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

Abstract

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...

Continental Transform Basins: Why Are They Asymmetric?

“…Yet many of the active basins dotting Turkey's North Anatolian fault (NAF; see Figure S1 in the electronic supplement to this Eos issue (http:// www .agu .org/ eos _ elec/)) are highly asym metric and thus appear to grow only in one direction along the fault. Asymmetric and unidirectional transform basins have also been described in Tierra del Fuego [Lodolo et al, 2003] and in central California [Wakabayashi, 2007].…”

Constraints on deformation of the Southern Andes since the Cretaceous from anisotropy of magnetic susceptibility

“…While it is commonly accepted that sinistral strike-slip faulting, mainly associated with the South America-Antarctica relative plate motion, affected the Fuegian Andes in the late Cenozoic (Lodolo et al, 2003;Rossello, 2005;Ghiglione and Ramos, 2005;Menichetti et al, 2008), the origin of dextral Miocene (16-10 Ma) strike-sip faults in the southern Patagonian Andes is less certain, and possibly associated to the Phoenix-Antarctica ridge subduction event (Thomson, 2002).…”

“…The Punta Gruesa strike-slip event defines the initial tectonic phase associated to the development of the MagallanesFagnano Fault Zone and the active strike-slip deformation (Cunningham, 1993(Cunningham, , 1995Lodolo et al, 2003;Eagles et al, 2005;Rossello, 2005). Then, subhorizontal units above lower Miocene beds mark the end of compressional tectonics in the southern Magallanes basin (Ghiglione, 2002;Ghiglione et al, 2010).…”

Fabrica Magnetica y Microestructuras a traves de los Andes de Tierra del Fuego, Argentina.

“…During the Cenozoic especially, a left-lateral wrench deformation affected the southern sector. The structures associated are mainly transtensional with extensional faults and the development of pull-apart basins along the main wrench faults (Lodolo et al, 2003;Menichetti et al, 2008). One of the main lineaments associated with this tectonic regime is the WNW-ESE 100 km long Valle Carbajal located in the central part of the Fuegian Andes (Cenni et al, 2006) that split the Andean chain in two sectors, i.e., the Sierra Alvear in the north and the Sierra Sorondo in the south (Fig.…”