PARTITIONING OF THE ANDEAN FORELAND BASIN DURING EASTWARD ADVANCE OF THRUSTING: CIANZO BASIN, PUNA-EASTERN CORDILLERA BOUNDARY, NORTHERN ARGENTINA

The fault-bounded Cianzo basin within the central Andean fold-thrust belt of northern Argentina (23-24°S) represents a Cenozoic depocenter between the Puna plateau and Eastern Cordillera. Analyses of fold-thrust cutoff relationships, nonmarine deposition, and sediment provenance help constrain the origin, interconnectedness, and subsequent uplift and exhumation of the Cianzo basin, a potential analogue for foreland basins in convergent retroarc systems that have been partitioned into intermontane hinterland basins. Structural mapping reveals a plunging syncline within the >6000-m-thick, upward coarsening Cenozoic clastic succession in the shared footwall of the north-striking, east-directed Cianzo thrust fault and transverse, northeast-striking Hornocal fault. Growth strata within upper Miocene levels indicate syncontractional sedimentation adjacent to the Hornocal fault.

Facies analyses and measured stratigraphic sections show upsection changes from (a) paleosol-rich, distal-fluvial sandstones (~400 m Paleocene-Eocene Santa Bárbara Subgroup) to (b) braided fluvial sandstones and mudstones (~1400 m upper Eocene–Oligocene Casa Grande Formation) to (c) distributary fluvial megafan sandstones and conglomerates (~3300 m upper Oligocene–Miocene Río Grande Formation) to (d) alluvial-fan conglomerates (~1600 m upper Miocene Pisungo Formation).

⁴⁰Ar/³⁹Ar geochronological results for interbedded tuffs indicate deposition of the Río Grande Formation from ~17 to 9 Ma. Sandstone petrographic results show distinct upsection trends in lithic and feldspar content, potentially distinguishing sources in the Western Cordillera magmatic arc from those in the Puna-Eastern Cordillera fold-thrust belt. In addition to growth stratal relationships and isotopic ages, conglomerate clast compositions reflect distinct lithologic variations constraining activation of the Cianzo thrust and concurrent reverse slip on the reactivated (inverted) Hornocal fault. Finally, detrital zircon U-Pb ages, paleocurrents, and facies patterns distinguish local from distal sources, revealing a systematic eastward advance of Eocene through upper Miocene fold-thrust deformation as the Andean foreland basin was partitioned into multiple intermontane hinterland basins.