EOS - Volume 83, n.1 January 2002

Anatomy of a continental transform fault in Tierra del Fuego

Authors
E. Lodolo, R. Geletti, P. Sterzai, Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Trieste, Italy; M. Menichetti, Istituto di Geodinamica e Sedimentologia, Università di Urbino, Italy; A. Tassone, H. Lippai, Instituto de Geofisica “Daniel Valencio”, Universidad de Buenos Aires, Argentina; J.-L. Hormaechea, Estación Astronómica Rio Grande, Argentina.
For additional information, contact E. Lodolo, Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS). E-mail: elodolo@ogs.trieste.it

A comprehensive suite of field surveys was carried out by a team of Italian and Argentinean scientists in the Tierra del Fuego region, the southern tip of South America, to investigate the 600-km-long Magallanes-Fagnano fault system (MFS), a transform-type margin developed on continental crust (Figure 1). The identification and analysis of the morphological and structural elements related to the MFS, and the understanding of mechanisms of slip along the fault, are the principal objectives of an ongoing project called TESAC (Tectonic Evolution of the South America-Scotia plate boundary during the Cenozoic), jointly managed by the Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS) of Trieste, and the Dpto. de Geologia, Universidad de Buenos Aires.

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, in two continental blocks. An integral part of the project is focused on the pull-apart basins, the elongated depocenters which generally form in correspondence of the principal displacement zone of the fault system. 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. The identification of the deep crustal features within a fault zone and the analyses of the sedimentary settings associated with strike-slip motion are thus key elements to reconstruct 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 which hides a significant part of the MFS - and the central-eastern 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 asymmetric basins formed within the principal displacement zone, in which the thick end of the depositional wedge abuts the transform fault, suggest simultaneous strike-slip motion and transform-normal extension, a common feature found in other continental transtensional environments.
The Tierra del Fuego is one of the few places on Earth which offers the opportunity to observe the surface and subsurface geological features related to a continental transform plate boundary, but it is one of the least known because its remoteness and the difficulties of access in most of its territory. The presence of large peat zones, widespread lagoons, forestry covers and the absence of paths, have always impeded the collection of geophysical and geological data on a regular basis along the strike of the transform segment.

TESAC logistics
Four separate campaigns, spanning the period February 1998 - December 2000, were realized both onshore and offshore in the Tierra del Fuego region. The group of geologists and geophysicists were logistically supported during the overall field operations by technicians of the Estación Astronómica Rio Grande, that realized the D-GPS data network, and supported the researchers for the acquisition of the 380 gravity and magnetic data points. The main effort was dedicated to reach some 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 for the first time realized in collaboration with the Prefectura Naval - Dpto. Lago Fagnano, which made available a Zodiac boat during the measurements. 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 two weeks preceding the cruise, which took place on October 1999, a portable multichannel seismic reflection equipment was installed onboard. It consisted of a 96-channels, 1.2-km-long solid-state analogue streamer, and two GI-guns (total volume 4 liters) for the acoustic energization. A total of about 900 km of high-resolution seismic profiles have been collected across the eastward projection of the MFS, and on the southern continental margin of the Isla Grande. This survey implemented previous seismic studies conducted by the research vessel OGS-Explora in the Magallanes Strait with the purpose to image the along-strike seismic structure of the MFS. In addition to the collected data, multi-spectral SPOT images, Synthetic Aperture Radar (SAR) frames and aerial photographs have helped in identifying on a regional scale the main morphostructural lineaments of the study area.

Wrench tectonics in the Tierra del Fuego region
The field geological mapping of the Isla Grande was preceded by a careful analysis of the remote-sensing images, and in particular of the aerial photos, in order 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 Nacionál N. 3, the only N-S arterial road of the Isla Grande, which connects Rio Grande and Ushuaia; along the northern shore of the Lago Fagnano and its surroundings; and, in some sectors of the Atlantic coast of the island, characterized by noticeable, sub-vertical cliffs.
Along the Ruta Nacionál 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 series of ESE-WNW-oriented folds and associated N-verging thrusts (i.e., the Magallanes fold and thrust belt). This system developed adjacent to the northern flank of the southernmost Andes in response to a mid-Cretaceous to Tertiary compressional phase, and involved also the Jurassic-Cretaceous assemblages of the Rocas Verdes marginal basin (Dalziel, 1989). Several wrenching shear zones have been identified along the northern shore of the Lago Fagnano, where the most active deformation occurs. The field evidence include shear bands, sigmoidal en-echelon tension gashes, and in some cases composite fluid inclusions in the deforming sedimentary beds. To the east of Lago Fagnano, there are morphological evidences of fault-related Quaternary activity, with linear truncation of river meanders, deflection of stream directions, and talus slopes. Some of these features are very recent, and include: 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 sub-vertical, S-dipping normal faults are present; a W-E scarp of about 1 m, associated with a gravel barrier; and, a sag pond of the Rio Turbio, the eastern tributary of the Lago Fagnano. They were probably created during the 1949, December 17 earthquake, described by Lomnitz (1970) in these terms: “Landslides occurred along the west 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 were recognized also in many of the steep outcrops which 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, because the absence of paths and the difficulty of access. The Dirección Provincial de Aeronáutica – Tierra del Fuego, got available a helicopter during part of the field survey along the coast to reach the most remote areas, and transport all the camp equipment. The field geological reconnaissance was mainly conducted during the low-standing tide intervals, that allow impressive exposition of the structures of the deformed sedimentary strata.

Lago Fagnano: a large pull-apart basin

The bathymetric map of the 105-km-long, E-W-trending Lago Fagnano, was realized utilizing seventeen 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 an 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 (Ben-Avraham and Zoback, 1992). Onshore, the strands of the MFS are well recognizable from the SPOT and Digital Elevation Model (DEM) maps, that show the presence of an ESE-WNW major lineament which 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, unconnected 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, morphologically less evident, reaches the coast to the east in correspondence of Cabo Colorado, where a complex deformational system include multilayer folds, thrust 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 in correspondence of the unconnected ends of the master segments of the MFS. Offshore, these gravity minima may be linked to the east with the major negative gravity anomaly that regionally extends E-W within the Falkland Trough, as seen from satellite-derived data.
The cross-section geometry of the MFS and associated basins, has been imaged by the seismic lines acquired by the TESAC project off the Atlantic coast of the Isla Grande, and those gathered during precedent R/V OGS-Explora Antarctic Campaigns in the central and western Magallanes Strait. The profile presented in Figure 4 shows a 8-km-wide asymmetric basin – comparable to the depression occupied by Lago Fagnano - which is bounded by a near-vertical discontinuity on one side reaching the sea floor (the transform segment), 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 transform-normal extension (Ben Avraham and Zoback, 1992). Comparable features are known for other transform environments, like as the Dead Sea rift, San Andreas fault, Polochic fault (Guatemala), El Pilar fault (off Venezuela), Lupa fault (Rukwa Lake, Tanzania).

Transtension along the Magallanes-Fagnano fault system
The data gathered both onshore and offshore support the interpretation that the MFS is remarkably transtensive in nature, and is structurally and temporally superposed on the older tectonic style of the Tierra del Fuego region (i.e., the Cretaceous-Tertiary contractional system of the Magallanes fold and thrust belt). It is constituted by diverse segments in a en-echelon arrangement, along which pull-apart depocenters have formed. The stress style that characterizes the MFS contrasts however with those of the North Scotia Ridge at the central-eastern part of the South America-Scotia plate boundary, where seismic profiles have documented N-S-directed convergence (Ludwig and Rabinowitz, 1982).

The evolution of the MFS is intimately related to the complex tectonic events responsible of the late-Oligocene development of the oceanic floor of the western Scotia Sea, which definitively led to the separation of Antarctica from the South America 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 in the Chilean side of Isla Grande (Klepeis, 1994b), indicate that these fault zones have accommodated left-lateral strike-slip motion which may have been present since Cretaceous time (Grunow et al., 1992). In a larger scale, the regional deformation mechanism has generated impressive topographic lineaments associated to strike-slip displacements in the central region of the Magallanes Strait, with abundant evidence of present activity (Winslow, 1982). These fault trends, that partly exploit early Tertiary and Cretaceous structural trends, represent the diverse segments constituting the MFS.

The present-day seismicity along the transform boundary is very low (<3.5 in magnitude) and shallow, as monitored in the period 1997-1999 by an array of portable broad-band seismic stations located in the Chilean territory, and by a permanent broad-band station installed near Ushuaia in 1996 (Vuan et al., 1999). Historical seismicity is however significant, as demonstrated by the 1949 event, and by a preceding, magnitude 7 earthquake, 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 re-occupation of D-GPS stations located in both the South America and Scotia sides of the fault system in the Isla Grande (Del Cogliano et al., 2000). The relative motion is partitioned along the diverse segments which make up the fault array, where the linkage and step-over geometry play an important role in the pull-apart system development.

Acknowledgments
Funds for this study were provided by the Italian Programma Nazionale di Ricerche in Antartide (PNRA), and partly by the Dirección Nacional del Antártico (DNA). L. Barbero, G. Connon and C. Ferrer (EARG) greatly contributed during the field operations. F. Coren, R. Vidmar and P. Vascotto (OGS) aided with processing of satellite-derived maps. Thanks are due to the Argentinean Navy, and particularly to Captain J.A. Gopcevich Canevari and crew of the research vessel A.R.A. Puerto Deseado (CONICET-SHIN) 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.

References
Ben-Avraham, Z. and M.D. Zoback, Transform-normal extension and asymmetric basins: an alternative to pull-apart models, Geology, 20, pp. 423-426, 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, Journ. Geoph. Res., 100, pp. 8257-8266, 1995.
Dalziel, I.W.D., Tectonics of the Scotia Arc, Antarctica, Field Trip Guide, T180, AGU, Washington D.C., 206 pp., 1989.
Del Cogliano, D., R. Perdomo, J.-L. Hormaechea, Desplazamiento entre placas tectónicas en Tierra del Fuego, Actas de la XX Reunión Científica de la AAGG, Mendoza, 2000.
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, Geolo. Soc. Am. Bull., 104, pp. 1497-1514, 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, Journ. Geoph. Res., 99, pp. 22,001-22,014, 1994b.
Lomnitz, C., Major earthquakes and tsunamis in Chile during the period 1535 to 1955, Geologische Rundschau, 59, pp. 938-960, 1970.
Ludwig, W.J. and P.D. Rabinowitz, The collision complex of the North Scotia Ridge, Journ. Geoph. Res., 87, pp. 3731-3740, 1982.
Vuan, A., R. Cazzaro, G. Costa, M. Russi, and G.F. Panza, S-wave velocity models in the Scotia Sea region, Antarctica, from non-linear inversion of Rayleigh waves dispersion, Pure and Applied Geophysics, 154, pp. 121-139, 1999.
Winslow, M.A., The structural evolution of the Magallanes Basin and neotectonics in the southernmost Andes, In: Antarctic Geoscience (C. Craddock, ed.), Madison, University of Winsconsin Press, pp. 143-154, 1982.
EOS – Tierra del Fuego