ANDEAN TECTONICS, FORELAND BASIN EVOLUTION, DRAINAGE REORGANIZATION, AND STRATIGRAPHIC HIATUSES (Invited Presentation)

The subduction-related tectonic history of the Andes is closely aligned with the evolution of its retroarc sedimentary basins. Ian W.D. Dalziel has appreciated the integration of structural and stratigraphic datasets, which likely reflects the influence of Robert H. Dott Jr. on Ian’s research in the southern Andes and Antarctica, and Ian’s experience with the utility (and pitfalls) of sediment provenance data in global plate reconstructions. Here several themes are highlighted in the context of Andean retroarc stratigraphic and structural records.

(1) The mid to Late Cretaceous onset of Andean shortening, as recognized by Dalziel (Dalziel et al., 1974, 1975, 2013; Dalziel, 1981, 1986), is well expressed in detrital zircon U-Pb geochronological records of Patagonia in southern Chile and Argentina. A continental-scale reorganization of sediment routing and river drainage patterns (“reversal in sedimentary polarity”) reflects the transition from an extensional or neutral tectonic regime to regional Andean shortening and uplift.

(2) This reconstruction is reinforced by the history of foreland basin sedimentation along much of the Andean margin, with rapid accumulation governed by flexural subsidence during Late Cretaceous-Cenozoic shortening and crustal thickening. However, phases of accelerated and decelerated foreland accommodation suggest complex time-space variations that belie most models of Andean-type mountain building.

(3) Fluctuations among extensional, neutral, and contractional conditions in retroarc regions remain underappreciated. Such alternating tectonic regimes may help resolve several anomalous features in the Andes, including evidence for diminished foreland subsidence or isostatic rebound, reduction of topography in the midst of the Andean orogeny, and along-strike variations in orogenic architecture and topography.

(4) Inversion of basins formed during separate phases of both Mesozoic and Cenozoic extension influenced orogenic architecture and induced complex recycling of basin fill.

(5) Stratigraphic hiatuses, in the form of regional disconformities with little to no angular discordance, present a unique challenge in that they commonly span 10-20 Myr. Potential formation mechanisms include: basement-involved foreland uplift, forebulge migration, eustatic sea level drop, regional isostatic uplift driven by slab window formation during ridge collision, and foreland abandonment and sediment bypass during tectonic quiescence.