The overall tectonic setting of the southernmost Chilean margin is driven by relative movements between three main plates: the Antarctic (AN), South America (SA) and Scotia (SC) plates. The AN-SC slip vector is oriented roughly E-NE and the plate boundary is approximately located along the Chile trench, while the triple junction (TJ) between the plates is interpreted to be a diffuse and unstable area ([Forsyth(1975)], [Cunningham (1993)]) located at the intersection of the Chile Trench and the seaward projection of the Magellan Fault around 51-52S (Fig. 1).
South of the TJ, the history of Late Cenozoic convergence is essentially divided in two phases. The first one took place between Nazca and South America plates, at a rate of about 90 mm/yr ([Cande and Leslie (1986)], [DeMets et al.(1990)]) and ending with the collision between the Chile-Ridge and the Chile-Trench that occurred between 10 and 14 Ma [Cande and Leslie (1986)]. The subsequent phase, from Late Miocene to present, is characterised by plate subduction between Antarctic and Scotia plates, that resumed after the ridge consumption at a considerably slower rate of about 10-20 mm/yr [DeMets et al.(1990)]. This sharp drop in convergence rate resulted from the northward migration of the Chile ridge along the margin to the site of the modern TJ (Fig. 1).
The consumption of a mid oceanic ridge below a subduction trench is a catastrophic event that should have disrupted and eroded a huge amount of material of the overriding plate, as reported by many authors in the present day TJ area [Cande and Leslie (1986)]. The present day accretionary wedge is interpreted to be post late Miocene in age [Polonia et al.(1999)] built up when subduction resumed after ridge consumption. The Paleozoic and early Mesozoic record of plate convergence is confined within the backstop. On the basis of indirect geophysical observations [Polonia et al.(1997)], the backstop has been interpreted to be constituted partly by the Jurassic Patagonian Batholith, and partly by the Paleo-Mesozoic accretionary wedge that constitute part of the continental basement of southern South America ([Forsyth(1982)], [Grunow eta al.(1992)]).
The 15-20 Ma old oceanic crust (anomalies 6-6a, see [Cande and Leslie (1986)]) of the Antarctic plate is being subducted at a variable angle from normal at 50-52S to highly oblique at 57S. This variation in subduction obliquity is related mainly to variation in the direction of the continental margin which shows a 90°bend in the Southernmost Andes between 50S and 56S. This sharp bend has been interpreted on the basis of paleomagnetic and structural data [Cunningham (1993)] as a product of tectonic rotation that affected this region since Mesozoic times.
At about 57S, the Chile Trench passes into the Shackleton Fracture Zone (SFZ). Little is known about the interaction between these two structures; [Herron et al.(1977)] assumed that the SFZ splays at its northern end into a series of northwest stepping faults that follow the continental margin. A gradual change from orthogonal subduction to transpressive and mainly transcurrent motions is likely too.
figure
Seismicity along the margin is low. The slow convergence rate across the Antarctic/South America or Antarctic/Scotia boundaries is probably the main reason for the observed low seismicity of the western margin of South America south of 46S [Herron et al.(1977)]. However, shortening and active accretion occur at the toe of the accretionary wedge where a thick section of sediment resting on the Antarctic plate approach the outer deformation front ([Polonia et al.(1997)], [Polonia et al.(1999)], [Rubio ate al.(2000)]). The earthquake that occurred near the southern tip of Tierra del Fuego in 1987, shows a thrust fault solution at a shallow depth (11 km), which indicates active underthrusting along a shallowly dipping nodal plain [Pelayo and Wiens(1989)].
Oblique convergence between plates, transcurrent motion, and tectonic rotation, make the geodynamic setting more complex than that of the central Andes. Contraction and uplift in the trench occurs at the same time that rifting is active on land. The Magellan strait, for instance, forms part of a Neogene rift system perpendicular to the orogen [Diraison et al.(1997)] and all major depressions onland are interpreted as rifts or half-graben developed during Neogene and still active today.
