The Manantiales Basin, Southern Central Andes (~32°S), 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°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 Río 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. The Andes of South America are one of Earth's largest mountain belts. The weight of these mountains flexes the South American continent downward, creating a depression where sediment accumulates, known as a foreland basin. In one area ∼140 km northwest of Mendoza, AR, rocks consisting of sediments that accumulated during an important phase of mountain building are visible at the surface. We studied these rocks to learn how the mountain belt developed. We found that the oldest sediments accumulated relatively far from the mountain front, which was then confined to Chile. Younger sediments were deposited mainly by small north‐ and south‐flowing river systems, salt flats, and lakes. However, the regional foreland basin instead contained mainly large, east‐flowing rivers at this time. This indicates that, rather than accumulating in the open area in front of the Andes, these sediments probably “ponded” behind topography that grew east of the basin. In turn, even younger deposits accumulated during uplift on the western basin margin. Combined with information from similar age rocks in the region, our results suggest that this segment of the Andes was created by progressive eastward growth interrupted by phases during which faults were activated “out of order.”.