Morphostructure of a transform margin in the Tierra del Fuego region: the Magallanes-Fagnano fault system

LODOLO E.; MENICHETTI, M.; TASSONE A.; GELETTI, R.

Siena. Italia

Workshop; Antartic Neotectonic" Workshop; 2001

Terra Antartica Publ

The Tierra del Fuego region represents one of the few places on Earth which offers the opportunity to observe the geological features related to a continental transform plate boundary. Continental crust boundaries are important to understand because their formation produces distinct geologic structures and generally manifest significant along-strike variations in geometry. The 400-km-long Magallanes-Fagnano fault system (MFS) runs from the western arm of the Magallanes Strait to the Atlantic offshore, and substantially splits the Tierra del Fuego Island in two continental blocks. It marks the trace of the transform margin on continent, and represents one of the major segments of the South America-Scotia strike-slip plate boundary. A comprehensive suite of field surveys was carried out from February 1998 to December 2000 in the Argentinean side of Tierra del Fuego region, as part of an ongoing project called TESAC (Tectonic Evolution of the South America-Scotia plate boundary during the Cenozoic). This research is designed to identify and analyze the structural elements related to the MFS both onshore and offshore, and to understand the role played by this segment of the plate boundary in the general scenario of the Tertiary tectonic evolution of the western Scotia Sea. Onshore measurements comprised geophysical data (DGPS-fixed gravity, magnetic and bathymetric profiles), and field structural and geological reconnaissance. A seismic survey was carried out off the Atlantic coast of the Island aboard the Argentinean R/V A.R.A. Puerto Deseado using a portable multichannel seismic reflection equipment. This survey implements previous seismic studies conducted by the R/V 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 tectonic lineaments characterizing the study area. The modern MFS accommodates the left-lateral relative motion between the South America and Scotia plates, and extends in both the Argentinean and Chilean territories of the Tierra del Fuego. A significant part of the fault system is hidden by the 110-km-long, east-west-trending Lago Fagnano, a major depression where important morphological elements related to the shear zone can be analyzed. The MFS impinges to the west with the southern Chile Trench, forming a sort of triple junction there, and to the east connects with the system of shallow banks and submarine ridges constituting the northern margin of the oceanic Scotia plate. 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 isolation of Antarctica from the other land masses. 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 (Wiens et al., 1998), and by a permanent broad-band station installed near Ushuaia (Argentinean Tierra del Fuego) in 1996 (Vuan et al., 1999), but historical seismicity is significant. The field geological mapping of the Tierra del Fuego, in combination with the regional structural framework of the central-eastern part of the Island gathered from the interpretation of remote-sensing images, have shown that the area is characterized by a sequence of ESE-WNW-trending asymmetric folds and associated N-verging thrusts (the Magallanes fold and thrust belt). This system developed adjacent to the northern flank of the southernmost Andes in response to mid-Cretaceous to Tertiary compressional phase, which involved also the Jurassic-Cretaceous Rocas Verdes marginal basin assemblages (Dalziel et al., 1974). Several wrenching shear zones have been identified along the northern shore of the Lago Fagnano and in some sectors of the steep cliffs on the Atlantic coast, which partially reactivate the thrusts of the Magallanes belt and where the most active deformation occurs. The field evidence of this activity is testified by brittle deformations, and in some cases by composite fluid inclusions in the deforming sedimentary successions. This tectonic activity also involves the Quaternary glacio-lacustrine sediments outcropping in the surrounding of the Lago Fagnano. A series of distinctive morphological trends seen on the SPOT and Digital Elevation Model (DEM) maps, and broadly trending ESE-WNW, align with the geological and structural patterns found in the field. A major lineament, associated with a narrow depression, traverses the central-eastern Tierra del Fuego from the eastern shore of the Lago Fagnano to the Atlantic coast. It is constituted by at least two segments in an en-echelon arrangement. The bathymetric map of the eastern part of the Lago Fagnano delineates the main submerged morphological expressions of the plate boundary in this sector of the Tierra del Fuego Island. The basin profile presents an highly asymmetric shape, with the steepest slope paralleling the northern shore, and reaching a maximum depth of 201 m. This trend follows the ESE-WNW narrow depression located onshore to the east, with a releasing sidestep located in correspondence of the eastern shore of Lago Fagnano. The seismic lines acquired by the TESAC project off the Atlantic coast of the Tierra del Fuego Island, and those gathered during precedent R/V OGS-Explora Antarctic Campaigns in the central and western Magallanes Strait, have imaged the cross-section geometry of the MFS. An asymmetric basin, bounded by a near-vertical discontinuity on one side (the transform segment), and a set of subsidiary normal faults on the other side, is mostly present in all profiles. The width of the basin is about 8 km, comparable to the depression occupied by the Lago Fagnano. The deepest sedimentary cover generally presents a fan-shaped geometry, and the thickness variations, and consequently slight changes of dip, indicate continued deposition during basin subsidence. This complex sedimentary architecture may reflect different tectonic mechanisms, in which periods of oblique extension can alternate with transform-normal extension. In this case, basin subsidence occurs because of extension in a direction normal to the regional strike of the transform at the same time that strike-slip motion is taking place (Ben Avraham and Zoback, 1992). Published kinematic analyses of fault populations near Monte Hope and the eastern end of Seno Almirantazgo (both in the Chilean side of the Island), indicate that these fault zones have accommodated left-lateral strike-slip motion with a probable component of normal slip on vertical - sub-vertical faults during Cenozoic time (Klepeis and Austin, 1997). Field work conducted in the Beagle Channel region (Cunningham, 1993) has also provided evidence for widespread ductile strike-slip faulting, which may have been however present since Cretaceous time. 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 (Fig. 1). Both onshore and offshore data 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 (i.e., the contractional system of the Magallanes fold and thrust belt), even if the displacement history of this fault system is still poorly known. The near parallelism among the younger and older lineaments suggests that the development of the transtensional structures may have reactivated pre-existing weakened zones formed by the Cretaceous-Tertiary shortening. The low seismicity recorded in correspondence of the transform fault 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 DGPS stations located in both the South America and Scotia sides of the fault system in the Tierra del Fuego (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.