Magallanes-Fagnano continental transform fault (Tierra del Fuego, southernmost South America)



[1] Multichannel seismic reflection profiles, gravity measurements, and bathymetric soundings, in conjunction with field geological reconnaissance and remote sensing images, reveal with unprecedented detail the morphostructure of a major segment of the South America–Scotia plate boundary in the Tierra del Fuego region. This segment, known as the Magallanes-Fagnano fault system, is a continental transform margin arranged in an en echelon geometry, along which prominent asymmetric basins were developed. Data acquired off the Atlantic coast of Isla Grande (the main island of Tierra del Fuego), in its central and eastern part, and in the central and western Magallanes Strait image the surface and subsurface structure of the transform fault and its associated basins. The Magallanes-Fagnano fault system is composed of distinct tectonic lineaments that are segments of the transform system and are represented by mostly near-vertical faults. In the Atlantic sector, the fault system trends broadly N70°E and seems to be composed by a single master fault, along which a highly asymmetric basin has formed. At around 63°W, the fault terminates by splaying into secondary normal faults that dissipate the horizontal displacement along the system. In the central eastern part of Isla Grande, the fault segments have been principally identified from analyses of remote sensing images on the basis of their morphological expression. These segments are located within river valleys and are generally associated with localized gravity minima. Lago Fagnano, a 105-km-long, E-W trending depression, is a large, mostly asymmetric pull-apart basin developed within the principal displacement zone of the Magallanes-Fagnano fault system. Restraining bends and overlapping step-over geometry characterize the central part of the Magallanes Strait. Along the western part of the fault system, in the vicinity of the Pacific entrance of the Magallanes Strait, asymmetric sedimentary basins have also developed. The sedimentary architecture of the basins formed within the principal displacement zone of the fault, in which the thick end of the depositional wedge abuts the transform segment, suggest simultaneous strike-slip motion and transform-normal extension, a common feature found in other continental transtensional environments. Strike-slip faulting in the Tierra del Fuego region is also documented along other prominent lineaments which parallel the Magallanes-Fagnano fault system. Along at least two of these lineaments, characterized by a remarkable morphological expression, widespread Quaternary activity occurs. The present-day motion between the South America and Scotia plates is slow (<5 mm/yr). Also the modern seismicity monitored in the Tierra del Fuego region is low (individual events <3.5 in magnitude). The low seismicity may be explained by the slow relative motion between plates and may be further affected by slip partitioning along the different segments which make up the Magallanes-Fagnano fault array, and along the subsidiary wrench lineaments that traverse the region.