The Manantiales Basin, Southern Central Andes (32S), Preserves a Record of Late Eocene-Miocene Episodic Growth of an East-Vergent Orogenic Wedge

The Manantiales basin contains >4 km of nonmarine sedimentary strata that accumulated at 31.75-32.5(degrees)S during construction of the High Andes. We report field and analytical data from the underexplored northern portion of this basin. The basin contains upper Eocene-middle Miocene strata that accumulated in back-bulge or distal foredeep through inner-wedge-top depozones of the Andean foreland basin as it migrated through this region. A revised accumulation history for the basin-filling Rio de los Patos and Chinches Formations supports a regional pattern of flexure in front of an east-vergent orogenic wedge. The former formation consists of eolian and localized fluviolacustrine deposits which accumulated between ca. 38 Ma and <= 34 Ma during thrust belt development in Chile. A subsequent <= 12 Myr hiatus may reflect passage of the flexural forebulge or cessation of subsidence during orogenic quiescence. The overlying Chinches Formation records a transition from the foredeep to wedge-top depozones. Foredeep deposits of east-flowing, meandering streams were incised prior to ca. 18 Ma, after which deposits of axial rivers, playas, and perennial lakes ponded in a depression behind orogenic topography to the east. After ca. 15 Ma, alluvial-fan deposits were syndepositionally deformed adjacent to growing thrust-belt structures along the western basin margin. Although the basin record supports a westward step in the locus of deformation during Early-Middle Miocene time, it conflicts with models involving west-vergence of the orogenic wedge. Rather, this pattern can be explained as out-of-sequence deformation alternating with wedge forward propagation, consistent with Coulomb wedge models incorporating syntectonic sedimentation.