“Thick - Skinned” Triangle Zone of the Precordillera Thrust Belt, Argentina

Most of the eastern boundary of the Albreta portion of the Canadian Rocky Mountains is formed by a triangle zone (Gordy and Frey, 1977). In this "classical triangle zone", the deformation is characterized by two thin-skinned thrust systems with opposing vergence (hinterland and foreland directed). We introduce here a different type of triangle zone which we call a "thick-skinned triangle zone" (Zapata and Allmendinger, 1993). In this case, one of the deformation systems is thick-skinned and the other thin-skinned. We analyze the structural evolution of this type of triangle zone in time and space, using as an example the Precordillera thrust belt of western Argentina. The thick-skinned triangle zone occurs where the Precordillera impinges on its foreland basin (known as Bermejo basin) at about 30° S latitude (Fig. 1). The surface geometry shows two thrust systems of opposing vergence: to the west are the thin-skinned thrusts of the Central Precordillera, and to the east occur the thick-skinned system of the Eastern Precordillera. The two systems are separated by a narrow valley (Matagusanos and Huaco valley). The easternmost thrust of the Central Precordillera is the Niquivil thrust, which is, along with the rest of the Central Precordillera thrusts, detached from a Cambro-Ordovician limestone (Los Azules Fm., San Juan Fm.). Along strike, the exposed geometry of the Niquivil thrust front changes from a faultbounded homoclinal section which dips between 30° and 45° to the west to an asymmetric eastverging fault-propagation fold known as Agua Hedionda anticline (Fig. 1). The Eastern Precordillera is composed of three asymmetric, west-verging anticlines; from west to east they are Niquivil, Las Salinas, and Bermejo. The latter is not exposed, but its geometry is well imaged on the seismic lines (Allmendinger et al., 1990). Evidence for thick-skinned deformation associated with this system comes from deep earthquakes (Smalley et. al, 1993) as well as from seismic interpretation (Allmendinger et. al, 1990). We defined the following segments of the triangle zone along strike (Fig. 1):