D-GPS stations redeployed on both the South America and Scotia sides of the fault system in the Isla Grande [Del Cogliano etal.,2000].the relative motion is partitioned along the diverse segments that make up the fault array, where the linkage and step-over geometry play an important role in pull-apart system development. Acknowledgments TESAC is jointly managed by the Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS) of Trieste and the Dpto. de Geologia, Universidad de Buenos Aires. Funds for this study were provided by the Italian Programma Nazionale di Ricerche in Antartide (PNRA) and partly by the Direccion Nacional del Antartico (DNA). L. Barbero, G. Connon, and C. Ferrer greatly contributed to the field operations. ECoren,R.Vidmar, and PVascotto (OGS) helped process satellite-derived maps.thanks are due to the Argentinean navy, and particularly to Captain J.AGopcevich Canevari and crew of the research vessel A.R.A. Puerto Deseado for their support during data acquisition at sea. We are also grateful to C. Maidana, D. Bravo, S. Prieto, and C. Silva for the invaluable help offered during the bathymetric survey Authors Lodolo, M. Menichetti,A. Tassone, R. Geletti, PSterzai, H. Lippai, and J.-L. Hormaechea For additional information, contact Emanuele Lodolo, Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS),Trieste, Italy elodolo@ogs.trieste.it References Ben-Avraham, Z., and M. D. Zoback,Transformnormal extension and asymmetric basins: An alternative to pull-apart models, Geology, 20, ,1992. Cunningham,W D., I.W D. Dalziel,T.-Y Lee, and L. A. Lawver, Southernmost South America-Antarctic Peninsula relative plate motions since 84 Ma: Implications for the tectonic evolution of the Scotia Arc region,./ Geophys. Res., 100, ,1995. Dalziel, I.W D., Tectonics of the Scotia Arc, Antarctica, Field Trip Guidejl80,206 pp.,agu,washington D.C., Del Cogliano, D., R. Perdomo, and J.-L. Hormaechea, Desplazamiento entre placas tectonicas en Tierra del Fuego, Actas de la XX Reunion Cientifica de la Asociacion Argentina de Geologia y Geofisica, Mendoza Grunow,A. M, I.W D. Dalziel.T. M. Harrison, and M.T Heizler, Structural geology and geochronology of subduction complexes along the margin of Gondwanaland: New data from the Antarctic Peninsula and southernmost Andes, Geol. Soc. Am. Bull, 104, ,1992. Klepeis, K. A.,The Magallanes and Deseado fault zones: Major segments of the South American- Scotia transform plate boundary in southernmost South America,Tierra del Fuego, J. Geophys. Res., 99,22,001-22,014,1994. Lomnitz, C, Major earthquakes and tsunamis in Chile during the period 1535 to 1955, Geologische Rundschau, 59, ,1970. Ludwig,WJ.,and PD. Rabinowitz,The collision complex of the North Scotia Ridge,i Geophys. Res., 87, ,1982. Vuan,A.,R.Cazzaro,G.Costa,M.Russi,and G. FPanza, S-wave velocity models in the Scotia Sea region, Antarctica, from non-linear inversion of Rayleigh waves dispersion,pureappl. Geophys., 154, ,1999. Winslow, M. A.,The structural evolution of the Magallanes Basin and neotectonics in the southernmost Andes, in Antarctic Geoscience, edited by C. Craddock, pp , University of Wisconsin Press, Madison, Initiative to Quantify Terrestrial Carbon Sources and Sinks PAGES 1,6-7 Questions related to the distribution and spatio-temporal dynamics of the terrestrial carbon fluxes are at the core of current scientific and policy debates. In recent years, the primary concern has been the increasing C0 2 content in the atmosphere, its effect on climate, and the associated role of terrestrial ecosystems in mitigating the increase and impact of climate change. However, terrestrial carbon dynamics is also closely related to biodiversity land degradation, and other pressing policy and assessment questions.yet at the global level, no system in place now can provide quantitative information about carbon sources and sinks systematically, reliably, and accurately An international effort aimed at changing this situation was initiated in 1999 under the auspices of the Integrated Global Observing Strategy Partnership (IGOS-P).The Terrestrial Carbon Observation (TCO) initiative is an effort by space and international organizations within IGOS-P to employ the best current observing tools and models to rapidly build up a global observing system for tracking carbon fluxes between the terrestrial ecosystems and the atmosphere or oceans. TCO planning has been led by the Global Terrestrial Observing System (GTOS), with Food and Agriculture

5 Eos,Vol. 83, No. 1,1 January 2002 Page 1 Fig. 1. This simplified tectonic sketch of the southern tip of South America shows the principal structural features mentioned in the text. Inset box shows the Digital Elevation Model presented in Figure 3. The stippled segment refers to the seismic profile in Figure 4. MFS = Magallanes- Fagnano fault system; LF = Lago Fagnano; MS = Magallanes Strait; CSP = Cabo San Pablo; CC = Cabo Colorado; CL = Cabo Leticia.

6 Eos,Vol. 83, No.1, 1 January 2002 ~'-"-- """"7"---r- Lago Fagnano Simplified Bathymetric Map -6...,r~g4 30' '-- -'-54 38' Fig. 2. This bathymetric map shows the central-eastern Lago Fagnano, which reaches a maximum water depth of202 m. The basin is highly asymmetric in shape, with the steepest slope along the northern shore. Page 5 NORTH t\ Page 5 Fig. 3. This relief image of the area comprising the eastern Lago Fagnano was obtained by superimposing the SPOT image onto the Digital Elevation Model map. The clearly recognizable master fault of the Magallanes-Fagnano fault system is associated with a linear valley occupied by the Rio Turbio.